Triple Junction Amorphous Silicon based Flexible Photovoltaic Submodules on Polyimide Substrates

Transcription

1 A Dissertation entitled Triple Junction Amorphous Silicon based Flexible Photovoltaic Submodules on Polyimide Substrates by Aarohi Vijh As partial fulfillment of the requirements for the Doctor of Philosophy Degree in Engineering Advisor: Dr. Xunming Deng Graduate School The University of Toledo July 2005

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3 An Abstract of Triple Junction Amorphous Silicon based Flexible Photovoltaic Submodules on Polyimide Substrates Aarohi Vijh Submitted in partial fulfillment of the requirements for the Doctor of Philosophy Degree in Engineering The University of Toledo July 2005 This dissertation provides the first detailed description of the fabrication of flexible, monolithically interconnected photovoltaic sub-modules based on triple-junction amorphous silicon (a-si) cell technology. There are several problems encountered when progressing from small area (0.25 cm 2 ) solar cells to series interconnected modules that are three orders of magnitude larger in area. This work involved the development of techniques required to overcome some of those problems, and the application of these in order to successfully fabricate triple-junction a-si based solar sub-modules 10cm 30cm (4 12 ) in size. The solar cells were fabricated on Kapton-VN and Upilex-S polyimide films. Polyimide films were chosen because polyimides have the highest thermal and dimensional stability of any polymer commercially available in film form. Chromium or molybdenum tie-coat layers were introduced between the polyimide and back-reflector films in order to improve film adhesion, which is otherwise unsatisfactory. An improved electrochemical shunt passivation process (light assisted shunt passivation) was developed ii

4 and used to passivate shunts in the cells. Shunt passivation is especially important for larger area solar cells. A combination of laser (dry) and wet chemical processing was used for the series interconnection. A laser-weld based interconnection scheme was chosen, for its compatibility with the shunt passivation process. The cells were encapsulated with the terpolymer THV (St. Gobain) and Tefzel (Dupont), using a vacuum laminator that was specially built for this purpose. To summarize, triple-junction-amorphous-silicon-based, series interconnected photovoltaic submodules of size 10 cm 30 cm were fabricated on lightweight and flexible substrates. The modules have an aperture area of approximately 200 cm 2. An initial AM1.5 efficiency of 4.75% (5.34% in natural sunlight) was obtained for a module with an aperture area of 204 cm 2. The specific power of this module is approximately 40 W/kg. iii

5 Acknowledgments I would like to express my gratitude towards my advisor, Prof. Xunming Deng. This work would not have been possible without the constant guidance, motivation and support that I received from him. I would like to thank Xiesen Yang for fabricating the back-reflector layers for all cells made during this research. I also thank Wenhui Du for his assistance with the deposition of the amorphous silicon layers. Special thanks are due to Prof. Alvin Compaan of the Department of Physics and Astronomy for help with this research and for his suggestions for improvement of this dissertation. I would also like to thank the other members of the UT photovoltaic research groups for discussions and help. The funding provided by the Air Force Research Laboratory, Kirtland AFB and the National Renewable Energy Laboratory is acknowledged. Last, but certainly not least, I thank my wife, Uma, for her love and support. iv

6 Contents Abstract ii Acknowledgments iv Contents v List of Figures ix List of Tables xii 1 Introduction Flexible Solar Cells Amorphous Silicon Solar Cells Previous Work Related work at the University of Toledo Layout Substrate Material Selection and Back-Reflector Film Adhesion Substrate Material Selection Film adhesion issues v

7 2.2.1 Upilex-S Kapton VN Tie-coats Gouldflex Conclusion Shunt Passivation Introduction Light-assisted shunt passivation for a-si cells with ITO top contacts Apparatus Shunt passivation process Comparison of shunt passivation with and without illumination Best parameters for light-assisted shunt passivation Conclusion Series Interconnection Interconnection schemes Laser Scribing Back-reflector scribing Amorphous silicon scribing ITO Scribing Alternative methods of ITO isolation Mechanical scribing Thermal decomposition of ITO vi

8 4.3.3 Electrochemical method Chemical etching Choice of interconnection scheme Cell length Encapsulation Introduction Vacuum Laminator Alternatives to EVA Sylgard THV Interconnected Cell Assembly and Module Fabrication Back Reflector Fabrication Triple-junction Amorphous Silicon Cell Deposition ITO Isolation and Shunt Passivation Laser Welding and Ink Application Lamination Measurement Performance Conclusions Factors limiting module performance Improving Specific Power vii

9 7.3 Future Work Performance, Yield and Processing Improvements Testing List of Publications 76 References 77 viii

10 List of Figures 1-1 Structure of a triple junction a-si solar cell Outgassing from polyimides when heated in vacuum GD Back reflector on Upilex-S with a-si triple cell on bottom half, showing peeling Sample# GD Triple junction a-si cells on Mo-coated Kapton VN Effect of chromium tie-coat on Al back reflector adhesion Poor adhesion of Ag/ZnO to Cr-coated Upilex-S Effect of Ni, Cr and Mo tie-coats on Ag back reflector adhesion Single junction a-si cells on Gouldflex I-V characteristics of sample# GD1128, with efficiency over 9.8% Apparatus used for light-assisted shunt passivation Results of shunt passivation on several 0.25 cm 2 cells I-V characteristics of a 0.25 cm 2 cell before and after light-assis ted shunt passivation ix

11 3-4 Comparison of shunt passivation performance with and without simultaneous illumination Effect of light during shunt passivation on quantum efficiency, and a photograph of passivation marks Average open circuit voltages before and after shunt passivation for various passivation conditions Three scribe scheme for series interconnection Fuji s SCAF interconnect scheme Sanyo s weld interconnect scheme ITFT s laser-weld based interconnection scheme Schematic of setup used for laser scribing Profile of a scribe through a single amorphous silicon n-i-p stack Isolation lines produced in a film of ITO on glass by decomposition of the ITO Isolation line produced electrochemically in an ITO top contact. Width is approximately 0.35 mm Amorphous silicon-germanium cell with classic three-scribe interconnect, after shunt passivation Interconnection scheme selected for the 4 12 modules IV characteristics of an interconnected cell made by laser welding Use of conductive paint to bridge resistive ITO over the back-reflector scribe x

12 4-13 IV characteristics of an interconnected cell made by laser welding, with conductive paint used to bridge resistive part of ITO Vacuum laminator Cure percentages for quick-cure EVA, as a function of time and temperature Lamination at 160 C and 145 C, showing trapped bubbles and no trapped bubbles respectively Transmission of tested encapsulants Patterning used for the interconnected cells. All dimensions in mm I-V characteristics of samples used in assembly of Module # J-V and P-V characteristics of Module # Comparison of I-V characteristics of samples used in assembly of Module # Photograph of Module # I-V and P-V characteristics of Module # External quantum efficiencies of triple junction cells on Kapton with 70nm ITO and 140 nm ITO xi

13 List of Tables 2.1 Physical properties of substrate and back reflector materials Series of light-assisted shunt passivation tests Laser scribing parameters Sputter Parameters Performance of Module # Performance of Module # Masses of various components of the photovoltaic sub-module xii

14 Chapter 1 Introduction 1.1 Flexible Solar Cells Solar cell research for terrestrial power generation applications is focused on developing production technology for large volume production at minimum cost. Such an approach is necessary if photovoltaics is to compete economically with conventional energy sources. Efficiency, weight and other performance factors therefore take a position secondary to cost per watt. Most photovoltaic panels manufactured today tend to be fabricated with heavy materials such as glass and are mounted in rigid frames. On the other hand, the area of space photovoltaics has traditionally been focused on performance with little or no regard to cost, and has preferred efficient single crystal based solar cells over less efficient ones made from amorphous or polycrystalline thin films. This is because it is important for spacecraft to be as light as possible to reduce launch costs. However, launch prices and budgets have been decreasing and 1

15 2 spacecraft power requirements have been increasing. Due to this fact, thin film cells may become competitive on the basis of their low cost and high power to weight ratio, even though they have lower efficiencies in absolute terms[redd03]. Thin film solar cells can be fabricated on materials such as plastic films or steel foil. Such solar cells can be lightweight as well as flexible. This second property would be invaluable in space applications, as it would allow solar arrays to be stowed in a minimum of volume. Since efficiencies of thin film solar cells have increased, thin film based photovoltaics may now be able to compete in the space market in terms of power-to-weight ratio and cost. Some of the material systems used for thin film solar cells are amorphous silicon, cadmium telluride and copper-indium-gallium-diselenide (CIGS). Other applications for flexible and lightweight solar arrays include portable power, and perhaps even solar cells integrated in clothing and other consumer products requiring flexibility. There has been much attention in recent times to the development of lightweight, flexible thin film solar cells based on amorphous silicon, cadmium telluride and CIGS materials[ichi01, Tiwa01, Kess04]. Flexible film cells promise high power/weight ratios and low stowage volume requirements, making them suitable for space and portable terrestrial applications. Amorphous silicon cells have also been shown to be resistant to degradation from radiation in the space environment[kuen00, Kaga00]. Further, a-si cells can be made at lower temperatures as compared to other thin film technologies and contain no toxic materials.

16 3 1.2 Amorphous Silicon Solar Cells Most solar cells manufactured today are made from crystalline silicon wafers, which have to be pulled from molten silicon. This makes them expensive to manufacture in terms of both energy and money. On the other hand, thin films of amorphous silicon can be deposited by plasma enhanced chemical vapor deposition (PECVD) at low temperatures, and these films can be used to make solar cells. PECVD can be used to deposit amorphous silicon over large areas (of the order of square meters). It would be difficult or impossible to make single crystals of that size. In crystalline silicon, each Si atom is bonded to four others, i.e. it has four-fold co-ordination, and the structure is periodic. Amorphous silicon (a-si), however, is disordered. Although the majority of Si atoms in a-si are four-fold coordinated, due to topological constraints a small fraction of Si atoms (<1%) does not have four other Si atoms to bond to, resulting in some unbonded, or dangling, Si bonds. These dangling bonds can trap photogenerated carriers, making the material not useful for photovoltaic applications. However, if hydrogen is introduced into the system during a-si deposition, the hydrogen atoms could bond to Si atoms which do not have fourfold co-ordination, thereby passivating the dangling bonds. Therefore, hydrogen is almost always added during a-si growth. In this document, a-si means a-si:h, or hydrogenated amorphous silicon. Apart from the fact that it can be deposited by PECVD, a-si also has the advantage that it is a non-direct band gap semiconductor [Coll05], unlike crystalline silicon. As a result, thinner layers of amorphous silicon can absorb a given amount of light,

17 4 Figure 1-1: Structure of a triple junction a-si solar cell [Deng02] as compared to crystalline silicon. The thinner the solar cell, the more flexible it will be. The following is a description of how a triple-junction amorphous silicon solar cell (see Fig. 1-1) can be made. First, a layer of aluminum is deposited on the substrate. This is followed by a layer of a transparent conductor such as zinc oxide. Together, these layers form the back reflector. The purpose of the back reflector is to reflect unabsorbed light back through the semiconductor layers. Together with total internal reflection from the ITO layer (further in this description), this reflection from the BR allows us to use thinner amorphous silicon layers. The purpose of the zinc oxide is to prevent diffusion of aluminum into the a-si layers. The ZnO can also provide some texture to the BR, enhancing total internal reflection at the ITO interface. The back reflector layers are deposited by RF sputtering.

18 5 After the back reflector, the amorphous silicon layers are deposited by plasma enhanced chemical vapor deposition (PECVD). There are nine major layers deposited in an n-i-p-n-i-p-n-i-p sequence (n=phosphorus doped, n-type; i=intrinsic; p=boron doped, p-type). The silicon in the intrinsic layers of the bottom and middle cells is alloyed with germanium to change the bandgap. The bandgaps for the intrinsic layers of the bottom, middle and top cells are 1.45 ev, 1.65 ev and 1.85 ev respectively. This allows these layers to absorb different portions of the solar spectrum. Finally, another transparent conducting oxide (TCO) layer (indium tin oxide, or ITO) is deposited by sputtering on top of the amorphous silicon layers. The purpose of this layer is to collect the photocurrent while allowing light to pass through to the a-si layers. See [Wang02] for details of this type of solar cell. The advantages of a triple junction a-si cell over a single or tandem cell are known [Deng02]. These cells have higher open circuit voltages and lower short circuit currents. Therefore, fewer cells need be connected to obtain a given voltage, and currents are lower, allowing one to use thinner TCO layers or longer cells without introducing excessive series resistance. 1.3 Previous Work As mentioned earlier, there has been a good deal of interest in recent times in the area of flexible photovoltaics. Flexible solar cell and modules using a-si, CdTe, and CIGS have been reported by various groups. Nishiwaki et al. [Nish95], describe a process for fabricating single junction a-si

19 6 modules on polymer substrates, and a method of monolithic series interconnection. In another report, Ichikawa et al. [Ichi01], describe an interconnection scheme developed by them as well as production technology that they have used for fabricating flexible a-si tandem (double-junction) cells. The fabrication of single junction and tandem flexible interconnected a-si cells and also described in papers by Ikeo et al. [Ikeo96] and Jeffrey et al. [Jeff93], respectively. Described in a paper by Tiwari et al. [Tiwa01] is the fabrication of flexible cadmium-telluride solar cells on polyimide films. Flexible CIGS cells on polymer and metal foil substrates have also been made. [Kess04] lists flexible cells made from CIGS and other chalcopyrite materials by various laboratories, on thin metal and polyimide substrates. A reference to monolithically integrated triple-junction a-si modules on polymer has been made in [Guha98]. Also in [Ichi01] the authors announce their intention to apply their roll-to-roll polymer-based tandem cell technology to triple-junction cells. Nevertheless, a full description of the fabrication of triple-junction amorphous silicon-based, monolithically interconnected submodules has not been reported. This dissertation reports in detail the fabrication of flexible, monolithically interconnected triple-junction amorphous silicon based cells and submodules. Issues of yield, film adhesion and monolithic interconnection have been examined. An improved method of electrochemical shunt passivation was developed, to protect which a patent application has been filed. As a result of this work, a 10 cm 30 cm ( 4 12 )size flexible submodule with AM1.5 efficiency of 4.75% (5.34% in sunlight) for an aperture area of 204 cm 2 and 40 W/kg specific power has been successfully fab-

20 7 ricated. Polyimide films were used as the substrates and the module was encapsulated using a fluoropolymer based material Related work at the University of Toledo The University of Toledo has all facilities and equipment required for the fabrication of high-efficiency amorphous and microcrystalline silicon based solar cells. A triple-junction solar cell with an initial efficiency of 12.7% (0.25 cm 2 ) has been fabricated at UT [Wang02]. This is the highest reported efficiency for this type of cell made by any academic group in the world. Small area lightweight and flexible solar cells have previously been fabricated on metal foils at UT. These were single-junction a-si (1.85 ev bandgap) cells fabricated directly on stainless steel foils as thin as 7.5 µm. On 7.5 µm thick substrates, these cells showed efficiencies of 6.5% (0.25 cm 2 ) and power-to-weight ratios of 1.08 kw/kg in unencapsulated form [Deng00]. In addition to a-si based cells, flexible and lightweight cadmium telluride based solar cells are also being fabricated at UT. 1.4 Layout The remainder of this dissertation is organized as follows. Chapter 2 discusses the choice of substrate material used for module fabrication. Also described is the problem of back-reflector film adhesion and how it was solved by the application of tie-coats. Chapter 3 describes an improved process for the passivation of shunts in

21 8 amorphous silicon solar cells. Discussed in Chapter 4 are methods of implementing monolithic series interconnection and the choice of interconnection scheme for the 4 12 modules. Details on the materials and method used for encapsulating the interconnected cells are in Chapter 5. Chapter 6 presents the complete sequence used for the fabrication of two 4 12 monolithically interconnected triple-junction amorphous silicon based photovoltaic submodules. The characterization and performance of these submodules is also presented there. Chapter 7 concludes the dissertation with a discussion of the performance of the submodules and suggestions for future work.

22 Chapter 2 Substrate Material Selection and Back-Reflector Film Adhesion 2.1 Substrate Material Selection Every solar cell is fabricated on a substrate, which provides mechanical support to the conducting and semiconducting layers that make up the cell. Triple-junction amorphous silicon cells are made by a vacuum processes involving temperatures over 300 C. This means that the substrate material used must be able to withstand high temperatures, and produce low outgassing/contamination in a vacuum system. The substrate material used for the deposition of flexible amorphous silicon solar cells must be flexible be able to withstand temperatures of 300 C or more 9

23 10 have low outgassing at high temperatures and in vacuum be mechanically strong have good adhesion to the solar cell layers Finally, it is easier to implement monolithic series interconnection of cells if the substrate used is insulating. The requirements of flexibility and insulating behavior mean that a plastic of some kind must be used. There are relatively few plastic materials that satisfy the requirement of high temperature stability. Polyimides are one such class of plastics. Other possible candidates are polybenzimidazoles (PBI) and polyamideimides (PAI), but these are not yet commercially available in sheet form [Kess04]. Polyimides are manufactured by reacting aromatic diamines with a dianhydride of an aromatic carboxylic acid, and then heating the resulting polyimide acid to around 400 C. They are commercially available in sheet form from manufacturers such as Dupont (Kapton, Apical) and Ube Industries (Upilex). Polyimide acids can also be spin coated onto a rigid substrate, cured, to produce a thin substrate material which can be peeled off after the solar cell has been deposited [Tiwa01]. The glass transition temperature (T g ) of polyimides can exceed 500 C. Four different types of polyimide films (Kapton E, Kapton HN, Kapton VN and Upilex-S) were heated in vacuum to check for excessive outgassing. The samples were placed in a vacuum chamber and the chamber pumped down to its base pressure. The temperature of the sample was then ramped up slowly and the chamber pressure was noted at frequently. Fig. 2-1 shows the observed pressure rises for (a) 125 mm

24 11 Figure 2-1: Outgassing from polyimides when heated in vacuum. thick Kapton VN and (b) 125 mm thick Upilex-S. Triple junction amorphous silicon cells were fabricated on Kapton VN and Upilex-S for further study. Out of the four polyimides, Kapton-E was found to have a considerable amount of outgassing in the test. Kapton HN, VN and Upilex-S showed an acceptable amount of outgassing. 2.2 Film adhesion issues Of the three candidates, Kapton VN and Upilex-S were readily available in the desired thickness of 125 µm. For the next test, we fabricated back reflectors and triple junction amorphous silicon cells on these two substrate materials.

25 12 Figure 2-2: GD Back reflector on Upilex-S with a-si triple cell on bottom half, showing peeling Upilex-S An aluminum/zinc-oxide back-reflector was made by RF sputtering on 4 x4, 125 µm thick Upilex-S substrate. A triple junction amorphous silicon cell was deposited on half of this back reflector. The other half was covered with a standard stainlesssteel based back-reflector as a control. The portion of the Upilex-S sample with a-si on it showed severe cracking and delamination minutes after deposition (see Fig. 2-2). A few days after deposition, the portion without a-si also developed cracks Kapton VN An Al/ZnO back reflector was also prepared on 125 µm thick Kapton VN, and a triple junction amorphous silicon cell was deposited on this substrate, followed by RF-sputtered ITO ( 70 nm). This cell remained intact for over a week. However, it developed cracks when immersed in water for cleaning, probably due to thermal stress caused by abrupt change in temperature upon immersion. The sample had

26 13 been heated with an IR lamp for the purpose of etching a pattern in the ITO and was hot when immersed in the water. 2.3 Tie-coats Thus, a major issue encountered during fabrication of triple junction a-si solar cells on polyimide substrates is the adhesion of the thin films to the substrates. In both cases, the film delamination was observed to be at the back reflector/polyimide interface. A likely cause of the cracking/peeling is internal stress in the back reflector layers. One possible reason that Kapton performed better may be due to its lower bulk modulus (2.75 GPa) as compared to Upilex-S (8.25 GPa). A lower bulk modulus material may be able to better relieve stress through expansion. However, the issue of metal film adhesion to polyimides is a complex one, and there may be other important factors that determine adhesion [Brow00]. Material CTE(10 6 K 1 ) Bulk Modulus (GPa) Comment Kapton VN T g =500 C Upilex S T g =550 C Aluminum Molybdenum Chromium Table 2.1: Physical properties of substrate and back reflector materials. (From [Kess04]) Tie-coats: Although Kapton 500 VN showed promise as a substrate material for flexible a-si cells, Al/ZnO back reflector films showed imperfect adhesion to it. We therefore evaluated various intermediate layers (tie-coats) to see if adhesion could be improved.

27 14 Figure 2-3: Sample# GD Triple junction a-si cells on Mo-coated Kapton VN Certain transition metals are known to have excellent adhesion to glass and other substrate materials. For example, chromium and nickel alloys are used as tie coats to promote the adhesion of copper to polyimides [Berg01, Sall89]. Chromium, molybdenum and zinc oxide were evaluated as tie-coats with our standard Al/ZnO backreflectors. Chromium, molybdenum and nickel were evaluated as tie-coats for Ag/ZnO based back reflectors. The procedure for evaluating the tie-coats was identical in all cases. We sputtered the tie-coat material onto a 4 x4 square of Kapton VN, then deposited the metal (Al or Ag)/ZnO back reflector. A triple junction amorphous silicon solar cell was then deposited by PECVD on this coated substrate. The triple-junction a-si cell with the chromium tie coat and Al/ZnO back reflector showed no signs of peeling or cracking after several immersions in water, and after several weeks, indicating that the Cr tie-coat did in fact promote adhesion. A similar effect was observed with tie coats of molybdenum (DC sputtered) or zinc oxide (RF sputtered) - there was no cracking or peeling observed after thermal cycling between room temperature and 80 C or immersion of a hot sample in water. See fig To

28 15 Figure 2-4: Effect of chromium tie-coat on Al back reflector adhesion. verify that it was indeed the tie-coat layer that improved adhesion, we performed an experiment in which only a portion of a 4 x4 Kapton substrate had a tie coat. A mask was used to cover one-third of the substrate and a layer of chromium was sputtered onto the exposed two-thirds. Then, aluminum was sputtered through another mask such that the aluminum and chromium overlapped vertically in the center of the substrate. Two different thicknesses of zinc-oxide were then sputtered onto the top and bottom halves of the substrate to complete the back reflector. A triple junction a-si cell was then fabricated on this using the standard process. After the cell was fabricated, the sample was immersed in cold water, upon which the portions without the chromium tie-coat developed cracks. It was also noted that amorphous silicon showed poor adhesion to chromium alone. There was no difference between the halves with 160 nm and 500 nm ZnO layers from the point of view of film adhesion, at least in this particular case. See Fig. 2-4 for a photograph of the sample. Owing to its higher conductivity and reflectance, silver can be a superior choice

29 16 Figure 2-5: Poor adhesion of Ag/ZnO to Cr-coated Upilex-S. of back-reflector material as compared to aluminum. However, it was found that like aluminum, silver too showed poor adhesion to Kapton VN or Upilex-S. Therefore, we also evaluated the adhesion promoting properties of chromium, molybdenum and nickel on a silver-based back reflector made on Kapton VN. It was found that chromium did not work well as a tie-coat for a silver based back reflector (see Fig. 2-5). For a more systematic study, half of a 4 4 sample was sputter-coated with nickel, and the remaining two quadrants were coated with chromium and molybdenum. An Ag/ZnO back reflector was made by RF sputtering and a triple-junction a-si cell was deposited by PECVD. The portions on Cr and Mo showed severe de-lamination of the back-reflector and a-si layers almost immediately after a-si deposition. In contrast, the portion with the nickel tie-coat remained relatively stable even upon immersion in cold water. see Fig. 2-6 for a photograph of the sample. Zinc-oxide will also be evaluated as a tie-coat for Ag-based back-reflectors in the future.

30 17 Figure 2-6: Effect of Ni (top half), Cr (bottom left) and Mo (bottom right) tie-coats on Ag back reflector adhesion Gouldflex We also evaluated copper-coated Kapton material (Gouldflex) manufactured by Gould Electronics (Eastlake, OH). Since this material already has a tie-coat as well as a conductive layer on a polyimide substrate, it had the potential to eliminate one step in our fabrication process. A single n-i-p amorphous silicon solar cell was fabricated on this material, after verifying that the outgassing from the material was not excessive. The film we tested consisted of a Kapton base with a sputtered chromium tie coat and electrodeposited copper layers. Since copper is known to be a fast diffuser in silicon, three quarters of the copper were coated with aluminum or/and zinc oxide before amorphous silicon deposition. Thus we had four combinations (Cu, Cu/Al, Cu/ZnO and Cu/Al/ZnO). The a-si film on bare Cu peeled off at the Cu/a-Si interface (see Fig. 2-7), and cells on the other three quarters of this sample showed extremely poor performance. It is likely that the copper diffused into a-si, causing shunting and

31 18 Figure 2-7: Single junction a-si cells on Gouldflex defects. Therefore, this material was eliminated as a candidate for the flexible cell substrate. 2.4 Conclusion It was found that Kapton VN and Upilex-S are suitable substrate materials for the fabrication of amorphous silicon solar cells. However, adhesion of back reflector and a-si layers to these is marginal unless an intermediate tie-coat layer is employed. We found that tie coats made from chromium, molybdenum and zinc oxide do promote adhesion of aluminum/zinc-oxide back-reflector layers to the Kapton substrate. Triple junction amorphous silicon cells were fabricated on these tie-coated substrates and were found to have good adhesion and conversion efficiencies. See Fig. 2-8 for IV characteristics of a cell from sample GD1128, which was made with a molybdenum tie-coat. We also found that a nickel tie coat performs better than one made from

32 19 Figure 2-8: I-V characteristics of sample# GD1128, with efficiency over 9.8%. chromium or molybdenum, if a silver reflective layer is used instead of aluminum. Amorphous silicon cells were also fabricated on commercially available copper-coated polyimide, but these were found to have poor performance, possibly due to diffusion of copper into the cells. Thus, chromium, nickel, molybdenum and zinc oxide were identified as suitable tie coat materials for improving the adhesion of back-reflector layers to polyimides. Chromium or molybdenum tie coats were employed in all samples that were subsequently fabricated. It should be noted that while the tie-coats improved adhesion to a satisfactory level over a temperature range of approximately 10 C to 80 C, no tests have been performed to evaluate adhesion outside this range. Satisfactory performance outside this range will be required for space solar cells and also for terrestrial solar cells during winter.

33 Chapter 3 Shunt Passivation 3.1 Introduction Photovoltaic devices of amorphous and microcrystalline silicon and germanium alloys are usually manufactured using glow discharge processes. Large area photovoltaic devices manufactured in this manner tend to contain current shunting defects that reduce the yield and performance of the devices. As the area of a solar cell increases, so does the probability of the presence of a current-shunting defect somewhere in the cell. In fact, as described by Tyan and Perez-Albuerne [Tyan84], the probability of a given region being unshunted decreases exponentially with its area. Shunting defects can occur due to flaws in the substrate, during solar cell deposition (e.g. due to dust in the system) or during handling of the solar cell at any point before it is encapsulated [Benn93]. Since even a single current shunting defect can detract from the performance of a large solar cell, it is essential to have a process to passivate, or neutralize the effect of, these shunts. 20

34 21 Various methods of reducing the effect of shunts have been devised, which work by disconnecting electrically the shunted regions from the rest of cell. This disconnection may be achieved by removing the electrode material surrounding the shunt or by converting it to an insulator by thermal decomposition or an electrochemical reaction. For example, thermal imaging of the surface of the solar cells may be used to reveal the location of shunts, since shunted areas tend to be hotter than the rest of the cell if current is forced through the cell as described by Breitenstein et al. [Brei99]. To isolate the shunts, electrode material can be etched away from the shunted areas thus identified. Another methods is to immerse the solar cell in an electrolyte and apply a voltage to it. Since current flow in shunted regions is greater, the electrode in those regions can be selectively removed, or transformed into an insulator by an electrochemical reaction. Electrochemical methods of shunt passivation are superior in that they do not require prior knowledge of the location of shunts. One such electrochemical method is described in a paper by Nath et al. [Nath88]. Regardless of the mechanism, a successful shunt passivation method must passivate a broad range of shunts, have a large process window, and must not affect unshunted regions. During the course of this research, a improved method of electrochemical shunt passivation for a-si photovoltaics was developed, which involves the simultaneous application of light and electrical bias to the solar cell, leading to a high degree of selectivity between shunted and unshunted regions.

35 Light-assisted shunt passivation for a-si cells with ITO top contacts A typical substrate-configuration triple junction a-si solar cell consists of a back electrode, a back reflector (silver/zinc oxide), a-si layers in a nipnipnip sequence, and a transparent and conductive front contact, usually ITO, to make contact to the top p-layer. A shunt is any direct path from the back contact to the front contact, through the semiconductor layers. The shunt can be passivated if the ITO front contact immediately surrounding the shunt is converted into a material of higher resistivity as described in the paper by Nath et al. [Nath88]. If the surface of the ITO is held at a negative potential of approximately 1 volt or more with reference to the aluminum chloride electrolyte, an electrochemical reaction occurs that converts the ITO into an insulator. Negative potentials of a smaller magnitude do not favor this reaction, nor do positive potentials. Due to this requirement of polarity, it turns out that the pin stack must be under forward bias during passivation. This can lead to current flow and conversion of ITO in unshunted regions, reducing the selectivity of the passivation process as described in certain U.S. patents [Swar83, Izu85, Kawa93]. However, if the sample is illuminated during passivation, the unshunted areas produce a photovoltage that actively opposes the applied electrical bias, thus preventing the loss of unshunted regions.

36 23 Figure 3-1: Apparatus used for light-assisted shunt passivation Apparatus The apparatus used for light-assisted shunt passivation is shown in Fig The shunted cell is immersed in a solution of aluminum chloride and illuminated through an aluminum mesh-electrode. A power supply is connected between the back electrode of the a-si cell and the aluminum mesh-electrode. The cell is then illuminated with the tungsten halogen lamp. Due to the illumination, the unshunted areas of the cell produce a photovoltage of approximately 2.2 V (for a triple junction a-si solar cell). i.e. the transparent conducting electrode near unshunted areas of the cell will be 2.2 volts positive with respect to the stainless steel back electrode. Depending on the severity of the shunts, the shunted areas either do not produce this photovoltage; or if they do, it is of a much smaller magnitude than 2.2 volts.

37 Shunt passivation process To perform light-assisted shunt passivation, a positive electrical bias of approximately 2 volts is applied to the counter electrode (aluminum mesh through which light penetrates), i.e. the stainless steel back contact of the cell is held negative with respect to the counter electrode by 2 volts. The polarity of the photovoltage produced by the unshunted portions of the cell is such as to oppose the electrical bias. For this reason, the voltage present at the ITO in those portions will be small, zero, or positive with respect to the electrolyte. Hence there is a negligibly slow or no reaction in those portions of the ITO. On the other hand, the shunted portions of the cell produce a smaller or no photovoltage. As a result, a large portion of the applied electrical bias voltage (2 V) appears across the ITO/electrolyte interface, and the polarity is such that the ITO becomes negative with respect to the electrolyte. Thus the passivation reaction proceeds at a high rate in the shunted regions. Once the ITO surrounding a shunted region is converted to an insulator, the shunt is effectively disconnected from the rest of the cell and the reaction in that area stops. In the absence of illumination, even the unshunted portions of the cell are under a small forward bias, there being little or no photovoltage to oppose the electrical bias. The applied bias voltage can lead to a small but possibly significant current flow in these regions. This current makes the ITO negative with respect to the electrolyte, and may be sufficient to cause conversion of the ITO even in the unshunted regions. This problem is made worse by the fact that thin film solar cells typically possess shunts with different levels of severity, ranging from weak shunts to dead shorts.

38 25 Hence, in the absence of illumination the optimal voltage for passivating shunts of a certain severity may be too high for very weak shunts or unshunted regions (causing unwanted conversion of ITO), or too low to passivate more severe shunts (leaving these unpassivated). In contrast, the process window for shunt passivation can be made wider in the presence of illumination by exploiting the fact that the expected open circuit voltage (V OC ) for a given type of cell does not change very much between production runs if deposition conditions are accurately controlled. Setting the operating point for the shunt passivation process to the expected V OC or slightly higher allows self-limiting passivation of almost all shunts while minimizing unwanted conversion of ITO. Figure 3-2 shows the extent of improvement in efficiency in 17, 0.25 cm 2 cells. All but one of the cells recovered to normal levels. Figure 3-3 shows the IV-characteristics before and after light-assisted shunt passivation of a 0.25 cm 2 triple-junction solar cell. In this case, a voltage bias of 2.2 V had been applied for 5 seconds. The halogen lamp illumination was approximately 1-sun and the electrolyte conductivity was 40 ms. 3.3 Comparison of shunt passivation with and without illumination Figure 3-4 shows the relative improvement in efficiency and open circuit voltage under room light at different applied biases for shunt passivation (a) with and (b) without illumination. The samples used were triple junction amorphous silicon solar

39 26 Figure 3-2: Results of shunt passivation on several 0.25 cm 2 cells. Figure 3-3: I-V characteristics of a 0.25 cm 2 cell before (left) and after (right) lightassisted shunt passivation.

40 27 Figure 3-4: Comparison of shunt passivation performance with (light) and without (dark) illumination. The samples used were triple junction a-si cells with areas of approximately 0.2 cm 2. cells, as fabricated. No shunting was intentionally induced. Each point represents an average of the results from three samples passivated at the same applied bias. Shunt passivation was carried out with applied biases of 1.2 to 2.6 volts. The graphs show that the improvement in open circuit voltage and efficiency are greater when the passivation reaction is carried out in the presence of illumination, at least at applied biases of 2 volts. At the same applied voltage (2 V), fill factors and room light open circuit voltages also showed greater improvement in the presence of illumination than in the absence of it. It may be noted that shunt passivation in the absence of illumination led to a negative improvement (i.e. worsening of the cell) in some cases. This is most likely due to unwanted conversion of ITO in good cells. To further compare the light-assisted shunt passivation process with the nonlight assisted type,two sets of triple-junction samples were shunt-passivated, using conditions that were determined to be optimal for each process. The cells produce an open circuit voltage of 2.15 V under 1-sun AM1.5 illumination if unshunted. The open circuit voltages of the cells were measured with AM1.5 illumination of two

41 28 (a) (b) Figure 3-5: The graph in (a) compares the open circuit voltages (relative to the maximum of 2.15 V) and external quantum efficiencies of triple-junction a-si cells which have been subjected to shunt passivation with and without light bias and 2 V and 3 V respectively (average values of 10 samples). Shunt passivation with light bias leads to both a high open circuit voltage as well as a higher quantum efficiency in the cells. (b) is a photograph showing the passivation marks for sample passivated in the absence (top) and presence (bottom) of light. In the presence of light, the area affected by shunt passivation is smaller, but passivation is still complete. different intensities before and after passivation. This illumination for the purpose of measurement is not to be confused with the bias illumination used during shunt passivation. The external quantum efficiencies at 400 nm of the cells in the region surrounding the shunt were also measured. Fig. 3-5(a) shows these results and Fig. 3-5(b) is a photograph of the shunted regions. The use of a light bias reduces the area that is affected by the shunt passivation process, leading to a higher quantum efficiency.

42 Best parameters for light-assisted shunt passivation A series of tests was also carried out in order to determine the best parameters for light-assisted shunt passivation of triple-junction a-si solar cells. The parameters that were varied were: bias voltage, electrolyte concentration, passivation time and intensity of the simultaneous illumination (See table 3.1). The samples used in the experiments were triple-junction a-si/a-sige/a-sige cells with ITO top contacts. They produced an open circuit voltage of 2.15 V under 1-sun AM1.5 illumination, when unshunted. Shunting was intentionally induced in these samples with two focused laser pulses from a Nd:YAG laser (532 nm), one strong and one weak. The pulses were applied a few millimeters apart. Sets of samples were passivated at each condition. A tungsten-halogen lamp was used for illumination. The open circuit voltages of the samples were measured before and after shunt passivation under AM1.5 illumination of two intensities: 2.5% of 1 sun and 20% of 1 sun. Neutral density filters were used to attenuate the light from the AM1.5 simulator. High open circuit voltage of a solar cell under such reduced light indicates the absence of shunts. Series Voltage Conductivity Time Intensity Bias Voltage Variable 43 ms/cm 10 s 1 sun Concentration 2 V Variable 10 s 1 sun Passivation Time 2 V 43 ms/cm Variable 1 sun Bias Light Intensity 2 V 43 ms/cm 10 s Variable Table 3.1: Series of light-assisted shunt passivation tests done in order to determine parameters for effective passivation of shunts in a-si solar cells. Results are shown in Fig. 3-6 Figure 3-6 shows the results of these tests. The bars represent average open circuit

43 30 voltages of the cells, relative to the maximum of 2.15 V, expressed as a percentage (2.15 V was the V OC of unshunted cells under 1-sun AM1.5 illumination). Relative open circuit voltages measured under 2.5% of 1 sun and 20% of 1 sun light intensities are graphed, before and after shunt passivation. Fig. 3-6(a) shows the effect of bias (a) (b) (c) (d) Figure 3-6: Results of the series of tests listed in Table 3.1. Shown are average open circuit voltages measured in reduced light of 2.5% of 1-sun and 20% of 1-sun intensity, before and after light assisted shunt passivation. One of four parameters - bias voltage, concentration, passivation time and bias light intensity - was varied, while the other three were kept constant. A fresh set of artificially shunted samples was used each time. voltage on the completeness of the passivation process. A broad range of voltages from 1.5 to 2.5 V was found to be acceptable. In general, however, higher voltages

44 31 may be expected to produce more complete passivation, other factors remaining the same. Fig. 3-6(b) shows the effect of electrolyte concentration (conductivity) on the effectiveness of the passivation process. Higher electrolyte concentrations (i.e. higher conductivities) produce more complete passivation than lower concentrations. Fig. 3-6(c) shows the effect of passivation time on the effectiveness of the shunt passivation process. From the figure, it is apparent that passivation times of as little as one second may be sufficient to produce a high degree of shunt passivation. The passivation reaction is self-limiting, so the use of times longer than necessary do not cause unwanted effects. From Fig. 3-6(d) it is clear that the shunt passivation is more complete in the case of bias illumination of 0.1 sun than with bias illumination of 1 sun, when the applied bias voltage is 2 V. High photovoltage in very weakly shunted areas may reduce the applied bias of 2 V in those areas to an extent that it is insufficient for full passivation. It can be seen in Fig. 3-6(a), however, that a voltage of 2.5 volts can produce complete passivation at a 1 sun illumination and Fig. 3-5(b) shows that the area affected is small even with a bias of 3 V. 3.5 Conclusion In conclusion, it was found that this type of shunt passivation reaction is more selective when the cell is illuminated, because the unshunted portions of the cell produce a voltage that actively opposes the one required for the passivation to take

45 32 place. This increases the acceptable range of the electrical bias for effective shunt passivation, allowing the process to passivate shunts with different levels of severity without causing unwanted conversion of ITO into a more electrically resistive material in the unshunted areas, maximizing yield and performance. Although we studied light assisted shunt passivation of triple junction amorphous silicon cells with ITO front contacts, the process may prove useful for single-junction solar cells as well as for cell made from other materials, as the principles will remain the same. Since the yield of solar cells fabricated on polyimides is invariably lower than on traditional substrates such as glass or steel, shunt passivation is even more important in this case. The light assisted shunt passivation process was used to passivate shunts in all the samples described in this document.

46 Chapter 4 Series Interconnection Solar cells are low voltage, high current devices. For example, a triple-junction amorphous silicon solar cell might produce an open circuit voltage (V OC ) of 2.3 V but a short circuit current of 8 ma/cm 2. That is, a single 1 m 2 cell would produce 80 A at short circuit. Transmission of such high currents results in considerable I 2 R losses unless conductors are made very thick. Hence, solar cells need to be connected in series to produce higher voltages and lower currents for a given power output. This reduces transmission losses and also increases the voltage to a more useable level. 4.1 Interconnection schemes Broadly, there are two approaches to cell interconnection: monolithic and external. In the external method, individual cells are fabricated and then connected in series and/or parallel using conducting wires. This approach requires greater postdeposition ( back-end ) processing as compared to the monolithic approach, wherein 33

47 34 Figure 4-1: Three scribe scheme for series interconnection (adapted from [Comp00]) the series connections are formed during the solar cell deposition sequence, not after it is complete. The back-end is the most labor-intensive part in most solar-cell manufacturing processes; any simplification here leads to a significant increase in productivity and decrease in costs. There are many schemes that can be employed for monolithic series interconnection, four of which are shown diagrammatically in Figs. 4-1 (Classic three-scribe), 4-2 (Fuji SCAF), 4-3 (Sanyo weld) and 4-4 (ITFT weld). Fig. 4-1 shows the classic three laser-scribe interconnect scheme for series interconnection, as might be applied to a-si solar cells on a plastic substrate. This scribing scheme is the simplest of all. The first scribe isolates strips of the back reflector (back contact). In order to achieve this, the buffer and metal layers must be completely removed without damage to the underlying substrate material. Amorphous silicon is deposited after this first scribing step. Next, a second scribe is made to open a via in the amorphous silicon layers - all the amorphous silicon layers must be removed without damage to the back reflector layers. After this step, the transparent conducting front contact (e.g.

48 35 Figure 4-2: Fuji s SCAF interconnect scheme [Ichi01] Figure 4-3: Sanyo s weld interconnect scheme [Nish95]

49 36 Figure 4-4: ITFT s laser-weld based interconnection scheme [Grim96]. ITO) is deposited. This material covers the entire front surface of the solar cell and penetrates the vias in the a-si layers to make electrical contact with the back reflector. A third scribe is then made in the ITO layer to complete the interconnected cell. This third scribe must remove the ITO layer without damage to the a-si layer. There are other possible interconnection schemes. Fig. 4-2 shows the scheme preferred by Fuji Electric for their tandem a-si cell line. The scheme uses sputtered metal contacts on both sides of a perforated plastic film. During sputtering, both sides get electrically connected through the perforations. Laser scribing is performed after deposition of all films to finish the interconnection. Details are given in [Ichi01]. Fig. 4-3 and Fig. 4-4 show the interconnection schemes used by Sanyo Electric and Iowa Thin Film Technologies (ITFT) respectively. They are similar - both use a line of insulating ink that acts as a laser beam stop. This prevents shunting of the cells during laser scribing of the ITO top contact (details on this shunting problem

50 37 in section 4.2.3). Regardless of the scheme used, the spacing between scribes needs to be kept to a minimum because this area of the solar cell does not contribute to electrical output - it is dead area. The smaller this area can be made, the higher the overall efficiency of the solar module will be. Due to the desirability of minimum dead area, laser scribing is considered the technique of choice. Scribe widths and spacings of the order of tens of microns can be attained with a laser. Further, laser scribing is fast, dry, and inexpensive, which makes it well suited to large-scale manufacture. 4.2 Laser Scribing Parameters were obtained for the laser scribing of two of the three basic layers of the flexible a-si solar cell: back reflector, amorphous silicon. Attempts were also made to laser scribe the ITO (TCO) layer. The laser used for the scribing was a Molectron MY32-10 flashlamp-pumped Nd:YAG laser with a pulse width of 15 ns and a maximum pulse repetition frequency of 20 Hz. The laser has outputs at 1064 nm (fundamental), 532 nm (frequency doubled) and 355 nm (frequency tripled). The 532 nm output was used for all three scribes, although the 1064 nm and 355 nm outputs were also tested for ITO scribing. The laser beam was directed through attenuating filters to allow finer control of the pulse energy density, especially at low energies. The attenuated beam was focused on to an X-Y stage, on which the sample was mounted. A cylindrical lens (f=8 cm) was used for scribing the back-reflector and a-si layers, while best results for the ITO scribe were obtained with a spherical

51 38 Figure 4-5: Schematic of setup used for laser scribing. lens (f=10 cm). Fig. 4-5 shows a diagram of the laser scribing setup Back-reflector scribing The back reflector is composed of one or more metallic layers covered with a layer of zinc oxide. The objective of laser scribing the back reflector is to create separated back contacts for the interconnected solar cell. All back reflector layers must be completely removed from the scribed region, while keeping the underlying polyimide intact. The scribing of the back reflector using the frequency-doubled output of a Molectron Nd:YAG laser (532 nm wavelength) was optimized. This laser has a pulse width of ns. Pulse energies of the order of 8 J/cm 2 were required to completely remove the back reflector layers composed of 200 nm of Al and 500 nm of ZnO, using a pulse repetition frequency (p.r.f.) of 10 Hz and a scan speed 2 mm/s. A cylindrical lens of focal length 15 cm was used.

52 39 Figure 4-6: Profile of a scribe through a single amorphous silicon n-i-p stack Amorphous silicon scribing The second set of layers to be scribed are the amorphous silicon layers. There are nine major a-si layers in a triple cell (nipnipnip). The objective of the second scribe is to completely remove the amorphous silicon layers without removing the back reflector layers. This step opens a via in the a-si layers. It was found that a pulse energy of 0.7 J/cm 2 is sufficient to open a via in the amorphous silicon nipnipnip structure (prf 10 Hz and scan speed 3 mm/s), using a cylindrical lens of focal length 15 cm. Since the silicon layers strongly absorb visible light, only a small pulse energy is required to scribe them, as described by Compaan et al. [Comp00]. Fig. 4-6 shows the profile of a scribe through a single amorphous silicon n-i-p stack. The scribe width is less than 50 microns.

53 ITO Scribing The third laser scribing step must remove the transparent conducting oxide layer without significant damage to the amorphous silicon layers underneath. Since the transparent conducting oxide absorbs visible wavelengths poorly, it has a high scribing threshold at 532 nm as described by Compaan et al. [Comp00]. Instead, the laser energy is absorbed by the amorphous silicon layers, leading to shunting of the cell. It should be possible, in theory, to scribe the ITO with a low pulse energy that leads to explosive ablation of a small portion of the a-si. However, highly precise focusing and low pulse to pulse laser energy variations will be required. Laser scribing of the ITO layer was attempted using 1064 nm, 532 nm and 355 nm wavelengths from a Nd:YAG laser. Best results were obtained with a wavelength of 532 nm, p.r.f. of 20 Hz, spherical focusing, scan speed 0.5 mm/s and an energy density on the order of 1 J/cm 2, using spherical focusing. With these parameters it was possible to produce isolation in the ITO top contact of a steel-substrate triple-junction a-si cell, with only mild shunting. However, it was not possible to achieve this with polyimidebased samples. This problem has also been noted by other research groups working in the flexible solar cell area. The schemes shown in Fig. 4-3 and 4-4 get around this problem by printing a layer of insulating ink under the line to be scribed. The ink acts as a beam stop, preventing shunting of the top contact to the back-reflector. However, there is an additional step required for printing the beam-stop line, and the ink needs to be vacuum compatible.

54 41 Table 4.1 lists parameters suitable for scribing the back-reflector, a-si and ITO layers. The parameters for the ITO layers are the ones for which best results were obtained on a stainless-steel substrate solar cell. It should be noted that the pulse energy densities are estimates based on the spot size and should be considered correct only to the order of magnitude. Layer Thickness Energy Density p.r.f. Scan Speed (nm) (J/cm 2 ) (Hz) (mm/s) Mo/Al/ZnO 500/200/ a-si ITO * Table 4.1: Laser scribing parameters 4.3 Alternative methods of ITO isolation As described in the preceding section, it is difficult to produce laser isolation scribes in ITO without causing shunting of the solar cell. Since ITO isolation lines are required by all the schemes discussed above, it was necessary to explore other methods of producing the ITO isolation scribe. From a brief survey of the literature, the following possible processes may be used to produce isolation lines in ITO (other than laser scribing) Mechanical scribing Mechanical scribing with a sharp tool to remove the ITO layer has reportedly been used with some success with other thin film technologies. It was found not to work well by us, perhaps because the amorphous silicon layers are too thin ( 500

55 42 Figure 4-7: Isolation lines produced in a film of ITO on glass. The wider line is produced with tip positive while the narrower line is produced with tip negative. Applied voltage was 4 volts in both cases. In both cases, resistance of the ITO across the scribe increased from < 100Ω to > 4 MΩ. nm). The scribing process tended to cause shunting Thermal decomposition of ITO When a large current is forced through a pin in contact with ITO, the ITO surrounding the point of contact is decomposed and becomes resistive [Hohn00]. This was tested and found to work well with ITO films on glass (see Fig 4-7), but did not work well with ITO contacts on solar cells, flexible or rigid. The intense local heat generated tends to weld the tip to the back reflector, producing a large shunt.

56 Electrochemical method As described in the chapter on shunt passivation, ITO can be transformed into an insulator electrochemically for the purpose of shunt passivation. The same reaction can be used to produce isolation lines in an ITO film [Ichi99]. The ITO is immersed in an electrolyte and a metallic pattern in placed parallel to the surface, close to it. An electrical bias is then applied between the metal pattern and the ITO. The ITO in the regions where metal is present is transformed into an insulator. This process was tried here, but it was found that the resolution is poor and the width of the line produced is difficult to control. The technique was modified to use a fiber-tip pen instead of the metallic pattern. The ITO surface is not immersed in electrolyte. Instead, the pen is filled with the electrolyte and an electrode is embedded in the tip. This pen is then used to draw isolation lines on the ITO surface. The resolution in this case is determined by the tip size and it is possible to produce isolation lines <0.5 mm in width using this technique. Fig. 4-8 shows a photograph of one such line produced with an AlCl 3 filled pen. A voltage of 3 V was applied between the pen-electrode and the ITO surface. A pen-plotter type of arrangement can be used to produce isolation lines in large cells at reasonable speeds. The plotter can easily be reprogrammed to produce a different pattern Chemical etching ITO can be etched with phosphoric acid at about 70 C, or with a solution consisting of 20% HCl and 5% HNO 3. Starch may be added to make a paste. This paste

57 44 Figure 4-8: Isolation line produced electrochemically in an ITO top contact. Width is approximately 0.35 mm. may then be applied in the desired areas by screen printing. Disadvantages of the screen printing method are relatively poor resolution and low flexibility - a new screen must be made every time the pattern is modified. Industrial ink-jet printers are now available that can deposit almost any solution or suspension onto a substrate with high speed, accuracy and resolution and with the minimum of waste. It should be possible to use an ink-jet printer to deposit either insulating ink or etchant solution to the solar cell. The printer can simply be reprogrammed if a new pattern is needed. Photolithography can also be used to define the pattern to be etched. Photolithography is a relatively expensive technique and can involve additional problems of waste disposal, but it can be used to produce high-quality etch lines in the ITO. For ITO removal, the use of a mask for exposure is not necessary - the photoresist may be directly exposed with a blue or UV laser scanned across the surface of the sample

58 45 (see section 6.3 for details). Since the spacing between ITO etch lines is typically 5 mm or more, photoresist need only be applied to the areas where a scribe is required, and the etchant applied within the resist area. Further, since the resolution required is low (100 microns is acceptable), it is possible to apply the photoresist with a brush or spray. Finally, the photoresist can be of a commercial grade, e.g. the types used in the printing and PCB industries. All the samples ultimately used for the 4 x12 submodule assemblies used photolithographic removal of ITO. Details are provided in section Choice of interconnection scheme The classic three-scribe interconnect is largely incompatible with the electrochemical shunt passivation process described in chapter 4. In that scheme, a via is formed in the amorphous silicon prior to ITO deposition. This via is an intentional low resistance path between the ITO and the back contact of the next cell in the series. However, to the shunt passivation process, this path appears as a strong shunt, and the ITO in the via region is converted to an insulator. This causes the series interconnection to be broken. This was noted in sample GD shown in Fig This sample had a single a-sige cell with a thick i-layer (1.6 ev bandgap). The total voltage produced by the sample under 1-sun illumination was only 0.3V, even though each segment produced a voltage close to 0.8 V. The short circuit current was only a few microamperes, indicating that the ITO in the via regions has been converted to an insulator. A possible workaround to this problem is to immerse only the active

59 46 Figure 4-9: Amorphous silicon-germanium cell with classic three-scribe interconnect, after shunt passivation. portion of the ITO front contact in the electrolyte during shunt passivation. This requires precise positioning of the cell and is therefore complicated. In view of these problems, it was decided to use a scheme in which the interconnect via or connection is fabricated last, after the ITO scribes have been produced and shunts passivated. The scheme shown in Fig.4-4 was chosen, with the difference that the ITO was chemically etched. The insulating ink/laser scribe method was not chosen due to concern that the ink would contaminate the vacuum chamber used for the subsequent step. Besides, any printing step used for applying insulating ink can also be used to print etchant instead. First, the back-reflector is scribed to divide it into cells of equal size as shown in Fig Next, the amorphous silicon and ITO layers are deposited by PECVD and sputtering. Isolation lines are then etched chemically in the ITO parallel to those in the back-reflector. At this stage, the cell may be shunt-passivated. After shunt passivation, laser pulses at relatively high power are applied in the region between the back-reflector and ITO scribes. This melts and recrystallizes the silicon and also

60 47 Figure 4-10: Interconnection scheme selected for the 4 12 modules melts the back-reflector, creating a low resistance path between the front and back contacts in this region, and thus connects the back contact of one cell to the front contact of the next. See Fig for a schematic diagram of the interconnection scheme. A Cr/Al/ZnO back-reflector was fabricated and scribed into six segments of 14 mm length each. A triple-junction interconnected cell was fabricated on this back-reflector. The ITO was etched and shunts passivated. Focused pulses from a 532 nm Nd:YAG laser ( 4 J/cm 2 ) were used to create weld spots for interconnection. A low resistance path if formed between the front and back contacts of the cell, most likely due to melting and recrystallization of the a-si layers caused by the high energy density of the laser pulses. The I-V characteristics of the sample were then measured and are shown in Fig From this IV curve, it is clear that the series resistance is excessive. It was subsequently found that the region of amorphous silicon above the back reflector scribe was severely shunted. This shunting is caused because the back-reflector scribe is not smooth and has jagged edges. The jagged edges remained even after the sample was cleaned after scribing with compressed nitrogen or with water in an ultrasonic bath. During shunt passivation, the ITO above this line is

61 48 Figure 4-11: IV characteristics of an interconnected cell made by laser welding. transformed into an insulator. The resistance of the interconnect can be reduced by applying a bridge of conducting ink as shown in Fig To test this method, another sample was in a similar manner, but a line of conductive paint was applied to bridge the resistive portion of the ITO. The IV characteristics of one half of this sample are shown in Fig Though this sample was somewhat shunted and did not perform well, series resistance was certainly reduced by the application of the conductive paint. The fill factor was approximately 35%. 4.5 Cell length Current generated by the cell must be collected by the ITO contact and must flow in the plane of the ITO layer until it reaches the laser welded via and flows to the next cell. Once there, it must flow through the back contact of that cell. To keep power loss

62 49 Figure 4-12: Use of conductive paint to bridge resistive ITO over the back-reflector scribe. Figure 4-13: IV characteristics of an interconnected cell made by laser welding, with conductive paint used to bridge resistive part of ITO. Series resistance is lower and fill factor is better than the sample in Fig. 4-11

63 50 to a minimum, the series resistance along this path must be minimized. The dominant component of this resistance is the resistance of the ITO film. The resistance of the ITO film can be reduced either by using a thicker layer of ITO, or reducing the length along which the current flows, or both. For maximum cell performance, the ITO coating must be of λ/4 thickness at the peak wavelength of the solar radiation. This reduces reflection at and near that wavelength, leading for maximum absorption of solar energy. This thickness turns out to be 70 nm and the sheet resistance of this thickness of ITO is 80Ω per square. A one square centimeter triple junction a-si cell is expected to produce approximately 8 ma. Since this current is distributed uniformly over the 1 cm 2 area, one can assume that half of this current passes through the sheet resistance. 4 ma through 80 Ω would give a voltage drop of 0.32 V, which is rather high when compared to the open circuit voltage of 2.2 V (about 15%). If the cell length is reduced to lower the resistance, dead area as a percentage of total area increases, because the dead area in the interconnect region is unchanged. Another option is to increase the thickness of the ITO in order to lower sheet resistance. The disadvantages of this approach are increased absorption in the ITO and destruction of the anti-reflection properties. However, since the interconnection process has not been optimized and the dead area is large (1.5 mm), it was chosen to use double the thickness of ITO (130 nm) and maintain a total cell length of 10 mm. Then the maximum anticipated loss is 0.16 V per cell. A 3 thickness of ITO would be expected to have the similar anti-reflection properties could be used, except that the shunt passivation process has not been optimized for thicker ITO films, and may not passivate shunts completely. Also, optical absorption would be even higher in the

64 51 thicker ITO. For further tests it was decided to use a thicker film of ITO as the top contact along with a cell length of 10 mm.

65 Chapter 5 Encapsulation 5.1 Introduction As the final assembly step, photovoltaic modules must be encapsulated in a transparent material to protect it from damage. The most commonly used encapsulant in ethyl-vinyl acetate (EVA) (see for example [Pern00]). The solar module to being encapsulated is placed between two sheets of un-cured EVA and laminated with heat and pressure in a vacuum laminator. EVA softens at about 60 C and starts to cure. Once cured, it forms a transparent layer that adheres well to the solar cells, protecting them from moisture, dust and other environmental factors. A glazing material can also be applied over the encapsulant to provide a smooth, hard surface finish. A layer of this material is placed over the encapsulant during lamination. The encapsulant bonds the glazing to the cell. An example glazing material is ethylenetetrafluoroethylene (ETFE), sold under brand names such as Tefzel (Dupont). ETFE softens around 150 C and at this temperature EVA can form a strong bond to it. 52

66 53 Figure 5-1: Vacuum laminator 5.2 Vacuum Laminator A vacuum laminator was designed and constructed for the encapsulation of the interconnected cells. The laminator consists of two rectangular shells (chambers) fabricated from stainless steel. The upper chamber is 6 x 14 x 1.5 while the lower one is 6 x 14 x 2.5 in size (W x L x D). A silicone rubber diaphragm is permanently attached to the open face of the upper chamber (see Fig. 5-1). Either chamber can be evacuated independently of the other. A mechanical pump is used for evacuation and a pressure of <1 torr can be attained. The lower chamber has a platform of adjustable height that is used to support the module being laminated. The platform is heated by a 300 W flexible heater in close contact with the platform. The lower chamber includes vacuum feedthroughs for a thermocouple and for heater power. A triac-based phase angle driver powers the heater. Temperature is set and monitored with an Omega CN9000 temperature controller and a type K thermocouple in contact with the platform top plate. The lamination sandwich is placed on the platform and the chamber closed by placing the top shell on it. Both chambers are evacuated

67 54 for minutes to remove moisture and air. This step helps reduce trapped bubbles in the laminated module. The upper chamber is then vented slowly to atmospheric pressure in order to apply pressure on the sandwich. The heater is turned on and the temperature held at 60 C for 5 minutes. At this temperature the EVA softens and flows, but does not cure. The temperature is then raised and held at 145 C for 15 minutes to cure the EVA (See Fig. 5-2 for EVA curing times). The sandwich is then allowed to cool to room temperature, the lower chamber vented and the laminated module removed. The edges are trimmed to remove excess Tefzel and EVA C was found to be the optimal temperature range for lamination. Higher temperatures cause degassing from one or more of the films (resulting in trapped bubbles), while curing takes too long at lower temperatures. See Fig. 5-3 for a photo. The EVA shows good adhesion to Kapton. Regular (LZ) Tefzel showed relatively poor adhesion to EVA at 145 C. However, good adhesion was obtained with Tefzel CLZ, which is supplied etched on one side for improved adhesion. EVA used was the 15420P/UF formulation from Specialized Technology Resources in an 18 mil (0.45 mm) thickness. 5.3 Alternatives to EVA A known problem with EVA is its tendency to turn yellow or brown when exposed to UV light, especially at elevated temperatures [Klem97]. This can be a greater issue for space photovoltaics, since the amount of UV outside the atmosphere is greater than on earth. Therefore, two alternative encapsulants were also tested: Sylgard 182 and

68 55 Figure 5-2: Cure percentages for quick-cure EVA, as a function of time and temperature. Figure 5-3: Lamination at 160 C (left) and 145 C(right), showing trapped bubbles and no trapped bubbles respectively.

69 56 Norton THV. Terrestrial PV manufacturers avoid these encapsulants because they are more expensive than EVA, but they may be affordable for space applications Sylgard 182 Sylgard 182 is a silicone elastomer, supplied in two parts that must be mixed in a 10:1 ratio. It is a clear liquid which cures to a flexible, optically clear, rubbery material of refractive index 1.4. The backbone of this material is a siloxane (-Si- O-Si-) chain. According to the manufacturer, Sylgard 182 can be heat cured in 45 minutes at 100 C, 20 minutes at 125 C, or in 10 minutes at 150 C. A test lamination was carried out using this encapsulant. A sheet of ETFE (Dupont Tefzel) slight larger than the dummy module being encapsulated was laid on the platform of the laminator. Sylgard 182 was mixed and poured onto the ETFE sheet. The dummy module was then laid on the ETFE and Sylgard 182 was poured onto it. A second piece of ETFE was then laid onto the module. Air bubbles were brushed out and the laminator closed. Lamination was performed under pressure at 145 C for 15 minutes. The laminated module showed very few trapped bubbles. However, ETFE/EVA adhesion was found to be better than ETFE/Sylgard 182 adhesion. The silicone elastomer is also available in a space qualified version (Dow Corning ) THV THV is a terpolymer composed of three different fluoropolymers: tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride. It is available commercially in film form

70 57 Figure 5-4: Transmission of tested encapsulants. and has a refractive index of Dummy modules and glass slides were laminated using 3-mil thick Saint Gobain Norton THV. 15 minutes at 145 C were found to be sufficient for satisfactory lamination. Adhesion-treated ETFE showed good adhesion to THV. Fluoropolymers are generally resistant to UV light, and THV is advertised as having high UV-resistance [DyneTHV]. Fig. 5-4 shows the optical transmission curves for STR 15420P/UF EVA, Sylgard 182 and Norton THV. ETFE/THV was used to encapsulate all modules during this research.

71 Chapter 6 Interconnected Cell Assembly and Module Fabrication This chapter describes the fabrication and testing of interconnected cells, assembly and lamination of 4 x 12 modules. 6.1 Back Reflector Fabrication Tie-coat and back-reflector sputtering: The back-reflectors used for the interconnected cells were made on Kapton VN or Upilex-S of 125 µm thickness, coated with molybdenum tie-coats. The back-reflectors themselves consisted of a layer of aluminum, followed by a layer of zinc-oxide. Parameters used for sputtering these layers are given in Table 6.1. The substrate to be coated was cleaned with Micro-90 detergent solution in an ultrasonic bath. It was then rinsed with deionized water and blow-dried with nitrogen. 58

72 59 Layer Temperature Pressure Ar Flow Power Time Thickness ( C) (mtorr) (sccm) (W) (min) (nm) Mo Al ZnO Table 6.1: Sputter Parameters The molybdenum tie-coats were fabricated by DC sputtering. The sample was heated in vacuum to a temperature of 150 C, then argon was flowed at 50 sccm. The pressure was maintained at 11 millitorrs. Sputtering was done with a power of 100 W DC for a period of six minutes. The sample was moved during the sputter process for better film uniformity. The sample was allowed to cool before unloading. The thickness of the Mo layer thus obtained was approximately 500 nm. RF sputtering (13.56 MHz) was used to sputter aluminum onto the Mo-coated sample. Power used was 100 W. The substrate temperature was maintained at 200 C. Argon flow was 30 sccm and pressure was 4 millitorrs. Total sputter time was 25 minutes. The thickness of the Al layer was 300 nm. After aluminum sputtering, the samples were transferred without vacuum break to the zinc-oxide sputter chamber. Zinc-oxide was RF sputtered at 250 C, 4 millitorrs pressure and 100 W RF power. Pressure was maintained at 4 millitorrs and Argon flow was 4 sccm. Sputtering was done for 24 minutes for a film thickness of 350 nm. Back-reflector laser scribe: The substrate with the back-reflector was then mounted on the X-Y stage of the laser scribing system. The 532 nm laser beam was focused onto the sample with a cylindrical lens. The back-reflector was then scribed into eight segments with a power density of approximately 8 J/cm 2, scan speed 1 mm/s and p.r.f. 10 Hz. One scribe was also made to divide the sample

73 60 into two halves as shown in Fig Then if one half of the sample were to suffer a catastrophic failure, the other half would still remain useable. After scribing, the sample was cleaned in an ultrasonic bath and dried. The resistance across scribes was measured to verify complete isolation. 6.2 Triple-junction Amorphous Silicon Cell Deposition The scribed substrate was loaded into the a-si PECVD system and the triplejunction a-si cell was fabricated. A mask was used to prevent a-si deposition on the edges of the sample. This was done to preserve an exposed back-contact area. The sample was then moved to the ITO sputter chamber without vacuum break. The parameters used for the ITO sputtering were: 4 sccm argon flow, 4 millitorrs pressure, 60 W RF power, 250 C temperature and 9 deposition time. The film thickness thus obtained was approximately 130 nm. 6.3 ITO Isolation and Shunt Passivation Shipley S1811 photoresist was applied to the sample with a soft brush. The sample was baked at 80 C for 5 minutes on a hot plate. The sample was then positioned on an X-Y stage which was scanned at 4 mm/s, while a diode-pumped, frequency tripled Nd:YVO 4 laser (Spectra-Physics J40-BL6-106Q), running at a pulse rate of 35 khz and power of 90 mw was used to expose the photoresist. A cylindrical lens was used

74 Figure 6-1: Patterning used for the interconnected cells. All dimensions in mm. 61

75 62 for focusing. The pattern is shown in Fig After exposure, the photoresist was developed for 60 seconds in MF-319 developer. Dilute sodium hydroxide solution can also be used as the developer. After rinsing with DI water, the sample was heated to 80 C on a hot plate and 40% phosphoric acid was applied to the surface. After 1 minute, the sample was rinsed in DI water to remove the acid and the photoresist was stripped off with acetone. The sample was rinsed in water and dried. This completed the ITO isolation step. At this point, there were 16 electrically separate cells on the sample. Light-assisted shunt passivation was then performed. Temporary electrical connections were made to the back contact of each of the 16 cells for the purpose of passivation. Electrical bias of 2.5V was applied for 10 s. A tungsten-halogen lamp was used for light bias. AlCl 3 solution of 40 ms/cm conductivity was used as the electrolyte. After shunt passivation, the open circuit voltages of each of the cells was measured in room light to verify that the cells were operational. 6.4 Laser Welding and Ink Application Once again, the sample was mounted on the X-Y stage. A spherical lens was used to focus the beam of the flashlamp-pumped Nd:YAG laser on to the space between the back-reflector scribes and the ITO etch lines. A power density of 8 J/cm 2, p.r.f. of 10 Hz and a scan rate of 10 mm/s was used to produce weld spots every millimeter. Next, a fine line of insulating ink was applied manually on top of the back-reflector scribe line. This was done to reduce the chance of shunting in this region. Finally, a

76 Illuminated J-V of samples used in Module #1 GD1282 GD1272 GD J (ma/cm 2 ) V (Volts) Figure 6-2: I-V characteristics of samples used in assembly of Module #1 line of graphite-based conductive paint (Electrodag 112) was applied manually over the insulating ink line to form a bridge, connecting electrically the ITO on both sides of the insulating ink line. The open-circuit voltage and short-circuit currents of both halves were then measured under 1-sun AM1.5 illumination. Two more samples were fabricated in the same manner. The I-V characteristics of the three samples are shown in Fig The three samples were then connected in series with strips of adhesive-backed copper foil. Silver-filled epoxy was applied to ensure good electrical contact between the back contact of the first segment in each sample and the copper tape. After verifying that the two halves of the module had the same open-circuit voltage, the two anodes and cathodes were connected together with copper wire, connecting the halves together in parallel. The halves could also have been connected in series instead of in parallel, to produce twice the open-circuit voltage and half the short-circuit current.

77 Lamination A 5 12 inch piece was cut from a sheet of 2-mil Dupont Tefzel CLZ. The piece was placed on the laminator platform with the side treated for adhesion face-up. A piece of 3-mil Saint Gobain Norton THV, slightly smaller than the Tefzel was placed on it. The assembled module was placed on the THV sheet. A sheet of THV was placed on the module and a sheet of Tefzel, treated side down, was placed on top. The laminator was closed and both chambers were evacuated for 20 minutes. A pressure of <1 torr was reached. The platform was then heated to 60 C and held at this temperature for 5 minutes to allow any water vapor to escape. At the end of this interval, the top chamber was vented slowly to apply pressure on the module. The temperature was then raised to 145 C and maintained at that point for 15 minutes. The heater was then turned off and the platform allowed to cool to room temperature. The bottom chamber was vented and the laminated module was taken out. 6.6 Measurement After lamination, the final I-V characteristics of the module were measured. The measurement was carried out in two ways. The I-V characteristics of individual cells were measured under an AM1.5 solar simulator. A composite I-V curve was then generated by adding the voltages produced by each cell at equal current points. This was necessary because a solar simulator capable of accommodating a 30 cm long sample was unavailable. The I-V and P-V characteristics are shown in Fig. 6-3 (labelled AM1.5 ). The I-V characteristics of the module were also measured in

78 J (ma/cm 2 ) AM1.5 J -4 AM1.5 P Sun J -5 Sun P Voltage (V) Figure 6-3: J-V and P-V characteristics of Module #1 P (mw) natural sunlight of intensity 0.92 suns as measured by a calibrated Si detector. The current has been normalized to a 1-sun intensity. These are also shown in Fig. 6-3 (labelled Sun ). Table 6.2 lists the performance parameters of the module under both illumination conditions. This module showed an AM1.5 efficiency of 3.9% (3.6% in sunlight) for an aperture area of 187 cm 2. The module showed a high series resistance (R S ) of 657 Ω and a low shunt resistance (R SH ) of 5.4 kω. This module was not really expected to perform well since only one of the component cells used has a good fill factor (see Fig Three more interconnected cells (GD1308, GD1347, GD1367) were fabricated, assembled into a module and laminated as described above. Figure 6-4 shows the current-voltage characteristics of the three samples used for the second module. Solar cells to be connected in series must have the same short circuit currents, else the performance of the module is limited by the component with the lowest short circuit

79 66 Parameter AM1.5 Sunlight Units Area cm 2 Voc V Jsc ma/cm 2 Isc ma Vm V Jm ma/cm 2 Im ma P mw Fill Factor % η % Table 6.2: Performance of Module #1 under AM1.5 standard illumination and in natural sunlight (intensity 0.92 suns, normalized to 1 sun) current. From Fig. 6-4, the sample GD1367 has a lower J SC than the other two samples by about 10%, which can be expected to cause a 10% reduction in the active area efficiency of the entire module. However, the average performance of these three samples is clearly better than of those used for the first module (Fig. 6-2). Most fabrication steps were done manually and the improvement in performance is purely due to more care in the handling of the samples. Handling is made difficult by the fact that the flexible cells tend to curl due to stresses in the various layers. However, in an automated manufacturing process (e.g. a roll-to-roll process), damage due to handling can be minimized. After testing, the three cells were connected in series with copper foil. Connections to individual cells were made accessible. The cells were then placed between layers of ETFE (Dupont Tefzel) as glazing and THV (from Saint Gobain) as encapsulant and encapsulated in a vacuum laminator at 145 C. A photograph of the finished module is shown in Fig. 6-5.

80 Illuminated J-V of samples used in Module #2 GD1308 Light GD1347 Light GD1367 Light 0 J (ma/cm 2 ) Voltage (V) Figure 6-4: Comparison of I-V characteristics of samples used in assembly of Module #2 Figure 6-5: Photograph of Module #2