Novel laser technologies for crystalline silicon solar cell production

Transcription

1 Invited Paper Novel laser technologies for crystalline silicon solar cell production Andreas Grohe*, Annerose Knorz, Jan Nekarda, Ulrich Jäger, Nicola Mingirulli, Ralf Preu Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstrasse 2, 791 Freiburg, Germany ABSTRACT Laser processes have penetrated into the crystalline silicon solar cell production market some time ago already, but are still far from reaching the status they probably will achieve one day. As the largest fraction of state-of-the-art production lines still produces conventional screen-printed aluminum back surface field (Al-BSF) cells, the applicability of lasers is currently limited mainly to the process step of laser edge isolation, while only a few other companies use lasers for groove formation (fabrication of laser grooved buried contact solar cells [1]) or via hole drilling [2]. Due to the contactless nature as well as the possibility to process a wide variety of materials with fine structures, lasers can be used for a vast field of production steps like ablating, melting and soldering different materials. Within this paper several applications of laser processes within the fabrication of various next-generation silicon solar cell structures are presented. These processes are for example laser via hole drilling, which is inevitable for MWT and EWT (metal and emitter wrap through) solar cells, LFC (laser-fired contacts) as a fast and easy approach for the production of passivated emitter and rear solar cells as well as laser ablation of dielectric layers and laser doping which offer the chance for industrial production of several different high efficiency solar cell structures. Keywords: MWT, EWT, LFC, laser edge isolation, photovoltaic, silicon solar cell, laser ablation, laser doping *Corresponding author: andreas.grohe@ise.fraunhofer.de; phone +49/(0)761/ ; fax +49/(0)761/ ; internet: 1. INTRODUCTION 1.1 Current status of silicon solar cell production The majority of the current cell production capacity produces Al-BSF solar cells. This cell structure is a fairly simple and cost-effective approach for mass production, but comprises some disadvantages. The process routine starts with a wet chemical removal of wire-saw induced crystal damage from the wafer surface and a simultaneous texture of the sample surface for decreasing the reflectivity. In the next step the emitter is formed using a phosphorous gas containing environment in order to form a phosphorous glass layer on the sample surface and subsequently driving the phosphorous into the silicon during a thermal diffusion step. After removing the hydrophilic phosphorous glass layer from the surface by a wet chemical HF dip, the light facing front side is coated by an approx. 70 nm thick silicon nitride layer using plasma enhanced chemical vapor deposition (PECVD) or physical vapor deposition (PVD) technologies. Within the following metallization sequence the front contact is deposited by screenprinting fine silver fingers, whereas the rear side is fully covered by screen-printed aluminum paste. In order to enable a soldering process during module fabrication, silver aluminum paste is printed in small areas on the rear side as well. Both front and rear contact is subsequently formed during a contact firing process. The glass frit within the silver front contact paste etches through the front side silicon nitride antireflection coating and contacts the emitter, while the aluminum paste alloys with the rear side silicon and forms a highly doped region at the silicon aluminum interface. As the front contact is still electrically connected with the rear contact via the emitter reaching around the wafer edges a laser induced groove around the edges isolates the contacts and prevents short cuts. As a share of ~ 40 % of the total module production costs is contributed by the silicon wafer price, the usage of thinner silicon wafers can save a significant amount of money. Therefore cell manufacturers continue to push the solar cell thickness of currently µm further down. Aside of some basic manufacturing steps like wet chemical etching, Laser-based Micro- and Nanopackaging and Assembly III, edited by Wilhelm Pfleging, Yongfeng Lu, Kunihiko Washio, Willem Hoving, Jun Amako, Proc. of SPIE Vol. 7202, 72020P 2009 SPIE CCC code: X/09/$18 doi:.1117/ Proc. of SPIE Vol P-1

2 thermal diffusion or deposition of dielectric layers, the production sequence for the above mentioned Al-BSF cell also comprises technologies like the contact formation process, which limit further developments towards higher efficiencies. 1.2 Optimization potentials of the current cell technology Most limitations of the Al-BSF cell can be linked to the metallization process and lead to electrical, optical and mechanical disadvantages: poor contact aspect ratio of ~ 1: high shading losses of the front metallization lower output current high series resistance within contact grid on front side (screen printing paste conducts only half as good as pure metal) high cross-section necessary leading to even more shading losses low contact fraction between front side metal and semiconductor in printed areas high series resistance contribution limitations on surface doping concentration in order to achieve good electrical contact between silicon and metallization low passivation quality of full area alloyed aluminum on rear side increased recombination rate low optical reflectance of rear side metallization for long wavelength range reduced internal reflection leads to reduced probability of charge carrier generation varying heat expansion coefficient of metal and silicon wafer bowing during final sintering step high breakage risk and limitation in minimal wafer thickness screen printing process depend on mechanical contact of screen with wafer higher breakage rate Due to a lot of effort in recent years these problems have been addressed and minimized. Nevertheless, next-generation cell concepts using different structures and processes feature technologies which help to overcome or at least limit most of these limitations. 1.3 Next generation cell structures In order to further decrease the cost rated power of the photovoltaic devices novel cell concepts which eliminate or at least reduce these disadvantages are necessary. Those cell structures feature additional dielectric passivation layers on the rear side for reduced recombination and better optical performance (passivated emitter and rear cell PERC concept [3]), improved front side metallization schemes (laser-grooved buried contact cell LGBC [4]) in order to minimize electrical and shading losses or finally the reduction or complete elimination of the front side metallization (metal / emitter wrapped through to the cells rear side using via holes MWT / EWT [5], [6]). These so-called back-contact cell types use via holes in order to produce a possible connection between front and rear side and therefore allow placing all metallization on the rear, enabling a simpler module interconnection. The via connection gets its conductivity either from metal, which is covering the walls of the holes, or by a doped emitter layer at the surface of the walls. As the conductivity of metal is some magnitudes higher than of doped silicon, fewer holes are necessary ( s instead of,000 s) for the MWT cell. Very high charge carrier diffusion lengths even allow for real single (back) side structures [7]. In this case no connection between front and rear side is needed. In Fig. 1 an overview over the most prominent cell structures is given. Fig. 1: (from left to right) Schematic drawings of different solar cell structures. The aluminum back surface field cell (Al-BSF, left) represents the majority of todays cell production capacity. Although one of the earliest cell designs introduced, the passivated and rear cell (PERC, second from left) is just about to be transferred from laboratory into production due to the lack of feasible technological possibilities for its cost effective production. In order to avoid shading losses of the front side metallization at the metal wrap-through cell (MWT, second from right) the collecting busbars are moved to the rear and connected with the front side as well as the collecting front side metal fingers by via-holes. A complete absence of metal on the front side is characteristic for the emitter wrap-through cell (EWT), where the current is transported to the rear side only through the emitter present in the via holes. If the material quality is high enough, the interdigitated back junction cell (IBC) represents the purest form of back contact solar cells. Proc. of SPIE Vol P-2

3 2. LASER ABLATION OF SILICON BULK MATERIAL In laser processing of crystalline silicon solar cells the machining of silicon itself obviously plays an important role. The structures applicable differ widely in their demands regarding geometrical properties: some require small but deep structures (e.g. via hole drilling), others larger and shallower ones (e.g. groove formation for subsequent metallization, edge isolation or marking applications). In Fig. 2 the absorption coefficient α and absorption length X L for silicon are shown. It can be seen that silicon shows reasonable absorption already in the visible wavelength range. Generally it is assumed that the photons lead to stimulation of electrons within the material over the band gap to higher states which thus transfer their energy to phonons to drop back to levels close to the band gap. Therefore they contribute strongly to the heating of the material. This makes clear that photon energies lower as the band gap, which is the case for laser light at a wavelength of 64 nm should not be absorbed very well. Reality though behaves different as the material changes its parameters strongly dependent on heat leading to a reasonable high absorption of near infrared light (NIR) in molten Si. Additionally, effects like impurities, crystal damages and interstitials and eventually more complex processes like multi-photon absorption lead to an initial generation of heat which thus creates more of these absorption centers making the material absorb in the end as well. This makes a decision regarding the optimal laser parameters very difficult, as effects like focus ability, available output power and cost of ownership have to be regarded in addition to the raw laser material interaction parameters. Nevertheless, this makes clear that at least with conventional laser parameters a cold ablation process can not take place. Therefore one has to keep the heat impact on the sample in mind when choosing the right parameter set for each application. absorption coefficient α [µm -1 ] α X L I I 0.6 I 0.7 I I 1.0 I wavelength λ [µm] absorption length X L [µm] Fig. 2: Absorption coefficient α and absorption length X L for silicon. The vertical lines represent the common wavelength of diode-pumped solid state lasers (fundamental 64 nm, frequency 532 nm, frequency 355 nm). 2.1 Via hole drilling Both wrap through concepts, namely the MWT and EWT cell structure, feature emitter vias that connect the front side emitter with the back side emitter. The development in the laser source market resulted in state-of-the-art laser processes which allow drilling a plurality of via-holes within a processing time appropriate for industrial application ([8], [9]). All via hole drilling processes studied so far induce damage in the Si crystal. As this damage is expected to be highly recombination active, the avoidance or at least the wet chemical removal of this laser-induced damage plays a major role in judging the via hole drilling process quality. In Fig. 3 (left) a SEM image of a cross section of a via after 12 min of KOH etching drilled in Czochralski (Cz) material from the right hand side is pictured. The cross section is rotated by 45 with regard to the wafer edges. The via hole surface structure depends on crystal direction, which is ascribed to the anisotropy of alkaline etching. Currently the only laser sources which offer high pulse energy and good beam qualities are q-switched diode-pumped solid state lasers in their fundamental wavelength. Therefore two possible systems with the potential of high throughput Proc. of SPIE Vol P-3

4 required for EWT-cells (.000 vias/s) were investigated in [9]: A flash lamp pumped Nd:YAG laser (KLS246 by LASAG, single pulse drilling at pulse energies of 80-0 mj and pulse lengths of µs process A) as well as a diode pumped Yb:YAG disc laser (Disc 0Q, multi pulse drilling with 7 to 15 pulses at pulse energies in the range of 3 mj and pulse lengths of about 1 µs). The last system was used in two different ways, once drilling each hole with a burst of multiple pulses ( process B), second in an on-the-fly -mode, where complete pattern of all holes drilled with a single shot are repeated various times ( process C). Here every pulse needs to be accurately positioned in order to be deposited on the same spot each repetition, which can be solved by state-of-the-art scanning heads and synchronized laser sources. Both damage and geometry of vias can influence the resulting lifetime. In Fig. 3 (right) microscope images of typical vias drilled with process A (first row) and process C (second row) are pictured in front and rear side view. The via drilled with process A exhibits a brighter ring at the via edge visible on the front side, which is ascribed to laser debris. For etching times up to 180 s a full ring is observable, whereat after 300 s only residuals can be observed, and at 460 s no residuals are observed. Process C does not exhibit a similar phenomenon for the etching times under investigation. Furthermore a different taper of the vias drilled with process A and C can be observed in Fig. 3 (right). The ratio of the radius on front and rear side process A ranges between 1.0 and 1.1, whereat the disc laser processes rather feature a value of 1.3 to 1.5. A higher value for taper can result in an enhanced deposition possibility e.g. of a subsequent passivation layer on the via walls and therefore be favorable. Rear Side Pross A 2Opm Process C Fig. 3: (left) SEM image of a via hole cross section in Cz-Material after 12 min etching in KOH-solution. Laser irradiation incident from right side. The cross section is rotated 45 to the wafer edge for better appearance. (right) Microscope images of typical vias drilled with a single pulse (first row) and with multiple pulses (second row) in front and rear side view (left column and right column). Lifetime measurements performed on various via drilled samples and etching durations give a good overview on the amount of laser-induced damage and its depth, as the etch rate of silicon in KOH is more or less linear. Again, the multipulse process shows a better performance than the single-pulse one. Vias drilled with process A still contribute to the over all recombination activity after 460 s of alkaline etching. Laser debris residuals have been observed for etching times up to 300 s. For process B and C the contribution becomes negligible for etching times 300 s respectively 240 s in 30% 80 C. The scanning electron microscopy (SEM) analysis of the via hole surface structure in Fig. 4 (left series) shows a changing appearance during the etching process. After drilling, the via-hole wall appears to be smooth and slightly wavy (left), and laser debris is visible on the wafer surface. After 60 s etching in aqueous solution of 30 % KOH aq, debris on the original wafer surface is nearly removed (middle). The via-hole wall exhibits brighter stripes in <1> directions, indicating that the anisotropy of the alkaline bath is beginning to play a role. Thus at this stage the material is of at least partly crystalline structure. After an etching time of 240 s (right) the surface topology in <1> and <0> direction is Proc. of SPIE Vol P-4

5 clearly different. The <0> part is rather smooth with few steps, whereas the <1> directions exhibit a corrugated surface and several steps. In a top view one would expect an octahedron of decreasing size moving from the laser incident side to the back. When plotting the measured radius of the laser entrance and output Fig. 4 (right) the (equivalent) radius vs. the etching time on the float zone (FZ) samples show an almost linear behavior, allowing the adjustment of the desired diameter by further etching if the process sequence enables it. radius [µm] 70 average r 0 laser entrance side 60 laser exit side linear fit t KOH [s] Fig. 4: (left series) SEM images of a via hole cross sections in Cz-Material. Directly after drilling (left) laser debris is present on the surface and the via-hole wall appears to be smooth. After an etching time (middle) as well as after t KOH = 240 s the anisotropy of the alkaline etch is visible. The measurement of the (equivalent) radius depending on the etching time (right) leads to an linear behavior. Future experiments will investigate further the effect of the pulse duration on the drilling efficiency and the laser induced damage. Although an avoidance of wet-chemical post-processing would be favorable, this value is already tolerable for most of the current MWT / EWT cell process variations. 2.2 Laser edge isolation and groove formation Laser edge isolation was one of the first laser processes introduced into crystalline silicon solar cell production []. It has quickly become the standard procedure for electrically isolating the emitter and base contact, which is present due to the residual emitter on the wafer edge and rear side surrounding non-metalized area. This electrical interconnection can be intercepted by a laser trench around the whole wafer close to the wafer edge. Modern laser edge isolation tools use various different technological approaches. Some use portal axis systems with flying optics, some scanning heads, both of which deliver a laser beam with speeds up to 1 m/s around the wafer. Further information on this process can be found in [11, 12]. 3. LASER ABLATION OF THIN LAYERS Most of the current industrially manufactured solar cells already feature thin layers, e.g. in the form of a front side antireflection coating consisting of approx. 70 nm silicon nitride deposited by PVD or PECVD technologies. Future cell concepts like the above mentioned ones furthermore include dielectric layers used for passivation or isolation purposes on the rear side as well. The metallization can consist of a thin layer as well. Therefore several processes for laser ablation of these thin layers are possible. 3.1 Surface structure issues In addition to non-optimal distribution of absorption between silicon and a covering dielectric layer one further problem arises when working with textured surfaces. Those surface textures are used to reduce the reflection primarily on the front side and are formed by wet chemical etching. When texturing monocrystalline wafers, the low etch rate of the Proc. of SPIE Vol P-5

6 <111> lattice structure is used to form random pyramids on the original <0> wafer surface by etching in KOH. Contrary to planar surfaces, where the ablation structure resembles the beam profile, one faces interference effects on a structured surface when penetrating it with laser radiation. This effect is analyzed in detail in [13]. By numerical simulations it can be found that due to these interferences the intensity along the tips and corners of the pyramids can lead to intensity amplifications with up to a factor of. This means on the other hand that due to these amplification effects first a homogeneous removal is not possible and second much lower pulse energies are needed to initially penetrate through the dielectric. 3.2 Ablation of dielectric materials The most common dielectric material used in silicon solar cell production by far is silicon nitride, but silicon oxide, silicon carbide and amorphous silicon have made their way at least into the laboratories as well. All materials are used either as passivation layers, for optical purposes or as sacrificial layers during diffusion and wet chemical processing. Silicon oxides are generally generated by wet or dry thermal growth which leads to pure SiO 2, but can also be deposited by means of PECVD. Nevertheless, the low refractive index of n SiO ~ 1.46 leads to a poor absorption of silicon oxide for all wavelengths available with conventional DPSS lasers. As reasonable absorption within the layer itself takes place below a wavelength of 200 nm, the direct processing of silicon oxide layers is very hard to accomplish. Silicon nitride on the other hand generally is tuned primarily by the silicon / nitrogen ratio to match the specific demands. In common solar cell production the front side anti-reflection coating (ARC) consists of SiN layers deposited by sputtering or PEVCD (batch or in-line process) with a refractive index in the range around n SiN ~ 2.1, which leads to a good optical match of the solar cell in the module and furthermore shows reasonable passivation qualities. Rear side applications often use silicon nitride with a higher refractive index (2.4 < n SiN < 2.8) in order to achieve even better passivation qualities, but compromise optical transparency. Most silicon nitrides show reasonable good absorption at 355 nm, making it directly absorbing for frequency-tripled solid state lasers. Furthermore different deposition methods lead to different densities, fine structure, melting points and additives like hydrogen etc., which influence the interaction of the laser process with the layer significantly. It can be seen that in order to achieve significant absorption within the dielectric layer the influence of the wavelength is more significant than the one of the pulse length, which is the case at least within specific regions. Therefore the use of frequency-tripled lasers at λ = 355 nm or less is preferred. On the other hand, shorter pulse durations limit the thermal impact on the sample, which favors lasers in the nanosecond and even more picosecond range. A comparison of different lasers for the selective ablation of silicon oxide layers can be found in [14]. SO far, no results with femtosecond lasers have been reported. It is assumed that they could show even better results, as the thermal influence is further decreased. Nevertheless, generally dielectric layers feature a lower absorption than the underlying silicon, making selective processing difficult to achieve without harming any underlying structures. Therefore it is beneficial to either adjust the introduced laser energy precisely or use structures, where minor laser induced damage is tolerable. One major application for the ablation of dielectric layers is the selective opening of the front side anti-reflection coating for subsequent metallization ([15], [16]). When moving away from screen-printing silver pastes, which is the current industry standard, towards novel approaches like ink jetting or electroplating, there is no possibility of using glass frit contents in the pastes any further. Therefore the anti-reflection coating can not be penetrated by the contact during a conventional firing step. The better aspect ratio and smaller contact structure of such a contact opening reduces losses in the short circuit current density j SC due to shadowing effects. Even though the total area coverage is smaller, the laser ablation of the anti reflection coating furthermore can increase the significant contact area fraction underneath the contacts compared to screen printing. In combination with contact metals like nickel with low work functions, a highly doped emitter is not further required. Therefore the open-circuit voltage U OC and thereby the efficiency potential is no longer limited by the emitter profile. Silicon nitride most probably will be used even for highest efficient solar cells due to the optical adjustment to the module glass. In [17] the potential of this process has been shown by ablating silicon nitride on solar cell test samples with an close to conventional front side but high-efficient rear side. The silicon nitride anti-reflection coating with a thickness of d SiN = 70 nm was laser ablated by a ns-pulsed laser at 355 nm on planar and textured surfaces. In order to compare the laser ablated openings with a highly sophisticated metallization scheme the references as well as the complete metallization was defined by photo lithography. It can be nicely seen that the laser ablated samples match the references pretty well and achieve a maximum efficiency of 19.1 % for the textured and 17.4 % for the planar surface. Here the use of optimized emitter profiles as well as a simpler metallization scheme could lead to very high efficiency levels with a fairly simple process. The results of the best solar cells in each group can be Proc. of SPIE Vol P-6

7 seen in Table 1. Newer experimental results will be published elsewhere soon, indicating even higher maximum efficiencies. Table 1: Illuminated IV measurement results of the best laser ablated solar cells in comparison to the best photo lithographically processed reference solar cells on textured and planar surfaces. The data represent the best results achieved so far. Sample Surface V OC J SC FF η [mv] [ma/cm²] [%] [%] laser ablated planar laser ablated textured reference (photo lithography) planar reference (photo lithography) textured Various dielectric layers can be ablated in a similar fashion in order to structure diffusion or wet chemical etching barriers. Generally, thermally grown silicon dioxide is used for this purpose, but other materials can be used as well. The applicability of this procedure has already been shown in [14, 18, 19]. 4. LASER DOPING / LASER ALLOYING One negative aspect of selective ablation, the thermal impact on the underlying substrate, is beneficial for other processes like laser doping and laser alloying. Both technologies use the generation of a liquid phase to mix materials, which results in a alloy of silicon with this material. By carefully choosing the additional element, localized doping profile variations can be achieved in an simple and fairly easy manner. 4.1 Laser doping The key feature of a solar cell is the p-n-junction, which is formed by thermal diffusion of phosphorous into a boron doped silicon wafer or vice versa. This emitter formation generally is performed in a tube furnace, where the silicon wafers are exposed to a POCl 3 ambient, which forms a phosphorous silicate glass (PSG) on the surface. At elevated temperatures around 900 C the dopant subsequently diffuses into the bulk material. By principle the formation of the PSG and therefore the emitter always takes place on the whole exposed wafer surface homogeneously. In order to achieve a selective structure various diffusion barriers can be used (see heading 2). They all have the disadvantage that they need to be applied and sometimes subsequently structured prior to diffusion as well as removed afterwards. Laser diffusion on the other hand enables the direct generation of variously doped regions due to the localized energy distribution of the laser beam. Therefore the specific demands of the different areas can be addressed individually, namely good metal-semiconductor-contact resistance underneath the metallization and low recombination in the current generating area. This enables the generation of different emitter structures in the lateral dimension, so called selective emitters, as well as interdigitated grid structure used for back contacted solar cells. The basic principle relies on a local melting of the silicon surface together with the overlying dopant. As the diffusion coefficient of phosphorous and boron into silicon in the liquid phase is several magnitudes higher than in the solid phase, an emitter can be formed within fractions of a second (see Fig. 5 left). The generated diffusion profile differs from the one formed by thermal diffusion in solid phase and can be influenced by choosing appropriate laser parameters, mainly pulse energy and pulse duration (see Fig. 5 right). Additionally, the resulting doping profile can be altered by varying the melt cycles of the system as well. In order to characterize emitter layers, one can measure the sheet resistance. This value is an easy accessible and fairly simple value and allows a quick comparison of different emitter layers. Although often used, unfortunately it only gives a limited part of the information, as the quality of the emitter also strongly depends on the distribution of the dopant within the silicon. Therefore direct measurements of the doping profiles, which can be obtained e.g. by secondary ion Proc. of SPIE Vol P-7

8 mass spectroscopy (SIMS) are much more accurate, but also more sophisticated. The selectively fabricated emitter area has to serve many purposes: the surface recombination has to be high enough to allow a good metal-semiconductor contact but low enough to minimize recombination. It also has to be deep enough to prevent the contact of short-cutting the emitter. Phosphorous containing layer silicon focussed Laser beam liquid phase solid phase P conc. N A [cm -3 ] 20 low high e P [J/cm ] Pitch [µm] high high low low high low depth z [µm] Fig. 5: (left) Principle sketch of the laser doping process. A focused beam is directed on a silicon wafer surface coated with a thin layer containing the dopant. By carefully melting silicon and dopant layer the diffusion takes place in the liquid phase. (right) Examples of secondary ion mass spectroscopy (SIMS) measurements of laser doped emitter profiles. As can be seen the profile depth and surface concentration can be influenced either by the pulse energy or the melt cycles (indicated as pitch, which leads to a certain overlap). A third possibility is a variation in pulse duration. Primary experiments have shown promising results with simplified test structures already (see Table 2). These 50x50 cm² cells manufactured on 1 Ω cm FZ material feature a homogeneous emitter with various sheet resistances on a planar surface. Underneath the fingers the emitter sheet resistance is lowered to values of R Sh ~ Ω/sq by using a frequency-doubled ns-laser with an optimized (homogenized) beam profile in order to be able to subsequently contact it by a standard screen printing process. The rear side features a conventional screen-printed Al-BSF. Table 2: Results of the first selectively doped solar cells on planar surfaces. Shown is a comparison of the best cells with homogeneous with the best cells with selective emitters. sample Emitter V OC J SC FF η [Ω/sq] [mv] [ma/cm²] [%] [%] homogeneous emitter selective emitter 90 / homogeneous emitter selective emitter 150 / As those first results already show, the short circuit current is limited to a value of ~ 33 ma/cm². This is caused by the absence of a front side surface texture and a non-optimal internal side reflection due to the polished surface with Al-BSF. Furthermore the laser doped areas are generated spacious in order to align the screen-printed contact properly. By Proc. of SPIE Vol P-8

9 switching to a more efficient rear side structure including an intermediate passivation layer between wafer and metal a gain in open circuit voltage should easily be possible. This limit might also be the reason why at least for the samples with 90 Ω/sq emitter no benefit is visible. The 150 Ω/sq emitter nevertheless proofs already the principal correctness of the approach as the drop in performance of the homogeneous cell is almost prevented by the selective emitter. Again, the achieved level probably is limited by the structure used. Latest cell results show a significant enhancement of the selective emitter compared to the conventional homogeneous one and will be presented elsewhere. 4.2 Laser-Fired Contacts The laser-fired contacts (LFC) technology [20], [21] is a simple method of implementing a passivated and locally contacted rear side into any solar cell structure without needing to additionally generate highly diffused areas underneath the contacts or creating local contact opening in the passivation layer. The whole layer system, consisting of a dielectric passivation layer as well as an aluminum metallization layer, is deposited subsequently without forming contact openings in between. The best results achieved so far were on a layer system consisting of ~ 0 nm thermally grown silicon dioxide followed by ~ 2 µm aluminum, but both layers can be varied widely [22, 23]. Solar cells with efficiencies well above 20 % have been manufactured using thermal silicon dioxide, PECVD as well as PVD silicon nitride, PECVD silicon carbide, PECVD amorphous silicon as well as combinations of different dielectrics as rear side passivation material. After applying both layers, the contact is established by locally alloying the aluminum through the dielectric layer into the silicon bulk material. By using aluminum on p-type doped silicon, a highly doped p + -region underneath the contact can be formed, which additionally lowers the contact resistance as well as the recombination [24]. Although the contacts with a diameter of ~ 0 µm are applied with a pitch of mm depending on the material, they only lead to a metalized area of a few percent, while the rest of the surface remains passivated. This leads to a superior rear side quality compared to a conventional, fully metalized aluminum screen printed rear side, which can be seen in the long wavelength range of the internal quantum efficiency (see Fig. 6 left). Here the absorption of the silicon bulk material is low enough to allow light to penetrate through the complete cell. A rise in reflection in this range indicates a better optical behavior at the rear side, as the photons get a second chance of being absorbed on their way back to the front side. This leads to an increased internal quantum efficiency close to the band gap of silicon. Due to the high absorption of IR light in the covering metal layer, there is no need to use frequency-multiplied laser systems. Although the process was developed on a system with a pulse length of < 0 ns, it can be performed with pulses of ~ 1 µm as well. Therefore conventional q-switched diode-pumped solid state lasers with a repetition rate between -30 khz and pulse energies in the range of 2 mj can be used for the process. The laser beam is scanned with high scanning velocities and a conventional scanning head over the sample. By choosing appropriate parameter combinations of scanning speed and pulse repetition rate, single pulses can be generated at the desired pitch. This approach leads to very short cycle times, e.g. a sample of mm 2 can be contacted in under two seconds by > points in a pitch of 1 mm. In this case the beam is moving with m/s over the sample at a repetition rate of khz, making the laser technology quite simple. The process works similar to conventional laser ablation. In the contact center some parts of the material evaporate or lead to plasma formation, which expels the liquid material in the outer contact region. Due to the Gaussian intensity distribution of the TEM 00 laser beam this leads to the formation of an inner contact area, where the aluminum is alloyed into the silicon and the electrical contact is established. In the outer contact area there is still a thin residual aluminum layer as well as the passivation present. It is assumed that this unfavorable contact shape leads to increased recombination and series resistance contribution in the outer contact area. Therefore a homogeneous flat top intensity distribution could be favorable (see Fig. 6 right). Proc. of SPIE Vol P-9

10 internal QE, Reflectance [%] Al-BSF LFC wavelength λ (nm) energy density F LFC [kj/m 2 ] 300 beam profile Gauss fit ± 61 kj/m 2 75 ± 61 kj/m radius r [µm] Fig. 6: (left) Comparison of the reflectance (open symbols) and the internal quantum efficiency (solid symbols) of a conventional solar cell with full area rear side metallization (Al-BSF) with one featuring a dielectrically passivated rear side and laser-fired contacts (LFC). (right) The LFC structure with its inner and outer contact area resembles the gaussian beam shape of the TEM 00 laser perfectly. As can be seen a flat top intensity distribution would be beneficial, as the additional intensity in the contact center as well as the lower intensity in the outer contact area probably lead to increased laser induced damage. 5. CONCLUSIONS AND OUTLOOK Various industrially feasible laser processes with the potential to further increase the efficiency of crystalline silicon solar cells as well as some underlying considerations for the choice of suited laser sources for these processes were presented. Via hole drilling has always been done by laser usage, but currently the cycle times seem to drop to levels which are low enough for a transfer into industrial environments. Primary experiments on laser doping led to promising results for selective emitter structures, while further adjustments to the sample structure should lead to a future increase of the efficiency and therefore proof the theoretical assumptions. With laser ablation of SiN anti-reflection coatings on high efficiency solar cell structures efficiencies of 19.1 % on textured and 17.4 % on planar surfaces have been shown demonstrating the high potential of this part of a novel metallization technology step as well. The industrial feasibility of the via hole drilling process as well as the LFC point contacting technology has been shown. Proc. of SPIE Vol P-

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