High Productive Deposited Mo Layers for Back Ohmic Contacts of Solar Cells

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1 Full Paper High Productive Deposited Mo Layers for Back Ohmic Contacts of Solar Cells Jens-Peter Heinß,* Frank Händel, Toni Meyer, Roland Würz In the paper, thin molybdenum (Mo) layers produced by magnetron sputtering [state of the art in production for photovoltaic applications (PV)] are compared with those produced by highrate electron beam (EB) deposition technology. Stainless steel and borofloat glass served as substrate materials. Mo layers deposited by DC-magnetron sputtering were produced as a reference and investigated by analysis of structure and specific electrical resistance. Alternative layers prepared by high-rate EB-deposition with a rate up to 240 nms 1 were characterised by inquests of mechanical properties, sheet resistance and cell efficiency. A strong dependency of specific electrical resistance on residual gas conditions was determined. The specific electrical resistance dropped from 18 to 11 mvcm. Compactness of Mo layers increased with implementation of plasma activation. The layer formation became denser and comparable to the magnetron sputtered Mo layers. Introduction Mo is the most important material used as back Ohmic contact in CIS and CIGS thin film solar cells. It forms a fairly reproducible, low-resistant contact to CIS/CIGS and moreover annealing at elevated temperatures seems to improve the contact. [1,2] The metallic back contact serves as substrate on which the absorber layer is deposited and affects its growth. The diffusion of Mo into the bulk material (CIS/CIGS) starts at 600 8C. [3] Relative stability at process temperatures, resistance to alloying with copper and indium, and a low contact resistance predestine Mo as back contact layer in CIS and CIGS thin film solar cells. [4,5] Sputtering Mo by a DC-magnetron technique reveals a correlation between sputtering gas pressure (argon) and developed residual stress in the deposited layer. [4,6] This is explained by the different deposition energies of the particles. Low pressure results in only small energy losses J.-P. Heinß, F. Händel, T. Meyer Fraunhofer-Institut für Elektronenstrahl- und Plasmatechnik (FEP), Winterbergstr. 28, Dresden, Germany Fax: þ49 (351) ; jens-peter.heinss@fep.fraunhofer.de R. Würz Zentrum für Sonnenenergie und Wasserstoff-Forschung Baden- Württemberg (ZSW), Industriestr. 6, Stuttgart, Germany of the particles on their way to the substrate and a dense microstructure with compressive stress. High pressure increases the density of argon particles and the rate of scattering with energy loss leading to a porous microstructure with tensile stress. [4] This article displays an alternative to the DC-magnetron sputtering technique by comparing its results with the high-rate EB deposition of Mo. Layers were deposited by both techniques and investigated afterwards. At first Mo layers were deposited by DC-magnetron sputtering. The structure of the layer was analysed by scanning electron microscopy (SEM), the roughness with an atomic force microscope (AFM) and the specific electrical resistance was derived from the measurement of the sheet resistance by a four-point probe system. The high-rate EB deposition of Mo on glass or metal depicts a very economical option. A beam power of 55 kw enables stationary deposition rates of about 120 nms 1. For comparison with the bulk material the mechanical properties hardness and elasticity modulus were analysed by nanoindentation, whereas the roughness was determined with an AFM. Also the specific electrical resistance was measured with a four-point probe system. A special procedure for conditioning the vacuum state of the experimental equipment was developed, which is explained later on. For further increase in the Mo layer compactness plasma activation of the Mo vapour was used ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: /ppap S29

2 J.-P. Heinß, F. Händel, T. Meyer, R. Würz during the evaporation. The process is called spotless arc activated deposition (SAD). [7] These deposited Mo layers were used for the preparation of complete CIGS solar cells. Thus, it was possible to give an impression of the quality of Mo back Ohmic contacts produced by SAD process for further PV applications through evaluation of the cell efficiency. Table 1. Process parameters for the deposition of Mo by DCmagnetron sputtering. DC-power Deposition time Argon flow Chamber pressure P/kW t/s F/sccm p/mbar Experimental Part A vacuum pre-treatment of the substrates was executed to improve the layer adhesion. The steel sheets were cleaned by magnetron assisted sputter etching and the glass sheets were heated by a radiation heater. The etching process was executed immediately before the deposition in the process chamber with an argon flow of 50 sccm (purity of %), which was connected with a pressure of mbar. The argon flow was stopped with the end of precleaning process. For the evaporationof the Mo an axial EB gun with a working range of up to 100 kw/40 kv was used. The Mo (purity of 99.9%) was placed in a water cooled crucible with a diameter of 160 mm. During the experiments with plasma activation of the Mo vapour, we installed an additional electrode above the crucible. This electrode acts electrically as an anode and the crucible as a cathode for a diffuse arc discharge during the evaporation of the Mo. The principle of the SAD is described in a previous publication. [7] The scheme of the SAD is to be seen in Figure 1. The Mo was deposited onto stainless steel sheets with a thickness of 1.5 mm and onto borofloat glass with a thickness of 5 mm. Both typesof substrates had a size of mmand were fixed to the same substrate holder during the process. For the deposition the substrates were located stationary in a height of 270 mm over the crucible. The experiments were carried out in a batch coater with a volume of about 1 m 3. The thickness of the Mo layers in a range between 0.5 and 1.5 mm was measured with a profilometer, glow discharge optical emission spectroscopy (GD- OES) and also was estimated gravimetrically. The sheet resistance of evaporated and sputtered layers was analysed by a four-point probe system and together with the layer thickness the specific electrical resistance was evaluated. To permit this measurement and due to the dimension of the substrate it was cut into smaller parts with a size of 60 mm 50 mm. By using DC-magnetron sputtering Mo was applied stationary on borofloat glass substrates. The float glass substrate Figure 1. Scheme of the SAD. (600 mm 400 mm 3 mm) was retained in front of the magnetron (500 mm 250 mm). It was located in the centre of the substrate and parallel to it. The distance between target and substrate equalled 90 mm. In order to achieve a film thickness of about 500 nm with a constant rate of 7 nms 1 a power of 10 kw was chosen. A pre-heating of the substrate did not take place and the sputtering gas was argon (Table 1). Afterwards the position dependent properties (specific electrical resistance and homogeneity of film thickness) were determined. A profilometer was used for the analysis of the film thickness and its distribution. Additionally the determination of the surface roughness occurred with an AFM on an area of 2.3 mm 2.3 mm (see Figure 11). Results and Discussion Properties of Deposited Mo Layers on Glass Produced by DC-Magnetron Sputtering The typical thickness distribution of the magnetron sputter technology was proved. The layer thickness strongly depends on the geometry of the magnetron arrangement. The highest rate you get at the intensive plasma torus. If the distance between target and substrate is longer than the distance between the two erosion furrows you get a broad layer thickness maximum. This configuration was used. So the layer thickness has a maximum in the centre of the substrate and declines to the outer regions. An area with a size of about 300 mm 200 mm in the middle of the substrate is homogenous with a variation less than 10% from the layer thickness maximum of 520 nm. A decreasing thickness results in an increasing sheet resistance. Furthermore the specific electrical resistance shows a dependency on the film thickness. This is shown in Figure 2, the solid line is a ledger line and does not represent a mathematical model. The minimum of 12.7 mvcm was achieved at a layer thickness of about 500 nm. As a conclusion one sees that for very thin layers in contrast to the bulk material the volume property specific electrical resistance looses its validity. It becomes strongly dependent on the film thickness. Consequently it is proved that a minimum layer thickness of about 300 nm is necessary to fabricate a well functional back Ohmic contact for CIGS solar cells. S30 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: /ppap

3 High Productive Deposited Mo Layers Figure 4. Ion currents of water vapour (mass number 18) measured by the mass spectrometer during the evaporation considering different vacuum conditionings. Figure 2. Specific electrical resistance in dependency on film thickness for sputtered Mo layers. Electron Beam Evaporation of Molybdenum At first the achievable deposition rates were checked by varying the EB power in a range between 40 and 80 kw. The relation is shown in Figure 3. Stationary rates between 50 and 240 nms 1 could be obtained. For most of the following experiments an average beam power of 55 kw was used implying a stationary deposition rate of 120 nms 1. The measurement of the specific electrical resistance of the Mo layers after the first deposition on glass substrates yielded values in a range of mvcm. This is remarkably high considering the resistance of 5.4 mv cm [8] ] known for Mo bulk material. The chemical analysis of these first samples by GD-OES yields oxygen contents of about 10 at. % and nitrogen contents of about 1 at. %, which were constant over the whole layer. That means the oxygen was incorporated during the deposition and caused by the vacuum conditions. By using a mass spectrometer the presence of different relevant gases like oxygen, nitrogen, argon and water vapour was detected in the residual gas and the process atmosphere. The ion current of the water, which is correlated to the water vapour concentration in the chamber, measured by the mass spectrometer is plotted against the process time in Figure 4. One sees the base level of the water concentration, its increase at the beginning of the evaporation and its light decrease at the end. The water vapour partial pressure corresponds to values between and mbar. For a better preconditioning a cold trap with liquid nitrogen was fixed inside the recipient. Furthermore the EB was used thrice each time with a duration of 5 min. Between the separate steps there was a break of about 15 min for an intermediate pumping. In Figure 4 one sees a strong decrease in the base level of the water vapour with vacuum conditioning in contrast to the situation without vacuum conditioning. After using the above described procedure of interval evaporation for the vacuum conditioning this low level of the water vapour concentration could nearly be kept constant during the whole process time. The evidence for an apparent tendency that low values of the specific electrical resistance could only be reached with oxygen poor Mo layers is given in Figure 5. The specific electrical resistance of the samples deposited by high-rate EB evaporation typically reached values of 18 mvcm. It was Figure 3. Dependency of the deposition rate of Mo on the adapted EB power. Figure 5. Reduction of the specific electrical resistance with decreasing oxygen concentration in the EB deposited Mo layers [sources: r bulk ¼ 5.4 mvcm, [8] r DC-sputtering ¼ 10.5 mvcm, [9] r DC-sputtering ¼ 12.7 mvcm (see Table 2)] ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim S31

4 J.-P. Heinß, F. Händel, T. Meyer, R. Würz possible to remarkably reduce the resistance with the described preconditioning of the batch coater equipment in contradiction to the early stage of the experiments. The oxygen content could be reduced to an amount of 8 at. %. The nitrogen content was determined in a region of 0.5 at. %, so the used cold trap did not induce an increased nitrogen level in the Mo-layers. The dotted line connecting the triangles is a ledger line and does not represent a mathematical model in Figure 5. Plasma Activated Electron Beam Evaporation of Molybdenum For the experiments with the plasma activated EB deposition in the SAD-mode an average value of the EB power of 55 kw was chosen. In contrast to the evaporation without plasma activation the deposition rate for this beam power was reduced from 120 to 60 nms 1 by adapting the spotless arc for the plasma assisted EB evaporation. In Figure 6 the characteristic curve for the spotless arc voltage is pictured. For arc currents below 250 A nearly constant low voltages of about 6 V were measured. With the increase in the current above 300 A the arc voltage increased linearly. The voltage of the diffuse arc was kept below 30 V in all experiments. The spotless arc current between the crucible and the electrode could be increased up to 550 A in a very stable way. The diffuse arc with the characteristic plasma glow could be observed in the region of the intensive evaporation in the middle of the melting bath. By applying a BIAS voltage of 50 V to the electrically isolated metal sheet during the deposition the ion current at the substrate was measured. The BIAS current density had an estimated maximum at 20 macm 2 (Figure 7). By considering the elementary charge the number of ionised particles per square unit, which reached the substrate, was estimated from the BIAS current, whereas the number of Mo atoms per square unit was calculated from the deposition rate. Both approximations use a series of simplifications, e.g. an ideal sticking coefficient, no Figure 7. Bias current density on dependency of the SAD-arc current. resputtering and no occurrence of secondary electrons. Under these conditions we evaluated the relation of the currents of the charged metal ions to the neutral vapour atoms to a value of about 30%. This represents an acceptable value for an effective influence onto the layer properties by using the above described plasma activation. By using the plasma activated EB evaporation a few glass and metal substrates were covered with Mo. The layer thickness was chosen in a range from 0.5 to 1 mm. In Figure 8 a depth profile for the concentration of different elements in a Mo layer is shown. At the surface of the Mo up to depth of 50 nm a high level of oxygen was detected. This is an indication for an oxidation process after the end of the evaporation. Over the whole deposited layer thickness the oxygen content decreases to low values of about 2 at. %. The samples deposited with plasma activated evaporation show lower oxygen contents up to 3 at. % in comparison to the normal evaporation with oxygen contents at a minimum of 8 at. %. The concentration of the elements of the stainless steel substrate (Fe, Ni, Cr) increases starting in a depth of 0.7 mm for that concrete sample. Figure 6. Characteristic curve of the SAD-Arc Voltage. Figure 8. Depth profile of the concentration of different elements in a Mo layer deposited by plasma activated EB evaporation. S32 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: /ppap

5 High Productive Deposited Mo Layers Figure 9. Decrease of the specific electrical resistance by applying the spotless arc during EB evaporation (SAD). The Mo covered glass substrates were used to estimate the sheet resistance of the Mo layers and combined with the layer thickness the specific electrical resistance was calculated. Increasing the adapted spotless arc during the plasma activated deposition results in a decreasing specific electrical resistance of the layers. This correlation isshowninfigure9foraconstantvalueoftheebpowerof 55 kw. With an arc current of 450 A a specific electrical resistance of 11 mv cm was attained. This proves a strong decrease in the specific electrical resistance in opposite to the EB evaporation without plasma activation described above. Likewise it is shown that plasma activated deposition leads to layers with less O 2 -content. It is suggested that the ion bombardment causes a resputtering of weakly bonded oxygen atoms during the deposition. Figure 10. (a), SEM fracture (Mo layer produced by EB evaporation without plasma activation); (b), SEM fracture (Mo layer produced by EB evaporated with SAD plasma activation); (c), SEM fracture (sputtered Mo layer). ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim S33

6 J.-P. Heinß, F. Händel, T. Meyer, R. Würz Figure 11. (a), Surface topography of a sputtered Mo layer determined by AFM. (b), Surface topography of a Mo layer deposited by EBdeposition determined by the AFM. Comparison The results of this analysis show the similarities of sputtered Mo layers and those deposited by EB evaporation. All SEM fractures indicate the same, a columnar structure for both deposition techniques, when sputtered Mo layers are deposited with a thickness of 0.5 mm (Figure 10). Also it is shown that the plasma activation during the evaporation results in an increasing compactness of the Mo layers. However, the sputtered layers seem to be the densest. The topography of Mo layers deposited by plasma activated EB evaporation can be characterised with the AFM profile shown in Figure 11. The comparison of the pictures of the AFM analysis in Figure 11a and b show different surface topographies of sputtered and evaporated Mo layers. The evaporated layers are more rough compared to the sputtered. However, the difference of 3 nm between the roughness values R a ¼ 2nm (sputtered) and R a ¼ 5 nm (evaporated) is very small. Considering the whole thickness of a thin film solar cell they are almost identical and probably insignificant in terms of the application. So, besides the value of the roughness, the hardness and also the specific electrical resistance of the layers deposited by us are nearly equal for both deposition technologies as Table 2 shows. For the molybdenum layers deposited by EB technology it was possible to drop the O 2 -content to the level demanded for industrial application. So the technology of EB evaporation tops the state of the art because of its highrate deposition (more than one order of magnitude higher deposition rate than DC-magnetron sputtering) with an equal application quality. Figure 12. Cell efficiency on dependency of sheet resistance for Cu(In,Ga)Se 2 absorbers on Mo layers prepared by plasma activated high-rate EB deposition. Table 2. Characteristic values of Mo layers deposited by DC-magnetron sputtering and high-rate EB technology. Process Rate Roughness Hardness O 2 -content Specific resistance R/nms 1 R a /nm H/GPa C/at. % r/mvcm DC-magnetron-sputtering Not estimated 13 Plasma activated evaporation S34 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: /ppap

7 High Productive Deposited Mo Layers Cell Preparation and Cell Efficiency Thin-film solar cells with the layer sequence Mo/Cu(In,- Ga)Se 2 /CdS/i-ZnO/n-ZnO on float glass are fabricated on Mo layers produced by high-rate EB evaporation technology. The Cu(In,Ga)Se 2 layers are grown in a pilot plant of the ZSW. This inline system is equipped with multiple copper, indium, gallium, and selenium evaporation sources and multiple substrate heaters distributed over two deposition chambers. The process and the technology is explained in a different article. [10] In this contribution we present average values of the cell efficiency of solar cells with Cu(In,Ga)Se 2 absorbers prepared by this system in Figure 12. We can report a maximum cell efficiency of 11% on 2.5 cm 5.0 cm substrates (without antireflection coating). The substrate was a float glass with a low NaO-content of about 2.5%. It is known that the diffusion of Na from the glass to the absorber increases the cell efficiency up to 5%. [11] To verify our results the ZSW fabricated solar cells on float glass (15% NaO) with an Al 2 O 3 buffer layer as a reference. Al 2 O 3 is well known as a diffusion barrier against Na and we were able to compare these samples with our own low NaO-content substrates. Since, the cell efficiency shows similar values in both cases, we expect its increase due to Na diffusion also with samples prepared by high-rate electron beam deposition on substrates with higher NaO-content. Conclusion Mo layers were evaluated in terms of applying them as back Ohmic contacts in thin film CIGS solar cells. The layers were deposited by DC-magnetron sputtering and by EB evaporation with and without plasma activation. A technique for the vacuum conditioning of a batch-coater was developed making it possible to obtain O 2 -contents with a minimum of 3%. However, this is less relevant for inline systems used in industrial production. A maximum deposition rate of 240 nms 1 was verified (30 times faster than sputter rate) as a specific electrical resistance of 18 mvcm was. Furthermore the plasma activation with a rate of 60 nms 1 resulted in a reduction of the specific electrical resistance to 11 mvcm. The developed Mo layers deposited by high-rate EB evaporation show nearly equal values for the characteristic properties when compared to the sputtered ones. As a reference CIGS solar cells were fabricated on Mo layers deposited by EB evaporation. This investigation allows the statement that layers deposited by EB evaporation are completely comparable to the state of the art (using low NaO-content glass). Future investigations should deal with the high-rate EB evaporation on floatglass with a NaO-content of 15%. Furthermore it should be possible to increase the EB power to attain higher deposition rates. Received: October 1, 2008; Accepted: March 12, 2009; DOI: /ppap Keywords: back Ohmic contacts; electron beam deposition; molybdenum; photovoltaic applications; plasma activation [1] S. Ashour, A. H. Moutinho, R. Matson, F. Abou-Elfotouh, Thin Solid Films , 129. [2] I. Moons, J. Electron. Mater. 1993, 22, 275. [3] S. Raud, M. A. Nicolet, Thin Solid Films 1991, 201, 361. [4] A. A. Kadam, N. G. Dhere, P. Holloway, E. Law, J. Vac. Sci. Technol. 2005, A23(4), [5] K. Granath, A. Rockett, M. Bodegard, C. Nender, L. Stolt, 13 th European Photovoltaic Solar Energy Conference, October 1995, Nice, France, Stephens & Associates, Felmersham 1995, p [6] J. H. Scofield, A. Duda, D. Albin, B. L. Ballard, P. K. Predecki, Thin Solid Films 1995, 260, 26. [7] K. Goedicke, B. Scheffel, S. Schiller, Surf. Coat. Technol. 1994, 68/69, [8] H. Kuchling, Handbook of Physics, Fachbuchverlag Leipzig, Carl Hanser Verlag München Wien [9] Personal information of Zentrum für Sonnenenergie und Wasserstoff-Forschung Baden-Württemberg (ZSW), [10] G. Voorwinden, P. Jackson, R. Kniese, Proceedings of the 22 th European Photovoltaic Solar Energy Conference, Milan, Italy 2007, p [11] A. Rockett, J. S. Britt, T. Gillespie, C. Marshall, M. M. Al Jassim, F. Hasoon, R. Matson, B. Basol, Thin Solid Films 2000, 372, ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim S35