Bucher, C.

Faÿ, S.

Graf, U.

Kroll, U.

Meier, J.

Shah, A.

Spitznagel, J.

Infoscience - École polytechnique fédérale de Lausanne

PROGRESS IN AMORPHOUS AND "MICROMORPH" SILICON SOLAR CELLSJ. Meier, U. Kroll, J. Spitznagel, S. Fay, C. Bucher, U. Graf, A. ShahInstitut de Microtechnique (IMT), Université de Neuchâtel,Rue A.L. Breguet 2, CH-2000 Neuchâtel (Switzerland)ABSTRACT: LP-CVD ZnO has been developed at IMT as front TCO for amorphous silicon p-i-n single-junction and“micromorph” (microcrystalline/amorphous silicon) tandem solar cells.  In comparison with SnO2 (Asahi type U),ZnO demonstrates superior light-trapping behaviour. Applying the monolithic series connection by laser-scribing,both for amorphous single-junction and micromorph tandem cells, modules have been fabricated on LP-CVD ZnO.After light-soaking, mini-modules, with an aperture efficiency of 8.5 % in the case of amorphous silicon cells, andof 9.8 % (initial 11%) in the case of micromorph cells, could be obtained. Micromorph tandem cells with anintermediate TCO reflector between the amorphous top and the microcrystalline bottom cell reveal an almost stableperformance with respect to light-soaking. A first corresponding module with the applied intermediate TCOreflector could be fabricated.Keywords: Thin Film, a-Si, Micro Crystalline Si, TCO Transparent Conducting Oxides, ZnO, PECVD1  INTRODUCTIONLight-trapping by transparent conductive oxides(TCO’s) plays a fundamental role for the efficiency ofthin-film silicon solar cells. The key factors necessaryfor a high quality TCO performance are hightransparency, high electrical conductivity and highscattering ability. A good TCO layer enhances theoptical absorption for amorphous, as well as formicrocrystalline silicon, thereby, allowing for areduction of the absorber thickness.IMT has, therefore, started developing its own "in-house" zinc oxide, prepared by low pressure chemicalvapour deposition (LP-CVD) [1-3]. This specific TCOhas notable advantages for thin-film solar cells ingeneral. ZnO itself is an abundant, low-cost material.The LP-CVD method is a simple process withdeposition rates of over 20 Å/sec making upscaling toareas of 1m2 easily achievable. The low processtemperatures of around 200 °C are entirely compatiblewith low-cost substrates (polymers, aluminium,..) andthe amorphous silicon PECVD (plasma enhancedchemical vapour deposition) fabrication technology. In previous studies we have already compared our“in-house” ZnO with the best commercially availableSnO2 applied as front TCO in p-i-n configurated solarcells [4-7]. In this paper we report on the furtherprogress of amorphous and micromorph solar cellsusing LP-CVD ZnO as front TCO.  State-of-art mini-module fabrication using the laser-scribing techniquefor both amorphous single-junction and micromorphtandem cells will be outlined.The high photocurrent potential ofmicrocrystalline silicon solar cells can be optimallyutilized with the application of an intermediatereflector between the amorphous silicon (a-Si:H) topand the microcrystalline silicon (µc-Si:H) bottom cell.This concept, introduced by IMT in 1996 [8], permitsan increase of the a-Si:H top cell current or a reductionof the top a-Si:H cell thickness to improve thestability of the tandem cell. By applying thisintermediate TCO reflector concept, Yamamoto et al [9,10] have recently demonstrated an initial cellefficiency of 14.5 % and declared as a goal the massproduction of modules with 12 % efficiency.Such “modified” micromorph tandem cells havebeen recently further investigated by IMT.2  EXPERIMENTALThe preparation of LP-CVD ZnO has beendescribed in previous studies [1-3]. The ZnO layerswere deposited on Schott glass AF45. The a-Si:H p-i-nand micromorph a-Si:H/µc-Si:H tandem solar cellswere deposited by VHF-PECVD (Very High FrequencyPlasma Enhanced Chemical Vapour Deposition) in a8x8 cm2 substrate size laboratory reactor. Thedeposition rates amounted to 5 Å/s for both theamorphous and the microcrystalline intrinsic layers.The cells and modules were characterised by anAM1.5 sun simulator (Wacom WXS-140S-10), and testcells were additionally analysed by spectral responsemeasurements. Amorphous silicon single-junction p-i-n test cells were independently characterised byNREL (National Renewable Energy Laboratory, USA).The stability of test cells and modules wasexamined under AM1.5 close conditions (1000 h at0.75/1 sun intensity and 50˚C cell temperature).3  RESULTS AND DISCUSSION3.1 Amorphous silicon p-i-n test cellsRecently,, we reported [4] on high stableefficiencies of 9 % for a-Si:H p-i-n cells when applyingLP-CVD ZnO as front TCO (initial efficiencies close to11 %). Fig. 1 shows the external quantum efficiency ofa typical amorphous p-i-n cell (0.25 µm thickness)deposited on LP-CVD ZnO. This cell was light-soakedover 700 h, however, at an intensity of 0.75 suninstead of one sun. NREL confirmed for this cell anefficiency of 9.5 % (area of 1 cm2).3.2 Micromorph silicon tandem test cellsIn order to reduce process time and cost weinvestigated the performance of micromorph tandemcells with µc-Si:H bottom cells of 2 µm thicknessusing our in-house LP-CVD ZnO as front TCO. Suchbottom cells were combined with amorphous top cells.Fig. 2 shows that high initial efficiencies of over 12 %could be achieved, leading to high open circuitvoltages of 1.4 volts. After light-soaking (1000 h)these cells show a stabilized efficiency of 10.8 %.3.3 Intermediate reflector in micromorph tandem cellsThe  spectral effect of such an intermediate TCOreflector on the a-Si:H top cell is shown in Fig. 3. Asinitially reported [8], the interlayer improves the top440PV in Europe - From PV Technology to Energy Solutions, 7-11 October 2002, Rome, Italy cell photocurrent. In the example of Fig. 3 fromapproximately 11 to 11.6 mA/cm2. For the spectralresponse of the µc-Si:H bottom cell we hithertoobserved a higher photocurrent drop that more thanoffsets the  improvement in the a-Si:H top cell. Furtherinvestigations with different TCO intermediate layerthicknesses and roughnesses need to be done now.Examination, under prolonged light-soaking, oftandem cells with an intermediate ZnO layer (Fig. 3)reveals a surprisingly high stability. As Table Ireflects, there is no significant change in I-Vcharacteristics after a period of over 1300 h.The tandem cell in Table I consists of a very thin a-Si:H top cell (<0.2 µ m), which combined with a µ c-Si:H bottom cell of only 1.8 µm thickness leads to alarge mismatch of the AM1.5 photocurrent (> 1mA/cm2). The fill factor is principally affected by thestable µc-Si:H bottom cell and less influenced bysmall alterations in the a-Si:H top cell (by light-induced degradation).The conductive TCO interlayer results in theconventional monolithic series connection to a shortcircuiting of the two component cells. By adding  afourth scribe, we suggest a new interconnectionscheme for  the segments, as shown in Fig. 4; here, inwhich the conductive interlayer is separated by the µc-Si:H bottom cell from the back contact.Proof of concept: In order to check the feasibility ofmicromorph modules with the intermediate TCOreflector we applied the suggested interconnection ofFig. 4. Hereby, the top and bottom cells involved, aswell as the TCO interlayer, were not optimised withrespect to high efficiency. The goal was to showwhether  the series interconnection concept  with 4laser pattern works or not. Results of our first module(Fig. 5, initial state) clearly show that theinterconnection scheme as suggested in Fig. 4may be apossible solution to overcome the shunting problemdue to the intermediate ZnO reflector in micromorphtandem large-area modules. This 7-segment modulepossesses a Voc-value that clearly is the sum of the 7individual open circuit voltages as well as areasonable fill factor of around 64 %. Furtherinvestigations are necessary to improve the I-Vcharacteristics of this type of micromorph modules. To what extent the efficiency of micromorphtandem cells with intermediate TCO reflectors can beimproved while keeping the µc-Si:H bottom cell thinenough is to be seen; furthermore, mass production ofmicromorph modules with intermediate TCO layersrequire additional deposition and scribing steps.Particle formation by the laser scribing of theamorphous top cell may cause additional shunting020406080100400 500 600 700 800QE [%] Wavelength [nm]Vo c  = 895 mVFF = 67 % = 9.5 %( A M 1 . 5 ) = 1 5 . 8 5Js c mA/cm2Figure 1: External quantum efficiency of anamorphous p-i-n solar cell of 0.25 µ m thickness onLP-CVD ZnO. The values given are deduced from aNREL data of this cell. The cell efficiency of 9.5 % wascertified by NREL. This cell was light-soaked during700 h at 50 °C, however, under 0.75 sun intensity.-10-500 0.5 1 1.5(η= 12.3 %)Current  [mA/cm2 ]Voltage  [V]light-soakedini t ia lVoc = 1.40VJ s c  = 11.55 mA/cm2FF = 66.6 % = 10.8 %A = 0.4 cm2Figure 2 : I-V characteristics (AM1.5) of a micromorphtandem test cell on LP-CVD ZnO in the initial state andafter 1000 h of light-soaking (1 sun @ 50 ºC). The µc-Si:H bottom cell has a thickness of 2 µm.00.20.40.60.81400 500 600 700 800QE [%] Wavelength [nm]a-Si:H/ZnO/µc-Si:HZnO intermediate reflectorno ZnOJsc = 11.63 mA/cm2Jsc = 10.98 mA/cm2Figure 3: Impact of the intermediate ZnO reflectorwithin the micromorph tandem configuration on thespectral response of the amorphous silicon top cell.Table I: Micromorph tandem cell with an intermediateZnO layer in the initial state and after 1300 h of light-soaking.cell state Voc (V) FF (%) Jsc (mA/cm2) η (%)initial 1.378 73.6 10.5 10.65light-soaked 1.418 72.1 10.5 10.73glass substratea-Si:Hµc-Si:Hbackcontact front TCOintermediatereflectorFigure 4: Proposed series interconnection for themicromorph tandem cell module with an intermediatereflector between the a-Si:H top and µc-Si:H bottomcell. 4 scribe pattering are applied instead of 3.441PV in Europe - From PV Technology to Energy Solutions, 7-11 October 2002, Rome, Italy problems, that will yet have to be overcome. 3.4 Amorphous silicon modulesLaser-patterned LP-CVD ZnO glass substrates havebeen used for the deposition of amorphous single-junction cells for the fabrication of “entirely”amorphous mini-modules, with integrated monolithicseries connection.Fig. 6 shows the AM1.5 I-V characteristics of a 12-segmented module with 24 cm2 aperture area in theinitial and stabilized state (1000 h of light-soaking). The lower Voc-value in the initial state may be anindication that some interface layers (at theinterconnections?) were subsequently improved bylight-soaking, a phenomena that is normally notobserved in basic test cells. The stabilized aperturemodule efficiency of 8.5 % shows a surprisingly highpotential for the simple amorphous single-junctioncell technology, in combination with a high-qualityfront TCO. Further improvements of our modules canbe realized by reducing the scribe losses andimproving the monolithic series connection so as toobtain initial fill factors of over 73 %, as usuallyobtained in test cells.3.5 Micromorph mini-modulesSuch modules have also been fabricated on glasssubstrates coated with LP-CVD ZnO. The AM1.5 I-Vcharacteristics of a 9-segmented module in the initialand light-soaked (after 1000 h) state is given in Fig. 7.The module shows initially an efficiency of 11 %,a fact that confirms the high quality of the seriesinterconnection we have developed. After light-soaking a stabilized aperture efficiency of 9.8 % couldbe measured.As with the amorphous single-junction module,the micromorph module does not exceed the value of71 % for its initial FF, whereas test cells show withinthe deposition area FF values that are typicallybetween 73 and 74 %. The variation of the segmentwidth in the range of 6 to 10 mm does not remarkablyaffect the FF of both the amorphous single-junctionmodules, nor that of the micromorph tandem module.We attribute the FF loss of our modules as comparedto our test cells, to the monolithic series connection.Improving the FF to values of 73 to 74 % and reducingscribe losses should make stable micromorph moduleswith over 10 % aperture module efficiency achievable.A further improvement in the micromorph module isnow clearly linked with an appropriate choice of theamorphous top cell that is implemented.4   CONCLUSIONSIn thin-film silicon solar cells with the p-i-nconfiguration LP-CVD ZnO applied as front-TCOimproves light-trapping, as compared to commerciallyavailable SnO2 (Asahi type U). IMT’s amorphoussingle-junction p-i-n solar cell technology wastransferred from stable 9 % efficient test cells to mini-modules with a stable aperture module efficiency of8.5 %, applying thereby the conventional seriesinterconnection technique by laser scribing. Thisresult suggests that the combination of a high-qualityTCO with a single-junction amorphous silicon cellallows for the simplification of module manufacturingwhile sustaining a high efficiency, reducing processtime and costs. LP-CVD ZnO in combination with a-1.2-0.8-0.400 2 4 6 8 10Current  [mA/cm2 ]Voltage  [V]light-soakedarea : 24 cm2i n i t i a lVo c = 10.6 V  (12x 883 mV)FF = 62.4 %(η= 9.8 %)aperture= 8.5%Figure 6:  AM1.5 I-V characteristics of a 11-segmenteda-Si:H single-junction module based on LP-CVD ZnOas front TCO. The module was light-soaked for 1000 h(1 sun @ 50 ºC).-1.2-0.8-0.400 2 4 6 8 10 12Current  [mA/cm2 ]Voltage  [V]light-soakedarea: 24 cm2initial(η= 11.0 %)Voc = 12.56V (9x1.396V)FF = 64.2 %aperture= 9.8 %Figure 7: AM1.5 I-V characteristics of a micromorphtandem cell module fabricated on LP-CVD ZnO-coatedglass in the initial and light-soaked state. The µc-Si:Hbottom cell has a thickness of 2 µm.-1.2-0.8-0.400 2 4 6 8Current  [mA/cm2 ]Voltage  [V]a-Si:H/ZnO/µc-Si:Harea: 18.8 cm2Voc = 8.7 V (7x1.243V)FF = 63.8 %aperture= 7.5 %Figure 5: First micromorph mini-module withintermediate ZnO reflector where the seriesinterconnection of Fig. 4 has been applied. Note, themicromorph tandem is here not optimised with respectto interlayer optimisation and with respect to currentmatching.442PV in Europe - From PV Technology to Energy Solutions, 7-11 October 2002, Rome, Italy simple single-junction deposition technology is astrong candidate to bring the cost of PV (in $/Wp)down.In connection with micromorph tandem cells, theuse of LP-CVD ZnO enables the reduction of the µc-Si:H bottom cell thickness to 2 µm while maintaininga high efficiency of 12.3 % in the initial and 10.8 % inthe light-soaked state. A successful implementation ofthe monolithic series connection by laser-scribingand the use of LP-CVD ZnO in micromorph tandemcells resulted in an aperture area module efficiency ofinitially 11 % and stabilized 9.8 %.  By improvingboth types of cells and by perfecting the seriesinterconnection, we should be able to obtain easily yeta further increase in module efficiency.Micromorph tandem test cells with intermediateZnO reflector layers between the amorphous top andmicrocrystalline bottom cell reveal a high stabilityunder prolonged light-soaking. We have achievedalready a high stable efficiency of 10.7 %, with a µc-Si:H bottom cell thickness of 1.8 µm, and now afurther improvement to values above 12 % isforeseeable at reasonable cell thicknesses. A firstmodule, incorporating our proposed novelinterconnection concept, designed to overcome cellshort-circuiting due to the intermediate reflector,could successfully be prepared. This first module, thatis not yet optimized already shows an initial apertureefficiency of 7.5 %.Low-cost, high-quality TCO layers and economicalmass-production fabrication processes (such as LP-CVD ZnO and VHF-PECVD) are applicable today inthin-film silicon modules; they are essential for thereduction for the high costs associated with PV. UsingLP-CVD ZnO amorphous single-junction andmicromorph tandem solar cells with the highlyreliable glass/TCO/p-i-n configuration will definitelylead in the near future to the production of moduleswith reasonably high stabilized efficiencies (around  8% or around 10 %, respectively, for the single-junctionand for the tandem case); thereby, this very samecombination will allow for significant reductions indeposition process times and in material costs,leading, thus, to attractively low costs for modulemanufacturing (1 $/Wp becomes a realistic andattainable value).AcknowledgmentsThis work is supported by the Swiss Federal EnergyOffice under Research Grant No. 36 487. We thank Dr.K. Emery from the National Renewable EnergyLaboratory for characterization of our cells and forhelpful discussions.References[1] S. Faÿ, S. Dubail, U. Kroll, J. Meier, Y. Ziegler, A.Shah, Proc. of 16th EU PVSEC (2000), p. 361.[2] J. Bauer, H. Calwer, P. Marklstorfer, P. Milla, F.W.Schulze, K.-D. Ufert, J. Non-Cryst. Solids 164-166(1993), p. 685.[3] R.G. Gordon, AIP Conf. Proc. 394, NREL/SNLPhotovoltaics Program Review, edited by C.E.Witt, M. Al-Jassim, J.M. Gee, AIP Press (New York1997), p. 39.[4] J. Meier, U. Kroll, S. Dubail, S. Golay. S. Faÿ, J.Dubail, A. Shah, Proc. of 28th IEEE PVSC, (2000)p. 746.[5] J. Meier, S. Dubail, S. Golay, U. Kroll, S. Faÿ, E.Vallat-Sauvain, L. Feitknecht, J. Dubail, A. Shah,to be publ. in Solar Energy Mat. & Solar Cells.[6] A.V. Shah, J. Meier, L. Feitknecht, E. Vallat-Sauvain, J. Bailat, U. Graf, S. Dubail, C. Droz, to bepubl. in Proc. of 17th EU PVSEC (Munich 2001).[7] J. Meier et al., Proc. of 29th IEEE PVSC (2002), tobe publ..[8] D. Fischer, S. Dubail, J.A. Anna Selvan, N. PellatonVaucher, R. Platz, Ch. Hof, U. Kroll, J. Meier, P.Torres, H. Keppner, N. Wyrsch, M. Goetz, A. Shah,K-D. Ufert, Proc. of 25 th IEEE PVSC (1996), p.1053.[9] K. Yamamoto et al., Techn. Digest of the Inter-national PVSEC-12 (Jeju 2001), p. 547.[10]K. Yamamoto et al., Proc.  29th IEEE PVSC (2002),to be publ..443PV in Europe - From PV Technology to Energy Solutions, 7-11 October 2002, Rome, Italy 

Progress in Amorphous and Micromorph Silicon Solar Cells

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Progress in Amorphous and Micromorph Silicon Solar Cells

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Infoscience - École polytechnique fédérale de Lausanne