Dubail, J.

Dubail, S.

Meier, J.

Shah, A.

Vallat-Sauvain, E.

Infoscience - École polytechnique fédérale de Lausanne

Microstructures of Microcrystalline Silicon Solar Cells Prepared by Very HighFrequency Glow-DischargeJ. Dubail, E. Vallat-Sauvain, J. Meier, S. Dubail and A. ShahInstitut de Microtechnique (IMT), A.L. Breguet 2, Université de Neuchâtel,CH-2000 Neuchâtel, Switzerland.ABSTRACT 	ﬀﬂﬁﬃ ! "#	$&%'()	*+,,)	)-.*,+ﬁ	,ﬁ0/213 4*55	6Electron Microcopy (TEM). At the micrometer scale, a difference in the grain structure isobserved close to the ZnO substrate: cracks are found in the cell deposited with the higher silaneconcentration. Surprisingly, the cell with the cracks close to the transparent conductive oxideshows the largest VOC (530 mV) and FF (68%). These first studies reveal that microstructures offully microcrystalline silicon devices may show a quite large variation with correspondingeffects in solar cell performance.INTRODUCTION738:9,;5<=?>@ABC>69EDGF	H;5<H;C8:IJBCA,KK	FL@>0I5F	KF	H,<6@NM H!OP#F	Q&7SR+F	I+AT;<6DFI5FL@=U@>VWTX<6Bﬃ<6Y<KLBﬃA,F	HM[Z#\SRA]I5<6;]>;^DAB_>;FA,K	`6a4XF&@NbF	K&DI+<b2B	XFI-DABﬃ>;FA,KcXAY^>]>,>@d=6;<6Ve@ﬂBC<ﬂbA]2;F	H6ABC>I(FL@-=fK	>!O gCh@HBﬃF	<6@ H6OPFQL7iA@90DFH;<6DG<6;TXNMA,OP#F	Q&7Sj H,OPFQL7eR^BﬃA@9>DkI(<K	A;lH6>,K	KI5m6T;<6YF9FL@=nBX^>;>]8N>(@H,<6h;A!=fFL@=0;5>,IhKLBﬃI[1-3]. While detailed characteristics of microcrystalline absorber material in terms of absorptionbehavior, transport properties, device performance and stability are available, only very little isknown about the internal structure of this material. Recently, we have reported, in a transmissionelectron microscopy (TEM) study of undoped µc-Si:H films deposited on glass substrates thatµc-Si:H shows a surprisingly large variation of internal microstructures [4, 5]. In this formerstudy we found that the morphology is strongly dependent on the hydrogen to silane dilutionratio used during deposition.On the other hand, it is known that the microstructure is also critically substrate-dependent,at least during the early stage of growth [6-8]. In case of entirely microcrystalline p-i-n devicesdeposited on a transparent conductive oxide (TCO), not only the microstucture but also thequality of the electrical contact is essential for obtaining satisfactory solar cell performance.Especially the initial crystalline growth of the µc-Si:H p-i interface on the TCO is critical for agood working device. Sofar, only very little has been reported on the microstructure of entire µc-Si:H cells. This present work is a first attempt to focus on the structure of microcrystalline p-i-ndevices by using TEM investigations. We show results of TEM cross-sections of two µc-Si:Hcells with conversion efficiencies of 6 and 7 %, respectively. These two devices were depositedunder slightly different deposition conditions, using in both cases the Very High FrequencyGlow Discharge technique (VHF-GD) [9].EXPERIMENTALCell depositionThis study is based on two microcrystalline single-junction p-i-n solar cells named Cell Aopqﬂrns6t	tcuSv6wixzytt|{ },~#	L~~zpNy}ys.o,Ł66epﬂ{3e3~6pNo6yŁ6pW[0rUtransparent conductive oxide) layer deposited by us on a glass substrate.The doped layers of both cells investigated were deposited using the same depositionparameters. Both layers were deposited at a plasma excitation frequency of 180 MHz.Microcrystalline growth was obtained using a silane concentration SiH4/(SiH4+H2) of 5.6 and5%, respectively, at a deposition temperature of 250°C. We observe that the hydrogen to silanedilution ratio critically affects solar cell performance [3]. As already reported on our filmdilution series deposited on glass substrates, this deposition parameter also influences themicrostructure [4, 5]. On the other hand, in a superstrate solar cell structure consisting off	,(	 n¡U¢l£¤¥	¤[¦ §,¤¨¥©Lª.«6¬	­®§(¯5¥L¬ﬃ¥	§,,°£¤¥±¥L¦¬C®¯²ﬃ,§6®¥	+²³,¯´®6µﬂ¬¶¬­®·-®!f¥L¦¦¥&¦¸³² §,¤¨#¥	©Lªdeposition. In this case, the condition of the substrates certainly very decisive for obtaining anoptimally grown p-i interface. The cells were characterised under AM 1.5 illumination and byspectral response measurements. Even if both cells were not optimised with respect to ultra-thinp-window or a high reflectivity of the back reflector, they still have conversion efficienciesbetween 6 and 7%. In this first attempt to investigate the microstructure of our cells, we aimed atchoosing cells deposited under rather similar conditions, but exhibiting different electricalcharacteristics. Table I. shows the properties of the cells investigated by TEM.Table I. Parameters of investigated solar cells.Cell parameter Cell A Cell BVOC [mV] 530 486JSC [mA/cm2] 16.6 22.5FF [%] 68 64¹ºL»½¼	¾5¿:ÀÁ^Â	ÃºÄÅÆ-À,Ç5Ç+È ÉÊ 1.4 5.4deposition rate [Å/s] 5.2 4.6TEM sample preparationBoth cells were prepared as cross-sections for TEM examination using a techniquedescribed in [10]. In order to achieve electron transparency, two pieces of the same specimenwere glued head to tail (see Fig. 1). An special instrument named "Tripod" permits one tomechanically polish the "sandwich", thereby obtaining a corner with an angle as low as 0.6-0.8°.At the edge of the corner one gets then a thin enough zone (few 100 nm), where electrontransmission occurs. After this preparation step, an ion-beam-assisted cleaning and polishingprocedure was applied. In our case, the advantage of this mostly mechanical preparationtechnique of thin-film cross-sections is the reduced ion-milling step [10].In this preliminary study, we present our TEM observations made at low magnification (<30000 x) in order to inspect the micrometric structure of our material. Bright field mode wasused as well as dark field (DF) imaging. The two first diffraction rings were used to form darkfield images. Indeed, we could qualitatively observe that the (220) diffraction rings yieldedbrighter DF images, indicating a (220) preferential growth orientation within both layers ofthe cells.RESULTS AND DISCUSSIONBoth cells shows a significant difference in the microstructure, although only a slightintentional change in the deposition parameter was applied (with respect to H2 dilution of silane).Figs. 2 and 3 represent the bright and dark field cross-sections of Cell A and Cell B, respectively.It has to be pointed out that the different characteristics of the two structures can be seen even atlower magnification.At the bottom of Figs. 2 and 3 we can observe the teeth-shaped zinc oxide substrate overwhich our cells were grown in the upwards direction. On the top of each picture one can observeË	ÌÍÎ,Ï(ÐÑÍÒÓÏ5Ô&ËCÍ,Ñ0Õ5Ó6Ö-×ØÌÙ-Í,Ï5Ï+ÓÚ-Ó6ÖÕ Û!ÐÜÔÝLÞÒÐÔÐzÙNÛ,Í,ßß	Ï5à,áJËÏ5Í6ÍâÏ-ËÌÎË¶ËÌÔ	Ï+ÏJÖÕÚﬃÎ,Û6ÍãÕÓ6Ö-×ØÌÙÍ,Ï5ÏãÔ	Ï-ÙÓ6Ëmuch affected by the initial roughness of the TCO substrate. The growth of the -layersmoothens the roughness of the substrate. This may have a beneficial effect on light-trappingproperties of the cell.The structure of the -layer consists of an aglomerate of small microcrystallites (darkspots in Fig. 2). The grains (i.e. aglomerates) have a diameter of several hundreds of nanometersand run across the whole thickness of the device. Close to the substrate, these aglomerates areloosely packed, resulting in visible cracks, appearing brighter on Fig. 2. These cracks mayconsist of an amorphous phase or may just be voids. Towards the top of the cell, themicrostructure becomes denser. Somewhat similar growth morphology has been observed for n-type layers deposited on flat crystalline silicon substrates [11].In Fig. 3, the microcrystallites within the aglomerates of Cell B are evidenced by the darkfield imaging conditions, where crystallites satisfying diffraction conditions appear brighter.Their diameter is of a few tens of nanometers, while their length depends on their locationwhithin the cell. Indeed, close to the ZnO substrate, the crystallites grow perpendicular to theZnO facets; their length is equal to several tens of nanometers. However further away, they growperpendicular to the average substrate plane, and their length can reach several micrometers.The characteristic features of both microstructures are sketched in Fig. 4.Figure 1. Sketch of a sample preparation for TEM observations.glueglassZnOp-i-nFigure 2. TEM bright field cross-section micrograph of cell A. Voids and/or cracks appearbrighter.Figure 3. TEM dark field cross-section micrograph of cell B. Microcrystallites best satisfyingdiffraction conditions appear bright.In contrast to Cell A (Fig. 2), cracks are not observed in Cell B. But the aglomerate size isdetermined in both cells by the substrate surface geometry. Indeed, as can be observed in Fig. 2,grain (aglomerate) boundaries start in the bottom valleys, as already observed by [6]. Thissuggests that in both cases aglomerates start to grow at the top of ZnO pyramids. If the presenceof cracks in Cell A is only related to the difference in deposition conditions (in our case, thehydrogen to silane dilution ratio) remains sofar unclear and needs further studies.Both cells exhibit sub-structure (microcrystallites) growing perpendicular to the ZnO facets[6]. In Cell A, the direction of microcrystallite growth remains constant over the whole cellthickness. In Cell B, which is more than three times thicker, the direction of crystallite growthchanges as the cell becomes thicker. Cell B exhibits a much more denser microstructure. Here,ä6åæèçêé ëíìî.ìçïzðä6ëòñ	óôõ4åö÷Låñﬃôðïì6ø!ôîﬀô0ä6ùúôðûô÷&åﬂùä6ñ	óNø!ô,ææ	ú+ì6üýþì,ø!ôåñ ß6ðì,÷&åúô(ðôåü÷	ø æì(ð^ñCäthe substrate. The diameter of these large grains is of the order of a few hundreds of nm; thegrains run all the way across the device.Electrical characteristics of cells A and B were given in Table I. As the cells have quitedifferent thicknesses, we cannot compare the current densities. Thus, our two cells can becompared only w.r.t. VOC and FF. Surprisingly, the VOC is higher in the Cell A, which exhibitsvoids at the ZnO/p-i interface. This suggests that a high VOC is due to features at lower scales(nanometer or atomic range) such as the quality of the p-i interface / contamination or the qualityof the passivation of the grain boundaries. Such features cannot be observed on our mediummagnification micrographs. On the other hand, the study of the microstructure and electrical		 ﬀﬂﬁﬃ	 "!#$  %'&($')$*+!,.-/!102!,.3	4$	$ !567-"7"78/$$3(observed in cells A and B) with device performance.CONCLUSIONSTwo microcrystalline silicon single-junction p-i-n solar cells with conversion efficiencies of6 and 7% have been prepared in cross-sections for TEM analysis. Medium magnificationobservation of both samples already permits one to draw the following three main conclusions:1. A small change in the deposition parameters and different cell thicknesses leads to quitedifferent microstructures.2. The surface topology of the substrate seems determinant at the early stage of growth for the9:<;>=/?@ ABC:D<EGFIHJ$:ﬂK9MLNOKP?QRH#A$J9=TSU=6:V=KW3:@X.W"YJWZFHJ$:ﬂK9J'H8=/J$V'[8JA$=KWZ:<K4WY=T\J]]=X^9?@>W"Y_=underlaying ZnO layer. We therefore suggest that the nucleation of each grain takes place atCell A Cell B1.4 µm5.4 µm µc-Si:HZnO200 µmFigure 4. Schematics of both TEM micrographs given above.the top of a zinc oxide pyramid. In Cell B, at the sub-grain (crystallites) scale, the initialgrowth occurs perpendicular to the ZnO facets, and becomes perpendicular to the substrateplane, i.e. parallel to the current path, as growth proceeds.3. In cell A, the presence of micrometric cracks between grains is not harmful for cell operationand the value of VOC is well over 500 mV.`bac"de2fgh4e2i$dikjc<gdlmill3no2plqlf4frdn'gsnTl"anTo+c"tMgfdl"gutl"ugn$dfjnvlc<gn twxcy<zGdf{"ig#tn${{"di$difunction of the silane dilution concentration used for deposition. This approach is encouraged bythe observation of microstructural variations of the material (i.e. at the micrometer scale) forsmall changes in the deposition parameters and variable cell thickness. At this stage, the effect ofmaterial microstructure on the electrical properties of the devices are still far from beingelucidated.ACKNOWLEDGMENTSJ.D. would like to thanks personnally Mrs. D. Laub from CIME-EPFL in Lausanne(Switzerland) for her help in preparing the samples for TEM observation, Dr. M. Dadras for hishelp for TEM manipulations and Mrs. M. Leboeuf for her help in printing the photographs. IMTexpresses its gratitude to Prof. F. Stöckli for granting access to the TEM facilities.We acknowledge financial support from the Swiss Federal Office for Energy BEW/OFENunder grants 2757 and 19431 as well as the Swiss National Science Fund under grant FN52337.REFERENCES[1] J. Meier, R. Flückiger, H. Keppner, A. Shah, Appl. Phys. Lett. 65, p. 860 (1994).[2] J. Meier, S. Dubail, R. Flückiger, D. Fischer, H. Keppner, A. Shah, Proc. of 1st WCPEC,p. 409, (1994).[3] J. Meier, E. Vallat-Sauvain, S. Dubail, U. Kroll, J. Dubail, S. Golay, L. Feitknecht, P.Torres, S. Fay, D. Fischer and A. Shah, to be publ. in Solar Energy Mat. and Solar Cells(presented at PVSEC-11, 1999).[4] E. Vallat-Sauvain, U. Kroll, J. Meier, A. Shah, J. Appl. Phys. 87, p. 3137 (2000).[5] E. Vallat-Sauvain, U. Kroll, J. Meier, N. Wyrsch and A. Shah, to be publ. in J. Non-Cryst.Solids (presented at ICAMS 18, 1999).[6] H. Yamamoto, M. Isomura, M. Kondo and A. Matsuda, Techn. Digest of PVSEC-11, p.231, (1999).[7] P. Torres, H. Keppner, R. Flückiger, J. Meier, A. Shah, Proc. 12th EPVSEC, p. 132 (1994).[8] J. Koh, Y. Lee, H. Fujwara, C.R. Wronski, R.W. Collings, Appl. Phys. Lett. 73, p. 1526(1998).[9] H. Curtins, N. Wyrsch, A. Shah, Electron. Lett. 23, p. 223 (1987).[10] J. Benedict, R. Anderson and S. J. Klepeis, Mat. Res. Soc. Symp. Proc. Vol. 254, pp. 121-140 (1992).[11] L. Houben, M. Luysberg, P. Hapke, R. Carius, F. Finger and H. Wagner, Phil. Mag. A77(6), pp. 1447-1460, (1998).

Microstructure of Microcrystalline Silicon Solar Cells Prepared by Very High Frequency Glow-Discharge

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Microstructure of Microcrystalline Silicon Solar Cells Prepared by Very High Frequency Glow-Discharge

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