Complete Report - University of New South Wales

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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTV oc(mV) J sc(mA/cm 2 ) FF (%) Eff (%) Rs (Ωcm 2 )479 20.1 66 6.3 4Measured 1 sun J-V results of1.8 µm thick planar e-beamcells with aligned bifacialmetallisation ([4.4.1.13,4.4.1.14]). The shunt resistanceof the cells is > 2000 Ωcm 2and has a negligible effect onthe performance.Table 4.4.1.10Schematic (left) and actual (right) views of a metallised e-beam cell (whitepaint BSR not shown).Figure 4.4.1.20Doping (cm -3 ) V oc(mV) FF (%) J sc(mA/cm 2 ) R s(Ωcm 2 ) Eff (%) pEff (%)1.7×10 15 413 64.1 19.2 2.0 5.09 5.55.7×10 15 418 61.9 17.6 3.4 4.67 5.31.0×10 17 449 62.5 6.5 7.9 2.16 2.58.3×10 17 403 54.4 4.0 19 1.04 1.3Performance parameters of e-beamcells with different absorberdoping densities.Table 4.4.1.11Measured values for Voc, pFF,J sc(EQE), and pEff as a functionof the absorber doping. J sciscalculated from EQE results.Jsc(EQE) and pEff were obtainedunder two different back surfacereflector (BSR) schemes: air (blackclosed squares) and white paint (redopen squares).Figure 4.4.1.21E-Beam cell metallisationThe e-beam cell metallisation scheme, illustratedin Figure 4.4.1.20, was developed in 2008 and allowbifacial illumination and versatile back surfacereflector (BSR) study. It features interdigitated Al linecontacts for both heavily doped layers, the emitterand BSF. The BSF electrodes only cover about 4% ofthe area. The emitter electrode is formed in a waythat avoids contact with the absorber layer entirely.Instead of plasma etching the groovesthrough the whole Si thickness, a timed etchstop is used such that a thinned emitter layerstill remains at the bottom of the grooves.Then, the glass-side comb-like electrode iscentred in the grooves leaving significantspace (>10 μm) between the edges of theelectrodes and the groove sidewalls. Aftermetallisation the entire cell rear surface isthen coated with a layer of a white paint asa diffuse BSR. The typical cell performanceparameters obtained with the describedmetallisation are listed in Table 4.4.1.10.E-Beam cell absorberdoping densityThe electronic properties of the absorberlayer, mainly the effective minority carrierdiffusion length and life-time, are expectedto depend strongly on the layer doping level,which thus can have a large effect on the cellperformance. After introducing a workingmetalisation scheme, an absorber dopingoptimisation experiment was conducted in orderto estimate this effect. All cells in the experimentwere intended to be nominally identical but withthe different doping densities in the absorber(about 2 μm thick, made on the SiN coated planarglass, and with the metallisation and the whitepaintBSR as described in the previous section).The dopant concentration was determined usingthe Z-analysis technique described elsewhere[4.4.1.27]. This technique has an advantage ofyielding the electrically active dopant density,which is most relevant to the cell performance. Thecells were characterised by Suns-Voc, EQE, lightJ-V measurements and the results are shown inTable 4.4.1.11. To account for the spectral mismatchbetween the light I-V tester and the AM1.5G solartest spectrum, which causes measurement artifacts,a conservative approach was used where the shortcircuitcurrent density during the J-V measurementswas adjusted to match those determined fromEQE measurements.The major observed effect of the doping densityis a sharp reduction of the cell current withincreased doping. It dominates a weaker effecton the cell voltage, where V ocrises slightly up todopant density of about 1e17/cm 3 before fallingsharply at the higher densities. The best cellefficiencies are obtained at the lowest active dopantconcentrations in the range between 1e15/cm 3 and5e15/cm 3 . Figure 4.4.1.21 shows the effect of thedoping densities on the key cell parameters.Role of back surface filedThe BSF is very important for the poly-Si solar cell.Besides transporting the holes to the metalliccontacts, it also repels the electrons from the rearcell surface thus reducing recombination at thisinterface. In fact, this effect is crucial for the poly-Sithin-film device as shown in Figure 4.4.1.22. Withoutthe BSF the V ocof the cell is reduced from 435 mV toonly 270 mV. Increasing the BSF thickness from 30to 90 nm decreases the sheet resistance by a factor3 (as expected) but does not impact the Voc of thedevice as presented in Figure 4.4.1.22. Increasingthe deposition rate of the BSF has a positive effecton the Voc. The Voc increases by 40 mV when thedeposition rate increases from 5 A/s to 10 A/s.The best efficiency reported previously for thepoly-Si cell with the identical metallization was5.2 % with a voltage of 435 mV [4.4.1.13]. Thedevelopment of the high rate BSF clearly improved46
Voc [mV]2 m thick film480BSF deposition rate465 5 [A/s]10 [A/s]450 2.5 [A/s]435420405+165 mV+40 mVJ sc[mA/cm 2 ]ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORT2702550 30 60 90BSF thickness [nm]20.017.515.012.510.07.55.02.5 No BSR 482 mV 68% 14.8mA/cm2 4.9%White paint 479mV 66% 20.1mA/cm2 6.3%0.00 50 100 150 200 250 300 350 400 450 500V oc[mV]Voc of 2 µm thick evaporated poly-Si solar cells withvarious BSF layers thickness and deposition rate.Figure 4.4.1.22I-V curves of 2 µm thick planar poly-Si thin film solarcells with and without white paint as back reflector.Figure 4.4.1.23the voltage. The I-V curves data of the solar cellswith and without a BSR are presented in Figure4.4.1.23. The poly-Si solar cells with a white paintBSR has a conversion efficiency of 6.3% withVoc=479 mV, FF=66% and Jsc=20.1 mA/cm 2 . TheBSR increases the Jsc by 5.3 mA/cm 2 and decreasesthe FF by 2% due to the increase of Jsc and (increaseof ohmic losses in the metallization).Light- trapping in e-beam poly-Sithin‐film cellsPoly-Si thin-film solar cells need effective lighttrappingto compensate for the moderateabsorption. This is typically achieved by glasssubstrate texturing for PECVD cells. However,evaporated poly-Si cells are not compatible withtextured glass due to low density material grown ontextured substrates as shown in Figure 4.4.1.24. Across-sectional FIB investigation of the evaporatedSi films on the textured glass reveals noticeablemorphological differences from the films on theplanar glass. On relatively smoother textures the Sifilm has apparently vague areas of lower materialdensity, or high defect density, extending from theinflexions in the texture features. Such defectiveareas further develop into microcracks or voidseither after the thermal treatment (SPC, RTA) oron the rougher textures. When the glass texturehas vertical or overhanging features, extendeddiscontinuities in the evaporated Si film are found.Most described defects are believed to be relatedto the columnar microstructure, which is typical forevaporated films. Clear experimental evidence forthe columnar microstructure and the low densityvoid network present in the SPC poly-Si films wasrecently reported [4.4.1.26].Such a defective structuralmorphology is formed due totwo inherent characteristicsof the evaporation, highdirectionality of the arrivingatomic flux and the adatom lowsurface mobility. The depositiononly takes place on the surfaceareas in direct line-of-sight ofthe evaporation source resulting in discontinuitiesin the areas shaded from the source either by thesurface topographical features or by the previouslydeposited atoms themselves. The earlier arrivedadatoms cast “shadows” for subsequently arrivingadatoms, thus creating a network of parallel highdensity columns surrounded by lower densitymaterial, as it is schematically shown in Figure4.4.1.25. Besides, in contrast to PECVD deposition,the evaporated adatoms have a sticking coefficientclose to unity; they do not have or receive theexcessive energy to move in any way fromthe place of their initial landing to find a morethermodynamically favorable position, thus leadingto morphological and possibly electronic defects.Possible approaches to reducing an extent of thedefect formation during deposition by evaporationis using smoother glass texturing and/or to texturethe Si film surface instead of, or in addition tosmooth glass textures or a completely differentapproach, the localized surface plasmon (LSP)enhanced light-trapping. The different structuresare presented in Figure 4.4.1.26.FIB cross section of an evaporated Sifilm on textured glass substrate. (a)is a grey-scale image, and (b) is thesame image but set to a black andwhite mode.Figure 4.4.1.24Schematic representation offormation of low density materialand voids during evaporation,where already adsorbed adatomscast shadows for newly arrivingadatoms thus forming parallelchannels filled with no or lowdensity material.Figure 4.4.1.25Schematic representationdifferent light trappingscheme. A) planar, B)textured glass, C) backtexture, D) nanoparticles.Figure 4.4.1.2647
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