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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTQuv [%]110100680 C 640 C 600 C9080706050403020E-beam10Hybrid0PECVD-105 50 500Annealing Time [min]Calculated random scatter absorption limit (blackdots) and measured optical absorption in 2 μm thickpoly-Si thin films on planar, AIT, and SAIT glass with airas the BSR.Figure 4.4.1.4Normalised UV crystallinity factor (Q uv) of PECVD,E-beam and Hybrid films as a function of the annealingtime at 600ºC, 640ºC and 680ºC.Figure 4.4.1.5AFM images of bare AIT (a) and SAIT(b) glass and SEM image of 2 μmthick poly-Si film on AIT glass (c).Figure 4.4.1.3texture is prepared by the UNSW-developed AITmethod [4.4.1.10] described below and consistingof an irregular rough array of sub-micron sizeddimples.The next step is the deposition of anapproximately 75 nm thick SiN film (refractiveindex ~2.1 at 633 nm) by PECVD at about 400°Cor by reactive sputtering at 200°C. The SiN filmacts both as an AR coating and a barrier layerfor contaminants from the glass. An a-Si n +/ p/p + precursor diode is then deposited by eithere-beam evaporation or PECVD on planar or AITglass respectively, followed by SPC at 600°C. Theresulting as-crystallised poly-Si diodes have Vocof only about 150-180 mV. A high temperaturetreatment is then performed at ~1000°C for 30-60s to activate the dopants and to anneal defects,which increases Voc up to 220-250 mV. The finalstep of the material preparation is hydrogenationin a remote hydrogen plasma to passivate theremaining defects resulting in Voc of 450~500 mVtypical for the finished cells.The cells are metallised using a combinationof photolithographic patterning with dielectricand metal deposition processes to create aninterdigitated electrode structure on the rear (Siside) of the cells. Typically, the BSF electrode of thePECVD cells covers the whole rear cell surface andthe cells can only be illuminated from the glassside [4.4.1.11, 4.4.1.12]. The BSF electrode of thee-beam cells consists of thin Al fingers covering only4% of the rear surface, thus allowing bifacial (i.e.illumination from both glass and Si side) operationof the cells [4.4.1.13, 4.4.1.14, 4.4.1.15]. The detailsof the cell metallisation are described in respectivesections below.The hybrid cells are made on planar glass with theSiN layer and the emitter deposited in the CSG Solarlarge area KAI PECVD tool and the absorber and BSFlayers in the UNSW e-beam evaporator. The filmsare processed into either individual 2 cm 2 large solarcells using the UNSW bifacial metalisation or solarmini-modules of 36 cm 2 area using the proprietaryCSG Solar metallisation technology describedelsewhere [4.4.1.4].4.4.1.4 Glass TexturingAn advanced glass texturing method wasdeveloped at UNSW and it is referred to as“aluminium induced texture” (AIT) [4.4.1.5, 4.4.1.10].A thin sacrificial Al film is deposited onto planarglass, followed by annealing at about ~600°C inan inert atmosphere. The anneal initiates a red-oxreaction whereby Al is oxidised to Al 2O 3and SiO 2from the glass is reduced to silicon, as follows:A surface texture is imparted to the glass accordingto the nucleation conditions provided during thisanneal. Subsequent wet-chemical etching removesthe reaction products from the glass surface andreveals the glass texture (Figure 4.4.1.3a). Furtherimprovement in the AIT process conditions led todevelopment of so-called “sub-micron AIT” (SAIT)glass with smaller and steeper feathers (Figure4.4.1.3b), which ensure even better light absorptionin a poly-Si film prepared on such texture (Figure4.4.1.4) [4.4.1.16].Typical atomic force microscope (AFM) images ofthe bare AIT and SAIT glass surfaces with the RMSroughness of 800 and 150 nm respectively and anSEM image of the AIT glass coated with a 2 μm thickpoly-Si film are shown in Figure 4.4.1.3.As shown in Figure 4.4.1.4, the absorption inthe 2 μm thick poly-Si films on the best AITand particularly SAIT glass in the 500~1000 nmwavelength interval is very close to the calculatedrandom scattering absorption limit, based onMonte Carlo ray tracing. It should be noted thatthe significantly higher absorption than thetheoretical limit at 1000 nm and above can beattributed to the enhanced parasitic absorption inthe glass and/or measurements errors as discussedelsewhere [4.4.1.11].40
Films Method Incubation Time [min] Crystallisation Time [min]600°C 640°C 680°C 600°C 640°C 680°COTM 300 50 16 780 140 29Raman 360 50 12 720 125 28XRD 270 62 16 1020 120 32UV-R 270 50 14 900 125 32PECVD Average 300 53 14 855 128 30OTM 300 35 11 720 90 23Raman 300 32 10 660 90 24XRD 270 31 12 1020 105 24UV-R 270 31 9 900 90 25E-beam Average 285 32 10 825 94 24OTM 300 45 14 780 130 28Raman 360 40 12 690 120 28XRD 270 60 15 1020 135 30UV-R 270 45 12 900 120 30Hybrid Average 300 48 13 848 126 29Generic equation [4.4.1.19] 660 108 22Incubation andcrystallisation timesfor Si films (solar cellstructures) deposited bydifferent techniques.Table 4.4.1.1ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORT4.4.1.5 Solid Phase CrystallisationSolid phase crystallisation (SPC) is a thermallyactivated process of transformation of a-Si into poly-Si, which is used for poly-Si thin-film cell fabrication.The SPC parameters, such as the exact temperatureprofile, as well as the film structure, such as thecomposition and thickness of the individuallayers, have effects on the resulting crystal andelectronic quality of poly-Si film and thus on the cellperformance. A detailed study of the SPC kineticswas conducted to clarify such effects. Severaltechniques – optical transmission microscopy(OTM), Raman, UV reflection (UV-R), and X-raydiffraction (XRD) spectroscopes, scanning electronmicroscopy (SEM) – were used to characterise theSPC kinetics and the crystal quality of the films.External Quantum Efficiency (EQE) and Suns-Vocmeasurements were used to characterise the cellperformance [4.4.1.17, 4.4.1.18].Representative SPC kinetics for Si films depositedby different techniques (PECVD, e-beam, andhybrid) at different temperatures based on the UV-Rmeasurements are shown in Figure 4.4.1.5. Similarso-called “S-curves” were obtained by the othercharacterisation techniques mentioned above. Thesummary of the incubation and full crystallisationtimes based on all film characterisation methods isgiven in Table 4.4.1.1.Based on the data from the table it is possibleto estimate the activation energies (Ea) for theincubation and crystal growth processes usingthe Arrhenius law. The Arrhenius plots are shownin Figure 4.4.1.6. The Ea calculated from the plotsare 3.0 and 3.3 eV (e-beam), and 2.7 and 3.2eV (PECVD), and 2.8 and 3.2 eV (hybrid) for theincubation and crystal growth respectively. TheEa for the crystal growth falls within the reported3.1-3.4 eV range, but the Ea of less than 3.0 eV forthe incubation is significantly smaller than ~3.4 eVpreviously found [4.4.1.20]. Most likely it is becausethe nucleation rate, to which the incubation timeis closely related, is enhanced by particular filmdeposition conditions and/or the particular filmstructure, such as the presence of the heavilydoped layers and interfaces.The average grain size in the e-beam poly-Si filmscrystallised at different temperatures was estimatedfrom the SEM images shown in Figure 4.4.1.7, wherethe grain boundaries were enhanced by Seccoln(1/time) [s -1 ]ln(1/time) [s -1 ]-6-7-8-9-10-7-8-9-10Annealing Temperature [ C]680 640 600IncubationCrystal growth-1112.0 12.5 13.0 13.51/kT [eV -1 ]Arrhenius plots for the incubationand crystal growth processes inPECVD (♦), E-beam (■) and Hybrid(●) films with the n+/p-/p+ solarcell structure.Figure 4.4.1.6E-beam = 3.01 eVHybrid = 2.82 eVPECVD = 2.72 eVE-beam = 3.31 eVHybrid = 3.17 eVPECVD = 3.18 eVFilmsPECVDE-beamHybridV ocJscAbsorber Doping Diffusion Length [nm]T [°C] [mV] [mA/cm 2 ] p.FF [%] p.Eff [%] [/cm 3 ]Emitter Absorber600 474 12.7 76 4.6 5.4×10 15 160 2400640 466 11.5 70 3.8 5.8×10 15 140 1800680 456 10.8 63 3.1 4.6×10 15 135 1500600 474 13.4 76 4.8 9.8×10 15 180 2150640 462 13.3 75 4.6 8.6×10 15 155 2100680 432 12.9 74 4.1 9.5×10 15 120 2000600 459 14.4 75 4.9 2.9×10 15 235 4000600 455 14.1 75 4.8 3.2×10 15 210 4000640 433 13.6 73 4.2 2.5×10 15 170 4000Performance parameters ofthe poly-Si thin film solar cellsobtained by integration of theEQE curves over the AM1.5Gsolar spectrum, from Suns-V ocmeasurements, and C-V analysis.All cells have an area of 2 cm 2 .Table 4.4.1.2(a)(b)(c)3 m3 m3 mSEM images of the fullycrystallized and Secco etchede-beam poly-Si films for differentSPC temperatures.Figure 4.4.1.741
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