Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTMetallisation on cell levelcompleted: top view of a 5x5 cm 2sample showing 4 individual cellsplus cross sectional view (SiN andsilica layers not shown) with theemitter and air side busbars.Figure 4.4.1.15A schematic air-side top view of amini-module using wire-bondedcell interconnection.Figure 4.4.4.161.210.80.60.40.2Deposition rate of a-Si:H as afunction of power for the SPCoptimisedPECVD process.Figure 4.4.1.13Raman spectra (normalised) of thepoly-Si films obtained by SPC of thestandard (pink) and high (blue) ratePECVD a-Si:H. The spectrum of c-Siis (black) shown for reference.Figure 4.4.1.14Absorber deposition rate,nm/minSuns-Voc results (the average of fivepoints) after RTA and hydrogenationof SPC poly-Si diodes on AITglass with PECVD a-Si:H absorberdeposited at 35 nm/min (“low rate”)250 nm/min (“high rate”).Table 4.4.1.71-Sun Voc,mVhi gh r at estd. rateS i wa f e r0505 510 515 520 525 530 535n=1 Voc,mVHigh rate PECVD of a-Sifilms for poly-Si cellsThe manufacturing of poly-Si thin-filmsolar cells is a commercially viable process[4.4.1.1]. However, the deposition ofthe precursor a-Si films by currentlyconventional PECVD has been identifiedas having high cost and a low throughputdue to its low deposition rate (25-35 nm/min) and the high equipment cost [4.4.1.3].Si evaporation by electron beam (e-beam)has been investigated as a higher rate andlower cost alternative. A challenge with e-beamevaporation is its directional nature (“line-of-sight”deposition), leading to non-conformal coatingof textured surfaces, which results inmicrostructural defects and the poortextured cell performance [4.4.1.25,4.4.1.26]. Another high rate process, whichin contrast to the evaporation can providea conformal coating of textures, is hot-wireCVD. However, no performance results havebeen reported for the SPC hot-wire CVDsolar cells, and it is also known that thehot-wire CVD a-SiH has very high nucleationrates [4.4.1.23], which may lead to poorcrystal and electronic quality of SPC poly-Si.A major part of research conducted on PECVD ofa-Si:H has been traditionally focused on minimisingthe defect density for a-Si:H solar cell applications.However, device-grade a-Si:H, which performs wellfor a-Si:H cells, is not necessarily the best precursormaterial for making poly-Si cells byn=2 Voc,mV35 494.6 ± 8.9 496 666250 512.4 ± 4.5 513 711SPC and a higher rate PECVD a-Si:Hmay also be suitable for producinga good electronic quality poly-Sifilms. Using a conventional 13.56MHz PECVD system and a processoptimised for the SPC application,much higher deposition rates up to 265 nm/min have been demonstrated as shown in Figure4.4.1.13 [4.4.1.7].Raman microprobe characterisation of the poly-Sifilms obtained after SPC from the high rate PECVDa-Si:H material confirms an identical crystal qualityto that of the standard rate poly-Si as shown inFigure 4.4.1.14.The poly-Si cell structures were fabricated withthe thin emitter and BSF layers deposited atthe standard rate and the ~2 µm thick absorberdeposited at the high rate of 250 nm/min in adifferent PECVD chamber. After the standardmaterial processing sequence described above, theVoc of the high rate cell was higher than the Voc ofthe standard rate cell as shown in Table 4.4.1.7.The current research in PECVD poly-Si cells isfocusing on developing advanced cells, whichcombine the best AIT glass for enhanced lighttrapping with the high rate PECVD Si films toachieve efficiencies exceeding 10%.PECVD cell metallisation – cellinterconnectionDuring fabrication of large size thin-film PVmodules, the as-deposited films are dividedinto smaller area unit cells, which are theninterconnected in series to reduce the resistiveohmic losses. A novel approach allowing a lowloss interconnection of individual thin-filmcells into a minimodule using wire-bondingwas recently developed in the UNSW thin filmgroup [4.4.1.9]. Figure 4.4.1.15 shows a samplecontaining four individual PLASMA cells prior to theinterconnection process.The interconnection method consists of laserscribing an isolation groove along the glass sidealuminium busbar to define the cells. Wire-bondsare then placed over this groove, connecting theair-side BSF busbar to the glass side emitter busbarof the adjacent cell as shown in Figure 4.4.4.16. Thismethod can be expanded by applying a conductivematerial (with a non-conductive adhesive) to thesurface of the cell, and wire-bonding the first andthe last cell busbars to the surface of the conductivematerial as also shown in the figure.44