Experimental quantification of useful and parasitic absorption of light in plasmon-enhanced thin silicon films for solar cells application

A combination of photocurrent and photothermal spectroscopic techniques is applied to experimentally quantify the useful and parasitic absorption of light in thin hydrogenated microcrystalline silicon (μc-Si:H) films incorporating optimized metal nanoparticle arrays, located at the rear surface, for improved light trapping via resonant plasmonic scattering. The photothermal technique accounts for the total absorptance and the photocurrent signal accounts only for the photons absorbed in the μc-Si:H layer (useful absorptance); therefore, the method allows for independent quantification of the useful and parasitic absorptance of the plasmonic (or any other) light trapping structure. We demonstrate that with a 0.9 μm thick absorber layer the optical losses related to the plasmonic light trapping in the whole structure are insignificant below 730 nm, above which they increase rapidly with increasing illumination wavelength. An average useful absorption of 43% and an average parasitic absorption of 19% over 400-1100 nm wavelength range is measured for μc-Si:H films deposited on optimized self-assembled Ag nanoparticles coupled with a flat mirror (plasmonic back reflector). For this sample, we demonstrate a significant broadband enhancement of the useful absorption resulting in the achievement of 91% of the maximum theoretical Lambertian limit of absorption

Variable light biasing method to measure component I–V characteristics of multi-junction solar cells

We presentanewtechniquetomeasurecomponentcurrent–voltage(I–V) curvesofindividual sub-cellsintegratedinamonolithicmulti-junctionsolarcell.Thisnewapproach,comparedtoall previouslyreportedones,iswellsuitedforthin-filmsiliconp–i–nstructureswheretheso-called shiftingapproximation,whichsupposesthatilluminationonlyshiftsthe I–V curve withoutchangingits shape, isnotvalid.Moreover,theproposedmethodisparticularlyresistanttoproblemsrelatedto electricalshunts.Theprincipleofthismethodliesincouplingthelevelofaselectivelightbiaswiththe level ofmeasuredelectricalcurrentinordertofixthevoltageofaselectedsub-cellwhilesweepingover the currentaxis.Whenoneofthesub-cellshasafixedvoltage,itisthenpossibletogetthe I–V characteristicsofthesecondone,shiftedbyafixedvoltagevalue.Thismeasurementprocedureis simple andrequiresnomodeling.Theaccuracyofthemethodisevaluatedbynumericalsimulationsof a thin-filmsiliconp–i–nphotodiode.Ourtechniqueisthensuccessfullyexperimentallytestedona speciallypreparedthree-terminalamorphous/microcrystallinesilicontandemsolarcell

High efficiency high rate microcrystalline silicon thin-film solar cells deposited at plasma excitation frequencies larger than 100MHzsola

Microcrystalline silicon thin-film solar cells were fabricated at high absorber layer deposition rates from 1.0 up to 2.5 nm/s. High efficiencies of 9.6% (1.0 nm/s) and 8.6% (2.5 nm/s) were achieved using a very high frequency (VHF) of 140 MHz for the deposition of all silicon layers (p–i–n). Using such a high frequency in the VHF band is unique in the field of thin-film silicon solar cells. The efficiencies obtained especially at very high rates belong to the highest reported efficiencies so far for this technology. This shows that VHF deposition with frequencies larger than 100 MHz is very well suited for a highly productive solar cell fabrication. The VHF power homogeneity problem can be solved by using for example the linear plasma source concept developed at FAP GmbH/TU-Dresden. We show that the efficiency at very high rates of 2.5 nm/s is limited by an increased crack formation in the absorber layer