Page - 89 - in Photovoltaic Materials and Electronic Devices

hydrogen enables passivation of the electronic defect states in a-Si:H and relaxes the a-Si:H network to improve electronic and structural properties. IR-absorption studieshaveshownthathydrogenina-Si:HisbondedasSi-Hn,withn=1,2,and 3[35,74]. IRfeatures in ε for the intrinsica-Si:Hfilmarehighlighted in the insetof Figure 10. Spectra in ε for a-Si:H in the n-i-p device configuration exhibited bending modesnear0.079eV(635.6cm´1)andastretchingmonohydride(Si-H)modearound 0.249eV(2008.3cm´1). Inadditionto theexpectedSi-Hmodes, thisa-Si:Hsample exhibitedanabsorptionmodecenteredaround0.106eV(854.9cm´1),whichcanbe attributed to the bending or scissoring Si-H2 dihydride mode. The peak centered ~2100 cm´1 assigned to the dihydride (Si-H2) or clustered hydrogen is not observed. Although Si-H and Si-H2 bonding modes were previously resolved in ellipsometric measurementsforothersamples[75],wecanresolveonlytheSi-Hpeakherepossibly duetoamuchloweramplitudeoftheSi-H2 peakorreducedsensitivitytothatfeature in this particular sample. In addition to mode deconvolution in ε, the Si-H2 mode is also not observed in the extinction coefficient, k, or the absorption coefficient,α, obtainedfrom ε. Theabsenceof thatpeakusuallyconfirmsthepresence of ordered denseSi:Hmaterial [76,77]. Theamplitudeof ε1 andtherelativelyhighamplitude of thenear IR to UVabsorption feature in ε2 indicate that this is densematerial and suitable forPVdevices. 4. SummaryandConclusions RTSEandIR-SEhavebeendemonstratedasausefulmetrologytechniquefor characterization of PECVD Si:H layers and components of the BR structure used in n-i-p a-Si:H solar cells. Growth evolution diagrams were developed for n-type, intrinsic, and p-type Si:H to identify the regions of optimized protocrystalline a-Si:H material for the respective thicknesses used in the solar cell configuration. IR to UV ex situ SE measurements and analysis were used to determine spectra in ε for Ag,ZnO, theZnO+Aginterface,andprotocrystalline intrinsica-Si:Hinthedevice configuration. Free carrier absorption in Ag and the ZnO + Ag interface, the particle plasmonfeature in theZnO/Aginterface,andfour IRphononmodes inZnOwere identified. Si-Hn bondingmodeswere identifiedin εobtainedfromintrinsica-Si:H prepared on a n-type a-Si:H coated BR. IR-SE has been demonstrated to be sensitive tobondingcharacteristicsofa-Si:Hlayers in thePVdeviceconfiguration. Overall, theresultsandanalysisproceduresdevelopedhereareapplicable tomoredirectly relating film properties, as obtained by non-destructive measurements in the PV deviceconfiguration,withvariations indeviceperformance. Acknowledgments: Wegratefullyacknowledgesupport fromtheUniversityofToledostart up funds and the Ohio Department of Development (ODOD) Ohio Research Scholar Program entitledNorthwestOhio Innovators inThinFilmPhotovoltaics,GrantNo. TECH09-025. 89