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Thin film p-i-n microcrystalline silicon (mu c-Si:H) solar cells were studied by photoluminescence (PL) spectroscopy and open circuit voltage, V-oc. By this means, the cause of the commonly observed increase of V-oc when the silane concentration in the gas phase for the preparation of the intrinsic absorber layers is raised is addressed. The hot wire chemical vapour deposition (HW-CVD) technique was used for the preparation of films and intrinsic absorber layers of solar cells with efficiencies higher than 9%. By monitoring V-oc and the PL energy, E-PL, as a function of temperature, information on the photogenerated carrier distributions and the splitting of the quasi-Fermi energies was gained and the effect of states in the gap was studied by comparison with results from a crystalline p-i-n diode. An increase of E-PL and V-oc for increasing SC and decreasing temperature is attributed to the shift of the excess carrier distributions to higher energies. It is proposed that this shift is limited by band tail states. It is proposed that increasing SC leads to a reduction of the density of band tail states, due to structural relaxation of the mu c-Si:H network by the presence of hydrogen or hydrogenated amorphous silicon.

A simple model is proposed to calculate photoluminescence (PL) spectra and open circuit voltages (V-oc) in thin film p-i-n microcrystalline silicon (mu c-Si:H) solar cells with different structural compositions, as a function of temperature. By using a new technique, namely voltage-modulated PL on solar cells, experimental data are obtained that can be directly compared with the model. The model is based on the distributions of electrons and holes in quasi-equilibrium conditions. Recombination between the two distributions determines the PL band (energy and width of the spectrum). A symmetrical density of states distribution (DOS) described by a superposition of a DOS like that in c-Si and band tail states for the conduction and valence bands is assumed, The best agreement between the model calculations and experimental results for two solar cells with different structural properties is obtained by using an E-o approximate to 0.031 eV for the slope of both exponential band tail states.

Opto-electronic properties of pc-Si:H films prepared by hot-wire/catalytic chemical vapor deposition (HWCVD) and very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) techniques with various silane concentrations (SC) have been investigated using Raman spectroscopy, the steady-state photocarrier grating technique (SSPG), and the steady-state photoconductivity (SSPC). A correlation between the minority carrier transport properties and the microstructure has been found, using the dependence of the diffusion length (L-d) on the SC and Raman intensity ratio (I-C(RS)) representing crystalline volume fractions. I-c(RS) changes from 0.22 to 0.77. L-d increases with increasing I-c(RS). It peaks around 0.5 with a maximum value of 270 nm, then decreases. Similar dependences of L-d on I-c(RS) were obtained for films prepared by both HWCVD and VHF-PECVD. However, the grating quality factor measured on highly crystalline HWCVD films is substantially smaller than that found for VHF-PECVD films, indicating a relatively higher surface roughness present in the highly crystalline HWCVD films.

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Gap states in a-SiGe:H alloys were examined by numerical simulations of sub-bandgap absorption spectra measured by the constant photocurrent method and photothermal deflection spectroscopy. In contrast to simple deconvolution methods our analysis uses occupation statistics and takes into account the condition of charge neutrality. The simulations yield information on the energy distribution and the charge state of the defects. The results reveal the coexistence of charged and neutral defects. The defect distributions are similar to those found in amorphous hydrogenated silicon. In the investigated range of compositions charged states dominate the defect density. Taking the position of the defect states as a reference level both band edges shift towards the defect states with decreasing band gap. In contrast to electron spin resonance measurements, no evidence for a distinction between Si-related and Ge-related defect states can be found in sub-bandgap absorption spectra. (C) 1998 Elsevier Science B.V. All rights reserved.