The effect of the amount of Na present during the 3-stage growth of CIGS at very low temperature T2 on polyimide (PI) foils is studied. While at higher growth temperatures Na seems to impede In-Ga interdifussion, at very low temperatures it appears to further the process. An increase in Voc for a higher Na concentration can be explained by a higher net carrier concentration as measured by drive level capacitance profiling. Admittance spectroscopy measurements show shallow defects when the Na concentration increases. These results suggest that the main role of Na could be the passivation of InCu donor deep defect, in agreement with Wei's theory. Efficiencies of up to 15.1 % (0.5 cm2 active area with antireflection coating) and 13.6%, 14.1% (1 cm2 total and active area respectively without antireflection coating) for nominal T2=420° C were achieved on PI substrates so far.

The objective of this work is to increase the open circuit voltage of CuGaSe2(CGS)-based solar cells without decreasing their efficiency. For that, the interface between the p-type CGS absorber and the n-type CdS/ZnO window layer is compared using three different recipes for the growth of the buffer layer. Results show the importance of the adaptation of the CdS buffer layer to the CuGaSe2 absorber film. A maximum open circuit voltage of 922 mV is achieved for the devices when using 60ºC as the chemical bath temperature and a low thiourea concentration. Drive-level capacitance profiling, external quantum efficiency and temperature dependent current-voltage measurements reveal a better quality of the CdS/CuGaSe2 interface for this buffer layer deposition conditions. Factors such as the larger depletion region width and the lower doping level, reducing the tunnelling component, are pointed out as responsible of the higher Voc.

Solar cells based on CuInS2 still suffer from Voc limitation compared to the value expected from the bandgap. The open circuit voltage can be enhanced via controlled doping of small amounts of Zinc or Magnesium (< 1 at. %), however the underlying physical reason for this improvement is not understood so far. We have systematically studied the effect of diffusion of Zn and Mg into the absorber layers with photoluminescence and quantum efficiency measurements. ZnO (ZnMgO, MgF2) has been deposited onto the absorber layers and photoluminescence (PL) spectra at room temperature were recorded as a function of the annealing temperature. The modified emission spectra reveal a defect related transition with a strong luminescence signal. At the same time the open-circuit voltage of the Zn (Mg) doped CuInS2 cells are around 100 mV higher than the values found for the undoped cells from the same device process. We interpret these results as a reduced defect state density at the surface and an increased bulk defect concentration induced by the incorporation of Zn or Mg. Comparison of external quantum efficiency (EQE) and I/U measurements with the PL results support observed changes of the interface/bulk properties.

We measure subgap absorption on n-type amorphous silicon using the “absolute” constant photocurrent method. We find that for typical monochromator probe beam intensities the measurement is not significantly influenced by lifetime changes. When the measurement is performed under light bias, an apparent increase in the defect absorption coefficient is observed, but no change in the photoexcitation threshold or spectral shape of the absorption band is seen. We show that this increase is likely due to a bias-light amplification of spectrally dependent lifetime changes. Our measurements suggest a larger electron capture cross section of positive valence band tail states compared to neutral dangling bonds.

We investigate capacitance transients under various experimental conditions on a number of different amorphous silicon Schottky barrier device structures. The samples differ mainly in their back contact configuration, which is non-ohmic for one type of sample and ohmic for the second type of samples. Anomalous transients can be observed for all samples regardless of the back contact if the depletion region is only partially collapsed by partial voltage pulsing. We find that samples without an ohmic back contact show a slowly rising capacitance during the filling pulse. These samples show anomalous capacitance transients for short filling pulses.

The structural memory model of slow defect relaxation in a-Si:H is extended to the limit of long defect filling times. The model was proposed in order to explain unusual, defect filling-time dependent capacitance transients that were observed for short defect filling times. For long defect filling pulses however, the experiments show normal charge emission transients that saturate into filling-time independent transients. We present two possibilities for explaining the approach to saturation within the structural memory model. Results of Monte Carlo simulations of the models are discussed.

The dependence of the light-induced degradation on the band gap and hydrogen content in Amorphous silicon and related alloys is discussed. A simple model is presented which predicts that the light-induced degradation should be strongly correlated with the optical gap. In particular, for materials with Egap smaller than approximately 1.2eV no degradation is expected. The implications and consequences of the model for the task of reducing metastability in Amorphous silicon alloys are discussed.

We have used transient and steady-state junction capacitance methods to gain information about the density of states in the mobility gap of silicon-germanium alloys a-Si1-xGex:H (0.25<×<1.0). With those measurements we are also able to differentiate between minority and majority carrier processes in the alloys. We find that the (μτ)p is considerably reduced from its value in a-Si:H, even for films with small germanium concentrations (x=0.25). The density of states deduced from the optical spectra shows two distinct features above and below midgap for all concentrations, which both increase with light-soaking. Light-soaking also sharply reduces the (μτ)p.

The effect of trace amounts of impurities such as carbon and nitrogen on the density of states in undoped a-Si:H has been investigated using drive-level capacitance profiling and transient photocurrent methods. The results indicate a significant correlation between impurity levels and the susceptibility to metastable state creation after light-soaking. We found this correlation to be independent of bandtail broadening and hydrogen content in the material.