Modulation spectroscopies for characterization of photovoltaic materials and devices

This work has presented new ways of using modulation spectroscopies and applied these to study samples. The main focus has been on continuouswave photoinduced absorption (cwPA). As a contact-free method, cwPA can help researchers in identifying and characterizing good candidates for solar cells without having to construct complete devices. Using an arbitrary reaction order δ, I have generalized the methods for characterizing recombination in cwPA-data. This yielded the ω1/(δ−1) dependence on modulation frequency ω for the quadrature at low frequencies with square-wavemodulation. This expression was used to distinguish between trap-assisted and twodimensional Langevin recombination in films of PBTTT:PC60BM with different blend ratios. I also showed that for trap-assisted recombination with an exponential trap distribution the expression depends on the characteristic energy Ech as ωkT/Ech.

Using a wide span of frequencies and sinusoidal modulation, I extracted generation rates and charge carrier lifetimes from cwPA for a range of temperatures. This revealed distinctly different temperature-dependent generation curves for P3HT:ICBA and PTB7:PCBM. I have described and motivated the procedures necessary to achieve reliable results at high frequencies, especially for slow photodetectors.

Furthermore, I have presented a new type of modulation spectroscopy, electromodulated-photoluminescence quantum efficiency,which can be used to gain access to the quasi Fermi level splitting at open circuit conditions. Combining one photovoltaic material with a range of different contact materials, we showed that the method measures the same QFLS over all devices, despite variations in the open-circuit voltage. The new method offers a way to characterize different losses occuring in solar cells, aiding researchers in optimizing device design.