cfaed Seminar Series

Organic and hybrid semiconductors offer interesting physical properties for a range of optoelectronic applications, including photovoltaics. For the optimisation of corresponding solar cells, a deeper understanding of the physical processes from light absorption to charge extraction is required.

First, I will discuss the charge carrier recombination in organic blend solar cells, which is usually the dominant loss mechanism in state-of-the-art devices. Optimised devices often show a donor-acceptor phase separation, which the influence on which on encounter-limited charge carrier recombination we investigated using kinetic Monte Carlo (KMC) simulations of pump-probe experiments. Using model bulk heterojunction morphologies, we could quantify the dependence of the recombination rate on domain size and charge carrier mobility. Unifying the Langevin and the minimum mobility model, we show that the mobility dependence of the recombination rate can be described using the power mean of the electron and hole mobilities with a domain size dependent exponent. Additionally, for domain sizes typical of organic photovoltaic devices, we find that the mobility dependence can be approximated well by the geometric mean.

Then, I will discuss the role of radiative recombination, which is the only loss mechanism in an ideal solar cell according to the Shockley-Queisser limit. The less ideal as solar cell becomes, the lower the yield of radiative recombination as compared to non-radiative loss processes, which implies that the overall recombination is increased. In other words, the maximum efficiency of any solar cell can be evaluated in terms of its corresponding ability to emit light. I will report on a joint study where we determined the radiative efficiency for methylammonium lead iodide perovskite solar cells. The emission from the perovskite devices is dominated by a sharp band-to-band transition that has a radiative efficiency clearly higher than that of an average OPV device. As a consequence, the perovskite have the benefit of retaining an open circuit voltage comparably close to its radiative limit, showing the promise of this solar cell type at least in terms of its power conversion efficiency.

The lecture takes place within the Goldberg prize giving ceremony.