Impact of energy alignment and morphology on the efficiency in inorganic–organic hybrid solar cells

Carrier transport in hybrid inorganic–organic solar cells has been studied by means of a two-dimensional drift-diffusion-based model including the generation and motion of excitons. The devices consist of a polymer serving as donor material and a semiconducting small-band gap inorganic component as acceptor material. For the first time it is taken into account that, in strong contrast to purely organic or inorganic cells, charge carriers can be generated at the heterojunction (due to dissociation of the donor excitons) and in the bulk of the acceptor material (due to band to band generation in the inorganic material). The efficiencies of devices were investigated dependent on (i) the donor–acceptor interface geometry, (ii) the transport level offsets at the heterojunction, and (iii) the energy barriers formed at the contacts. For each case, a detailed analysis of the behavior is given. We demonstrate that, depending on the particular scenario, each of these three factors can be responsible for profoundly reduced efficiencies and pronounced s-shaped sections in the I–V curves. Moreover, we show that each of the investigated factors may give rise to equally serious efficiency losses. However, it is not possible to identify a dominant effect. Depending on the particular combination, the efficiency can vary by two orders of magnitudes. In order to avoid such losses, our theoretical assessment reveals that suitable material combinations are required to form (i) ohmic contacts, (ii) preclude formation of isolated islands or nanoparticles during growth, and (iii) possess a hole-blocking offset in the transport levels at the heterojunction.