Revealing the recombination dynamics in squaraine-based bulk heterojunction solar cells

摘要

We combine steady-state with transient optoelectronic characterization methods to understand the operation of photovoltaic devices based on a benchmark model squaraine blended with a fullerene acceptor. These devices suffer from a gradual decrease in the fill factor when increasing the active layer thickness and incident light intensity. Using transient photocurrent, transient photovoltage, and bias-assisted charge extraction measurements, we show that the fill factor deteriorates due to slow charge carrier collection competing with bimolecular recombination. Under normal operating conditions, we find a bimolecular recombination rate constant of ~10~(-17) m~3 s~(-1), which corresponds to a reduction of one to two orders of magnitude compared to the Langevin model.