Exploring the open-circuit voltage of organic solar cells under low temperature

Open-circuit voltage (VOC) in organic solar cells (OSCs) is currently still not well-understood. A generally acceptable view is that VOC is mainly determined by the energy level offset between donor and acceptor materials. Recently in ternary blend OSCs, VOC is found to be dependent on the blend composition. But contrary to expectation, this dependence is not a simple linear relationship, which adds complications to understanding on VOC. Here, in order to figure out the origin of VOC, we performed a series of experiments on both binary and ternary blend OSCs in a wide temperature range from 15 K to 300 K. It is observed that the devices behave like Schottky barrier (SB) diode. By fitting the experimental results with SB diode model, the detailed device parameters of ternary blend OSCs are extracted and it is found that VOC is determined by the energetics of organic molecules and metal at the cathode interface, and the inhomogeneity of the SB also play a great role in the origin of VOC at low temperatures. This work not only paves the way to deep understanding on the origin of VOC, but also opens a door to further exploring the general working principle of OSCs.