Open‐Circuit Voltage and Effective Gap of Organic Solar Cells

The open‐circuit voltage ( V OC ) of an organic solar cell is limited by the donor‐acceptor material system. The effective gap E g eff between the electron affinity of the acceptor and the ionization potential of the donor is usually regarded as the upper limit for V OC , which is only reached for T → 0 K. This relation is confirmed for a number of small‐molecule bulk heterojunction p‐i‐n type solar cells by varying the temperature and illumination intensity. With high precision, the low temperature limit of V OC is identical to E g eff . Furthermore, the influence of the hole transport material in a p‐doped hole transport layer and the donor‐acceptor mixing ratio on this limit V 0 is found to be negligible. Varying the active material system, the quantitative relation between V 0 and E g eff is found to be identity. A comparison of V 0 in a series of nine different donor‐acceptor material combinations opens a pathway to quantitatively determine the ionization potential of a donor material or the electron affinity of an acceptor material.