Slow Relaxation of Photogenerated Charge Carriers Boosts Open-Circuit Voltage of Organic Solar Cells

Tanvi Upreti; Sebastian Wilken; Huotian Zhang; Martijn Kemerink

doi:10.1021/acs.jpclett.1c02235

Slow Relaxation of Photogenerated Charge Carriers Boosts Open-Circuit Voltage of Organic Solar Cells

J Phys Chem Lett. 2021 Oct 14;12(40):9874-9881. doi: 10.1021/acs.jpclett.1c02235. Epub 2021 Oct 5.

Authors

Tanvi Upreti^{1

2}, Sebastian Wilken^{1

3}, Huotian Zhang⁴, Martijn Kemerink^{1

2}

Affiliations

¹ Complex Materials and Devices, Department of Physics, Chemistry and Biology (IFM), Linköping University, 581 83 Linköping, Sweden.
² Centre for Advanced Materials, Heidelberg University, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany.
³ Physics, Faculty of Science and Engineering, Åbo Akademi University, Porthansgatan 3, 20500 Turku, Finland.
⁴ Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology (IFM), Linköping University, 581 83 Linköping, Sweden.

Abstract

Among the parameters determining the efficiency of an organic solar cell, the open-circuit voltage (V_OC) is the one with most room for improvement. Existing models for the description of V_OC assume that photogenerated charge carriers are thermalized. Here, we demonstrate that quasi-equilibrium concepts cannot fully describe V_OC of disordered organic devices. For two representative donor:acceptor blends, it is shown that V_OC is actually 0.1-0.2 V higher than it would be if the system was in thermodynamic equilibrium. Extensive numerical modeling reveals that the excess energy is mainly due to incomplete relaxation in the disorder-broadened density of states. These findings indicate that organic solar cells work as nonequilibrium devices, in which part of the photon excess energy is harvested in the form of an enhanced V_OC.