In the well-known work by Shockley and Queisser [1] a fundamental efficiency limit for inorganic solar cells was derived. Giebink et al. [2] enhanced the SQ limit for excitonic solar cells based on the second law of thermodynamics. The high exciton binding energy in organic materials leads to a decrease in the thermodynamic efficiency. The work by Giebink et al. considers the optical processes of light absorption and exciton dissociation, however, the electrical part has been treated to be ideal with the assumption of an infinite charge carrier mobility and zero recombination. The significance of both geminate and non-geminate recombination of charge carriers in organic solar cells has been shown [3], which is caused by the low permittivity and low charge carrier mobility. Thus, the ideal treatment of the electrical properties is a substantial simplification leading to an overestimation of the performance of organic solar cells.

In our work, we present a thermodynamic treatment of the electrical processes inside organic solar cells using kinetic Monte Carlo simulations. Our study of the thermodynamic properties is based on the theoretical framework of stochastic thermodynamics [3]. Herewith, thermodynamic properties are evaluated on the level of particle trajectories and fluctuations. So far, this framework has not been adapted apart from single particle devices [4,5] to the best of our knowledge.  As an exemplary system a bulk-heterojunction consisting of P3HT:PCBM is studied. In addition to [2], electrical properties such as charge transport, recombination, injection and collection at the contacts are included. Charge transport is modeled as a thermally assisted hopping process which leads to heat dissipation into the environment being in thermal equilibrium. Thermodynamic properties such as heat and efficiency are studied, and the contribution of each process to the losses is distinguished.

Based on this work, strategies for optimizing solar cells may be deduced. The presented model allows a detailed thermodynamic characterization and enhances existing thermodynamic limits for organic solar cells.

[1] Shockley, W., and Queisser, H.J. J. Appl. Phys. 32.3 (1961): 510-519.
[2] Giebink, N. C., et al. Phys. Rev. B 83.19 (2011): 195326.
[3] Albes, T., and Gagliardi, A. Phys. Chem. Chem. Phys. 19.31 (2017): 20974-20983.
[3] Ito, S., and Sagawa, T. Phys. Rev. Lett. 111.18 (2013): 180603.
[4] Seifert, U. Rep. Prog. Phys. 75.12 (2012): 126001.
[5] Rutten, B., et al. Phys. Rev. B 80.23 (2009): 235122