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In recent works, we have used transient absorption and ultrafast charge extraction measurements to show that photocreated charges do not thermalize in the disordered density of states that is typical of organic semiconductors. [1] Here, we discuss how this impacts the performance of the device and how the slow thermalization might be used to boost the open circuit voltage Voc and the fill factor FF. The basis of this work is a calibrated kinetic Monte Carlo (kMC) model that we compare to a wide range of experiments and to drift-diffusion calculations. [2] Specifically, comparing the kMC model to both DD simulations and to an analytical model based on Rau’s reciprocity model, [3] shows that the non-equilibrium nature of the photogenerated charge carriers leads to a 0.1-0.2 eV enhancement of Voc as compared to a situation where thermalization is instantaneous. [4] In IV-curves, this is directly visible from the difference between the curves that are calculated by DD and kMC using the same parameters. Interestingly, carefully chosen composition gradients can be used to further enhance Voc and the FF. [5]
REFERENCES:
[1] A. Melianas and M. Kemerink, Photogenerated Charge Transport in Organic Electronic Materials: Experiments Confirmed by Simulations, Advanced Materials 31, 1806004 (2019).
[2] S. Wilken, T. Upreti, A. Melianas, S. Dahlström, G. Persson, E. Olsson, R. Österbacka, and M. Kemerink, Experimentally Calibrated Kinetic Monte Carlo Model Reproduces Organic Solar Cell Current-Voltage Curve, Sol. RRL 4, 2000029 (2020).
[3] T. Kirchartz, J. Nelson, and U. Rau, Reciprocity between Charge Injection and Extraction and Its Influence on the Interpretation of Electroluminescence Spectra in Organic Solar Cells, Phys. Rev. Applied 5, 054003 (2016).
[4] T. Upreti, S. Wilken, H. Zhang, and M. Kemerink, Slow Relaxation of Photogenerated Charge Carriers Boosts Open-Circuit Voltage of Organic Solar Cells, J. Phys. Chem. Lett. 8 (2021).
[5] O. Andersson and M. Kemerink, Enhancing Open-Circuit Voltage in Gradient Organic Solar Cells by Rectifying Thermalization Losses, Solar RRL 4, 2000400 (n.d.).
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