Since the average photon energy of solar radiation is well above the band gap energy of most photovoltaic materials, the role of “hot” states in the generation and extraction of free charge carriers is subject of ongoing discussion. Recent experimental work provided firm proof for free carrier formation to proceed mainly via thermalized interfacial charge transfer states in organic bulk heterojunction solar cells [1]. Also, the JV-characteristics of organic solar cells could be well reproduced with steady state transport and recombination data [2], questioning a severe contribution of hot pathways to the photon-to-electron conversion in a working device. On the other hand, recent TPC and TAS data suggested photogenerated carriers to become extracted from the active layer before attaining thermal equilibrium [3]. 

Here we address the question whether and how thermalization of photogenerated charge determines the dynamics of non-geminate recombination under application relevant conditions. We do this by applying state of the art charge extraction methods to organic and hybrid devices of different composition and architecture. For the well-crystallized P3HT:PCBM blend, transient recombination data are consistently explained by bimolecular recombination with a time-independent recombination coefficient k₂, ruling out significant thermalization effects at the relevant time scale. In contrast, the very same measurements performed on the high performance amorphous PCDTBT:PCBM blend reveal a pronounced slow-down of non-geminate recombination, which is attributed to the continuous thermalization of carriers in a inhomogeneously broadened DOS [4]. This decay is shown to proceed throughout the entire time scale studied here (from nanoseconds to microseconds), until all charge carriers have either been extracted or recombined. Interestingly, planar organometal halide perovskite solar cells exhibit a rather similar recombination dynamics, but now being due to rapid extraction of photogenerated charge out of the high mobility perovskite layer [5]. Our measurements prove non-geminate recombination to be of dispersive nature in several organic and hybrid solar cells, with important consequences for the accurate modeling of their steady state performance data.

[1] K. Vandewal et al., Nat. Mater. 13, 63 (2014)

[2] D. Bartesaghi, I. del Carmen Pérez, J. Kniepert, S. Roland, M. Turbiez, D. Neher, L. J. A. Koster, Nat. Commun. 6, 7083 (2014) 

[3] A. Melianas, F. Etzold, T.J. Savenije, F. Laquai, O. Inganäs, M. Kemerink, Nat. Commun. 6, 8778 (2015)

[4] J. Kurpiers, D. Neher, Scientific Reports, submitted 

[5] A. Paulke, S. D. Stranks, J. Kniepert, J. Kurpiers, C. M. Wolff, N. Schön, H. J. Snaith, T.J.K. Brenner, D. Neher, Appl. Phys. Letter., under revision