Forth and Back and in Between: Free Charge Formation and Recombination in Organic Solar Cells

The invention of new materials combined with an improved knowledge of structure-property relationships of organic donor-acceptor blends led to an impressive improvement in their energy conversion efficiency to 17 % [1]. This success seems to contradict the simple view that the long range Coulomb interaction between electrons and holes in organic semiconductors causes inefficient formation but efficient recombination of free charge. An important characteristic of organic solar cells is that they comprise at least two organic components of different chemical structure, introducing a large complexity of the morphology and electron landscape of the active layer.

In this talk, I will present results regarding the generation and recombination of free charges in selected bulk heterojunction solar cells, with particular focus on the role of the interfacial CT state. We show that free charge formation proceeds predominately through low energy CT states, ruling out the predominance of a hot CT dissociation pathway, and that the same states dominate the subsequent recombination [2]. For fullerene-based solar cells with a low donor content, we find that the efficiency of charge generation is limited by the same mechanism that limits the VOC, namely non-radiative recombination of the CT states via vibronic coupling [3]. Notably, the rate of this recombination process obeys the classical energy gap law, implying that donor-acceptor blends benefit from a higher CT energy through longer CT lifetimes and more efficient photocurrent generation [4]. Consistent with this result, we observe that devices suffer from inefficient CT dissociation also through a higher rate of non-geminate recombination [5]. As a consequence, it’s only the systems with very efficient charge generation and very fast CT dissociation that the free carrier recombination is strongly suppressed, irrespective of the details of the spin statistics. We, finally, present recent results on a highly efficient polymer:NFA blend, where we find a surprisingly low activation energy for free charge generation, despite a low energy offset at the heterojunction [6]. These results highlight the importance of a comprehensive understanding of the energy landscape, and how it affects the pathway from the bound CT exciton to the spatially separated electron-hole pair.

[1] Lin, Y.; Adilbekova, B.; Firdaus, Y.; Yengel, E.; Faber, H.; Sajjad, M.; Zheng, X.; Yarali, E.; Seitkhan, A.; Bakr, O. M.; et al. 17% Efficient Organic Solar Cells Based on Liquid Exfoliated WS2 as a Replacement for PEDOT:PSS. Adv. Mater. 2019, 31.

[2] Kurpiers, J.; Ferron, T.; Roland, S.; Jakoby, M.; Thiede, T.; Jaiser, F.; Albrecht, S.; Janietz, S.; Collins, B. A.; Howard, I. A.; et al. Probing the Pathways of Free Charge Generation in Organic Bulk Heterojunction Solar Cells. Nat. Commun. 2018, 9, 2038.

[3] Benduhn, J.; Tvingstedt, K.; Piersimoni, F.; Ullbrich, S.; Fan, Y.; Tropiano, M.; McGarry, K. A.; Zeika, O.; Riede, M. K.; Douglas, C. J.; et al. Intrinsic Non-Radiative Voltage Losses in Fullerene-Based Organic Solar Cells. Nat. Energy 2017, 2, 17053.

[4] Collado-Fregoso, E.; Pugliese, S. N.; Wojcik, M.; Benduhn, J.; Bar-Or, E.; Perdigón Toro, L.; Hörmann, U.; Spoltore, D.; Vandewal, K.; Hodgkiss, J. M.; et al. Energy-Gap Law for Photocurrent Generation in Fullerene-Based Organic Solar Cells: The Case of Low-Donor-Content Blends. J. Am. Chem. Soc. 2019, 141, 2329–2341.

[5] Shoaee, S.; Armin, A.; Stolterfoht, M.; Hosseini, S. M.; Kurpiers, J.; Neher, D. Decoding Charge Recombination through Charge Generation in Organic Solar Cells. Sol. RRL 2019, 1900184, 1900184.

[6] Perdigón-Toro. L. et al., Adv. Mater., accepted for publication