Unequal Partnership: Asymmetric Roles of Polymeric Donor and Fullerene Acceptor in Generating Free Charge

Abstract

Natural photosynthetic complexes accomplish the rapid conversion of photoexcitations into spatially separated electrons and holes through precise hierarchical ordering of chromophores and redox centers. In contrast, organic photovoltaic (OPV) cells are poorly ordered, utilize only two different chemical potentials, and the same materials that absorb light must also transport charge; yet, some OPV blends achieve near-perfect quantum efficiency. Here we perform electronic structure calculations on large clusters of functionalized fullerenes of different size and ordering, predicting several features of the charge generation process, outside the framework of conventional theories but clearly observed in ultrafast electro-optical experiments described herein. We show that it is the resonant coupling of photogenerated singlet excitons to a high-energy manifold of fullerene electronic states that enables efficient charge generation, bypassing localized charge-transfer states. In contrast to conventional views, our findings suggest that fullerene cluster size, concentration, and dimensionality control charge generation efficiency, independent of exciton delocalization.

Morphology, Temperature, and Field Dependence of Charge Separation in High-Efficiency Solar Cells Based on Alternating Polyquinoxaline Copolymer

“…Our results question the thermodynamic model, that proposes that there is a balance between lowest CT state and separated charges. Instead, the results agree well with both the dissociation during cooling model 42 and the quantum mechanical model 24,41 , that emphasize the importance of energetic disorder or charge delocalization in the charge separation process. With increasing fullerene content (from the 3:1 to the 2:5), an increasing amount of charge carriers delocalizes in the fullerene clusters, contributing to the enhanced device performance.…”

“…It is well known that in a majority of polymer/fullerene blends, 24 including TQ1:PC71BM, 52 an increase of the acceptor fraction has a positive effect on the local morphology and charge extraction.…”

“…On the opposite, the local morphology, particularly ordering and aggregation effects, seem to play a critical role for the efficiency of long-range charge separation. 24,49 There is less consensus on the field dependence of charge separation. While most ultrafast (sub-ns) studies reported minor effects of the applied electric field, 24 experiments on longer time scales demonstrated both strong and weak field influences on the charge separation.…”

Temperature Dependence of Charge Carrier Generation in Organic Photovoltaics

“…Some alternative explanations for efficient CT state separation include the contribution from high local mobility and disorder, which account for the cooling of charge carriers due to hopping within the disordered density of states [14][15][16][17][18]. Very recently, a quantum mechanical model suggests that charge delocalization might play a key role in ultrafast free charge carrier generation [19][20][21][22][23][24]. Delocalization results in a large initial electron-hole distance in highly efficient blends and hence small CT state binding energy [20,21].…”

A Tetraperylene Diimides Based 3D Nonfullerene Acceptor for Efficient Organic Photovoltaics

“…DFT calculations indicate that tetra‐PDI can be seen as a molecule mimics fullerene derivatives in terms of π‐surface exposure and electron affinity (C 60 with a diameter of 7.1 Å, Figure 1c,f) if alkyl chains are not taken into account. Structurally, the 3D‐like tetra‐PDI possesses certain advantages over the planar ones that can allow charge separation and transport from different directions15, 16 and can inhibit the commonly observed excimer formation at the D–A interfaces 17, 18…”