Insights into the Charge-Transfer Mechanism of Organic Photovoltaics: Effect of Domain Size

Gabriele Boschetto,Chris-Kriton Skylaris,Michal Krompiec

doi:10.1021/acs.jpcc.8b03409

Gabriele Boschetto, Chris-Kriton Skylaris + Show 1 more

Open Access

https://doi.org/10.1021/acs.jpcc.8b03409

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Abstract

A great effort has been devoted into understanding the mechanisms of charge generation and charge separation processes in bulk heterojunction solar cells, with the aim of improving their performance. Theoretical methods, such as density functional theory (DFT), have been used to shed light into these complex processes, but the computational cost associated with the simulations limits the model size and thus its accuracy with respect to real heterojunctions. To overcome this limitation, a linear-scaling reformulation of time-dependent DFT is employed, allowing to move beyond the simple polymer–fullerene models and to consider larger complexes composed of more than a single oligomer chain and numerous fullerene molecules. In this work, the interaction between an analogue of PBTZT-stat-BDTT-8, a high-performance D–A statistical copolymer developed by Merck, and phenyl-C61-butyric acid methyl ester is explored, with a focus on (i) the effect of the size of the polymer’s acceptor (A) blocks and (ii) the effect of the domain size. Results suggest that large acceptor blocks enhance the probability of a charge transfer (CT) to occur and that CT states are more significantly affected by the size of the polymer rather than the fullerene phase. Evidence of long-range CT states in the low-energy part of the excited-state manifold is also observed.

Highlights

Organic photovoltaic (OPV) devices have attracted considerable interest in the last 15−20 years, mainly due to their relatively low manufacturing costs, high flexibility, and the possibility of roll-to-roll processing, which enables high- and cheap-throughput device production; OPVs work in both diffuse light and indoor, making them extremely promising as a light harvesting technology.[1,2]Schematically, a typical OPV module consists of an active layer sandwiched between an anode and a cathode and composed of a blend of electron donor and electron acceptor materials
Charge separation may happen via coupling of excited CTn states with the CS state, at present it is still not clear whether these hot states, as well as the excess energy, are important for charge generation: on the one hand, it has been argued that hot charge transfer (CT) states, being in general more delocalized, are useful for charge separation because of the significant decrease in the Coulomb interaction between the electron and the hole and that a large excess energy is beneficial for an efficient charge dissociation;[4−7] on the other hand, this view has been challenged by experiments that showed
For the final and main part of this study we focused on the 5A complex and we increased the complexity of the system: we multiplied in turn the size of the fullerene domain and the size of the polymer domain to investigate how far the charge-transfer states can extend in the PCBM crystal and in the polymer domain, respectively, and if this has any effect on the interface energetics

Summary

Introduction

A typical OPV module consists of an active layer sandwiched between an anode and a cathode and composed of a blend of electron donor and electron acceptor materials This kind of device architecture, which at present is the state of the art in OPVs, is known as the bulk heterojunction (BHJ), first reported by Yu et al in 1995.3 In BHJ devices, the donor material is typically a π-conjugated polymer (P), ideally with a good charge-carrier mobility, while the acceptor material is often a fullerene (F) derivative, e.g., phenyl-C61-butyric acid methyl ester (PC61BM or PCBM). The interfacial CT state can recombine or dissociate: in the first case, depending on the state spin, the electron and the hole can recombine to form the donor ground state, S0, or decay into the donor triplet exciton state, T1; in the second case, if the hole−electron Coulomb attraction is overcome, the charge-separated (CS) state is generated (step iv). Charge separation may happen via coupling of excited CTn states with the CS state, at present it is still not clear whether these hot states, as well as the excess energy (i.e., the energy dissipated during the S1 → CT1 relaxation process), are important for charge generation: on the one hand, it has been argued that hot CT states, being in general more delocalized, are useful for charge separation because of the significant decrease in the Coulomb interaction between the electron and the hole and that a large excess energy is beneficial for an efficient charge dissociation;[4−7] on the other hand, this view has been challenged by experiments that showed

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