Efficient free charge generation in organic photovoltaics (OPVs) generally relies on the energetic offset at a donor (D) and an acceptor (A) interface. The interfacial area is maximised by blending the components into a bulk heterojunction (BHJ). While the BHJ morphology has undoubtedly proven effective to increase the power conversion efficiency in OPVs, the physical blending of components often leads to morphological degradation due to phase separation, resulting in poor performance stability. A promising approach to tackle the stability issue is the use of single-component macromolecular semiconductors that contain tethered D-A units [1,2]. The incorporation of Y-type acceptors into conjugated block copolymers (CBC) has recently led to significant improvements in conversion efficiency, reaching 15% in the best cases, while maintaining good performance stability [2,3,4]. Interestingly, in some cases CBC based devices have demonstrated superior performance over BHJ devices containing the same segments. This raises the question of how photocurrent and voltage generation are controlled in these materials and how constraining the heterojunction into chemically bonded D-A pairs influences the processes of charge pair generation and recombination. Factors that may influence these processes in CBCs in comparison with BHJs are the stronger coupling, more controlled domain sizes, and reduced configurational disorder at the interface.
To explore this question, we compare the PV performance of a series of CBCs with different interfacial energetics, with a series of polymer-D:small-molecule-A (PSM) BHJs and polymer-D:polymer-A (PP) BHJs constructed from the same fundamental building blocks. While the CBC series probes the impact of interface energetics, comparison with the two BHJ equivalents allow us to explore the impact of covalent bonding of the D-A heterojunction compared to a non-bonded D-A pairs, as well as the impact of the microstructure in the acceptor phase. When employing these materials in OPV devices, we find a consistent trade-off between current generation and non-radiative voltage losses across molecular architectures and across energetic offsets. Compared to the other microstructures, CBCs show low non-radiative losses, however, at the cost of reduced charge generation, which we attribute to limited exciton dissociation efficiency. We attempt to explain these trends at a molecular level, by simulating the configurational phase space of bonded and non-bonded D-A structures using molecular dynamics. We study the properties of the thermodynamic ensembles of local excited states and charge transfer (CT) state, where the latter can either occur through-space or through-bond. We uncover the relationship between the interfacial energetic offset and the CT state energy, and find that conjugated heterojunctions lead to bright, high-energy CT states that hybridise strongly with the Frenkel exciton. The landscape of excited states explains the trends in charge generation and voltage losses and allows us to propose design principles to further improve CBC photovoltaic devices.