Impact of Acceptor Quadrupole Moment on Charge Generation and Recombination in Blends of IDT‐Based Non‐Fullerene Acceptors with PCE10 as Donor Polymer

Abstract

Advancing non-fullerene acceptor (NFA) organic photovoltaics requires the mitigation of the efficiency-limiting processes. Acceptor end-group and sidechain engineering are two handles to tune properties, and a better understanding of their specific impact on the photophysics could facilitate a more guided acceptor design. Here, the device performance, energetic landscape, and photophysics of rhodanine and dicyanovinyl end-capped IDT-based NFAs, namely, O-IDTBR and O-IDTBCN, in PCE10-based solar cells are compared by transient optical and electro-optical spectroscopy techniques and density functional theory calculations. It is revealed how the acceptors' quadrupole moments affect the interfacial energetic landscape, in turn causing differences in exciton quenching, charge dissociation efficiencies, and geminate versus non-geminate recombination losses. More precisely, it is found that the open circuit voltage (V OC ) is controlled by the acceptors' electron affinity (EA), while geminate and non-geminate recombination, and the field dependence of charge generation, rely on the acceptors' quadrupole moments. The kinetic parameters and yields of all processes are determined, and it is demonstrated that they can reproduce the performance differences of the devices' current-voltage characteristics in carrier drift-diffusion simulations. The results provide insight into the impact of the energetic landscape, specifically the role of the quadrupole moment of the acceptor, beyond trivial considerations of the donor-acceptor energy offsets.

Efficiency Limits in Wide‐Bandgap Ge‐Containing Donor Polymer:Nonfullerene Acceptor Bulk Heterojunction Solar Cells

“…The measured figures of merit of the devices are shown in Table 1 ; they indicate that the main difference in the PCEs stems from the differences in J SC and FF, which are both higher for the O‐IDTBCN‐based device, whereas the V OC is larger for the O‐IDTBR‐based device, likely due to the difference in EA of both acceptors. [ 22 ] In Figure 1d, the EQE spectra of both OPV devices are shown. The photoresponse is significantly larger for the O‐IDTBCN‐based device and it extends up to 800 nm.…”

“…We note that no spectral shift over time was observed, indicating that only charge‐induced absorption contributes to the Δ T/T signal, and (polymer) triplet state formation was not observed, unlike recently shown for PCE10:O‐IDTBR blends. [ 22 ]…”

“…The quadrupole moments of the acceptors have recently been determined by us using density functional theory calculations, and their impact on charge generation and recombination has been discussed very recently. [ 22 ] Briefly, the CT state dissociation is more efficient in the O‐IDTBCN system, thus less geminate recombination is observed, as the CT state binding energy is lowered by the larger quadrupole moment. In contrast, the lower quadrupole moment of O‐IDTBR causes less efficient CT state dissociation, therefore facilitating geminate recombination as also experimentally confirmed by the TA measurements reported here.…”

“…[ 21 ] Both acceptors differ in their energetic landscape, more precisely not only in their IE and electron affinity (EA) but also in their quadrupole moments, as recently reported by us. [ 22 ] Using the novel wide‐bandgap germanium‐containing donor polymer, we investigated the performance difference in bulk heterojunction (BHJ) solar cells, and moreover identified the efficiency‐limiting loss processes by time‐resolved spectroscopy. More specifically, a marked difference in J SC and FF was observed, both significantly larger for the O‐IDTBCN‐based solar cells, while the V OC was found to be lower for the O‐IDTBCN‐based solar cells in line with a recent study using PCE10 as donor polymer.…”

“…More specifically, a marked difference in J SC and FF was observed, both significantly larger for the O‐IDTBCN‐based solar cells, while the V OC was found to be lower for the O‐IDTBCN‐based solar cells in line with a recent study using PCE10 as donor polymer. [ 22 ] First, transient absorption (TA) spectroscopy showed a considerably increased geminate recombination (≈38%) in O‐IDTBR‐based solar cells, which explains the comparably low J SC , as it adds significant carrier recombination loss, thereby limiting the device performance. Second, our time‐resolved photoluminescence (TR‐PL) measurements revealed incomplete PL quenching as another efficiency‐limiting factor, leading to low J SC .…”