Evidence of Molecular Structure Dependent Charge Transfer between Isoindigo-Based Polymers and Fullerene

Abstract

The effects of the oligothiophene length of two thiophene-isoindigo copolymers on film morphology, charge transfer, and photovoltaic device performance are reported. Despite the similarities in their repeat unit structures, the two polymers show distinctly different film morphologies and photovoltaic performance upon blending with PC71BM. We found that there is a significant increase in the dielectric constant of the photoactive film upon blending fullerene with the polymer that exhibits a higher power conversion efficiency. Blend photoluminescence transients revealed a fast dissociation route in the better performing polymer followed by a slower decay. The fast decay in transient PL is attributed to a higher charge transfer efficiency when blending with the fullerene. We suggest that the charge transfer efficiency is determined not only by the microscopic morphology but also whether the polymer can accommodate the fullerene molecules in close proximity to the acceptor moiety to facilitate electronic coupling between the isoindigo acceptor and the fullerene molecule. We propose that the fast decay component seen in transient PL for the better performing polymer, along with the increase in dielectric constant, is a signature of enhanced electronic coupling between the polymer and the fullerene. The enhanced electronic coupling is thought to originate from a polymer chemical structure which allows the fullerene molecules to come to closer proximity for more efficient charge transfer.

Effect of Polymer–Fullerene Interaction on the Dielectric Properties of the Blend

“…We used two thiophene–isoindigo copolymers, poly(terthiophene‐ co ‐isoindigo) P(T3‐iI) and poly(thiophene‐ co ‐isoindigo) P(T1‐iI), to study the effects of the oligothiophene length on device performance and dielectric properties. We previously showed that, in devices made using P(T3‐iI):PC 71 BM, we see no large‐scale phase separation and a high PCE of 6.1%, whereas in devices made using P(T1‐iI):PC 71 BM large scale phase separation exists in the active layer and the PCE remains at 2.6% . In addition to device performance and film morphology, dielectric constant was also found to be affected by the oligothiophene length with the P(T3‐iI):PC 71 BM blend having a dielectric constant of 4.8 ± 0.1 and P(T1‐iI):PC 71 BM a dielectric constant of 4.2 ± 0.2.…”

Critical Role of Polymer Aggregation and Miscibility in Nonfullerene‐Based Organic Photovoltaics

“…To further investigate the charge generation dynamics in the films studied herein, transient PL measurements were also carried out. It has been previously shown that in many cases when blending a polymer donor with an electron acceptor a biexponential decay is observed in the transient PL spectrum . A fast initial decay on a sub‐50 ps timescale is assigned to nonradiative decay of excitons to the charge‐transfer (CT) states formed at the D‐A interface, which in turn leads to fast exciton dissociation.…”

Donor Conjugated Polymers with Polar Side Chain Groups: The Role of Dielectric Constant and Energetic Disorder on Photovoltaic Performance

“…Systems based on cyano‐containing polymers show higher dielectric constants (4.8 ± 0.2 for DTS‐TPD‐C8‐CN and 4.7 ± 0.2 for DTS‐TPD‐C4‐CN) than those based on the noncyano‐containing parent polymers (4.0 ± 0.1 for DTS‐TPD‐C8 and 3.5 ± 0.1 for DTS‐TPD‐C4). The effect of blending on dielectric constant has been investigated in our previous studies . It has been found that the blend dielectric constant is determined by polymer–fullerene interaction resulting in a change in the dielectric environment of the blend, which leads to a significant increase in dielectric constant in the blend upon blending the polymers with fullerene.…”