Using Ultralow Dosages of Electron Acceptor to Reveal the Early Stage Donor–Acceptor Electronic Interactions in Bulk Heterojunction Blends

Abstract

Tuning the donor–acceptor (D–A) weight ratio is an essential step to optimize the performance of a bulk heterojunction (BHJ) solar cell. The unoptimized regime with a low acceptor concentration is generally unexplored despite it may reveal the early stage electronic D–A interactions. In this study, PTB7:PC71BM is used to examine factors that limit the device performance in unoptimized regime. The key limiting factor is the creation of traps and localized states originated from fullerene molecules. Photothermal deflection spectroscopy is used to quantify the trap density. Starting with pristine PTB7, addition of small concentration of fullerene increases the electron trap density and lowers the electron mobility. When the D–A weight ratio reaches 1:0.1, fullerene percolation occurs. There is an abrupt drop in trap density and simultaneously a six orders of magnitude increase in the electron mobility. Furthermore, the fill factors of the corresponding photovoltaic devices are found to anticorrelate with the trap density. This study reveals that electron trapping is the key limiting factor for unoptimized BHJ solar cells in low fullerene regime.

Impact of Nonfullerene Molecular Architecture on Charge Generation, Transport, and Morphology in PTB7‐Th‐Based Organic Solar Cells

“…The corresponding energetic disorder values for blend films are summarized in Table 2 . Typically, the energetic disorder may vary from 70 to 120 meV, depending on the different polymer:acceptor systems, donor/acceptor ratio, and morphology . The high energetic disorder of 94.8 ± 2.2 meV in PTB7‐Th:SF‐PDI4 reflects that a broadening distribution of electron hopping site manifold is probably due to the interruption of SF‐PDI4‐based network for electron transport.…”

“…Based on the above results, we propose that both the electronic coupling between the donor polymer and the acceptors and the nanoscale morphology are the origins of the differences in charge generation and transport efficiencies in these three donor–acceptor systems. As illustrated in Figure , the PC 71 BM molecules have a spherical structure and this unique structure enables effective electronic coupling between PC 71 BM and the thieno[3,4‐b]thiophene (TT) unit in PTB7‐Th, which is a key factor determining the fullerene‐based solar cell performance . In addition to the donor acceptor interaction in the blend, the nanoscale morphology is another factor determining the device performance.…”

Near-infrared and visible light dual-mode organic photodetectors

“…The dc bias used in the transient photoresponses measurements was controlled by the RIGOL DP821A dc power supply. report (26). The charge transport mobility measurement reveals the electron trapping nature of the binary blend P3HT:PC 70 BM (100:1) and ternary blend P3HT:PTB7-Th:PC 70 BM (70:30:1) organic active layers in the PM OPDs.…”

Recent Advances in Morphology Optimization for Organic Photovoltaics

“…c) Schematic illustration describing the percolation of fullerene domains: hopping via polymer sites, hopping via polymer sites in the presence of a small amount of fullerene, hopping via both host and guest sites, and hopping via guest sites. a–c) Reproduced with permission . Copyright 2017, Wiley‐VCH.…”

“…Ho et al studied how the donor:acceptor composition affects the photovoltaic performance of solar cells with the PTB7:PC 70 BM blend system . The addition of ultralow dosages of PC 70 BM to the blend was performed to reveal the early stages of the development of the electronic relationship between PTB7 and PC 70 BM (Figure a–c).…”