Favorable Mixing Thermodynamics in Ternary Polymer Blends for Realizing High Efficiency Plastic Solar Cells

Abstract

Ternary blends with broad spectral absorption have the potential to increase charge generation in organic solar cells but feature additional complexity due to limited intermixing and electronic mismatch. Here, a model system comprising the polymers poly[5,5‐bis(2‐butyloctyl)‐(2,2‐bithiophene)‐4,4‐dicarboxylate‐alt‐5,5‐2,2‐bithiophene] (PDCBT) and PTB7‐Th and PC70BM as an electron accepting unit is presented. The power conversion efficiency (PCE) of the ternary system clearly surpasses the performance of either of the binary systems. The photophysics is governed by a fast energy transfer process from PDCBT to PTB7‐Th, followed by electron transfer at the PTB7‐Th:fullerene interface. The morphological motif in the ternary blend is characterized by polymer fibers. Based on a combination of photophysical analysis, GIWAXS measurements and calculation of the intermolecular parameter, the latter indicating a very favorable molecular affinity between PDCBT and PTB7‐Th, it is proposed that an efficient charge generation mechanism is possible because PTB7‐Th predominantly orients around PDCBT filaments, allowing energy to be effectively relayed from PDCBT to PTB7‐Th. Fullerene can be replaced by a nonfullerene acceptor without sacrifices in charge generation, achieving a PCE above 11%. These results support the idea that thermodynamic mixing and energetics of the polymer–polymer interface are critical design parameter for realizing highly efficient ternary solar cells with variable electron acceptors.

Simultaneous enhanced efficiency and thermal stability in organic solar cells from a polymer acceptor additive

“…Several effective approaches such as suitably reducing the crystallinity of photovoltaic polymers 31,32 or fullerenes 24,33 , selecting suitable D/A pairs 34,35 , incorporating a "molecular lock" 16,36 , and cross-linking between D/A components in the active layers have been developed to solve thermal instability of BHJ blends 22,37,38 . Among them, blending host-active layers with additives emerges as a simple and general approach to achieve the optimal blend morphology, and also improves the electronic performance and environmental stability in OSCs [39][40][41] . The capability of packing motifs at elevated temperatures and preserving close intermolecular interactions is imperative but still lacking, especially for high-performance NFA systems 9,14,42,43 .…”

Enhanced Hole Transport in Ternary Blend Polymer Solar Cells

“…We therefore conclude that PTB7-Th domains are hole transport channels and PTB7-Th hole polarons serve as hole carriers in the PTB7-Th/PDCBT/PCBM ternary blend film. [26] The second one is hole transfer from PDCBT to PTB7-Th domains as reported for other blends. Here, we propose two possible explanations.…”

“…[34][35][36] Very recently, a fast energy transfer from PDCBT to PTB7-Th has been reported for the PTB7-Th/PDCBT/PCBM ternary blend. [26] The second one is hole transfer from PDCBT to PTB7-Th domains as reported for other blends. [28,37] As shown in Figure 6, the HOMO energy level was evaluated to be À 5.0 eV for PTB7-Th and À 5.1 eV for PDCBT films by the photoelectron emission yield spectroscopy.…”

“…Next, we estimated hole mobility in ternary and binary blend films by measuring space-charge limited current (SCLC). [26] In order to assign hole polarons in PTB7-Th/PDCBT/PCBM ternary blends, we measured transient absorption spectra of binary and ternary blend films. Figure 2 shows the absorption spectra of PTB7-Th, PDCBT, and PCBM neat films.…”

“…A recent study has also shown similar improvements both in J SC and FF for PTB7-Th/PDCBT/PCBM ternary blend polymer solar cells. [26] In order to assign hole polarons in PTB7-Th/PDCBT/PCBM ternary blends, we measured transient absorption spectra of binary and ternary blend films. Figure 3 shows the transient absorption spectra of each binary blend film.…”