Wide‐Bandgap Donor–Acceptor Copolymer Based on BDTTz Donor and TPD Acceptor for Polymer Solar Cells Using Fullerene and Nonfullerene Acceptors

Abstract

Polymer solar cells (PSCs) based on bulk heterojunction (BHJ), consisting of a mix of donor (D) and acceptor (A) organic semiconductors, have gained a lot of interest due to their numerous advantages such as flexibility, semitransparency, low cost, and potential for large-scale fabrications. [1][2][3][4][5][6][7][8][9] Moreover, with the emergence of nonfullerene small-molecule acceptors (NFSMAs) [10][11][12][13][14] and interface engineering and optimization of the BHJ active layer, the PSCs have attained the power conversion efficiency (PCE) in the range of 18-19%. [15][16][17][18][19] It is predicted that the PCE can achieve up to 20% with the proper selection of donor [20,21] and NFSMAs and makes these PSCs attractive for commercialization in future. [22,23] Generally, most of the efficient NFSMAs used for PSCs exhibit absorption profile ranging from 650 to 950 nm and deeper frontier molecular energy levels. Therefore, it is highly prospective to design the copolymer donors which must have a high absorption coefficient below 650 nm, that is, complementary to the narrow-bandgap NFSMAs as well as the deeper highest occupied molecular orbital (HOMO) energy level. This will lead to improvement of the light harvesting efficiency of the BHJ active layer to get more short-circuit current ( J SC ), and the deeper HOMO level of copolymer helps to achieve the high open circuit voltage (V OC ) of the PSCs.The concept of the D-A approach has been utilized to design the conjugated copolymers with different bandgaps and energy levels. [24][25][26] In most of the D-A copolymers used for high performance, PSCs contain benzodithiophene (BDT) as donor unit along with different acceptor units and show excellent photovoltaic performance when paired with fullerene derivatives as acceptors in the BHJ active layer of the PSCs. [27][28][29] However, the absorption spectra of D-A copolymers particularly PTB7-Th used as the donor for fullerene-based polymer solar cells significantly overlap with most of the narrow-bandgap NFSMAs. It is challenging to achieve simultaneously both high J SC and V OC for PSCs which limit the PCE. Therefore, to harvest the photons from a wide range of solar spectrum, that is, from 300 to 950 nm, there is a demand