Abstract:

Organic solar cells (OSCs) are a promising next-generation photovoltaic technology that can be used to harvest clean and renewable solar energy. OSCs are typically composed of donor:acceptor blends as photo-active materials. Compared to the conventional inorganic silicon solar cells, OSCs are suitable for large-scale production using roll-to-roll technology, promising low-cost and the potential to avoid environmental pollution. The last few years have witnessed the rapid development of OSCs. The power conversion efficiencies (PCEs) of OSCs have surpassed ~14%–16%, benefiting from the design of novel materials, optimization of blend morphology, and deep understanding of the charge generation mechanism. Currently, the most widely used processing solvents for preparing high-efficient OSCs are chlorinated or aromatic solvents including chlorobenzene, dichlorobenzene, and chloroform, which are highly detrimental to the environment and human health, and may not be utilized for future in industry. Thus, replacing these highly toxic solvents with environmentally friendly alternatives called "green solvents" is an important topic in OSC research. Herein, poly[(2, 6-(4, 8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1, 2-b:4, 5-b′]dithiophene)-co-(1, 3-di(5-thiophene-2-yl)-5, 7-bis(2-ethylhexyl)benzo[1, 2-c:4, 5-c′]dithiophene-4, 8-dione)] (PBDB-T) was used as a reference material to design and synthesize a novel conjugated polymer (PBDB-DT) by extending the alkyl side chains and enlarging the conjugated side groups. The thermal stability of the polymer donor was examined via thermogravimetric analysis, showing that the polymers exhibit very good stability at > 400 ℃. Importantly, PBDB-DT exhibits good solubility in low-toxic solvent tetrahydrofuran (THF) due to its longer alkyl side chains, and shows a strong aggregation effect in THF due to the larger conjugated side groups. A favorable PCE of 10.2% was achieved for the THF-processed PBDB-DT:IT-M based OSC device. In contrast, PBDB-T has limited solubility in THF. The solar cell device based on PBDB-T:IT-M delivered a moderate PCE of 6.41%. The investigation of blend morphology via atomic force microscope suggested that the PBDB-DT:IT-M has a smooth surface, which is favorable for charge generation and transport. These results demonstrate that molecular optimization is a promising strategy to modulate the solubility and achieve high efficiency for organic photovoltaic materials processed using green solvents.

Key words: Conjugated polymer, Non-fullerene acceptor, Organic solar cell, Green solvent, Molecular design