Recent progress in high efficiency polymer solar cells by rational design and energy level tuning of low bandgap copolymers with various electron-withdrawing units

• Collect recent five-year publications on low bandgap semiconducting polymers, showing the PCE higher than 6%.
• Analyze the factors to control Jsc and Voc of PSCs in terms of chemical structure of conjugated polymers.
• Propose the strategy to achieve both high Voc and high Jsc of polymer based solar cells.

AbstractThis review collects recent five-year publications on low bandgap semiconducting polymers, which are composed of electron donor (D) and electron acceptor (A) units, exhibiting the power conversion efficiency (PCE) higher than 6%. When the photovoltaic performances of different types of D−A semiconducting copolymers are compared after the copolymers are classified into several categories according to the type of A-units, it is realized that diketopyrrolopyrrole (DPP)-based copolymers exhibit high JSCs owing to low bandgaps and low VOCs due to high-lying HOMO levels, while thienopyrroledione (TPD)-based copolymers exhibit high VOCs due to their deep HOMO levels and low JSCs because of wide bandgaps. Benzothiadiazole- and thienothiophene-based copolymers show intermediate values of VOC and JSC between DPP- and TPD-based ones. For further enhancement of photovoltaic performance, DPP-based copolymers may be designed to have deeper HOMO level with the minimum widening of bandgap while TPD-based polymers may be designed to have lower bandgap with the minimum rise of HOMO level. Hence, the energy level tuning must be considered so as to minimize the adverse effect.

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