Potassium-neutralized perylene derivative (K4PTC) and rGO-K4PTC composite as effective and inexpensive electron transport layers for polymer solar cells

• The K4PTC and its composite (rGO-K4PTC) are used as alternatives to the Ca ETL.
• K4PTC and rGO-K4PTC exhibit low WF of 4.0 eV, matching the LUMO level of acceptors.
• P3HT and PTB7-th-based devices with respective ETLs yield increases of ∼35% in PCE.
• PSCs with the rGO-K4PTC ETL exhibit better stability under degradation tests in air.

The polymer solar cell (PSC) with Ca/Al electrode always suffers from low stability mainly due to the incorporation of oxygen and moisture-sensitive Ca electron-transport interlayer (ETL). To alleviate this problem, air-stable alternatives to Ca ETL are highly desired. Herein, we report two solution-processable, air-stable, effective and inexpensive ETLs based on potassium-neutralized perylene tetracarboxylic derivative (K4PTC) and its rGO composite (rGO-K4PTC), respectively. These ETL materials were facilely prepared and characterized by means of UV-vis, FL, FTIR, XPS and UPS. Importantly, both ETLs exhibited a low work function (WF) of 4.0 eV, which well matches the LUMO level of fullerene acceptors and allows their use as ETL in PSCs. As a result, the P3HT and PTB7-th-based devices with respective ETL remarkably outperformed those without ETL yielding increases of ∼35% in power conversion efficiencies (PCEs), which indicates good electron-transporting capabilities of K4PTC and rGO-K4PTC interlayers. The high-performance PSCs with the ETL gave average PCEs of 6.17–6.18% (for PTB7-th:PC61BM-based devices) and 7.26% (for PTB7-th:PC71BM-based devices), respectively, fairly comparable to those of Ca/Al devices (6.50% and 7.50%). Furthermore, the rGO-K4PTC device exhibited stability higher than that of the K4PTC device probably due to the fact that the rGO-K4PTC layer can provide more efficient protection for the active layer against degradation. Thus, rGO-K4PTC layer might be more suitable for real applications as compared to the K4PTC layer.

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