Zwitterion Nondetergent Sulfobetaine-Modified SnO as an Efficient Electron Transport Layer for Inverted Organic Solar Cells.

Tin oxide (SnO) has been widely accepted as an effective electron transport layer (ETL) for optoelectronic devices because of its outstanding electro-optical properties such as its suitable band energy levels, high electron mobility, and high transparency. Here, we report a simple but effective interfacial engineering strategy to achieve highly efficient and stable inverted organic solar cells (iOSCs) via a low-temperature solution process and an SnO ETL modified by zwitterion nondetergent sulfobetaine 3-(4--butyl-1-pyridinio)-1-propanesulfonate (NDSB-256-4T). We found that NDSB-256-4T helps reduce the work function of SnO, resulting in more efficient electron extraction and transport to the cathode of iOSCs. NDSB-256-4T also passivates the defects in SnO, which serves as recombination centers that greatly reduce the device performance of iOSCs. In addition, NDSB-256-4T provides the better interfacial contact between SnO and the active layer. Thus, a higher power conversion efficiency (PCE) and longer device stability of iOSCs are expected for a combination of SnO and NDSB-256-4T than for devices based on SnO only. With these enhanced interfacial properties, P3HT:PCBM-based iOSCs using SnO/NDSB-256-4T (0.2 mg/mL) as an ETL showed both a higher average PCE of 3.72%, which is 33% higher than devices using SnO only (2.79%) and excellent device stability (over 90% of the initial PCE remained after storing 5 weeks in ambient air without encapsulation). In an extended application of the PTB7-Th:PCBM systems, we achieved an impressive average PCE of 8.22% with SnO/NDSB-256-4T (0.2 mg/mL) as the ETL, while devices based on SnO exhibited an average PCE of only 4.45%. Thus, the use of zwitterion to modify SnO ETL is a promising way to obtain both highly efficient and stable iOSCs.