Zwitterion Nondetergent Sulfobetaine-Modified SnO(2) as an Efficient Electron Transport Layer for Inverted Organic Solar Cells

[Image: see text] Tin oxide (SnO(2)) 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(2) ETL modified by zwitterion nondetergent sulfobetaine 3-(4-tert-butyl-1-pyridinio)-1-propanesulfonate (NDSB-256-4T). We found that NDSB-256-4T helps reduce the work function of SnO(2), resulting in more efficient electron extraction and transport to the cathode of iOSCs. NDSB-256-4T also passivates the defects in SnO(2), 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(2) and the active layer. Thus, a higher power conversion efficiency (PCE) and longer device stability of iOSCs are expected for a combination of SnO(2) and NDSB-256-4T than for devices based on SnO(2) only. With these enhanced interfacial properties, P3HT:PC(60)BM-based iOSCs using SnO(2)/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(2) 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:PC(70)BM systems, we achieved an impressive average PCE of 8.22% with SnO(2)/NDSB-256-4T (0.2 mg/mL) as the ETL, while devices based on SnO(2) exhibited an average PCE of only 4.45%. Thus, the use of zwitterion to modify SnO(2) ETL is a promising way to obtain both highly efficient and stable iOSCs.