Zwitterion Nondetergent Sulfobetaine-Modified SnO₂ as an Efficient Electron Transport Layer for Inverted Organic Solar Cells

Tin oxide (SnO₂) 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 lowerature solution process and an SnO₂ 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 SnO₂, resulting in more efficient electron extraction and transport to the cathode of iOSCs. NDSB-256-4T also passivates the defects in SnO₂, which serves as recombination centers that greatly reduce the device performance of iOSCs. In addition, NDSB-256-4T provides the better interfacial contact between SnO₂ and the active layer. Thus, a higher power conversion efficiency (PCE) and longer device stability of iOSCs are expected for a combination of SnO₂ and NDSB-256-4T than for devices based on SnO₂ only. With these enhanced interfacial properties, P3HT:PC₆₀BM-based iOSCs using SnO₂/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 SnO₂ 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:PC₇₀BM systems, we achieved an impressive average PCE of 8.22% with SnO₂/NDSB-256-4T (0.2 mg/mL) as the ETL, while devices based on SnO₂ exhibited an average PCE of only 4.45%. Thus, the use of zwitterion to modify SnO₂ ETL is a promising way to obtain both highly efficient and stable iOSCs.