Mitigating Detrimental Effect of Self‐Doping Near the Anode in Highly Efficient Organic Solar Cells

• Published in: Advanced functional materials 2023-04, Vol.33 (16), p.n/a
• Main Authors: Kim, Yong Ryun, Sandberg, Oskar J., Zeiske, Stefan, Burwell, Gregory, Riley, Drew B., Meredith, Paul, Armin, Ardalan
• Format: Article
• Language: English
• Subjects: Chemical reactions; Diffusion layers; Doping; Electric contacts; Energy conversion efficiency; Heterojunctions; hole transport layer; Interface stability; interfacial doping; interfacial recombination; Materials science; Monolayers; organic solar cells; Phosphonic acids; Photoelectric effect; Photovoltaic cells; Polystyrene resins; Self-assembly; Solar cells
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.202300147

Poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been one of the most established hole transport layers (HTL) in organic solar cells (OSCs) for several decades. However, the presence of PSS− ions is known to deteriorate device performance via a number of mechanisms including diffusion to the HTL‐active layer interface and unwanted local chemical reactions. In this study, it is shown that PSS− ions can also result in local p‐doping in the high efficiency donor:non‐fullerene acceptor blends – resulting in photocurrent loss. To address these issues, a facile and effective approach is reported to improve the OSC performance through a two‐component hole transport layer (HTL) consisting of a self‐assembled monolayer of 2PACz ([2‐(9H‐Carbazol‐9‐yl)ethyl]phosphonic acid) and PEDOT:PSS. The power conversion efficiency (PCE) of 17.1% using devices with PEDOT:PSS HTL improved to 17.7% when the PEDOT:PSS/2PACz two‐component HTL is used. The improved performance is attributed to the overlaid 2PACz layer preventing the formation of an intermixed p‐doped PSS− ion rich region (≈5–10 nm) at the bulk heterojunction‐HTL contact interface, resulting in decreased recombination losses and improved stability. Moreover, the 2PACz monolayer is also found to reduce electrical shunts that ultimately yield improved performance in large area devices with PCE enhanced from 12.3% to 13.3% in 1 cm2 cells. PEDOT:PSS – the most widely used hole transport layer (HTL) in organic solar cells – is found to induce local p‐doping near the HTL interface inside the photoactive layer, resulting in photocurrent loss. This loss can be mitigated by adding a self‐assembled monolayer of 2PACz on PEDOT:PSS, ultimately improving the device performance.