Prof Henry Snaith FRS

Abstract:

Utilizing molecular bridges presents a promising means to enhance the performance of perovskite solar cells (PSCs). However, concurrently bridging the perovskite absorber and its two adjacent interfaces remains a significant challenge that is yet to be achieved. Here, we construct dual molecular bridges at perovskite heterointerfaces, enabled by a self-organizing additive of 4-fluoro-phenethylammonium formate (4-F-PEAFa) and a synthesized hole transporter of [2-(7H-dibenzo[c, g]carbazol-7-yl)ethyl]phosphonic acid (DBZ-2PACz). The molecular bridges spanning two interfaces lead to the formation of an ‘integral carrier transport pathway’, mitigating both non-radiative recombination and charge-transport losses in the fabricated PSC devices. We thus achieve a champion power conversion efficiency (PCE) of 26.0% (25.6% certified) in inverted PSCs, accompanied by an exceptionally high fill factor of 0.87 (maximum 0.88 from the certified devices, 97% of its Shockley–Queisser limit) and a low ideality factor of 1.06. The unencapsulated devices retain 96% of their PCEs after aging at 85°C for 2200 h and 90% after maximum power point tracking at an elevated temperature of 50°C for 973 h.

Abstract:

Due to minimal material use and low-cost processing, next-generation thin film solar cells represent a promising alternative to traditional crystalline silicon solar cells. Among these, metal-halide perovskite solar cells have seen significant improvements in power conversion efficiency and are now on the verge of market entry. However, most efficient and stable perovskite solar cells contain lead in the perovskite absorber layer, along with indium and silver in their electrodes. This study demonstrates an environmentally benign recycling process for recovering all three elements from end-of-life perovskite solar cells. In short, the process consists of mechanical dismantling (milling), aqueous extraction/purification of PbI2, and acid extraction and purification of indium and silver by nanofiltration. After the quantitative recovery of lead as PbI2 (95 ± 5%), indium and silver were dissolved using nitric acid with recovery rates of 87 ± 7% for both metals. Life cycle assessment calculations were used to determine optimal conditions in terms of minimal environmental impact per gram of extracted element. After acid extraction, nanofiltration was employed using both custom-made layer-by-layer membranes and commercially available acid-resistant flat sheet membranes to separate indium from silver. Using an optimized membrane design, indium was almost entirely retained (96.9 ± 0.4%) using a layer-by-layer membrane at 50% permeate recovery. Hence, a twofold concentration of indium was achieved over the course of the filtration. In contrast, silver was not retained (retention of -7.6 ± 6.3%), resulting in a dilute Ag permeate. Using the commercial flat sheet membrane resulted in similar retention rates, with 98.5 ± 0.4% for indium and 5.8 ± 11.6% for silver. However, this came at the expense of considerably higher operating pressure (25 bar vs 5 bar) and lower flux (6 L/m2h vs 30 L/m2h), resulting in higher energy demand (72 Wh/L vs 9 Wh/L). Therefore, layer-by-layer membrane filtration proved to be the superior method for element recovery from perovskite photovoltaic devices. This study has shown that combining hydrometallurgical processing (aqueous and acidic extraction) with layer-by-layer membrane filtration offers an efficient and environmentally benign approach for metal recovery from end-of-life solar cells. Since indium and silver are also key elements for other thin film photovoltaic applications, layer-by-layer membrane filtration may represent a platform technology for future photovoltaic panel recycling.

Abstract:

Perovskite solar cells achieve high power conversion efficiencies but usually rely on vacuum-deposited metallic contacts, leading to high material costs for noble metals and stability issues for more reactive metals. Carbon-based materials offer a cost-effective and potentially more stable alternative. The vast majority of carbon-electrode PSCs use the negative-intrinsic-positive (n-i-p) or “hole-transport-layer-free” architectures. Here, we present a systematic study to assess the compatibility of “inverted”, p-i-n configuration PSC contact layers with carbon top electrodes. We identify incompatibilities between common electron transport layers and the carbon electrode deposition process and previously unobserved semiconducting properties in carbon electrodes with unique implications for charge extraction and electronic behavior. To overcome these issues, we introduce a double-layer atomic layer deposited tin oxide (SnO2) and Poly­(2,3-dihydrothieno-1,4-dioxin)-poly­(styrenesulfonate) (PEDOT:PSS), yielding up to 16.1% PCE and a retained 94% performance after 500 h of outdoor aging. The study is a crucial step forward for printable, metal-electrode-free, and evaporation-free perovskite PV technologies.