Snaith group

Abstract:

Thanks to their superior bandgap tunability and high absorption coefficient, metal halide perovskites demonstrate high potential for fabricating multijunction photovoltaics capable of achieving power conversion efficiencies surpassing the radiative efficiency limit of single-junction solar cells[1],[2]. One of the key challenges currently facing all-perovskite multijunction photovoltaics is the low quality of the narrow bandgap (~1.25 eV) mixed tin-lead perovskite films used as the rear absorber. At this conference, we will present our recent investigations on the mixed tin−lead perovskites covering the control of the Sn(II) oxidation[3], interface carrier extraction[4], and in-situ surface reaction[5], as well as the understanding of the solution chemistry and resultant crystallization[6], aiming to generate a global picture toward the comprehensive understanding of this material and its photovoltaic devices. As a result, we have obtained efficiencies of over 23.9% for the single-junction tin−lead perovskite devices, with an open circuit voltage of up to 0.91 V. Building on optimizations of neat lead perovskites, we then showcase the successful integration of these improved mixed tin-lead perovskites into double-, triple-, and quadruple-junction tandem solar cells, achieving efficiencies exceeding 29%, 28%, and 27%, respectively. In addition, we will propose promising strategies for enhancing the light and temperature stability of the involved perovskite subcells, aiming to improve the reliability of efficient all-perovskite multijunction photovoltaics. Furthermore, we will also share insights and recent progress achieved in perovskite-on-silicon multijunction cells.

Abstract:

Transparent conductive films are key components of many optoelectronic devices but are often made from either scarce or brittle materials like indium tin oxide. Carbon nanotube-polymer films offer an abundant and flexible alternative. Here, we report how the dimensions of the carbon nanotube raw material affect their thin film performance and thickness yield when processed with the polymer ethylene-vinyl acetate. We perform chemical doping with several halogenated metals and find the electron affinity of the metal to be a good indicator of p-doping effectiveness. We identify CuCl2 as low-cost alternative to the established gold chloride dopants. Optimising the dopant deposition method allows us to reduce the effect of doping on the optical transmittance. Percolation analysis of our films demonstrates that optimized single-walled carbon nanotube-ethylene-vinyl acetate films show no sign of percolation effects down to thicknesses of 5 nm. Finally, we produce transparent touch-sensitive devices. Comparing several of these devices, we find a linear relationship between the sheet resistance and the on/off ratio of the touch sensing that can be used to determine a threshold film thickness. Using doped carbon nanotube-ethylene-vinyl acetate films increases the on/off ratio and allows us to fabricate touch-sensitive devices with an on/off ratio of 10 at 95% optical transmittance. This clearly demonstrates the potential of these films for transparent touch-sensitive applications.

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.