Design and synthesis of multi-metal oxides as efficient hole transport layers for high-performance perovskite solar cells

Abstract

Perovskite solar cells (PSCs) have become a new star among the photovoltaic community due to the exclusive features of long carrier diffusion length (> 100 nm), high optical absorption coefficient (> 10 4 cm-1), and low exciton binding energy (< 40 meV). Since the first application of perovskite materials as photoactive layers in dye-sensitized solar cells in 2009, there are plentiful researches on PSCs from device efficiency to stability. Currently, the certified power conversion efficiency (PCE) has up to 25% and the reported stability has reached 1500 h light illumination (maintained 89% of its original PCE). Carrier transport layers (CTLs), including hole and electron transport layers, offer vital functions in realizing high-performance and stable PSCs. Even though organic CTLs (such as spiro-OMeTAD) have been intensively investigated in PSCs, their high-cost, sophisticated synthesizing process, and intrinsic instability will hinder the further progress of PSCs in commercial applications. Multi-metal oxides belong to a family of inorganic materials that have attracted tremendous attention because of their tunable optical and electrical properties, superior stability, and low cost. Here, multi-metal oxides (MMO) are oxides consisting of two or more metallic components. Our researches are about the exploiting of new multi-metal oxides materials and fabricating low-temperature and solution-processed HTLs for highly efficient and stable PSCs: (1) We proposed a novel synthesis method of controllable deamination of cobalt-ammonium complexes in the existence of Ni(OH)2 for synthesizing NiCo2O4 nanoparticles (NPs). When adopting the compact and smooth NiCo2O4 film as HTL in inverted PSCs, a higher PCE (about 18.23%) and better device stability than that of PEDOT:PSS based PSCs was achieved, which attributed to the enlarged perovskite grains, effective charge extraction and transportation, and suppressed recombination. (2) We introduced an azeotrope promoted approach for synthesizing In:CuCrO2 NPs as an HTL for high-performance PSCs. This approach can simplify the synthesis process, enable Indium doping in CuCrO2 successfully, and realize treatment-free HTL. Moreover, thanks to the wisely selection of Indium dopants, the In:CuCrO2 HTL presented strengthen p-type semiconducting trait and weaken d-d transition absorption, resulting in the increased transparency and conductivity simultaneously. Finally, In:CuCrO2 HTL based inverted PSCs exhibited the best PCE of 20.54%, which is better than that of pristine CuCrO2 HTL (PCE of 18.37%). (3) We demonstrated low-temperature solution-processed copper and lithium codoped NiOx NPs as an efficient HTL. (Li, Cu):NiOx HTL based inverted PSCs exhibited a champion PCE of 20.83% with negligible hysteresis due to the features of pin-hole free and uniform morphology, suppressed interface recombination, enlarged perovskite crystals, and enhanced hole extraction and transporting ability. Furthermore, (Li, Cu):NiOx HTL based PSCs retained 95% of the previous PCE after storing in inert condition for 60 days. (4) We synthesized fine size CuGaO2 nanoplates by using Cr doping and choline chloride surfactant (named as Cr:CuGaO2-CC) and fabricated 3D structured Cr:CuGaO2-CC/NiOx composite film as an efficient HTL. A higher performance inverted PSCs based on Cr:CuGaO2-CC/NiOx HTL (>20% PCE) than that of NiOx control devices was achieved due to the increased conductivity, well-alignment energy band, as well as high-quality perovskite film.