Low-temperature and solution-processed metal oxides as carrier transport layers for high-performance photovoltaic devices

Abstract

Organic solar cells (SCs) and perovskite SCs are emerging photovoltaic technologies with great potential to explore sustainable and green solar energy for future energy demand. Their remarkable progress in the past few years demonstrates strong possibility of practical application. In the multilayered organic/perovskite SCs, carrier transport layer (CTL) is an indispensable component to efficiently extract photogenerated carriers for achieving high power conversion efficiency (PCE) and long-term stability. Solution processed metal oxides with the features of low cost, stable, high mobility and good transparency are promising CTL materials. To develop metal oxides as efficient CTLs, there are still challenges, such as synthesis of ultra-small and ligand free metal oxide nanocrystals, film formation, interface contact engineering, and etc. In this thesis, we aim to develop highly performed metal oxide based CTLs by focusing on nanocomposite, nanocrystal synthesis, and film deposition. Firstly, we demonstrated a new nanocomposites as hole transport layer (HTL) with low energy consumption by realizing all room-temperature solution processes from the synthesis of the composite nanocrystals to the deposition of high-quality HTL. To obtain desirable electrical properties without using post-treatment, controlling solution acidity during the conversion of precipitant is adopted to achieve a component controllable nanocomposite of maghemite and iron hydroxide, which contributes to in-situ tunable work function from 4.70 to 5.16 eV. Simultaneously, ultra-small size (6–10 nm) and surfactant-free enable high-quality films can be formed with post-treatment. As a result, higher performance of PCE and stability has been demonstrated by the nanocomposite based HTL than that of PEDOT:PSS in organic SCs. Secondly, we proposed a concept of constructing a hypocrystalline intermediate to develop a general approach with ability to synthesize a series of ternary metal oxides (TMO). Four typical TMOs were successfully synthesized with size of sub-ten nanometer and good dispersibility. Particularly, a guideline of this general method is summarized based on the understandings about the impact of metal ion intercalation as well as water and anion coordination on the hypocrystalline intermediate. More importantly, TMO nanoparticles prepared by the general method exhibit excellent ability for forming high-quality (smooth and good-coverage) films for HTL application. As an example, organic/perovskite SCs using TMO films as HTL have been demonstrated with better performance including stability and PCE than that of the control devices. Finally, we demonstrated a chelation scheme to form amorphous tin oxide (amp-SnOx) film as compact electron transport layer (ETL) for inverted perovskite SCs with low-temperature solution process. Acetic acid was used in the hydrolytic process to form a chelating intermediate that can favor the formation of compact amp-SnOx film atop the perovskite layer after low-temperature post-treatment. To enhance the electrical contact between perovskite layer and ETL for better electron extraction, an interface modification additive of 5-aminovaleric acid hydroiodide was adopted into the precursor solution to promote the interface connection by chemical interaction. With this compact amp-SnO2 ETL and a good electrical contact, inverted perovskite SCs have achieved high performance of 15.98% in PCE for reversed scan and 300-hour photo-stability without obvious PCE degradation.