Development of new inorganic p-type materials for perovskite solar cells

Abstract

ABSTRACT: Photovoltaic (PV) energy is one the most promising alternatives to replace fossil fuels as main electricity source and decrease greenhouse effect caused by related CO2 emissions. Perovskite solar cells (PSC) is a third generation PV technology which has revolutionized this field because of its extremely fast increase in power conversion efficiency (PCE). Remarkably, PSC PCE has achieved values >24% being comparable with polycrystalline silicon and thin film PV (CdTe, CIGS) so that it has become in a real alternative to traditional silicon PV. In order to reach a commercialization quality level, PSC need to improve its long-term stability, PCE of large devices and cost-effectiveness. It has been widely recognized that charge selective layers are crucial components of PSC structure for managing of photogenerated charges. Specifically, hole-transporting materials (HTM) have a strong influence on device performance, stability and cost. However, they represent one of the major bottleneck for PSC commercialization owing to unstable and expensive organic molecules materials often employed. In this framework, inorganic p-type semiconductors are a promising option to overcome issues related to organic HTM because of their intrinsic good properties as hole selective contacts and ambient stability. Nevertheless, relative few inorganic materials have been explored for this function so that they have not reached organic counterparts’ performance. Thus, it is mandatory to explore and optimize new inorganic alternatives for HTM in PSC. Accordingly, in this thesis two inorganic p-type semiconductors, namely, copper sulfide and nickel oxide, have been applied in two different PSC architectures by three distinct approaches. First, copper sulfide thin films (CuxS) were fabricated by spray pyrolysis technique and applied as semi-transparent electrode in planar p-i-n PSC. Morphological and optoelectronic properties of CuxS were correlated with device performance. In the second approach, copper sulfide was synthesized in the form of nanoparticles (CuS NPs) and colloidal dispersions in non-polar solvents were obtained. Subsequently, the CuS NPs were applied by spin-coating technique in a mesoscopic n-i-p architecture, acting as sole HTM and exhibiting efficiencies over 13%. Third, hydrophobic nickel oxide (ho-NiOx) nanocrystals were synthesized and corresponding colloidal dispersions were obtained. Then, n-i-p planar PSC were fabricated employing ho-NiOx as sole HTM which was deposited by solution-process spin-coating. Remarkably, PCE as high as 12.7% and impressive high-humidity air stability was observed. Namely, PCE retention over 90% was exhibited by ho-NiOx-based PSC for more than 1000 h. The optoelectronic properties, energy band alignment and interface phenomena are studied and discussed in detailed for all the obtained semiconductors and devices.