Planar low-temperature n-i-p perovskite solar cell: from concept to up-scaling and stability

The research activity, started in 2015, is mainly focused on the development of highly efficient perovskite solar cell based on a planar n-i-p structure. In particular, the aim of this thesis is the development of a low temperature process in contrast with the high temperature processing made on mesoscopic structure. In the first part of next chapter, I will evaluate the use of different materials as ETL in order to fabricate the PSC on planar structure without using high temperature steps. In the first chapter will be introduced the main properties, key points and issues of the perovskite absorbing material. The introduction continues showing the planar architecture and the different materials employed. As last point is presented the most used scale-up processes needed to fabricate efficient planar Perovskite Solar Modules. The second chapter will focus on the fabrication of high-performance low-T planar PSCs. Initially I will characterize two widely employed inorganic ETL used in PSCs, the TiO2 and the SnO2. The former generally shows good performances when treated at high temperatures or when it is formed by using chemical or physical depositions such as CVD or PVD. Instead the SnO2 layer is generally optimized by using low temperature steps and solution processing. The main action is related to the increase of the PV performance by using a CH3NH3PbI3 perovskite deposited via Solvent Engineering method in N2 controlled environment. The main optimization will be made on the SnO2 ETL deposited by low-T solution process. In particular, I will introduce an UV-assisted low-temperature annealing approach in order to improve the film properties and pinholes-free formation of the SnO2 layer. A deep investigation by Synchrotron Radiation induced XPS is made to proof the beneficial role of UV-curing on the SnO2 film formation. In a second step we will evaluate the impact of the deposition environment in the perovskite film made in N2 or in air by using morphological and electrical characterization. The role of moisture will be discussed by checking the PV performances when the perovskite layer is deposited at different Relative Humidity Levels. Successively, we will fabricate a multi-cation perovskite layer to enhance the PCE and the stability of the Low-T planar PSCs, obtaining a maximum efficiency of 19.2% on small area (0.09 cm2 defined by a shadow mask). In chapter 3 the single and multi-cation PSCs will be tested on larger active areas, thanks to the fabrication of single cell with active area of 1 cm2 and modules (from 15 cm2 to 47 cm2 of active area). The results of the upscaling process will show the obtainment of an impressive PCE of 15% on the planar Low-T PSMs (AA = 47 cm2 ) thanks to the optimization of the perovskite deposition and layout by using fully laser assisted ablations. A comparative preliminary study based on different deposition technique for the ETL layer on such large area will be discussed. Finally, promising PCE of 14% is achieved by using blade coating approach. In chapter 4 we will evaluate long-term stability of planar PSCs. We will analyse and compare the results by following the ISOS measurement protocols introduced for the new-generation PV. The standard tests to take in consideration are the ISOS-D-1 where the devices are kept in dark condition at RT with a constant RH, in our case 20-40%; ISOS-T-1 consist in a thermal stress at 85°C at 20- 30% of RH and the ISOS-L-1 where the PSCs are tested under continuous light soaking. The impact of the sealing will be evaluated, including the encapsulation of large area modules. The main topic is focused on the understanding of the detrimental factors related to the long-term stability of the device. Factors like perovskite composition, n-side interfaces, hysteresis presence and electrode diffusion will be discussed as main detrimental factors of the device performance. The comparison will be based to the evaluation of the different achieved T80, the time when the initial PCE is reduced of the 20%. The results obtained working on the main detrimental factors of the planar PSCs lead in a T80 longer than 4000 h during the Shelf life test and an enhancement on thermal stability of 800%.