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Photoexcitation Dynamics and Direct Monitoring of Photovoltaic Processes of Solid-State Hybrid Organic-Inorganic Perovskite Solar Cells

Periodic Reporting for period 1 - PHOTOPEROVSKITES (Photoexcitation Dynamics and Direct Monitoring of Photovoltaic Processes of Solid-State Hybrid Organic-Inorganic Perovskite Solar Cells)

Reporting period: 2017-09-01 to 2019-08-31

The project has achieved most of its objectives and milestones for the period, with relatively minor deviations. The photophysical studies of simple solution- processed high quality perovskite thin films and partial heterostructures were carried out and the kinetics of the charge transfer were compared for NiO and PEDOT:PSS. Using state of the art cathodoluminescence hyperspectral imaging, highly spatially resolved ~50 nm information on composition heterogeneity and local strain were obtained for the first time in the promising optoelectronic halide perovskites of CsPbBr3 and CsPbBr3:KI. Novel charge transporting layers, (both metal oxide, and organic semiconductor based) were developed to improve the shelf-life stability of the perovskite solar cells and to enable the fabrication of perovskite solar cells on the flexible substrates. A novel method towards the faster optimisation of charge transporting layers for efficient perovskite solar cells was developed through surface photovoltage measurements. The novel applicability of the fabricated perovskite solar cells in their indoor light harvesting is demonstrated through interface engineering and generated enough electrical power to power the sensors on the Internet of things applications.
I developed consistently high performing hybrid perovskite solar cells in p-i-n and n-i-p configuration with power conversion efficiency ≈16%. High quality thin films of different compositions of hybrid perovskites were prepared by solution processing and their photovoltaic properties were studied. Charge extraction kinetics of the most widely used perovskite photoactive layer of CH3NH3PbI3 to the charge transport layers of NiO and PEDOT: PSS were investigated using time-resolved photoluminescence and transient absorption measurements. The study revealed that in the most commonly used hole extraction layer of PEDOT:PSS, at illumination intensities of 1 Sun, the charge extraction is interface limited giving a power conversion efficiency of only 7.5 % where as in NiO, a faster charge extraction, on the time scale of 300 ps is occurring with achieved PCE of ≈ 13%.
Because of the simple solution processing method used for the fabrication of the hybrid perovskites, multiple length scales of heterogeneity in properties exist in these materials. These heterogeneities can adversely impact the stability and the performance of perovskite photovoltaic devices made from these materials. It is a real challenge to identify the origin and spatial distribution of these grain-to-grain inhomogeneities, and their correlation with functional properties. Using cathodoluminescence hyperspectral imaging method I have identified the heterogeneity in light emission from the promising optoelectronic materials of CsPbBr3 and CsPbBr3:KI with a high spatial resolution (~50 nm) and related this to the composition and the local strain. Our study showed that grain boundaries are under compressive strain and are associated with higher non-radiative recombination losses. Also, in the mixed halide composition especially the iodide rich phases exist in the grain boundary regions. Our study provided the first visual evidence for the existence of nanoscale heterogeneity in composition, both laterally and vertically in CsPbBr3 and its mixed halide composition.
Charge extraction layers that can be processed at temperatures compatible with flexible substrates such as polyethylene terephthalate (PET) were developed. The metal oxide-based charge transport layer of NiO (for hole extraction), SnO2 and ZnO (electron extraction layers) were developed to obtain efficient charge extraction for both p-i-n and n-i-p device architectures. Compared to the commonly used hole extraction layer (HEL) of PEDOT:PSS, hybrid perovskite solar cells with NiO nanoparticles as HELs showed enhanced shelf-life stability by preventing the permeation of moisture into the active layer. The low temperature processability (at 100 oC) of NiO permitted the fabrication of flexible perovskite solar cells with power conversion efficiency close to 10 %. A new methodology of transport layer optimization using surface photovoltage measurement of partial heterostructures of perovskite solar cells was established. In collaboration with the University of Glasgow, BODIPY based novel hole transporting materials were developed which has the potential to be employed in both p-i-n and n-i-p configuration with appropriate surface passivation using PFN molecules. This novel cost-effective hole transporting material can replace the expensive and complexly prepared Spiro-MeOTAD but without compromising on power conversion efficiency (≈ 15 %). In collaboration with Chemists from the School of Chemistry, University of St Andrews, I have contributed in the development of lead-free hybrid perovskites, such as CH3NH3Bi2I9, microstructural and photophysical properties of A-site cation aliovalent non-stoichiometric (CH3NH3)1-2x(H3NC2H4NH3)PbI3, and CH3NH3PbBr3-xClx. In collaboration with the University of Edinburgh, I have contributed to the development of perylene diimide based novel electron transporting layers for hybrid perovskite solar cells

The novel charge transporting layers developed is beneficial tow
The new insights of charge extraction kinetics at the buried heterointerfaces developed through this project will enhance the development of highly efficient charge transporting layers for the hybrid perovskite based optoelectronic devices such as LEDs and photodetectors. The low temperature charge transporting layers developed through the project will accelerate the fabrication of printable perovskite solar cells and their easier integration with the portable electronic devices. In the Si-perovskite tandem photovoltaic technology, the p-i-n device architecture is mostly used and the low temperature processed NiO can replace the currently physical vapor deposited NiO interlayers. The new knowledge developed on the nanoscale composition heterogeneity and the local strain in halide perovskites using cathodoluminescence hyperspectral imaging will lead to their development of stable and robust optoelectronic devices. The promising indoor light harvesting demonstrated for the fabricated solar cells will advance the robust and resilient autonomous energy infrastructure inside the building required for disruptive technologies such as Internet of Things and wearable devices. In addition, my project results have great implications in the development of energy efficient ‘smart’ buildings.
Device performance of the NiO HEL based perovskites to that of commonly used PEDOT:PSS HEL.