Current Research
With increasing demand for energy consumption, there is heavy dependence on fossil fuels. Fossil fuels are limited resources, thereby increasing the risk of energy crisis in the future. The alternative way is to harness renewable energy sources efficiently, such as solar energy. To date, various types of solar cell technologies based on silicon or chalcopyrite (CIGS) absorber have been commercialized. In recent years, novel concepts such as the hybrid organic-inorganic perovskite solar cells (PSC) have been developed as next-generation emerging photovoltaics. The perovskite materials have a general chemical formula of ABX3. A-site generally consists of methylammonium, formamidinum, cesium, and B-site consist of the metal cation (Sn, Pb), and X denotes halides. PSCs stand out among other emerging light absorber films due to their rapid evolution of efficiency and ease of scale-up fabrication. However, improving the operational stability and efficiency of PSCs still remains a key objective to enable the final breakthrough of this technology.
The main topic of this research project is to use atomic layer deposition (ALD) to tailor the interfaces for improved charge carrier extraction from the perovskite absorber, thereby the ultimate enhancement of solar cell efficiency. This type of interface design not only aims at making the device more efficient but also to enhance stability by forming a protective buffer layer. One of the major concerns with the perovskite solar cell is it’s limited environmental and thermal stability, especially the extensively researched MAPbI3, but also still for the more recently employed triple cation mixed-halide variants. In current developments, the use of compact oxides directly grown over the perovskite material has led to significant improvement in the stability and performance of the solar cells. In a few demonstrations, oxide layers such as aluminum oxide, titanium oxides were implemented directly on top of perovskite layers using spin coating, sputtering, thermal evaporation, and ALD. However, not all techniques are suited equally well; sputter deposition for instance usually damages the perovskite layer due to high energy ion bombardment. Furthermore, the thermal instability of the perovskite materials limits the post-annealing/processing temperature for the oxide growth while using spin coating and thermal evaporation. In contrast, the ALD technique is quite promising for growing oxide layers directly on top of perovskite at a lower temperature.
This new approach of interface design could be crucial in solving the stability issue of perovskite-based technologies, yet the lack of a precise understanding of the chemical and physical processes at the interface caused a roadblock to effectively utilize this approach and hence call for dedicated fundamental research as proposed here. To summarise, the ALD technique will be used and optimized in this doctoral thesis work to grow an oxide buffer layer on top of the perovskite material. The interface chemistry, structure, morphology, and the charge transfer mechanism will be studied and analyzed using advanced characterization techniques such as ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), Kelvin probe force microscope (KPFM), photoluminescence(PL), X-ray diffraction (XRD), and scanning electron microscopy (SEM) and, etc.
The Ph.D. project is funded within the scope of the “Interfaces and Hybrid Materials for PV (InHyMat-PV)” project, which has been granted in the “Make Our Planet Great Again” program initiative by the French President Emmanuel Macron to combat the impending climate crisis. This research work will also be done in collaboration with several research institutes in Europe and the USA, including the National Renewable Energy Lab (NREL), USA, and the Helmholtz Zentrum Berlin (HZB), Germany.
“Often, it may be said that the interface is the device“
– Nobel laureate Herbert Kroemer
Nitin Mallik 