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Materials, methods and concepts for 21st century solar cells

  • Bowman, Alan
Publication Date
Apr 12, 2021
Apollo - University of Cambridge Repository
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This thesis focuses on the development of the next generation of solar panels. The motivation behind this work is explained in Chapter 1. Chapter 2 gives a background to solar panels, semiconductors and thesis relevant materials while main experimental and theoretical tools used are introduced in Chapter 3. Spectroscopic measurements are used to better understand recombination and passivation in halide perovskites in Chapters 4 and 5. Specifically, low-bandgap halide perovskites are studied in Chapter 4. They are demonstrated to have significantly longer lifetimes and higher photoluminescence quantum efficiencies (PLQEs) than has previously been observed. Furthermore, zinc iodide is shown to increase these materials’ tolerance to oxygen substantially. It is shown that zinc iodide increases the mixing of lead and tin, removing tin rich (oxygen sensitive) clumps from the surface of the material. Chapter 5 introduces a method to obtain ratios between recombination rates in luminescent semiconductors rapidly using PLQE measurements. Extracted rates agree well with those from transient absorption spectroscopy. It is also demonstrated that non-radiative second order processes exist in halide perovskites and cannot be explained by parasitic absorption alone. This approach will allow for faster screening of solar cell absorber materials. Chapters 6 and 7 shed new light on the role of re-emitted photons in solar cells. Photon recycling, especially its relationship to controllable parameters, is quantified in single junction solar cells and light emitting diodes (LEDs) in Chapter 6. Photon recycling and device performance are both shown to improve for increased absorber thickness, better back reflection and reduced charge trapping. However, photon recycling reduces and device performance increases for better light management. Photon recycling is also found to be significantly more important in LEDs than solar cells at operating voltages. Chapter 7 calculates the limiting efficiency of two-absorber layer all-halide perovskite and halide perovskite-silicon tandem solar cells using measured recombination rates as 40.8 % and 42.0 %. Luminescence coupling (the emission of light from the high-bandgap layer and its re-absorption in the lowbandgap layer) is found to be important in both these devices at experimentally achievable charge trapping rates. This process relaxes current matching requirements, giving tandems better spectral tolerance and allowing for lower bandgap, more stable halide perovskites to be used as the high-bandgap absorber in tandems. Chapter 8 explores a halide perovskite/singlet fission material interface. Experiments screening for triplet transfer from singlet fission materials to halide perovskites are presented. Triplet transfer was not observed in any experiment. The interface was modelled using density functional theory; its formation energy is found to be weak and triplets are shown to remain strongly localised on tetracene, even at a clean interface optimal for triplet transfer. This goes some way to explain the experimental lack of triplet transfer. Finally, Chapter 9 summarises all findings and suggests future research directions. All the work presented herein helps to pave the way towards a future with cheap, high efficiency solar panels. / Oppenheimer studentship; Wintin studentship; Centre for Doctoral Training for New and Sustainable Photovoltaics

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