Recently, European Commission announced a set of policy initiatives called European Green Deal which aims at net zero greenhouse gases in Europe by 2050, and photovoltaic technologies are expected to play a significant role in sustainable energy generation. Although halide perovskites have been highlighted as a class of the most promising materials for solar cells due to a combination of low-cost solution processibility and high power conversion efficiency, non-radiative losses and chemical decomposition originated from defects in the perovskite films prevent them from being commercialised successfully. The primary goal of this project is to unveil defect-strain relationships and to provide a way to exploit as an extra lever for defect control. To attain the goal, how microscopic structural changes at halide perovskite lattice sites affect performance of macroscopic solar cells needs to be understood, which cannot be done from a single research technique. A multidisciplinary approach - various spectroscopy and microscopy measurements as stepping stones in between the two end points - is, thus, an essential part of this ambitiously proposed project. The applicant (Dr Young-Kwang Jung) will perform state-of-the-art first-principles materials simulation techniques to identify responses of (i) native point defects; (ii) extrinsic dopants; and (iii) extended defects to lattice strain in halide perovskite solar cells, which will be complemented by cutting edge experimental validation from the host (Dr Samuel D. Stranks and the StraksLab at the University of Cambridge). The results of this project will provide practical solutions to improve the device efficiency and stability, which will accelerate commercialisation of halide perovskite solar cells in the public market. Consequently, this work will contribute the world-wide movement for zero-carbon energy generation, and ultimately, will alleviate the global warming.