As climate change accelerates, increased reliance on renewable energy sources is necessary. One particularly promising renewable energy technology is metal halide perovskite (MHP)-based photovoltaics, which can provide lightweight, low-cost power sources for commercial and residential, disaster relief, military, and space applications. MHPs exhibit similar efficiencies to more established technologies such as silicon photovoltaics, but with lower production cost and waste. However, MHPs have intrinsic mechanical and chemical instabilities that currently prevent commercial viability. This project will be focused on addressing these instabilities through developing an understanding of how stresses that form in the MHPs during fabrication influence performance and stability, and how these effects can be mitigated through directed growth or stress modulation by using organic ligands (or additives). The fundamental scientific knowledge produced by this project will benefit society by elucidating the relationship between lattice strain, stability, and performance in solution-processable organic-inorganic hybrid materials, which can be used to reduce the material use and cost, improving access to renewable energy. In parallel, we will develop and present ?research readiness? seminars for marginalized students via established University of Arizona programs. In these seminars, we will also launch a new video outreach pilot program?Scientists Like Me?designed to increase self-concept as a researcher in these communities. Finally, we will build upon these interrelated efforts to inform curriculum development for a cornerstone course for the first of its kind Renewable Energy Science and Engineering minor?which will accelerate the dissemination of this knowledge to our communities and help train a more diverse workforce for renewable energy. This project will utilize organic molecules to control lattice strain in metal halide perovskites to modulate the optoelectronic behavior of these systems. In particular, we will focus on three interrelated questions: (1) How do lattice strain gradients influence defect migration and localization? (2) How does the molecular structure of additives in perovskite inks affect nucleation, growth, and orientation of crystallites? and (3) How can the molecular structure of additives influence lattice strains in perovskites? Successfully answering these questions will be a significant step towards realizing scalable production of stable high-performance solution-processable devices, enabling low-cost and transformative applications such as renewable energy, energy-efficient lighting and displays, and portable and wearable sensors for health care. We will synergistically use benchtop and computational experiments to inform research pathways and build towards a goal of understanding how molecular additives and their interactions at MHP surfaces can influence stability and performance through strain modulation. Furthermore, we will develop broad design rules for additive selection based on specific chemistries and stoichiometries of a range of perovskite compositions. The need for these broad design rules has become increasingly important given the ever-expanding library of metal halide perovskite compositions, each with their own complex and varied surface chemistries. Finally, we will implement a new strategy to improve the stability and performance of perovskite devices by informed design and selection of molecular additives to modulate the film stresses to control defect migration, charge transport, and band gap. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.