Grain boundaries and the photoresponse of epitaxial <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mrow> <mml:mi>La</mml:mi> <mml:mi>Fe</mml:mi> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> <mml:mn>3

Centrosymmetric ${\mathrm{La}\mathrm{Fe}\mathrm{O}}_{3}$ (LFO) shows a photovoltaic effect when sandwiched between asymmetric electrodes, which stems from the built-in potential induced by the different work functions of the top and bottom electrodes, interfacial polar discontinuity, or even flexoelectric fields. However, the complex microstructure of epitaxial LFO film contributes to its photoresponse differently depending on the crystallographic domain landscape and the nature of the grain boundaries originating between them. In this work, to minimize the presence of strain gradients and isolate the substrate-induced lattice mismatch effects on microstructure and photoresponse, ultrathin (10 nm) LFO films are grown on substrates with different compressive and tensile in-plane strain. Structural characterization allows us to disentangle the contributions of the domain distribution and strain state to the measured photocurrent of the LFO films. Four-dimensional scanning transmission electron microscopy orientation mapping proves a strong correlation between the strain imposed by the substrate, the LFO domain distribution, and the photocurrent performance. Films grown on substrates imposing a compressive strain [${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ (STO) (001), $({\mathrm{La}\mathrm{Al}\mathrm{O}}_{3}{)}_{0.3}({\mathrm{Sr}}_{2}{\mathrm{Ta}\mathrm{Al}\mathrm{O}}_{6}{)}_{0.7}$(001), and ${\mathrm{La}\mathrm{Al}\mathrm{O}}_{3}$(001)] exhibit predominantly twin boundaries with low disorder, whereas films grown on substrates imposing tensile strain (${\mathrm{Dy}\mathrm{Sc}\mathrm{O}}_{3}$ (DSO) (110)) display grain boundaries formed by crystallites with orthogonal c-axis orientations, which we hypothesize are associated with the observed enhanced photocurrent. Further validation is provided by LFO films grown on STO(110), where controlled grain boundary formation replicates the crystallite texture observed in films grown on DSO(110) and concomitantly show a large short-circuit photocurrent (${J}_{SC}$ \ensuremath{\approx} 50 mA/${\mathrm{cm}}^{2}$). This work highlights grain boundary engineering as a strategy for optimizing the photoelectronic properties of oxide thin films, as well as an experimental approach applicable to other orthorhombic Pbnm systems to investigate the performance of perovskite-based optoelectronic devices through crystallographic domain configurations.