Photo-electrochemical (PEC) devices allow for converting solar energy into chemical energy and for the production of energetically dense solar fuels. Light absorption, charge separation and transport, electrochemical reactions, and ionic transport are required in such devices, all processes happening simultaneously. PEC devices - compared to competing, conventional PV-electrolysis systems - offer the promise of less complexity in design and implementation and more flexibility in their use. Nevertheless, PEC devices' economic and performance competitiveness is not well understood, given their low technology readiness level. No study has considered accurate multi-physical, multi-scale, and multi-dimensional performance models, degradation aspects, and uncertainty in the performance and cost metrics. Addressing some of these unknowns and focusing on the conversion of solar energy into two different solar fuels (H2 from water and CO from CO2), the objective of this thesis is threefold: (i) conduct a system-level techno-economic analysis based on a systematic and physical performance model (including degradation), and address uncertainty via a probabilistic approach (Monte Carlo (MC) method); (ii) based on the insight gained from the techno-economic analysis, identify most promising design and operational principles, substantiated by experimental investigation of an example case to assess practical feasibility; and (iii) develop two intricate multi-dimensional, multi-physical models: one for an innovative PEC device designed to operate with water vapor, and the other for a PEC device utilizing concentrated solar light engaged in the conversion of CO2 to CO. Overall, this thesis provides a combination of experimental demonstration and simulation tools to conduct feasibility studies, predict costs, and provide design guidelines and operational conditions for PEC devices in diverse electrochemical processes. This scope extends the use of PEC devices beyond the traditional liquid water splitting reaction, encompassing applications such as water vapor splitting, energy storage, and CO2 reduction.