Direct solar water splitting appears to be a promising route to produce hydrogen, avoiding competition for electricity with other important economic uses. Halogenated hybrid perovskites recently enabled the demonstration of efficient and potentially low-cost photoelectrochemical cells and PV-coupled electrolysers, reaching high efficiencies but so far limited to a small active area of a few mm2, in the case of perovskite/silicon tandem solar cells. Here, we show the added value of integrating a thermal exchanger into the system thanks to additive manufacturing, providing a thermally integrated photoelectrochemical cell (IPEC) with performance doubled compared to the device without any heat exchanger (from 3.3 to 8% STH). In addition, we develop a modular approach to scale-up this concept from 7.6 to 342 cm2, highlighting statistical variations in the efficiency of single integrated photoelectrochemical cells and their origin. We conduct an outdoor stability test for 72 hours, achieving a STH performance of 6.3%, and investigate the causes of device degradation through the easy disassembly of the integrated photoelectrochemical devices. We identify the interface between the perovskite layer and p-layer as critical for achieving stable photoelectrochemical devices integrating perovskite/silicon tandem solar cells.