We present successful growth of high quality carbon nitride based copolymer (CNBC) films on various substrates (glass, FTO, TiO₂) through solvothermal approaches with post-annealing. Structural characterizations of the films reveal the polymeric carbon nitride network consisting of tri-s-triazines and s-triazines bridged by different functional groups. The strong bonding between CNBC films and the substrates via covalent linkage ensures intimate contact and smooth interfacial charge transport. As such, the CNBC films exhibit excellent photoelectrochemical performance, which is attributed to strong visible light harvesting, excellent conductivity, and efficient charge separation and transport. Further, we demonstrate that similar solvothermal approach can be also used to create CNBC films on TiO₂ surface, and the resulted CNBC/TiO₂ junction allows for vectorial charge transfer, giving greatly enhanced photocurrent responses. Our studies provide a general solvothermal-based strategy for in-situ growing metal-free films on various substrates towards efficient photoelectrochemical water splitting.