Zn₂SnO₄ octahedron nanoparticles/Cu₄Bi₄S₉ (ZTO/CBS) and ZTO octahedron nanoparticles/CBS–graphene nanoplatelets (ZTO/CBS–GNs), as well as two types of bulk heterojunction (BHJ) solar cells with high flexibility were prepared on stainless steel meshes (SSMs). The improved photovoltaic responses of CBS–GNs and ZTO/CBS–GNs with the incorporation of graphene nanoplatelets were determined using surface photovoltage spectroscopy (SPS). The signals of time-resolved fluorescence response (TFR) and transient surface photovoltage (TPV) can provide more detailed information on the transition, separation and transport of photogenerated carriers. Here, the ZTO/CBS–GNs cell exhibits a superior performance and the highest efficiency is 8.6%. The multichannel separation process from the TPV signals indicates that the macro-photoelectric signals can be attributed to photogenerated charges separated at the interface of CBS/GNs, rather than CBS/ZTO. The comprehensive signals of SPS, TFR and TPV not only provide insights into the transition, separation and shifting of photogenerated carriers, but also promote the exploration of charge dynamics from the picosecond to second range. Importantly, the BHJ flexible solar cells with excellent efficiency and facile, scalable production present significant potential for practical application.