Perovskite solar cells (PSC) have attracted much attention in recent years due to the rapid progress on the performance of PSC to attain PCE over 22 % within 5 years. Our group follows this trend working on PSC but with emphasizing on three major directions (i) lead-free PSC, (ii) carbon-based mesoscopic PSC and (iii) planar heterojunction PSC with the p-contact layer using graphene oxide (GO) and reduced GO (rGO) derivatives. For the tin-rich perovskites, we designed and synthesized alloyed Sn-Pb mixed halide perovskites by dipping the precursor solutions on the mesoporous films with the TiO₂/Al₂O₃/C configuration to form solar cells free of organic HTM. The photovoltaic performance was further improved on adding 30 mol % of tin fluoride (SnF₂) with device configuration FTO/TiO₂/Al₂O₃/NiO/C producing best power conversion efficiency 5.2 % with great reproducibility and intrinsic stability. To make a stable pure tin perovskite free of lead, we varied SnCl₂/SnBr₂ ratios to yield tin perovskites with three halides (I, Br and Cl) co-crystallized inside the tin-perovskite crystals according to the stoichiometric ratios of the precursors. A device with MASnIBr_1.8Cl_0.2 (SnCl₂ = 10 %) showed the best photovoltaic performance with PCE 3.2 % with great reproducibility and long-term stability. We also investigated inverted planar heterojynction (PHJ) PSC with PEDOT:PSS, GO and rGO as potential p-contact electrodes. The devices made of 2D or 2D/3D hybrid tin perovskite solar cells will be introduced and discussed. Electrochemical impedance spectra (EIS), transient photoelectric decays and transient PL decays were performed to understand the charge transfer mechanism in related to their device performances.