We are developing organic thin-film solar cells utilizing low-weighted molecular semiconductors. Bulk hetero junction solar cells utilizing soluble phthalocyanine derivative, C6PcH2 (1,4,8,11,15,18,22,25-octahexylphthalocyanine) were investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH2 and PCBM, and active layer thickness dependence was investigated, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO3 hole transport buffer layer between the positive electrode and active layer, the fill factor and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solarcell was also fabricated utilizing active layer materials of C6PcH2 and P3HT and the interlayer of LiF/Al/MoO3 structure, and a high open-circuit voltage of 1.27 V was achieved.