This chapter deals with the nanostructured organic bulk heterojunction solar cells. More than three decades of research on organic solar cells based on π-conjugated materials has led to steadily increasing efficiency of solar cells. This chapter focuses on small molecular organic materials and their solar cells comprise three sections: (1) photophysical properties of organic p-conjugated materials; (2) characteristics of simple bulk heterojunction solar cells; and (3) characteristics of hybrid-type heterojunction solar cells. The hybrid bulk heterojunction (HH) solar cell was shown to have a high conversion efficiency compared to the heterojunction (PH) and bulk heterojunction (BH) solar cells. Moreover, there is a possibility that applying anti-reflecting coatings to the tandem cell could increase efficiency to over 6%. This chapter explains the design of the BH solar cell structure that has been discussed so far by focusing on the exciton diffusion and charge carrier collection efficiencies only. The product of the four key factors, ηA × ηED ×ηCT × ηCC, determines the conversion efficiency of an organic solar cell. Therefore, all four quantum efficiencies must be optimized to achieve conversion efficiency in excess of 10-15%. New donor and acceptor materials must be tailored to have optimum energy levels of highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO and LUMO) so that the materials can absorb a broader solar-spectral region and have lower exciton-binding energy. This continuous and fundamental research will mark a new epoch in organic solar cells, which are expected to offer low-cost solar energy conversion and be environmental friendly.
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