Synthesis of Benzoporphyrin-Diketopyrrolopyrrole Conjugates and Application for Organic Solar Cells

Organic solar cells (OSC) have great attractive attention as key technology for next energy harvesting source. They could be fabricated for low cost, large area and flexible devices. However, power-conversion efficiency (PCE) of OSC is limited compared with the inorganic materials-base solar cell. Therefore, OSC requires new organic functional dyes for OSC for the improvement of PCEs. Benzoporphyrin (BP) has a expanded structure with four benzannulation at pyrrolic β-position on porphyrin core. BP has a highly planer structure and excellent semiconducting properties. Ono and colleagues　have developed the thermal-convertible precursors of BP, namely bicycle[2,2,2]octadiene-fused porphyrin (CP). CP can be converted to BP under heating conditions by retro-Diels-Alder reaction. This thermal conversion method allowed us to fabricate the multi-layered OSCs by solution process. However, the nature of BP has the problems and limitations for improvement of the PCE because BP has weak absorption in the NIR regions and the HOMO level of BP is higher than that of ideal donor materials for OSC. It is known that improvement of the PCE is necessary for the wide-range absorption in the visible to NIR regions and appropriated HOMO and LUMO levels.  In this presentation, we will report the design, synthesis and properties of the benzoporphyrin-diketopyrrlopyrrole conjugates (BP–DPP) for the p-type material of the PCEs. The absorption spectrum of BP–DPP was measured as thin film on glass substrate. The absorption of BP­–DPP shows broader absorption with four peak tops from visible to NIR regions.  We have fabricated the bulk heterojunction (BHJ) type OSC with BP–DPP as p-type material and PC₆₁BM as n-type material. The PCEs of 4.0% for BP–DPP:PC₆₁BM was achieved with short-circuit current (J_SC) of 12.47 mA cm^-2, open circuit voltage (V_oc) of 0.68 V, and fill factor (FF) of 0.47.We will also report the evaluations of film structures of these devices by AFM and XRD measurements.