Role of interfacial charge transfer state in charge generation and recombination in low-bandgap polymer solar cell

The charge carrier dynamics in blend films of poly[2,6-(4,4-bis(2- ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1, 3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C ₆₁-butyric acid methyl ester (PCBM) was studied by transient absorption spectroscopy in order to address the origin of limited external quantum efficiency (EQE) of this solar cell compared to that of a benchmark solar cell composed of regioregular poly(3-hexythiphene) (RR-P3HT) and PCBM. Upon photoexcitation, PCPDTBT singlet excitons promptly convert to the interfacial charge transfer (CT) state that is a Coulombically bound charge pair of PCPDTBT polaron and PCBM anion at the heterojunction with almost 100% efficiency in a picosecond. In other words, the exciton diffusion efficiency η _ED and charge transfer efficiency η _CT are 100% in this blend, which are higher than and comparable to those of the RR-P3HT/PCBM solar cell, respectively. On a time scale of nanoseconds, 70% of the PCPDTBT bound polarons are dissociated into free charge carriers, and the others recombine geminately to the ground state through the CT state. The charge dissociation efficiency η _CD = 70% is lower than that of RR-P3HT/PCBM solar cells. The PCPDTBT dissociated polarons recombine bimolecularly on a time scale of nano- to microseconds with a charge lifetime of ∼10 ^-7 s, which is shorter than that observed for RR-P3HT/PCBM blends. In summary, the lower charge dissociation efficiency and shorter charge lifetime are the limiting factors for the photovoltaic performance of PCPDTBT/PCBM solar cells. Furthermore, the origin of such limitation is also discussed in terms of the charge dissociation and recombination through the interfacial CT state in PCPDTBT/PCBM blends.