Singlet fission in low bandgap pi-conjugated polymers for organic photovoltaic applications

Singlet fission (SF) is a process that occurs in some organic semiconductors whereby the photoexcited singlet exciton (SE) undergoes internal conversion to a multiexciton triplet-triplet (TT) state, which subsequently splits into two independent triplet excitons. This process was first observed in crystalline acenes (most notably pentacene) in the 1960s. Renewed interest on singlet fission has been seen dramatically increased in recent years because of its potential in harvesting charges from the triplet excitons in organic photovoltaic cells, thereby doubling the photocurrents. It was shown that the cell external quantum efficiency may exceed 100%, and thus it could potentially overcome the Shockley-Queisser PV efficiency limit under the sun illumination. In this work, we used various optical techniques in our research arsenal to uncover the intrachain singlet fission in a new class of OPV materials, namely low bandgap pi-conjugated polymers, which was used as the electron donor in bulk hetero-junction solar cells. These copolymers produced a record high power conversion efficiency of ~ 8% in an optimum OPV device. Particularly, we introduced two new novel techniques, the nanosecond to millisecond transient photo-induced absorption and transient magneto-photoinduced absorption, dubbed t-PA and t-MPA, respectively, to unravel the population exchange between the singlet exciton and triplet pair (TT) state, which is a new quantum state constituted by two correlated triplet excitons. Using the t-PA in picosecond time domain, we detected the TT state that appears simultaneously with the singlet exciton SE within 300fs time resolution of our experimental setup. The picosecond t-MPA technique further elucidates the nature of TT state, showing its coupling to the SE through their spin exchange interaction with the interaction strength as large as ~30mT. Using the t-MPA together with the ns t-PA, we found that the TT state later separates into two uncorrelated triplets in microsecond time domain. In the copolymers/PC71BM blend, which was used as the active layer in OPV devices, the TT state dissociates, by the unique spin conserved process, into one polaron pair in triplet configuration, PP_T ; leaving behind one triplet on the copolymer chains within 20ps. The PP_T could either dissociate into free charges to generate photocurrents in cell devices or recombine back to triplet excitons. Here we observed the “back reaction”, PP_T --> triplets, in nanosecond time regime, which we identify as a loss mechanism for charge photogeneration in solar cell devices. We also introduce a method to reduce the carrier loss mechanism by the “back reaction” of PP into triplet excitons on the copolymer chains, by adding spin ½ radicals; this method may be especially suitable for copolymer-based OPV cells.