Photophysical Properties and Photovoltaic Performance of Sensitizers with a Bipyrimidine Acceptor

Molecular engineering is a crucial strategy for improving the photovoltaic performance of dye-sensitized solar cells(DSSCs). Despite the common use of the donor-π bridge-acceptor architecture in designing sensitizers, the underlying structure-performance relationship remains not fully understood. In this study, we synthesized and characterized three sensitizers: MOTP-Pyc, MOS_(2)P-Pyc, and MOTS_(2)P-Pyc, all featuring a bipyrimidine acceptor. Absorption spectra, cyclic voltammetry, and transient photoluminescence spectra reveal a photo-induced electron transfer(PET) process in the excited sensitizers. Electron spin resonance spectroscopy confirmed the presence of charge-separated states. The varying donor and π-bridge structures among the three sensitizers led to differences in their conjugation effect, influencing light absorption abilities and PET processes and ultimately impacting the photovoltaic performance. Among the synthesized sensitizers, MOTP-Pyc demonstrated a DSSC efficiency of 3.04%. Introducing an additional thienothiophene block into the π-bridge improved the DSSC efficiency to 4.47% for MOTS_(2)P-Pyc. Conversely, replacing the phenyl group with a thienothiophene block reduced DSSC efficiency to 2.14% for MOS_(2)P-Pyc. Given the proton-accepting ability of the bipyrimidine module, we treated the dye-sensitized TiO_(2) photoanodes with hydroiodic acid(HI), significantly broadening the light absorption range. This treatment greatly enhanced the short-circuit current density of DSSCs owing to the enhanced electron-withdrawing ability of the acceptor. Consequently, the HI-treated MOTS_(2)P-Pyc-based DSSCs achieved the highest power conversion efficiency of 7.12%, comparable to that of the N719 dye at 7.09%. This work reveals the positive role of bipyrimidine in the design of organic sensitizers for DSSC applications.

Singlet relaxation dynamics and long triplet lifetimes of thiophene-coupled perylene diimides dyads:New insights for high efficiency organic solar cells

In the active layer of organic solar cells(OSCs),the lifetime of triplet excitons is one of the decisive factors in the diffusion length and therefore has important impact on the power conversion efficiency of the devices.Herein,we have investigated singlet excited state relaxation dynamics and their triplet exciton lifetimes of two thiophene-coupled perylene diimides(PDI)dyads(2 PDI-Th and fused-2 PDI-Th),in order to provide a unique explanation in depth on their different performances in OSC devices.From the transient absorption(TA)spectra,the singlet excitons of 2 PDI-Th form excimers in the time scale of 1.5 ps.Then the excimers go into the triplet state via intersystem crossing(ISC).In fused-2 PDI-Th,triplet excitons are generated directly from the singlet excited excitons via the efficient ISC.Density functional theory(DFT)calculations further support the formation of excimers.DFT results indicate that 2 PDI-Th exhibits an H-typed molecular configuration which is beneficial to form the excimers,while fused-2 PDITh gives a twisted X-shaped configuration in the optimized ground and excited state.In steady-state emission spectra,2 PDI-Th shows abroad and featureless spectral characteristics of the excimers with a decay time of 840 ps,which is much shorter than those of PDI(5.5 ns)and fused-2 PDI-Th(3.3 ns).The triplet lifetime(67 ms)of fused-2 PDI-Th is factor of 3 longer than that of 2 PDI-Th(22 ms).These results demonstrate that ring-fused structure is an efficient strategy to eliminate excimer formation and prolong the lifetime of triplet excitons,which provides a new insight for design of optoelectronic molecules for high efficiency organic solar cells.