Excited-State Photophysics in a Low Band Gap Polymer with High Photovoltaic Efficiency

Abstract

We report an experimental investigation of a newly synthesized polymer, PNDT-DTPyT, with a 1.55 eV band gap tuned for optimal photovoltaic performance. Several time-resolved optical spectroscopies are used to examine fundamental photophysics in pure polymer films. Clear signatures of a distribution in photoexcitation sizes are found in steady state absorption and fluorescence measurements conducted from 100 to 300 K. Electronic structure calculations are combined with an empirical model to establish that, on average, the excitations span less than two repeat units because of strong interactions with the surrounding environment. These strong vibronic interactions are attributed to the charge transfer nature of the excitation, which involves a significant change in the spatial charge distribution. Femtosecond transient grating (TG) measurements find energy transfer processes occurring with roughly 1 and 10 ps time constants. This distinction in time scales most likely has a geometric origin (e.g., processes involving stacked and nonstacked neighbors). Fluorescence quenching and TG experiments show that the addition of PCBM to blends composed of both PNDT-DTPyT and PCBM correlates with an increase in the charge separation efficiency following photoexcitation. This increase in charge separation efficiency is caused by a reduction in the average distance that photoexcitations in the polymer must traverse before injecting electrons into PCBM. Photovoltaic power conversion efficiencies measured for the films are consistent with this interpretation, where a maximum efficiency of 4.1% is found for the 1:1 blend of polymer and PCBM. Overall, the present study covers several fundamental processes taking place in photovoltaic devices from the primary events surrounding the creation of photoexcitations in the polymer to their deactivation at the PCBM interface.

Synthesis and properties of dithienometallole-pyridinochalcogenadiazole alternate polymers

“…Pyridinochalcogenadiazole PTA and PSeA with a fused pyridine ring would be more electron-deficient than the benzoannulated analogues, and thus, a stronger donor-acceptor interaction was anticipated in the present polymers. [20][21][22] Indeed, both the DTS-and DTG-PTA alternate polymers synthesized in this study showed absorption bands at approximately 740 nm in solution, which were red-shifted from those of pDTS-BTA and pDTG-BTA with the same substituents on the center metals, as reported previously. 8,10,18…”

“…Chlorobenzene and ether used as the reaction solvents were distilled from CaH 2 and sodium/benzophenone ketyl, respectively, and stored over activated molecular sieves until use. The monomers DTSSn 2 , 4 DTGSn 2 16 and PTABr 2 21,22 were prepared as reported in the literature. Nuclear magnetic resonance (NMR) spectra were recorded on Varian 400-MR and System-500 spectrometers (Agilent Inc., Santa Clara, CA, USA).…”

Incorporation of Thiadiazole Derivatives as π-Spacer to Construct Efficient Metal-Free Organic Dye Sensitizers for Dye-Sensitized Solar Cells: A Theoretical Study

“…Previous experiments have demonstrated that the optical absorption of the organic dyes is red-shifted on extension of the π conjugation by the incorporation of electron-deficient segments [22,24]. You and coworkers have synthesized a series of narrow band gap polymers incorporating NDT units for organic solar cells [29][30][31][32]. For example, recently they constructed a library of polymers employing the NDT and BT units as the weak donor and the strong acceptor, respectively.…”

“…A smaller HOMO-LUMO gap is needed to achieve a higher JSC, while the low HOMO energy level should still be maintained for efficient dye regeneration. You and coworkers have reported that the LUMO of donor-acceptor copolymers largely locates on the acceptor moiety, and the bandgap can be reduced by incorporating a electron deficient unit to lower the LUMO while at the same time maintain the HOMO energy level [29][30][31][32].…”

Pretargeted radioimmunoscintigraphy in patients with primary colorectal cancer using a bispecific anticarcinoembryonic antigen CEA X anti‐di‐diethylenetriaminepentaacetic acid F(ab′)2 antibody

“…To exploit high‐performance photovoltaic materials, great efforts have been devoted to low‐bandgap polymers and broad‐absorption polymers 7, 8. Although significant advances have been achieved in improving the PCEs of PSCs by using new active layer materials,9–16 there is still much to do before the realization of practical applications of PSCs. Limiting factors include the narrow absorption band of conjugated polymers, the low photon–electron conversion rate, the unmatched molecular energy levels, and the low‐charge‐carrier mobility of the polymer photovoltaic materials.…”