Donor-π-acceptor conjugated polymers form the material basis for high power conversion efficiencies in organic solar cells. Large dipole moment change upon photoexcitation via intramolecular charge transfer in donor-π-acceptor backbone is conjectured to facilitate efficient charge-carrier generation. However, the primary structural changes that drive ultrafast charge transfer step have remained elusive thereby limiting a rational structure-function correlation for such copolymers. Here we use structure-sensitive femtosecond stimulated Raman spectroscopy to demonstrate that π-bridge torsion forms the primary reaction coordinate for intramolecular charge transfer in donor-π-acceptor copolymers. Resonance-selective Raman snapshots of exciton relaxation reveal rich vibrational dynamics of the bridge modes associated with backbone planarization within 400 fs, leading to hot intramolecular charge transfer state formation while subsequent cooling dynamics of backbone-centric modes probe the charge transfer relaxation. Our work establishes a phenomenological gating role of bridge torsions in determining the fundamental timescale and energy of photogenerated carriers, and therefore opens up dynamics-based guidelines for fabricating energy-efficient organic photovoltaics.

In this work, we have rationally designed and synthesized a novel thiophene-diketopyrrolopyrrole (TDPP)-vinyl-based dimer. We have investigated the optical and electronic properties and have probed the photophysical dynamics using transient absorption to investigate the possibility of singlet exciton fission. These revealed extremely rapid decay to the ground state (<50 ps), which we confirm is due to intramolecular excitonic processes rather than large-scale conformational change enabled by the vinyl linker. In all cases, the main excited state appears to be "dark", suggesting rapid internal conversion into a dark 2A-type singlet state. We found no evidence of triplet formation in TDPP-V-TDPP under direct photoexcitation. This may be a consequence of significant singlet stabilization in the dimer, bringing it below the energy needed to form two triplets. Our studies on this model compound set valuable lessons for design of novel triplet-forming materials and highlight the need for more broadly applicable design principles.

The formation of two triplet excitons at the cost of one photon via singlet exciton fission in organic semiconductors can potentially enhance the photocurrent in photovoltaic devices. However, the role of spin density distribution in driving this photophysical process has been unclear until now. Here we present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. We synthetically modulate the spin density distribution in a series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Using femtosecond transient absorption spectroscopy, we find that efficient iSEF is only observed for the phenyl-derivative in ~2.4 ps while absent in the other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character.

Abstract:N-type organic semiconductors are prone to oxidation upon exposed to ambient conditions. Herein, we report design and synthesis of diketopyrrolopyrrole (DPP) based oligomers for ambipolar organic thin film transistors (OFETs) with excellent air and bias stability at ambient conditions. The cyclic voltammetry measurements reveal exceptional electrochemical stability during the redox cycle of oligomers. Structural properties including aggregation, crystallinity and morphology in thin film were investigated by UV-visible spectroscopy, atomic force microscopy (AFM), thin film X-ray diffraction (XRD) and grazing incidence small angle X-ray scattering (GISAXS) measurements. AFM reveals morphological changes induced by different processing conditions whereas GISAXS measurements show increase in the population of face-on oriented crystallites in films subjected to a combination of solvent and thermal treatments. These measurements also highlight the significance of chalcogen atom from sulphur to selenium on the photophysical, optical, electronic and solid-state properties of DPP-DPP oligomers. Charge carrier mobilities of the oligomers were investigated by fabricating top-gate bottom-contact (TG-BC) thin-film transistors by annealing the thin films under various conditions. Combined solvent and thermal annealing of DPP-DPP oligomer thin films results in consistent electron mobilities as high as ~0.2 cm 2 V -1 s -1 with an on/off ratio exceeding 10 4 . Field-effect behaviour was retained for up to ~ 4 week which illustrates remarkable air and bias stability. This work paves the way towards the development of n-type DPP-DPP based oligomers exhibiting retention of field effect behaviour with superior stability at ambient conditions.

The use of a ternary blend is a promising strategy to enhance the power conversion efficiency of organic solar cells. However, an active layer thickness of ∼100 nm is typically required to achieve optimized performance in ternary blend organic solar cells. The efficiency of a thicker ternary blend film is limited by the low exciton diffusion length and charge carrier mobility of organic semiconductors, which leads to significant energy loss. In this work, we have employed a thick layer (∼300 nm) of ternary blend, featuring a donor–acceptor type diketopyrrolopyrrole (2DPP-BDT) based small molecule along with P3HT and PC71BM and established the role of Förster resonance energy transfer (FRET) to improve the power conversion efficiency (PCE). A dramatic enhancement (27%) in PCE was observed for the ternary blend organic solar cell compared to the binary blend solar cell containing P3HT:PC71BM as active layer. The performance enhancement is attributed to extended light absorption by the ternary blend photoactive layer, which emphasizes the contribution of 2DPP-BDT to harvest photons in the near-IR region of the solar spectrum. FRET between P3HT and 2DPP-BDT is found to be crucial in the exciton dissociation process. Steady state and transient absorption spectroscopy unambiguously established the role of FRET to enhance the device performance. This work highlights the significance of FRET to improve the performance of ternary blend organic solar cells.