Search

Non-fullerene acceptors have recently attracted tremendous interest due to their potential as alternatives to fullerene derivatives in bulk-heterojunction solar cells. Nevertheless, physical understanding of charge carrier generation and transfer mechanism that occurred at the interface between the non-fullerene molecule and donor polymer is still behind their enhanced photovoltaic performance. Here we report examples of a non-planar perylene dimer (TP) as an electron acceptor and achieve a power conversion efficiency of 6.29% in a fullerene-free solar cell. Photoluminescence (PL) measurements show high quenching efficiency driven by the excitons of both conjugated polymer and TP molecule, respectively, indicating efficient electron and hole transfer, which can support a highly intermixed phase of blends measured by atomic force microscopy (AFM) and grazing incident wide-angle X-ray diffraction (GIWAXS). Femtosecond transient absorption spectroscopy (fs-TAS) reveals that the fast exciton dissociation process from TP molecule to donor polymer contributes to additionally increasing current density, leading to stronger incident photon to current efficiency in the visible region. [ABSTRACT FROM AUTHOR]

Herein the morphology and exciton/charge carrier dynamics in bulk heterojunctions (BHJs) of the donor polymer PTQ10 and molecular acceptor IDIC are investigated. PTQ10:IDIC BHJs are shown to be particularly promising for low cost organic solar cells (OSCs). It is found that both PTQ10 and IDIC show remarkably high crystallinity in optimized BHJs, with GIWAXS data indicating pi‐pi stacking coherence lengths of up to 8 nm. Exciton‐exciton annihilation studies indicate long exciton diffusion lengths for both neat materials (19 nm for PTQ10 and 9.5 nm for IDIC), enabling efficient exciton separation with half lives of 1 and 3 ps, despite the high degree of phase segregation in this blend. Transient absorption data indicate exciton separation leads to the formation of two spectrally distinct species, assigned to interfacial charge transfer (CT) states and separated charges. CT state decay is correlated with the appearance of additional separate charges, indicating relatively efficient CT state dissociation, attributed to the high crystallinity of this blend. The results emphasize the potential for high material crystallinity to enhance charge separation and collection in OSCs, but also that long exciton diffusion lengths are likely to be essential for efficient exciton separation in such high crystallinity devices. [ABSTRACT FROM AUTHOR]

Abstract: Nonfullerene acceptors (NFAs) in blends with highly crystalline donor polymers have been shown to yield particularly high device voltage outputs, but typically more modest quantum yields for photocurrent generation as well as often lower fill factors (FF). In this study, we employ transient optical and optoelectronic analysis to elucidate the factors determining device photocurrent and FF in blends of the highly crystalline donor polymer PffBT4T‐2OD with the promising NFA FBR or the more widely studied fullerene acceptor PC71BM. Geminate recombination losses, as measured by ultrafast transient absorption spectroscopy, are observed to be significantly higher for PffBT4T‐2OD:FBR blends. This is assigned to the smaller LUMO‐LUMO offset of the PffBT4T‐2OD:FBR blends relative to PffBT4T‐2OD:PC71BM, resulting in the lower photocurrent generation efficiency obtained with FBR. Employing time delayed charge extraction measurements, these geminate recombination losses are observed to be field dependent, resulting in the lower FF observed with PffBT4T‐2OD:FBR devices. These data therefore provide a detailed understanding of the impact of acceptor design, and particularly acceptor energetics, on organic solar cell performance. Our study concludes with a discussion of the implications of these results for the design of NFAs in organic solar cells. [ABSTRACT FROM AUTHOR]

Two alkoxynaphthalene-based conjugated polymers, poly((5,5-E-alpha-((2-thienyl)methylene)-2-thiophene)-alt-2,6-[(1,5-didecyloxy)naphthalene])) (PBTDN) and poly((5,5-E-alpha-((2-thienyl)methylene)-2-thiopheneacetonitrile)-alt-2,6-[(1,5-didecyloxy)naphthalene])) (PBTADN), are used as second electron donor materials in a poly(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b′)dithiophene-2,6-diyl-alt-(4-(2-ethylhexyloyl)-thieno(3,4-b)thiophene))-2,6-diyl):[6,6]-phenyl-C 71 -butyric acid methyl ester (PBDTTT-C: PC 71 BM) binary blend for organic ternary solar cells. The PBTADN:PBDTTT-C:PC 71 BM blends yields complementary absorption spectra as well as a better molecular structure ordering and more balanced charge mobility, which together provided excellent charge transport properties with reduced recombination compared to the PBTDN:PBDTTT-C: PC 71 BM blends. The optimized PBTADN:PBDTTT-C: PC 71 BM blend solar cells yield higher fill factor (FF) values and better efficiencies compared to the PBDTTT-C: PC 71 BM binary blend solar cells. The best photovoltaic performances in the PBTADN:PBDTTT-C:PC 71 BM blend solar cells are characterized by a current density ( J SC ) of 16.3 mAcm −2 , an open circuit voltage ( V OC ) of 0.73 V, a FF of 53.3%, and a power conversion efficiency (PCE) of 6.9% under AM1.5G illumination. The use of a processing additive, 3 vol% of 1,3-diiodopropane (DIP), increases the PCEs of the organic ternary solar cells to values as high as 7.2% with a high FF of 59.1%. [ABSTRACT FROM AUTHOR]

ABSTRACT Two novel polymeric semiconductor materials based on naphtho[2,1- b:3,4- b']dithiophene (NDT), PNDT-TTT and PNDT-TET, were designed and synthesized. These synthesized polymers were tested in bulk heterojunction solar cells as blends with the acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). PNDT-TTT contained tri-thiophene units, and PNDT-TET contained bi-thiophene units coupled by ethylenic linkages. Comparison to the properties of PNDT-T, which contained single thiophene units, these polymers exhibit red-shifted absorption spectra as a result of the enhanced conjugation lengths. These effects resulted in high short circuit currents ( JSC) in the organic solar cells. The PNDT-TET- and PNDT-TTT-based devices exhibited considerably better photovoltaic performances, with power conversion efficiencies of 3.5 and 3.3%, respectively, compared to the PNDT-T-based device (1.3%). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4742-4751 [ABSTRACT FROM AUTHOR]

Three classes of quinoxaline (Qx)-based donor-acceptor (D-A)-type copolymers, poly[thiophene-2,5-diyl- alt-2,3-bis(4-(octyloxy)phenyl-quinoxaline-5,8-diyl] P(T-Qx), poly{4,8-bis(2-ethylhexyloxy)benzo[1,2- b:4,5- b′]dithiophene-2,6-diyl- alt-2,3-bis(4-(octyloxy)phenyl-quinoxaline-5,8-diy} P(BDT-Qx), and poly{4,8-bis(2-ethylhexyloxy)benzo[1,2- b:4,5- b′]dithiophene-2,6-diyl- alt-(5′,8′-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)-quinoxaline-5,5-diyl} P(BDT-DTQx), were synthesized via a Stille coupling reaction. The Qx unit was functionalized at the 2- and 3-positions with 4-(octyloxy)phenyl to provide good solubility and to reduce the steric hindrance. The absorption spectra of the Qx-containing copolymers could be tuned by incorporating three different electron-donating moieties. Among these, P(T-Qx) acted as an electron donor and yielded a high-performance solar cell by assuming a rigid planar structure, confirmed by differential scanning calorimetry, UV-vis spectrophotometer, and density functional theory study. In contrast, the P(BDT-Qx)-based solar cell displayed a lower power conversion efficiency (PCE) with a large torsional angle (34.7°) between the BDT and Qx units. The BDT unit in the P(BDT-DTQx) backbone acted as a linker and interfered with the formation of charge complexes or quinoidal electronic conformations in a polymer chain. The PCEs of the polymer solar cells based on these copolymers, in combination with [6,6]-phenyl C70 butyric acid methyl ester (PC71BM), were 3.3% [P(T-Qx)], 1.9% [P(BDT-Qx)], and 2.3% [P(BDT-DTQx)], respectively, under AM 1.5G illumination (100 mW cm−2). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013 [ABSTRACT FROM AUTHOR]