Enhancement of power conversion efficiency of PTB7:PCBM-based solar cells by gate bias

We designed and fabricated poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7): [6,6]-phenyl-C70-butyric-acid-methyl-ester (PC₇₀BM)–based solar cells with gate electrodes, which can introduce an additional electric field within the devices just as in organic thin film transistors (OTFTs). Our proposed realize the simple and convenient modulation of electric field within the device, and power conversion efficiency (PCE) of 8.1% is reached at 2.0 V gate bias, significantly higher than the PCE of 6.8% at the case of no gate structure. By calculating the carrier mobility and the rate of exciton dissociation efficiency in detail, the role of electric field to the exciton dissociation and carrier transport was investigated, respectively. Meanwhile, the feasibility of the proposed device structure in practical application was discussed. The results suggest that such a gate structure has a great of prospects in achieving high efficiency polymer solar cells.     

Enhanced carrier dynamics of PTB7:PC71BM based bulk heterojunction organic solar cells by the incorporation of formic acid

Formic acid (FA) was used as a novel additive in bulk heterojunction (BHJ) solar cells, which contains blends of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl]] (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). The effect of FA on the performance of PTB7:PC₇₁BM based BHJ solar cells is investigated. By the incorporation of FA, the device with the ratio of 6 vol % shows the best power conversion efficiency (PCE) of 9.04%, along with the short-circuit current density (J_sc), open-circuit voltage (V_oc), and fill factor (FF) being 24.11 mA/cm², 0.72 V, and 52.11%, respectively. Experimental results suggest that FA has a strong influence on charge carrier dynamics with a significant increase in J_sc by ∼65% and the dramatically enhanced PCE is mainly due to the increase of absorption and exciton generation of the active layers and the improved charge-carrier mobility of the devices.     

Visibly transparent conjugated polymers based on non-alternant cyclopenta-fused emeraldicene for polymer solar cells

In this study, we synthesized two emeraldicene (EMD)-based conjugated polymers, PBTEMD and PFEMD, through polymerization of 4,7-di(thien-2-yl)benzo[c][1,2,5]thiadiazole and 9,9-bis(2-ethylhexyl)-9H-fluorene, respectively. We then blended these EMD-derived polymers (as electron-donating materials) with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) in the active layers of polymer solar cells (PSCs) and investigate their optoelectronic properties and related photovoltaic performance. To best of our knowledge, this study is the first to use EMD derivatives for PSC applications. We compared the molecular structures, absorption behavior, energy levels, thermal properties, and thermal stability of these two polymers to determine their suitability for use in PSCs. The main absorption of PFEMD was in the near-IR spectrum (600–800 nm). We observed a transparency of greater than 80% for the blend film of PFEMD having a thickness of 95 nm; the constructed device exhibited a power conversion efficiency (PCE) of 2.5% and the transparent PFEMD:PC₆₁BM-derived device exhibited a PCE of 1.2% under AM 1.5 G irradiation (100 mW cm⁻²). We observed a significant improvement in thermal stability for the device incorporating the additive crosslinker TBT-N₃; it retained approximately 60% of its initial PCE after accelerated heating (150 °C) for 18 h. In contrast, the PCE of the corresponding normal device decayed to 0.01% of its initial value.     

Synthesis,optical and electrochemical properties new donor–acceptor (D–A) copolymers based on benzo[1,2-b:3,4-b′:6,5-b″] trithiophene donor and different acceptor units: Application as donor for photovoltaic devices

Two new conjugated D–A polymers P3 (PBTT-d-BTT) and P4 (PBTT-d-TPD) based on same benzo[1,2-b:3,4-b′:6,5-b″] trithiophene (BTT) donor and different acceptors monomers 5,8-dibromo-2-dodecanoylbenzo[1,2-b:3,4-b′:6,5-b″] trithiophene (d-BTT), and 1,3-dibromo-5-(2-ethylhexyl)thieno[3,4]pyrrol-4,6-dione (d-TPD) respectively, were synthesized by Stille cross-coupling reaction and characterized by gel permeation chromatography (GPC), ¹H NMR, UV–Vis absorption, thermal analysis and electrochemical cyclic voltammetry (CV) tests. Photovoltaic properties of the polymers were studied by using the polymers as donor and PC₇₁BM as acceptor with a weight ratio of polymer:PC₇₁BM 1:1, 1:2 and 1:2.5. The optimized photovoltaic device was fabricated with an active layer of a blend P3:PC₇₁BM and P4:PC₇₁BM with a blend ratio of 1:2 showed PCE 3.16% and 2.42%, respectively under illumination of AM 1.5 at 100 mW/cm² with solar simulator. The PCE of the device based on P3:PC₇₁BM processed with DIO/o-DCB has been further improved up to 4.64% with J_sc of 10.52 mA/cm² and FF of 0.58 attributed to the increase in crystalline nature of active layer and more balanced charge transport in the device, induced by DIO additive.     

Luminescent cathode buffer layer for enhanced power conversion efficiency and stability of bulk-heterojunction solar cells

We report high photovoltaic efficiency of over 9% in solution-processed, small-molecule (SPSM) 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c]1,2,5]thiadiazole) p-DTS(FBTTh₂)₂:[6-6]-phenyl C70 butyric acid methyl ester (PC₇₀BM) blend based inverted BHJ solar cell by incorporating luminescent zinc oxide doped with sodium (ZnO:Na) quantum dots (QD) (l-ZnO) as a cathode buffer layer (CBL) in inverted bulk-heterojunction (BHJ) solar cells for the first time. The l-ZnO absorbs ultraviolet (UV) light and down-converts it to visible light. The l-ZnO layer's emission overlaps significantly with the absorption of p-DTS(FBTTh₂)₂, leading to an enhanced absorption by p-DTS(FBTTh₂)₂. This resulted in a significant enhancement of photo-current from 15.4 to 17.27 mA/cm² and efficiency from 8% to 9.2% for ZnO and l-ZnO based devices, respectively. This is among one of the highest efficiency values reported so far in the case of SPSM based single junction BHJ solar cells. The luminescent ZnO layer also protects the active layer from UV-induced degradation as solar cells show high stability under constant solar light illumination retaining more than 90% (∼28 h) of its initial efficiency, whereas BHJ solar cells without the luminescent ZnO layer degraded to ∼50% of its initial value under same conditions. Since ZnO is an essential part of inverted organic solar cells, the luminescent l-ZnO CBL has great potential in inverted organic solar cells.     

Green Ir(III) complexes with multifunctional ancillary ligands for highly efficient solution-processed phosphorescence organic light-emitting diodes with high current efficiency

Two new highly efficient green emitting heteroleptic Ir(III) complexes, namely, bis[5-(2-ethylhexyl)-8-(trifluoromethyl)benzo[c][1,5]naphthyridin-6(5H)-one]iridium-4-((3,5-di(9H-carbazol-9-yl)benzyl)oxy)picolinate (Ir-HT) and bis[5-(2-ethylhexyl)-8-(trifluoromethyl) benzo[c][1,5]naphthayridin-6(5H)-one]iridium-4-((4-(5-phenyl-1,3,4-oxadiazol-2-yl)benzyl) oxy)picolinate (Ir-ET) were designed and synthesized for solution-processed phosphorescence organic light-emitting diodes (PHOLEDs). These new Ir(III) complexes are based on amide-bridged trifluoromethyl (-CF₃) substituted phenylpyridine unit as main ligand and 1,3-bis(N-carbazolyl)benzene (mCP) unit and 1,3,4-oxadiazole (OXD) unit functionalized picolinate (pic) as an ancillary ligand. These multifunctional groups were attached into the 4-position of pic ancillary ligands via ether linkage. Interestingly, the solution-processed PHOLED device using Ir-HT as a dopant exhibited a maximum external quantum efficiency (EQE_max) of 20.92% and a maximum current efficiency (CE_max) of 64.04 cd A⁻¹. Whereas PHOLED device using Ir-ET displayed a EQE_max of 20.68% and a CE_max of 65.02 cd A⁻¹. This is one of best CE with high EQE for green Ir(III) complexes via solution-processed PHOLEDs using multifunctional ancillary ligands so far.     

Efficient semitransparent organic solar cells with good color perception and good color rendering by blade coating

This paper proposes high efficiency semitransparent organic solar cells (OSCs) with good color perception and good color rendering using blade coating technique. We investigate four different polymer blends and first fabricate small area devices with active area of 0.04 cm², followed by large area devices with active area of 10.8 cm². Two of the polymer blends, 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene:6,6-phenyl C71-butyric acid methyl ester (PBDTTT-CT:PC₇₁BM) and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′] dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]:PC₇₁BM (PBDTTT-EFT:PC₇₁BM) show promising results. For small area devices, semitransparent PBDTTT-CT:PC₇₁BM and semitransparent PBDTTT-EFT:PC₇₁BM achieve a power conversion efficiency (PCE) of 5.2% (opaque PCE = 7.5%) and 5.6% (opaque PCE = 9.4%) respectively. For large area devices, they are found to produce a PCE of 3.8% (opaque PCE = 4.2%) and 5.3% (opaque PCE = 5.9%) respectively. Based on the CIE 1931 chromaticity diagram, semitransparent PBDTTT-CT:PC₇₁BM and semitransparent PBDTTT-EFT:PC₇₁BM are located very close to the standard illuminant D65, indicating good color perception. As for color rendering, they demonstrate high color rendering index (CRI) of 95.4 and 87.1 respectively. These combined high performances indicate high-quality transmitted light, which is suitable for window application.     

Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor type polymers

Direct arylation polymerization (DAP) is emerging as a promising green, cheap, simple, and efficient environment friendly method for synthesizing conjugated polymers without involving any organometallic reagent. We report fluorene based novel cross-conjugated alternate and random copolymers for polymer solar cells (PSCs), which were synthesized by DAP and/or Yamamoto polymerization under appropriate reaction conditions to obtain high molecular weight. These cross-conjugated polymers possess absorption maxima in the range of 490–520 nm and have narrow band gap (1.7–2.05 eV) which is suitable for bulk heterojuntion (BHJ) type organic solar cells. Among the synthesized polymers, the highest number average molecular weight (M_n) i.e. 43.1 kg mol⁻¹ was obtained for polymer P2b (poly((9H-fluoren-9-ylidene)methylene)bis((2-ethylhexyl)sulfane)-alt-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)), and so good polymeric films were formed for P2b. Thus, BHJ films were prepared for P2b for device performance studies and the morphology of these films was studied by atomic force microscopy (AFM). Polymer P2b was blended with the fullerene derivative [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) in different ratios and under the illumination of solar simulator with Air Mass global (AM 1.5G) irradiated at 100 mW cm⁻². Power conversion efficiency (PCE) of 1.4% has been achieved for BHJs in ratio of 1:2 of P2b: PC₇₁BM in simply processed devices. This result indicates that cross-conjugated polymers can be tapped as potential donors for BHJs as the PCE obtained is the highest among this type of cross-conjugated polymers.     

Interfacial layer material derived from dialkylviologen and sol–gel chemistry for polymer solar cells

Sol–gel processible organosilicate material based on dialkylviologen (1,1-(bis-trimethoxysilane)-[4,4′]bipyridium dibromide (bis-trimethoxypropylsilane)-yl-viologen, PV-Si) was synthesized and used as an interfacial layer material for polymer solar cells based on poly(3-hexylthiophene): [6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM). PV-Si is very good soluble in polar protic solvents because of two pyrinium bromide salts and PV-Si pre-polymer can be easily prepared by sol–gel chemistry under the mild acidic conditions. From the ultraviolet spectroscopy (UPS) study, the reduction of the work function of Al and ITO is observed by the formation of interface dipole, which is induced by the thin film of thermally cured PV-Si pre-polymer (cPV-Si) at 180 °C. The open circuit voltage (V_oc) of conventional type polymer solar cell (CPSC) with a structure of ITO/active layer (P3HT:PCBM)/cPV-Si(<5 nm)/Al is 0.58 V, which is higher than the CPSC without cPV-Si (0.55 V). This indicates that the favorable interface dipole is generated by the thin film of cPV-Si. Besides, the power conversion efficiency (PCE) of CPSC with cPV-Si reaches at 2.90%, which is higher than that of the device without cPV-Si (2.69%). Surprisingly, the PCE and the short circuit current (J_sc) of inverted type polymer solar cell (IPSC) with a structure of ITO/cPV-Si (<5 nm)/active layer/WO₃/Ag are 2.83% and ?9.19 mA/cm², respectively, which are higher than those of the device with ZnO (2.51% and ?8.63 mA/cm²) as an electron transporting/injecting layer. This is due to that the work function of ITO is also reduced by the formation of interface dipole. The IPSC with cPV-Si as an interfacial layer (IFL) shows very good rectification and a contact property as well. From the results, the thin layer of cPV-Si is potential material for an IFL for either CPSC or IPSC. Especially, ZnO can be replaced by cPV-Si because of their improved device performances and pretty low processing temperature.     

Perylene diimide-benzodithiophene D-A copolymers as acceptor in all-polymer solar cells

Two n-type conjugated D-A copolymers with perylene diimide (PDI) as acceptor unit and benzodithiophene (BDT) as donor unit, P(PDI-BDT-Ph) and P(PDI-BDT-Th), were synthesized and applied as electron acceptor in all-polymer solar cells (all-PSCs). P(PDI-BDT-Ph) and P(PDI-BDT-Th) films exhibit similar absorption spectra in the visible region with optical bandgap (E_g) of 1.65 eV and 1.55 eV respectively, and the identical LUMO level of −3.89 eV. The all-PSCs based on P(PDI-BDT-Ph) as acceptor and PTB7-Th as donor demonstrated a power conversion efficiency (PCE) of 4.31% with a short-circuit current density (J_sc) of 11.94 mA cm⁻², an open-circuit voltage (V_oc) of 0.81 V, and a fill factor (FF) of 44.49%. By contrast, the corresponding all-PSCs with P(PDI-BDT-Th) as acceptor showed a relative lower PCE of 3.58% with a J_sc of 11.36 mA cm⁻², V_oc of 0.79 V, and FF of 40.00%.     

Air-processed high performance ternary blend solar cell based on PTB7-Th:PCDTBT:PC70BM

Ternary bulk heterojunctions (BHJs) are promising candidates that can improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs). In this paper, a ternary OSC with two donors, including one wide bandgap polymer poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), one low bandgap polymer Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th), and one acceptor [6,6]-phenyl C70 butyric acid methyl ester (PC₇₀BM), is fabricated in atmospheric conditions. By incorporating a 20% content of PCDTBT, an optimized PCE of 7.86% for ternary OSC is characterized by a short-circuit current density (J_sc) of 15.21 mA cm⁻², a fill factor of 69.70% and an open-circuit voltage (V_oc) of 0.74 V. The V_oc values increased steadily from 0.73 to 0.86 V as the increase of PCDTBT fraction, which indicates that the V_oc of ternary OSC is not limited by the smallest one of the corresponding binary OSC. We show that the J_sc of the ternary OSC is better than those of the binary OSC in virtue of the complementary polymer absorption and cascade energy levels, as well as optimized morphology of the ternary system. Furthermore, the lifetime of the devices with PCDTBT is greatly enhanced. This work indicates that two donors (PTB7-Th/PCDTBT) ternary BHJs system provide a simple and effective method to improve the performance and also the stability of OSCs.     

A solution-processable star-shaped molecule for high-performance organic solar cells via alkyl chain engineering and solvent additive

A new star-shaped D–π–A molecule, tris{4-[5′′-(1,1-dicyanobut-1-en-2-yl)-2,2′-bithiophen-5-yl]phenyl}amine N(Ph-2T-DCN-Et)₃, with high efficiency potential for photovoltaic applications was synthesized. As compared to its analogue S(TPA-bT-DCN), it showed stronger absorption in the region of 350–450 nm and a lower lying highest occupied molecular energy level (HOMO). Solution-processed organic solar cells (OSCs) based on a blend of N(Ph-2T-DCN-Et)₃ and PC₇₀BM resulted in a high PCE of 3.1% without any post-treatment. The PCE of N(Ph-2T-DCN-Et)₃ based solar cells was further improved to 3.6% under simulated AM 1.5 by addition of a new additive 4-bromoanisole (BrAni).     

Morphological study of polymer/fullerene interfaces via benzene–PC70BM interaction

We designed and synthesized a series of highly soluble random copolymers (P(BDTT-PhC8TPD-TPD)) comprising a two-dimensional (2D) electron-rich 2-ethylhexylthiophene-substituted benzo[1,2-b:4,5-b′]dithiophene (M1) electron-rich unit and various ratios of two side chain-functionalized thieno[3,4-c]-pyrrole-4,6-diones (M2 and M3) as planar electron-deficient units. The morphologies and photovoltaic performances of the bulk heterojunction blend films were examined, revealing concentration-dependent competitive benzene–benzene and benzene–fullerene interactions. A bulk heterojunction solar cell prepared using 10 mol% M2 (RP1) and PC₇₀BM exhibited a power conversion efficiency (PCE) of 4.9%, representing a significant improvement over the PCE (1.1%) obtained from parent copolymer (P1). The PCE in these cells was strongly associated with the nanoscale morphology of the BHJ film, which depended on the higher miscibility of RP1 with PC₇₀BM compared to RP2 (M2: 20 mol%), and RP3 (M2: 30 mol%).     

2,2-Dicyanovinyl-end-capped oligothiophenes as electron acceptor in solution processed bulk-heterojunction organic solar cells

Three 2,2-dicyanovinyl (DCV) end-capped A-π-D-π-A type oligothiophenes (DCV-OTs) containing dithieno[3,2-b:2′,3′-d]silole (DTSi), cyclopenta[1,2-b:3,4-b′]dithiophene (DTCP) or dithieno[3,2-b:2′,3′-d]pyrrole (DTPy) unit as the central donor part, mono-thiophene as the π-conjugation bridge were synthesized. The absorption spectroscopies, cyclic voltammetry of these compounds were characterized. Results showed that all these compounds have intensive absorption band over 500–680 nm with a LUMO energy level around −3.80 eV, which is slightly higher than that of [6,6]phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM, E_LUMO = −4.01 eV), but lower than that of poly(3-hexylthiophene) (P3HT, E_LUMO = −2.91 eV). Solution processed bulk heterojunction “all-thiophene” solar cells using P3HT as electron donor and the above mentioned oligothiophenes as electron acceptor were fabricated and tested. The highest power conversion efficiency (PCE) of 1.31% was achieved for DTSi-cored compound DTSi(THDCV)₂, whereas PTB7:DTSi(THDCV)₂ based device showed slightly higher PCE of 1.56%. Electron mobilities of these three compounds were measured to be around 10⁻⁵ cm² V⁻¹ s⁻¹ by space charge limited current method, which is much lower than that of PC₆₁BM, and was considered as one of the reason for the low photovoltaic performance.     

Stepwise co-sensitization as a useful tool for enhancement of power conversion efficiency of dye-sensitized solar cells: The case of an unsymmetrical porphyrin dyad and a metal-free organic dye

A tertiary arylamine compound (DC), which contains a terminal cyano-acetic group in one of its aryl groups, and an unsymmetrical porphyrin dyad of the type Zn[Porph]-L-H₂[Porph] (ZnP-H₂P), where Zn[Porph] and H₂[Porph] are metallated and free-base porphyrin units, respectively, and L is a bridging triazine group functionalized with a glycine moiety, and were synthesized and used for the fabrication of co-sensitized dye-sensitized solar cells (DSSCs). The photophysical and electronic properties of the two compounds revealed spectral absorption features and frontier orbital energy levels that are appropriate for use in DSSCs. Following a stepwise co-sensitization procedure, by immersing the TiO₂ electrode in separate solutions of the dyes in different sequence, two co-sensitized solar cells were obtained: devices C (ZnP-H₂P/DC) and D (DC/ZnP-H₂P).The two solar cells were found to exhibit power conversion efficiencies (PCEs) of 6.16% and 4.80%, respectively. The higher PCE value of device C, which is also higher than that of the individually sensitized devices based on the ZnP-H₂P and DC dyes, is attributed to enhanced photovoltaic parameters, i.e. short circuit current (J_sc = 11.72 mA/cm²), open circuit voltage (V_oc = 0.72 V), fill factor (FF = 0.73), as it is revealed by photovoltaic measurements (J–V curves) and by incident photon to current conversion efficiency (IPCE) spectra of the devices, and to a higher total dye loading. The overall performance of device C was further improved up to 7.68% (with J_sc = 13.45 mA/cm², V_oc = 0.76 V, and FF = 0.75), when a formic acid treated TiO₂ ZnP-H₂P co-sensitized photoanode was employed (device E). The increased PCE value of device E has been attributed to an enhanced J_sc value (=13.45 mA/cm²), which resulted from an increased dye loading, and an enhanced V_oc value (=0.76 V), attributed to an upward shift and increased of electron density in the TiO₂ CB. Furthermore, dark current and electrochemical impedance spectra (EIS) of device E revealed an enhanced electron transport rate in the formic acid treated TiO₂ photoanode, suppressed electron recombination at the photoanode/dye/electrolyte interface, as well as shorter electron transport time (τ_d), and longer electron lifetime (τ_e).     

Effect of substituents of twisted benzodiperylenediimides on non-fullerene solar cells

Twisted benzodiperylenediimides (TBDPDI) with large rigid conjugated core and strong absorption is regarded as an excellent acceptor in non-fullerene solar cells. Since side chains of semiconductors play a crucial role in the solar cells, TBDPDI acceptors with different side chains (1-ethylpropyl, C5; 2-ethylhexyl, C8; 1-pentylhexyl, C11; 2-octyldodecyl, C20; 1-undecyldodecyl, C23) were synthesized. In solution, TBDPDI compounds (C5, C11, and C23) with alkyl chains branched at 1-position show significantly different absorption profiles and fluorescence intensity with those (C8 and C20) branched at 2-position, due to stronger aggregation of the latter. Nevertheless, alkyl chains have little effect on the molecular orbital energy levels and optical band gaps, as verified by cyclic voltammetry and solid state absorption. Due to their complementary absorption and matchable energy levels with donor of PCE10, these acceptors and PCE10 were used together to fabricate bulk heterojunction (BHJ) solar cells. Because of inferior phase separation with large domain size around 100 nm and bulky insulated side chains, acceptors (C20 and C23) with long alkyl chains have the low electron mobility (μ_e) around 10⁻⁸ cm² V⁻¹ s⁻¹ and the low power conversion efficiency (PCE) of solar cells. TBDPDI (C11) with 1-pentylhexyl gives the highest PCE of 5.0% under the optimized condition, which is attributed to proper phase separation with domain size around 20 nm and highest μ_e of 10⁻⁶ cm² V⁻¹ s⁻¹.     

Two new A-D-A type small molecule acceptors based on C2v-symmetric dithienocyclopentaspiro[fluorene-9,9′-xanthene] core for polymer solar cells

A C_2v-symmetric core, dithienocyclopentaspiro[fluorene-9,9′-xanthene], was used as the central block for the first time to design and synthesize A-D-A type small molecule acceptors for nonfullerene polymer solar cells (PSCs), and two new small molecule acceptors of TSFX-2F and TSFX-4F were synthesized based on the C_2v-symmetric core. The two TSFX-based acceptors show high thermal stability, strong absorption in the wavelength region of 550–750 nm and appropriate energy levels. The PSCs with the broad bandgap polymer J71 as donor and TSFX-2F as acceptor demonstrated power conversion efficiency (PCE) of 9.42% with open circuit voltage (V_oc) of 0.89 V, short circuit current density (J_sc) of 15.27 mA cm⁻² and fill factor (FF) of 69.30%, while the PSC based on J71:TSFX-4F shows a PCE of 8.47% with V_oc of 0.83 V, J_sc of 15.48 mA cm⁻² and FF of 66.16%. The higher V_oc of the PSC based on J71: TSFX-2F is benefitted from the up-shifted LUMO energy level of the TSFX-2F acceptor, and its higher FF can be ascribed to the higher and more balanced hole and electron mobilities of the J71: TSFX-2F active layer. This work demonstrates that the new C_2v-symmetric building block is a promising central D-unit for the design and synthesis of new structured norfullerene acceptors for high-performance PSCs.     

Copoly[6,6′-bis(9-(2-ethylhexyl)carbazole-3-yl)/thieno-(2,5-b)thiophenylidenevinylene]-based photorefractive composite: A comparative study on conjugated and non-conjugated polymer systems

Photoconductivity and birefringence are two important factors that affect performances of photorefractive devices. Here, the optical properties of this composite based on copoly[6,6′-bis(9-(2-ethylhexyl)carbazole-3-yl)/thieno-(2,5-b)thiophenylidenevinylene] as photoconducting material, 2-[3-[(E)-2(piperidine)-1-ethenyl]-5,5-dimethyl]-2-cyclohexenyliden]malononitrile as nonlinear optical chromophore, butyl benzyl phthalate as plasticizer and C₆₀ as photosensitizer, has been compared to those of the corresponding non-π-conjugated polymer composite. The 50-μm thick photorefractive composite showed a diffraction efficiency 37.2% at 50 V/μm, which corresponded to a Δn of 2.62 × 10^?3. When we compare the speeds of the current and the non-conjugated analogue composites, the current composite has about a factor of seven larger speeds due to the π-conjugated polymer backbone. We also discuss and present simple explanation of the observed effect.     

Phenylimidazole-based homoleptic iridium(III) compounds for blue phosphorescent organic light-emitting diodes with high efficiency and long lifetime

In order to obtain triplet emitters with high stability and efficiency, three homoleptic iridium(III) compounds — specifically, Ir(tpim)₃ (1), Ir(mtpim)₃ (2), and Ir(itpim)₃ (3), where tpim = 1-([1,1′:3′,1″-terphenyl]-2′-yl)-2-(4-fluorophenyl)-1H-imidazole, mtpim = 2-(4-fluorophenyl)-1-(5′-methyl-[1,1′:3′,1″-terphenyl]-2′-yl)-1H-imidazole, and itpim = 2-(4-fluorophenyl)-1-(5′-isopropyl-[1,1′:3′,1″-terphenyl]-2′-yl)-1H-imidazole — were prepared by one-pot reaction of the corresponding phenylimidazole ligand with an Ir(I) complex as a starting material. Compounds 1–3 emit bright sky-blue phosphorescence with λ_max = 459–463 nm and phosphorescent quantum efficiencies of 0.38–0.50. Multi-layer phosphorescent organic light-emitting diodes using compounds 1–3 as the triplet emitters and mCBP (3,3-di(9H-carbazol-9-yl)biphenyl) as the host have been fabricated. Compound 3 doped in the emissive layer demonstrate external quantum efficiency as high as 20.1% at 1000 cd/m². In addition, the device based on compound 1 as an emitter shows a stable lifetime greater than 300 h at 1000 cd/m², which is one of the best results concerning the device lifetime.