Large Band Gap Polymers Research Articles

Influence of the terminal group on optoelectronic properties of fused-ring nonfullerene acceptors with ethylhexyl side chain

Three fused-ring nonfullerene electron acceptors are designed and synthesized by combining side chain effect and terminal group modification. The 2-ethylhexyl substituent is employed to replace the commonly used n-hexyl or n-hexylphenyl side chains. Three acceptor units, 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (IC), 2-(6-oxo-5,6-dihydro-4H-cyclopenta [c]thiophen-4-ylidene) malononitrile (CPTCN), and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC–2Cl) are used as the terminal groups and the three fused-ring nonfullerene acceptors, IDTC8-IC, IDTC8-Th, and IDTC8-4Cl are developed, respectively. The thermal, photophysical, electrochemical and photovoltaic properties of the three acceptors are systematically studied in view of the influence of the side chain and the terminal group. The three acceptors exhibit strong absorption ranging from 500 nm to 720 nm in solution with high molar extinction coefficients. The large bandgap polymer PM6 is chosen as the donor to blend with the acceptors, due to the complementary absorption bands and well-matched energy levels. The optimized organic solar cell based on PM6:IDTC8-IC exhibits a power conversion efficiency (PCE) of 8.56% with an open-circuit voltage (VOC) of 0.98 V, a short-circuit current density (JSC) of 13.93 mA cm−2, and a fill factor (FF) of 62.42%. The PM6:IDTC8-4Cl device demonstrates the best performance with a PCE of 9.79%, a VOC of 0.82 V, a JSC of 17.40 mA cm−2, and a FF of 68.67%. The impacts of carrier mobility, film morphology, and molecular packing properties of the blend films on photovoltaic performances are systematically investigated. The results imply that the synergistic effect of alkyl chains on the central core and modification of the terminal groups yields improved photovoltaic performance of nonfullerene acceptors.

Organic Solar Cells with 18% Efficiency Enabled by an Alloy Acceptor: A Two-in-One Strategy.

The trade-off between the open-circuit voltage (Voc ) and short-circuit current density (Jsc ) has become the core of current organic photovoltaic research, and realizing the minimum energy offsets that can guarantee effective charge generation is strongly desired for high-performance systems. Herein, a high-performance ternary solar cell with a power conversion efficiency of over 18% using a large-bandgap polymer donor, PM6, and a small-bandgap alloy acceptor containing two structurally similar nonfullerene acceptors (Y6 and AQx-3) is reported. This system can take full advantage of solar irradiation and forms a favorable morphology. By varying the ratio of the two acceptors, delicate regulation of the energy levels of the alloy acceptor is achieved, thereby affecting the charge dynamics in the devices. The optimal ternary device exhibits more efficient hole transfer and exciton separation than the PM6:AQx-3-based system and reduced energy loss compared with the PM6:Y6-based system, contributing to better performance. Such a "two-in-one" alloy strategy, which synergizes two highly compatible acceptors, provides a promising path for boosting the photovoltaic performance of devices.

Design of All-Fused-Ring Electron Acceptors with High Thermal, Chemical, and Photochemical Stability for Organic Photovoltaics

High-performance donor–acceptor electron acceptors containing 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN)-type terminals are labile toward photooxidation and basic conditions, and ...

Transparent Hole-Transporting Frameworks: A Unique Strategy to Design High-Performance Semitransparent Organic Photovoltaics.

Thanks to the nature of molecular orbitals, the absorption spectra of organic semiconductors are not continuous like those in traditional inorganic semiconductors, which offers a unique application of organic photovoltaics (OPVs): semitransparent OPVs. Recently, the exciting progress of materials design has promoted the development of semitransparent OPVs. However, in the perspective of device engineering, almost all reported works reduce the thickness of back/reflected electrode to obtain high average visible transmittance (AVT), which is a trade-off between power conversion efficiency (PCE) and the transmittance of the whole solar spectrum (visible and infrared), and therefore limit the further development. Herein, a unique strategy of "transparent hole-transporting frameworks" is proposed. A hole-transporting large-bandgap polymer (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA)) is employed to partially replace polymer donors in the active layer of PBDB-T/Y1. PTAA is a p-type polymer with a large bandgap of 2.9eV; the partial substitution of PBDB-T by PTAA reduces the absorption of the active layer only in the visible region, keeping the hole-transporting pathways as well as the optimized film morphology. As a result, semitransparent OPVs with PCEs of 12% and AVTs of 20% are achieved, both on rigid and flexible substrates. To demonstrate the generality, this strategy is also used in three different active layers.

Shorter alkyl chain in thieno[3,4-c]pyrrole-4,6-dione (TPD)-based large bandgap polymer donors – Yield efficient non-fullerene polymer solar cells

Typically, conjugated polymers are composed of conjugated backbones and alkyl side chains. In this contribution, a cost-effective strategy of tailoring the length of alkyl side chain is utilized to design high-performing thieno[3,4-c]pyrrole-4,6-dione (TPD)-based large bandgap polymer donors PBDT-BiTPD(Cχ) (χ = 48, 52, 56), in which χ represents the alkyl side chain length in term of the total carbon number. A combination of light absorption, device, and morphology examinations make clear that the shorter alkyl side chains yield (i) higher crystallinity and more predominant face-on crystallite orientation in their neat and BHJ blend films, (ii) higher charge mobilities (6.7 × 10−4 cm2 V−1 s−1 for C48 vs. 3.2 × 10−4 cm2 V−1 s−1 for C56), and negligible charge recombination, consequently, (iii) significantly improved fill-factor (FF) and short current (JSC), while almost the same open circuit voltage (VOC) of ca. 0.82 V in their corresponding BHJ devices. In parallel, as alkyl side chain lengths decrease from C56 to C48, power conversion efficiencies (PCEs) increased from 7.8% for C56 to 11.1% for C52, and further to 14.1% for C48 in their BHJ solar cells made with a narrow bandgap non-fullerene acceptor Y6. This systematic study declares that shortening the side chain, if providing appropriate solubility in device solution processing solvents, is of essential significance for developing high-performing polymer donors and further improving device photovoltaic performance.

Pronounced Dependence of All-Polymer Solar Cells Photovoltaic Performance on the Alkyl Substituent Patterns in Large Bandgap Polymer Donors.

For all-polymer solar cells which are composed of polymer donors and polymer acceptors, the effect of alkyl side chains on photovoltaic performance is a matter of some debate, and this effect remains difficult to forecast. In this concise contribution, we demonstrate that three alkyls namely branched alkyl 2-butyloctyl (2BO), long linear alkyl n-dodecyl (C12), and double-short linear alkyl n-hexyls (DC6) incorporated into the side chains of large bandgap polymer donor PBDT-TTz can induce considerable, of significance, and different electronic, optical, and morphological parameters. Systematic studies shed light on the critical role of the double-short linear alkyl n-hexyls (DC6) in (i) producing large ionization potential value, (ii) increasing propensity of the polymer to order along the π-stacking direction, (iii) generating polymer crystallites with more preferential "face-on" orientation, consequently, (iv) improvement of carriers transportation, (v) suppression of charge recombination, (vi) reduction of energy loss in all-polymer devices. In parallel, we unearth that the PBDT-TTz with double-short linear alkyl n-hexyls (DC6) represents the highest efficiency of 8.3 %, whereas, the other two PBDT-TTz analogues (2BO, C12) yield efficiencies of less than 3 % in optimized all-polymer solar cells. Though branched or long linear alkyl side chains (2BO, C12) have been applied to provide the solution processability of conjugated polymers, motifs bearing multiple short linear alkyl substituents (DC6) are proved critical to the development of high performing polymers.

Efficient near ultraviolet organic light-emitting diode based on PVK material doped with BCPO molecules

Near ultraviolet organic light-emitting diodes (NUV-OLEDs) have a wide range of applications. However, when large band-gap polymer of PVK was used as the emitting material, its poor electron transportation capability resulted in a large amount of charge accumulation on both sides of light-emitting layer/electron-transporting layer interface of the device, hence generating intense electroplex emission and seriously reducing the efficiency of NUV light-emitting. In order to solve this issue, BCPO molecules were doped in PVK with certain concentrations, forming PVK:BCPO mixing light-emitting layers. As compared with pure PVK, electron transportation capability was apparently increased in PVK:BCPO mixing layers. On one hand, charge accumulation at the interface was substantially weakened, and the electroplex emission was inhibited. On the other hand, transportations of electron and hole carriers became more balanced in light-emitting layer. Both of these improvements effectively promoted device performance. At appropriate BCPO doping concentration, the external quantum efficiency of PVK:BCPO-based NUV-OLEDs reached the maximum value of ∼2.6 %. Further experimental progress would bring it closer to the practical application in future.

Synergistic Effects of Polymer Donor Backbone Fluorination and Nitrogenation Translate into Efficient Non-Fullerene Bulk-Heterojunction Polymer Solar Cells.

State-of-the-art non-fullerene bulk-heterojunction (BHJ) polymer solar cells outperform the more extensively studied polymer-fullerene BHJ solar cells in terms of efficiency, thermal-, and photostability. Considering the strong light absorption in the near-infrared region (600-1000 nm) for most of the efficient acceptors, the exploration of high-performing large band gap (LBG) polymer donors with complementary optical absorption ranging from 400 to 700 nm remains critical. In this work, the strategy of concurrently incorporating fluorine (-F) and unsaturated nitrogen (-N) substituents along the polymer backbones is used to develop the LBG polymer donor PB[N][F]. Results show that the F- and N-substituted polymer donor PB[N][F] realizes up to 14.4% efficiency in BHJ photovoltaic devices when paired with a benchmark molecule acceptor Y6, which largely outperforms the analogues PB with an efficiency of only 3.6% and PB[N] with an efficiency of 11.8%. Systematic examinations show that synergistic effects of polymer backbone fluorination and nitrogenation can significantly increase ionization potential values, improve charge transport, and reduce bimolecular recombination and trap-assisted recombination in the PB[N][F]:Y6 BHJ system. Importantly, our study shows that the F- and N-substituted conjugated polymers are promising electron-donor materials for solution-processed non-fullerene BHJ solar cells.

Revealing the Critical Role of the HOMO Alignment on Maximizing Current Extraction and Suppressing Energy Loss in Organic Solar Cells.

SummaryFor state-of-the-art organic solar cells (OSCs) consisting of a large-bandgap polymer donor and a near-infrared (NIR) molecular acceptor, the control of the HOMO offset is the key to simultaneously achieve small energy loss (Eloss) and high photocurrent. However, the relationship between HOMO offsets and the efficiency for hole separation is quite elusive so far, which requires a comprehensive understanding on how small the driving force can effectively perform the charge separation while obtaining a high photovoltage to ensure high OSC performance. By designing a new family of ZITI-X NIR acceptors (X = S, C, N) with a high structural similarity and matching them with polymer donor J71 forming reduced HOMO offsets, we systematically investigated and established the relationship among the photovoltaic performance, energy loss, and hole-transfer kinetics. We achieved the highest PCEavgs of 14.05 ± 0.21% in a ternary system (J71:ZITI-C:ZITI-N) that best optimize the balance between driving force and energy loss.

Trap-Assisted Charge Injection into Large Bandgap Polymer Semiconductors.

The trap-assisted charge injection in polyfluorene-poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) model systems with an Al or Al/LiF cathode is investigated. We find that inserting 1.3 nm LiF increases electron and hole injections simultaneously and the increase of holes is greater than electrons. The evolution of internal interfaces within polymer light-emitting diodes is observed by transmission electron microscopy, which reveals that the introduction of LiF improves the interface stability at both the cathode (cathode/polymer) and the anode (indium tin oxide (ITO)/PEDOT:PSS). Above-mentioned experimental results have been compared to the numerical simulations with a revised Davids model and potential physical mechanisms for the trap-assisted charge injection are discussed.

Trifluoromethyl-Substituted Large Band-Gap Polytriphenylamines for Polymer Solar Cells with High Open-Circuit Voltages.

Two large band-gap polymers (PTPACF and PTPA2CF) based on polytriphenylamine derivatives with the introduction of electron-withdrawing trifluoromethyl groups were designed and prepared by Suzuki polycondensation reaction. The chemical structures, thermal, optical and electrochemical properties were characterized in detail. From the UV-visible absorption spectra, the PTPACF and PTPA2CF showed the optical band gaps of 2.01 and 2.07 eV, respectively. The cyclic voltammetry (CV) measurement displayed the deep highest occupied molecular orbital (HOMO) energy levels of −5.33 and −5.38 eV for PTPACF and PTPA2CF, respectively. The hole mobilities, determined by field-effect transistor characterization, were 2.5 × 10−3 and 1.1 × 10−3 cm2 V−1 S−1 for PTPACF and PTPA2CF, respectively. The polymer solar cells (PSCs) were tested under the conventional device structure of ITO/PEDOT:PSS/polymer:PC71BM/PFN/Al. All of the PSCs showed the high open circuit voltages (Vocs) with the values approaching 1 V. The PTPACF and PTPA2CF based PSCs gave the power conversion efficiencies (PCEs) of 3.24% and 2.40%, respectively. Hence, it is a reliable methodology to develop high-performance large band-gap polymer donors with high Vocs through the feasible side-chain modification.

A Twisted Thieno[3,4-b]thiophene-Based Electron Acceptor Featuring a 14-π-Electron Indenoindene Core for High-Performance Organic Photovoltaics.

With an indenoindene core, a new thieno[3,4-b]thiophene-based small-molecule electron acceptor, 2,2'-((2Z,2'Z)-((6,6'-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene-2,7-diyl)bis(2-octylthieno[3,4-b]thiophene-6,4-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (NITI), is successfully designed and synthesized. Compared with 12-π-electron fluorene, a carbon-bridged biphenylene with an axial symmetry, indenoindene, a carbon-bridged E-stilbene with a centrosymmetry, shows elongated π-conjugation with 14 π-electrons and one more sp3 carbon bridge, which may increase the tunability of electronic structure and film morphology. Despite its twisted molecular framework, NITI shows a low optical bandgap of 1.49 eV in thin film and a high molar extinction coefficient of 1.90 × 105 m-1 cm-1 in solution. By matching NITI with a large-bandgap polymer donor, an extraordinary power conversion efficiency of 12.74% is achieved, which is among the best performance so far reported for fullerene-free organic photovoltaics and is inspiring for the design of new electron acceptors.

Isomeric Effects of Solution Processed Ladder‐Type Non‐Fullerene Electron Acceptors

The role of electronic structure and thin film morphology is investigated in determining charge transfer and electron coupling due to orbital interactions in two isomeric non‐fullerene acceptors with the structure of acceptor‐donor‐acceptor. Differences are found in the distribution of electron density of the highest occupied molecular and lowest unoccupied molecular orbitals, whose bonding interactions result in improved intermolecular interactions and hence, molecular stacking. When combined with a large band gap polymer donor, solution‐processed organic photovoltaic cells are demonstrated with power conversion efficiencies as high as 10.5 ± 0.4%, and with absorption extending to wavelengths of 800 nm. Due to strong internal organization driven by the planar molecular structure and strong intermolecular interactions, no post‐deposition processing such as solvent vapor or thermal annealing is required. To our knowledge, these are the highest efficiencies for as‐cast solution‐based devices employing non‐fullerene acceptors.

Visualization of trap dilution in polyfluorene based light-emitting diodes

The effects of electron trapping and resulting trap-assisted recombination can be strongly suppressed by diluting a semiconducting polymer with a large band gap polymer such as polystyrene. Polyfluorene (PFO)-based light-emitting diodes (PLEDs) are an excellent model system to visualize this trap dilution effect. The blue emission from the pristine PFO backbone is accompanied by a broad, featureless green emission band originating from monomeric ketone defects that act as an electron trap. We demonstrate that the green emission from radiative trap-assisted recombination at these ketone defects is nearly eliminated upon dilution. The ratio between bimolecular blue emission and trap-assisted green emission as a function of dilution is shown to be in quantitative agreement with model predictions.

A fused-ring based electron acceptor for efficient non-fullerene polymer solar cells with small HOMO offset

A non-fullerene electron acceptor bearing a novel backbone with fused 10-heterocyclic ring (indacenodithiopheno-indacenodiselenophene), denoted by IDTIDSe-IC is developed for fullerene free polymer solar cells. IDTIDSe-IC exhibits a low band gap (Eg=1.52eV) and strong absorption in the 600–850nm region. Combining with a large band gap polymer J51 (Eg=1.91eV) as donor, broad absorption coverage from 300nm to 800nm is obtained due to complementary absorption of J51 and IDTIDSe-IC, which enables a high PCE of 8.02% with a VOC of 0.91V, a JSC of 15.16mA/cm2 and a FF of 58.0% in the corresponding PSCs. Moreover, the EQE of 50–65% is achieved in the absorption range of IDTIDSe-IC with only about 0.1eV HOMO difference between J51 and IDTIDSe-IC.

Electroluminescence and cathodoluminescence from polyethylene and polypropylene films: Spectra reconstruction from elementary components and underlying mechanisms

The mechanisms of electroluminescence from large band gap polymers used as insulation in electric components are still under debate. It becomes important to unravel the underlying physics of the emission because of increasing thermo-electric stress and a possible relationship between electroluminescence and field withstand. We report herein on the cathodoluminescence spectra of polyethylene and polypropylene films as a way to uncover the nature of its contributions to electroluminescence emission. It is shown that spectra from the two materials are structured around four elementary components, each of them being associated with a specific process contributing to the overall emission with different weights depending on excitation conditions and on materials. The cathodoluminescence and electroluminescence spectra of each material are reconstructed from the four spectral components and their relative contribution are discussed. It is shown that electroluminescence from polyethylene and polypropylene has the same origin pointing towards generic mechanisms in both.

Effects of the incorporation of an additional pyrrolo[3,4-c]pyrrole-1,3-dione unit on the repeating unit of highly efficient large band gap polymers containing benzodithiophene and pyrrolo[3,4-c]pyrrole-1,3-dione derivatives

Property modulation of 2,5-dioctylpyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (DPPD)-based high energy converting wide band gap polymers, P(BDT-TDPPDT) and P(BDTT-TDPPDT), was studied via the incorporation of an additional DPPD unit on their repeating units. A new electron acceptor (BDPPD) unit containing two DPPD units was prepared and copolymerized with the distannyl derivatives of benzodithiophene (BDT) or 2D-conjugated benzodithiophene (BDTT) to provide two new polymers, P(BDT-BDPPD) and P(BDTT-BDPPD). The optical band gaps and highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels of P(BDT-BDPPD) and P(BDTT-BDPPD) were 2.16 eV, 2.08 eV and −5.37 eV/‒3.21 eV, −5.44 eV/‒3.36 eV, respectively. The hole mobilities of P(BDT-BDPPD) and P(BDTT-BDPPD) were in the order of 10−4 cm2V−1 s−1. The polymer solar cells (PSCs) prepared with the configuration of ITO/PEDOT:PSS/P(BDT-BDPPD) or P(BDTT-BDPPD):PC70BM/Al gave maximum power conversion efficiencies (PCEs) of 2.74% and 3.63%, respectively. The insertion of a BDPPD unit instead of a TDPPDT unit on the DPPD-based polymer backbone did not affect the optical and electrochemical properties considerably. On the other hand, the new polymers, P(BDT-BDPPD) and P(BDTT-BDPPD), resulted in improved photovoltaic performances compared to the reported polymers, P(BDT-TDPPDT) and P(BDTT-TDPPDT), for the devices prepared without additives.

Side-chain engineering of diindenocarbazole-based large bandgap copolymers toward high performance polymer solar cells

Diindenocarbazole-based large bandgap polymers were designed and synthesized as short wavelength absorbing materials for PSCs exhibiting an efficiency up to 7.34% and a Voc as large as 0.95 V.

Photocurrent enhancement of an efficient large band gap polymer incorporating benzodithiophene and weak electron accepting pyrrolo[3,4−c]pyrrole−1,3−dione derivatives via the insertion of a strong electron accepting thieno[3,4−b]thiophene unit

The opto-electrical, charge transport and photovoltaic properties of the highly efficient large band gap polymer, P(BDT-TDPPDT), containing electron rich 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT) and a weak electron accepting 2,5-dioctyl-4,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (TDPPDT) units were modulated by the incorporation of a relatively strong electron deficient thieno[3,4-b]thiophene (TT) unit on its backbone. A new random copolymer (RP2) was prepared by polymerizing the BDT derivative with TDPPDT and TT derivatives at a 2:1:1 ratio. The estimated optical band gap (Eg) and highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO) energy levels of RP2 were 1.70 eV and −5.30 eV/−3.60 eV, respectively. The determined hole mobility of RP2 was 4.0 × 10−4 cm2V −1s−1. The polymer solar cells (PSCs) prepared with a simple device configuration of ITO/PEDOT:PSS/RP2:PC70BM/Al offered a maximum power conversion efficiency (PCE) of 5.12% with an open circuit voltage (Voc) of 0.75 V, a current density (Jsc) of 12.99 mA/cm2, and a fill factor (FF) of 53%. The incorporation of a TT unit on the P(BDT-TDDPT) backbone was found to reduce the band gap by 0.41 eV and increase slightly the hole mobility of the resulting copolymer, RP2. Consequently, the PSC prepared from RP2 showed the enhanced photocurrent compared with that of the PSC prepared from P(BDT-TDPPDT).

High‐Performance Non‐Fullerene Polymer Solar Cells Based on a Pair of Donor–Acceptor Materials with Complementary Absorption Properties

A 7.3% efficiency non-fullerene polymer solar cell is realized by combining a large-bandgap polymer PffT2-FTAZ-2DT with a small-bandgap acceptor IEIC. The complementary absorption of donor polymer and small-molecule acceptor is responsible for the high-performance of the solar-cell device. This work provides important guidance to improve the performance of non-fullerene polymer solar cells.