Wide-bandgap Polymer Research Articles

Dramatically different photovoltaic effect induced by siloxane-terminated combinatory side chain in polymer solar cells

Abstract Combinatory side chains have attracted much attention for molecular engineering in recent years. In this work, siloxane-terminated alkylthiol thiophene as a new combinatory side chain was developed to construct a new wide band-gap polymer CY1. Interestingly, the polymer CY1 exhibited preferential compatibility with nonfullerene acceptor rather than with fullerene acceptor, as revealed by Flory-Huggins interaction factor. The power conversion efficiency of the device based on CY1:IDIC was much higher than that based on CY1:PC71BM even with the additive. The contrasting performance is mainly ascribed to the different morphology induced by siloxane-terminated combinatory side chain on the polymer backbone. Our preliminary result demonstrates that a suitable combinatory side chain should be valuable to tune the compatibility between polymer donor and nonfullerene acceptor and finally promote the photovoltaic performance.

Morphology optimization via molecular weight tuning of donor polymer enables all-polymer solar cells with simultaneously improved performance and stability

Organic solar cells with excellent device performance and stability are essential for both academic and industrial purposes. The molecular weight (Mn) of donor polymers plays a critical role in determining the photovoltaic properties of all-polymer solar cells (all-PSCs). Despite Mn of donor polymers has pronounced effects on microstructure morphology, photovoltaic performance and stability of all-PSCs are rarely analyzed and understood in high-performance devices. In this study, we systematically investigate the influence of Mn of donor polymers on the device performance metrics and device stability of all-PSCs comprising of a wide-bandgap polymer donor PTzBI-Si and a polymer acceptor N2200. All-PSCs based on high Mn of PTzBI-Si exhibit an excellent PCE of 11.5% with improved stability, which is clearly superior to that of all-PSCs based on low Mn of PTzBI-Si. The photovoltaic performance difference caused by Mn of PTzBI-Si is systematically characterized and analysed to establish the morphology-property-performance correlation. The findings demonstrated in our model system suggest that tuning the Mn of donor polymer could be a facile and promising strategy to simultaneously enhance the efficiency and stability of all-PSCs.

Impact of the Siloxane-Terminated Side Chain on Photovoltaic Performances of the Dithienylbenzodithiophene-Difluorobenzotriazole-Based Wide Band Gap Polymer Donor in Non-Fullerene Polymer Solar Cells.

To thoroughly disclose the role of the siloxane-terminated side chain with different substituent positions, three difluorobenzotriazole-dithienylbenzodithiophene (FTAZ-BDTT)-based polymers PBZ-1Si, PBZ-2Si, and PBZ-3Si with the siloxane-terminated side chain on the FTAZ unit (PBZ-1Si), on the BDTT unit (PBZ-2Si), and both on BDTT and FTAZ units (PBZ-3Si), respectively, were synthesized. The different side chain substitutions have slight influences on absorption behavior, thermal stability, and frontier molecular orbitals but have shown a great effect on the aggregation of the polymers. Grazing-incidence wide-angle X-ray scattering measurements reveal that, relative to PBZ-1Si with branched alkyl on the BDTT unit, polymers PBZ-2Si and PBZ-3Si, bearing the siloxane-terminated side chains on the BDTT unit, exhibit smaller π-π stacking distances and larger crystal coherence lengths, suggesting that adopting the siloxane-terminated side chain on the BDTT unit can promote the interchain π-π interaction and the ordering of molecular packing. With IT-M as the non-fullerene acceptor, among the three polymers, the PBZ-2Si-based active layer possesses the highest ordered crystals for both polymers and IT-M as well as the purest domain, which affords efficient exciton dissociation, the most balanced hole-electron transport, and reduced recombination, leading to the highest short-circuit current density (Jsc) and fill factor (FF) and then the highest power conversion efficiency (PCE) of 11.14%. In contrast, PBZ-1Si- and PBZ-3Si-based devices show lower PCEs of 8.98 and 9.92%, respectively. Moreover, PBZ-2Si:IT-M also exhibits good thickness tolerance, and its thick active layer of 240 nm shows the most limited decrease of efficiency after 77 days of storage, supplying good potential for mass fabrication. Our work suggests that the fine pairing of a siloxane-terminated side chain and an alkyl side chain is beneficial for the optimizing of a conjugated polymer donor toward high-performance non-fullerene polymer solar cells.

High-Performance Nonfullerene Organic Photovoltaic Cells Using a TPD-Based Wide Bandgap Donor Polymer

Organic photovoltaics (OPVs) consisting of a wide bandgap polymer donor and a nonfullerene acceptor (NFA) have received attention because they can effectively overcome the weaknesses of efficiency ...

Chain Engineering of Benzodifuran-Based Wide-Bandgap Polymers for Efficient Non-Fullerene Polymer Solar Cells.

Two novel benzo[1,2-b:4,5-b' ]difuran (BDF)-based wide-bandgap polymers, PBDFT-FBz and PBDFF-FBz, featuring a difluorobenzotriazole (FBz) acceptor unit, are designed and synthesized. The first attempt through main-chain engineering to alter thiophene units to furan units in the main chain of PBDFT-FBz, and further side-chain engineering eliminate the 2-ethylthiophenyl side chains of PBDFT-FBz by 2-ethylfuryl side chains to generate the "all-furan" polymer PBDFF-FBz. By taking the benefit of the oxygen atom in furan, both PBDFT-FBz and PBDFF-FBz exhibit lower HOMO energy levels and enhanced polymer chain interactions compared to their benzo[1,2-b:4,5-b' ]dithiophene (BDT)-based counterparts. As a result, while applying both polymers in non-fullerene polymer solar cells with non-fullerene acceptor m-ITIC, both devices exhibit highly promising photovoltaic performance. The power conversion efficiency (PCE) in the PBDFT-FBz device reaches 7.57% with increased open circuit voltage (Voc ) and fill factor (FF) compared to the PCE of 5.98% in its BDT counterpart (J52). A further increased PCE is obtained (8.79%) in the PBDFF-FBz:m-ITIC device, which shows ≈47% enhancement in device performance compared to that of J52. The large increase in photovoltaic performance is attributed to the lower-lying HOMO energy levels and better chain interactions in these BDF-based polymers.

Star-Shaped Fused-Ring Electron Acceptors with a C3h-Symmetric and Electron-Rich Benzotri(cyclopentadithiophene) Core for Efficient Nonfullerene Organic Solar Cells.

Classical fused-ring electron acceptors (FREAs) with a linear acceptor-donor-acceptor (A-D-A) architecture continuously break records of power conversion efficiency (PCE) in nonfullerene organic solar cells. In contrast, the development of star-shaped FREAs still lags behind. Herein, a new C3h-symmetric and electron-rich core, benzotri(cyclopentadithiophene) (BTCDT) in which the central benzo[1,2-b:3,4-b':5,6-b″]trithiophene fused with three outer thiophenes via three cyclopentadienyl rings, is synthesized and used for the construction of star-shaped FREAs (BTCDT-IC and BTCDT-ICF). Owing to the strong electron-donating ability of the BTCDT unit, both acceptors exhibit the effective intramolecular charge transfer, leading to the strong absorption in the region of 500-800 nm with narrow band gaps below 1.70 eV as well as suitable highest occupied molecular orbital and lowest unoccupied molecular orbital levels. Compared with nonfluorinated BTCDT-IC, fluorinated BTCDT-ICF red-shifts the absorption peak to 688 nm and reduces the band gap to 1.62 eV, which induces a broader external quantum efficiency (EQE) response ranging from 300 to 800 nm and a higher maximum EQE of 70% while blending with a wide band gap polymer donor J61. The J61:the BTCDT-ICF blend film exhibits more suitable phase morphology compared with the J61:BTCDT-IC blend film, which is responsible for the enhanced EQE value, increased short-circuit current density(JSC), and fill factor (FF) in organic solar cell devices. As a result, the J61:BTCDT-ICF-based device yields a best PCE of 8.11% with a high JSC of 16.93 mA cm-2 and a high FF of 65.6%, demonstrating that the BTCDT-based star-shaped FREAs hold great potential for nonfullerene organic solar cells.

Realizing Efficient Charge/Energy Transfer and Charge Extraction in Fullerene-Free Organic Photovoltaics via a Versatile Third Component.

Solution-processed organic photovoltaics (OPVs) based on bulk-heterojunctions have gained significant attention to alleviate the increasing demend of fossil fuel in the past two decades. OPVs combined of a wide bandgap polymer donor and a narrow bandgap nonfullerene acceptor show potential to achieve high performance. However, there are still two reasons to limit the OPVs performance. One, although this combination can expand from the ultraviolet to the near-infrared region, the overall external quantum efficiency of the device suffers low values. The other one is the low open-circuit voltage (VOC) of devices resulting from the relatively downshifted lowest unoccupied molecular orbital (LUMO) of the narrow bandgap. Herein, the approach to select and incorporate a versatile third component into the active layer is reported. A third component with a bandgap larger than that of the acceptor, and absorption spectra and LUMO levels lying within that of the donor and acceptor, is demonstrated to be effective to conquer these issues. As a result, the power conversion efficiencies (PCEs) are enhanced by the elevated short-circuit current and VOC; the champion PCEs are 11.1% and 13.1% for PTB7-Th:IEICO-4F based and PBDB-T:Y1 based solar cells, respectively.

A new non-fullerene acceptor based on the heptacyclic benzotriazole unit for efficient organic solar cells

Abstract A new non-fullerene electron acceptor with A-DA'D-A structure, namely, Y9, featuring a ladder-type heptacyclic fused ring (3-undecyl-thieno [2′,':4,5]thieno[3,2-b]pyrrole units with benzotriazole) as a core unit, is designed and synthesized. The new acceptor exhibits a broad and efficient absorption and extends into the near-infrared (NIR) region with a low optical bandgap of 1.36 eV. Efficient organic solar cells (OSCs) are fabricated with the wide bandgap polymer PBDB-T as polymer donor, showing high power conversion efficiency (PCE) of 13.26%. These results demonstrate that Y9 is a promising NIR non-fullerene acceptor for high-performance OSCs.

Efficient Non-Fullerene Organic Solar Cells Based on a Wide-Bandgap Polymer Donor Containing an Alkylthiophenyl-Substituted Benzodithiophene Moiety.

Two wide-bandgap polymer donors containing an alkylthiophenyl substituted benzo[1,2-b : 4,5-b']dithiophene moiety, namely PTZPO and PTZPS, were designed and synthesized. Both polymers exhibit relatively wide optical bandgap of 1.95 V with similar absorption profiles. The polymer PTZPS with alkylthiophenyl substituted benzo[1,2-b : 4,5-b']dithiophene units showed enhanced light-harvesting capabilities, leading to improved short-circuit current densities. The PTZPS : ITIC film shows more appreciable film morphology and phase separation than the film composed of a blend of ITIC with alkoxyl substitutions containing copolymer PTZPO, which facilitates exciton dissociation and charge transport. The PTZPS : ITIC-based non-fullerene organic solar cells show clearly improved short-circuit current density and an impressively high power conversion efficiency of more than 11 %. These observations demonstrate the great promise of using PTZPS as electron-donating materials for high-performance non-fullerene organic solar cells.

A chlorinated non-fullerene acceptor for efficient polymer solar cells

Improving the performance and reducing the manufacturing costs are the main directions for the development of organic solar cells in the future. Here, the strategy that uses chemical structure modification to optimize the photoelectric properties is reported. A new narrow bandgap (1.30 eV) chlorinated non-fullerene electron acceptor (Y15), based on benzo[d][1,2,3] triazole with two 3-undecyl-thieno[2′,3′:4,5] thieno[3,2-b] pyrrole fused -7-heterocyclic ring, with absorption edge extending to the near-infrared (NIR) region, namely A-DA'D-A type structure, is designed and synthesized. Its electrochemical and optoelectronic properties are systematically investigated. Benefitting from its NIR light harvesting, the fabricated photovoltaic devices based on Y15 deliver a high power conversion efficiency (PCE) of 14.13%, when blending with a wide bandgap polymer donor PM6. Our results show that the A-DA'D-A type molecular design and application of near-infrared electron acceptors have the potential to further improve the PCE of polymer solar cells (PSCs).

Indoor Organic Photovoltaics: Highly Efficient Indoor Organic Photovoltaics with Spectrally Matched Fluorinated Phenylene‐Alkoxybenzothiadiazole‐Based Wide Bandgap Polymers (Adv. Funct. Mater. 27/2019)

Indoor Organic Photovoltaics: Highly Efficient Indoor Organic Photovoltaics with Spectrally Matched Fluorinated Phenylene‐Alkoxybenzothiadiazole‐Based Wide Bandgap Polymers (Adv. Funct. Mater. 27/2019)

Characterization of organic solar cells using semiconducting polymers with different bandgaps

Abstract Polymer-based organic solar cells are of great interest as they can be produced with low-cost techniques and also have many interesting features such as flexibility, graded transparency, easy integration, and lightness. However, conventional wide bandgap polymers used for the light-absorbing layer significantly affect the power conversion efficiency of organic solar cells because they collect sunlight in a given spectrum range and due to their limited stability. Therefore, in this study, polymers with different bandgaps were used, which could allow for the production of more stable and efficient organic solar cells: P3HT as the wide bandgap polymer, and PTB7 and PCDTBT as low bandgap polymers. These polymers with different bandgaps were combined with PCBM to obtain increased efficiency and optimum photoactive layer in the organic solar cell. The obtained devices were characterized by measuring optical, photoelectrical, and morphological properties. Solar cells using the PTB7 and PCDTBT polymers had more rough surfaces than the reference cell using P3HT. The use of low-bandgap polymers improved I sc significantly, and when combined with P3HT, a higher V oc was obtained.

Enhanced Hole Transport in Ternary Blend Polymer Solar Cells.

Recently, ternary blend polymer solar cells have attracted great attention to improve a short-circuit current density (JSC ) effectively, because complementary absorption bands can harvest the solar light over a wide wavelength range from visible to near-IR region. Interestingly, some ternary blend solar cells have shown improvements not only in JSC but also in fill factor (FF). Previously, we also reported that a ternary blend solar cell based on a low-bandgap polymer (PTB7-Th), a wide-bandgap polymer (PDCBT), and a fullerene derivative (PCBM) exhibited a higher FF than their binary analogues. Herein, we study charge transport in PTB7-Th/PDCBT/PCBM ternary blend films to address the origin of the improvement in FF. We found that hole polarons are located in PTB7-Th domains and their mobility is enhanced in the ternary blend film.

Efficient Polymer Solar Cells Having High Open-Circuit Voltage and Low Energy Loss Enabled by a Main-Chain Twisted Small Molecular Acceptor

A new main-chain twisted small molecular acceptor with nonhalogenated end groups (i-IEICO) is designed and synthesized. In contrast to its planar analogue IECIO, i-IEICO possesses an obviously twisted backbone, leading to significant hypsochromic shift in film absorption, slight enhancement in solution extinction coefficient, and significantly elevated molecular energy level. Benefited from these features, i-IEICO is matched well with two wide band gap polymer donor materials (J52 and PBDB-T) both in absorption spectra and molecular energy levels. Relative to the planar-molecule IEICO-based devices, the open-circuit voltage ( VOC), short-circuit current density, and fill factor of the i-IEICO-based devices are simultaneously improved, giving rising to a 10.48% (with J52) and 8.79% (with PBDB-T) power conversion efficiency, respectively. Moreover, J52:i-IEICO device exhibits a high VOC of 0.96 V accompanied by a small energy loss of 0.64 eV, which can be further improved to 1.01 V and 0.59 eV for the PBDB-T-based device. The obtained VOC of i-IEICO-based devices are among one of the highest values of either J52 or PBDB-T-based binary devices, suggesting the effectiveness of main-chain twisted strategy coupled with end-group modification to achieve highly efficient nonfullerene acceptors with low energy loss and high VOC.

Highly Efficient Indoor Organic Photovoltaics with Spectrally Matched Fluorinated Phenylene‐Alkoxybenzothiadiazole‐Based Wide Bandgap Polymers

AbstractThe unique electro‐optical features of organic photovoltaics (OPVs) have led to their use in applications that focus on indoor energy harvesters. Various adoptable photoactive materials with distinct spectral absorption windows offer enormous potential for their use under various indoor light sources. An in‐depth study on the performance optimization of indoor OPVs is conducted using various photoactive materials with different spectral absorption ranges. Among the materials, the fluorinated phenylene‐alkoxybenzothiadiazole‐based wide bandgap polymer—poly[(5,6‐bis(2‐hexyldecyloxy)benzo[c][1,2,5]thiadiazole‐4,7‐diyl)‐alt‐(5,50‐(2,5‐difluoro‐1,4‐phenylene)bis(thiophen‐2‐yl))] (PDTBTBz‐2Fanti)‐contained photoactive layer—exhibits a superior spectrum matching with indoor lights, particularly a light‐emitting diode (LED), which results in an excellent power absorption ratio. These optical properties contribute to the state‐of‐the‐art performance of the PDTBTBz‐2Fanti:[6,6]‐phenyl‐C71 butyric acid methyl ester (PC71BM)‐based OPV with an unprecedented high power‐conversion efficiency (PCE) of 23.1% under a 1000 lx LED. Finally, its indoor photovoltaic performance is observed to be better than that of an interdigitated‐back‐contact‐based silicon photovoltaic (PCE of 16.3%).

A new approach for preparation of ternary bulk-heterojunction using dual-feed ultrasonic spray for organic solar cells

Ternary bulk heterojunctions using a wide band gap polymer P3HT and a narrow band gap polymer PTB7 as donors and ICBA as acceptor have been fabricated using a novel dual-feed approach in an ultrasonic spray. The binary blend of two donor polymers in a BHJ formed by a conventional method in which the two donor polymers are mixed intimately in a common solvent may lead to unfavourable phase morphology due to unfavourable interaction between them and puts constraints on the composition of the polymer blend in the BHJ. The unfavourable interaction may cause severe molecular disorder leading to charge recombination trap site, and thereby decreases the device performance. In the new method proposed in this paper intimate mixing of two donor polymers is avoided, which removes the constraint on the fraction of second donor in the first. Moreover, it also results in better polymer ordering leading to significantly higher hole mobility compared to that achieved with conventional method. The steady state photoluminescence study depicts more efficient FRET in P3HT:PTB7 binary blend prepared with dual channel method as compared to that prepared with single channel mode. With this method, ternary BHJ can be prepared even with higher concentration of the third component to have maximum power absorption, without compromising its optoelectronic properties. Although the superior properties of BHJ formed with the new dual-feed method did not reflected in similar performance in device functioning, with better choice of the donors and acceptor and with more control on nanoscale morphology through selection of proper solvents the device performance may improve to acceptable level. This report opens a new way of BHJ preparation in ternary organic solar cell fabrication.

Low-bandgap non-fullerene acceptors based on selenophene π spacer and alkylated indaceno[1,2-b:5,6-b′]dithiophene for organic solar cells

Three non-fullerene acceptors, i.e. IDT2SeC2C4, IDT2SeC2C42F and IDT2SeC2C44F, with selenophene π bridge between central alkylated indaceno[1,2-b:5,6-b′]dithiophene (IDT) unit and terminal 1,1-dicyanomethylene-3-indanones (ICs) with 0–2 fluorine substituents were synthesized. These molecules presented low optical band gaps in the range of 1.30–1.44 eV. Moreover, compared with the counterparts with phenyl side chains on IDT unit, the alkyl-based molecules achieved higher electron mobility owing to the more ordered and closer π-π stacking. Organic solar cells (OSCs) based on the non-fullerene acceptors were fabricated with the wide band gap polymer PBDB-T as donor. Attributed to the desired morphology with more obvious fibril-like structures, the devices based on IDT2SeC2C42F and IDT2SeC2C44F with fluorinated IC terminals delivered high power conversion efficiencies (PCEs) over 10%, whereas the devices based on DT2SeC2C4 showed a lower PCE of 8.92%.

Achieving over 16% efficiency for single-junction organic solar cells

To achieve high photovoltaic performance of bulk hetero-junction organic solar cells (OSCs), a range of critical factors including absorption profiles, energy level alignment, charge carrier mobility and miscibility of donor and acceptor materials should be carefully considered. For electron-donating materials, the deep highest occupied molecular orbital (HOMO) energy level that is beneficial for high open-circuit voltage is much appreciated. However, a new issue in charge transfer emerges when matching such a donor with an acceptor that has a shallower HOMO energy level. More to this point, the chemical strategies used to enhance the absorption coefficient of acceptors may lead to increased molecular crystallinity, and thus result in less controllable phase-separation of photoactive layer. Therefore, to realize balanced photovoltaic parameters, the donor-acceptor combinations should simultaneously address the absorption spectra, energy levels, and film morphologies. Here, we selected two non-fullerene acceptors, namely BTPT-4F and BTPTT-4F, to match with a wide-bandgap polymer donor P2F-EHp consisting of an imide-functionalized benzotriazole moiety, as these materials presented complementary absorption and well-matched energy levels. By delicately optimizing the blend film morphology, we demonstrated an unprecedented power conversion efficiency of over 16% for the device based on P2F-EHp:BTPTT-4F, suggesting the great promise of materials matching toward high-performance OSCs.

Asymmetric Wide-Bandgap Polymers Simultaneously Improve the Open-Circuit Voltage and Short-Circuit Current for Organic Photovoltaics.

A trade-off between open-circuit voltage (V OC ) and high short-circuit (J SC ) becomes one of the most vital problems limiting further improvement in polymer solar cells' (PSCs) efficiency. In this work, two asymmetric polymer donors PBDT-F-2TC and PBDT-SF-2TC are designed and synthesized. When blended with a state-of-the-art acceptor IT-4F with low lowest-unoccupied molecular orbital level, simultaneously high V OC (up to 0.94V) and J SC (up to 20.73mA cm-2 ) are obtained for both copolymers. Note that the V OC value of 0.94V is the highest value of PSCs based on IT-4F reported so far. The simultaneously improved V OC and J SC in resulting devices are discovered from the deep highest-occupied molecular orbital levels (-5.5 to -5.7eV) and the hyperchromic effect of the polymers, the small driving force, and the small energy loss during the charge transfer, due to the synergistic effect of asymmetric carboxylate unit and fluorine/sulfur atoms. More importantly, thanks to the asymmetric 2TC, both PBDT-F-2TC- and PBDT-SF-2TC-based PSCs can be successfully processed by non-halogenated solvent 1,2,4-trimethylbenzene (TMB) to yield device efficiencies of 10.29% and 10.39%, respectively, which are the maximum values for non-fullerene PSCs fabricated using the eco-friendly solvent TMB.

A 3-Fluoro-4-hexylthiophene-Based Wide Bandgap Donor Polymer for 10.9% Efficiency Eco-Friendly Nonfullerene Organic Solar Cells.

Nonfullerene organic solar cells (NFOSCs) are attracting increasing academic and industrial interest due to their potential uses for flexible and lightweight products using low-cost roll-to-roll technology. In this work, two wide bandgap (WBG) polymers, namely P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6, are designed and synthesized using benzodithiophene (BDT) derivatives. Good oxidation stability and high solubility are achieved by simultaneously introducing fluorine and alkyl chains to a single thiophene (Th) unit. Solid P(fTh-2DBDT)-C6 films present WBG optical absorption, suitable frontier orbital levels, and strong π-π stacking effects. In addition, P(fTh-2DBDT)-C6 exhibits good solubility in both halogenated and nonhalogenated solvents, suggesting its suitability as donor polymer for NFOSCs. The P(fTh-2DBDT)-C6:3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(5-hexylthienyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC-Th) based device processed using chlorobenzene/1,8-diiodooctane (CB/DIO) exhibits a remarkably high power conversion efficiency (PCE) of 11.1%. Moreover, P(fTh-2DBDT)-C6:ITIC-Th reaches a high PCE of 10.9% when processed using eco-friendly solvents, such as o-xylene/diphenyl ether (DPE). The cell processed using CB/DIO maintains 100% efficiency after 1272 h, while that processed using o-xylene/DPE presents a 101% increase in efficiency after 768 h and excellent long-term stability. The results of this study demonstrate that simultaneous fluorination and alkylation are effective methods for designing donor polymers appropriate for high-performance NFOSCs.