Non-fullerene Polymer Solar Cells Research Articles

Highly efficient near-infrared and semitransparent polymer solar cells based on an ultra-narrow bandgap nonfullerene acceptor

Non-fullerene polymer solar cells based on a low bandgap polymer PTB7-Th and an ultra-narrow bandgap acceptor ACS8 exhibited an optimal PCE of 13.2%, indicating that the blend of PTB7-Th/ACS8 has potential for the practical applications of PSCs.

High-efficiency non-fullerene polymer solar cell fabricated by a simple process using new conjugated terpolymers

A series of conjugated terpolymers bearing weak and strong accepting units were synthesized. Their optical and electrochemical properties and device performance can be easily tuned by controlling molar ratio of two accepting units.

An extraordinary cyclohexylmethyl side chain dominating polymeric donor packing patterns and energy levels for efficient non-fullerene polymer solar cells

A bulky cyclic side group admirably optimize the optical, electrochemical and crystalline characteristics of the donor polymers via its remarkable control on their polarities.

Perylene diimide-based cathode interfacial materials: adjustable molecular structures and conformation, optimized film morphology, and much improved performance of non-fullerene polymer solar cells

Through the regulation of PDI derivatives, the optimized photovoltaic performance was achieved by the adjustable molecular packing and film morphology.

Amorphous electron donors with controllable morphology for non-fullerene polymer solar cells

Amorphous polymeric donor, electron acceptor and donor:acceptor blend films for high performance polymer solar cells.

A wide bandgap conjugated polymer donor based on alkoxyl-fluorophenyl substituted benzodithiophene for high performance non-fullerene polymer solar cells

Through side chain fluorination strategy, the PCE was improved from 6.2% for POPB-based device to 11.7% for PFOPB-based device.

Effects of incorporated pyrazine on the interchain packing and photovoltaic properties of wide-bandgap D–A polymers for non-fullerene polymer solar cells

The impact of using pyrazine as an acceptor core for designing donor–acceptor (D–A) based wide bandgap polymers for application in non-fullerene solar cells was evaluated.

Dithienothiapyran: An Excellent Donor Block for Building High-Performance Copolymers in Nonfullerene Polymer Solar Cells.

In this work, a new but excellent donor block dithienothiapyran (DTTP) was developed for constructing highly efficient wide band gap copolymer donors. Compared to dithienopyran (DTP), DTTP features weaken electron-donating ability and more planar-conjugated backbones. Polymer-fluorinated benzotriazole (FBTA) based on DTTP exhibits lower highest occupied molecular orbital level (-5.30 vs -5.21 eV), higher molar extinction coefficient (1.54 × 105 vs 8.65 × 104 M-1 cm-1), and better crystallinity than -FBTA based on DTP, thus producing a higher device performance of 10.51% in binary blend nonfullerene polymer solar cells (NF-PSCs) blended with IT-M. To improve the absorption strength of PDTTP-FBTA: devices in the shorter wavelength range and further optimize the blend morphology, a small molecule of , which has strong absorption at short wavelength (300-600 nm), was incorporated. Finally, the performance of the ternary blends was successfully enhanced to 11.57% and a very high fill factor of 76.5%. Our work provided a new but excellent donor block for building high-performance conjugated copolymers to achieve highly efficient NF-PSCs.

Molecular Order Control of Non-fullerene Acceptors for High-Efficiency Polymer Solar Cells

Summary Adjusting molecular ordering, orientation, and nanoscale morphology within the photoactive layer of polymer:non-fullerene organic solar cells is crucial in achieving high power-conversion efficiency (PCE). Herein, we demonstrate that the molecular ordering and orientation of the n-type small-molecule acceptor COi8DFIC can be tuned from flat-on and edge-on lamellar crystalline to H- and J-type π-π stacking during the solution-casting process, resulting in broadened photon absorption and fine phase separation with the electron donor PTB7-Th. This favorable morphology with face-on π-π stacked electron donors and acceptors promotes efficient exciton dissociation at the donor/acceptor interface, together with enhanced and balanced carrier mobility. The enhanced short-circuit current density and fill factor lead to the achievement of a maximum PCE of 13.8% in binary, single-junction PTB7-Th:COi8DFIC non-fullerene polymer solar cells while also exhibiting superior stability.

High-Efficiency Nonfullerene Polymer Solar Cells with Band gap and Absorption Tunable Donor/Acceptor Random Copolymers.

Energy level alignment between a donor and an acceptor has a critical role in determining the open-circuit voltage ( VOC) in polymer solar cells (PSCs). Also, broad absorption of the photoactive layer is required to generate a high photocurrent. Herein, non-fullerene PSCs with D/A random copolymers and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d:2',3'- d']- s-indaceno[1,2- b:5,6- b']dithiophene (ITIC) has been demonstrated. The D/A random copolymers are composed of a 2-ethylhexylthienyl-substituted benzo[1,2- b:4,5- b']dithiophene (BDT) donor unit (D) and a fluorinated thieno[3,4- b]thiophene (TT-F) acceptor unit (A). By controlling the D/A unit ratio in the polymer backbone, it is possible to modulate both the energy levels and absorption spectra of random copolymers. As the ratio of the donor unit in the polymer back bone increases, the highest occupied molecular orbital energy level is located deeper, leading to higher VOC. Also, the absorption spectra of random copolymers become blue-shifted with an increase of the donor unit ratio; it compensates the weak absorption region of ITIC. This complementary absorption enhances the photocurrent, leading to higher power conversion efficiency (PCE). Because of the optimization of the D/A ratio of random copolymers, a notable PCE of 10.27% can be achieved in PSCs with D5A and ITIC.

Phthalimide-Based High Mobility Polymer Semiconductors for Efficient Nonfullerene Solar Cells with Power Conversion Efficiencies over 13.

Highly efficient nonfullerene polymer solar cells (PSCs) are developed based on two new phthalimide‐based polymers phthalimide‐difluorobenzothiadiazole (PhI‐ffBT) and fluorinated phthalimide‐ffBT (ffPhI‐ffBT). Compared to all high‐performance polymers reported, which are exclusively based on benzo[1,2‐b:4,5‐b′]dithiophene (BDT), both PhI‐ffBT and ffPhI‐ffBT are BDT‐free and feature a D‐A1‐D‐A2 type backbone. Incorporating a second acceptor unit difluorobenzothiadiazole leads to polymers with low‐lying highest occupied molecular orbital levels (≈−5.6 eV) and a complementary absorption with the narrow bandgap nonfullerene acceptor IT‐4F. Moreover, these BDT‐free polymers show substantially higher hole mobilities than BDT‐based polymers, which are beneficial to charge transport and extraction in solar cells. The PSCs containing difluorinated phthalimide‐based polymer ffPhI‐ffBT achieve a substantial PCE of 12.74% and a large V oc of 0.94 V, and the PSCs containing phthalimide‐based polymer PhI‐ffBT show a further increased PCE of 13.31% with a higher J sc of 19.41 mA cm−2 and a larger fill factor of 0.76. The 13.31% PCE is the highest value except the widely studied BDT‐based polymers and is also the highest among all benzothiadiazole‐based polymers. The results demonstrate that phthalimides are excellent building blocks for enabling donor polymers with the state‐of‐the‐art performance in nonfullerene PSCs and the BDT is not necessary for constructing such donor polymers.

New-structure perylene diimide oligomers by the linkage of the bay- and imide-position for nonfullerene solar cells

A new family of perylene diimide (PDI) oligomers, T-PDI and H-PDI, were designed and synthesized via dehydration condensation reactions with two PDI monomers linked together at bay- and imide-positions. The as-cast non-fullerene polymer solar cells (PSCs) based on H-PDI as acceptor achieved a power conversion efficiency (PCE) of 1.91%. In comparison with the device based on H-PDI, the as-cast T-PDI based PSC gave a PCE of 3.50% with higher open-circuit voltage (VOC), short-circuit density (JSC) and fill factor (FF). The higher VOC of the PSC based on T-PDI was consistent well with its shallower LUMO energy level compared with that of H-PDI. Meanwhile, the higher JSC and FF for the T-PDI-based device should be mainly attributed to the stronger light absorption, more balanced carrier transport and favorable morphology relative to those of the device based on H-PDI. These results reveal the relationship between the PDI oligomers and the photovoltaic performance.

Retarding the Crystallization of a Nonfullerene Electron Acceptor for High‐Performance Polymer Solar Cells

AbstractDeveloping a fundamental understanding of the molecular order within the photoactive layer, and the influence therein of solution casting conditions, is a key factor in obtaining high power conversation efficiency (PCE) polymer solar cells. Herein, the molecular order in PBDB‐T:INPIC‐4F nonfullerene solar cells is tuned by control of the molecular organization time during film casting, and the crucial role of retarding the crystallization of INPIC‐4F in achieving high performance is demonstrated. When PBDB‐T:INPIC‐4F is cast with the presence of solvent vapor to prolong the organization time, INPIC‐4F molecules form spherulites with a polycrystalline structure, resulting in large phase separation and device efficiency below 10%. On the contrary, casting the film on a hot substrate is effective in suppressing the formation of the polycrystalline structure, and encourages face‐on π−π stacking of INPIC‐4F. This molecular transformation of INPIC‐4F significantly enhances the absorption ability of INPIC‐4F at long wavelengths and facilitates a fine phase separation to support efficient exciton dissociation and balanced charge transport, leading to the achievement of a maximum PCE of 13.1%. This work provides a rational guide for optimizing nonfullerene polymer solar cells consisting of highly crystallizable small molecular electron acceptors.

Steric Engineering of Alkylthiolation Side Chains to Finely Tune Miscibility in Nonfullerene Polymer Solar Cells

AbstractMorphology and miscibility control are still a great challenge in polymer solar cells. Despite physical tools being applied, chemical strategies are still limited and complex. To finely tune blend miscibility to obtain optimized morphology, chemical steric engineering is proposed to systemically investigate its effects on optical and electronic properties, especially on a balance between crystallinity and miscibility. By changing the alkylthiol side chain orientation different steric effects are realized in three different polymers. Surprisingly, the photovoltaic device of the polymerPTBB‐m with middle steric structure affords a better power conversion efficiency, over 12%, compared to those of the polymers PTBB‐o and PTBB‐p with large or small steric structures, which could be attributed to a more balanced blend miscibility without sacrificing charge‐carrier transport. Space charge‐limited current, atomic force microscopy, grazing incidence wide angle X‐ray scattering, and resonant soft X‐ray scattering measurements show that the steric engineering of alkylthiol side chains can have significant impacts on polymer aggregation properties, blend miscibility, and photovoltaic performances. More important, the control of miscibility via the simple chemical approach has preliminarily proved its great potential and will pave a new avenue for optimizing the blend morphology.

Isomery-Dependent Miscibility Enables High-Performance All-Small-Molecule Solar Cells.

Nonfullerene polymer solar cells develop quickly. However, nonfullerene small-molecule solar cells (NF-SMSCs) still show relatively inferior performance, attributing to the lack of comprehensive understanding of the structure-performance relationship. To address this issue, two isomeric small-molecule acceptors, NBDTP-Fout and NBDTP-Fin , with varied oxygen position in the benzodi(thienopyran) (BDTP) core are designed and synthesized. When blended with molecular donor BDT3TR-SF, devices based on the two isomeric acceptors show disparate photovoltaic performance. Fabricated with an eco-friendly processing solvent (tetrahydrofuran), the BDT3TR-SF:NBDTP-Fout blend delivers a high power conversion efficiency of 11.2%, ranked to the top values reported to date, while the BDT3TR-SF:NBDTP-Fin blend almost shows no photovoltaic response (0.02%). With detailed investigations on inherent optoelectronic processes as well as morphological evolution, this performance disparity is correlated to the interfacial tension of the two combinations and concludes that proper interfacial tension is a key factor for effective phase separation, optimal blend morphology, and superior performance, which can be achieved by the "isomerization" design on molecular acceptors. This work reveals the importance of modulating the materials miscibility by interfacial-tension-oriented molecular design, which provides a general guideline toward efficient NF-SMSCs.

Using ternary blend as a strategy to improve the driving force for charge transfer and facilitate electron transport in polymer solar cells

Abstract The recently emerged non-fullerene small molecular acceptors can enhance the light absorption and increase the open-circuit voltage in polymer solar cells (PSCs); however, sometimes a decrease in the driving force for charge transfer from donor to acceptor is accompanied. Here, we reported a ternary blend strategy by utilizing fullerene derivative (PC71BM) as a third component to improve the driving force for charge transfer, the charge transport, and the film morphology. Accordingly, the ternary blend PSCs demonstrate a much enhanced power conversion efficiency (PCE) with an optimized value of 9.18% and a good fill factor (FF) of 63%. In comparison, the non-fullerene PSCs without the third component only show a PCE of 6.90% and an FF value of 56%.

Side-Chain-Promoted Benzodithiophene-based Conjugated Polymers toward Striking Enhancement of Photovoltaic Properties for Polymer Solar Cells.

In this work, we have reported a highly efficient photovoltaic material, PBDTTz-SBP, by fine-tuning the side chains of the benzodithiophene (BDT) unit. With the replacement of alkoxy chains with alkylthio chains, a large increase in power conversion efficiency (PCE) was realized. Non-fullerene polymer solar cells (PSCs) without any post-treatment generate an optimal PCE of up to 12.09%, with a high VOC of 0.914 V, JSC of 18.52 mA cm-2, and fill factor of 71.43%. Notably, the efficiency of a PBDTTz-SBP-based solar cell was about 1.31-fold of the PCE (9.20%) of its counterpart based on the polymer, PBDTTz-BP, with alkoxy chains, indicating the striking modulation effect of side-chain engineering. Although VOC and JSC were lower than those of non-fullerene devices, the PSCs with PC71BM as the acceptor exhibited a fairly high fill factor of up to 76.69%, affording a moderate PCE. Our work reported a highly efficient polymer solar cell with a PCE of 12.09% and clearly demonstrated the great tuning effect of alkylthio chains on photovoltaic performance.

Semitransparent ternary nonfullerene polymer solar cells exhibiting 9.40% efficiency and 24.6% average visible transmittance

Efficient semitransparent ternary polymer solar cells (PSCs) are fabricated with narrow band gap material PTB7-Th as donor, two narrow band gap nonfullerene materials BDTThIT-4F and IEICO-4F as one alloyed acceptor. Firstly, the content of IEICO-4F in acceptors is optimized in opaque PSCs with 100 nm Ag as electrode, power conversion efficiency (PCE) of the optimized opaque PSCs arrives to 12.03% with 50 wt% IEICO-4F in the alloyed acceptor. The 1 nm Au/10 nm Ag is designed to replace 100 nm Ag as semitransparent electrode. The semitransparent electrode exhibits high transmittance in visible light range and low transmittance in long wavelength range, which is beneficial to improve photon harvesting of active layers prepared with narrow band gap materials. The semitransparent ternary PSCs exhibit a PCE of 9.40% with 24.6% average visible transmittance from 370 nm to 740 nm, which should be among the highest values for semitransparent PSCs. This work indicates that ternary strategy may have great potential in preparing efficient semitransparent PSCs with two narrow band gap nonfullerene materials.

Overcoming the trade-off between Voc and Jsc: Asymmetric chloro-substituted two-dimensional benzo[1,2-b:4,5-b′]dithiophene-based polymer solar cells

To achieve the high PCE and simultaneously overcome the trade-off between open circuit voltage (VOC) and short-circuit current density (JSC), the reasonable design of the donor material is still a great challenge in non-fullerene polymer solar cells (PSCs). Herein, a pair of asymmetric 2D BDT-based wide band gap polymers, named PBPTBz-1Cl and PBPTBz-2Cl, respectively, were designed and synthesized through simultaneously introducing a chloro-substituted thiophene and another alkoxylphenyl groups as side chains onto the same BDT unit. As expected, the polymers PBPTBz-1Cl and PBPTBz-2Cl exhibit the deeper HOMO levels of −5.32 and −5.41 eV, respectively, compared with the HOMO value of −5.27 eV for chlorine-free substituted polymer PBPTBz-0Cl. Therefore, the PBPTBz-2Cl:IT-M based PSCs exhibited a higher Voc of 0.99 V, with an enhanced Jsc of 14.92 mA cm−2 and PCE of 7.18%. The Voc, Jsc and PCE were simultaneously enhanced from PBPTBz-0Cl:IT-M to PBPTBz-1Cl:IT-M, then to PBPTBz-2Cl:IT-M based PSCs successively. These results demonstrate that the challenge of the trade-off between Voc and Jsc could be overcome through the asymmetric strategy of the chloro-substituted thiophene and alkylphenyl groups as different side chains onto the same BDT unit. Hence, considering the low cost of starting materials and convenient introduction of chlorine atoms, the asymmetric chlorination of the side chain of BDT unit would be the great potential for achieving the high-performance PSCs.

The progress of non-fullerene small molecular acceptors for high efficiency polymer solar cells

In the past few decades, polymer solar cells (PSCs) have gained great progress. And fullerene PSCs occupy a dominant position in the early stage of the development of PSCs. Recently, non-fullerene PSCs have gained more attention because of their advantages, such as good absorption in the visible and near infra-red (NIR) region, high tunability of energy levels, better complementary absorption spectra with donors. Thus, numerous efforts have focused on the synthesis and modulation of small molecular acceptor (SMAs). And lots of non-fullerene PSCs have achieved higher power conversion efficiencies than fullerene PSCs. This short review focuses on the progress of non-fullerene small-molecule acceptors for high efficiency PSCs in recent years. And it would provide new ideas for designing high efficiency PSCs.