Efficient Polymer Solar Cells Research Articles

Efficient Polymer Solar Cells Enabled by A-DA′D-A Type Acceptors with Alkoxypheny-Substituted Quinoxaline as the Fused-Ring Core

Efficient Polymer Solar Cells Enabled by A-DA′D-A Type Acceptors with Alkoxypheny-Substituted Quinoxaline as the Fused-Ring Core

Over 18.1% Efficiency of Layer-by-Layer Polymer Solar Cells by Enhancing Exciton Utilization near the ITO Electrode.

In this work, layer-by-layer (LbL) polymer solar cells (PSCs) are constructed without/with the incorporation of a dissociation strengthening layer (DSL) on the basis of the wide-bandgap donor D18-Cl, as well as the narrow-bandgap nonfullerene acceptor Y6. The efficiency of LbL PSCs is enhanced from 17.62 to 18.15% through introducing a DSL, originating from the enhanced dissociation of D18-Cl excitons near the ITO electrode. Meanwhile, the interfacial energy between D18-Cl and Y6 layers is decreased by incorporating a DSL, which should facilitate molecular interdiffusion for more adequate exciton dissociation in LbL active layers. This work offers a simple and resultful way for realizing power conversion efficiency (PCE) improvement of LbL PSCs with maximized exciton utilization in LbL active layers. The universality of the DSL incorporation strategy on performance improvement can be further confirmed with a boosted PCE from 17.39 to 18.03% or from 17.13 to 17.61% for D18-Cl/L8-BO- or D18-Cl/N3-based LbL PSCs by incorporating a DSL.

Smartly Optimizing Crystallinity, Compatibility, and Morphology for Polymer Solar Cells by Small Molecule Acceptor with Unique 2D‐EDOT Side Chain

AbstractA desired morphology is essential for achieving efficient polymer solar cells. Donors and acceptors with appropriate crystallization can lead to a suitable phase‐separated morphology for effective photocurrent generation process. Inspired by the success of Y6 acceptors and the 2D side chain engineering on popular polymer donors and small molecule acceptors, the usage of unique 2D 3,4‐ethylene dioxythiophene (EDOT) side chains on Y6 to regulate its crystallinity, compatibility, and thus the related blend morphology is explored. In this study, two molecules of BTP‐EDOT‐4F and BTP‐EDOT‐4Cl with such unique 2D EDOT side chains are designed and synthesized. Due to the advantage of EDOT side chain, when these molecules are blended with PM6, the decent power conversion efficiencies (PCEs) of 16.78% and 15.87% are obtained. Furthermore, BTP‐EDOT‐4F is selected as the third component and added into PM6:L8‐BO binary system to form ternary blends. The optimized crystallinity, compatibility, and morphology of such ternary blend are discovered in the presence of BTP‐EDOT‐4F, which enables efficient exciton dissociation and charge transport as well as decreased recombination, resulting in higher short circuit current density (Jsc) and fill factor. Finally, the outstanding PCE of 18.56% is achieved in ternary blends containing PM6, L8‐BO, and BTP‐EDOT‐4F.

Y-Type Non-Fullerene Acceptors with Outer Branched Side Chains and Inner Cyclohexane Side Chains for 19.36% Efficiency Polymer Solar Cells.

Raising the lowest unoccupied molecular orbital (LUMO) energy level of Y-type non-fullerene acceptors can increase the open-circuit voltage (Voc ) and thus the photovoltaic performance of the current top performing polymer solar cells (PSCs). One of the viable routes is demonstrated by the successful Y6 derivative of L8-BO with the branched alkyl chains at the outer side. This will introduce steric hindrance and reduce intermolecular aggregation, thus open up the bandgap and raise the LUMO energy level. To take further advantages of the steric hindrance influence on optoelectronic properties of Y6 derivatives, two Y-type non-fullerene acceptors of BTP-Cy-4F and BTP-Cy-4Cl are designed and synthesized by adopting outer branched side chains and inner cyclohexane side chains. An outstanding Voc of 0.937V is achieved in the D18:BTP-Cy-4F binary blend devices along with a power conversion efficiency (PCE) of 18.52%. With the addition of BTP-eC9 to extend the absorption spectral coverage, a remarkable PCE of 19.36% is realized finally in the related ternary blend devices, which is one of the highest values for single-junction PSCs at present. The results illustrate the great potential of cyclohexane side chains in constructing high-performance non-fullerene acceptors and their PSCs.

Super aligned carbon nanotubes for interfacial modification of hole transport layer in polymer solar cells

This work presents a facile and innovative method for the insertion of super aligned carbon nanotubes (CNTs) as sheets over and within poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as an efficient hole transport layer (HTL) in the inverted type polymer solar cell. CNTs sheets drawn from the vertically grown CNTs arrays were extracted via a sharp-edge blade and were directly transferred to the surface of PEDOT:PSS followed by the deposition of thermally evaporated silver electrode. All the characteristic photovoltaic parameters of the device have been improved with the insertion of special patterened CNTs sheets as compared to the standards device (without CNTs sheets). Power conversion efficiency (PCE) improved from 2.14% to 3.69% with the increased short circuit current density from 9.12 mAcm−2 to 11.58 mAcm−2 and fill factor from 0.48 to 0.58, respectively. Scanning electron microscopy (SEM) images revealed the successful insertion of CNTs without any destructive impact on the integrity of CNTs structure. Moreover, the performance and stability of the device have also been optimized by increasing the thickness of CNTs sheets. Electrochemical impedance spectroscopy (EIS) showed the reduction in charge transfer resistance on replacement of CNTs sheets-modified PEDOT:PSS as HTL as compared to simple PEDOT:PSS, which may be attributed to the creation of ordered steps which provided cascade routes for the better charge (holes) collection. Facile modification of the anode materials with improved device performance for designing of new device architecture will help scale-up production of low-cost, stable, and efficient polymer solar cells.

19.28% Efficiency and Stable Polymer Solar Cells Enabled by Introducing an NIR‐Absorbing Guest Acceptor

AbstractHere, a near‐infrared (NIR)‐absorbing small‐molecule acceptor (SMA) Y‐SeNF with strong intermolecular interaction and crystallinity is developed by combining selenophene‐fused core with naphthalene‐containing end‐group, and then as a custom‐tailor guest acceptor is incorporated into the binary PM6:L8‐BO host system. Y‐SeNF shows a 65 nm red‐shifted absorption compared to L8‐BO. Thanks to the strong crystallinity and intermolecular interaction of Y‐SeNF, the morphology of PM6:L8‐BO:Y‐SeNF can be precisely regulated by introducing Y‐SeNF, achieving improved charge‐transporting and suppressed non‐radiative energy loss. Consequently, ternary polymer solar cells (PSCs) offer an impressive device efficiency of 19.28% with both high photovoltage (0.873 V) and photocurrent (27.88 mA cm−2), which is one of the highest efficiencies in reported single‐junction PSCs. Notably, ternary PSC has excellent stability under maximum‐power‐point tracking for even over 200 h, which is better than its parental binary devices. The study provides a novel strategy to construct NIR‐absorbing SMA for efficient and stable PSCs toward practical applications.

Compatible Solution‐Processed Interface Materials for Improved Efficiency of Polymer Solar Cells

AbstractThe electron transport layer (ETL) in an organic solar cell is one of the main components that play a crucial role in the extraction of charges, improving efficiency, and increasing the lifetime of the solar cells. Herein, solution‐processed PBDTTT‐C‐T:PC71BM‐based organic solar cells are fabricated using conjugated PDINO molecules, sol‐gel derived under stoichiometric titanium oxide (TiOx), and a mixture of the same as an ETL. For PBDTTT‐C‐T:PC71BM‐based organic solar cells, a blend of organic‐inorganic ETLs demonstrates reduced bimolecular recombination and trap‐assisted recombination than a single ETL of either two materials. Furthermore, in both, fullerene and nonfullerene systems, the efficiency of the devices employing the blend ETL as compared to the single ETLs show some performance improvement. The strategy of integrating compatible organic and inorganic interface materials to improve device efficiency and lifetime simultaneously, and demonstrate the universality of different systems, has potential significance for the commercial development of organic solar cells.

A polymer donor based on difluoro-quinoxaline with a naphthalimide substituent unit exhibits a low-lying HOMO level for efficient non-fullerene polymer solar cells.

The design and synthesis of polymer donors with a low-lying highest occupied molecular orbital (HOMO) level are crucial for increasing open-circuit voltages (VOC) and achieving high-performance non-fullerene polymer solar cells. Here, we developed two copolymers using non-fluorinated or fluorinated thienyl-conjugated benzodithiophenes as electron donor units, and difluoro-quinoxaline with a naphthalimide substituent (DNB) as the electron acceptor unit. These copolymers, namely PDNB and PDNB-2F, exhibited deep HOMO levels owing to the strong electron-withdrawing ability of the naphthalimide substituent. Density-functional theory calculations demonstrated that the skeletons of the two copolymers featured good coplanarity. Owing to the fluorination, PDNB-2F displayed an increased absorption coefficient and deeper HOMO level than PDNB. Moreover, the blended film based on PDNB-2F:Y6 demonstrated enhanced carrier mobility, decreased bimolecular recombination as well as favorable phase-separation regions. Consequently, the PDNB-2F:Y6-based device yielded a superior power conversion efficiency (PCE) of 12.18%, whereas the device based on PDNB:Y6 showed a comparatively lower PCE of 8.83%. These results indicate that difluoro-quinoxaline with a naphthalimide substituent is a prospective electron-deficient building block to develop donor polymers with low-lying HOMO levels to achieve efficient non-fullerene polymer solar cells.

Ionic Ir(III) complex-interfacial layer for efficient carrier collection via induced electric dipole.

The power conversion efficiency (PCE) of polymer solar cells (PSCs) has recently reached >19% through the development of photoactive materials, particularly non-fullerene acceptors. Interfacial layers (ILs) have been another essential factor in optimizing device charge extraction. In this study, we propose a series of ILs, in which ionic iridium(III) (Ir(III)) complexes of different alkali metal cations (Li+, Na+, and K+) enhance the charge collection efficiency between zinc oxide and active layers through an induced internal electric field. The anionic coordinate sphere and counter-cations of the Ir(III) complexes are distributed according to the operating voltage of the PSCs, causing electric dipoles that enhance the internal electric field and charge collection efficiency. Ion species migration in the ILs is confirmed using electrochemical impedance spectroscopy. The PCE of the PM6:Y6-based PSCs was improved from 14.0% to 15.6% by introducing an IL (Ir-K+). Furthermore, the stability of PSCs containing ionic Ir(III) complexes is enhanced significantly under ultraviolet (UV) light and AM 1.5 G one-sun irradiation owing to the intense UV absorption capacity and photo durability of the ILs. A device containing the Ir(III) complex-based ILs retained ∼60% of its initial PCE after UV irradiation, whereas the control device retained only ∼20%.

Photovoltaic performances of two alternating polymers having meta-octyloxy-phenyl modified dithieno[3,2-f:2′,3′-h]quinoxaline unit

Two donor-acceptor (D-A)-type alternative conjugated polymers, namely Z2 and Z3, based on meta-octyloxy-phenyl modified dithieno[3,2-f:2′,3′-h]quinoxaline segment as the electron-deficient unit, thiophene and 2,3-dioctylthienyl substituted benzo[1,2-b:4,5-b’]dithiophene as the electron-giving segment were synthesized, which were used as donors to fabricate polymer solar cells (PSCs) with a small molecule electron-acceptor material i-IEICO-4F. With wide-bandgaps of 1.95 eV for Z2 and 2.02 eV for Z3, both of the two polymers provide complementary absorption with i-IEICO-4F. Moreover, Z3 shows deepened molecular frontier orbitals compared to Z2. Polymer solar cells based on Z2 and i-IEICO-4F acquired a moderate efficiency of 6.46%. While the device of Z3:i-IEICO-4F possessed exhibited better charge acquisition and generation properties, suppressed charge recombination probability, higher and more balanced hole and electron mobilities, and superior molecular orientation and crystallinity, thus achieving simultaneously improved short-circuit current density (JSC, 16.76 mA cm−2), VOC (0.936 V), fill factor (FF, 53.93%), and the resultant high PCE (8.46%). This work is of great significance for obtaining highly efficient PSCs with alkyoxy-phenyl modified dithieno[3,2-f:2′,3′-h]quinoxaline unit as the electron-attractive fused-ring core in wide bandgap polymers.

Recent advances in polymer and perovskite based third-generation solar cell devices

This review article begins with a comparative overview of the configurations, materials, fabrication methods, and energy conversion efficiency of polymer and perovskite solar cells' photovoltaic performances. Firstly, there has been a significant increase in the adoption of solar cells due to the growing need for renewable and clean energy. Among all photovoltaic technologies, polymer and perovskite solar cells have garnered considerable attention owing to their potential to be affordable, lightweight, easy to manufacture, and quick charging. Research on polymer solar cells has spanned over two decades, whereas the use of perovskite solar cells is just eight years old. This expanding topic offers a wealth of information to readers interested in polymers and perovskite. As a basic comparison, the best power conversion efficiency results are 21.6 percent for a 1 cm2 perovskite solar cell and 15.2 percent for polymer solar cells. Finally, this article recommends necessary improvements and future research areas in polymer and perovskite solar cells.

A Versatile and Low-Cost Polymer Donor Based on 4-Chlorothiazole for Highly Efficient Polymer Solar Cells.

Benefiting from the emergence of narrow-band-gap small-molecule acceptors (SMAs), especially "Y" series, the power conversion efficiency (PCE) of polymer solar cells (PSCs) is rapidly improved. However, polymer donors with high efficiency, easy synthesis, and good universality are relatively scarce except PBDB-TF and D18. Herein, two polymer donors are designed and synthesized based on 4-chlorothiazole derivatives with simple structures, namely PTz3Cl and PBTTz3Cl. The OSCs based on PBTTz3Cl with slightly weaker intermolecular forces in comparison to PTz3Cl exhibits a decent PCE of 18.38% in blending with SMA L8-BO, owing to its strong donor/acceptor interaction with L8-BO, which shapes suitable phase separation morphology. Further research finds that PBTTz3Cl can exhibit excellent photovoltaic performances with various SMA materials, highlighting its universality. Based on this, ternary PSCs are designed where BTP-eC9 is introduced as a guest into the PBTTz3Cl:L8-BO host system. Thanks to further optimal blend morphology and more balanced charge transport, the PCE is improved up to 19.12%, which is among the highest values for PSCs. This work provides a new design of low-cost electron-deficient units for constructing highly versatile, high-performance polymer donors.

Elucidating the Chain-Extension Effect on the Exciton-Dissociation Mechanism through an Intra- or Interchain Polaron-Pair State in Push–Pull Conjugated Polymers

We elucidated chain-extension effects of a benzodithiophene (BDT) and thienopyrroledione-based push–pull conjugated polymer (CP) on its exciton-dissociation mechanism within aggregate systems using transient absoption spectroscopy. The side-group extension CP with benzothiophene on the BDT unit induced H-type excitons with excess energy owing to decreased chain stiffness. This led to interchain polaron-pair (PP)-mediated exciton dissociation. The stiff side-group extended with thienothiophene on the BDT unit also induced H-type excitons, but the decreased energy and breadth of the density of states suppressed the interchain PP-mediated exciton dissociation. The main-chain-extension CP with two thiophenes on either side of the BDT unit has a curved structure disturbing the interchain packing. Thus, the driving force of exciton dissociation between the chains decreased, leading to intrachain PP-mediated exciton dissociation. Our findings can facilitate the development of novel CPs to further increase the efficiencies of polymer solar cells.

Tethered Small-Molecule Acceptors Simultaneously Enhance the Efficiency and Stability of Polymer Solar Cells.

For polymer solar cells (PSCs), the mixture of polymer donors and small-molecule acceptors (SMAs) is fine-tuned to realize a favorable kinetically trapped morphology and thus a commercially viable device efficiency. However, the thermodynamic relaxation of the mixed domains within the blend raises concerns related to the long-term operational stability of the devices, especially in the record-holding Y-series SMAs. Here, a new class of dimeric Y6-based SMAs tethered with differential flexible spacers is reported to regulate their aggregation and relaxation behavior. In their polymer blends with PM6, it is found that they favor an improved structural order relative to that of Y6 counterpart. Most importantly, the tethered SMAs show large glass transition temperatures to suppress the thermodynamic relaxation in mixed domains. For the high-performing dimeric blend, an unprecedented open circuit voltage of 0.87V is realized with a conversion efficiency of 17.85%, while those of regular Y6-base devices only reach 0.84V and 16.93%, respectively. Most importantly, the dimer-based device possesses substantially reduced burn-in efficiency loss, retaining more than 80% of the initial efficiency after operating at the maximum power point under continuous illumination for 700h. The tethering approach provides a new direction to develop PSCs with high efficiency and excellent operating stability.

18.66% Efficiency of Polymer Solar Cells Employing Two Nonfullerene Acceptors with Fluorine or Chlorine Substitution

Organic semiconducting materials with fluorine or chlorine substitution are commonly designed to prepare efficient polymer solar cells (PSCs) through finely modulating the energy levels and absorption spectra. Herein, two small molecular acceptors with fluorine substitution L8‐BO or chlorine substitution eC9‐2Cl are selected to prepare ternary PSCs with PM6 as polymer donor. The optimal ternary PSCs exhibit a power conversion efficiency of 18.66%, benefiting from the simultaneously increased open‐circuit voltage (VOC) of 0.89 V, short‐circuit current density (JSC) of 26.63 mA cm−2, and fill factor of 78.74% when the content of chlorine substitution eC9‐2Cl is about 20 wt% in acceptors. Fluorinated and chlorinated substitutions can improve the VOC or JSC of the corresponding binary PSCs, which can be recombined into efficient ternary PSCs through optimizing their content in acceptors. Chlorinated materials have special crystalline conditions due to the specific features of the chlorine atoms with large atomic radius and the empty 3d orbits compared to the corresponding fluorinated materials. Adding an appropriate amount of the chlorine substitution eC9‐2Cl further enhances the crystallization and intermolecular interaction of the ternary PSCs, which is beneficial for charge transport in the active layer and for device performance improvement.

Simple Approach for Synthesizing a Fluorinated Polymer Donor Enables Promoted Efficiency in Polymer Solar Cells

Simple Approach for Synthesizing a Fluorinated Polymer Donor Enables Promoted Efficiency in Polymer Solar Cells

Terpolymer Donor with Inside Alkyl Substituents on Thiophene π-Bridges toward Thiazolothiazole A2 -Unit Enables 18.21% Efficiency of Polymer Solar Cells.

PM6 is a widely used D-A copolymer donor in the polymer solar cells (PSCs). Incorporating second electron-withdrawing (A2 ) units into PM6 backbone by ternary D-A1 -D-A2 random copolymerization strategy is an effective approach to further improve its photovoltaic performance. Here, the authors synthesize the PM6-based terpolymers by introducing thiazolothiazole as the A2 units connecting with thiophene π-bridges attaching alkyl substituent towards the A2 unit (PMT-CT) or towards D-unit (PMT-FT), and study the effect of the alkyl substituent position on the photovoltaic performance of them. Two terpolymers PMT-FT-10 and PMT-CT-10 are obtained by incorporating 10% A2 units in the terpolymers. The film of PMT-CT-10 shows slightly up-shifted highest occupied molecular orbital (HOMO) energy levels while better co-planar structure than that of PMT-FT-10. Meanwhile, the PMT-CT-10:Y6 blend film exhibits better molecular packing properties, more proper phase separation and more balanced hole and electron mobilities, which are beneficial to more efficient exciton dissociation, efficient charge transport and weaker bimolecular recombination. Consequently, the PMT-CT-10 based PSCs obtain the highest power conversion efficiency of 18.21%. The results indicate that side chain position on the thiophene π-bridges influence the device performance of the terpolymer donors, and PMT-CT-10 is a high efficiency polymer donor for the PSCs.

Intrinsically Stretchable and Non‐Halogenated Solvent Processed Polymer Solar Cells Enabled by Hydrophilic Spacer‐Incorporated Polymers

AbstractBlends of polymer donors (PDs) and small molecule acceptors (SMAs) have afforded highly efficient polymer solar cells (PSCs). However, most of the efficient PSCs are processed using toxic halogenated solvents, and they are mechanically fragile. Here, a new series of PDs by incorporating a hydrophilic oligo(ethylene glycol) flexible spacer (OEG‐FS) is developed, and efficient PSCs with a high power conversion efficiency (PCE) of 17.74% processed by a non‐halogenated solvent are demonstrated. Importantly, the incorporation of these OEG‐FSs into the PDs significantly increases the mechanical robustness and ductility of resulting PSCs, making them suitable for application as stretchable devices. The OEG‐FS alleviates excessive backbone rigidity of the PDs while enhancing their pre‐aggregation in the non‐halogenated solvent. In addition, the OEG‐FS in the PDs enhances PD‐SMA interfacial interactions and improves blend morphology, resulting in efficient charge generation and mechanical stress dissipation. The resulting PSCs demonstrate a superior PCE (17.74%) and high crack‐onset strain (COS = 10.50%), outperforming the PSCs without OEG (PCE = 15.64% and COS = 2.99%). Importantly, intrinsically stretchable (IS) PSCs containing the PD featuring OEG‐FS exhibit a high PCE (12.05%) and stretchability (maintaining 80% of the initial PCE after 22% strain), demonstrating their viability for wearable applications.

The principles of selecting green solvent additives for optimizing the phase separation structure of polymer solar cells based on PTB7:PC71BM

Processing solvent additives is an effective approach to improve the performance of polymer solar cells (PSCs). However, traditional additives, such as 1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO), are halogenated solvent, which is not environmentally friendly. Here, we investigated the device performance of PTB7:PC71BM blend films processed with three aromatic ethers, CN and DIO. The results indicate that aromatic ethers have a higher device performance improvement than halogenated solvents. Using green solvent 1,2-dimethoxybenzene as the processing additive, the efficiency of PSCs was improved from 2.83% to 6.18%. We proposed two criteria to choose green solvent additives: (i) good solvent for PC71BM and nonsolvent for PTB7, (ii) higher boiling point than the host solvent, but not too high (about 70 ℃ higher is enough). The X-ray diffraction results indicated that PTB7 does not crystallize in the blend films, and thus the formation of phase separation morphology is primarily through liquid–liquid (L-L) phase separation. The structure analyses of the blend films demonstrated that solvent additives with poor solubility to PTB7 are beneficial in eliminating PC71BM sea-island domains and in promoting the formation of bi-continuous phase separation morphology, while nonsolvent additives with relatively lower boiling point could prevent PC71BM from moving to the bottom layer of the blend films during the drying process and thus the resulting films have a better vertical phase separation structure. The selection principles of green solvent additives proposed in this study can be further used to optimize the active layer structure of other PSCs occurred upon L-L phase separation.

Improved efficiency of polymer solar cells via utilizing Alcohol-Soluble Small Molecules N-(2-Carboxyethyl)iminodi acetic acid as cathode interfacial layer

The cathode modification is vital for improving device performance of polymer solar cells (PSCs). Here, N-(2-Carboxyethyl)iminodiacetic acid (C7H11NO6) layers with different concentrations were deposited on the photoactive layer PTB7-Th:PC71BM film, and used as the cathode interface modification material for the conventional device structure of PSCs. By varying the concentration of C7H11NO6, the effect on the performance of PSCs was investigated. It was found that the efficiency of the photovoltaic device was reached as high as 8.86% at a C7H11NO6 concentration of 1 mg mL−1, which was about 56% higher than the performance of the reference device. Among them, the C7H11NO6 film acts as an effective cathode interface layers, forming an interfacial dipole on the surface of the PTB7-Th:PC71BM film to increase the built-in potential and promote photogenerated electron extraction and transport.