High-performance Polymer Solar Cells Research Articles

High-Performance Ternary Polymer Solar Cells Enabled by Using a Branched Polymer as the Guest Donor

High-Performance Ternary Polymer Solar Cells Enabled by Using a Branched Polymer as the Guest Donor

Unveiling the Influence of Linkers on Conformations of Oligomeric Acceptors for High-Performance Polymer Solar Cells.

Conformational isomerism of organic photovoltaic materials has a profound impact on their molecular packing and therefore performance of polymer solar cells (PSCs). However, the conformations of oligomeric acceptors (OAs) are mostly predicted by simulations rather than experimental determinations. Herein, the stereochemical S-shaped structure of two dimeric-type acceptor molecules, V-DYIC and V-DYIC-4F, is first confirmed with different end groups (IC for V-DYIC and IC-2F for V-DYIC-4F), incorporating vinylene linkage for connecting the distinct state-of-the-art small molecule acceptor Y-segments. Through the synthetic control of fluorination sites adjacent to the vinyl-linker, S-shaped the conformation by NMR experiments is validated. Compared to the O-shaped dimer, S-shaped conformation results in enhanced lamellar order and reduced nonradiative recombination losses. The optimal acceptor, V-DYIC-4F, achieved a champion efficiency of 18.10% with the lowest energy loss of 0.556eV in its devices paired with PM6 due to their efficient carrier transport, and suppressed recombination compared to other devices, being attributed to the synergistic effect of conformation and end group fluorination. The insights gained in this work contribute valuable knowledge of both synthetic control and structural determination of OAs, providing strategic design guidelines for the future development of dimeric acceptors toward high-efficiency PSCs.

High-performance nonfullerene polymer solar cells based on chlorinated quinoxaline copolymer with a high short-circuit current density

In organic solar cells, the absorption range and extinction coefficient of the active layer not only affect the charge separation and carrier transfer efficiency of excitons, but also influence the JSC, and thus the device efficiency. Herein, an efficient nonfullerene polymer solar cells (NF–PSCs) based on a medium-bandgap (MBG) polymer donor PBDTTS-TClQx comprising chlorinethiophene quinoxaline (Qx) unit and a small molecule nonfullerene acceptor (SM-NFA) Y6 is developed. The PBDTTS-TClQx shows a strong absorption in the wavelength region of 330∼750 nm with an optical band gaps (Egopt) of 1.68 eV, which is well complementary with that of Y6 (1.33 eV) and facilitates achieving of high short-circuit current (JSC) in PSCs. As a result, the PBDTTS-TClQx:Y6-based PSCs achieved a power conversion efficiency (PCE) of 14.28% with a JSC of 25.9 mA cm−2. The JSC of 25.9 mA cm−2 achieved is among the highest reported for Qx-based polymer donors in PSCs.

Charge photogeneration and recombination dynamics in non-fullerene solar cells based on a polymer donor with a mono-fluorinated π-bridge.

Non-symmetric fluorine substitution is an important route for designing π-conjugated polymers for high-performance solar cells. However, excited state dynamics in a non-symmetric fluorinated polymer and solar cells are poorly understood. In this work, excited state dynamics in a neat mono-fluorinated polymer (PTzBI-dF) film and blend films (PTzBI-dF:Y6) were studied using time-resolved spectroscopy. For the neat donor film, we found the activation energy of the PTzBI-dF film transiting from radiative states to non-radiative states to be ∼58 meV. Besides, we found that both exciton diffusion coefficient and exciton diffusion length in the PTzBI-dF film are higher than those in the PTzBI-Cl film, which has no fluorinated π-bridge. The high exciton diffusion length of the PTzBI-dF film benefits from its long exciton lifetime (∼327.8 ps), which is much longer than that of the polymer donors such as P3HT and PM6. For PTzBI-dF:Y6 blend films, we found that the energy level offsets of the HOMO (0.13 eV) and LUMO (0.23 eV) are sufficient to dissociate the acceptor and donor excitons, respectively. Besides, we found that carrier recombination in PTzBI-dF:Y6 and PTzBI-Cl:Y6 blend films are dominated by bimolecular recombination processes, and thermal annealing treatment has a weak influence on the charge photogeneration and recombination process of the PTzBI-dF:Y6 film at an ultrafast time scale. Thus, this work is helpful for understanding photoelectrical conversion processes in non-symmetric fluorinated polymer-based solar cells.

Designing dithieno-benzodithiophene-based small molecule donors for thickness-tolerant and large-scale polymer solar cells

The small molecule donor SD62-doping strategy with an excellent universality is beneficial to fabricating thickness-tolerant and large-scale high-performance polymer solar cells and solar modules.

Constructing High-Performance Ternary Device Using Analogous Polymer Donors.

Both ternary copolymerization and ternary blending are effective methods to fine-tune polymer structure and manipulate thin-film morphology to improve device performance. In this work, three D-A-A-A (D: donor, A: acceptor) terpolymer donors (FY1, FY2, and FY3) are synthesized by introducing BDD (1,3-bis(2-ethylhexyl)-5,7-di(thiophen-2-yl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione) units into the D-A alternating copolymer PM6 backbone. Owing to the promoted conjugated planarity and excellent absorption of BDD, the obtained terpolymers display an extended absorption range and enhanced π-π stacking orientation, which is a promising third component in ternary device. As a result, the optimal FY1:PM6:BTP-eC9-based ternary device afforded an impressive power conversion efficiency (PCE) as high as 18.52%, owing to the efficient charge transport, negligible energy loss, and suitable domain size. The result provides an efficient method to obtain high-performance polymer solar cells by using analogous polymer donors in ternary device.

On the Conformation of Dimeric Acceptors and Their Polymer Solar Cells with Efficiency over 18 .

The determination of molecular conformations of oligomeric acceptors (OAs) and their impact on molecular packing are crucial for understanding the photovoltaic performance of their resulting polymer solar cells (PSCs) but have not been well studied yet. Herein, we synthesized two dimeric acceptor materials, DIBP3F-Se and DIBP3F-S, which bridged two segments of Y6-derivatives by selenophene and thiophene, respectively. Theoretical simulation and experimental 1D and 2D NMR spectroscopic studies prove that both dimers exhibit O-shaped conformations other than S- or U-shaped counter-ones. Notably, this O-shaped conformation is likely governed by a distinctive "conformational lock" mechanism, arising from the intensified intramolecular π-π interactions among their two terminal groups within the dimers. PSCs based on DIBP3F-Se deliver a maximum efficiency of 18.09 %, outperforming DIBP3F-S-based cells (16.11 %) and ranking among the highest efficiencies for OA-based PSCs. This work demonstrates a facile method to obtain OA conformations and highlights the potential of dimeric acceptors for high-performance PSCs.

Improvement of photovoltaic properties of benzo[1,2-b:4,5-b′]difuran-conjugated polymer by side-chain modification

The development of polymer solar cells (PSCs) for the donor materials based on benzo[1,2-b:4,5-b′]dithiophene (BDT) has significantly boosted the power conversion efficiency (PCE). However, the PCE of polymer donor materials for benzo[1,2-b:4,5-b′]difuran (BDF)-based lags far behind that of their BDT analogs. To further explore efficient copolymers based on BDF units, a two-dimensional (2D) side-chain strategy was proposed to investigate the atom-changing effects on the copolymer donors for the properties of electron and optical. In this study, we designed and synthesized three new BDF-based copolymer donor materials, named PBDF-C, PBDF-O, and PBDF-S. Owing to the balanced charge transport and favorable phase separation of PBDF-S:Y6, a high PCE of 13.4%, a short-circuit current ( J sc ) of 25.48 mA cm −2 , an open-circuit voltage ( V oc ) of 0.721 V, and a fill factor (FF) of 72.6% was obtained. This research demonstrates that the BDF building block has great potential for constructing conjugated copolymer donors for high-performance PSCs and that 2D side-chain modification is a facile approach for designing high-performance BDF-based copolymer materials.

Low-Cost and High-Performance Polymer Solar Cells with Efficiency Insensitive to Active-Layer Thickness

Low-Cost and High-Performance Polymer Solar Cells with Efficiency Insensitive to Active-Layer Thickness

Low-Cost and High-Performance Polymer Solar Cells with Efficiency Insensitive to Active Layer Thickness

Low-Cost and High-Performance Polymer Solar Cells with Efficiency Insensitive to Active Layer Thickness

Facile Synthesis of Key Building Blocks of D18 Series Conjugated Polymers for High-Performance Polymer Solar Cells

D18 and its derivatives (such as D18-Cl) newly emerged as high-performance conjugated polymers for bulk heterojunction (BHJ) solar cells, with efficiency breaching 17% when paired with small molecule acceptors such as Y6 and its derivatives. However, the complex synthesis of D18 series has limited their accessibility to the research community. We redesigned the original synthetic scheme of D18, particularly the key monomer, 5,8-bis(5-bromo-4-(2-butyloctyl)thiophen-2-yl)dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2-c][1,2,5]thiadiazole (Br2-T2-DTBT), and replaced a few key steps in the original scheme that were hard to reproduce and/or low yielding with simple to operate and high yielding reactions. Carother’s equation was successfully applied to control/tune the molar mass of D18-Cl, and three representative molar masses (high, medium, and low) of D18-Cl were subject to device testing. It is discovered that a medium molar mass (56 kg/mol) D18-Cl offered the highest performance (average 16.6%) when paired with Y6 as the acceptor in a typical BHJ device configuration, consistent with the literature report. This optimized synthesis of D18 series offers great synthetic tunability, and the modular nature/optimized key reactions will facilitate the design and synthesis of conjugated polymers for optoelectronics.

Phosphotungstate-Based Anode Interfacial Material for Constructing High-Performance Polymer Solar Cells with a Fill Factor over 80.

In the field of organic solar cells (OSCs), the interfacial layer plays the role of enhancing carrier extraction/transportation, inhibiting their recombination, etc. In contrast to the wide variety of cathode interfacial materials with good modification ability, much less effort has been reported for anode interfacial materials. In this study, we report a polyoxometalate-based inorganic molecular cluster, zinc phosphotungstate (Zn3P2W24O80, denoted ZnPW), as an anode interfacial layer. Based on the PM6/EH-HD-4F/L8-BO-F ternary system, the device with ZnPW modification achieved a high power conversion efficiency (PCE) and a fill factor of up to 18.67 and 80.29%, respectively, which are higher than the counterpart device (PCE of 18.01%) with PEDOT/PSS as the anode interfacial layer. Detailed studies revealed that under the modification of ZnPW, the devices obtained promoted light absorption and suitable energy level matching between the active layer and the electrode, reduced contact resistance, and suppressed charge recombination. In addition, the ZnPW-modified devices had improved photostability and storage stability compared to PEDOT/PSS-modified devices. Our work shows that the polyoxometalate-based inorganic nanocluster ZnPW has great advantages in enhancing the device performance and stability of OSCs.

Dramatic improvement in the stability and mechanism of high-performance inverted polymer solar cells featuring a solution-processed buffer layer.

In this study, we demonstrate inverted PTB7:PC71BM polymer solar cells (PSCs) featuring a solution-processed s-MoO3 hole transport layer (HTL) that can, after thermal aging at 85 °C, retain their initial power conversion efficiency (PCE) for at least 2200 h. The T80 lifetimes of the PSCs incorporating the novel s-MoO3 HTL were up to ten times greater than those currently reported for PTB7- or low-band-gap polymer:PCBM PSCs, the result of the inhibition of burn-in losses and long-term degradation under various heat-equivalent testing conditions. We used X-ray photoelectron spectroscopy (XPS) to study devices containing thermally deposited t-MoO3 and s-MoO3 HTLs and obtain a mechanistic understanding of how the robust HTL is formed and how it prevented the PSCs from undergoing thermal degradation. Heat tests revealed that the mechanisms of thermal inter-diffusion and interaction of various elements within active layer/HTL/Ag electrodes controlled by the s-MoO3 HTL were dramatically different from those controlled by the t-MoO3 HTL. The new prevention mechanism revealed here can provide the conceptual strategy for designing the buffer layer in the future. The PCEs of PSCs featuring s-MoO3 HTLs, measured in damp-heat (65 °C/65% RH; 85 °C per air) and light soaking tests, confirmed their excellent stability. Such solution-processed MoO3 HTLs appear to have great potential as replacements for commonly used t-MoO3 HTLs.

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.

Side-chain engineering of wide-bandgap copolymers based on two different electron-deficient units for high-performance polymer solar cells

Novel wide-bandgap polymers based on two different electron-deficient units are designed and synthesized for nonfullerene solar cells with a best PCE of 12.56%.

Morphology Optimization of the Photoactive Layer through Crystallinity and Miscibility Regulation for High-performance Polymer Solar Cells.

On the premise of strongly crystalline materials involved, it is a challenge to control the phase separation of bulk-heterojunction donor/acceptor active layer to fabricate high-performance polymer solar cells (PSCs). Herein, we develop a molecular design strategy of the third component to synthesize three guest materials (namely BTPT, BTP-Th, and BTP-2Th) to address this issue. We investigate and reveal the effect of crystallinity and miscibility of the third component in controlling the phase separation of Y6-derivatives-based blend film. As a result, a remarkable power-conversion efficiency of 18.53 % is obtained in the ternary PSC based on PTQ10 : m-BTP-PhC6 with BTP-Th as the third component, which is a significant improvement with regard to the efficiency of 17.22 % for the control binary device. Our study offers a molecular design strategy to develop a third component for building ternary PSCs in terms of crystallinity and miscibility regulation.

17% efficiency for linear-shaped ADA-type nonfullerene acceptors enabled by 3D reticulated molecular packing

The fundamental principles governing photovoltaic properties of nonfullerene acceptors (NFAs) are essential for developing high-performance polymer solar cells. In this work, using three heteroheptacene-based acceptors (M-series) as model compounds, we systematically study the implication of heteroatoms in the heteroheptacene core on the molecular packing, morphology, optoelectronic, photophysical, and photovoltaic properties of the NFAs. It is found that replacing the oxygen atoms at the inner positions of the heteroheptacene core with sulfur atoms leads to the molecular packing mode change from a linear two-dimensional (2D) brickwork structure to a three-dimensional (3D) reticulated motif, which facilitates the formation of a desirable active layer with nanostructured phase separation morphology and improved charge transport. Meanwhile, the down-shifted highest occupied molecular orbital energy level of M36 induced by the sulfur substitution, matches better with that of polymer donor PM6 thereby leading to more efficient exciton dissociation and charge transfer. As a result, the best-performing photovoltaic device based on M36 affords an outstanding efficiency of 17%, which is among the highest values reported for all the ADA-type NFAs. Our work reveals the important role of the heteroatoms in the heteroheptacene core in constructing the 3D network of ADA-type NFAs, and the structure-property relationships herein shall provide an important guidance for designing high-performance NFAs.

Composition-Tolerant Terpolymers for Efficient, Nonhalogenated Solvent-Processed Polymer Solar Cells

The design of terpolymers is a compelling strategy to improve the polymer solar cell (PSC) performance. However, the terpolymer composition at which the power conversion efficiency (PCE) of associated PSCs is typically optimized generally falls within a very narrow range, complicating the reproducible fabrication of optimal PSCs. In this study, a series of D–A1–D–A2 type random terpolymers (DTTz-X, where X = 10–60) is designed with structurally similar A1 and A2 accepting units. The sulfur atom of the benzothiadiazole subunit in the A1 (DTBT) unit is replaced by a nitrogen atom in the A2 (DTTz) unit, enabling the introduction of an alkyl solubilizing group while maintaining the overall structural properties of the terpolymer system. Consequently, the DTTz-X PDs maintain good optoelectronic properties at various A1/A2 ratios, while their processability in nonhalogenated solvents is significantly enhanced. Accordingly, the PSC performance of the terpolymer system shows good composition tolerance; i.e., the terpolymer system affords PSCs with PCEs exceeding 15% (up to 16.4%) over a broad range of DTTz compositions (10–40 mol %). This study establishes a useful design strategy for the development of efficient terpolymer donors for reproducible and eco-friendly fabrication of high-performance PSCs.

Improving the Performance of Polymer Solar Cells with Benzo[ghi]perylenetriimide-Based Small-Molecules as Interfacial Layers.

In this study, we prepared three benzo[ghi]perylenetriimide (BPTI) conjugated molecules as electron-transporting surface-modifying layers for polymer solar cells (PSCs). These three BPTI derivatives differed in the nature of their terminal functionalities, featuring butylamine (C3NH2), propylammonium iodide (C3NH3I), and butyldimethylamine (C3DMA) units, respectively. We evaluated the optoelectronic properties of PTB7-Th: PC71BM blends modified with these interfacial layers, as well as the performance of resulting PSCs. We used UV–Vis spectroscopy, atomic force microscopy, surface energy analysis, ultraviolet photoelectron spectroscopy, and photoelectric flow measurements to examine the phenomena behind the changes in the optoelectronic behavior of these blend films. The presence of a BPTI derivative changed the energy band alignment at the ZnO–active layer interface, leading to the ZnO film behaving more efficiently as an electron-extraction electrode. Modifying the ZnO surface with the BPTI-C3NH3I derivative resulted in a best power conversion efficiency (PCE) of 10.2 ± 0.53% for the PTB7-Th:PC71BM PSC (cf. PCE of the control device of 9.1 ± 0.13%). In addition, modification of a PM6:Y6:PCBM PSC with the BPTI-C3NH3I derivative increased its PCE from 15.6 ± 0.25% to 16.5 ± 0.18%. Thus, BPTI derivatives appear to have potential as IFLs when developing high-performance PSCs, and might also be applicable in other optoelectronic devices.

Semitransparent polymer solar cell/triboelectric nanogenerator hybrid systems: Synergistic solar and raindrop energy conversion for window-integrated applications

Development of photovoltaic (PV)-derived hybrid power systems can overcome the weather-dependent electricity production and increase the amount of dispatchable renewable energy generation. Herein, monolithic hybrid devices are developed via rational integration of high-performance semitransparent polymer solar cells (ST-PSCs) and liquid−solid triboelectric nanogenerators (TENGs). High-performance PSCs with efficiencies of 17.4% for rigid and 15.7% for flexible devices are achieved. Further electrode modifications and integration of transparent TENGs synergistically balance the above-bandgap photon harvesting and transparency in a broad wavelength range (380 − 1000 nm), yet significantly reduce the transmittance in the near-infrared wavelength range (1000 − 2500 nm) of hybrid devices. The hybrid devices simultaneously provide high visible light transparency, good color fidelity, efficient heat resistance and possibility to integrate on rigid and flexible substrates. The hybrid devices attain a high solar conversion efficiency of 10.1% under 1 sun, indicating efficient light-to-electricity conversion (a maximum electrical power output: 101 W m−2) on sunny days. The hybrid devices can also generate a maximum electrical power output of 2.62 W m−2 through waterdrop energy conversion, implying complementary green electricity production on rainy days. The controlled ambient temperature and specific transmittance windows provided by the hybrid devices sustain plant growth and highlight their great potential in agricultural applications. Gratifyingly, this work demonstrates the first example of ST-PSC/TENG hybrid systems for scaling up renewable power generation in different weather conditions, considering architectural and agricultural applications.