Showing papers on "Organic solar cell published in 2020"

High-efficiency organic solar cells with low non-radiative recombination loss and low energetic disorder

Abstract: Energy loss within organic solar cells (OSCs) is undesirable as it reduces cell efficiency1–4. In particular, non-radiative recombination loss3 and energetic disorder5, which are closely related to the tail states below the band edge and the overall photon energy loss, need to be minimized to improve cell performance. Here, we report how the use of a small-molecule acceptor with torsion-free molecular conformation can achieve a very low degree of energetic disorder and mitigate energy loss in OSCs. The resulting single-junction OSC has an energy loss due to non-radiative recombination of just 0.17 eV and a high power conversion efficiency of up to 16.54% (certified as 15.89% by the National Renewable Energy Laboratory). The findings take studies of organic photovoltaics deeper into a new regime, beyond the limits of energetic disorder and large energy offset for charge generation. An organic solar cell designed with minimal energetic disorder exhibits very low energy loss due to non-radiative recombination and highly efficient operation.

Over 17% efficiency ternary organic solar cells enabled by two non-fullerene acceptors working in an alloy-like model

Abstract: Nowadays, organic solar cells (OSCs) with Y6 and its derivatives as electron acceptors provide the highest efficiencies among the studied binary OSCs. To further improve the performances of OSCs, the fabrication of ternary OSCs (TOSCs) is a convenient strategy. Essentially, morphology control and the trade-off between voltage and photocurrent are the main critical issues in TOSCs. Herein, we address these problems by constructing TOSCs where an alloy-like composite is formed between Y6 and a newly designed derivative, BTP-M. Employing an electron-pushing methyl substituent as a replacement for the electron-withdrawing F atoms on Y6, BTP-M shows higher energy levels and lower crystallinity than Y6. As a result, the obtained Y6:BTP-M alloy can simultaneously optimize energy levels to reduce energy loss as well as the morphologies of the active layers to favor photocurrent generation, leading to an enhanced open-circuit voltage (Voc) of 0.875 V together with a larger short-circuit current density (Jsc) of 26.56 mA cm−2 for TOSCs based on the polymer donor PM6 and Y6:BTP-M acceptor alloy. Consequently, a best efficiency of 17.03% is achieved for the corresponding TOSCs, which is among the best values for single-junction OSCs. In addition, our TOSCs also exhibit good thickness tolerance, and can reach 14.23% efficiency even though the active layer is as thick as 300 nm.

New Phase for Organic Solar Cell Research: Emergence of Y-Series Electron Acceptors and Their Perspectives

Abstract: With the recent emergence of a new class of high-performance nonfullerene acceptors (NFAs), organic solar cells (OSCs) have entered a new phase of research featuring high power conversion efficienc...

Self-assembled Monolayer Enables HTL-free Organic Solar Cells with 18% Efficiency and Improved Operational Stability

Abstract: We report on bulk-heterojunction (BHJ) organic photovoltaics (OPVs) based on the self-assembled monolayer (SAM) 2PACz as a hole-selective interlayer functionalized directly onto the indium tin oxid

Cathode engineering with perylene-diimide interlayer enabling over 17% efficiency single-junction organic solar cells.

Abstract: In organic solar cells (OSCs), cathode interfacial materials are generally designed with highly polar groups to increase the capability of lowering the work function of cathode. However, the strong polar group could result in a high surface energy and poor physical contact at the active layer surface, posing a challenge for interlayer engineering to address the trade-off between device stability and efficiency. Herein, we report a hydrogen-bonding interfacial material, aliphatic amine-functionalized perylene-diimide (PDINN), which simultaneously down-shifts the work function of the air stable cathodes (silver and copper), and maintains good interfacial contact with the active layer. The OSCs based on PDINN engineered silver-cathode demonstrate a high power conversion efficiency of 17.23% (certified value 16.77% by NREL) and high stability. Our results indicate that PDINN is an effective cathode interfacial material and interlayer engineering via suitable intermolecular interactions is a feasible approach to improve device performance of OSCs. It is desired to design cathode interfacial layers to simultaneously improve the efficiency and stability of organic solar cells and tune the cathode properties. Here, Yao et al. develop such interfacial layers for the best donor-acceptor system and achieve a high certified efficiency close to 17%.

Delocalization of exciton and electron wavefunction in non-fullerene acceptor molecules enables efficient organic solar cells

Abstract: A major challenge for organic solar cell (OSC) research is how to minimize the tradeoff between voltage loss and charge generation. In early 2019, we reported a non-fullerene acceptor (named Y6) that can simultaneously achieve high external quantum efficiency and low voltage loss for OSC. Here, we use a combination of experimental and theoretical modeling to reveal the structure-property-performance relationships of this state-of-the-art OSC system. We find that the distinctive π–π molecular packing of Y6 not only exists in molecular single crystals but also in thin films. Importantly, such molecular packing leads to (i) the formation of delocalized and emissive excitons that enable small non-radiative voltage loss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces that significantly reduces the Coulomb attraction between interfacial electron-hole pairs. These properties are critical in enabling highly efficient charge generation in OSC systems with negligible donor-acceptor energy offset. Y6, as a non-fullerene acceptor for organic solar cells, has attracted intensive attention because of the low voltage loss and high charge generation efficiency. Here, Zhang et al. find that the delocalization of exciton and electron wavefunction due to strong π-π packing of Y6 is the key for the high performance.

Graphdiyne Derivative as Multifunctional Solid Additive in Binary Organic Solar Cells with 17.3% Efficiency and High Reproductivity

Abstract: Morphology tuning of the blend film in organic solar cells (OSCs) is a key approach to improve device efficiencies. Among various strategies, solid additive is proposed as a simple and new way to enable morphology tuning. However, there exist few solid additives reported to meet such expectations. Herein, chlorine-functionalized graphdiyne (GCl) is successfully applied as a multifunctional solid additive to fine-tune the morphology and improve device efficiency as well as reproductivity for the first time. Compared with 15.6% efficiency for control devices, a record high efficiency of 17.3% with the certified one of 17.1% is obtained along with the simultaneous increase of short-circuit current (Jsc ) and fill factor (FF), displaying the state-of-the-art binary organic solar cells at present. The redshift of the film absorption, enhanced crystallinity, prominent phase separation, improved mobility, and decreased charge recombination synergistically account for the increase of Jsc and FF after introducing GCl into the blend film. Moreover, the addition of GCl dramatically reduces batch-to-batch variations benefiting mass production owing to the nonvolatile property of GCl. All these results confirm the efficacy of GCl to enhance device performance, demonstrating a promising application of GCl as a multifunctional solid additive in the field of OSCs.

Tuning the electron-deficient core of a non-fullerene acceptor to achieve over 17% efficiency in a single-junction organic solar cell

Abstract: Finding effective molecular design strategies to enable efficient charge generation and small energy loss is among the long-standing challenges in developing high performance non-fullerene organic solar cells (OSCs). Recently, we reported Y-series non-fullerene acceptors with an electron-deficient-core-based fused structure (typically Y6), opening a new door to achieve high external quantum efficiency (∼80%) while maintaining low energy loss (∼0.57 eV). On this basis, further reducing the energy losses and ultimately improving the performance of OSCs has become a research hotspot. In this paper, we design and synthesize a new member of the Y-series acceptor family, Y18, which adopts a fused benzotriazole segment with unique luminescence properties as its electron-deficient core. Compared to Y6, the benzotriazole-based acceptor Y18 exhibits extended optical absorption and higher voltage. Consequently, the device delivers a promising power conversion efficiency of 16.52% with a very low energy loss of 0.53 eV. Further device optimization by exploiting a ternary blend strategy allowed us to achieve a high efficiency of 17.11% (certified as 16.76% by NREL). Y18 may become one of the most important candidate materials for its broader absorption spectra and higher voltage of Y18 (compared to Y6) in the OSCs field.

Improving open-circuit voltage by a chlorinated polymer donor endows binary organic solar cells efficiencies over 17%

Abstract: Power conversion efficiency (PCE) of single-junction polymer solar cells (PSCs) has made a remarkable breakthrough recently. Plenty of work was reported to achieve PCEs higher than 16% derived from the PM6:Y6 binary system. To further increase the PCEs of binary OSCs incorporating small molecular acceptor (SMA) Y6, we substituted PM6 with PM7 due to the deeper highest occupied molecular orbital (HOMO) of PM7. Consequently, the PM7:Y6 has achieved PCEs as high as 17.0% by the hot-cast method, due to the improved open-circuit voltage ( V OC). Compared with PM6, the lower HOMO of PM7 increases the gap between E LUMO-donor and E HOMO-acceptor, which is proportional to V OC. This research provides a high PCE for single-junction binary PSCs, which is meaningful for device fabrication related to PM7 and commercialization of PSCs.

Subtle Molecular Tailoring Induces Significant Morphology Optimization Enabling over 16% Efficiency Organic Solar Cells with Efficient Charge Generation.

Abstract: Manipulating charge generation in a broad spectral region has proved to be crucial for nonfullerene-electron-acceptor-based organic solar cells (OSCs). 16.64% high efficiency binary OSCs are achieved through the use of a novel electron acceptor AQx-2 with quinoxaline-containing fused core and PBDB-TF as donor. The significant increase in photovoltaic performance of AQx-2 based devices is obtained merely by a subtle tailoring in molecular structure of its analogue AQx-1. Combining the detailed morphology and transient absorption spectroscopy analyses, a good structure-morphology-property relationship is established. The stronger π-π interaction results in efficient electron hopping and balanced electron and hole mobilities attributed to good charge transport. Moreover, the reduced phase separation morphology of AQx-2-based bulk heterojunction blend boosts hole transfer and suppresses geminate recombination. Such success in molecule design and precise morphology optimization may lead to next-generation high-performance OSCs.

Progress in Stability of Organic Solar Cells

Abstract: The organic solar cell (OSC) is a promising emerging low-cost thin film photovoltaics technology. The power conversion efficiency (PCE) of OSCs has overpassed 16% for single junction and 17% for organic-organic tandem solar cells with the development of low bandgap organic materials synthesis and device processing technology. The main barrier of commercial use of OSCs is the poor stability of devices. Herein, the factors limiting the stability of OSCs are summarized. The limiting stability factors are oxygen, water, irradiation, heating, metastable morphology, diffusion of electrodes and buffer layers materials, and mechanical stress. The recent progress in strategies to increase the stability of OSCs is surveyed, such as material design, device engineering of active layers, employing inverted geometry, optimizing buffer layers, using stable electrodes and encapsulation materials. The International Summit on Organic Photovoltaic Stability guidelines are also discussed. The potential research strategies to achieve the required device stability and efficiency are highlighted, rendering possible pathways to facilitate the viable commercialization of OSCs.

A Simple n-Dopant Derived from Diquat Boosts the Efficiency of Organic Solar Cells to 18.3%

Abstract: Molecular doping has recently been shown to improve the operating characteristics of organic photovoltaics (OPVs). Here, we prepare neutral Diquat (DQ) and use it as n-dopant to improve the perform...

Barrierless Free Charge Generation in the High-Performance PM6:Y6 Bulk Heterojunction Non-Fullerene Solar Cell

Abstract: Organic solar cells are currently experiencing a second golden age thanks to the development of novel non-fullerene acceptors (NFAs). Surprisingly, some of these blends exhibit high efficiencies despite a low energy offset at the heterojunction. Herein, free charge generation in the high-performance blend of the donor polymer PM6 with the NFA Y6 is thoroughly investigated as a function of internal field, temperature and excitation energy. Results show that photocurrent generation is essentially barrierless with near-unity efficiency, regardless of excitation energy. Efficient charge separation is maintained over a wide temperature range, down to 100 K, despite the small driving force for charge generation. Studies on a blend with a low concentration of the NFA, measurements of the energetic disorder, and theoretical modeling suggest that CT state dissociation is assisted by the electrostatic interfacial field which for Y6 is large enough to compensate the Coulomb dissociation barrier.

Selenium Heterocyclic Electron Acceptor with Small Urbach Energy for As-Cast High-Performance Organic Solar Cells.

Abstract: Typical organic photovoltaic materials show high Urbach energies (ca. 25-50 meV), which is considerably higher than those of their inorganic counterparts and limits further improvement in the device efficiency of organic solar cells (OSCs). In this study, we introduce a facile method of selenium substitution to reduce the Urbach energy of organic photovoltaic materials to 20.4 meV (Y6Se), which is the lowest value reported for high-performance organic photovoltaic materials and very close to those (ca. 15 meV) of typical inorganic/hybrid semiconductors, such as crystalline silicon, gallium nitride, and lead-halide perovskite. Next, OSCs based on Y6Se showed 17.7% efficiency, which is among the best results for OSCs and the record efficiency of as-cast single junction OSCs to date.

Asymmetric Electron Acceptors for High-Efficiency and Low-Energy-Loss Organic Photovoltaics.

Abstract: Low energy loss and efficient charge separation under small driving forces are the prerequisites for realizing high power conversion efficiency (PCE) in organic photovoltaics (OPVs). Here, a new molecular design of nonfullerene acceptors (NFAs) is proposed to address above two issues simultaneously by introducing asymmetric terminals. Two NFAs, BTP-S1 and BTP-S2, are constructed by introducing halogenated indandione (A1 ) and 3-dicyanomethylene-1-indanone (A2 ) as two different conjugated terminals on the central fused core (D), wherein they share the same backbone as well-known NFA Y6, but at different terminals. Such asymmetric NFAs with A1 -D-A2 structure exhibit superior photovoltaic properties when blended with polymer donor PM6. Energy loss analysis reveals that asymmetric molecule BTP-S2 with six chlorine atoms attached at the terminals enables the corresponding devices to give an outstanding electroluminescence quantum efficiency of 2.3 × 10-2 %, one order of magnitude higher than devices based on symmetric Y6 (4.4 × 10-3 %), thus significantly lowering the nonradiative loss and energy loss of the corresponding devices. Besides, asymmetric BTP-S1 and BTP-S2 with multiple halogen atoms at the terminals exhibit fast hole transfer to the donor PM6. As a result, OPVs based on the PM6:BTP-S2 blend realize a PCE of 16.37%, higher than that (15.79%) of PM6:Y6-based OPVs. A further optimization of the ternary blend (PM6:Y6:BTP-S2) results in a best PCE of 17.43%, which is among the highest efficiencies for single-junction OPVs. This work provides an effective approach to simultaneously lower the energy loss and promote the charge separation of OPVs by molecular design strategy.

A Non-fullerene Acceptor with Enhanced Intermolecular π-Core Interaction for High-Performance Organic Solar Cells.

Abstract: Understanding the molecular structure and self-assembly of thiadiazole-derived non-fullerene acceptors (NFAs) is very critical for elucidating the origin of their extraordinary charge generation an...

A-DA′D-A non-fullerene acceptors for high-performance organic solar cells

Abstract: Since the world-record power conversion efficiency of 15.7% was achieved for organic solar cells (OSCs) in 2019, the newly developed non-fullerene acceptor (NFA) Y6 with an A-DA′D-A structure (A denotes an electron-accepting moiety, D denotes an electron-donating moiety) has attracted increasing attention. Subsequently, many new A-DA′D-A NFAs have been designed and synthesized, and the A-DA′D-A NFAs have played a significant role in the development of high-performance non-fullerene organic solar cells (NF-OSCs). Compared with the classical A-D-A-type acceptors, A-DA′D-A NFAs contain an electron-deficient core (such as benzothiadiazole (BT), benzotriazole (BTA), quinoxaline (Qx), or their derivatives) in the ladder-type fused rings to fine-tune the energy levels, broaden light absorption and achieve higher electron mobility of the NFAs. This review emphasizes the recent progress on these emerging A-DA′D-A (including Y-series) NFAs. The synthetic methods of DA′D-fused rings are introduced. The relationships between the chemical structure of the A-DA′D-A NFAs and the photovoltaic performance of the corresponding OSCs are summarized and discussed. Finally, issues and prospects for further improving photovoltaic performance of the OSCs are also proposed.

Optimized active layer morphology toward efficient and polymer batch insensitive organic solar cells.

Abstract: Morphology control in laboratory and industry setting remains as a major challenge for organic solar cells (OSCs) due to the difference in film-drying kinetics between spin coating and the printing process. A two-step sequential deposition method is developed to control the active layer morphology. A conjugated polymer that self-assembles into a well-defined fibril structure is used as the first layer, and then a non-fullerene acceptor is introduced into the fibril mesh as the second layer to form an optimal morphology. A benefit of the combined fibril network morphology and non-fullerene acceptor properties was that a high efficiency of 16.5% (certified as 16.1%) was achieved. The preformed fibril network layer and the sequentially deposited non-fullerene acceptor form a robust morphology that is insensitive to the polymer batches, solving a notorious issue in OSCs. Such progress demonstrates that the utilization of polymer fibril networks in a sequential deposition process is a promising approach towards the fabrication of high-efficiency OSCs. Reliably controlling the morphology in organic solar cells is desired for up-scaling. Here Weng et al. combine the advantages of the fibril network donor and the state of the art Y6 acceptor in a two-step approach to deliver a high efficiency of 16% without batch-to-batch variation.

15.34% efficiency all-small-molecule organic solar cells with an improved fill factor enabled by a fullerene additive

Abstract: Solution processed organic solar cells (OSCs) composed of all small molecules (ASM) are promising for production on an industrial scale owing to the properties of small molecules, such as well-defined chemical structures, high purity of materials, and outstanding repeatability from batch to batch synthesis. Remarkably, ASM OSCs with power conversion efficiency (PCE) beyond 13% were achieved by structure improvement of the electron donor and choosing Y6 as the electron acceptor. However, the fill factor (FF) is an obstacle that limits the further improvement of the PCE for these ASM OSCs. Herein, we focus on the FF improvement of recently reported ASM OSCs with BTR-Cl:Y6 as the active layer by miscibility-induced active layer morphology optimization. The incorporation of fullerene derivatives, which have good miscibility with both BTR-Cl and Y6, results in reduced bimolecular recombination and thus improved FF. In particular, when ca. 5 wt% of PC71BM was added in the active layer, a FF of 77.11% was achieved without sacrificing the open circuit voltage (VOC) and the short circuit current density (JSC), leading to a record PCE of 15.34% (certified at 14.7%) for ASM OSCs. We found that the optimized device showed comparable charge extraction, longer charge carrier lifetime, and slower bimolecular recombination rate compared with those of the control devices (w/o fullerene). Our results demonstrate that the miscibility driven regulation of active layer morphology by incorporation of a fullerene derivative delicately optimizes the active layer microstructures and improves the device performance, which brings vibrancy to OSC research.

Random terpolymer based on thiophene-thiazolothiazole unit enabling efficient non-fullerene organic solar cells.

Abstract: Developing a high-performance donor polymer is critical for achieving efficient non-fullerene organic solar cells (OSCs). Currently, most high-efficiency OSCs are based on a donor polymer named PM6, unfortunately, whose performance is highly sensitive to its molecular weight and thus has significant batch-to-batch variations. Here we report a donor polymer (named PM1) based on a random ternary polymerization strategy that enables highly efficient non-fullerene OSCs with efficiencies reaching 17.6%. Importantly, the PM1 polymer exhibits excellent batch-to-batch reproducibility. By including 20% of a weak electron-withdrawing thiophene-thiazolothiazole (TTz) into the PM6 polymer backbone, the resulting polymer (PM1) can maintain the positive effects (such as downshifted energy level and reduced miscibility) while minimize the negative ones (including reduced temperature-dependent aggregation property). With higher performance and greater synthesis reproducibility, the PM1 polymer has the promise to become the work-horse material for the non-fullerene OSC community. The batch reproducibility of polymer donor materials limits the performance of polymer solar cells. Here Wu et al. develop a polymer donor PM1 by random terpolymerization strategy with a high efficiency of 17.6% in the device and excellent batch-to-batch reproducibility.

Over 14.5% efficiency and 71.6% fill factor of ternary organic solar cells with 300 nm thick active layers

Abstract: The ternary strategy exhibits great potential in optimizing the photon harvesting and phase separation of active layers. In this work, non-fullerene MF1 was selected as the third component to prepare efficient ternary organic solar cells (OSCs) by finely optimizing the MF1 content in the acceptors. The optimized power conversion efficiency (PCE) of 15.31% is achieved in the ternary OSCs with 20 wt% MF1 content in the acceptors and 100 nm active layer thickness, also exhibiting a relatively high fill factor (FF) of 78.05%. The relatively high FF indicates efficient charge transport and collection in the optimized ternary OSCs, which should be beneficial to achieve efficient thick-film OSCs. It is highlighted that a PCE of 14.57% is achieved in the optimized ternary OSCs with 300 nm thick active layers compatible with the roll-to-roll (R2R) large-scale printing process. To date, high performance thick-film ternary non-fullerene OSCs have seldom been reported. This work indicates that the thick-film ternary strategy has great potential in achieving efficient large-scale OSCs.

n-Type Molecular Photovoltaic Materials: Design Strategies and Device Applications.

Abstract: The use of photovoltaic technologies has been regarded as a promising approach for converting solar energy to electricity and mitigating the energy crisis, and among these, organic photovoltaics (O...

Two compatible polymer donors contribute synergistically for ternary organic solar cells with 17.53% efficiency

Abstract: A ternary strategy has been demonstrated as a promising method to further boost the performance of organic solar cells (OSCs). Herein, an efficient polymer donor S3 was synthesized and incorporated into a PM6:Y6 system to fabricate ternary OSCs. S3 possesses complementary absorption spectra and good compatibility with PM6, which is beneficial to fine-tune the photon harvesting and morphology of the ternary blend films, resulting in simultaneous enhancement of the short-circuit current density (JSC) and fill factor (FF). In addition, the highest occupied molecular orbital (HOMO) energy level of S3 is slightly lower than that of PM6, which enables lower nonradiative energy loss in ternary OSCs compared with that of PM6-based binary OSCs, leading to higher open-circuit voltage (VOC). The optimized ternary OSCs with 20 wt% S3 in the donors achieve a PCE of 17.53%, which should be among the highest values of ternary OSCs. This work provides an effective approach to fabricate high-performance ternary OSCs by synergizing two well-matched polymer donors.

The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets

Abstract: Organic solar cells utilize an energy-level offset to generate free charge carriers. Although a very small energy-level offset increases the open-circuit voltage, it remains unclear how exactly charge generation is affected. Here we investigate organic solar cell blends with highest occupied molecular orbital energy-level offsets (∆EHOMO) between the donor and acceptor that range from 0 to 300 meV. We demonstrate that exciton quenching at a negligible ∆EHOMO takes place on timescales that approach the exciton lifetime of the pristine materials, which drastically limits the external quantum efficiency. We quantitatively describe this finding via the Boltzmann stationary-state equilibrium between charge-transfer states and excitons and further reveal a long exciton lifetime to be decisive in maintaining an efficient charge generation at a negligible ∆EHOMO. Moreover, the Boltzmann equilibrium quantitatively describes the major reduction in non-radiative voltage losses at a very small ∆EHOMO. Ultimately, highly luminescent near-infrared emitters with very long exciton lifetimes are suggested to enable highly efficient organic solar cells. Donor–acceptor systems with low energy-level offset enable high power efficiency in organic solar cells yet it is unclear what drives charge generation. Classen et al. show that long exciton lifetimes enable efficient exciton splitting and thus generation of free charges while also suppressing voltage losses.