Cost-Effective Cathode Interlayer Material for Scalable Organic Photovoltaic Cells.

Organic photovoltaic (OPV) cells have demonstrated remarkable success on the laboratory scale. However, the lack of cathode interlayer materials for large-scale production still limits their practical application. Here, we rationally designed and synthesized a cathode interlayer, named NDI-Ph. Benefiting from their well-modulated work function and self-doping effect, NDI-Ph-based binary OPV cells achieve an excellent power conversion efficiency (PCE) of 19.1%. NDI-Ph can be easily synthesized on a 100 g scale with a low cost of 1.96 $ g-1 using low-cost raw materials and a simple postprocessing method. In addition, the insensitivity to the film thickness of NDI-Ph enables it to maintain a high PCE at various coating speeds and solution concentrations, demonstrating excellent adaptability for high-throughput OPV cell manufacturing. As a result, a module with 21.9 cm2 active area achieves a remarkable PCEactive of 15.8%, underscoring the prospects of NDI-Ph in the large-scale production of OPV cells.

Molecular Design of Fully Nonfused Acceptors for Efficient Organic Photovoltaic Cells.

The nonfused thiophene-benzene-thiophene (TBT) unit offers advantages in obtaining low-cost organic photovoltaic (OPV) materials due to its simple structure. However, OPV cells, including TBT-based acceptors, exhibit significantly lower energy conversion efficiencies. Here, we introduce a novel approach involving the design and synthesis of three TBT-based acceptors by substituting different position-branched side chains on the TBT unit. In comparison to TBT-10 and TBT-11, TBT-13, which exclusively incorporates α-position branched side chains with a large steric hindrance, demonstrates a more planar and stable conformation. When blended with the donor PBQx-TF, TBT-13-based blend film achieves favorable π-π stacking and aggregation characteristics, resulting in excellent charge transfer performance in the corresponding device. Due to the simultaneous enhancements in short-circuit current density and fill factor, the TBT-13-based OPV cell obtains an outstanding efficiency of 16.1%, marking the highest value for the cells based on fully nonfused acceptors. Our work provides a practical molecular design strategy for high-performance and low-cost OPV materials.

Reducing Depletion Region Width at Electrode Interface via a Hole-transport Layer for Over 18% Efficiency in Organic Solar Cells.

The large depletion region width at the electrode interface may cause serious energy loss in charge collection of organic solar cells (OSCs), depressing the open-circuit voltage and power conversion efficiency (PCE). Herein, a pH neutral solution-processed conjugated polyelectrolyte PIDT-F:IMC as hole transport layer (HTL) to reduce the depletion region width in efficient OSCs is developed. By utilizing "mutual doping" strategy, the doping density of PIDT-F:IMC is increased by more than two orders of magnitude, which significantly reduces the depletion region width at the anode interface from 55 to 7.4 nm, playing an effective role in decreasing the energy loss in hole collection. It is also revealed that the optimal thickness of HTL should be consistent with the depletion region width for achieving the minimum energy loss. The OSC modified by PIDT-F:IMC shows a high PCE of 18.2%, along with an amazing fill factor of 0.79. Moreover, a PCE of 16.5% is achieved in the 1 cm2 OSC by using a blade-coated PIDT-F:IMC HTL, indicating the good compatibility of PIDT-F:IMC with large-area processing technology. The PIDT-F:IMC-modified OCS exhibits a lifetime of 400 h under operational conditions, which is ten times longer than that of the PEDOT:PSS device.

Morphology Control for Efficient Nonfused Acceptor-Based Organic Photovoltaic Cells.

Non-fused electron acceptors have huge advantages in fabricating low-cost organic photovoltaic (OPV) cells. However, morphology control is a challenge as non-fused C─C single bonds bring more molecular conformations. Here, by selecting two typical polymer donors, PBDB-TF and PBQx-TF, the blend morphologies and its impacts on the power conversion efficiencies (PCEs) of non-fused acceptor-based OPV cells are studied. A selenium-containing non-fused acceptor named ASe-5 is designed. The results suggest that PBQx-TF has a lower miscibility with ASe-5 when compared with PBDB-TF. Additionally, the polymer networks may form earlier in the PBQx-TF:ASe-5 blend film due to stronger preaggregation performance, leading to a more obvious phase separation. The PBQx-TF:ASe-5 blend film shows faster charge transfer and suppressed charge recombination. As a result, the PBQx-TF:ASe-5-based device records a good PCE of 14.7% with a higher fill factor (FF) of 0.744, while the PBDB-TF:ASe-5-based device only obtains a moderate PCE of 12.3% with a relatively low FF of 0.662. The work demonstrates that the selection of donors plays a crucial role in controlling the blend morphology and thus improving the PCEs of non-fused acceptor-based OPV cells.

Iodinated Electron Acceptor with Significantly Extended Exciton Diffusion Length for Efficient Organic Photovoltaic Cells.

Iodination has unlocked new potentials in organic photovoltaics (OPVs). A newly designed and synthesized iodinated non-fullerene acceptor, BO-4I, showcases exceptional excitation delocalization property with the exciton diffusion length increased to 80 nm. The enhanced electron delocalization property is attributed to the larger atomic radius and electron orbit of the iodine atom, which facilitates the formation of intra-moiety excitations in the acceptor phase. This effectively circumvents the charge transfer state-related recombination mechanisms, leading to a substantial reduction in non-radiative energy loss (ΔEnr ). As a result, OPV cell based on PBDB-TF : BO-4I achieves an impressive efficiency of 18.9 % with a notable ΔEnr of 0.189 eV, markedly surpassing their fluorinated counterparts. This contribution highlights the pivotal role of iodination in reducing energy loss, thereby affirming its potential as a key strategy in the development of advanced next-generation OPV cells.

Completely Non-Fused Low-Cost Acceptor Enables Organic Photovoltaic Cells with 17 % Efficiency.

To meet the industrial requirements of organic photovoltaic (OPV) cells, it is imperative to accelerate the development of cost-effective materials. Thiophene-benzene-thiophene central unit-based acceptors possess the advantage of low synthetic cost, while their power conversion efficiency (PCE) is relatively low. Here, by incorporating a para-substituted benzene unit and 1st-position branched alkoxy chains with large steric hindrance, a completely non-fused non-fullerene acceptor, TBT-26, was designed and synthesized. The narrow band gap of 1.38 eV ensures the effective utilization of sunlight. The favorable phase separation morphology of TBT-26-based blend film facilitates the efficient exciton dissociation and charge transport in corresponding OPV cell. Therefore, the TBT-26-based small-area cell achieves an impressive PCE of 17.0 %, which is the highest value of completely non-fused OPV cells. Additionally, we successfully demonstrated the scalability of this design by fabricating a 28.8 cm2 module with a high PCE of 14.3 %. Overall, our work provides a practical molecular design strategy for developing high-performance and low-cost acceptors, paving the way for industrial applications of OPV technology.

Pyrrole-Based Fully Non-fused Acceptor for Efficient and Stable Organic Solar Cells.

Organic solar cells (OSCs) are still suffering from the low light utilization and unstable under ultraviolet irradiation. To tackle these challenges, we design and synthesize a non-fused acceptor based on 1-(2-butyloctyl)-1H-pyrrole as π-bridge unit, denoted as GS70, which serves as active layer in the front-cell for constructing tandem OSCs with a parallel configuration. Benefiting from the well-complementary absorption spectra with the rear-cell, GS70-based parallel tandem OSCs exhibit an improved photoelectron response over the range between 600-700 nm, yielding a high short-circuit current density of 28.4 mA cm-2. The improvement in light utilization translates to a power conversion efficiency of 19.4 %, the highest value among all parallel tandem OSCs. Notably, owing to the intrinsic stability of GS70, the manufactured parallel tandem OSCs retain 84.9 % of their initial PCE after continuous illumination for 1000 hours. Overall, this work offers novel insight into the molecular design of low-cost and stability non-fused acceptors, emphasizing the importance of adopting a parallel tandem configuration for achieving efficient light harvesting and improved photostability in OSCs.

Optimizing Molecular Packing via Steric Hindrance for Reducing Non-Radiative Recombination in Organic Solar Cells.

Innovative molecule design strategy holds promise for the development of next-generation acceptor materials for efficient organic solar cells with low non-radiative energy loss (ΔEnr). In this study, we designed and prepared three novel acceptors, namely BTP-Biso, BTP-Bme and BTP-B, with sterically structured triisopropylbenzene, trimethylbenzene and benzene as side chains inserted into the shoulder of the central core. The progressively enlarged steric hindrance from BTP-B to BTP-Bme and BTP-Biso induces suppressed intramolecular rotation and altered the molecule packing mode in their aggregation states, leading to significant changes in absorption spectra and energy levels. By regulating the intermolecular π-π interactions, BTP-Bme possesses relatively reduced non-radiative recombination rate and extended exciton diffusion lengths. The binary device based on PB2 : BTP-Bme exhibits an impressive power conversion efficiency (PCE) of 18.5 % with a low ΔEnr of 0.19 eV. Furthermore, the ternary device comprising PB2 : PBDB-TF : BTP-Bme achieves an outstanding PCE of 19.3 %. The molecule design strategy in this study proposed new perspectives for developing high-performance acceptors with low ΔEnr in OSCs.

Achieving High Fill Factor in Organic Photovoltaic Cells by Tuning Molecular Electrostatic Potential Fluctuation.

In the field of organic photovoltaics (OPVs), significant progress has been made in tailoring molecular structures to enhance the open-circuit voltage and the short-circuit current density. However, there remains a crucial gap in the development of coordinated material design strategies focused on improving the fill factor (FF). Here, we introduce a molecular design strategy that incorporates electrostatic potential fluctuation to design organic photovoltaic materials. By reducing the fluctuation amplitude of IT-4F, we synthesized a new acceptor named ITOC6-4F. When using PBQx-TF as a donor, the ITOC6-4F-based cell shows a markedly low recombination rate constant of 0.66×10-14 cm3 s-1 and demonstrates an outstanding FF of 0.816, both of which are new records for binary OPV cells. Also, we find that a small fluctuation amplitude could decrease the energetic disorder of OPV cells, reducing energy loss. Finally, the ITOC6-4F-based cell creates the highest efficiency of 16.0 % among medium-gap OPV cells. Our work holds a vital implication for guiding the design of high-performance OPV materials.

A Wide Bandgap Acceptor with Large Dielectric Constant and High Electrostatic Potential Values for Efficient Organic Photovoltaic Cells.

Low-bandgap materials have achieved rapid development and promoted the enhancement of power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, the design of wide-bandgap non-fullerene acceptors (WBG-NFAs), required by indoor applications and tandem cells, has been lagging far behind the development of OPV technologies. Here, we designed and synthesized two NFAs named ITCC-Cl and TIDC-Cl by finely optimizing ITCC. In contrast with ITCC and ITCC-Cl, TIDC-Cl can maintain a wider bandgap and a higher electrostatic potential simultaneously. When blending with the donor PB2, the highest dielectric constant is also obtained in TIDC-Cl-based films, enabling efficient charge generation. Therefore, the PB2:TIDC-Cl-based cell possessed a high PCE of 13.8% with an excellent fill factor (FF) of 78.2% under the air mass 1.5G (AM 1.5G) condition. Furthermore, an exciting PCE of 27.1% can be accomplished in the PB2:TIDC-Cl system under the illumination of 500 lux (2700 K light-emitting diode). Combined with the theoretical simulation, the tandem OPV cell based on TIDC-Cl was fabricated and exhibited an excellent PCE of 20.0%.

Self-Doped n-Type Quinoidal Compounds with Good Air Stability and High Electrical Conductivity for Organic Electronics.

Air stable n-type conductive molecules with high electrical conductivities and excellent device performance have important applications in organic electronics, but their synthesis remains challenging. Herein, we report three self-doped n-type conductive molecules, designated QnNs, with a closed-shell quinoidal backbone and alkyl amino chains of different lengths. The QnNs are self-doped by intermolecular electron transfer from the amino groups to the quinoidal backbone. This process is ascertained unambiguously by experiments and theoretical calculations. The use of a quinoidal structure effectively improves the self-doping level, and thus increases the electrical conductivity of self-doped n-type conductive molecules achieved by a closed-shell structure from<10-4  S cm-1 to>0.03 S cm-1 . Furthermore, the closed-shell quinoidal structure results in good air stability of the QnNs, with half-lives>73 days; and Q4N shows an electrical conductivity of 0.019 S cm-1 even after exposure to air for 120 days. When applying Q6N as the cathode interlayer in organic solar cells (OSCs), an outstanding power conversion efficiency of up to 18.2 % was obtained, which represents one the best results in binary OSCs.

Design of a Fully Non-Fused Bulk Heterojunction toward Efficient and Low-Cost Organic Photovoltaics.

To modulate the miscibility between donor and acceptor materials both possessing fully non-fused ring structures, a series of electron acceptors (A4T-16, A4T-31 and A4T-32) with different polar functional substituents were synthesized and investigated. The three acceptors show good planarity, high conformational stability, complementary absorption and energy levels with the non-fused polymer donor (PTVT-BT). Among them, A4T-32 possesses the strongest polar functional group and shows the highest surface energy, which facilitates morphological modulation in the bulk heterojunction (BHJ) blend. Benefiting from the proper morphology control method, an impressive power conversion efficiency (PCE) of approaching 16.0 % and a superior fill factor over 0.795 are achieved in the PTVT-BT : A4T-32-based organic photovoltaic cells with superior photoactive materials price advantage, which represent the highest value for the cells based on the non-fused blend films. Notably, this cell maintains ≈84 % of its initial PCE after nearly 2000 h under the continuous simulated 1-sun-illumination. In addition, the flexible PTVT-BT : A4T-32-based cells were fabricated and delivered a decent PCE of 14.6 %. This work provides an effective molecular design strategy for the non-fused non-fullerene acceptors (NFAs) from the aspect of bulk morphology control in fully non-fused BHJ layers, which is crucial for their practical applications.

Thermodynamic Phase Transition of Three-Dimensional Solid Additives Guiding Molecular Assembly for Efficient Organic Solar Cells.

Fine-tuning the thermodynamic self-assembly of molecules via volatile solid additives has emerged to be an effective way to construct high-performance organic solar cells. Here, three-dimensional structured solid molecules have been designed and applied to facilitate the formation of organized molecular assembly in the active layer. By means of systematic theory analyses and film-morphology characterizations based on four solid candidates, we preselected the optimal one, 4-fluoro-N,N-diphenylaniline (FPA), which possesses good volatility and strong charge polarization. The three-dimensional solids can induce molecular packing in active layers via strong intermolecular interactions and subsequently provide sufficient space for the self-reassembly of active layers during the thermodynamic transition process. Benefitting from the optimized morphology with improved charge transport and reduced energy disorder in the FPA-processed devices, high efficiencies of over 19 % were achieved. The strategy of three-dimensional additives inducing ordered self-assembly structure represents a practical approach for rational morphology control in highly efficient devices, contributing to deeper insights into the structural design of efficient volatile solid additives.

Low-Cost Fully Non-fused Ring Acceptor Enables Efficient Organic Photovoltaic Modules for Multi-Scene Applications.

Organic photovoltaic (OPV) cells, with highly tunable light-response ranges, offer significant potential for use in driving low-power consumption off-grid electronics in multi-scenarios. However, development of photoactive layer materials that can meet simultaneously the requirements of diverse irradiation conditions is a still challenging task. Herein, a low-cost fully non-fused acceptor (denoted as GS60) featuring well-matched absorption spectra with solar, scattered light and artificial light radiation was designed and synthesized. Systematic characterizations revealed that GS60 possessed outstanding photoelectron properties and ideal morphology, which resulted in reduced voltage loss and suppressed charge recombination. By blending with a non-fused ring polymer PTVT-T, the as-obtained GS60 based OPV cells achieved a good power conversion efficiency (PCE) of 14.1 %, a high value for the cells based on non-fused ring bulk heterojunction. Besides, manufactured large-area OPV modules based on PTVT-T:GS60 yielded PCEs of 11.2 %, 11.8 %, 12.1 %, 23.1 %, and 20.3 % under irradiation of AM 1.5G, natural light of cloudy weather, natural light in shadow, laser and indoor, respectively. The PTVT-T:GS60 devices exhibited considerable potential in terms of improving photostability and reducing material cost. Overall, this work provides novel insight into the molecular design of low-cost non-fused ring acceptors, and extended potential of medium band gap acceptors based OPV cells used in various application scenarios.

Recent Advances in Single-Junction Organic Solar Cells.

Single-junction organic solar cells (OSCs) have made significant progress in recent years. Innovations in material design and device optimization have improved the power conversion efficiencies to over 19 %. In this Minireview, based on recent advances, we discuss molecular design strategies to tune the absorption spectrum, energy level, and intermolecular aggregation as well as highlight the role of molecular electrostatic potential in decreasing energy loss. Then, we introduce the latest progress in four types of OSCs composed of different donor:acceptor combinations: polymer donor:small-molecule acceptor, all-polymer, all-small-molecule, and small-molecule donor:polymer acceptor. Finally, the challenges of OSCs in practical applications, including material cost, stability, and multi-function integration, are discussed.

Design of Near-Infrared Nonfullerene Acceptor with Ultralow Nonradiative Voltage Loss for High-Performance Semitransparent Ternary Organic Solar Cells.

Semitransparent organic solar cells (ST-OSCs) are considered as one of the most valuable applications of OSCs and a strong contender in the market. However, the optical band gap of current high-performance ST-OSCs is still not low enough to achieve the optimal balance between power conversion efficiency (PCE) and average visible transmittance (AVT). An N-substituted asymmetric nonfullerene acceptor SN with over 40 nm bathochromically shifted absorption compared to Y6 was designed and synthesized, based on which the device with PM6 as donor obtained a PCE of 14.3 %, accompanied with a nonradiative voltage loss as low as 0.15 eV. Meanwhile, ternary devices with the addition of SN into PM6 : Y6 can achieve a PCE of 17.5 % with an unchanged open-circuit voltage and improved short-circuit current. Benefiting from extended NIR absorption and lowered voltage loss, ST-OSCs based on PM6 : SN : Y6 were fabricated and the optimized device demonstrated a PCE of 14.0 % at an AVT of 20.2 %, which is the highest PCE at an AVT over 20 %.

Double-Cable Conjugated Polymers with Pendent Near-Infrared Electron Acceptors for Single-Component Organic Solar Cells.

Double-cable conjugated polymers with near-infrared (NIR) electron acceptors are synthesized for use in single-component organic solar cells (SCOSCs). Through the development of a judicious synthetic pathway, the highly sensitive nature of the 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC)-based electron acceptors in basic and protonic solvents is overcome. In addition, an asymmetric design motif is adopted to optimize the packing of donor and acceptor segments, enhancing charge separation efficiency. As such, the new double-cable polymers are successfully applied in SCOSCs, providing an efficiency of over 10 % with a broad photo response from 300 to 850 nm and exhibiting excellent thermal/light stability. These results demonstrate the powerful design of NIR-acceptor-based double-cable polymers and will enable SCOSCs to enter a new stage.

Multi-Functional Solid Additive Induced Favorable Vertical Phase Separation and Ordered Molecular Packing for Highly Efficient Layer-by-Layer Organic Solar Cells.

Layer-by-layer (LBL) deposition strategy enabling favorable vertical phase distributions has been regarded as promising candidates for constructing high-efficient organic photovoltaic (OPV) cells. However, solid additives with the merits of good stability and reproducibility have been rarely used to fine-tune the morphology of the LBL films for improved efficiency and stability. Herein, hierarchical morphology control in LBL OPV is achieved via a dual functional solid additive. Series of LBL devices are fabricated by introducing the solid additive individually or simultaneously to the donor or acceptor layer to clarify the functions of additives. Additive in the donor layer can facilitate the formation of preferable vertical component distribution, and that in the acceptor layer will enhance the molecular crystallinity for better charge transport properties. The optimized morphology ultimately contributed to high PCEs of 16.4% and 17.4% in the binary and quaternary LBL devices. This reported method provides an alternative way to controllably manipulate the morphology of LBL OPV cells.

Simultaneous Improvement of Efficiency and Stability of Organic Photovoltaic Cells by using a Cross-Linkable Fullerene Derivative.

Improving power conversion efficiencies (PCEs) and stability are two main tasks for organic photovoltaic (OPV) cells. In the past few years, although the PCE of the OPV cells has been considerably improved, the research on device stability is limited. Herein, a cross-linkable material, cross-linked [6,6]-phenyl-C61-butyric styryl dendron ester (c-PCBSD), is applied as an interfacial modification layer on the surface of zinc oxide and as the third component into the PBDB-TF:Y6-based OPV cells to enhance photovoltaic performance and long-term stability. The PCE of the OPV cells that underwent the two-step modification increased from 15.1 to 16.1%. In particular, such OPV cells exhibited much better stability under both thermal and air conditions because of the decreased number of interfacial defects and stable interfacial and active layer morphologies. The results demonstrated that the introduction of a cross-linkable fullerene derivative into the interfacial and active layers is a feasible method to improve the PCE and stability of OPV cells.

Recent Progress in Chlorinated Organic Photovoltaic Materials.

ConspectusOver the past few years, the development of new materials has contributed to rapid increases in the power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells to over 17%, showing great potential for the commercialization of this technology in the near future. At this stage, designing new materials with superior performance and low cost simultaneously is of crucial importance. Chlorinated materials are emerging as new stars with very high PCEs, creating a molecular design trend to replace the most popular fluorinated materials. For example, by using chlorinated non-fullerene acceptors, we recently got a record PCE of 17% for single-junction OPV cells. Firmly based on recent advances, herein we focus on the topic of chlorinated OPV materials, aiming to provide a guideline for further molecular design.In this Account, first, on the basis of most fundamental features of the Cl atom, we highlight the features of chlorinated materials compared with their fluorinated counterparts: (1) Chlorination is more efficient than fluorination in modulating the optical and electrical properties of OPV materials. In many cases, chlorinated materials show lower energy levels and broader absorption spectra than their fluorinated counterparts, which contribute higher output voltages and current densities in the resulting photovoltaic devices. (2) Cl has a large atomic size than F. On one hand, enhanced overlap of π electrons is beneficial for enhancing the intermolecular packing and crystalline property and thus improving the charge transport. On the other hand, if Cl is introduced inappropriately in the backbone or side chain, this feature will cause a more twisted π plane and larger steric hindrance, having negative impacts on the photovoltaic performance of the corresponding materials. (3) Importantly, chlorination is usually chemically cheaper in synthesis, which has the potential to decrease the material cost of OPV cells. Then, we provide a concise review of chlorinated OPV materials, including polymeric and small-molecule donors and non-fullerene acceptors. The photovoltaic performance in various types of OPV cells using chlorinated materials, such as single-junction, tandem, semitransparent, and indoor-light photovoltaic cells is also discussed. For instance, ultranarrow-band-gap chlorinated acceptors can be used to construct highly efficient color-semitransparent OPV cells, and the wide-band-gap chlorinated materials show great potential for fabricating indoor-light photovoltaic devices. Finally, we briefly discuss current questions related to chlorinated OPV materials and highlight the significance of chlorination in future development.