Interface Engineering for Highly Efficient Organic Solar Cells

Exploiting Bis-Sulfonimide Featuring Multiple d-pπ Bonds to Construct Interlayers for Organic Solar Cells

Herein, bis-sulfonimide (BSI), characterized by multiple d-pπ bonds rather than typical p-pπ bonds, is unprecedently utilized as a general and extendable building block to develop a series of multifunctional cathode interlayer materials (CIMs) for organic solar cells (OSCs). An illustrative CIM, BSIz-TT-PDI, demonstrates favorable alcohol processability, superior work function tunability, appropriate energy levels, strong self-doping effect, and decent crystallinity. These attributes contribute to its high conductivity exceeding 5×10-3 S/cm, as well as precise optimization of the interfacial connection between the active layer and metal cathode. Therefore, BSIz-TT-PDI-based OSCs delivers an outstanding efficiency of 18.08 % using PM6:Y6 active layer while retaining 84 % of its initial performance after tracking at the maximum power point under continuous illumination for 1100 hours. Additionally, the devices maintain over 94 % of the optimal performance across a film thickness range of BSIz-TT-PDI from 5 to 90 nm. Moreover, BSIz-TT-PDI exhibits high compatibility with various active layers, enabling a record efficiency of 19.80 % with the PM6:BTP-eC9:L8-BO active layer. This work not only introduces a new library of water/alcohol-soluble n-type semiconductors containing BSI, while also pioneers the creation of thickness-insensitive CIMs for stable and efficient OSCs by integrating electron-withdrawing components with d-pπ bonds.

Textured Inorganic Perovskite Interlayer Enhances Carrier Extraction for Organic Solar Cells

The PEDOT:PSS has been utilized extensively as a hole transport layer (HTL) in organic solar cells (OSCs) due to its excellent compatibility with various bulk heterojunction (BHJ) active layers. However, its intrinsically low electrical conductivity and suboptimal surface morphology limit hole extraction, ultimately constraining the performance of OSCs. To address this, we constructed an advanced heterojunction interface by introducing a wide-bandgap perovskite (CsPbBr3) interlayer between the PEDOT:PSS and BHJ. The textured CsPbBr3 interlayer serves as an efficient hole transport modifier by enhancing extraction and transport efficiency, while simultaneously functioning as an energy donor via Förster resonance energy transfer (FRET) and as a photosensitizer capable of generating photocarriers independently through its intrinsic optoelectronic properties. This synergetic enhancement of charge generation, extraction, and transport properties resulted in an increase in the power conversion efficiency (PCE) of PM6:Y6-based OSCs from 16.80% to 17.74%, along with improved photocurrent and fill factor (FF). The universality of this approach was further demonstrated in state-of-the-art PM6:BTP-eC9:L8-BO systems, achieving a PCE of 19.02%. Our work elucidates the multifunctional role of CsPbBr3 in managing interfacial properties, presenting a feasible interface engineering strategy to achieve high-performance OSCs.

Vertical Component Distributions in Organic Solar Cells Controlled by Photocrosslinking and Layer-by-Layer Deposition

The vertical component distribution is investigated in bulk-heterojunction (BHJ) type organic solar cells (OSCs) by combining photocrosslinking of donor polymers with layer-by-layer (LbL) deposition of acceptor molecules. Different concentrations of a tetradiazirine photocrosslinker controlled the crosslinker density of the polymer films, which in turn influenced the permeation behavior of acceptor molecules during LbL deposition. Time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), and grazing incidence wide-angle X-ray scattering (GIWAXS) analyses revealed the effect of crosslinker density on the vertical distribution of donor and acceptor materials. Increasing crosslinker density during LbL processing produces distinct bilayer-like structures, with each layer having different component ratios. OSC performance is optimized at lower crosslink densities with the uniformly mixed structure, while higher densities reduce the donor-acceptor interface, thereby decreasing power conversion efficiency from 12.6% (0.3 wt.%) to 4.48% (2.0 wt.%). These findings challenge the previous assumption that molecular permeation during LbL deposition naturally results in continuous component gradients or p-i-n structures, which are proposed as an advantage of the LbL method over traditional BHJ structures.

Organic Solar Cell with Efficiency of 20.49% Enabled by Solid Additive and Non-Halogenated Solvent

Recently, benzene-based solid additives (BSAs) have emerged as pivotal components in modulating the morphology of the blend film in organic solar cells (OSCs). However, since almost all substituents on BSAs are weak electron-withdrawing groups and contain halogen atoms, the study of BSAs with non-halogenated strong electron-withdrawing groups has received little attention. Herein, an additive strategy is proposed, involving the incorporation of non-halogenated strong electron-withdrawing groups on the benzene ring. An effective BSA, 4-nitro-benzonitrile (NBN), is selected to boost the efficiency of devices. The results demonstrate that the NBN-treated device exhibits enhanced light absorption, superior charge transport performance, mitigated charge recombination, and more optimal morphology compared to the additive-free OSC. Consequently, the D18:BTP-eC9+NBN-based binary device and D18:L8-BO:BTP-eC9+NBN-based ternary OSC processed by non-halogenated solvent achieved outstanding efficiencies of 20.22% and 20.49%, respectively. Furthermore, the universality of NBN is also confirmed in different active layer systems. In conclusion, this work demonstrates that the introduction of non-halogenated strong electron-absorbing moieties on the benzene ring is a promising approach to design BSAs, which can tune the film morphology and achieve highly efficient devices, and has certain guiding significance for the development of BSAs.

Dominant Face-On Oriented Perylene-Diimide Interlayers for High-Performance Organic Solar Cells

Electron transport properties of cathode interlayers are crucial to high-performance organic solar cells (OSCs). We propose a novel approach to enhance electron transport of cathode interlayers through controlling a preferential face-on molecular orientation of non-ionic perylene-diimide- (PDI) based cathode interlayers with restricted n-doping effects. 1-(2,5,8-trioxadec-10-yl)-1,2,3-triazole (TOT) units as bulky and extended side chains were incorporated into brominated-PDIs via click chemistry to yield PDIBr-TOT. TOT side chains impart PDI-based interlayers with a dominant face-on orientation, meanwhile leading to a negligible doping effect due to their weak electron-donating properties. Impressively, at a slight doping level, higher electron mobility is gained through efficient vertical charge transport channels built by preferred face-on molecular orientations of PDIBr-TOT, beating the results acquired through strong doping effects of traditional PDIBr-N with an edge-on orientation. Thus, PDIBr-TOT can suppress exciton recombination and lower the surface energies for good contact with active layers, consequently leading to increases in fill factor and short-circuit current. Integrating PDIBr-TOT with various active layers, a remarkable efficiency of 19.52 % is obtained. Moreover, device stability is enhanced by restrained doping effects. Modulating face-on orientations of cathode interlayers prescribed here will encourage further innovative designs of high-performance cathode interlayers towards OSC advances.

Progress and Developments in the Fabrication and Characterization of Metal Halide Perovskites for Photovoltaic Applications

Metal halide perovskites have emerged as a groundbreaking material class for photovoltaic applications, owing to their exceptional optoelectronic properties, tunable bandgap, and cost-effective fabrication processes. This review offers a comprehensive analysis of recent advancements in synthesis, structural engineering, and characterization of metal halide perovskites for efficient solar energy conversion. We explore a range of fabrication techniques, including solution processing, vapor deposition, and nanostructuring, emphasizing their impact on material stability, efficiency, and scalability. Additionally, we discuss key characterization methods, such as X-ray diffraction, electron microscopy, impedance spectroscopy, and optical analysis, that provide insights into the structural, electrical, and optical properties of these materials. Despite significant progress, challenges related to long-term stability, degradation mechanisms, and environmental sustainability persist. This review delves into current strategies for enhancing the durability and performance of perovskite-based photovoltaics and highlights emerging trends in device integration and commercialization. Finally, we provide future perspectives on optimizing material design and overcoming existing limitations to guide continued research in this rapidly advancing field.

Effects of Self-Assembled Polymer-Based Hole Transport Monolayer on Organic Photovoltaics

In this study, the first attempt is made to implement conjugated polymer-based self-assembled monolayer (SAM), poly[3-(6-carboxyhexyl) thiophene-2,5-diyl] (P3HT-COOH), is implemented as the hole transport layer (HTL) in fabricatiing organic photovoltaics (OPVs). The scanning tunneling microscopy (STM) results show that those P3HT-COOH molecules with periodic carboxylic acid anchoring groups pack periodically on the indium tin oxide (ITO) surface and form a monolayer. Further, this monolayer is smooth and dense with a polar feature that minimizes defects, forms an excellent interface with the photoactive layer, and tunes the work function of ITO beneficial for hole extraction. OPVs with this P3HT-COOH polar monolayer as HTL exhibit an improved exciton dissociation rate, enhanced polymer crystallinity of the photoactive layer with increased hole mobility for more balanced charge transport, reduced trap state density, and weaker bimolecular recombination with larger recombination resistance. The improved charge transport properties lead to a ≈9% increment in power conversion efficiency (PCE) of OPVs relative to those using well-known PEDOT:PSS as HTLs. Additionally, the hydrophobic feature of P3HT-COOH SAM stabilizes the OPVs with residual PCE of ≈80% even after 5.3 months. The proposed approach is very useful and timely and provides a key step for developing optoelectronic devices with high-efficiency, high stability, and cost-effective production.

Transparent and Conductive Polyimide-Ionene Hybrid Interlayers for High Performance and Cost-Effective Semitransparent Organic Solar Cells

The contradiction between high transmittance and favorable conductivity poses a great challenge in developing effective cathode interlayer (CIL) materials with sufficient thickness tolerance, which hinders the further advancement of organic solar cells (OSCs). Herein, a completely new class of alcohol processable polyimide-ionene hybrids (PIIHs) is proposed by melding pyromellitic diimide (PMD) subunits into imidazolium-based ionenes backbone covalently. These PIIHs, named PMD-DI and PMD-PD, boast high transparency, suitable energy levels, and decent conductivity. A higher PMD content endows PMD-PD with improved work function tunability, electrical properties, and crystallinity, enabling PMD-PD as CIL material with excellent thickness-insensitive characteristics, while simultaneously improving device stability significantly. Furthermore, PMD-PD also exhibits good compatibility with various electrodes and active layers, offering solar cell efficiencies of up to 19.91% and 19.29% with Ag and Cu cathodes, respectively. More importantly, the application of PMD-PD can improve the performance of semi-transparent OSCs without losing transmittance, thereby drastically enhancing the light utilization efficiency to 4.04% with an ultrathin, low-cost Cu cathode, that competes with leading optical modulation-free semitransparent OSCs with expensive Ag cathodes. This work opens a pathway to realize transparent and conductive interlayers by strategic molecular design, leading to highly efficient, stable, and cost-effective OSCs suitable for diverse applications.

Enhancing the photovoltaic properties of phenylsulfonyl carbazole-based materials by incorporating a thiophene ring and end-capped acceptors for organic solar cells: a DFT approach

In the present study, phenylsulfonyl carbazole-based organic chromophores, abbreviated as PSCD1-PSCD6, were designed through tailoring the terminal group of a PSCR chromophore. Quantum chemical studies were carried out using the M06/6-311G(d,p) functional to understand the electronic, structural, chemical, and optical properties of the title chromophores. All the derivatives exhibited reduced band gaps with ΔE = 2.742-3.025 eV and significant bathochromic shifts with λ max = 496.891-545.009 nm compared with PSCR. DOS and TDM investigations revealed that the central acceptor moiety plays a crucial role in charge transfer. The minimal binding energy values for PSCD1-PSCD6 indicated a greater rate of exciton dissociation and more effective charge transfer than PSCR. The studied compounds exhibited open-circuit voltages (V oc) ranging from 1.015 to 1.720 V. PSCD4 showed a significantly reduced band gap of 2.742 eV and a red-shifted absorption maximum of 545.009 nm, among all the studied chromophores. These findings suggest that all the designed organic chromophores might be utilized as reasonable photovoltaic materials.

An n-Doping Cross-Linkable Quinoidal Compound as an Electron Transport Material for Fully Stretchable Inverted Organic Solar Cells

The photocatalytic activity and inherent brittleness of ZnO, which is commonly used as an electron transport layer (ETL) in inverted organic solar cells (OSCs), have impeded advancements in device stability and the development of fully stretchable OSCs. In this study, an intrinsically stretchable ETL for inverted OSCs through a side-chain cross-linking strategy has been developed. Specifically, cross-linking between bromine atoms on the side chains of a quinoidal compound and the amino groups in polyethylenimine resulted in a film, designated QBr-PEI-50, with high electrical conductivity (0.049 S/m) and excellent stretchability (crack-onset strain>45 %). When used as the ETL in inverted OSCs, QBr-PEI-50 was markedly superior to ZnO in terms of device performance and stability, yielding a power conversion efficiency (PCE) of 18.27 % and a T80 lifetime exceeding 10000 h. Moreover, incorporation of QBr-PEI-50 in fully stretchable inverted OSCs yielded a PCE of 14.01 %, and 80 % of the initial PCE was maintained after 21 % strain, showcasing its potential for wearable electronics.

Highly Conductive Alcohol-Processable n-Type Conducting Polymer Enabled by Finely Tuned Electrostatic Interactions for Green Organic Electronics

Solution-processable conducting polymers open up a new era in organic electronics, fundamentally altering the processing methods of electronic devices. P-type conducting polymers, exemplified by aqueous solution-processed poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS), have been successfully commercialized. However, the performance of electron-transporting (n-type) materials remains considerably poorer. One of the primary challenges lies in striking a balance between conductivity and solvent processability. At present, most n-type conducting polymers necessitate toxic solvents for processing, which contradicts environmentally sustainable principles and impedes their potential for large-scale industrial applications. Herein, we developed an alcohol-processable high-performance n-type conducting polymer, poly(3,7-dihydrobenzo[1,2-b : 4,5-b']difuran-2,6-dione): poly(2-ethyl-2-oxazoline) (PBFDO : PEOx), which utilized electrostatic interactions between PEOx and PBFDO to simultaneously achieve high conductivity and alcohol-processability. The PBFDO : PEOx films exhibited remarkable electrical conductivity exceeding 1000 S cm-1 with outstanding stability even at temperatures up to 250 °C, establishing it as a prominent green solvent-processed n-type conducting polymer that rivals the most advanced p-type counterparts. Various applications including organic thermoelectric, electrochemical transistor, and electrochromic devices were showcased, highlighting the broad potential of PBFDO : PEOx in advancing green organic electronics.

High-Performance and Stable Perovskite/Organic Tandem Solar Cells Enabled by Interconnecting Layer Engineering

Perovskite/organic tandem solar cells (PO-TSCs) have recently attracted increasing attention due to their high efficiency and excellent stability. The interconnecting layer (ICL) is of great importance for the performance of PO-TSCs. The charge transport layer (CTL) and the charge recombination layer (CRL) that form the ICL should be carefully designed to enhance charge carrier extraction and promote charge carrier recombination balance from the two subcells. Here, we propose an effective strategy to optimize the ICL by using [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) to modify the poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) as the hole transport layer (HTL) in the ICL. It is found that the coverage state of 2PACz on the PEDOT:PSS significantly affects the performance of PO-TSCs and can be regulated by adjusting the concentration of the 2PACz solution. The PEDOT:PSS/2PACz structure facilitates effective charge carrier extraction from the organic solar cells to the CRL. Herein, for the PO-TSCs, this strategy results in an efficient and balanced charge carrier recombination in the ICL and also allows a thinner PEDOT:PSS with reduced parasitic absorption. As a result, the PO-TSC achieves a power conversion efficiency (PCE) of 25.26%, much higher than the control device (PCE of 23.57%), and better stability. This work demonstrates an effective approach to achieving high-performance PO-TSCs through ICL engineering.

Thienyltriazine Triamides: Thickness Insensitive Interlayer Materials Featuring Fine-Tuned Optoelectronic and Aggregation Characters for Efficient Organic Solar Cells

A novel class of thienyltriazine triamides (TTTAs) was facile synthesized and firstly used as cathode interlayers (CILs) for organic solar cells (OSCs). By utilizing different aromatic arms and pendant polar groups, their optoelectronic properties and aggregation behaviors were effectively modulated. The combination of thienyltriazine (TT) core, naphthylamide arm and imidazole pendant group endows TT-N-M with suitable energy levels, intensified work function tunability, higher conductivity, and well-balanced crystallinity and film-forming ability, boosting both the performance and stability of OSCs significantly. Remarkably, the solar cell efficiency remains stable at around 90 % of the optimal efficiency even as the interlayer thickness varied from 5 to 95 nm, demonstrating its insensitivity to thickness. Moreover, TT-N-M exhibits compatibility with various active layer systems, achieving a maximum efficiency of 19.60 % for single-junction solar cell. Its exceptional tolerance to thickness fluctuations and performance establishes a new benchmark for multi-armed CIL-based OSCs, also positioning them among the most high-performing CIL materials documented thus far. This work not only broadens the scope of CIL materials for OSCs but also offers deep insights into design strategies and structure-properties relationships, being beneficial for the future development of more efficient CIL materials for organic optoelectronic applications.

Progress in the Stability of Small Molecule Acceptor-Based Organic Solar Cells

Significant advancements in power conversion efficiency have been achieved in organic solar cells with small molecule acceptors. However, stability remains a primary challenge, impeding their widespread adoption in renewable energy applications. This review summarizes the degradation of different layers within the device structure in organic solar cells under varying conditions, including light, heat, moisture, and oxygen. For the photoactive layers, the chemical degradation pathways of polymer donors and small molecule acceptors are examined in detail, alongside the morphological stability of the bulk heterojunction structure, which plays a crucial role in device performance. The degradation mechanisms of commonly used anode and cathode interlayers and electrodes are addressed, as these layers significantly influence overall device efficiency and stability. Mitigation methods for the identified degradation mechanisms are provided in each section to offer practical insights for improving device longevity. Finally, an outlook presents the remaining challenges in achieving long-term stability, emphasizing research directions that require further investigation to enhance the reliability and performance of organic solar cells in real-world applications.

Designed Polar Cosolvent-Prepared Zinc Oxide Film for Efficient and Stable Inverted Organic Solar Cells

Here, a simple method of applying dimethylformamide (DMF) as cosolvent in the sol-gel technology is used to improve the quality of ZnO bulk films. First-principles calculations show that with the addition of polar solvent DMF, the adsorption energy (Eads) between the solvent and Zn(OH)₂ increases from -1.42 to -1.74 eV, which can stabilize the existence of Zn(OH)₂, thereby promoting the ZnO synthesis. Besides, the elimination of amine residues in the DMF-ZnO film significantly suppress the photocatalytic activity induced by amine-induced coordination or redox reactions. Inverted organic solar cells (OSCs) based on PM6:Y6 and PM6:BTP-eC9 achieves impressive power conversion efficiencies (PCE) of 17.58 and 18.14%, respectively. Furthermore, benefiting from the reduced defects of bulk ZnO, pseudo-bilayer bulk heterojunction (PBHJ) devices based on the optimized ZnO film exhibited superior stability, the PM6:Y6 devices based on DMF-ZnO ETLs can maintain 90.28% of their initial PCE after 1000 h of thermal aging at 85 °C, and 80.98% of their initial PCE after 168 h of UV aging. This simple solvent optimization strategy can significantly improve the charge transport of ZnO bulk films, making it a reliable strategy for the preparation of electron transport layers in OSCs.

Configurational Isomerization-Induced Orientation Switching: Non-Fused Ring Dipodal Phosphonic Acids as Hole-Extraction Layers for Efficient Organic Solar Cells

Phosphonic acid (PA) self-assembled molecules have recently emerged as efficient hole-extraction layers (HELs) for organic solar cells (OSCs). However, the structural effects of PAs on their self-assembly behaviors on indium tin oxide (ITO) and thus photovoltaic performance remain obscure. Herein, we present a novel class of PAs, namely "non-fused ring dipodal phosphonic acids" (NFR-DPAs), featuring simple and malleable non-fused ring backbones and dipodal phosphonic acid anchoring groups. The efficacy of configurational isomerism in modulating the photoelectronic properties and switching molecular orientation of PAs atop electrodes results in distinct substrate surface energy and electronic characteristics. The NFR-DPA with linear (C2h symmetry) and brominated backbone exhibits favorable face-on orientation and enhanced work function modification capability compared to its angular (C2v symmetry) and non-brominated counterparts. This makes it versatile HELs in mitigating interfacial resistance for energy barrier-free hole collection, and affording optimal active layer morphology, which results in an impressive efficiency of 19.11 % with a low voltage loss of 0.52 V for binary OSC devices and an excellent efficiency of 19.66 % for ternary OSC devices. This study presents a new dimension to design PA-based HELs for high-performance OSCs.

Non-Ionic Perylene-Diimide Polymer as Universal Cathode Interlayer for Conventional, Inverted, and Blade-Coated Organic Solar Cells

As a class of predominantly used cathode interlayers (CILs) in organic solar cells (OSCs), perylene-diimide (PDI)-based polymers exhibit intriguing characteristics of excellent charge transporting capacity and suitable energy levels. Despite that, PDI-based CILs with satisfied film-forming ability and adequate solvent resistance are rather rare, which not only limits the further advance of OSC performances but also hinders the practical use of PDI CILs. Herein, we designed and synthesized two non-conjugated PDI polymers for achieving high power conversion efficiency (PCE) in diverse types of OSCs. The utilization of oligo (ethylene glycol) (OEG) linkage enhanced the n-doping effect of PDI polymers, leading to an improved ability of the CIL to reduce work function and improve electron transporting capability. Moreover, the introduction of the non-ionic OEG chain effectively improve the wetting property and solvent resistance of PDI polymers, so the PPDINN CIL can withstand diverse processing conditions in fabricating different OSCs, including conventional, inverted and blade-coated devices. The binary OSC with conventional structure using PPDINN CIL showed a PCE of 18.6 %, along with an improved device stability. Besides, PPDINN is compatible with the large-area blade-coating technique, and a PCE of 16.6 % was achieved in the 1-cm2 OSC where a blade-coated PPDINN was used.

Magnetic Nanocomposite Modified Hybrid Hole-Transport Layer for Constructing Organic Solar Cells with High Efficiencies

An interface modification layer holds paramount significance in reducing interface carrier recombination and improving the ohmic contact between the active layer and the electrode in organic solar cells (OSCs). Modifying or doping the widely used hole-transport layer (HTL) PEDOT:PSS to adjust the work function, conductivity, and acidity has become a common strategy for achieving high-performance OSCs. Metal oxides and two-dimensional materials as secondary dopants into PEDOT:PSS, respectively, as well as a replacement of PEDOT:PSS both exhibit immense potential for achieving high-performance OSCs due to their excellent electrical properties. Herein, we report a method utilizing a Fe3O4/GO magnetic nanocomposite as a secondary dopant for PEDOT:PSS to modulate its inherent properties for constructing high-efficiency OSCs. The magnetic nanocomposite hybrid HTL exhibits a suitable optical transmittance and higher work function. Meanwhile, it is found that the addition of Fe3O4/GO magnetic nanoparticles expands the domain of PEDOT and enhances the phase separation between PEDOT and PSS segments, thereby improving the conductivity of PEDOT:PSS. By fine-tuning the doping ratio of a Fe3O4/GO magnetic nanocomposite in PEDOT:PSS, the best power conversion efficiency of OSCs based on PM6:L8-BO was up to 18.91%. The notable enhancement of the device's performance was due to the enhanced hole mobility and the improved charge extraction, further complemented by the decreased likelihood of interface recombination brought about by the hybrid HTL. Compared with PEDOT:PSS-based OSCs, an enhanced stability of the hybrid HTL-based device was also obtained. In addition, the diverse adaptability of the hybrid HTL was demonstrated in enhancing the performance of OSCs that are based on PM6:Y6 and PBDB-T:ITIC. The effectiveness and versatility of a magnetic nanocomposite hybrid HTL present opportunities for achieving high-performance OSCs.

Design Strategy for the Synthesis of Self-Doped n-Type Molecules

n-Type organic conductive molecules play a significant role in organic electronics. Self-doping can increase the carrier concentration within the materials to improve the conductivity without the need for additional intentional dopants. This review focuses on the various strategies employed in the synthesis of self-doped n-type molecules, and provides an overview of the doping mechanisms. By elucidating these mechanisms, the review aims to establish the relationship between molecular structure and electronic properties. Furthermore, the review outlines the current applications of self-doped n-type molecules in the field of organic electronics, highlighting their performance and potential in various devices. It also offers insights into the future development of self-doped materials.

Polyfluoride Acceptor with Limited Molecular Diffusion Enables Efficient and Stable Ternary Organic Solar Cells

Due to the slow diffusion of photovoltaic molecules, in particular, small-molecule acceptors (SMAs), under light and heating, the morphology of the active layer in organic solar cells (OSCs) prefers to deviate from the favorably metastable status, leading to the challenge of stability during long-term operation. Employing materials with a high glass transition temperature (Tg) as the third component to suppress molecular diffusion is an efficient method to achieve the balance of efficiency and stability of OSCs. Herein, a dimerized small-molecule acceptor denoted as F6D is synthesized by introducing a polyfluoride moiety as the linker to enhance the Tg. Benefitting from a rational molecular design, F6D not only exhibits a higher Tg, complementary absorption, and cascade energy levels with the host materials of the polymer donor PM6 and the SMA Y6 but also has excellent miscibility and multiple intermolecular interactions with Y6. As a result, a champion power conversion efficiency of 17.52% is achieved in the optimal PM6:Y6:F6D-based device. More importantly, the ternary device exhibits superior stability under continuous heating and lighting compared with the binary device.

Dual-Use Self-Assembled Monolayer Controlling Charge Carrier Extraction in Organic Solar Cells

The development and use of interface materials are essential to the continued advancement of organic solar cells (OSCs) performance. Self-assembled monolayer (SAM) materials have drawn attention because of their simple structure and affordable price. Due to their unique properties, they may be used in inverted devices as a modification layer for modifying ZnO or as a hole transport layer (HTL) in place of typical poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) in conventional devices. In this work, zinc oxide (ZnO) is modified using five structurally similar SAM materials. This resulted in a smoother surface, a decrease in work function, a suppression of charge recombination, and an increase in device efficiency and photostability. In addition, they can introduced asfor hole extraction layer between the active layer and MoO3, enabling the use of the same material at several functional layers in the same device. Through systematic orthogonal evaluation, it is shown that some SAM/active layer/SAM combinations still offered device efficiencies comparable to ZnO/SAM, but with improved device' photostability. This study may provide recommendations for future SAM material's design and development as well as a strategy for boosting device performance by using the same material across both sides of the photoactive layer in OSCs.

Electron Transporting Polymeric Materials with Partial Quaternization for High-Performance Organic Solar Cells

Efficient cathode interfacial layers (CILs) have become a crucial component of organic solar cells (OSCs). Charge extraction barriers, interfacial trap states, and significant transport resistance may be induced due to the unfavorable cathode interlayer, limiting the device performance. In this study, poly(4-vinylpyridine) is used as the CIL for OSCs, and a new type of CIL named P4VP-I is synthesized through the quaternization strategy. Compared to P4VP, P4VP-I CIL exhibits enhanced conductivity and optimized work function. OSCs employing the P4VP-I ETL demonstrate prolonged carrier lifetime, suppressed charge recombination, and achieve higher power conversion efficiencies (PCE) than the commonly used ETLs such as PFN-Br and Phen-NaDPO.

N-Type Small Molecule Electron Transport Layers with Excellent Surface Energy and Moisture Resistance Siloxane for Non-Fullerene Organic Solar Cells

Electron transport layers (ETLs) generally contain polar groups for enhancing performance and reducing the work function. Nevertheless, the polar group with high surface energy may cause inferior interfacial compatibility, which challenges the ETLs to balance stability and performance. Here, two conjugated small molecules of ETLs with low surface energy siloxane, namely PDI-Si and PDIN-Si, are synthesized. The siloxane with low surface energy not only enhances the interfacial compatibility between ETLs and active layers but also improves the moisture-proof stability of the device. Impressively, the amine-functionalized PDIN-Si can simultaneously exhibit conspicuous n-type self-doping properties and outstanding moisture-proof stability. The optimization of interfacial contact and morphology enables the PM6:Y6-based OSC with PDIN-Si to achieve a power conversion efficiency (PCE) of 15.87%, which is slightly superior to that of classical ETL PDINO devices (15.27%), and when the PDIN-Si film thickness reaches 28 nm, the PCE remains at 13.19% (≈83%), which indicates that PDIN-Si has satisfactory thickness insensitivity to facilitate roll-to-roll processing. Excitingly, after 120 h of storage in an environment with humidity above 45%, the unencapsulated device with PDIN-Si as ETL remains at 75% of the initial PCE value, while the device with PDINO as ETL is only 50%.

A Three-Dimensional Non-Fullerene Acceptor with Contorted Hexabenzocoronene and Perylenediimide for Organic Solar Cells

Although fullerene derivatives such as [6,6]-phenyl-C61/C71-butyric acid methyl ester (PC61BM/PC71BM) have dominated the the photoactive acceptor materials in bulk heterojunction organic solar cells (OSCs) for decades, they have several drawbacks such as weak absorption, limited structural tunability, prone to aggregation, and high costs of production. Constructing non-fullerene small molecules with three-dimensional (3D) molecular geometry is one of the strategies to replace fullerenes in OSCs. In this study, a 3D molecule, contorted hexa-cata-hexabenzocoronene tetra perylenediimide (HBC-4-PDI), was designed and synthesized. HBC-4-PDI shows a wide and strong light absorption in the whole UV-vis region as well as suitable energy levels as an acceptor for OSCs. More importantly, the 3D construction effectively reduced the self-aggregation of c-HBC, leading to an appropriate scale phase separation of the blend film morphology in OSCs. A preliminary power conversion efficiency of 2.70 % with a champion open-circuit voltage of 1.06 V was obtained in OSCs with HBC-4-PDI as the acceptor, which was the highest among the previously reported OSCs based on c-HBC derivatives. The results indicated that HBC-4-PDI may serve as a good non-fullerene acceptor for OSCs.

Chemisorption-Induced Robust and Homogeneous Tungsten Disulfide Interlayer Enables Stable PEDOT-Free Organic Solar Cells with Over 19% Efficiency

Construction of a high-quality charge transport layer (CTL) with intimate contact with the substrate via tailored interface engineering is crucial to increase the overall charge transfer kinetics and stability for a bulk-heterojunction (BHJ) organic solar cell (OSC). Here, we demonstrate a surface chemistry strategy to achieve a homogeneous composite hole transport layer (C-HTL) with robust substrate contact by self-assembling two-dimensional tungsten disulfide (WS2) nanosheets on a thin molybdenum oxide (MoO3) film-evaporated indium tin oxide (ITO) substrate. It is found that over such a well-defined C-HTL, WS2 is homogeneously tethered on the ITO/MoO3 substrate stemming from the strong electronic coupling interaction between the building blocks, which enables a favorable interfacial configuration in terms of uniformity. As a result, the D18:L8-BO-based OSC with C-HTL exhibits a power conversion efficiency (PCE) of 19.23%, an 11% improvement over the WS2-based control device, and the highest efficiency among single-junction PEDOT-free binary BHJ OSCs.

High-Performance All-Small-Molecule Organic Solar Cells Fabricated via Halogen-Free Preparation Process

Small-molecule organic photovoltaic materials attract more attention attributing to their precisely defined structure, ease of synthesis, and reduced batch-to-batch variations. The majority of all-small-molecule organic solar cells (ASM-OSCs) have traditionally relied on halogenated solvents for dissolving photovoltaic materials as well as used for the additives or solvent vapor annealing. However, these halogen-based processes pose risks to the environment and human health, potentially impeding future commercial production. Herein, we conducted an investigation into the impact of various nonhalogen solvents on the performance of the devices. By selecting the high boiling point solvent toluene, we achieved a desirable phase separation and stable morphology characterized by fibrous crystals within the blend film. Consequently, the power conversion efficiencies of 14.4 and 11.7% were obtained from H31:Y6-based small-area (0.04 cm2) and large-area (1 cm2) devices with steady performance, respectively. This study successfully demonstrated the fabrication of ASM-OSCs without employing halogenated solvent processes, thus offering promising prospects for the commercial production of ASM-OSCs.

Novel Polyelectrolytes Based on Naphthalene Diimide with Different Counteranions for Cathode Interlayers in Polymer Solar Cells

We synthesized novel polyelectrolytes based on naphthalene diimide with quaternary amine featuring hydroxyl groups at the side chain, along with different counteranions (PF-NDIN-Br-OH and PF-NDIN-I-OH) for polymer solar cell (PSC) application as the interlayer. The polyelectrolytes establish a beneficial interface dipole through the ionic moieties and synergistic effects arising from the hydroxyl groups located at the side chain. Incorporating polyelectrolytes as the cathode interlayer resulted in an enhancement of the power conversion efficiency (PCE). The PCE of the device with PF-NDIN-Br-OH increased from 8.96% to 9.51% compared to the ZnO-only device. The best PCE was obtained with the device based on PF-NDIN-I-OH, up to 9.59% resulting from the Jsc enhancement. This outcome implies a correlation between the performance of the device and the synergistic effects observed in polyelectrolytes containing hydroxyl groups in the side chain, along with larger anions when employed in PSCs.

Structure Influence of Amine-Containing Additives on the Solution State and Out-of-Plane Conductivity of PEDOT:PSS for Efficient Organic Solar Cells

Additives are extensively explored for improving PEDOT:PSS performances mainly through the removal of excess PSS and as a secondary dopant. In this work, amine-containing additives are introduced to PEDOT:PSS solutions as processing additives where the interactions to the PSS are anticipated through electrostatic interactions. Such interactions affected solution property where the increased viscosity is found to significantly increase the out-of-plane conductivity of the PEDOT:PSS thin films. Organic solar cells adopting these additive-assisted processed PEDOT:PSS layers as hole transporting layers (HTL) showed the improved device performances that resulted from the reduced series resistance provided by the PEDOT:PSS HTL. A top power conversion efficiency of 18.28% is achieved with para-phenylenediamine (PPD) additive in the PEDOT:PSS HTL, which is 3.5% higher compared to devices with neat PEDOT:PSS thin film as the HTL.

The central core size effect in quinoxaline-based non-fullerene acceptors for high VOC organic solar cells

For organic solar cells (OSCs), obtaining a high open circuit voltage (VOC) is often accompanied by the sacrifice of the circuit current density (JSC) and filling factor (FF), and it is difficult to strike a balance between VOC and JSC × FF. The trade-off of these parameters is often the critical factor limiting the improvement of the power conversion efficiency (PCE). Extended backbone conjugation and side chain engineering of non-fullerene acceptors (NFAs) are effective strategies to optimize the performance of OSCs. Herein, based on the quinoxaline central core and branched alkyl chains at the β position of the thiophene unit, we designed and synthesized three NFAs with different sized cores. Interestingly, Qx-BO-3 with a smaller central core showed better planarity and more appropriate crystallinity. As a result, PM6:Qx-BO-3-based devices obtained more suitable phase separation, more efficient exciton dissociation, and charge transport properties. Therefore, the OSCs based on PM6:Qx-BO-3 yielded an outstanding PCE of 17.03%, significantly higher than the devices based on PM6:Qx-BO-1 (10.57%) and PM6:Qx-BO-2 (11.34%) although the latter two devices have lower VOC losses. These results indicated that fine-tuning the central core size can effectively optimize the molecular geometry of NFAs and the film morphology of OSCs. This work provides an effective method for designing high-performance NFA-OSCs with high VOCs.

Robust and Sustainable Indium Anode Leading to Efficient and Stable Organic Solar Cells

The fast degradation of the charge-extraction interface at indium tin oxide (ITO) poses a significant obstacle to achieving long-term stability for organic solar cells (OSCs). Herein, a sustainable approach for recycling non-sustainable indium to construct efficient and stable OSCs and scale-up modules is developed. It is revealed that the recovered indium chloride (InCl3 ) from indium oxide waste can be applied as an effective hole-selective interfacial layer for the ITO electrode (noted as InCl3 -ITO anode) through simple aqueous fabrication, facilitating not only energy level alignment to photoactive blends but also mitigating parasitic absorption and charge recombination losses of the corresponding OSCs. As a result, OSCs and modules consisting of InCl3 -ITO anodes achieve remarkable power conversion efficiencies (PCEs) of 18.92% and 15.20% (active area of 18.73 cm2 ), respectively. More importantly, the InCl3 -ITO anode can significantly extend the thermal stability of derived OSCs, with an extrapolated T80 lifetime of ≈10 000 h.

A rare case of brominated small molecule acceptors for high-efficiency organic solar cells

Given that bromine possesses similar properties but extra merits of easily synthesizing and polarizing comparing to homomorphic fluorine and chlorine, it is quite surprising very rare high-performance brominated small molecule acceptors have been reported. This may be caused by undesirable film morphologies stemming from relatively larger steric hindrance and excessive crystallinity of bromides. To maximize the advantages of bromides while circumventing weaknesses, three acceptors (CH20, CH21 and CH22) are constructed with stepwise brominating on central units rather than conventional end groups, thus enhancing intermolecular packing, crystallinity and dielectric constant of them without damaging the favorable intermolecular packing through end groups. Consequently, PM6:CH22-based binary organic solar cells render the highest efficiency of 19.06% for brominated acceptors, more excitingly, a record-breaking efficiency of 15.70% when further thickening active layers to ~500 nm. By exhibiting such a rare high-performance brominated acceptor, our work highlights the great potential for achieving record-breaking organic solar cells through delicately brominating.

Influence of Traps and Lorentz Force on Charge Transport in Organic Semiconductors

Charge transport characteristics in organic semiconductor devices become altered in the presence of traps due to defects or impurities in the semiconductors. These traps can lead to a decrease in charge carrier mobility and an increase in recombination rates, thereby ultimately affecting the overall performance of the device. It is therefore important to understand and mitigate the impact of traps on organic semiconductor devices. In this contribution, the influence of the capture and release times of trap states, recombination rates, and the Lorentz force on the net charge of a low-mobility organic semiconductor was determined using the finite element method (FEM) and Hall effect method through numerical simulations. The findings suggest that increasing magnetic fields had a lesser impact on net charge at constant capture and release times of trap states. On the other hand, by increasing the capture time of trap states at a constant magnetic field and fixed release time, the net charge extracted from the semiconductor device increased with increasing capture time. Moreover, the net charge extracted from the semiconductor device was nearly four and eight times greater in the case of the non-Langevin recombination rates of 0.01 and 0.001, respectively, when compared to the Langevin rate. These results imply that the non-Langevin recombination rate can significantly enhance the performance of semiconductor devices, particularly in applications that require efficient charge extraction. These findings pave the way for the development of more efficient and cost-effective electronic devices with improved charge transport properties and higher power conversion efficiencies, thus further opening up new avenues for research and innovation in this area of modern semiconductor technology.

Cathode Interlayer Based on Naphthalene Diimide: A Modification Strategy for Zinc-Oxide-Free Inverted Organic Solar Cells

Perylene diimide with ammonium oxide as a terminal group (named PDIN-O) is a well-known cathode interlayer in conventional-type organic solar cells (OSCs). Since naphthalene diimide exhibits a lower LUMO level than perylene diimide, we chose it as a core to further control the LUMO level of the materials. Small molecules (SMs) produce a beneficial interfacial dipole by the end of ionic functionality at the side chain of naphthalene diimide. With the active layer based on a nonfullerene acceptor (PM6:Y6BO), the power conversion efficiency (PCE) is enhanced by utilizing SMs as cathode interlayers. We discovered that the inverted-type OSC with naphthalene diimide with oxide as a counteranion (NDIN-O) shows poor thermal stability, which can cause irreversible damage to the interlayer-cathode contact, leading to poor PCE (11.1%). To overcome the disadvantage, we introduce NDIN-Br and NDIN-I with a higher decomposition temperature. An excellent PCE of 14.6% was achieved with the device based on NDIN-Br as an interlayer, which is almost the same as the PCE of the ZnO-based device (15.0%). The device based on NDIN-I without the ZnO layer exhibits an improved PCE of 15.4%, which is slightly higher than the ZnO-based device. The result offers a replacement of the ZnO interlayer, which is necessary to carefully manage the sol-gel transition by annealing temperatures as high as 200 °C and leading to low-cost manufacture of OSCs.