High-Efficiency and Mechanically Robust All-Polymer Organic Photovoltaic Cells Enabled by Optimized Fibril Network Morphology

Control of molecular aggregation structures towards flexible organic photovoltaics

Flexible organic photovoltaics (OPVs) utilizing conjugated polymers have shown considerable promise in the field of wearable electronic devices. Although active-layer materials featuring extensive conjugated structures demonstrate good electron and optical properties, they often suffer from brittleness, which poses a significant challenge to the advancement of flexible OPVs. The aggregation structure of molecules within the active layer is pivotal in determining its mechanical properties, particularly its stretchability. Recently, researchers have employed a variety of strategies to manipulate the molecular aggregation structure within the active layer to enhance its tensile properties. This review first categorizes the aggregation structures of molecules across different scales, ranging from small to large (including molecular arrangement, chain entanglement, crystallization, phase separation, and semi-interpenetrating networks) and elucidates the mechanisms by which tensile performance can be improved. Subsequently, it summarizes the methodologies for regulating the molecular aggregation structures at various scales. Finally, the review discusses the ongoing development of flexible OPVs to provide valuable insights for researchers in the field.

Novel thiophene[3,4-b]thiophene-based polymer acceptors for high-performance all-polymer solar cells

This work developed three thiophene[3,4-b]thiophene-based polymer acceptors using a polymerized small-molecule acceptor strategy. The thiophene π-bridge integration and optimized side-chains enhanced the device performance. The PM6:PYF-EF binary devices achieved 17.07% PCE, which further reached 18.62% in the PM6:PY-IT:PYF-EF ternary blends.

Unexpected MoO3/Al Interfacial Reaction Lowering the Performance of Organic Solar Cells upon Thermal Annealing and Methods for Suppression

Understanding the degradation mechanism and improving the thermal stability of organic solar cells are essential for this new photovoltaic technology. In this work, we found that the high-performance polymer solar cells suffer from significant performance decay upon thermal annealing at 150 °C owing to the fast decay of VOC and FF. We demonstrated that the thermal annealing process leads to a severe chemical reaction of MoO3 with Al, forming an Al2O3 barrier layer at the MoO3/Al interface, which lowers the built-in potential (Vbi) of the cells and consequently reduces charge collection efficiency. Inserting a thin C60 interlayer between MoO3/Al slows the chemical reaction of MoO3 with Al, which ensures a high Vbi and charge collection efficiency for the annealed MoO3/C60/Al cells. Such a protection effect of the C60 layer in improving device performance against thermal annealing was also confirmed for cells with different polymer photoactive layers and metal electrodes, demonstrating the generality of the interfacial degradation of the cells and the protection effect of the C60 layer. Finally, we demonstrated that the inverted polymer solar cells with the C60-modified anode showed almost no performance decay upon high-temperature hot-press encapsulation, demonstrating excellent heat tolerance of this new device structure.

High-Speed Slot-Die Coating with Donor-Priority Rapid Aggregation Kinetics for Improved Morphology and Efficiency in Ecofriendly Organic Solar Cells

Solution-processable organic solar cells (OSCs) represent a promising renewable photovoltaic technology with significant potential for eco-compatible production. While high power conversion efficiencies (PCEs) have been achieved in OSCs, scaling this technology for high-throughput manufacturing remains challenging. Key reason lies in the lack of efficient control strategies for the complex and long-duration morphology evolution during high-speed coating process with ecofriendly solvents. Here, a donor-priority rapid aggregation process (DP-RAP) scheme is proposed to solve this issue by adjusting the aggregation kinetics of donor and acceptor components. DP-RAP enables blends with a nanoscale fiber network structure and favorable crystallinity, which contributes to balanced carrier transport and reduced recombination losses. As a result, the PCE is improved from 14.3% (reference) to 17.4% (DP-RAP) for ultra-high speed coated PM6:BTP-eC9 devices in atmosphere, which is one of the highest values for non-halogenated solvent-processed solar cells at coating speeds of 500 mm s-1. Moreover, the DP-RAP based devices remain a stable PCE of approximately 17.4% across a broad range of coating speeds (20-500 mm s-1), illustrating its tolerance to the varied manufacturing conditions. This work highlights a promising avenue for the high-speed, ecofriendly production of efficient OSCs, pushing the boundaries of practical manufacturing in renewable energy technologies.

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.

Tuning Active Layer Morphology via Ternary Copolymerization with an Asymmetric Benzothiazole Unit as the Third Component to Enhance Photovoltaic Performance

Controlling the morphology and phase separation of the active layer is of great significance for the development of efficient organic solar cells. This study employs ternary copolymerization to optimize the phase separation and surface morphology of the active layer. Three terpolymers (PMz-5, PMz-10, and PMz-20) are synthesized by incorporating an asymmetric benzo[d]thiazole (BTz) unit as the third component to suppress the strong aggregation of the PM6:L8-BO blend film. Compared to the PM6 blend film, terpolymer blend films exhibit improved surface morphology, an appropriate phase separation scale, and, thus, an enhanced device performance. Furthermore, the incorporation of a BTz unit can lower the highest occupied molecular orbital levels of three terpolymers, which is conducive to charge transport. The champion device PMz-5:L8-BO system achieves an efficiency of 16.57%, featuring a high open-circuit voltage of 0.886 V, a high short-circuit current density of 25.49 mA cm-2, and a remarkable fill factor of 73.35%. This work provides an efficient and straightforward ternary copolymerization strategy that can be used to enhance the device performance.

All-Polymer Bulk-Heterojunction Enables Stable Monolithic Perovskite/Organic Tandem Solar Cells with High Efficiency

Perovskite-based tandem solar cells (PTSCs) are promising for achieving higher efficiency limits, making them promising candidates for energy supply. However, the commercialization in complex scenarios necessitate extreme stability and reliability of tandem devices, particularly in ambient conditions. Herein, the use of a high-efficiency and air-stable quaternary all-polymer bulk heterojunction (BHJ) is pioneered to optimize spectral absorption, facilitate charge transport, and suppress exciton recombination, resulting in 18.0% of power conversion efficiency (PCE) in the organic subcell. The resultant monolithic perovskite/organic tandem solar cell (POTSC) delivers an impressive PCE of 24.8%, with minimal efficiency distribution and negligible hysteresis. Ambient stability tests on tandem devices reveal  outstanding ambient stability, which is attributed to the reduced increase in exciton recombination. Remarkably, the unencapsulated tandem device maintained 88% of its initial efficiency after exposure to air for 500 h. The superior stability is owing to the enhanced resistance of the hydrophobic all-polymer BHJ to water and oxygen, thereby protecting the perovskite active layer. This work provides a novel approach from an organic perspective for achieving superior efficiency and stability in POTSC devices and holds promise for future real-world applications in the field of tandem solar cells.

Optimizing Branching Linkers in Dimerized Acceptors for Enhanced Efficiency and Stability in Organic Solar Cells

Most high-performing dimerized acceptors are based on Y-series precursors with superior conjugated π-backbones. The utilization of branch-connected dimerized acceptors can fully leverage the four end groups to enhance molecular packing, thereby potentially improving both the stability of organic solar cells (OSCs) while maintaining high power conversion efficiency (PCE). Therefore, optimizing the linker is critical to fully realizing their potential in improving device performance. In this study, three dimerized acceptors are synthesized with conjugated and conjugation-break linkers in the branching direction to systematically investigate the effects of different linker structures on molecular properties and device performance. By introducing an appropriate flexible chain, favorable solubility, and superior morphology are achieved, which facilitates charge generation and transport while suppressing recombination. As a result, the OSC based on dYTAT-C6-F exhibits a significantly improved PCE of 18.08%, the highest among dimerized acceptors with linkers in the branching direction. Additionally, the OSC based on dYTAT-C6-F demonstrates a T80 lifetime of 1840 h. These results indicate that conjugation breakages can tune molecular solubility, aggregation, and carrier mobility and that optimizing the linker length further improves these characteristics. The findings highlight the significant potential of engineering linkers in the branching direction to achieve outstanding OSC performance.

Reducing Non-Radiative Energy Losses in Non-Fullerene Organic Solar Cells

With the rapid advancement of non-fullerene acceptors (NFAs), the power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed the 20 % threshold, highlighting their considerable potential as next-generation energy conversion devices. In comparison to inorganic or perovskite solar cells, the open-circuit voltage (Voc) of OSCs is constrained by substantial non-radiative energy losses (ΔEnr), leading to values notably below those anticipated by the Shockley-Queisser limit. In OSCs, non-radiative energy losses are intimately associated with the electroluminescent quantum efficiency (EQEEL) of charge transfer states, which is in turn directly affected by the photoluminescence quantum yield (PLQY) of acceptor materials. Consequently, enhancing the PLQY of low-bandgap acceptor materials has emerged as a pivotal strategy to effectively mitigate ΔEnr. This review article delves into the intrinsic correlation between molecular structure and PLQY from the vantage point of acceptor material design. It further explores methodologies for designing acceptor materials exhibiting high PLQY, with the ultimate goal of realizing OSCs that combine high efficiency with minimal ΔEnr.

Advancing High-Performance Organic Solar Cells with Carbazole-Modified 2PACz for Scalable Large-Area Fabrication

The self-assembling molecule 2PACz tends to aggregate in thin films, which negatively impacts the performance of organic solar cells (OSCs) when used as a hole-transporting layer (HTL), particularly in large-area devices. To overcome this, a binary conjugated molecular system incorporating carbazole (Cz), which shares a similar backbone with 2PACz, is introduced. Despite the strong aggregation tendencies of 2PACz and Cz individually, their blend forms homogeneous films due to hydrogen bonding interactions between the two molecules. These interactions suppress 2PACz aggregation, resulting in smooth and well-ordered films. Devices with the modified HTL show significantly enhanced charge transfer, achieving a power conversion efficiency (PCE) of 20.10%, a fill factor of 80.3%, and a short-circuit current of 28.98 mA cm- 2, outperforming those with unmodified 2PACz. Large-area devices (1.0 cm2) with the modified HTL achieve a record-high PCE of 18.56% and a retention rate of 92.7%, compared to 43% for devices with 2PACz. These findings highlight the potential of carbazole-modified 2PACz to improve both efficiency and stability in OSCs, offering a promising strategy for high-performance HTL development.

Exploring the Origin of High Thermal Stability of the Performance of Pseudo-Quaternary All-Polymer Solar Cells

As all-polymer solar cells (all-PSCs) have achieved impressive power conversion efficiencies (PCEs), extending their lifetime under long-term operation is also increasingly important. To address this issue, in this study, a new pseudo-quaternary blend composed of conjugated block copolymer donors and acceptors, PM6-b-TT:b-PYT, is introduced as the active layer for all-PSCs. Compared to the all-PSC based on the traditional binary blend, PM6:BTTP-T, those based on pseudo-quaternary active layer exhibited significantly improved thermal stability after thermal annealing under harsh conditions of 150 °C in an ambient atmosphere. More importantly, to elucidate the morphological stability of the pseudo-quaternary active layer, visible evidence of the thin film's surface and internal structure is carefully investigated by multiple advanced techniques. After extended thermal stress at 150 °C, the binary bulk heterojunction (BHJ) films exhibit excessive polymer chain aggregation, phase separation of the polymers, and increased surface roughness, forming bulk charge traps and increasing the exciton recombination. Meanwhile, the pseudo-quaternary BHJ films maintain the crystallinity and nanostructure of the active layer, improving the stability of the all-PSCs. Overall, this study provides a detailed understanding of the long-term stability of high-efficiency all-PSCs, offering key insights into the polymer section and proposing promising polymer structures for the long-term stability of all-PSCs.

Synergistic Multimodal Energy Dissipation Enhances Certified Efficiency of Flexible Organic Photovoltaics beyond 19

All-polymer organic solar cells (OSCs) have shown unparalleled application potential in the field of flexible wearable electronics in recent years due to the excellent mechanical and photovoltaic properties. However, the small molecule acceptors after polymerization in still retain some mechanical and aggregation properties of the small molecule, falling short of the ductility requirements for flexible devices. Here, based on the multimodal energy dissipation theory, the mechanical and photovoltaic properties of flexible devices are co-enhanced by adding the thermoplastic elastomer material (polyurethane, PU) to the PM6:PBQx-TF:PY-IT-based active layer films. The construction of multi-fiber network structure and the decrease of films' residual stresses contribute to the enhancement of carrier transport properties and the decrease of defect state density. Eventually, the PCE (power conversion efficiency) of 19.40% is achieved on the flexible devices with an effective area of 0.102 cm2, and the third-party certified PCE reaches 19.07%, which is the highest PCE for flexible OSCs currently available. To further validate the potential of this strategy for large-area module applications, the 25-cm2-based flexible and super-flexible modules are prepared with the PCEs of 15.48% and 14.61%, respectively, and demonstration applications are implemented.

Synthesis and Characterization of Copolymers with Fluorene-di-2-thienyl-2,1,3-benzothiadiazole Units for Application in Optoelectronic Devices

Conjugated donor-acceptor (D-A) copolymers are widely used in optoelectronic devices due to their influence on the resulting properties. This study focuses on the synthesis and characterization of the conjugated D-A copolymer constructed with fluorene and di-2-thienyl-2,1,3-benzothiadiazole units, resulting in Poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4,7-di(2-thienyl)-2,1,3-benzothiadiazole)] (PFDTBT). The synthesis associated with reaction times of 48 and 24 h, the latter incorporating the phase-transfer catalyst Aliquat 336, was investigated. The modified conditions produced copolymers with higher molar masses (Mw > 20,000 g/mol), improved thermal stability and red emission at 649 nm. Furthermore, the resulting D-A copolymers exhibited uniform morphology with low surface roughness (P2-Ra: 0.77 nm). These improved properties highlight the potential of D-A copolymers based on PFDTBT for various optoelectronic applications, including photovoltaics, light-emitting devices, transistors and biological markers in the form of quantum dots.

Boosting organic solar cell efficiency via tailored end-group modifications of novel non-fused ring electron acceptors

In this study, we designed and synthesized two NFREAs, 2BTh-3F and 2BTh-CN, incorporating distinct substituents to modulate their electron-withdrawing properties. We meticulously explore the distinct impacts of these substituents on NFREA performance. Our investigation revealed that the introduction of 3,5-difluoro-4-cyanophenyl in 2BTh-CN significantly enhanced electron withdrawal and intramolecular charge transfer, leading to a red-shifted absorption spectrum and optimized energy levels. Consequently, organic solar cells (OSCs) utilizing 2BTh-CN demonstrate a notable power conversion efficiency (PCE) of 15.07%, outperforming those employing 2BTh-3F (PCE of 9.34%). Moreover, by incorporating 2BTh-CN into the D18:2BTh-C2 system as a third component, we achieve a PCE exceeding 17% in a high-performing ternary OSC, ranking among the most efficient NFREA-based OSCs reported to date. Overall, our study underscores the potential of deliberate design and optimization of non-fused ring acceptor molecular structures to attain outstanding photovoltaic performance.

Constructing Multiscale Fibrous Morphology to Achieve 20% Efficiency Organic Solar Cells by Mixing High and Low Molecular Weight D18

This study underscores the significance of precisely manipulating the morphology of the active layer in organic solar cells (OSCs). By blending polymer donors of D18 with varying molecular weights, a multiscale interpenetrating fiber network structure within the active layer is successfully created. The introduction of 10% low molecular weight D18 (LW-D18) into high molecular weight D18 (HW-D18) produces MIX-D18, which exhibits an extended exciton diffusion distance and orderly molecular stacking. Devices utilizing MIX-D18 demonstrate superior electron and hole transport, improves exciton dissociation, enhances charge collection efficiency, and reduces trap-assisted recombination compared to the other two materials. Through the use of the nonfullerene acceptor L8-BO, a remarkable power conversion efficiency (PCE) of 20.0% is achieved. This methodology, which integrates the favorable attributes of high and low molecular weight polymers, opens a new avenue for enhancing the performance of OSCs.

Approaching 20% Efficiency in Ortho-Xylene Processed Organic Solar Cells by a Benzo[a]phenazine-Core-Based 3D Network Acceptor with Large Electronic Coupling and Long Exciton Diffusion Length

High-performance organic solar cells often rely on halogen-containing solvents, which restrict the photovoltaic industry. Therefore, it is imperative to develop efficient organic photovoltaic materials compatible with halogen-free solvents. Herein, a series of benzo[a]phenazine (BP)-core-based small-molecule acceptors (SMAs) achieved through an isomerization chlorination strategy is presented, comprising unchlorinated NA1, 10-chlorine substituted NA2, 8-chlorine substituted NA3, and 7-chlorine substituted NA4. Theoretical simulations highlight NA3's superior orbit overlap length and tight molecular packing, attributed to interactions between the end group and BP unit. Furthermore, NA3 demonstrates dense 3D network structures and a record electronic coupling of 104.5 meV. These characteristics empower the ortho-xylene (o-XY) processed PM6:NA3 device with superior power conversion efficiency (PCE) of 18.94%, surpassing PM6:NA1 (15.34%), PM6:NA2 (7.18%), and PM6:NA4 (16.02%). Notably, the significantly lower PCE in the PM6:NA2 device is attributed to excessive self-aggregation characteristics of NA2 in o-XY. Importantly, the incorporation of D18-Cl into the PM6:NA3 binary blend enhances crystallographic ordering and increases the exciton diffusion length of the donor phase, resulting in a ternary device efficiency of 19.75% (certified as 19.39%). These findings underscore the significance of incorporating new electron-deficient units in the design of efficient SMAs tailored for environmentally benign solvent processing of OSCs.

Unraveling the Solution Aggregation Structures and Processing Resiliency of High-Efficiency Organic Photovoltaic Blends

The solution aggregation structure of conjugated polymers is crucial to the morphology and resultant optoelectronic properties of organic electronics and is of considerable interest in the field. Precise characterizations of the solution aggregation structures of organic photovoltaic (OPV) blends and their temperature-dependent variations remain challenging. In this work, the temperature-dependent solution aggregation structures of three representative high-efficiency OPV blends using small-angle X-ray/neutron scattering are systematically probed. Three cases of solution processing resiliency are elucidated in state-of-the-art OPV blends. The exceptional processing resiliency of high-efficiency PBQx-TF blends can be attributed to the minimal changes in the multiscale solution aggregation structure at elevated temperatures. Importantly, a new parameter, the percentage of acceptors distributed within polymer aggregates (Ф), for the first time in OPV blend solution, establishes a direct correlation between Ф and performance is quantified. The device performance is well correlated with the Kuhn length of the cylinder related to polymer aggregates L1 at the small scale and the Ф at the large scale. Optimal device performance is achieved with L1 at ≈30 nm and Ф within the range of 60 ± 5%. This study represents a significant advancement in the aggregation structure research of organic electronics.

Efficient and Stable All-Polymer Solar Cells Enabled by Dual Working Mechanism

Ternary strategy with integration characteristics and adaptability is a simple and effective method for blooming of the performance of photovoltaic devices. Herein, a novel wideband gap polymer donor PBB2-Hs is synthesized as the guest component to optimize all-polymer solar cells (all-PSCs). High-energy photon absorption and long exciton lifetime of PBB2-Hs constitute efficient energy transfer. Good miscibility and cascade energy levels promote the formation of alloy-like structure between PBB2-Hs and host system. The dual working mechanisms greatly improve photon capture and charge transfer in active layers. Additionally, the introduction of PBB2-Hs also optimizes the ordered molecular stacking of acceptors and suppresses molecular peristalsis. Upon adding 15 wt% PBB2-Hs, the ternary all-PSC achieved a champion efficiency of 17.66%, and can still maintain 82% photostability (24 h) and 91% storage stability (1000 h) of the original PCE. Moreover, the strong molecular stacking and entanglement between PBB2-Hs and the host material increased the elongation at break of ternary blend film by 1.6 times (16.2%), allowing the flexible device to maintain 83% of the original efficiency after 800 bends (R = 5 mm). This work highlights the effectiveness of guest polymer on simultaneously improving photovoltaic performance, photostability and mechanical stability in all-PSCs.

Dimerized Nonfused Electron Acceptor Based on a Thieno[3,4-c]pyrrole-4,6-dione Core for Organic Solar Cells

In this work, the first dimerized nonfused electron acceptor (NFEA), based on thieno[3,4-c]pyrrole-4,6-dione as the core, has been designed and synthesized. The dimerized acceptor and its single counterpart exhibit similar energy levels but different absorption spectra due to their distinct aggregation behavior. The dimerized acceptor-based organic solar cells (OSCs) demonstrate a higher power conversion efficiency of 11.05%, accompanied by enhanced thermal stability. This improvement is attributed to the enhancement of the short-circuit current density and fill factor, along with an increase in the glass transition temperature. Characterizations of exciton dynamics and film morphology reveal that a dimerized acceptor-based device possesses an enhanced exciton dissociation efficiency and a well-established charge transport pathway, explaining its improved photovoltaic performance. All these results indicate that the dimerized NFEA as a promising candidate can achieve efficiency-stability-cost balance in OSCs.

Recent Progress in Semitransparent Organic Solar Cells: Photoabsorbent Materials and Design Strategies

The increasing energy demands of the global community can be met with solar energy. Solution-processed organic solar cells have seen great progress in power conversion efficiencies (PCEs). Semitransparent organic solar cells (ST-OSCs) have made enormous progress in recent years and have been considered one of the most promising solar cell technologies for applications in building-integrated windows, agricultural greenhouses, and wearable energy resources. Therefore, through the synergistic efforts of transparent electrodes, engineering in near-infrared photoabsorbent materials, and device engineering, high-performance ST-OSCs have developed, and PCE and average visible transmittance reach over 10% and 40%, respectively. In this review, we present the recent progress in photoabsorbent material engineering and strategies for enhancing the performance of ST-OSCs to help researchers gain a better understanding of structure-property-performance relationships. To conclude, new design concepts in material engineering and outlook are proposed to facilitate the further development of high-performance ST-OSCs.

Tunable Donor Aggregation Dominance in a Ternary Matrix of All-Polymer Blends with Improved Efficiency and Stability

Using two structurally similar polymer acceptors in constructing high-efficiency ternary all-polymer solar cells is a widely acknowledged strategy; however, the focus thus far has not been on how polymer acceptor(s) would tune the aggregation of polymer donors, and furthermore film morphology and device performance (efficiency and stability). Herein, it is reported that matching of the celebrity acceptor PY-IT and the donor PBQx-TCl results in enhanced H-aggregation in PBQx-TCl, which can be finely tuned by controlling the amount of the second acceptor PY-IV. Consequently, the efficiency-optimized PY-IV weight ratio (0.2/1.2) leads to a state-of-the-art power conversion efficiency of 18.81%, wherein light-illuminated operational stability is also enhanced along with well-protected thermal stability. Such enhancements in the efficiency and operational and thermal stabilities of solar cells can be attributed to morphology optimization and the desired glass transition temperature of the target active layer based on comprehensive characterization. In addition to being a high-power conversion efficiency case for all-polymer solar cells, these enhancements are also a successful attempt for using combined acceptors to tune donor aggregation toward optimal morphology, which provides a theoretical basis for the construction of other types of organic photovoltaics beyond all-polymer solar cells.

Advancements in Photovoltaic Cell Materials: Silicon, Organic, and Perovskite Solar Cells

The evolution of photovoltaic cells is intrinsically linked to advancements in the materials from which they are fabricated. This review paper provides an in-depth analysis of the latest developments in silicon-based, organic, and perovskite solar cells, which are at the forefront of photovoltaic research. We scrutinize the unique characteristics, advantages, and limitations of each material class, emphasizing their contributions to efficiency, stability, and commercial viability. Silicon-based cells are explored for their enduring relevance and recent innovations in crystalline structures. Organic photovoltaic cells are examined for their flexibility and potential for low-cost production, while perovskites are highlighted for their remarkable efficiency gains and ease of fabrication. The paper also addresses the challenges of material stability, scalability, and environmental impact, offering a balanced perspective on the current state and future potential of these material technologies.

The role of interfacial donor-acceptor percolation in efficient and stable all-polymer solar cells

Polymerization of Y6-type acceptor molecules leads to bulk-heterojunction organic solar cells with both high power-conversion efficiency and device stability, but the underlying mechanism remains unclear. Here we show that the exciton recombination dynamics of polymerized Y6-type acceptors (Y6-PAs) strongly depends on the degree of aggregation. While the fast exciton recombination rate in aggregated Y6-PA competes with electron-hole separation at the donor-acceptor (D-A) interface, the much-suppressed exciton recombination rate in dispersed Y6-PA is sufficient to allow efficient free charge generation. Indeed, our experimental results and theoretical simulations reveal that Y6-PAs have larger miscibility with the donor polymer than Y6-type small molecular acceptors, leading to D-A percolation that effectively prevents the formation of Y6-PA aggregates at the interface. Besides enabling high charge generation efficiency, the interfacial D-A percolation also improves the thermodynamic stability of the blend morphology, as evident by the reduced device "burn-in" loss upon solar illumination.

Regulating Phase Separation Kinetics for High-Efficiency and Mechanically Robust All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) possess excellent operation stability and mechanical robustness than other types of organic solar cells, thereby attracting considerable attention for wearable flexible electron devices. However, the power conversion efficiencies (PCEs) of all-PSCs are still lagging behind those of small-molecule-acceptor-based systems owing to the limitation of photoactive materials and unsatisfactory blend morphology. In this work, a novel terpolymer, denoted as PBDB-TFCl (poly4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b″]dithiophene-1,3-bis(2-ethylhexyl)-5,7-di(thiophen-2-yl)-4H,8H-benzo[1,2-c:4,5-c″]dithiophene-4,8-dione-4,8-bis(4-chloro-5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene), is used as an electron donor coupled with a ternary strategy to optimize the performance of all-PSCs. The addition of PBDB-TCl unit deepens the highest occupied molecular orbital energy level, reducing voltage losses. Moreover, the introduction of the guest donor (D18-Cl) effectively regulates the phase-transition kinetics of PBDB-TFCl:D18-Cl:PY-IT during the film formation, leading to ideal size of aggregations and enhanced crystallinity. PBDB-TFCl:D18-Cl:PY-IT devices exhibit a PCE of 18.6% (certified as 18.3%), judged as the highest value so far obtained with all-PSCs. Besides, based on the ternary active layer, the manufactured 36 cm2 flexible modules exhibit a PCE of 15.1%. Meanwhile, the ternary PSCs exhibit superior photostability and mechanical stability. In summary, the proposed strategy, based on molecular design and the ternary strategy, allows optimization of the all-polymer blend morphology and improvement of the photovoltaic performance for stable large-scale flexible PSCs.

18.6% Efficiency All-Polymer Solar Cells Enabled by a Wide Bandgap Polymer Donor Based on Benzo[1,2-d:4,5-d']bisthiazole

The limited selection of wide bandgap polymer donors for all-polymer solar cells (all-PSCs) is a bottleneck problem restricting their further development and remains poorly studied. Herein, a new wide bandgap polymer, namely PBBTz-Cl, is designed and synthesized by bridging the benzobisthiazole acceptor block and chlorinated benzodithiophene donor block with thiophene units for application as an electron donor in all-PSCs. PBBTz-Cl not only possesses wide bandgap and deep energy levels but also displays strong absorption, high-planar structure, and good crystallinity, making it a promising candidate for application as a polymer donor in organic solar cells. When paired with the narrow bandgap polymer acceptor PY-IT, a fibril-like morphology forms, which facilitates exciton dissociation and charge transport, contributing to a power conversion efficiency (PCE) of 17.15% of the corresponding all-PSCs. Moreover, when introducing another crystalline polymer acceptor BTP-2T2F into the PBBTz-Cl:PY-IT host blend, the absorption ditch in the range of 600-750 nm is filled, and the blend morphology is further optimized with the trap density reducing. As a result, the ternary blend all-PSCs achieve a significantly improved PCE of 18.60%, which is among the highest values for all-PSCs to date.

Delayed Crystallization Kinetics Allowing High-Efficiency All-Polymer Photovoltaics with Superior Upscaled Manufacturing

Though encouraging performance is achieved in small-area organic photovoltaics (OPVs), reducing efficiency loss when evoluted to large-area modules is an important but unsolved issue. Considering that polymer materials show benefits in film-forming processability and mechanical robustness, a high-efficiency all-polymer OPV module is demonstrated in this work. First, a ternary blend consisting of two polymer donors, PM6 and PBQx-TCl, and one polymer acceptor, PY-IT, is developed, with which triplet state recombination is suppressed for a reduced energy loss, thus allowing a higher voltage; and donor-acceptor miscibility is compromised for enhanced charge transport, thus resulting in improved photocurrent and fill factor; all these contribute to a champion efficiency of 19% for all-polymer OPVs. Second, the delayed crystallization kinetics from solution to film solidification is achieved that gives a longer operation time window for optimized blend morphology in large-area module, thus relieving the loss of fill factor and allowing a record efficiency of 16.26% on an upscaled module with an area of 19.3 cm2 . Besides, this all-polymer system also shows excellent mechanical stability. This work demonstrates that all-polymer ternary systems are capable of solving the upscaled manufacturing issue, thereby enabling high-efficiency OPV modules.

19.32% Efficiency Polymer Solar Cells Enabled by Fine-Tuning Stacking Modes of Y-Type Molecule Acceptors: Synergistic Bromine and Fluorine Substitution of the End Groups

The success of Y6-type nonfullerene small molecule acceptors (NF-SMAs) in polymer solar cells (PSCs) can be attributed to their unique honeycomb stacking style, which leads to favorable thin-film morphologies. The intermolecular interactions related to the crystallization tendency of these NF-SMAs is closely governed by their electron accepting end groups. For example, the high performance Y6 derivative L8-BO (BTP-4F) presents three types of stacking modes in contrast to two stacking modes of Y6. Hence, it is ultimately interesting to obtain more insight on the packing properties and the preferences influenced by chemical modifications such as end group engineering. This work designs and synthesizes asymmetric and symmetric L8-BO derivatives with brominated end groups and explores the stacking preferences in various modes. The asymmetric BTP-3FBr displays an optimized crystallization tendency and thin film morphology, leading to a decent power conversion efficiency (PCE) of 18.34% in binary devices and a top PCE of 19.32% in ternary devices containing 15 wt% IDIC as the second acceptor.

Constructing High-Performance Ternary Device Using Analogous Polymer Donors

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

Advances in organic photovoltaic cells: a comprehensive review of materials, technologies, and performance

This paper provides a comprehensive overview of organic photovoltaic (OPV) cells, including their materials, technologies, and performance. In this context, the historical evolution of PV cell technology is explored, and the classification of PV production technologies is presented, along with a comparative analysis of first, second, and third-generation solar cells. A classification and comparison of PV cells based on materials used is also provided. The working principles and device structures of OPV cells are examined, and a brief comparison between device structures is made, highlighting their advantages, disadvantages, and key features. The various parts of OPV cells are discussed, and their performance, efficiency, and electrical characteristics are reviewed. A detailed SWOT analysis is conducted, identifying promising strengths and opportunities, as well as challenges and threats to the technology. The paper indicates that OPV cells have the potential to revolutionize the solar energy industry due to their low production costs, and ability to produce thin, flexible solar cells. However, challenges such as lower efficiency, durability, and technological limitations still exist. Despite these challenges, the tunability and versatility of organic materials offer promise for future success. The paper concludes by suggesting that future research should focus on addressing the identified challenges and developing new materials and technologies that can further improve the performance and efficiency of OPV cells.