Design rules for minimizing voltage losses in high-efficiency organic solar 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.

Endothermic Charge Separation Occurs Spontaneously in Non-Fullerene Acceptor/Polymer Bulk Heterojunction

Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved a power conversion efficiency close to 20%. These NFA OPVs can generate free carriers efficiently despite a very small energy level offset at the donor/acceptor interface. Why these NFAs can enable efficient charge separation (CS) with low energy losses remains an open question. Here, the CS process in the PM6:Y6 bulk heterojunction is probed by time-resolved two-photon photoemission spectroscopy. It is found that the CS, the conversion from bound charge transfer (CT) excitons to free carriers, is an endothermic process with an enthalpy barrier of 0.15 eV. The CS can occur spontaneously despite being an endothermic process, which implies that it is driven by entropy. It is further argued that the morphology of the PM6:Y6 film and the anisotropic electron delocalization restrict the electron and hole wavefunctions within the CT exciton such that they can primarily contact each other through point-like junctions. This configuration can maximize the entropic driving force.

Recent progress and prospects of dimer and multimer acceptors for efficient and stable polymer solar cells

High power conversion efficiency (PCE) and long-term stability are essential prerequisites for the commercialization of polymer solar cells (PSCs). Small-molecule acceptors (SMAs) are core materials that have led to recent, rapid increases in the PCEs of the PSCs. However, a critical limitation of the resulting PSCs is their poor long-term stability. Blend morphology degradation from rapid diffusion of SMAs with low glass transition temperatures (Tgs) is considered the main cause of the poor long-term stability of the PSCs. The recent emergence of oligomerized SMAs (OSMAs), composed of two or more repeating SMA units (i.e., dimerized and trimerized SMAs), has shown great promise in overcoming these challenges. This innovation in material design has enabled OSMA-based PSCs to reach impressive PCEs near 19% and exceptional long-term stability. In this review, we summarize the evolution of OSMAs, including their research background and recent progress in molecular design. In particular, we discuss the mechanisms for high PCE and stability of OSMA-based PSCs and suggest useful design guidelines for high-performance OSMAs. Furthermore, we reflect on the existing hurdles and future directions for OSMA materials towards achieving commercially viable PSCs with high PCEs and operational stabilities.

Isomerization of Benzothiadiazole Yields a Promising Polymer Donor and Organic Solar Cells with Efficiency of 19.0

The exploration of high-performance and low-cost wide-bandgap polymer donors remains critical to achieve high-efficiency nonfullerene organic solar cells (OSCs) beyond current thresholds. Herein, the 1,2,3-benzothiadiazole (iBT), which is an isomer of 2,1,3-benzothiadiazole (BT), is used to design wide-bandgap polymer donor PiBT. The PiBT-based solar cells reach efficiency of 19.0%, which is one of the highest efficiencies in binary OSCs. Systemic studies show that isomerization of BT to iBT can finely regulate the polymers' photoelectric properties including i) increasing the extinction coefficient and photon harvest, ii) downshifting the highest occupied molecular orbital energy levels, iii) improving the coplanarity of polymer backbones, iv) offering good thermodynamic miscibility with acceptors. Consequently, the PiBT:Y6 bulk heterojunction (BHJ) device simultaneously reaches advantageous nanoscale morphology, efficient exciton generation and dissociation, fast charge transportation, and suppressed charge recombination, leading to larger VOC of 0.87 V, higher JSC of 28.2 mA cm-2, greater fill factor of 77.3%, and thus higher efficiency of 19.0%, while the analog-PBT-based OSCs reach efficiency of only 12.9%. Moreover, the key intermediate iBT can be easily afforded from industry chemicals via two-step procedure. Overall, this contribution highlights that iBT is a promising motif for designing high-performance polymer donors.

Optimization of Charge Management and Energy Loss in All-Small-Molecule Organic Solar Cells

All-small-molecule organic solar cells (ASM-OSCs) have received tremendous attention in recent decades because of their advantages over their polymer counterparts. These advantages include well-defined chemical structures, easy purification, and negligible batch-to-batch variation. Remarkable progress with a power conversion efficiency (PCE) of over 17% has recently been achieved with improved charge management (FF × JSC) and reduced energy loss (Eloss). Morphology control is the key factor in the progress of ASM-OSCs, which remains a significant challenge because of the similarities in the molecular structures of the donors and acceptors. In this review, the effective strategies for charge management and/or Eloss reduction from the perspective of effective morphology control are summarized. The aim is to provide practical insights and guidance for material design and device optimization to promote further development of ASM-OSCs to a level where they can compete with or even surpass the efficiency of polymer solar cells.

A2-A1-D-A1-A2-Type Nonfullerene Acceptors

The A2-A1-D-A1-A2-type molecules consist of one electron-donating (D) core flanked by two electron-accepting units (A1 and A2) and have emerged as an essential branch of nonfullerene acceptors (NFAs). These molecules generally possess higher molecular energy levels and wider optical bandgaps compared with those of the classic A-D-A- and A-DA'D-A-type NFAs, owing to the attenuated intramolecular charge transfer effect. These characteristics make them compelling choices for the fabrication of high-voltage organic photovoltaics (OPVs), ternary OPVs, and indoor OPVs. Herein, the recent progress in the A2-A1-D-A1-A2-type NFAs are reviewed, including the molecular engineering, structure-property relationships, voltage loss (Vloss), device stability, and photovoltaic performance of binary, ternary, and indoor OPVs. Finally, the challenges and provided prospects are discussed for the further development of this type of NFAs.

High Performance As-Cast Organic Solar Cells Enabled by a Refined Double-Fibril Network Morphology and Improved Dielectric Constant of Active Layer

High performance organic solar cells (OSCs) are usually realized by using post-treatment and/or additive, which can induce the formation of metastable morphology, leading to unfavorable device stability. In terms of the industrial production, the development of high efficiency as-cast OSCs is crucially important, but it remains a great challenge to obtain appropriate active layer morphology and high power conversion efficiency (PCE). Here, efficient as-cast OSCs are constructed via introducing a new polymer acceptor PY-TPT with a high dielectric constant into the D18:L8-BO blend to form a double-fibril network morphology. Besides, the incorporation of PY-TPT enables an enhanced dielectric constant and lower exciton binding energy of active layer. Therefore, efficient exciton dissociation and charge transport are realized in D18:L8-BO:PY-TPT-based device, affording a record-high PCE of 18.60% and excellent photostability in absence of post-treatment. Moreover, green solvent-processed devices, thick-film (300 nm) devices, and module (16.60 cm2) are fabricated, which show PCEs of 17.45%, 17.54%, and 13.84%, respectively. This work brings new insight into the construction of efficient as-cast devices, pushing forward the practical application of OSCs.

A 3,3'-Difluoro-2,2'-Bithiophene Based Donor Polymer Realizing High Efficiency (>17%) Single Junction Binary Organic Solar Cells

In this study, two novel donor-acceptor (D-A) copolymers are designed and synthesized, DTBT-2T and DTBT-2T2F with 2,2'-bithiophene or 3,3'-difluoro-2,2'-bithiophene as the donor unit and dithienobenzothiadiazole as the acceptor unit, and used them as donor materials in non-fullerene organic solar cells (OSCs). Due to enhanced planarity of polymer chains resulted by the intramolecular F···S noncovalent interactions, the incorporation of 3,3'-difluoro-2,2'-bithiophene unit instead of 2,2'-bithiophene into the polymers can enhance their molecular packing, crystallinity and hole mobility. The DTBT-2T:L8-BO based binary OSCs deliver a power conversion efficiency (PCE) of only 9.71% with a Voc of 0.78 V, a Jsc of 20.69 mA cm-2 , and an FF of 59.67%. Moreover, the introduction of fluoro atoms can lower the highest occupied molecular orbital levels. As a result, DTBT-2T2F:L8-BO based single-junction binary OSCs exhibited less recombination loss, more balanced charge mobility, and more favorable morphology, resulting in an impressive PCE of 17.03% with a higher Voc of 0.89 V, a Jsc of 25.40 mA cm-2, and an FF of 75.74%. These results indicate that 3,3'-difluoro-2,2'-bithiophene unit can be used as an effective building block to synthesize high performance polymer donor materials. This work greatly expands the selection range of donor units for constructing high-performance polymers.

Guest Acceptors with Lower Electrostatic Potential in Ternary Organic Solar Cells for Minimizing Voltage Losses

The ternary strategy, in which one guest component is introduced into one host binary system, is considered to be one of the most effective ways to realize high-efficiency organic solar cells (OSCs). To date, there is no efficient method to predict the effectiveness of guest components in ternary OSCs. Herein, three guest compositions (i.e., ANF-1, ANF-2 and ANF-3) with different electrostatic potential (ESP) are designed and synthesized by modulating the electron-withdrawing ability of the terminal groups through density functional theory simulations. The effects of the introduction of guest component into the host system (D18:N3) on the photovoltaic properties are investigated. The theoretical and experimental studies provide a key rule for guest acceptor in ternary OSCs to improve the open-circuit voltage, that is, the larger ESP difference between the guest and host acceptor, the stronger the intermolecular interactions and the higher the miscibility, which improves the luminescent efficiency of the blend film and the electroluminescence quantum yield (EQEEL) of the device by reducing the aggregation-caused-quenching, thereby effectively decreasing the non-radiative voltage loss of ternary OSCs. This work will greatly contribute to the development of highly efficient guest components, thereby promoting the rapid breakthrough of the 20% efficiency bottleneck for single-junction OSCs.

The Role of Grain Boundaries on Ion Migration and Charge Recombination in Halide Perovskites

Grain boundaries (GBs) have a significant role in polycrystalline perovskite solar cells (PSCs). However, there is ongoing debate regarding the impact of GBs on the performance and long-term stability of PSCs. Employing the first-principles molecular dynamics for perovskites, the iodine vacancy defect migrations both in bulk and at GBs are investigated. i) The positive iodine vacancy (VI +) is found that have both lower formation energy (1.4 eV) and activation energy (0.18 eV) than those of neutral iodine vacancy (VI), statistically. It indicated the VI + acts as the dominant migrated iodine vacancy rather than VI; ii) the iodine vacancy at GBs has ≈0.48 eV higher activation energy than those in bulk, which leads to the accumulation of iodine vacancy at GBs; iii) the presence of VI + result in a 3-fold increase in charge recombination ratio at GBs, compared to pristine PSCs. Based on quantum molecular dynamics statistical results, which are consistent with experimental measurements, insights into iodine vacancy migration both at GBs and in the bulk are gained. This understanding can be valuable for defects engineering related to ion migration, in order to improve the long-term stability and promote the performance of PSCs.

Tuning polymer-backbone coplanarity and conformational order to achieve high-performance printed all-polymer solar cells

All-polymer solar cells (all-PSCs) offer improved morphological and mechanical stability compared with those containing small-molecule-acceptors (SMAs). They can be processed with a broader range of conditions, making them desirable for printing techniques. In this study, we report a high-performance polymer acceptor design based on bithiazole linker (PY-BTz) that are on par with SMAs. We demonstrate that bithiazole induces a more coplanar and ordered conformation compared to bithiophene due to the synergistic effect of non-covalent backbone planarization and reduced steric encumbrances. As a result, PY-BTz shows a significantly higher efficiency of 16.4% in comparison to the polymer acceptors based on commonly used thiophene-based linkers (i.e., PY-2T, 9.8%). Detailed analyses reveal that this improvement is associated with enhanced conjugation along the backbone and closer interchain π-stacking, resulting in higher charge mobilities, suppressed charge recombination, and reduced energetic disorder. Remarkably, an efficiency of 14.7% is realized for all-PSCs that are solution-sheared in ambient conditions, which is among the highest for devices prepared under conditions relevant to scalable printing techniques. This work uncovers a strategy for promoting backbone conjugation and planarization in emerging polymer acceptors that can lead to superior all-PSCs.

Rational molecular and device design enables organic solar cells approaching 20% efficiency

For organic solar cells to be competitive, the light-absorbing molecules should simultaneously satisfy multiple key requirements, including weak-absorption charge transfer state, high dielectric constant, suitable surface energy, proper crystallinity, etc. However, the systematic design rule in molecules to achieve the abovementioned goals is rarely studied. In this work, guided by theoretical calculation, we present a rational design of non-fullerene acceptor o-BTP-eC9, with distinct photoelectric properties compared to benchmark BTP-eC9. o-BTP-eC9 based device has uplifted charge transfer state, therefore significantly reducing the energy loss by 41 meV and showing excellent power conversion efficiency of 18.7%. Moreover, the new guest acceptor o-BTP-eC9 has excellent miscibility, crystallinity, and energy level compatibility with BTP-eC9, which enables an efficiency of 19.9% (19.5% certified) in PM6:BTP-C9:o-BTP-eC9 based ternary system with enhanced operational stability.

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.

Charge photogeneration dynamics in non-fullerene polymer solar cells with fluorinated and non-fluorinated acceptors

In this work, charge photogeneration and recombination processes of PM6:IDIC-4F and PM6:IDIC blend films were investigated by the steady-state and time-resolved spectroscopies, as well as the time-dependent density functional theory calculations. The peaks in absorption and photoluminescence (PL) spectra of IDIC and IDIC-4F solutions were assigned by combining the experiment and the simulation of UV-vis absorption and PL spectra. For neat acceptor films, the exciton diffusion length of neat IDIC and IDIC-4F films was estimated as ∼28.9 and ∼19.9 nm, respectively. For PM6-based blend films, we find that the fluorine substitution engineering on the IDIC acceptor material can increase the phase separate size of acceptor material in blend films, resulting in the reduction of dissociation efficiencies of acceptor excitons. In addition, we find that the charge recombination in PM6:IDIC-4F is dominated by bimolecular recombination, in comparison to geminate type carrier recombination in PM6:IDIC blend films. In addition, we find that thermal annealing treatment has a weak influence on carrier recombination but slightly reduces the exciton dissociation efficiency of acceptor in PM6:IDIC blend films, leading to a slightly reduced power conversion efficiency of PM6:IDIC solar cells. These results may shed light on the design of high-performance semiconductor molecules for application in solar cells.

Noncovalent Interaction Boosts Performance and Stability of Organic Solar Cells Based on Giant-Molecule Acceptors

Designing giant-molecule acceptors is deemed as an up-and-coming strategy to construct stable organic solar cells (OSCs) with high performance. Herein, two giant dimeric acceptors, namely, DYV and DYFV, have been designed and synthesized by linking two Y-series derivatives with a vinyl unit. DYFV exhibits more red-shifted absorption, down-shifted energy levels, and enhanced intermolecular packing than DYV because the intramolecular noncovalent interaction (H···F) of DYFV leads to better coplanarity of the backbone. The D18:DYFV film owns a distinct nanofibrous nanophase separation structure, a more dominant face-on orientation, and more balanced carrier mobilities. Therefore, the D18:DYFV OSC achieves a higher photoelectron conversion efficiency of 17.88% and a longer-term stability with a t80 over 45,000 h compared with the D18:DYV device. The study demonstrates that the intramolecular noncovalent interaction is a superior strategy to design giant-molecule acceptors and boost the photovoltaic performance and stability of the OSCs.

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.

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.

Maximizing Performance and Stability of Organic Solar Cells at Low Driving Force for Charge Separation

Thanks to the development of novel electron acceptor materials, the power conversion efficiencies (PCE) of organic photovoltaic (OPV) devices are now approaching 20%. Further improvement of PCE is complicated by the need for a driving force to split strongly bound excitons into free charges, causing voltage losses. This review discusses recent approaches to finding efficient OPV systems with minimal driving force, combining near unity quantum efficiency (maximum short circuit currents) with optimal energy efficiency (maximum open circuit voltages). The authors discuss apparently contradicting results on the amount of exciton binding in recent literature, and approaches to harmonize the findings. A comprehensive view is then presented on motifs providing a driving force for charge separation, namely hybridization at the donor:acceptor interface and polarization effects in the bulk, of which quadrupole moments (electrostatics) play a leading role. Apart from controlling the energies of the involved states, these motifs also control the dynamics of recombination processes, which are essential to avoid voltage and fill factor losses. Importantly, all motifs are shown to depend on both molecular structure and process conditions. The resulting high dimensional search space advocates for high throughput (HT) workflows. The final part of the review presents recent HT studies finding consolidated structure-property relationships in OPV films and devices from various deposition methods, from research to industrial upscaling.

Alkoxythiophene-Directed Fibrillization of Polymer Donor for Efficient Organic Solar Cells

Realizing fibrillar molecular framework is highly encouraged in organic solar cells (OSCs) due to the merit of efficient charge carrier transport. This is however mainly achieved via the chemical structural design of photovoltaic semiconductors. In this work, through the utilization of three alkoxythiophene additives, T-2OMe, T-OEH, and T-2OEH, the intermolecular interactions among a series of BDT-type polymer donors, i.e., PM6, D18, PBDB-T, and PTB7-Th, are tuned to self-assemble into nanofibrils during solution casting. X-ray technique and molecular dynamics simulation reveal that the alkoxythiophene with (2-ethylhexyl)oxy (─OEH) chains can attach on the 2-ethylhexyl (EH) chains of these polymer donors and promote their self-assembly into 1D nanofibrils, in their neat films as well as photovoltaic blends with L8-BO. By adapting these fibrillar polymer donors to construct pseudo-bulk heterojunction (P-BHJ) OSCs via layer-by-layer deposition, generally improved device performance is seen, with power conversion efficiencies enhanced from 18.2% to 19.2% (certified 18.96%) and from 17.9% to 18.7% for the PM6/L8-BO and D18/L8-BO devices, respectively. This work provides a physical approach to promote the fibrillar charge transport channels for efficient photovoltaics.

Magnetic-biased chiral molecules enabling highly oriented photovoltaic perovskites

The interaction between sites A, B and X with passivation molecules is restricted when the conventional passivation strategy is applied in perovskite (ABX3) photovoltaics. Fortunately, the revolving A-site presents an opportunity to strengthen this interaction by utilizing an external field. Herein, we propose a novel approach to achieving an ordered magnetic dipole moment, which is regulated by a magnetic field via the coupling effect between the chiral passivation molecule and the A-site (formamidine ion) in perovskites. This strategy can increase the molecular interaction energy by approximately four times and ensure a well-ordered molecular arrangement. The quality of the deposited perovskite film is significantly optimized with inhibited nonradiative recombination. It manages to reduce the open-circuit voltage loss of photovoltaic devices to 360 mV and increase the power conversion efficiency to 25.22%. This finding provides a new insight into the exploration of A-sites in perovskites and offers a novel route to improving the device performance of perovskite photovoltaics.

Manipulating Vertical Phase Separation Enables Pseudoplanar Heterojunction Organic Solar Cells Over 19% Efficiency via Ternary Polymerization

Controlling vertical phase separation of the active layer to enable efficient exciton dissociation and charge carrier transport is crucial to boost power conversion efficiencies (PCEs) of pseudoplanar heterojunction (PPHJ) organic solar cells (OSCs). However, how to optimize the vertical phase separation of PPHJ OSCs via molecule design is rarely reported yet. Herein, ternary polymerization strategy is employed to develop a series of polymer donors, DL1-DL4, and regulate their solubility, molecular aggregation, molecular orientation, and miscibility, thus efficiently manipulating vertical phase separation in PPHJ OSCs. Among them, DL1 not only has enhanced solubility, inhibited molecular aggregation and partial edge-on orientation to facilitate acceptor molecules, Y6, to permeate into polymer layer and increase donor/acceptor interfaces, but also sustains high crystallinity and appropriate miscibility with Y6 to acquire ordered molecular packing, thus achieving optimized vertical phase separation to well juggle exciton dissociation and charge transport in PPHJ devices. Therefore, DL1/Y6 based PPHJ OSCs gain the best exciton dissociation probability, highest charge carrier mobilities and weakest charge recombination, and thus afford an impressive PCE of 19.10%, which is the record value for terpolymer donors. It demonstrates that ternary polymerization is an efficient method to optimize vertical phase separation in PPHJ OSCs for high PCEs.

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.

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.

A-D-A Type Nonfullerene Acceptors Synthesized by Core Segmentation and Isomerization for Realizing Organic Solar Cells with Low Nonradiative Energy Loss

Reducing non-radiative recombination energy loss (ΔEnonrad ) in organic solar cells (OSCs) has been considered an effective method to improve device efficiency. In this study, the backbone of PTBTT-4F/4Cl is divided into D1-D2-D3 segments and reconstructed. The isomerized TPBTT-4F/4Cl obtains stronger intramolecular charge transfer (ICT), thus leading to elevated highest occupied molecular orbital (HOMO) energy level and reduced bandgap (Eg ). According to ELoss  = Eg- qVOC , the reduced Eg and enhanced open circuit voltage (VOC ) result in lower ELoss , indicating that ELoss has been effectively suppressed in the TPBTT-4F/4Cl based devices. Furthermore, compared to PTBTT derivatives, the isomeric TPBTT derivatives exhibit more planar molecular structure and closer intermolecular stacking, thus affording higher crystallinity of the neat films. Therefore, the reduced energy disorder and corresponding lower Urbach energy (Eu ) of the TPBTT-4F/4Cl blend films lead to low ELoss and high charge-carrier mobility of the devices. As a result, benefitting from synergetic control of molecular stacking and energetic offsets, a maximum power conversion efficiency (PCE) of 15.72% is realized from TPBTT-4F based devices, along with a reduced ΔEnonrad of 0.276 eV. This work demonstrates a rational method of suppressing VOC loss and improving the device performance through molecular design engineering by core segmentation and isomerization.

Over 18% Efficiency Ternary Organic Solar Cells with 300 nm Thick Active Layer Enabled by an Oligomeric Acceptor

The development of high-efficiency thickness-insensitive organic solar cells (OSCs) is crucially important for the mass production of solar panels. However, increasing the active layer thickness usually induces a substantial loss in efficiency. Herein, a ternary strategy in which an oligomer DY-TF is incorporated into PM6:L8-BO system as a guest component is adopted to break this dilemma. The S···F intramolecular noncovalent interactions in the backbone endow DY-TF with a high planarity. Upon the addition of DY-TF, the crystallinity of the blend is effectively improved, leading to increased charge carrier mobility, which is highly desirable in the fabrication of thick-film devices. As a result, thin-film PM6:L8-BO:DY-TF-based device (110 nm) shows a power conversion efficiency (PCE) of 19.13%. Impressively, when the active layer thickness increases to 300 nm, an efficiency of 18.23% (certified as 17.8%) is achieved, representing the highest efficiency reported for 300 nm thick OSCs thus far. Additionally, blade-coated thick device (300 nm) delivers a promising PCE of 17.38%. This work brings new insights into the construction of efficient OSCs with high thickness tolerance, showing great potential for roll-to-roll printing of large-area solar cells.

Macrocyclic Encapsulation in a Non-fused Tetrathiophene Acceptor for Efficient Organic Solar Cells with High Short-Circuit Current Density

Non-fullerene acceptors have shown great promise for organic solar cells (OSCs). However, challenges in achieving high efficiency molecular system with conformational unicity and effective molecular stacking remain. In this study, we present a new design of non-fused tetrathiophene acceptor R4T-1 via employing the encapsulation of tetrathiophene with macrocyclic ring. The single crystal structure analysis reveals that cyclic alkyl side chains can perfectly encapsulate the central part of molecule and generate a conformational stable and planar molecular backbone. Whereas, the control 4T-5 without the encapsulation restriction displays cis- and twisted conformation. As a result, R4T-1 based OSCs achieved an outstanding power conversion efficiency (PCE) exceeding 15.10 % with a high short-circuit current density (Jsc ) of 25.48 mA/cm2 , which is significantly improved by ≈30 % in relative to that of the control. Our findings demonstrate that the macrocyclic encapsulation strategy could assist fully non-fused electron acceptors (FNEAs) to achieve a high photovoltaic performance and pave a new way for FNEAs design.

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.

Propeller vs Quasi-Planar 6-Cantilever Small Molecular Platforms with Extremely Two-Dimensional Conjugated Extension

Two exotic 6-cantilever small molecular platforms, characteristic of quite different molecular configurations of propeller and quasi-plane, are established by extremely two-dimensional conjugated extension. When applied in small molecular acceptors, the only two cases of CH25 and CH26 that could contain six terminals and such broad conjugated backbones have been afforded thus far, rendering featured absorptions, small reorganization and exciton binding energies. Moreover, their distinctive but completely different molecular geometries result in sharply contrasting nanoscale film morphologies. Finally, CH26 contributes to the best device efficiency of 15.41 % among acceptors with six terminals, demonstrating two pioneered yet highly promising 6-cantilever molecular innovation platforms.

Characterizing Spatial and Energetic Distributions of Trap States Toward Highly Efficient Perovskite Photovoltaics

Due to their greater opt electric performance, perovskite photovoltaics (PVs) present huge potential to be commercialized. Perovskite PV's high theoretical efficiency expands the available development area. The passivation of defects in perovskite films is crucial for approaching the theoretical limit. In addition to creating efficient passivation techniques, it is essential to direct the passivation approach by getting precise and real-time information on the trap states through measurements. Therefore, it is necessary to establish quantitative characterization methods for the trap states in energy and 3D spaces. The authors cover the characterization of the spatial and energy distributions of trap states in this article with an eye toward high-efficiency perovskite photovoltaics. After going over the strategies that have been created for characterizing and evaluating trap states, the authors will concentrate on how to direct the creative development of characterization techniques for trap states assessment and highlight the opportunities and challenges of future development. The 3D space and energy distribution mappings of trap states are anticipated to be realized. The review will give key guiding importance for further approaching the theoretical efficiency of perovskite photovoltaics, offering some future research direction and technological assistance for the development of appropriate targeted passivation technologies.

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.

Comparative Study of the Optical and Electronic Properties of Y6 Derivatives: A Theoretical Study

A series of Y-series nonfullerene acceptors (Y-NFAs) including symmetric acceptors (Y6 and TTY6) as well as asymmetric acceptors (KY6, TY6, and KTY6) have been constructed, and the electronic structure, electronic properties, and excited-state properties have been comparatively studied. The optoelectronic properties, interfacial charge-transfer (CT) mechanism, and interfacial CT rate for the solar cells composed of PM6 as the donor and Y6 derivatives as the acceptors are investigated further. We show that asymmetric Y6 derivatives have high molecular planarity, strong and wide absorption spectra, and large intramolecular charge transfer (ICT). For the solar cells, the complexes of Y6 derivatives show increased open-circuit voltage, larger fill factor, and smaller energy loss compared to Y6. In addition, the complexes of Y6 derivatives have more charge-transfer states than Y6 in the low-energy region, such that there are multiple ways for CT generations, such as hot excitation, intermolecular electric field (IEF), and direct excitation. The detailed CT mechanism as well as interfacial CT rate depends on the type of complexes, and all Y6 derivatives have a similar magnitude of charge-transfer rate to the one of Y6. This work not only reveals the differences in performance between symmetric and asymmetric NFA but also reveals that proper terminal tuning is an effective way to improve photovoltaic properties.

Facile, Versatile and Stepwise Synthesis of High-Performance Oligomer Acceptors for Stable Organic Solar Cells

Oligomer acceptors have recently emerged as promising photovoltaic materials for achieving high power conversion efficiency (PCE) and long-term stability in organic solar cells (OSCs). However, the limited availability of diverse acceptors, resulting from the sole synthetic approach, has hindered their potential for future industrialization. In this study, we present a facile and effective stepwise approach that utilizes two consecutive Stille coupling reactions for the synthesis of oligomer acceptors. To demonstrate the feasibility of the novel approach, we successfully synthesize a trimer acceptor, Tri-Y6-OD, and further systematically investigate the impact of oligomerization on device performance and stability. The results reveal that this approach has significant advantages compared to the conventional method, including reduced formation of unwanted by-products and lower difficulties in purification. Remarkably, the OSC based on PM6 : Tri-Y6-OD achieves an impressive PCE of 18.03 % and maintains 80 % of the initial PCE (T80 ) for 1523 h under illumination, surpassing the performance of the corresponding small molecule acceptor Y6-OD-based device. Furthermore, the versatility of the synthetic strategy in obtaining diverse acceptors is further demonstrated. Overall, our findings provide a facile, versatile and stepwise way for synthesizing oligomer acceptors, thereby facilitating the development of stable and efficient OSCs.

Eliminating the Imbalanced Mobility Bottlenecks via Reshaping Internal Potential Distribution in Organic Photovoltaics

The imbalanced carrier mobility remains a bottleneck for performance breakthrough in even those organic solar cells (OSCs) with recorded power conversion efficiencies (PCEs). Herein, a counter electrode doping strategy is proposed to reshape the internal potential distribution, which targets to extract the low mobility carriers at far end. Device simulations reveal that the key of this strategy is to partially dope the active layer with a certain depth, therefore it strengthens the electric field for low mobility carriers near counter electrode region while avoids zeroing the electric field near collection electrode region. Taking advantage of these, PCE enhancements are obtained from 15.4% to 16.2% and from 16.9% to 18.0%, respectively, via cathode p-doping and anode n-doping. Extending its application from opaque to semitransparent devices, the PCE of dilute cell rises from 10.5% to 12.1%, with a high light utilization efficiency (LUE) of 3.5%. The findings provide practical solutions to the core device physical problem in OSCs.

Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells

Organic solar cells benefit from non-fullerene acceptors (NFA) due to their high absorption coefficients, tunable frontier energy levels, and optical gaps, as well as their relatively high luminescence quantum efficiencies as compared to fullerenes. Those merits result in high yields of charge generation at a low or negligible energetic offset at the donor/NFA heterojunction, with efficiencies over 19% achieved for single-junction devices. Pushing this value significantly over 20% requires an increase in open-circuit voltage, which is currently still well below the thermodynamic limit. This can only be achieved by reducing non-radiative recombination, and hereby increasing the electroluminescence quantum efficiency of the photo-active layer. Here, current understanding of the origin of non-radiative decay, as well as an accurate quantification of the associated voltage losses are summarized. Promising strategies for suppressing these losses are highlighted, with focus on new material design, optimization of donor-acceptor combination, and blend morphology. This review aims at guiding researchers in their quest to find future solar harvesting donor-acceptor blends, which combine a high yield of exciton dissociation with a high yield of radiative free carrier recombination and low voltage losses, hereby closing the efficiency gap with inorganic and perovskite photovoltaics.

Impact of Electrostatic Interaction on Non-radiative Recombination Energy Losses in Organic Solar Cells Based on Asymmetric Acceptors

Reducing non-radiative recombination energy loss (ΔE3 ) is one key to boosting the efficiency of organic solar cells. Although the recent studies have indicated that the Y-series asymmetric acceptors-based devices featured relatively low ΔE3 , the understanding of the energy loss mechanism derived from molecular structure change is still lagging behind. Herein, two asymmetric acceptors named BTP-Cl and BTP-2Cl with different terminals were synthesized to make a clear comparative study with the symmetric acceptor BTP-0Cl. Our results suggest that asymmetric acceptors exhibit a larger difference of electrostatic potential (ESP) in terminals and semi-molecular dipole moment, which contributes to form a stronger π-π interaction. Besides, the experimental and theoretical studies reveal that a lower ESP-induced intermolecular interaction can reduce the distribution of PM6 near the interface to enhance the built-in potential and decrease the charge transfer state ratio for asymmetric acceptors. Therefore, the devices achieve a higher exciton dissociation efficiency and lower ΔE3 . This work establishes a structure-performance relationship and provides a new perspective to understand the state-of-the-art asymmetric acceptors.

The Energy Structure of Spin States in Reducing the Nonradiative Voltage Loss in Organic Solar Cells

In organic solar cells (OSCs), the nonradiative voltage loss (ΔVnr) has been identified as a critical factor for the relatively lower open-circuit voltage. Under open-circuit conditions, most of the charge recombination processes occur via the triplet exciton state, underscoring the importance of the energy structures concerning the local exciton (LE) and charge transfer (CT) spin states. In this Letter, we propose a five-state model to explore the spin state energy structures to reduce ΔVnr. Our calculations reveal that, to minimize ΔVnr, the spin singlet state for LE should possess a lower energy than the triplet state, ES1 < ET1. In contrast, the energies of the CT spin states have a negligible effect on ΔVnr. We identify the best energy structure as ES1 < ET1 ∼ ECT1/CT3. Moreover, our calculations demonstrate that strong couplings between these spin states, particularly involving spin flip, can effectively mitigate ΔVnr. These findings present novel insights for the advancement of OSCs.

Air-Knife-Assisted Spray Coating of Organic Solar Cells

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have risen dramatically since the introduction of the "Y-series" of non-fullerene acceptors. However, the demonstration of rapid scalable deposition techniques to deposit such systems is rare. Here, for the first time, we demonstrate the deposition of a Y-series-based system using ultrasonic spray coating─a technique with the potential for significantly faster deposition speeds than most traditional meniscus-based methods. Through the use of an air-knife to rapidly remove the casting solvent, we can overcome film reticulation, allowing the drying dynamics to be controlled without the use of solvent additives, heating the substrate, or heating the casting solution. The air-knife also facilitates the use of a non-halogenated, low-toxicity solvent, resulting in industrially relevant, spray-coated PM6:DTY6 devices with PCEs of up to 14.1%. We also highlight the obstacles for scalable coating of Y-series-based solar cells, in particular the influence of slower drying times on blend morphology and crystallinity. This work demonstrates the compatibility of ultrasonic spray coating, and use of an air-knife, with high-speed, roll-to-roll OSC manufacturing techniques.

Suppressing electron-phonon coupling in organic photovoltaics for high-efficiency power conversion

The nonradiative energy loss (∆Enr) is a critical factor to limit the efficiency of organic solar cells. Generally, strong electron-phonon coupling induced by molecular motion generates fast nonradiative decay and causes high ∆Enr. How to restrict molecular motion and achieve a low ∆Enr is a sticking point. Herein, the free volume ratio (FVR) is proposed as an indicator to evaluate molecular motion, providing new molecular design rationale to suppress nonradiative decay. Theoretical and experimental results indicate proper proliferation of alkyl side-chain can decrease FVR and restrict molecular motion, leading to reduced electron-phonon coupling while maintaining ideal nanomorphology. The reduced FVR and favorable morphology are simultaneously obtained in AQx-6 with pinpoint alkyl chain proliferation, achieving a high PCE of 18.6% with optimized VOC, JSC and FF. Our study discovered aggregation-state regulation is of great importance to the reduction of electron-phonon coupling, which paves the way to high-efficiency OSCs.

Mitigating the Trade-Off between Non-Radiative Recombination and Charge Transport to Enable Efficient Ternary Organic Solar Cells

Ternary organic solar cells (OSCs) have attracted intensive studies due to their promising potential for attaining high-performing photovoltaics, whereas there has been an opening challenge in minimizing the open circuit voltage (Voc) loss while retaining the optimal carrier extraction in the multiple mixture absorbers. Here, we systemically investigate a ternary absorber comprised of two acceptors and a donor, in which the resultant Voc and fill factor are varied and determined by the ratios of acceptor components as a result of the unbalance of non-radiative recombination rates and charge transport. The transient absorption spectroscopy and electroluminescence techniques verify two distinguishable charge-transfer (CT) states in the ternary absorber, and the mismatch of non-radiative recombination rates of those two CT states is demonstrated to be associated with the Voc deficit, whilst the high-emissive acceptor molecule delivers inferior electron mobility, resulting in poor charge transport and a subpar fill factor. These findings enable us to optimize the mixture configuration for attaining the maximal-performing devices. Our results not only provide insight into maximizing the photovoltage of organic solar cells but can also motivate researchers to further unravel the photophysical mechanisms underlying the intermolecular electronic states of organic semiconductors.

Charge-Transfer State Dissociation Efficiency Can Limit Free Charge Generation in Low-Offset Organic Solar Cells

We investigate the charge-generation processes limiting the performance of low-offset organic bulk-heterojunction solar cells by studying a series of newly synthesized PBDB-T-derivative donor polymers whose ionisation energy (IE) is tuned via functional group (difluorination or cyanation) and backbone (thiophene or selenophene bridge) modifications. When blended with the acceptor Y6, the series present heterojunction donor-acceptor IE offsets (ΔEIE) ranging from 0.22 to 0.59 eV. As expected, small ΔEIE decrease nonradiative voltage losses but severely suppresses photocurrent generation. We explore the origin of this reduced charge-generation efficiency at low ΔEIE through a combination of opto-electronic and spectroscopic measurements and molecular and device-level modeling. We find that, in addition to the expected decrease in local exciton dissociation efficiency, reducing ΔEIE also strongly reduces the charge transfer (CT) state dissociation efficiency, demonstrating that poor CT-state dissociation can limit the performance of low-offset heterojunction solar cells.

Improving the efficiency of ternary organic solar cells by reducing energy loss

Effectively reducing the voltage loss in organic solar cells (OSCs) is critical to improving the power conversion efficiency (PCE) of OSCs. In this study, highly efficient ternary OSCs were constructed by adding a non-fullerene acceptor Qx2 with a high open-circuit voltage (VOC) and low energy loss (Eloss) into PM6:m-BTP-PhC6 based binary devices. The third component Qx2 shows slightly complementary absorption with m-BTP-PhC6 and also optimizes the molecular packing, orientation, and morphology of the active layer. Moreover, the incorporation of Qx2 reduced the energetic disorder and improved the electroluminescence quantum efficiency, which suppresses the Eloss and further leads to a higher VOC than the PM6:m-BTP-PhC6 binary blend. Consequently, synergetic enhancements of VOC, short circuit current (JSC), and fill factor (FF) are realized, resulting in the PCE of 18.60%. This work shows that the selection of the appropriate third component has positive implications for reducing Eloss and improving the PCE of OSCs.

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%.

Tandem organic solar cells with 20.6% efficiency enabled by reduced voltage losses

Large voltage losses are the main obstacle for achieving high efficiency in organic solar cells (OSCs). Here we construct ternary OSCs by introducing an asymmetric small molecule acceptor AITC into PBDB-TCl : BTP-eC9 system and demonstrate the effectiveness in simultaneously decreasing energy disorder and non-radiative voltage losses. It is found that the introduction of AITC can modify domain size and increase the degree of crystallinity, which enhances open-circuit voltage and power conversion efficiency (19.1%, certified as 18.9%). Inspiringly, an output efficiency of 20.6% of the constructed tandem OSCs based on PBDB-TCl : AITC : BTP-eC9 ternary active layer output a recorded efficiency of 20.6% (certified as 20.3%), which is the highest value in OSCs field to date. This work demonstrates that decreasing the voltage losses by ternary strategy and constructing of tandem architecture are effective approaches towards improving photovoltaic performance.

Halogen-Free Donor Polymers Based on Dicyanobenzotriazole with Low Energy Loss and High Efficiency in Organic Solar Cells

Halogenation of organic semiconductors is an efficient strategy for improving the performance of organic solar cells (OSCs), while the introduction of halogens usually involves complex synthetic process and serious environment pollution problems. Herein, three halogen-free ternary copolymer donors (PCNx, x = 3, 4, 5) based on electron-withdrawing dicyanobenzotriazole are reported. When blended with the Y6, PCN3 with strong interchain interactions results in appropriate crystallinity and thermodynamic miscibility of the blend film. Grazing-incidence wide-angle X-ray scattering measurements indicate that PCN3 has more ordered arrangement and stronger π-π stacking than previous PCN2. Fourier-transform photocurrent spectroscopy and external quantum efficiency of electroluminescence measurements show that PCN3-based OSCs have lower energy loss than PCN2, which leads to their higher open-circuit voltage (0.873 V). The device based on PCN3 reaches power conversion efficiency (PCE) of 15.33% in binary OSCs, one of the highest values for OSCs with halogen-free donor polymers. The PCE of 17.80% and 18.10% are obtained in PM6:PCN3:Y6 and PM6:PCN3:BTP-eC9 ternary devices, much higher than those of PM6:Y6 (16.31%) and PM6:BTP-eC9 (17.33%) devices. Additionally, this ternary OSCs exhibit superior stability compared to binary host system. This work gives a promising path for halogen-free donor polymers to achieve low energy loss and high PCE.

Compromising Charge Generation and Recombination of Organic Photovoltaics with Mixed Diluent Strategy for Certified 19.4% Efficiency

The ternary blend is demonstrated as an effective strategy to promote the device performance of organic photovoltaics (OPVs) due to the dilution effect. While the compromise between the charge generation and recombination remains a challenge. Here, a mixed diluent strategy for further improving the device efficiency of OPV is proposed. Specifically, the high-performance OPV system with a polymer donor, i.e., PM6, and a nonfullerene acceptor (NFA), i.e., BTP-eC9, is diluted by the mixed diluents, which involve a high bandgap NFA of BTP-S17 and a low bandgap NFA of BTP-S16 (similar with that of the BTP-eC9). The BTP-S17 of better miscibility with BTP-eC9 can dramatically enhance the open-circuit voltage (V_OC ), while the BTP-S16 maximizes the charge generation or the short-circuit current density (J_SC ). The interplay of BTP-17 and BTP-S16 enables better compromise between charge generation and recombination, thus leading to a high device performance of 19.76% (certified 19.41%), which is the best among single-junction OPVs. Further analysis on carrier dynamics validates the efficacy of mixed diluents for balancing charge generation and recombination, which can be further attributed to the more diverse energetic landscapes and improved morphology. Therefore, this work provides an effective strategy for high-performance OPV for further commercialization.

Improved photovoltaic performance and robustness of all-polymer solar cells enabled by a polyfullerene guest acceptor

Fullerene acceptors typically possess excellent electron-transporting properties and can work as guest components in ternary organic solar cells to enhance the charge extraction and efficiencies. However, conventional fullerene small molecules typically suffer from undesirable segregation and dimerization, thus limiting their applications in organic solar cells. Herein we report the use of a poly(fullerene-alt-xylene) acceptor (PFBO-C12) as guest component enables a significant efficiency increase from 16.9% for binary cells to 18.0% for ternary all-polymer solar cells. Ultrafast optic and optoelectronic studies unveil that PFBO-C12 can facilitate hole transfer and suppress charge recombination. Morphological investigations show that the ternary blends maintain a favorable morphology with high crystallinity and smaller domain size. Meanwhile, the introduction of PFBO-C12 reduces voltage loss and enables all-polymer solar cells with excellent light stability and mechanical durability in flexible devices. This work demonstrates that introducing polyfullerenes as guest components is an effective approach to achieving highly efficient ternary all-polymer solar cells with good stability and mechanical robustness.

Subgap Absorption in Organic Semiconductors

Organic semiconductors have found a broad range of application in areas such as light emission, photovoltaics, and optoelectronics. The active components in such devices are based on molecular and polymeric organic semiconductors, where the density of states is generally determined by the disordered nature of the molecular solid rather than energy bands. Inevitably, there exist states within the energy gap which may include tail states, deep traps caused by unavoidable impurities and defects, as well as intermolecular states due to (radiative) charge transfer states. In this Perspective, we first summarize methods to determine the absorption features due to the subgap states. We then explain how subgap states can be parametrized based upon the subgap spectral line shapes. We finally describe the role of subgap states in the performance metrics of organic semiconductor devices from a thermodynamic viewpoint.

The Dynamics of Delocalized Excitations in Organic Solar Cells with Nonfullerene Acceptors

Recently, the performance of organic solar cells has been markedly improved benefiting from the development of nonfullerene acceptors (NFAs) with acceptor-donor-acceptor structures. Arising from the intermolecular electronic interactions between the electron donating and accepting units, intramoiety and interfacial delocalized excitations make a substantial contribution to the photocurrent generation. In this Perspective, we discuss recent studies on the excited-state dynamics responsible for the working mechanism in NFA-based organic solar cells and emphasize the dynamics of delocalized excitations in charge generation and recombination processes. The intramoiety delocalized excitations in NFAs enable charge separation without forming interfacial charge-transfer excitons first, allowing efficient photocharge generation in planar heterojunctions with reduced interfacial energy loss. We suggest a few research directions in elucidating the performance-limited processes toward the further optimization of NFA-based devices.

Organic photovoltaics: The current challenges

Organic photovoltaics are remarkably close to reaching a landmark power conversion efficiency of 20%. Given the current urgent concerns regarding climate change, research into renewable energy solutions is crucially important. In this perspective article, we highlight several key aspects of organic photovoltaics, ranging from fundamental understanding to implementation, that need to be addressed to ensure the success of this promising technology. We cover the intriguing ability of some acceptors to undergo efficient charge photogeneration in the absence of an energetic driving force and the effects of the resulting state hybridization. We explore one of the primary loss mechanisms of organic photovoltaics-non-radiative voltage losses-and the influence of the energy gap law. Triplet states are becoming increasingly relevant owing to their presence in even the most efficient non-fullerene blends, and we assess their role as both a loss mechanism and a potential strategy to enhance efficiency. Finally, two ways in which the implementation of organic photovoltaics can be simplified are addressed. The standard bulk heterojunction architecture could be superseded by either single material photovoltaics or sequentially deposited heterojunctions, and the attributes of both are considered. While several important challenges still lie ahead for organic photovoltaics, their future is, indeed, bright.

A Nitroxide Radical Conjugated Polymer as an Additive to Reduce Nonradiative Energy Loss in Organic Solar Cells

Nonfullerene-acceptor-based organic solar cells (NFA-OSCs) are now set off to the 20% power conversion efficiency milestone. To achieve this, minimizing all loss channels, including nonradiative photovoltage losses, seems a necessity. Nonradiative recombination, to a great extent, is known to be an inherent material property due to vibrationally induced decay of charge-transfer (CT) states or their back electron transfer to the triplet excitons. Herein, it is shown that the use of a new conjugated nitroxide radical polymer with 2,2,6,6-tetramethyl piperidine-1-oxyl side groups (GDTA) as an additive results in an improvement of the photovoltaic performance of NFA-OSCs based on different active layer materials. Upon the addition of GDTA, the open-circuit voltage (V_OC ), fill factor (FF), and short-circuit current density (J_SC ) improve simultaneously. This approach is applied to several material systems including state-of-the-art donor/acceptor pairs showing improvement from 15.8% to 17.6% (in the case of PM6:Y6) and from 17.5% to 18.3% (for PM6:BTP-eC9). Then, the possible reasons behind the observed improvements are discussed. The results point toward the suppression of the CT state to triplet excitons loss channel. This work presents a facile, promising, and generic approach to further improve the performance of NFA-OSCs.