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All-polymer solar cells (all-PSCs) based on polymerized small molecular acceptors (PSMAs) have made significant progress recently. Here, we synthesize two A-DA’D-A small molecule acceptor based PSMAs of PS-Se with benzo[c][1,2,5]thiadiazole A’-core and PN-Se with benzotriazole A’-core, for the studies of the effect of molecular structure on the photovoltaic performance of the PSMAs. The two PSMAs possess broad absorption with PN-Se showing more red-shifted absorption than PS-Se and suitable electronic energy levels for the application as polymer acceptors in the all-PSCs with PBDB-T as polymer donor. Cryogenic transmission electron microscopy visualizes the aggregation behavior of the PBDB-T donor and the PSMA in their solutions. In addition, a bicontinuous-interpenetrating network in the PBDB-T:PN-Se blend film with aggregation size of 10~20 nm is clearly observed by the photoinduced force microscopy. The desirable morphology of the PBDB-T:PN-Se active layer leads its all-PSC showing higher power conversion efficiency of 16.16%., Through development of non-fullerene acceptors, OPVs have reached efficiencies of 18%, yet the inadequate operational lifetime still poses a challenge for the commercialisation. Here, the authors investigate the origin of instability of NFA solar cells, and propose some strategies to mitigate this issue.

Summary As the power conversion efficiency of organic photovoltaic has been dramatically improved to over 18%, achieving long-term stability is now crucial for applications of this promising photovoltaic technology. Among the high-efficiency systems, most are using BTP-4F and its analogs as acceptors. Herein, we determine the thermal transition temperatures (Tg) of seven BTP analogs to develop a structure-Tg framework. Our results point out an unresolved molecular design conundrum on how to simultaneously achieve high performance and intrinsic stability with BTP-based acceptors. We also show that PC71BM has miscibility above the percolation threshold in PM6 and can maintain local charge percolation and improved stability in ternary devices. However, PC71BM is not miscible with BTP-C3-4F and unfavorable vertical gradients that develop during aging still degrade performance. This points to a second thermodynamic conundrum. A compound with differential miscibility in the donor polymer can only impact percolation, and a compound with differential miscibility with the BTP only impacts diffusion.

Rapid increase in the power conversion efficiency of organic solar cells (OSCs) has been achieved with the development of non-fullerene small-molecule acceptors (NF-SMAs). Although the morphological stability of these NF-SMA devices critically affects their intrinsic lifetime, their fundamental intermolecular interactions and how they govern property–function relations and morphological stability of OSCs remain elusive. Here, we discover that the diffusion of an NF-SMA into the donor polymer exhibits Arrhenius behaviour and that the activation energy Ea scales linearly with the enthalpic interaction parameters χH between the polymer and the NF-SMA. Consequently, the thermodynamically most unstable, hypo-miscible systems (high χ) are the most kinetically stabilized. We relate the differences in Ea to measured and selectively simulated molecular self-interaction properties of the constituent materials and develop quantitative property–function relations that link thermal and mechanical characteristics of the NF-SMA and polymer to predict relative diffusion properties and thus morphological stability. Studies on the morphology stability of polymer donor–small-molecule acceptor blends relevant to solar cell stability reveal relationships between their intermolecular interactions and the thermodynamic, kinetic, thermal and mechanical properties.

In developing organic semiconductor polymers for photovoltaic applications, chlorine substitution has become an effective strategy in replacing fluorine substitution to overcome the drawbacks of low yield and high cost, commonly associated with fluorination. In general, several molecular positions are available for chlorination. To obtain a clear understanding of the impact of chlorine substitution on the intrinsic polymer properties, an investigation of structure-property relationships is necessary. Herein, four donor-acceptor type polymers with the same conjugated backbone and flexible alkyl chains, but with chlorine atoms in different positions, are employed to systematically investigate the effect of the site of chlorination on the optoelectronic properties and photovoltaic performance. Substitution of fluorine by chlorine in the backbone slightly increases open circuit voltage (Voc) and fill factor (FF) of the solar cells but causes a loss of short-circuit current density (Jsc). The introduction of chlorine in the conjugated side chains, however, significantly improves Voc, FF, and power conversion efficiency, benefiting from a lower HOMO energy level, efficient and well-balanced transport properties, and superior nanoscale morphology. This journal is

Combining the acceptor–donor–acceptor-type fused ring-based molecular architecture into a polymeric backbone is a promising strategy to design polymer acceptors for high-performance all-polymer sol...

Donor polymer number-average molar mass (Mn) has long been known to influence organic photovoltaic (OPV) performance via changes in both the polymer properties and the resulting bulk heterojunction...

With the recent remarkable advances in the efficiency of organic solar cells, the need to distill key structure–property relationships for semiconducting materials cannot be understated. The fundam...

Currently, high-performance organic solar cells (OSCs) are mainly composed of narrow band gap (NBG) nonfullerene acceptors and medium band gap (MBG) polymer donors, whereas the solar cells based on...

Conjugated polymers have proven to be an important class of materials for flexible and stretchable electronics. To ensure long-term thermal and mechanical stability of associated devices, there is ...

Summary Polythiophene (PT) and its derivatives have attracted long-standing attention in the organic photovoltaic community for their low cost and high scalability of synthesis. However, due to the lack of rational guidelines in controlling morphology and matching materials, the power conversion efficiencies (PCEs) based on PTs reported so far are generally below 10%. Here, we establish the first-ever relationship between miscibility, morphology, and device performance of binary blends, based on various nonfullerene acceptors (ITIC-Th1, ITIC, IT4F, IDIC, and Y6) and a PT derivative named PDCBT-Cl by scattering and calorimetric characterizations. Benefiting from a properly quenched mixed phase, PDCBT-Cl:ITIC-Th1 system shows the best efficiency of over 12%. Conversely, the blend of PDCBT-Cl and the star acceptor Y6 remained in a homogeneous state due to their high miscibility, resulting in abysmal performance with PCE of 0.5%. Specific guidelines are also proposed to remediate the performance of PDCBT-Cl:Y6, which are crucial for advancing their practical applications.

Y6-type non-fullerene acceptors (NFAs) with an acceptor–donor–acceptor′–donor–acceptor (A–D–A′–D–A) structure have been very popular in the field of organic solar cells (OSCs) due to their excellent performances. In this study, two novel NFAs, BTPS-4F and BTPS-4Cl were designed by incorporating undecylthio side chains into a thienothiophene moiety and connecting it to two different halogenated 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile end groups (2F-IC and 2Cl-IC) respectively. When blended with a donor polymer, PM6, BTPS-4F-based devices achieved a high power conversion efficiency (PCE) of up to 16.2% with an open-circuit voltage (VOC) of 0.82 V, a short-circuit current density (JSC) of 25.2 mA cm−2 and a fill factor (FF) of 0.78, while BTPS-4Cl-based devices achieved an inferior PCE of 13.5%. This is the first time alkylthio chains are employed on Y6-like NFAs to achieve high-performance OSCs. Subsequent characterization showed that the upshifted energy level of BTPS-4F and the better intermolecular packing in PM6:BTPS-4F blends are the major reasons for the enhanced performance of BTPS-4F-based devices over BTPS-4Cl-based ones. This work provides high-performance non-fullerene acceptors for OSCs and demonstrates a promising molecular design strategy that can effectively regulate their energy levels and morphology.

Power conversion efficiencies (PCEs) in organic solar cells (OSCs) have rapidly improved in the last 5 years owing largely to the development of novel small-molecule acceptors and polymer donors with several systems’ performances exceeding 17%. A key factor for these materials implementation into industrial relevant devices is their active layer thickness tolerance as solar cell performances are typically reported with thicknesses on the order of 100–150 nm, but thicker films (ca. 300 nm) are needed for printing and roll-to-roll processing. In this report, two PM7 isomeric derivatives were synthesized featuring a chlorinated benzodithiophene and ester functionalized terthiophene moieties for the incorporation into non-fullerene OSCs. The fundamental difference between the two isomeric polymers is the location of the ester side chains where the PM7 D1 esters are located on the outer thiophene units, whereas the esters on PM7 D2 are located on the central thiophene unit. This simple modification produced polymers with similar absorption profiles, electrochemical onsets, charge carrier mobilities when blended with ITIC-4F, and grazing-incidence wide-angle X-ray scattering patterns. Thin-film (100 nm) OSCs were fabricated resulting in average PCEs of 11.6% for PM7, 12.1% for PM7 D1, and 9.9% for PM7 D2 when blended with ITIC-4F. In contrast, large differences are observed in PCE when the active layer thickness is increased to 180 nm resulting in a decrease in average PCE for PM7 D2 (5.3%), whereas PM7 D1 was able to retain a 11.9% average PCE. The difference in active layer thickness tolerance between PM7 D1 and PM7 D2 is rationalized by extracting the energetic disorder (σ) for hole transport using temperature dependent space-charge limited current studies. In the end, this study conveys how small changes to polymer structure, such as side chain placement, may have a small effect on thin-film polymer properties and device performance, but significant differences are realized when charges are transported over longer distances (thicker films, >150 nm).

Low cost photovoltaic materials and green solvent processing are important issues for commercial application of organic solar cells (OSCs). Here, we fabricated high-performance OSCs based on the low-cost conjugated polymer PTQ10 as a donor and the small molecule n-type organic semiconductor HO-IDIC-2F as an acceptor, using non-halogen tetrahydrofuran (THF) as the processing solvent. The power conversion efficiency (PCE) of the as-cast OSCs reached 12.20%, which is comparable with the ones processed with chloroform (CF) (12.43%). Thermal stability of the active layer was also studied by thermal treatment at 100 °C for 8 hours. As a result, the active layer morphology remained unchanged and the PCE of the devices still reached 12.13%, indicating a good thermal-stability of the PTQ10:HO-IDIC-2F blend film. As is commonly known, for OSCs fabricated on larger scales, the layer-by-layer (LL) method generally provides more advantages than the traditional D–A blend solution-processing method. Therefore, to test the compatibility of the THF solvent in the LL method, we fabricated blade-coated LL devices with THF as a single type of solvent for both the donor and acceptor, and the PCE could reach up to 11.85%. The surprisingly high performance of the blade-coated OSC based on the green solvent indicates that PTQ10 is a promising donor to be processed in green solvent for LL technology in the future.

Core engineering of small molecule acceptors (SMAs) is crucially important for enhancing device efficiency for nonfullerene organic solar cells (NF-OSCs). The most commonly used SMAs (e.g., ITIC) utilize indacenodithieno[3,2-b]thiophene (IDTT) as the central core, which has restricted their absorption ranges due to the weak electron-donating ability and short conjugation length. Here, we fused two electron-rich units, namely cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) and dithieno[3,2-b:2′,3′-d]pyran (DTPR), into the cores for constructing low-bandgap SMAs. The resulting CPDT-4Cl and DTPR-4Cl molecules exhibit extended nonacyclic central cores and strengthened intramolecular transfer (ICT) effect, resulting in red-shifted absorption (up to ∼950 nm) and up-shifted HOMO levels compared with IDTT-4Cl. Consequently, the NF-OSCs based on PTB7-Th:CPDT-4Cl and PTB7-Th:DTPR-4Cl achieved higher PCEs of 12.15% and 10.75%, respectively, than those of the PTB7-Th:IDTT-4Cl ones (7.70%). Notably, high short-circuit current densities (JSC) of 23–25 mA cm−2 were obtained by the CPDT-4Cl and DTPR-4Cl-based devices, indicating the great potential of the electron-donating CPDT and DTPR as promising building blocks to construct high-performance low-bandgap SMAs.

Small molecular acceptors (SMAs) have gained extensive research attention as they offer many attractive features and enable highly efficient organic solar cells (OSCs) that cannot be achieved using fullerene acceptors. Recently, a new SMA named Y6 was reported, yielding high-performance OSCs with an efficiency of 15.7%. This report has inspired the OSC community to study the structure–property relationship and further modify this important class of materials. In this work, we used the selenium (Se) substitution strategy and developed two new Y6-type SMAs to study the effect of Se atoms on materials properties and device performances. It is found that the introduction of Se atoms can red-shift the absorption spectra and enhance the aggregation of the resulting SMAs. Interestingly, the variations in the substitution positions of Se atoms induce different intramolecular charge transfer within the SMAs. Se substitution at the benzothiadiazole ring is more effective than that at the thienothiophene rings, leading to the increased short-circuit current density (JSC) and higher efficiencies of over 16%. This contribution suggests that appropriate Se substitution is a promising method for optimizing the absorption and aggregation of Y6-type SMAs, thus enhancing their OSC performances.

The rapid increase in the power conversion efficiency of organic solar cells (OSCs) during the last few years has been achieved through the development of non-fullerene small-molecule acceptors (NF-SMAs). Stability is now becoming a pressing concern. The presentation will discuss how intermolecular interactions govern morphological stability, specifically, how the diffusion of an NF-SMA exhibits Arrhenius behavior with an activation energy that scales linearly with the enthalpic Flory-Huggins interaction parameter. Consequently, the thermodynamically most unstable systems (high interaction parameter) are the most kinetically stabilized. In short, unfavorable interactions can enable stability. The activation energy is shown to scale with the glass transition temperature (Tg) of the NF-SMA and mechanical characteristics of the polymer (elastic modulus) of the polymers [1]. This allows predicting relative diffusion properties and thus morphological stability from simple analytical measurements or molecular dynamic simulations. Unfortunately, the star acceptor Y6 and its analogs have low glass transition temperatures, are highly diffusive and thus yield unstable morphologies. In contrast, IEICO-4F is a stable, high Tg material. The relationship of the Tg to the chemical structure is not yet understood except in the most general terms within a homologous series. The side-chains needed to provide solubility are the enemy of stability. Shorter side-chains are better for stability but often results in difficulties in processing. The impact of the molecular core on the Tg is not yet understood. A new approach to molecular design or novel stabilization strategies are needed if devices are to have high performance and high stability at the same time.

Summary The field of organic solar cells has seen rapid developments after the report of a high-efficiency (15.7%) small molecule acceptor (SMA) named Y6. In this paper, we design and synthesize a family of SMAs with an aromatic backbone identical to that of Y6 but with different alkyl chains to investigate the influence of alkyl chains on the properties and performance of the SMAs. First, we show that it is beneficial to use branched alkyl chains on the nitrogen atoms of the pyrrole motif of the Y6. In addition, the branching position of the alkyl chains also has a major influence on material and device properties. The SMA with 3rd-position branched alkyl chains (named N3) exhibits optimal solubility and electronic and morphological properties, thus yielding the best performance. Further device optimization using a ternary strategy allows us to achieve a high efficiency of 16.74% (and a certified efficiency of 16.42%).

The design of polymer structures has played a vital role in improving the efficiency of organic solar cells (OSCs). A common approach to increase solar cell efficiency is to add a specific substitu...

D–A conjugated polymers have played critical roles in recently reported nonfullerene acceptors-based polymer solar cells (NF-PSCs) with high performance. Although the molecular design of the D–A po...

End group engineering is critical for developing A-D-A-type non-fullerene acceptors with suitable absorption properties and high crystallinity for organic solar cells. Here, we report the synthesis of a novel chlorinated thiophene end group and its application in A-D-A-type acceptors. When combined with a tetraoctyl-substituted indacenodithieno[3,2-b]thiophene core, the resulting C8-ITCC-Cl acceptor exhibited red-shifted absorption and downshifted energy levels relative to those of the nonchlorinated analogue, C8-ITCC, due to the strong electron-withdrawing ability of the chlorine atoms as revealed by theoretical calculations. Moreover, the noncovalent interactions induced by the chlorine atoms enabled improved lamellar and π–π stacking of C8-ITCC-Cl, leading to a higher electron mobility of 6.6 × 10–4 cm2 V–1 s–1 and a fill factor of 73% of the PBDB-TF:C8-ITCC-Cl devices. Consequently, a high power conversion efficiency of 12.7% was achieved by the C8-ITCC-Cl-based devices, which outperformed the C8-ITCC...

Two polymers J75 and J76 with selenophene instead of thiophene on the conjugated side chain of benzodithiophene (BDT) unit or π bridges of polymer J71 were designed and synthesized, for investigating the effect of selenophene substitution on the photovoltaic performance of the conjugated polymer donors in comparison with J71. The selenophene π bridges in J76 can narrow optical band gap and red-shift absorption of the polymer film by ca. 25 nm, but the highest occupied molecular orbital (HOMO) energy level (EHOMO) of J76 is up-shifted slightly by 0.04 eV. Two typical electron acceptors of fullerene derivative PC71BM and the nonfullerene acceptor m-ITIC were used to investigate photovoltaic performance of the polymer donors. For the PC71BM-based polymer solar cells (PSCs), J76 with selenophene π bridges shows the best power-conversion efficiency (PCE) of 8.40% in comparison with the J71-based device (PCE = 6.79%), benefitted from the red-shifted absorption, larger coherence length, purer average domains, an...

Two-dimensional (2D) organic−inorganic hybrid perovskites (OIHPs) have showed impressive stability, compared to their three-dimensional (3D) counterparts. However, tuning the chemical structure of the organic cations to simultaneously improve the device performance and stability of 2D OIHP solar cells is rarely reported. Here, we demonstrate that by introducing a classic noncovalent aryl-perfluoroaryl interaction, 2D OIHP solar cells with 1:1 mixed phenethylammonium (PEA) and perfluorophenethylammonium (F5-PEA) can achieve an efficiency of >10% with much enhanced stability using a simple deposition at low temperature without using any additives. The competing effects of surface morphology and crystal orientation with an increased amount of F5-PEA result in the highest efficiency at a 1:1 ratio, while single-crystal studies reveal the expected aryl-perfluoroaryl interaction, accounting for the highest device stability of 2D OIHP solar cell at 1:1 ratio as well. This work provides an example where tuning the interactions of organic cations via molecular engineering can have a profound effect on device performance and stability of 2D OIHP solar cells.

Fluorination of the donor polymer or non-fullerene acceptor (NFA) in an organic photovoltaic device is an effective method to improve device efficiency. Although there have been many studies on donor polymer fluorination, blends containing both a fluorinated donor and fluorinated NFA have rarely been reported. In this study, we use two donor polymers (4′-FT-HTAZ and 4′-FT-FTAZ) and two NFAs (ITIC-Th and ITIC-Th1) with different amounts of fluorine (from 2F to 6F) to investigate how the degree of fluorination in a blend impacts device performance. We find that fluorinating the NFA leads to a higher short-circuit current density (Jsc) and fill factor (FF), however, the open-circuit voltage (Voc) is decreased due to a depressed lowest unoccupied molecular orbital (LUMO) level. Adding additional fluorine to the donor polymer does not have a large effect on the Jsc or FF, but it does lead to an improved Voc. By fluorinating the NFA and having more fluorine on the donor polymer, we obtain both a high Jsc and Voc simultaneously, leading to a power conversion efficiency over 10% in the case of 4′-FT-FTAZ:ITIC-Th1, which has the most amount of fluorine (6F).

In addition to the innovation of nonfullerene acceptors, the development of highly efficient nonfullerene organic solar cells requires the design of new polymer donors and fundamental understanding...

Summary The general lack of knowing the quench depth and the convolution with key kinetic factors has confounded deeper understanding of the respective importance of these factors in the morphology development of organic solar cells. Here, we determine the quench depth of a high-efficiency system and delineate the need to kinetically quench the mixed domains to a composition close to the percolation threshold. Importantly, the ability to achieve such a quench is very sensitive to structural parameters in polymer solar cells (PSCs) of the polymer PBDB-TF. Only the highest-molecular-weight polymer is able of earlier liquid-solid transition to “lock in” a high-performing PSC morphology with a composition above the miscibility limit and with an efficiency of over 13%. Systems with deep quench depths are therefore sensitive to molecular weight and the kinetic factors of the casting, likely impacting fabrication yield and reliability. They also need to be vitrified for stable performance.

Here we demonstrate efficient all-polymer solar cells (all-PSCs) based on a polymer acceptor named PFBDT-IDTIC. By combining PFBDT-IDTIC with a fluorinated donor polymer (PM6), a high power conversion efficiency of 10.3% can be achieved, which is the highest value reported to date for single-junction all-PSCs. This performance can be attributed to its good absorption property (absorption coefficient: 2.74 × 105 cm–1) and high electron mobility of PFBDT-IDTIC. It is also found that the choice of donor polymer has major impacts on the performance of the cell. By replacing PBDB-T with its fluorinated counterpart, PM6, the VOC, JSC, and FF of the devices were all improved, which can be attributed to the deeper HOMO level of PM6 and more crystalline and pure domains of the active layer blends. Our study provides a promising polymer acceptor for all-PSCs and also shows that selecting a matching donor polymer is important in achieving the optimal all-PSC performance.

Indacenodithiophene (IDT) is a promising building block for designing organic semiconductors. In this work, a new decacyclic building block (m-IDTIDT) with a double IDT unit and meta-alkyl-phenyl substitutions was synthesized and used for designing a new non-fullerene electron acceptor (m-IDTIDT-FIC). Compared to p-IDTIDT-IC, m-IDTIDT-FIC has a smaller π–π distance, larger crystalline coherence length, and higher electron mobility. Thus, polymer solar cells (PSCs) based on PCE10:m-IDTIDT-FIC can achieve a higher PCE (8.27%) than PCE10:p-IDTIDT-IC-based PSCs (6.48%). In addition, PSCs based on J71:m-IDTIDT-FIC show better photovoltaic performances than PSCs based on PCE10:m-IDTIDT-FIC. This is because the J71:m-IDTIDT-FIC blend film has more balanced charge transport ability, a smooth surface, and higher relative domain purity than the PCE10:m-IDTIDT-FIC blend film. PSCs based on J71:m-IDTIDT-FIC exhibited the best photovoltaic performances with a PCE of 11.32%, an open-circuit voltage (Voc) of 0.92 V, a short-circuit current (Jsc) of 18.01 mA cm−2, and a fill factor (FF) of 68.30%. These results indicate that m-IDTIDT with meta-alkyl-phenyl substitutions is a promising IDT-based building block for high-performance non-fullerene electron acceptors.

Nonfullerene all-small molecule organic solar cells (OSCs) have attracted significant attention due to their competitive performance compared to the well-studied polymer-based OSCs. Small molecules are considered to be materials of choice for industrial scale OSC production due to their relatively high purity, reproducibility, and well-defined molecular weights. Despite the recent progress in all-small molecule solar cells, the mesoscale morphology of these devices remains elusive. A fundamental understanding of the structure–property relationships of these systems is thus critical for further device performance enhancement. Here, we report a dramatic change in device performance of a nonfullerene all-small molecule bulk heterojunction system upon changing the side chains of the electron donor small molecules. Morphology analysis via resonant soft X-ray scattering shows that the average composition variation and the size of the smaller domains of the all-small molecule blends are the most influential factors that derive the photogeneration and collection efficiency of charges in efficient all-small molecule OSCs. The observed side-chain and processing initiated morphological changes resulted in distinct device efficiencies in these systems. Despite the fact that side chains are mainly meant to facilitate material dissolution, the current study clearly shows that they can also impose a significant effect on material organization and photovoltaic properties. The findings will give a clear understanding of the role of side chains and processing in material organization and device functionality, and the knowledge gained in this study is quite crucial in designing next-generation materials and devices.

High-performance organic semiconductors that can be processed with environmentally benign solvents are highly desirable for printable optoelectronics. Herein, four acceptor–donor–acceptor conjugated molecules, i.e., DRTT-T, DRTT-R, DRTT-OR and DRTT, with 3-ethylrhodanine as acceptor terminal units and 2,5-bis(4,8-di(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophen-2-yl)thieno[3,2-b]thiophene derivatives as donor units were synthesized. 5-(2-Ethylhexyl)thiophen-2-yl, 2-ethylhexyl and 2-ethylhexyloxy were introduced at the β-positions of the central thieno[3,2-b]thiophene (TT) units in DRTT-T, DRTT-R and DRTT-OR, respectively, and unsubstituted TT was used as the central unit in DRTT. As revealed by density functional theory calculations, DRTT-OR and DRTT adopt an almost planar geometry, while DRTT-T and DRTT-R have “twisted” backbones due to the introduction of bulky substituents on TT units. Differing from DRTT-OR and DRTT which are only well soluble in chlorinated solvents such as chloroform, DRTT-T and DRTT-R also show high solubility in “greener” solvents, including toluene and tetrahydrofuran (THF). Non-fullerene small molecule (NFSM) organic solar cells (OSCs) were fabricated with these molecules as donor materials. The molecules (DRTT-T and DRTT-R) with twisted backbones displayed remarkably higher device performance compared to more planar ones (DRTT-OR and DRTT), attributed to the formation of ordered face-on microstructures with π–π stacking distances of 3.7–3.8 A and interpenetration networks of donor and acceptor components in the blend films based on DRTT-T and DRTT-R. Most importantly, the power conversion efficiencies (PCEs) of DRTT-T and DRTT-R based devices processed with THF reached 9.37% and 10.45%, respectively. This study demonstrates that “twisting” conjugated backbones is an appropriate strategy to design eco-friendly solvent processable organic semiconductors for high-efficiency OSCs.

All-polymer solar cells (all-PSCs) with a p-type conjugated polymer as a donor and an n-type conjugated polymer as an acceptor have attracted increasing attention recently due to their good film-forming properties, excellent flexibility and high morphological stability. Herein, we synthesized a new n-type conjugated copolymer LA03 based on naphthalene diimide (NDI) and benzotriazole (BTZ) with thiophene π-bridges, which is a structure modification of N2200 by inserting a benzotriazole unit between its two thiophene units. Compared with N2200, LA03 exhibits blue-shifted absorption and a higher lowest unoccupied molecular orbital (LUMO) energy level. The LA03-based all-PSCs with wide-band-gap polymer PBDB-T as a donor, achieved a power conversion efficiency (PCE) of 6.49%, with a high open circuit voltage (Voc) of 0.937 V, which is improved in comparison with the PBDB-T:N2200-based all-PSCs with a PCE of 5.85% and a Voc of 0.812 V. The higher Voc of the LA03-based devices is benefitted from the up-shifted LUMO energy level of the LA03 acceptor. The results indicate that inserting a benzotriazole unit into the main chain of N2200 is an effective way to upshift its LUMO energy level and increase the Voc of the all-PSCs.

Adsorption of the acceptor material tetracyanoquinodimethane can control optoelectronic properties of MoS2 by accepting defect generated excess negative charge from the surface that would otherwise interfere with radiative decay processes. Angle Resolved Photoelectron Spectroscopy measurements show that the MoS2 band structure near the Γ point shifts rigidly upward by ∼0.2 eV for a complete surface coverage of acceptor species as expected for an upward Fermi level shift due to charge transfer to the TCNQ. The molecular adsorbate orbitals visible in photoemission are indicative of an anionic species, consistent with interfacial charge transfer but without evidence for hybrid states arising from covalent adsorbate-surface interactions. Thus, our interface studies support the notion that molecular adsorbates are a useful tool for controlling optoelectronic functionality in 2D materials without fundamentally modifying their favorable band structures.

Newly developed fused-ring electron acceptors (FREAs) have proven to be an effective class of materials for extending the absorption window and boosting the efficiency of organic photovoltaics (OPVs). While numerous acceptors have been developed, there is surprisingly little structural diversity among high performance FREAs in literature. Of the high efficiency electron acceptors reported, the vast majority utilize derivatives of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) as the acceptor moiety. It has been postulated that the high electron mobility exhibited by FREA molecules with INCN end groups is a result of close π-π stacking between the neighboring planar INCN groups, forming an effective charge transport pathway between molecules. To explore this as a design rationale for electron acceptors, we synthesized a new fused-ring electron acceptor, IDTCF, which has methyl substituents out of plane to the conjugated acceptor backbone. These methyl groups hinder packing and expand the π-π stacking distance by ∼1 Å, but have little impact on the optical or electrochemical properties of the individual FREA molecule. The extra steric hindrance from the out of plane methyl substituents restricts packing and results in large amounts of geminate recombination, thus degrading the device performance. Our results show that intermolecular interactions (especially π-π stacking between end groups) play a crucial role in performance of FREAs. We demonstrated that the planarity of the acceptor unit is of paramount importance as even minor deviations in end group distance are enough to disrupt crystallinity and cripple device performance.

Conjugated polymers have a long history of exploration and use in organic solar cells, and over the last twenty-five years, marked increases in the solar cell efficiency have been achieved. However, the synthetic complexity of these materials has also drastically increased, which makes the scalability of the highest-efficiency materials difficult. If conjugated polymers could be designed to exhibit both high efficiency and straightforward synthesis, the road to commercial reality would be more achievable. For that reason, a new synthetic approach was designed towards PTQ10 (=poly[(thiophene)-alt-(6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline)]). The new synthetic approach to make PTQ10 brought a significant reduction in cost (1/7th the original) and could also easily accommodate different side chains to move towards green processing solvents. Furthermore, high-efficiency organic solar cells were demonstrated with a PTQ10:Y6 blend exhibiting approximately 15 % efficiency.

Silicon phthalocyanines (SiPcs) have shown great potential as n-type or ambipolar organic semiconductors in organic thin-film transistors (OTFTs) and organic photovoltaics. Although properly designed SiPcs rival current state-of-the-art n-type organic semiconducting materials, relatively few structure−property relationships have been established to determine the impact of axial substituents on OTFT performance, hindering the intelligent design of the next generation of SiPcs. To address this omission, we have developed structure−property relationships for vapor-deposited SiPcs with phenoxy axial substituents. In addition to thorough electrical characterization of bottom-gate top-contact OTFTs, we extensively investigated SiPc thin films using X-ray diffraction, atomic force microscopy (AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and density functional theory (DFT) modeling. OTFT performance, including relative electron mobility (μe) of materials, was in general agreement with values obtained through DFT modeling including reorganization energy. Another significant trend observed from device performance was that increasing the electron-withdrawing character of the axial pendant groups led to a reduction in threshold voltage (VT) from 47.9 to 21.1 V. This was corroborated by DFT modeling, which predicted that VT decreases with the square of the dipole induced at the interface between the SiPc pendant and substrate. Discrepancies between modeling predictions and experimental results can be explained through analysis of thin-film morphology and orientation by AFM and GIWAXS. Our results demonstrate that a combination of DFT modeling to select prospective candidate materials, combined with appropriate processing conditions to deposit molecules with a favorable thin-film morphology in an “edge-on” orientation relative to the substrate, yields high-performance n-type SiPc-based OTFTs.

Organic solar cells are composed of electron donating and accepting organic semiconductors. Whilst a significant palette of donors has been developed over three decades, until recently only a small number of acceptors have proven capable of delivering high power conversion efficiencies. In particular the fullerenes have dominated the landscape. In this perspective, the emergence of a family of materials-the non-fullerene acceptors (NFAs) is described. These have delivered a discontinuous advance in cell efficiencies, with the significant milestone of 20% now in sight. Intensive international efforts in synthetic chemistry have established clear design rules for molecular engineering enabling an ever-expanding number of high efficiency candidates. However, these materials challenge the accepted wisdom of how organic solar cells work and force new thinking in areas such as morphology, charge generation and recombination. This perspective provides a historical context for the development of NFAs, and also addresses current thinking in these areas plus considers important manufacturability criteria. There is no doubt that the NFAs have propelled organic solar cell technology to the efficiencies necessary for a viable commercial technology-but how far can they be pushed, and will they also deliver on equally important metrics such as stability? The work was supported by the Ser Cymru II Program through the Welsh Government, European Regional Development Fund, Welsh European Funding Office, and Swansea University strategic initiative in Sustainable Advanced Materials. A.A. is a Ser Cymru II Rising Star Fellow and P.M. is a Ser Cymru II National Research Chair. T.W. acknowledges financial support from the National Natural Science Foundation of China (21774097). Z.X. and L.D. thank National Key Research and Development Program of China (2017YFA0206600) and National Natural Science Foundation of China (51773045, 21772030, 51922032, 21961160720) for financial support. J.N. thanks the European Research Council for support under the European Union's Horizon 2020 research and innovation program (grant agreement No 742708). Armin, A; Meredith, P (corresponding author), Swansea Univ, Dept Phys, Sustainable Adv Mat Ser SAM, Singleton Pk, Swansea SA2 8PP, W Glam, Wales. ardalan.armin@swansea.ac.uk; paul.meredith@swansea.ac.uk

Polymerization sites of small molecule acceptors (SMAs) play vital roles in determining device performance of all-polymer solar cells (all-PSCs). Different from our recent work about fluoro- and bromo- co-modified end group of IC-FBr (a mixture of IC-FBr1 and IC-FBr2), in this paper, we synthesized and purified two regiospecific fluoro- and bromo- substituted end groups (IC-FBr-o & IC-FBr-m), which were then employed to construct two regio-regular polymer acceptors named PYF-T-o and PYF-T-m, respectively. In comparison with its isomeric counterparts named PYF-T-m with different conjugated coupling sites, PYF-T-o exhibits stronger and bathochromic absorption to achieve better photon harvesting. Meanwhile, PYF-T-o adopts more ordered inter-chain packing and suitable phase separation after blending with the donor polymer PM6, which resulted in suppressed charge recombination and efficient charge transport. Strikingly, we observed a dramatic performance difference between the two isomeric polymer acceptors PYF-T-o and PYF-T-m. While devices based on PM6:PYF-T-o can yield power conversion efficiency (PCE) of 15.2 %, devices based on PM6:PYF-T-m only show poor efficiencies of 1.4 %. This work demonstrates the success of configuration-unique fluorinated end groups in designing high-performance regular polymer acceptors, which provides guidelines towards developing all-PSCs with better efficiencies.

Organic solar cells based on D18:Y6 recently exhibited a record power conversion efficiency of over 18%. We have initially studied the molecular packing and thermodynamic properties of three D18:NFA blends by employing synchrotron X-ray techniques, secondary ion mass spectrometry and UV-vis miscibility measurements. The D18 polymer exhibits strong chain extension in films, which is beneficial to charge transport. Miscibility and other characterizations explain the disparate performance of three systems and the processing procedures. Our contribution reveals several unique property–performance relations of D18-based photovoltaic devices and help guide design or fabrication of yet higher efficiency organic solar cells.

Recently, the efficiencies of nonfullerene organic photovoltaics (OPVs) have surpassed 18%. The realization of these high-efficiency OPVs is based on the use of push-pull type conjugated polymer donors, which are costly and not scalable. In contrast, polythiophenes (PTs) hold great promises in cost and scalability, rendering them alternatives to the push-pull type donors for commercial applications. Here, we reveal the crucial role of miscibility and crystallinity in determining the performance of PT:nonfullerene systems. Our study underscores the need for nonfullerene acceptors with much lower miscibility in matching PTs and provides design rules for higher efficiency PT:nonfullerene solar cells.

Regulating molecular structure to optimize the active layer morphology is of considerable significance for improving the power conversion efficiencies (PCEs) in organic solar cells (OSCs). Herein, we demonstrated a simple ternary copolymerization approach to develop a terpolymer donor PM6-Tz20 by incorporating the 5,5'-dithienyl-2,2'-bithiazole (DTBTz, 20 mol%) unit into the backbone of PM6 (PM6-Tz00). This method can effectively tailor the molecular orientation and aggregation of the polymer, and then optimize the active layer morphology and the corresponding physical processes of devices, ultimately boosting FF and then PCE. Hence, the PM6-Tz20: Y6-based OSCs achieved a PCE of up to 17.1% with a significantly enhanced FF of 0.77. Using Ag (220 nm) instead of Al (100 nm) as cathode, the champion PCE was further improved to 17.6%. This work provides a simple and effective molecular design strategy to optimize the active layer morphology of OSCs for improving photovoltaic performance.

Developing high-performance donor polymers is important for nonfullerene organic solar cells (NF-OSCs), as state-of-the-art nonfullerene acceptors can only perform well if they are coupled with a matching donor with suitable energy levels. However, there are very limited choices of donor polymers for NF-OSCs, and the most commonly used ones are polymers named PM6 and PM7, which suffer from several problems. First, the performance of these polymers (particularly PM7) relies on precise control of their molecular weights. Also, their optimal morphology is extremely sensitive to any structural modification. In this work, a family of donor polymers is developed based on a random polymerization strategy. These polymers can achieve well-controlled morphology and high-performance with a variety of chemical structures and molecular weights. The polymer donors are D-A1-D-A2-type random copolymers in which the D and A1 units are monomers originating from PM6 or PM7, while the A2 unit comprises an electron-deficient core flanked by two thiophene rings with branched alkyl chains. Consequently, multiple cases of highly efficient NF-OSCs are achieved with efficiencies between 16.0% and 17.1%. As the electron-deficient cores can be changed to many other structural units, the strategy can easily expand the choices of high-performance donor polymers for NF-OSCs.

High-performance low-band-gap polymer semiconductors are visibly colored, making them unsuitable for transparent and imperceptible electronics without reducing film thickness to the nanoscale range. Herein, we demonstrate polymer/insulator blends exhibiting favorable miscibility that improves the transparency and carrier transport in an organic field-effect transistor (OFET) device. The mesoscale structures leading to more efficient charge transport in ultrathin films relevant to the realization of transparent and flexible electronic applications are explored based on thermodynamic material interaction principles in conjunction with optical and morphological studies. By blending the commodity polymer polystyrene (PS) with two high-performing polymers, PDPP3T and P (NDI2OD-T2) (known as N2200), a drastic difference in morphology and fiber network are observed due to considerable differences in the degree of thermodynamic interaction between the conjugated polymers and PS. Intrinsic material interaction behavior establishes a long-range intermolecular interaction in the PDPP3T polymer fibrillar network dispersed in the majority (80%) PS matrix resulting in a ca. 3-fold increased transistor hole mobility of 1.15 cm2 V-1 s-1 (highest = 1.5 cm2 V-1 s-1) as compared to the pristine material, while PS barely affects the electron mobility in N2200. These basic findings provide important guidelines to achieve high mobility in transparent OFETs.

Some fundamental questions in the organic solar cell (OSC) community are related to the role of bulk and interfacial morphology on key processes such as charge generation, recombination, and extraction that dictate power conversion efficiencies (PCEs). The challenges with answering these questions arise due to the difficulty in accurately controlling, as well as comprehensively characterizing the morphology in bulk-heterojunction (BHJ) OSC blends. In this work, large variations in the interfacial and bulk morphologies of different low molecular weight fraction (LMWF) PM6:Y6 blends were detected despite the blends being fabricated from ostensibly the same building blocks. A drop in PCE from ∼15% to ∼5% was observed when the concentration of LMWFs of the PM6 polymer was increased from 1% to 52%. The drop in PCEs is found to be due to the lowering of the short-circuit current density (JSC) and fill-factor (FF) values as a result of compromised charge generation efficiencies, increased bulk trap densities, reduced charge transport, and inefficient charge extraction. The origin of the high device performance in the 1% LMWF blend is rationalized by the favorable bulk and interfacial morphological features, resolved from four techniques at sub-nanometer to sub-micrometer length scales. First, the closer donor:acceptor (D:A) interactions, smaller D and A domains, and increased D:A interfacial area facilitate ultrafast electron and hole transfer at the D:A interface. Second, the better long-range ordering and optimal phase separation of the D:A regions lead to superior charge transport and extraction.

Molecular packing, crystallinity, and texture of semiconducting polymers are often critical to performance. Although frame-works exist to quantify the ordering, interpretations are often just qualitative, resulting in imprecise and liberal use of terminology. Here, we reemphasize the continuity of the degree of molecular ordering and advocate that a more nuanced and consistent terminology is used with regards to crystallinity, semicyrstallinity, paracrystallinity, crystallite/aggregate, and related characteristics. We are motivated in part by our own imprecise and inconsistent use of terminology and the need to have a primer or tutorial reference to teach new group members. We show that a deeper understanding can be achieved by combining grazing-incidence wide-angle X-ray scattering and differential scanning calorimetry. We classify a broad range of representative polymers into four proposed categories based on the quantitative analysis of molecular order based on the paracrystalline disorder parameter (g). A small database is presented for over 10 representative conjugated and insulating polymers ranging from amorphous to semicrystalline. Finally, we outline the challenges to rationally design perfect polymer crystals and propose a new molecular design approach that envisions conceptual molecular grafting that is akin to strained and unstrained hetero-epitaxy in classic (compound) semiconductors thin film growth., Comment: Review article, 37 pages, 9 figures

Compared to conjugated polymers, small-molecule organic semiconductors present negligible batch-to-batch variations, but presently provide comparatively low power conversion efficiencies (PCEs) in small-molecular organic solar cells (SM-OSCs), mainly due to suboptimal nanomorphology. Achieving precise control of the nanomorphology remains challenging. Here, two new small-molecular donors H13 and H14, created by fluorine and chlorine substitution of the original donor molecule H11, are presented that exhibit a similar or higher degree of crystallinity/aggregation and improved open-circuit voltage with IDIC-4F as acceptor. Due to kinetic and thermodynamic reasons, H13-based blend films possess relatively unfavorable molecular packing and morphology. In contrast, annealed H14-based blends exhibit favorable characteristics, i.e., the highest degree of aggregation with the smallest paracrystalline π–π distortions and a nanomorphology with relatively pure domains, all of which enable generating and collecting charges more efficiently. As a result, blends with H13 give a similar PCE (10.3%) as those made with H11 (10.4%), while annealed H14-based SM-OSCs have a significantly higher PCE (12.1%). Presently this represents the highest efficiency for SM-OSCs using IDIC-4F as acceptor. The results demonstrate that precise control of phase separation can be achieved by fine-tuning the molecular structure and film formation conditions, improving PCE and providing guidance for morphology design.

Polymer conductors that are solution-processable provide an opportunity to realize low-cost organic electronics. However, coating sequential layers can be hindered by poor surface wetting or dissolution of underlying layers. This has led to the use of transfer printing where solid film inks are transferred from a donor substrate to partially fabricated devices using a stamp. This approach typically requires favorable adhesion differences between the stamp, ink, and receiving substrate. Here, we present a shear-assisted organic printing (SHARP) technique that employs a shear load on a post-less polydimethylsiloxane (PDMS) elastomer stamp to print large-area polymer films that can overcome large unfavorable adhesion differences between the stamp and receiving substrate. We explore the limits of this process by transfer printing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films with varied formulation that tune the adhesive fracture energy. Using this platform, we show that the SHARP process is able to overcome a 10-fold unfavorable adhesion differential without the use of a patterned PDMS stamp, enabling large-area printing. The SHARP approach is then used to print PEDOT:PSS films in the fabrication of high-performance semitransparent organic solar cells.

Side-chain engineering of conjugated polymer donor materials is an important way for improving photovoltaic performances of polymer solar cells (PSCs). On the basis of the polymer J61 synthesized in our group, here, we design and synthesize three new 2D-conjugated polymers J62, J63, and J64 with different types of side chains to further investigate the effect of side chain on their physicochemical and photovoltaic properties. With the narrow bandgap n-type organic semiconductor (n-OS) ITIC as acceptor, the optimized PSCs based on polymer donor of J62 with linear octyl, J63 with linear unsaturated hexylene, and J64 with cyclohexane side chains display power conversion efficiency (PCE) of 10.81%, 8.13%, and 8.59%, respectively. After thermal treatment at 200 °C for 2 h on the active layer,the PCE of the PSC based on J63 still keeps 92% of the original value, which verifies that the cross-linking of the polymer can improve the thermal stability of PSCs. Morphological studies show that the active layer based ...

Polymer acceptors based on perylene diimide (PDI) with three symmetrical S-heteroacene backbone units of different sizes were synthesized for use in all-polymer solar cells (all-PSCs). The effects of varying the size of the heteroacene unit on the backbone of the PDI polymer are evident in the absorption spectra and their energy level offsets, which are correlated with their morphological and photovoltaic properties. These newly synthesized polymers were employed as acceptors with the polymer poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)] (PPDT2FBT) as the donor in all-PSCs that were found to exhibit modest power conversion efficiencies. The variations in photovoltaic properties of all-PSCs are investigated by characterizing the charge generation, carrier mobilities and recombination. The morphological disorder at the polymer/PPDT2FBT interface and average composition variations are revealed by using grazing incidence wide angle X-ray scattering (GIWAXS) and resonance soft X-ray scattering (R-SoXS) characterizations.

Non-fullerene acceptors (NFAs) have been demonstrated to be promising candidates for highly efficient organic photovoltaic (OPV) devices. The tunability of absorption characteristics of NFAs can be used to make OPVs with complementary donor-acceptor absorption to cover a broad range of the solar spectrum. However, both charge transfer from donor to acceptor moieties and energy (energy) transfer from high-bandgap to low-bandgap materials are possible in such structures. Here, we show that when charge transfer and exciton transfer processes are both present, the coexistence of excitons in both domains can cause a loss mechanism. Charge separation of excitons in a low-bandgap material is hindered due to exciton population in the larger bandgap acceptor domains. Our results further show that excitons in low-bandgap material should have a relatively long lifetime compared to the transfer time of excitons from higher bandgap material in order to contribute to the charge separation. These observations provide significant guidance for design and development of new materials in OPV applications.

The effects of fused-ring regiochemistry on the physicochemical and photovoltaic properties of n-type organic semiconductor (n-OS) acceptors are investigated. Two n-OS isomers TPTC and TPTIC were prepared with different oxygen positions in the central fused-ring unit of the acceptor molecules: oxygen is connected with benzene in TPTC and it is connected with two thiophenes in TPTIC. It is found that TPTC tends to cause excessive self-aggregation with several different packing motifs or polymorphs, while TPTIC with compact alkyl chains forms well-defined crystals. The electron mobility of TPTC, which is measured by the space-charge-limited current (SCLC) method, is much lower than that of TPTIC. When blending these acceptors with the polymer PTQ10, excessive self-aggregation of TPTC leads to large phase separation and exhibits little change after thermal annealing treatment, while the intermolecular interaction in TPTIC is appropriate to achieve suitable phase separation in its blend films with PTQ10, and the stacking of both crystallites was obviously improved after thermal annealing. Thus the PSCs with TPTIC as the acceptor show a much higher power conversion efficiency (PCE) of 10.42%, in comparison with that (1.97%) of the device with TPTC as the acceptor. These results indicate that the regiochemistry of the n-OS acceptors greatly influences the aggregation behavior of the molecules, which strongly affects the performance of the PSCs, and the structure–property relationship of the materials with the regiochemistry could guide the development of high performance n-OS acceptors.

Reducing the voltage loss is considered a promising route to improve the performance of organic solar cells (OSCs). In this work, two donor polymers PT4FB and PTT4FB based on a tetra-fluorinated benzene building block are synthesized and applied to non-fullerene OSC devices, which achieve high open-circuit voltage (VOC). When PT4FB is combined with IDIC, the blend exhibits large fiber-like domains (size ∼39 nm) due to the excessive aggregation of PT4FB, which results in a relatively low short-circuit current (JSC) of 14.06 mA cm−2 and a moderate power conversion efficiency (PCE) of 9.81%. To weaken the aggregation of PT4FB, we replace the alkyl chains on the polymer backbone with alkylthiophene side chains to afford a novel donor polymer PTT4FB with reduced packing and crystallinity. As a result, the PTT4FB:IDIC-based blend shows significantly smaller domain sizes (24 nm) than the PT4FB-based device, contributing to a remarkably increased JSC and an overall higher PCE of 10.60%. The synergistic effects of abundant fluorination and alkylthiophene side chains can be utilized as a general method to tune the morphological and energetic properties when designing novel donor polymers to achieve both high VOC and favorable morphology for efficient non-fullerene OSCs.