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Chemical degradation and morphology failure of ink-processed organic solar cells are now extensively studied. In contrast, the general problem that inks prepared via thermal and mechanical agitation degrade and age rapidly at room temperature has yet to be delineated as a commercialization bottleneck and resolved. This study unveils the intrinsic aging of common polymer:nonfullerene acceptor (NFA) binary inks and the impact of electro-optically active component additives on ink shelf life. As a result, we developed an effective approach to slow down the ink aging by employing an additive (i.e. PCBM variants) with high miscibility with the polymer and NFA. It is inferred that the PCBMs in the inks acts as a co-solvent and slows down the polymer and possibly the NFA pre-aggregation, preventing the formation of large domains in the films. At the same time, the PCBMs dissolved in the polymer-rich phase of the devices can maintain the electron percolations and hence benefit charge creation and collection. The method of introducing a hyper-miscible third component, that is, with concentration above the percolation threshold, to improve ink shelf life and maintain the percolation is delineated for the first time. It represents a synergistic approach to promote the scale-up of ink-processed organic solar cells. [ABSTRACT FROM AUTHOR]

"Y-series" small-molecule acceptors (SMAs) have attracted extensive research interest as they boost the power conversion efficiencies of organic solar cells (OSCs) and research communities have begun to concentrate on the structure–property relationship and further structural optimization of this "A–DA′D–A" type SMA. In this study, we developed a novel Y-series SMA, called Y6–Se–O, by adopting benzoselenadiazole as the central core and alkoxy side chains on the β-position of thiophene moieties. The alkoxy substitution yields obvious effects on frontier orbitals of molecules and thus open-circuit voltage (VOC) of the corresponding devices. The lowest unoccupied molecular orbital (LUMO) level of Y6–Se–O is up-shifted compared to that of Y6–Se, therefore, a higher VOC of 0.94 V was attained. Additionally, when mixed with the donor polymer PM6, the Y6–Se–O-based blend films can maintain optimal morphology with stronger crystallinity and reasonable phase segregation, which leads to efficient charge dissociation, suppressed recombination, and balanced charge transportation. As a result, the PM6:Y6–Se–O-based devices reached a better efficiency of 17.5%. Our study demonstrated that the introduction of alkoxy side chains is an effective approach to tuning the optoelectrical properties of SMAs and facilitates both VOC and efficiencies. [ABSTRACT FROM AUTHOR]

Conjugated polymers are widely used as photoactive and transport layers in organic and hybrid photovoltaics (PV), where the energetics of polymers are a key design criterion. Here, we show that significant variations in terminal molecular ordering between top and bottom surfaces of a wide range of conjugated polymer films can result in sizable interfacial ionization energy (IE) differences by as much as 0.33 eV, which has significant impact on organic and hybrid PV devices. Such tunability is surprisingly seen even in nominally amorphous polymers. We devise a strategy leveraging wet and dry laminations to form donor–acceptor planar heterojunction (PHJ) devices using exposed and buried surfaces of donor polymers and demonstrate meaningful influence over the open circuit voltage (VOC) by up to 0.32 V. We use this insight to devise a controlled intermixing approach which yields superior VOC and JSC to conventional bulk heterojunction devices by leveraging the disordered interface to maximize VOC and the greater aggregation of the donor to increase the JSC. We go on to demonstrate how judicious control of polymer surface IE benefits charge extraction in colloidal quantum dot PV devices in the role of hole transport layers. Our results show that polymer interfacial and bulk properties are both critical to the functionality of optoelectronic devices and should both be given prime consideration when designing heterojunction devices. [ABSTRACT FROM AUTHOR]

Molecular innovation is highly important to achieve highly efficient small-molecule organic solar cells (SMOSCs). Herein, we report two small-molecule donors, namely, B3T-T and B3T-P, which differ only in their conjugated side chains: the former with a thienyl group and the latter with a phenyl unit. Surprisingly, both small-molecule donors show distinct electron density distribution and electrostatic potential along the conjugated backbone. B3T-P has a much higher dipole moment (0.920 D) than that of B3T-T (0.237 D). In SMOSCs, the B3T-T:BO-4Cl-based device shows a decent power conversion efficiency (PCE) of 11.1%. In contrast, the B3T-P:BO-4Cl-based device gives an outstanding PCE of 15.2%, which is one of the highest values among SMOSCs. Compared to the B3T-P-based device, although the B3T-T-based device has a large enough driving force for exciton separation and an extremely low non-radiative recombination voltage loss (0.168 V) for achieving high open-circuit voltage, the large domain size (63 nm) and low domain purity in the B3T-T:BO-4Cl-based device results in a relatively low short-circuit current density and fill factor, thus giving a low PCE. This result may pave the way to rationally design SM donors for highly efficient SMOSCs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 Å 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Proper molecular packing is key to improving the performance of non-fullerene organic solar cells (OSCs) based on perylene diimides (PDI). Herein, we implement a novel concept of introducing intramolecular π–π stacking for the design of PDI-based acceptors. We synthesized and systematically studied two isomeric PDI-tetramers with different functionalization positions (α- and β-positions) of naphthalene, and thus different extents of intramolecular packing. Interestingly, both PDI molecules exhibit two π–π stacking peaks with intramolecular and intermolecular stacking distances of 3.7–3.8 Å and 4.6–4.7 Å, respectively. Most importantly, the α-positioned PDI shows stronger intramolecular π-stacked features as evidenced by UV-Vis and photoluminescence, and morphological measurements. Consequently, the α-positioned PDI achieved a higher device efficiency of 8.6% with an open-circuit voltage of 1.15 V and a reasonably high fill factor of 60%. Our work highlights the importance of intramolecular π–π stacking in the design of novel PDI-based SMAs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Building on our previous works that compared the efficacy of terpolymers vs. ternary blends in improving the performance of bulk heterojunction organic solar cells, the final piece of this series of studies focuses on comparing terpolymer and ternary blends constructed with two polymers with structurally similar backbones (monoCNTAZ and FTAZ) yet markedly different open circuit voltage (Voc) values. Terpolymers and ternary blends of five different ratios were studied and the results demonstrate that while the overall performance of both the systems is similar, the ternary blends exhibit higher short circuit current (Jsc) values, while the terpolymers exhibit higher Voc values. Investigation of the charge transfer state using low-energy external quantum efficiency (EQE) indicates that the ternary blends are governed by a parallel-like mechanism, while the terpolymer does not follow this mechanism. The key morphological difference between the systems, as elucidated by resonance soft X-ray scattering (RSoXS), is the slightly smaller size (∼60 nm) of domains in the ternary blends compared to that of the terpolymer (∼80 nm), which may affect exciton harvesting in the terpolymer system and lead to lower Jsc values. In addition, a lower driving force for the formation of charge transfer (CT) state is also likely to contribute to the lower Jsc values in the terpolymer system. All together, the data show that structurally similar (perhaps even miscible) polymers still exhibit key differences in performance when paired in terpolymers vs. ternary blends and allow us to further illuminate the underlying mechanisms of such complex systems. [ABSTRACT FROM AUTHOR]

To minimize the voltage loss of non-fullerene organic solar cells (OSCs), it is important to modulate the energy levels of active materials and thus the photovoltage of the device. In this paper, we report a simple and effective approach to tune the energy levels of a state-of-the-art polymer P3TEA by switching the position of alkyl side chains and carboxylate substituents on the polymer backbone. The resulting polymer P3TAE exhibits a deep highest occupied molecular orbital (HOMO) level, contributing to a high open circuit voltage (VOC) of 1.20 V and a small voltage loss of 0.54 V when it is blended with a small molecule acceptor (SMA) FTTB-PDI4. Despite a small charge separation driving force, the P3TAE:FTTB-PDI4 blend exhibits efficient charge extraction, supported by relatively high external quantum efficiency (EQE) (∼60%) in the corresponding device. In addition, the P3TAE:FTTB-PDI4 blend shows relatively high electron mobility and domain purity, leading to a high fill factor (FF) in the device. As a result, the P3TAE:FTTB-PDI4-based solar cell exhibits a power conversion efficiency (PCE) of 8.10%, which is one of the highest achieved performances for single-junction OSCs with VOC higher than 1.20 V. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The performance of the highly efficient PBDB-T : ITIC system with 11.25% power conversion efficiency is degraded significantly to 4.35% when ITIC is blended with a version of PBDB, dubbed PBDB-O, which has only a minor side-chain modification. We explored the reasons for this difference and investigated the impact of morphology, molecular packing and mobility on carrier lifetime and performance. We employed transient absorption spectroscopy to investigate the hole and electron carrier dynamics in each of the blends and observed significant differences in geminate recombination. In order to understand the influence of morphology and mobility on the carrier dynamics and recombination, we utilized grazing incident wide-angle X-ray scattering (GIWAXS), resonant soft X-ray scattering (RSoXS) and diode mobility measurements. Our results indicate that the difference in performance is difficult to explain with the small differences observed in morphology and packing, and seems to be dominated by the changes in intrinsic mobility that the side-chain modification engenders, which impact both charge creation and extraction. The results highlight the difficulties of predicting the impact of synthetic structural modifications on performance and on a specific device-relevant parameter. [ABSTRACT FROM AUTHOR]

So far, the most successful and widely used solvents in polymer solar cells (PSCs) are chlorinated solvents like chloroform (CF), 1,2-dichlorobenzene (DCB) and chlorobenzene (CB), which are highly detrimental to human health and the environment. In this work, by the approach of manipulating flexible and conjugated side groups of a widely used polymer donor material (PBDB-T), two new polymers named PBDB-T-BO and PBDB-BzT are designed and synthesized to improve the solubility and fine-tune the aggregation property in tetrahydrofuran (THF), a much more benign solvent. As a result, an outstanding PCE of 12.10% was achieved with the THF-processed PBDB-BzT:IT-M device. The result in this work demonstrates the importance to modulate the aggregation effect of the polymer in solution by inserting conjugated groups and also suggests a feasible method to convert the main processing solvent of a highly efficient non-fullerene (NF)-PSC from CB into THF. [ABSTRACT FROM AUTHOR]

Donor polymer fluorination has proven to be an effective method to improve the power conversion efficiency of fullerene-based polymer solar cells (PSCs). However, this fluorine effect has not been well-studied in systems containing new, non-fullerene acceptors (NFAs). Here, we investigate the impact of donor polymer fluorination in NFA-based solar cells by fabricating devices with either a fluorinated conjugated polymer (FTAZ) or its non-fluorinated counterpart (HTAZ) as the donor polymer and a small molecule NFA (ITIC) as the acceptor. We found that, similar to fullerene-based devices, fluorination leads to an increased open circuit voltage (Voc) from the lowered HOMO level and improved fill factor (FF) from the higher charge carrier mobility. More importantly, donor polymer fluorination in this NFA-based system also led to a large increase in short circuit current (Jsc), which stems from the improved charge transport and extraction in the fluorinated device. This study demonstrates that fluorination is also advantageous in NFA-based PSCs and may improve performance to a higher extent than in fullerene-based PSCs. In the context of other recent reports on demonstrating higher photovoltaic device efficiencies with fluorinated materials, fluorination appears to be a valuable strategy in the design and synthesis of future donors and acceptors for PSCs. [ABSTRACT FROM AUTHOR]

Non-fullerene organic solar cells (NF-OSCs) require donor polymers with different morphological properties from those used in fullerene devices to achieve optimal cell performance. In this paper, we report a random donor polymer (PTFB-M) constructed from an asymmetric donor unit (T–FB–T-M), which can effectively tune the morphology and thus enhance the performance of NF-OSCs. Compared with its analog polymer PTFB-P based on a C2 symmetric monomer, the asymmetric T–FB–T-M unit introduces some randomness in the PTFB-M polymer yielding several beneficial effects. Firstly, although the neat PTFB-M film exhibits slightly reduced crystallinity and hole mobility compared to PTFB-P, it can, to our surprise, better maintain its crystallinity when blended with non-fullerene acceptors, hence yielding NF-OSCs with higher hole mobility and fill factors (FF) compared to devices based on PTFB-P. In addition, PTFB-M also exhibits smaller and more favorable domain sizes in NF-OSCs, leading to higher external quantum efficiency (EQE) and short circuit current density (Jsc). As a result, when combined with a small molecule acceptor (SMA) ITIC-Th, PTFB-M yields a power conversion efficiency (PCE) of 10.4%, whereas the PCE is only 8.4% for PTFB-P:ITIC-Th-based cells. This provides a useful approach to tune the morphology of donor polymers and to enhance the performance of NF-OSCs. [ABSTRACT FROM AUTHOR]

Numerous articles on thermal annealing (TA) and solvent vapor annealing (SVA) treatments have shown that both strategies effectively improve bulk morphology, reduce carrier recombination and thus improve photovoltaic performance of bulk heterojunction (BHJ) organic solar cells (OSCs). In previous work, we found that both TA and SVA treated devices based on a blend composed of a 3-ethyl-2-thioxothiazolidin-4-one containing molecule as donor (named DRCN5T) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) as acceptor show the similar photovoltaic performance, but their bulk microstructures are difference. Here employing in situ photoluminescence (PL) setup and X-ray scattering technologies, we found that the early stages of SVA remarkably affect the surface of film, and in contrast TA affects the whole bulk. Meanwhile, a plurality of experimental results all further confirm the different thermodynamics and kinetics of morphology evolution processed either by TA treatment or SVA treatment. Importantly, it was found that the SVA-treated film showed the increased photo-degradation in devices as compared to the TA-treated layer, resulting from obvious bulk morphology changes under illumination over 500 hours. The aim is to provide comprehensive insight into the influence of TA and SVA on time morphological evolution and degradation in OSCs. [ABSTRACT FROM AUTHOR]

The performance of polymer–fullerene bulk heterojunction (BHJ) solar cells is highly affected by the morphology of the blend film. Though the structure of the BHJ morphology is well-understood, the relationship between relevant material interactions and BHJ morphological evolutions is poorly understood and seldom explored. In this report, we discuss the impact of polymer molecular weight (MW) on thermodynamic and kinetic phenomena which have a drastic influence on the nanoscale BHJ morphology. The blend film comprises a highly aggregating low bandgap diketopyrrolopyrrole-based polymer PDPP3T and the fullerene electron acceptor molecule PC71BM, cast from a 1,2-dichlorobenzene (DCB) solution with and without the additive 1,8-diiodooctane (DIO). The pair-wise interactions among the components were evaluated by Flory–Huggins interaction parameters (χ). The BHJ blend of PDPP3T and PC71BM exhibited liquid–liquid (L–L) phase separation as a result of strong polymer–fullerene interactions (large χPDPP3T–fullerene) in DCB solution. In contrast, addition of 3% DIO into the DCB solution is found to stimulate polymer aggregation which gives rise to liquid–solid (L–S) phase separation. Large χPDPP3T–fullerene and χPDPP3T–solvent were observed as a result of increasing polymer MW, and these changes promote strong L–L phase separation and polymer aggregation in the blend solution, respectively. The latter interactions have led to low molecular mobility, with an end result of reduced crystallinity and smaller domain size of the BHJ films. The observed dramatic MW-dependent morphological changes were also manifested in solar cell outputs as well as charge carrier dynamics calculated by transient absorption measurements. [ABSTRACT FROM AUTHOR]

To address the challenges of reliability and facile processability of a benchmark polymer PTB7-Th, one of the most prevailing donor materials utilized in organic solar cells, we have systematically investigated the correlations among chemical structure, processing, morphology and device performance. Our study reveals that the regioregularity of PTB7-Th plays a crucial role in polymer properties as well as the fabrication process of devices. The regioregular polymer entirely consisting of favourable repeat units is capable of realizing high power conversion efficiency (＞10%) in organic solar cells without using any solvent additive and tedious processing treatments. More importantly, the device efficiency based on this regioregular polymer is insensitive over a broad range of film thickness (from 100 nm to ＞400 nm). This will be very advantageous for manufacturing highly efficient and stable polymer solar cells by high throughput fabrication processes. [ABSTRACT FROM AUTHOR]

Non-fullerene acceptors (NFAs) are becoming a serious contender to fullerene-based electron acceptors in organic photovoltaics, due to their structural versatility and easily tunable optical and electronic properties. However, NFA-based solar cells often have a decreased short-circuit current (Jsc) and fill factor (FF) compared to their fullerene-based counterparts. Here, we investigate the fundamental causes of this decrease in the performance of solar cells using a non-fullerene acceptor (SF-PDI2) paired with two polymer donors, FTAZ and PyCNTAZ, compared with their fullerene-based counterparts. Through a number of experimental techniques and morphological studies, we show that the SF-PDI2-based solar cells suffer from insufficient charge generation, transport, and collection when compared with the PCBM-based solar cells. The SF-PDI2-based solar cells show increased bimolecular recombination, which, together with other recombination loss mechanisms in these cells, causes a significant decrease in their Jsc and FF. Notably, the less pure domains, low electron mobility (on the order of 10−5 cm2 V−1 s−1), and imbalanced mobility (in regard to the hole mobility) further explain the low FF. On the other hand, the higher open-circuit voltage (Voc) in the SF-PDI2 devices is mainly due to the increase in the CT state energy. It is worth mentioning that the PyCNTAZ-based devices show an ultralow charge separation energy (ΔECS), close to 0 eV. Our results demonstrate that further increasing the mobility (both of electrons and holes) in these NFA-based solar cells would be a viable approach to further enhance the efficiency of these new types of solar cells, ideally, without losing the high Voc of such cells. [ABSTRACT FROM AUTHOR]

One of the most employed morphology optimization methods to design better performing organic photovoltaic devices is ink formulation engineering with additives. In this work, by employing a suboptimal host solvent mixture and 1,8-diiodooctane (DIO) as a very optimal solvent for both components in poly-thieno[3,2b]thiophene-diketopyrrolopyrrole-co-thiophene (DPP-TT-T)-based bulk-heterojunctions (BHJ), an alternative, previously unknown mechanism of additive behavior on BHJ microstructure formation is presented. In situ characterization methods involving grazing incidence X-ray diffraction, white-light reflectometry, laser light scattering and photoluminescence during film drying reveal that the microstructure formation under the influence of DIO is led towards thermodynamic equilibrium during host solvent drying, and the kinetics of morphology formation (i.e. polymer crystallization, fullerene aggregation…) are controlled dominantly by the additive during its evaporation. Ex situ X-ray-based characterization methods, such as scanning transmission X-ray microspectroscopy (STXM) and resonant soft X-ray scattering (R-SoXS), additionally reveal that the microstructure of dried films favors smaller domain sizes with purer domains, smaller fullerene aggregates, bimodal polymer crystallization relative to the substrate and more face-on molecular orientation relative to the donor/acceptor interface, which at the end lead to better performing devices with power conversion efficiencies ranging from 1.25% to 4.68%. [ABSTRACT FROM AUTHOR]

Here a series of isoindigo (ID) and quaterthiophene (T4)-based donor–acceptor copolymers are synthesized and compared. The polymer with fluorination on the donor unit exhibits the strongest extent of temperature-dependent aggregation, which leads to a higher hole mobility of the polymer and PSCs with efficiencies up to 7.0% without using any processing additives. Our results provide important insights into how fluorination affects the aggregation properties and performance of isoindigo-based polymers. [ABSTRACT FROM AUTHOR]

We observe thermally stable growth of fcc(111) films of fullerene-C60 on top of crystalline, flat-lying, CuPc film structures on graphite using combined grazing incidence wide-angle X-ray scattering and atomic force microscopy. Such a morphology is nearly ideal for bilayer films of C60/CuPc in solar cells. Very similar crystallinity and morphology is observed when varying the film thickness of either material from 5 nm up to more than 100 nm, suggesting that this advantageous solar cell morphology is not only robust, but may actually be preferred compared to the typical mounded fullerene morphology often seen on standing, edge-on aromatic film substrates. The large, ordered domains observed in both materials and the cofacial interface should greatly increase charge carrier mobility leading to increased fill factor. We envision that bilayer films, such as described here, grown on very high quality graphene as a transparent conducting electrode would simultaneously optimize crystallinity and molecular orientation for peak solar cell performance. [ABSTRACT FROM AUTHOR]

Thermal annealing and additive processing are employed and compared using alkoxy substituted (QxO) and extended π conjugated alkythienyl substituted (QxT) benzo[1,2-b:4,5 b′]dithiophene based heterojunction (BHJ) solar cells. The characteristic median length of the morphology, average composition fluctuations, interface structure, crystallinity and molecular miscibility are investigated based on these two processes. Our results suggest that focusing on single structural, morphological or thermodynamic measurements is not sufficient to explain differences in device performance. In the current work, no blends are close to the ideal morphology containing either domains that are too large, too mixed or too pure. An optimization strategy is proposed to improve those devices. Importantly, we find that domain size and relative domain purity are overall correlated with molecular miscibility, i.e. the more immiscible system induces larger and purer domains irrespective of the processing and even in non-equilibrium structures. This indicates that the relative domain size and purity, and device performance can be potentially predicted by the donor–acceptor molecular miscibility, a factor not yet widely considered when designing new materials for BHJ devices. [ABSTRACT FROM AUTHOR]