Optimizing Alkyl Side Chains in Difluorobenzene-Rhodanine Small-Molecule Acceptors for Organic Solar Cells.

A series of small molecules, T-2FB-T-ORH, T-2FB-T-BORH, and T-2FB-T-HDRH, were synthesized to have a thiophene-flanked difluorobenzene (T-2FB-T) core and alkyl-substituted rhodanine (RH) end groups for their use as nonfullerene acceptors (NFAs) in organic solar cells (OSCs). Octyl, 2-butyloctyl (BO), and 2-hexyldecyl (HD) alkyl side chains were introduced into RHs to control the material's physical properties based on the length and size of the alkyl chains. The optical properties of the three NFAs were found to be almost the same, irrespective of the alkyl chain length, whereas the molecular crystallinity and material solubility significantly differed depending on the alkyl side chains. Owing to the sufficient solubility of T-2FB-T-HDRH, OSCs based on PTB7-Th and T-2FB-T-HDRH were fabricated. A power conversion efficiency of up to 4.49% was obtained by solvent vapor annealing (SVA). The AFM study revealed that improved charge mobility and a smooth and homogeneous film morphology without excessive aggregation could be obtained in the SVA-treated film.

Morphology Control for Efficient Nonfused Acceptor-Based Organic Photovoltaic Cells.

Non-fused electron acceptors have huge advantages in fabricating low-cost organic photovoltaic (OPV) cells. However, morphology control is a challenge as non-fused C─C single bonds bring more molecular conformations. Here, by selecting two typical polymer donors, PBDB-TF and PBQx-TF, the blend morphologies and its impacts on the power conversion efficiencies (PCEs) of non-fused acceptor-based OPV cells are studied. A selenium-containing non-fused acceptor named ASe-5 is designed. The results suggest that PBQx-TF has a lower miscibility with ASe-5 when compared with PBDB-TF. Additionally, the polymer networks may form earlier in the PBQx-TF:ASe-5 blend film due to stronger preaggregation performance, leading to a more obvious phase separation. The PBQx-TF:ASe-5 blend film shows faster charge transfer and suppressed charge recombination. As a result, the PBQx-TF:ASe-5-based device records a good PCE of 14.7% with a higher fill factor (FF) of 0.744, while the PBDB-TF:ASe-5-based device only obtains a moderate PCE of 12.3% with a relatively low FF of 0.662. The work demonstrates that the selection of donors plays a crucial role in controlling the blend morphology and thus improving the PCEs of non-fused acceptor-based OPV cells.

Design of Chlorinated Indaceno[1,2-b:5,6-b']dithiophene Acceptors toward Efficient Organic Photovoltaics.

Chlorinated modifications have been extensively employed to modulate the optoelectronic properties of π-conjugated materials. Herein, the Cl substitution in designing nonfullerene acceptors (NFAs) with various bandgaps is studied. Four narrow-bandgap electron acceptors (GS-40, GS-41, GS-42, and GS-43) were synthesized by tuning the electrostatic potential distributions of the molecular conjugated backbones. The optical absorption onset of these NFAs ranges from 900 to 1030 nm. Compared to the nonchlorinated analogue, the introduction of Cl atoms on the core of indaceno[1,2-b:5,6-b'] dithiophene (IDT) and π spacer results in an upward shift of the lowest unoccupied molecular orbital levels and induces a blue shift in the absorption spectra of the NFAs. This alteration facilitates achieving appropriate energy-level alignment and favorable bulk heterojunction morphology when blended with the widely used donor PBDB-TF. The PBDB-TF:GS-43-based solar cells show an optimal power conversion efficiency of 13.3%. This work suggests the potential of employing chlorine-modified IDT and thiophene units as fundamental building blocks for developing high-performance photoactive materials.

Trifluoromethyl-Substituted Conjugated Random Terpolymers Enable High-Performance Small and Large-Area Organic Solar Cells Using Halogen-Free Solvent.

The advancement of non-fullerene acceptors with crescent-shaped geometry has led to the need for polymer donor improvements. Additionally, there is potential to enhance the photovoltaic parameters in high-efficiency organic solar cells (OSCs). The random copolymerization method is a straightforward and effective strategy to further optimize photoactive morphology and enhance device performance. However, finding a suitable third component in terpolymers remains a crucial challenge. In this study, a series of terpolymer donors (PTF3, PTF5, PTF10, PTF20, and PTF50) is synthesized by introducing varying amounts of the trifluoromethyl-substituted unit (CF3) into the PM6 polymer backbone. Even subtle changes in the CF3 content can significantly enhance all the photovoltaic parameters due to the optimized energy levels, molecular aggregation/miscibility, and bulk-heterojunction morphology of the photoactive materials. Thus, the best binary OSC based on the PTF5:Y6-BO achieves an outstanding power conversion efficiency (PCE) of 18.2% in the unit cell and a PCE of 11.6% in the sub-module device (aperture size: 54.45 cm2 ), when using halogen-free solvent o-xylene. This work showcases the remarkable potential of the easily accessible CF3 unit as a key constituent in the construction of terpolymer donors in high-performance OSCs.

Morphology and Performance Enhancement through the Strong Passivation Effect of Amphoteric Ions in Tin-based Perovskite Solar Cells.

Despite the optoelectronic similarities between tin and lead halide perovskites, the performance of tin-based perovskite solar cells remains far behind, with the highest reported efficiency to date being ≈14%. This is highly correlated to the instability of tin halide perovskite, as well as the rapid crystallization behavior in perovskite film formation. In this work, l-Asparagine as a zwitterion plays a dual role in controlling the nucleation/crystallization process and improving the morphology of perovskite film. Furthermore, tin perovskites with l-Asparagine show more favorable energy-level matching, enhancing the charge extraction and minimizing the charge recombination, leading to an enhanced power conversion efficiency of 13.31% (from 10.54% without l-Asparagine) with remarkable stability. These results are also in good agreement with the density functional theory calculations. This work not only provides a facile and efficient approach to controlling the crystallization and morphology of perovskite film but also offers guidelines for further improved performance of tin-based perovskite electronic devices.

Non-Fullerene Acceptors with Benzodithiophene-Based Fused Planar Ring Cores for Organic Solar Cells.

Fused aromatic rings are widely employed in organic solar cell (OSC) materials due to their planarity and rigidity. Here, we designed and synthesized four two-dimensional non-fullerene acceptors, D6-4F, D6-4Cl, DTT-4F, and DTT-4Cl, based on two new fused planar ring structures of f-DTBDT-C6 and f-DTTBDT. Owing to the desirable phase separation formed in the blend films and the higher energy levels induced by the extra alkyl groups, PM6:D6-4F-based devices achieved a high VOC = 0.91 V with PCE = 11.10%, FF = 68.54%, and JSC = 17.75 mA/cm2. Because of the longer π-conjugation of the f-DTTBDT core with nine fused rings, DTT-4F and DTT-4Cl showed high molar extinction coefficients and broad absorption bands that enhanced the current density of OSCs. Finally, the PM6:DTT-4F-based devices achieved a JSC = 19.82 mA/cm2 with PCE = 9.68%, VOC = 0.83 V, and FF = 58.85%.

Large Steric Hindrance Enhanced Molecular Planarity for Low-Cost Non-Fused Electron Acceptors.

Designing efficient non-fused ring electron acceptors is of great importance in decreasing the material cost of organic photovoltaic cells (OPVs). It is a challenge to construct a planar molecular skeleton in non-fused molecules as there are many torsions between adjacent units. Here, we design two non-fused electron acceptors based on bithieno[3,2-b]thiophene units as core structures and study the impact of steric hindrance of substituents on molecular planarity. We use 2,4,6-triisopropylphenyl and 4-hexylphenyl groups to prepare ATTP-1 and ATTP-2, respectively. Our results suggest that the enhanced steric hindrance is beneficial for obtaining a more planar molecular configuration, which significantly increases the optical absorption and charge transport properties. The power conversion efficiency (PCE) of PBDB-TF:ATTP-1 combination (11.3%) is superior to that of PBDB-TF:ATTP-2 combination (3.7%). In addition, an impressive PCE of 10.7% is recorded in ATTP-1-based devices when a low-cost polythiophene donor PDCBT is used, which is an outstanding value in OPVs fabricated by non-fused donor/acceptor combinations. Our work demonstrates that modulation of the steric hindrance effect is of great significance to control the molecular planarity and thus obtain excellent photovoltaic performance of low-cost non-fused electron acceptors.

Unusual Foreign Body, a Spoon, in the Esophagus of a Middle-Aged Female: A Case Report and Review of the Literature.

In adults, a large metal foreign body in the esophagus is rarely seen and is usually caused accidentally. Here, we have described an unusual case of foreign body (spoon) in the esophagus of an adult patient. A 48-year-old woman initially presented to the emergency department with marked dysphagia, drooling, and radiating pain to the chest. She had swallowed a spoon while trying to vomit because of feeling sick. Radiological images revealed a spoon stuck in the esophagus. The edge of the spoon was grabbed with forceps and safely extracted under hypnic anesthesia. No esophageal perforation was detected on evaluation with esophagography using Gastrografin on the next day. This case highlights an unusual situation in an adult patient showing long nonfood-type foreign body in the esophagus. It is important that an appropriate workup and removal of foreign body is performed according to the location and type.

Ladder-Type Fused Benzodithiophene Extended along the Short-Axis Direction as a New Donor Building Block for Efficient Organic Solar Cells.

Ladder-type fused aromatic systems are important core structures of small molecule acceptors for organic solar cells (OSCs). In this study, a new ladder-type donor building block, based on the benzo[1,2-b:4,5-b']dithiophene (BDT) unit where the 3,7 positions of the BDT thiophene rings and the 3' position of the thiophene rings of the vertical BDT were fused to construct a seven-ring core structure named f-DTBDT, was investigated. In the f-DTBDT structure, the fusion of the BDT core and the thiophene rings at the 4,8 positions of BDT constrains all of the aromatic rings in a coplanar structure. The newly designed f-DTBDT was successfully employed as a core donor building block and conjugated with three electron-withdrawing acceptors (2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2HIC), 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2FIC), and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2ClIC)) as acceptor-donor-acceptor (A-D-A)-type acceptor materials for OSCs. Characterization results showed that the three synthesized A-D-A acceptors of DTBDT-IC, DTBDT-4F, and DTBDT-4Cl have high absorption behavior in the vis-NIR region as result of an intramolecular charge transfer interaction engendered by f-DTBDT and the ending group. The absorption regions of the acceptors were complementary with that of polymer PM6. Also, the frontier orbital energy levels of the new acceptors and wide-band-gap PM6 are well matched. Bulk heterojunction OSCs were fabricated using PM6 and the acceptors, and the highest power conversion efficiency (PCE) of 10.15% was obtained when using PM6:DTBDT-4F as the active layer.

Alkyl-Side-Chain Engineering of Nonfused Nonfullerene Acceptors with Simultaneously Improved Material Solubility and Device Performance for Organic Solar Cells.

Two nonfullerene small molecules, TBTT-BORH and TBTT-ORH, which have the same thiophene-benzothiadiazole-thiophene (TBTT) core flanked with butyloctyl (BO)- and octyl (O)-substituted rhodanines (RHs) at both ends, respectively, are developed as electron acceptors for organic solar cells (OSCs). The difference between the alkyl groups introduced into TBTT-BORH and TBTT-ORH strongly influence the intermolecular aggregation in the film state. Differential scanning calorimetry and UV-vis absorption studies reveal that TBTT-ORH exhibited stronger molecular aggregation behavior than TBTT-BORH. On the contrary, the material solubility is greatly improved by the introduction of a BO group in TBTT-BORH, and the inevitably low molecular interaction and packing ability of the as-cast TBTT-BORH film can be effectively increased by a solvent-vapor annealing (SVA) treatment. OSCs based on the two acceptors and PTB7-Th as a polymer donor are fabricated owing to their complementary absorption and sufficient energy-level offsets. The best power conversion efficiency of 8.33% is obtained with the SVA-treated TBTT-BORH device, where, together with a high open-circuit voltage of 1.02 V, the charge-carrier mobility and the short-circuit current density were greatly improved by the SVA treatment to levels comparable to those of the TBTT-ORH device because of the suppressed charge recombination and improved film morphology. In this work, the simultaneous improvement of both material solubility and device performance is achieved through alkyl side-chain engineering to balance the trade-offs among material solubility/crystallinity/device performance.

Highly Efficient and Photostable Ternary Organic Solar Cells Enabled by the Combination of Non-Fullerene and Fullerene Acceptors with Thienopyrrolodione-based Polymer Donors.

Two polymer donors, PFBDT-8ttTPD and PClBDT-8ttTPD, consisting of halogenated thiophene-substituted benzo[1,2-b:4,5-b']dithiophene and alkyl-substituted thieno[3,2-b]thiophene linked thieno[3,4-c]pyrrole-4,6(5H)-dione, were designed and synthesized for the evaluation of photovoltaic performances. The fabricated IT-4F-based organic solar cells (OSCs) exhibited maximum power conversion efficiency (PCE) values of 12.81 and 11.12% for PFBDT-8ttTPD and PClBDT-8ttTPD, respectively. Furthermore, PFBDT-8ttTPD:Y6 showed significantly improved PCE (15.05%) due to the extended light harvesting in the broad solar spectrum, whereas the PClBDT-8ttTPD:Y6 displayed relatively low PCE (10.02%). A ternary system incorporating PC71BM as the third component into bulk-heterojuction composites (PFBDT-8ttPTD:non-fullerene) was investigated with the aim of utilizing the advantages of PC71BM. As a result, PFBDT-8ttTPD:IT-4F:PC71BM exhibited an improved PCE (13.67%) compared to that of the corresponding binary OSC. In particular, ternary OSC of PFBDT-8ttTPD:Y6:PC71BM showed outstanding photovoltaic performance (PCE = 16.43%) as well as photostability, retaining approximately 80% of the initial PCE after 500 h under continuous illumination. The introduction of a small amount of PC71BM resulted in favorable and dense molecular packing with improved crystallinity as well as enhanced charge carrier mobility for efficient OSC.

Alkylated Indacenodithiophene-Based Non-fullerene Acceptors with Extended π-Conjugation for High-Performance Large-Area Organic Solar Cells.

In this work, a series of A-D-A'-D-A-type electron acceptors based on alkylated indacenodithiophene (C8IDT), dicyanated thiophene-flanked 2,1,3-benzothiadiazole (CNDTBT), and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (FINCN) are synthesized in order to investigate the effect of substituents on their photovoltaic properties. The corresponding CNDTBT-C8IDT-INCN and CNDTBT-C8IDT-FINCN acceptors vary in their optical, electrochemical, morphological, and charge transport properties. The fluorinated-INCN-based acceptor (CNDTBT-C8IDT-FINCN) exhibits lower energy levels, improved absorptivity, narrower π-π spacing, and prominent fibrillar structures when it is blended with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo [1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PBDB-T). CNDTBT-C8IDT-FINCN exhibits a high power conversion efficiency (PCE) of 12.33% due to its high and well-balanced charge carrier mobility and distinct face-on orientation. Furthermore, large-area organic solar cells (OSCs) (active area: 55.45 cm2) with CNDTBT-C8IDT-FINCN exhibit a high PCE of 9.21%. This result demonstrates that CNDTBT-C8IDT-FINCN is a suitable and promising electron acceptor for large-area OSCs.

Performance data of CH3NH3PbI3 inverted planar perovskite solar cells via ammonium halide additives.

The data provided in this data set is the study of organic-inorganic hybrid perovskite solar cells fabricated through incorporating the small amounts of ammonium halide NH4X (X = F, Cl, Br, I) additives into a CH3NH3PbI3 (MAPbI3) perovskite solution and is published as "High-Performance CH3NH3PbI3 Inverted Planar Perovskite Solar Cells via Ammonium Halide Additives", available in Journal of Industrial and Engineering Chemistry [1]. A compact and uniform perovskite absorber layer with large perovskite crystalline grains, is realized by simply incorporating small amounts of additives into precursor solutions, and utilizing the anti-solvent engineering technique to control the nucleation and growth of perovskite crystal, turning out the enhanced device efficiency (NH4F: 14.88 ± 0.33%, NH4Cl: 16.63 ± 0.21%, NH4Br: 16.64 ± 0.35%, and NH4I: 17.28 ± 0.15%) compared to that of a reference MAPbI3 device (Ref.: 12.95 ± 0.48%). In addition, this simple technique of ammonium halide addition to precursor solutions increase the device reproducibility as well as long term stability.

Synthesis of ITIC Derivatives with Extended π-Conjugation as Non-Fullerene Acceptors for Organic Solar Cells.

The realization of printed organic solar cells (OSCs) as a commercial technology is dependent on the development of high-performance photovoltaic materials suitable for large-scale device manufacture. In this study, the design, synthesis, and characterization of a series of A-D-A'-D-A-type molecular acceptors based on indacenodithienothiophene (IDTT) and thiophene-flanked 2,1,3-benzothiadiazole (DTBT) are reported. The synthesized molecular acceptors showed broader absorption ranges and narrower band gap energies than those of well-known 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno [2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC)-based molecular acceptors. Furthermore, the synthesized acceptors could tune the frontier molecular orbital energy levels, dipole moments, and their crystallinities by introducing fluorine (F) atoms and cyano (CN) groups on DTBT as a core A' unit. The cyano-substituted DTBT-based molecular acceptor (CNDTBT-IDTT-FINCN) showed a strong molar absorptivity and dipole moment, high hole/electron charge mobilities, and a favorable face-on orientation using films blended with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PBDB-T). An inverted organic photovoltaic (OPV) device using CNDTBT-IDTT-FINCN exhibits a power conversion efficiency (PCE) of 9.13% when using PBDB-T as a donor material in small cells (0.12 cm2). Sub-module devices with an active area of 55.45 cm2 are fabricated using bar-coating and exhibit PCEs of up to 7.50%. This demonstration of a high-efficiency large-area device makes CNDTBT-IDTT-FINCN a suitable and promising candidate for printed OPV devices.

Simple and Versatile Non-Fullerene Acceptor Based on Benzothiadiazole and Rhodanine for Organic Solar Cells.

Most non-fullerene acceptors (NFAs) are designed in a complex planar molecular conformation containing fused aromatic rings in high-efficiency organic solar cells (OSCs). To obtain the final molecules, however, numerous synthetic steps are necessary. In this work, a novel simple-structured NFA containing alkoxy-substituted benzothiadiazole and a rhodanine end group (BTDT2R) is designed and synthesized. We also investigate the photovoltaic properties of BTDT2R-based OSCs employing representative polymer donors (wide band gap and high-crystalline P3HT, medium band gap and semicrystalline PPDT2FBT, and narrow band gap and low-crystalline PTB7-Th) to compare the performance capabilities of fullerene acceptor-based OSCs, which are well matched with various polymer donors. OSCs based on P3HT:BTDT2R, PPDT2FBT:BTDT2R, and PTB7-Th:BTDT2R achieved efficiency as high as 5.09, 6.90, and 8.19%, respectively. Importantly, photoactive films incorporating different forms of optical and molecular ordering characteristics exhibit favorable morphologies by means of solvent vapor annealing. This work suggests that the new n-type organic semiconductor developed here is highly promising as a universal NFA that can be paired with various polymer donors with different optical and crystalline properties.

Amine-Based Interfacial Engineering in Solution-Processed Organic and Perovskite Solar Cells.

Solution-processed organic solar cells (OSCs) and hybrid perovskite solar cells (PvSCs) generally require appropriate transparent electrode with a low work function, which improves the electron extraction, increases the built-in potential, and suppresses charge recombinations. Hence, interfacial modifiers between the cathode and the photoactive layer play a significant role in OSCs and PvSCs, as they provide suitable energy-level alignment, leading to desirable charge carrier selectivity and suppressing charge carrier recombinations at the interfaces. Here, we present a comprehensive study of the energy-level mapping between a transparent electrode and photoactive layers to enhance the electron-transport ability by introducing amine-based interfacial modifiers (ABIMs). Among the ABIMs, polyethylenimine ethoxylated (PEIE) incorporating inverted OSCs shows enhanced power conversion efficiencies (PCEs) from 0.32 to 9.83% due to large interfacial dipole moments, leading to a well-aligned energy level between the cathode and the photoactive layer. Furthermore, we explore the versatility of the PEIE ABIM by employing different photoactive layers with fullerene derivatives, a nonfullerene acceptor, and a perovskite layer. Promisingly, inverted nonfullerene OSCs and planar n-i-p PvSCs with PEIE ABIM show outstanding PCEs of 11.88 and 17.15%, respectively.

Stable P3HT: amorphous non-fullerene solar cells with a high open-circuit voltage of 1 V and efficiency of 4.

A non-fullerene small molecule acceptor, SF-HR composed of 3D-shaped spirobifluorene and hexyl rhodanine, was synthesized for use in bulk heterojunction organic solar cells (OSCs). It possesses harmonious molecular aggregation between the donor and acceptor, due to the interesting diagonal molecular shape of SF-HR. Furthermore, the energy level of SF-HR matches well with that of the donor polymer, poly(3-hexyl thiophene) (P3HT) in this system which can affect efficient charge transfer and transport properties. As a result, OSCs made from a P3HT:SF-HR photoactive layer exhibited a power conversion efficiency rate of 4.01% with a high V OC of 1.00 V, a J SC value of 8.23 mA cm-2, and a FF value of 49%. Moreover, the P3HT:SF-HR film showed superior thermal and photo-stability to P3HT:PC71BM. These results indicate that SF-HR is specialized as a non-fullerene acceptor for use in high-performance OSCs.

High-Efficiency Nonfullerene Polymer Solar Cells with Band gap and Absorption Tunable Donor/Acceptor Random Copolymers.

Energy level alignment between a donor and an acceptor has a critical role in determining the open-circuit voltage ( VOC) in polymer solar cells (PSCs). Also, broad absorption of the photoactive layer is required to generate a high photocurrent. Herein, non-fullerene PSCs with D/A random copolymers and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d:2',3'- d']- s-indaceno[1,2- b:5,6- b']dithiophene (ITIC) has been demonstrated. The D/A random copolymers are composed of a 2-ethylhexylthienyl-substituted benzo[1,2- b:4,5- b']dithiophene (BDT) donor unit (D) and a fluorinated thieno[3,4- b]thiophene (TT-F) acceptor unit (A). By controlling the D/A unit ratio in the polymer backbone, it is possible to modulate both the energy levels and absorption spectra of random copolymers. As the ratio of the donor unit in the polymer back bone increases, the highest occupied molecular orbital energy level is located deeper, leading to higher VOC. Also, the absorption spectra of random copolymers become blue-shifted with an increase of the donor unit ratio; it compensates the weak absorption region of ITIC. This complementary absorption enhances the photocurrent, leading to higher power conversion efficiency (PCE). Because of the optimization of the D/A ratio of random copolymers, a notable PCE of 10.27% can be achieved in PSCs with D5A and ITIC.

Effective Molecular Engineering Approach for Employing a Halogen-Free Solvent for the Fabrication of Solution-Processed Small-Molecule Solar Cells.

To utilize the potential of small-molecule-based organic solar cells, proper designs of the photoactive materials which result in reasonable performance in a halogen-free solvent system and thickness tolerance over a range are required. One of the best approaches to achieve these requirements is via the molecular engineering of small-molecule electron donors. Here, we have modified a previously reported dithienobenzodithiophene (DTBDT)-based small molecule (SM1) via the dimerization approach, that is, the insertion of an additional DTBDT into the main backbone of the small molecule (SM2). An SM1-based photoactive film showed severe pinhole formation throughout the film when processed with a halogen-free o-xylene solvent. On the other hand, the modified small-molecule SM2 formed an excellent pinhole-free film when processed with the o-xylene solvent. Because of the dimerization of the DTBDT in the SM2 core, highly crystalline films with compact lamellae and an enhanced donor/acceptor interdigitation were formed, and all of these factors led to a high efficiency of 8.64% with chloroform and 8.37% with the o-xylene solvent systems. To the best of our knowledge, this study represents one of the best results with the SM donor and fullerene derivative acceptor materials that have shown the device performance with halogen-free solvents.

Three-Dimensional Observation of a Light-Soaked Photoreactant Layer in BTR:PCBM Solar Cells Treated with/without Solvent Vapor Annealing.

A key challenge to the commercialization of solution-processed solar cells is a proper understanding of the morphological variations during long periods, particularly under light-soaking conditions. Many research groups have competitively reported solvent vapor annealing (SVA)-treated small-molecule devices with efficiency rates exceeding 11%; however, their light-soaking effects have been rarely studied. Here, we investigate the morphological changes in the light-soaked devices with/without SVA treatments depending on the illumination time via three-dimensional observations. From the results, we found that the trends of morphological variations differ in the surface and bulk parts of the active film and that the difference is closely related to the device performance capabilities. Therefore, our research will enhance the underlying knowledge of the light-soaking effect on active morphologies over long term.