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Organic solar cells have obtained a prodigious amount of attention in photovoltaic research due to their unique features of light weight, low cost, eco-friendliness, and semitransparency. A rising trend in this field is the development of all-small-molecules organic solar cells (ASM-OSCs) due to their merits of excellent batch-to-batch reproducibility, well-defined structures, and simple purification. Among the numerous organic photovoltaic (OPV) materials, benzodithiophene (BDT)-based small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking the 17% efficiency barrier in single-junction OPV devices, indicating the significant potential of this class of materials in commercial photovoltaic applications. This review specially focuses on up-to-date information about improvements in BDT-based ASM-OSCs since 2011 and provides an outlook on the most significant challenges that remain in the field. We believe there will be more exciting BDT-based photovoltaic materials and devices developed in the near future. [ABSTRACT FROM AUTHOR]

Efficient charge transfer between organic semiconductors and electrode materials at electrode interfaces is essential for achieving high-performance organic optoelectronic devices. For efficient charge injection and extraction at the electrode interface, an interlayer is usually introduced between the organic active layer and electrode. Here, a simple and effective approach for further improving charge transfer at the organic active layer–interlayer interface was presented. Treatment of the zinc oxide (ZnO) interlayer, a commonly used n-type interlayer, with a fullerene-based self-assembled monolayer (SAM) effectively improved electron transfer at the organic–ZnO interface, without affecting the morphology and crystalline structure of the organic active layer on the cathode interlayer. Furthermore, this treatment reduced charge recombination in the device, attributed to the improved charge extraction and reduction of undesirable ZnO-donor polymer contacts. The photocurrent density and power conversion efficiency of organic solar cells employing the fullerene-SAM-treated interlayer were ~10% higher than those of the device employing the nontreated interlayer. This improvement arises from the enhanced electron extraction and reduced charge recombination. [ABSTRACT FROM AUTHOR]

The effect of the cross‐coupling catalyst tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) on the performance of a model organic bulk‐heterojunction solar cell composed of a blend of poly([2,6′‐4,8‐di(5‐ethylhexylthienyl)benzo[1,2‐b;3,3‐b]dithiophene]{3‐fluoro‐2[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl}) (PTB7‐Th) donor and 3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6,7‐difluoro)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (IOTIC‐4F) non‐fullerene acceptor is investigated. The effect of intentional addition of different amounts of Pd(PPh3)4 on morphology, free charge carrier generation, non‐geminate bulk trap‐ and surface trap‐assisted recombination as well as bimolecular recombination and charge extraction is quantified. This work shows that free charge carrier generation is affected significantly, while the impact of Pd(PPh3)4 on non‐geminate recombination processes is limited because the catalyst does not facilitate efficient trap‐assisted recombination. The studied system shows substantial robustness towards the addition of Pd(PPh3)4 in small amounts. [ABSTRACT FROM AUTHOR]

The relationship of the temperature–light intensity dependence of open‐circuit voltage Voc in nonfullerene‐based organic solar cells with their material characteristics and multimechanism recombination parameters is described. The systematic variation of the effective bandgap Eg,eff and the electrode layers allows the observation of different relative contributions of bimolecular, bulk, and surface trap‐assisted recombination mechanisms. The complementary advantages of the analytical model and the established voltage‐impedance spectroscopy technique provide a useful tool to quantify multimechanism recombination parameters, effective density of states Nc, and energetic disorder σ in organic solar cells under operating conditions. The validity of the proposed model to understand the temperature and light intensity dependent of Voc is shown by applying it to four different donor:nonfullerene acceptor blend systems with conventional or inverted device architectures. [ABSTRACT FROM AUTHOR]

Three symmetrically difluorinated organic semiconductors (namely D5T2F‐P, D5T2F‐S, and D5T2F‐T) containing rhodanine‐flanked pentathiophene structures are synthesized and used as donors in all‐small‐molecule organic solar cells (ASM‐OSCs) prepared with the small‐molecule acceptor 2,2′‐((2Z,2′Z)‐((4,4,9,9‐tetrahexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (IDIC‐4F). The different substitutional positions of fluorine atoms (–F) in the conjugated backbone of the donor molecule lead to various material and photovoltaic properties being exhibited. Among the three isomers, the centrally fluorinated D5T2F‐P exhibits a redshifted absorption spectrum, downshifted highest occupied molecular orbital (HOMO) energy level, and improved miscibility with IDIC‐4F in the blend films, all of which result in superior device performance. The power conversion efficiency (PCE) of the ASM‐OSCs based on D5T2F‐P:IDIC‐4F reaches an impressive value of 9.36% with an open‐circuit voltage (VOC) value of 0.86 V and a short‐circuit current density (JSC) value of 16.94 mA cm−2, whereas those of D5T2F‐S (6.11%) and D5T2F‐T (5.42%) are much lower. In comparison, an ASM‐OSC based on the nonfluorinated analogue DRCN5T fabricated under the same conditions exhibits poorer performance (8.03% with IDIC‐4F), revealing 16% enhancement in the PCE achieved through backbone fluorination. The PCE of 9.36% may be one of the highest efficiencies of oligothiophene‐based ASM‐OSCs reported in the literature to date. [ABSTRACT FROM AUTHOR]

Abstract: A series of narrow bandgap electron acceptors is designed and synthesized for efficient near‐infrared (NIR) organic solar cells. Extending π‐conjugation of donor frameworks leads to an intense intramolecular charge transfer, resulting in broad absorption profiles with band edge reaching 950 nm. When blended with an electron donor polymer PTB7‐Th, IOTIC‐2F exhibits efficient charge transfer even with a small energetic offset, so as to achieve a large photogenerated current over 22 mA cm−2 with small energy losses (≈0.49 eV) in solar cell devices. With an intense NIR absorbance, PTB7‐Th:IOTIC‐2F‐based cells achieve a power conversion efficiency of 12.1% with good visible transparency (52% transmittance from 370 to 740 nm). Analysis of film morphology reveals that processing with solvent additives enhances crystalline features of acceptor components, while keeping an appropriate level of donor:acceptor intermixing in the binary blends. The incorporation of the third component, ITIC‐2F, into the PTB7‐Th:IOTIC‐2F blends increases the device efficiency up to 12.9%. The improvement is assigned to the cascaded energy‐level structure and desirable nanoscale phase separation of the ternary blends, which is beneficial to the photocurrent generation. This work provides an efficient molecular design strategy to optimize nonfullerene acceptor properties for efficient NIR organic photovoltaics. [ABSTRACT FROM AUTHOR]

Abstract: Two narrow bandgap non‐fullerene acceptors (NBG‐NFAs), namely, COTIC‐4F and SiOTIC‐4F, are designed and synthesized for the fabrication of efficient near‐infrared organic solar cells (OSCs). The chemical structures of the NBG‐NFAs contain a D′‐D‐D′ electron‐rich internal core based on a cyclopentadithiophene (or dithienosilole) (D) and alkoxythienyl (D′) core, end‐capped with the highly electron‐deficient unit 2‐(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile (A), ultimately providing a A‐D′‐D‐D′‐A molecular configuration that enhances the intramolecular charge transfer characteristics of the excited states. One can thereby reduce the optical bandgap (Egopt) to as low as ≈1.10 eV, one of the smallest values for NFAs reported to date. In bulk‐heterojunction (BHJ) OSCs, NBG‐NFA blends with the polymer donor PTB7‐Th yield power conversion efficiencies (PCE) of up to 9.0%, which is particularly high when compared against a range of NBG BHJ blends. Most significantly, it is found that, despite the small energy loss (Egopt − eVOC) of 0.52 eV, the PTB7‐Th/NBG‐NFA bulk heterojunction blends can yield short‐circuit current densities of up to 22.8 mA cm−2, suggesting that the design and application of NBG‐NFA materials have substantial potential to further improve the PCE of OSCs. [ABSTRACT FROM AUTHOR]

Interfacial energetics determines the performance of organic photovoltaic (OPV) cells based on a thin film of organic semiconductor blends. Here, an approach to modulating the 'carrier selectivity' at the charge collecting interfaces and the consequent variations in the nongeminate charge carrier recombination dynamics in OPV devices are demonstrated. A ferroelectric blend interfacial layer composed of a solution-processable ferroelectric poly­mer and a wide bandgap semiconductor is introduced as a tunable electron selective layer in inverted OPV devices with non-Ohmic contact electrodes. The direct rendering of dipole alignment within the ferroelectric blend layer is found to increase the carrier selectivity of the charge collecting interfaces up to two orders of magnitude. Transient photovoltaic analyses reveal that the increase of carrier selectivity significantly reduces the diffusion and recombination among minority carriers in the vicinity of the electrodes, giving rise to the 85% increased charge carrier lifetime. Furthermore, the carrier-selective charge extraction leads to the constitution of the internal potential within the devices, even with energetically identical cathodes and anodes. With these carrier-selectivity-controlled interlayers, the devices based on various photoactive materials commonly display significant increments in the device performances, especially with the high fill factor of up to 0.76 under optimized conditions. [ABSTRACT FROM AUTHOR]

A series of four polymers containing benzo[1,2-b:4,5-b′]dithiophene (BDT) and 5,6-difluoro-4,7-diiodobenzo[c][1,2,5]thiadiazole (2FBT), PBDT2FBT, PBDT2FBT-O, PBDT2FBT-T, and PBDT2FBT-T-O, are synthesized with their four different side chains, alkyl-, alkoxy-, alkylthienyl-, and alkoxythienyl. Experimental results and theoretical calculations show that the molecular tuning of the side chains simultaneously influences the solubilities, energy levels, light absorption, surface tension, and intermolecular packing of the resulting polymers by altering their molecular coplanarity and electron affinity. The polymer solar cell (PSC) based on a blend of PBDT2FBT-T/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibits the best photovoltaic performance of the four PBDT2FBT derivatives, with a high open-circuit voltage of 0.98 V and a power conversion efficiency of 6.37%, without any processing additives, post-treatments, or optical spacers. Furthermore, PBDT2FBT-T-O, which has a novel side chain alkoxythienyl, showed promising properties with the most red-shifted absorption and strong intermolecular packing property in solid state. This study provides insight into molecular design and fabrication strategies via structural tuning of the side chains of conjugated polymers for achieving highly efficient PSCs. [ABSTRACT FROM AUTHOR]

In article number 1500802, Kilwon Cho and co‐workers report charge‐carrier‐selective extraction through ferroelectric‐semiconductor hybrid interlayers in organic solar cells. The electrically polarized ferroelectric domains provide directional charge carrier extraction even with non‐Ohmic cathodes, thereby significantly reducing non‐geminate charge carrier recombination within the organic solar cells. [ABSTRACT FROM AUTHOR]

Structural optimization of the side chains of conjugated polymers is essential for highly efficient organic solar cells. In article number 1400087, Kilwon Cho and co‐workers describe the importance of side chains by investigating polymer solar cells based on PBDT2FBT low‐bandgap polymers containing four types of side chains. The effects of the side chains on the optoelectronic and morphological properties of the bulk‐heterojunction photoactive layer are explored and found to be strongly correlated with the power conversion efficiency of the organic photovoltaic devices. [ABSTRACT FROM AUTHOR]

Solution-processed organic solar cells (OSCs) have great promise for next-generation solar energy conversion technologies. Innovative acceptors are constantly being designed to improve device efficiency. In this work, we designed and synthesized a series of A–π–D–π–A type non-fullerene acceptor (NFA) molecules, named IB-TOT, IB-TOM, IB-IOO, and IB-IOM for their applications in OSCs. All molecules showed high thermal stability, broad photoabsorption, and relatively lower highest occupied molecular orbital (HOMO) energy levels from −5.48 eV to −5.58 eV. The optimized OSCs based on these NFAs and PTB7-Th donor deliver power conversion efficiencies (PCEs) of 8.19%, 5.50%, 2.42%, and 3.10% for IB-TOT, IB-TOM, IB-IOO, and IB-IOM molecules, respectively. The higher PCE of IB-TOT-based OSCs is attributed to their excellent miscibility and superior interpenetrating network morphology compared to the other three compounds. These results demonstrate that using the 3-ethylrhodanine acceptor unit to construct the NFA molecule is a successful approach for outstanding achievement in OSCs. • Four IDT-based NFA molecules were designed and synthesized. • The impact of various end-groups on device performance was fully discussed. • An empirical relationship between the end groups was established. • The best-performing OSCs achieved a maximum PCE of 8.19%. • The work promotes future OSC performance enhancement. [ABSTRACT FROM AUTHOR]