Thienoisoindigo-Based Dopant-Free Hole Transporting Material for Efficient p–i–n Perovskite Solar Cells with the Grain Size in Micrometer Scale

In this study, a series of 2,2′ and 3,3′ substituted thienoisoindigo (TII)-based small molecules (H3–H7) were synthesized by using 1,3-di­(9H-carbazol-9-yl) benzene, N-phenylcarbazole, triphenylamine, and benzene as electron donor (D) at the periphery, while TII as electron acceptor (A) at the core. The highest occupied molecular orbital energy levels of H3–H7 range from −5.31 to −5.43 eV, while their lowest unoccupied molecular orbital energy levels range from −3.43 to −3.59 eV. Under AM 1.5 condition, the perovskite solar cell (PSC) with inverted p–i–n device structure using H7 as the dopant-free hole transporting material achieved a power conversion efficiency (PCE) of 12.1%, which is comparable to that using PEDOT:PSS as the hole transporting material (12.0%). Under an argon atmosphere, the PCE of H7-based PSC did not decay within 168 h, and it can retain 86.3% of its original PCE after 1000 h. The morphology study revealed that the film of H3–H7 was smooth and hydrophobic, while the perovskite film spin-coated on H3–H7 film was uniform with the grain size in micrometer scale. Although the time-resolved photoluminescence spectra of the perovskite films suggested that the hole extraction capability of H7 is weaker than that of PEDOT:PSS, the improved film morphology of the film in H7-based PSC accounts for its comparable PCE to PEDOT:PSS-based PSC

Low-Bandgap Poly(Thiophene-Phenylene-Thiophene) Derivatives with Broaden Absorption Spectra for Use in High-Performance Bulk-Heterojunction Polymer Solar Cells

Two low-bandgap (LGB) conjugated polymers (P1 and P2) based on thiophene-phenylene-thiophene (TPT) with adequate energy levels have been designed and synthesized for application in bulk-heterojunction polymer solar cells (PSCs). The absorption spectral, electrochemical, field effect hole mobility and photovoltaic properties of LGB TPT derivatives are investigated and compared with poly(3-hexylthiophene) (P3HT). Photophysical studies reveal bandgaps of 1.76 eV for P1 and 1.70 eV for P2, which could effectively harvest broader solar spectrum. In addition, the thin film absorption coefficients of P1 and P2 are 1.6 × 105 cm−1 (λ ≈ 520 nm) and 1.4 × 105 cm−1 (λ ≈ 590 nm), respectively. Electrochemical studies indicate desirable HOMO/LUMO levels that enable a high open circuit voltage while blending them with fullerene derivatives as electron acceptors. Furthermore, both materials show sufficient hole mobility (3.4 × 10−3 cm2/Vs for P2) allowing efficient charge extraction and a good fill-factor for PSC application. High-performance power conversion efficiency (PCE) of 4.4% is obtained under simulated solar light AM 1.5 G (100 mW/cm2) from PSC device with an active layer containing 25 wt% P2 and 75 wt% [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), which is superior to that of the analogous P3HT cell (3.9%) under the same experimental condition

Highly Efficient Inverted Organic Photovoltaics Containing Aliphatic Hyperbranched Polymers as Cathode Modified Layers

In this study, we found that interfacial layers (IFLs) based on wholly aliphatic hyperbranched poly­(amido acid)­s (HBPAs) with interior tertiary amido groups can increase the performance of organic photovoltaics (OPVs) substantially. The performance of constructed devices having the layered configuration glass/indium tin oxide (ITO)/ZnO (with or without IFL)/active layer/MoO₃/Ag were enhanced when containing the studied aliphatic HBPAAs, the result of increases in the short circuit current. The presence of the IFL caused the ZnO layers to function more efficiently as electron-selective electrodes. The power conversion efficiencies of the devices incorporating PTB7/PC₇₁BM (from 7.1 to 7.8%) and PffBT4T–2OD/PC₇₁BM (from 7.8 to 8.7%) increased because of physisorption of the aliphatic HBPAAs, thereby changing the ZnO film’s surface energy and altering the active layer’s morphology. We processed these HBPA-based IFLs in air from solution, providing a simple method for the preparation of solution-processable inverted OPVs

Highly Efficient P3HT: C60 Solar Cell Free of Annealing Process

All conjugated C60-containing block copolymers (BCPs) based on quasi-living Grignard metathesis (GRIM) polymerization have been designed and synthesized for application in polymer solar cells (PSCs). The C60-containing BCP can induce the formation of a self-organization nanostructure of P3HT domain. Moreover, this C60-containing BCP serves as a compatibilizer to reduce the interfacial tension between the P3HT and C60, thus help establishing a moderate phase-separated morphology with crystalline P3HT and C60 domain. The performance up to 2.56%(AM 1.5G irradiation (100 mW/cm2)) of a P3HT:C60 device can be achieved by using C60–BCP as additive without any post-treatment

Photovoltaic Performance Enhancement of Perovskite Solar Cells Using Polyimide and Polyamic Acid as Additives

Poly­(amic acid) (PAA) and polyimide (PI) can interact with Pb2+ and methylammonium halide by forming Lewis acid–base adducts and hydrogen bonds, respectively. These interactions can passivate perovskite (PVSK) defects and enhance PVSK solar cell (PSC) performance. Here, PAA and PI polymers were used as PSC additives by using p-i-n PSC [ITO/NiOx/CH3NH3PbI3/with or without PAA or PI/PC61BM/BCP/Ag], and PVSK’s interactions with PAA or PI were explored through X-ray photoelectron, UV–visible, photoluminescence (PL), and time-resolved PL spectroscopies. In additive-derived PVSKs, defects passivation increased PL intensity and carrier lifetime. Field emission scanning electron microscopy revealed increased grain size, suggesting decreased grain boundary defects in PAA-derived PVSK. Moreover, 0.0497 mg/mL PAA/PVSK had high power conversion efficiency (14.16% ± 0.54% in control devices vs 16.80% ± 0.63%; highest = 17.85%). PAA/PVSK displayed excellent shelf life stability, with efficiency maintained at 16.57% ± 0.75% after storage in Ar-filled glovebox for >500 h

On the Air Stability of <i>n</i>-Channel Organic Field-Effect Transistors: A Theoretical Study of Adiabatic Electron Affinities of Organic Semiconductors

In an air-stable n-channel organic field-effect transistor (OFET), the charge carrier (i.e., the radical anion of an organic semiconductor (OSC)) has to be stable enough against ambient oxidants such as O2 and H2O. It has been suggested that OSCs with large enough electron affinity (EA) will possess air-stable charge carriers, but extensive correlation between air stability and EA has not been established. We have studied 47 existing n-channel OSCs with different molecular core structures and device configurations. A correlation between calculated adiabatic EA and air stability was established, and the threshold value found at the B3LYP/6-31+G*//B3LYP/6-31G** level for air stability was ca. 2.8 eV. This information provides a foundation for theoretical screening of potential n-channel OFETs before their synthesis and facilitates the discussion of the complex device degradation mechanism. Analysis of EAs of derivatives of perylenetetracarboxylic diimide (PDI) and naphthalenetetracarboxylic diimide (NDI) also sheds light on the roles of various substituents

High-Performance Ternary Organic Photovoltaics Incorporating Small-Molecule Acceptors with an Unfused-Ring Core

Organic photovoltaics (OPVs) have made enormous progress in recent years, benefiting from the rapid development of non-fullerene acceptors (NFAs). Most high-performance NFAs, however, have featured π-conjugated backbones with large-fused core structures, increasing the complexity and cost of their synthesis and limiting their practical commercialization. In this study, we synthesized a series of acceptor–donor–acceptor-configured small-molecule acceptors (NTCPDTCN, NTCPDTID, and NTCPDT2F) based on a core structure featuring a naphthobisthiadiazole (NT) group and two cyclopenta[2,1-b,3,4-b′]dithiophene (CPDT) groups as the unfused-ring central unit and equipping malononitrile (CN), 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (ID), and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2F) groups as terminal groups. When blended with PM7, the NTCPDTCN-containing binary device displayed a high open-circuit voltage (VOC) of 1.04 V, without self-aggregation, as well as the best device performance. When blended with PM6:Y6, the ternary NTCPDTCN-, NTCPDTID-, and NTCPDT2F-based OPVs provided power conversion efficiencies of 15.4 ± 0.09, 14.6 ± 0.15, and 16.0 ± 0.08%, respectively. NTCPDT2F provided complementary absorption and allowed fine-tuning of the blend morphology, resulting in suppression of charge recombination and improvements in charge generation and collection, thereby achieving the highest device performance. Thus, our findings might provide some directions for developing high-performance ternary OPVs through the introduction of unfused-ring small-molecule acceptors

Synthesis of Eco-Friendly CuInS₂ Quantum Dot-Sensitized Solar Cells by a Combined Ex Situ/in Situ Growth Approach

A cadmium-free CuInS₂ quantum dot (QD)-sensitized solar cell (QDSC) has been fabricated by taking advantage of the ex situ synthesis approach for fabricating highly crystalline QDs and the in situ successive ionic-layer adsorption and reaction (SILAR) approach for achieving high surface coverage of QDs. The ex situ synthesized CuInS₂ QDs can be rendered water soluble through a simple and rapid two-step method under the assistance of ultrasonication. This approach allows a stepwise ligand change from the insertion of a foreign ligand to ligand replacement, which preserves the long-term stability of colloidal solutions for more than 1 month. Furthermore, the resulting QDs can be utilized as sensitizers in QDSCs, and such a QDSC can deliver a power conversion efficiency (PCE) of 0.64%. Using the SILAR process, in situ CuInS₂ QDs could be preferentially grown epitaxially on the pre-existing seeds of ex situ synthesized CuInS₂ QDs. The results indicated that the CuInS₂ QDSC fabricated by the combined ex situ/in situ growth process exhibited a PCE of 1.84% (short-circuit current density = 7.72 mA cm^–2, open-circuit voltage = 570 mV, and fill factor = 41.8%), which is higher than the PCEs of CuInS₂ QDSCs fabricated by ex situ and in situ growth processes, respectively. The relative efficiencies of electrons injected by the combined ex situ/in situ growth approach were higher than those of ex situ synthesized CuInS₂ QDs deposited on TiO₂ films, as determined by emission-decay kinetic measurements. The incident photon-to-current conversion efficiency has been determined, and electrochemical impedance spectroscopy has been carried out to investigate the photovoltaic behavior and charge-transfer resistance of the QDSCs. The results suggest that the combined synergetic effects of in situ and ex situ CuInS₂ QD growth facilitate more electron injection from the QD sensitizers into TiO₂

Hybrid Solar Cells with Prescribed Nanoscale Morphologies Based on Hyperbranched Semiconductor Nanocrystals

In recent years, the search to develop large-area solar cells at low cost has led to research on photovoltaic (PV) systems based on nanocomposites containing conjugated polymers. These composite films can be synthesized and processed at lower costs and with greater versatility than the solid state inorganic semiconductors that comprise today's solar cells. However, the best nanocomposite solar cells are based on a complex architecture, consisting of a fine blend of interpenetrating and percolating donor and acceptor materials. Cell performance is strongly dependent on blend morphology, and solution-based fabrication techniques often result in uncontrolled and irreproducible blends, whose composite morphologies are difficult to characterize accurately. Here we incorporate three-dimensional hyperbranched colloidal semiconductor nanocrystals in solution-processed hybrid organic−inorganic solar cells, yielding reproducible and controlled nanoscale morphology