Organic solar cells (OSCs) can directly convert the sunlight into electrical energy and present some advantages, such as low cost, light weight, flexibility, semitransparency, and roll‐to‐roll large‐area fabrication. Due to the short diffusion length of exciton (≈10 nm) in organic semiconductor materials, the ideal nanoscale phase separation in the active layer is one of the crucial factors for achieving efficient exciton dissociation and charge transport. The morphology of the active layer is mainly determined by the nature of donors and acceptors (e.g., solubility, crystallinity, and miscibility), the film processing, the device configuration, and so on. In general, it is very hard to obtain ideal morphology in the as‐cast films. Therefore, it is usually essential to take measures to achieve the active layer with good molecular stacking, proper domain size, high domain purity, and suitable vertical phase separation. In this review, recent developments in morphology control and morphology characterization are summarized and analyzed. This review might help the community to decipher active layer morphology at multiple length scales and to achieve ideal morphology toward high‐performance OSCs.

Morphology Control in Organic Solar Cells

Tungsten oxide (WO3) is prepared by a low-temperature ultrasonic spray pyrolysis method in air atmosphere, and it is used as an anode buffer layer (ABL) for organic solar cells (OSCs). The properties of the WO3 transition metal oxide material as well as the mechanism of ultrasonic spray pyrolysis processes are investigated. The results show that the ultrasonic spray pyrolysized WO3 ABL exhibits low roughness, matched energy level, and high conductivity, which results in high charge transport efficiency and suppressive recombination in OSCs. As a result, compared to the OSCs based on vacuum thermal evaporated WO3, a higher power conversion efficiency of 3.63% is reached with low-temperature ultrasonic spray pyrolysized WO3 ABL. Furthermore, the mostly spray-coated OSCs with large area was fabricated, which has a power conversion efficiency of ~1%. This work significantly enhances our understanding of the preparation and application of low temperature-processed WO3, and highlights the potential of large area, all spray coated OSCs for sustainable commercial fabrication.

Low-Temperature Preparation of Tungsten Oxide Anode Buffer Layer via Ultrasonic Spray Pyrolysis Method for Large-Area Organic Solar Cells

Copper phthalocyanine (CuPc)-based organic thin film transistors (OTFTs) with various thicknesses of organic semiconductor were fabricated, and the corresponding gas sensing performance was systematically evaluated. In situ measurements of OTFTs under nitrogen dioxide (NO 2 ) exposure showed a remarkable sensitivity enhancement from 7% to 241% by simply decreasing CuPc film thickness from 40 to 10 nm. CuPc films were analyzed by atomic force microscopy and grazing incidence X-ray diffraction. The results showed that thinner CuPc film, which enhances the interplay of CuPc film and NO 2 analyte, is beneficial for NO 2 diffusion and smaller CuPc grain formation, resulting in considerable sensing performance improvement.

Thickness modulation on semiconductor towards high performance gas sensors based on organic thin film transistors

From Binary to Ternary Solvent: Morphology Fine-tuning of D/A Blends in PDPP3T-based Polymer Solar Cells

A novel polyelectrolyte containing triazine (TAZ) and benzodithiophene (BDT) scaffolds with polar phosphine oxide (P═O) and quaternary ammonium ions as pendant groups, respectively, in the polymer backbone (PBTAZPOBr) was synthesized to use it as a cathode interfacial layer (CIL) for polymer solar cell (PSC) application. Owing to the high electron affinity of the TAZ unit and P═O group, PBTAZPOBr could behave as an effective electron transport material. Due to the polar quaternary ammonium and P═O groups, the interfacial dipole moment created by PBTAZPOBr substantially reduced the work function of the metal cathode to afford better energy alignment in the device, thus enabling electron extraction and reducing recombination of excitons at the photoactive layer/cathode interface. Consequently, the PSC devices based on the poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl]:[6,6]-phenyl-C71-butyric acid methyl ester (P...

Triazine-based Polyelectrolyte as an Efficient Cathode Interfacial Material for Polymer Solar Cells

The effects of different binary-solvent treatments on the performance of polymer solar cells (PSCs) with thieno [3, 4-b]thiophene/benzodithiophene (PTB7):(6, 6)-phenyl- ${\rm{C}}_{{71}}$ -butyric ac-id methyl ester ( ${\hbox{PC}}_{{71}}{\hbox{BM}}$ ) as active layers are systematically investigated. The binary-solvents are obtained by mixing a primary solvent of methanol (MeOH) with a secondary solvent comprising of 2-chlorophenol (2-CP), dimethylsulfoxide (DMSO), or o-xylene. By analyzing the variation of photovoltaic performance and the nanostructure morphology of active layers, we find that both polarity and boiling point of the secondary solvent are playing an important role in controlling the nanostructure morphology of PTB7: ${\hbox{PC}}_{{71}}{\hbox{BM}}$ blends. In particular, 2-CP and DMSO with higher polarity and boiling point mixed with MeOH could significantly increase the power conversion efficiency from 6.47% (MeOH-treated device) to 7.81% and 7.19%, respectively. The improved photovoltaic performance of binary-solvent-treated PSCs is mainly due to the enhanced light absorption, optimized blend morphology, and increased charge carrier mobilities, resulting in efficient exciton generation, effective charge separation, and balanced charge transport. Moreover, the impedance spectroscopy results indicate that the binary-solvent treatment can achieve electrically ideal interfacial capacitance and, accordingly, suppress the bimolecular recombination in the blends.

Improving the Efficiency of Bulk Heterojunction Polymer Solar Cells Via Binary-Solvent Treatment

A smart ternary strategy was designed to improve the performance of ternary polymer solar cells (TPSCs) by doping high-mobility conjugated polymer poly{2, 2'-(2,5-bis(2-octyldodecyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3, 4-c]pyrrole-1, 4-diyl)dithieno[3, 2-b]thiophene-5,5'-diyl-alt-thiophen-2,5-diyl} (PDPPBTT) into the dominating poly({4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b′] dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethy-lhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7):[6,6]-phenyl C71 -butyric acid methyl ester (PC71BM) system. The enhanced short-circuit current and stable fill factor were simultaneously obtained. The champion power conversion efficiency of TPSCs with only 0.5 wt% PDPPBTT arrived to 7.89% with 21.6% improvement compared with the binary device, which was mainly attributed to good absorption in the infrared region, appropriate energy cascade alignment, efficient Forster resonance energy transfer from PTB7 to PDPPBTT, and more balanced hole–electron mobility. Moreover, as a third component, PDPPBTT enhanced the crystallinity and optimized the morphology of ternary blends. It was also found that the incorporation of PDPPBTT increased the charge carrier lifetime, ensuring efficient charge carrier transport and collection with suppressed bimolecular recombination.

Realization of Performance Enhancement in Ternary Polymer Solar Cells by Broad Absorption, Efficient Energy Transfer, and Balanced Charge Carrier Mobility

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Improving the Efficiency of Bulk Heterojunction Polymer Solar Cells Via Binary-Solvent Treatment

Realization of Performance Enhancement in Ternary Polymer Solar Cells by Broad Absorption, Efficient Energy Transfer, and Balanced Charge Carrier Mobility