Effects of different polar solvents for solvent vapor annealing treatment on the performance of polymer solar cells

The effects of different polar solvents on the performance of solvent vapor annealing treated polymer solar cell (PSC) with a structure of ITO/ZnO/PTB7: PC₇₁BM/MoO₃/Ag was systematically investigated by applying different polar solvents, including methanol, ethanol, dimethylsulfoxide, acetone and isopropanol. By analyzing the variation of PSC performance and the morphology of active layer, we found that both the solubility parameters (Δ) and viscosity of solvent were playing an important role in controlling the morphology of PTB7: PC₇₁BM blend. Especially, the PSC treated by methanol with high Δ and low viscosity exhibited a remarkable enhancement of power conversion efficiency from 6.55% to 8.13%. The performance improvement was mainly due to the formation of the nanoscale crystallization of PTB7: PC₇₁BM blend and the moderated aggregation of PC₇₁BM, resulting in efficient charge separation, balanced charge transport and suppressed charge recombination.     

Improving performance of polymer solar cells based on PSBTBT/PC71BM via controlled solvent vapor annealing

Controlled solvent vapor annealing (C-SVA) is a powerful tool to control the morphology for high performance polymer solar cells (PSCs). In this work, the PSCs employed a blend of poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) is used to show this case. The solar cells upon C-SVA give Power Conversion Efficiency (PCE) of 5.40%, in contrast to 4.14% for the pristine and 4.70% for the thermally annealed devices. The increased PSBTBT concentration on the bottom surface of the C-SVA treated film favors charge carriers transportation to the anode, which contributes to the increased hole mobility of the photoactive layer and thus the device performance.     

Using d-limonene as the non-aromatic and non-chlorinated solvent for the fabrications of high performance polymer light-emitting diodes and field-effect transistors

Aiming to environment protection, green solvents are crucial for commercialization of solution-processed optoelectronic devices. In this work, d-limonene, a natural product, was introduced as the non-aromatic and non-chlorinated solvent for processing of polymer light-emitting diodes (PLEDs) and organic field effect transistors (OFETs). It was found that d-limonene could be a good solvent for a blue-emitting polyfluorene-based random copolymer for PLEDs and an alternating copolymer FBT-Th₄(1,4) with high hole mobility (μ_h) for OFETs. In comparisons to routine solvent-casted films of the two conjugated polymers, the resulting d-limonene-deposited films could show comparable film qualities, based on UV–vis absorption spectra and observations by atomic force microscopy (AFM). With d-limonene as the processing solvent, efficient blue PLEDs with CIE coordinates of (0.16, 0.16), maximum external quantum efficiency of 3.57%, and luminous efficiency of 3.66 cd/A, and OFETs with outstanding μ_h of 1.06 cm² (V s)⁻¹ were demonstrated. Our results suggest that d-limonene would be a promising non-aromatic and non-chlorinated solvent for solution processing of conjugated polymers and molecules for optoelectronic device applications.     

Effects of microwave-assisted annealing on the morphology and electrical performance of semiconducting polymer thin films

Organic field-effect transistors (OFETs) based on p-channel polymer semiconductors such as poly(3-hexyl)thiophene (P3HT) and 30-diketopyrrolopyrrole-selenophene vinylene selenophene (30-DPP-SVS) were fabricated using a microwave (MW) irradiation process for thermal annealing. The influence of MW annealing was investigated based on microstructural characterizations such as X-ray diffraction (XRD) and atomic force microscopy (AFM). MW annealing not only shortened the annealing time, but also produced enhanced device performance including higher on/off ratio, lower threshold voltage, and higher field-effect mobility in comparison with the traditional annealing method. These microstructural analyses revealed that annealing by MW irradiation enhances the crystallinity and molecular orientation in the polymer thin films in a short time, thereby improving the electrical performance effectively. Our results suggest that MW-assisted annealing is a simple and viable method for enhancing OFET performance.     

Solvent-vapor induced self-assembly of a conjugated polymer: A correlation between solvent nature and transistor performance

A systematical investigation on solvent-vapor annealing in polymer thin film transistors is performed using a thiazolothiazole-bithiazole conjugated polymer as the active layer. Film morphology, packing order and device performance are closely related to polarity and solubility parameter of the annealing solvent and annealing time. The formation of highly ordered and closely connected fibrillar domains is realized by using a solvent with similar solubility parameter and polarity to the conjugated polymer. Field-effect transistors based on pristine polymer films exhibit a highest charge carrier mobility of 0.0067 cm² V⁻¹ s⁻¹. After solvent vapor annealing with THF for 48 h, the mobility boosts up to 0.075 cm² V⁻¹ s⁻¹. This correlation between solvent polarity, solubility parameter and film morphology, packing order and mobility provides a useful guideline towards high performance polymer thin film transistors with solvent-vapor annealing method.