High-performance solution-processable ZnO thin films for use as electron-transporting layers (ETLs) of inverted-structured polymer solar cells (I-PSCs) are developed via a low-temperature annealing (<200 C) sol-gel process. The properties of the low-temperature-annealed ZnO (L-ZnO) thin films (used as ETLs) are optimized based on the evaluation of the roles of the internal nanocrystal (NC) orientation and filmsurface morphology in charge transport/transfer in I-PSCs. The low-temperature annealing conditions (dynamic annealing or static annealing) could be successfully manipulated to alter the NC orientation of the L-ZnO films, whereas tactical control of the precursor-coating conditions enabled the embedding of nanoripples on the film surfaces. Suppression of the preferential (002) plane NC orientation of the L-ZnO layers is beneficial for charge transport in I-PSCs; these devices should be evaluated in a manner different from field-effect transistors (FETs). The performance of ETLs is further enhanced by the development of nanoripple-embedded L-ZnO film surfaces, which furnish an increased area for contact with the active layers. The I-PSCs fabricated using the optimized L-ZnO films display a >20% higher power-conversion efficiency (PCE) than those employing the conventional L-ZnO films for a range of active materials including poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PC 60 BM) and poly(thienothiophene-co-benzodithiophenes)7-F20 (PTB7-F20)/phenyl-C71-butyric acid methyl ester (PC 71 BM) blends. A PCE of 6.42% is achieved for the I-PSCs using the optimized L-ZnO films and PTB7-F20/PC 71 BM blends as the ETL and active materials, respectively. This study presents a universal method for optimizing sol-gel-driven ZnO-based ETLs, whilst the low-temperature processability and long-term stability of the developed ETLs are beneficial for the commercialization of I-PSCs.
Polymer-based field-effect transistors are fabricated using the gas-assisted spray technique, and their performance is considerably improved when a solvent-assisted post-treatment method, solvent sprayed overlayer (SSO), is used. The SSO method is a unique treatment that can facilitate chain packing to increase crystallinity within the sprayed polymer layers, which inherently have a kinetically trapped amorphous chain morphology with lack of crystallinity due to rapid solvent evaporation. The device performance was drastically improved after SSO relative to conventional post-treatment, thermal annealing (TA). This occurred because SSO can rearrange the polymer chains into a dominantly edge-on crystal orientation, which is preferential for charge transport, whereas TA increases the crystallinity without rearrangement of the crystal orientation resulting in a complex of edge-on and face-on. The development of edge-on crystal domains after SSO within the active layers was responsible for the significant improvement in performance. The SSO is a simple and effective post-treatment method that validates the use of spray process and holds promise for use in other high-throughput processes for OFETs fabrication.
The effect of surface characteristics of dielectric layers on the molecular orientation and device performance of sprayed organic field-effect transistors (OFETs) obtained by a novel solvent-assisted post-treatment, called the solvent-sprayed overlayer (SSO) method, were investigated. The OFETs were fabricated by the spray method using regioregular poly(3-hexylthiophene) (RR-P3HT) as an active material. The SSO treatment was applied on the as-sprayed active layers to arrange the molecular ordering. Bare thin SiO(2) layers and octadecyltrichlorosilane (OTS)-treated SiO(2) (OTS-SiO(2)) were employed as the dielectric materials. The resulting chain orientation, crystallinity, and device performance were correlated as a function of SSO treatment and dielectric layers. The intrinsic limitation of spray methods for polymer film formation was overcome regardless of the type of dielectric layer using the SSO treatment. The orientation direction of RR-P3HT was controlled by SSO treatment to an edge-on dominant orientation that is preferential for charge transport, regardless of the type of dielectric layer. The crystal growth was further enhanced on the OTS-SiO(2) layers because of the reduced nucleation sites. These effects were successfully reflected in the device performance, including an orders-of-magnitude increase in charge mobility. The SSO method is a powerful external treatment method for reorienting the molecular ordering of solidified active films of OFETs to the preferential edge-on packing. The growth of crystals was further optimized by controlling the surface characteristics of the dielectric layers. The purpose of this study was to find the full capabilities of the SSO treatment method that will facilitate the development of high-throughput, large-area organic electronic device manufacturing.
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