We have demonstrated the influence of evaporation‐induced flow in a single droplet on the crystalline microstructure and film morphology of an ink‐jet‐printed organic semiconductor, 6,13‐bis((triisopropylsilylethynyl) pentacene (TIPS_PEN), by varying the composition of the solvent mixture. The ringlike deposits induced by outward convective flow in the droplets have a randomly oriented crystalline structure. The addition of dichlorobenzene as an evaporation control agent results in a homogeneous film morphology due to slow evaporation, but the molecular orientation of the film is undesirable in that it is similar to that of the ring‐deposited films. However, self‐aligned TIPS_PEN crystals with highly ordered crystalline structures were successfully produced when dodecane was added. Dodecane has a high boiling point and a low surface tension, and its addition to the solvent results in a recirculation flow in the droplets that is induced by a Marangoni flow (surface‐tension‐driven flow), which arises during the drying processes in the direction opposite to the convective flow. The field‐effect transistors fabricated with these self‐aligned crystals via ink‐jet printing exhibit significantly improved performance with an average effective field‐effect mobility of 0.12 cm2 V–1 s–1. These results demonstrate that with the choice of appropriate solvent ink‐jet printing is an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct‐write fabrication of high‐performance organic electronics.

Bulk heterojunction solar cells based on blends of poly(3‐hexylthiophene) (P3HT) and phenyl‐C61‐butyric acid methyl ester (PC61BM) are fabricated using self‐assembled P3HT nanowires in a marginal solvent without post‐treatments. The interconnected network structures of self‐organized P3HT nanowires create continuous percolation pathways through the active layer and contribute to enhanced carrier mobility. The morphology and photovoltaic properties are studied as a function of ageing time of the P3HT precursor solution. Optimal photovoltaic properties are found at 60 h ageing time, which increases both light absorption and charge balance. Multilayered solar cells with a compositionally graded structure are fabriacted using preformed P3HT nanowires by inserting a pure P3HT donor phase onto the hole‐collecting electrode. Applying optimized annealing conditions to the P3HT buffer layer achieves an enhanced hole mobility and a power conversion efficiency of 3.94%. The introduction of a compositionally graded device structure, which contains a P3HT‐only region, reduces charge recombination and electron injection to the indium tin oxide (ITO) electrode and enhances the device properties. These results demonstrate that preformed semiconductor nanowires and compositionally graded structures constitute a promising approach to the control of bulk heterojunction morphology and charge‐carrier mobility.

Synthesis of a size series of colloidal ZnTe/ZnSe (core/shell) quantum dots (QDs) is reported. Because of the unique Type-II characters, their emission can range over an extended wavelength regime, showing photoluminescence (PL) from blue to amber. The PL lifetime measures as long as 77 ns, which clearly indicates the Type-II characteristics. ZnTe/ZnSe (Core/Shell) QDs can be further passivated by ZnS layers, rendered in water, while preserving the optical and chemical stabilities and thus proved their potentials toward “nontoxic” biological or medical applications that are free from concerns regarding heavy-metal leakage. ZnTe/ZnSe Type-II QD/polymer hybrid organic solar cells are also showcased, promising environmentally friendly photovoltaic devices. ZnTe/ZnSe Type-II QD incorporated photovoltaic devices show 11 times higher power conversion efficiency, when compared to that of the control ZnSe QD devices. This results from the Type-II characteristic broad QD absorption up to extended wavelengths and the spatially separated Type-II excitons, which can enhance the carrier extractions. We believe that ZnTe/ZnSe-based Type-II band engineering can open many new possibilities as exploiting the safe material choice.

The effect of the solubility of the annealing solvent on the performance of poly(3-hexylthiophene) (P3HT): C61-butyric acid methyl ester (PCBM) solar cells is studied. The short-circuit current (J sc ) and the fill factor (FF) increase remarkably, regardless of the type of annealing solvent, whereas a reduction of the open-circuit voltage (V oc ) (of 0.1-0.2 V) is observed after solvent annealing. Interestingly, both the value of J sc and the power conversion efficiency (PCE) are higher for the poor-solvent-annealed devices than for the good-solventannealed ones. A good solvent vapor induces better self-organization of P3HT than a poor solvent vapor. However, the exciton loss increases due to excessive phase separation. A study of the space-charge-limited current (SCLC) reveals no significant differences between the carrier mobilities of good-and poor-solventannealed devices. Furthermore, the measured photocurrent suggests that the space charges no longer limit the values of J sc and FF for all the solvent-annealed devices. These results indicate that the higher J sc and PCE values obtained for the poor-solvent-annealed devices can be attributed to the optimized phase separation of the active layers, which induces balanced carrier mobility and minimum exciton loss.

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