Melt-processing of small molecule organic photovoltaics via bulk heterojunction compatibilization.

Melt-processing of organic semiconductors (OSCs) is a promising environmentally-friendly technique that can alleviate dependence on toxic chlorinated solvents. While melt-processed single-component OSC devices (e.g. field-effect-transistors) have been demonstrated, multi-component bulk heterojunctions (BHJs) for organic photovoltaics (OPVs) remain a challenge. Herein, we demonstrate a strategy that affords tunable BHJ phase segregation and domain sizes from a single-phase homogeneous melt by employing strongly-crystalline small-molecule OSCs together with a customized molecular compatibilizing (MCP) additive. An optimized photoactive BHJ with 50 wt% MCP achieved a device power conversion efficiency of ca. 1% after melting the active layer at 240 °C (15 min, followed by slow cooling) before deposition of the top electrode. BHJ morphology characterization using atomic force and Kelvin probe microscopy, X-ray diffraction, and photo-luminescence measurements further demonstrate the trade-off between free charge generation and transport with respect to MCP loading in the BHJ. In addition, a functional OPV was also obtained from the melt-processing of dispersed micron-sized solid BHJ particles into a smooth and homogeneous thin-film by using the MCP approach. These results demonstrate that molecular compatibilization is a key prerequisite for further developments towards true solvent-free melt-processed BHJ OPV systems.

Amorphous Ternary Charge-Cascade Molecules for Bulk Heterojunction Photovoltaics.

Ternary bulk heterojunctions with cascade-type energy-level configurations are of significant interest for further improving the power conversion efficiency (PCE) of organic solar cells. However, controlling the self-assembly in solution-processed ternary blends remains a key challenge. Herein, we leverage the ability to control the crystallinity of molecular semiconductors via a spiro linker to demonstrate a simple strategy suggested to drive the self-assembly of an ideal charge-cascade morphology. Spirobifluorene (SF) derivatives with optimized energy levels from diketopyrrolopyrrole (DPP) or perylenediimide (PDI) components, coded as SF-(DPP)4 and SF-(PDI)4, are synthesized and investigated for application as ternary components in the host blend of poly(3-hexylthiophene-2,5-diyl):[6,6]phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Differential scanning calorimetry and X-ray/electron diffraction studies suggest that at low loadings (up to 5 wt %) the ternary component does not perturb crystallization of the donor:acceptor host blend. In photovoltaic devices, up to 36% improvement in the PCE (from 2.5% to 3.5%) is found when 1 wt % of either SF-(DPP)4 or SF-(PDI)4 is added, and this is attributed to an increase in the fill factor and open-circuit voltage, while at higher loadings, the PCE decreased because of a lower short-circuit current density. A comparison of the quantum efficiency measurements [where light absorption of SF-(DPP)4 was found to give up to 95% internal conversion] suggests that improvement due to enhanced light absorption or to better exciton harvesting via resonance energy transfer is unlikely. These data, together with the crystallinity results, support the inference that the SF compounds are excluded to the donor:acceptor interface by crystallization of the host blend. This conclusion is further supported by impedance spectroscopy and a longer measured charge-carrier lifetime in the ternary blend.

Robust Hierarchically Structured Biphasic Ambipolar Oxide Photoelectrodes for Light-Driven Chemical Regulation and Switchable Logic Applications.

Tunable ambipolar photoelectrochemical behavior emerges from microdomains of nanostructured p-type CuFeO2 and n-type Fe2 O3 that arise from a single facile solution-processed thin film. The switchable operation of this system is controlled by chemical, optical, or electronic inputs with a uniquely high photocurrent response (on the order of 1 mA cm-2 ), suitable for robust practical application as an oxygen photoregulator.

Enhancing the Thermal Stability of Solution-Processed Small-Molecule Semiconductor Thin Films Using a Flexible Linker Approach.

Using flexible aliphatic chains to link conjugated molecular semiconductors affords a polymeric material that possesses defined conjugated segments but extended covalent connectivity, which enhances crystallinity and thermal stability in field-effect transistors and bulk heterojunction solar-cell devices when used as an additive.

Templating Sol-Gel Hematite Films with Sacrificial Copper Oxide: Enhancing Photoanode Performance with Nanostructure and Oxygen Vacancies.

Nanostructuring hematite films is a critical step for enhancing photoelectrochemical performance by circumventing the intrinsic limitations on minority carrier transport. Herein, we present a novel sol-gel approach that affords nanostructured hematite films by including CuO as sacrificial templating agent. First, by annealing in air at 450 °C a film comprising an intimate mixture of CuO and Fe2O3 nanoparticles is obtained. The subsequent treatment with NaCl and annealing at 700 °C under Argon reveals a nanostructured highly crystalline hematite film devoid of copper. Photoelectrochemical investigations reveal that the incorporation of CuO as templating agent and the inert conditions employed during the annealing play a crucial role in the performance of the hematite electrodes. Mott-Schottky analysis shows a higher donor concentration when annealing in inert conditions, and even higher when combined with the NaCl treatment. These findings agree well with the presence of an oxygen-deficient shell on the material's surface evidenced by FT-IR and XPS measurements. Likewise, the incorporation of the CuO enhances the photocurrent obtained at 1.23 V from 0.55 to 0.8 mA·cm(-2) because of an improved nanostructure. Optimized films demonstrate an incident photon-to-current efficiency (IPCE) of 52% at 380 nm when applying 1.23 V versus RHE, and a faradaic efficiency for water splitting close to unity.

Rediscovering a key interface in dye-sensitized solar cells: guanidinium and iodine competition for binding sites at the dye/electrolyte surface.

We propose a new mechanism by which the common electrolyte additive guanidinium thiocyanate (GdmSCN) improves efficiency in dye-sensitized solar cells (DSSCs). We demonstrate that binding of Gdm(+) to TiO2 is weak and does not passivate recombination sites on the TiO2 surface as has been previously claimed. Instead, we show that Gdm(+) binds strongly to the N719 and D131 dyes and probably to many similar compounds. The binding of Gdm(+) competes with iodine binding to the same molecule, reducing the surface concentration of dye-I2 complexes. This in turn reduces the electron/iodine recombination rate constant, which increases the collection efficiency and thus the photocurrent. We further observe that GdmNO3 can increase efficiency more than the current Gdm(+) source, GdmSCN, at least in some DSSCs. Overall, the results point to an improved paradigm for DSSC operation and development. The TiO2/electrolyte surface has long been held to be the key interface in DSSCs. We now assert that the dye layer/electrolyte interaction is at least, and probably more, important.