Morphology stabilization strategies for small-molecule bulk heterojunction photovoltaics

The greater crystallinity of solution-processed small-molecule organic semiconductors, compared to their polymer counterparts, renders the bulk heterojunction (BHJ) more susceptible to phase separation under thermal stress, decreasing device performance. Here we demonstrate and compare strategies to stabilize the donor: acceptor BHJ in DPP(TBFu)(2):PC61BM solar cells using molecular additives designed to either afford compatiblization (CP) of the bulk heterojunction, or to in situ link (ISL) the components using a functional azide group. Both additives were found to stop phase segregation of the BHJ under thermal stress. At 5 wt% loading the ISL additive prevents phase segregation, while altering the azide reaction mechanism by using UV-induced linking versus thermal induced linking was found to significantly affect the device performance. Including 5 wt% of the CP additive slowed phase segregation and devices retained 80% of their optimum performance after 3000 min of thermal treatment at 110 degrees C (compared to 50% with the control). The CP additive at 10 wt% changed drastically the kinetics of phase segregation leading to devices with no decrease in performance over 3000 min thermal treatment. Thin film morphology characterization together with photoluminescence and impedance spectroscopy give further insight into the performance differences between the additives. These results reinforce the conclusion that the compatiblization method is the most promising strategy to engineer highly-efficient thermally-stable organic photovoltaics based on solution-processed small molecules

Autodecomposition Approach for the Low-Temperature Mesostructuring of Nanocrystal Semiconductor Electrodes

Templating nanocrystals into mesoporous electrodes using an organic porogen typically requires thermal oxidation at >400 degrees C to completely remove the organic material and afford good electronic properties. These oxidation conditions are incompatible with many NC materials. Here, we demonstrate that nitrocellulose can afford mesoporous CdS and CZTS nanocrystal thin film electrodes at only 250-300 degrees C in air or under argon. Remarkable control over the surface area and the average pore size (20-100 nm) in thin films are demonstrated by varying the ratio of preformed nanocrystals and nitrocellulose. Moreover the mesoporous electrodes exhibit excellent optoelectronic properties. Photoelectrochemical performance is enhanced due to the increased active surface area, showing an 8-fold photocurrent increase over compact nanocrystal films, and an IPCE over 70%

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-C-61-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

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

A specially-designed molecular compatibilizer enables bulk-heterojunction organic photovoltaic devices that tolerate melt processing thus reducing the need for toxic solvents.</p

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 Fe2O3 that arise from a single facile solution-processed thin fi lm. 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

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 degrees C a film comprising an intimate mixture of CuO and Fe2O3 nanopartides is obtained. The subsequent treatment with NaCl and annealing at 700 degrees 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

A Bottom-Up Approach toward All-Solution-Processed High-Efficiency Cu(In,Ga)S

The development of solution-processable routes to prepare efficient photoelectrodes for water splitting is highly desirable to reduce manufacturing costs. Recently, sulfide chalcopyrites (Cu(In,Ga)S-2) have attracted attention as photocathodes for hydrogen evolution owing to their outstanding optoelectronic properties and their band gap-wider than their selenide counterparts-which can potentially increase the attainable photovoltage. A straightforward and all-solution-processable approach for the fabrication of highly efficient photocathodes based on Cu(In,Ga)S-2 is reported for the first time. It is demonstrated that semiconductor nanocrystals can be successfully employed as building blocks to prepare phase-pure microcrystalline thin films by incorporating different additives (Sb, Bi, Mg) that promote the coalescence of the nanocrystals during annealing. Importantly, the grain size is directly correlated to improved charge transport for Sb and Bi additives, but it is shown that secondary effects can be detrimental to performance even with large grains (for Mg). For optimized electrodes, the sequential deposition of thin layers of n-type CdS and TiO2 by solution-based methods, and platinum as an electrocatalyst, leads to stable photocurrents saturating at 8.0 mA cm(-2) and onsetting at similar to 0.6 V versus RHE under AM 1.5G illumination for CuInS2 films. Electrodes prepared by our method rival the state-of-the-art performance for these materials