Electron transport in acceptor-sensitized polymer-oxide solar cells: the importance of surface dipoles and electron cascade effects

Tailored Interface Energetics for Efficient Charge Separation in Metal Oxide-Polymer Solar Cells

Hybrid organic-inorganic heterointerfaces in solar cells suffer from inefficient charge separation yet the origin of performance limitations are widely unknown. In this work, we focus on the role of metal oxide-polymer interface energetics in a charge generation process. For this purpose, we present novel benzothiadiazole based thiophene oligomers that tailor the surface energetics of the inorganic acceptor TiO₂ systematically. In a simple bilayer structure with the donor polymer poly(3-hexylthiophene) (P3HT), we are able to improve the charge generation process considerably. By means of an electronic characterization of solar cell devices in combination with ultrafast broadband transient absorption spectroscopy, we demonstrate that this remarkable improvement in performance originates from reduced recombination of localized charge transfer states. In this context, fundamental design rules for interlayers are revealed, which assist the charge separation at organic-inorganic interfaces. Beside acting as a physical spacer in between electrons and holes, interlayers should offer (1) a large energy offset to drive exciton dissociation, (2) a push-pull building block to reduce the Coulomb binding energy of charge transfer states and (3) an energy cascade to limit carrier back diffusion towards the interface.

DNA mediated electrocatalytic enhancement of α-Fe2O3–PEDOT–C-MoS2hybrid nanostructures for riboflavin detection on screen printed electrode

A facile synthesis of DNA wrapped iron oxide nanorods and PEDOT (poly(3,4-ethylenedioxythiophene)) nanospheres on carbon supported MoS₂(C-MoS₂) is reported for riboflavin (RF) sensing.

Dipole-induced conductivity enhancement by n-type inclusion in a p-type system: α-Fe₂O₃-PEDOT:PSS nanocomposites

Hematite (α-Fe2O3) nanoparticles of two different shapes but of same size (ca. 40 nm) were dispersed in PEDOT:PSS matrices in various concentration ranges (0-7 wt%) to study the consequent changes in conductivity in the dark and under solar illumination conditions. Within a distinct range of concentration, a distinct increase in the conductivity was observed for both spherical and cubical particle population. We ascribed this effect to the generalized Poole-Frenkel theory of conduction in conjunction with the basic depletion width properties of heterojunctions and electrostatic dipole moments, and verified our assumptions through data fitting. A difference in conductivity between sphere- and cube-based α-Fe2O3-PEDOT:PSS nanocomposites was also observed and ascribed to the electrostatic edge effect on the nanoparticles. The dispersion of α-Fe2O3 nanocrystals was confirmed by high-resolution electron microscopy, whereas the electrical properties and modulations thereof were followed by recording current-voltage characteristics.

Solid-state photogalvanic dye-sensitized solar cells

Photogalvanic cells are photoelectrochemical systems wherein the semiconductor electrode is not a participant in primary photoinduced charge formation. The discovery of photoelectrochemical systems that successfully exploit secondary (thermal) electron injection at dye-semiconductor interfaces may enable studies of electron transfer at minimal driving force for electron injection into the semiconductor. In this study, we have examined thermal electron transfer from molecular sensitizers to nanostructured semiconductor electrodes composed of titanium dioxide nanorods by means of transient spectroscopy and the assembly and testing of photoelectrochemical cells. Electron-accepting molecular dyes have been studied alongside an arylamine electron donor. Thermal injection is estimated for a naphthacenequinone radical anion as a multiexponential decay process with initial decay lifetimes of 6 and 27 ps. The ambient electric field present during charge separation at a surface-adsorbed dye monolayer causes Stark shifts of the radical ion pair absorbance peaks that confounded kinetic estimation of thermal injection for a fullerene sensitizer. Electron-accepting dyes that operate by thermal injection into titanium dioxide function better in solid-state photoelectrochemical cells than in liquid-junction cells due to the kinetic advantage of solid-state cells with respect to photoinduced acceptor-quenching to form the necessary radical anion sensitizers.

Rethinking band bending at the P3HT-TiO(2) interface

The advancement of solar cell technology necessitates a detailed understanding of material heterojunctions and their interfacial properties. In hybrid bulk heterojunction solar cells (HBHJs), light-absorbing conjugated polymers are often interfaced with films of nanostructured TiO2 as a cheaper alternative to conventional inorganic solar cells. The mechanism of photovoltaic action requires photoelectrons in the polymer to transfer into the TiO2, and therefore, polymers are designed with lowest unoccupied molecular orbital (LUMO) levels higher in energy than the conduction band of TiO2 for thermodynamically favorable electron transfer. Currently, the energy level values used to guide solar cell design are referenced from the separated materials, neglecting the fact that upon heterojunction formation material energetics are altered. With spectroelectrochemistry, we discovered that spontaneous charge transfer occurs upon heterojunction formation between poly(3-hexylthiophene) (P3HT) and nanocrystalline TiO2. It was determined that deep trap states (0.5 eV below the conduction band of TiO2) accept electrons from P3HT and form hole polarons in the polymer. This equilibrium charge separation alters energetics through the formation of interfacial dipoles and results in band bending that inhibits desired photoelectron injection into TiO2, limiting HBHJ solar cell performance. X-ray photoelectron spectroscopic studies quantified the resultant vacuum level offset to be 0.8 eV. Further spectroelectrochemical studies indicate that 0.1 eV of this offset occurs in TiO2, whereas the balance occurs in P3HT. New guidelines for improved photocurrent are proposed by tuning the energetics of the heterojunction to reverse the direction of the interfacial dipole, enhancing photoelectron injection.

Influence of seeding and bath conditions in hydrothermal growth of very thin (∼20 nm) single-crystalline rutile TiO₂ nanorod films

New seeding conditions have been examined for the hydrothermal growth of single-crystalline rutile TiO₂ nanorods. Rutile nanorods of ∼20 nm diameter are grown from seed layers consisting of either (A) TiO₂ or MnOOH nanocrystals deposited from suspension, or (B) a continuous sheet of TiO₂. These seed layers are more effective for seeding the growth of rutile nanorods compared to the use of bare F-SnO₂ substrates. The TiO₂ sheet seeding allows lower concentration of titanium alkoxide precursor relative to previously reported procedures, but fusion of the resulting TiO₂ nanorods into bundles occurs at higher precursor concentration and/or longer growth duration. Performance of polymer-oxide solar cells prepared using these nanorods shows a dependence on the extent of bundling as well as rod height.