High-performance perovskite-polymer hybrid solar cells via electronic coupling with fullerene monolayers.

A plethora of solution-processed materials have been developed for solar cell applications. Hybrid solar cells based on light absorbing semiconducting polymers infiltrated into mesoporous TiO2 are an interesting concept, but generating charge at the polymer-metal oxide heterojunction is challenging. Metal-organic perovskite absorbers have recently shown remarkable efficiencies but currently lack the range of color tunability of organics. Here, we have combined a fullerene self-assembled monolayer (C60SAM) functionalized mesoporous titania, a perovskite absorber (CH3NH3PbI3-xClx), and a light absorbing polymer hole-conductor, P3HT, to realize a 6.7% efficient hybrid solar cell. We find that photoexcitations in both the perovskite and the polymer undergo very efficient electron transfer to the C60SAM. The C60SAM acts as an electron acceptor but inhibits further electron transfer into the TiO2 mesostructure due to energy level misalignment and poor electronic coupling. Thermalized electrons from the C60SAM are then transported through the perovskite phase. This strategy allows a reduction of energy loss, while still employing a "mesoporous electron acceptor", representing an exciting and versatile route forward for hybrid photovoltaics incorporating light-absorbing polymers. Finally, we show that we can use the C60SAM functionalization of mesoporous TiO2 to achieve an 11.7% perovskite-sensitized solar cell using Spiro-OMeTAD as a transparent hole transporter.

The real TiO2/HTM interface of solid-state dye solar cells: role of trapped states from a multiscale modelling perspective.

In this paper we present a multiscale simulation of charge transport in a solid-state dye-sensitized solar cell, where the real morphology between TiO2 and the hole transport material is included. The geometry of the interface is obtained from an electron tomography measurement and imported in a simulation software. Charge distribution, electric field and current densities are computed using the drift-diffusion model. We use this approach to investigate the electrostatic effect of trap states at the interface between the electron and hole transport materials. The simulations show that when the trapped electrons are not screened by external additives, the dynamics of holes is perturbed. Holes accumulate at the interface, enhancing recombination and reducing cell performance.

Trap-induced losses in hybrid photovoltaics.

We investigate the loss mechanisms in hybrid photovoltaics based on blends of poly(3-hexylthiophene) with CdSe nanocrystals of various sizes. By combining the spectroscopic and electrical measurements on working devices as well as films, we identify that high trap-mediated recombination is responsible for the loss of photogenerated charge carriers in devices with small nanocrystals. In addition, we demonstrate that the reduced open-circuit voltage for devices with small nanocrystals is also caused by the traps.

Hyperbranched quasi-1D nanostructures for solid-state dye-sensitized solar cells.

In this work we demonstrate hyperbranched nanostructures, grown by pulsed laser deposition, composed of one-dimensional anatase single crystals assembled in arrays of high aspect ratio hierarchical mesostructures. The proposed growth mechanism relies on a two-step process: self-assembly from the gas phase of amorphous TiO2 clusters in a forest of tree-shaped hierarchical mesostructures with high aspect ratio; oriented crystallization of the branches upon thermal treatment. Structural and morphological characteristics can be optimized to achieve both high specific surface area for optimal dye uptake and broadband light scattering thanks to the microscopic feature size. Solid-state dye sensitized solar cells fabricated with arrays of hyperbranched TiO2 nanostructures on FTO-glass sensitized with D102 dye showed a significant 66% increase in efficiency with respect to a reference mesoporous photoanode and reached a maximum efficiency of 3.96% (among the highest reported for this system). This result was achieved mainly thanks to an increase in photogenerated current directly resulting from improved light harvesting efficiency of the hierarchical photoanode. The proposed photoanode overcomes typical limitations of 1D TiO2 nanostructures applied to ss-DSC and emerges as a promising foundation for next-generation high-efficiency solid-state devices comprosed of dyes, polymers, or quantum dots as sensitizers.

Photoinduced charge transfer and efficient solar energy conversion in a blend of a red polyfluorene copolymer with CdSe nanoparticles.

We present measurements of charge transfer and the photovoltaic effect in a blend of the alternating polyfluorene copolymer poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)) with branched CdSe nanoparticles. Quasi-steady-state photoinduced absorption measurements identified a long-lived charged species that formed after photoexcitation at room temperature. Photovoltaic devices based on this blend system showed a spectral response extending to 650 nm and gave a solar power conversion efficiency of 2.4% under Air Mass 1.5 Global (AM1.5G) conditions.