Bulk Heterojunction versus Diffused Bilayer: The Role of Device Geometry in Solution p-Doped Polymer-Based Solar Cells

We exploit the effect of molecular p-type doping of P3HT in diffused bilayer (DB) polymer solar cells. In this alternative device geometry, the p-doping is accomplished in solution by blending the F₄-TCNQ with P3HT. The p-doping both increases the film conductivity and reduces the potential barrier at the interface with the electrode. This results in an excellent power conversion efficiency of 4.02%, which is an improvement of ∼48% over the p-doped standard bulk heterojunction (BHJ) device. Combined <i>V</i>_OC–light intensity dependence measurements and Kelvin probe force microscopy reveal that the DB device configuration is particularly advantageous, if compared to the conventional BHJ, because it enables optimization of the donor and acceptor layers independently to minimize the effect of trapping and to fully exploit the improved transport properties

Synthesis and Photovoltaic Properties of Regioregular Head-to-Head Substituted Thiophene Hexadecamers

We describe the expedient synthesis of regioregular thiophene hexadecamers head-to-head (hh) substituted with hexyl and hexylthio grous. The synthesis was carried out by means of a sequence of ultrasound-assisted selective monobrominations and microwave-assisted Suzuki reactions using 4,4,5,5-tetramethyl-1,3,2-dioxaborolane in THF:water. The hexadecamers, which are very soluble in organic solvents, were investigated in solution and thin film by a variety of techniques (UV, PL, CV, X-ray diffraction, FET charge mobility, SKFM) with the aim of elucidating the effect of the sulfur spacer on morphology and functional properties. We show that the sulfur spacer compensates for the decrease in π–π conjugation caused by the hh regiochemistry and that the λ_max value and redox potentials of the S-alkyl-substituted hexadecamer are similar to those of head-to-tail substituted poly­(3-hexylthiophene). Measurements in field effect transistor devices showed that the alkylthio-substituted hexadecamer is a p-type semiconductor while the alkyl-substituted counterpart in the same conditions is not electroactive. Scanning Kelvin force microscopy measurements showed that a blend of the alkylthio-substituted hexadecamer with PCBM displays photovoltaic behavior under illumination. In agreement with this, a bulk heterojunction cell fabricated employing the same blend displayed near 1.5% conversion efficiency without addition of additives or device optimization

Colloidal Arenethiolate-Capped PbS Quantum Dots: Optoelectronic Properties, Self-Assembly, and Application in Solution-Cast Photovoltaics

Suitable postsynthesis surface modification of lead-chalcogenide quantum dots (QDs) is crucial to enable their integration in photovoltaic devices. Here we exploit arenethiolate anions to completely replace pristine oleate ligands on PbS QDs in the solution phase, thus preserving the colloidal stability of QDs and allowing their solution-based processability into photoconductive thin films. Complete QD surface modification relies on the stronger acidic character of arenethiols compared to that of alkanethiols and is demonstrated by FTIR and UV–vis–NIR absorption spectroscopy analyses, which provide quantitative evaluation of stoichiometry and thermodynamic stability of the resulting system. Arenethiolate ligands induce a noticeable reduction of the optical band gap of PbS QDs, which is described and explained by charge transfer interactions occurring at the organic/inorganic interface that relax exciton confinement, and a large increase of QD molar absorption coefficient, achieved through the conjugated moiety of the replacing ligands. In addition, surface modification in the solution phase promotes switching of the symmetry of PbS QD self-assembled superlattices from hexagonal to cubic close packing, which is accompanied by further reduction of the optical band gap, ascribed to inter-QD exciton delocalization and dielectric effects, together with a drastic improvement of the charge transport properties in PbS QD solids. As a result, smooth dense-packed thin films of arenethiolate-capped PbS QDs can be integrated in heterojunction solar cells via a single solution-processing step. Such single PbS QD layers exhibit abated cracking upon thermal or chemical postdeposition treatment, and the corresponding devices generate remarkable photocurrent densities and overall efficiencies, thus representing an effective strategy toward low-cost processing for QD-based photovoltaics

Three-Dimensional Self-Assembly of Networked Branched TiO₂ Nanocrystal Scaffolds for Efficient Room-Temperature Processed Depleted Bulk Heterojunction Solar Cells

In this work, we report on ∼4% power conversion efficiency (PCE) depleted bulk heterojunction (DBH) solar cells based on a high-quality electrode with a three-dimensional nanoscale architecture purposely designed so as to maximize light absorption and charge collection. The newly conceived architecture comprises a mesoporous electron-collecting film made of networked anisotropic metal-oxide nanostructures, which accommodates visible-to-infrared light harvesting quantum dots within the recessed regions of its volume. The three-dimensional electrodes were self-assembled by spin-coating a solution of colloidal branched anatase TiO₂ NCs (BNC), followed by photocatalytic removal of the native organic capping from their surface by a mild UV-light treatment and filling with small PbS NCs via infiltration. The PCE ∼ 4% of our TiO₂ BNC/PbS QD DBH solar cell features an enhancement of 84% over the performance obtained for a planar device fabricated under the same conditions. Overall, the DBH device fabrication procedure is entirely carried out under mild processing conditions at room temperature, thus holding promise for low-cost and large-scale manufacturing