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 UVvisNIR 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.

One of the most fascinating characteristics of perovskite solar cells (PSCs) is the retrieved obtainment of outstanding photovoltaic (PV) performances withstanding important device configuration variations. Here we have analyzed CH3NH3PbI3-xClx in planar or in mesostructured (MS) configurations employing both titania and alumina scaffolds fully infiltrated with perovskite material or presenting an overstanding layer. The use of the MS scaffold induces to the perovskite different structural properties in terms of grain size preferential orientation and unit cell volume in comparison to the ones of the material grown with no constraints as we have found out by X-ray diffraction analyses. We have studied the effect of the PSC configuration on photoinduced absorption and time-resolved photoluminescence complementary techniques that allow studying charge photogeneration and recombination. We have estimated electron diffusion length in the considered configurations observing a decrease when the material is confined in the MS scaffold with respect to a planar architecture. However the presence of perovskite overlayer allows an overall recovering of long diffusion lengths explaining the record PV performances obtained with a device configuration bearing both the mesostructure and a perovskite overlayer. Our results suggest that performance in devices with perovskite overlayer is mainly ruled by the overlayer whereas the mesoporous layer influences the contact properties.

The role of chloride in the MAPbI(3_x)Cl(x) perovskite is still limitedly understood, albeit subjected of much debate. Here, we present a combined angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and first-principles DFT modeling to investigate the MAPbI(3-x)Cl(x)/TiO2 interface. AR-XPS analyses carried out on ad hoc designed bilayers of MAPbI(3-x)Cl(x) perovskite deposited onto a flat TiO2 substrate reveal that the chloride is preferentially located in close proximity to the perovskite/TiO2 interface. DFT calculations indicate the preferential location of chloride at the TiO2 interface compared to the bulk perovskite due to an increased chloride TiO2 surface affinity. Furthermore, our calculations clearly demonstrate an interfacial chloride-induced band bending, creating a directional 'electron funnel' that may improve the charge collection efficiency of the device and possibly affecting also recombination pathways. Our findings represent a step forward to the rationalization of the peculiar properties of mixed halide perovskite, allowing one to further address material and device design issues.

Here we conceive an innovative nanocomposite to endow hybrid perovskites with the easy processability of polymers providing a tool to control film quality and material crystallinity. We verify that the employed semiconducting polymer poly[2-methoxy-5-(2-ethylhexyloxy)-14-phenylenevinylene] (MEH-PPV) controls the self-assembly of CH3NH3PbI3 (MAPbI(3)) crystalline domains and favors the deposition of a very smooth and homogenous layer in one straightforward step. This idea offers a new paradigm for the implementation of polymer/perovskite nanocomposites towards versatile optoelectronic devices combined with the feasibility of mass production. As a proof-of-concept we propose the application of such nanocomposite in polymer solar cell architecture demonstrating a power conversion efficiency up to 3% to date the highest reported for MEH-PPV. On-purpose designed polymers are expected to suit the nanocomposite properties for the integration in diverse optoelectronic devices via facile processing condition.

Charge generation and transport in (CH3NH3) Pbl(3-x)Cl(x) sensitized mesostructured solar cells are investigated. A highly efficient charge generation is directly proven by time correlated single photon counting analysis. Photoinduced absorption and transient photo-voltage investigations depict double charge recombination dynamics. To explain the high device performances according to those spectroscopic observations, we suggest the existence of two complementary paths for electron transport, involving either TiO2 or perovskite matrixes.

We propose a new approach for converting light energy into electrical energy based on the photogeneration of nano-dipoles at donor-acceptor interfaces. The nano-dipoles are oriented in space so as to contribute to a collective polarization that induces a potential difference across the material sandwiched between electrodes. A current is detected in the external circuit upon illumination. Such a device would exploit many advantages of organic semiconductors and get rid of the main limitation namely transport. We provide a proof of concept and we discuss the ideal limit of the device based on numerical simulations. This provides design guidelines to the achievement of best performances. Simulations show that the proposed device can be an appealing opportunity with giant conversion efficiency provided some technological issues are overcome.

In recent years the use of nanocrystals (NCs) as inorganic semiconductors in conjugated polymer-based hybrid solar cells (HSCs) results as a challenging target for researchers because of their high electron mobility and good mechanical robustness compared to fullerene derivatives, as well as their capability of harvesting photons in the visible to near-infrared region of the solar spectrum.Unfortunately HSCs still suffer from a major drawback, that is a poor control over the organic donor and inorganic acceptor domain size distributions. The domain size of the donor and acceptor materials should be comparable to the exciton diffusion length in order to increase the probability of exciton dissociation, and to improve charge carrier transport and collection at the device electrodes. The readiness of inorganic NCs to macrophase separate from non-polar conjugated polymers at higher loading concentrations, prevents the building of a controllable interpenetrating percolation network.A suitable nanostructuring compatibilizer can induce selective intermolecular interactions, leading to morphological order associated to an improvement in the interface processes, which consequently drive a strong rising in the performances of the device by decreasing recombination losses. For this purpose, block copolymers are particularly significant because they can self-assemble through phase separation by rationally tailoring the blocks using functional moieties with different physical-chemical behaviours.We report on the synthesis of a rod-coil diblock copolymer based on poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), a low band gap copolymer, and a coil block of poly(4-vinylpyridine) able to interact with semiconducting CdSe NCs with the aim of using it as coordinating material in the preparation of PCPDTBT/CdSe NCs-based hybrid solar cells.

Hybrid halide perovskites represent one of the most promising solutions toward the fabrication of all solid nanostructured solar cells, with improved efficiency and long-term stability. This article aims at investigating the structural properties of iodide/chloride mixed-halide perovskites and correlating them with their photovoltaic performances. We found out that, independent of the components ratio in the precursor solution, Cl incorporation in an iodide-based structure, is possible only at relatively low concentration levels (below 3-4%). However, even if the material band gap remains substantially unchanged, the Cl doping dramatically improves the charge transport within the perovskite layer, explaining the outstanding performances of meso-superstructured solar cells based on this material.

Hybrid composites obtained upon blending conjugated polymers and colloidal semiconductor nanocrystals are regarded as attractive photoactive materials for optoelectronic applications. Here it is demonstrated that tailoring nanocrystal surface chemistry permits to control non-covalent and electronic interactions between organic and inorganic components. The pending moieties of organic ligands at the nanocrystal surface are shown to not merely confer colloidal stability while hindering charge separation and transport, but drastically impact morphology of hybrid composites during formation from blend solutions. The relevance of this approach to photovoltaic applications is demonstrated for composites based on poly(3-hexylthiophene) and lead sulfide nanocrystals, considered as inadequate until this report, which enable the fabrication of hybrid solar cells displaying a power conversion efficiency that reaches 3%. By investigating (quasi) steady-state and time-resolved photo-induced processes in the nanocomposites and their constituents, it is ascertained that electron transfer occurs at the hybrid interface yielding long-lived separated charge carriers, whereas interfacial hole transfer appears hindered. Here a reliable alternative aiming to gain control over macroscopic optoelectronic properties of polymer/nanocrystal composites by mediating their non-covalent interactions via ligands' pending moieties is provided, thus opening new possibilities towards efficient solution-processed hybrid solar cells.

Hybrid halide perovskites have emerged as promising active constituents of next generation solution processable optoelectronic devices. During their assembling process perovskite components undergo very complex dynamic equilibria starting in solution and progressing throughout film formation. Finding a methodology to control and affect these equilibria responsible for the unique morphological diversity observed in perovskite films constitutes a fundamental step towards a reproducible material processability. Here we propose the exploitation of polymer matrices as cooperative assembling components of novel perovskite CH3NH3PbI3:polymer composites in which the control of the chemical interactions in solution allows a predictable tuning of the final film morphology. We reveal that the nature of the interactions between perovskite precursors and polymer functional groups probed by Nuclear Magnetic Resonance (NMR) spectroscopy and Dynamic Light Scattering (DLS) techniques allows the control of aggregates in solution whose characteristics are strictly maintained in the solid film and permits the formation of nanostructures that are inaccessible to conventional perovskite depositions. These results demonstrate how the fundamental chemistry of perovskite precursors in solution has a paramount influence on controlling and monitoring the final morphology of CH3NH3PbI3 (MAPbI3) thin films foreseeing the possibility of designing perovskite:polymer composites targeting diverse optoelectronic applications.

Herein we describe the realization of nanowalled polymeric microtubes through a novel andversatile approach combining the layer-by-layer (LbL) deposition technique, the self-rolling ofhybrid polymer/semiconductor microtubes and the subsequent removal of the semiconductortemplate. The realized channels were characterized in detail using scanning electron and atomicforce microscopes. Additionally, we report on the incorporation of a dye molecule within thenanowalls of such microtubes, demonstrating a distribution of the fluorescence signalthroughout the whole channel volume. This approach offers the possibility to tailor theproperties of micro/nanotubes in terms of size, wall thickness and composition, thus enablingtheir employment for several applications.

A spectroscopic investigation focusing on the charge generation and transport in inverted p-type perovskite-based mesoscopic (Ms) solar cells is provided in this report. Nanocrystalline nickel oxide and PCBM are employed respectively as hole transporting scaffold and hole blocking layer to sandwich a perovskite light harvester. An efficient hole transfer process from perovskite to nickel oxide is assessed through time-resolved photoluminescence and photoinduced absorption analyses for both the employed absorbing species namely MAPbI(3-x)Cl(x) and MAPbI(3). A striking relevant difference between p-type and n-type perovskite-based solar cells emerges from the study.

Tetrapod-shaped CdSe(core)/CdTe(arms) colloidal nanocrystals, capped with alkylphosphonic acids or pyridine, were reacted with various small molecules (acetic acid, hydrazine and chlorosilane) which induced their tip-to-tip assembly into soluble networks. These networks were subsequently processed into films by drop casting and their photoconductive properties were studied. We observed that films prepared from tetrapods coated with phosphonic acids were not photoconductive, but tip-to-tip networks of the same tetrapods exhibited appreciable photocurrents. On the other hand, films prepared from tetrapods coated with pyridine instead of phosphonic acids were already highly photoconductive even if the nanocrystals were not joined tip-to-tip. Based on the current-voltage behavior under light we infer that the tunneling between tetrapods is the dominant charge transport mechanism. In all the samples, chemically-induced assembly into networks tended to reduce the average tunneling barrier. Additionally, pyridine-coated tetrapods and the tip-to-tip networks made out of them were tested as active materials in hybrid photovoltaic devices. Overall, we introduce an approach to chemically-induced tip-to-tip assembly of tetrapods into solution processable networks and demonstrate the enhancement of electronic coupling of tetrapods by various ligand exchange procedures.

A highly dense and uniform layer of Au nanoparticles (NPs) on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film has been produced by the pulsed laser deposition (PLD) technique toward the production of an improved efficiency photovoltaic device. The advantage of PLD over other techniques is the easy and precise control of the Au NPs size and spatial distribution, without needing of further NP surface functionalization. The efficiency enhancement factor related to Au NPs doping has been evaluated in a solar cell based on poly-(3-hexylthiophene):[6,6]-phenyl-C-61-butyric acid methyl ester (P3HT:PCBM) diffused bilayer. The short-circuit current density, J (SC), increases by 18 % and the power conversion efficiency by 22 %, respectively, in comparison with an equivalent device without Au NPs. The optical and morphological properties of the Au NPs layer have been selected in order to evaluate the contribution of the surface plasmon resonance as enhancement factor of the solar cell efficiency, in a range size where light scattering is negligible.

Hybrid nanocomposites (HCs) obtained by blend solutions of conjugated polymers and colloidal semiconductor nanocrystals are among the most promising materials to be exploited in solution-processed photovoltaic applications. The comprehension of the operating principles of solar cells based on HCs thus represents a crucial step toward the rational engineering of high performing photovoltaic devices. Here we investigate the effect of conjugated polymers on hybrid solar cell performances by taking advantage from an optimized morphology of the HCs comprising lead sulfide quantum dots (PbS QDs). Uncommonly, we find that larger photocurrent densities are achieved by HCs incorporating wide-bandgap polymers. A combination of spectroscopic and electro-optical measurements suggests that wide-bandgap polymers promote efficient charge/exciton transfer processes and hinder the population of midgap states on PbS QDs. Our linings underline the key role of the polymer in HC-based solar cells in the activation/deactivation of charge transfer/loss pathways.

We have developed a room-temperature solution processing approach to integrate colloidal anatase titanium dioxide nanorods (TiO2 NRs) and lead sulfide quantum dots (PbS QDs) into a heterostructured p-n junction device. To this aim we have exploited a post-deposition treatment to remove surface-adsorbed ligands by means of UV-light-irradiation of TiO2 NRs and a dilute acid treatment of PbS QDs. Here we report a systematic study on the optimization of the post-deposition treatments and device fabrication. Our approach is fully compatible with plastic device technology and is potentially useful for the integration of crystalline TiO2 as active component into disparate solar cell architectures and organic optoelectronic devices.

Suitable post-synthesis surface modification of lead-chalcogenide quantum dots (QDs) is crucial to enable their integration in photovoltaic devices. We have developed a solution-phase ligand exchange strategy that exploits arenethiolate anions to replace the pristine oleate ligands on PbS QDs, while preserving the long-term colloidal stability of QDs and allowing their solution-based processability into photoconductive thin-films. Complete QD surface modification is demonstrated by IR spectroscopy analysis, whereas UV-Vis-NIR Absorption Spectroscopy provides quantitative evaluation of stoichiometry and thermodynamic stability of the resulting system. Arenethiolate ligands permit to reduce the inter-particle distance in PbS QD solids, leading to a drastic improvement of the photoinduced charge transport properties. Therefore, smooth dense-packed thin-films of arenethiolate-capped PbS QDs obtained via a single solution-processing step are integrated in heterojunction solar cells: such devices generate remarkable photocurrent densities (14 mA cm(-2)) and overall efficiencies (1.85%), which are outstanding for a single PbS QD layer. Solution-phase surface modification of QDs thus represents an effective intermediate step towards low-cost processing for all-inorganic and hybrid organic/inorganic QD-based photovoltaics.

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 pi-pi conjugation caused by the hh regiochemistry and that the lambda(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.

Conjugated polymers are characterized by the delocalization of ?-electrons responsible of their electrical properties and structural rigidity. The manipulation of the chemical structures through the introduction of flexible chains allows to influence the morphology controlling the nanosegregation of the material.The use of nanocrystals (NCs) of inorganic semiconductors in "hybrid solar cells" offers high electron mobility enhancing the performance of the devices. The control of the active layer morphology at the nanoscale level ensures the further improve of the efficiency of the cells.The goal of this work is the evaluation of two approaches for the synthesis of a rod-coil copolymer based on poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] rod-like block (electrondonor) and a coil block of poly(4-vinylpyridine) able to interact with semiconducting CdSe NCs (electronacceptor) with the aim of using it as both active and coordinating material for nanocomposites preparation with over 90% of NCs.

In this work, we report on similar to 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 similar to 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.