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

The Influence of Dopant Concentration on Optical-Electrical Features of Quantum Dot-Sensitized Solar Cell

In this study, TiO₂/CdS/Cd_xCu_1−xSe, TiO₂/CdS/Cd_xMn_1−xSe, and TiO₂/CdS/Cd_xAg_2−2xSe thin films were synthesized by chemical bath deposition for the fabrication of photoanode in quantum-dot-sensitized solar cells. As a result, the structural properties of the thin films have been studied by X-ray diffraction, which confirmed the zinc Blende structure in the samples. The optical films were researched by their experimental absorption spectra with different doping concentrations. Those results were combined with the Tauc correlation to estimate the absorption density, the band gap energy, valence band and conduction band positions, steepness parameter, and electron–phonon interaction. Furthermore, the electrical features, electrochemical impedance spectrum and photocurrent density curves were carried out. The result was used to explain the enhancing performance efficiency.

Competitive Adsorption of ZrO2 Nanoparticle and Alkali Cations (Li+-Cs+) on Muscovite (001)

We studied the adsorption behavior of ZrO₂ nanoparticles on a muscovite (001) surface in the presence of cations from the alkali series (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺). The results of X-ray reflectivity, i.e., specular crystal truncation rod and resonant anomalous X-ray reflectivity in combination with AFM images, show that the sorption of ZrO₂ nanoparticles is significantly affected by the binding mode of alkali ions on the muscovite (001) surface. From solutions containing alkali ions binding as outer sphere surface complexes (i.e., Li⁺ and Na⁺), higher uptake of Zr⁴⁺ is observed corresponding to the binding of larger nanoparticles, which relatively easily replace the loosely bound alkali ions. However, Zr⁴⁺ uptake in solutions containing alkali ions binding as inner sphere surface complexes (i.e., K⁺, Rb⁺, and Cs⁺) is significantly lower, and smaller nanoparticles are found at the interface. In addition, the uptake of Zr⁴⁺ in the presence of inner sphere bound cations displays a strong linear relationship with the hydration energy of the coexisting alkali ion. The linear trend can be interpreted as competitive adsorption between ZrO₂ nanoparticles and inner sphere bound alkali cations, which are replaced on the surface and undergo rehydration after release to the solution. The rehydration of alkali ions gives rise to a large energy gain, which dominates the reaction energy of the competitive adsorption process. The competitive adsorption mechanism of ZrO₂ nanoparticles and alkali ions is discussed comprehensively to highlight the potential relationship between the hydration effect of alkali ions and the effect of charge density of the nanoparticles.

Correlation between CdSe QD Synthesis, Post-Synthetic Treatment, and BHJ Hybrid Solar Cell Performance

In this publication we show that the procedure to synthesize nanocrystals and the post-synthetic nanocrystal ligand sphere treatment have a great influence not only on the immediate performance of hybrid bulk heterojunction solar cells, but also on their thermal, long-term, and air stability. We herein demonstrate this for the particular case of spherical CdSe nanocrystals, post-synthetically treated with a hexanoic acid based treatment. We observe an influence from the duration of this post-synthetic treatment on the nanocrystal ligand sphere size, and also on the solar cell performance. By tuning the post-synthetic treatment to a certain degree, optimal device performance can be achieved. Moreover, we show how to effectively adapt the post-synthetic nanocrystal treatment protocol to different nanocrystal synthesis batches, hence increasing the reproducibility of hybrid nanocrystal:polymer bulk-heterojunction solar cells, which usually suffers due to the fluctuations in nanocrystal quality of different synthesis batches and synthesis procedures.

A high performance organic photovoltaic utilizing PEDOT:PSS and graphene oxide

Interface engineering may lead to a high performance organic photovoltaic as well as long lifetime.

Improved efficiency of bulk heterojunction hybrid solar cells by utilizing CdSe quantum dot-graphene nanocomposites

We present a significant efficiency enhancement of hybrid bulk heterojunction solar cells by utilizing CdSe quantum dots attached to reduced graphene oxide (rGO) as the electron accepting phase, blended with the PCPDTBT polymer. The quantum dot attachment to rGO was achieved following a self-assembly approach, recently developed, using thiolated reduced graphene oxide (TrGO) to form a TrGO-CdSe nanocomposite. Therefore, we are able to obtain TrGO-CdSe quantum dot/PCPDTBT bulk-heterojunction hybrid solar cells with power conversion efficiencies of up to 4.2%, compared with up to 3% for CdSe quantum dot/PCPDTBT devices. The improvement is mainly due to an increase of the open-circuit voltage from 0.55 V to 0.72 V. We found evidence for a significant change in the heterojunction donor-acceptor blend nanomorphology, observable by a more vertical alignment of the TrGO-quantum dot nanocomposites in the z-direction and a different nanophase separation in the x-y direction compared to the quantum dot only containing device. Moreover, an improved charge extraction and trap state reduction were observed for TrGO containing hybrid solar cells.

Conjugated polymer/nanocrystal nanocomposites for renewable energy applications in photovoltaics and photocatalysis

Conjugated polymer/nanocrystal composites have attracted much attention for use in renewable energy applications because of their versatile and synergistic optical and electronic properties. Upon absorbing photons, charge separation occurs in the nanocrystals, generating electrons and holes for photocurrent flow or reduction/oxidation (redox) reactions under proper conditions. Incorporating these nanocrystals into conjugated polymers can complement the visible light absorption range of the polymers for photovoltaics applications or allow the polymers to sensitize or immobilize the nanocrystals for photocatalysis. Here, the current developments of conjugated polymer/nanocrystal nanocomposites for bulk heterojunction-type photovoltaics incorporating Cd- and Pb-based nanocrystals or quantum dots are reviewed. The effects of manipulating the organic ligands and the concentration of the nanocrystal precursor, critical factors that affect the shape and aggregation of the nanocrystals, are also discussed. In the conclusion, the mechanisms through which conjugated polymers can sensitize semiconductor nanocrystals (TiO2 , ZnO) to ensure efficient charge separation, as well as how they can support immobilized nanocrystals for use in photocatalysis, are addressed.

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.

Mechanism for strong binding of CdSe quantum dots to multiwall carbon nanotubes for solar energy harvesting

As hybrid nanomaterials have myriad of applications in modern technology, different functionalization strategies are being intensely sought for preparing nanocomposites with tunable properties and structures. Multi-Walled Carbon Nanotube (MWNT)/CdSe Quantum Dot (QD) heterostructures serve as an important example for an active component of solar cells. The attachment mechanism of CdSe QDs and MWNTs is known to affect the charge transfer between them and consequently to alter the efficiency of solar cell devices. In this study, we present a novel method that enables the exchange of some of the organic capping agents on the QDs with carboxyl functionalized MWNTs upon ultrasonication. This produces a ligand-free covalent attachment of the QDs to the MWNTs. EXAFS characterization reveals direct bond formation between the CdSe QDs and the MWNTs. The amount of oleic acid exchanged is quantified by temperature-programmed decomposition; the results indicate that roughly half of the oleic acid is removed from the QDs upon functionalized MWNT addition. Additionally, we characterize the optical and structural properties of the QD-MWNT heterostructures and investigate how these properties are affected by the attachment. The steady state photoluminescence response of QDs is completely quenched. The lifetime of the PL of the QDs measured with time resolved photoluminescence shows a significant decrease after they are covalently bonded to functionalized MWNTs, suggesting a fast charge transfer between QDs and MWNTs. Our theoretical calculations are consistent with and support these experimental findings and provide microscopic models for the QD binding mechanisms.

Nanochemistry and nanomaterials for photovoltaics

Nanochemistry and nanomaterials provide numerous opportunities for a new generation of photovoltaics with high solar energy conversion efficiencies at low fabrication cost. Quantum-confined nanomaterials and polymer-inorganic nanocomposites can be tailored to harvest sun light over a broad range of the spectrum, while plasmonic structures offer effective ways to reduce the thickness of light-absorbing layers. Multiple exciton generation, singlet exciton fission, photon down-conversion, and photon up-conversion realized in nanostructures, create significant interest for harvesting underutilized ultraviolet and currently unutilized infrared photons. Nanochemical interface engineering of nanoparticle surfaces and junction-interfaces enable enhanced charge separation and collection. In this review, we survey these recent advances employed to introduce new concepts for improving the solar energy conversion efficiency, and reduce the device fabrication cost in photovoltaic technologies. The review concludes with a summary of contributions already made by nanochemistry. It then describes the challenges and opportunities in photovoltaics where the chemical community can play a vital role.

High performance photoswitches based on flexible and amorphous D-A polymer nanowires

A fluorene-based donor-acceptor conjugated polymer is synthesized and the polymer nanowires are successfully prepared with high quality and large scale using a simple and practical template dipping method. These amorphous polymer nanowires are flexible and show excellent photoconductive properties with reliable reproducibility. The individual nanowire photoswitches exhibit a responsivity as high as 1700 mA W(-1) and an on/off ratio as high as 2000 under a light intensity of 5.76 mW cm(-2) and a driving voltage of 40 V.

Strong carrier lifetime enhancement in GaAs nanowires coated with semiconducting polymer

The ultrafast charge carrier dynamics in GaAs/conjugated polymer type II heterojunctions are investigated using time-resolved photoluminescence spectroscopy at 10 K. By probing the photoluminescence at the band edge of GaAs, we observe strong carrier lifetime enhancement for nanowires blended with semiconducting polymers. The enhancement is found to depend crucially on the ionization potential of the polymers with respect to the Fermi energy level at the surface of the GaAs nanowires. We attribute these effects to electron doping by the polymer which reduces the unsaturated surface-state density in GaAs. We find that when the surface of nanowires is terminated by native oxide, the electron injection across the interface is greatly reduced and such surface doping is absent. Our results suggest that surface engineering via π-conjugated polymers can substantially improve the carrier lifetime in nanowire hybrid heterojunctions with applications in photovoltaics and nanoscale photodetectors.

Understanding polycarbazole-based polymer:CdSe hybrid solar cells

We report for the first time the fabrication and characterization of organic-inorganic bulk heterojunction (BHJ) hybrid solar cells made of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and pyridine-capped CdSe nanorods. By optimizing both CdSe loading and active layer film thickness, the power conversion efficiencies (PCEs) of PCDTBT:CdSe hybrid solar cells were able to reach 2%, with PCDTBT:CdSe devices displaying an open-circuit voltage (V(OC )) that is 35% higher than P3HT:CdSe devices due to the deeper HOMO level of PCDTBT polymer. The performance of PCDTBT:CdSe devices is limited by its morphology and also its lower LUMO energy offset compared to P3HT:CdSe devices. Hence, the performance of PCDTBT:CdSe solar cells could be further improved by modifying the morphology of the films and also by including an interlayer to generate a built-in voltage to encourage exciton dissociation. Our results suggest that PCDTBT could be a viable alternative to P3HT as an electron donor in hybrid BHJ solar cells for high photovoltage application.

Improved photovoltaic performances by post-deposition acidic treatments on tetrapod shaped colloidal nanocrystal solids

The ligand exchange reaction with pyridine is the standard procedure for the integration of colloidal semiconductor nanocrystals (NCs) in photovoltaic devices; however, for large sized and irregularly shaped branched NCs, such as CdSe@CdTe tetrapods, this procedure can lead to a considerable waste of materials and the aggregation of NCs in the colloidal solution, therefore resulting in the formation of an inhomogeneous film and low device performances. Here, we report on alternative post-deposition treatments with carboxylic acids on films of CdSe@CdTe tetrapod shaped NCs. This approach guarantees the removal of the insulating surfactant, necessary to obtain good charge transport among NCs, while preserving the film integrity. We perform a complete characterization of the nanocrystalline films treated with different carboxylic acids and demonstrate the successful integration of such films in photovoltaic devices, showing a doubled efficiency with respect to the standard ligand exchange procedure. Our approach represents a general route towards the development of NC based devices with improved performances and minimized waste of material.

Inorganic-organic hybrid solar cell: bridging quantum dots to conjugated polymer nanowires

Quantum dots show great promise for fabrication of hybrid bulk heterojunction solar cells with enhanced power conversion efficiency, yet controlling the morphology and interface structure on the nanometer length scale is challenging. Here, we demonstrate quantum dot-based hybrid solar cells with improved electronic interaction between donor and acceptor components, resulting in significant improvement in short-circuit current and open-circuit voltage. CdS quantum dots were bound onto crystalline P3HT nanowires through solvent-assisted grafting and ligand exchange, leading to controlled organic-inorganic phase separation and an improved maximum power conversion efficiency of 4.1% under AM 1.5 solar illumination. Our approach can be applied to a wide range of quantum dots and polymer hybrids and is compatible with solution processing, thereby offering a general scheme for improving the efficiency of nanocrystal hybrid solar cells.

Aqueous-solution-processed hybrid solar cells from poly(1,4-naphthalenevinylene) and CdTe nanocrystals

Poly(1,4-naphthalenevinylene), prepared from a water-soluble precursor, was used to fabricate hybrid solar cells by blending with water-soluble CdTe nanocrystals (NCs) to act as the photoactive layer. In composites with CdTe NCs as the electron acceptors in a bulk heterojunction configuration, the devices exhibited a short-circuit current density of -6.14 mA/cm(2), an open-circuit voltage of 0.44 V, a fill factor of 0.32, and a power conversion efficiency of 0.86% under AM1.5G conditions. Because the devices were fabricated from water-soluble materials, the procedure was generally simple and environmentally friendly in comparison to the conventional devices fabricated from oil-soluble materials.

Conjugated polymers/semiconductor nanocrystals hybrid materials--preparation, electrical transport properties and applications

This critical review discusses specific preparation and characterization methods applied to hybrid materials consisting of π-conjugated polymers (or oligomers) and semiconductor nanocrystals. These materials are of great importance in the quickly growing field of hybrid organic/inorganic electronics since they can serve as active components of photovoltaic cells, light emitting diodes, photodetectors and other devices. The electronic energy levels of the organic and inorganic components of the hybrid can be tuned individually and thin hybrid films can be processed using low cost solution based techniques. However, the interface between the hybrid components and the morphology of the hybrid directly influences the generation, separation and transport of charge carriers and those parameters are not easy to control. Therefore a large variety of different approaches for assembling the building blocks--conjugated polymers and semiconductor nanocrystals--has been developed. They range from their simple blending through various grafting procedures to methods exploiting specific non-covalent interactions between both components, induced by their tailor-made functionalization. In the first part of this review, we discuss the preparation of the building blocks (nanocrystals and polymers) and the strategies for their assembly into hybrid materials' thin films. In the second part, we focus on the charge carriers' generation and their transport within the hybrids. Finally, we summarize the performances of solar cells using conjugated polymer/semiconductor nanocrystals hybrids and give perspectives for future developments.

Nanostructured hybrid polymer-inorganic solar cell active layers formed by controllable in situ growth of semiconducting sulfide networks

Nanostructured composites of inorganic and organic materials are attracting extensive interest for electronic and optoelectronic device applications. In this paper, we introduce a general method for the fabrication of metal sulfide nanoparticle/polymer films employing a low-cost and low temperature route compatible with large-scale device manufacturing. Our approach is based upon the controlled in situ thermal decomposition of a solution processable metal xanthate precursor complex in a semiconducting polymer film. To demonstrate the versatility of our method, we fabricate a CdS/P3HT nanocomposite film and show that the metal sulfide network inside the polymer film assists in the absorption of visible light and enables the achievement of high yields of charge photogeneration at the CdS/P3HT heterojunction. Photovoltaic devices based upon such nanocomposite films show solar light to electrical energy conversion efficiencies of 0.7% under full AM1.5 illumination and 1.2% under 10% incident power, demonstrating the potential of such nanocomposite films for low-cost photovoltaic devices.

Quantum Dots and Their Multimodal Applications: A Review

Semiconducting quantum dots, whose particle sizes are in the nanometer range, have very unusual properties. The quantum dots have band gaps that depend in a complicated fashion upon a number of factors, described in the article. Processing-structure-properties-performance relationships are reviewed for compound semiconducting quantum dots. Various methods for synthesizing these quantum dots are discussed, as well as their resulting properties. Quantum states and confinement of their excitons may shift their optical absorption and emission energies. Such effects are important for tuning their luminescence stimulated by photons (photoluminescence) or electric field (electroluminescence). In this article, decoupling of quantum effects on excitation and emission are described, along with the use of quantum dots as sensitizers in phosphors. In addition, we reviewed the multimodal applications of quantum dots, including in electroluminescence device, solar cell and biological imaging.

Efficient bulk heterojunction solar cells with poly[2,7-(9,9-dihexylfluorene)-alt-bithiophene] and 6,6-phenyl C61 butyric acid methyl ester blends and their application in tandem cells

We present herein efficient bulk heterojunction (BHJ) solar cells via mixing poly[2,7-(9,9-dihexylfluorene)-alt-bithiophene] (F6T2) and 6,6-phenyl C61 butyric acid methyl ester (PCBM) with variable weight ratios. The photo-physics and morphology of F6T2:PCBM blend films and the electrical characteristics of their corresponding single cells were studied in details by changing PCBM concentration. The complete photoluminescence quenching of F6T2 emission occurs with only a small fraction of PCBM blended, demonstrating effective photoinduced charge transfer between F6T2 and PCBM. Morphology images from atomic force microscopy and scanning electron microscopy (SEM) reveal that the phase separation in F6T2:PCBM blend films becomes pronounced with the increase of PCBM concentration, resulting in the increased fill factor from 25.2% (1:1) to 56.9% (1:6). A SEM image also shows the phase separation is within the range of 10 - 20 nm. With the optimized F6T2:PCBM weight ratio (1:2), the single cell exhibits a highest power conversion efficiency of 2.46% due to the balance of light absorption and charge transport. Finally, the polymer-small molecule tandem cells are constructed using F6T2:PCBM BHJ as the bottom cell and copper phthalocyanine (CuPc):fullerene (C(60)) as the top cell. The open-circuit voltage (V(oc)) of tandem cell (1.27 V) is equal to the summation of the V(oc) values of the bottom cell (0.86 V) and the top cell (0.43 V).

Photovoltaic devices with a low band gap polymer and CdSe nanostructures exceeding 3% efficiency

We report the fabrication and measurement of solar cells approaching a power conversion efficiency of 3.2% using a low band gap conjugated polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] and CdSe nanoparticles. These devices exhibit an external quantum efficiency (EQE) of >30% in a broad range of 350-800 nm with a maximum EQE of 55% in a range of 630-720 nm. We also present certified device efficiencies of 3.13% under AM 1.5 illumination.

Rational design, synthesis, and optical properties of film-forming, near-infrared absorbing, and fluorescent chromophores with multidonors and large heterocyclic acceptors

A new series of film-forming, low-bandgap chromophores (1 a,b and 2 a,b) were rationally designed with aid of a computational study, and then synthesized and characterized. To realize absorption and emission above the 1000 nm wavelength, the molecular design focuses on lowering the LUMO level by fusing common heterocyclic units into a large conjugated core that acts an electron acceptor and increasing the charge transfer by attaching the multiple electron-donating groups at the appropriate positions of the acceptor core. The chromophores have bandgap levels of 1.27-0.71 eV, and accordingly absorb at 746-1003 nm and emit at 1035-1290 nm in solution. By design, the relatively high molecular weight (up to 2400 g mol(-1)) and non-coplanar structure allow these near-infrared (NIR) chromophores to be readily spin-coated as uniform thin films and doped with other organic semiconductors for potential device applications. Doping with [6,6]-phenyl-C(61) butyric acid methyl ester leads to a red shift in the absorption only for 1 a and 2 a. An interesting NIR electrochromism was found for 2 a, with absorption being turned on at 1034 nm when electrochemically switched (at 1000 mV) from its neutral state to a radical cation state. Furthermore, a large Stokes shift (256-318 nm) is also unique for this multidonor-acceptor type of chromophore, indicating a significant structural difference between the ground state and the excited state. Photoluminescence of the film of 2 a was further probed at variable temperatures and the results strongly suggest that the restriction of bond rotations certainly helps to diminish non-radiative decay and thus enhance the luminescence of these large chromophores.

Superstructures of PbS nanocrystals in a conjugated polymer and the aligning role of oxidation

We present a method to directly align PbS nanocrystals in micron-sized superstructures within a conjugated polymer. First, lead sulfide nanocrystals are directly synthesized in a MEH-PPV suspension via a single pot, surfactant-free method. Post-synthesis precipitation of the composite solution involving mild oxidation of the nanocrystals results in the formation of nanocrystal-polymer and nanocrystal-oxide superstructures. Detailed TEM is used to study the crystallographic nature of these structures and the roles of polymer and lead sulfate. An epitaxial relationship between lead sulfide and lead sulfate at the nanoscale is shown, giving insight into the oxidation rates of the PbS nanocrystals' facets.

The effect of three-dimensional morphology on the efficiency of hybrid polymer solar cells

The efficiency of polymer solar cells critically depends on the intimacy of mixing of the donor and acceptor semiconductors used in these devices to create charges and on the presence of unhindered percolation pathways in the individual components to transport holes and electrons. The visualization of these bulk heterojunction morphologies in three dimensions has been challenging and has hampered progress in this area. Here, we spatially resolve the morphology of 2%-efficient hybrid solar cells consisting of poly(3-hexylthiophene) as the donor and ZnO as the acceptor in the nanometre range by electron tomography. The morphology is statistically analysed for spherical contact distance and percolation pathways. Together with solving the three-dimensional exciton-diffusion equation, a consistent and quantitative correlation between solar-cell performance, photophysical data and the three-dimensional morphology has been obtained for devices with different layer thicknesses that enables differentiating between generation and transport as limiting factors to performance.

Stepwise Self-Assembly of P3HT/CdSe Hybrid Nanowires with Enhanced Photoconductivity

A facile approach to prepare poly(3-hexylthiophene) (P3HT)/cadmium selenide quantum dot (CdSe QD) hybrid coaxial nanowires by a stepwise self-assembly process is reported. P3HT nanowires of ≈20 nm diameter are first prepared by self-assembly in a poor solvent such as cyclohexanone, and then as-prepared CdSe QDs are deposited compactly onto the P3HT nanowires by non-covalent interactions between P3HT and CdSe. When illuminated with white light, the hybrid nanowires show enhanced photoconductivity compared with the pristine P3HT nanowires and the blended nanocomposites.

Synthesis of CdSe-TiO2 nanocomposites and their applications to TiO2 sensitized solar cells

CdSe-TiO(2) nanocomposites were synthesized via aminolysis of Ti-oleate complexes in the presence of CdSe nanocrystals, and their application as sensitizers for TiO(2) solar cells was investigated. The formation of CdSe-TiO(2) nanocomposites was confirmed using transmission electron microscopy and Raman spectroscopy. The emission spectrum of CdSe-TiO(2) nanocomposites revealed photoinduced charge separation at the CdSe-TiO(2) interface of the composite. The photocurrent-voltage properties of CdSe-TiO(2)-sensitized TiO(2) particle films compared favorably with those of CdSe-sensitized TiO(2) films. Evidence was also found indicating that the TiO(2) component of the composite protects CdSe against degradation during film annealing.