Hybrid Bulk Heterojunction Research Articles

Core/shell-shaped CdSe/PbS nanotetrapods for efficient organic–inorganic hybrid solar cells

Core/shell-shaped CdSe/PbS nanotetrapod (NT), a novel nanostructure, has been synthesized and incorporated as an electron acceptor in organic–inorganic hybrid bulk-heterojunction (HBH) solar cells with poly(3-hexylthiophene) (P3HT) acting as an electron donor. The composite NT shows an homogeneous decoration of 4 nm PbS quantum dots on the CdSe NT surface, forming an inorganic heterojunction with a type-II band alignment. Compared to pure CdSe NT, the core/shell-shaped NT demonstrates an improved performance on splitting photogenerated exciton in P3HT. The charge reverse transfer (leakage or recombination) at the heterojunction interface is suppressed due to a potential barrier that is as high as 0.5 eV for CdSe/PbS NT. Furthermore, charge transport and collection are also enhanced through the spatially isolated charge channels of P3HT (for holes) and the CdSe core (for electrons). The efficient exciton dissociation and charge collection greatly contribute to the photovoltaic performance improvement. Compared with the P3HT:CdSe hybrid solar cell, a remarkable efficiency enhancement of 76% is achieved by incorporating the P3HT:CdSe/PbS hybrid with a donor–acceptor mass ratio of 1 : 6.

Structural and optoelectronic properties of hybrid bulk-heterojunction materials based on conjugated small molecules and mesostructured TiO2

Improved hybrid bulk-heterojunction materials was fabricated by spin-casting a benchmark conjugated small molecule, namely, 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP(TBFu)2), into mesostructured TiO2. Due to both a reduced molecular size and less hydrophobic nature of the conjugated molecules (relative to conjugated polymers), homogeneous and improved infiltration into the mesoporous TiO2 are achieved without the need for pre-treatment of the TiO2. Remarkably, this small molecule can realize loadings of up to 25% of the total pore volume—2.5× the typical loadings achieved for conjugated polymers. The small molecule loading was determined using dynamic secondary ion mass spectroscopy and absorption spectroscopy. Further characterization such as charge transfer and nanoscale conducting atomic force microscopy helps to demonstrate the promise and viability of small molecule donors for hybrid optoelectronic devices.

Near-infrared-sensitive bulk heterojunction solar cells using nanostructured hybrid composites of HgTe quantum dots and a low-bandgap polymer

Near-infrared (NIR)-sensitive hybrid bulk heterojunction solar cells were developed using NIR-absorbing HgTe quantum dots (QDs) and a low-bandgap polymer, poly[2,6-(4,4′-bis-(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-alt-4,7(2,1,3-benzothiadiazole)] (PSBTBT). Hybrid composites of HgTe QDs and PSBTBT facilitate broad-range exploitation of the solar spectrum and efficient carrier dissociation prior to recombination. Nanostructures were formed on the surface of the hybrid composite via a nanoimprinting process using an anodic aluminum oxide (AAO) mold. This contributes to optical light scattering for efficient utilization of light up to the NIR region and enlarged photoactive layer–electrode interfacial areas for improving charge extraction, increasing the overall efficiency from 1.09 to 1.41%.

Improved photovoltaic performance of hybrid solar cells based on silicon nanowire and P3HT

The properties of organic/inorganic poly(3-hexylthiophene) (P3HT): silicon nanowires nanocomposite films and nanocomposite based solar cells as a function of SiNWs concentration and the solvent used for the film fabrication were studied. We demonstrate that the performance of these devices is highly dependent on these parameters. A detailed study of the effects that active layer thickness has on the photovoltaic performances has also been performed for bulk heterojunction hybrid solar cells. Photoluminescence spectroscopy (PL) shows the existence of a critical SiNWs concentration of about 15wt% for PL quenching corresponding to the most efficient charge pair separation. Photoluminescence responses were correlated with the topography (AFM) of the thin films. The photovoltaic effect of ITO/PEDOT:PSS/SiNWs:P3HT/Al was studied by current–voltage (I–V) measurements under illumination and interpreted on the basis of the charge transfer differences resulting from the morphologies. By optimizing all the physical parameters listed above we fabricated devices with PCE of 0.08%, which is the highest efficiency reported so far for this system.

Rethinking Band Bending at the P3HT–TiO2 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.

Direct Electronic Property Imaging of a Nanocrystal-Based Photovoltaic Device by Electron Beam-Induced Current via Scanning Electron Microscopy.

Scanning electron microscopy (SEM) electron beam-induced current (EBIC) studies were performed on the cross-section of a nanocrystal-based hybrid bulk heterojunction photovoltaic device. Using these techniques, the short circuit carrier collection efficiencies are mapped with a better than 100 nm resolution. Electronically deficient and proficient regions within the photoactive layer are determined. The results show that only a fraction of the CdSe nanorod:P3HT layer (P3HT = poly-3(hexylthiophene)) at the Al cathode interface shows primary collection of charged carriers, in which the photoactivity decreases exponentially away from the interface. The recombination losses of the photoactive layer away from this interface prove that the limiting factor of the device is the inability for electrons to percolate between nanoparticles; to alleviate this problem, an interparticle network that conducts the electrons from one nanorod to the next must be established. Furthermore, the EBIC technique applied to the nanocrystalline device used in this study is the first measurement of its kind and can be applied toward other similar architectures.

Modular Fabrication of Hybrid Bulk Heterojunction Solar Cells Based on Breakwater-like CdSe Tetrapod Nanocrystal Network Infused with P3HT

We demonstrate the modular fabrication of nanocrystal/polymer hybrid bulk heterojunction solar cells based on breakwater-like CdSe tetrapod (TP) nanocrystal networks infused with poly(3-hexylthiophene) (P3HT). This fabrication method consists of sequential steps for forming the hybrid active layers: the assembly of a breakwater-like CdSe TP network followed by nanocrystal surface modification and the infusion of semiconducting polymers. Such a modular approach enables the independent control of the nanoscopic morphology and surface chemistry of the nanocrystals, which are generally known to exhibit complex correlations, in a reproducible manner. Using these devices, the influence of the passivation ligands on solar cell characteristics could be clarified from temperature-dependent solar cell experiments. We found that a 2-fold increase in the short-circuit current with 1-hexylamine ligands, compared with the value based on pyridine ligands, originates from the reduced depth of trap states, minimizing the ...

Broadband-absorbing hybrid solar cells with efficiency greater than 3% based on a bulk heterojunction of PbS quantum dots and a low-bandgap polymer

Currently existing common conjugated polymer:PbS quantum dot (QD)-based hybrid bulk heterojunction (BHJ) solar cells show efficiencies of less than 1% owing to improper bandgap alignment and poor coupling at the hybrid material interfaces. However, herein we report that PbS QD-based hybrid BHJ solar cells provide greatly increased efficiencies of more than 3%, which is attributed to the employment of a new kind of donor polymer poly[2,6-(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-alt-4,7(2,1,3-benzothiadiazole) (PSBTBT), the optimization of donor–acceptor (D–A) band-offsets by strategically changing the diameter of PbS QDs in the donor matrix, and the modification of hybrid material interfaces via a chemical treatment of ligands around the QD surface. The optimized hybrid nanocomposite device performs well in good combination between the low-bandgap polymer and near-infrared (NIR)-absorbing PbS QDs, and it enables a broad spectral response from UV to NIR under an energetically favorable type-II heterojunction system, leading to high efficiencies of up to 3.48% under an air mass 1.5G illumination. The efficiency is higher than 3.39%, which corresponds to the efficiency value for the purely organic device fabricated in this study by utilizing [6,6]-phenyl-C71-butyric acid methyl ester (PCBM), which is the most widely studied electron acceptor in BHJ systems. These findings suggest that our hybrid BHJ blends are very promising, not only for use as energy conversion platforms to substitute all-organic or all-inorganic systems, but also for optoelectronic devices that require a broad spectral reaction.

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.

Highly efficient hybrid solar cells with tunable dipole at the donor-acceptor interface.

Effects of molecular dipole at the conjugated polymer-nanocrystal interface on the energy level alignment, the exciton dissociation process, and consequently the photovoltaic performance of 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):CdSe quantum dot bulk heterojunction hybrid solar cells are systemically studied. Power conversion efficiency up to 4.0% is achieved when 4-fluorobenzenethiol is used for ligand exchange.

Hybrid bulk heterojunction solar cells based on poly (3-hexylthiophene) and Z907-modified ZnO nanorods

This work presents hybrid bulk heterojunction solar cells based on blends of Z907-modified ZnO nanorods and poly (3-hexylthiophene) (P3HT). ZnO nanorods were synthesized by a low temperature hydrothermal method and chemically modified with Z907 through a reflux process. Transmission electron microscope, X-ray diffractometer, infrared spectrometer, UV–vis spectrophotometer, fluorescence spectrometer and photovoltaic device measurements were used to study the morphology, structure, dye binding mode, light absorption, charge transfer and device performance of the structures consisting of P3HT polymer in contact with Z907-modified ZnO nanorods. Results show that Z907 grafting improves the compatibility between ZnO nanorods and P3HT and has not substantially altered the crystal structure of ZnO nanorods. In addition, each Z907 molecule is chemically grafted onto the ZnO surface with two carboxylic acid groups in a unidentate binding mode. Moreover, Z907 modification enlarges the visible light absorption range of ZnO/P3HT composite film and enhances the charge transfer efficiency at the P3HT/ZnO interface. The power conversion efficiency of the P3HT/Z907–ZnO solar cells at a Z907 concentration of 0.3mM under 100mW/cm2 of AM 1.5 illumination is 0.21%.

Low‐Cost Solar Cell Based on a Composite of Silicon Nanowires and a Highly Conductive Nonphotoactive Polymer

Hybrid bulk heterojunction solar cells based on inorganic-nanowire (NW)/conjugated-polymer composites have been shown to achieve a power conversion efficiency (PCE) of 1–3 %, which is much lower than theoretically expected. This work addresses the short exciton diffusion lengths in common photoactive polymers with a new solar-cell design that has a PCE of 4.68 % (see scheme; ITO=indium tin oxide, G-PEDOT:PSS=glycerol-doped poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate); E=electropolymerized; a=amorphous).

Effects of ZnO Nanoparticles on P3HT:PCBM Organic Solar Cells with DMF-Modulated PEDOT:PSS Buffer Layers

In this study, we investigated hybrid bulk heterojunction organic solar cells containing ZnO nanoparticles blended with poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) and having poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) or N,N-dimethylformamide (DMF)-modulated PEDOT:PSS buffer layers. The reference cell, which had a P3HT:PCBM active layer sandwiched between ITO\PEDOT:PSS and LiF\Al electrodes, exhibited an efficiency of 1.55%. The ZnO nanoparticle-doped active layer (ITO\PEDOT:PSS(DMF) \ZnO:P3HT:PCBM\LiF\Al) exhibited a higher efficiency of 3.39% due to the modulated PEDOT:PSS buffer layer with low resistivity and the hybrid active layer containing ZnO nanoparticles. Here, we demonstrate that the low resistivity of the PEDOT:PSS layer can improve the Jsc value of hybrid solar cells, and ZnO nanoparticles can enhance the Voc value of organic solar cells.

Hybrid Bulk Heterojunction Solar Cells Based on the Cooperative Interaction of Liquid Crystals within Quantum Dots and Diblock Copolymers

In this article, the conjugated rod-rod polythiophene diblock copolymers comprising a regioregular poly(3-hexylthiophene) (P3HT) segment and a side-chain liquid-crystalline polythiophene segment bearing cyanobiphenyl mesogenic pendants (PTcbp), polythiophene-b-poly{3-[10-(4'-cyanobiphenyloxy)decyl]thiophene} (P3HT-b-PTcbp), were rationally designed and synthesized. It was observed that the diblock copolymers could self-assemble into high crystalline and oriented nanofibrils upon 1,2-dichlorobenzene solvent vapor annealing, originating from the crystallization of two segments and the orientation of cyanobiphenyl side-chain mesogens. Hybrid bulk heterojunction (BHJ) solar cells were then fabricated using P3HT-b-PTcbp as electron donors and ZnO and CdS quantum dots (QDs) modified by 4'-hydroxy-[1,1'-biphenyl]-4-carbonitrile (cbp) liquid-crystalline ligands (cbp@ZnO and cbp@CdS) as electron acceptors. The interaction between the cbp ligands on the surface of ZnO and CdS QDs and cyanobiphenyl side-chain mesogens of diblock copolymers promoted the cooperative self-assembly and controllable well-dispersion of QDs in the polymer matrix and, as a consequence, yielded an intimately contacted polymer-QD nanocomposites. The power conversion efficiency (PCE) of the device based on P3HT-b-PTcbp/cbp@ZnO hybrids was improved by 2.6 times compared with that of P3HT/ZnO hybrids from 0.58 to 0.97. In addition, an overall PCE of a homologous device based on the P3HT-b-PTcbp/cbp@CdS hybrid active layer reached 2.3%. The research paved the way for the further development of high-efficiency hybrid BHJ solar cells by introducing block copolymer nanofibrils with favored crystalline domain orientations and liquid-crystalline organization properties.

Nanotetrapods: quantum dot hybrid for bulk heterojunction solar cells.

Hybrid thin film solar cell based on all-inorganic nanoparticles is a new member in the family of photovoltaic devices. In this work, a novel and performance-efficient inorganic hybrid nanostructure with continuous charge transportation and collection channels is demonstrated by introducing CdTe nanotetropods (NTs) and CdSe quantum dots (QDs). Hybrid morphology is characterized, demonstrating an interpenetration and compacted contact of NTs and QDs. Electrical measurements show enhanced charge transfer at the hybrid bulk heterojunction interface of NTs and QDs after ligand exchange which accordingly improves the performance of solar cells. Photovoltaic and light response tests exhibit a combined optic-electric contribution from both CdTe NTs and CdSe QDs through a formation of interpercolation in morphology as well as a type II energy level distribution. The NT and QD hybrid bulk heterojunction is applicable and promising in other highly efficient photovoltaic materials such as PbS QDs.

Hybrid bulk-heterojunction solar cells based on all inorganic nanoparticles

Hybrid bulk-heterojunction (HBH) nanostructure enables high excitons splitting efficiency by virtue of increasing contact area in the active layer. In this work, we have demonstrated a HBH solar cell based on all inorganic p-type and n-type nanoparticles. In order to facilitate charge transportation and collection after efficient excitons splitting, nanotetrapod shaped CdTe is used as p-type semiconductor and CdSe quantum dot as n-type material. The operation mechanism of HBH solar cells is demonstrated to be a donor–acceptor (D–A) model based on splitting of excitons. Besides the morphology and absorption characterization, photoluminescence quenching behavior is researched to show the excitons splitting process. Solar cells with HBH structure show promising photovoltaic performance, which benefits from the percolated donor–acceptor networks in the hybrid film. Our preliminary research shed light on the new development of solution based nanoparticles thin film solar cells.

Optimizing ZnO nanoparticle surface for bulk heterojunction hybrid solar cells

The performance of hybrid solar cells composed of polymer and ZnO is mainly hindered by the defects of ZnO. Here, we investigate the effects of ZnO nanoparticle surface modification with poly(ethylene oxide) (PEO) on the performance of bulk heterojunction hybrid solar cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and ZnO nanoparticles. The reference device using ZnO nanoparticles as electron acceptor shows an open-circuit voltage (VOC) of 0.83V, a short-circuit current (JSC) of 3.00mA/cm2, a fill factor (FF) of 0.46 and a power conversion efficiency (PCE) of 1.15%. After modification with very small amount of PEO, the PCE will be enhanced, which is attributed to less surface traps of ZnO nanoparticles with PEO modification. With optimized PEO (0.05%) modified ZnO nanoparticles as electron acceptors, the device typically shows a VOC of 0.86V, a JSC of 3.84mA/cm2, a FF of 0.51 and a PCE of 1.68% due to less recombination loss of carriers, smaller series resistance, and improved electrical coupling between ZnO nanoparticle and MEH-PPV. However, further increase of PEO content to 0.3% will deteriorate device performance.

Inverted hybrid solar cells based on pyrite FeS2 nanocrystals in P3HT:PCBM with enhanced photocurrent and air-stability

Pyrite iron disulfide (FeS2) is an earth-abundant and environmentally benign semiconductor with low procurement costs and advantageous optoelectronic properties for photovoltaic (PV) applications. Here, pyrite FeS2 nanocrystals (NCs) synthesized with trioctylphosphine oxide (TOPO) additives are deployed in ternary hybrid bulk-heterojunction (BHJ) solar cells with an inverted architecture. The band gap (Eg) of the NCs is estimated to be 1.3 and 1.7eV using UV–vis–NIR absorption and cyclic voltammetry, respectively. A dramatic increase of the oxidized iron species in the NCs is shown by X-ray photoelectron spectroscopy (XPS), which may cause the elevated Eg, according to recent density functional theory calculations. Three device performance regimes are observed when the pyrite NC concentration is varied from 0 to 4wt% that appear linked to microstructure transitions in the BHJ active layer. While addition of FeS2 NCs consistently increases photocurrent, the open-circuit voltage (VOC) and fill factor (FF) decreases in the devices with higher pyrite NC concentrations, possibly due to increased current leakage. The inverted ternary hybrid solar cells demonstrate great air-stability after aging 28 days exposed in air. Due to the inverted structure, exposure in air actually improves the VOC and FF significantly resulting in an average 28% increase in power conversion efficiency of the ternary hybrid devices compared to the control binary device with no FeS2. The photocurrent enhancement and air-stability demonstrated by this inverted design offer a promising architecture for future pyrite FeS2 NC-based devices. Furthermore, the NC characterizations presented here provide insight to advance pyrite's development as a sustainable PV material.

Efficient Electron Collection in Hybrid Polymer Solar Cells: In-Situ-Generated ZnO/Poly(3-hexylthiophene) Scaffolded by a TiO2 Nanorod Array.

A nanoarchitectural hybrid polymer solar cell, integrating the ordered and the bulk heterojunction hybrid polymer solar cells, is fabricated by infiltrating the diethylzinc/poly(3-hexylthiophene) (P3HT) solution into the interstices of the TiO2 nanorod (NR) array. An inorganic network composed of tiny ZnO nanocrystals is constructed in the in-situ-generated hybrid within the interstice of the single-crystalline TiO2 NRs. The TiO2 NR array, which possesses a longer electron lifetime and an appropriate electron-transport rate, serves not only as an electron transporter/collector extended from fluorine-doped tin oxide (FTO) electrode to sustain the efficient electron collection but also as a scaffold to hold the sufficient amount of ZnO/P3HT hybrid. The in-situ-generated ZnO/P3HT hybrid layer with superior charge separation efficiency can therefore be thickened in the presence of a TiO2 NR array for increasing the light-harvesting efficiency. A notable efficiency of 2.46% is therefore attained in the TiO2 NR-ZnO/P3HT hybrid solar cell.

Effects of metal oxide as an anode interlayer for organic photovoltaics

In this study, polymer:fullerene bulk-heterojunction hybrid solar cells with the structure indium tin oxide (ITO)/nickel oxide (NiO)/poly (3-hexylthiophene) (P3HT):[6, 6]-phenyl C61-butyric(PCBM):titania (TiO2):platinum (Pt) nanoparticles (NPs)/Ca/Al were fabricated. The effects of a p-type NiO thin layer deposited by thermal evaporation between the active layer P3HT:PCBM:TiO2:Pt and ITO on cell performance were examined. The results show that the NiO interfacial layer between the ITO and active layer can increase the efficiency and stability of the prepared hybrid solar cells. The optimum cell performance by ITO/NiO(5nm)/P3HT:PCBM:TiO2 (15wt.%):Pt (0.03wt.%)/Ca/Al (best cell structure) is an open-circuit voltage (Voc)=0.61V, short circuit current density (Jsc)=6.22mA/cm2, fill factor (FF)=54.8%, and η=2.1%.