Hybrid Bulk Heterojunction Solar Cells Research Articles

Pn-Junction nanorods in a polymer matrix: A paradigm shift from conventional hybrid bulk-heterojunction solar cells

We report introduction of pn-junction nanorods in a polymer matrix to form hybrid bulk-heterojunction (BHJ) solar cells. This is a paradigm shift from conventional hybrid solar cells, where quantum dots or nanorods of an inorganic semiconductor and a conjugated polymer form a BHJ. The pn-junctions were formed in n-type CdS nanorods through a controlled cationic exchange process; here p-type Cu2S formed from one end of CdS due to selective reactivity of crystalline planes of the nanorods. Due to an epitaxial attachment between Cu2S and CdS semiconductors in the nanorod, a depletion region hence formed in the pn-junction, which is a classical example of type-II band-alignment at the interface. The junction separated charge carriers under illumination through a drift of minority carriers across the depletion region. Hybrid BHJs based on Cu2S|CdS pn-junction nanorods in a conventional polymer matrix therefore acted as efficient solar cells as compared to similar BHJ devices with nanorods of individual materials, that is Cu2S or CdS. We varied the relative lengths of p- and n-sections of the pn-junctions that in turn controlled charge separation and carrier transport processes to optimize the solar cell performance.

Schottky diodes between Bi2S3 nanorods and metal nanoparticles in a polymer matrix as hybrid bulk-heterojunction solar cells

We report the use of metal-semiconductor Schottky junctions in a conjugated polymer matrix as solar cells. The Schottky diodes, which were formed between Bi2S3 nanorods and gold nanoparticles, efficiently dissociated photogenerated excitons. The bulk-heterojunction (BHJ) devices based on such metal-semiconductor Schottky diodes in a polymer matrix therefore acted as an efficient solar cell as compared to the devices based on only the semiconductor nanorods in the polymer matrix or when gold nanoparticles were added separately to the BHJs. In the latter device, gold nanoparticles offered plasmonic enhancement due to an increased cross-section of optical absorption. We report growth and characteristics of the Schottky junctions formed through an intimate contact between Bi2S3 nanorods and gold nanoparticles. We also report fabrication and characterization of BHJ solar cells based on such heterojunctions. We highlight the benefit of using metal-semiconductor Schottky diodes over only inorganic semiconductor nanorods or quantum dots in a polymer matrix in forming hybrid BHJ solar cells.

Efficient and low cost inverted hybrid bulk heterojunction solar cells

In this study, hybrid bulk heterojunction solar cells with an inverted architecture were fabricated using solution processing techniques. ZnO and TiO2 nanoparticles were then incorporated to the active layer which primarily consists of poly (3-hexylthiophene) (P3HT) and [6,6] phenyl-C61-butyric acid methyl ester (PCBM). The devices were prepared with varying ratios of PCBM to nanostructured inorganic oxides (ZnO and TiO2) while keeping a fixed amount of P3HT. Various characterization techniques were used to understand the effect of metal oxide nanoparticles on structure, morphology, and performance of resulting devices. It was observed that incorporating an optimum amount of nanoparticles to the active layer increased charge carrier mobility, surface roughness of the active layer, and absorption in visible region leading to a significant increase in power conversion efficiency. However, a significant agglomeration of nanoparticles was observed as their ratio relative to PCBM increased and they completely agglomerated in the absence of the fullerene derivative.

Improvement in PbS-based Hybrid Bulk-Heterojunction Solar Cells through Band Alignment via Bismuth Doping in the Nanocrystals.

We introduce dopants in lead sulfide (PbS) quantum dots (QDs) in forming hybrid bulk-heterojunction (BHJ) solar cells. Because an increase in the content of bismuth as dopants in PbS QDs transforms the intrinsic p-type semiconductor into an n-type one, the band alignment between a conjugated polymer and the doped QDs changes upon doping affecting performance of BHJ solar cells. From scanning tunneling spectroscopy (STS) of the doped QDs, we observe a shift in their Fermi energy leading to formation of a type II band alignment in the polymer:doped-QD interface. We also show that the dopants improve electron-conduction process through the QDs. With the dopants controlling both band alignments at the interface and the conduction process, we show that the dopant concentration in QDs influences open-circuit voltage unfavorably and short-circuit current in a beneficial manner. The device performance of hybrid BHJ solar cells is hence maximized at an optimum concentration of bismuth in PbS QDs.

Enhanced photovoltaic performance of inverted hybrid bulk-heterojunction solar cells using TiO 2 /reduced graphene oxide films as electron transport layers

In this study, we investigated inverted hybrid bulk-heterojunction solar cells with the following configuration: fluorine-doped tin oxide (FTO) |TiO2/RGO|P3HT:PC61BM|V2O5 or PEDOT:PSS|Ag. The TiO2/GO dispersions were prepared by sol-gel method, employing titanium isopropoxide and graphene oxide (GO) as starting materials. The GO concentration was varied from 0.1 to 4.0 wt%. The corresponding dispersions were spin-coated onto FTO substrates and a thermal treatment was performed to remove organic materials and to reduce GO to reduced graphene oxide (RGO). The TiO2/RGO films were characterized by x-ray diffraction, Raman spectroscopy, and microscopy techniques. Atomic force microscopy (AFM) images showed that the addition of RGO significantly changes the morphology of the TiO2 films, with loss of uniformity and increase in surface roughness. Independent of the use of V2O5 or PEDOT: PSS films as the hole transport layer, the incorporation of 2.0 wt% of RGO into TiO2 films was the optimal concentration for the best organic photovoltaic performance. The solar cells based on TiO2/RGO (2.0 wt%) electrode exhibited a ∼22.3% and ∼28.9% short circuit current density (Jsc) and a power conversion efficiency enhancement, respectively, if compared with the devices based on pure TiO2 films. Kelvin probe force microscopy images suggest that the incorporation of RGO into TiO2 films can promote the appearance of regions with different charge dissipation capacities.

Top-down Strategy toward Versatile Graphene Quantum Dots for Organic/Inorganic Hybrid Solar Cells

Metal oxide nanocrystals have been pursued for various applications in photovoltaics as a buffer layer. However, it remains a challenging task to adjust their energy levels to achieve a better match of the donor–acceptor system. Herein, we report the fabrication of graphene quantum dots (GQDs) with bright blue photoluminescence by a top-down strategy based on laser fragmentation with posthydrothermal treatment. The GQDs demonstrate appropriate energy level positions and are used as an intermediate buffer layer between TiO2 and P3HT to form a cascade energy level architecture. The introduction of the GQDs into a bulk heterojunction hybrid solar cell has led to an enhancement of the power conversion efficiency.

Synthesis of Carboxylated Graphene Oxide–CdS Nanocomposite and Its Application on Photovoltaic Devices

Abstract Carboxylated graphene oxide–CdS nanocomposite (GO-COOH-CdS) was successfully prepared by a facile mixing method in tetrahydrofuran (THF) and used to fabricate photovoltaic devices. The as-prepared nanocomposite was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD analysis further confirmed that CdS nanoparticles have been effectively intercalated into the GO-COOH stacks. The prepared GO-COOH-CdS nanocomposite has been applied for fabrication of hybrid bulk heterojunction photovoltaic devices by using the poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as an electron donor in two different weight ratios with GO-COOH-CdS nanocomposite, as an electron acceptor. The effective electron transfer from MEH-PPV to GO-COOH-CdS was confirmed by the significant photoluminescence (PL) quenching of MEH-PPV. According to the obtained results, the photovoltaic devices based on conjugated polymer-GO-COOH-CdS have the potential to open up new production technologies for hybrid bulk heterojunction solar cells based on GO–semiconductor composites.

Effects of surface ligands on energetic disorder and charge transport of P3HT:CdSe hybrid solar cells

In this work, the effects of surface ligands of cadmium selenide (CdSe) nanocrystals on energetic disorder and charge transport of poly (3‐hexylthiophene‐2,5‐diyl):cadmium selenide (P3HT:CdSe) hybrid bulk heterojunction solar cells were studied. Photo‐induced current transient spectroscopy (PICTS) was employed to quantitatively characterize the trap state energies in P3HT:CdSe hybrid solar cells from pyridine (Py) and tert‐butylthiol (tBT) treated CdSe nanocrystals. A data processing workflow to extract the trap emission spectra was implemented in the form of solving an inverse problem with regularization techniques. We observed significant differences in the trap state distribution in P3HT:CdSe(tBT) compared to P3HT:CdSe(Py) solar cells, and we found that devices based on P3HT:CdSe(Py) had deeper level trap states. Additionally, transient photoconductivity measurements were performed on P3HT:CdSe solar cells for the purpose of examining the effects of ligands on charge transport. The results were self‐consistent with PICTS analysis in that P3HT:CdSe(tBT) devices exhibited a greater than three‐fold enhancement in carrier mobility relative to P3HT:CdSe(Py) devices, which we attributed to a decreased depth of trap states.

The effect of TiO2 surface modification on the photovoltaic properties of hybrid bulk heterojunction solar cells based on MEH-PPV/CdS/TiO2 active layer

AbstractIn this work, we present the synthesis details of uniform shape and size-controlled titanium dioxide (TiO

High-efficiency polymer–PbS hybrid solar cells via molecular engineering

We successfully designed a series of new polymers to further boost the efficiency of polymer–PbS QD hybrid bulk heterojunction solar cells. We also systematically investigated the effect of polymer structures and quantum dot size on the device performance.

Colloidal CuZnSnSe4−xSx nanocrystals for hybrid solar cells

We report the synthesis of different colloidal CZTSe4−xSx nanocrystals and their performance in [6,6]-phenyl C61 butyric acid methyl ester (PCBM) based organic/inorganic hybrid bulk heterojunction solar cells. Synthesis of colloidal CuZnSnSe4−xSx were performed and characterized by XRD, TEM, SEM, UV–Visible absorption techniques. Electrochemical and photovoltaic properties were investigated. The best device concept, ITO/PEDOT:PSS/CZTS:PCBM blend (1:10)/Al, showed 280 μA/cm2 short circuit current, Isc with 300mV open circuit voltage Voc, and fill factor FF of 0.38.

Optical and morphological studies of transition metal doped ZnO nanorods and their applications in hybrid bulk heterojunction solar cells

One of the challenges in improving the efficiency of hybrid solar cells is to remove crystal defects from the semiconducting nanoparticles. Here we report the synthesis, characterization and applications of transition metal doped ZnO nanorods in hybrid bulk heterojunction solar cells. MnxZn1−xO and NixZn1−xO with dopant concentrations ranging from x=0.01–0.10 were successfully synthesized by a novel facile solution-processed wet chemical method, which gave better yield at low cost and temperature. The morphological and optical properties of the material were investigated by Transmission electron microscopy (TEM), UV–Visible and Fluorescence spectroscopies. TEM measurements confirmed the reduction in size of nanorods upon doping. UV–Visible spectra of doped nanorods showed blue shift with respect to the reference undoped ZnO. The defect emissions were completely disappeared from the fluorescence spectra upon doping at high concentrations. These doped nanorods in combination with P3HT were employed in the hybrid solar cells which gave better current densities than their corresponding undoped counterparts.

Elaboration of Conductive Polymer thin films (P3HT/PCBM) by spin coating method – Application for hybrid organic solar cell

The nanostructured polymer-fullerene thin film is one of the most prominent materials to make the hybrid bulk heterojunction solar cell (BHJ) with high conversion efficiency. Especially when the blend of P3HT and PCBM was used as the donor and acceptor materials. The properties of P3HT and PCBM layer in solar cell has been much studied and considered as high performance systems. One of the important factors for the high performance device is the fabrication of photo active layer with the appropriate thickness and morphology. In the ideal case, the intermolecular distance between the polymer and fullerene should be approximately 10-20 nm (the exciton diffusion distance) giving an area of extensive contact between the two phases. After the dissociation of carriers, the two polymer phases should create the path way for carriers to reach electrodes. But so far, this ideal configuration has not been published. In this work, we’ve elaborated the P3HT and PCBM photo active layers by spin coating methodon glass substrates covered ITO electrode. The rotational velocity was determined to get the necessary effective thickness of the polymer film. The annealing effect on structure, optical and electrical properties of the polymer thin film with different content of PCBM were also investigated. The experiments show the best device on electrode ITO has I-V characteristic as a photodiode and short circuit current (Isc) aboutmili-Ampere. These results demonstrate convincingly that polymer layers elaborated by spin coating method can result a good performance of the device.

Enhancement of the photovoltaic performance in P3HT: PbS hybrid solar cells using small size PbS quantum dots

Different approaches of surface modification of the quantum dots (QDs), namely, solution-phase (octylamine, octanethiol) and post-deposition (acetic acid, 1,4-benzenedithiol) ligand exchange were used in the fabrication of hybrid bulk heterojunction solar cell containing poly (3-hexylthiophene) (P3HT) and small (2.4 nm) PbS QDs. We show that replacing oleic acid by shorter chain ligands improves the figures of merit of the solar cells. This can possibly be attributed to a combination of a reduced thickness of the barrier for electron transfer and an optimized phase separation. The best results were obtained for post-deposition ligand exchange by 1,4-benzenedithiol, which improves the power conversion efficiency of solar cells based on a bulk heterojunction of lead sulfide (PbS) QDs and P3HT up to two orders of magnitude over previously reported hybrid cells based on a bulk heterojunction of P3HT:PbS QDs, where the QDs are capped by acetic acid ligands. The optimal performance was obtained for solar cells with 69 wt. % PbS QDs. Besides the ligand effects, the improvement was attributed to the formation of an energetically favorable bulk heterojunction with P3HT, when small size (2.4 nm) PbS QDs were used. Dark current density-voltage (J-V) measurements carried out on the device provided insight into the working mechanism: the comparison between the dark J-V characteristics of the bench mark system P3HT:PCBM and the P3HT:PbS blends allows us to conclude that a larger leakage current and a more efficient recombination are the major factors responsible for the larger losses in the hybrid system.

Fabrication of polymer/cadmium sulfide hybrid solar cells [P3HT:CdS and PCPDTBT:CdS] by spray deposition

This paper investigates fabrication of surfactant free CdS nanoparticles (NPs) and application in the fabrication of P3HT:CdS and PCPDTBT:CdS bulk-heterojunction hybrid solar cells using high-throughput, large-area, low cost spray deposition technique. Both the hybrid active layers and hole transport layers are deposited by spray technique. The CdS/Poly(3-hexylthiophene-2,5-diyl) (P3HT) and CdS/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) hybrid devices are fabricated by spray deposition process at optimized conditions (i.e. film thickness, spray solution volume, distance between sample and spray nozzle, substrate temperature, etc.). The power conversion efficiency of η=0.6% and 1.02% is obtained for P3HT:CdS and PCPDTBT:CdS hybrid devices, respectively. Spray coating holds significant promise as a technique capable of fabricating large-area, high performance hybrid solar cells.

The Effect of Band-Bending on Hybrid Bulk Heterojunction Solar Cell Performance

Hydrid bulk heterojunction solar cells (HBHJs) consist of nanostructured metal oxide (TiO2 and ZnO) acceptors and polymer light absorbing donors. To date, these promising architectures have only achieved moderate efficiency due to surprisingly low short circuit photocurrents. To elucidate the underlying properties that result in this low current, a detailed understanding of interfacial properties is required. Herein, we quantify a vacuum level offset of 0.8 eV between the polymer and the TiO2 created by spontaneous charge transfer upon heterojunction formation. This offset imparts a barrier to charge separation at the TiO2-polymer interface and explains the poor performance of cells to date. New materials for improved photocurrent are proposed by tuning the energetics of the heterojunction.

Interpenetrated Inorganic Hybrids for Efficiency Enhancement of PbS Quantum Dot Solar Cells

Photovoltaic performance improvement of a PbS quantum dot (QD) based hybrid bulk-heterojunction solar cell is achieved by modulating the interpenetration and percolation of the two hybrid phases. The PbS QD:CdSe nanotetrapod hybrid is demonstrated to be more effective at extracting charge carriers compared to PbS:CdSe QD, which favors a 44% overall increase in conversion efficiency compared to traditional PbS QD solar cells.

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.

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.