Effect of cadmium sulphide quantum dot processing and post thermal annealing on P3HT/PCBM photovoltaic device

The present study demonstrates the effect on photovoltaic performance of poly(3-hexylthiophene) (P3HT) on doping of cadmium sulphide (CdS) quantum dots (QDs). The P3HT/CdS nanocomposite shows a 10 nm blue shift in the UV-vis absorption relative to the pristine P3HT. The blue shift in the absorption of the P3HT/CdS nanocomposite can be assigned to the quantum confinement effect from the CdS nanoparticles. Significant PL quenching was observed for the nanocomposite films, attributed to additional decaying paths of the excited electrons through the CdS. Solar cell performance of pure P3HT and dispersed with CdS QDs have been studied in the device configuration viz indium tin oxide (ITO)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/Al and ITO/PEDOT:PSS/ P3HT:CdS:PCBM/Al, respectively. Incorporation of CdS QDs in the P3HT matrix results in the enhancement in the device efficiency (?) of the solar cell from 0.45 to 0.87%. Postproduction thermal annealing at 150 °C for 30 min improves device performance due to enhancement in the device parameters like FF, V_OC and improvement in contact between active layer and Al.     

Optimization of CdSe layer on modified ZnO hierarchical spheres by spin-SILAR for efficient CdS/CdSe co-sensitized solar cells

In this study, after CdS quantum dots sensitized ZnO hierarchical spheres (ZnO HS), we used a simple process to deposit CdSe QDs on ZnO by spin-coating-based SILAR, and applied to photoanodes of quantum dots-sensitized solar cells. Before CdS and CdSe QDs deposition, the ZnO HS photoanodes were modified by Zn(CH₃COO)₂·2H₂O methanol solution to further enhance the open-circuit voltage and power conversion efficiency (PCE). The program of modifying photoanodes and the number of CdSe spin-SILAR cycles are evaluated on the optical and electrochemical properties of the cells. As a result, a high energy conversion efficiency of 2.49 % was obtained by using modified ZnO HS/CdS photoanode under AM 1.5 illumination of 100 mW cm^?2. And further decorated by the CdSe QDs, the ZnO HS/CdS/CdSe cell achieved a PCE of 5.36 % due to the modification of ZnO HS nanostructure, the enhanced absorption in the visible region, the lower recombination reaction and higher electron lifetime.     

Inverted organic solar cells with ZnO nanowalls prepared using wet chemical etching in a KOH solution

We report on the photovoltaic (PV) performances of inverted organic solar cells (IOSCs) that were fabricated from PCBM:P3HT polymer with a ZnO thin film and ZnO nanowalls as electron transport and hole block layers. ZnO thin film on ITO/glass substrate was deposited using a simply aqueous solution route. ZnO nanowall structures were obtained via wet chemical etching of ZnO thin films in a KOH solution. The power conversion efficiency (PCE) of the IOSC with ZnO nanowalls was significantly improved by 44% from 1.254% to 1.811% compared to that of the IOSC with ZnO thin film. The short circuit current in IOSCs fabricated with the ZnO nanowalls was increased mainly due to the increase in the charge transport interface area, as a result of enhancement in the PCE. This work suggests a method for fabricating efficient PV devices with a larger charge transport area for future prospects.     

Enhanced performance of Al2O3 coated ZnO nanorods in CdS/CdSe quantum dot-sensitized solar cell

CdS/CdSe quantum dot-sensitized solar cells (QDSCs) based on ZnO nanorods, 4.55 μm in length, were studied. Many studies have shown that the performance of QDSCs is limited by a recombination process. Therefore, the interface layer was fabricated on the surface of the ZnO nanorods to retard recombination at the interface between the semiconductor and electrolyte. Overall, the performance of the QDSCs was improved by a surface coating of aluminum isopropoxide (Al₂O₃) on the ZnO nanorod, which facilitates a decrease in electron recombination and increased adsorption of CdS/CdSe QDs on the ZnO nanorods.     

Hybrid photovoltaic devices of polymer and ZnO nanofiber composites

Organic semiconductor-based photovoltaic devices offer the promise of a low-cost photovoltaic technology that could be manufactured via large-scale, roll-to-roll printing techniques. Existing organic photovoltaic devices have currently achieved solar power conversion efficiencies greater than 3%. Although encouraging, the reasons higher efficiencies have not been achieved are poor overlap between the absorption spectrum of the organic chromophores and the solar spectrum, non-ideal band alignment between the donor and acceptor species, and low charge carrier mobilities resulting from the disordered nature of organic semiconductors. To address the latter issues, we are investigating the development of nanostructured oxide/conjugated polymer composite photovoltaic (PV) devices. These composites can take advantage of the high electron mobilities attainable in oxide semiconductors and can be fabricated using low-temperature solution-based growth techniques. Additionally, the morphology of the composite can be controlled in a systematic way through control of the nanostructured oxide growth. ZnO nanostructures that are vertically aligned with respect to the substrate have been grown. Here we discuss the fabrication of such nanostructures and present results from ZnO nanofiber/poly(3-hexylthiophene) (P3HT) composite PV devices. The best performance with this cell structure produced an open circuit voltage (V_oc) of 440 mV, a short circuit current density (J_sc) of 2.2 mA/cm², a fill factor (FF) of 0.56, and a conversion efficiency (η) of 0.53%. Incorporation of a blend of P3HT and (6,6)-phenyl C₆₁ butyric acid methyl ester (PCBM) into the ZnO nanofibers produced enhanced performance with a V_oc of 475 mV, J_sc of 10.0 mA/cm², FF of 0.43, and η of 2.03%. The power efficiency is limited in these devices by the large fiber spacing and the reduced V_oc.     

Hole transport in organic field-effect transistors with active poly(3-hexylthiophene) layer containing CdSe quantum dots

Hybrid field-effect transistors (FETs) based on poly(3-hexylthiophene) (P3HT) containing CdSe quantum dots (QDs) were fabricated. The effect of the concentration of QDs on charge transport in the hybrid material was studied. The influence of the QDs capping ligand on charge transport parameters was investigated by replacing the conventional trioctylphosphine oxide (TOPO) surfactant with pyridine to provide closer contact between the organic and inorganic components. Electrical parameters of FETs with an active layer made of P3HT:CdSe QDs blend were determined, showing field-effect hole mobilities up to 1.1×10^?4 cm²/Vs. Incorporation of TOPO covered CdSe QDs decreased the charge carrier mobility while the pyridine covered CdSe QDs did not alter this transport parameter significantly.     

ZnO-CdS core-shell quantum dots sensitized solar cell: influence of crystalline and amorphous CdS structures in photovoltaic performance

ZnO nanoparticles (NPs) coated with amorphous and crystalline CdS quantum dots (QDs) were successfully synthesized through chemical bath deposition (CBD) process. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been utilized to characterize the samples morphology and structural properties. The conduction band of CdS QDs is much higher than the ZnO conduction band facilitates electron transfer process through cascade system. The thickness and crystallinity of the CdS QDs coated on ZnO NPs critically controls the electron diffusion length and photovoltaic performance of the solar cell. The red shift from 506 to 524 nm, increased optical absorption in the UV-visible range and electron diffusion length limited by the thickness of the amorphous/crystalline CdS QDs coated on ZnO NPs film, influences the performance of the QDs sensitized solar cell (QDSSCs) under one sun illumination intensity (AM 1.5, 100 mW/cm2). The results discuss the CBD process controlled growth of CdS QDs on ZnO NPs and its influence on the photovoltaic performance of QDSSCs.     

Improving conversion efficiency of CdS quantum dots-sensitized TiO2 nanotube arrays by doping with Zn and decorating with ZnO nanoparticles

The Zn-doped TiO₂ nanotube arrays (TNTs) decorated with ZnO nanoparticles have been prepared via electrochemical anodization and immersing method. Furthermore, the CdS quantum dots (QDs) were deposited on the prepared Zn-doped TNTs-ZnO thin films by chemical bath deposition (CBD) method to fabricate the CdS QDs-sensitized Zn-doped TNTs-ZnO photoelectrodes. The nanostructure, morphology, optical properties and electrochemical properties of the CdS/Zn-doped TNTs-ZnO photoelectrode with comparison to those of the CdS/TNTs photoelectrodes were investigated. It has been found that the Zn-doped TNTs-ZnO photoelectrodes significantly increased the UV–vis light absorption of the CdS/Zn-doped TNTs-ZnO photoelectrodes and reduced the charge recombination at the surfaces of the CdS/Zn-doped TNTs-ZnO photoelectrodes. As a consequence, when the Zn-doped TNTs-ZnO film was adopted instead of the plain TNTs film, the light-chemical energy conversion efficiency of the CdS/Zn-doped TNTs-ZnO photoelectrode was much improved compared with the CdS/TNTs photoelectrode. A maximum energy conversion efficiency achieved for the CdS/Zn-doped TNTs-ZnO photoelectrode is 3.86%, which is a 17% improvement compared with the maximum energy conversion efficiency of 3.29% achieved for the CdS/TNTs photoelectrodes.     

Growth of ZnO nanowires on fibers for one-dimensional flexible quantum dot-sensitized solar cells

One-dimensional flexible solar cells were fabricated through vertical growth of ZnO nanowires on freestanding carbon fibers and subsequent deposition of CdS quantum dots (QDs). Under light illumination, excitons were generated in the CdS QDs and dissociated in the ZnO/CdS interface. Photoelectrochemical characterization indicates that fiber quantum dot-sensitized solar cells (QDSCs) could effectively absorb visible light and convert it to electric energy. The photoelectrochemical performance was enhanced after the deposition of a ZnS passivating layer on the CF/ZnO/CdS surface. The highest conversion efficiency of about 0.006% was achieved by the fiber QDSCs. A higher conversion efficiency was expected to be achieved after some important parameters and cell structure were optimized and improved.     

Characteristics of n-type ZnO nanorods on top of p-type poly(3-hexylthiophene) heterojunction by solution-based growth

We report that ZnO nanorods (NRs) are grown on an organic layer of poly(3-hexylthiophene) (P3HT) using a modified seeding layer. Thus, ZnO NRs/P3HT heterojunction light-emitting diodes could be fabricated using the hydrothermal method, in which ZnO acts as an n-type material and P3HT as a p-type material. The ZnO NRs improve the electron transportation in the devices. A three-fold enhancement of current density of the device is observed due to the NRs formed on the P3HT. The electroluminescence (EL) of the optimized ZnO-based device is 1.5 times larger than that without NRs. The influence of the P3HT thickness for the EL spectrum is also discussed.     

Surface Stoichiometry Control of Colloidal Heterostructured Quantum Dots for High-Performance Photoelectrochemical Hydrogen Generation

Manipulating the separation and transfer behaviors of charges has long been pursued for promoting the photoelectrochemical (PEC) hydrogen generation based on II–VI quantum dot (QDs), but remains challenging due to the lack of effective strategies. Herein, a facile strategy is reported to regulate the recombination and transfer of interfacial charges through tuning the surface stoichiometry of heterostructured QDs. Using this method, it is demonstrated that the PEC cells based on CdSe-(Se_xS_1−x)₄-(CdS)₂ core/shell QDs with a proper S_surface/Cd_surface ratio exhibits a remarkably improved photocurrent density (≈18.4 mA cm⁻² under one sun illumination), superior to the PEC cells based on QDs with Cd-rich or excessive S-rich surface. In-depth electrochemical and spectroscopic characterizations reveal the critical role (hole traps) of surface S atoms in suppressing the recombination of photogenerated charges, and further attribute the inferior performance of excessive S-rich QDs to the impeded charge transfer from QDs to TiO₂ and electrolyte. This work puts forward a simple surface engineering strategy for improving the performance of QDs PEC cells, providing an efficient method to guide the surface design of QDs for their applications in other optoelectronic devices.     

Quantum dot sensitized solar cell based on poly (3-hexyl thiophene)/CdSe nanocomposites

The optoelectronic properties of P3HT–CdSe nanocomposites prepared by insitu chemical oxidative polymerization were studied. CdSe QDs were synthesized by hot injection method using tri octyl phosphine oxide (TOPO) as capping ligand whereas the P3HT polymer was prepared by chemical oxidative polymerization. FTIR studies confirmed the regioregularity of the P3HT and revealed the chemical interaction of P3HT and CdSe in nanocomposite. Absorption studies showed blue shift for the nanocomposites as compare to pristine P3HT, the electron transfer from conducting polymer to the CdSe was detected by the measurements of quenching of photoluminescence from conducting polymer after the addition of semiconductor nano crystals which confirmed that an optimum amount of nanoparticles provide networking in hybrid composites. The optimal result for device prepared by P3HT–CdSe nanocomposites was open circuit voltage (V_oc) 0.5, short circuit current density (J_sc) 0.66, Fill factor (FF) 0.6855 and efficiency (η) 0.22%.     

Plasmonic Ag@Oxide Nanoprisms for Enhanced Performance of Organic Solar Cells

Localized surface plasmon resonance (LSPR), light scattering, and lowering the series resistance of noble metal nanoparticles (NPs) provide positive effect on the performance of photovoltaic device. However, the exciton recombination on the noble metal NPs accompanying above influences will deteriorate the performance of device. In this report, surface‐modified Ag@oxide (TiO₂ or SiO₂) nanoprisms with 1–2 nm shell thickness are developed. The thin film composed of P3HT/Ag@oxides and P3HT:PCBM/Ag@oxides is investigated by absorption, photoluminescence (PL), and transient absorption spectroscopy. The results show a significant absorption, PL enhancement, and long‐lived photogenerated polaron in the P3HT/Ag@TiO₂ film, indicating the increase of photogenerated exciton population by LSPR of Ag nanoprisms. In the case of P3HT/Ag nanoprisms, partial PL quench and relatively short‐lived photogenerated polaron are observed. That indicates that the oxides layer can effectively avoid the exciton recombination. When the Ag@oxide nanoprisms are introduced into the active layer of P3HT:PCBM photovoltaic devices, about 31% of power conversion efficiency enhancement is obtained relative to the reference cell. All these results indicate that Ag@oxides can enhance the performance of the cell, at the same time the ultrathin oxide shell prevents from the exciton recombination.     

Toward efficient carbon nanotube/P3HT solar cells: active layer morphology, electrical, and optical properties

We demonstrate single-walled carbon nanotube (SWCNT)/P3HT polymer bulk heterojunction solar cells with an AM1.5 efficiency of 0.72%, significantly higher than previously reported (0.05%). A key step in achieving high efficiency is the utilization of semiconducting SWCNTs coated with an ordered P3HT layer to enhance the charge separation and transport in the device active layer. Electrical characteristics of devices with SWCNT concentrations up to 40 wt % were measured and are shown to be strongly dependent on the SWCNT loading. A maximum open circuit voltage was measured for SWCNT concentration of 3 wt % with a value of 1.04 V, higher than expected based on the interface band alignment. Modeling of the open-circuit voltage suggests that despite the large carrier mobility in SWCNTs device power conversion efficiency is governed by carrier recombination. Optical characterization shows that only SWCNT with diameter of 1.3-1.4 nm can contribute to the photocurrent with internal quantum efficiency up to 26%. Our results advance the fundamental understanding and improve the design of efficient polymer/SWCNTs solar cells.     

Enhancement of gas sensing properties of CdS nanowire/ZnO nanosphere composite materials at room temperature by visible-light activation

CdS nanowire/ZnO nanosphere materials (CdS/ZnO) with hierarchical structure were synthesized by a three-step solvothermal process. XRD, FESEM and TEM analysis confirmed the growth of ZnO nanospheres on the surface of CdS nanowires (NWs). The transient photovoltage (TPV) measurements revealed that the interface between CdS and ZnO can inhibit the recombination of photogenerated excess carriers and prolong the lifetime of excess carriers in CdS/ZnO materials. Moreover, the CdS/ZnO materials exhibit a dramatic improvement in optoelectronic performance and visible-light-irradiation gas sensing activity, which gave 1 order of magnitude larger than that of CdS NWs in response to formaldehyde. The enhancement of sensing properties is attributed to the interfacial transport of excess carriers.     

Formation of phenyl-C<Subscript>61</Subscript>-butyric acid methyl ester nanoscale aggregates after supercritical carbon dioxide annealing

In this study, we successfully developed a novel method to create [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) nanoscale aggregates using supercritical carbon dioxide (scCO₂) annealing and fabricated bulk heterojunction (BHJ) solar cells with the nanoscale PCBM to improve device performance. PCBM forms nanoscale aggregates with a size of approximately 70 nm after scCO₂ annealing at 11 MPa and 50 °C for 60 min. However, PCBM remains amorphous after thermal annealing (TA) at 150 °C for 5 min. The morphology, structure, and crystallinity of poly(3-hexylthiophene) (P3HT) in the scCO₂-treated P3HT film are nearly the same as those in the TA-treated P3HT film. In the P3HT/PCBM blend, the formation of PCBM nanoscale aggregates by scCO₂ treatment decreases the disturbance for P3HT crystallization and improves diffusion and regular packing of P3HT molecular chains. This increases the crystallinity of P3HT so that it becomes higher than that in the TA-treated blend film. The nanoscale aggregates of PCBM and the higher crystallinity of P3HT give the scCO₂-treated P3HT/PCBM BHJ solar cells a maximum power conversion efficiency (PCE) of 2.74%, which is much higher than that of the as-cast device (PCE is 1.70%) and a little higher than the highest PCE (2.64%) of thermally annealed devices. These results indicate that scCO₂ is an effective, mild, and environmental method to modulate the nanoscale aggregates of PCBM and to improve the PCE of BHJ solar cells. However, the size of the PCBM aggregates is a little larger than the most suitable size of the exciton diffusion length, leading to limited improvement of the PCE.     

Cellulose acetate fibers covered by CdS nanoparticles for hybrid solar cell applications

In this work cellulose acetate (CA) fibers with a diameter of approximately 1 μm were immersed in a cadmium sulfide (CdS) precursor solution. After 3 h the original white color CA fibers became yellow and maintained the same form, suggesting the deposition of CdS on fiber surface. SEM images showed that CA fibers were covered by uniformly sized CdS nanoparticles of approximately 100 nm. XRD and optical absorption spectra indicated that they contained mostly cubic crystalline phase with the optical band gap of 2.43 eV. CdS coated CA fibers, called CdS(CA) fibers, were dispersed in a polar dispersant (dimethyl sulfoxide, DMSO) and then mixed with a poly(3-hexylthiophene) (P3HT) solution in a non-polar solvent (dichlorobenzene, DCB). The mixture was cast onto a transparent conductive glass substrate (Indium–Tin–Oxide, ITO), and after solvent evaporation a thin layer of CdS(CA)–P3HT composite was formed. It is observed that the volume relation between the polar dispersant and non-polar solvent influences the solubility of the P3HT product in the composite coating and the photovoltaic performance of the corresponding cell as well. The mass ratio between CdS(CA) fibers and P3HT in the composite layer affects the optical absorption of the composite. The best photovoltaic performance was obtained in CdS(CA)–P3HT based cells with a volume relation between DCB and DMSO of 3.5–1, a mass ratio between CdS(CA) and P3HT of 1:1, and a rapid drying process for composite coatings.     

The Impact of Grain Alignment of the Electron Transporting Layer on the Performance of Inverted Bulk Heterojunction Solar Cells

This report presents a new strategy for improving solar cell power conversion efficiencies (PCEs) through grain alignment and morphology control of the ZnO electron transport layer (ETL) prepared by radio frequency (RF) magnetron sputtering. The systematic control over the ETL's grain alignment and thickness is shown, by varying the deposition pressure and operating substrate temperature during the deposition. Notably, a high PCE of 6.9%, short circuit current density (J_sc) of 12.8 mA cm^?2, open circuit voltage (V_oc) of 910 mV, and fill factor of 59% are demonstrated using the poly(benzo[1,2‐b:4,5‐b′]dithiophene–thieno[3,4‐c]pyrrole‐4,6‐dione):[6,6]‐phenyl‐C₇₁‐butyric acid methyl ester polymer blend with ETLs prepared at room temperature exhibiting oriented and aligned rod‐like ZnO grains. Increasing the deposition temperature during the ZnO sputtering induces morphological cleavage of the rod‐like ZnO grains and therefore reduced conductivity from 7.2 × 10^?13 to ≈1.7 × 10^?14 S m^?1 and PCE from 6.9% to 4.28%. An investigation of the charge carrier dynamics by femtosecond (fs) transient absorption spectroscopy with broadband capability reveals clear evidence of faster carrier recombination for a ZnO layer deposited at higher temperature, which is consistent with the conductivity and device performance.     

Low temperature aqueous chemical synthesis of CdS sensitized ZnO nanorods

Cadmium sulfide nanoparticles (CNPs) sensitized zinc oxide nanorod arrays (ZNRs) were synthesized in the two step deposition process at relatively low temperature. The vertically aligned ZNRs were grown on the conducting glass substrates (FTO) using aqueous chemical method, followed by the deposition of CNPs at 70 °C using chemical bath deposition (CBD) technique. The samples were characterized by optical absorption, X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). Further, the photoelectrochemical (PEC) performance of ZNRs with and without CNPs sensitization was tested in Na₂S-NaOH-S and Na₂SO₄ electrolyte, respectively. When the CNPs are coated on the ZNRs, the optical absorption is enhanced and band edge is shifted towards visible region (525 nm) as compared with ZNRs (375 nm). The sample sensitized with CNPs shows higher photoelectrochemical (PEC) performance with maximum short circuit current of (I_sc) 2.60 mA/cm².     

CeO2 quantum dot functionalized ZnO nanorods photoanode for DSSC applications

Cerium oxide quantum dots (CeO₂ QDs) decorated zinc oxide nanorods (ZnO NRs) heterostructures were grown by a combination of solvothermal and chemical bath deposition methods and used for dye sensitized solar cell (DSSC) applications. Transmission electron microscope images showed the formation of CeO₂/ZnO NRs, where ~5 nm CeO₂ QDs were decorated on ZnO NRs having 1–2.5 μm length and 100–150 nm width. Photoluminescence spectra showed the significant increase in UV emission after decoration of ZnO NRs with CeO₂ QDs. DSSC results revealed that the ZnO NRs with CeO₂ QDs leads to an increase in the open circuit voltage and fill factor and exhibited a maximum efficiency of 2.65 %, which was 2.01 times higher than that of unmodified ZnO NRs. The decoration of CeO₂ QDs on the ZnO NRs surface may lead to the formation of barrier layer and hindered the back electron transfer and thereby high light harvesting efficiency.