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Performance Improvement Strategies for Quantum Dot Sensitized Solar Cells: A Review

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Abstract

Development of a new low-cost technologies for high-efficient quantum dot sensitized solar cells (QDSCs) is an effective way to solve current energy and environmental problems. Over the past few years QDSCs employing inorganic semiconductor nanocrystals as the light harvesters, introduced in 2010 with the power conversion efficiency (PCE) of 5% has aroused much attention since the recent breakthrough PCE reports over 12%. In this review, the strategies to obtain highly efficient QDSCs are discussed in details from the viewpoints of QD sensitizers, properties of photoanode films, counter electrodes, redox electrolytes, and the interfacial passivation method. Furthermore, the limitations and further prospects have also been discussed how to fabricate QDSCs devices with high efficiency and excellent stability. It is expected that these goals can be realized by virtue of the above effective strategies. We believe that this review will not only offers a theoretical basis and technical support for further industrial application of QDSCs, but also provides inspiration and references for improvement performance of other types of solar cells.

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A new simple strategy based on TiO2@reduced graphene oxide (rGO) composites for QDSSCs is demonstrated in this study. We prepare TiO2@rGO-2 photoanodes in which graphene was obtained through hydrothermal and high-temperature reduction. This task is conducted to make TiO2@rGO composite be uniformly mixed together. The SILAR method is applied to the QDs adsorption of TiO2@rGO composites. Compared with that of TiO2@rGO-1 photoanodes, the complete reduction properties of TiO2@rGO-2 photoanodes result in a remarkable increase in the photovoltaic characteristics of solar cells. The relevant performance characteristics of QDSSCs are observed, and the current–voltage curve results show CdS/TiO2@rGO-2 and Mn²⁺–CdS/TiO2@rGO-2 photoelectric conversion efficiencies of 1.51% and 2.54%, respectively. These measured values are almost 10% higher than those of cells with TiO2@rGO-1 as photoanode, in fact. Electrochemical impedance testing results show that the effective secondary reduction of graphene leads to this meaningful enhancement.
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The feasibility of utilizing CdSe/ZnS quantum dots (QDs) in liquid scintillator radioluminescent nuclear batteries to improve battery performance was studied. The peak position of the radioluminescence emission spectra of liquid scintillator can be regulated by controlling the QD components. This method is suitable for obtaining a satisfactory spectral matching between fluorescence materials and photovoltaic devices to increase the output performance of the battery. In the experiment, CdSe/ZnS QDs were introduced into Emulsifier‐Safe liquid scintillator, and the output properties of radioluminescent nuclear batteries were investigated via X‐ray. Results indicate that the battery with 15 mg CdSe/ZnS QDs generated the best electric power under different tube voltages. To analyze the X‐ray radioluminescence effects of the liquid scintillator, the radioluminescence spectra of the Emulsifier‐Safe with and without CdSe/ZnS QDs were measured and compared. The spectral matching degree between the Emulsifier‐Safe with different concentrations of CdSe/ZnS QDs and the GaAs device was also analyzed by considering luminescence utilization in batteries. This framework can serve as a guide for the development of a radioluminescent material system for long‐lasting, high‐performance power supplies.
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The main component in photoelectrochemical (PEC) water splitting technology is the semiconductor, which its electronic structure has an essential effect on the final performance of the cell. Morphology plays a significant role in the electronic properties of semiconductors. The relationship between the electron and optical properties of nanostructures and their surface morphology is a subject that is less addressed in scientific papers. This review attempts to detail the impact of morphology on nanostructures' properties, such as light absorption and charge carrier transport. Creating a direct path for electron transport, reducing charge recombination, and increasing light scattering are some of the things that are directly related to the surface morphology of nanostructures. The present review aims to provide general information about synthesized nanostructures and their advantages and disadvantages to help readers create new ideas and innovate in this field.
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Urgent requirements for high-efficiency and low-cost photovoltaic devices are constantly pushing forward the development of the emerging solar cells. Currently, organic solar cells (OSCs) and perovskites solar cells (PSCs) were considerable as the most likely commercialized solar cells in the short-term period. Enormous optimization strategies towards optimizing the devices efficiency and stability have been developed. It is noteworthy that the well-known small-sized quantum dots (QDs), have been explored as the additional components in OSCs and PSCs, and yielded the rather modest amelioration of devices performance. Herein, we reviewed the recent advances in strategically integrating all kinds of QDs (consisting of metal chalcogenides based QDs, perovskite QDs, InP based QDs, carbon QDs, graphene QDs, black phosphorus QDs, and other emerging two-dimensional QDs) in association with relevant performance enhancement of OSCs and PSCs. In view of each type of QDs, we mainly emphasized their involved devices configuration, integration location, and physical mechanism. Additionally, the fundamental structures, operation principles and analogies/distinctions of OSCs and PSCs were briefly outlined. Finally, the existing challenges and future prospect based on QDs integrated OSCs and PSCs were listed out.
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Considering the minus points of Polysulfide redox electrolyte we explored Lanthanum Strontium Manganite to see the possibility of its usage in Quantum dot sensitized solar cells (QDSSCs) as electrolyte. Variations in material morphology synthesized via different methods and its effect on device performance and charge transfer dynamics across counter electrode are studied. The ceramic acts as a good passivation layer reducing the back transfer of electrons resulting in greater Voc and higher stability of cells. This study opens scope for use of such ceramics to completely replace the liquid electrolyte.
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The search for non-toxic and non-heavy metal absorbers for solar cells is attracting lot of attention and this research has led to development of many non-toxic nanocrystal absorbers that have...
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CuS is widely used as counter electrode material in quantum dot sensitized solar cells (QDSSCs) due to its good catalytic activity in polysulfide electrolyte. To improve the performance of CuS counter electrode, carbon black (CB) with fast electron transport structure and large surface area was used as scaffolding material in this work. Carbon black/copper sulfide composite counter electrode (CB/CuS CE) was prepared and its performance was evaluated by coupling with TiO2/CdS photoanode. The results unveiled the significant role of carbon black in enhancing the power conversion efficiency of QDSSC. CdS quantum dots (QDs) were deposited on TiO2 photoanode by the successive ionic layer adsorption and reaction (SILAR) method. The XRD and TEM results showed the quantum dot CdS deposition on TiO2 photoanode. UV–Visible spectra of CdS QDs sensitized TiO2 clearly revealed the redshift in the light-absorbing edge position with enhanced solar light absorption. The counter electrode was prepared by depositing CB/copper sulfide composite film on FTO by spin coating followed by thermal sulfidation methods. XRD, HRTEM, XPS and RAMAN spectroscopy techniques clearly showed the crystalline phases, morphology and chemical composition of carbon black/copper sulfide (CB/CuS) composite material. The CB/CuS composite counter electrode exhibited improved electrocatalytic activity, better current density and enhanced power conversion efficiency than bare CuS and bare CB counter electrodes independently. The superior performance was synergistically contributed by the high surface area, high electron conductivity and high electrocatalytic activity of CB and CuS nanoparticles, respectively, in the composite CE material. Graphical abstract
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Charge transfer mechanism in ZnO nanorods grown on aluminium doped zinc oxide substrates using a single step hydrothermal method was investigated as a function of ZnO nanorod (NR) growth temperature/nanorod’s length. Band-to-band transition and the defect state transition in the prepared nanorods was confirmed using photoluminescence spectroscopy. The increase in active sites up to an optimum growth temperature/nanorod’s length results in an increase of current density and photoconversion efficiency of the prepared NR. But with the further increase in the latter, the current density decreases due to the recombination of electrons. Nanorod grown at the optimized condition exhibited a current density of 0.82 mA/cm² and a photoconversion efficiency η of 0.53 %. Electrochemical impedance spectroscopy and open-circuit photovoltage decay (OCPVD) measurement were employed to determine the charge transfer resistanceRct, double layer capacitance (Cdl) and the recombination time τ of zinc oxide nanorods of various length. The nanorod prepared at the optimized condition of 140 oC having an average length of 2.4 μm had superior current density and the better charge transfer mechanism. The study confirmed that this was due to the low charge transfer resistance Rct=1300Ω value and better recombination time τ=2.38s for the sample, which makes ZnO nanorods a promising candidate to be used as a photoanode in quantum dot sensitized solar cells.
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Semiconductor quantum dots (QDs) are nanocrystals whose excitons are bound in 3D space. Owning to their remarkable quantum confinement effect, QDs exhibit a discontinuous electronic energy level structure similar to that of atoms, leading to novel physical, optical, and electrical properties for various optoelectronic device applications including solar cells. Near‐infrared photoactive narrow bandgap (NBG) QDs can maximize the use of solar energy through the quantum size effect, offering a good opportunity for designing highly efficient wide‐spectrum responsive solar cells. This review analyzes the recent research progress of NBG QDs as light absorbing materials in solar cells. The critical elaboration of the latest achievements both in material design and device optimization for NBG QD‐based solar cells (QDSCs), including QD synthesis and film fabrication, design of device configuration, classification of NBG QDs and their photovoltaic performance, strategies for performance improvements is focused upon. The current challenges and perspectives for the further advance of NBG QDSCs are also discussed. Near‐infrared (NIR) photoactive semiconductor quantum dots (QDs) play a critical role for designing efficient wide‐spectrum solar cells. This review provides a comprehensive analysis of the latest achievements of NIR QDs used for solar cells, including the classification of QDs and their photovoltaic performance, various strategies for performance improvements, and the challenges and perspectives for the future advances.
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In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.
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A low cost H3PW12O40 (PW12)/CoS2 complex is prepared and used as a counter electrode (CE) to combine with sandwich quantum dot sensitized solar cells (QDSSCs) composed of a TiO2/CdS/CdSe/ZnS photoanode and polysulfide electrolyte to study their photovoltaic properties via a simple hydrothermal method. Under standard simulated sunlight, the photoelectric conversion efficiency (PCE) of 2%PW12 (PW12-2/CoS2) doped CEs was 6.29%, which was significantly 67.7% higher than those of QDSSCs based on undoped CoS2 CEs (3.75%). Due to the introduction of PW12, the nanoparticles forming the hollow structure of CoS2 changed from regular octahedra to rough nanoparticles, which increase the active sites. At the same time, the work function of CoS2 decorated with PW12 is decreased. This study and discovery demonstrate that POMs can be used to optimize CE materials and improve the photoelectric conversion efficiency of QDSSCs, which provide an experimental and theoretical basis for subsequent investigations.
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Influence of citric acid on the photovoltaic properties of the CdS quantum dot-sensitized TiO2 solar cells (QDSSCs) was studied. Tethering of citric acid molecules with both TiO2 and CdS quantum dots (QDs) was confirmed by Fourier transform infrared spectroscopy technique. High-resolution transmission electron microscopic studies revealed that QDs with average size of ~4.5 nm, were tethered with TiO2 nanoparticles of diameter ~40 nm. Presence of Cd, S, C, Ti and O elements in the composite photoanode and their uniform distribution throughout the photoanode were confirmed by energy dispersive X-ray spectroscopy measurements. QDSSCs fabricated with pristine TiO2 photoanode exhibited a short circuit current density (JSC) of 5.80 mA cm−2 and an overall power conversion efficiency (η) of 1.10%, whereas solar cells made with citric acid-treated, photoanode-exhibited a JSC of 8.20 mA cm−2 with 1.50% efficiency under 100 mW cm−2 (AM 1.5) light illumination. This is an impressive 60% increase in the JSC and ~36% enhancement in the overall power conversion efficiency. Interfacial resistance of QDSSCs is estimated by using electrochemical impedance spectroscopy revealed that citric acid treatment enhanced both the electron injection to the conduction band of the TiO2 from the CdS as well as the overall charge transfer of the device, while decreasing the recombination of the photo-generated electrons with their holes in the electrolyte.
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Ternary I-III-VI (such as CuInSe2 and AgInS2, etc.) quantum dots (QDs) have been increasingly studied in the field of photoelectric conversion applications. Here, we prepare a relatively less toxic Cu-In-Sn-Se (CISSe) QD by an organic high-temperature hot injection method, and this material is subsequently applied as a functional sensitizer to produce quantum dot-sensitized solar cells (QDSSCs). Due to the effect of Sn doping and ZnS passivation, the TiO2/CISSe/ZnS photoanode obtains the greatest composite resistance (985.5 Ω·cm²) and highest electron lifetime (120.23 ms) among the experimentally compared materials, showing the further inhibition of charge recombination when compared to bare Cu-In-Se (CISe) solar cells while effectively enhancing the collection efficiency of photogenerated electrons. Using CISSe/ZnS quantum dots as a sensitizer, the power conversion efficiency (PCE) of the QDSSC reaches 6.7%, of which Voc, Jsc and FF reach 0.559 V, 22.93 mA/cm² and 0.52, respectively. It provides a new possibility for the development of QDSSCs.
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In recent years, low-cost spinel sulfides as counter electrode materials for quantum dot sensitized solar cells (QDSSCs) have gradually become a research hotspot in this field. In this study, CuCo2S4/reduced graphene oxide (RGO) composite is successfully prepared via a simple and low-cost solvothermal method, which was used as counter electrode (CE) and assembled with CdS/CdSe photoanode to form QDSSCs device. In comparison with other materials, the power conversion efficiency of CuCo2S4/RGO CE reaches 6.59% and is conspicuously superior to that of pure CuCo2S4 and Cu2S/brass CE, benefiting from that the peculiar structure of the CuCo2S4 hollow spherical shell loaded on RGO nanosheets shortens electron migration paths and enables efficient transportation of electrons. Furthermore, the corrosion resistance of CEs has been prominently enhanced compared with Cu2S/brass CE. Thus, CuCo2S4/RGO composite can be primely acknowledged as promising CE materials for competitive high-performance QDSSCs and offer the possibility for high efficiency and energy saving of QDSSCs in the future.
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A second spin-coating process was employed for CuInS2 quantum dot (QD)-sensitized TiO2 nanowire-based solar cells, which is anticipated to increase the QD loading amount of photoelectrodes. And the photoelectrodes had been modulated by the quantum dot dispersion concentration and spin-coating cycles. The optical absorption spectra and photoluminescence spectra of different photoelectrodes were investigated, which had exhibited the larger QD loading amount and better charge separation property of photoelectrodes after the second spin-coating process. Meanwhile, a net connection structure had been formed between each nanowire by the suitable QD loading amount of the photoelectrodes, which had simultaneously provided more paths for charge transfer of the solar cells. By optimization, the CuInS2 QD-sensitized TiO2 nanowire solar cells prepared from QD dispersion concentration of 30 mg∙mL−1 and two spin-coating cycles had exhibited higher current density value, which had enhanced the photovoltaic conversion efficiency from 3.9 to 5.03%.
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In this research, two novel multi-layer photoelectrodes were fabricated and applied in quantum dot sensitized solar cells (QDSCs). The double layer scaffolds were composed of a TiO2 nanocrystalline sub-layer covered with an over-grown TiO2 nanorods (NRs) or ex-synthesized TiO2 hollow spheres (HSs) film. The light sensitization was carried out by deposition of separated PbS, CdS and CdSe quantum dots (QDs) layers. Finally, two electron blocking/passivating films of ZnS and SiO2 were formed for better photovoltaic (PV) performance. The syntheses of TiO2 nanocrystals (NCs), HSs and NRs were performed and tuned through different hydrothermal methods. The depositions of the PbS, CdS and ZnS layers were carried out by conventional successive ionic layer adsorption and reaction (SILAR) process. The CdSe and SiO2 films were also formed using a modified, effective chemical bath deposition (CBD) and sol–gel techniques, respectively. The PbS sensitizing film was additionally utilized for higher PV efficiency and the number of deposition cycles(X) was altered for the purpose of optimization. For the cell with novel TiO2NCs/TiO2NRs/PbS(X)/CdS/CdSe/ZnS/SiO2, X = 0–3, photoanode, the highest efficiency was about 5.5% and achieved for X = 2. This power conversion efficiency (PCE) was enhanced about 28% compared to that of the PbS free photoelectrode. The other improved TiO2 NCs/TiO2HSs/PbS(X)/CdS/CdSe/ZnS/SiO2, X = 0–3, photoanode was also utilized with a light scattering HSs film. The maximum efficiency was achieved about 6.5% for the extra PbS(2) sensitizing layer. This PCE was increased about 20% compared to the PbS free photoelectrode and 51% compared to that of the QDSC with the PbS free, NRs included photoanode. The highest efficiency was attributed to the co-sensitization and optimized PbS sensitizing layer together with the effective light scattering and utilization of double passivating layers.
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CuInS2 (CIS) nanoparticles have unique chemical, toxicological and optoelectronic properties that favor their technological applications. In the present work we report a novel one step biomimetic method for the aqueous synthesis of CIS nanoparticles, that is also low cost and environmentally friendly. This biomimetic method uses only CuSO4 and InCl3 as precursor salts, and the biological molecule glutathione as sulfur donor and stabilizer of the nanoparticles (NPs). The reaction is performed at low temperatures, under aerobic conditions and atmospheric pressure. CIS nanoparticles produced by our biomimetic method exhibit fluorescence emission between 650-700 nm when excited at 500 nm. A size between 10-15 nm was determined by Dynamic light scattering (DLS) and corroborated by electron transmission microscopy. X-ray diffraction analysis (XRD) confirmed the crystalline structure of the CIS NPs produced. Energy Dispersive X-Ray Spectroscopy (EDX) analyses revealed the presence of Cu, In, and S in a 0.6: 1.4: 2 ratio, which has been reported for other CIS NPs in literature. No cytotoxicity of CIS NPs was observed in human OKT6/TERT2 cells and bacteria. Besides, the potential application of biomimetic CIS NPs as photosensitizers in quantum dots sensitized solar cells (QDSSCs) was confirmed. The biocompatibility, spectroscopic properties, and energy harvesting performance in solar cells of the CIS NPs produced by our biomimetic method make them suitable for their use in different biotechnological applications.
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The dye‐sensitized solar cell (DSSC), a third‐generation photovoltaic technology, has gained considerable attention since the achievement of around 7% efficiency in 1991. To reduce the cost of the commonly used platinum (Pt) counter electrode (CE), different materials with good electrocatalytic activities have been applied as CEs for DSSCs. Recently, transition metal chalcogenides, such as metal sulfides, metal selenides, and metal tellurides, have been investigated because of their low cost, unique electrocatalytic performance, and electronic structure similar to Pt. However, more efforts remain to be made on the mechanism and application of these metal chalcogenides as CEs for DSSCs. Herein, an overview and guidelines are given on recent advances in binary and multinary metal chalcogenides used as CEs in DSSCs. The synthesis techniques and the effect of morphology optimization and stoichiometric ratios are briefly described. The development of composites made of metal chalcogenides combined with highly electrocatalytic materials, especially carbon‐based materials, is also briefly discussed. Some suggestions and methods to improve the power conversion efficiencies (PCEs) of DSSCs designed with metal chalcogenides as CEs are also provided. Metal chalcogenides (M x E y ; E = S, Se, and Te) receive special attention in various fields thanks to their impressive properties. However, more should be done on their application as counter electrodes (CEs) for Pt‐free dye‐sensitized solar cells (DSSCs). Herein, an overview and guidelines are provided on recent advances made on binary and multinary metal chalcogenides as CEs for DSSCs.
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High light-harvesting efficiency and low interfacial charge transfer loss are essential for the fabrication of high-efficiency quantum dot-based solar cells (QDSCs). Increasing the thickness of mesoporous TiO2 films can improve the loading of pre-synthesized QDs on the film and enhance the absorbance of photoanode, but commonly accompanied by the increase in the unfavorable charge recombination due to prolonged electron transmission paths. Herein, we systematically studied the influence of the balance between QD loading and TiO2 film thickness on the performance of QDSCs. It is found that the relative thin photoanode prepared by the cationic surfactant-assisted multiple deposition procedure has achieved a high QD loading which is comparable to that of the thick photoanode commonly used. Under AM 1.5G illumination, Zn−Cu−In−Se and Zn−Cu−In−S based QDSCs with optimized 11.8 μm photoanodes show the PCE of 10.03% and 8.53%, respectively, which are comparable to the corresponding highest PCE of Zn−Cu−In−Se and Zn−Cu−In−S QDSCs (9.74% and 8.75%) with over 25.0 μm photoanodes. Similarly, an impressive PCE of 6.14% was obtained for the CdSe based QDSCs with a 4.1 μm photoanode, which is slightly lower than the best PCE (7.05%) of reference CdSe QDSCs with 18.1 μm photoanode.
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The nitrogen-doped carbon supported Cu nanoparticles composite materials ([email protected]) have been recognized as the excellent counter electrode catalysts for quantum dot-sensitized solar cells (QDSCs). Herein, the [email protected] composite materials were synthesized via facile one-step pyrolysis of nitrogen-containing Cu-MOFs precursor. By controlling the reaction temperature from 600 to 1000 °C, the [email protected] (x is the pyrolysis temperature) catalysts are obtained. An ordered nitrogen-doped graphitized carbon lattice around Cu nanoparticles of the [email protected] improves the conductivity of counter electrodes (CEs), and uniform doping of nitrogen in the carbon enhances the wettability of the carbon material. The [email protected]/FTO CEs prepared exhibit higher electrocatalytic activity than both pristine N-C/FTO and [email protected]/FTO CEs, which could be ascribed to the synergistic effect between abundant CuxS catalytic active sites and nitrogen-doped graphitic carbon. It is noted that the [email protected]/FTO CEs also show superior catalytic activity compared to [email protected]/FTO CEs due to the higher specific surface area and porosity of the carbon skeleton. As a result, the QDSCs based on [email protected]/FTO CEs achieved the highest PCE of 8.63%, which is slightly higher than that of [email protected]/FTO CEs (8.20%) and [email protected] CEs (8.37%), but significantly higher than that of N-C/FTO CEs (6.52%).
Article
Constructing the highly efficient and stable quasi-solid-state even solid-state devices is one of the most significant tendency in the field of quantum dots sensitized solar cells (QDSCs). Herein, highly efficient and stable quasi-solid-state (QS)-QDSCs devices were fabricated via synergistically combining the TiO2-sol interconnecting-modified photoanodes with alginate hydrogel (AH)-assisted electrolyte films. TiO2-sol binding agent could incorporate into the surface and voids of TiO2 particles, which increased the surface area and roughness for QDs loading and induced better connection between the neighboring particles. Meanwhile, the AH-assisted QS-electrolyte films possessed the crosslinking hydrogel network structure and exhibited the perfect interfacial contact with the modified TiO2 surface, thus enhancing the ion transport and redox reaction of polysulfide couple. Benefited from the synergistic effect of TiO2-sol modified photoanodes and alginate hydrogel (AH)-assisted electrolytes, a champion power conversion efficiency (PCE) of 8.87% for model ZnCuInSe (ZCISe) based QS-QDSCs was achieved with an enhancement of near 10% related to that of the devices based the pristine devices (8.01%), and very close to that of liquid-junction QDSCs (9.06%). Moreover, the constructed QS-QDSCs devices exhibit the excellent stability. This work thus provides a facile and effective strategy to achieve the QS-QDSCs devices with both of high efficiency and good stability.
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As an essential working part in quantum dots sensitized solar cells (QDSCs), the counter electrode (CE) performs the function of extracting electrons form external circuit and catalyzing the reduction of electrolyte. To improve photovoltaic performance of cells, a new novel composite CE composed of graphitic C3N4 (g-C3N4) nanosheets and CuS nanoparticles is prepared on FTO glass via chemical bath deposition method. CuS nanoparticles can be successfully deposited on g-C3N4 nanosheets according to XRD, EDX, SEM and TEM results. A fast electron transport network can be constructed in 3D architecture formed with g-C3N4 nanosheets and CuS nanoparticles, which can afford multi-direction channels for electron transport and accessible catalytic active sites for reduction of polysulfide electrolyte. The synergistic effect of composite CE with continuous conductive network structure shows the excellent catalytic activity and fast electron mobilization. Electrochemical impedance spectrum reveals that the electrochemical property of g-C3N4/CuS composite CE in QDSCs has been changed significantly with the different composition of g-C3N4 and CuS, which is utilized to analyze why QDSC based on CN/30CuS CE shows the best photovoltaic properties of PCE of 5.10%. The new g-C3N4/CuS composite CE with the merits of low cost, easy processing and considerable photovoltaic performance is beneficial to large scale commercial use of three-generation solar cells.
Article
Developing cost-effective solid-state electrolyte is a urgent issue and a common request for (photo) electrochemical cells. In this work, a series of organic ionic conductors based on the S-substituted benzothiophenium salts have been synthesized via a simple and low-cost method. Research on the thermal behavior, optical absorption and ionic conductivity shows that S-methylbenzothiophenium tetrafluoroborate ([MBT]BF4) among them have the good thermal stability, high ionic conductivity and no absorption in the visible region. And the solid-state electrolyte using [MBT]BF4 as the matrix exhibits the good interfacial compatibility with photoanode. Applying this solid-state electrolyte into the quantum-dot sensitized solar cell (QDSSCs), the device achieves an open circuit voltage of 0.71 V, a short circuit current density of 20.73 mW cm⁻², and a power conversion efficiency of 5.49% which is higher than those previously reported for the solid-state QDSSCs.
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Bandgap-tunable alloyed CdS0.12Se0.88 quantum dot (QD) sensitizers on TiO2 photoanode with ZnSe/ZnS passivation layer (denoted as CSS(ZSS)) was successfully synthesized for the first time by a facile one-pot successive ionic layer adsorption and reaction (SILAR) process from a cationic solution containing Zn2+ and Cd2+, and an anionic solution containing Se2- and S2-. A high power conversion efficiency (PCE) up to 6.14% (Jsc=20.4 mA/cm2, Voc=578 mV, FF=0.52) was achieved, which is almost doubled the efficiency of 3.40 % for the quantum dot sensitized solar cells (QDSCs) without Zn2+ feeding in the cationic solution. The results indicated that the light absorbance enhancement as well as the optical bandgap variation (from 2.13 eV to 1.89 eV) effectively promoted the light harvesting, leading to an increased photocurrent density. A careful control of the molar ratio of Zn/Cd by the SILAR cycles played a vital role determining Jsc and Voc, and the possible explanation and mechanisms are discussed. The charge recombination at the interface between QDs and electrolyte was also elaborated.
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In the present study, we synthesized TiO2 nanofibers (NFs) by electrospinning technique and they were subject to solvosonication process using glycerol as a pore forming agent to produce porous TiO2 NFs. The prepared porous TiO2 NFs are seen to improve the light harvesting capability as a result of enhanced light scattering inside the TiO2 NFs and offer a high surface area for maximum adsorption of pre-synthesized CdSe (∼4 nm) QDs. The FE-SEM and BET analysis were performed to confirm the surface texture and surface area of porous TiO2 NFs, respectively. Finally, QDSSCs were fabricated using these porous TiO2 NFs sensitized with CdSe QDs as the photoanode, Cu2S nanoparticles as the counter electrode and polysulfide redox couple (S²⁻/Sx²⁻) as the electrolyte. The porous TiO2 NFs obtained by solvosonication at the time duration of 90 min has enhanced photocurrent density (Jsc) of 9.21 mA/cm² with high power conversion efficiency (η) of 2.15% than the conventional TiO2 NFs (η ≈ 1.50%).
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Alloyed structures of quantum dot light-harvesting materials favor the suppression of unwanted charge recombination as well as acceleration of the charge extraction and therefore the improvement of photovoltaic performance of the resulting solar cell devices. Herein, the advantages of Zn–Cu–In–S (ZCIS) alloy QD serving as light-harvesting sensitizer materials in the construction of quantum dot-sensitized solar cells (QDSCs) were compared with core/shell structured CIS/ZnS, as well as pristine CIS QDs. The built QDSCs with alloyed Zn–Cu–In–S QDs as photosensitizer achieved an average power conversion efficiency (PCE) of 8.47% (Voc = 0.613 V, Jsc = 22.62 mA cm⁻², FF = 0.610) under AM 1.5G one sun irradiation, which was enhanced by 21%, and 82% in comparison to those of CIS/ZnS, and CIS based solar cells, respectively. In comparison to cell device assembled by the plain CIS and core/shell structured CIS/ZnS, the enhanced photovoltaic performance in ZCIS QDSCs is mainly ascribed to the faster photon generated electron injection rate from QD into TiO2 substrate, and the effective restraint of charge recombination, as confirmed by incident photon-to-current conversion efficiency (IPCE), open-circuit voltage decay (OCVD), as well as electrochemical impedance spectroscopy (EIS) measurements.
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A methylcellulose–polysulfide gel polymer electrolyte has been prepared for application in quantum dot-sensitized solar cells (QDSSCs) having the configuration FTO/TiO2/CdS/ZnS/SiO2/electrolyte/Pt(cathode). The electrolyte with the composition of 30.66 wt.% methylcellulose, 67.44 wt.% Na2S, and 1.90 wt.% sulfur exhibits the highest conductivity of 0.183 S cm−1 with the lowest activation energy of 6.14 kJ mol−1. CdS quantum dot sensitizers have been deposited on TiO2 film via the successive ionic layer absorption and reaction (SILAR) method. The QDSSC fabricated using the highest conducting electrolyte and CdS QD prepared with five SILAR cycles exhibits a power conversion efficiency (PCE) of 0.78%. After deposition of zinc sulfide (ZnS) and silicon dioxide SiO2 passivation layers, the PCE of the QDSSC with photoanode arrangement of TiO2/CdS(5)/ZnS(2)/SiO2 increased to 1.42%, an improvement in performance by 82%.
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A high efficiency quantum dot sensitized solar cell (QDSC) based on Ag NPs decorated TiO2/ZnO nanorod arrays (NAs) photoelectrode has been constructed. Not only does the incorporation of Ag NPs to TiO2/ZnO NAs photoelectrode increase light harvesting efficiency and facilitate exciton dissociation, but also decrease surface charge recombination and prolong electron lifetime, which collectively contribute to improving the Jsc of the CdS/CdSe QDs co-sensitized solar cells. The direct contact of Ag NPs with TiO2 NPs is undergoing Fermi level alignment, thus the apparent Fermi level is supposed to trigger an upward shift of more negative potential, which result in an increase the Voc of the QDSCs. As a result, the power conversion efficiency of the QDSCs with Ag NPs decorated TiO2/ZnO NAs photoelectrode reached 5.92%, which is higher than the efficiency of 4.80% for the solar cells without Ag NPs or a ~22 % enhancement.
Article
Colloidal quantum dots (QDs) are semiconductor nanocrystals which exhibit discrete energy levels. They are promising building blocks for optoelectronic devices, thanks to their tunable band structure. Here, we explore a nanoengineering approach to highlight the influence of an alloyed interface on the optical and electronic properties of CdSe/(CdS)6 “giant” core/shell (CS) QDs by introducing CdSexS1-x interfacial layers between core and shell. By incorporating of CdSexS1-x interfacial layers, CdSe/(CdSexS1-x)4/(CdS)2 (x = 0.5) core/shell (CSA1) QDs exhibit a broader absorption response towards longer wavelength and higher electron-hole transfer rate due to favorable electronic band alignment with respect to CS QDs, as confirmed by optical absorption, photoluminescence (PL) and transient fluorescence spectroscopic measurements. In addition, simulations of spatial probability distributions show that the interface layer enhances electron-hole spatial overlap. As a result, CSA1 QDs sensitized solar cells (QDSCs) yield a maximum photoconversion efficiency (PCE) of 5.52%, which is 79% higher than QDSCs based on reference CS QDs. To fully demonstrate the structural interface engineering approach, the CdSexS1-x interfacial layers were further engineered by tailoring the selenium (Se) and sulfur (S) molar ratios during in situ growth of each interfacial layer. This graded alloyed CdSe/(CdSexS1-x)5/(CdS)1 (x = 0.9–0.1) core/shell (CSA2) QDs show a further broadening of the absorption spectrum, higher carrier transport rate and modified confinement potential with respect to CSA1 QDs as well as reference CS QDs, yielding a PCE of 7.14%. Our findings define a promising approach to improve the performance of QDSCs and other optoelectronic devices based on CS QDs.
Article
Quantum dot-sensitized solar cells (QDSCs) have emerged as a promising candidate for next-generation solar cells due to the distinct optoelectronic features of quantum dot (QD) light-harvesting materials, such as high light, thermal, and moisture stability, facilely tunable absorption range, high absorption coefficient, multiple exciton generation possibility, and solution processability as well as their facile fabrication and low-cost availability. In recent years, we have witnessed a dramatic boost in the power conversion efficiency (PCE) of QDSCs from 5% to nearly 13%, which is comparable to other kinds of emerging solar cells. Both the exploration of new QD light-harvesting materials and interface engineering have contributed to this fantastically fast improvement. The outstanding development trend of QDSCs indicates their great potential as a promising candidate for next-generation photovoltaic cells. In this review article, we present a comprehensive overview of the development of QDSCs, including: (1) the fundamental principles, (2) a history of the brief evolution of QDSCs, (3) the key materials in QDSCs, (4) recombination control, and (5) stability issues. Finally, some directions that can further promote the development of QDSCs in the future are proposed to help readers grasp the challenges and opportunities for obtaining high-efficiency QDSCs.
Article
Quantum dot (QD) sensitization of TiO2 is a powerful method to improve its performance as a photoanode material in solar energy conversion. The efficiency of sensitization depends strongly on the rate of interfacial electron transfer (ET) from the QDs to TiO2. To understand the key factors affecting the ET, arene substituted (ortho, meta and para) bifunctional linkers with single or double aromatic rings were employed to link CdSe QDs to TiO2 and control the strength of their interaction as well as the rate of interfacial ET. Interestingly, the para-substituted aromatic linker, 4-mercaptophenylacetic acid (4MBA) with the longest distance between the carboxyl and thiol groups, shows the best photoelectrochemical (PEC) performance, when compared to ortho-(2-mercaptophenylacetic acid, 2MBA), meta-substituted aromatic linkers (3-mercaptophenylacetic acid, 3MBA). Two other bifunctional linkers with double aromatic rings, 4’-mercapto-[1,1’-biphenyl]-4-carboxylic acid (4M1B4A) and 6-mercapto-2-naphthioc acid (6M2NA), were also studied for comparison. Ultrafast transient absorption (TA) spectroscopy was used to study the exciton dynamics in CdSe QDs and determine the interfacial ET rate constant (kET). The kET results are consistent with the trend of PEC measurements in that 4MBA shows the highest kET. To gain further insight into the ET mechanism, we performed density functional theory (DFT) calculations to examine the intrinsic properties of the linkers. The results revealed that the favorable wave function distribution of the molecular orbitals of 4MBA and 4M1B4A are responsible for the higher interfacial ET rate and PEC performance due to better interfacial coupling, a factor that dominates over distance. The present study provides important new insight into the mechanism of interfacial ET using aromatic bifunctional linkers, which is useful in designing QD sensitized semiconductor metal oxide nanostructures for applications including photovoltaics and solar fuel generation.
Article
Full‐spectrum solar energy utilization is the ultimate goal of high‐performance photovoltaic devices. However, the present approaches to enhance sunlight harvesting in the cost‐effective quantum dot–sensitized solar cells mainly focus on the use of high‐frequency photons with the long‐wavelength sunlight being left behind. Here, a full‐spectrum solar cell architecture is proposed and the near‐infrared light–enhanced cell performance is demonstrated with a plasmonic and electrocatalytic dual‐function CuS nanostructure electrode. In the CdS/CdSe quantum dot–sensitized solar cells, an enhancement factor as high as 15% in power conversion efficiency is obtained for the device with near‐infrared part of 1‐sun light irradiating from the counter electrode side and ultraviolet–visible part incidence from the photoanode side. Electrochemical characterizations show that the enhanced electrocatalytic activity toward polysulfide reduction is attributed to the better device performance. This may be due to the plasmon‐induced photothermal effect and interfacial energy transfer from the counter electrode under the near‐infrared light, which accelerate the preceding chemical reactions for polysulfide reduction and improve the charge transfer at the electrode–electrolyte interface. This strategy provides an alternative way to achieve a full‐spectrum liquid‐junction solar cell via the integration of plasmon‐enhanced electrocatalysis into photovoltaics. A full‐spectrum solar cell architecture is demonstrated by integrating the near‐infrared light–enhanced electrocatalysis into the ultraviolet–visible light photovoltaics. An enhancement factor as high as 15% in power conversion efficiency is achieved via rational design of a semiconductor–plasmonic and electrocatalytic nanostructure electrode, which is due to the plasmon‐induced photothermal effect and interfacial energy transfer under near‐infrared light excitation.
Article
The unique properties of II-VI semiconductor nanocrystals such as superior light absorption, size dependent optoelectronic properties, solution processability and interesting photophysics prompted quantum dot sensitized solar cells (QDSSCs) as promising candidates for next generation photovoltaic (PV) technology. The QDSSCs have advantages such as low-cost device fabrication, multiple exciton generation and possibilities to push over the theoretical power conversion efficiency (PCE) limit of 32%. In spite of dedicated research efforts to enhance the PCE, optimize individual solar cell components and better understanding of the underlying science, QDSSCs have unfortunately lacked behind promise due to shortcomings in the fabrication process and with the QDs themselves. In this feature article, we briefly discuss the QDSSC concepts and mechanisms of the charge carrier recombination pathways that occur at multiple interfaces viz. (i) metal oxide (MO)/QDs (ii) MO/QDs/electrolyte and (iii) counter electrode (CE)/electrolyte. The so far developed rational strategies to minimize/block these charge recombination pathways are elaborated. The review concludes with a discussion on the present challenges to fabricate efficient devices and the future prospects of QDSSCs.
Article
Quantum dot sensitized solar cells (QDSCs) have been considered as a promising candidate for the low-cost, high efficiency third generation photovoltaic solar cells. In the past few years, QDSCs have witnessed tremendous progress with an increase of power conversion efficiency from less than 5% in 2010 to 12.57% in 2017. Both photoanodes and counter electrodes of QDSCs have been extensively studied and reviewed in previous reports, while little attention has been paid on the electrolyte system. Herein, we present a comprehensive review on recent advances of electrolyte in QDSCs, with a conclusion and future prospects section.
Article
In this paper, a novel kind of functional NH2-rich silica nanoparticle (A-SiO2) as an electrolyte additive is reported, which is employed to assemble high-efficiency quasi-solid-state dye-sensitized solar cells (DSCs) and quantum dot sensitized solar cells (QDSCs), while the additional solidifying character of A-SiO2 makes it superior to the common additives. It is found that the A-SiO2 nanoparticle as an additive for ionic-liquid electrolyte can significantly improve the photovoltaic performance of quasi-solid-state DSCs, especially the open-circuit photovoltage (Voc) and fill factor (FF) through (1) negatively shifting the TiO2 conduction band (CB) edge, (2) effectively facilitating the ions transport and (3) remarkably inhibiting the charge recombination. Notably, DSC fabricated using the A-SiO2 based ionic-liquid gel electrolytes achieves a power conversion efficiency (PCE) of 7.30% under 1 sun illumination (AM 1.5 G, 100 mW cm⁻²), which is higher than that of DSC with the ionic-liquid electrolyte employing N-methylbenzimidazole (NMBI) and Guanidinium thiocyanate (GuNCS) as additives (PCE = 6.23%). Moreover, the A-SiO2 additive is of the universality in organic electrolytes for DSCs and polysulfide electrolytes for QDSCs. The PCE of CdS/CdSe co-sensitized QDSCs using A-SiO2 additives is improved by 34.9% due to the enhancement of short-circuit current density (Jsc) and Voc, resulting in a champion PCE of 7.11%, which is one of the best results for CdS/CdSe co-sensitized QDSCs.
Article
To pursue electron-generation stability with no sacrifice of photovoltaic performance has been a persistent objective for all kinds of solar cells. Here, we demonstrate the experimental realization of this objective by quasi-solid-state quantum dot-sensitized solar cells from a series of conducting gel electrolytes composed of polyacrylamide (PAAm) matrix and conductive polymers [polyaniline (PANi), polypyrrole (PPy) or polythiophene (PT)]. The reduction of Sx²⁻ occurred in both interface and three dimensional framework of conducting gel electrolyte as a result of the electrical conduction of PANi, PPy and PT toward refluxed electrons from external circuit to Pt electrode. The resulting solar cells can yield the solar-to-electrical conversion efficiency of 2.33%, 2.25% and 1.80% for PANi, PPy and PT based gel electrolytes, respectively. Those solar cells possessed much higher efficiency than that of 1.74% based on pure PAAm gel electrolyte owing to the enhanced kinetics for Sx²⁻ ↔ S²⁻ conversion. More importantly, the stability of quasi-solid-state solar cell is significantly advanced, arising from the localization of liquid electrolyte into the three dimensional framework and therefore reduced leakage and volatilization.
Article
Some quantum dot-sensitized solar cells (QDSSCs) were fabricated by doping Al³⁺ ions into the TiO2 and addition of amine (ethylglycinate hydrochloride) to the CdS solution in order to improve their efficiency. Results indicated that the photovoltaic parameters of the optimized cell were boosted compared with those of the reference cell which was free of Al³⁺ QDs and amine indicating 68, 73 and 10.40% improvements in efficiency, current density and open-circuit voltage, respectively. The electrochemical impedance spectra (EIS) proved that the electron life time was the most improved by 1.60 times in the optimized cell (τe = 37.58 ms) compared with that of the cell made only using pure TiO2 (τe = 23.52 ms). Thus, addition of 0.3%wt Al³⁺ as a dopant was appropriate to attain suitable photocurrent efficiency for the QDSSCs because it could be used in a minimum amount to improve the electron transport, drop the recombination and increase the cell efficiency.
Article
Novel approaches to boost quantum dot solar cell (QDSC) efficiencies are in demand. Here, three strategies are used: (i) a hydrothermally synthesized TiO2-multiwalled carbon nanotubes (MWCNTs) composite instead of conventional TiO2, (ii) a counter electrode (CE) that has not been applied to QDSCs till date, namely, tin sulfide (SnS) nanoparticles (NPs) coated over a conductive carbon (C)-fabric, and (iii) a quasi-solid-state gel electrolyte composed of S²⁻, an inert polymer and TiO2 nanoparticles as opposed to a polysulfide solution based hole transport layer. MWCNTs by the virtue of their high electrical conductivity and a suitably positioned Fermi level (below the conduction bands of TiO2 and PbS) allow fast photogenerated electron injection to the external circuit, and this is confirmed by the higher efficiency of 6.3% achieved for a TiO2-MWCNTs/PbS/ZnS based (champion) cell, compared to the corresponding TiO2/PbS/ZnS based cell (4.45%). Nanoscale current map analysis of TiO2 and TiO2-MWCNTs reveal the presence of narrowly spaced highly conducting domains in the latter, which equips it with an average current carrying capability greater by a few orders of magnitude. Electron transport and recombination resistances are lower and higher respectively for the TiO2-MWCNTs/PbS/ZnS cell relative to the TiO2/PbS/ZnS cell, thus leading to a high performance cell. The efficacy of SnS/C-fabric as a CE is confirmed from the higher efficiency achieved in cells with this CE compared to C-fabric based cells. Lower charge transfer and diffusional resistances, slower photovoltage decay, high electrical conductance and lower redox potential impart high catalytic activity to the SnS/C-fabric assembly for sulfide reduction and thus endow the TiO2-MWCNTs/PbS/ZnS cell with a high open circuit voltage (0.9 V) and a large short circuit current density (~20 mA cm⁻²). This study attempts to unravel how simple strategies can amplify QDSC performances.
Article
The undesired charge recombination loss occurring at photoanode/electrolyte interfaces as well as high redox potential of the current commonly used polysulfide redox couple electrolyte restrain the photovoltaic performance, especially the open-circuit potential (Voc), of quantum dot sensitized solar cells (QDSCs). Herein, a valid and facile method to improve the performance of QDSCs is presented by modifying the polysulfide electrolyte with addition of tetraethyl orthosilicate (TEOS). This approach is effective to a series of QDSC systems including the most commonly studied CdSe, CdSeTe, as well as Zn-Cu-In-Se (ZCISe) QDSCs. Experimental results indicate that with the use of 6 vol% TEOS additive in pristine polysulfide electrolyte at staying time of 24 h, a remarkable enhancement of conversion efficiency from 11.75% to 12.34% was obtained in ZCISe QDSCs. This photovoltaic performance is believed to be among the best results for all kinds of QD based solar cells. The intrinsic mechanism for the performance improvement by TEOS additive was verified by electrochemical impedance spectroscopy (EIS) and open-circuit voltage decay (OCVD) measurements.
Article
A novel solid-state electrolyte based on 1,3-dihexylbenzimidazolium ([DHexBIm]) cation combined with the Br-, BF4- or SCN- anions are used in the CdS/CdSe sensitized quantum dot sensitized solar cells (QDSSCs). A power conversion efficiency of 4.26 % was achieved with a fill factor of 56.44 % using the [DHexBIm][SCN]-based electrolyte. X-ray powder diffraction analysis reveals that [DHexBIm][SCN] is crystalline with a much higher symmetry than the other two salts. Differential scanning calorimetry and thermal gravimetric analysis of [DHexBIm][SCN] reveals a solid-solid phase transition at 50°C together with a high thermal stability. Current density-Voltage (J-V) tests, Tafel-polarization plots, cyclic voltammetry curves and electrochemical impedance spectroscopic methods were used to characterize QDSSCs and to probe the mechanism of device performance. These studies show that the good ionic conductivity and the low RCE value lead to the good short-current density (Jsc) of 12.58 mA•cm-2, which is the major effect factor for the high power conversion efficiency observed. In stability test, this device shows excellent long-term stability, maintaining 67% of the initial efficiency after 504 h.
Article
The insulating nature of organic ligands containing long hydrocarbon tail brings forward serious limitations for pre-synthesized quantum dots (QDs) in photovoltaic applications. Replacing the initial organic hydrocarbon chain ligands with simple, cheap, and small inorganic ligands is regarded as an efficient strategy to improve the performance of the resulting solar cells. Herein, thiosulfate (S2O32-), and sulfide (S2-) were employed as ligand exchange reagents to get access to the inorganic ligand S2O32-, and S2- capped CdSe QDs. The obtained inorganic ligand capped QDs together with the initial oleylamine capped QDs were used as light-harvesting materials in the construction of quantum dot sensitized solar cells (QDSCs). Photovoltaic results indicate that thiosulfate capped QDs give excellent power conversion efficiency (PCE) of 6.11% under full one sun illumination, which is remarkably higher than those of sulfide (3.36%) and OAm capped QDs (0.84%), and is comparable to the state-of-art value based on mercaptocarboxylic acid capped QDs. Photoluminescence (PL) decay characterization demonstrates that thiosulfate based QDSCs have a much faster electron injection rate from QD to TiO2 substrate in comparison with those of sulfide and OAm based QDSCs. Electrochemical impedance spectroscopy (EIS) results indicate that higher charge recombination resistance between potoanode and eletrolyte interfaces were observed in the thiosulfate based cells. To our best konwledge, this is the first time for the application of thiosulfate capped QDs in the construction of efficient QDSCs. This will lend a new perspective to improve the photovoltaic performance of QDSCs furthermore.
Article
Quantum dot-sensitized solar cells (QDSCs), as a promising candidate of cost-effective photoelectrochemical solar cells, have attracted much attention due to their characteristic properties such as processability in low cost, feasibility to control light absorption spectrum in a wide region, and possibility of multiple electron generation with a theoretical conversion efficiency up to 44%. QDSCs have analogous structures to dye-sensitized solar cells (DSCs) typically consisting of semiconductor photoanodes sensitized with quantum dots (QDs), redox electrolytes, and counter electrodes (CEs). Much effort has been dedicated to optimizing each component of QDSCs for higher device performance. In this review, recent advances of photoanodes with various architectures, QDs with tunable band gaps, electrolytes in liquid, quasi-solid or solid state, and CEs with great electrocatalytic activity for QDSCs will be highlighted. We aim to elaborate the rational strategies in material design for QDSC applications. Finally, the conclusion and future prospects emphasize the key developments and remaining challenges for QDSCs.
Article
In this article, we report CdS based quantum dot sensitized solar cells (QDSSCs) with methylcellulose–polysulfide gel polymer electrolytes (GPEs) and platinum (Pt), gold (Au) or lead sulphide (PbS) counter electrodes. The optimized GPE has the composition of 30.7 wt.% methylcellulose, 67.4 wt.% Na2S, and 1.9 wt.% sulfur with an ionic conductivity of 0.18 S cm⁻¹ at room temperature. The QDSSCs fabricated with Pt counter electrode exhibits a power conversion efficiency (PCE) of 1.42% with a short-circuit current density (Jsc) of 7.30 mA cm⁻², open circuit voltage (Voc) of 0.56 V and fill factor (FF) of 0.34. When Au is used as the counter electrode, the performance of the cell declined slightly with a PCE of 1.30%, Jsc of 10.59 mA cm⁻², Voc of 0.47 V and FF of 0.26. With PbS as counter electrode, the PCE of the QDSSC enhanced to 2.90% with Jsc of 9.61 mA cm⁻², Voc of 0.60 V and FF of 0.50. This corresponds to an efficiency enhancement of 104%. The good performance of PbS is attributed to the better catalytic activity of PbS for the electron transfer at the counter electrode/electrolyte interface.
Article
Considering the balance of the hole diffusion length and the loading quantity of quantum-dots, the rutile TiO2 nanorod array with the length of 600 nm, the diameter of 20 nm, and the areal density of 500 μm⁻² is successfully prepared by the hydrothermal method using the aqueous grown solution of 38 mM titanium isopropoxide and 6 M hydrochloric acid at 170 °C for 105 min. The compact PbS quantum-dot thin film on the TiO2 nanorod array is firstly obtained by the spin-coating-assisted successive ionic layer absorption and reaction with using 1,2-ethanedithiol (EDT). The result reveals that the strong interaction between lead and EDT is very important to control the crystallite size of PbS quantum-dots and obtain the compact PbS quantum-dot thin film on the TiO2 nanorod array. The all solid-state sensitized solar cell with the combination of the short-length, high-density TiO2 nanorod array and the compact PbS quantum-dot thin film achieves the photoelectric conversion efficiency of 4.10%, along with an open-circuit voltage of 0.52 V, a short-circuit photocurrent density of 13.56 mA cm⁻² and a fill factor of 0.58.
Article
The rapid development of modern electronics has given rise to a higher demand for flexible and wearable energy sources. Flexible transparent conducting electrodes (TCEs) are one of the essential components of flexible/wearable thin-film solar cells (SCs). In this regard, we present highly transparent and conducting CuS-nanosheet (NS) networks with an optimized sheet resistance (Rs) as low as 50 Ω sq(-1) at 85% transmittance as a counter electrode (CE) for flexible quantum-dot solar cells (QDSCs). The CuS NS network electrode exhibits remarkable mechanical flexibility under bending tests compared to traditional ITO/plastic substrates and sputtered CuS films. Herein, CuS NS networks not only served as conducting films for collecting electrons from the external circuit, but also served as superior catalysts for reducing polysulfide (S(2-)/Sx(2-)) electrolytes. A power conversion efficiency (PCE) up to 3.25% was achieved for the QDSCs employing CuS NS networks as CEs, which was much higher than those of the devices based on Pt networks and sputtered CuS films. We believe that such CuS network TCEs with high flexibility, transparency, conductivity and catalytic activity could be widely used in making wearable electronic products.
Article
The exploration of catalyst materials for counter electrode (CE) in quantum dot sensitized solar cells (QDSCs) that have both high electrocatalytic activity and low charge transfer resistance is always significant yet challenging. In this work, we report the incorporation of nitrogen heteroatoms into carbon lattices leading to nitrogen doped mesoporous carbon (N-MC) materials with superior catalytic activity when used as CE in Zn-Cu-In-Se QDSCs. A series of N-MC materials with different nitrogen contents were synthesized by colloidal silica nanocasting method. Electrochemical measurements revealed that the N-MC with nitrogen content of 8.58 wt % exhibited the strongest activity in catalyzing the reduction of polysulfide redox couple (Sn2-/S2-) and therefore the corresponding QDSC device showed the best photovoltaic performance with an average power conversion efficiency (PCE) of 12.23% and a certified PCE of 12.07% under one full sun illumination, which is a new PCE record for quantum dot based solar cells.
Article
Quantum dot sensitized solar cell (QDSSC) has been considered as a promising candidate for the low-cost third-generation photovoltaics due to the unique optoelectronic properties of quantum dot light absorbers. Over the past years, QDSSCs have witnessed tremendous progress with a rapid rising of the power conversion efficiency from sub-5% in 2010 to 11.6% in 2016. Herein, we present a comprehensively review on the recent progresses in QDSSCs with an emphasis on the design and fabrication of three-dimensional (3D) nanostructured electrodes for efficient photoanodes and counter electrodes (CEs). By increasing QD loading at photoanode and catalyst loading at CEs, enlarging solid-liquid interface to reduce charge transfer resistance, facilitating charger transport and mass transfer, and enhancing the light harvesting, 3D nanostructured electrodes have demonstrated their promising potentials for the construction of efficient photoanodes and CEs. Together with the efforts on the surface engineering to inhibit the interfacial charge recombination and the exploration of efficient quantum dot absorbers and electrolytes, such 3D nanostructured photoanodes and CEs will open up great opportunities to achieve high-performance QDSSCs with industrially appealing PCEs and stability for practical applications.
Article
It reported a novel and simple method for the first time to prepare TiO2 hierarchical porous film (THPF) using ultrastable foams as a soft template to construct porous structures. Moreover, dodecanol as one foam component was creatively used as solvent during the synthesis of CdSe quantum dots (QDs) to decrease reaction temperature and simplify precipitation process. The result showed that hierarchical pores in scale of microns introduced by foams were regarded to benefit for high coverage and unimodal distribution of QDs on the surface of THPF to increase the efficiencies of light-harvesting, charge-collection and charge-transfer. The increased efficiencies caused an enhancement in quantum efficiency of the cell and thus remarkably increased the short circuit current density (Jsc). In addition, the decrease of charge recombination resulted in the increase of the open circuit voltage (Voc) as well. The QDSSC based on THPF exhibited about 2-fold higher power conversion efficiency (η = 2.20%, Jsc = 13.82 mA cm⁻², Voc = 0.572 V) than that of TiO2 nanoparticles film (TNF) (η = 1.06%, Jsc = 6.70 mA cm⁻², Voc = 0.505 V). It provided a basis to use foams both as soft template and carrier to realize simultaneously construction and in-situ sensitization of photoanode in further work.
Article
Increasing QD loading amount on photoanode and suppressing charge recombination are prerequisite for high efficiency quantum dot sensitized solar cells (QDSCs). Herein, a facile technique for enhancing loading amount of QDs on photoanode and therefore improving the photovoltaic performance of the resultant cell devices is developed by pipetting metal salt aqueous solutions on TiO2 film electrode and then evaporating at elevated temperature. The effect of different metal salt solutions were investigated and experimental results indicated that the isoelectric point (IEP) of metal ions influenced the loading amount of QDs and consequently the photovoltaic performance of the resultant cell devices. The influence of anions was also investigated, and the results indicated that anions of strong acid made no difference, while acetate anion hampered the performance of solar cells. Infrared spectroscopy confirmed the formation of oxyhydroxides, whose behavior was responsible for QD loading amount and thus solar cell performance. Suppressed charge recombination based on Mg2+ treatment under optimal conditions was confirmed by impedance spectroscopy as well as transient photovoltage decay measurement. Combined with high QD loading amount and retarded charge recombination, the champion cell based on Mg2+ treatment exhibited an efficiency of 9.73% (Jsc = 27.28 mA/cm2, Voc = 0.609 V, FF = 0.585) under AM 1.5 G full one sun irradiation. The obtained efficiency was one of the best performances for liquid-junction QDSCs, which exhibited a 10% improvement over the untreated cells with the highest efficiency of 8.85%.
Article
CdTe quantum dots (QDs) with a narrow band gap and high conduction band (CB) edge provide great potential for the fabrication of QD sensitized solar cells (QDSCs). Herein, a convenient aqueous route was adopted to synthesize CdTe core QDs for the construction of type-II core/shell QD sensitizers. First, mercaptopropionic acid (MPA) capped water-soluble CdTe QDs were synthesized in aqueous media. After tethering the CdTe QDs onto a mesoporous TiO2 photoanode, CdTe/CdS and CdTe/CdSexS1-x type-II core/shell QD sensitizers were formed by post-depositing CdS and CdSexS1-x shell materials over the photoanodes via a successive ionic layer adsorption and reaction processes (SILAR), respectively. A wider light harvesting range together with a wider photoelectronic response range was observed in the resultant CdTe/CdS and CdTe/CdSexS1-x core/shell QD based solar cells relative to plain CdTe cells. Simultaneously, suppressed charge recombination processes has also been confirmed by impedance spectroscopy (IS), and open-circuit voltage decay (OCVD) characterizations. Consequently, compared to the plain CdTe QDSCs, the power conversion efficiencies (PCEs) for CdTe/CdS and CdTe/CdSeS were enhanced by 22 and 35%, respectively. With the optimization of CdSexS1-x shell thickness, a PCE of 7.24% under the illumination of one full sun was obtained.
Article
Over the past few decades, various types of solar cells provided alternative ways for solar energy conversion. Among them, quantum dot-sensitized solar cells (QDSCs) have gained significant interest due to the advantages of quantum dots (QDs) including easy fabrication, multiple exciton generation, band-gap energy controllability and high absorption coefficient. A QDSC consists of a metal oxide photoanode, QDs, electrolyte and a counter electrode (CE). In comparison with the photoanode and QDs, the CE has not been paid much attention. As an essential part of QDSCs, the CE plays an important role in the charge transport and collection of the device. Here, the recent progress in the development of CEs is reviewed, and the key issues for the materials, structures and performance evaluation of CEs are also addressed.
Article
For quantum dot sensitized solar cells (QDSCs), optimizing interfacial structures by specifically developing new interfacial modification methods to minimize recombination associated with photo-generated carrier transportation and collection is an effective way to achieve highly efficient devices. In this respect, fumed SiO2 nanoparticles have been used as a polysulfide electrolyte additive for improving the interface in a TiO2/QDs/electrolyte for the first time. It was found that the fill factor (FF), open-circuit voltage (Voc) and the cell performance of the devices are significantly enhanced. Additionally, a power conversion efficiency (PCE) of 11.23% is achieved, which is one of the highest efficiencies for liquid-junction QDSCs. Furthermore, the electron transport and recombination processes in the CdSexTe1-x QDSCs with the SiO2 modified electrolyte have been investigated. This revealed that the existence of SiO2 nanoparticles in the electrolyte can create an energy barrier for the recombination between photo-generated electrons from the QDs as well as the recombination between the electrolyte and the injected electrons from TiO2. It is encouraging that CdSexTe1-x QDSCs in SiO2 modified electrolytes can reach a higher electron collection efficiency (98%) and longer electron lifetime. This work provides a simple and convenient method to modify the TiO2/QDs/electrolyte interfaces of QDSCs.
Article
Although the highest power conversion efficiency (PCE) of quantum dot sensitized solar cells (QDSCs) has been continuously updated recently, the stability limited by the volatilization and the leakage of liquid electrolytes remain main challenges for the application of QDSCs. To construct a quasi-solid-state QDSC, a new gel electrolyte was developed by solidifying a polysulfide aqueous solution using sodium carboxymethylcellulose (CMC-Na) with superabsorbent and water-holding capability. With a high mobility of ions in the three dimensional porous network provided, the gel electrolyte obtained exhibits a satisfactory conductivity compared to common liquid polysulfide electrolytes. Through the strong coordination between carboxylate groups on CMC-Na polymer chains and metal ions of the photoanode surface, the gel electrolyte prepared also shows good contact with the surfaces of mesoporous photoanodes. As a result, an impressive PCE of 9.21%, among the highest efficiency of quasi-solid-state and solid-state QDSCs reported, was obtained for gel electrolyte based QDSCs with CdSeTe as the photosensitizer and Cu2S as the counter electrode. Furthermore, the stability of the resultant QDSC device is improved significantly.
Article
Colloidal copper indium sulfide (CIS) nanocrystals (NCs) are Pb- and Cd-free alternatives for use as absorbers in quantum dot solar cells. In a heterojunction with TiO2, non-annealed ligand-exchanged CIS NCs form solar cells yielding a meager power conversion efficiency (PCE) of 0.15%, with photocurrents plummeting far below predicted values from absorption. Decreasing the amount of zinc during post-treatment leads to improved mobility but marginal improvement in device performance (PCE = 0.30%). By incorporating CIS into a porous TiO2 network, we saw an overall drastic improvement in device performance, reaching a PCE of 1.16%, mainly from an increase in short circuit current density (Jsc) and fill factor (FF) and a 10-fold increase in internal quantum efficiency (IQE). We have determined that by moving from a bilayer to a bulk heterojunction architecture, we have reduced the trap-assisted recombination as seen in changes in the ideality factor, the intensity dependence of the photocurrent and transient photocurrent (TPC) and photovoltage (TPV) characteristics.
Article
In this study, the graphene sheets produced by supercritical CO2 exfoliation of graphite were used to improve the photovoltaic performance of the CdS quantum dot-sensitized solar cells (QDSSCs). The zinc titanium mixed metal oxides (MMO) based on layered double hydroxide (LDH) precursor and the graphene/MMO hybrid materials were used as photoanodes of the CdS QDSSCs, respectively. The presence of graphene in the photoanodes was confirmed by Raman spectroscopy, X-ray diffraction (XRD) and Energy-dispersive X-ray spectroscopy (EDS). The influence of graphene concentration on the performance of CdS QDSSCs was studied by electrochemical method. The addition of graphene enhanced QDs adsorption properties and lowered internal resistance, so the QDSSCs displayed higher power conversion efficiency (PCE). Accordingly, the highest PCE of the QDSSCs based on graphene/Zn-Ti MMO photoanode reached 0.44% and increased 37.5% in compared with that based on plain Zn-Ti MMO working electrodes.
Article
In this manuscript, a novel hierarchical ZnO nanosheets branched electrospun TiO2 non-woven fabric film is prepared and used as a photoanode for quantum dot-sensitized solar cells (QDSSCs). Porous ZnO nanosheets grown on a TiO2 nanofiber are synthesized via simple hydrothermal reaction followed by a calcination process, which is proven to provide both large surface areas for QDs loading and superior light-scattering capability. Detailed photoelectrochemical measurements, including UV-vis diffuse reflectance and absorption spectra, electrochemical impedance spectra (EIS), intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS), etc., reveal that, compared with a bare TiO2 nanofiber film, the hierarchical fibrous film exhibits enhanced light-harvesting efficiency, suppressed electron recombination and superior charge-collection efficiency, thus leading to both improved short-circuit current density (Jsc) and open-circuit voltage (Voc). Therefore, the conversion efficiency of the solar cell is greatly improved from 2.37% for a bare TiO2 nanofiber film to 3.05%. A Cu2S counter electrode can help to yield a conversion efficiency as high as 4.21%.
Article
The mean power conversion efficiency (PCE) of quantum-dot-sensitized solar cells (QDSCs) is mainly limited by the low photovoltage and fill factor (FF), which are derived from the high redox potential of polysulfide electrolyte and the poor catalytic activity of the counter electrode (CE), respectively. Herein, we report that this problem is overcome by adopting Ti mesh supported mesoporous carbon (MC/Ti) CE. The confined area in Ti mesh substrate not only offers robust carbon film with submillimeter thickness to ensure high catalytic capacity, but also provides an efficient three-dimension electrical tunnel with better conductivity than state-of-art Cu2S/FTO CE. More importantly, the MC/Ti CE can down shift the redox potential of polysulfide electrolyte to promote high photovoltage. In all, MC/Ti CEs boost PCE of CdSe0.65Te0.35 QDSCs to a certified record of 11.16% (Jsc = 20.68 mA/cm(2), Voc = 0.798 V, FF = 0.677), an improvement of 24% related to previous record. This work thus paves a way for further improvement of performance of QDSCs.
Article
Quantum dot (QD) has emerged as a promising agent in the field of solar energy conversion due to its distinct size-dependent optoelectronic characteristics. As next generation solar cell having facile and low cost fabrication techniques, quantum dot sensitized solar cell (QDSSC) has great potential to meet global demand for clean energy. In this type of solar cell architecture, high performance is expected due to multiple exciton generation effect and tuning of energy band gap in QD. As reported photo conversion efficiency of QDSSC is still less than dye sensitized solar cells, more research on optimization of material selection and material engineering is required. In this review paper, structure & working of QDSSC along with interfacial charge transportation mechanism is presented. Various fabrication techniques related to QDSSCs have been briefly described. This article focuses on recent advances in photoanode of QDSSC. It includes various aspects of photoanode such as structural, morphological effect, carbon related materials, doping effect in metal oxides, alternative photo-anodic materials, surface treatment, blocking layer, new types of sensitizer etc. Issues related to photoanode corrosion and stability are reviewed. Limitations and future prospects have been discussed to fabricate more efficient and stable quantum dot sensitized solar cell.
Article
Two-dimensional (2D) anatase TiO2 nanosheets (TiO2-NSs) with exposed (001) crystal planes were obtained via a simple one-pot hydrothermal route. And they were utilized to fabricate efficient CdSe quantum dot sensitized solar cells (QDSSCs) for the first time. A power conversion efficiency (PCE) of 5.01% was attained with the TiO2-NS based cell, which is 63% higher than that of the TiO2 nanoparticle (TiO2-NP) based cell under the same conditions. The increased photovoltaic performance mainly profits from more quantum-dot (QD) loading onto TiO2-NSs due to large pore size and the strong absorption ability with the carboxylate linker of colloidal quantum dots capped using bifunctional linker molecules in the TiO2-NSs with exposed (001) crystal planes, which lead to an enhancement of light harvesting efficiency and thus significantly enhanced short-circuit photocurrent. Furthermore, the good crystallization, large particle size and low surface area of the TiO2-NSs also result in fewer defects and provide efficient electron transfer pathways and prolong electron lifetime. Meanwhile, the large pore size of the TiO2-NS photoanode will also diminish the infiltration resistance of the electrolyte, which is helpful to regenerate the quantum-dots (QDs). The excellent properties of the 2D anatase TiO2 nanosheet (TiO2-NS) with exposed (001) crystal planes make it a promising candidate as a photoanode material for highly efficient QDSSCs.