Performance Of Quantum Dot Solar Cells Research Articles

Tailoring Energy Structure of Low‐Toxic Ternary Ag−Bi−S Quantum Dots through Solution‐Phase Synthesis for Quantum‐Dot‐Sensitized Solar Cells

AbstractLow‐toxic multinary semiconductor quantum dots (QDs) showing a photoresponse in a wide wavelength range from visible to near‐IR wavelength regions have been intensively investigated for fabricating efficient solar light conversion systems. Recently, AgBiS2 QDs have attracted much attention for their application to improve the performance of QD solar cells because they have a large absorption coefficient in the visible and near‐IR regions. In this report, we describe solution‐phase preparation of ternary Ag−Bi−S QDs with different sizes and with different Ag/Bi ratios. The average size of resulting spherical Ag−Bi−S QDs was controllable from 2.7 to 8.1 nm by adjusting the reaction temperature from 373 to 473 K, while the Ag fraction of obtained QDs also increased with an increase in particle size. The absorption onset wavelength shifted from 850 to 1200 nm in the near‐IR region as the particle size increased. AgBiS2 QDs with an almost stoichiometric composition, obtained at reaction temperatures of 423 K and 473 K, contained no deep intragap states and exhibited a p‐type semiconductor behavior, while QDs prepared at a reaction temperature of 393 K or lower had non‐stoichiometric Ag‐deficient compositions producing intragap defect states. Sensitized solar cells fabricated with stoichiometric AgBiS2 QDs exhibited a photoresponse in visible and near‐IR wavelength regions, the optimal PCE being 0.74% with AgBiS2 QDs of 6.2 nm in diameter that were prepared at 423 K.

Band and interface engineering for quantum dot solar cells

<p indent=0mm>Semiconductor quantum dots (QDs) have received tremendous attention for solar cells owing to their excellent physical and chemical properties. In quantum dot solar cells (QDSCs), the low charge separation efficiency and low charge collection efficiency are two critical factors that greatly restrict their photovoltaic performance. Band engineering and interface engineering are of great significance for accelerating the separation of photogenerated electron-hole pairs and improving the efficiency of carrier collection to improve the performance of QDSCs. This article reviews the applications of band engineering and interfaces engineering for performance optimization from the perspectives of materials, films, and devices. Firstly, the features of QDs and the working principle and device structure of QDSCs are introduced. Secondly, based on the latest research progress of QDSCs, we overview the specific strategies of the band and interface engineering as well as their positive roles in improving the device performance. Finally, the correlation between band and interface issues and the further research direction of the band and interface engineering for QDSCs prospects are discussed.

Matrix Manipulation of Directly‐Synthesized PbS Quantum Dot Inks Enabled by Coordination Engineering

AbstractThe direct‐synthesis of conductive PbS quantum dot (QD) ink is facile, scalable, and low‐cost, boosting the future commercialization of optoelectronics based on colloidal QDs. However, manipulating the QD matrix structures still is a challenge, which limits the corresponding QD solar cell performance. Here, for the first time a coordination‐engineering strategy to finely adjust the matrix thickness around the QDs is presented, in which halogen salts are introduced into the reaction to convert the excessive insulating lead iodide into soluble iodoplumbate species. As a result, the obtained QD film exhibits shrunk insulating shells, leading to higher charge carrier transport and superior surface passivation compared to the control devices. A significantly improved power‐conversion efficiency from 10.52% to 12.12% can be achieved after the matrix engineering. Therefore, the work shows high significance in promoting the practical application of directly synthesized PbS QD inks in large‐area low‐cost optoelectronic devices.

Facet Control for Trap‐State Suppression in Colloidal Quantum Dot Solids

AbstractTrap states in colloidal quantum dot (QD) solids significantly affect the performance of QD solar cells, because they limit the open‐circuit voltage and short circuit current. The {100} facets of PbS QDs are important origins of trap states due to their weak or missing passivation. However, previous investigations focused on synthesis, ligand exchange, or passivation approaches and ignored the control of {100} facets for a given dot size. Herein, trap states are suppressed from the source via facet control of PbS QDs. The {100} facets of ≈3 nm PbS QDs are minimized by tuning the balance between the growth kinetics and thermodynamics in the synthesis. The PbS QDs synthesized at a relatively low temperature with a high oversaturation follow a kinetics‐dominated growth, producing nearly octahedral nanoparticles terminated mostly by {111} facets. In contrast, the PbS QDs synthesized at a relatively high temperature follow a thermodynamics‐dominated growth. Thus, a spherical shape is preferred, producing truncated octahedral nanoparticles with more {100} facets. Compared to PbS QDs from thermodynamics‐dominated growth, the PbS QDs with less {100} facets show fewer trap states in the QD solids, leading to a better photovoltaic device performance with a power conversion efficiency of 11.5%.

Plasmonic Effects of Dual-Metal Nanoparticle Layers for High-Performance Quantum Dot Solar Cells

To improve quantum dot solar cell performance, it is crucial to make efficient use of the available incident sunlight to ensure that the absorption is maximized. The ability of metal nanoparticles to concentrate incident sunlight via plasmon resonance can enhance the overall absorption of photovoltaic cells due to the strong confinement that results from near-field coupling or far-field scattering plasmonic effects. Therefore, to simultaneously and synergistically utilize both plasmonic effects, the placement of different plasmonic nanostructures at the appropriate locations in the device structure is also critical. Here, we introduce two different plasmonic nanoparticles, Au and Ag, to a colloidal PbS quantum dot heterojunction at the top and bottom interface of the electrodes for further improvement of the absorption in the visible and near-infrared spectral regions. The Ag nanoparticles exhibit strong scattering whereas the Au nanoparticles exhibit an intense optical effect in the wavelength region where the absorption of light of the PbS quantum dot is strongest. It is found that these dual-plasmon layers provide significantly improved short-circuit current and power conversion efficiency without any form of trade-off in terms of the fill factor and open-circuit voltage, which may result from the indirect contact between the plasmonic nanoparticles and colloidal quantum dot films.

Temperature dependent photovoltaic performance of TiO2/PbS heterojunction quantum dot solar cells

A planner heterojunction quantum dot solar cells (QDSCs) structure of FTO/TiO2/PbS-EMII/PbS-EDT/Au is fabricated via layer-by-layer spin coating method, and then the temperature dependent photovoltaic performance of QDSCs is studied. The results indicate that the environment temperature has great influence on the current density-voltage (J-V) characteristics of quantum dot solar cell. The short-circuit photocurrent density (JSC), open-circuit voltage (VOC) and fill factor (FF) are all increased when the temperature decreases from 353 K to 253 K. A top value of power conversation efficiency (PCE, 9.78%) is obtained for the QDSCs when the environment temperature is lowered to 253 K, with a VOC of 0.63 V, JSC of 33.1 mA/cm2 and FF of 0.47, which is 33% above the PCE at room temperature (7.34%). In conclusion, it is necessary to cool the device for keeping the high efficiency operation of solar cell.

Spray‐Coated Colloidal Perovskite Quantum Dot Films for Highly Efficient Solar Cells

AbstractA fully automated spray‐coated technology with ultrathin‐film purification is exploited for the commercial large‐scale solution‐based processing of colloidal inorganic perovskite CsPbI3 quantum dot (QD) films toward solar cells. This process is in the air outside the glove box. To further improve the performance of QD solar cells, the short‐chain ligand of phenyltrimethylammonium bromide (PTABr) with a benzene group is introduced to partially substitute for the original long‐chain ligands of the colloidal QD surface (namely PTABr‐CsPbI3). This process not only enhances the carrier charge mobility within the QD film due to shortening length between adjacent QDs, but also passivates the halide vacancy defects of QD by Br− from PTABr. The colloidal QD solar cells show a power conversion efficiency (PCE) of 11.2% with an open voltage of 1.11 V, a short current density of 14.4 mA cm−2, and a fill factor of 0.70. Due to the hydrophobic surface chemistry of the PTABr–CsPbI3 film, the solar cell can maintain 80% of the initial PCE in ambient conditions for one month without any encapsulation. Such a low‐cost and efficient spray‐coating technology also offers an avenue to the film fabrication of colloidal nanocrystals for electronic devices.

Surface Plasmon Resonance Enhancement of PbS Quantum Dot-Sensitized Solar Cells

Lead sulfide (PbS)-sensitized quantum dot solar cells (QDSC) were fabricated using TiO2 and TiO2–Au plasmonic nanocomposite films by successive ionic layer adsorption and reaction (SILAR) method. The average size of gold nanoparticles (GNPs) used for fabricating nanocomposite films was ~ 15 nm. Thin plasmonic QDSC, with a film thickness of 10 µm, showed an increase of ~ 11% in photocurrent and ~ 6% in overall energy conversion efficiency compared to the device without GNPs. The improved performance of QDSCs is attributed to the increased absorption due to the plasmonic near-field effects of the incorporated GNPs. High-efficiency PbS/CdS-co-sensitized thick cells with 16 µm bilayer TiO2 also showed improvement in photocurrent and efficiency. The results show that the plasmonic-enhanced absorption can be used to augment efficiency of QDSC devices in much the same fashion as that of dye-sensitized solar cells.

Correlating Charge-Carrier Dynamics with Efficiency in Quantum-Dot Solar Cells: Can Excitonics Lead to Highly Efficient Devices?

The photovoltaic performance of quantum-dot solar cells strongly depends on the charge-carrier relaxation and recombination processes, which need to be modulated in a favorable way to obtain maximum efficiency. Recently, significant efforts have been devoted to investigate the carrier dynamics of nanocrystal sensitizers, both in solution and deposited on TiO2 photoanodes, with the aim to correlate the excitonics with solar-energy conversion efficiency. This Minireview summarizes some proof of the concepts that efficiency can be directly correlated to the exciton dynamics of quantum-dot solar cells. The presented findings are based on CdSeS alloy, CdSe/CdS core/shell, Au/CdSe nanohybrids, and Mn-doped CdZnSSe nanocrystals, where the favourable excitonic processes are optimized to enhance the efficiency. Future prospects and limitations are addressed as well.

Ultrathin ALD coating on TiO2 photoanodes with enhanced quantum dot loading and charge collection in quantum dots sensitized solar cells

TiO2 nanocrystals are widely used in photoanodes for quantum dot solar cells (QDSCs) owing to their chemical stability and suitable energy band structure. However, surface defects and grain boundaries of TiO2 nanocrystals photoanodes allow high surface charge recombination, which limits the performance of QDSCs. In this work, an ultrathin TiO2 layer is introduced to the surface of TiO2 photoanodes by atomic layer deposition (ALD). The ultrathin layer not only reduces the surface defects of TiO2 nanoparticles and strengthens the connection between adjacent nanoparticles to suppress the charge recombination for improving the electron collection efficiency (ηcc), but also increases the surface energy of photoanodes to load more quantum dots (QDs) for enhancing the light harvesting efficiency (LHE). As a result, the solar cell based on CdS/CdSe QDs with ALD treatment exhibits an efficiency of 5.07% that is much higher than that of the cells without modification (4.03%).

Tuning band alignment by CdS layers using a SILAR method to enhance TiO2/CdS/CdSe quantum-dot solar-cell performance.

We report tuning band alignment by optimized CdS layers using a SILAR method to achieve the recorded best performance with about 6% PCE in TiO2/CdS/CdSe QDSSCs. Combining experimental and theoretical studies, we find that a better lattices match between CdS and TiO2 assists the growth of CdSe, and the combined effect of charge transfer and surface dipole moment at the TiO2/CdS/CdSe interface shifts the energy levels of TiO2 upward and increases Voc of the solar cells. More importantly, the band gap of CdS buffer layers is sensitive to the distortion induced by lattice mismatch and numbers of CdS layers. For example, the barrier for charge transfer disappears when there are more than 4 layers of CdS, facilitating the charge injection from CdSe to TiO2.

Nd2(S, Se, Te)3 Colloidal Quantum Dots: Synthesis, Energy Level Alignment, Charge Transfer Dynamics, and Their Applications to Solar Cells

Novel and less toxic quantum dot (QD) semiconductors are desired for developing environmentally benign colloidal quantum dot solar cells. Here, the synthesis of novel lead/cadmium‐free neodymium chalcogenide Nd2(S, Se, Te)3 QDs via solution‐processed method is reported for the first time. The results show that small‐bandgap semiconductor QDs with a narrow size distribution ranging from 2 to 8 nm can be produced, and the wide absorption band can be achieved by the redshift owing to the size quantization effect by controlling the initial loading of chalcogenide precursors. By analyzing the band structure of QDs and the energy level alignment between QDs and TiO2, the influence of energy offset between the conduction band edges of QDs and TiO2 on the charge transfer dynamics and photovoltaic performance of QD solar cells (QDSCs) is investigated. It is revealed that among the three types of QDs studied, Nd2Se3 QDSCs with the smallest energy offset exhibit the best performances and a decent power conversion efficiency of 3.19% is achieved. This work clearly demonstrates the promising potentials of novel rare earth chalcogenide quantum dots in photovoltaic applications.

Passivation Using Molecular Halides Increases Quantum Dot Solar Cell Performance.

A solution-based passivation scheme is developed featuring the use of molecular iodine and PbS colloidal quantum dots (CQDs). The improved passivation translates into a longer carrier diffusion length in the solid film. This allows thicker solar-cell devices to be built while preserving efficient charge collection, leading to a certified power conversion efficiency of 9.9%, which is a new record in CQD solar cells.

InAs/InGaP quantum dot solar cells with an AlGaAs interlayer

We studied the growth of InAs/InGaP quantum dot (QD) solar cells with near ideal bandgaps for intermediate band solar cells. Using a solid-source molecular beam epitaxy system, the evolution of InAs QDs grown on an InGaP buffer layer was examined as a function of InAs coverage and growth temperature. QDs were initiated with defective clusters for InAs deposition as small as 0.3 monolayer. The cross diffusion of In atoms and As–P exchange between InAs and the InGaP buffer layer could be responsible for the formation of coherent QDs and large defective clusters. An AlGaAs interlayer was inserted between InAs and the InGaP buffer layer to prevent the surface exchange reactions between InAs and InGaP. As a result, InAs QDs with a density of 1×1011cm−2 have been demonstrated with stronger photoluminescence emission compared to those with the GaAs interlayer. This growth technique was applied to fabricate InAs/InGaP QD solar cells. Post-growth annealing treatment was also investigated to improve the performance of QD solar cells.

InAs/GaAs quantum dot solar cell with an AlAs cap layer

We report the effects of the deposition of an AlAs cap layer (CL) over InAs quantum dots (QDs) on the performance of QD solar cells (QDSCs). The growth of AlAs CL over InAs QDs led to the elimination of the wetting layer absorption and hence the enhancement of the open-current voltage, Voc, of a 20-layer InAs/GaAs QDSC from 0.69 V to 0.79 V. Despite a slight reduction in short-circuit current, Jsc, for the QDSC with AlAs CL, the enhancement of the Voc is enough to ensure that its efficiency is higher than the QDSC without AlAs CL.

Quantum Dot Solar Cells Prepared Using Electrophoretic Deposition

Quantum dot solar cells (QDSCs) have attracted great interest as a next-generation solar energy conversion system. 1 While the conversion efficiency of QDSCs has recently exceeded 5%, they still lag behind their dye-sensitized solar cell analogues. The performance of QDSCs is governed by many factors such as their light harvesting capacity, interfacial charge recombination, the characteristics of the electrolytes, and the electrocatalytic activity of the counter electrode. 2 Among them, the anode characteristics (i.e., light absorption and charge recombination) are intimately associated with the deposition method of the QD sensitizer. Therefore, the development of an effective deposition method is of great importance for highly efficient solar cell fabrication. Commonly used deposition methods include chemical bath deposition (CBD), successive ionic layer adsorption and reaction (SILAR), a molecular-linker based approach (MBA), and direct adsorption (DA). 3 CBD and SILAR are

On the missing links in quantum dot solar cells: a DFT study on fluorophore oxidation and reduction processes in sensitized solar cells

The possibility of achieving many electrons per absorbed photon of sufficient energy by quantum dots (QDs) drives the motivation to build high performance quantum dot solar cells (QDSCs). Although performance of dye-sensitized solar cells (DSCs), with similar device configuration as that of QDSCs, has significantly improved in the last two decades QDSCs are yet to demonstrate impressive device performances despite the remarkable features of QDs as light harvesters. We investigated the fundamental differences in the optical properties of QDs and dyes using DFT calculations to get insights on the inferior performance of QDSCs. The CdSe QDs and the ruthenium bipyridyl dicarboxylic acid dye (N3) were used as typical examples in this study. Based on a generalized equation of state correlating material properties and photoconversion efficiency, we calculated ground and excited state properties of these absorbers at the B3LYP/lanl2dz level of DFT and analyzed them on the basis of the device performance. Five missing links have been identified in the study which provides numerous insights into building high efficiency QDSCs. They are (i) fundamental differences in the emitting states of the QDs in the strong and weak confinement regimes were observed, which explained successfully the performance differences; (ii) the crucial role of bifunctional ligands that bind the QDs and the photo-electrode was identified; in most cases use of bifunctional ligands does not lead to a QD enabled widening of the absorption of the photo-electrode; (iii) wide QDs size distribution further hinders efficient electron injections; (iv) wide absorption cross-section of QDs favours photon harvesting; and (v) the role of redox potential of the electrolyte in the QD reduction process.

Improving the performance of colloidal quantum-dot-sensitized solar cells

Solar cells based on a mesoporous structure ofTiO2 and the polysulfide redox electrolyte were prepared by direct adsorption of colloidal CdSequantum dot light absorbers onto the oxide without any particular linker. Severalfactors cooperate to improve the performance of quantum-dot-sensitized solarcells: an open structure of the wide bandgap electron collector, which facilitates ahigher covering of the internal surface with the sensitizer, a surface passivation ofTiO2 to reduce recombination and improved counter electrode materials. As aresult, solar cells of 1.83% efficiency under full 1 sun illumination intensityhave been obtained. Despite a relatively large short circuit current (Jsc = 7.13 mA cm−2) and opencircuit voltage (Voc = 0.53 V), the colloidal quantum dot solar cell performance is still limited by a low fill factor of0.50, which is believed to arise from charge transfer of photogenerated electrons to theaqueous electrolyte.