Boosting the Performance of Environmentally Friendly Quantum Dot‐Sensitized Solar Cells over 13% Efficiency by Dual Sensitizers with Cascade Energy Structure

Generally, high light‐harvesting efficiency, electron‐injection efficiency, and charge‐collection efficiency are the prerequisites for high‐efficiency quantum‐dot‐sensitized solar cells (QDSCs). However, it is fairly difficult for a single QD sensitizer to meet these three requirements simultaneously. It is demonstrated that these parameters can be felicitously balanced by a cosensitization strategy through the adoption of environmental‐friendly Zn–Cu–In–Se and Zn–Cu–In–S dual QD sensitizers with cascade energy structure. Experimental results indicate that: i) the combination of the dual QDs can improve the light‐harvesting capability of the cells, especially in the visible light window; ii) the cosensitization approach can facilitate electron injection, benefitting from the cascade energy structure of the two QD sensitizers employed; iii) the charge‐collection efficiency can be remarkably enhanced by the suppressed charge‐recombination process due to the improved QD coverage on TiO₂. Consequently, this cosensitization strategy delivers a new certified efficiency record of 12.98% for liquid‐junction QDSCs under AM 1.5G 1 sun irradiation. Moreover, the constructed cells exhibit good stability in a high‐humidity environment.     

Hydrothermal Growth of TiO2 Nanorod Arrays and In Situ Conversion to Nanotube Arrays for Highly Efficient Quantum Dot‐Sensitized Solar Cells

TiO₂ nanorod (NR) and nanotube (NT) arrays grown on transparent conductive substrates are attractive electrode for solar cells. In this paper, TiO₂ NR arrays are hydrothermally grown on FTO substrate, and are in situ converted into NT arrays by hydrothermally etching. The TiO₂ NR arrays are reported as single crystalline, but the TiO₂ NR arrays are demonstrated to be polycrystalline with a bundle of 2–5 nm single crystalline nanocolumns grown along [001] throughout the whole NR from bottom to top. TiO₂ NRs can be converted to NTs by hydrothermal selective etching of the (001) core and remaining the inert sidewall of (110) face. A growth mechanism of the NR and NT arrays is proposed. Quantum dot‐sensitized solar cells (QDSCs) are fabricated by coating CdSe QDs on to the TiO₂ arrays. After conversion from NRs to NTs, more QDs can be filled in the NTs and the energy conversion efficiency of the QDSCs almost double.     

Fabrication of quantum dot-sensitized solar cells with multilayer TiO2/PbS(X)/CdS/CdSe/ZnS/SiO2 photoanode and optimization of the PbS nanocrystalline layer

In this work, two multilayer photoanode structures of TiO₂/PbS(X)/CdS/ZnS/SiO₂ and TiO₂/PbS(X)/CdS/CdSe/ZnS/SiO₂ were fabricated and applied in quantum dot-sensitized solar cells (QDSCs). Then, the effect of PbS QDs layer on the photovoltaic performance of corresponding cells was investigated. The sensitization was carried out by PbS and CdS QDs layers deposited on TiO₂ scaffold through successive ionic layer adsorption and reaction (SILAR) method. The CdSe QDs film was also formed by a fast, modified chemical bath deposition (CBD) approach. Two passivating ZnS and SiO₂ layers were finally deposited by SILAR and CBD methods, respectively. It was shown that the reference cell with TiO₂/CdS/ZnS/SiO₂ photoanode demonstrated a power conversion efficiency (PCE) of 3.0%. This efficiency was increased to 4.0% for the QDSC with TiO₂/PbS(2)/CdS/ZnS/SiO₂ photoelectrode. This was due to the co-absorption of incident light by low-bandgap PbS nanocrystalline film and also the CdS QDs layer and well transport of the charge carriers. For the CdSe included QDSCs, the PbS-free reference cell represented a PCE of 4.1%. This efficiency was improved to 5.1% for the optimized cell with TiO₂/PbS(2)/CdS/CdSe/ZnS/SiO₂ photoelectrode. The maximized efficiency was enhanced about 25% and 70% compared to the PbS-free reference cells with and without the CdSe QDs layer.

     

CdSe quantum dot-sensitized Au/TiO2 hybrid mesoporous films and their enhanced photoelectrochemical performance

Novel CdSe quantum dot (QD)-sensitized Au/TiO₂ hybrid mesoporous films have been designed, fabricated, and evaluated for photoelectrochemical (PEC) applications. The Au/TiO₂ hybrid structures were made by assembly of Au and TiO₂ nanoparticles (NPs). A chemical bath deposition method was applied to deposit CdSe QDs on TiO₂ NP films with and without Au NPs embedded. We observed significant enhancements in photocurrent for the film with Au NPs, in the entire spectral region we studied (350–600 nm). Incident-photon-to-current efficiency (IPCE) data revealed an average enhancement of 50%, and the enhancement was more significant at short wavelength. This substantially improved PEC performance is tentatively attributed to the increased light absorption of CdSe QDs due to light scattering by Au NPs. Interestingly, without QD sensitization, the Au NPs quenched the photocurrent of TiO₂ films, due to the dominance of electron trapping over light scattering by Au NPs. The results suggest that metal NPs are potentially useful for improving the photoresponse in PEC cells and possibly in other devices such as solar cells based on QD-sensitized metal oxide nanostructured films. This work demonstrates that metal NPs can serve as light scattering centers, besides functioning as photo-sensitizers and electron traps. The function of metal NPs in a particular nanocomposite film is strongly dependent on their structure and morphology.      

Absorption Enhancement in “Giant” Core/Alloyed‐Shell Quantum Dots for Luminescent Solar Concentrator

Luminescent solar concentrators (LSCs) can potentially reduce the cost of solar cells by decreasing the photoactive area of the device and boosting the photoconversion efficiency (PCE). This study demonstrates the application of “giant” CdSe/Cd_xPb_1–xS core/shell quantum dots (QDs) as light harvesters in high performance LSCs with over 1.15% PCE. Pb addition is critical to maximize PCE. First, this study synthesizes “giant” CdSe/Cd_xPb_1–xS QDs with high quantum yield (40%), narrow size distribution (<10%), and stable photoluminescence in a wide temperature range (100–300 K). Subsequently these thick alloyed‐shell QDs are embedded in a polymer matrix, resulting in a highly transparent composite with absorption spectrum covering the range 300–600 nm, and are applied as active material for prototype LSCs. The latter exhibits a 15% enhancement in efficiency with respect to 1% PCE of the pure‐CdS‐shelled QDs. This study attributes this increase to the contribution of Pb doping. The results demonstrate a straightforward approach to enhance light absorption in “giant” QDs by metal doping, indicating a promising route to broaden the absorption spectrum and increase the efficiency of LSCs.     

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

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

Enhanced photovoltaic performance of solar cell based on front-side illuminated CdSe/CdS double-sensitized TiO2 nanotube arrays electrode

Free-standing TiO₂ nanotube (NT) arrays have been prepared by a two-step anodization method. These translucent TiO₂ NT arrays can be transferred to the fluorine-doped tin oxide glass substrates to form front-side illuminated TiO₂ NT electrodes. The TiO₂ NT electrodes were double-sensitized by CdSe/CdS quantum dots (QDs) through successive ionic layer adsorption and reaction (SILAR) process. The absorption range of the TiO₂ NT electrode was extended from ~380 to 700 nm after sensitization with CdSe/CdS QDs. The SILAR cycles were investigated to find out the best combination of CdS and CdSe QDs for photovoltaic performance. The power conversion efficiency of 2.42 % was achieved by the CdSe(10)/CdS(8)/TiO₂ NT solar cell. A further improved efficiency of 2.57 % was obtained with two cycles of ZnS overlayer on the CdSe(10)/CdS(8)/TiO₂ NT electrode, which is 45.19 % higher than that of back-side illuminated solar cell. Furthermore, the ZnS(2)/CdSe(10)/CdS(8)/TiO₂ NT solar cell possesses a higher stability than CdSe(10)/CdS(8)/TiO₂ NT solar cell during the same period. The better photovoltaic performance of the ZnS(2)/CdSe(10)/CdS(8)/TiO₂ NT solar cell has demonstrated the promising value to design quantum dots-sensitized solar cells with double-sensitized front-side illuminated TiO₂ NT arrays strategy.     

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

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

Hydrothermal growth of double-layer TiO2 nanostructure film for quantum dot sensitized solar cells

A double-layer (DL) film with a TiO₂ nanosheet-layer on a layer of TiO₂ nanorod-array, was synthesized on a transparent conductive fluorine-doped tin oxide substrate by a two-step hydrothermal method. Starting from the precursors of NaSeSO₃, CdSO₄ and the complex of N(CH₂COOK)₃, CdSe quantum dots (QDs) were grown on the DL-TiO₂ substrate by chemical bath deposition method. The samples were characterized by X-ray diffraction, Scanning electron microscopy, Energy dispersion spectroscopy, and their optical scattering property was measured by light reflection spectrometry. Some CdSe QDs sensitized DL-TiO₂ films serve as the photoanodes, were assembled into solar cell devices and their photovoltaic performance were also characterized. The short circuit current and open-circuit voltage of the solar cells range from 0.75 to 4.05 mA/cm² and 0.20 − 0.42 V under the illumination of one sun (AM1.5, 100 mW/cm²), respectively. The photocurrent density of the DL-TiO₂ film is five times higher than that of a bare TiO₂ nanorod array photoelectrode cell.     

Bandgap Tuning with Thermal Residual Stresses Induced in a Quantum Dot

Lattice distortion induced by residual stresses can alter electronic and mechanical properties of materials significantly. Herein, a novel way of the bandgap tuning in a quantum dot (QD) by lattice distortion is presented using 4‐nm‐sized CdS QDs grown on a TiO₂ particle as an application example. The bandgap tuning (from 2.74 eV to 2.49 eV) of a CdS QD is achieved by suitably adjusting the degree of lattice distortion in a QD via the tensile residual stresses which arise from the difference in thermal expansion coefficients between CdS and TiO₂. The idea of bandgap tuning is then applied to QD‐sensitized solar cells, achieving ≈60% increase in the power conversion efficiency by controlling the degree of thermal residual stress. Since the present methodology is not limited to a specific QD system, it will potentially pave a way to unexplored quantum effects in various QD‐based applications.     

Ultrafast Flame Annealing of TiO2 Paste for Fabricating Dye‐Sensitized and Perovskite Solar Cells with Enhanced Efficiency

Mesoporous TiO₂ nanoparticle (NP) films are broadly used as electrodes in photoelectrochemical cells, dye‐sensitized solar cells (DSSCs), and perovskite solar cells (PSCs). State‐of‐the‐art mesoporous TiO₂ NP films for these solar cells are fabricated by annealing TiO₂ paste‐coated fluorine‐doped tin oxide glass in a box furnace at 500 °C for ≈30 min. Here, the use of a nontraditional reactor, i.e., flame, is reported for the high throughput and ultrafast annealing of TiO₂ paste (≈1 min). This flame‐annealing method, compared to conventional furnace annealing, exhibits three distinct benefits. First, flame removes polymeric binders in the initial TiO₂ paste more completely because of its high temperature (≈1000 °C). Second, flame induces strong interconnections between TiO₂ nanoparticles without affecting the underlying transparent conducting oxide substrate. Third, the flame‐induced carbothermic reduction on the TiO₂ surface facilitates charge injection from the dye/perovskite to TiO₂. Consequently, when the flame‐annealed mesoporous TiO₂ film is used to fabricate DSSCs and PSCs, both exhibit enhanced charge transport and higher power conversion efficiencies than those fabricated using furnace‐annealed TiO₂ films. Finally, when the ultrafast flame‐annealing method is combined with a fast dye‐coating method to fabricate DSSC devices, its total fabrication time is reduced from over 3 h to ≈10 min.     

Correlation between crystal growth and photosensitization of nanostructured TiO2 electrodes using supporting Ti substrates by self-assembled CdSe quantum dots

Crystal growth of semiconductor quantum dots (QDs) adsorbed on nanostructured TiO₂ electrodes is important not only for crystallographic studies but also for improving the photovoltaic efficiency of semiconductor-sensitized solar cells. In this study, nanostructured TiO₂ electrodes using supporting Ti substrates were prepared. These electrodes are then adsorbed with self-assembled CdSe QDs as photosensitizers to investigate the crystal growth and photoelectrochemical current properties. Average diameters of the CdSe QDs can be estimated from optical absorption spectra by using photoacoustic (PA) technique. PA technique is a powerful tool for evaluating the optical absorption of opaque and scattered samples because of the detection by photothermal phenomenon. When the adsorption time increases, the CdSe QDs diameter increases and then shows saturation for all the cases. Normal solution growth plus suppression (negative growth) contributions can be derived by PA spectroscopic analysis. Both of them depend on adsorption temperatures for CdSe QDs formation. Photosensitization of the nanostructured TiO₂ electrodes in the visible region resulting from CdSe QDs deposition can be clearly observed. Incident photon to current conversion efficiency (IPCE) of CdSe QDs adsorbed at high temperature formation is smaller than that adsorbed at low temperature one, indicating the increase of recombination centers with increasing adsorption temperature. This implies that negative growth, or dissolving effect, produces much more recombination centers inside of CdSe QDs and/or interface between the QDs and TiO₂.     

Hybrid photovoltaic cells with II–VI quantum dot sensitizers fabricated by layer-by-layer deposition of water-soluble components

We present the fabrication of all solid state heterojunction photovoltaic devices consisting of TiO₂ films sensitized by colloidal CdSe and CdTe quantum dots (QDs) and a hole transport layer of the conjugated polymer poly(9,9-dioctyl-fluorene-co-N-(4-butylenphenyl)diphenylamine). The sensitized films were prepared by alternating the layer-by-layer deposition of TiO₂ nanoparticles, water-soluble semiconductor QDs and polycations. Photovoltaic devices sandwiched between fluorinated tin oxide and gold electrodes showed a high rectification ratio and photovoltages of up to 1.15 V. Effective sensitization was observed for CdSe QDs, while incorporated CdTe QDs apparently had no such effect. These findings are explained by confinement effects in QDs.     

Enhancing the Performance of Perovskite Solar Cells by Hybridizing SnS Quantum Dots with CH3NH3PbI3

The combination of perovskite solar cells and quantum dot solar cells has significant potential due to the complementary nature of the two constituent materials. In this study, solar cells (SCs) with a hybrid CH₃NH₃PbI₃/SnS quantum dots (QDs) absorber layer are fabricated by a facile and universal in situ crystallization method, enabling easy embedding of the QDs in perovskite layer. Compared with SCs based on CH₃NH₃PbI₃, SCs using CH₃NH₃PbI₃/SnS QDs hybrid films as absorber achieves a 25% enhancement in efficiency, giving rise to an efficiency of 16.8%. The performance improvement can be attributed to the improved crystallinity of the absorber, enhanced photo‐induced carriers' separation and transport within the absorber layer, and improved incident light utilization. The generality of the methods used in this work paves a universal pathway for preparing other perovskite/QDs hybrid materials and the synthesis of entire nontoxic perovskite/QDs hybrid structure.     

In Situ Passivation for Efficient PbS Quantum Dot Solar Cells by Precursor Engineering

Current efforts on lead sulfide quantum dot (PbS QD) solar cells are mostly paid to the device architecture engineering and postsynthetic surface modification, while very rare work regarding the optimization of PbS synthesis is reported. Here, PbS QDs are successfully synthesized using PbO and PbAc₂ · 3H₂O as the lead sources. QD solar cells based on PbAc‐PbS have demonstrated a high power conversion efficiency (PCE) of 10.82% (and independently certificated values of 10.62%), which is significantly higher than the PCE of 9.39% for PbO‐PbS QD based ones. For the first time, systematic investigations are carried out on the effect of lead precursor engineering on the device performance. It is revealed that acetate can act as an efficient capping ligands together with oleic acid, providing better surface coverage and replace some of the harmful hydroxyl (OH) ligands during the synthesis. Then the acetate on the surface can be exchanged by iodide and lead to desired passivation. This work demonstrates that the precursor engineering has great potential in performance improvement. It is also pointed out that the initial synthesis is an often neglected but critical stage and has abundant room for optimization to further improve the quality of the resultant QDs, leading to breakthrough efficiency.     

Bifacial illuminated PbS quantum dot-sensitized solar cells with translucent CuS counter electrodes

This paper reports the optimization of the TiO₂ photoanode and the fabrication of bifacial illuminated PbS quantum dot-sensitized solar cells (QDSSCs) with translucent CuS counter electrodes. TiO₂ photoanode is prepared by introducing a compact TiO₂ layer between FTO substrate and TiO₂ film with titanium diisopropoxide bis(acetylacetonate) in 1-butanol and post-treatment with TiCl₄ aqueous solution; then, PbS quantum dots (QDs) are deposited on the surface of TiO₂ film by successive ionic layer adsorption and reaction method; also, by means of control of the TiO₂ surface charge, QD density of TiO₂ film is improved by adding triethanolamine into the cationic precursor solution. Using this optimized TiO₂ photoanode and a translucent CuS counter electrode, a bifacial illuminated PbS QDSSC is fabricated. The preparation conditions are optimized and the surface morphology and electrochemical properties of TiO₂ photoanodes are characterized. The bifacial illuminated PbS QDSSC achieves a power conversion efficiency of 2.16 %, which is increased by 48.97 % compared with the single illuminated QDSSC.     

Hierarchical Rutile TiO2 Flower Cluster‐Based High Efficiency Dye‐Sensitized Solar Cells via Direct Hydrothermal Growth on Conducting Substrates

Dye‐sensitized solar cells (DSSCs) based on hierarchical rutile TiO₂ flower clusters prepared by a facile, one‐pot hydrothermal process exhibit a high efficiency. Complex yet appealing rutile TiO₂ flower films are, for the first time, directly hydrothermally grown on a transparent conducting fluorine‐doped tin oxide (FTO) substrate. The thickness and density of as‐grown flower clusters can be readily tuned by tailoring growth parameters, such as growth time, the addition of cations of different valence and size, initial concentrations of precursor and cation, growth temperature, and acidity. Notably, the small lattice mismatch between the FTO substrate and rutile TiO₂ renders the epitaxial growth of a compact rutile TiO₂ layer on the FTO glass. Intriguingly, these TiO₂ flower clusters can then be exploited as photoanodes to produce DSSCs, yielding a power conversion efficiency of 2.94% despite their rutile nature, which is further increased to 4.07% upon the TiCl₄ treatment.     

Highly Efficient Inverted Structural Quantum Dot Solar Cells

Highly efficient PbS colloidal quantum dot (QD) solar cells based on an inverted structure have been missing for a long time. The bottlenecks are the construction of an effective p–n heterojunction at the illumination side with smooth band alignment and the absence of serious interface carrier recombination. Here, solution‐processed nickel oxide (NiO) as the p‐type layer and lead sulfide (PbS) QDs with iodide ligand as the n‐type layer are explored to build a p–n heterojunction at the illumination side. The large depletion region in the QD layer at the illumination side leads to high photocurrent. Interface carrier recombination at the interface is effectively prohibited by inserting a layer of slightly doped p‐type QDs with 1,2‐ethanedithiol as ligands, leading to improved voltage of the device. Based on this graded device structure design, the efficiency of inverted structural heterojunction PbS QD solar cells is improved to 9.7%, one time higher than the highest efficiency achieved before.     

Room‐Temperature Triple‐Ligand Surface Engineering Synergistically Boosts Ink Stability,Recombination Dynamics,and Charge Injection toward EQE‐11.6% Perovskite QLEDs

Developing low‐cost and high‐quality quantum dots (QDs) or nanocrystals (NCs) and their corresponding efficient light‐emitting diodes (LEDs) is crucial for the next‐generation ultra‐high‐definition flexible displays. Here, there is a report on a room‐temperature triple‐ligand surface engineering strategy to play the synergistic role of short ligands of tetraoctylammonium bromide (TOAB), didodecyldimethylammonium bromide (DDAB), and octanoic acid (OTAc) toward “ideal” perovskite QDs with a high photoluminescence quantum yield (PLQY) of >90%, unity radiative decay in its intrinsic channel, stable ink characteristics, and effective charge injection and transportation in QD films, resulting in the highly efficient QD‐based LEDs (QLEDs). Furthermore, the QD films with less nonradiative recombination centers exhibit improved PL properties with a PLQY of 61% through dopant engineering in A‐site. The robustness of such properties is demonstrated by the fabrication of green electroluminescent LEDs based on CsPbBr₃ QDs with the peak external quantum efficiency (EQE) of 11.6%, and the corresponding peak internal quantum efficiency (IQE) and power efficiency are 52.2% and 44.65 lm W^?1, respectively, which are the most‐efficient perovskite QLEDs with colloidal CsPbBr₃ QDs as emitters up to now. These results demonstrate that the as‐obtained QD inks have a wide range application in future high‐definition QD displays and high‐quality lightings.     

Goethite Quantum Dots as Multifunctional Additives for Highly Efficient and Stable Perovskite Solar Cells

Minimization of defects and ion migration in organic–inorganic lead halide perovskite films is desirable for obtaining photovoltaic devices with high power conversion efficiency (PCE) and long‐term stability. However, achieving this target is still a challenge due to the lack of efficient multifunctional passivators. Herein, to address this issue, n‐type goethite (FeOOH) quantum dots (QDs) are introduced into the perovskite light‐absorption layer for achieving efficient and stable perovskite solar cells (PSCs). It is found that the iron, oxygen, and hydroxyl of FeOOH QDs can interact with iodine, lead, and methylamine, respectively. As a result, the crystallization kinetics process can be retarded, thereby resulting in high quality perovskite films with large grain size. Meanwhile, the trap states of perovskite can be effectively passivated via interaction with the under‐coordinated metal (Pb) cations, halide (I) anions on the perovskite crystal surface. Consequently, the PSCs with FeOOH QDs achieve a high efficiency close to 20% with negligible hysteresis. Most strikingly, the long‐term stability of PSCs is significantly enhanced. Furthermore, compared with the CH₃NH₃PbI₃‐based device, a higher PCE of 21.0% is achieved for the device assembled with a Cs_0.05FA_0.81MA_0.14PbBr_0.45I_2.55 perovskite layer.