Surface-Modified Graphene Oxide/Lead Sulfide Hybrid Film-Forming Ink for High-Efficiency Bulk Nano-Heterojunction Colloidal Quantum Dot Solar Cells

Solution-processed colloidal quantum dot solar cells(CQDSCs) is a promising candidate for new generation solar cells.To obtain stable and high performance lead sulfide(PbS)-based CQDSCs,high carrier mobility and low non-radiative recombination center density in the PbS CQDs active layer are required.In order to effectively improve the carrier mobility in PbS CQDs layer of CQDSCs,butylamine(BTA)-modified graphene oxide(BTA@GO) is first utilized in PbS-PbX2(X=I-,Br-) CQDs ink to deposit the active layer of CQDSCs through one-step spin-coating method.Such surface treatment of GO dramatically upholds the intrinsic superior hole transfer peculiarity of GO and attenuates the hydrophilicity of GO in order to allow for its good dispersibility in ink solvent.The introduction of B TA@GO in CQDs layer can build up a bulk nano-heterojunction architecture,which provides a smooth charge carrier transport channel in turn improves the carrier mobility and conductivity,extends the carriers lifetime and reduces the trap density of PbS-PbX2 CQDs film.Finally,the BTA@GO/PbS-PbX2 hybrid CQDs film-based relatively large-area(0.35 cm2) CQDSCs shows a champion power conversion efficiency of 11.7% which is increased by 23.1% compared with the control device.

Higher open-circuit voltage set by cobalt redox shuttle in SnO_2 nanofibers-sensitized CdTe quantum dot solar cells

In this study, we report an efficient CdTe-SnOquantum dot（QD） solar cell fabricated by heat-assisted drop-casting of hydrothermally synthesized CdTe QDs on electrospun SnOnanofibers. The as-prepared QDs and SnOnanofibers were characterized by dynamic light scattering（DLS）, UV–Vis spectroscopy,photoluminescence（PL） spectra, X-ray diffraction（XRD） and transmission electron microscopy（TEM）. The SnOnanofibers deposited on fluorine-doped tin oxide（SnO） and sensitized with the CdTe QDs were assembled into a solar cell by sandwiching against a platinum（Pt） counter electrode in presence of cobalt electrolyte. The efficiency of cells was investigated by anchoring QDs of varying sizes on SnO. The best photovoltaic performance of an overall power conversion efficiency of 1.10%, an open-circuit voltage（Voc）of 0.80 V, and a photocurrent density（JSC） of 3.70 m A/cmwere obtained for cells with SnOthickness of5–6 μm and cell area of 0.25 cmunder standard 1 Sun illumination（100 m W/cm）. The efficiency was investigated for the same systems under polysulfide electrolyte as well for a comparison.

Improving the Performance of PbS Quantum Dot Solar Cells by Optimizing ZnO Window Layer

Comparing with hot researches in absorber layer,window layer has attracted less attention in PbS quantum dot solar cells(QD SCs). Actually, the window layer plays a key role in exciton separation, charge drifting, and so on.Herein, ZnO window layer was systematically investigated for its roles in QD SCs performance. The physical mechanism of improved performance was also explored. It was found that the optimized ZnO films with appropriate thickness and doping concentration can balance the optical and electrical properties, and its energy band align well with the absorber layer for efficient charge extraction. Further characterizations demonstrated that the window layer optimization can help to reduce the surface defects, improve the heterojunction quality, as well as extend the depletion width. Compared with the control devices, the optimized devices have obtained an efficiency of 6.7% with an enhanced V_(oc) of 18%, J_(sc) of 21%, FF of 10%, and power conversion efficiency of 58%. The present work suggests a useful strategy to improve the device performance by optimizing the window layer besides the absorber layer.

Recent progress of colloidal quantum dot based solar cells

Colloidal quantum dot （CQD） solar cells have attracted great interest due to their low cost and superior photo-electric properties. Remarkable improvements in cell performances of both quantum dot sensitized solar cells （QDSCs） and FbX （X = S, Se） based CQD solar cells have been achieved in recent years, and the power conversion efficiencies （PCEs） ex- ceeding 12% were reported so far. In this review, we will focus on the recent progress in CQD solar cells. We firstly summarize the advance of CQD sensitizer materials and the strategies for enhancing carrier collection efficiency in QD- SCs, including developing multi-component alloyed CQDs and core-shell structured CQDs, as well as various methods to suppress interfacial carrier recombination. Then, we discuss the device architecture development of PbX CQD based solar cells and surface/interface passivation methods to increase light absorption and carrier extraction efficiencies. Finally, a short summary, challenge, and perspective are given.

CdS Quantum Dots-sensitized TiO_2 Nanotube Arrays for Solar Cells

CdS quantum dots（QDs） sensitized TiO2 nanotube arrays photoelectrodes were investigated for their photovoltaic performance of quantum dots-sensitized solar cells. The highly ordered TiO2 nanotube arrays（TNAs） were synthesized on Ti foils by anodic oxidation method. Then CdS quantum dots were deposited onto the TiO2 nanotube arrays by successive ionic layer absorption and reaction（SILAR） method to serve as the sensitizers. Cd（NO3）2 and Na2S were used as the precursor materials of Cd＋ and S2- ions, respectively. It is found that the CdS QDs sensitizer may significantly increase the light response of TiO2 nanotube arrays. With increasing CdS QDs deposition cycles, the visible light response increases. Maximum photocurrent was obtained for the QDs that have an absorption peak at about 500 nm. Under AM 1.5 G illuminations（100 mW cm^-2）, a 4.85 mA/cm^2 short circuit current density was achieved, and the maximium energy conversion efficiency of the asprepared CdS QDs-sensitized TNAs solar cells was obtained as high as 0.81% at five SILAR cycles.

Composite Semiconductor Quantum Dots CdSe/CdS Co-sensitized TiO_2 Nanorod Array Solar Cells

CdSe/CdS semiconductor quantum dots co-sensitized TiO2 nanorod array was fabricated on the transparent conductive fluorine-doped tin oxide （FTO） substrate using the hydrothermal and successive ionic layer adsorption and reaction （SILAR） process. The structural and morphological properties of the samples were characterized by X-ray diffraction （XRD）, field-emission scanning electron microscopy （FESEM）, and transmission electron microscopy （TEM）. The results indicate that CdSe/CdS QDs are uniformly coated on the surface of the TiO2 nanorods. The shift of light absorption edge was monitored by taking UV-visible absorption spectra. Compared with the absorption spectra of the TiO2 nanorod array, deposition of CdSe/CdS QDs shifts the absorption edge to the higher wavelength. The enhanced light absorption in the visible-light region of CdSe/CdS/TiO2 nanorod array indicates that CdSe/CdS layers can act as co-sensitizers in quantum dots sensitized solar cells （QDSSCs）. By optimizing the CdSe layer deposition cycles, a photocurrent of 5.78 mA/cm2, an open circuit photovoltage of 0.469 V and a conversion efficiency of 1.34 % were obtained under an illumination of 100 mw/cm2.

Decrease of back recombination rate in CdS quantum dots sensitized solar cells using reduced graphene oxide

The photovoltaic performance of CdS quantum dots sensitized solar cells （QDSSCs） using the 0.2 wt% of reduced graphene oxide and TiO2 nanoparticles （RGO＋TiO2 nanocomposite） photoanode is investigated. CdS QDs are adsorbed onto RGO＋TiO2 nanocomposite films by the successive ionic layer adsorption and reaction （SILAR） technique for several cycles. The current density-voltage （J-V） characteristic curves of the assembled QDSSCs are measured at AM1.5 simulated sunlight. The optimal photovoltaic performance for CdS QDSSC was achieved for six SILAR cycles. Solar cells based on the RGO＋TiO2 nanocomposite photoanode achieve a 33% increase in conversion efficiency （η） compared with those based on plain TiO2 nanoparticle （NP） photoanodes. The electron back recombination rates decrease significantly for CdS QDSSCs based on RGO＋TiO2 nanocomposite photoanodes. The lifetime constant （τ） for CdS QDSSC based on the RGO＋TiO2 nanocomposite photoanode is at least one order of magnitude larger than that based on the bare TiO2NPs photoanode.

200-nm long TiO_2 nanorod arrays for efficient solid-state Pb S quantum dot-sensitized solar cellsR

To ensure the infiltration of spiro-OMeTAD into the quantum dot-sensitized photoanode and to consider the limit of the hole diffusion length in the spiro-OMeTAD layer, a rutile TiO2 nanorod array with a length of 200 nm, a diameter of 20 nm and an areal density of 720 ram 2 was successfully prepared using a hydrothermal method with an aqueous-grown solution of 38 mM titanium isopropoxide and 6 M hydrochloric acid at 170 ℃ for 75 min. PbS quantum dots were deposited by a spin coating-assisted successive ionic layer adsorption and reaction （spin-SILAR）, and all solid-state PbS quantum dot-sensitized TiO2 nanorod array solar cells were fabricated using spiro-OMeTAD as electrolytes. The results revealed that the average crystal size of PbS quantum dots was -78 nm using Pb（NO3）2 as the lead source and remain unchanged with the increase of the number of spin-SILAR cycles. The all solid-state PbS quantum dot-sensitized TiO2 nanorod array solar cells with spin-SILAR cycle numbers of 20, 30 and 40 achieved the photoelectric conversion efficiencies of 3.74%, 4.12% and 3.11%, respectively, under AM 1.5 G illumination （100 mW/cm2）.

Efficient quantum dot sensitized solar cells via improved loading amount management

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 TiO2films 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 TiO2film 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.

Theoretical investigation of some parameters into the behavior of quantum dot solar cells

The main goal of this paper is to determine the accurate values of two parameters namely the surface generation–recombination rate and the average total number of electrons density generated in the i-region. These values will enhance the performance of quantum dot solar cells（QDSCs）. In order to determine these values, this paper concentrates on the optical generation lifetime, the recombination lifetime, and the effective density state in QDs. Furthermore, these parameters are studied in relation with the average total number of electrons density. The values of the surface generation–recombination rate are found to be negative, which implies that the generation process is dominant in the absorption quantum dot region. Consequently, induced photocurrent density relation with device parameters is determined. The results ensure that QDSCs can have higher response photocurrent and then improve the power conversion efficiency. Moreover, the peak value of the average total number of electrons density is achieved at the UV range and is extended to the visible range, which is adequate for space and ground solar applications.

Novel Hybrid Ligands for Passivating Pb S Colloidal Quantum Dots to Enhance the Performance of Solar Cells

We developed novel hybrid ligands to passivate Pb S colloidal quantum dots(CQDs),and two kinds of solar cells based on as-synthesized CQDs were fabricated to verify the passivation effects of the ligands.It was found that the ligands strongly affected the optical and electrical properties of CQDs,and the performances of solar cells were enhanced strongly.The optimized hybrid ligands,oleic amine/octyl-phosphine acid/Cd Cl2improved power conversion efficiency(PCE)to much higher of 3.72%for Schottky diode cell and 5.04%for p–n junction cell.These results may be beneficial to design passivation strategy for low-cost and high-performance CQDs solar cells.

TiO_2 hierarchical pores/nanorod arrays composite film as photoanode for quantum dot-sensitized solar cells

Power conversion efficiency(PCE) of quantum dot-sensitized solar cells(QDSSCs) was boosted in a TiO_2 composite film(TCSF) with delicate design in structure where TiO_2 hierarchical porous film(THPF) situated on the top of TiO_2 nanorod arrays film(TNAF). In this case, TNAF could supply efficient scattering centers for high light harvesting and direct electrical pathways for fast electron transfer while the THPF could offer porous channels for loading high quantity of previously synthetized quantum dots(QDs) and facilitate the penetration of electrolyte. Meanwhile, in this specific configuration, the presence of anatase–rutile heterojunction at the interface could help the rutile TNAF layer to efficiently collect photo-injected electrons from the anatase THPF layer thus suppressing the recombination of electrons and holes in electrolyte. The results showed that the PCE of QDSSC based on the TNAF photoanode was about 1.4-fold higher(η = 3.05%, J_(sc)= 15.86 m A cm^(-2), V_(oc)= 0.602 V, FF = 0.319) than that of device based on pure THPF(η = 2.20%, J_(sc)= 13.82 m A cm^(-2), V_(oc)= 0.572 V, FF = 0.278).

Selenium cooperated polysulfide electrolyte for efficiency enhancement of quantum dot-sensitized solar cells

The modification of polysulfide electrolyte with additives has been demonstrated as an effective way to improve the photovoltaic performance of quantum dot-sensitized solar cells(QDSCs). Most of these additives can inhibit the charge recombination processes at photoanode/electrolyte interface and favor the improvement of V oc of cell devices. Herein, we showed that the incorporation of elemental selenium(Se) in polysulfide electrolyte to form polyselenosulfide species can notably improve the performance of QDSCs. Unlike previous reports, we present here an integrated investigation of the effects of polyselenosulfide species in polysulfide electrolyte on the photovoltaic performance of QDSCs from both of the photoanode and counter electrode(CE) aspects. Electrochemical impedance spectroscopy(IS) and opencircuit voltage-decay(OCVD) measurements demonstrated that the introduction of Se into polysulfide electrolyte can not only retard charge recombination at photoanode/electrolyte interface, but also reduce the charge transfer resistance at CE/electrolyte interface, resulting in the improvement of J sc and FF values. Consequently, the average efficiency of Zn-Cu-In-Se QDSCs was improved from 9.26% to 9.78% under AM 1.5 G full one sun illumination.

The effect of multi-intermediate bands on the behavior of an InAs_(1-x)N_x/GaAs(1-y)Sb_y quantum dot solar cell

A mathematical model of quantum dot intermediate band solar cells （QDIBSCs） is investigated using two intermediate bands （IBs）. These two IBs arise from the quantum dot （QD） semiconductor material within the bandgap energy. Some parameters such as the width of the QD （WQD） and the barrier thickness or the inter-dot distances between the QDs （BT） are studied to show their influence on the performance of the QDIBSC. The timeindependent Schr6dinger equation, which is solved using the Kronig-Penney model, is used to determine the posi- tion and bandwidth energies of the two IBs. In our proposed model, the cubic shape of the QDs from InAs0.9N0.1 and the barrier or host semiconductor material from GaAs0.98Sb0.02 are utilized. It is shown from the results obtained that changing the parameters WQD and Bx has more influence on the bandwidth energy for the first IB, A 1, than in the case of the second IB, A2. The optimum power conversion efficiencies （PCEs） of the QDIBSCs with two IBs for the model under study are 58.01% and 73.55% at 1 sun and maximum solar concentration, respectively. One can observe that, in the case of the two IBs, an improvement of the PCE is achieved.

A series of conducting gel electrolytes for quasi-solid-state quantum dot-sensitized solar cells with boosted electron transfer processes

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 solarto-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.

ZnSe quantum dots downshifting layer for perovskite solar cells

To date, the instability of organometal halide perovskite solar cells（PSCs） has become the focus issue that limits the development and long-term application of PSCs. Both the ultraviolet（UV） rays in sunlight and moisture in air can significantly accelerate the disintegration of the perovskite. Here, we introduced a Zn Se quantum dots layer as downshifting materials, which was spin-coated onto the backside of PSCs.This layer converted the UV rays into visible light to prevent the destruction of PSCs as well as increase the light harvesting of the perovskite layer. Under the UV irradiation in the moisture ambient（40%）, the destruction speed of the unencapsulated perovskite films were also delayed evidently. In addition, the power conversion efficiency（PCE） of the PSCs was increased from 16.6% to 17.3% due to the increase of the visible light absorbance of the perovskite.

Simply synthesized TiO_2 nanorods as an effective scattering layer for quantum dot sensitized solar cells

TiO2 nanorod layers are synthesized by simple chemical oxidation of Ti substrates. Diffuse reflectance spectroscopy measurements show effective light scattering properties originating from nanorods with length scales on the order of one micron. The films are sensitized with CdSe quantum dots （QDs） by successive ionic layer adsorption and reaction （SILAR） and integrated as a photoanode in quantum dot sensitized solar cells （QDSCs）. Incorporating nanorods in photoanode structures provided 4- to 8-fold enhancement in light scattering, which leads to a high power conversion efficiency, 3.03% （Voc = 497 mV, Jsc = 11.32 mA/cm2, FF = 0.54）, in optimized structures. High efficiency can he obtained just by tuning the photoanode structure without further treatments, which will make this system a promising nanostructure for efficient quantum dot sensitized solar cells.

Photovoltaic Properties of CdSe Quantum Dot Sensitized Inverse Opal TiO₂Solar Cells: The Effect of TiCl₄Post Treatment

Recently, semiconductor quantum dot (QD) sensitized solar cells (QDSSCs) are expected to achieve higher conversion efficiency because of the large light absorption coefficient and multiple exciton generation in QDs. The morphology of TiO2 electrode is one of the most important factors in QDSSCs. Inverse opal (IO) TiO2 electrode, which has periodic mesoporous structure, is useful for QDSSCs because of better penetration of electrolyte than conventional nanoparticulate TiO2 electrode. In addition, the ordered three dimensional structure of IO-TiO2 would be better for electron transport. We have found that open circuit voltage Voc of QDSSCs with IO-TiO2 electrodes was much higher (0.2 V) than that with nanoparticulate TiO2 electrodes. But short circuit current density Jsc was lower in the case of IO-TiO2 electrodes because of the smaller surface area of IO-TiO2. In this study, for increasing surface area of IO-TiO2, we applied TiCl4 post treatment on IO-TiO2 and investigated the effect of the post treatment on photovoltaic properties of CdSe QD sensitized IO-TiO2 solar cells. It was found that Jsc could be enhanced due to TiCl4 post treatment, but decreased again for more than one cycle treatment, which indicates excess post treatment may lead to worse penetration of electrolyte. Our results indicate that the appropriate post treatment can improve the energy conversion efficiency of the QDSSCs.

CdHgTe Quantum Dots Sensitized Solar Cell with Using of Titanium Dioxide Nanotubes

The sensitization of TiO2 nanotubes with CdHgTe quantum dots (QDs) was applied by using the direct dispersion technique. The CdHgTe-QDs were fabricated with different Hg% ratio in organic medium for controlling their particle size. While TiO2 nanotubes (NTs) were fabricated by anodization technique. The QDs and NTs were characterized using SEM, TEM and UV-VIS spectrophotometer. In this work, the photovoltaic parameters of the quantum dots sensitized solar cell (QDSSC) depend mainly on the Hg% ratio in the QDs. The most efficient QDSSC was obtained at 25% of Hg ratio with Jsc of 4 mA/cm2, Voc of 0.63 V, FF of 0.32 and efficiency of 0.81%.