The photovoltaic performance of alloyed CdTexS1−x quantum dots sensitized solar cells

The photovoltaic performance of alloyed CdTe_xS_1−x quantum dots (QDs) sensitized solar cells (QDSSCs) as a function of tuning the band gap of alloyed CdTe_xS_1−x QDs is studied. The tuning of band gap was carried out through controlling the molar ratio (x) of QDs. Presynthesized alloyed CdTe_xS_1−x QDs of different x values (0, 0.2, 0.4, 0.6, 0.8, and 1) were deposited by direct adsorption (DA) technique onto a layer of TiO₂ nanoparticles (NPs) to serve as sensitizers for the solar cells. The characteristic parameters of the assembled QDSSCs were measured under AM 1.5 sun illumination, and show that CdTe_xS_1−x QDs has better photovoltaic performance than pure CdTe QDs or CdS QDs. The maximum values of J_sc (1.54 mA/cm²) and η (0.31%) were obtained for x=0.6. However, The open circuit voltages (V_oc) approximately constant (0.46±0.02 V) for all alloyed CdTe_xS_1−x QDSSCs. It is only dictated by the conduction band (CB) level of TiO₂ nanoparticles (NPs) and the valance band (VB) of the electrolyte.     

CdS and PbS quantum dots co-sensitized TiO2 nanorod arrays with improved performance for solar cells application

In this paper, we report a novel CdS and PbS quantum dots (QDs) co-sensitized TiO₂ nanorod arrays photoelectrode for quantum dots sensitized solar cells (QDSSCs). TiO₂ film consisting of free-standing single crystal nanorods with several microns high and 90–100 nm in diameter were deposited on a conducting glass (SnO₂:FTO) substrate by hydrothermal method. Then CdS/PbS QDs were deposited in turn on TiO₂ nanorods by facile SILAR technique. The FTO/TiO₂/CdS/PbS, used as photoelectrode in QDSSCs, produced a light to electric power conversion efficiency (Eff) of 2.0% under AM 1.5 illumination (100% sun), which shows the best power conversion efficiency compared with single CdS or PbS sensitized QDSSCs. One dimension TiO₂ nanorod provides continuous charge carrier transport pathways without dead ends. The stepwise structure of the band edges favored the electron injection and the hole-recovery for both CdS and PbS layers in photoelectrode, which may gave a high electric power conversion efficiency. The facile preparation and low cost nature of the proposed method and structure make it has a bright application prospects in photovoltaic areas in the future.     

Sb2(SxSe1−x)3 sensitized solar cells prepared by solution deposition methods

Solid state semiconductor sensitized solar cells are a very active research subject in emerging photovoltaic technologies. In this work, heterojunctions of antimony sulfide-selenide (Sb₂(S_xSe_1−x)₃) solid solution as the absorbing material and cadmium sulfide coated titanium dioxide (TiO₂/CdS) as the electron conductor have been developed with solution deposition methods such as spin-coating, successive ionic layer adsorption and reaction (SILAR), and chemical bath deposition. In particular, CdS has been deposited on mesoporous TiO₂ layers by SILAR deposition, followed by the chemical deposition of Sb₂(S_xSe_1−x)₃. It was found that by increasing the number of CdS SILAR deposition, both the open circuit voltage V_oc and the short circuit current density J_sc of the Sb₂(S_xSe_1−x)₃ sensitized solar cells had been increased from 153 to 434 mV and 0.77–9.73 mA/cm², respectively. This improvement was attributed to the fact that the presence of the CdS on TiO₂ surface reduces the formation of undesired Sb₂O₃ and promotes a better nucleation of the Sb₂(S_xSe_1−x)₃ during the chemical bath deposition. The best result was obtained for the solar cell with 30 cycles of CdS which produced a V_oc of 434 mV, a J_sc of 9.73 mA/cm², and a power conversion efficiency of 1.69% under AM1.5 G solar radiation.     

Size-dependent polymer/CuInS2 solar cells with tunable synthesis of CuInS2 quantum dots

This paper reports the size-dependent performance in polymer/CuInS₂ solar cells with tunable synthesis of chalcopyrite CuInS₂ quantum dots (QDs) by the solvothermal method. The CuInS₂ QDs of 3.2–5.4 nm in size are fine tuned by the reaction time in the solvothermal process with the slow supply of In³⁺ ions during the crystallization, and the band gaps increased with QDs sizes decreasing according to the results from the characterization of sizes, morphologies, component elements, valence states and band gaps of CuInS₂ QDs. We fabricated MEH-PPV/CuInS₂ solar cells, and the photoactive layer of device displayed size-dependent light-harvesting, charge separation and transport ability. Moreover, the solar cells exhibit size-dependent short circuit current (J_sc) and open circuit voltage (V_oc), with higher performance in both J_sc and V_oc for smaller CuInS₂ QDs, resulting in the maximum power conversion efficiency of ca. 0.12% under the monochromic illumination at 470 nm; CuInS₂ QDs actually serve as an effective electron acceptor material for the MEH-PPV/CuInS₂ solar cells with the wide spectral response extending from 300 to 900 nm.     

Low-cost TiO2/Sb2(S,Se)3 heterojunction thin film solar cell fabricated by sol-gel and chemical bath deposition

Low cost TiO₂/ Sb₂(S, Se)₃ heterojunction thin film solar cell are prepared successfully by using sol-gel and chemical bath deposition. At first, TiO₂ thin film is prepared on the ITO-coated glass substrate by a simple sol-gel and dip-coating method. Subsequently, Sb₂(S, Se)₃ film is fabricated on TiO₂ by selenizing the Sb₂S₃ film prepared by chemical bath deposition (CBD). The heat-treated process of TiO₂ and Sb₂(S, Se)₃ films has been discussed, respectively. After being heat-treated at 550 °C for TiO₂ and 290 °C for Sb₂(S, Se)₃ films, the photovoltaic devices are completed with the conductive graphite as electrode. The J-V characteristics of TiO₂/ Sb₂(S, Se)₃ solar cell are measured and the open circuit voltage (V_oc) of this cell is about 350 mV.     

Influence of compact TiO2 layer on the photovoltaic characteristics of the organometal halide perovskite-based solar cells

A series of perovskite-based solar cells were fabricated wherein a compact layer (CL) of TiO₂ of varying thickness (0–390 nm) was introduced by spray pyrolysis deposition between fluorine-doped tin oxide (FTO) electrode and TiO₂ nanoparticle layer in perovskite-based solar cells. Investigations of the CL thickness-dependent current density–voltage (J–V) characteristics, dark current, and open circuit voltage (V_oc) decays showed a similar trend for thickness dependence. A CL thickness of 90 nm afforded the perovskite-based solar cell with the maximum power conversion efficiency (η, 3.17%). Furthermore, two additional devices, perovskite-based solar cell omitting hole transporting materials layer and cell without the TiO₂ nanoparticles, were designed and fabricated to study the influence of the CL thickness on different electron transport paths in perovskite-based solar cells. Solar cells devoid of TiO₂ nanoparticles, but with perovskite and organic hole-transport materials (HTMs), exhibited sustained improvement in photovoltaic performances with increase in the thickness of CL, which is in contrast to the behavior of classical perovskite-based solar cell and common solid state solar cell which showed optimal photovoltaic performances when the thickness of CL is 90 nm. These observations suggested that TiO₂ nanoparticles play a significant role in electron transport in perovskite-based solar cells.     

Effect of sol–gel MgO spin-coating on the performance of TiO2-based dye-sensitized solar cells

This paper is concerned with the improvement of dye-sensitized solar cell (DSSC) efficiency upon MgO post-treatment of the TiO₂ electrode. A simple sol–gel technique, involving magnesium acetate as precursor, ethanol as solvent and nitric acid as stabilizer, is applied to prepare a solution of suspended MgO nanoparticles. A single drop of MgO sol at 0.1 M precursor concentration was spin-coated at 3000 rpm for 30 s onto the TiO₂ electrode and sintered at 500 K for 1 h. Dye-loading using N3-dye was applied for 6 h. An increase in the average efficiency of the DSSC from 2.5% to 3.9% (over 50% enhancement) was recorded. Measurements of the dark I–V characteristics, the open circuit voltage decays, the SEM images and the dye absorbance spectra, for both uncoated and MgO-coated electrodes were examined. The improvement of the DSSC efficiency was attributed to an upward shift of the TiO₂ flat band energy and a reduction of the rate of back-transport and recombination.     

High performance dye-sensitized solar cells (DSSCs) achieved via electrophoretic technique by optimizing of photoelectrode properties

This paper describes a simple method utilizing electrophoretic deposition (EPD) of commercial P25 nanoparticles (NPs) films on fluoride-doped tin oxide (FTO) substrate. In this process, voltage and the number of deposition cycles are well controlled to achieve TiO₂ film thickness of around 1.5–26 μm, without any mechanical compression processing. The experimental results indicate that the TiO₂ film thickness plays an important role as the photoelectrode in DSSCs because it adsorbs a large number of dye molecules which are responsible for electrons supply. Furthermore, it was found that effects of the bulk traps and surface states within the TiO₂ films on the recombination of the photo-injected electrons (electron–hole pairs) strongly depend on the TiO₂ electrode annealing temperature. Finally, a DSSC with a 24 μm thick TiO₂ film and annealed at 500 °C produced the highest conversion efficiency (η=6.56%, I_SC=16.4, V_OC=0.72, FF=0.55) with an incident solar energy of 100 mW/cm².     

Thin films of silica imbedded silicon and germanium quantum dots by solution processing

Hydrophilic silicon (0.9 nm) and germanium (2.7 nm) quantum dots (QDs), synthesized utilizing micelles to control particle size, were coated with silica using liquid phase deposition. The use of dodecyltrimethylammonium bromide as a surfactant yielded uniform spheres (Si@SiO₂=57 nm; Ge@SiO₂=32 nm), which could then be arrayed in three dimensions using a vertical deposition method on quartz plates. The silica coated QDs were characterized by UV–visible spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron microscopy. The thin films were characterized by UV–visible spectroscopy, scanning electron microscopy, and the measurement of a photocurrent.     

Enhance the performance of quantum dot-sensitized solar cell by manganese-doped ZnS films as a passivation layer

To make quantum dot-sensitized solar cells (QDSSCs) more attractive, it is necessary to achieve higher power conversion efficiency. A novel Mn-doped ZnS has been successfully fabricated on CdS/CdSe quantum dots (QDs) by simple successive ion layer adsorption and reaction (SILAR) technique. The Mn-doped ZnS is used as a passivation layer in the QDSSCs. The performance of the QDSSCs was examined in detail using sulfide/polysulfide electrolyte with a Pt or copper sulfide (CuS) counter electrode. Here we demonstrated, the fabricated Mn-doped ZnS QDs shows an improved V_oc (0.65 V) compared to bare ZnS QDs (0.60 V). The QDSSC based on a photoanode with Mn-doped ZnS (10 wt% of Zn) shows higher J_sc (15.32 mA cm⁻²) and power conversion efficiency (4.18%) compared to the bare ZnS photoanode (2.90%) under AM 1.5 G one sun illumination. We explore the reasons for this enhancement and demonstrated that it is caused by improved passivation of the ZnS surface by Mn ions, leading to a lower recombination of photo-injected electrons with the electrolyte. The effect of Cu ions in ZnS has been investigated by UV–Vis spectra and current density–voltage analysis.     

Application research of CdS: Eu3+ quantum dots-sensitized TiO2 nanotube solar cells

Trivalent Eu³⁺-doped CdS quantum dot (CdS: Eu³⁺ QD)-sensitized TiO₂ nanotube arrays (TNTAs) solar cells are prepared by using the direct adsorption method. The influences of sensitization time, sensitization temperature, and Eu³⁺ ion concentrations are investigated systematically. The photo-current of the CdS: Eu³⁺ QDs/TiO₂ nanotubes appear at the main absorption region of 320–480 nm, and the maximum incident photon to the current conversion efficiency (IPCE) value is 21% at 430 nm when the sensitization condition is 4% doping Eu³⁺ concentration, 60 °C sensitization temperature, 8 h sensitization time. Compared with the un-doped CdS QD-sensitized TNTAs, the conversion efficiency and IPCE of CdS: Eu³⁺ QDs/TNTAs are two times and three times than that of un-doped CdS QDs sensitized TNTAs. This scenario exhibits the potential applications of rare earth elements in QD-sensitized solar cells.     

Fabrication of inverted pyramid structure by Cesium Chloride self-assembly lithography for silicon solar cell

Inverted pyramids were fabricated through a method combining cesium chloride (CsCl) self-assembly technology and anisotropy corrosion of silicon solar cells. Ti film with nanoporous masks was formed by lift-off the CsCl nanoislands for the inverted pyramids. The pyramids were then formed by anisotropy corrosion of alkaline solution. The average diameter and morphology of the pyramids were controlled by varying the average diameter of CsCl nanoislands from 400 nm to 1.5 µm and by varying the etching time of alkaline solution from 2 to 8 min. The inverted-pyramid texture could suppress reflection to below 10% at wavelengths from 400 to 1000 nm, which was much lower than that of planar wafer. A solar cell fabricated from the pyramids had higher short-circuit current density (J_sc) and photovoltaic conversion efficiency (PCE) compared with those of planar solar cells for the good antireflection property. The solar cell showed a PCE of 15.25%, a J_sc of 38.35 mA/cm², and an open-circuit voltage of 555.7 mV.     

Quantum dot based photocatalytic decolorization as an efficient and green strategy for the removal of anionic dye

In this study, zinc sulfide (ZnS) quantum dots (QDs), as a pure and iron doped were synthesized using a chemical precipitation method in aqueous solution, in the presence of 2-hydroxyethanthiol as a capping agent. Optical and structural properties of the pure and Fe³⁺ ion doped (i.e. Zn_(1−x)Fe_xS) QDs were investigated by various techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV–vis absorption spectroscopy. In addition, the prepared ZnS QDs, as pure and doped with various mole fractions of Fe³⁺ ion were used for photodecolorization of thymol blue (TB) as an anionic and pollutant dye, under UV light irradiation. In photodecolorization studies, the effect of main experimental parameters on the decolorization efficiency (DE) of QDs as green nano-semiconductors were studied and optimized. The results indicated that DE is affected by some parameters such as dopant mole fraction; the pH of initial dye solution, nanophotocatalyst amount, irradiation time, and initial dye concentration. The maximum decolorization efficiency of 100% and 99.88% was obtained at pH 9.5 of dye solution for 10 ppm of TB dye, in the presence of 10 mg of undoped and doped (for 5% of Fe³⁺) ZnS QDs, respectively.     

Low-temperature formation of self-assembled Ge quantum dots on Si(100) under high carbon mediation via solid-phase epitaxy

CMOS-compatible low-temperature formation of self-assembled Ge quantum dots (QDs) by carbon (C) mediation via a solid-phase epitaxy (SPE) has been demonstrated. The samples were prepared by a solid-source molecular beam epitaxy (MBE) system. C and Ge were successively deposited on Si(100) at 200 °C and Ge/C/Si heterostructure was annealed in the MBE chamber. Sparse Volmer-Weber mode Ge dots without a wetting layer were formed for C coverage (θ_C) of 0.25 and 0.5 ML by lowering SPE temperature (T_S) to 450 °C, but small and dense Stranski-Krastanov (SK)-mode Ge QDs with the wetting layer were obtained with increasing C coverage of 0.75 ML even at 450 °C. From the investigation of SPE temperature effect on Ge QD formation for θ_C of 0.75 ML, SK-mode Ge QDs of about 10 nm in diameter and of about 4.5×10¹¹ cm⁻² in density were formed at T_S≥400 °C. The wetting layer of SK-mode QDs was almost constant 0.2-nm thick at T_S≥450 °C. Measurements of chemical binding states of C in Ge QDs and at Ge/Si interface revealed that a large amount of C–Ge bonds were formed in the wetting layer for high C coverage, and the formation of C–Ge bonds, together with the formation of C–Si bonds, enabled the low-temperature formation of small and dense Ge QDs. These results suggest that the C-mediated solid-phase epitaxy is effective to form small and dense SK-mode QDs at low temperature.     

Development of SnS quantum dot solar cells by SILAR method

SnS quantum dot solar cell is fabricated by Successive Ionic Layer Adsorption and Reaction (SILAR) method. SnS layer is optimized by different SILAR cycles of deposition. The particle size increased with the increase in number of SILAR cycles. Cu₂S coated FTO is used as counter electrode against the conventional Platinum electrode. On comparison with a cell having a counter electrode–electrolyte combination of Platinum–Iodine, Cu₂S–polysulfide combination is found to improve both the short circuit current and fill factor of the solar cell. A maximum efficiency of 0.54% is obtained with an open circuit voltage of 311 mV and short circuit current density of 4.86 mA/cm².     

Stepwise co-sensitization as a useful tool for enhancement of power conversion efficiency of dye-sensitized solar cells: The case of an unsymmetrical porphyrin dyad and a metal-free organic dye

A tertiary arylamine compound (DC), which contains a terminal cyano-acetic group in one of its aryl groups, and an unsymmetrical porphyrin dyad of the type Zn[Porph]-L-H₂[Porph] (ZnP-H₂P), where Zn[Porph] and H₂[Porph] are metallated and free-base porphyrin units, respectively, and L is a bridging triazine group functionalized with a glycine moiety, and were synthesized and used for the fabrication of co-sensitized dye-sensitized solar cells (DSSCs). The photophysical and electronic properties of the two compounds revealed spectral absorption features and frontier orbital energy levels that are appropriate for use in DSSCs. Following a stepwise co-sensitization procedure, by immersing the TiO₂ electrode in separate solutions of the dyes in different sequence, two co-sensitized solar cells were obtained: devices C (ZnP-H₂P/DC) and D (DC/ZnP-H₂P).The two solar cells were found to exhibit power conversion efficiencies (PCEs) of 6.16% and 4.80%, respectively. The higher PCE value of device C, which is also higher than that of the individually sensitized devices based on the ZnP-H₂P and DC dyes, is attributed to enhanced photovoltaic parameters, i.e. short circuit current (J_sc = 11.72 mA/cm²), open circuit voltage (V_oc = 0.72 V), fill factor (FF = 0.73), as it is revealed by photovoltaic measurements (J–V curves) and by incident photon to current conversion efficiency (IPCE) spectra of the devices, and to a higher total dye loading. The overall performance of device C was further improved up to 7.68% (with J_sc = 13.45 mA/cm², V_oc = 0.76 V, and FF = 0.75), when a formic acid treated TiO₂ ZnP-H₂P co-sensitized photoanode was employed (device E). The increased PCE value of device E has been attributed to an enhanced J_sc value (=13.45 mA/cm²), which resulted from an increased dye loading, and an enhanced V_oc value (=0.76 V), attributed to an upward shift and increased of electron density in the TiO₂ CB. Furthermore, dark current and electrochemical impedance spectra (EIS) of device E revealed an enhanced electron transport rate in the formic acid treated TiO₂ photoanode, suppressed electron recombination at the photoanode/dye/electrolyte interface, as well as shorter electron transport time (τ_d), and longer electron lifetime (τ_e).     

Application of sputter-deposited amorphous and anatase TiO2 as electron-collecting layers in inverted organic photovoltaics

We demonstrate the deposition of amorphous and anatase TiO₂ on indium tin oxide (ITO) substrates via the process of sputtering, and the use of these materials as electron-collecting layers (ECLs) in inverted-type organic photovoltaics (OPVs). Anatase TiO₂ was obtained via vacuum-annealing of as-deposited amorphous TiO₂ at 300 °C. No deterioration of optical and electrical properties of ITO was observed after both sputter-deposition of TiO₂ and annealing process. The anatase TiO₂ proved to be an effective ECL when employed in inverted OPVs using bulk heterojunction photoactive layer of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, achieving a power conversion efficiency of 3.3% (J_SC = 9.0 mA cm^?2, V_OC = 0.62 V and FF = 0.60).     

Improved efficiency of multicrystalline silicon solar cells by TiO2 antireflection coatings derived by APCVD process

This paper reports about the adaptation of the chemical vapor deposition (CVD) thin films technology to the fabrication process of multicrystalline silicon solar cells as a simple, low cost and very effective technology for efficiency device improvement by reducing reflection and improving the light-generated current. In this contribution, the higher reflection of a mc-Si solar cell surface is strongly reduced by the deposition of TiO₂ antireflection coating (ARC) on the front using the atmospheric pressure chemical vapor deposition method (APCVD). The surface morphology and elemental composition of the TiO₂ antireflective layers were revealed using scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy . The reflectivity was then reduced from 35% to 8.6% leading to the increase of the short circuit current J_sc which was 33.86 mA/cm² with a benefit of 5.23 mA/cm² (surface area=25 cm²) compared to the reference cell (without ARC). This simple and low cost technology induces a 14.26% conversion efficiency which is a gain of +3% absolute in comparison to the reference cell. The LBIC measurements of a typical multicrystalline cell confirmed the uniformity of the photocurrent distribution throughout the device. These results are encouraging and prove the effectiveness of the APCVD method for efficiency enhancement in silicon solar cells.     

Development of highly conducting n-type micro-crystalline silicon oxide thin film and its application in high efficiency amorphous silicon solar cell

Wide band gap and highly conducting n-type nano-crystalline silicon film can have multiple roles in thin film solar cell. We prepared phosphorus doped micro-crystalline silicon oxide films (n-μc-SiO:H) of varying crystalline volume fraction (X_c) and applied some of the selected films in device fabrication, so that it plays the roles of n-layer and back reflector in p-i-n type solar cells. It is generally understood that a higher hydrogen dilution is needed to prepare micro-crystalline silicon, but in case of the n-μc-SiO:H an optimized hydrogen dilution was found suitable for higher X_c. Observed X_c of these films mostly decreased with increased plasma power (for pressure<2.0 Torr), increased gas pressure, flow rate of oxygen source gas and flow rates of PH₃>0.08 sccm. In order to determine deposition conditions for optimized opto-electronic and structural characteristics of the n-μc-SiO:H film, the gas flow rates, plasma power, deposition pressure and substrate temperature were varied. In these films, the X_c, dark conductivity (σ_d) and activation energy (E_a) remained within the range of 0–50%, 3.5×10⁻¹⁰ S/cm to 9.1 S/cm and 0.71 eV to 0.02 eV, respectively. Low power (30 W) and optimized flow rates of H₂ (500 sccm), CO₂ (5 sccm), PH₃ (0.08 sccm) showed the best properties of the n-μc-SiO:H layers and an improved performance of a solar cell. The photovoltaic parameters of one of the cells were as follows, open circuit voltage (V_oc), short circuit current density (J_sc), fill-factor (FF), and photovoltaic conversion efficiency (η) were 950 mV, 15 mA/cm², 64.5% and 9.2% respectively.     

Performance analysis of dye solar cell with additional TiO2 layer under different light Intensities

Deployment of dye solar cells (DSCs) for building integration application would require a highly efficient solar cell that work well in diffused light. In order to improve the efficiency of dye solar cell, an additional layer of ultrathin anatase titanium dioxide (TiO₂) has been deposited for strengthening the adhesion of the porous TiO₂-based photo electrode to the conductive transparent substrate, which can lead to an enhancement in electron transportation. Fabricated cells of 1 cm² area were tested under different light intensities (100, 33 and 10 mW cm⁻²) and characterized by scanning electron microscopy (SEM), Raman spectroscopy and electrochemical impedance spectroscopy (EIS). Analysis showed an increment in overall quantum conversion efficiency (η), as high as 35% compared to the standard cell without the additional layer of TiO₂. EIS analysis has proven that the additional ultrathin anatase layer has improved the collection efficiency (Φ_COLL) as the result of the enhancement in both electron transport and lifetime within the porous TiO₂ film which translated into better conversion efficiency of DSCs.