Calculation of band offsets at the CdS/SnS heterojunction

SnS is a promising material for heterojunction solar cells, but the energy band alignment is not known for SnS-based heterojunctions. In this study, the energy band offset at the CdS/SnS heterojunction is calculated using the first principle, density-functional, pseudopotential method. A procedure analogous to that used in the core-level photoemission spectroscopy is adopted to calculate the band offset. The 4d core-level difference between Cd and Sn was estimated from the energy calculation of a superstructure consisting of zincblende CdS and rock-salt or zincblende SnS. The calculated valence-band offset is 0.1 eV when the rock-salt SnS is assumed and 0.84 eV when the zincblende SnS is assumed.     

Spectral characteristics of a-Si:H/c-Si heterostructures

We have examined the current-voltage characteristic of i-a-Si:H/n-c-Si heterojunction Schottky solar cells in the dark and under different illumination (spectrum AM 1.5) intensities as well as the voltage- and temperature-dependent spectral response of these devices. The photo-current from the crystalline silicon depends on both voltage and temperature due to their influence on the impact of the band offsets. From our measurements of the spectral response we conclude that there is a small conduction band offset ΔE_c and a large valence band offset ΔE_v at the i-a-Si:H/n-c-Si heterojunction. The large valence band offset inhibits the collection of photogenerated holes from the crystalline silicon under normal photovoltaic conditions. Thus, the current-voltage characteristic under illumination between V = 0 V and V = V_oc is caused only by the photocurrent from the a-Si:H layer.     

Fabrication of vacuum-evaporated SnS/CdS heterojunction for PV applications

SnS/CdS heterojunction is a promising system for the fabrication of thin film solar cells. In our work, thin film SnS/CdS heterojunction was prepared by evaporating CdS and SnS films. The photovoltaic properties of the heterojunction were investigated with posttreatment of the window material treatment by CdCl₂ for grain size enlargement. I–V characteristics in dark and at light were taken and figures of merit were evaluated. The efficiency with and without window layer treatment were about 0.08% and 0.05%, respectively, under 100 mW/cm² intensity. To the best of our knowledge so far there has been no report on vacuum-evaporated SnS-based heterojunction with window material treatment by CdCl₂.     

Control of conduction band offset in wide-gap Cu(In,Ga)Se2 solar cells

The effects of conduction band offset of window/Cu(In,Ga)Se₂ (CIGS) layers in wide-gap CIGS based solar cells are investigated. In order to control the conduction band offset, a Zn_1−xMg_xO film was utilized as the window layer. We fabricated CIGS solar cells consisting of an ITO/Zn_1−xMg_xO/CdS/CIGS/Mo/glass structure with various CIGS band gaps (E_g≈0.97–1.43 eV). The solar cells with CIGS band gaps wider than 1.15 eV showed higher open circuit voltages and fill factors than those of conventional ZnO/CdS/CIGS solar cells. The improvement is attributed to the reduction of the CdS/CIGS interface recombination, and it is also supported by the theoretical analysis using device simulation.     

Polymer impinged CdS/CuInSe2 solar cell

In the present paper we report, effect of conjugated polymer (polyaniline) impinging in nanostructured CdS/CuInSe₂ heterojunction thin film solar cell. The heterojunction architecture for the solar cell is achieved by sandwiching the conjugated conducting polymer in n and p type of wide band gap semiconducting material by multilayer chemical deposition methods onto the ITO coated glass substrate at room temperature. The obtained multilayer thin film heterojunction of ITO/CdS/Polymer/CuInSe₂/Ag has been characterized for structural, compositional, optical and solar cell characteristics by illuminating it to 100 mW/cm² intensity light source. The X-ray diffraction pattern (XRD) confirms formation of CdS/CuInSe₂ phase while on polymer impinging the crystallite size observed to be increased from 13 to 19 nm. The compositional analysis by energy dispersive X-ray spectra (EDAX) supports presence of expected elements in the heterojunction. The energy band gap calculated from absorbance spectra shows significant shift in its value from polymer and CdS/CuInSe₂ band gap. I–V analysis shows increase in conversion efficiency from 0.26 in CdS/CuInSe₂ to 0.55% in CdS/Polymer/CuInSe₂ heterojunction upon illumination.     

Theoretical analysis of the effect of conduction band offset of window/CIS layers on performance of CIS solar cells using device simulation

One of the most important factors of CdS leading to high performance in Cu(In,Ga)Se₂ (CIGS) solar cells is appropriation of the conduction band offset of CdS/CIGS layers. However, it is not clearly explained. In this study, device modeling and simulation were conducted to explain the effect of conduction band offset of window/CIGS layers on performance of CIGS solar cells. As a result of calculation, excellent performance can be obtained when the conduction band of window layer positions higher by 0–0.4 eV than that of CIGS.     

White radiation upon breakdown of an nSi— pCdiamond heterojunction and upon limiting efficiency of a silicon solar element

White radiation was registered upon breakdown of nSi— pC_diamond heterojunction resulting from collision ionization and recombination of electron-hole pairs without thermalization of an excess energy of electrons. The probabilities of collision ionization and radiation transitions of photoelectrons in the region of a conduction band generated by photons with energy above 2E _g and of enhancement of the limiting efficiency of a silicon solar element are discussed.     

Noble metal free efficient photocatalytic hydrogen generation by TaON/CdS semiconductor nanocomposites under natural sunlight

Tantalum oxynitride have narrow band gap and its band potentials are suitable for visible light induced hydrogen generation. However, due to fast electron-hole recombination, the efficiency of photocatalytic hydrogen evolution reaction is very low. Herein, we have synthesized semiconductor heterojunction photocatalyst, i.e., TaON/CdS with suitable band positions by a simple precipitation method. Ratio between two semiconductors is optimized to obtain maximum hydrogen evolution. XRD, XPS and TEM analysis demonstrate the formation of heterojunction between these semiconductors. Among the synthesized catalysts, 3% TaON/CdS heterostructure exhibits the highest hydrogen evolution activity with H₂ production rate of 7.5 mmol h⁻¹ under natural solar light, whereas the rate is 11 mmol h⁻¹ under the visible light generated by xenon (Xe) lamp without the addition of any noble metal as the co-catalyst. The CdS and 3% TaON/CdS nanomaterials show an AQE of 5.1% and 12.2%, respectively. Combination of Mott-Schottky, UPS and DR UV–visible spectroscopy studies revealed the formation of S scheme semiconductor heterojunction between these nanomaterials with valence, conduction band positions, i.e., 1.46, −0.78 eV for CdS and 2.19, −0.66 eV for TaON, respectively. These band positions help in efficient e-h pair separation to produce hydrogen from water.     

Facile synthesis of titanium dioxide‐cadmium sulfide nanocomposite using pulsed laser ablation in liquid and its performance in photovoltaic and photocatalytic applications

Narrow band gap semiconductors like cadmium sulfide (CdS) are being applied as an agent to reduce the band gap of metal oxide semiconductors like titanium dioxide (TiO₂). In order to obtain a TiO₂/CdS nanocomposite with reduced electron‐hole recombination and improved stability, we coupled 10%, 20%, and 40% by weight of CdS with TiO₂ in this work using pulsed laser ablation in liquid technique. Here, 532 nm wavelength generated from neodymium‐doped yttrium aluminum garnet laser was directed into the TiO₂/CdS mixture prepared in a colloid form to produce the TiO₂/CdS nanocomposites. The effect of the CdS concentration on the performance of the obtained nanocomposite in a dye‐sensitized solar cell and photocatalytic degradation of methyl orange in water was studied in detail. However, the nanocomposite with 10% percentage weight of CdS in anatase TiO₂ showed the best performance as compared with pure TiO₂, and the photoconversion efficiency of the dye‐sensitized solar cell was improved from 0.6% to 4.3%, while the percentage of methyl orange degraded was enhanced from 58% to 82% after 36 min irradiation using ultraviolet–visible light. This improvement in photovoltaic and photodegradation properties is due to limited electron hole recombination rate, higher conduction of charge carriers, their longer lifetime during the photocatalytic process, improved ultraviolet–visible light activity, reduced photocorrosion, and improved pore size. Copyright © 2017 John Wiley & Sons, Ltd.     

Analysis of heterointerface recombination by Zn1−xMgxO for window layer of Cu(In,Ga)Se2 solar cells

An adjustment of a conduction band offset (CBO) of a window/absorber heterointerface is important for high efficiency Cu(In,Ga)Se₂ (CIGS) solar cells. In this study, the heterointerface recombination was characterized by the reduction of the thickness of a CdS layer and the adjustment of a CBO value by a Zn_1−xMg_xO (ZMO) layer. In ZnO/CdS/CIGS solar cells, open-circuit voltage (V_oc) and shunt resistance (R_sh) decreased with reducing the CdS thickness. In constant, significant reductions of V_oc and R_sh were not observed in ZMO/CdS/CIGS solar cells. With decreasing the CdS thickness, the CBO of (ZnO or ZMO)/CIGS become dominant for recombination. Also, the dominant mechanisms of recombination of the CIGS solar cells are discussed by the estimation of an activation energy obtained from temperature-dependent current-voltage measurements.     

Impact of Zn1-xMgxO:Al transparent electrode for buffer-less Cu(In,Ga)Se2 solar cells

Buffer layers such as CdS and ZnS are used in high efficiency Cu(In,Ga)Se₂ (CIGS) thin film solar cells. Eliminating buffer layer is attractive to realize low-cost thin film solar cells by reducing fabrication process. However, the elimination of the buffer layers leads to shunting due to the interface recombination between transparent conductive oxide (TCO) and CIGS layers. To reduce the interface recombination, the control of conduction band offset (CBO) is effective. In this study, we fabricated Zn_1−xMg_xO:Al (ZMO:Al) as the TCO for the CBO control. ZMO:Al was prepared by co-sputtering of ZnO:Al₂O₃ (ZnO:Al) and MgO:Al₂O₃ targets. ZMO:Al shows high transmittance in visible region and the band gap energy widen with the addition of Mg to ZnO:Al. Buffer-less CIGS solar cells with an Al/NiCr/TCO/CIGS/Mo/soda-lime glass structure using ZMO:Al and ZnO:Al were fabricated. For comparison, ZnO/CdS buffered cell was also fabricated. Current density-voltage characteristics of the devices showed the cell with ZMO:Al film achieved higher efficiency compared to the buffer-less cell with ZnO:Al. This result suggested that the control of CBO is important to reduce interface recombination between TCO layer and CIGS absorber.     

Simulation and study of the influence of the buffer intrinsic layer, back-surface field, densities of interface defects, resistivity of p-type silicon substrate and transparent conductive oxide on heterojunction with intrinsic thin-layer (HIT) solar cell

The influence of various parameters such as buffer intrinsic layers, back-surface fields, densities of interface defects (D_it), the resistivity of p-type silicon substrates (ρ) and then work function of transparent conductive oxide (?_TCO) on heterojunction with intrinsic thin-layer (HIT) solar cell performance was investigated using software simulation. Automat for the simulation of heterostructures (AFORS-HET) software was used for that purpose. Our results indicate that band bending, which is determined by the band offsets at the buffer intrinsic/c-Si and/or the c-Si/back-surface field heterointerface, could be critical to solar cell performance. The effect of band bending on solar cell performance and the dependence of cell performance on ρ and ?_TCO were investigated in detail. Eventually, suggestive design parameters for HIT solar cell fabrication are proposed.     

Photovoltaic cells based on pulsed electrochemically deposited SnS and photochemically deposited CdS and Cd1−xZnxS

CdS/SnS and Cd_1−xZn_xS/SnS solar cells were fabricated. SnS films were deposited by the pulsed electrochemical deposition method using an aqueous solution containing SnSO₄ and Na₂S₂O₃. CdS and Cd_1−xZn_xS window layers were deposited by using the photochemical deposition method using an aqueous solution containing CdSO₄, ZnSO₄ and Na₂S₂O₃. Both the techniques were simple, economical and advantageous for fabricating cheap solar cells. The fabricated cells showed rectification characteristics. The photovoltaic properties were measured under AM 1.5 illumination. The cells with the Cd_1−xZn_xS window layer show larger photocurrent than those with the CdS window layer.     

The effect of heteropolyacids and isopolyacids on the properties of chemically bath deposited CdS thin films

The deposition of CdS films on ITO/glass substrates from a chemical bath containing cadmium acetate, ammonia, ammonium acetate and thiourea has been carried out with and without small amounts of heteropolyacids (HPA) (phosphotungstic acid (PTA): H₃[PW₁₂O₄₀], silicotungstic acid (STA): H₄[SiW₁₂O₄₀], phosphomolybdic acid (PMA): H₃[PMo₁₂O₄₀]) and isopolyacids (IPA) (tungstic acid (TA): H₂WO₄ and molybdic acid (MA): H₂MoO₄) for different deposition times. The chemical, morphological, structural and optical properties of the films have been determined. The composition in sulphur and in cadmium of the films’ surface and volume was determined for various HPA and IPA used in the deposition bath. The HPA and IPA which give the thickest film with the biggest grain size were deduced. The optical transmission at 400 nm of CdS films deposited with STA at short time (20 min) (50%) is higher than those of CdS deposited at longer time (6 h) (7%). The optical transmission of CdS deposited with STA at short time is higher (50%) than that of CdS deposited without STA (20%). The performances of heterojunctions CdS/CdTe solar cells fabricated from CdS films deposited with and without STA and CdTe films deposited without STA have been determined. It was shown that the CdS/CdTe heterojunction solar cells fabricated from CdS films deposited with STA exhibited better photon collection efficiency and solar cell efficiency (η=6%) than CdS/CdTe heterojunction solar cells fabricated from CdS films deposited without STA (η=3.3%).     

Photoelectrochemical characteristics of brush plated tin sulfide thin films

Thin films of tin sulfide find wide applications in optoelectronic devices and window materials for heterojunction solar cells. Thin films of p-SnS were brush plated onto tin oxide coated glass substrates from aqueous solution containing SnCl₂ and Na₂S₂O₃. Deposits have been characterized with XRD and SEM for structural analysis. Hot probe method showed invariably p-type nature for all the brush plated SnS films. The variation of space charge capacitance, C_sc, with applied potential, V, was recorded for the PEC cell with p-SnS/Fe³⁺, Fe²⁺/Pt system. The spectral response of the PEC cell formed with SnS photoelectrode was studied and reported.     

Performance enhancement of dye‐sensitized solar cells via cosensitization of ruthenizer Z907 and organic sensitizer SQ2

Cosensitization is a highly effective technique to enhance the photovoltaic performance of a dye‐sensitized solar cell. The main objective of this work is to improve the performance of dye‐sensitized solar cell using cosensitization approach and investigation of the effect of the organic cosensitizer concentration on the power conversion efficiency of the fabricated solar cell devices. In this work, Z907, a ruthenium dye, has been cosensitized with SQ2, an organic sensitizer, and an overall efficiency of 7.83% has been achieved. The fabricated solar cells were evaluated using UV‐Vis spectroscopy, current‐voltage (I‐V) characteristics, and electrochemical impedance spectroscopy analysis. Our results clearly indicate that the concentration of organic cosensitizer strongly affects the photovoltaic performance of fabricated solar cells. Upon optimization, the cell fabricated with 0.3 mM Z907 + 0.2 mM SQ2 dye solution demonstrated J_sc (mA/cm²) = 21.38, V_oc (mV) = 698.37, FF (%) = 52.46, and power conversion efficiency of η (%)  = 7.83 under standard AM1.5G 1 sun illumination (100 mW/cm²). It was observed that the efficiency of cosensitized solar cells is significantly superior than that of individual sensitized solar cells (Z907 [η  = 5.08%] and SQ2 [η  = 1.39%]). This enhancement in efficiency could be attributed to the lower electron‐hole recombination rate, decrease in competitive absorption of I^?/I^?₃, and less dye aggregation because of the synergistic effect in cosensitized solar cells.     

Optimal energy offsets for organic solar cells containing a donor/acceptor pair

Optimal energy levels and offsets of an organic donor/acceptor binary-type solar cell have been analyzed using the classic Marcus electron transfer theory to identify the most efficient photo-induced charge separation. This study reveals that, an exciton quenching parameter (Y_eq) yields one optimal donor/acceptor energy offset where the photo-induced exciton is converted to charges most efficiently, and a recombination quenching parameter (Y_rq) yields a second optimal donor/acceptor energy offset where the ratio of charge separation rate constant over charge recombination rate constant becomes maximum. Another energy offset is also identified where charge recombination becomes most severe. This information would be useful for evaluating and fine tuning frontier energy levels of a donor/acceptor pair for optimum solar cell applications.     

Influence of growth and microstructure of chemical deposited CdS films on the electrical properties of CdS/CdTe solar cells

Thin film n-CdS/p-CdTe solar cells were prepared from chemical bath deposited CdS and electrodeposited CdTe layers. The microstructural and some electrical properties of these layers were studied and connection with photovoltaic performance of the cells was shown. Especially, adherence of the CdS films and the quality of heterojunction interface manifesting themselves in the value of the open-circuit voltage U_oc depend on the cadmium precursor used for CdS deposition and on whether pH buffered conditions were applied or not. The number of CdS layers in the cells needed to obtain U_oc of about 700 mV is connected with the CdS deposition conditions.     

Photovoltaic properties of SnS based solar cells

Polycrystalline thin films of tin sulphide have been synthesised using spray pyrolysis. The layers grown at a temperature of 350 °C had the orthorhombic crystal structure with a strong (1 1 1) preferred orientation. The films had resistivities 30 Ω cm with an optical energy band gap (E_g) of 1.32 eV. Heterojunction solar cells were fabricated using sprayed SnS as the absorber layer and indium doped cadmium sulphide as the window layer and the devices were characterised to evaluate the junction properties as well as the solar cell performance. The current transport across the junction has been modelled as a combination of tunnelling and recombination. The best devices had solar conversion efficiencies of 1.3% with a quantum efficiency of 70%.     

Efficient photocatalytic hydrogen evolution with high-crystallinity and noble metal-free red phosphorus-CdS nanorods

The photocatalytic production of H₂ by low-cost semiconductors is a promising approach to store solar energy. Photocatalysts with heterojunctions convert visible light into H₂ faster because of more efficient charge separation. The morphology, the structure, and the crystallinity are additional factors to consider when developing a photocatalyst. Here, highly-crystalline CdS nanorod (NR) were synthesized by a facile one-pot process. Under visible light, pure CdS NR produced H₂ 2.1 times faster than conventional CdS nanoparticles (NP). CdS NR were then combined with the semiconductor red phosphorus (RPh). A CdS NR-based heterojunction photocatalyst with RPh_5% had an excellent photocatalytic H₂ evolution rate of 11.72 mmol g⁻¹ h⁻¹, which was 3.6 times higher than pure CdS NR. The apparent quantum efficiency of RPh_5%/CdS NR was 19.57%. Furthermore, RPh_5%/CdS NR exhibited a superior photogenerated charge separation efficiency and was stable with little photocorrosion compared to CdS NP showing the high potential of this heterojunction photocatalyst.