CdS Thickness Research Articles

Apparent quantum efficiency effects under forward bias in MZO/CdS/CdTe solar cells

The apparent quantum efficiency measurement under forward bias can be used to analyze the window layer inter-diffusion, the photocurrent loss mechanism and the battery back-contact barrier height. In this article, quantum efficiency measurement and apparent quantum efficiency measurement under forward bias are performed on the MgxZn1−xO/CdS/CdTe solar cells. The results show that the introduction of MZO thin film is benefit to reduce CdS thickness which improves the short-wavelength response of quantum efficiency. The inter-diffusion occurs at the interface between CdS and CdTe, where CdS0.844Te0.156 polycrystalline thin film possibly forms. However, there might be an obvious electron barrier at the conduction band interface between CdS0.844Te0.156 and MZO, which reduces the carrier collection efficiency. In addition, more photo-generated carriers are collected with increasing forward bias in the space charge region of the back contact barrier to generate a current opposite to the primary photocurrent. So the AQE response for the cell without the back contact decreases monotonously in the wavelength range of 300–600 nm and has a lower negative peak in the range of 800–900 nm when the forward bias exceeds 0.6 V.

Importance of CdS buffer layer thickness on Cu2ZnSnS4-based solar cell efficiency

Cu2ZnSnS4 (CZTS) thin films were grown on Mo-coated soda lime glass (SLG) substrates by the sulfurization of DC magnetron-sputtered Zn, Sn and Cu metallic precursors under a sulfur atmosphere at 550 °C for 45 min. Understanding the composition and structure of the CZTS absorber layer is necessary to obtain efficient solar cells. With this aim, x-ray diffractometry, Raman spectroscopy, scanning electron microscopy, energy dispersive spectroscopy and x-ray photoelectron spectroscopy were used to investigate the CZTS absorber layers. CZTS absorber films were obtained and found to be Cu-poor and Zn-rich in composition, which are both qualities desired for efficient solar cells. CdS was used as a buffer layer and was grown by the chemical bath deposition technique. The optical properties of CdS films on SLG were searched for using a spectroscopic ellipsometer and the results revealed that the bandgap increases with film thickness increment. CZTS-based solar cells with different CdS buffer layer thicknesses were prepared using a SLG/Mo/CZTS/CdS/ZnO/AZO solar cell configuration. The influence of the CdS buffer layer thickness on the performance of the CZTS solar cells was investigated. Device analysis showed that electrical characteristics of solar cells strongly depend on the buffer layer’s thickness. Highly pronounced changes in VOC, fill factor and JSC parameters, which are the main efficiency limiting factors, with changing buffer layer thicknesses were observed. Our experiments confirmed that decreasing the CdS thickness improved the efficiency of CZTS solar cells down to the lowest thickness limit.

Influence of P+-ZnTe back surface contact on photovoltaic performance of ZnTe based solar cells

In order to improve photovoltaic performance of solar cells based on ZnTe thin films two device structures have been proposed and its photovoltaic parameters have been numerically simulated using Solar Cell Capacitance Simulator software. The first one is the ZnO/CdS/ZnTe conventional structure and the second one is the ZnO/CdS/ZnTe/P+-ZnTe structure with a P+-ZnTe layer inserted at the back surface of ZnTe active layer to produce a back surface field effect which could reduce back carrier recombination and thus increase the photovoltaic conversion efficiency of cells. The effect of ZnO, CdS and ZnTe layer thicknesses and the P+-ZnTe added layer and its thickness have been optimized for producing maximum working parameters such as: open-circuit voltage Voc, short-circuit current density Jsc, fill factor FF, photovoltaic conversion efficiency η. The solar cell with ZnTe/P+-ZnTe junction showed remarkably higher conversion efficiency over the conventional solar cell based on ZnTe layer and the conversion efficiency of the ZnO/CdS/ZnTe/P+-ZnTe solar cell was found to be dependent on ZnTe and P+-ZnTe layer thicknesses. The optimization of ZnTe, CdS and ZnTe layers and the inserting of P+-ZnTe back surface layer results in an enhancement of the energy conversion efficiency since its maximum has increased from 10% for ZnO, CdS and ZnTe layer thicknesses of 0.05, 0.08 and 2 µm, respectively to 13.37% when ZnO, CdS, ZnTe and P+-ZnTe layer thicknesses are closed to 0.03, 0.03, 0.5 and 0.1 µm, respectively. Furthermore, the highest calculated output parameters have been Jsc = 9.35 mA/cm2, Voc = 1.81 V, η = 13.37% and FF = 79.05% achieved with ZnO, CdS, ZnTe, and P+-ZnTe layer thicknesses about 0.03, 0.03, 0.5 and 0.1 µm, respectively. Finally, the spectral response in the long-wavelength region for ZnO/CdS/ZnTe solar cells has decreased at the increase of back surface recombination velocity. However, it has exhibited a red shift and showed no dependence of back surface recombination velocity for ZnO/CdS/ZnTe/P + -ZnTe solar cells.

Theoretical study of ZnS/CdS bi-layer for thin-film CdTe solar cell

The performance of CdTe solar cells is strongly limited by the thickness of CdS window layer. A higher short-circuit current density might be achieved by decreasing the thickness of CdS layer as a result of reducing the absorption losses that take place in this layer. However, it is difficult to obtain uniform and pin-hole free CdS layers thinner than 50 nm. This problem can be solved through increasing the band gap of the window layer by adding a wide band gap semiconductor such as ZnS. In this work, bi-layer ZnS/CdS film was studied as an improved window layer of ITO/ZnS/CdS/CdTe solar cell. The total thickness of ZnS/CdS layer was taken about 60 nm. The effect of optical losses due to reflection at different interfaces in the cell and absorption in ITO, ZnS, CdS as well as the recombination loss have been studied. Finally, the effects of the recombination losses in the space-charge region and the reflectivity from the back contact were taken into accounts. The results revealed that the optical losses of 23% were achieved at 60 nm thickness of CdS and theses losses minimized to 18% when ZnS layer of 30 nm thickness was added to CdS layer. The minimum optical and recombination losses of about 26% were obtained at 1 ns of electron life-time and ∼0.4 μm width of the space-charge region. The maximum efficiency of 18.5% was achieved for ITO/CdS/CdTe cell and the efficiency increased up to 20% for ITO/ZnS/CdS/CdTe cell.

Thermodynamic Model for Quantum Dot Assemblies Formed Because of Charge Transfer

Two initially neutral semiconductor quantum dots with appropriate band offsets can participate in a ground state charge transfer process. The charge transfer manifests itself in the form of bleaching of optical transitions and also causes the quantum dots to precipitate from solution, giving rise to assemblies with unusual properties. As this represents a postsynthetic modification of the electronic structure of quantum dots, it holds tremendous potential for improving the characteristics of quantum dot devices. Here, we study the dependencies of the properties of these assemblies on the structure of the participating quantum dots. In particular, we find that for assemblies formed out of Cu:CdS and ZnTe/CdS quantum dots, the composition of the assembly varies from 1:1.26 to 1:0.23 ZnTe/CdS to Cu:CdS as the shell thickness of CdS in ZnTe/CdS is increased. In contrast, the composition changes from 1:1.1 to 1:15 for PbSe/CdSe and Cu:CdS quantum dots, as the size of the PbSe core is increased. These observations are explained on the basis of a phenomenological thermodynamic model. The applicability of thermodynamics to this example of self-assembly is verified empirically.

Efficiency Enhancement of Kesterite Cu2ZnSnS4 Solar Cells via Solution-Processed Ultrathin Tin Oxide Intermediate Layer at Absorber/Buffer Interface

The ultrathin SnO2 intermediate layer deposited by a successive ionic layer adsorption and reaction (SILAR) method was introduced into the heterointerface between p-type Cu2ZnSnS4 (CZTS) absorber and n-type CdS buffer for interface defect passivation in kesterite thin film solar cells. CZTS solar cells with SnO2 intermediate layers show higher open circuit voltage (Voc) of 657 mV and fill factor (FF) of 62.8%, compared to its counterpart cells without the SnO2 intermediate layer, which have Voc of 638 mV and FF of 52.4%, resulting in improvement in the overall efficiency from 6.82% to 8.47%. The mitigation of the Voc deficit and the improvement of FF are believed to result from the integrated effects of CZTS/CdS heterointerface passivation, shunt blocking, and band alignments. The passivation effect is further affirmed by the improved carrier lifetime. Furthermore, external quantum efficiency profiles show a strengthened blue optical response which is contributed by the decrease of CdS thickness. This wor...

Evidence for Chemical and Electronic Nonuniformities in the Formation of the Interface of RbF-Treated Cu(In,Ga)Se2 with CdS.

We report on the initial stages of CdS buffer layer formation on Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers subjected to rubidium fluoride (RbF) postdeposition treatment (PDT). A detailed characterization of the CIGSe/CdS interface for different chemical bath deposition (CBD) times of the CdS layer is obtained from spatially resolved atomic and Kelvin probe force microscopy and laterally integrating X-ray spectroscopies. The observed spatial inhomogeneity in the interface's structural, chemical, and electronic properties of samples undergoing up to 3 min of CBD treatments is indicative of a complex interface formation including an incomplete coverage and/or nonuniform composition of the buffer layer. It is expected that this result impacts solar cell performance, in particular when reducing the CdS layer thickness (e.g., in an attempt to increase the collection in the ultraviolet wavelength region). Our work provides important findings on the absorber/buffer interface formation and reveals the underlying mechanism for limitations in the reduction of the CdS thickness, even when an alkali PDT is applied to the CIGSe absorber.

Structural and Optoelectronic Properties of β‐In2S3 Thin Films to be Applied on Cadmium Reduced Solar Cells

In2S3 thin films are prepared by chemical bath deposition (CBD) technique to be applied as buffer layer in CdTe solar cells. CdTe photovoltaic devices are developed using In2S3 as “standard buffer layer” in order to reduce the CdS thickness used as window material. It is important to examine potential thin films in a prospective life cycle study, focusing on direct costs, resource availability, and environmental impacts. Open and closed CBD system influence on the In2S3 physical properties is analyzed. Stable tetragonal β‐In2S3 phase was confirmed by X‐ray diffraction. Electrical properties were determined by four‐point probe technique obtaining a resistivity value of 102 Ω cm. CdTe solar cells performance was studied by measuring J–V characteristics and spectral quantum efficiencies. These results reveal In2S3 thin films as buffer layer reduce the cadmium quantity used in solar cells manufacture and improve their current collection in blue wavelength region (300–500 nm).

Chemically Deposited CdS Buffer/Kesterite Cu2ZnSnS4 Solar Cells: Relationship between CdS Thickness and Device Performance.

Earth-abundant, copper-zinc-tin-sulfide (CZTS), kesterite, is an attractive absorber material for thin-film solar cells (TFSCs). However, the open-circuit voltage deficit (Voc-deficit) resulting from a high recombination rate at the buffer/absorber interface is one of the major challenges that must be overcome to improve the performance of kesterite-based TFSCs. In this paper, we demonstrate the relationship between device parameters and performances for chemically deposited CdS buffer/CZTS-based heterojunction TFSCs as a function of buffer layer thickness, which could change the CdS/CZTS interface conditions such as conduction band or valence band offsets, to gain deeper insight and understanding about the Voc-deficit behavior from a high recombination rate at the CdS buffer/kesterite interface. Experimental results show that device parameters and performances are strongly dependent on the CdS buffer thickness. We postulate two meaningful consequences: (i) Device parameters were improved up to a CdS buffer thickness of 70 nm, whereas they deteriorated at a thicker CdS buffer layer. The Voc-deficit in the solar cells improved up to a CdS buffer thickness of 92 nm and then deteriorated at a thicker CdS buffer layer. (ii) The minimum values of the device parameters were obtained at 70 nm CdS thickness in the CZTS TFSCs. Finally, the highest conversion efficiency of 8.77% (Voc: 494 mV, Jsc: 34.54 mA/cm2, and FF: 51%) is obtained by applying a 70 nm thick CdS buffer to the Cu2ZnSn(S,Se)4 absorber layer.

Effects of annealing on Cu2ZnSnS4 thin films prepared on Mo substrate and the fabrication of solar cells

Cu2ZnSnS4 (CZTS) thin films on Mo substrate were obtained by a simple sol–gel process and subsequent sulfurization. To decrease the impact of the back contact to CZTS thin film solar cells, the pre-annealing and sulfurization process was controlled. Based on the sol–gel method, CZTS thin film solar cells with a structure of ZnO:Al/i-ZnO/CdS/CZTS/Mo/glass have been fabricated. The best power conversion efficiency (PCE) of CZTS solar cells was up to 4.28% under the optimized condition: the sulfurization temperature of 570 °C and CdS thickness of 50 nm.

Three-Dimensional Plasmonic Nanostructure Design for Boosting Photoelectrochemical Activity.

Plasmonic nanostructures have been widely incorporated into different semiconductor materials to improve solar energy conversion. An important point is how to manipulate the incident light so that more light can be efficiently scattered and absorbed within the semiconductors. Here, by using a tunable three-dimensional Au pillar/truncated-pyramid (PTP) array as a plasmonic coupler, a superior optical absorption of about 95% within a wide wavelength range is demonstrated from an assembled CdS/Au PTP photoanode. Based on incident photon to current efficiency measurements and the corresponding finite difference time domain simulations, it is concluded that the enhancement is mainly attributed to an appropriate spectral complementation between surface plasmon resonance modes and photonic modes in the Au PTP structure over the operational spectrum. Because both of them are wavelength-dependent, the Au PTP profile and CdS thickness are further adjusted to take full advantage of the complementary effect, and subsequently, an angle-independent photocurrent with an enhancement of about 400% was obtained. The designed plasmonic PTP nanostructure of Au is highly robust, and it could be easily extended to other plasmonic metals equipped with semiconductor thin films for photovoltaic and photoelectrochemical cells.

The comparison of optical and photodetector properties of Zn, Al and Sn doped and undoped CdS thin films via chemical bath deposition

We deposit undoped CdS thin films as well as doped CdS thin films with Zn, Al, and Sn via chemical bath deposition method. We then investigate the optical and photodetector properties of these two. Our results show that the particle size of CdS:Zn thin film is smaller than undoped CdS thin film. In addition, the CdS:Sn thin film shows a special morphology which is a combination of spherical and elongated particles. CdS:Sn thin film has a higher surface to volume ratio in comparison to undoped CdS thin film. We find out that the thickness of CdS:Zn, CdS:Al, CdS:Sn and undoped CdS thin film are 380, 350, 190 and 320 nm, respectively. The optical band gap of CdS:Zn, CdS:Al, CdS:Sn and undoped CdS thin film are 2.49, 2.42, 2.78, and 2.53 eV, respectively. Moeover, photosensivity of CdS:Sn thin film (Iph/Id) is (approximately) 904 at bias voltage of 10 V, which is higher than the other thin films. This phenomenon is related to lower sulfur vacancy. Finally, we propose that alloying with tin could be effective in eliminating cadmium vacancy and hence improving the detector performance.

ZnO/CdS nanorod arrays decorated by layered double hydroxides for efficient solar water oxidation

Developing efficient photoanode without either hole scavenger or noble-metal co-catalyst is extremely important for solar water oxidation. However, in most cases, hole scavengers such as Na2S and/or Na2SO3 are usually used to inhibit carrier recombination while blocking the real water oxidation process. To avoid this, non-noble metal co-catalyst, that is, CoNi layered double hydroxides (LDHs), has been successfully deposited onto ZnO/CdS core-shell nanorod arrays for efficient photoelectrochemical water oxidation in KOH aqueous solution. By changing the deposition cycle numbers from 10 to 25 for CdS and tuning the electrodeposition charge quantities from 0.1 to 0.4 C for CoNi LDHs layers, the effect of thickness or loading of CdS and CoNi LDHs on light absorption and photocurrent was systematically studied. A transformation from M(OH)2 to MOOH (M═Co, Ni) is found when the photoexcited holes in CdS are transferred to CoNi LDHs, making the co-catalyst more active than before. These high active M3+ sites, combined with the high absorption over visible light, fast-charge transfer along the 1D ZnO tunnels, and low-charge transfer resistance at photoanode/electrolyte interfaces, endow the ZnO/CdS/CoNi LDHs nanoarrays photoanode with a photocurrent of 2.87 mA/cm2 at 1.0 VRHE and incident photon-to-current efficiency of 36% at 450 nm. The work provides an effective strategy toward efficient solar water oxidation free of hole scavengers. Copyright © 2017 John Wiley & Sons, Ltd.

Effect of CdS deposition temperature for thermally grown Cu(In,Ga)Se2 and two-step chalcogenized Cu(In,Ga)(S,Se)2 absorbers on solar cell performances

CdS buffer layer of varying thickness ranging from 23 to 58 nm deposited at different substrate temperature were prepared as n-type junction partner for thermally grown Cu(In,Ga)Se2 and two-step chalcogenized Cu(In,Ga)(S,Se)2 photovoltaic absorber films and the effect of deposition temperature and time on the CdS growth behavior and solar cell performance were evaluated. High deposition temperature resulted in a thicker CdS layer and more importantly lower density and shallower depth of open voids, which attributed to the improved open-circuit voltage and fill factor due to reduced interface recombination. The solar cell efficiency of thermally grown absorber saturated at about 30 nm thickness of CdS, while that of chalcogenized absorber gradually increased with CdS thickness up to 60 nm without significant loss of short-circuit current density.

Inverted polymer solar cells using CdS fabricated by thermal decomposition of cadmium xanthate precursor as electron transporting layer

In this manuscript, CdS thin films fabricated by thermal decomposition of the solution processable cadmium xanthate precursor were used as electron transporting layer (ETL) in inverted polymer solar cells (IPSCs) with device structure ITO/CdS/poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PC61BM)/MoO3/Ag. The effects of CdS thickness and the annealing temperature of cadmium xanthate precursor on the photovoltaic performance of IPSCs were investigated. The highest power conversion efficiency (PCE) value of 3.22% was obtained when CdS thin film thickness was 10nm and the annealing temperature of cadmium xanthate precursor was 160°C. This value is slightly higher than that of the IPSC with device structure ITO/ZnO/P3HT:PC61BM/MoO3/Ag using sol-gel derived ZnO thin films as ETL. The results indicate that CdS films deposited by thermal decomposition of metal xanthate precursor are excellent ETL for IPSCs.

Effect of thickness: a case study of electrodeposited CdS in CdS/CdTe based photovoltaic devices

The effect of electrodeposition technique on CdS thickness incorporated in CdS/CdTe-based solar cell has been investigated using all-electrodeposited g/FTO/n-CdS/n-CdTe/p-CdTe multilayer device configuration. The optical, morphological and structural properties of the electroplated CdS were investigated for CdS thicknesses between 50 and 200 nm. The observed CdS bandgap ranges between 2.42 and 2.46 eV. The morphological analysis shows full coverage of underlying g/FTO substrate for all CdS thicknesses except for the 50 nm which shows the presence of gap in-between grains. The structural analysis shows a preferred orientation of H(101) for all the CdS thicknesses except the 50 nm thick CdS which shows either a weak crystallinity or an amorphous nature. The fabricated solar cell shows a maximum conversion efficiency of ~11 % using CdS thickness ranging between 100 and 150 nm. These results show that although low CdS thickness is desirable for photovoltaic application, the effect of nucleation mechanism of deposition technique should be taken into consideration.

Boosting Efficiency and Stability of Perovskite Solar Cells with CdS Inserted at TiO2/Perovskite Interface

Organic–inorganic hybrid perovskite solar cells have undergone an unprecedented development as the next‐generation photovoltaic devices in recent years. The power conversion efficiency and stability are the key factors attracting great attentions from both academic and industrial communities. Here, a nonoxide CdS layer is inserted between TiO2 and perovskite to passivate the TiO2 interface in planar‐type perovskite solar cells. After introducing the CdS layer, significantly enhanced air stability and suppressed recombination between the trapped electrons and perovskite related to inverse transport are observed. At the optimum CdS thickness, a champion power conversion efficiency of 14.26% is achieved compared with 10.31% for the reference CdS‐free devices. This impressive efficiency also surpasses that of previously reported perovskite solar cells based on CdS. The largely improved performance is ascribed to the increased Fermi level of the electron transport layer, more efficient charge transport, and lower recombination rates.

Modeling of optical losses in graphene contacted CIGS solar cells

For the first time, an optical model is applied to superstrate configuration of CdS/CIGS thin film solar cells with graphene front/back contact (FC/BC) to simulate the loss in current density and efficiency. Graphene shows to be a great candidate to replace with the metallic BC transparent conductive oxides as the front electrode. Our model is based on the refractive index and extinction coefficient and takes into account the reflection and absorption in interfaces and layer’s thickness, respectively. CIGS cells with graphene as front electrode have a lower current density and efficiency than the one with graphene BC. However, the bifacial configuration shows a higher current density and efficiency, mostly because of a higher transmission rate. The interference effect was observed in simulation of transmission rate of hybrid cells representing that graphene can cause multiple reflection. We simulated the device parameters versus the ZnO layer’s thickness, which is essential for high quality interfaces. However, the simulation results are also consistent when CdS thickness is replaced with inorganic ZnO.

One-sided rectifying p–n junction diodes fabricated from n-CdS and p-ZnTe:Te semiconductors

The fabrication of a one-sided p–n hetero-junction (HJ) diodes have been successfully carried out using both p-type ZnTe and n-CdS semiconductors. Chemical bath deposition (CBD) and electrodeposition (ED) techniques have been used in the deposition of n-CdS and p-ZnTe layers respectively. Before the fabrication of the one-sided p–n HJ diodes, the electrical properties of glass/FTO/p-ZnTe/Al and glass/FTO/n-CdS/Au rectifying structures were separately studied using capacitance–voltage (C–V) technique so as to determine the doping density of each of the thin films. The results from C–V analyses showed that p-ZnTe is moderately doped with an acceptor density of 3.55 × 1015 cm−3 while n-CdS is heavily doped with a donor density of 9.00 × 1019 cm−3. The heavy doping of n-CdS and moderate doping of p-ZnTe will make the interface between n-CdS and p-ZnTe thin films a one-sided n+p diode. Therefore, to fabricate the CdS/ZnTe hetero-structure, it was ensured that approximately same thickness of CdS and ZnTe thin films being used in the initial experiment to study the electrical properties of glass/FTO/n-CdS/Au and glass/FTO/p-ZnTe/Al were also used in the development of the one-sided n+p junction diodes to obtain more accurate results. The electronic properties of the device structure were studied using both current–voltage (I-V) and C–V measurement techniques. The I–V results show that the one-sided n+p HJ diodes possess good rectifying quality with a series resistance (Rs) of ∼35 Ω and rectification factors exceeding 102.7 under dark condition. The results of the C–V analyses showed that the acceptor density of the one-sided n+p HJ diode is of the order of 1015 cm−3 while the donor density is of the order of 1018 cm−3. The results obtained from this analysis still showed the moderate doping of p-ZnTe and the degenerate nature of n-CdS.

Simulation and optimization of CdS-n/Cu2ZnSnS4 structure for solar cell applications

In this work, the performance of solar cell based on CdS-n/Cu2ZnSnS4-p hetero-junction is numerically simulated. The aim of the study is to investigate the influence of thickness, defects density and bandgap energy of absorber layer CZTS and the thickness of the buffer layer CdS of the solar cell on electrical parameters Jsc, Voc, FF and efficiency η of the cell. The results of our simulation allowed us to optimize the parameters above mentioned in order to get the best efficiency at the optimal band gap which corresponds to the maximum of the solar spectrum with optimal values of the electrical performances of the cell. This results lead to develop CZTS solar cells with high efficiency and low cost and give a help full indication for fabrication process.