CdS Side Research Articles

Expanding the Notch Region by Adjusting the Copper Growth Profile for High-Efficiency Flexible Cu(In,Ga)Se2 Solar Cells.

Flexible CIGS solar cells, with their adjustable band gap for future flexible tandem solar cells and flexibility for roll-to-roll manufacturing, have the potential to be used in a wide range of applications. However, flexible CIGS solar cells are always manufactured at relatively low temperatures, where Cu diffusion has a substantial impact on the CIGS surface state and defect formation. To address these issues, we designed a new CIGS growth profile in this work by carefully examining the effects of different locations of excess Cu in the third stage of the CIGS deposition profile. The results showed that adding more Cu to the middle part of the third stage can enhance the crystal quality, expand the GGI grading notch region, move the GGI minimum to the CdS side, cause a JSC rise, achieve proper band alignment at the interface, and then enhance the FF. By taking advantage of these advantages, the photoelectric conversion efficiency (PCE) is significantly raised. An impressive improvement of 32.6% over the initial efficiency of 13.8% has been attained, yielding a high efficiency of 18.3% and providing a strong basis for the development of flexible CIGS solar cells.

The role of CdS doping in improving SWIR photovoltaic and photoconductive responses in solution grown CdS/PbS heterojunctions

Low cost short wavelength infrared (SWIR) photovoltaic (PV) detectors and solar cells are of very great interest, yet the main production technology today is based on costly epitaxial growth of InGaAs layers. In this study, layers of p-type, quantum confined (QC) PbS nano-domains (NDs) structure that were engineered to absorb SWIR light at 1550 nm (Eg = 0.8 eV) were fabricated from solution using the chemical bath deposition (CBD) technique. The layers were grown on top of two different n-type CdS intermediate layers (Eg = 2.4 eV) using two different CBD protocols on fluoride tin oxide (FTO) substrates. Two types of CdS/PbS heterojunction were obtained to serve as SWIR PV detectors. The two resulting devices showed similar photoluminescence behavior, but a profoundly different electrical response to SWIR illumination. One type of CdS/PbS heterojunction exhibited a PV response to SWIR light, while the other demonstrated a photo-response to SWIR light only under an applied bias. To clarify this intriguing phenomenon, and since the only difference between the two heterojunctions could be the doping level of the CdS layer, we measured the doping level of this layer by means of the surface photo voltage (SPV). This yielded different polarizations for the two devices, indicating different doping levels of the CdS for the two different fabrication protocols, which was also confirmed by Hall Effect measurements. We performed current voltage measurements under super bandgap illumination, with respect to CdS, and got an electrical response indicating a barrier free for holes transfer from the CdS to the PbS. The results indicate that the different response does, indeed, originate from variations in the band structures at the interface of the CdS/PbS heterojunction due to the different doping levels of the CdS. We found that, unlike solar cells or visible light detectors having similar structure, in SWIR photodetectors, a type I heterojunction is formed having a barrier at the interface that limits the injection of the photo-exited electrons from the QC-PbS to the CdS side. Higher n-doped CdS generates a narrow depletion region on the CdS side, with a spike like barrier that is narrow enough to enable tunneling current, leading to a PV current. Our results show that an external quantum efficiency (EQE) of ∼2% and an internal quantum efficiency (IQE) of ∼20% can be obtained, at zero bias, for CBD grown SWIR sensitive CdS/PbS-NDs heterojunctions.

Hierarchically CdS–Ag2S nanocomposites for efficient photocatalytic H2 production

Hierarchically nanostructured CdS composed of 4.7 nm-thick self-assembled ultrathin nanosheets was synthesised through a microwave-assisted solvothermal method. Ag2S nanoparticles (NPs) were deposited at the edge of the CdS nanosheets by an in situ ion exchange strategy. The hierarchically CdS–Ag2S nanocomposites exhibited a high visible light photocatalytic H2 evolution rate of 375.6 μmol h−1 g−1, which was 11.5 times higher than that of pure CdS. Given the difference in work functions between CdS and Ag2S, electrons diffused from the CdS side to the Ag2S side until the Fermi levels align after their contact. When the CdS–Ag2S was illuminated, the photogenerated electrons on the conduction band of the CdS further migrated to Ag2S. Considering the lower overpotential of Ag2S, the electrons more easily participated in the reduction of protons. Meanwhile, the holes on the valence band of CdS reacted with the hole sacrificial agent (triethanolamine). In this process, the photogenerated electron–hole pairs realised effective separation. The introduction of Ag2S also enhanced the utilisation of infrared light and increased the temperature of CdS surface.

Mapping Charge Distribution in Single PbS Core - CdS Arm Nano-Multipod Heterostructures by Off-Axis Electron Holography.

We synthesized PbS core-CdS arm nanomultipod heterostructures (NMHs) that exhibit PbS{111}/CdS{0002} epitaxial relations. The PbS-CdS interface is chemically sharp as determined by aberration corrected transmission electron microscopy (TEM) and compared to density functional theory (DFT) calculations. Ensemble fluorescence measurements show quenching of the optical signal from the CdS arms indicating charge separation due to the heterojunction with PbS. A finite-element three-dimensional (3D) calculation of the Poisson equation shows a type-I heterojunction, which would prevent recombination in the CdS arm after optical excitation. To examine charge redistribution, we used off-axis electron holography (OAEH) in the TEM to map the electrostatic potential across an individual heterojunction. Indeed, a built-in potential of 500 mV is estimated across the junction, though as opposed to the thermal equilibrium calculations significant accumulation of positive charge at the CdS side of the interface is detected. We conclude that the NMH multipod geometry prevents efficient removal of generated charge carriers by the high energy electrons of the TEM. Simulations of generated electron-hole pairs in the insulated CdS arm of the NMH indeed show charge accumulation in agreement with the experimental measurements. Thus, we show that OAEH can be used as a complementary methodology to ensemble measurements by mapping the charge distribution in single NMHs with complex geometries.

Trilayer CdS/carbon nanofiber (CNF) mat/Pt-TiO2 composite structures for solar hydrogen production: Effects of CNF mat thickness

Solar H2 production by photocatalytic water splitting is a promising technology that permits direct H2 production from the clean and abundant resources of water and solar light. We successfully fabricated trilayer heterostructures of CdS/carbon nanofiber (CNF) mat/Pt-deposited TiO2 (Pt-TiO2) for solar H2 production. The CNF mat was prepared by electrospinning and carbonization. CdS and Pt-TiO2 were coated on the front and back of the CNF mat by doctor blading. Under visible-light irradiation on the CdS side, the addition of the Pt-TiO2 coating improved the H2 production by a factor of 3.4. This suggests that the H2 production reaction could occur on Pt-TiO2, which is not active under visible irradiation; therefore, the CNF mat could act as an efficient photogenerated electron-transfer mediator from CdS to Pt-TiO2. The H2 production rates of the trilayer CdS/CNF/Pt-TiO2 heterostructures were strongly affected by the thickness of the CNF mat and the carbonization temperatures used in production, which affect the resistance of the CNF mat between the CdS and Pt-TiO2 sides. The results clearly demonstrated that the CNF acted as an efficient electron-transfer mediator as well as a support material.

CdS1−yTey thin films: Production and bandgap investigation

In CdS/CdTe solar cells it is necessary to determine the efficiency limitations related with the intermixing at the interface between the CdS window layer and CdTe absorber layer. So understanding the properties of the solid solution (CdSxTe1−x on the CdTe side which is CdTe rich and CdS1−yTey on the CdS side which is CdS rich) that is always formed in this region is essential. We produced thin films of CdS1−yTey solid solution-which is CdS rich – by first producing CdS:In thin films on glass substrates by the spray pyrolysis technique and then annealing the films in nitrogen atmosphere at 400°C in the presence of Te vapor. We are the first who produce this solid solution by this simple and low cost method. The composition and morphology of the films were determined by energy dispersive X-ray detection (EDAX) measurements and scanning electron microscopy (SEM) observations respectively. Eight values of y in the range 0⩽y⩽0.2845 were obtained. The transmittance was measured and used to investigate the optical bandgap energy by using the second derivative of the absorbance. It is found that the films show a single hexagonal phase for y⩽0.0852 and then a mixed (hexagonal and cubic) phase for 0.0997⩽y⩽0.2845. Bandgap energies in the range 2.259⩽Eg⩽2.528eV were obtained. Urbach tailing in the bandgap was also investigated.

Structural, optical and photoconductive properties of electron beam evaporated CdSxSe1‐x films

AbstractCdSxSe1‐x films were deposited by the electron beam evaporation technique on glass substrates at different temperatures in the range 30 – 300 °C using the laboratory synthesized powders of different composition. The films exhibited hexagonal structure and the lattice parameters shifted from CdSe to CdS side as the composition changed from CdSe to CdS side. The bandgap of the films increased from 1.68 to 2.41 eV as the concentration of CdS increased. The root‐mean‐roughness (RMS) values are 3.4, 2.6, 1.2 and 0.6 nm as the composition of the films shifted towards CdS side. The conductivity varies from 30 Ωcm‐1 to 480 Ωcm‐1 as the ‘x’ value increases from 0 to 1. The films exhibited photosensitivity. The PL spectrum shifts towards lower energies with decreasing x, due to the decrease of the fundamental gap with Se composition. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pulse plated CdS xTe 1−x films and their properties

CdS x Te 1− x films were deposited on titanium and conducting glass substrates at room temperature using 0.25 M cadmium sulphate, the concentration of sodium thiosulphate and TeO 2 dissolved in sodium hydroxide was varied in the range of 0.01–0.05 M. The as deposited films exhibited hexagonal structure irrespective of the composition. The FWHM maximum of the x-ray diffraction peaks were found to decrease with increase of duty cycle. The optical energy gap values are in the range of 1.54–2.32 eV for films of different composition, it is observed that the band gap shifts towards CdS side as the concentration of CdS in the films increase. XPS studies indicated the formation of CdSTe solid solution. The grain size increases from 11.54 to 99.40 nm as the value of x increases from 0.2 to 0.8. The surface roughness is found to increase from 0.22 to 2.50 nm as the value of ‘ x’ increases from 0.2 to 0.8. The resistivity is found to vary from 53 to 8 ohm cm as the ‘ x’ value decreases from 1 to 0.

Diffusion length in CdTe by measurement of photovoltage spectra in CdS/CdTe solar cells

Modified method of surface photovoltage (SPV), spectral response measurement, and constant photocurrent method (CPM) were applied to thin film CdS/CdTe solar cells with the aim of finding diffusion length of minority carriers (L) in the CdTe material. The SPV signal was theoretically calculated without constraints of absorption coefficients for the incident radiation and thickness of the sample assuming one space charge region (SCR) located on the CdS/CdTe interface. In addition to the diffusion length, the SPV is a function of the surface recombination velocity and the parameters of the SCR, which complicates the evaluation. Illuminating the back side of the solar cell (without ohmic contact) we obtain a photovoltage spectrum predominantly influenced by the diffusion length. On the other hand, the standard measurement using light penetrating from the CdS side strongly depends on the thickness of the SCR. The small signal approximation model presented here successfully explains both measured spectra and permits extraction of the diffusion length of minority carriers and thickness of the SCR in CdTe absorber. The CPM is used for determination of absorption coefficients in the CdTe layer. The absorption of this material depends on its preparation and must be known for correct evaluation of experimental data.

Improvements in stoichiometry and stability of p-CuxS in thin-film CdS solar cells

To counter the problem of instability, irreproducibility, and low stoichiometry of p-CuxS layers in thin-film CdS solar cells, a new field-assisted chemiplating process of barrier formation is described. Using the dc-forming potential as a first-order variable, nominal deviation from Cu2S composition without any dependence on external process parameters has been obtained. The stability of higher composition CuxS is attributed to the formation of a Cd++, rich barrier layer at the growing CuxS interface towards the CdS side, as inferred from the spectral response and Auger Cu and Cd compositional depth profile of the solar cells.

The lateral photovoltaic effect in CdS-Cu2S heterojunction solar cell

The lateral photovoltaic effect has been observed in CdS-Cu2S thin-film solar cells. The effect is more pronounced on the CdS side than on the Cu2S side of the cells. On the CdS side, where the contacts were formed by soldering Cu wire by indium and then applying Ag paint, the photovoltage developed were found to increase as the point of illumination was moved towards the contact. The spectral response of photovoltage for coevaporated cells shows a peak atλ=0.5μm (2.45 eV). But for topotaxial cells two peaks, one atλ=0.5μm and the other atλ=0.65μm (1.89eV) were observed. A band model has been proposed for the heat-treated optimized cells.

The electron-voltaic effect in CdS/Cu 2S heterojunctions

An investigation of energy conversion in CdS/Cu 2S thin film heterojunctions has shown that the devices appear to operate differently in the photovoltaic and electron-voltaic modes, the response in the latter mode occurring mainly on the CdS side of the junction. With a view to the manufacture of compact long-life reliable microgenerators the electron-voltaic mode has been studied using 6–15 keV electrons, 75–400 keV electrons and β-emitting radioactive sources such as tritium, promethium and thallium. Greater efficiencies for electron-voltaic energy conversion were obtained when the cells were fabricated by a ‘dry’ barrier method rather than by a conventional ‘wet’ barrier technique. Tritium was found to be an ideal source for electron—voltaic energy conversion in the particular CdS/Cu 2S heterojunctions studied.

Electrical and optoelectrical properties of CdS-Ge heterojunctions

The electrical and optoelectrical characteristics are presented for n-CdS/ p-Ge heterojunctions fabricated by depositing Ge onto (0001) faces of CdS monocrystals. The experimental results are interpreted in terms of an energy band model proposed for this heterojunction. It is found that a Schottky barrier is present on the CdS side of the interface. With an In (injecting) contact to the CdS and an ohmic contact to the Ge, the device acts as a photoconductor in which the current is space-charge-limited for forward bias (Ge positive, CdS negative). For the opposite bias polarity the photoconductivity is reduced substantially and the current is limited by tunneling through the barrier spike in the conduction band. The photoconductivity at room temperature results from electrons being excited in the CdS bulk by photons of 2·34 eV energy. For forward bias the photocurrent is found to be proportional to the light intensity at moderate voltages, as for a normal photoconductor. However, at low voltages the photocurrent is found to be proportional to the two-thirds power of the light intensity.

Growth and properties of steeply graded Zn fCd 1-fS crystals

Steeply graded mixed crystals of Zn fCd 1-fS were grown by a vapor phase deposition of CdS onto cleaved slices of ZnS followed by a period of interdiffusion. This produced a graded region between 50 and 100 μm wide with a bandgap gradient d E g/d x varying between 10 1 and 10 2 eV/cm. The diffusion coefficient was determined as a function of position and extrapolates to 10 -10 cm 2/sec for Zn diffusion in CdS and 5 x 10 -12 cm 2/sec for Cd diffusion in ZnS at 1100°C. The crystals have good photoluminescent properties. With UV excitation, the ZnS side shows blue emission at 77 K while the CdS side has red photoluminescence. By irradiating the graded region, the peak of the photoluminescence shifts as a function of the wavelength of excitation. The crystals do not exhibit measurable electroluminescence or dependence of the photoluminescence on applied electrical field. A low minority carrier lifetime is explained by the high density of deep acceptors.