Donors In CdS Research Articles

Numerically investigating the AZO/Cu2O heterojunction solar cell using ZnO/CdS buffer layer

Aluminium (Al)-doped zinc oxide (ZnO)/copper oxide (AZO/Cu2O) heterojunction solar cell is analyzed by a 1-D solar cell capacitance simulator (SCAPS-1D). The influence of single (CdS) and double (ZnO and CdS) buffer layers on the performance of AZO/Cu2O heterojunction solar cells is studied. The effects of device parameters such as Cu2O acceptor and CdS donor doping concentrations, external quantum efficiency, interface states density, metal work function, absorber and buffer layer thicknesses, and working temperature on both single and double buffer layer solar cell are carried out. The optimized solar cell structure of single buffer AZO/CdS/Cu2O/Au and double buffer AZO/ZnO/CdS/Cu2O/Au results in the power conversion efficiency (PCE) of 10.47 and 10.60 %, short circuit current density (JSC) of 11.33 and 11.37 mA cm−2, open-circuit voltage (Voc) of 1.1381 and 1.1405 V and fill factor (FF) of 81.21 and 81.74 %, respectively under illumination. Our findings reveal that the PCE of AZO/Cu2O based heterojunction thin-film solar cell is enhanced through the usage of a double buffer (ZnO/CdS) layer in comparison to the single (CdS) buffer layer.

Energy transfer kinetics in Basic Fuchsin dye sensitized CdS quantum dots

Sensitization of QDs (Quantum Dots) with a dye of good spectral overlap maximizes and extends the absorption range of incident photons. Interactions of Cadmium Sulfide Quantum Dots (CdS QDs) with Basic Fuchsin (BF) dye has been investigated in the present work using steady state and time resolved fluorescence emission techniques. The analysis has led to the inference that fluorescence resonance (Förster type) energy transfer (FRET) is the mechanism behind the fluorescence quenching of CdS QDs in the presence of dye molecules. An efficient energy transfer (~90%) from CdS donor to BF dye acceptor was observed and the FRET parameters such as Förster distance and spectral overlap integral were determined. The non-radiative energy transfer mechanism in QDs-dye binary mixtures holds great promise in analytical chemistry, biodetection system and in light harvesting systems.

Enhancement of optical properties in Neutral Red Dye through energy transfer from CdS Quantum Dots

We report FRET (Fluorescence Resonance Energy Transfer) between CdS QDs (Quantum Dots) and NR (Neutral Red) dye acceptors using steady-state and time-resolved spectroscopy. The results reveal an enhancement in the NR emission intensity due to energy transfer from CdS donor to the NR dye acceptor which is discussed in light of Förster’s theory. The outcome of the present work provides a platform for designing biochemical sensors and bioimaging with the unique properties of QDs. This study also shows the feasibility of using QD- dye pairs in FRET-based solar cell applications.

Control of Donor Concentration in <italic>n</italic>-Type Buffer Layer for High-Efficiency Cu(In,Ga)Se 2 Solar Cells

To improve the conversion efficiency of Cu(In,Ga)Se2 (CIGS) solar cells, we clarified the correlation between the recombination at CdS/CIGS interface and donor concentration in a CdS buffer layer, and propose a technique to control the donor concentration in CdS by In doping. A theoretical analysis was performed for CIGS solar cells with Al/ZnO:B/i-ZnO/CdS/CIGS structure. Open-circuit voltage and fill factor increased with CdS donor concentration. Increasing the CdS donor concentration, the hole-carrier density at the CdS/CIGS interface decreases, leading to the suppression of interfacial recombination. To verify the In-doping effect, CdS films covered with a thin In2 S3 layer were evaluated after an air-annealing process. Carrier concentration increased from 2.4 × 10 15 cm−3 in nonannealed CdS films to 1.1 × 1016 cm−3 in In-doped CdS films with air annealing at 150 °C, indicating that In doping is useful to control the donor concentration in CdS. Finally, In-doping treatment was applied on the CdS buffer layer of CIGS solar cells. Using the nonannealed CdS, efficiency was 15.7%, whereas 17.0% was obtained with the In-doped CdS, which represents an absolute gain of 1.6%. These results indicate that the control of donor concentration in the CdS buffer layer contributes to the reduction of CdS/CIGS interfacial recombination and improves the CIGS solar cells efficiency.

Loss mechanisms influence on Cu2ZnSnS4/CdS-based thin film solar cell performance

One of the most important issues in kesterite Cu2ZnSnS4 (CZTS)-based thin film solar cells is low open circuit voltage, which is mainly related to loss mechanisms that take place in both CZTS bulk material and CdS/CZTS interface. A device model for CZTS/CdS solar cell which takes into account loss mechanisms influence on solar cell performance is presented. The simulation results showed that our model is able to reproduce experimental observations reported for CZTS/CdS-based solar cells with the highest conversion efficiencies, measured under room temperature and AM1.5 intensity. The comparison of simulation results to experimental observations demonstrated that among the different loss mechanisms, trap-assisted tunneling losses are the major hurdle to boost open circuit voltage. Under this loss mechanism, a solar cell efficiency enhancement up to 10.2% with CdS donor concentration decrease was reached. Finally, the possible path toward a further solar cell efficiency improvement is discussed.

Intermixing at the absorber-buffer layer interface in thin-film solar cells: The electronic effects of point defects in Cu(In,Ga)(Se,S)2 and Cu2ZnSn(Se,S)4 devices

We investigate point defects in the buffer layers CdS and ZnS that may arise from intermixing with Cu(In,Ga)(S,Se)2 (CIGS) or Cu2ZnSn(S,Se)4 (CZTS) absorber layers in thin-film photovoltaics. Using hybrid functional calculations, we characterize the electrical and optical behavior of Cu, In, Ga, Se, Sn, Zn, Na, and K impurities in the buffer. We find that In and Ga substituted on the cation site act as shallow donors in CdS and tend to enhance the prevailing n-type conductivity at the interface facilitated by Cd incorporation in CIGS, whereas they are deep donors in ZnS and will be less effective dopants. Substitutional In and Ga can favorably form complexes with cation vacancies (A-centers) which may contribute to the “red kink” effect observed in some CIGS-based devices. For CZTS absorbers, we find that Zn and Sn defects substituting on the buffer cation site are electrically inactive in n-type buffers and will not supplement the donor doping at the interface as in CIGS/CdS or ZnS devices. Sn may also preferentially incorporate on the S site as a deep acceptor in n-type ZnS, which suggests possible concerns with absorber-related interfacial compensation in CZTS devices with ZnS-derived buffers. Cu, Na, and K impurities are found to all have the same qualitative behavior, most favorably acting as compensating acceptors when substituting on the cation site. Our results suggest one beneficial role of K and Na incorporation in CIGS or CZTS devices is the partial passivation of vacancy-related centers in CdS and ZnS buffers, rendering them less effective interfacial hole traps and recombination centers.

Electron polarization and formation probability of bound muonium in CdS and Si

Positive muons implanted in CdS and Si create electrically active defect centers that mimic and model the contrasting donor states of interstitial hydrogen in these two materials---a shallow donor in CdS and a deep donor in Si. In the present experiment the polarization of the muonium electron and the formation probability of these states are investigated. We find that the polarization is rather weak and strongly deviates from the polarization expected for a paramagnetic center at low temperatures and high magnetic fields. We assume that the polarization is built up in a precursor stage and is not completed at the time of conversion to the observed final state. The polarization depends on the purity of the samples and is larger for samples with a higher concentration of defects. In addition, the branching ratio between paramagnetic muonium and diamagnetic muon depends on the purity of the samples and on the strength of the magnetic field. These different findings are interpreted in the following model: after implantation, the muon comes to rest at an interstitial site in the unrelaxed lattice. This site is unstable and relaxes to the final configuration. The total energy along the transition path and the strength of the single electron binding to the positive muon during the transformation determine the parameters of the experiment.

Improvement in the light emission characteristics of CdS:Cu/CdS diodes

p-CdS:Cu/n-CdS thin-film diodes showed light emission of bright green by deposition of a CdS layer at a high substrate temperature and by the use of an ITO/SnO 2-coated glass substrate. XPS analysis showed that indium diffused into the CdS:Cu layer from the ITO layer at the ITO/CdS:Cu interface but that indium did not diffuse into the CdS:Cu layer at the ITO/SnO 2/CdS:Cu interface. Indium compensates the acceptor of Cu as a donor in CdS. The tin oxide layer prevented diffusion of In into CdS. Again, the deposition of CdS at a high substrate temperature may promote activation of Cu as an acceptor, and the Cu may form shallow acceptor levels. Therefore, it is thought that the bright green emission from the ITO/SnO 2-based diodes is due to shallow Cu acceptor levels.

A theoretical model for the front region of cadmium telluride solar cells

Recently, the modelling investigation on thin film chalcogenide solar cells has been focused on the front region of the devices as a possible source of fill factor losses through a light dependent barrier modulation. We present a model based on a modulated barrier photodiode in series with the main junction, which consistently explains apparent quantum efficiency measurements and, light and dark IV characteristics of CdTe devices. This bulk barrier forms in the front region of the device as a consequence of an increased compensation of donors in CdS, which can induce crossover and in extreme cases rollover in the IV curves. The internal resistance due to this majority carrier barrier, which depends on the irradiance level, is an important limiting factor for cell efficiency, inherent to the cell structure. Numerical simulation gives consistent results with the observed measurement characteristics.

Compensating defects and electrical activation of donors in CdS

Electrical deactivation of donors in CdS was studied by using Perturbed γγ Angular correlation (PAC) spectroscopy and temperature dependence of Hall mobility, resistivity, and free-carrier concentration. PAC spectra and electrical properties were monitored as a function of thermal treatment either under S or Cd pressure in a temperature range from RT to 1073 K. For samples annealed above 800 K under S pressure, Hall effect showed increased electrical compensation whereas PAC detected spontaneous creation of cadmium vacancies, VCd, via the formation of (InCd–VCd) pairs. The increase of the concentration of compensating acceptors, determined from electrical measurements, precisely correlates with the increase of the concentration of (InCd–VCd) pairs found by means of PAC. In contrast to that, thermal treatment under Cd pressure up to 1073 K does not provoke the formation of compensating native defects up to [In] >1019/cm3.

Study of microscopic mechanisms of electrical compensation of donors in CdS by fast diffusors (Cu, Ag, or Au)

The goal of this study was to identify microscopic causes of the extensive changes in electrical properties observed in CdS crystal when doped with some fast-diffusers (FD), Cu, Ag or Au. The compensation of donors and the dynamics of in-diffusion was quantitatively determined by C–V measurements, whereas the changes at the atomic level were followed by perturbed angular correlation (PAC) spectroscopy. The penetration of FDs, as deduced from C–V, correlated well with changes observed in PAC, which manifest as new defects and/or as the increase of the concentration of (VCd–InCd) pairs. Arguments are given that the new defects observed by PAC in CdS:Ag belong to (InCd+–VCd2−–Agi+)o and similar complexes in Cu. No indication of such complexes was observed in Au doped CdS. In CdS:Au practically all donors are passivated directly by pairing with VCd trapped at the nearest Cd site, whereas in Ag or Cu-doped CdS most of the donors are electrically compensated from the distance by FDCd acceptors. Our results suggest a complex microscopic mechanism by which the FD in-diffusion into CdS creates highly compensated semi-insulating material.

Visible light emission from CdS nanocrystals

Room temperature green and red light emission under ultra violet radiation have been observed from CdS nanocrystals. The chemical route used suggested a size distribution of nanocrystallites and was confirmed by atomic force microscopy analysis. The red band photoluminescence is ascribed to the presence of CdO phase as donor in CdS.

The Photoionization in Anisotropic Polar Semiconductors

The photoionization has been considered as an optical transition from the impurity ground state to a continuum state belonging to a conduction band in semiconductors. The dependence of the photoionization cross-section on photon energy is calculated for a shallow donor impurity taking into account the effect of the electron mass anisotropy. The interaction between the electron and the longitudinal optical (LO) phonon is taken into account using Fröhlich's coupling and applying the Lee-Low-Pines transformation. The cross-sections involving zero or one phonon are evaluated for a shallow donor in CdS and it is shown that the cross-section is very sensitive to the electron mass anisotropy, the electron–phonon coupling, and their combined effect.

Defects in CdS: In detected by perturbed angular correlation spectroscopy (PAC)

The local lattice environment of the donor In in CdS is investigated measuring the electric-field gradient at the site of the radioactive probe atom 111In by the perturbed γγ angular correlation technique. It is shown that implantation of In into CdS with subsequent annealing drives 100% of the In atoms to Cd lattice sites. Diffusion of In into CdS under S overpressure results in the formation of In Cd-V Cd pairs which seem to be responsible for the self-compensation of In donors in CdS.

The Pseudopotential Approach to the Shallow Donor Problem in II–VI Semiconductors

AbstractThe problem of chemical shift of shallow donor states in compound semiconductors is investigated in the framework of pseudopotential theory. The “first principle” pseudo‐Hamiltonian including the electron–phonon interaction for shallow donor is obtained and the one‐band pseudo effective mass approximation is discussed. As an example the ionization energies for several donors in CdS and ZnSe are calculated. A good correspondence between the theory and experimental data for impurities in cation site is found.

Optically detected magnetic resonance from a complex donor in CdS

A new optically-detected magnetic resonance at g = 1.885 for H| c, associated with a broad emission band at 2.17 eV, has been observed. The 2.17 eV band is attributed to donor-acceptor recombination involving a complex double donor, and the optically-detected magnetic resonance effect to spin-dependent electron transfer from a shallow donor to the complex donor.

Detection of the EPR and spin lattice relaxation of donors in CdS by Faraday rotation

The Faraday rotation associated with the donors in CdS is found to be a very efficient tool to study magnetic resonance allowing measurement of the spin lattice relaxation time. By comparing two different concentrations it is shown that interaction between donors becomes important well before the onset of the Mott transition.

Magnetic properties of an amorphous antiferromagnet

We present measurements and a theoretical analysis of the magnetic properties of dilute neutral (In) donors in CdS, which we demonstrate to be a well-characterized example of an amorphous antiferromagnet with spins S=1/2. High-quality data for relative values of sample magnetization have been obtained by the Faraday rotation method at temperatures ranging from 55 mK to 2.5 K and fields up to 15 kG. Data are given for crystals with neutral donor concentrations of 8⋅1016/cm3 and 2.1⋅1017/cm3. The susceptibility, χ, increases monotonically with decreasing T with no evidence of ordering, even though the median exchange coupling is ∼7 K. A model Hamiltonian is derived by scaling calculations of direct exchange for the H2 molecule to the Bohr radius and effective Rydberg of the donor wave functions. A sample of such a Hamiltonian is generated in a computer simulation and solved approximately by dividing the system into clusters (and isolated spins) according to a coupling threshold and solving the cluster Hamiltonians exactly. Inter-cluster interactions are included in a molecular field approximation. Excellent agreement with experiment is found, and χ is shown to be dominated by clusters, both even and odd, at all temperatures.

Magnetic Behavior of anS=12Amorphous Antiferromagnet

The magnetic behavior of bound shallow donors in CdS, a model $S=\frac{1}{2}$ amorphous antiferromagnet, has been measured by a Faraday rotation technique down to $T\ensuremath{\sim}50$ mK. The susceptibility, $\ensuremath{\chi}$, and $\frac{d\ensuremath{\chi}}{\mathrm{dT}}$ as well increase monotonically with decreasing $T$ with no sign of an ordering transition even though the median exchange interaction of a spin is \ensuremath{\sim}7 K. The results agree with a hydrogeniclike exchange model which is solved in terms of exactly diagonalized weakly interacting clusters.

Molecular model for exciton-impurity complexes in CdS and CdSe

A theory of the exciton bound to a neutral or ionized donor in CdS and CdSe is presented. The high anisotropy of the hole mass is taken into account. The theoretical results are compared with the available experimental data.