X. Mathew

Photo-induced current transient spectroscopic study of the traps in CdTe

Photo-induced current transient spectroscopy is an efficient technique for the detection and identification of traps in semiconductors. This technique has been used to determine the traps in various semiconductors. CdTe is one of the leading candidates for the development of thin film solar cells; hence, the detection of trap levels in CdTe is of interest for the photovoltaic researchers. In this review special emphasis is given to the photo-induced current transient spectroscopy (PICTS) technique applied in the case of CdTe. The PICTS technique, theory as well as a collection of the majority of the traps in CdTe reported in the literature are presented.

Development of CdTe thin films on flexible substrates—a review

In the recent years there has been an increased interest in photovoltaic structures on lightweight flexible substrates. Photovoltaic structures on lightweight substrates have several advantages over the heavy glass-based structures in both terrestrial and space applications. CdTe is one of the leading candidates for the solar cells due to its optimum band gap and the variety of film preparation methods. The development of CdTe thin films on flexible substrates is discussed. The film growth and characterization are reviewed.

Optical properties of electrodeposited CdTe thin films

Abstract The absorption coefficient spectra of the elctrodeposited CdTe thin films were analyzed and compared with that of the single crystal. Pinhole-free thin films facilitated the analysis of the high-energy regions of the absorption coefficient spectra. The various allowed direct and indirect transitions were detected successively by subtracting the extrapolated values of the lower-energy transitions. The effect of heat treatment on the optical transitions were analyzed with films annealed at 300°C in air, argon and CdCl 2 .The direct band gap of the electrodeposited films decreased with increasing film thickness and approaches the value of the single crystal. The films annealed at different environments show slightly lower value for the band gap. Annealing in argon caused significant change in the optical transition spectra.

Does the band gap calculated from the photocurrent of Schottky devices lead to erroneous results? Analysis for CdTe

In recent years there has been an increased interest in flexible, lightweight photovoltaic modules based on thin metallic substrates. This paper reports some optoelectronic properties of electrodeposited CdTe thin films grown onto lightweight stainless steel (SS) foils. The optoelectronic properties were investigated with Schottky barriers of Au/CdTe/SS structure. The influence of the built-in potential of the Schottky junction on the bulk and the interface recombination of the photo-generated minority carriers is explained with the existing models. The voltage-dependent collection functions influence the photocurrent of the devices in both short- and long-wavelength regions of the spectrum. It is observed that in the photovoltaic mode the contribution due to the collection functions depends on the open-circuit voltage of the device. Au/CdTe Schottky devices, having higher open-circuit voltage, exhibit a better response in the long wavelength region. This is due to the efficient collection of the carriers generated in the bulk of the film and in such devices the contribution from the bulk collection function is higher. The enhancement in the bulk collection function causes a shift in the response of the device to higher wavelengths giving lower values for the calculated band gap. Due to this dependence of the long wavelength response on the open-circuit voltage of the devices, the band gap calculated from the photocurrent of different Schottky devices gives different values for the band gap of the material. Thus the method of calculating the band gap from the photocurrent of Schottky devices can lead to erroneous conclusions regarding the band gap of the material.

Opto-electronic properties of an Au/CdTe device

Au/CdTe Schottky devices were used to investigate various opto-electronic parameters such as open circuit voltage, short circuit current, fill factor, efficiency, spectral response, activation energy and the band gap of CdTe. The dependence of various device parameters on illumination and temperature has been investigated. The activation energy measurements revealed two trapping levels with energies 0.31 eV and 0.53 eV under 0.8 V bias. These levels showed voltage dependence and the zero-field values of the activation energy were estimated to be 0.49 eV and 0.66 eV, respectively. Devices with higher open circuit voltage showed an enhanced photo response in the long wavelength region. The dependence of the long wavelength cut-off on the open circuit voltage of the devices was explained as being due to the influence of the bulk collection function. The absolute zero value of the band gap was estimated to be 1.61 eV.

Structural modifications of SnO2 due to the incorporation of Fe into the lattice

Recently there is an increased interest in developing magnetic semiconductors due to their promising applications in spintronics. The semiconductors can be made ferromagnetic by doping with transition-metal ions. In this paper, the results of our studies using x-ray diffraction (XRD) and IR and Raman spectroscopic techniques on the effect of Fe doping on the structural properties of SnO2 are presented. The XRD results showed that the doping affects the structure and the lattice constants decrease as doping concentration increases, reaches a minimum, and again increases. The doped samples are under compressive strain and the strain is maximum for the sample doped with 3% (at. %) Fe. The grain size of the nanoparticles decreases from 42nm in undoped SnO2 to 26nm in Sn0.90Fe0.10O2. It was observed that the preferred orientation is along the (101) direction and both texture coefficient and preferential orientation show a dependence on doping level. The Raman spectra showed clear evidences of the change in gra...

A CdTe/PMeT photovoltaic structure formed by electrodeposition and processing

CdTe thin films were electrodeposited from an ethylene-glyco-based bath by the galvanostatic method. As-deposited and tellurized films were characterized by structural, optoelectronic and photoelectrochemical methods. The film stoichiometry improved after tellurization of the film at 300°C by a technique called chemical vapor transport by Gas (CVTG) in a tubular furnace. Tellurized films showed near stoichiometry with p-type conductivity in the bulk and n-type surface conductivity. Schottky barrier type photovoltaic junctions were obtained using a heavily doped PMeT (poly-3(methylthiophene), prepared by electropolymerization, displaying nearly metallic behavior, and CdTe obtained by electrodeposition. A solar to electrical conversion efficiency of the order of 1% was obtained in the case of PMeT/CdTe junction.

Development of a semitransparent CdMgTe/CdS top cell for applications in tandem solar cells

The band gap of Cd1−xMgxTe can be easily tuned from 1.48 to 3.5 eV (x = 0 to 1), and hence this material is a potential candidate for developing the wide band gap top cell in tandem solar cells. In this paper, we present an all-sputtered Cd1−xMgxTe/CdS top cell developed on commercial SnO2:F-coated (Tec-7) glass substrates with sheet resistance of ~7Ω/. The Cd1−xMgxTe (x = 0.05, 0.21, 0.26) films were deposited by RF sputtering at deposition temperatures in the range 250–300 °C. The films were characterized for structure, morphology and optical properties. Raman spectroscopic studies showed that the lattice perturbations increase with Mg content in the film. We have explored cadmium chloride vapor annealing of Cd1−xMgxTe thin films, and found that the film composition has a significant influence on stability. Films with a lesser amount of Mg show better stability during vapor chloride treatments at 387 °C. Longer annealing durations resulted in a decrease in band gap of the films. Recrystallization due to the vapor chloride treatments were studied using transmittance, XRD, AFM and Raman spectroscopy. Prototype devices of the configuration Tec-7/CdS/Cd0.95Mg0.05Te/metal show promising photovoltaic characteristics such as 630 mV open circuit voltage and 17 mAcm−2 short circuit current.

Enhanced charge carrier mobility and lifetime suppress hysteresis and improve efficiency in planar perovskite solar cells

Perovskite solar cells (PSCs) are very promising lab-scale technologies to deliver inexpensive solar electricity. Low-temperature, planar PSCs are of particularly interest for large-scale deployment due to their inherent suitability for flexible substrates and potential for silicon/perovskite tandems. So far, planar PSCs have been prone to large current–voltage hysteresis and low stabilized power output due to a number of issues associated with this kind of device configuration. We find that the suppression of the yellow-phase impurity (∂-FAPbI3) present in formamidium-based perovskites, by RbI addition, contributes to low hysteresis, higher charge carrier mobility, long-lived carrier lifetimes and a champion stabilized power output of 20.3% using SnOx as the electron selective contact. We study the effects of these impurities on the transient behavior that defines hysteresis and its relation to ionic movement. In addition, we find that the formation of a RbPbI3 phase does not significantly affect the charge carrier lifetimes and consequently the performance of the devices. This brings new physical insights onto the role of different impurities in perovskite solar cells, which make these materials so remarkable.

Photoelectrochemical characterization of porous Si

Abstract The photoelectrochemical measurements of p-Si and porous silicon in 0.1 M H2SO4 were done. The band gap of Si is a bit low for efficient hydrogen production. The silicon was made porous to improve the efficiency of hydrogen production. The porous silicon photocathodes show a substantial improvement in the hydrogen production compared to that of p-Si photocathodes. Studies were done by applying different light flux on the porous silicon.