Y. Y. Proskuryakov

Impedance spectroscopy of unetched CdTe/CdS solar cells—equivalent circuit analysis

A detailed study of electric and structural properties has been carried out on CdTe/CdS solar cells which deliberately were not subjected to etching by a nitric-phosphoric (NP) or bromine-methanol (Br-Me) acids, conventionally employed for the formation of Te-rich layer before back contacting. In the previous work [J. Appl. Phys. 101, 014505 (2007)] we have shown that cells that were not etched provide more extensive information on sample/material properties than the etched ones, as analyzed by admittance spectroscopy. Although seemingly being able to describe the distribution of defect energy levels, the admittance spectroscopy approach has a significant drawback because the underlying theoretical formulation does not take into account the frequency-dependent contribution from the back contact together with its influence on the trap contributions. In this work we use an alternative methodology for analysis of impedance data measured in dark conditions, which applies an equivalent circuit model to the exp...

Impedance spectroscopy of thin-film CdTe/CdS solar cells under varied illumination

The electrical properties of CdTe/CdS solar cells grown by metal organic chemical vapor deposition were investigated by a technique of impedance measurements under varied intensity of AM1.5 illumination. A generalized impedance model was developed and applied to a series of CdTe/CdS cells with variations in structure and doping. The light measurements were compared to the conventional ac measurements in dark under varied dc bias, using the same methodology for equivalent circuit analysis in both cases. Detailed information on the properties of the device structure was obtained, including the properties of the main p-n junction under light, minority carrier lifetime, back contact, as well as the effect of the blocking ZnO layer incorporated between the transparent conductor and CdS layers. In particular, the comparison between samples with different chemical concentrations of As has shown that the total device impedance and the series resistance are strongly increased at lower As densities, resulting in th...

Highly Arsenic Doped CdTe Layers for the Back Contacts of CdTe Solar Cells

In an effort to overcome the lack of a suitable metal as an ohmic back contact for CdTe solar cells, a study was carried out on the potential for using a highly arsenic (As) doped CdTe layer with metallization. The deposition of full CdTe/CdS devices, including the highly doped CdTe:As and the CdCl2 treatment, were carried out by metal organic chemical vapour deposition (MOCVD), in an all-in-one process with no etching being necessary. They were characterized and compared to control devices prepared using conventional bromine-methanol back contact etching. SIMS and C-V profiling results indicated that arsenic concentrations of up to 1.5 × 1019 at·cm-3 were incorporated in the CdTe. Current-voltage (J-V) characteristics showed strong improvements, particularly in the open-circuit voltage (Voc) and series resistance (Rs): With a 250 nm thick doped layer, the series resistance was reduced from 9.8 Ω·cm2 to 1.6 Ω·cm2 for a contact area of 0.25 cm2; the J-V curves displayed no rollover, while the Voc increased by up to 70 mV (~ 12 % rise). Preliminary XRD data show that there may be an As2Te3 layer at the CdTe surface which could be contributing to the low barrier height of this contact.

Admittance spectroscopy of CdTe/CdS solar cells subjected to varied nitric-phosphoric etching conditions

In this work we investigate the electric and structural properties of CdTe∕CdS solar cells subjected to a nitric-phosphoric (NP) acid etching procedure, employed for the formation of a Te-rich layer before back contacting. The etching time is used as the only variable parameter in the study, while admittance spectroscopy is employed for the characterization of the cells’ electric properties as well as for the analysis of the defect energy levels. Particular attention was also given to the characteristics of unetched devices and it is shown that despite the larger height of back-contact barrier such samples show well defined admittance spectra, as well as allow for extraction of as much as five defect levels in the range of 0.08–0.9eV above the valence band. In contrast, admittance characteristics of the etched samples show a decrease of the number of the detectable trap levels with increasing etching time. (Hence it is usual for only one or two trap levels to be reported in the literature for finished dev...

Comparative study of trap densities of states in CdTe∕CdS solar cells

Density of deep and shallow states has been investigated in three different kinds of CdTe∕CdS samples, two of which were grown by metal-organic chemical vapor deposition (MOCVD) and one by close-space sublimation (CSS) methods. The MOCVD samples were p doped by As and grown either with or without a ZnO buffer layer between the transparent conductor and CdS layers. Capacitance-voltage, admittance spectroscopy, and quantum efficiency measurements show pronounced effects of As doping and ZnO incorporation. It is found that A centers and vacancies of Cd, usually observed in CSS devices, are absent in the defect spectra of MOCVD samples.

Evaluation of the chemical, electronic and optoelectronic properties of γ-CuCl thin films and their fabrication on Si substrates

CuCl is a I–VII semiconductor material with a direct band gap of ~3.4 eV. It exhibits a zincblende structure (γ-phase) at low temperatures, up to ~680 K. Unlike GaN, ZnO and related materials, CuCl has a relatively low lattice mismatch with Si (<0.4%) and a large excitonic binding energy (~190 meV). This suggests the possibility of the fabrication of excitonic-based blue/UV optoelectronic devices on Si with relatively low threading dislocation densities. In this study, CuCl has been deposited and examined as a candidate material for the fabrication of these devices. X-ray diffraction (XRD) measurements confirmed that the deposited films were preferentially oriented in the (1 1 1) plane. Room temperature photoluminescence measurements reveal a strong Z3 free exciton peak (3.232 eV). Both steady state dc and ac impedance spectroscopy experiments suggested that the deposited CuCl is a mixed ionic–electronic semiconductor material. An electronic conductivity of the order of 2.3 × 10−7 S cm−1 was deduced to be in coexistence with Cu+ ionic conductivity using irreversible electrodes (Au), while a total conductivity of the order of 6.5 × 10−7 S cm−1 was obtained using reversible electrodes (Cu) at room temperature. Further to this, we have identified some of the challenges in fabricating an optoelectronic device based on a CuCl/Si hybrid platform and propose some possible solutions.

Impact of cadmium-rich back surfaces on cadmium chloride treatment and device performance in close space sublimation deposited CdTe solar cells

The cool-down process following close-space sublimation deposition of CdTe films has been found to induce the formation of a Cd-rich layer at the back surface. It has also been shown that this layer may subsequently block the in-diffusion of CdCl2 used in the post-growth activation step of solar cell devices. Various routes to correct this have been investigated, with the most effective being found to be pre-etching of the as-grown surfaces prior to doping with a nitric-phosphoric acid etch. This leads to increase in the amount of chlorine and oxygen in-diffusion owing to the formation of a Te-rich layer and this improves device efficiency from ∼2% to ∼10%. Incorporation of the etching step into device processing for CdTe/CdS devices with a ZnO buffer layer and two-stage CdTe growth allowed for fabrication of cells having 13.6% efficiency (1 cm2). The devices were grown on NSG TEC™ C15, an SnO2:F multilayer on low iron soda lime glass.

Nucleation and Grain Boundaries in Thin Film CdTe/CdS Solar Cells

The early stage formation mechanisms operating during the sublimation growth of CdTe films on CdS has been evaluated using a growth interrupt methodology for deposition under 100 Torr of N 2 . Key stages of the growth were identified and are discussed in terms of the processes of island nucleation, island growth/coalescence, channel formation and secondary nucleation that have been reported for other materials systems. It was demonstrated that the grain size could be manipulated by means of controlling the gas pressure in the range 2 – 200 Torr, with the grain diameter increasing with pressure linearly as D (μm) = 0.027(± 0.011) × P (Torr) + 0.90(± 0.31). For a series of solar cells made using such material, the performance parameters were seen to increase with grain size up to a plateau corresponding to grains of ∼4 μm in this case. Equivalent circuit parameters for resistive components arising from grain boundaries, and the contact to the CdTe, were measured. It is considered that grain boundary barriers in CdTe are harmful to PV performance, and that the plateau in performance occurs when the grain size is increased to the level where the contact resistance is greater than that due to grain boundaries.