15.3% efficient graded bandgap solar cells fabricated using electroplated CdS and CdTe thin films

Making use of previously designed and experimentally tested results of graded bandgap devices, and the comprehensive electrodeposition of semiconducting materials knowledge, a three layer n-n-p device structure was fabricated and tested for their electronic properties and solar cell performance. Glass/FTO/n-CdS/n-CdTe/p-CdTe/Au devices were fabricated and studied as a first step towards development of graded bandgap devices using electroplated materials. Efficiencies up to 15.3% were observed for lab-scale small devices

The effect on CdS/CdTe solar cell conversion efficiency of the presence of fluorine in the usual CdCl2 treatment of CdTe

The addition of CdF2 to the CdCl2 solution used in the well-known CdCl2 treatment of CdS/CdTe solar cells has been observed to drastically improve the conversion efficiency of fully fabricated CdS/CdTe solar cells. The observed improvement is as a result of further enhancement of structural and optoelectronic properties of the CdCl2+CdF2-treated CdTe layers compared to the CdCl2-treated CdTe layers. A set of CdS/CdTe samples were grown by electrochemical deposition under different conditions and each sample was divided into two. One set of these parts was treated with only CdCl2 solution while the other set was treated with the CdCl2+CdF2 mixture. Both sets were annealed at 450°C for 15min and the solar cells completed by evaporating Au back contact on the CdTe layers after chemical etching. The results of the device measurements show drastic improvements in all the solar cell parameters (open-circuit voltage, short-circuit current density, fill factor and conversion efficiency) for the devices treated with CdCl2+CdF2 mixture compared to those treated with only CdCl2. The conversion efficiencies increased from (1.0-3.0)% with CdCl2 treatment to about (5.0-7.0)% with CdCl2+CdF2 treatment. The observed improvements have been attributed to the unique properties of fluorine

Electrochemical deposition of CdTe semiconductor thin films for solar cell application using two-electrode and three-electrode configurations: a comparative study

Thin films of CdTe semiconductor were electrochemically deposited using two-electrode and three-electrode configurations in potentiostatic mode for comparison. Cadmium sulphate and tellurium dioxide were used as cadmium and tellurium sources respectively. The layers obtained using both configurations exhibit similar structural, optical and electrical properties with no specific dependence on any particular electrode configuration used. These results indicate that electrochemical deposition (electrodeposition) of CdTe, and semiconductors in general, can equally be carried out using two-electrode system as well as the conventional three-electrode system without compromising the essential qualities of the materials produced. The results also highlight the advantages of the two-electrode configuration in process simplification, cost reduction and removal of a possible impurity source in the growth system especially as the reference electrode ages. Keywords: Electrochemical deposition; two-electrode system; three-electrode system; CdTe; thin-films; solar cells

The effects of anode material type on the optoelectronic properties of electroplated CdTe thin films and the implications for photovoltaic application

The effects of the type of anode material on the properties of electrodeposited CdTe thin films for photovoltaic application have been studied. Cathodic electrodeposition of two sets of CdTe thin films on glass/fluorine-doped tin oxide (FTO) was carried out in two-electrode configuration using graphite and platinum anodes. Optical absorption spectra of films grown with graphite anode displayed significant spread across the deposition potentials compared to those grown with platinum anode. Photoelectrochemical cell result shows that the CdTe grown with graphite anode became p-type after post-deposition annealing with prior CdCl2 treatment, as a result of carbon incorporation into the films, while those grown with platinum anode remained n-type after annealing. A review of recent photoluminescence characterization of some of these CdTe films reveals the persistence of a defect level at (0.97–0.99) eV below the conduction band in the bandgap of CdTe grown with graphite anode after annealing while films grown with platinum anode showed the absence of this defect level. This confirms the impact of carbon incorporation into CdTe. Solar cell made with CdTe grown with platinum anode produced better conversion efficiency compared to that made with CdTe grown using graphite anode, underlining the impact of anode type in electrodeposition

Investigating the effect of GaCl3 incorporation into the usual CdCl2 treatment on CdTe-based solar cell device structures

The incorporation of GaCl3 into the usual CdCl2 post-deposition treatment solution of CdTe-based solar cells has been investigated. Both CdS and CdTe layers used in this work were prepared by electroplating technique and they are n-type in electrical conduction as observed from photoelectrochemical cell measurement technique. Before applying the chemical treatments to the device structures, the effect of GaCl3 incorporation into the usual CdCl2 treatment was first studied on the structural and optical properties of CdTe thin films. The results of the optical properties show that the bandgap of CdTe thin films treated with a mixture of GaCl3 + CdCl2 is closer to that of bulk CdTe layers than the ones treated only with CdCl2 solution. The structural properties also showed that CdTe thin films treated with GaCl3 + CdCl2 is more crystalline than CdTe thin films treated only with CdCl2 solution. The addition of GaCl3 into the CdCl2 solution have been seen to drastically enhance the solar-to-electric conversion efficiency of CdS/CdTe based solar cells. One of the effects of Ga incorporation into the usual CdCl2 treatment was seen in the series resistance reduction which ultimately leads to enhancement in the observed short-circuit current density, fill factor and overall solar cell efficiency. For the glass/FTO/n-CdS/n-CdTe device structures, the cell efficiencies were observed in the range 1.9–2.1% after being treated with CdCl2 solution only. When treated with CdCl2 + GaCl3, the efficiency increased to 6.1–6.4%. Subsequent study on multi-junction graded bandgap solar cells using the GaCl3 + CdCl2 chemical solution for the surface treatment of glass/FTO/n-ZnS/n-CdS/n-CdTe device structures results in solar cell efficiency >10%

Investigating the electronic properties of multi-junction ZnS/CdS/CdTe graded bandgap solar cells

The fabrication of multi-junction graded bandgap solar cells have been successfully implemented by electroplating three binary compound semiconductors from II-VI family. The three semiconductor materials grown by electroplating techniques are ZnS, CdS and CdTe thin films. The electrical conductivity type and energy bandgap of each of the three semiconductors were determined using photoelectrochemical (PEC) cell measurement and UV-Vis spectrophotometry techniques respectively. The PEC cell results show that all the three semiconductor materials have n-type electrical conductivity. These two material characterisation techniques were considered in this paper in order to establish the relevant energy band diagram for device results, analysis and interpretation. Solar cells with the device structure glass/FTO/n-ZnS/n-CdS/n-CdTe/Au were then fabricated and characterised using current-voltage (I-V) and capacitance-voltage (C-V) techniques. From the I-V characteristics measurement, the fabricated device structures yielded an open circuit voltage (Voc) of 670 mV, short circuit current density (Jsc) of 41.5 mAcm-2 and fill-factor (FF) of 0.46 resulting in ∼12.8% efficiency when measured at room temperature under AM1.5 illumination conditions. The device structure showed an excellent rectification factor (RF) of 104.3 and ideality factor (n) of 1.88. The results obtained from the C-V measurement also showed that the device structures have a moderate doping level of 5.2×1015 cm-3

Investigation of electronic quality of electrodeposited cadmium sulphide layers from thiourea precursor for use in large area electronics

CdS layers used in thin film solar cells and other electronic devices are usually grown by wet chemical methods using CdCl2 as the Cadmium source and either Na2S2O3, NH4S2O3 or NH2CSNH2 as Sulphur sources. Obviously, one of the sulphur precursors should produce more suitable CdS layers required to give the highest performing devices. This can only be achieved by comprehensive experimental work on growth and characterisation of CdS layers from the above mentioned sulphur sources. This paper presents the results observed on CdS layers grown by electrodepositing using two electrode configuration and thiourea as the sulphur precursor. X-ray diffraction (XRD), Raman spectroscopy, optical absorption, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and photoelectrochemical (PEC) cell methods have been used to characterise the material properties. In order to test and study the electronic device quality of the layers, ohmic and rectifying contacts were fabricated on the electroplated layers. Schottky barriers, formed on the layers were also compared with previously reported work on Chemical Bath Deposited CBD-CdS layers and bulk single crystals of CdS. Comparatively, Schottky diodes fabricated on electroplated CdS layers using two-electrode system and thiourea precursor exhibit excellent electronic properties suitable for electronic devices such as thin film solar panels and large area display devices

Fabrication of CdS/CdTe-based thin film solar cells using an electrochemical technique

Thin film solar cells based on cadmium telluride (CdTe) are complex devices which have great potential for achieving high conversion efficiencies. Lack of understanding in materials issues and device physics slows down the rapid progress of these devices. This paper combines relevant results from the literature with new results from a research programme based on electro-plated CdS and CdTe. A wide range of analytical techniques was used to investigate the materials and device structures. It has been experimentally found that n-, i- and p-type CdTe can be grown easily by electroplating. These material layers consist of nano- and micro-rod type or columnar type grains, growing normal to the substrate. Stoichiometric materials exhibit the highest crystallinity and resistivity, and layers grown closer to these conditions show n - p or p - n conversion upon heat treatment. The general trend of CdCl2 treatment is to gradually change the CdTe material’s n-type electrical property towards i-type or p-type conduction. This work also identifies a rapid structural transition of CdTe layer at 385 ± 5 °C and a slow structural transition at higher temperatures when annealed or grown at high temperature. The second transition occurs after 430 °C and requires more work to understand this gradual transition. This work also identifies the existence of two different solar cell configurations for CdS/CdTe which creates a complex situation. Finally, the paper presents the way forward with next generation CdTe-based solar cells utilising low-cost materials in their columnar nature in graded bandgap structures. These devices could absorb UV, visible and IR radiation from the solar spectrum and combine impact ionisation and impurity photovoltaic (PV) effect as well as making use of IR photons from the surroundings when fully optimised