CdCl2 Activation Research Articles

Absorber texture and the efficiency of polycrystalline thin film CdTe solar cells

A range of microstructural changes occur during the deposition and activation of CdTe based thin film solar cells. In particular, the cadmium chloride (CdCl2) activation treatment results in wholesale recrystallisation which transforms the conversion efficiency of the solar cell. One of the noticeable effects is the change of preferred orientation of the CdTe absorber. Highly orientated [111] texture is observed in as deposited or under-treated CdTe based devices. Optimized activation results in a more randomized texture and the [111] preferred texture component is significantly weakened. In this paper we use Electron Backscatter Diffraction to characterise absorber cross-sections. The focus is on how randomization of the absorber texture reflects device performance. We have had access to a range of CdTe devices using a variety of deposition techniques. We have observed a clear pattern that shows that devices with a highly orientated [111] texture have poor efficiency. Devices with a randomized texture have much higher efficiency. Here we illustrate this empirical correlation using devices deposited by Metal Organic Chemical Vapour Deposition with a range of efficiencies from 13.1 % to 17 %. We have also included the analysis of an absorber from a 18.7 % high efficiency CdSeTe/CdTe device to show that texture is similarly important in these advanced devices. We have been able to quantify the effect of texture by using multiples of uniform density or (MUD) values from the inverse pole figures. MUD figures close to 1 correlate with highest efficiency. Although the random texture of the absorber microstructure is only one of several important process factors, it appears to be a necessary feature for highest efficiency CdTe-based polycrystalline solar cells.

Exploring the impact of different annealing conditions on physical properties of CdSeTe thin films for solar cell absorber layer applications

The perovskite and Silicon based single junction solar cells certainly showed higher efficiency but the perovskite solar cells are highly degradable and unstable in environment whereas conventional Silicon solar cells are relatively costly. The CdTe absorber based thin film solar cells have attained efficiency of greater than 22% where doping of Selenium into the CdTe absorber layer seems to improve the photovoltaic performance. The CdCl2 treatment is proven to be critical process which induces the grain growth and recrystallization in CdSeTe films. Thus herein, air and CdCl2 annealing is performed over thermally evaporated CdSeTe thin films to seek out their applicability as an effective absorber for Cd-based thin film photovoltaics. The XRD diffractograms exposed the appearance of cubic phase with (111) preferred orientation where crystallite size is increased with annealing temperature. The optical absorbance is found to be altered with temperature of annealing and Tauc’s plots exhibited that bandgap is found to be changed in the range of 1.40–1.52 eV with increasing annealing temperature. Electrical analysis divulges Ohmic character of films where electrical resistivity and conductivity are affected by nature and temperature of post-deposition annealing. The EDS patterns confirmed the deposition of CdSeTe thin films and post deposition CdCl2 activation treatment. The FESEM images demonstrated alteration in surface morphology by grain growth where the grain size of CdSeTe films is increased from 70 nm to 235 nm with CdCl2 annealing temperature. The findings confirm that annealed CdSeTe thin films showed demanding behavior for solar cell devices as prominent light harvesting absorber layers.

Phase transition correlated grain growth in CdSe thin films: Annealing evolution to cadmium chloride activation

In order to explore a potential absorber layer, an impact of post-deposition CdCl2 activation on thermally evaporated CdSe thin films is undertaken where physical characteristics of CdSe thin films are performed at 200 °C, 300 °C and 400 °C temperatures. The crystallographic characteristics reveal that all CdSe films show polycrystalline behavior and having mixture of hexagonal and cubic phases where (111)C phase is changed into (103)H phase at temperature of 400 °C. The crystallite size and resistivity are altered from 27 to 44 nm and from 1.60 × 102 to 1.89 × 102 Ωcm with CdCl2 activation, respectively. The absorbance and intensity of PL emission peaks are enhanced with CdCl2 activation. AFM images show hill like topography where roughness and average grain size are increased from 75.83 nm to 143.8 nm and from 36 nm to 53 nm, respectively. FESEM images demonstrated explicit grain growth with activation. The integration of Cl and successful growth of CdSe films are confirmed by EDS patterns.

Chlorine activated stacking fault removal mechanism in thin film CdTe solar cells: the missing piece

The conversion efficiency of as-deposited, CdTe solar cells is poor and typically less than 5%. A CdCl2 activation treatment increases this to up to 22%. Studies have shown that stacking faults (SFs) are removed and the grain boundaries (GBs) are decorated with chlorine. Thus, SF removal and device efficiency are strongly correlated but whether this is direct or indirect has not been established. Here we explain the passivation responsible for the increase in efficiency but also crucially elucidate the associated SF removal mechanism. The effect of chlorine on a model system containing a SF and two GBs is investigated using density functional theory. The proposed SF removal mechanisms are feasible at the 400 ∘C treatment temperature. It is concluded that the efficiency increase is due to electronic effects in the GBs while SF removal is a by-product of the saturation of the GB with chlorine but is a key signal that sufficient chlorine is present for passivation to occur.

Back-Surface Passivation of CdTe Solar Cells Using Solution-Processed Oxidized Aluminum.

Although back-surface passivation plays an important role in high-efficiency photovoltaics, it has not yet been definitively demonstrated for CdTe. Here, we present a solution-based process, which achieves passivation and improved electrical performance when very small amounts of oxidized Al3+ species are deposited at the back surface of CdTe devices. The open circuit voltage (Voc) is increased and the fill factor (FF) and photoconversion efficiency (PCE) are optimized when the total amount added corresponds to ∼1 monolayer, suggesting that the passivation is surface specific. Addition of further Al3+ species, present in a sparse alumina-like layer, causes the FF and PCE to drop as the interface layer becomes blocking to current flow. The optimized deposit increases the average baseline PCE for both Cu-free devices and devices where Cu is present as a dopant. The greatest improvement is found when the Al3+ species are deposited prior to the CdCl2 activation step and Cu is employed. In this case, the best-cell efficiency was improved from 12.6 to 14.4%. Time-resolved photoluminescence measurements at the back surface and quantum efficiency measurements performed at the maximum power point indicate that the performance enhancement is due to a reduction in the interface recombination current at the back surface.

A study on CdCl2 activation of CBD-CdS films

A study on CdCl2 activation of CBD-CdS films

Properties of Arsenic-Doped ZnTe Thin Films as a Back Contact for CdTe Solar Cells.

As-doped polycrystalline ZnTe layers grown by metalorganic chemical vapor deposition (MOCVD) have been investigated as a back contact for CdTe solar cells. While undoped ZnTe films were essentially insulating, the doped layers showed significant rise in conductivity with increasing As concentration. High p-type carrier densities up 4.5 × 1018 cm−3 was measured by the Hall-effect in heavily doped ZnTe:As films, displaying electrical properties comparable to epitaxial ZnTe single crystalline thin films in the literature. Device incorporation with as-deposited ZnTe:As yielded lower photovoltaic (PV) performance compared to reference devices, due to losses in the open-circuit potential (VOC) and fill factor (FF) related to reducing p-type doping density (NA) in the absorber layer. Some minor recovery observed in absorber doping following a Cl-free post–ZnTe:As deposition anneal in hydrogen at 420 °C contributed to a slight improvement in VOC and NA, highlighting the significance of back contact activation. A mild CdCl2 activation process on the ZnTe:As back contact layer via a sacrificial CdS cap layer has been assessed to suppress Zn losses, which occur in the case of standard CdCl2 anneal treatments (CHT) via formation of volatile ZnCl2. The CdS sacrificial cap was effective in minimising the Zn loss. Compared to untreated and non-capped, mild CHT processed ZnTe:As back contacted devices, mild CHT with a CdS barrier showed the highest recovery in absorber doping and an ~10 mV gain in VOC, with the best cell efficiency approaching the baseline devices.

CdS barrier to minimize Zn loss during CdCl2 treatment of Cd-Zn-Te absorbers

A major challenge in the fabrication of high band gap II–VI polycrystalline solar cells is to preserve the original composition of the absorber after the CdCl2 activation treatment. In this study, a method is demonstrated to maintain the Cd-Zn-Te alloy absorber composition during its exposure to the CdCl2 treatment. A thin film of CdS was applied as a barrier on the back surface of the high band gap polycrystalline Cd(1−x)ZnxTe (x = 20% by atomic ratio, corresponding band gap 1.72 eV) before the CdCl2 treatment. Using transmission electron microscopy and energy dispersive spectroscopy, it was observed that the composition of Cd-Zn-Te was maintained after the CdCl2 treatment. The devices fabricated after removing the thin film of CdS, exhibited diode-like behavior. A significant increase in the quantum efficiency near the short wavelength region was observed, and the band gap of Cd(1−x)ZnxTe was maintained.

Development of CdCl2 Activation to Minimize Zn Loss from Sputtered Cd1-xZnxTe Thin Films for Use in Tandem Solar Cells

ABSTRACTIncreasing the band gap of cadmium telluride (CdTe) from 1.48 eV to > 2 eV can be achieved by alloying CdTe with ZnTe. Like CdTe, the alloyed films are expected to allow for low cost production, suggesting that Cd1-xZnxTe could be an ideal top cell for mass produced tandem devices. However, the CdCl2 activation of the alloyed films results in a significant loss of Zn, thereby reducing the bandgap. In this study, we demonstrate a novel CdCl2 activation method that does not result in significant Zn loss. By performing the activation step in a closed, inert environment we are able to avoid oxidation of the Zn in the Cd1-xZnxTe film; furthermore, by including sacrificial Zn in the container, an overpressure of ZnCl2 forms limiting the amount of ZnCl2 formed in the film. Both x-ray diffraction, optical measurements, and Auger spectroscopy show that the CdCl2 treatment with no flowing gas minimizes the loss of Zn from the CZT alloy.

Structural and chemical evolution of the CdS:O window layer during individual CdTe solar cell processing steps

Oxygenated cadmium sulfide (CdS:O) is often used as the n-type window layer in high-performance CdTe heterojunction solar cells. The as-deposited layer prepared by reactive sputtering is XRD amorphous, with a bulk composition of CdS0.8O1.2. Recently it was shown that this layer undergoes significant transformation during device fabrication, but the roles of the individual high temperature processing steps was unclear. In this work high resolution transmission electron microscopy coupled to elemental analysis was used to understand the evolution of the heterojunction region through the individual high temperature fabrication steps of CdTe deposition, CdCl2 activation, and back contact activation. It is found that during CdTe deposition by close spaced sublimation at 600 °C the CdS:O film undergoes recrystallization, accompanied by a significant (∼30%) reduction in thickness. It is observed that oxygen segregates during this step, forming a bi-layer morphology consisting of nanocrystalline CdS adjacent to the tin oxide contact and an oxygen-rich layer adjacent to the CdTe absorber. This bilayer structure is then lost during the 400 °C CdCl2 treatment where the film transforms into a heterogeneous structure with cadmium sulfate clusters distributed randomly throughout the window layer. The thickness of window layer remains essentially unchanged after CdCl2 treatment, but a ∼25 nm graded interfacial layer between CdTe and the window region is formed. Finally, the rapid thermal processing step used to activate the back contact was found to have a negligible impact on the structure or composition of the heterojunction region.

Effects of Cd1-xZnxS alloy composition and post-deposition air anneal on ultra-thin CdTe solar cells produced by MOCVD

Ultra-thin CdTe:As/Cd1-xZnxS photovoltaic solar cells with an absorber thickness of 0.5 μm were deposited by metal-organic chemical vapour deposition on indium tin oxide coated boro-aluminosilicate substrates. The Zn precursor concentration was varied to compensate for Zn leaching effects after CdCl2 activation treatment. Analysis of the solar cell composition and structure by X-ray photoelectron spectroscopy depth profiling and X-ray diffraction showed that higher concentrations of Zn in the Cd1-xZnxS window layer resulted in suppression of S diffusion across the CdTe/Cd1-xZnxS interface after CdCl2 activation treatment. Excessive Zn content in the Cd1-xZnxS alloy preserved the spectral response in the blue region of the solar spectrum, but increased series resistance for the solar cells. A modest increase in the Zn content of the Cd1-xZnxS alloy together with a post-deposition air anneal resulted in an improved blue response and an enhanced open circuit voltage and fill factor. This device yielded a mean efficiency of 8.3% over 8 cells (0.25 cm2 cell area) and best cell efficiency of 8.8%.

Close spaced sublimation deposition of CdTe layers with process gas oxygen for thin film solar cells

Close spaced sublimation (CSS) is one of the leading technologies for deposition of the absorber layer in CdTe solar cells. For large scale production CSS is a dynamic process where the substrates are moved above the crucible at a close distance. In this study oxygen as a process gas is injected during dynamic CSS deposition. Solar cells are prepared from these layers by CdCl2 activation, copper doping and back contact deposition. The samples are investigated using current voltage measurement, quantum efficiency, scanning electron microscopy of cross sections and deep level transient spectroscopy.The results show that the precise introduction of oxygen can improve the solar cell efficiency. The reasons for this effect are discussed in this study.

Comparison of high efficiency flexible CdTe solar cells on different substrates at low temperature deposition

In this paper we present for the first time flexible CdTe solar cells deposited on ultra-thin glass using a low temperature process and compared the results to CdTe cells deposited on polyimides. The effects of the different substrates on the morphology and crystal structure of CdTe absorber layers, as well as on the efficiency of solar cells were investigated by means of different characterization techniques. Atomic force microscopy measurements showed that the CdTe layers deposited on different substrates have a similar morphology. XRD analysis was used to study the structure and the orientation of the crystallite of the CdTe layers on the different substrates. Micro-Raman spectroscopy provided evidence of both the TO phonon of CdTe and the A1 phonon of Te, whose peak position is somehow dependent from the substrate material. CdCl2 treatments change the crystal orientation of the CdTe layers, with the loss of preferential orientation. In the case of the polyimide substrate it has been seen that its transparency decreases after CdCl2 activation treatment, resulting in a parasitic light absorbance and in lower photo-current values of the devices. Moreover strain of the CdTe surface is observed for cells deposited on polymers. Cells on ultra-thin glass do not show these issues, confirming the glass to be an interesting alternative substrate for flexible CdTe solar cells.

Arsenite and Cadmium Activate MAPK/ERK via Membrane Estrogen Receptors and G-Protein Coupled Estrogen Receptor Signaling in Human Lung Adenocarcinoma Cells.

Epidemiological evidence indicates that cadmium and arsenic exposure increase lung cancer risk. Cadmium and arsenic are environmental contaminants that act as endocrine disruptors (EDs) by activating estrogen receptors (ERs) in breast and other cancer cell lines but their activity as EDs in lung cancer is untested. Here, we examined the effect of cadmium chloride (CdCl2) and sodium arsenite (NaAsO2) on the proliferation of human lung adenocarcinoma cell lines. Results demonstrated that both CdCl2 and NaAsO2 stimulated cell proliferation at environmentally relevant nM concentrations in a similar manner to 17β-estradiol (E2) in H1793, H2073, and H1944 cells but not in H1792 or H1299 cells. Further studies in H1793 cells showed that 100 nM CdCl2 and NaAsO2 rapidly stimulated mitogen-activated protein kinase (MAPK, extracellular-signal-regulated kinases) phosphorylation with a peak detected at 15 min. Inhibitor studies suggest that rapid MAPK phosphorylation by NaAsO2, CdCl2, and E2 involves ER, Src, epidermal growth factor receptor, and G-protein coupled ER (GPER) in a pertussis toxin-sensitive pathway. CdCl2 and E2 activation of MAPK may also involve ERβ. This study supports the involvement of membrane ER and GPER signaling in mediating cellular responses to environmentally relevant nM concentrations of CdCl2 and NaAsO2 in lung adenocarcinoma cells.

Photosensitive cadmium telluride thin-film field-effect transistors.

We report on the graphene-seeded growth and fabrication of photosensitive Cadmium telluride (CdTe)/graphene hybrid field-effect transistors (FETs) subjected to a post-growth activation process. CdTe thin films were selectively grown on pre-defined graphene, and their morphological, electrical and optoelectronic properties were systemically analyzed before and after the CdCl2 activation process. CdCl2-activated CdTe FETs showed p-type behavior with improved electrical features, including higher electrical conductivity (reduced sheet resistance from 1.09 × 10(9) to 5.55 × 10(7) Ω/sq.), higher mobility (from 0.025 to 0.20 cm2/(V·s)), and faster rise time (from 1.23 to 0.43 s). A post-growth activation process is essential to fabricate high-performance photosensitive CdTe/graphene hybrid devices.

Current Enhancement of CdTe-Based Solar Cells

We report on the realization of CdTe solar cell photocurrent enhancement using an n-type CdSe heterojunction partner sputtered on commercial SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :F coated soda-lime glass substrates. With high-temperature close-space sublimation CdTe deposition followed by CdCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> activation, this thin-film stack allows for substantial interdiffusion at the CdSe/CdTe interface facilitating a CdSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> alloy formation. The bowing effect causes a reduced optical bandgap of the alloyed absorber layer and, therefore, leads to current enhancement in the long-wavelength region and a decrease in open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ). To overcome the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> loss and maintain a high short-circuit current (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> ), the CdTe cell configuration has been modified using combined CdS:O/CdSe window layers. The new device structure has demonstrated enhanced collection from both short-and long-wavelength regions as well as a V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> improvement. With an optimized synthesis process, a small-area cell using CdS:O/CdSe window layer showed an efficiency of 15.2% with a V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> of 831 mV, a J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> of 26.3 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and a fill factor of 69.5%, measured under an AM1.5 illumination without antireflection coating. The results provide new directions for further improvement of CdTe-based solar cells.

Fabrication of Semi-Transparent Photovoltaic Cell by a Cost-Effective Technique

Semi-transparent inorganic thin film PV cells have been fabricated using n-type (CdS) and p-type (CdTe) semiconductors. Large area devices which can be used as windows and skylights in buildings can be fabricated using cost effective solution processes. The device structure is Glass/TCO/CdTe/CdS/TCO. Chemically stable CdS and CdTe layers are deposited at temperatures 353 K to 373 K (80 °C to 100 °C) under controlled pH. The CdCl2 activation is carried out followed by air annealing. The p-n junction is formed by sintering the device at 673 K to 723 K (400 °C to 450 °C). The characterization of cells is carried out using XRD, SEM, AFM, and UV–Visible spectroscopy. The thickness of the cell is ~600 nm. The band gap values are 2.40 eV for CdS and 1.36 eV for CdTe with transmittance of about 70 pct in the visible region. Under 1.5 AM solar spectrum, V oc, and I sc of the initial device are 3.56e−01 V and 6.20e−04 A, respectively.

Properties of sputtered CdS and CdTe films and performance of CdTe solar cells as a function of annealing temperature

Thin film PV devices were fabricated in FTO/CdS/CdTe/Cu/Au superstrate configuration by RF sputtering of CdS and CdTe films, and CdCl2 activation process was performed at 370, 380 and 390°C, respectively. Morphological studies indicated for the first time that there were numerous holes randomly distributed at the as-deposited CdS/CdTe interface; although the holes were reduced after CdCl2 annealing, the p–n junction was greatly improved. Further, the new concept of solar spectrum window width (ΔEg) for the solar cells was demonstrated, and the ΔEg of CdTe solar cells was found to be extended with increasing annealing temperature. Moreover, the efficiency of CdTe solar cells was dramatically improved from 0.02% to 12.02% as a function of annealing temperature, and the values of J0 and A were reduced with the optimal values (7.2×10−7mA/cm2 and 1.87), indicating the primary carrier recombination of the best device was Shockley–Real–Hall mechanism in the space charge region.

Influence of CdCl2 activation treatment on ultra-thin Cd1−xZnxS/CdTe solar cells

Ultra-thin CdTe photovoltaic solar cells with an absorber thickness of 0.5μm were produced by metal organic chemical vapour deposition onto indium tin oxide coated boroaluminosilicate glass. A wide band gap Cd1−xZnxS alloy window layer was employed to improve spectral response in the blue region of the solar spectrum. X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy were used to monitor changes in the chemical composition and microstructure of the Cd1−xZnxS/CdTe solar cell after varying the post-deposition CdCl2 activation treatment time and annealing temperature. The CdCl2 treatment leached Zn from the Cd1−xZnxS layer causing a redshift in the spectral response onset of window absorption. S diffusion occurred across the Cd1−xZnxS/CdTe interface, which was more pronounced as the CdCl2 treatment was increased. A CdTe1−ySy alloy was formed at the interface, which thickened with CdCl2 treatment time. Small concentrations of S (up to 2at.%) were observed throughout the CdTe layer as the degree of CdCl2 treatment was increased. Greater S diffusion across the Cd1−xZnxS/CdTe interface caused the device open-circuit voltage (Voc) to increase. The higher Voc is attributed to enhanced strain relaxation and associated reduction of defects in the interface region as well as the increase in CdTe grain size.

A comparative study of microstructural stability and sulphur diffusion in CdS/CdTe photovoltaic devices

The role of CdCl2 activation in the production of high quality CdTe-based photovoltaic devices remains a subject of much debate. In this study, CdTe-based cells produced in three independent laboratories using different device fabrication technologies are investigated before and after CdCl2 activation with regard to structural changes (recrystallisation and grain growth) and sulphur out-diffusion. Using scanning transmission electron microscopy (STEM) and x-ray diffraction it is demonstrated that CdCl2 activation of the investigated cells produces no statistical structural changes to the CdTe. Additionally, energy dispersive spectrometry (EDS) performed in the STEM on the same samples illustrates that the change in sulphur diffusion following activation is more limited than expected from previous studies; no change is detectable when the thermal budget for CdTe deposition is significantly greater than that for activation. This suggests that the efficiency enhancement during CdCl2 treatment is not due to sulphur out-diffusion. Lastly, cathodoluminescence microscopy is used to demonstrate in two dimensions how sulphur diffuses into a model sample and the results are found to be consistent with STEM–EDS. Some spectroscopic evidence for enhanced sulphur diffusion along grain boundaries is also observed.