Cu-related recombination in CdS/CdTe solar cells

Cu used in the back contact of CdS/CdTe solar cells is known to improve contact behavior and open-circuit voltage. A study of devices made with varying Cu amounts confirmed these observations. However, Cu was also found to be deleterious to current collection. Time-resolved photoluminescence measurements of CdTe devices show that carrier lifetime decreased with increased Cu concentration. Drive-level-capacitance-profiling and low-temperature photoluminescence suggest this decrease in lifetime was associated with increased recombination center density introduced by Cu in the CdTe layer. The resulting impact of increased Cu on device performance was a voltage-dependent collection of photogenerated carriers that reduced fill-factor.     

Towards ultra-thin CdTe solar cells using MOCVD

A study was made on very thin CdTe absorber < 1 µm layers to investigate limitations in CdTe collection efficiency. Metal organic chemical vapour deposition (MOCVD) was used to deposit cadmium sulfide (CdS), cadmium zinc sulfide (Cd_0.9Zn_0.1S) and cadmium telluride (CdTe). Improvements in photon collection in the blue, where the absorption length is shorter, have been achieved using a wider band gap Cd_0.9Zn_0.1S ternary alloy to replace CdS as the window layer. Solar cell capacitance simulator (SCAPS) modelling software [M. Burgelman, P. Nollet, S. Degrave, Thin Solid Films, 361-362 (2000) 527-532] has been used to calculate device parameters as a function of the absorber layer thickness (controlled by in situ using laser reflectometry). One feature of the MOCVD grown devices is the apparent absence of pin-holes, demonstrated by growth of an ultra-thin absorber (200 nm) with conversion efficiency of nearly 4%.     

Thin-film solar cells

The rapid progress that is being made with inorganic thin-film photovoltaic (PV) technologies, both in the laboratory and in industry, is reviewed. While amorphous silicon based PV modules have been around for more than 20 years, recent industrial developments include the first polycrystalline silicon thin-film solar cells on glass and the first tandem solar cells based on stacks of amorphous and microcrystalline silicon films (“micromorph cells”). Significant thin-film PV production levels are also being set up for cadmium telluride and copper indium diselenide.     

Pathways to thin absorbers in CdTe solar cells

We present approaches to reduce the absorber thickness of CdTe solar cells. The investigations were done with CdTe absorber films deposited by the close-space sublimation (CSS) technique. Using these CdTe films, complete solar cells were produced in our own laboratory. The absorber thickness as the crucial parameter was varied between 1 and 11 µm in these experiments. It is analyzed how process steps following the CdTe layer deposition influence the structure of the absorber films as well as the solar cell properties. Three ways of back contact formation are compared. These include (i) the wet chemical etching of the CdTe surface, (ii) a plasma etching step, and (iii) the vacuum deposition of a thin intermediate copper layer. In the latter case, voids and shunts related to preferential etching at grain boundaries are avoided admitting the use of thinner absorber films. Thus, the solar-cell efficiencies were increased from below 9% to more than 10% while the CdTe film thickness was reduced from 11 µm to less than 4 µm.     

Dependence of carrier lifetime on Cu-contacting temperature and ZnTe:Cu thickness in CdS/CdTe thin film solar cells

Cu diffusion from a ZnTe:Cu contact interface can increase the net acceptor concentration in the CdTe layer of a CdS/CdTe photovoltaic solar cell. This reduces the space-charge width (W_d) of the junction and enhances current collection and open-circuit voltage. Here we study the effect of Cu concentration in the CdTe layer on carrier lifetime (τ) using time-resolved photoluminescence measurements of ZnTe:Cu/Ti-contacted CdTe devices. Measurements show that if the ZnTe:Cu layer thickness remains constant and contact temperature is varied, τ increases significantly above its as-deposited value when the contacting temperature is in a range that has been shown to yield high-performance devices (~ 280° to ~ 320 °C). However, when the contacting temperature is maintained near an optimum value and the ZnTe:Cu thickness is varied, τ decreases with ZnTe:Cu thickness.     

CdTe thin film solar cells with reduced CdS film thickness

A study was performed to reduce the CdS film thickness in CdTe thin film solar cells to minimize losses in quantum efficiency. Using close space sublimation deposition for CdS and CdTe a maximum efficiency of ~ 9.5% was obtained with the standard CdS film thickness of ~ 160 nm. Reduction of the film CdS thickness to less than 100 nm leads to poor cell performance with ~ 5% efficiency, mainly due to a lower open circuit voltage. An alternative approach has been tested to reduce the CdS film thickness (~ 80 nm) by depositing a CdS double layer. The first CdS layer was deposited at high substrate temperature in the range of 520-540 °C and the second CdS layer was deposited at low substrate temperature of ~ 250 °C. The cell prepared using a CdS double layer show better performance with cell efficiency over 10%. Quantum efficiency measurement confirmed that the improvement in the device performance is due to the reduction in CdS film thickness. The effect of double layer structure on cell performance is also observed with chemical bath deposited CdS using fluorine doped SnO₂ as substrate.     

The formation of different phases of CuxTe and their effects on CdTe/CdS solar cells

Material studies and device applications of Cu_xTe in an NREL-developed CdTe solar cell structured as glass/Cd₂SnO₄/ZnSnO_x/CdS/CdTe are presented. The Cu_xTe primary back contact was formed by evaporating a Cu layer with various thicknesses at room temperature on HNO₃/H₃PO₄ (NP) solution etched CdTe layer. A post-annealing was then followed. The structural evolution and electrical properties of Cu_xTe were investigated. Cu/Te ratio and post-annealing temperature are two processing parameters in this study. The Cu_xTe phases are mainly controlled by the Cu/Te ratio. After a post-annealing at a low temperature, such as 100 °C, no Cu_xTe phase transformation from its as-deposited phase was observed. A post-annealing treatment at a higher temperature, such as 250 °C, can reveal the stoichiometric Cu_xTe phases based on the Cu/Te ratio used in the devices. But a post-annealing at a further higher temperature, such as 400 °C, resulted in a complicated Cu_xTe phase appearance. CuTe, Cu_1.4Te, and Cu₂Te are three major phases detected by X-ray diffraction (XRD) for different Cu thickness application annealed at 250 °C. Application of Cu thicker than 60 nm degrades open-circuit voltage (V_oc) and shunting resistance (R_sh), but increases series resistance (R_s). The correlation between device performance and the Cu_xTe back contact illustrates that the process used for forming the Cu₂Te back contact failed to produce good fill factor (FF) and also introduced higher barrier height. The best device was observed for a back contact with a mixed Cu_1.4Te and CuTe phases.     

Role of thermal treatment on the luminescence properties of CdTe thin films for photovoltaic applications

The efficiency of CdTe based solar cells is strongly enhanced by a thermal treatment in HCF₂Cl ambient. CdTe thin films deposited on CdS/ZnO/ITO/glass by Closed Space Sublimation before and after the annealing are characterised. The CdTe morphology is studied by atomic force microscopy and scanning electron microscopy. In the treated films the non-homogeneous distribution of the grain size disappears, in addition an increasing of the dimensions of the grains is observed. Cathodoluminescence analyses show a remarkable difference in the spectra between the treated and untreated structures. A strong increase in the intensity of the 1.4 eV band is observed by increasing the HCF₂Cl content. A model of the electronic levels inside the CdTe band gap, due to incorporation of Cl (or F) is proposed.     

CdTe photoelectrochemical solar cells—chemical modification of surfaces

Chemical modification of bothn andp type CdTe has been found to improve the performance and stability of PEC solar cells. The surfaces, modified by Ru³⁺, have been examined by a variety of techniques. Modification results in enhanced barrier height at the surface due to the formation of a passivating oxide layer.     

Initial and degraded performance of thin film CdTe solar cell devices as a function of copper at the back contact

Copper performs an important role in obtaining high-performance thin-film CdTe solar cell devices. Both initial performance and performance after stress depends strongly on the total copper content at the back-contact, the Cd to Te ratio on the backside, the etching process, and the way the copper is activated. With regard to getting high open circuit voltage a small amount of Cu seems sufficient upon the right anneal treatment. However, regarding open circuit voltage degradation for stressed devices there seems to be an optimum amount of Cu. Te-enrichment does not seem to have a big impact on device stability.     

Investigation of the excitonic luminescence band of CdTe solar cells by photoluminescence and photoluminescence excitation spectroscopy

The excitonic luminescence band of polycrystalline cadmium telluride layers has been investigated by Photoluminescence (PL) and Photoluminescence excitation spectroscopy (PLE). CdTe was deposited by means of close space sublimation and the samples were activated by different chlorine containing compounds, i.e. cadmium chloride, hydrochloric acid, and sodium chloride as well as by simple air activation or received no post deposition treatment. In the PL spectra, four different peaks within the excitonic luminescence band were resolved. These include the free-exciton peak and two transitions of excitons bound to defects. Furthermore, free excitons and band to band transitions were detected by means of PLE. The PL and PLE spectra are discussed with respect to the post deposition treatments.     

Metal-organic chemical vapor deposition of ultra-thin photovoltaic devices using a pyrite based p-i-n structure

Ultra-thin photovoltaic (PV) devices were produced by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD) incorporating a highly absorbing intermediate sulphurised FeS_x layer into a CdS/CdTe structure. X-ray diffraction (XRD) confirmed a transitional phase change to pyrite FeS₂ after post growth sulphur (S) annealing of the FeS_x layer between 400 °C and 500 °C. Devices using a superstrate configuration incorporating a sulphurised or non-sulphurised FeS_x layer were compared to p-n devices with only a CdS/CdTe structure. Devices with sulphurised FeS_x layers performed least efficiently, even though pyrite fractions were present. Rutherford back scattering (RBS) confirmed deterioration of the CdS/FeS_x interface due to S inter-diffusion during the annealing process.     

Doping profiles in CdTe/CdS thin film solar cells

CdS/CdTe thin film solar cells showing comparable properties as commercial cells have been prepared by close space sublimation (CSS) in our own laboratory. We characterised the cells by capacitance-voltage profiling (C-V), thermal admittance spectroscopy (TAS), and thermally stimulated capacitance measurements (TSCAP). The doping profiles of the CdTe layer obtained by C-V measurements confirm the well known rise in dopant concentration with increasing depth if the usual evaluation procedure is employed. However, the TAS and TSCAP measurements reveal deep acceptors in the CdTe layer with a large concentration exceeding that of the shallow dopants. Under these conditions, C-V measurements are shown to yield an apparently rising dopant concentration even for homogeneous doping. A combined simulation of doping profiles measured at different temperatures using a fixed and uniform shallow and deep doping fits well to measured doping concentration. These results indicate how to get reliable information on the shallow dopant concentration.     

The role of transparent conducting oxides in metal organic chemical vapour deposition of CdTe/CdS Photovoltaic solar cells

A systematic study is made between the relationship of Cd_0.9Zn_0.1S/CdTe photovoltaic (PV) device properties for three different commercial transparent conducting oxide (TCO) materials and some experimental CdO to determine the role of the TCO in device performance. The resistance contribution from the TCO was measured after depositing the gold contact architectures directly onto the TCOs. These were compared with the Cd_0.9Zn_0.1S/CdTe device properties using the same contact arrangements. Series resistance for the commercial TCOs correlated with their sheet resistance and gave good agreement with the PV device series resistance for the indium tin oxide (ITO) and fluorine doped tin oxide (FTO) 15 Ω/Sq. superstrates. The devices on the thicker FTO 7 Ω/sq superstrates were dominated by a low shunt resistance, which was attributed to the rough surface morphology causing micro-shorts. The device layers on the CdO substrate delaminated but devices were successfully made for ultra-thin CdTe (0.8 μm thick) and compared favourably with the comparable device on ITO. From the measurements on these TCOs it was possible to deduce the back contact resistance and gave an average value of 2 Ω.cm². The correlation of fill factor with series resistance has been compared with the predictions of a 1-D device model and shows excellent agreement. For high efficiency devices the combined series resistance from the TCO and back contact need to be less than 1 Ω.cm².     

Phase control of CuxTe film and its effects on CdS/CdTe solar cell

Phase control is critical for achieving high-performance CdTe cells when Cu_xTe is used as a back-contact for CdTe cells. Cu_xTe phases are mainly controlled by the Cu/Te ratio, and they can also be affected by post-heat-treatment temperature. Although Cu₂Te has the highest conductivity, it is unstable and provides more Cu diffusion into the CdS and CdTe films. Cu diffusion into the CdS causes “cross-over”, and Cu diffusion into the CdTe film creates Cu-related defects that lower photogenerated carrier lifetime and result in voltage-dependent collection. A “recontact” experiment clearly indicated that the mechanism giving rise to “roll-over” is the formation of Cu-related oxides, rather than the loss of Cu on the back-contact.     

Annealing effects on the chemical deposited CdS films and the electrical properties of CdS/CdTe solar cells

CdS layers grown by chemical bath deposition (CBD) are annealed in the oxygen and argon-hydrogen atmosphere respectively. It has been found that the open circuit voltage of the CdS/CdTe solar cell increases when the CBD CdS is annealed with oxygen before the deposition of CdTe by close spaced sublimation (CSS), while the performance of the solar cell decreases when the CBD CdS is annealed with argon-hydrogen. Electronic properties of the CdS films are investigated using X-ray photo-electron spectroscopy (XPS), which indicates that the Fermi level is shifting closer to the conduction band after annealing in the oxygen and consequently a higher open circuit voltage of the solar cell can be obtained.     

Ordered polycrystalline thin films for high performance CdTe/CdS solar cells

An ordered polycrystalline approach is proposed to overcome fundamental problems associated with random polycrystalline thin films, namely grain boundaries and inhomogeneity. The approach consists of two main steps: (1) the deposition of a patterned growth mask and (2) the selective-area deposition of the ordered polycrystals. The ordered polycrystalline approach was investigated using the CdTe/CdS material system. Experimental results demonstrate that SiO₂ and Si₃N₄ are effective growth masks and that temperature is a dominant parameter for selective-area deposition. PL and XRD characterization indicates that the ordered polycrystalline technique has the potential for improving the crystal quality and order of polycrystalline CdTe thin films. The approach appears to be fairly general and could be applied to other material systems.     

Solar cells with Sb2S3 absorber films

CdS/Sb₂S₃/PbS structures were prepared by sequential chemical deposition of CdS, Sb₂S₃ and PbS thin films on TEC-8 (Pilkington) transparent electrically conductive SnO₂ (TCO) coatings. CdS thin films (100 nm) were deposited with hexagonal structure from Cd-citrate bath and of cubic structure from Cd-ammine/triethanolamine bath. Sb₂S₃ thin films were deposited at 40 °C from a solution mixture of potassium antimony tartrate, triethanolamine, ammonia and thioacetamide(TA) or at 1 to 10 °C from a mixture of antimony trichloride and thiosulfate (TS). These films were made photoconductive by heating at temperatures 250 to 300 °C. When heated in the presence of a chemically deposited Se thin film of 300 nm, a solid solution Sb₂S_1.8Se_1.2 resulted. PbS thin films of 100-200 nm thickness were deposited on the TCO/CdS/Sb₂S₃ or TCO/CdS/Sb₂S_1.8Se_1.2 structure. Graphite paint was applied on the PbS film prior to applying a silver epoxy paint. The cell structures were of area 0.4 cm². The best results reported here is for a cell: TCO/CdS(hex-100 nm)/Sb₂S₃(TS-100 nm)/PbS(200 nm) with open circuit voltage (V_oc) 640 mV, short circuit current density 3.73 mA/cm², fill factor 0.29, and conversion efficiency 0.7% under 1000 Wm^− 2 sunlight. Four series-connected cells of area 1 cm² each gave V_oc of 2 V and short circuit current of 1.15 mA.     

Effect of CdCl2 annealing treatment on thin CdS films prepared by chemical bath deposition

In order to study the CdS recrystallization mechanism, a comparative study was carried out on thin films prepared by chemical bath deposition. The CdS films were annealed in air with or without a CdCl₂ coating layer. In-situ Raman spectra obtained during the annealing showed that both the air- and the CdCl₂-annealing did not cause rearrangement of the neighboring atoms in the CdS clusters below ~ 300 °C. CdS thin film was partially oxidated to CdO and CdSO₄ on the cluster surface when annealed in air. The oxides and the sulfur stoichiometric deficiency prevented the clusters to coalesce at higher temperatures. Coating thin CdS film with a thin CdCl₂ layer protected it from oxidation during annealing in air and promoted formation of Cl_S and V_Cd point defects in CdS. The anti-oxidation was attributed to the incorporation of a significant amount of Cl into CdS to form the Cl_S, which prevented the oxygen in-diffusion and chemical bonding during the annealing. The anti-oxidation at the CdS nano-crystalline surface and the point defects formed in the CdS promoted coalescence of the neighboring clusters without the need of long-range redistribution of the atoms. Large CdS grains with good crystalline quality formed through recrystallization during the CdCl₂ heat treatment, which provided the solid basis for the subsequent CdTe growth and high efficient CdS/CdTe solar cell fabrication.