The effect of impurities on the doping and VOC of CdTe/CdS thin film solar cells

The effect of introducing impurities in CdTe, namely antimony (Sb) and oxygen (O), on the net carrier concentration in CdS/CdTe solar cells and on their open-circuit voltage (V_OC) has been investigated. Oxygen was introduced in the CdTe films during the deposition of this layer by the close-spaced sublimation process. The total pressure was held constant at 1330 Pa (N₂ and O₂). The amount of oxygen was varied by varying its partial pressure. Antimony was introduced into CdTe using a post-deposition diffusion process. Following the deposition of CdTe a thin film (a few nm) of Sb was deposited onto the CdTe surface and subsequently heat-treated to cause in-diffusion of Sb. The temperature and time during the diffusion process were varied in the range of 300-525 °C and 20-160 min respectively. In both instances it was possible to vary (increase) the doping concentration in CdTe. The increase in doping was accompanied by an increase in V_OC. However, in all instances the doping in CdTe reached a maximum value, beyond which further increases were not possible leading to saturation in V_OC. The highest V_OC measured was similar to state-of-the-art values in the range of 800-830 mV, and the highest doping concentration measured was in the 10¹⁶ cm^− 3 range.     

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.     

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.     

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.     

Physical properties of Bi doped CdTe thin films grown by CSVT and their influence on the CdS/CdTe solar cells PV-properties

The physical properties of Bi doped CdTe films, grown on glass substrates by the Closed Space Transport Vapour (CSVT) method, from different Bi doped CdTe powders are presented. The CdTe:Bi films were characterized using Photoluminescence, Hall effect, X-Ray diffraction, SEM and Photoconductivity measurements. Moreover, CdS/CdTe:Bi solar cells were made and their characteristics like short circuit current density (J_sc), open circuit voltage (V_OC), fill factor (FF) and efficiency (η) were determined. These devices were fabricated from Bi doped CdTe layers deposited on CdS with the same growth conditions than those used for the single CdTe:Bi layers. A correlation between the CdS/CdTe:Bi solar cell characteristics and the physical properties of the Bi doped CdTe thin films are presented and discussed.     

C-V calculations in CdS/CdTe thin films solar cells

Polycrystalline thin film CdS/CdTe heterojunction solar cells are important candidates for large scale photovoltaic applications. In this work we use a C-V (capacitance vs. voltage) theoretical method for the determination of the interface charge density σ and band discontinuity ΔEv of the CdS/CdTe heterojunction. The methodology is based on three cardinal equations: i) line up of the bands relative to the common Fermi level (at equilibrium) or the quasi-Fermi level (when voltage is applied), ii) charge neutrality and iii) the total capacitance of the heterostructure. We used CdS/CdTe solar cells, grown in our laboratory by the chemical bath deposition (for CdS film) and the close space vapor transport (for CdTe film) techniques. The interface parameters σ, and ΔEv are determined from C-V fitting between the calculated and the measured curve. The methodology presented in this study is general and can be applied to semiconductor-semiconductor and semimetal-semiconductor heterojunctions.     

Back contact formation in thin cadmium telluride solar cells

We present a model describing the undesired roll-over which is a well-known phenomenon in the current-voltage characteristics of CdTe solar cells. Therein, the roll-over is ascribed to a Schottky barrier at the back contact which is effective as a reverse diode. The formation of this barrier is investigated depending on the CdTe absorber thickness as well as on the employed back contact metal. Computer simulations of the energy band diagram reveal that the back contact barrier can be reduced and even eliminated for sufficiently thin absorbers. The reason is the spatial overlap between the space-charge regions of the p-n heterojunction with the one of the back contact. This behaviour correlates with experimental current-voltage data of solar cells with a simple gold back contact. In the latter, the roll-over is considerable for absorbers with 3 to 5 μm thickness, diminishes when the absorber thickness is reduced and finally vanishes when the absorber thickness is approximately 1 μm. The investigations show that thickness reduction can be employed in order to suppress the roll-over phenomenon in CdTe solar cells.     

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.     

Dependence of efficiency of thin-film CdS/CdTe solar cell on parameters of absorber layer and barrier structure

Dependences of the open-circuit voltage, short-circuit current, fill factor, and efficiency of a CdS/CdTe solar cell on the resistivity and thickness of the p-CdTe absorber layer, the noncompensated acceptor concentration N_a-N_d, and carrier lifetime τ in CdTe, are investigated, and optimization of these parameters in order to improve the solar cell efficiency is performed. It has been shown that the observed low efficiency of CdS/CdTe solar cells is caused by the too short electron lifetime in the range of 10^− 10-10^− 9 s and too thin (3-5 µm) CdTe layer currently used for fabrication of CdTe/CdS solar cells. To achieve an efficiency of 28-30%, the resistivity and thickness of the CdTe absorber layer, the noncompensated acceptor concentration, and carrier lifetime should be ∼ 0.1 Ω·cm, ≥ 20-30 µm, ≥ 10¹⁶ cm^− 3, and ≥ 10^− 6 s, respectively.     

Study of in situ CdCl2 treatment on CSS deposited CdTe films and CdS/CdTe solar cells

Effect of in situ CdCl₂ treatment on the morphological, structural and electrical properties of CdTe films as well as on solar cell characteristics of CdS/CdTe junction has been investigated. XRD measurements show that the presence of CdCl₂ vapours induces 111 oriented growth in the CdTe films. CdCl₂ concentration required for this oriented growth is found to be directly proportional to the substrate temperature. SEM measurements show enhanced grain growth in the presence of CdCl₂. Spectral response of the CdCl₂ treated CdS/CdTe solar cells shows an enhanced CdS diffusion in to the CdTe, which results in an improved spectral response in UV range and a consequent reduction in the interface states density. A drastic reduction in the deep levels due to the CdCl₂ treatment, as seen in the photo-capacitance studies, has results in CdS/CdTe solar cells having efficiency >8%.     

Lead antimony sulfides as potential solar absorbers for thin film solar cells

In an effort to prepare thin films of novel semiconductor materials that contain only cost effective, abundant, and relatively less-toxic materials, lead antimony sulfides films have been prepared. Herein, we report the thin film preparation of semseyite (Pb₉Sb₈S₂₁) via annealing of precursor films under sulfur vapor. Pb/Sb alloy precursor films suffered substantial changes in stoichiometry and produced rough films, whereas precursor films composed of multilayers of PbS and amorphous (Sb,S) produced smooth and compact phase-pure films composed of fine grains. Optical measurements indicated a direct band gap of 1.93 eV and a strong absorption coefficient of 1.0 × 10⁵ cm^− 1.     

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.     

Study of buried junction and uniformity effects in CdTe/CdS solar cells using a combined OBIC and EQE apparatus

A study of junction position and uniformity in CdTe/CdS solar cells is reported in which the influence of excluding oxygen from the CdS layers was investigated. The samples were characterised with an optical beam induced current instrument capable of mapping the cell response in the range 400-900 µm at a resolution of 12.5 µm — either as a map or a quantum efficiency spectrum. For oxygen-free CdS, the junctions were always buried in the CdTe — at a depth presumed to be controlled by the chloride treatment. If CdS:O is used then shallow junctions result, indicating that such layers have a role in doping the devices. The wavelength dependence of the spatial uniformity of the cell's responses is also discussed.     

Studies of key technologies for large area CdTe thin film solar cells

The structure and main manufacturing technologies of CdTe film solar cells of large area are reviewed. Among the technologies, some have been developed for application in a pilot manufacturing line. The high resistant SnO₂ (HRT) thin films have been fabricated by PECVD. The effects of annealing on the structure and properties have been studied. A surface etching process of CdTe in low temperature and lower concentration of nitric acid has been developed. The Cd_1 − xZn_xTe ternary compound films have been studied. In order to improve the back contact layer, Cd_0.4Zn_0.6Te layer with 1.8 eV band gap as a substitute for ZnTe layer is introduced in CdTe cells. The effects of the technologies on performance of CdTe cells and feasibility of application in the modules are discussed.     

An effective method of Cu incorporation in CdTe solar cells for improved stability

Thin film CdTe solar cells of the superstrate configuration have been fabricated in order to study the effect of Cu on device stability. The study focused on two distinct sets of solar cells: in one set of devices Cu was introduced during the formation of the back contact, by sputtering a small thickness of Cu onto the CdTe surface prior to the application of a graphite electrode; for the second set of devices Cu was introduced in CdS by briefly immersing the CdS films in a CuCl solution prior to the deposition of CdTe with the back contact electrode being sputtered Mo. The solar cells were light soaked under approximately AM1.5 conditions for nearly 700 h during 4 h ON/4 h OFF cycles. Device degradation correlated well with the amount of Cu for the devices with Cu in the back contact. Cells with larger amounts of Cu exhibited larger degradation, suggesting that the amount of Cu utilized during the back contact formation must be minimized. On the other hand, a number of devices fabricated without any Cu in the back contact, but with Cu in the CdS, exhibited nearly no degradation during the light soaking process suggesting that in addition to the amount of Cu used for the fabrication of CdTe cells, the method of incorporating this element is also critical in achieving long term device stability.     

Unravelling complex nature of CdS/CdTe based thin film solar cells

Thin film solar cells based on CdS/CdTe hetero-structure has shown a drastic improvement changing from 16.5 to 22.1% efficiency during a short period of time from ~2013 to ~2016. This has happened in the industrial environment and the open research in this field has stagnated over a period of two decades prior to ~2013. Most of the issues of this hetero-structure were not clear to the photovoltaic (PV) community and research efforts should be directed to unravel its complex nature. Issues related to materials, post-growth treatment, chemical etching prior to metallisation and associated device physics are the main areas needing deeper understanding in order to further develop this device. After a comprehensive research programme in both academia and in industry on these materials, surfaces and interfaces and fully fabricated devices over a period of over three decades by the main author, the current knowledge as understood today, on all above mentioned complex issues are presented in this paper. Full understanding of this structure will enable PV developers to further improve the conversion efficiency beyond 22.1% for CdS/CdTe based solar cells.     

Nitric-phosphoric acid etching effects on the surface chemical composition of CdTe thin film

Nitric-phosphoric (NP) acid etching has been regarded as one of the most effective methods for the formation of low resistance back contact with the metal electrode in CdTe based photovoltaic cells. We studied CdTe back surfaces and the changes with time of exposure to NP acid with x-ray photoelectron spectroscopy (XPS), and atomic force microscopy. Strong etching dependence on the back surface chemical composition, and surface roughness, was observed. In order to study the effect of the NP acid etching on surface degradation, the sample was left in open ambient condition for three weeks and XPS measurement in combination with ion sputtering was performed on unetched and highly etched parts. The difference in depth profiles of the NP acid etched and unetched CdTe surface has been discussed.     

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.     

Manufacturing of CdTe thin film photovoltaic modules

The technology to fabricate CdTe/CdS thin film solar cells can be considered mature for a large-scale production of CdTe-based modules. Several reasons contribute to demonstrate this assertion: a stable efficiency of 16.5% has been demonstrated for 1 cm² laboratory cell and it is expected that an efficiency of 12% can be obtained for 0.6 × 1.2 m² modules; low cost soda lime float glass can be used as a substrate; the amount of source material is at least 100 times less than that used for single crystal modules and is a negligible part of the overall cost. The fabrication process can be completely automated and a production yield of one module every 2 min can be obtained, which implies a production cost substantially less than 1€/W_P. A further cost reduction will render this kind of energy production competitive with the energy obtained from fossil fuels by approaching the so-called grid-parity. Some new companies have recently announced the start of production or plan to do so in the near future. Many of these plants are located in Germany, some in the USA. In Italy, a new company has been constituted in 2008, with the aim of building a factory with a capacity of 18 MW/year. In this article, we will describe and compare the basic principles of CdTe solar cells and modules. We will include an overview of the potentials of these technologies and of the R&D issues under investigation. This paper describes how the large-area mass production of CdTe solar modules is realized in the Italian factory and presents a worldwide overview of the current production activities.