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(a) Normalized CL spectra of non-contacted CdTe illustrating shift of DAP emission with increasing beam current at various levels shown in (b). (b-c) Effect of electron-beam current on emission spectrum (b) prior to and (c) after contacting.
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The present model for current transport at the CdTe/p-ZnTe:Cu/metal back contact assumes that the CdTe and ZnTe valence bands align, while current transport at a highly doped ZnTe and a metal interface proceeds by tunneling. To test part of this model, we have investigated the electrical and material properties of CdS/CdTe devices where the outer m...
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Thin film CdS/CdTe solar cells are promising candidates for large-scale photovoltaics. The CdSxTe1-x solid solution is always formed at the interface between the CdS window layer and CdTe absorber layer due to the intermixing at the interface region. It is confirmed that this interdiffusion process significantly affects the device performance. This...
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... ZnTe can be doped with various materials such as Cu [81,82], Ni [83], or Ag [84] to improve its electrical properties and further enhance device performance. The recorded efficiency of doping materials can be seen in Table 3. Cu doping of ZnTe has proven to effectively reduce resistivity and enhance conductivity, making it a promising candidate for improving CdTe solar cell performance [85][86][87][88]. Cu is a p-type dopant, which means it introduces positive charge carriers (holes) into semiconductor materials [89]. ...
Recent advancements in CdTe solar cell technology have introduced the integration of flexible substrates, providing lightweight and adaptable energy solutions for various applications. Some of the notable applications of flexible solar photovoltaic technology include building integrated photovoltaic systems (BIPV), transportation, aerospace, satellites, etc. However, despite this advancement, certain issues regarding metal and p-CdTe remained unresolved. Besides, the fabrication of a full-working device on flexible glass is challenging and requires special consideration due to the unstable morphology and structural properties of deposited film on ultra-thin glass substrates. The existing gap in knowledge about the vast potential of flexible CdTe solar cells on UTG substrates and their possible applications blocks their full capacity utilization. Hence, this comprehensive review paper exclusively concentrates on the obstacles associated with the implementation of CdTe solar cells on UTG substrates with a potential back surface field (BSF) layer. The significance of this study lies in its meticulous identification and analysis of the substantial challenges associated with integrating flexible CdTe onto UTG substrates and leveraging Cu-doped ZnTe as a potential BSF layer to enhance the performance of flexible CdTe solar cells.
... Developing a low resistance and stable back contact for CdS/CdTe thin-film photovoltaic (PV) devices is an important goal for the CdTe community. [1] Studies at the National Renewable Energy Laboratory (NREL) have shown that the required contact parameters can be achieved by incorporating a Cu-doped ZnTe interface layer between the CdTe absorber and a Ti outer metal contact. [2] This contacting process enables low-resistance tunneling through a narrow barrier between the ZnTe:Cu and Ti metal, because ZnTe can be doped to high acceptor levels. ...
... [2] This contacting process enables low-resistance tunneling through a narrow barrier between the ZnTe:Cu and Ti metal, because ZnTe can be doped to high acceptor levels. [1,3] Also, the nearly perfect alignment of the CdTe and ZnTe valence bands allows hole transport between the CdTe and ZnTe layers. [1,4] ZnTe:Cu films used in this study were deposited by r.f. ...
... [1,3] Also, the nearly perfect alignment of the CdTe and ZnTe valence bands allows hole transport between the CdTe and ZnTe layers. [1,4] ZnTe:Cu films used in this study were deposited by r.f. magnetron sputtering. ...
A back contact containing a sputtered ZnTe:Cu interface layer can produce high-performing thin-film CdS/CdTe photovoltaic devices. We have found that small changes in ZnTe:Cu sputtering target fabrication processes affect the properties of the ZnTe:Cu films, which affect the performance of the resulting devices. Different target manufacturing techniques were investigated to study changes in ZnTe:Cu film properties and how they impact device performance. Compositional, optical, and electrical properties of films made from different target recipes were studied. It was found that the amount of oxygen in the targets and films is strongly linked to changes in material properties, especially in band tailing and optical bandgap.
... Typical CIFLs are thin layers of ZnTe:Cu, HgTe:Cu, CuTe, or (chemically formed) Te through which a thin layer (∼1-2 nm) of pure Cu is diffused. On top of the CIFL is deposited metal(s) that reduces lateral resistivity and may also be chosen to limit moisture ingress and/or provide functionality to enhance stability [51,52]. ...
The chapter reviews the history, development, and present processes used to fabricate thin-film, CdTe-based photovoltaic (PV) devices. It is intended for readers who are generally familiar with the operation and material aspects of PV devices but desire a deeper understanding of the process sequences used in CdTe PV technology. The discussion identifies why certain processes may have commercial production advantages and how the various process steps can interact with each other to affect device performance and reliability. The chapter concludes with a discussion of considerations of large-area CdTe PV deployment including issues related to material availability and energy-payback time.
... Because the uncompensated acceptor density (NA-ND) in the ZnTe:Cu is generally larger than that in CdTe, a detrimental back-contact barrier does not result, enabling low-resistance current transport at this interface. The high NA-ND of ZnTe:Cu, and beneficial reactions between ZnTe:Cu and Ti [3], yields transport at this interface dominated by low-resistance tunneling. For optimum processing conditions, these contact attributes combine to yield devices demonstrating nearly ideal behavior (i.e., light current-voltage [LIV] performance shows very little "crossover" or "rollover"). ...
We study the performance of CdS/CdTe thin-film devices contacted with ZnTe:Cu/Ti of various thickness at a higher-than-optimum temperature of ~360degC. At this temperature, optimum device performance requires the same thickness of ZnTe:Cu as for similar contacts formed at a lower temperature of 320degC. C-V analysis indicates that a ZnTe:Cu layer thickness of <~0.5 mum does not yield the degree of CdTe net acceptor concentration necessary to reduce space charge width to its optimum value for n-p device operation. The thickest ZnTe:Cu layer investigated (1mum) yields the highest CdTe net acceptor concentration, lowest value of J<sub>o</sub>, and highest V<sub>oc</sub>. However, performance is limited for this device by poor fill factor. We suggest poor fill factor is due to Cu-related acceptors compensating donors in CdS
The following sections are included:* Introduction * Brief history of CdTe PV devices * Initial attempts towards commercial modules * Review of present commercial industry/device designs * General CdTe material properties * Layer-specific process description for superstrate CdTe devices * Where is the junction? * Considerations for large-scale deployment * Conclusions * Acknowledgements * References
NREL and Corning Incorporated have collaborated on a project to investigate the effect of increasing CdTe deposition temperature on device performance. CdTe deposition temperatures are generally limited by the thermal properties of the glass superstrate. Soda-lime glass is frequently used in commercial production of CdTe, but the low strain point (~515°C) requires deposition temperatures of 550°C or below. While the CdTe industry has enjoyed great success with material grown at these relatively low temperatures, there may be significant benefits to higher deposition temperatures enabled by a high strain point glass. To demonstrate the efficiency benefits of a CdTe cell fabricated at higher deposition temperatures, it is necessary to re-optimize the device fabrication process steps for devices made with CdTe films at each deposition temperature. Using Corning, Inc.'s new engineered high-strain-point glass superstrate, we developed a fabrication process optimized for CdTe films deposited at 550° and 600°C. Here, we report details of the fabrication processes that resulted in an absolute efficiency gain of 1.2% for devices fabricated with 600°C CdTe deposition temperature versus 550°C.
Fluorine-doped tin oxide (SnO2:F or FTO) is the most widely used transparent conducting oxide (TCO) in CdTe solar cells due to its low cost, chemical resistance, and thermal stability. Commercial SnO2:F generally has a sheet resistance between 8–15 Ω/sq and fairly high free-carrier absorption due to heavy doping and incorporation of unintentional impurities. We produce very high quality SnO2:F by metal-organic chemical vapor deposition using tetramethyltin (TMT), oxygen, and bromotrifluromethane (CBrF3). TMT is rarely used commercially due to its toxicity, but it is the only chlorine-free organo-tin precursor that is suitable for our reactor. These precursors consistently yield films with mobilities greater than 30 cm2/V-s and can produce SnO2:F with mobility up to 50 cm2/V-s. However, this process cannot be commercialized because CBrF3 (along with many fluorine sources) is banned under the Kyoto Protocol due to its high global warming potential. Here, we investigate the effect of varying the fluorine dopant source on the carrier concentration and mobility in the films. We have produced high mobility films (>25 cm2/V-s) using alternate fluorine precursors including F2, several hydro-fluorinated-ethers, and SF6. Here, we will report on the efficacy of these molecules and other dopants in order to produce high performance SnO2:F with sheet resistances between 10–20 Ω/sq. We will discuss the impact of the novel F dopants on CdTe device performance.
The objective of this research is to develop cheaper and more efficient photoelectrochemical (PEC) cells for the production of highly pure hydrogen and oxygen by water splitting. FSEC PV Materials Lab has developed PEC set up consisting of two thin film photovoltaic (PV) cells, a RuS2 photoanode for efficient oxygen evolution and a platinum cathode for hydrogen evolution. A p-type transparent-conducting layer is prepared at the back of PV cell to transmit unabsorbed infrared photons onto the photoanode for efficient oxygen evolution. This paper presents the preparation and characterization of p- type ZnTe:Cu transparent conducting back layer and PEC cell.
An impurity migration model for systems with material interfaces is applied to Cu migration in CdTe solar cells. In the model, diffusion fluxes are calculated from the Cu chemical potential gradient. Inputs to the model include Cu diffusivities, solubilities, and segregation enthalpies in CdTe, CdS and contact materials. The model yields transient and equilibrium Cu distributions in CdTe devices during device processing and under field-deployed conditions. Preliminary results for Cu migration in CdTe photovoltaic devices using available diffusivity and solubility data from the literature show that Cu segregates in the CdS, a phenomenon that is commonly observed in devices after back-contact processing and/or stress conditions.
