Optimization of vapor post‐deposition processing for evaporated CdS/CdTe solar cells

The effects of thermal annealing in conjunction with CdCl₂ vapor heat treatment on the properties of CdTe/CdS thin films and devices deposited by physical vapor deposition are reported. Results are compared for three treatment variations: high‐temperature anneal only, high‐temperature anneal followed by CdCl₂ vapor heat treatment and CdCl₂ vapor heat treatment only. X‐ray diffraction, transmission electron microscopy and scanning electron microscopy show improved crystallographic properties of the CdTe film and reduced CdS/CdTe interdiffusion when a high‐temperature anneal is used prior to CdCl₂ treatment. The CdTe/CdS solar cells fabricated using an anneal at 550°C in argon prior to the CdCl₂ vapor heat treatment exhibited improved electrical characteristics compared to cells fabricated with no anneal step, yielding an open‐circuit voltage exceeding 850 mV. Copyright © 1999 John Wiley & Sons, Ltd.     

CdTe solar cell in a novel configuration

Polycrystalline thin‐film CdTe/CdS solar cells have been developed in a configuration in which a transparent conducting layer of indium tin oxide (ITO) has been used for the first time as a back electrical contact on p‐CdTe. Solar cells of 7·9% efficiency were developed on SnO_x:F‐coated glass substrates with a low‐temperature (<450°C) high‐vacuum evaporation method. After the CdCl₂ annealing treatment of the CdTe/CdS stack, a bromine methanol solution was used for etching the CdTe surface prior to the ITO deposition. The unique features of this solar cell with both front and back contacts being transparent and conducting are that the cell can be illuminated from either or both sides simultaneously like a ‘bi‐facial’ cell, and it can be used in tandem solar cells. The solar cells with transparent conducting oxide back contact show long‐term stable performance under accelerated test conditions. Copyright © 2004 John Wiley & Sons, Ltd.     

13·9%‐efficient CdTe polycrystalline thin‐film solar cells with an infrared transmission of ∼50%

To fabricate a high‐efficiency polycrystalline thin‐film tandem cell, the most critical work is to make a high‐efficiency top cell ( > 15%) with high bandgap (E_g = 1·5–1·8 eV) and high transmission (T > 70%) in the near‐infrared (NIR) wavelength region. The CdTe cell is one of the candidates for the top cell, because CdTe state‐of‐the‐art single‐junction devices with efficiencies of more than 16% are available, although its bandgap (1·48 eV) is slightly lower for a top cell in a current‐matched dual‐junction device. In this paper, we focus on the development of a: (1) thin, low‐bandgap Cu_xTe transparent back‐contact; and (2) modified CdTe device structure, including three novel materials: cadmium stannate transparent conducting oxide (TCO), ZnSnO_x buffer layer, and nanocrystalline CdS:O window layer developed at NREL, as well as the high‐quality CdTe film, to improve transmission in the NIR region while maintaining high device efficiency. We have achieved an NREL‐confirmed 13·9%‐efficient CdTe transparent solar cell with an infrared transmission of ∼50% and a CdTe/CIS polycrystalline mechanically stacked thin‐film tandem cell with an NREL‐confirmed efficiency of 15·3%. Copyright © 2005 John Wiley & Sons, Ltd.     

Recrystallization and sulfur diffusion in CdCl2-treated CdTe/CdS thin films

The role of CdCl₂ in prompting recrystallization, grain growth and interdiffusion between CdS and CdTe layers in physical vapor-deposited CdS/CdTe thin-film solar cells is presented. Several CdTe/CdS thin-film samples with different CdTe film thicknesses were treated in air at 415°C for different times with and without a surface coating of CdCl₂. The samples were characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffractometry and optical absorption. The results show that CdCl₂ treatment enhances the recrystallization and diffusion processes, leading to a compositional variation within the CdTe layer due to diffusion of sulfur from the CdS. The highest sulfur concentrations observed after 30 min treatments with CdCl₂ at 415°C are near the solubility limit for sulfur in CdTe. The compositional distributions indicated by x-ray diffraction measurements of samples with different CdTe thickness show that the S-rich CdTe_1−xS_x region lies near the CdTe/CdS interface. A multiple-step mixing process must be inferred to account for the diffraction profiles obtained. © 1997 John Wiley & Sons, Ltd.     

A novel CdCl2 treatment for glass/SnO2/CBD-CdS/CdTe solar cell

Residual CdCl₂ in chemical bath deposited (CBD) CdS layer was utilized to observe grain growth in CdTe layer for glass/SnO₂/CBD-CdS/CdTe structures. The above as-deposited composite films were subjected to rapid thermal annealing (RTA) for observing grain growth and subsequent cell fabrication. The films were characterized by studying their microstructural and compositional properties. Interfacial mixing behavior was studied by secondary ion mass spectroscopy (SIMS) measurements which showed a slight interfacial diffusion of the CdS layer into the CdTe layer. Performance of a photovoltaic (PV) cell structure with non-optimized thickness of the CdTe and CdS layers obtained by this technique was studied. Carrier life time was obtained from V_oc decay measurement. Photoinduced charge separation observed in this glass/SnO₂/CBD–CdS/CdTe structure was associated with an increase in the dielectric constant and a decrease in the device resistance.     

Flexible CdTe solar cells on polymer films

Lightweight and flexible CdTe/CdS solar cells on polyimide films have been developed in a ‘superstrate configuration’ where the light is absorbed in CdTe after passing through the polyimide substrate. The average optical transmission of the approximately 10‐μm‐thin spin‐coated polyimide substrate layer is more than ∼75% for wavelengths above 550 nm. RF magnetron sputtering was used to grow transparent conducting ZnO:Al layers on polyimide films. CdTe/CdS layers were grown by evaporation of compounds, and a CdCl₂ annealing treatment was applied for the recrystallization and junction activation. Solar cells of 8·6% efficiency with V_oc = 763 mV, I_sc = 20·3 mA/cm² and FF = 55·7% were obtained. Copyright © 2001 John Wiley & Sons, Ltd.     

Contact wetting angle as a characterization technique for processing CdTe/CdS solar cells

The feasibility of measuring contact wetting angles to characterize processing induced changes to thin film semiconductors in CdTe/CdS solar cells is evaluated. The contact angles of water and formamide are used to determine the polar and dispersive surface energies of the thin films using two analysis methods. Changes in surface energies resulting from processing are correlated to changes in surface chemistry and structure detected by glancing incidence X‐ray diffraction (GIXRD), X‐ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Surface energies are evaluated for sputtered In₂O₃:SnO₂, chemical surface‐deposited CdS, and physical vapor‐deposited (PVD) CdTe thin films under as‐deposited and treated conditions. Treatments include thermal anneal in air, argon, and CdCl₂ ambient as well as surface etching. Indium tin oxide (ITO) and CdS films exhibit increased polar surface energy corresponding to enhanced crystallization of surfaces resulting from processing and increasing CdS growth temperature. Native oxidation of PVD CdTe (111)‐oriented film surfaces occurs rapidly and is readily detected by changes in contact angle. Surface energies of PVD (111)‐oriented CdTe stored under various humidities prior to processing are energetically similar due to native oxidation. The polar energy of CdTe surfaces is affected by the addition or removal of crystalline surface oxides during film processing. Copyright © 2006 John Wiley & Sons, Ltd.     

Interface Engineering for Both Cathode and Anode Enables Low‐Cost Highly Efficient Solution‐Processed CdTe Nanocrystal Solar Cells

Low‐cost solution‐processed CdTe nanocrystal (NC) solar cells always suffer from a high interface energy barrier and unbalanced hole/electron transport as well as anisotropic atom diffusion on the CdTe surface due to the limited amount of hole/electron interface materials or the difficulty in interface processing. In this work, a novel strategy is first adopted with gradient electron transport layer (CdS/CdSe) modification in the cathode and a new crosslinkable hole transport polymer (P‐TPA) implantation in the anode. The carrier recombination at interfaces is greatly decreased and thus the carrier collection is increased. Moreover, the light harvesting is improved both in short and long wavelength regions, making J_sc and V_oc increase simultaneously. A champion solar cell shows a very high power conversion efficiency of 9.2% and an outstanding J_sc of 25.31 mA cm^?2, which are among the highest values for all solution‐processed CdTe NC solar cells with a superstrate structure, and the latter value is even higher than that of traditional thick CdTe thin‐film solar cells (2 µm) via the high temperature close space sublimation method. This work demonstrates that facile surface modifications in both the cathode and anode with stepped extraction and organic–inorganic hybridization are very promising in constructing next‐generation highly efficient NC photovoltaic devices.     

Application of copper thiocyanate for high open‐circuit voltages of CdTe solar cells

Copper thiocyanate (CuSCN) has proven to be a low‐cost, efficient hole‐transporting material for the emerging organic–inorganic perovskite solar cells. Herein, we report that CuSCN can also be applied to CdTe thin‐film solar cells to achieve high open‐circuit voltages (V_OCs). By optimizing the thickness of the thermally evaporated CuSCN films, CdTe cells fabricated by close space sublimation in the superstrate configuration have achieved V_OCs as high as 872 mV, which is about 20–25 mV higher than the highest V_OC for the reference cells using the standard Cu/Au back contacts. CuSCN is a wide bandgap p‐type conductor with a conduction band higher than that of CdTe, leading to a conduction band offset that reflects electrons in CdTe, partially explaining the improved V_OCs. However, due to the low conductivity of CuSCN, CdTe cells using CuSCN/Au back contacts exhibited slightly lower fill factors than the cells using Cu/Au back contacts. With optimized CdS:O window layers, the power conversion efficiency of the best CdTe cell, using CuSCN/Au back contact, is 14.7%: slightly lower than that of the best cell (15.2%) using Cu/Au back contact. Copyright © 2015 John Wiley & Sons, Ltd.     

Cross‐sectional mapping of hole concentrations as a function of copper treatment in CdTe photo‐voltaic devices

The carrier density and carrier density distribution within CdTe solar cells were studied with scanning capacitance microscopy (SCM). The CdTe solar cells were studied after every process step and as a function of varying copper treatment conditions. It was found that the CdTe film is practically undoped after deposition and after CdCl₂ treatment, while after the copper step the carrier density distribution is non‐uniform with a mixture of p‐type and intrinsic grains in the CdTe film. These SCM observations were also confirmed with device performance data as well as capacitance–voltage measurements and Van der Pauw Hall measurements. Copyright © 2014 John Wiley & Sons, Ltd.     

Deposition and doping of CdS/CdTe thin film solar cells

1% oxygen is incorporated into both CdS and CdTe layers through RF sputtering of CdS/CdTe thin film solar cells. The optical and electrical parameters of the oxygenated and O₂-free devices are compared after CdCl₂ treatment and annealing in ambient Ar and/or air. The effects of ambient annealing on the electrical and optical properties of the films are investigated using current-voltage characterization, field emission scanning electron microscopy, X-ray diffraction, and optical transmission spectroscopy. The 1% oxygen content can slightly increase the grain size while the crystallinity does not change. Annealing in ambient Ar can increase the transmission rate of the oxygenated devices.     

Beam injection methods for characterizing thin‐film solar cells

II–VI and I–III–VI solar cells are promising for future thin‐film photovoltaics. In this paper, the roles of electron‐beam‐induced current (EBIC) and cathodoluminescence in evaluating the influence of interfaces on those solar cells are reviewed. CdTe and Cu(In,Ga)Se₂ (CIGS) are the absorbers of the cells investigated. For CdTe/CdS solar cells, a detailed study has been conducted of the effects of grain boundaries and the Te/CdTe or ZnTe:Cu/CdTe interfaces for back‐contacting. For CIGS solar cells, we have investigated different buffer layer schemes, showing that these interfaces are critical in the definition of the mechanisms for carrier collection. Copyright © 2002 John Wiley & Sons, Ltd.     

Direct evidence of enhanced chlorine segregation at grain boundaries in polycrystalline CdTe thin films via three‐dimensional TOF‐SIMS imaging

For more than 25 years, the CdTe photovoltaic research and manufacturing communities have been subjecting CdTe materials to a CdCl₂ treatment or activation step to improve performance. However, little work has been carried out using imaging to elucidate the spatial distribution of chlorine in the CdTe devices after this treatment. This work addresses fundamental questions about the spatial distribution of chlorine in the CdTe absorber material after a CdCl₂ treatment comparable to industrial practices. We used a state‐of‐the‐art, time‐of‐flight secondary ion mass spectrometer (ION‐TOF GmbH) (Muenster, Germany) with a lateral resolution of about 80 nm to complete three‐dimensional depth‐profiling and imaging of two CdTe devices. The results clearly demonstrate enhanced chlorine concentration along grain boundaries, supporting the hypothesis that chlorine plays an important role in passivating grain boundaries in CdTe solar cells. The results are discussed in terms of possible passivation mechanisms, and the effect of chlorine on grain interiors and grain boundaries. The data are also used to estimate the free energy of segregation of chlorine to grain boundaries in CdTe. Copyright © 2014 John Wiley & Sons, Ltd.     

Stable CdTe thin film solar cells with a MoOx back‐contact buffer layer

MoO_x thin films were employed as a buffer layer in the back contact of CdTe solar cells. A monograined CdS layer was employed as the window layer to reduce light absorption. The insertion of a MoO_x buffer layer in the back contact greatly reduced the Schottky barrier leading to increased fill factor and open‐circuit voltage. A CdTe solar cell, with an efficiency as high as 14.2%, was fabricated. The use of a MoO_x buffer layer made it possible to fabricate high‐efficient CdTe solar cell with much less Cu in the back contact, thus greatly enhancing the cell stability. Copyright © 2015 John Wiley & Sons, Ltd.     

Shunting problems due to sub‐micron pinholes in evaporated solid‐phase crystallised poly‐Si thin‐film solar cells on glass

Recent progress in the metallisation of poly‐silicon thin‐film solar cells on glass, created by solid phase crystallisation (SPC) of evaporated amorphous silicon (EVA), revealed that shunting through sub‐micron holes (density 100–200 mm⁻²) in the films causes severe shunting problems when the air‐side metal contact is deposited onto these diodes, by creating effective shunting paths between the two highly doped layers of EVA cells. We present evidence of these pinholes by optical transmission and focussed ion beam (FIB) microscopic images and confirm the point‐like pinhole shunts using lock‐in thermographic images. The latter revealed that the Al rear electrode induces strong ohmic shunts below the grid lines and a high density of weak non‐linear shunts away from the grid lines. Two distinctly different approaches are shown to reduce the shunting problem to a negligible level: (i) to contact only a small fraction of the rear Si surface via a point contacting scheme, whereby the metal layer needs to be thin (<1 µm) and the fractional area coverage small (<5%), and (ii) to deposit line contacts in a bifacial interdigitated scheme, whereby a thick layer of metal is deposited followed by a wet‐chemical etching step that effectively reduces shunting by preferentially etching away the shunting paths. Test devices with an area of 1 cm² achieve pseudo fill factors ( pFF ) of above 75% and diode ideality factors of below 1·3, demonstrating that the proposed methods are well suited for the metallisation of the rear surface of EVA solar cells. Copyright © 2008 John Wiley & Sons, Ltd.     

Microstructural and optical characterization of CdTe(211)B/ZnTe/Si(211) grown by molecular beam epitaxy

CdTe layers have been grown by molecular beam epitaxy on 3 inch nominal Si(211) under various conditions to study the effect of growth parameters on the structural quality. The microstructure of several samples was investigated by high resolution transmission electron microscopy (HRTEM). The orientation of the CdTe layers was affected strongly by the ZnTe buffer deposition temperature. Both single domain CdTe(133)B and CdTe(211)B were obtained by selective growth of ZnTe buffer layers at different temperatures. We demonstrated that thin ZnTe buffer layers (<2 nm) are sufficient to maintain the (211) orientation. CdTe deposited at ∼300°C grows with its normal lattice parameter from the onset of growth, demonstrating the effective strain accommodation of the buffer layer. The low tilt angle (<1°) between CdTe[211] and Si[211] indicates that high miscut Si(211) substrates are unnecessary. From low temperature photoluminescence, it is shown that Cd-substituted Li is the main residual impurity in the CdTe layer. In addition, deep emission bands are attributed to the presence of As_Te and Ag_Cd acceptors. There is no evidence that copper plays a role in the impurity contamination of the samples.     

CdTe/CdS thin film solar cells grown in substrate configuration

The ability to grow efficient CdTe/CdS solar cells in substrate configuration would not only allow for the use of non‐transparent and flexible substrates but also enable a better control of junction formation. Yet, the problems of barrier formation at the back contact as well as the formation of a p–n junction with reduced recombination losses have to be solved. In this work, CdTe/CdS solar cells in substrate configuration were developed, and the results on different combinations of back contact materials are presented. The Cu content in the electrical back contact was found to be a crucial parameter for the optimal CdCl₂‐treatment procedure. For Cu‐free cells, two activation treatments were applied, whereas Cu‐containing cells were only treated once after the CdTe deposition. A recrystallization behavior of the CdTe layer upon its activation similar to superstrate configuration was found; however, no CdTe–CdS intermixing could be observed when the layers were treated consecutively. Remarkably high V_OC and fill factor of 768 mV and 68.6%, respectively, were achieved using a combination of MoO₃, Te, and Cu as back contact buffer layer resulting in 11.3% conversion efficiency. With a Cu‐free MoO₃/Te buffer material, a V_OC of 733 mV, a fill factor of 62.3%, and an efficiency of 10.0% were obtained. Copyright © 2012 John Wiley & Sons, Ltd.     

Copper (I) Oxide (Cu2O) based back contact for p‐i‐n CdTe solar cells

In this paper a promising solution for the notorious problem of manufacturing a stable low ohmic back contact of a CdTe thin film superstrate solar cell is presented without using elemental copper. Instead we have used a Cu₂O layer inserted between the CdTe absorber and metal contact (Au). In contrast to the barrier free band alignment gained by using the transitivity rules, XPS measurements show a barrier in the valence band of the Cu₂O layers directly after deposition, which results in a low performing JV curve. The contact can be improved by a short thermal treatment resulting in efficiencies superior to copper based contacts for standard CdS/CdTe hetero junction solar cells prepared on commercial glass/FTO substrates. By replacing the CdS window layer with a CdS:O buffer layer efficiencies of >15% could be achieved. Copyright © 2016 John Wiley & Sons, Ltd.     

Modeling extremely thin absorber solar cells for optimized design

Extremely thin absorber (ETA) solar cells based on inorganic semiconductors are theoretically analyzed by a model that considers the absorber as being a pin junction, with tunneling‐assisted defect recombination. Tunneling recombination turns out to be very important in ETA solar cells, owing to the high electrical fields in the absorber, which establishes a minimum thickness for the absorber layer, which is calculated to be around 15 and 20 nm for CdTe and CuInS₂, respectively. Nevertheless, 15% efficient CdTe and CuInS₂ ETA solar cells are possible, even at low diffusion lengths down to 10 nm. Additionally, the modeling provides optimum values for the thickness and number of absorber layers for CdTe and CuInS₂ ETA solar cells, as a function of the minority‐carrier diffusion length and diffusion constant, for cells with and without light trapping. The calculations predict that light trapping serves two purposes: to enhance the cell efficiency by up to 5% absolute, and to use a simpler structure compared with the situation without light‐trapping. Copyright © 2004 John Wiley & Sons, Ltd.     

Progression of metalorganic chemical vapour‐deposited CdTe thin‐film PV devices towards modules

This paper reports important developments achieved with CdTe thin‐film photovoltaic devices produced using metalorganic chemical vapour deposition at atmospheric pressure. In particular, attention was paid to understand the enhancements in solar cell conversion efficiency, to develop the cell design, and assess scalability towards modules. Improvements in the device performance were achieved by optimising the high‐transparency window layer (Cd_0.3Zn_0.7S) and a device‐activation anneal. These increased the fill factor and open‐circuit voltage to 77 ± 1% and 785 ± 7 mV, respectively, compared with 69 ± 3% and 710 ± 10 mV for previous baseline devices with no anneal and thicker Cd_0.3Zn_0.7S. The enhancement in these parameters is associated with the two fold to three fold increase in the net acceptor density of CdTe upon air annealing and a decrease in the back contact barrier height from 0.24 ± 0.01 to 0.16 ± 0.02 eV. The optimum thickness of the window layer for maximum photocurrent was 150 nm. The cell size was scaled from 0.25 to 2 cm² in order to assess its impact on the device series resistance and fill factor. Finally, micro‐module devices utilising series‐connected 2‐cm² sub‐cells were fabricated using a combination of laser and mechanical scribing techniques. An initial module‐to‐cell efficiency ratio of 0.9 was demonstrated for a six‐cell module with the use of the improved device structure and processing. Prospects for CdTe photovoltaic modules grown by metalorganic chemical vapour deposition are commented on. Copyright © 2015 John Wiley & Sons, Ltd.