This roadmap highlights pathways to expand the CdTe module manufacturing capacity per year to 100 GWDC by 2030 by improving Te extraction from existing supply chains, minimizing Te usage in modules by leveraging thinner absorbers, and focusing research efforts in key areas to improve module efficiencies. Both scientific and supply chain innovations will be necessary to maintain the high compound annual growth rate of the CdTe photovoltaic (PV) industry and cement its role as a key technology for multi-TW-scale PV deployment.

Polycrystalline CdTe-based thin film solar cells are at the forefront of commercial thin film photovoltaic module products, with production levels approaching 20 GW per year and record cell efficiencies of 22.4 % . However, the open circuit voltage (VOC) remains below 900 mV, limiting further progress. Prior work on single crystal devices has demonstrated VOC over 1000 mV, suggesting there is a need to understand the factors limiting VOC of polycrystalline devices by developing a better understanding of single crystal devices. In this work, molecular beam epitaxy (MBE)-grown single crystal CdTe and CdSexTe1-x layers have been used for such studies. Sivananthan laboratories is working on a joint effort, the CdTe Accelerator Consortium (CTAC)* funded by the DOE, with other photovoltaic groups, to improve the device performance of polycrystalline CdTe devices to achieve their true potential. One of the major identified issues that limits performance is the low activation of dopants such as arsenic, which is well below 10%. Previous studies on arsenic doping in single crystal CdTe have demonstrated 50% activation with hole concentrations of -- 5 x 1016 cm-3. Recently, Sivananthan Laboratories has attained> 70% activation with hole concentrations of 7 x 1016 cm-3 and 20% activation with hole concentrations of > 2 x 1017 cm-3. Both these results represent records in activation and hole concentration respectively. Utilizing these advanced doping methods for single-crystal Cd'Te, we will systematically introduce grain boundaries in MBE-grown CdTe layers by epitaxial growth on polycrystalline templates from CTAC partners and study the effects of grain boundaries on the doping properties of polycrystalline layers in order to optimize As incorporation and improve activation beyond 10 % .

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.

The cadmium sulfide (CdS)/cadmium telluride (CdTe) heterojunction is a promising material combination for the development of cost efficient solar cells to meet the world's future energy demand. This study examined the effects of the surface roughness of six different layers, such as FTO, SnO2 buffered FTO, thick and thin CdS layers deposited on these buffered and unbuffered FTO, on the photovoltaic performance of the corresponding CdS/CdTe solar cells. The morphologies of these surfaces were examined by atomic force microscopy (AFM). The short circuit current densities and fill factors of the devices were improved significantly when the SnO2 buffer layer was introduced between the FTO and CdS layer. AFM images showed that surface roughness of the FTO coated glass substrates decreased when a buffer layer was present on FTO. The short circuit current densities and hence the external quantum efficiencies were improved further when the thickness of the CdS layer was reduced. This was attributed to the reduced filtering effect of the CdS layer. The optimized device showed an external quantum efficiency of more than 85% at the maximum absorption wavelengths of CdTe and an overall power conversion efficiency of more than 14.5% under an air mass (AM) 1.5 irradiation (100 mW cm(-2), 1 sun).

We have investigated in situ p-type doping of ZnTe and CdZnTe on Si(211) by molecular beam epitaxy using a radiofrequency (RF)-nitrogen plasma source for application to multijunction II–VI-based solar cells. CdZnTe would be used as a wide-gap top cell in a monolithic multijunction device, and ZnTe or CdZnTe could be used for the p-side of tunnel junctions. Highly p-type material is required for producing the high-quality tunnel junctions crucial for maintaining current flow, and p-doping of order 1017 cm−3 is required for the generation of a large built-in potential in the absorber region of solar cells. Our uniformly doped films exhibited good Hall characteristics, especially considering the large lattice mismatch between Si and either ZnTe or CdZnTe. Crystal quality was examined by x-ray diffraction. Nitrogen incorporation was examined as a function of the source-gas dilution with argon. A sample with layers of CdZnTe doped using 1% to 100% nitrogen was grown on nominally undoped CdZnTe and analyzed using secondary-ion mass spectrometry. The nitrogen incorporation differed by only a factor of 10, despite the factor of 100 difference in the nitrogen concentration in the plasma, indicating a saturation effect.

In this work we present a correlation between the structural and electro-optical properties of grain boundaries in CdTe thin films deposited by vapor transport technique. We were able to identify different types of grain boundaries using electron backscatter diffraction (EBSD), and investigated their recombination properties by cathodoluminescence (CL). The objective is to investigate the existence of “good” and “bad” boundaries in CdTe thin films, which will provide guidance for the growth of better films in the future. The crystallographic orientation, grain size, and relative fraction of different boundaries were determined by EBSD. For the comparison study, the grain boundaries were colored according to their character, and compared to the CL spectra. By applying focused ion beam (FIB) marks, we were able to analyze CL and EBSD maps taken at exactly the same areas. We present a correlation between the types of boundaries with recombination.

In this work, we study CdTe thin films used in CdTe/CdS solar cells with a substrate configuration, which allows for better control in forming the junction, and the possibility for using flexible non-transparent substrates. We studied the properties of CdTe films grown at 450° and 550°C, with and without a CuxTe layer, and before and after CdCl2 treatment. We analyzed the structural and electro-optical properties using electron backscatter diffraction (EBSD), cathodoluminescence (CL) and X-ray diffraction (XRD), and investigated how the film structure, stress, and defect structure changes with the different growth conditions.

We describe a new time-resolved photoluminescence (TRPL) analysis method for the determination of minority carrier lifetime tau(B). This analysis is based on subbandgap excitation (two-photon excitation, or 2PE) and allows selective lifetime determination at the surface or in the bulk of semiconductor absorbers. We show that for single-crystal CdTe, tau(B) could be determined even if surface recombination velocity is >10(5) cm s(-1). Two-photon excitation TRPL measurements indicate that radiative lifetime in undoped CdTe is >> 66 ns. We also compare one-photon excitation (1PE) and 2PE TRPL data for polycrystalline CdS/CdTe thin films.

We study the effects of Cu composition on the CdTe/ZnTe:Cu back contact and the bulk CdTe. For the back contact, its potential barrier decreases with Cu concentration while its saturation current density increases. For the bulk CdTe, the hole density increases with Cu concentration. We identify a Cu-related deep level at ∼0.55 eV whose concentration is significant when the Cu concentration is high. The device performance, which initially improves with Cu concentration then decreases, reflects the interplay between the positive influences (reducing the back contact potential barrier while increasing the saturation current density of the back contact and hole density in CdTe bulk) and negative influences (increasing deep levels in CdTe) of Cu.

Recent studies of thin-film CdS/CdTe photovoltaic (PV) devices have suggested that a significantly higher device performance will not be achieved unless recombination in the CdTe is reduced. Although some control of CdTe recombination has been achieved historically through the careful incorporation of oxygen, chlorine, and copper, we believe a more promising avenue to higher device performance is by controlling the defects in the as-deposited CdTe. This is supported by theoretical studies that suggest much of the improvement associated with oxygen, chlorine, and copper is due to the interaction of these species with intrinsic defects related to cadmium and tellurium vacancies, interstitials, and anti-sites in the bulk as well as within the grain boundary regions. Several research projects at NREL are currently focused on altering CdTe deposition and post-deposition processes to allow for enhanced control of the as-deposited intrinsic defects. This paper discusses initial results in which process changes expected to alter the as-deposited defects are also observed to affect junction evolution and device functionality.

We have investigated the microelectrical properties of CdTe thin films using scanning Kelvin probe force microscopy (SKPFM) and scanning spreading resistance microscopy (SSRM). Two films with the configurations of substrate and superstrate were subjected to the characterization studies. The electrical potential and resistance were properly mapped with the substrate film but not with the superstrate film because the underlying CdS/CdTe junction largely impacted the characterizations. The higher SKPFM potential on grain boundaries (GBs) of the substrate film than on the grain surface indicates positively charged GBs and upward band bending around the GB; therefore, the GBs are either depleted or inverted. The SSRM resistance mapping on this film shows nonuniformities and features that are associated with the grain structure and facets. However, the GBs do not exhibit distinct characteristic resistance. Comparing the low resistance channel along the GBs of high-performance CIGS films, the SSRM mapping of CdTe supports depletion of the GBs. In SSRM measurement, it is critical to adequately indent the probe to the film, and to apply a bias voltage larger than the onset voltage of the probe/film barrier, so that the contact resistance is minimized and that the local spreading resistance of CdTe film beneath the probe is measured.

Thin-film polycrystalline semiconductors are currently at the forefront of inexpensive large-area solar cell and integrated circuit technologies because of their reduced processing and substrate selection constraints. Understanding the extent to which structural and electronic defects influence carrier transport in these materials is critical to controlling the optoelectronic properties, yet many measurement techniques are only capable of indirectly probing their effects. Here we apply a novel photoluminescence imaging technique to directly observe the low temperature diffusion of photocarriers through and across defect states in polycrystalline CdTe thin films. Our measurements show that an inhomogeneous distribution of localized defect states mediates long-range hole transport across multiple grain boundaries to locations exceeding 10 μm from the point of photogeneration. These results provide new insight into the key role deep trap states have in low temperature carrier transport in polycrystalline CdTe by revealing their propensity to act as networks for hopping conduction. Understanding the role of defects on semiconductor carrier transport should help improve their performance in devices. Using photoluminescence techniques, Alberi et al. image the carrier diffusion in polycrystalline CdTe and find that long-range transport is mediated by the distribution of defect states.

CdS/CdTe photovoltaic cells were grown in a homemade close-spaced sublimation equipment where all steps of cell fabrication are carried out continuously, without breaking the vacuum. The best efficiency achieved was 7%. Some devices partially prepared in this system were finished (heat treatment and back contact processing) at the National Renewable Energy Laboratory and showed efficiencies in the range of 10–11%, due to higher values of fill factor and photocurrent. Comparison between the procedures followed in the two laboratories identified some critical issues in the CSS process carried out continuously, such as the temperature of the heat treatment and the nature of the back contact.

In this work we investigate the effects of a CdCl2 heat treatment on the interface of CdTe/CdS heterojunction solar cells using Rietveld analysis of X-ray diffraction patterns. Although the Rietveld method is an important tool for the study of materials by X-ray diffraction, there have been few reports of its use in thin film analysis. The results showed the occurrence of interdiffusion in the CdS–CdTe boundary, with substitution of tellurium by sulfur in the CdTe lattice that resulted in a CdSxTe1 − x alloy. The sulfur content of the alloy was greater than in previous studies. This is attributed to the low oxygen concentration during processing and the strong (111) texture of the CdTe films.

Most work on CdTe-based solar cells has focused on devices with a superstrate structure. This focus is due to the early success of the superstrate structure in producing high-efficiency cells, problems of suitable ohmic contacts for lightly doped CdTe, and the simplicity of the structure for manufacturing. The development of the CdCl 2 heat treatment boosted CdTe technology and perpetuated the use of the superstrate structure. However, despite the beneficial attributes of the superstrate structure, devices with a substrate structure are attractive both commercially and scientifically. The substrate structure eliminates the need for transparent superstrates and thus allows the use of flexible metal and possibly plastic substrates. From a scientific perspective, it allows better control in forming the junction and direct access to the junction for detailed analysis. Research on such devices has been limited. The efficiency of these devices has been limited to around 8% due to low open-circuit voltage (V oc ) and fill factor. In this paper, we present our recent device development efforts at NREL on substrate-structure CdTe devices. We have found that processing parameters required to fabricate high-efficiency substrate CdTe PV devices differ from those necessary for traditional superstrate CdTe devices. We have worked on a variety of contact materials including Cu-doped ZnTe and Cu x Te. We will present a comparative analysis of the performance of these contacts. In addition, we have studied the influence of fabrication parameters on junction properties. We will present an overview of our development work, which has led to CdTe devices with V oc values of more than 860 mV and NREL-confirmed efficiencies approaching 11%.

Since the days of the technology's conception, concerns have been voiced over potential supply constraints of Tellurium that could limit the large-scale deployment of the Cadmium Telluride (CdTe) solar photovoltaic technology. Because any potential supply-demand imbalance created by a Tellurium constraint would manifest itself in the form of a price increase (a trend that was already seen prior to the 2012 downturn in PV manufacturing), we have rigorously examined the sensitivity of total CdTe module manufacturing prices to the price of this minor metal. We found that module manufacturers could conceivably absorb a gradual increase in Te prices up to an order of magnitude higher than what was typical for 2011 without significantly compromising their near to mid-term competitive position within the PV industry (viewed here to be a $0.70/W module price)—if the pace of improvements in module power conversion efficiencies and reductions in the CdTe layer thickness is rapid enough. Realizing gains in module-area efficiencies while, at the same time, also reducing the CdTe thickness is certainly technically challenging and merits its own line of research. However, in order to accommodate up to an order of magnitude increase in Te prices while still keeping the cost of the active layer to a reasonable $0.15/W range, we find that the cost benefits gained by reducing the absorber layer thickness are expected to be as significant as those provided by efficiency gains alone. Realizing the optimistic target of 18% efficient modules with 1.0μm of CdTe could reduce the Te material intensity from today's requirement of around 74 metric tonnes (MT) per GW to 17MT/GW; even so, we estimate that CdTe PV is likely to be material constrained to around 10GW of annual production by 2020 unless new sources of Tellurium—beyond traditional copper byproduct sources at the current 55% recovery rate—come online. The economics of this mineral are such that a higher price offering is a necessary precondition in order to motivate enhanced recovery rates from copper mining, and for future direct mining projects.

Recently, CdTe photovoltaic (PV) devices fabricated in the nonstandard substrate configuration have attracted increasing interest because of their potential compatibility with flexible substrates such as metal foils and polymer films. This compatibility could lead to the suitability of CdTe for roll-to-roll processing and building-integrated PV. Currently, however, the efficiencies of substrate CdTe devices reported in the literature are significantly lower (~6%-8%) than those of high-performance superstrate devices (~17%) because of significantly lower open-circuit voltage (V oc ) and fill factor (FF). In our recent device development efforts, we have found that processing parameters required to fabricate high-efficiency substrate CdTe PV devices differ from those necessary for traditional superstrate CdTe devices. Here, we investigate how oxygen incorporation in the CdTe deposition, CdCl 2 heat treatment, CdS deposition, and post-deposition heat treatment affect device characteristics through their effects on the junction. By adjusting whether oxygen is incorporated during these processing steps, we have achieved V oc values greater than 860 mV and efficiencies greater than 10%.

Este trabalho mostra o desempenho de celulas solares de CdS/CdTe produzidas em um equipamento fabricado em laboratorio. Neste sistema, as etapas de producao sao conduzidas continuamente, via processos secos, sem quebra de vacuo. As eficiencias alcancadas foram da ordem de 7%. Estas celulas foram comparadas com celulas inacabadas, fabricadas neste mesmo sistema, mas que foram finalizadas (tratamento termico e contato de fundo) no NREL. Neste caso, as eficiencias alcancaram valores de ate 10%. Os resultados ajudaram a identificar as etapas criticas do processo conduzido no IME. Sugestoes para otimizar estas etapas sao propostas em funcao destes resultados.