Polycrystalline CdTe Solar Cells Research Articles

Thermomechanical Cleave of Polycrystalline CdTe Solar Cells and its Applications: A Review

One of the primary research challenges for cadmium telluride (CdTe) solar cells is addressing its open‐circuit voltage (VOC) deficit. While theoretical studies and single crystal work show VOC > 1 V is possible, devices remain stubbornly low at ≈800–900 mV. As absorber opto‐electronic properties (e.g., hole density, carrier lifetime) are improved, device modeling suggests that interfaces become limiting. Because CdTe‐based devices are typically grown in the superstrate configuration, the back interface is relatively accessible for manipulation and study, while the front interface (i.e., the heterojunction region) is buried under microns of material and inaccessible. NREL has developed a novel technique to thermomechanically cleave polycrystalline CdTe device stacks directly at the front interface, enabling characterization and controlled manipulation of this important region. Herein, recent work, primarily from NREL, will be reviewed, including considerations for achieving successful delamination; key scientific discoveries about the front interface that have been enabled by this technique; and practical applications, such as flexible, low‐cost solar with high power‐to‐weight ratio.

Band Bending at CdTe Solar Cell Contacts: Correlating Electro‐Optical and X‐Ray Photoelectron Spectroscopy Analyses of Thin Film Solar Cells

With the semiconductor bulk properties reaching target values for highly efficient solar cells, efforts are applied to reduce losses at solar cell interfaces and contacts. Advances in understanding back contacts in thin‐film polycrystalline CdTe solar cells, a leading thin‐film PV technology, are reported. By using X‐Ray photoelectron spectroscopy, Kelvin probe spectroscopy, time‐ and energy‐resolved photoluminescence, defects at the back contact are analyzed. Densities of recombination centers and charged defects that induce near‐back‐contact band bending, both resulting in recombination losses, were estimated. Electro‐optical and surface analysis results are integrated into a device model, simulating the performance of CdSeTe/CdTe solar cells with 902 mV open circuit voltage.

Exceeding 20% efficiency with in situ group V doping in polycrystalline CdTe solar cells

CdTe-based solar technology has achieved one of the lowest levelized costs of electricity among all energy sources as well as state-of-the-art field stability. Yet, there is still ample headroom to improve. For decades, mainstream technology has combined fast CdTe deposition with a CdCl2 anneal and Cu doping. The resulting defect chemistry is strongly compensated and limits the useful hole density to ~1014 cm−3, creating a ceiling for fill factor, photovoltage and efficiency. In addition, Cu easily changes energy states and diffuses spatially, creating a risk of instabilities that must be managed with care. Here, we demonstrate a significant shift by doping polycrystalline CdSexTe1 − x and CdTe films with As while removing Cu entirely from the solar cell. The absorber majority-carrier density is increased by orders of magnitude to 1016–1017 cm−3 without compromising the lifetime, and is coupled with a high photocurrent greater than 30 mA cm−2. We demonstrate pathways for fast dopant incorporation in polycrystalline thin films, improved stability and 20.8% solar cell efficiency. CdTe solar cells have relied for decades on copper, which creates limited hole density, stability issues and a ceiling for voltage and efficiency. Now, Metzger et al. demonstrate As-doped Cu-free polycrystalline CdTe cells with enhanced hole density and dopant stability, achieving 20.8% efficiency.

Study of thin film poly-crystalline CdTe solar cells presenting high acceptor concentrations achieved by in-situ arsenic doping

Doping of CdTe using Group-V elements (As, P, and Sb) has gained interest in pursuit of increasing the cell voltage of CdTe thin film solar devices. Studies on bulk CdTe crystals have shown that much higher acceptor concentration than the traditional copper treatment is possible with As, P or Sb, enabled by high process temperature and/or rapid thermal quenching under Cd overpressure. We report a comprehensive study on in-situ As doping of poly-crystalline CdTe solar cells by MOCVD, whereby high acceptor densities, approaching 3 × 1016 cm−3 were achieved at low growth temperature of 390 °C. No As segregation could be detected at grain boundaries, even for 1019 As cm−3. A shallow acceptor level (+0.1 eV) due to AsTe substitutional doping and deep-level defects were observed at elevated As concentrations. Devices with variable As doping were analysed. Narrowing of the depletion layer, enhancement of bulk recombination, and reduction in device current and red response, albeit a small near infrared gain due to optical gap reduction, were observed at high concentrations. Device modelling indicated that the properties of the n-type window layer and associated interfacial recombination velocity are highly critical when the absorber doping is relatively high, demonstrating a route for obtaining high cell voltage.

Low temperature growth and extrinsic doping of mono-crystalline and polycrystalline II-VI solar cells by MBE

An investigation of low temperature MBE growth of crystalline Cd(1-x)Zn(x)Te with 0 ≤x ≤ 1, in the range 220 ℃≤Ts ≤ 320 ℃, and extrinsic doping is conducted. This results in the construction of homo-junction crystalline solar cells on lattice-mismatched 211B and 111B GaAs substrates. Doping and composition studies of crystalline layers are used as a guide for low-temperature, high growth-rate, MBE grown polycrystalline CdTe solar cells on various n-type emitters and different back-contacts. The best resulting devices reach efficiencies of 14.7% under AM 1.5 g, without an anti-reflection coating.

Effect of CdCl2 heat treatment on ZnTe back electron reflector layer in thin film CdTe solar cells

The effect of CdCl2 heat treatment on the ZnTe layer used as the back electron reflector layer in polycrystalline CdTe solar cells was investigated. A thin film of ZnTe was grown on top of polycrystalline CdTe layer using metalorganic vapor phase epitaxy, which was then subjected to CdCl2 heat treatment. The effect was examined using various characterization techniques such as X-ray diffraction, energy dispersive X-ray spectroscopy and scanning electron microscope images and it was found that the ZnTe layer transformed into CdTe and ZnO due to the treatment. This was verified by performing CdCl2 heat treatment on pure ZnTe thin layer directly grown on glass which also showed the conversion of ZnTe to CdTe and ZnO. To study the effect of this transformation on final device performance, thin film CdTe solar cell structures were fabricated with ZnTe electron reflector layer with and without the CdCl2 heat treatment. The device without the ZnTe layer seemed to perform better than the devices with ZnTe layer and the CdCl2 heat treatment.

Numerical Simulation of Copper Migration in Single Crystal CdTe

With better CdTe material properties and more effective contacts, the compensated Cu doping is crucial for enhancing the performance of CdTe solar cells. Moreover, the metastable behavior of CdTe devices is associated with Cu migration. To understand the formation of Cu acceptors in CdTe and Cu-related metastabilities, we developed a one-dimensional diffusion-reaction simulator to study the kinetics of Cu-related defects in CdTe material. A detailed kinetic model that explicitly describes the evolutions of point defects was developed and applied for the first time. Migration of intrinsic and Cu-related extrinsic defects in single crystal CdTe (sx-CdTe) bulk has been investigated in time-space domain self-consistently with free carrier transport. Cu profiles achieved from simulations match with the experimental profiles quantitatively. The simulation results provide explanations of limited Cu incorporation (also known as the Cu solubility limits) and the development of Cu compensation in course of annealing and cooling processes. Although the one-dimensional simulation has intrinsic limitations when applied to poly-crystalline CdTe solar cells, the results suggest that the approach still has strong potential in better understanding the performance and the metastabilities of poly-crystalline CdTe photovoltaic devices.

Electron and hole drift mobility measurements on thin film CdTe solar cells

We report electron and hole drift mobilities in thin film polycrystalline CdTe solar cells based on photocarrier time-of-flight measurements. For a deposition process similar to that used for high-efficiency cells, the electron drift mobilities are in the range of 10−1–100 cm2/V s, and holes are in the range of 100–101 cm2/V s. The electron drift mobilities are about a thousand times smaller than those measured in single crystal CdTe with time-of-flight; the hole mobilities are about ten times smaller. Cells were examined before and after a vapor phase treatment with CdCl2; treatment had little effect on the hole drift mobility, but decreased the electron mobility. We are able to exclude bandtail trapping and dispersion as a mechanism for the small drift mobilities in thin film CdTe, but the actual mechanism reducing the mobilities from the single crystal values is not known.

7.5L: Late‐News Paper: Cathodoluminescence Quantum Efficiency of Quantum Dot Thin Films

AbstractA thin film of quantum dots (QD) was used to visualize the local photo‐response of polycrystalline CdTe solar cells by down‐converting an electron beam of high energy to photons of visible light. The efficient photon generation in the QD film is compared to cathodoluminescence of the high‐purity bulk semiconductors and phosphor.

Tailoring Impurity Distribution in Polycrystalline CdTe Solar Cells for Enhanced Minority Carrier Lifetime

Passivation of grain boundaries (GBs) and interfaces to suppress recombination and to improve minority carrier lifetime (MCLT) is essential for the functionality of devices based on polycrystalline materials. Improvement of MCLT is believed to be a very promising way to bring CdTe solar cells to the next efficiency level. However, which parameters significantly affect MCLT is not well understood. Here, high‐efficiency CdTe solar cells in an unconventional inverted structure are used to approach this issue. Advanced characterization tools such as secondary ion mass spectroscopy 3D chemical imaging, atom probe tomography, and X‐ray photoelectron spectroscopy are used to detect small amounts of impurities at GBs and are synergetically used together with time resolved photoluminescence measurements to correlate impurity distribution with electronic properties in CdTe solar cells. MCLT increases by an order of magnitude upon sulfur diffusion along GBs of the CdTe layer, which can occur by an elemental exchange with oxygen. Chlorine segregates at GBs and at the CdS/CdTe interface and bonding to cadmium and tellurium is indicated. CdTe solar cells in the inverted structure are presented with a certified efficiency of 13.5%. The results give guidance to further improve the performance of CdTe solar cells.

A comprehensive picture of Cu doping in CdTe solar cells

The importance of Cu for CdTe solar cell absorber doping has been increasingly recognized in recent years. Currently different models are being discussed how to understand the case of CuCd substitutional doping in polycrystalline CdTe solar cells. In this work, an understanding is developed, which is based on a low concentration deep acceptor doped CdTe layer (Na ∼ 5 × 1014 cm−3, Ea ∼ 300 meV above the valence band). Despite their non-shallow nature, CuCd acceptors are fully or at least heavily (>30%) ionized. The low hole concentration in CdTe (∼1 × 1014 cm−3) originates directly from low Cu solubility in CdTe bulk material and is not caused by partial ionization or compensation as proposed by earlier models. The three to four orders of magnitude difference between bulk acceptor concentration and average Cu concentration in polycrystalline CdTe is attributed to grain boundary segregation of Cu. Our model is derived from substrate and superstrate CdTe solar cell measurements, controlled CdTe doping and quenching, Hall Effect measurements of CdTe films, numerical and analytical calculations, and a broad literature survey. Based on these results, routes to improve the conversion efficiency of CdTe solar cells are discussed.

Simulation of Current Transport in Polycrystalline CdTe Solar Cells

Polycrystalline thin-film CdTe solar cells have demonstrated laboratory efficiency exceeding 17% and are nowadays a commercial technology (albeit with somewhat lower efficiencies). The standard process features a poorly understood recrystallization step, obtained by annealing with a source of chlorine. This study uses two-dimensional numerical modeling to investigate current transport inside the polycrystalline CdTe absorber with and without recrystallization effects [increase of grain size and donor ClTe states at grain boundaries (GBs)]. Solving the Poisson equation and the drift–diffusion model for transport with Fermi statistics, while treating the optical problem by the one-dimensional transfer matrix method and complex refractive indexes, this study shows that: (i) in a columnar absorber (i.e., one where only vertical GBs exist), the presence of ClTe donor traps at GBs results in a dip in the band profiles that effectively serves as an electron collector, significantly increasing the short-circuit current and efficiency compared with nondecorated GBs; (ii) while the same dip acts as a hole barrier and thus can be expected to block holes from flowing when horizontal GBs are present, under illuminated conditions electron collection at GBs reduces the dip enough to allow substantial hole flow, and the cell performance is only moderately affected.

Time-resolved photoluminescence studies of CdTe solar cells

We show that time-resolved photoluminescence measurements of completed polycrystalline CdTe solar cells provide a measure of recombination near the CdTe/CdS metallurgical interface that is strongly correlated to the open-circuit voltage in spite of complex carrier dynamics in the junction region. Oxygen in the growth ambient during close-spaced sublimation generally reduces this recombination rate; grain size does not have a strong effect.

Spatial uniformity of minority-carrier lifetime in polycrystalline CdTe solar cells

Time-resolved photoluminescence was used to map recombination lifetimes in polycrystalline CdS/CdTe solar cells. Typical lifetime profiles indicated spatial variation by factors of 2–3 across 1 cm dimensions. Correlated device efficiency measurements indicated that spatial regions of lifetime minima dominated the open-circuit voltage.

Growth and process optimization of CdTe and CdZnTe polycrystalline films for high efficiency solar cells

Abstract Polycrystalline CdTe solar cells with efficiencies of approximately 10% were achieved by metal organic chemical vapor deposition growth of CdTe on glass/SnO 2 /CdS substrates. An in situ pre-heat treatment of the CdS substrate at 450 °C in an H 2 ambient was found to be essential for high performance devices because it removes oxygen-related defects on the CdS surface. This heat treatment also results in a cadmium-deficient CdS surface which may, in part, limit the CdTe cell efficiency to 10% owing to cadmium vacancy related interface defects. The CdCl 2 treatment used in CdTe cell processing was found to promote grain growth, reduce series resistance and interface state density, and change to dominant current transport mechanism from thermally assisted tunneling and recombination via interface states to recombination in the depletion region. These beneficial effects resulted in an increase in the CdTe/CdS cell efficiency from around 2% to approximately 9%. In addition to the CdTe cells, polycrystalline 1.7 eV CdZnTe films were grown by molecular beam epitaxy for tandem cell applications. CdZnTe/CdS cells processed using the standard CdTe cell fabrication procedure resulted in 4.4% efficiency, high series resistance, and a band gap shift to 1.55 eV. Formation of ZnO at and near the CdZnTe surface was found to be the source of high contact resistance. A saturated dichromate etch instead of the Br 2 :CH 3 OH etch prior to contact deposition was found to solve the contact resistance problem. The CdCl 2 treatment was identified to be the cause of the observed band gap shift owing to the preferred formation of ZnCl 2 . A model for the band gap shift along with a possible solution using an overpressure of ZnCl 2 in the annealing ambient is proposed. Development of a sintering aid which promotes grain growth and preserves the optimum 1.7 eV band gap is shown to be the key successful wide gap CdZnTe cells.

U. S. Polycrystalline Thin Film Solar Cells Program

ABSTRACTThe Polycrystalline Thin Film Solar Cells Program, part of the United States National Photovoltaic Program, performs R&D on copper indium diselenide and cadmium telluride thin films. The objective of the Program is to support research to develop cells and modules that meet the U.S. Department of Energy's long-term goals by achieving high efficiencies (15% - 20%), low-cost ($50/M2), and long-time reliability (30 years). The importance of work in this area is due to the fact that the polycrystalline thin-film CuInSe2 and CdTe solar cells and modules have made rapid advances. They have become the leading thin films for PV in terms of efficiency and stability. The U.S. Department of Energy has increased its funding through an initiative through the Solar Energy Research Institute in CuInSe2and CdTe with subcontracts to start in Spring 1990.

CdTe solar cells and photovoltaic heterojunctions in II–VI compounds

This paper describes the principal features of polycrystalline CdTe solar cells and their method of fabrication. These cells are prepared such that a p-type copper telluride surface film is an integral part of the photovoltaic junction. Some description is also given of single-crystal cells where the junction is fabricated similarly to that of the films. Solar conversion efficiencies up to 6 per cent have been obtained in film cells, and 7 1/2 per cent for single-crystal ones.The junction is interpreted as a heterojunction between p-type copper telluride and n-type cadmium telluride, the transition region extending well into the CdTe side. The copper telluride film plays a minor part in the absorption of radiation for the production of electron-hole pairs. The method of fabrication, experimental results, and present interpretation are compared to that existing for other CdTe and CdS solar cells.