Photovoltaic junctions on polycrystalline cdte
Thin, polycrystalline, cadmium telluride solar cells
CADMIUM TELLURIDE↗
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Doping CdTe and CdSeTe for higher efficiency
Thin film cadmium telluride is one of the most successful photovoltaic technologies on the market today. Second only to silicon in yearly output and accounting for 40% of U.S. utility-scale photovoltaic installation, CdTe is known for its ease of manufacture, ideal bandgap, and low levelized cost of energy. Despite its commercial success, CdTe underperforms compared to its theoretical potential. The current world record CdTe device is only 21.0% compared to a theoretical maximum of 33.1%. This significant discrepancy in efficiencies can mostly be attributed to the poor open-circuit voltage of CdTe devices. Compared to silicon technologies, CdTe has a large voltage deficiency, exceeding 250 mV.
14 SOLAR ENERGY↗
Microwave Photoconductance Spectrometer for Thin-Film Deposited Semiconductor Materials
A 10GHz microwave reflectometer was developed to measure the mobility-lifetime product in thin film semiconductors. Development focused on perovskite films (an emerging photovoltaic material) as well as CdTe (an established photovoltaic material).
47 OTHER INSTRUMENTATION↗
Sputter-deposited CdMgTe for rear contact to CdSeTe/CdTe solar cells
CdTe-based photovoltaic device performance improvement is predominantly limited by open-circuit voltage. This work approaches the voltage deficit challenge through the incorporation of a cadmium magnesium telluride (CdMgTe) electron reflector layer at the back of 1.5- μm CdSeTe/CdTe absorbers to reduce back-surface recombination through a conduction band offset. A sputtered CdMgTe electron reflector layer permitted low fabrication temperatures and circumvented the intolerance of CdMgTe to thermal processing present in traditional close-space sublimation (CSS)-deposited CdMgTe devices. Here, we demonstrate that sputtered CdMgTe simultaneously promotes the retention of chlorine passivation and magnesium which is not feasible with CSS fabrication. We also show the importance of the Cu and Te back contact order in CdMgTe devices. Through the optimization of the sputtered CdMgTe substrate temperature, doping, and back structure configuration, 1.5 μm CdSeTe/CdTe devices with a sputtered CdMgTe layer achieved an efficiency of 16.0%, significantly higher than with CSS CdMgTe, and demonstrated partial enactment of electron-reflector behavior.
14 SOLAR ENERGY↗
Operando X-ray Nanocharacterization of Polycrystalline Thin Film Modules
Non-destructive measurement techniques, with high spatial resolution, capable of correlating composition and structure with device properties, are few and far between. For the case of polycrystalline and inhomogeneous materials, the added challenge is that large sampling areas are necessary in order to have a statistical representation of the specimen under study. For the study of grain cores and grain boundaries in polycrystalline solar absorbers this is of particular importance since their dissimilar behavior and variability throughout the samples makes it difficult to draw conclusions and ultimately optimize the materials and devices. Our approach to use state-of-the-art x-ray microscopy to study full module stacks under operating conditions is unique. Understanding the effects of elemental and charge migration under full encapsulation and operating conditions will allow CIGS and CdTe manufactures and in particular our partners in this proposal to modify materials, stacks, architectures, and processing steps to minimize the performance losses through time.
14 SOLAR ENERGY↗
Doping of CdTe Using CuCl2 Solution for Highly Efficient Photovoltaic Devices
Here, we report doping of cadmium telluride (CdTe) thin film solar cells using copper (II) chloride (CuCl 2 ) solution in deionized water. The CdTe thin films were immersed in 0.1 mmol CuCl 2 solution in water and annealed in the temperature range of 180-240 degrees C for various time durations. The devices were completed using thermally evaporated gold (Au) and measured under simulated AM1.5G spectrum. The average device efficiency of CuCl 2 treated CdTe devices increased to 13.7 % (best cell reaching 14.0%) with significant improvement on open-circuit voltage (VOC ) as compared with 12.4% (best cell 12.7%) for thermally evaporated Cu doped CdTe devices. For CuCl 2 treated CdTe devices, we found higher photoluminescence (PL) intensity and longer carrier life time at room temperature indicating solution based doping offers better control of Cu doping density as compared with evaporated Cu.
14 SOLAR ENERGY↗
Dual-Wavelength Time-Resolved Photoluminescence Study of CdSe x Te 1-x Surface Passivation via Mg y Zn 1-y O and Al 2 O 3
Voltage loss is currently one of the biggest challenges facing cadmium telluride (CdTe) based photovoltaics. Determining the location(s) of major voltage loss within the device stack (e.g., front/back interface, grain boundaries) is therefore of primary interest. Here, we present a custom-built time-resolved photoluminescence system with two excitation wavelengths - 670 (standard) and 405 nm - to probe the device stack at depths of approximately 130 and 35 nm, respectively; their comparison helps differentiate interface and bulk contributions to carrier lifetime. We apply this system to examine the passivation effect of two significant recent advances in CdTe: the incorporation of Se to form graded CdSe x Te 1-x and the replacement of CdS with Mg y Zn 1-y O. It is found that x = 0.2 Se is required to obtain lifetime improvements, primarily in the bulk. Additionally, evidence for trapping at the Mg y Zn 1-y O/CdSe x Te 1-x interface was observed. Furthermore, this indicates further work is required to sufficiently passivate the front interface.
CdSeTe↗
Investigating the Role of Copper in Arsenic Doped CdSeTe Photovoltaic Devices
As part of NREL's development of arsenic doped CdSeTe devices, co-doping with copper has become a common practice and, despite little difference in carrier concentration (often ~1016 cm-3), co-doped devices regularly show improved Voc. Given the critical importance of improving Voc, we are investigating this trend with a wide variety of characterization techniques including SEM, EBSD, SIMS, PL, CL, XPS, TRPL, JVT, KPFM, and DLTS. Together these indicate that Cu facilitates improved absorber-buffer interface properties and potentially improved bulk absorber characteristics, though specific mechanisms have not yet been determined. Despite the improved performance of co-doped devices, Voc is still far below its potential, and we explore the possibility that this limitation is the result of low buffer doping in conjunction with high absorber doping.
arsenic doping↗
Cross-Cutting Metrology Tools for In Operando Characterization of Carrier Dynamics in Photovoltaic Devices (Final Technical Report)
Understanding the nature of recombination and its dependence on defects and interfaces is essential for engineering materials and contacts for higher V oc and photovoltaic (PV) efficiency. Time-resolved photoluminescence (TRPL) has conventionally been used to evaluate recombination, but not all materials are strongly emissive or otherwise suitable. Time-resolved terahertz spectroscopy is a pump – probe method that presents valuable complementary information, wherein an optical pump pulse photoexcites carriers within the absorber and the transient photoconductivity is probed with a terahertz pulse. Until now, experimental constraints have prohibited the use of terahertz probes to interrogate full device stacks, and measurements were instead made on exfoliated films or films grown on unconventional substrates. However, interfaces are critical to the behavior of photoexcited carriers in solar cells, and the substrates themselves often influence the film growth and bulk properties. Therefore, it is important to probe the behavior of PV absorbers as close as possible to their normal operating conditions. Here we have developed cross-cutting metrology that enable in operando characterization of carrier dynamics and recombination mechanisms in working PV devices. Through a combination of complementary non-contact TRTS and TRPL experiments and modeling, we have obtained key parameters including photoexcited carrier bulk lifetime and interface and back surface recombination velocities with greater precision and accuracy than can be achieved with conventional TRPL alone. By varying pulsed photoexcitation conditions, the contributions of interface and bulk recombination mechanisms can be determined. We have developed and refined this characterization approach by investigating CdTe PVs, which is a well-established technology but with significant margin for further improvements in efficiency. Results were published in Journal of Applied Physics (DOI: 10.1063/5.0064730) and Proceedings of the IEEE PVSC (DOI: 10.1109/PVSC43889.2021.9518559). After validating the approach, we have applied it to correlate recombination rates to processing conditions and material properties, including composition and defects as well as interfaces and surface treatments. Such feedback can inform processing and design choices to enable higher PV efficiency, not only for CdTe, but also CdSe x Te 1-x , CIGS, perovskites, kesterites, and future technologies. A perspective on predicting solar cell performance from terahertz spectroscopy was published in Advanced Energy Materials (DOI: 10.1002/aenm.202102776). The cross-cutting metrology tools developed here will enable determination of the locus and mechanism of performance-limiting recombination in thin film PV devices. This feedback can guide the engineering of PV devices with higher V oc and efficiency, thereby leading to reductions in levelized cost of electricity to meet the SunShot 2030 target of $0.03/kWh for utility-scale PV. Lower costs will enable rapid expansion of renewable electricity generation that is nearly free of carbon emissions. Reducing carbon emissions of the electricity sector is one of the most important solutions to climate change. The project’s specific focus on CdTe PV can benefit First Solar, the US company that is the global leader in CdTe technology, through both technology and workforce development.
14 SOLAR ENERGY↗
Resilience of High-Efficiency CdSeTe:As and CdSeTe:Cu Solar Cells to Proton Irradiation
Power generation in space is currently dominated by expensive III-V multi-junction photovoltaic (PV) devices and cheap crystalline silicon (c-Si) PV devices. Both of these technologies degrade rapidly in proton-radiation-rich space environments, such as the Van Allen belt. The present study of cadmium selenide telluride- (CdSeTe-) based PV devices exposed to 150-to-1500 keV proton irradiation with fluences up to 9 x 1013 cm-2 reveal a more radiation-hard alternative to III-V and c-Si PV technologies. We report on measurement and analysis of current density vs voltage (JV), external quantum efficiency (EQE), external radiative efficiency, and capacitance vs voltage (CV) characteristics. JV characteristics of As-doped CdSeTe devices show 80% remaining power conversion efficiency (PCE) relative to unexposed controls when exposed to 650 keV protons at a fluence of 1012 cm-2. Under these same irradiation conditions, Cu-doped CdSeTe devices demonstrate an even better 95% PCE retention compared with unirradiated control devices. Evidence of radiation-induced absorber p-type doping compensation is observed in the glass-side EQE at 0 V and CV characteristics of most of the irradiated CdSeTe:As devices, but clear compensation is evident only for the most heavily irradiated CdSeTe:Cu devices. Elevated blue-green photocurrent in the film-side 0 V EQE suggests a buried junction in the most heavily irradiated CdSeTe:As devices. Although CdSeTe:Cu devices are the more resilient of the CdSeTe structures, both CdSeTe-based technologies are radiation-hard when compared to c-Si and III-V multi-junction PV.
14 SOLAR ENERGY↗
Design and simulation of n -type solar cells based on an iodine-doped CdTe absorber using SCAPS-1D
The performance of conventional p-type CdTe solar cells has plateaued in recent years, motivating exploration of n-type absorbers. Here, we evaluate a homojunction solar cell employing iodine-doped CdTe (CdTe:I) as the absorber in a Ti 3 C 2 T x MXene/p-CdTe:As/n-CdTe:I/indium structure through SCAPS-1D simulations and prototype devices. Optimized simulations predict efficiencies above 25% for thin CdTe:I absorbers (~0.7 μm). In contrast, the first prototype achieved only ~1.36% efficiency with V OC = 0.48 V, J SC = 6.45 mA/cm 2 and FF = 43.8%. When the simulation is adjusted to match the actual device structure, and the effective illumination is reduced to account for front-side light loss, the predicted V OC (0.48 V) and JSC (6.74 mA/cm 2 ) closely reproduce the experimental values. This suggests that the device performance is primarily limited by reduced front-side photon transmission and other material non-idealities. These results highlight the promise of iodine-doped n-type CdTe and identify clear pathways for further efficiency improvement.
14 SOLAR ENERGY↗
A comprehensive material study of CdSeTe films deposited with differing selenium compositions
The addition of selenium into CdTe to create the ternary alloy CdSeTe has been one of the most impactful advancements to CdTe-based photovoltaics in the last decade. CdSeTe/CdTe bilayer device structures have enabled a gain in short-circuit current due to the narrower bandgap of the alloy, with minimal to no loss in voltage. Intensity of photoluminescence and time-resolved photoluminescence measurements suggest this is due to an increase in carrier lifetime and concomitant greater fraction of radiative vs non-radiative recombination events which allows for a reduction in the voltage deficit. Here, in this work, we study the properties of as-deposited and CdCl 2 -treated CdSeTe films deposited by close-space sublimation under varying conditions from CdSeTe source charges with both 20 and 40 mol% CdSe. We find that the selenium content in the deposited films are substantially reduced from that of the source material. Additionally, deposition temperature, particularly that of the substrate, considerably affects the grain size, crystallinity, and photoluminescence of the material, illustrating the importance of source material selection and process optimization. Finally, we present evidence that the source material, and therefore the properties of the deposited films, change over time as the source material is used.
36 MATERIALS SCIENCE↗
Sputtered p-Type Cu x Zn 1– x S Back Contact to CdTe Solar Cells
As thin-film cadmium telluride (CdTe) solar cells gain prominence, one particular challenge is optimizing contacts and their interfaces to transfer charge without losses in efficiency. Back contact recombination is still significant and will prevent CdTe solar technology from reaching its full potential in device efficiency, and transparent back contacts have not been developed for bifacial solar technology or multijunction solar cells. To address these challenges, here we investigate sputtered Cu x Zn 1– x S as a p-type semi-transparent back contact material to thin-film polycrystalline CdTe solar cells at Cu concentrations x = 0.30, 0.45, and 0.60. This material is selected for its high hole conductivity (160–2120 S cm –1 ), wide optical band gap (2.25–2.75 eV), and variable ionization potential (approximately 6–7 eV) that can be aligned to that of CdTe. We report that without device optimization, CdTe solar cells with these Cu x Zn 1– x S back contacts perform as well as control cells with standard ZnTe:Cu back contacts. We observe no reduction in external quantum efficiency, low contact barrier heights of approximately 0.3 eV, and carrier lifetimes on par with those of baseline CdTe. These cells are relatively stable over one year in air, with V OC and efficiency of the x = 0.30 cell decreasing by only 1 and 3%, respectively. Using scanning electron microscopy and scanning transmission electron microscopy to investigate the Cu x Zn 1– x S/CdTe interface, we demonstrate that the Cu x Zn 1– x S layer segregates into a bilayer of Cu-Te-S and Zn-Cd-S, and thermodynamic reaction calculations support these findings. Despite its bilayer formation, the back contact still functions well. This investigation explains some of the physical mechanisms governing the device stack, inspires future work to understand interfacial chemistry and charge transfer, and elicits optimization to achieve higher-efficiency CdTe cells.
14 SOLAR ENERGY↗
Group-V Dopant Incorporation for High CdSeTe/CdTe Hole Density and Lifetime
CdTe-based photovoltaic device performance can be improved by increasing both hole density and carrier lifetime simultaneously. In this work, we discuss sources and thin film processes to achieve incorporation and high hole density in conjunction with lifetime using group-V dopants.
14 SOLAR ENERGY↗
Novel n-type Device Architectures to Achieve 1 Volt V OC in Thin Film CdTe cells
The main goal of this project was to develop a novel thin film CdTe solar cell architecture based on an n-type absorber, which can overcome performance limitations associated with the typical p-Cd(Se)Te state-of-the-art architecture. Even today, the performance of the Cd(Se)Te cell continues to be limited by its open- circuit voltage, which is directly related to the net doping of the absorber. (currently @ approx. 900 mV for CdTe/CST device configuration). The proposed polycrystalline thin- film solar cell architecture was based on n-type CdTe (and CdSeTe) absorbers to overcome this limitation, since n-type CdTe absorbers offer advantages over p-type: (i) n-type dopability in CdTe is higher (than p), enabling higher doping and therefore higher V OC (ii) using a wide bandgap material as the p-contact, the n-CdTe cell can become a “bi-facial” cell, with the potential to increase energy yield up to 25% depending on environmental and mounting conditions.
14 SOLAR ENERGY↗
Understanding the Role of CdTe in Polycrystalline CdSe x Te 1– x /CdTe-Graded Bilayer Photovoltaic Devices
Grading of bandgap by alloying CdTe with selenium to form a CdSe x Te 1–x /CdTe-graded bilayer device has led to a device efficiency over 19%. A CdSe x Te 1–x absorber would increase the short-circuit current due to its lower bandgap but at the expense of open-circuit voltage. It has been demonstrated that adding a CdTe layer at the back of such a CdSe x Te 1–x film reduces the voltage deficit caused by the lower bandgap of absorber from selenium alloying while maintaining the higher short-circuit current. This leads to a photovoltaic device that draws advantage from both materials with an efficiency greater than either of them. In this report there is a detailed account using device data, ultraviolet photoelectron spectroscopy, electron microscopy, and first-principles density functional theory modeling is provided, which shows that CdTe acts as an electron reflector for CdSe x Te 1–x .
32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
Growth and Characterization of Arsenic-Doped CdTe 1-x Se x Single Crystals Grown by the Cd-Solvent Traveling Heater Method
The photovoltaic performance of CdTe solar cells is mainly limited by low doping and short minority carrier lifetime. Group-V element doping and Se-alloying have a significant impact on tuning these fundamental CdTe properties. In this paper, we report the growth of p-type As-doped, Cd-rich CdTe 1-x Se x single crystals using metallic Cd as the solvent in the traveling-heater method. The structural and electrical properties of CdTe1-xSex are examined for different Se concentrations. CdTe 1-x Se x single crystals (0 ≤ x ≤ 0.5) with zincblende structure indicate homogeneous composition. The 10 17 cm-3 As-doping activation efficiency can be maintained at close to 50% for x ≤ 0.2. Se alloying leads to bulk minority carrier lifetime exceeding 30 ns for samples doped near 1017 cm -3 . These results help us to overcome the current roadblocks in device performance.
14 SOLAR ENERGY↗