Magnesium zinc oxide (MZO) is a promising front contact material for CdTe solar cells. Due to its higher band gap than traditional CdS, MZO can reduce parasitic absorption to significantly increase ...short-circuit current density while also providing a benefit of conduction band offset tuning through Mg:Zn ratio optimization. MZO has been successfully implemented into CdTe devices, however its stability has been of concern. The MZO stability issue has been attributed to the presence of oxygen in the CdTe device processing ambient, leading to double-diode behavior (S-kink) in the current density-voltage curves. Here we report on MZO thin films deposited by reactive co-sputtering. The reactively co-sputtered MZO thin films have encouraging stability, show no significant variation in work function of the surface over a period of 6 months, as measured by Kelvin probe. Energy conversion efficiencies of around 16% have been achieved both with and without presence of oxygen in device processing ambients across multiple research facilities. These efficiencies should be possible to increase further by tuning of the thin film deposition and device processing parameters, especially through optimization of the back contact.
•Combinatorial synthesis of MZO by reactive sputtering.•Identification of optimal composition as a function of architecture and process.•MZO stable with respect to time and processing with oxygen.•High efficiency devices achieved in multiple laboratories.
•CdSeTe films deposited by sublimation have reduced Se content compared to source.•Crystallinity, photoluminescence, and morphology are affected by substrate temperature.•Source material is not ...stable, results in varying film composition/deposition rate.
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, we study the properties of as-deposited and CdCl2-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.
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 CdCl2-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.
The voltage of CdTe-based solar cells has remained conspicuously low despite years of efforts focused directly on its improvement. The efforts here have been primarily in increasing the equilibrium ...carrier concentration of the CdTe or its alloys which are used to absorb the light. This direction has been guided by a theory of solar cells that views the cell only as a single p/n junction. The modelling which has been used to confirm this as an appropriate direction indicated that with a moderate carrier lifetime, relatively small front interface recombination velocity, and large equilibrium carrier concentration in the absorber, efficiencies greater than the current record of 22.1% will be possible with open circuit voltages reaching over 1V. However, cells with these properties have been measured and increases in Voc and efficiency have not been attained. In the c-Si community, notably, the “passivation – selectivity” framework has been developed. In particular, it rejects the view that a singular p/n junction is responsible for the function of a solar cell. Instead, this framework operates with the understanding that the potential in the cell which can be turned into useful electrical energy and an increase in open circuit voltage comes only from the excess carriers generated by sunlight forcing a deviation from the equilibrium condition. As such there are two main components: 1) passivation – which refers to the recombination behavior in the cell and development of a large internal potential difference and 2) selectivity – which refers to the asymmetry of conduction in the cell that allows for production of a unidirectional current and an external voltage approaching that within the cell. This framework tends to break the cell into 3, sometimes overlapping, regions: an absorber region that is used to produce as large a potential difference as possible, and two contact regions in which the transport properties are modified to prefer transport of one carrier or the other. Here this framework is applied to CdTe-based solar cells to determine what limits current cells and how to overcome these limitations.In the investigation of passivation, first the electron contact interface is evaluated, resulting in the determination that this interface is not currently limiting the recombination in the cell. As a result, the current baseline is compared to structures hypothesized to provide improvement in the recombination behavior. It is found that cells with CdSeTe as the only material in the bulk exhibit more ideal recombination behavior when compared to a CdSeTe/CdTe structure as is currently used. This comparison demonstrates a pathway for cells to overcome their current limitation due to recombination, with the possibility of reaching up to 25% efficiency and 970 mV Voc with the material that currently is produced at CSU. A native oxide of TeOx is found to passivate the surface, reducing the rate non-radiative recombination, and forms during dry air exposure, providing a pathway to passivate contacts that would be ideal if not for the recombination at the interface. In the investigation related to selectivity, the electron contact is evaluated and it is demonstrated that MgZnO is appropriately selective when deposited with the correct conditions. It therefore is expected that hole selectivity is the primary loss to open circuit voltage in structures determined to have longer excess carrier lifetimes and large radiative efficiencies. Efforts to investigate novel routes to hole selectivity by use of heterojunction contacts are presented. Such routes did not yield improvements in cell Voc and efficiency, and through this work it was determined that a major source of selectivity losses in these cells is the high resistance to hole transport through the bulk semiconductor. Increasing hole concentration or thinning the absorber provide pathways to overcome this specific limitation, but it is modelled that such cells will require structures with hole selective materials that internally cause a reduction of electron current to see improvement in Voc and efficiency.
Magnesium zinc oxide (MZO, MgxZn1-xO) is a leading emitter for CdTe-based solar cells due to its transparency and the ability to tune its conduction band offset with the absorber. Devices employing ...alloyed cadmium selenide telluride (CST, CdSeyTe1-y) absorbers achieved high efficiency (>19%) using MZO deposited by reactive sputtering over a broad composition range (3.68–3.92 eV, x: 0.20–0.35). Minimal differences in implied and measured open circuit voltage indicate that the contacts are well passivated and highly selective across the spectrum of MZO employed. Device performance insensitivity to MZO composition, which is not observed in CdTe devices, is attributed to the formation of an oxygenated interface layer. Se volatility creates a group VI deficiency at the interface that drives O migration from the MZO into the absorber. This introduces conductivity in the emitter not present in its as-deposited state, contributing to the exceptional performance observed. It is shown that the quality of device passivation depends on the oxidation state of the as-deposited MZO such that intelligent control and management of the reactive sputtering process is required.
•State-of-the-art MZO|CST devices with 19.5% champion.•Performance statistically independent of MZO composition.•High quality passivation confirmed by implied voltage measurements.•Highly oxidized interface between MZO and CST is revealed.•Sensitive to the oxidation state of the as-deposited MZO.
Magnesium zinc oxide (MZO, MgxZn1-xO) is a leading emitter for CdTe-based solar cells due to its transparency and the ability to tune its conduction band offset with the absorber. Devices employing ...alloyed cadmium selenide telluride (CST, CdSeyTe1-y) absorbers achieved high efficiency (>19%) using MZO deposited by reactive sputtering over a broad composition range (3.68–3.92 eV, x: 0.20–0.35). Minimal differences in implied and measured open circuit voltage indicate that the contacts are well passivated and highly selective across the spectrum of MZO employed. Device performance insensitivity to MZO composition, which is not observed in CdTe devices, is attributed to the formation of an oxygenated interface layer. Se volatility creates a group VI deficiency at the interface that drives O migration from the MZO into the absorber. This introduces conductivity in the emitter not present in its as-deposited state, contributing to the exceptional performance observed. It is shown that the quality of device passivation depends on the oxidation state of the as-deposited MZO such that intelligent control and management of the reactive sputtering process is required.
Mg x Zn 1- x O (MZO) shows great promise to replace CdS as a buffer layer in CdTe-based solar cells. It is more transparent, and the MZO bandgap and electron density can be tuned, thus providing ...flexibility in controlling the conduction band offsets and recombination rates between transparent conductive oxide/MZO and MZO/CdSeTe interfaces. Integrating this material into solar cell devices has been frustrated by the common observation of abnormal current-voltage curves. Simulations indicate that this anomalous behavior can be attributed to front interface barrier effects. Experiments demonstrate that this common MZO interface problem can be resolved experimentally by surface preparation, preheat steps, and removing oxygen during absorber deposition and CdCl 2 treatment. Oxygen during the cell fabrication process is likely to alter MZO properties and MZO/CdSeTe band alignment. After addressing these interface issues and modest optimization, devices with high short-circuit density of 29 mA/cm 2 and efficiency above 16% are demonstrated.
Most contemporary device models predict that an acceptor concentration of at least 1016 cm−3 is required to reach an open circuit voltage of 1 V in polycrystalline CdTe-based solar cells. While ...copper has traditionally been used as the de facto p-type dopant in polycrystalline cadmium telluride (CdTe) and cadmium selenide telluride (CdSeTe), reaching high acceptor concentrations has proved to be challenging in such devices due to significant dopant compensation. The acceptor concentration in copper-doped CdTe and CdSeTe typically ranges from 1013 to 1015 cm−3 and routinely exhibit low external radiative efficiencies below 0.01%, limiting their implied voltage (i.e., quasi-Fermi level splitting) to approximately 900 mV. As an alternative to copper, this work explores the use of arsenic as a p-type dopant for CdTe and CdSeTe. Using a novel technique in which a thin layer of arsenic-containing material is deposited and used as a reservoir for arsenic to diffuse into a front layer of previously undoped material, this contribution demonstrates that high external radiative efficiencies are achievable, a direct result of combined high acceptor concentrations and long minority-carrier lifetimes in the absorber. This leads to improved implied voltages, and indicates that As-doping represents a promising pathway towards improving the external voltage of CdSeTe/CdTe solar cells.
•As-deposited Cd(Se)Te:As exhibits low dopant activation and hole density as measured by CV.•The amount of arsenic incorporation is insensitive to the amount of cadmium overpressure present during fabrication (within the ranges tested).•An aggressive CdCl2 treatment is used to diffuse arsenic from as-deposited to material into a front layer of initial undoped CdSeTe.•By diffusing arsenic into CdSeTe, hole concentrations of 1015-1016 cm−3 and excess carrier lifetimes of greater than 1 μs are achieved.•Diffused-arsenic samples have implied voltages greater than the current record VOCs for polycrystalline CdTe-based solar cells.
There are few doping concentration measurement techniques that are contactless and usable for all semiconductors. In this article, we demonstrate the use of injection-dependent quasi-steady-state ...photoluminescence to simultaneously measure the external radiative efficiency, minority-carrier lifetime, and activated dopant concentration of solar cell absorber layers. We first demonstrate this measurement on Si, for which established lifetime and doping measurement techniques exist, and determine a doping density of 4.2 × 10 15 cm −3 , which is within a factor of 2 from the 2.1 × 10 15 cm −3 value calculated from quasi-steady state photoconductance. Then, we demonstrate the use of the technique to measure doping concentrations of CdSeTe over two orders of magnitude and of perovskite down to nearly 10 14 cm −3 -materials that are much more difficult to accurately assess with other techniques.
PDF
