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.     

CuxS背接触层制备及在碲化镉薄膜太阳电池中应用研究

本文采用化学水浴法沉积CuxS薄膜,通过改变Cu元素比例研究其对碲化镉电池效率的影响。研究表明化学水浴法沉积的CuxS是非晶的,采用适当退火条件可以使其晶化,随着退火温度的提高,薄膜变得致密且结晶明显。CuxS薄膜厚度对电池性能有很大的影响,结果表明,随着CuxS薄膜厚度增加,电池性能先增加后减少。薄膜厚度为75nm时,CdS/CdTe电池性能最佳,达到了最高转化效率(η)为12.19%,填充因子(FF)为68.82%,开路电压(Voc)为820mV。     

An investigation of photocurrent loss due to reflectance and absorption in CdTe/CdS heterojunction solar cells

An attempt is made to understand, quantify, and reduce the reflectance and photocurrent loss in CdTe solar cells. Model calculations are performed to determine the optimum thicknesses of CdS and SnO₂ films and anti-reflection (AR) coating on glass that can minimize the reflectance and enhance the performance of CdTe/CdS/SnO₂/glass solar cells. Photocurrent loss due to absorption in CdS films is also calculated as a function of CdS thickness. It was found that the current loss due to reflectance ando absorption is more sensitive to the CdS film when its thickness falls below 1500Å. Model calculations show that reducing the CdS thickness from 1500 to 600Å increases short-circuit current density ( J_sc) by 3 mA/cm² because of reduced reflectance as well as absorption. Further decrease in CdS thickness below 600Å increases reflectance but results in higher J_sc, because current gain due to reduced absorption in thin CdS offsets the current loss due to higher reflectance. Model calculations also indicate that J_sc is not sensitive to SnO₂ thickness above 4000Å. Finally, an optimum thickness for single layer MgF₂ AR coating on glass was calculated to be 1100Å, which should provide an additional increase of 0.7 mA/cm² in J_sc. Some of these results are also experimentally validated in this paper.     

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.     

Features of the light current-voltage characteristics of bifacial solar cells based on thin CdTe layers

It is shown that the features of the illuminated current-voltage characteristics of bifacial solar cells glass/SnO₂:F/CdS/CdTe/Cu/ITO with a thin base layer are associated with the photovoltaic effect at the back contact. An equivalent circuit of the device structure under study, which takes into account the existence of two illuminated diodes—a frontal diode (main separating barrier) and a diode at a back contact—is suggested.     

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.     

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.     

太阳电池中CdS薄膜的制备及其性能的研究

分别采用化学池沉积(CBD)和真空蒸发法,在三种衬底(玻片、ITO玻片、SnO2玻片)上沉积CdS薄膜,并利用扫描电镜(SEM)、透射光谱、X射线衍射(XRD)等方法对沉积膜进行了测试分析,同时阐述了两种不同方法下CdS膜的生长沉积机制。     

Influence of the layer parameters on the performance of the CdTe solar cells

Influence of the layer parameters on the performances of the CdTe solar cells is analyzed by SCAPS-1D. The ZnO:Al film shows a high efficiency than SnO2:F. Moreover, the thinner window layer and lower defect density of CdS films are the factor in the enhancement of the short-circuit current density. As well, to increase the open-circuit voltage, the responsible factors are low defect density of the absorbing layer CdTe and high metal work function. For the low cost of cell production, ultrathin film CdTe cells are used with a back surface field (BSF) between CdTe and back contact, such as PbTe. Further, the simulation results show that the conversion efficiency of 19.28% can be obtained for the cell with 1-μm-thick CdTe, 0.1-μm-thick PbTe and 30-nm-thick CdS.     

High-Efficiency Polycrystalline CdS/CdTe Solar Cells on Buffered Commercial TCO-Coated Glass

Multiple polycrystalline CdS/CdTe solar cells with efficiencies greater than 15% were produced on buffered, commercially available Pilkington TEC Glass at EPIR Technologies, Inc. (EPIR, Bolingbrook, IL) and verified by the National Renewable Energy Laboratory (NREL). n-CdS and p-CdTe were grown by chemical bath deposition (CBD) and close space sublimation, respectively. Samples with sputter-deposited CdS were also investigated. Initial results indicate that this is a viable dry-process alternative to CBD for production-scale processing. Published results for polycrystalline CdS/CdTe solar cells with high efficiencies are typically based on cells using research-grade transparent conducting oxides (TCOs) requiring high-temperature processing inconducive to low-cost manufacturing. EPIR’s results for cells on commercial glass were obtained by implementing a high-resistivity SnO₂ buffer layer and by optimizing the CdS window layer thickness. The high-resistivity buffer layer prevents the formation of CdTe-TCO junctions, thereby maintaining a high open-circuit voltage and fill factor, whereas using a thin CdS layer reduces absorption losses and improves the short-circuit current density. EPIR’s best device demonstrated an NREL-verified efficiency of 15.3%. The mean efficiency of hundreds of cells produced with a buffer layer between December 2010 and June 2011 is 14.4%. Quantum efficiency results are presented to demonstrate EPIR’s progress toward NREL’s best-published results.     

Ultrathin CdTe Solar Cells with MoO3−x /Au Back Contacts

We report on the fabrication and characterization of CdTe thin-film solar cells with Cu-free MoO_3?x /Au back contacts. CdTe solar cells with sputtered CdTe absorbers of thicknesses from 0.5 to 1.75 μm were fabricated on Pilkington SnO₂:F/SnO₂-coated soda–lime glasses coated with a 60- to 80-nm sputtered CdS layer. The MoO_3?x /Au back contact layers were deposited by thermal evaporation. The incorporation of MoO_3?x layer was found to improve the open circuit voltage (V _OC) but reduce the fill factor of the ultrathin CdTe cells. The V _OC was found to increase as the CdTe thickness increased.     

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.     

Factors Affecting the Stability of CdTe/CdS Solar Cells Deduced from Stress Tests at Elevated Temperature

CdTe/CdS solar cells were subjected to heat stress at 200 °C in the dark under different environments (in N₂ and in air), and under illumination (in N₂). We postulate that two independent mechanisms can explain degradation phenomena in these cells: i) Excessive Cu doping of CdS: Accumulation of Cu in the CdS with stress, in the presence of Cl, will increase the photoconductivity of CdS. With limited amounts of Cu in CdS, this does NOT affect the photovoltaic behavior, but explains the crossover of light/dark current–voltage (J–V) curves. Overdoping of CdS with Cu can be detrimental to cell performance by creating deep acceptor states, acting as recombination centers, and compensating donor states. Under illumination, the barrier to Cu cations at the cell junction is reduced, and, therefore, Cu accumulation in the CdS is enhanced. Recovery of light‐stress induced degradation in CdTe/CdS cells in the dark is explained by dissociation of the acceptor defects. ii) Back contact barrier: Oxidation of the CdTe back surface in O₂/H₂O‐containing environment to form an insulating oxide results in a back‐contact barrier. This barrier is expressed by a rollover in the J–V curve. Humidity is an important factor in air‐induced degradation, as it accelerates the oxide formation. Heat treatment in the dark in inert atmosphere can stabilize the cells against certain causes of degradation, by completing the back contact anneal.     

The effect of oxygen on interface microstructure evolution in CdS/CdTe solar cells

Microstructural changes at the CdS/CdTe solar cell interface where close‐spaced sublimation (CSS) is used as the growth technique to deposit the p‐type CdTe absorber layer are studied by systematic layer characterization at various stages during heterojunction growth. CdS layers grown by both chemical bath deposition (CBD) and CSS provide a basis for determining the effects of CdS crystallinity, grain size, and oxygen content on the subsequent CdTe layer. As‐grown CBD CdS films exhibit small grains and variations in optical properties attributed to film impurities. In contrast, CSS yields CdS films with good crystallinity, larger grains, and nearly ideal optical properties. The hexagonal nature of CSS‐grown CdS is seen to nucleate hexagonal CdTe during the initial stages of CdTe film growth. Cubic CdS deposited by CBD in contrast promotes cubic CdTe nucleation. Oxygen anneals in the latter case can aid hexagonal CdTe nucleation. Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) of the CdS/CdTe interface show CdS‐dependent differences in interdiffusion at the interface. This interdiffusion appears to be determined by the oxygen level in the CdS. When low‐oxygen‐containing CSS CdS films are used, sulfur diffusion is substantial, leading to significant consumption of the CdS layer. When these same films are annealed in oxygen, the consumption is reduced. Te diffusion into the CdS layer is also observed to decrease with oxygen anneals. Optical modeling shows that Te alloying with the CdS layer can greatly reduce the short‐circuit current of CdS/CdTe devices. Copyright © 2002 John Wiley & Sons, Ltd.     

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.     

Structural mechanisms of optimization of the photoelectric properties of CdS/CdTe thin-film heterostructures

Comparative studies of the effect of chloride treatment of CdS/CdTe thin-film heterostructures on the output characteristics of ITO/CdS/CdTe/Cu/Au solar cells and the crystal structure of their base CdTe layer are carried out. Structural mechanisms determining variation in the efficiency of photoelectric processes in ITO/CdS/CdTe/Cu/Au thin-film solar cells produced by varying the thickness of the CdCl₂ layer during the chloride treatment are suggested. It is shown for the first time by X-ray diffractometry that the metastable hexagonal CdTe phase transforms into a stable cubic modification during the chloride treatment. This circumstance provides a substantial improvement in the photoelectric properties of CdS/CdTe thin-film heterostructures.     

CdS/CuInSe2多晶薄膜太阳电池参数分析

利用Rothwarf模型,通过理论计算,讨论了多晶CdS/CuInSe2薄膜太阳电池的掺杂浓度、厚度和晶粒尺寸对太阳电池转换效率的影响.结果表明太阳电池的吸收层掺杂浓度存在一个最佳值;晶粒半径、太阳电池厚度对电池效率有显著影响,但达到一定值后,对效率的影响可以忽略.     

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.     

Optimization of CdTe thin‐film solar cell efficiency using a sputtered,oxygenated CdS window layer

A major source of loss in cadmium sulfide/cadmium telluride (CdS/CdTe) solar cells results from light absorbed in the CdS window layer, which is not converted to electrical current. This film can be made more transparent by oxygen incorporation during sputter deposition at ambient temperature. Prior to this work, this material has not produced high‐efficiency devices on tin oxide‐coated soda‐lime‐glass substrates used industrially. Numerous devices were fabricated over a variety of process conditions to produce an optimized device. Although the material does not show a consistent increase in band gap with oxygenation, absorption in this layer can be virtually eliminated over the relevant spectrum, leading to an increase in short‐circuit current. Meanwhile, fill factor is maintained, and open‐circuit voltage increases relative to baseline devices with sublimated CdS. The trend of device parameters with oxygenation and thickness is consistent with an increasing conduction band offset at the window/CdTe interface. Optimization considering both initial efficiency and stability resulted in a National Renewable Energy Laboratory verified 15.2%‐efficient cell on 3.2‐mm soda‐lime glass. This window material was shown to be compatible with SnO₂‐based transparent conducting oxide and high resistance transparent coated substrates using in‐line compatible processes. Copyright © 2015 John Wiley & Sons, Ltd     

Spectral response inhomogeneities in large-area solar cells

A set-up for studying spatial variations of spectral response of solar cells is described. Spectral measurements have been used to calculate solar cell parameters like diffusion length of minority carriers, junction depth and barrier height of the junction. Results on polycrystalline silicon, thin film CdS/Cu₂S and SnO₂/SiO_x/n-Si solar cells are reported and discussed.