Investigation of the excitonic luminescence band of CdTe solar cells by photoluminescence and photoluminescence excitation spectroscopy

The excitonic luminescence band of polycrystalline cadmium telluride layers has been investigated by Photoluminescence (PL) and Photoluminescence excitation spectroscopy (PLE). CdTe was deposited by means of close space sublimation and the samples were activated by different chlorine containing compounds, i.e. cadmium chloride, hydrochloric acid, and sodium chloride as well as by simple air activation or received no post deposition treatment. In the PL spectra, four different peaks within the excitonic luminescence band were resolved. These include the free-exciton peak and two transitions of excitons bound to defects. Furthermore, free excitons and band to band transitions were detected by means of PLE. The PL and PLE spectra are discussed with respect to the post deposition treatments.     

Melt growth of CdTe single crystals with controlled deviation from stoichiometry

CdTe single crystals were grown by an improved vertical Bridgman method with the Cd or Te reservoir chamber at the lower part of the growth tube. The reservoir is provided for regulating the partial pressures exactly and in consequence, for controlling the stoichiometry of the grown crystals. The effectiveness of the proposed method was confirmed from the electrical conductivities of the grown crystals which changed drastically depending on the controlled partial pressures.     

Phase control of CuxTe film and its effects on CdS/CdTe solar cell

Phase control is critical for achieving high-performance CdTe cells when Cu_xTe is used as a back-contact for CdTe cells. Cu_xTe phases are mainly controlled by the Cu/Te ratio, and they can also be affected by post-heat-treatment temperature. Although Cu₂Te has the highest conductivity, it is unstable and provides more Cu diffusion into the CdS and CdTe films. Cu diffusion into the CdS causes “cross-over”, and Cu diffusion into the CdTe film creates Cu-related defects that lower photogenerated carrier lifetime and result in voltage-dependent collection. A “recontact” experiment clearly indicated that the mechanism giving rise to “roll-over” is the formation of Cu-related oxides, rather than the loss of Cu on the back-contact.     

Studies on preparation and characterization of indium doped zinc oxide films by chemical spray deposition

The preparation of indium doped zinc oxide films is discussed. Variation of structural, electrical and optical properties of the films with zinc acetate concentration and indium concentration in the solution are investigated. XRD studies have shown a change in preferential orientation from (002) to (101) crystal plane with increase in indium dopant concentration. Films deposited at optimum conditions have a low resistivity of 1.33 x 10^-4Ωm with 94% transmittance at 550 nm. SEM studies have shown smooth polycrystalline morphology of the films. Figure of merit is evaluated from electrical resistivity and transmittance data.     

Study of the physical properties of Bi doped CdTe thin films deposited by close space vapour transport

Bi doped cadmium telluride (CdTe:Bi) thin films were grown on glass-substrates by the close space vapour transport method. CdTe:Bi crystals grown by the vertical Bridgman method, varying the nominal Bi concentration in the range between 1 × 10¹⁷ and 8 × 10¹⁸ cm^− 3, were used in powder form for CdTe:Bi thin film deposition. Dark conductivity and photoconductivity measurements in the 90-300 K temperature range and determination by photoacoustic spectroscopy of the optical-absorption coefficient of the films in the 1.0 to 2.4 eV spectral region were carried out. The influence of Bi doping levels upon the intergrain barrier height and other associated grain boundary parameters of the polycrystalline CdTe:Bi thin films were determined from electrical, optical and morphological characterization.     

退火对Cr3+离子掺杂Al2O3薄膜的结构及发光性能影响

用脉冲激光沉积技术在Si(100)基底上制备了纯Al2O3、掺杂浓度为0.3%、1%(质量分数)的Cr3+∶Al2O3薄膜。制备态的薄膜为立方γ-Al2O3结构,经800℃真空条件下退火1h样品的结晶度有所提高,呈现α-Al2O3相与γ-Al2O3相的衍射峰。薄膜基本保持了靶材中原有各元素成分比例,平均粒径为250nm,形貌为条形。与Al2O3粉体相比,制备态薄膜在386nm处的发光峰强度明显提高。这可归因于薄膜中氧空位的增加使双氧空位吸收电子所产生的F2+色心浓度提高。薄膜经真空退火后在332、398nm附近的发光峰强度明显增强,这是由于薄膜中氧空位的增加提高了F+、F色心浓度。与此同时,制备态薄膜在386nm附近发光峰经退火后由386nm蓝移至381nm,可归因于退火后制备态薄膜的内应力得到了释放。1%(质量分数)Cr3+掺杂薄膜在646、694nm出现Cr3+离子由4 T2能级跃迁至4 A2能级及由E-能级跃迁至4 A2能级产生的荧光发光峰。     

The formation of different phases of CuxTe and their effects on CdTe/CdS solar cells

Material studies and device applications of Cu_xTe in an NREL-developed CdTe solar cell structured as glass/Cd₂SnO₄/ZnSnO_x/CdS/CdTe are presented. The Cu_xTe primary back contact was formed by evaporating a Cu layer with various thicknesses at room temperature on HNO₃/H₃PO₄ (NP) solution etched CdTe layer. A post-annealing was then followed. The structural evolution and electrical properties of Cu_xTe were investigated. Cu/Te ratio and post-annealing temperature are two processing parameters in this study. The Cu_xTe phases are mainly controlled by the Cu/Te ratio. After a post-annealing at a low temperature, such as 100 °C, no Cu_xTe phase transformation from its as-deposited phase was observed. A post-annealing treatment at a higher temperature, such as 250 °C, can reveal the stoichiometric Cu_xTe phases based on the Cu/Te ratio used in the devices. But a post-annealing at a further higher temperature, such as 400 °C, resulted in a complicated Cu_xTe phase appearance. CuTe, Cu_1.4Te, and Cu₂Te are three major phases detected by X-ray diffraction (XRD) for different Cu thickness application annealed at 250 °C. Application of Cu thicker than 60 nm degrades open-circuit voltage (V_oc) and shunting resistance (R_sh), but increases series resistance (R_s). The correlation between device performance and the Cu_xTe back contact illustrates that the process used for forming the Cu₂Te back contact failed to produce good fill factor (FF) and also introduced higher barrier height. The best device was observed for a back contact with a mixed Cu_1.4Te and CuTe phases.     

Dependence of carrier lifetime on Cu-contacting temperature and ZnTe:Cu thickness in CdS/CdTe thin film solar cells

Cu diffusion from a ZnTe:Cu contact interface can increase the net acceptor concentration in the CdTe layer of a CdS/CdTe photovoltaic solar cell. This reduces the space-charge width (W_d) of the junction and enhances current collection and open-circuit voltage. Here we study the effect of Cu concentration in the CdTe layer on carrier lifetime (τ) using time-resolved photoluminescence measurements of ZnTe:Cu/Ti-contacted CdTe devices. Measurements show that if the ZnTe:Cu layer thickness remains constant and contact temperature is varied, τ increases significantly above its as-deposited value when the contacting temperature is in a range that has been shown to yield high-performance devices (~ 280° to ~ 320 °C). However, when the contacting temperature is maintained near an optimum value and the ZnTe:Cu thickness is varied, τ decreases with ZnTe:Cu thickness.     

铈掺杂多孔硅的形貌和光致发光研究

采用电化学方法在多孔硅中掺杂了稀土铈(Ce)元素.利用原子力显微镜表征了多孔硅和Ce掺杂多孔硅的表面形貌,采用荧光分光计对样品的光致发光(PL)特性进行了研究.多孔硅样品在480nm波长激发下PL谱上观察到两个发光峰,分别位于572和650nm;通过光致发光激发谱测量,得到位于572、650nm的发光峰对应的最佳激发波长分别为380和477nm.Ce掺杂多孔硅样品在480nm波长激发下,PL谱上只显示出多孔硅原有的发光增强;而在380nm波长激发下的PL谱上不仅显示多孔硅原有的发光增强,而且还出现了新的发光峰位于517nm.认为这分别是Ce3 与nc-Si发生了能量传递和Ce掺杂引入了新的发光中心所造成的.     

Influence of electrodeposition potential and heat treatment on structural properties of CdTe films

A modified Butler-Volmer model is developed to predict the potential of perfect stoichiometry (PPS) for electrodeposition. CdTe thin films are deposited in an acidic solution; their electrodeposition mechanism is investigated using cyclic voltammetry. Calculated and experimental PPS values exhibit good agreement. At PPS, well-connected granular CdTe thin films can be deposited; these are predicted to be intrinsic, and slightly p-type due to cadmium vacancies. Type conversion occurs only because of defect redistribution and local defect reactions after annealing; the converted n-type layer exhibits lower resistivity and higher mobility. A film annealed at 350 °C exhibits excellent crystallization.     

Effects of high-temperature annealing on ultra-thin CdTe solar cells

High-temperature annealing (HTA), a process step prior to vapor cadmium chloride (VCC) treatment, has been found to be useful for improving the crystallinity of CdTe films and the efficiency of ultra-thin CdTe solar cells. Scanning electron microscopy, optical absorption, photoluminescence measurements and analyses on photoluminescence results using spectral deconvolution reveal that the additional HTA step produces substantial grain growth and reduces grain boundary defects. It also reduces excessive sulfur diffusion across the junction that can occur during the VCC treatment. The HTA step helps to produce pinhole-free CdTe films and reduce electrical shorts in ultra-thin CdTe solar cells. An efficiency of about 11.6% has been demonstrated for ultra-thin CdS/CdTe solar cells processed with HTA step.     

Role of thermal treatment on the luminescence properties of CdTe thin films for photovoltaic applications

The efficiency of CdTe based solar cells is strongly enhanced by a thermal treatment in HCF₂Cl ambient. CdTe thin films deposited on CdS/ZnO/ITO/glass by Closed Space Sublimation before and after the annealing are characterised. The CdTe morphology is studied by atomic force microscopy and scanning electron microscopy. In the treated films the non-homogeneous distribution of the grain size disappears, in addition an increasing of the dimensions of the grains is observed. Cathodoluminescence analyses show a remarkable difference in the spectra between the treated and untreated structures. A strong increase in the intensity of the 1.4 eV band is observed by increasing the HCF₂Cl content. A model of the electronic levels inside the CdTe band gap, due to incorporation of Cl (or F) is proposed.     

Effect of back-contact barrier on thin-film CdTe solar cells

The presence of a back-contact barrier affects the current–voltage characteristics of thin-film CdS/CdTe/metal solar cells primarily by impeding hole transport, a current-limiting effect commonly referred to as “rollover.” In this work, the CdS/CdTe solar cell with a CdTe/metal back-contact barrier is modeled by two opposite polarity diodes in series. Analytic simulations are fitted to the measured current–voltage curve, the voltage distribution between the two diodes is shown under different conditions, and the back-contact barrier height is extracted. Room-temperature barrier heights exceeding 0.5 eV will generally result in significant fill-factor reduction.     

Initial and degraded performance of thin film CdTe solar cell devices as a function of copper at the back contact

Copper performs an important role in obtaining high-performance thin-film CdTe solar cell devices. Both initial performance and performance after stress depends strongly on the total copper content at the back-contact, the Cd to Te ratio on the backside, the etching process, and the way the copper is activated. With regard to getting high open circuit voltage a small amount of Cu seems sufficient upon the right anneal treatment. However, regarding open circuit voltage degradation for stressed devices there seems to be an optimum amount of Cu. Te-enrichment does not seem to have a big impact on device stability.     

Thickness-dependence of stoichiometry and microstructure characteristics in correlation with conductivity type of CdTe films

CdTe films were prepared by physical vapour deposition on a substrate at room temperature (RT) as well as on a cold (LT) one using low deposition rate. The thickness-dependence of stoichiometry revealed an abrupt decrease in the Cd/Te ratio as the thickness increases. Change of thickness did not affect the type of observed (111) crystallographic texture, only the degree of preferred orientation is enhanced as the film grows. The internal strain was negligible while the crystallite size increased rapidly at small thickness (up to 400 nm), and less thickness dependence was observed with further film growth. However, thickness dependence of lattice parameters showed a minimum and a maximum at approximately 300 nm in the case of RT and LT, respectively. The observed change in conductivity from n- to p-type and its vital correlation with the stoichiometry and structural characteristics were presented. Based on thickness dependence of stoichiometry and lattice parameters as well as the conductivity type, formation and annihilation of lattice defects were considered.     

The adsorption of O on (001) and (111) CdTe surfaces: A first-principles study

Density-functional theory and a pseudopotential plane-wave approach are employed to study the structural and electronic properties of oxygen on CdTe (001) and (111) surfaces. The energetically favored configuration for oxygen adsorption on CdTe (001) is that where the O adatoms are located at the Cd-terminated B site, while O adatoms are at the Cd-terminated H₃ site of CdTe (111) surface among the structures examined. Some possible surface defects are also examined on CdTe (001) and (111) surfaces. Oxygen can easily diffuse into the CdTe substrate at the (111) surface compared with the (001) surface.     

Growth regimes of CdTe deposited by close-spaced sublimation for application in thin film solar cells

We have systematically investigated the growth of CdTe thin films by Close Spaced Sublimation (CSS). Thin films of CdTe were deposited onto CdS substrates held at temperatures in the range 250 to 550 °C. The effect of substrate temperature and evaporation rate on structure and surface morphology of CdTe films were investigated. Up to 450 °C substrate temperature the growth rate was almost constant and decreased exponentially for higher temperatures. The structures of the CdTe films were determined by XRD and a strong (111) orientation was observed within the temperature range 250 °C-470 °C. Above 470 °C the texture changed to mostly (311) and (220) orientations. Surface morphology and grain size of CdTe growth was determined with AFM and SEM. The morphology of the layers showed three major modes: Columnar grains with a diameter of 0.2 μm and a length of 6 μm for temperatures from 250 °C to 350 °C, pyramidal grains with a diameter of 0.5-1.5 μm up to 470°C and irregular shaped grains with a diameter of 5-10 μm for temperatures up to 550 °C. The roughness increased linearly from 15 nm to 220 nm within the substrate temperature range.     

Large-energy-shift photon upconversion in degenerately doped InP nanowires by direct excitation into the electron gas

Realizing photon upconversion in nanostructures is important for many next- generation applications such as biological labelling, infrared detectors and solar cells. In particular nanowires are attractive for optoelectronics because they can easily be electrically contacted. Here we demonstrate photon upconversion with a large energy shift in highly n-doped InP nanowires. Crucially, the mechanism responsible for the upconversion in our system does not rely on multi-photon absorption via intermediate states, thus eliminating the need for high photon fluxes to achieve upconversion. The demonstrated upconversion paves the way for utilizing nanowires--with their inherent flexibility such as electrical contactability and the ability to position individual nanowires--for photon upconversion devices also at low photon fluxes, possibly down to the single photon level in optimised structures.     

Formation of embedded indium nitride and indium oxide nanoclusters in silica samples sequentially implanted with indium and nitrogen ions

This article discusses the formation of embedded indium nitride (InN) nanoclusters (NCs) in silica matrix through sequential implantation of 890?keV In²⁺ and 140?keV N⁺ ions. The implanted samples were subjected to post-implantation annealing at 500°C in nitrogen atmosphere. Investigations carried out on the implanted samples using glancing incidence X-ray diffraction, high-resolution transmission electron microscopy and Raman spectroscopy gave clear evidence for the formation of InN nanoclusters. Alongside with InN NCs, we also notice the presence of indium oxide nanoclusters in the sample.     

Electrochemical deposition of PbSe and CdTe nanoparticles onto p-Si(100) wafers and into nanopores in SiO2/Si(100) structure

Electrochemical deposition of PbSe and CdTe nanoparticles onto p-Si(100) wafers and into nanopores in SiO₂ layer grown thermally on p-Si(100) substrates (SiO₂/Si(100) structure) under illumination was studied. To produce nanopores we used SiO₂ layer with tracks developed by irradiation of 350 MeV Au ions. Pores structure was formed by chemical etching of the irradiated SiO₂ layer in dilute HF. The pores were shaped like truncated cones with a base diameter of 200 and 250 nm and height of 200 nm. Photoelectrochemical deposition of PbSe and CdTe was carried out with constant cathodic potentials from the water solutions containing Pb²⁺, Cd²⁺ cations and H₂SeO₃, H₂TeO₃ acids. The potentials applied were more positive than the equilibrium E_{Meⁿ⁺/Me⁰} redox potential. The underpotential deposition of Pb (or Cd) occurs only onto the co-deposited Se (or Te) atoms due to electrons photogenerated in Si substrate. The average sizes of PbSe and CdTe particles electrodeposited onto Si(100) wafers were dependent on the duration of electrodeposition, changing from 50 to 200 nm for PbSe and 30–80 nm for CdTe. According to XRD data, PbSe particles formed at ambient conditions had the crystalline structure. To deposit CdTe nanoparticles we used an electrolyte heated up to 80 °C. Variations in the electrodeposition time enabled one to form either separate chalcogenide particles or a polycrystalline film-like layer in case of electrodeposition onto p-Si(100) wafers and to control the degree of pore filling in case of electrodeposition into the nanopores of SiO₂/Si(100) structure.