Dependence of efficiency of thin-film CdS/CdTe solar cell on parameters of absorber layer and barrier structure

Dependences of the open-circuit voltage, short-circuit current, fill factor, and efficiency of a CdS/CdTe solar cell on the resistivity and thickness of the p-CdTe absorber layer, the noncompensated acceptor concentration N_a-N_d, and carrier lifetime τ in CdTe, are investigated, and optimization of these parameters in order to improve the solar cell efficiency is performed. It has been shown that the observed low efficiency of CdS/CdTe solar cells is caused by the too short electron lifetime in the range of 10^− 10-10^− 9 s and too thin (3-5 µm) CdTe layer currently used for fabrication of CdTe/CdS solar cells. To achieve an efficiency of 28-30%, the resistivity and thickness of the CdTe absorber layer, the noncompensated acceptor concentration, and carrier lifetime should be ∼ 0.1 Ω·cm, ≥ 20-30 µm, ≥ 10¹⁶ cm^− 3, and ≥ 10^− 6 s, respectively.     

Optical properties of swift ion beam irradiated CdTe thin films

This paper reports the effect of swift (80 MeV) oxygen (O⁺⁶) ion irradiation on the optical properties of CdTe thin films grown by conventional thermal evaporation on glass substrates. The films are found to be slightly Te-rich in composition and irradiation results no change in the elemental composition. The optical constants such as refractive index (n), absorption coefficient (α) and the optical band gap energy show significant variation in their values with increase in ion fluence. Upon irradiation the band gap energy decreased from a value of 1.53 eV to 1.46 eV whereas the refractive index (n) increased from 2.38 to 3.12 at λ = 850 nm. The photoluminescence spectrum shows high density of native defects whose density strongly depends on the ion fluence. Both analyses indicate considerable defect production after swift ion beam irradiation.     

Studies of key technologies for large area CdTe thin film solar cells

The structure and main manufacturing technologies of CdTe film solar cells of large area are reviewed. Among the technologies, some have been developed for application in a pilot manufacturing line. The high resistant SnO₂ (HRT) thin films have been fabricated by PECVD. The effects of annealing on the structure and properties have been studied. A surface etching process of CdTe in low temperature and lower concentration of nitric acid has been developed. The Cd_1 − xZn_xTe ternary compound films have been studied. In order to improve the back contact layer, Cd_0.4Zn_0.6Te layer with 1.8 eV band gap as a substitute for ZnTe layer is introduced in CdTe cells. The effects of the technologies on performance of CdTe cells and feasibility of application in the modules are discussed.     

CdTe thin film technology: Leading thin film PV into the future

CdTe is a near perfect material for PV application with a direct band gap of ～1.5 eV that is closely matched to the terrestrial solar spectrum and a high optical absorption coefficient where less than 1 μm thickness is adequate to absorb the incident light. CdTe thin film solar cell and module technology has validated the economies of scale that were projected for thin film PV technologies since the early 1980s where manufacturing costs are now below $0.84 with module efficiencies of 11.1%. Additionally, the low-temperature coefficient of CdTe modules results in a high annualized output. A critical issue for CdTe manufacturers is that there is not a clear pathway to increase the module performance to 15% or beyond based on current laboratory results and efficiency improvements will require fundamental improvements in the CdTe semiconductor properties and/or developing an alternative device structure.     

Requirements imposed on the electrical properties of the absorber layer in CdTe-based solar cells

The requirements for minimizing the electric losses in the CdTe layer in CdS/CdTe thin-film solar cells are discussed. It is shown that for achieving the total absorption of the radiation and to avoid electrical losses, the separation between Fermi level and the valance band should not be more than 0.3 eV. In order to fix the Fermi level near the valence band, it is necessary to dope with acceptor impurities which can introduce shallow level in the band gap with a concentration considerably exceeding the concentration of native impurities and defects (10¹⁵–10¹⁶ cm⁻³). Taking into account the fact that the location of the Fermi level in the bandgap of a semiconductor depends on the degree of compensation, the energy of ionization of the impurity should not be greater than 0.05–0.15 eV.

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Comparison of optical properties of Si and ZnO/CdTe core/shell nanowire arrays

The systematic computations of the short-circuit current density have been performed for Si and ZnO/CdTe core shell nanowire arrays of 1 μm height in order to optimize the structural morphology in terms of nanowire diameter and period. It is found that the best structural configuration for Si leading to the ideal short-circuit current density of 19.6 mA/cm² is achieved for a nanowire diameter and period of 315 nm and 350 nm, respectively. In case of ZnO/CdTe, the ideal short circuit current density is of 24.0 mA/cm², the nanowire diameter and period is of 210 nm and 350 nm, respectively. It is shown that the optimal configuration is more compact in the case of Si nanowire arrays than in the case of ZnO/CdTe nanowire arrays. Since Si has a smaller absorption coefficient than CdTe, a larger amount of material is needed and thus more compact nanowire arrays are required. It is also revealed that core–shell nanowire arrays made of ZnO/CdTe more efficiently absorb light than that of Si, making this device a good candidate for the next generation of nanostructured solar cells.     

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.     

Structural and electronic properties of CdTe1-xSex films and their application in solar cells

The performance improvement of conventional CdTe solar cells is mainly limited by doping concentration and minority carrier life time. Alloying CdTe with an isovalent element changes its properties, for example its band gap and behaviour of dopants, which has a significant impact on its performance as a solar cell absorber. In this work, the structural, optical, and electronic properties of CdTe_1-xSe_x films are examined for different Se concentrations. The band gap of this compound changes with composition with a minimum of 1.40 eV for x = 0.3. We show that with increasing x, the lattice constant of CdTe_1-xSe_x decreases, which can influence the solubility of dopants. We find that alloying CdTe with Se changes the effect of Cu doping on the p-type conductivity in CdTe_1-xSe_x, reducing the achievable charge carrier concentration with increasing x. Using a front surface CdTe_1-xSe_x layer, compositional, structural and electronic grading is introduced to solar cells. The efficiency is increased, mostly due to an increase in the short-circuit current density caused by a combination of lower band gap and a better interface between the absorber and window layer, despite a loss in the open-circuit voltage caused by the lower band gap and reduced charge carrier concentration.     

Characteristics of n-Cd0.9 Zn0.1S/p-CdTe heterojunctions

CdTe thin films were prepared by thermal evaporation under a vacuum of 10⁻⁶ Torr and with a deposition rate of about 60 nm/min. X-ray diffraction studies of the as-deposited films revealed polycrystalline films with cubic structure. The effect of heat treatment with or without CdCl₂ enhances the grain size and improves the crystallinity of the films. Moreover, the activation energy decreases upon heat treatment with or without CdCl₂ for CdTe thin films. The optical spectra of CdTe films show interference oscillations indicating the good optical quality of these films. The calculated energy gap decreases with or without CdCl₂ treatments. The current-voltage and capacitance-voltage characteristics for dark and illuminated three junction cells are measured. By analysing these measurements the different junction parameters are obtained and the effect of CdCl₂ treatment on the performance of the heterojunctions is investigated.     

Size dependent electron transfer from CdTe quantum dots linked to TiO2 thin films in quantum dot sensitized solar cells

In this present study, we demonstrate the size dependent charge transfer from CdTe quantum dots (QDs) into TiO₂ substrate and relate this charge transfer to the actual behavior of a CdTe sensitized solar cell. CdTe QDs was synthesized using mercaptopropionic acid as the capping agent. The conduction band offset for TiO₂ and CdTe QDs indicates thermodynamically favorable band edge positions for smaller QDs for the electron-transfer at the QD–TiO₂ interface. Time-resolved emission studies were carried out for CdTe QD on glass and CdTe QD on TiO₂ substrates. Results on the quenching of QD luminescence, which relates to the transfer kinetics of electrons from the QD to the TiO₂ film, showed that at the smaller QD sizes the transfer kinetics are much more rapid than at the larger sizes. I–V characteristics of quantum dot sensitized solar cells (QDSSC) with different sized QDs were also investigated indicating higher current densities at smaller QD sizes consistent with the charge transfer results. The maximum injection rate constant and photocurrent were obtained for 2.5 nm CdTe QDs. We have been able to construct a solar cell with reasonable characteristics (V_oc = 0.8 V, J_sc = 1 mA cm⁻², FF = 60%, η = 0.5%).     

Flexible cadmium telluride/cadmium sulphide thin film solar cells on mica substrate

Polycrystalline thin film II–VI compound semiconductors of cadmium sulfide (CdS) and cadmium telluride (CdTe) are the leading materials for the development of cost effective and reliable photovoltaic systems. The two important properties of these materials are its nearness to the ideal band gap for photovoltaic conversion efficiency and they have high optical absorption coefficients. Usually thin film solar cells are made by hetero-junction of p-type CdTe with n-type CdS partner window layer. In this article, we have deposited CdTe films on mica substrates using thermal evaporation technique and CdTe/CdS junction were developed by depositing a thin layer of CdS on to the CdTe substrate from chemical bath deposition method. The device was characterized by current voltage and photocurrent spectroscopy technique prior to the deposition of the transparent conducting layer. The devices were annealed in air at different temperatures and found that the device annealed at 673?K had better photovoltaic parameters. The efficiency of a typical device under 50?mW?cm^?2 illumination was estimated as 4%.     

Photovoltaic properties of low-band-gap fluorene-based donor-acceptor copolymers

The photovoltaic properties of three fluorene-thiophene-based donor-acceptor copolymers with low band gap and reasonably high hole mobility were studied in copolymer/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk-heterojunction photovoltaic cells. The copolymers were poly[2,7-(9,9′-dihexylfluorene)-alt-2,3-dimethyl-5,7-dithien-2-yl-quinoxaline] (PFDDTQ) (band gap = 1.94 eV; mobility = 2.83 × 10^− 5 cm² V^− 1 s^− 1), poly[2,7-(9,9′-dihexylfluorene)-alt-4,7-dithien-2-yl-2,1,3-benzothiadiazole] (PFDTBT) (band gap = 1.82 eV; mobility = 4.71 × 10^− 5 cm² V^− 1 s^− 1) and poly[2,7-(9,9′-dihexylfluorene)-alt-2,3-dimethyl-5,7-dithien-2-yl-thieno[3,4-b] pyrazine] (PFDDTTP) (band gap = 1.68 eV; mobility = 1.18 × 10^− 4 cm² V^− 1 s^− 1). The order in the short-circuit current density and power-conversion efficiency of the photovoltaic cells was PFDTBT > PFDDTQ > PFDDTTP, which contradicted the order in the band gap and mobility. The short-circuit current density and power-conversion efficiency (PCE) coincided instead with the order in the mobility of the copolymer/PCBM blend, where the mobility was increased for PFDTBT and PFDDTQ owing to the charge transfer with PCBM, but was decreased for PFDDTTP due to phase separation resulting from the strong intermolecular interactions of PFDDTTP. With its high blended mobility and low band gap, PFDTBT achieved a PCE of 1.1%.     

Optical and electrical characterization of CdS thin films

Thin CdS films have been grown by chemical bath (CdCl₂, thiourea, ammonia) deposition (CBD) on SnO₂ (TO)-coated glass substrate for use as window materials in CdS/CdTe solar cells. High-resolution transmission electron microscopy revealed grains with an average size of 10 nm. The structure was predominantly hexagonal with a high density of stacking faults. The film crystallinity improved with annealing in air. Annealing in a CdCl₂ flux increased the grain size considerably and reduced the density of stacking faults. The optical transmission of the as-deposited films indicated a band gap energy of 2.41 eV. Annealing in air reduced the band gap by 0.1 eV. Annealing in CdCl₂ led to a sharper optical absorption edge that remained at 2.41 eV. Similar band gap values were obtained by photocurrent spectroscopy and electroabsorption spectroscopy (EEA) using an electrolyte contact. EEA spectra were broad for the as-deposited and air-annealed samples, but narrower for the CdCl₂-annealed films, reflecting the reduction in stacking fault density. Donor densities of ca. 10¹⁷ cm ^–3 were derived from the film/electrolyte junction capacitance.     

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.     

Cu-related recombination in CdS/CdTe solar cells

Cu used in the back contact of CdS/CdTe solar cells is known to improve contact behavior and open-circuit voltage. A study of devices made with varying Cu amounts confirmed these observations. However, Cu was also found to be deleterious to current collection. Time-resolved photoluminescence measurements of CdTe devices show that carrier lifetime decreased with increased Cu concentration. Drive-level-capacitance-profiling and low-temperature photoluminescence suggest this decrease in lifetime was associated with increased recombination center density introduced by Cu in the CdTe layer. The resulting impact of increased Cu on device performance was a voltage-dependent collection of photogenerated carriers that reduced fill-factor.     

Highly-Directional,Highly-Efficient Solution-Processed Light-Emitting Diodes of All-Face-Down Oriented Colloidal Quantum Well Self-Assembly

Semiconductor colloidal quantum wells (CQWs) provide anisotropic emission behavior originating from their anisotropic optical transition dipole moments (TDMs). Here, solution-processed colloidal quantum well light-emitting diodes (CQW-LEDs) of a single all-face-down oriented self-assembled monolayer (SAM) film of CQWs that collectively enable a supreme level of IP TDMs at 92% in the ensemble emission are shown. This significantly enhances the outcoupling efficiency from 22% (of standard randomly-oriented emitters) to 34% (of face-down oriented emitters) in the LED. As a result, the external quantum efficiency reaches a record high level of 18.1% for the solution-processed type of CQW-LEDs, putting their efficiency performance on par with the hybrid organic-inorganic evaporation-based CQW-LEDs and all other best solution-processed LEDs. This SAM-CQW-LED architecture allows for a high maximum brightness of 19,800 cd m⁻² with a long operational lifetime of 247 h at 100 cd m⁻² as well as a stable saturated deep-red emission (651 nm) with a low turn-on voltage of 1.7 eV at a current density of 1 mA cm⁻² and a high J₉₀ of 99.58 mA cm⁻². These findings indicate the effectiveness of oriented self-assembly of CQWs as an electrically-driven emissive layer in improving outcoupling and external quantum efficiencies in the CQW-LEDs.     

Direct current electrical conduction mechanism in plasma polymerized thin films of tetraethylorthosilicate

The plasma polymerized tetraethylorthosilicate (PPTEOS) thin films were deposited on to glass substrates at room temperature by a parallel plate capacitively coupled glow discharge reactor. The current density-voltage (J-V) characteristics of PPTEOS thin films of different thicknesses have been observed at different temperatures in the voltage region from 0.2 to 15 V. In the J-V curves two slopes were observed — one in the lower voltage region and another in the higher voltage region. The voltage dependence of current density at the higher voltage region indicates that the mechanism of conduction in PPTEOS thin films is space charge limited conduction. The carrier mobility, the free carrier density and the total trap density have been calculated out to be about 2.80 × 10^− 15m²V^− 1s^− 1, 1.50 × 10²²m^− 3 and 4.16 × 10³³m^− 3 respectively from the observed data. The activation energies are estimated to be about 0.13 ± 0.05 and 0.46 ± 0.07 eV in the lower and higher temperature regions respectively for an applied voltage of 2 V and 0.09 ± 0.03 and 0.43 ± 0.10 eV in the lower and higher temperature regions respectively for an applied voltage of 14 V. The conduction in PPTEOS may be dominated by hopping of carriers between the localized states at the low temperature and thermally excited carriers from energy levels within the band gap in the vicinity of high temperature.     

Zn1-xMgxO能带结构及作为窗口层的CdTe薄膜太阳电池的SCAPS仿真应用

采用第一性原理广义梯度近似+U(GGA+U)方法计算了纤锌矿结构Zn_1-xMg_xO(ZMO)(0≤x≤0.25)合金的能带结构。计算表明: 随着Mg组分增加, ZMO化合物的导带底及费米能级均向真空能级方向移动, 带隙增宽。基于理论计算得到ZMO的能带结构参数, 使用SCAPS软件对ZMO作窗口层的CdTe薄膜太阳电池的性能进行了仿真模拟, 并将研究结果与CdS作窗口层的CdTe太阳电池的性能进行了比较。结果表明: Mg在ZMO中的含量0≤x≤0.125时, ZMO/CdTe太阳电池具有比CdS/CdTe太阳电池更高的开路电压和短路电流密度; ZMO的导带底高出CdTe导带底约0.13 eV时, CdTe薄膜太阳电池的转换效率最高, 达到18.29%。这些结果为高效率碲化镉薄膜太阳电池的结构设计和器件制备提供了理论指导。     

The effect of impurities on the doping and VOC of CdTe/CdS thin film solar cells

The effect of introducing impurities in CdTe, namely antimony (Sb) and oxygen (O), on the net carrier concentration in CdS/CdTe solar cells and on their open-circuit voltage (V_OC) has been investigated. Oxygen was introduced in the CdTe films during the deposition of this layer by the close-spaced sublimation process. The total pressure was held constant at 1330 Pa (N₂ and O₂). The amount of oxygen was varied by varying its partial pressure. Antimony was introduced into CdTe using a post-deposition diffusion process. Following the deposition of CdTe a thin film (a few nm) of Sb was deposited onto the CdTe surface and subsequently heat-treated to cause in-diffusion of Sb. The temperature and time during the diffusion process were varied in the range of 300-525 °C and 20-160 min respectively. In both instances it was possible to vary (increase) the doping concentration in CdTe. The increase in doping was accompanied by an increase in V_OC. However, in all instances the doping in CdTe reached a maximum value, beyond which further increases were not possible leading to saturation in V_OC. The highest V_OC measured was similar to state-of-the-art values in the range of 800-830 mV, and the highest doping concentration measured was in the 10¹⁶ cm^− 3 range.     

Characterization of nanocrystalline cadmium telluride thin films grown by successive ionic layer adsorption and reaction (SILAR) method

Structural, electrical and optical characteristics of CdTe thin films prepared by a chemical deposition method, successive ionic layer adsorption and reaction (SILAR), are described. For deposition of CdTe thin films, cadmium acetate was used as cationic and sodium tellurite as anionic precursor in aqueous medium. In this process hydrazine hydrate is used as reducing agent and NH₄OH as the catalytic for the decomposition of hydrazine. By conducting several trials optimization of the adsorption, reaction and rinsing time duration for CdTe thin film deposition was done. In this paper the structural, optical and electrical properties of CdTe film are reported. The XRD pattern shows that films are nanocrystalline in nature. The resistivity is found to be of the order of 411 × 10³ Ω-cm at 523 K temperature with an activation energy of ∼ 0.2 eV. The optical absorption studies show that films have direct band gap (1.41 eV).