Monocrystalline CdTe Research Articles

Recent progress in the stabilization of low band-gap black-phase iodide perovskite solar cells.

All-inorganic and organic-inorganic hybrid perovskite solar cells (PSCs) have taken a quantum leap owing to their high performance and low-cost solution processability. Their efficiency has been dramatically increased up to ∼26%, matching the conventional inorganic photovoltaics like monocrystalline Si (26.1%), polycrystalline Si (21.6%), CdTe (22.1%), and CIGS (22.3%). Such outstanding performance has been achieved due to their excellent optoelectronic properties, such as a direct bandgap in the visible region, a very high absorption coefficient, a long charge-carrier diffusion length, and ambipolar carrier transport characteristics. FAPbI3 (FA = formamidinium) and CsPbI3 perovskites among the pool of perovskites are recommended for solar cell applications because they meet all the requirements for photovoltaic applications. However, the fundamental problem of these perovskites is that their photoactive black phase is highly unstable under ambient conditions due to small and large sizes of Cs+ and FA+ ions, respectively. The instability of the black phase of these perovskites hinders their applications in photovoltaic devices as a high-quality light absorber layer. Several approaches have been employed to prevent the formation of the photo-inactive yellow phase or to enhance the stability of the black phase of perovskites, such as dimensional and compositional engineering, the addition of external additives, and dimensional engineering. This perspective summarizes the various methods for stabilizing the black phase of CsPbI3 and FAPbI3 perovskites at room temperature as well as their application in photovoltaic devices.

Spectral impact on PV performance in mid-latitude sunny inland sites: Experimental vs. modelled results

Abstract The influence of spectral irradiance on the performance of different PV materials—amorphous silicon, CdTe, CIS, and monocrystalline silicon—is studied in two cities assumed representative of mid-latitude inland sunny sites not affected by extreme atmospheric characteristics. Experimental and modelled spectral mismatch factors at different timescales—instantaneous, monthly, and annual—were derived from a) outdoor spectra recorded over a 24-month experimental campaign conducted in Madrid and Jaen (Spain) and b) synthetic spectra computed using SMARTS and input atmospheric data from AERONET. The experimental annual spectral gains presented here lie within ±1% for all PV materials under scrutiny in both sites, in close agreement with modelled results. In practical terms, this may lead to simpler PV modelling approaches by neglecting spectral effects on an annual basis in sites with spectral characteristics similar to Madrid's and Jaen's. Modelled spectral impacts on PV performance previously reported for these two cities are qualitatively well aligned with our findings. Interestingly, absolute differences between the results regarding the annual spectral influence on monocrystalline silicon and CdTe in both sites obtained by means of a model based on satellite-based spectrally resolved irradiance data and the experimental results for these materials presented here stay below 0.3%.

Loss Analysis of Monocrystalline CdTe Solar Cells With 20% Active-Area Efficiency

We report a monocrystalline CdTe/MgCdTe double-heterostructure solar cell with an a-Si:H hole contact and an ITO/SiO x electrode stack. Similar designs have achieved high open-circuit voltages, but low short-circuit current densities and fill factors have limited the cell efficiencies. We investigate the origin of these losses, and, in addressing some of them, achieve a maximum total-area efficiency of 18.5% and active-area efficiency of 20.3% measured under AM1.5G illumination. Additional cells have been measured with open-circuit voltages of up to 1.11 V, while still maintaining respectable fill factors and no rollover. The lack of rollover, either before or after open circuit, confirms the potential of this approach to reach very high efficiencies. The optical losses within the device are quantified and analyzed across the spectrum to determine if there exists potential for increased current generation through a reduction in parasitic absorption. In addition, fitting the current–voltage characteristic reveals that the leading cause of the less-than-ideal fill factor is series resistance, which contributes a 7% absolute loss.

Monocrystalline CdTe/MgCdTe Double-Heterostructure Solar Cells With ZnTe Hole Contacts

Monocrystalline p-ZnTe/i-MgCdTe/n-CdTe/n-MgC-dTe double-heterostructure solar cells are grown through a combination of molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD) deposition techniques using two different dopants within the ZnTe contact layer. The recombination at the ZnTe/CdTe heterointerface is believed to be suppressed by the use of a double heterostructure with an intrinsic MgCdTe passivation layer. A comparison of the steady-state photoluminescence intensity of these cells with record- $V_{{\rm{oc}}}$ monocrystalline CdTe solar cells indicates the performance potential of devices with ZnTe contacts, while increases in the internal quantum efficiency demonstrate the benefit of using ZnTe over these previously demonstrated contacts. Solar cells utilizing a copper-doped ZnTe hole contact show promise in terms of built-in voltage but do not realize that potential in terms of $V_{{\rm{oc}}}$ with a power conversion efficiency of 9.4% and a $V_{{\rm{oc}}}$ of 819 mV. Solar cells utilizing an arsenic-doped ZnTe hole contact exhibit the highest power conversion efficiency, reaching 14.08% with an open-circuit voltage of 867 mV.

Monocrystalline CdTe solar cells with open-circuit voltage over 1 V and efficiency of 17%

CdTe solar cells have not reached their full efficiency potential, in part because their voltages have lagged behind those of cells made from other semiconductors. Zhao et al. report monocrystalline CdTe solar cells with open-circuit voltages up to 1.1 V and conversion efficiencies up to 17%.

Influence of the Ion Coordination Number on Cation Exchange Reactions with Copper Telluride Nanocrystals.

Cu2–xTe nanocubes were used as starting seeds to access metal telluride nanocrystals by cation exchanges at room temperature. The coordination number of the entering cations was found to play an important role in dictating the reaction pathways. The exchanges with tetrahedrally coordinated cations (i.e., with coordination number 4), such as Cd2+ or Hg2+, yielded monocrystalline CdTe or HgTe nanocrystals with Cu2–xTe/CdTe or Cu2–xTe/HgTe Janus-like heterostructures as intermediates. The formation of Janus-like architectures was attributed to the high diffusion rate of the relatively small tetrahedrally coordinated cations, which could rapidly diffuse in the Cu2–xTe NCs and nucleate the CdTe (or HgTe) phase in a preferred region of the host structure. Also, with both Cd2+ and Hg2+ ions the exchange led to wurtzite CdTe and HgTe phases rather than the more stable zinc-blende ones, indicating that the anion framework of the starting Cu2–xTe particles could be more easily deformed to match the anion framework of the metastable wurtzite structures. As hexagonal HgTe had never been reported to date, this represents another case of metastable new phases that can only be accessed by cation exchange. On the other hand, the exchanges involving octahedrally coordinated ions (i.e., with coordination number 6), such as Pb2+ or Sn2+, yielded rock-salt polycrystalline PbTe or SnTe nanocrystals with Cu2–xTe@PbTe or Cu2–xTe@SnTe core@shell architectures at the early stages of the exchange process. In this case, the octahedrally coordinated ions are probably too large to diffuse easily through the Cu2–xTe structure: their limited diffusion rate restricts their initial reaction to the surface of the nanocrystals, where cation exchange is initiated unselectively, leading to core@shell architectures. Interestingly, these heterostructures were found to be metastable as they evolved to stable Janus-like architectures if annealed at 200 °C under vacuum.

Electrical and Optical Properties of n-Type Indium-Doped CdTe/Mg0.46Cd0.54Te Double Heterostructures

CdTe/Mg0.46Cd0.54Te double heterostructures with n-type In doping concentrations, varied from 1 × 1016 to 7 × 1018 cm−3, have been grown on InSb substrates using molecular beam epitaxy. Secondary ion mass spectroscopy measurements show strong diffusion of In from the InSb substrate to the CdTe buffer layer, while the In concentration is constant in the CdTe layer between the two Mg 0.46Cd0.54Te barrier layers. Capacitance–voltage measurements show that the dopants are 100% ionized for the doping concentration range from 1 × 1016 to 1 × 1018 cm −3. The carrier lifetime decreases with increasing doping concentration (from 0.73 μs for an unintentionally doped sample to 0.74 ns for a 1 × 1018 cm−3 doped sample) due to the decrease of both radiative and nonradiative lifetimes. Decent carrier lifetimes are achieved (∼100 ns) between 1 × 1016 and 1 × 1017 cm−3 doping levels, which is beneficial for developing n-type monocrystalline CdTe solar cells, photodetectors, and other optoelectronic devices. The strongest photoluminescence intensity is observed when the doping concentration is 1 × 1017 cm −3, which corresponds to the highest internal quantum efficiency.

Power rating of photovoltaic modules including validation of procedures to implement IEC 61853‐1 on solar simulators and under natural sunlight

ABSTRACTThis paper describes the measurement of photovoltaic module performance over a range of temperatures and irradiances according to the international standard IEC 61853 Part 1. The purpose of this work is to assess the reproducibility of power matrix measurements obtained using two methods specified in the standard: under natural sunlight with a tracker, and with a solar simulator. A comparison of results using the third principal method (under natural sunlight without tracker) is also summarised for completeness. The same measurement techniques have been employed to measure four modules of different technologies, namely mono and poly crystalline Si, CdTe and CIS. The method used to vary the irradiance in the natural sunlight with tracker and solar simulator approaches is based on un‐calibrated mesh filters. The uniformity and effect on spectrum of the mesh filters have been studied, and the impact of these on the measurements estimated. Measurements from all methods are compared over as much of the ranges as possible. The results show that for all modules, the reproducibility is within the estimated measurement uncertainty. The suitability of the different methods is discussed in light of the results and the limitations of the various methods as applied to different modules technologies. On the basis of the results, parts of IEC 61853 Part 1 will be introduced into the ISO 17025 laboratory accreditation at the European Solar Test Installation (ESTI). Copyright © 2013 John Wiley & Sons, Ltd.

ASSESSMENT OF THE POSSIBILITIES OF BUILDING INTEGRATED PV SYSTEMS OF 1 kW ELECTRICITY GENERATION IN BANJA LUKA

The paper focuses on the analysis and assessment of the possibilities of building integrated PV (BIPV) systems of 1 kW electricity generation in Banja Luka in the Republic of Srpska. Special attention was paid to average monthly sum of global solar irradiation per square meter received by horizontally, optimally and vertically inclined and south-oriented solar modules of 1kW BIPV system and total for year electricity production from 1 kW BIPV system with horizontally, optimally and vertically inclined and south-oriented solar modules of different materials for Banja Luka, obtained by PVGIS. In addition, comparison was made of total for year electricity production of 1kW BIPV system with horizontal, optimal and vertical inclination and south-oriented solar modules of monocrystalline silicon, CdTe and CIS in Banja Luka. The research estimated the proportion of BIPV systems power generation according to different types of PV modules and installation methods. The data could serve as a useful reference for the application of BIPV systems in buildings.

ASSESSMENT OF THE POSSIBILITIES OF BUILDING INTEGRATED PV SYSTEMS OF 1 kW ELECTRICITY GENERATION IN BANJA LUKA

The paper focuses on the analysis and assessment of the possibilities of building integrated PV (BIPV) systems of 1 kW electricity generation in Banja Luka in the Republic of Srpska. Special attention was paid to average monthly sum of global solar irradiation per square meter received by horizontally, optimally and vertically inclined and south-oriented solar modules of 1kW BIPV system and total for year electricity production from 1 kW BIPV system with horizontally, optimally and vertically inclined and south-oriented solar modules of different materials for Banja Luka, obtained by PVGIS. In addition, comparison was made of total for year electricity production of 1kW BIPV system with horizontal, optimal and vertical inclination and south-oriented solar modules of monocrystalline silicon, CdTe and CIS in Banja Luka. The research estimated the proportion of BIPV systems power generation according to different types of PV modules and installation methods. The data could serve as a useful reference for the application of BIPV systems in buildings.

The study of influence of doping by iodine or gallium on the lifetime of photogenerated current carriers in CdTe

This study investigated how doping by iodine or gallium affects processes associated with current carriers by undertaking a kinetics investigation of electron loss processes in the CdTe samples prepared by direct ampoule method. Owing to these processes, the current carriers generated by light can be trapped or recombined after a thermalization process and fail to reach electrodes during their lifetime and adversely impact the photovoltaic performance of solar cells. The microwave photoconductivity method provides an effective means of obtaining direct data on the lifetime and energy distribution of traps for current carriers. The obtained lifetimes in doped materials were compared to those in initial polycrystalline CdTe and monocrystalline CdTe. The experimental results indicate that the free electron lifetimes become higher after the doping. We found that the lifetime in the doped polycrystalline samples was equal (for I-doped samples) or even higher (for Ga-doped samples) than in CdTe monocrystals. The model based on the self compensation effect was suggested for the explanation of the experimental results.

APPLICATION OF SOLAR CELLS MADE OF DIFFERENT MATERIALS IN 1 MW PV SOLAR PLANTS IN BANJA LUKA

The paper outlines the energy efficiencies of the fixed, one-axis and dual– axis tracking 1 MW PV solar plant with monocrystalline silicon, thin film CdTe and CuInSe2 (CIS) solar cells in Banja Luka. Special attention is paid to physical characteristics of monocrystalline silicon solar cells, CdTe and CIS solar cells and their application in PV solar plants. For the calculation of the energy efficiency of PV solar plants PVGIS program was used.

Spectroscopic analysis of defects in chlorine doped polycrystalline CdTe

The effects of Cl doping of thick polycrystalline CdTe layers grown by close space sublimation on their crystalline structure and the compensation and passivation processes have been investigated. From an extensive low temperature photoluminescence study, it is shown that, in polycrystalline CdTe:Cl, the main compensation processes are similar to those occurring in monocrystalline CdTe:Cl. However, specific compensation processes are also evidenced in polycrystalline CdTe: Defect complexes can be formed between Cl atoms and impurities in the vicinity of extended defects such as dislocations, twin boundaries, and grain boundaries. Furthermore, using low temperature cathodoluminescence imaging, chlorine induced passivation processes are proposed for defects such as double positioning twin boundaries within the grains. On the structural side, a bimodal distribution of the grain size is found: While small grains (5μm in diameter) are predominant at low Cl doping (around 2×1017at.cm−3), larger grains (40μm in diameter) prevail at higher doping levels (around 6.2×1017at.cm−3), with the proportion of small versus large grain decreasing as the doping level increases. Interestingly, while the radiative emission of small grains is mainly attributed to compensating complexes implying chlorine and Cd vacancies, in larger grains, the luminescence comes from deeper levels most probably associated with compensating complexes implying residual impurities.

Vibrational spectra of hydrogenated CdTe

Infrared spectroscopy may deliver important information on defects created in CdTe monocrystals grown from Cd and Te components purified in a hydrogen atmosphere before the crystallization process. Reflectivity spectra on different CdTe samples were collected with a FTIR spectrometer using a standard Hg lamp and the IR synchrotron radiation source of the DAFNE synchrotron facility at Frascati. In the 500–2000 cm–1 frequency region, the characteristic vibrational spectra of oxygen and hydrogen bounded to Cd and Te in the monocrystalline CdTe were observed. Moreover the rich fine structure around the CdTe phonon frequency range has been resolved and the preliminary assignment of several lines has been performed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Smart Heterostructures Based on Solid Solution ZnCdHgTe

ABSTRACTCurrent – voltage (IVC) and capacitance – voltage (CVC) of heterostructures (Cd, Zn)Te/ZnCdHgTe are studied for the first time. Thin films ZnxCdyHg1-x-yTe were grown on monocrystalline (111) CdTe and ZnTe substrates by PLE technology. Deposition was carried out on substrates held at temperatures near 290 K. The thickness of investigated films was estimated to be about 5 μm. Electric characteristics of the as-grown structures were examined under T = 77–290 K in the wide range of applied bias. All investigated samples have demonstrated diode-like IVC and CVC under test signal frequency f = 1 kHz. Heterostructures CdTe/ZnCdHgTe have exhibited a room temperature photosensitivity in spectral range 0.50–0.65 μm.

Electrical and photoelectrical behaviour of CdTe structures

The electrical and photoelectrical behaviour of Au/n-CdTe junctions prepared on CdTe monocrystalline substrates and CdTe epitaxial layers grown on n + GaAs substrates were studied. The electrical and photoresponse properties depended very strongly on the parameters of the compensated high-resistive layer at the CdTe surface formed by annealing during preparation.

Growth, structure and composition of electrodeposited CdTe thin films for solar cells

An organic electrolytic deposition process for preparation of stoichiometric CdTe film for solar cell applications is described. Increase in deposition current modifies crystalline growth phase from hexagonal to cubic and surface microstructure from lateral crystallites to nodular grains. Initial surface morphology affects the etched and annealed microstructure of CdTe. Monocrystalline CdTe film growth occurs at low deposition currents < 0.25 mA/cm 2 due to low nucleation rate and surface diffusion of adions on the substrate to growth sites. Morphological and structural changes are caused by differential activity of adions at different facets of crystallites in CdTe during growth. Lesser defect density in CdTe enables insitu doping and p — conversion by phosphorous atoms. An upper limit of 20 at% of dopant in a deposition bath is set by nonstoichiometry of CdTe film due to concurrent high deposition of phosphorous resulting in depletion of Cd. P dopants occupy Te sites at concentration < 1 × 10 −5 Molar as inferred by the disappearing of the Auger Te(NOO) line and analysis of the Te(MNN) line shape. Substitutional location of P during growth results in CdTe films of p-type with resistivities 20–30 ohm · cm in the as-deposited state, without OZ annealing. Further reduction in resistivity is prevented by formation of P CD and P iP CD compensating defects in CdTe.

Infrared contrast of inclusions in CdTe

CdTe crystals have been grown by the Bridgman method with excess Cd and Te. Transmission infrared (IR) microscopy revealed inclusions in the monocrystalline CdTe matrix. Energy dispersive X-ray (EDX) analysis has been performed to determine their composition. Cd inclusions show a very specific contrast in the IR microscope. This contrast is discussed in terms of dislocations due to stress relief on account of a volume misfit between a Cd inclusion and the CdTe matrix.

Monoclinic deformation and tilting of epitaxial CdTe films on GaAs at 25–400°C

Monocrystalline CdTe films of (1¯1¯1¯) orientation were deposited on GaAs (100) with a vicinal angle of ≈2°. The orientation and monoclinic distortion of the films vary at 25–400°C because of the large differences in lattice constants and thermal expansion coefficients. X-ray diffraction measurements by the Bond method show a monoclinic distortion and a tilting with respect to the substrate by 1.33°–1.44°.

Reflectivity Spectra of CdTe Implanted at Room and Liquid Nitrogen Temperature

The fundamental reflectivity spectra of monocrystalline CdTe, implanted with Ag ions at room temperature and with Er ions at liquid nitrogen temperature, are investigated in the 0.5-6.0 eV energy range. The analysis of the obtained spectra leads to the suggestion that temperature of implantation influences the obtained results much more decisively than values of other parameters. For the implantation carried out at 300 K no significant changes in reflectivity spectra are observed regardless of magnitude of the ion dose (up to 5 x 1015 /cm2 ) and this fact, in our opinion, is due to the self-annealing effect. For samples implanted at temperature 77 K with comparable doses of ions, however, the characteristic changes of shape and intensity of reflection coefficient spectra appear. The manner of this changes gives evidence that temperature 77 K is low enough to make the radiation induced lattice defects stable (frozen-in) which are responsible for the observed behaviour of CdTe fundamental reflectivity spectra. PACS numbers: 61.80.Jh, 61.70.Tm, 78.20.Ci