CdTe PV Research Articles

Performance Analysis of 0.4–1.2-μm CdTe Solar Cells

Thin-film cadmium telluride (CdTe) solar cells with an absorber thickness range of 0.4–1.2 μm were fabricated by close-space sublimation on a transparent MgZnO window layer. Microscopy cross sections showed that a significant fraction of the CdTe crystallites extended throughout the absorber layer, and capacitance measurements demonstrated that the absorbers were depleted into forward bias. Current and fill factor loss analyses were used to identify dominant loss mechanisms in the devices as a function of absorber thickness, and photoluminescence measurements were increasingly influenced by back-surface recombination as the absorber was made thinner. The thinner CdTe devices showed modest current and fill-factor reduction but overall good diode quality, and the highest efficiency of 15% was achieved with 1.2-μm CdTe.

Copper doping of Sb2S3: fabrication, properties, and photovoltaic application

Sb2S3 solar cells are lagging behind conventional thin-film solar cells such as silicon solar cells and cadmium telluride solar cells in the power conversion efficiency (PCE). One of the most prominent problems is that the carrier concentration of Sb2S3 is relatively low. In order to increase the carrier concentration, elemental Cu was doped into Sb2S3 film by radio-frequency (RF) magnetron sputtering. We proved that Cu was doped into Sb2S3 films and mainly anchored with sulfur in the form of copper chalcogenide species at the surface and grain boundaries of Sb2S3. The doping of Cu essentially affects the physical and electrical properties of RF-sputtered Sb2S3 films such as the optical band gap, crystallinity, chemical composition, morphology, and carrier concentration. Specially, the electronic carrier concentration is remarkably increased from 6.28 × 109 to 6.06 × 1010 cm−3 and the Fermi level is also significantly uplifted after prudent doping with Cu. Planar solar cells based on RF-sputtered Cu-doped Sb2S3 absorber deliver an increased PCE of 1.13% and show good stability. This research proves that doping of Cu is an alternative and effective way to improve the electronic property of Sb2S3 films and enhance the performance of Sb2S3 solar cells.

Radiation hardness of cadmium telluride solar cells in proton therapy beam mode.

We evaluated the durability of cadmium telluride (CdTe) solar cells upon proton beam irradiation as well as the possibility of achieving a dosimeter usable in proton beam therapy by applying 100 MeV of pencil beam scanning (PBS) irradiation. Specifically, a 100 MeV proton PBS beam was applied at irradiation doses of 0, 1012, 1013, 1014, and 1015 cm-2. According to the results, the remaining factors (defined as the ratio of the degraded value to the initial value) of open-circuit voltage (Voc), short-circuit current (Jsc), fill-factor (FF), and efficiency (ƞ) which are solar cell performance parameters, were approximately 89%, 44%, 69%, and 30%, respectively, compared to those of the reference cell (without irradiation) at the highest dose of 1×1015 cm-2. In particular, the conversion efficiency, which is the main factor, was approximately 70% of that of the reference cell even at a high fluence of 1×1014 cm-2. In addition, we observed the projected range of the hydrogen atoms based on the PBS beam energy using the Tool for Particle Simulation software and assessed the amount of fluence accumulated in a CdTe cell. As the energy increased, the fluence accumulated inside the cell tended to decrease owing to the characteristics of the Bragg peak of the proton. Thus, the radiation damage to the cell induced by the proton beam was reduced. The results of this study are expected to provide valuable reference information for research on dosimetry sensors composed of thin-film solar cells, serving as the basis for future application in proton beam therapy with CdTe solar cells.

Degradation of Mg-doped zinc oxide buffer layers in thin film CdTe solar cells

Cadmium Sulphide is the conventional n-type buffer layer used in thin film Cadmium Telluride solar cells. It is well known that Cadmium Sulphide causes optical losses and sulphur diffuses into the absorber during high temperature activation. Sputter-deposited Mg-doped ZnO (MZO) has been shown to be an attractive buffer layer for Cadmium Telluride solar cells due to its transparency and tuneable band gap. It is also stable to high temperature processing and avoids diffusion of elements into the cadmium telluride absorber during the cadmium chloride activation treatment. However, degradation is observed in solar cells incorporating MZO buffer layers. Analysis of the MZO film surface potential has revealed significant fluctuations in the thin film work function once the layer is exposed to the atmosphere following deposition. These fluctuations are attributed to the high reactivity to water vapour of the MgO contained in the MZO films. This has been analysed using X-ray Photoelectron Spectroscopy to determine corresponding changes in the surface chemistry. The Zinc Oxide component is relatively stable, but the analysis shows that MgO forms a Mg(OH)2 layer on the MZO surface which forms a secondary barrier at the MZO/CdTe interface and/or at the interface between MZO and the Fluorine-doped SnO2. This affects the Fill Factor and as a consequence it degrades the conversion efficiency.

Comprehensive assessment of advanced solar facade: thermal, optical and economic assessment

In order to improve the enegy saving in building, development of glazing system has been widely conducted. CdTe PV window system has the feature of energy generation and adjustment on thermal and visual comfort, in this paper, the thermal performance of a glazing system integrated with semi-transparent CdTe solar cells with different transparency (10% and 50%) under different temperature conditions were investigated by experimental measurement (undertaken in a large climate chamber). The U-value of each PV window has been calculated. Moreover, the spectral transmittance within visible has been measured by spectrometer. This study provided comprehensive fundamental information of different CdTe PV window system for their further investigation on energy and visual comfort.

Impact of average photon energy on spectral gain and loss of various-type PV technologies at different locations

Spectral grain and loss (spectral gain&loss) of several-type PV technologies (amorphous silicon (a-Si), perovskite (perov), CdTe, CuInSe2 (CIS), multi-crystalline silicon (mc-Si), single-crystalline silicon back-contact (BC), single-crystalline silicon (sc-Si), and heterostructure-with-intrinsic-thin-layer (HIT)) was investigated in different places (Kusatsu city, Tsukuba city, and Miyazaki city in Japan) in a year. Spectral gain&loss is defined as a ratio of short-circuit current (ISC) corrected by solar irradiance (Irr) for PV module at an average photon energy (APE) to its ISC under standard test condition. The blue-rich spectra with APE over 1.88 eV yield spectral gain (spectral gain&loss over 1) for CdTe, perov, and a-Si PV technologies owing to large band-gap energy (Eg) values of 1.47, 1.60, and 1.80 eV, respectively. On the other hand, red-rich spectra with APE below 1.88 eV lead to spectral gain for CIS, mc-Si, BC, sc-Si, and HIT PV technologies with smaller Eg values of 1.21, 1.13, 1.17, 1.16, and 1.09 eV, respectively. Moreover, since average APE values in Kusatsu city, Tsukuba city, and Miyazaki city are 1.931, 1.900, and 1.899 eV, respectively, a-Si, perov, and CdTe PV technologies are suitable in term of spectral response. The spectral gain&loss of PV modules compared with sc-Si PV module is moreover discussed.

Improving the performance of cadmium telluride solar cell (CdTe) with different buffer layers

In this paper, the performance of the buffer layer of Cadmium Telluride (CdTe) thin film solar cell was optimized using SCAPS software. At first, five different buffer layers including CdS, In2S3, ZnO, ZnSe and ZnS with variable thicknesses from 10 to 100 nm have been replaced in the structure of the solar cell and it has been observed. As the thickness of the buffer layer is increased, the efficiency decreases.Due to the increase of thickness light absorption in the structure is increased and less light is absobed in CdTe layer. The reason behinf this incident is described physically. Then the performance of the solar cell with different buffer layers is compared, and the effect of each of the layers on the four main parameters of the solar cell has been investigated. Simulation results show that the ZnS buffer layer has a higher efficiency in all thicknesses than other materials which originates from the fact that the bandgap of this material is bigger than other materials.

Performance Evaluation of a Building Integrated PV (BIPV) at Heriot-Watt University Malaysia

Building Integrated Photovoltaics (BIPV) is a promising technology that combines the generation of energy into the building structure where the modules act as building material or blend completely with architectural design. This study analyses the performance characteristics of a proposed BIPV system for Heriot-Watt University Malaysia building, the system is designed with aim to utilize the curved roof area of the building. The available roof area of 7725 m2 is curved with varying tilt angles from 1.5° to 26°. The environmental plugin Ladybug integrated within Grasshopper was used to visualize and estimate the energy potential from the roof surface in Rhinoceros 3D modeling software. Additionally, detailed system simulations are conducted with PVSyst software. Eight proposed system variants of different PV technologies and modules types are studied with capacities of 411.8 kW to 1085.6 kW. The recommended system has a size of 1085.5 kW and utilizes Thin-film CdTe PV modules. The system generates 1415 MWh annually with a performance ratio of 84.9%, which saves 62.8% of the electricity bill and has an estimated cost of 901 thousand USD. Installation of the proposed system should preserve the aesthetical value of the building’s roof and satisfy BIPV rules.

Author Correction: Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells

Author Correction: Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells

Analysis of the daylight performance of window integrated photovoltaics systems

Integrating photovoltaics into windows provides the possibility of including an additional function of energy production to a conventional building fenestration component. There is no doubt that electrical power can be generated on-site. However, the effect of PV windows on the indoor luminous environment of the space served by them has not been comprehensively researched. This paper investigated the daylight performance of integrating four types of photovoltaics (semi-transparent thin film Cadmium telluride (CdTe) solar cells with 10% and 50% transparency, crystalline silicon solar cells with and without crossed compound parabolic concentrators (CCPC)) to a window of a typical south-facing office under different Window-to-Wall Ratios (WWRs). Annual useful daylight illuminance (UDI), daylight uniformity ratio (UR) and daylight glare probability (DGP) have been analysed based on dynamic simulation using RADIANCE. The simulation results show that windows integrated with crystalline silicon cells and CCPC optics have the potential to provide best daylight availability when compared with standard double glazed windows and other tested PV window prototypes, if it is applied to rooms with large WWRs (e.g. 60% or 75% WWR) at high latitudes (e.g. city of Harbin). Its application also improves the uniformity of daylight spatial distribution and eliminates the risk of glare. Semi-transparent CdTe PV window with 10% transparency can also improve the percentage of working hours that fall into UDI 500–2000lux range, however, it will result in the most sharp illuminance contrasts within the room. Applying all of these tested PV windows can effectively reduce the possibility of glare.

Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells

Electricity produced by cadmium telluride (CdTe) photovoltaic modules is the lowest-cost electricity in the solar industry, and now undercuts fossil fuel-based sources in many regions of the world. This is due to recent efficiency gains brought about by alloying selenium into the CdTe absorber, which has taken cell efficiency from 19.5% to its current record of 22.1%. Although the addition of selenium is known to reduce the bandgap of the absorber material, and hence increase the cell short-circuit current, this effect alone does not explain the performance improvement. Here, by means of cathodoluminescence and secondary ion mass spectrometry, we show that selenium enables higher luminescence efficiency and longer diffusion lengths in the alloyed material, indicating that selenium passivates critical defects in the bulk of the absorber layer. This passivation effect explains the record-breaking performance of selenium-alloyed CdTe devices, and provides a route for further efficiency improvement that can result in even lower costs for solar-generated electricity. Selenium in cadmium telluride solar cells is known to allow bandgap engineering, thus enabling highly efficient devices. Here, Fiducia et al. show that selenium also plays a role in passivating defects in the absorber layer.

Comprehensive evaluation of window-integrated semi-transparent PV for building daylight performance

Building-integrated semi-transparent photovoltaic windows (PV windows) have been considered as a potential candidate to replace conventional windows to improve building energy efficiency and hence reduce carbon emissions. With the integration of PV windows, the indoor luminous environment may be significantly affected. The presence of solar cells may cause undesirable shading, low illuminance levels and affect colour quality of the transmitted daylight. Therefore, it is important to comprehensively assess daylight performance of PV windows to ensure a comfortable luminous environment. In this study, the daylight performance of Cadmium telluride (CdTe) PV window with four different transparencies (i.e. 20%, 30%, 40% and 50%) applied to a cellular office space has been assessed in terms of daylight quantity and daylight quality. RADIANCE was selected to predict the annual daylight performance through advanced dynamic metrics including Useful Daylight Illuminance (UDI), simplified Daylight Glare Probability (DGPs) and Illuminance Uniformity (Uo). Correlated Colour Temperature (CCT) and Colour Rendering Index (CRI), which are two attributes to characterise the colour quality of transmitted daylight were used to evaluate performance of the selected PV windows. CCT and CRI were calculated under three CIE standard daylight scenarios (CCT of 4000 K, 6500 K and 25000 K respectively). It is found that CdTe PV windows can significantly improve the homogeneity of daylight distribution on a task area located close to the window and reduce the risk of daylight glare when compared with the performance of a conventional double glazing. Moreover, the recommended CCT (i.e. 3000–7500 K) can be achieved with the employment of CdTe PV windows under 4000 K and 6500 K daylight scenarios. All of the CdTe PV windows examined were able to maintain CRI at a comfortable level i.e. above 90 under the three daylight scenarios.

Controlling Band Alignment at the Back Interface of Cadmium Telluride Solar Cells using ZnTe and Te Buffer Layers

ABSTRACTFormation of a low barrier back contact plays a critical role in improving the photoconversion efficiency of the CdTe solar cells. Incorporating a buffer layer to minimize the band bending at the back of the CdTe device can significantly lower the barrier for the hole current, improving open circuit voltage (VOC) and the fill factor. Over the past years, researchers have incorporated the both ZnTe and Te as buffer layers to improve CdTe device performance. Here we compare device performance using these two materials as buffer layers at the back of CdTe devices. We show that using Te in contact to CdTe results in higher performance than using ZnTe in contact to the CdTe. Low temperature current density-voltage measurements show that Te results is a lower barrier with CdTe than ZnTe, indicating that Te has better band alignment, resulting in less downward bending in the CdTe at the back interface, than ZnTe does.

Integrated CdTe PV glazing into windows: energy and daylight performance for different window-to-wall ratio

When integrated photovoltaics into building windows, the solar cells will absorb a fraction of solar radiation that hit on the window surface to generate electrical power and thus obstruct the solar energy and natural daylight that would have penetrated into inside space. In this paper, a window system integrated with thin film CdTe solar cells with 10% transparency was electrically characterised by Sandia model. The annual energy performance of a typical office with this PV window applied to different façade design was investigated using EnergyPlus under five typical climatic conditions in China. The dynamic daylight performance of the office has been assessed using RADIANCE. The result shows that when compared to a conventional double glazed system, the application of PV window can result in significant energy saving potential if the office has a relative large window-to-wall ratio (i.e. WWR≥45%). The simulation results also show that this PV window offers better daylight performance than conventional double-glazing.

Evaluation of the colour properties of CdTe PV windows

Building-Integrated Photovoltaic Window can potentially improve building energy efficiency. However, with the integration of solar cells, the colour properties of daylight passing through PV windows may be affected. It is important to explore the colour properties of the transmitted daylight to ensure a comfort luminous environment for its occupants. In this study, the spectral transmittances of four types CdTe PV window have been measured, subsequently the colour properties of the tested windows are examined and evaluated by Correlated Colour Temperature (CCT) and Colour Rendering Index (CRI). It was found that CdTe PV windows can achieve recommended CCT (i.e. 3300-5000K) when the daylight source is in the range of 4000K and 6500K. The general CRI of all the CdTe PV windows can be maintained in a comfort range, which is higher than 90, under different daylight scenarios.

Progress in Major Thin-film Solar Cells: Growth Technologies, Layer Materials and Efficiencies

Thin film solar cells are desirable due to minimal material usage, cost effective synthesis processes and a promising trend in efficiency rise. In this review paper, remarkable progresses of five major types of thin film solar cell (TFSC) including amorphous silicon (a-Si) solar cell, copper indium gallium selenide (CIGS) solar cell, copper zinc tin sulfide (CZTS) solar cell, cadmium telluride (CdTe) solar cell and dye-sensitized solar cell (DSSC) have been presented from their inception to the state-of-the-art development. Cell configurations, different layers of these cells, their growth procedures, function and modification for working solar cells have also been explored. Critical issues that limit the performance of these cells as well as current scenario have also been addressed. Finally, a summary of this work has been presented as a comparative study among the five major types of TFSCs in terms of the state-of-the-art data for structural, optical, and electrical properties, material availability, toxicity, stability and highest efficiency for helping readers decipher the major challenges for popularization and commercialization of this technology.

Numerical modeling of CdTe solar cells thin film investigation by using PC1D model

PurposeThe purpose of this paper is to study the effect of individual layers of cadmium telluride (CdTe) solar cell to improve the efficiency of the photovoltaic cell.Design/methodology/approachTo improve the performances of CdTe thin-film solar cells, the thickness of CdTe and cadmium sulfide (CdS) have been modified separately. High-efficiency ultra-thin CdTe solar cell with ZnTe layer as back surface field (BSF) was achieved. The CdTe solar cell is under AM1.5 g illumination using a one-dimensional (1-D) model, i.e. personal computer one dimensional (PC1D).FindingsThe highest conversion efficiency of about 15.3 per cent was achieved for ultrathin CdTe solar cell with a ZnTe BSF layer. The results of simulation were compared with experimental and analytical results by other researchers.Originality/valueIn this paper, according to the authors’ knowledge ZnO:Al/CdS/CdTe/ZnTe is simulated by PC1D model for the first time and is compared with experimental result (ZnO:Al/CdS/CdTe). The results show a suitable performance.

Performance evaluation of semi-transparent CdTe thin film PV window applying on commercial buildings in Hong Kong

The energy performances of c-Si PV windows and a-Si PV windows have been investigated comprehensively worldwide. However, CdTe thin film PV windows which are supposed to achieve better performance due to its higher efficiency are rarely studied. This study evaluated the energy performance of semi-transparent CdTe thin film PV window applying on commercial buildings. The results show that CdTe PV windows have large energy saving potential in Hong Kong, and the annual energy generation of per unit CdTe PV windows is 52.3kWh/m2. Compared to the common window and a-Si PV window, the saving in net energy consumption is 19.6% and 15.3%, respectively.

Ultrathin CdTe solar cells with absorber layer thinner than 0.2 microns

In this study, ultrathin Cadmium telluride (CdTe) solar cells with absorber thickness from 50 to 200 nm were fabricated. The short-circuit current (JSC) and open-circuit voltage (VOC) were found to decrease significantly with the thickness of absorber layer decreasing. The decrease of the JSC was mainly because of the insufficient light absorption. Even so, the JSC was still found to be 8.2 mA/cm2, which was about 32% of that of a normal CdTe solar cell when the thickness of absorber layer was reduced to ∼1% of that of a normal CdS/CdTe solar cell, i.e. 50 nm. The reasons, which caused the decrease of VOC, were also discussed in this study. The dark current–voltage characteristics were analyzed and the contribution of ohmic shunting current to the total leakage current was found to increase with the thickness of CdTe absorber layer decreasing. The device characteristics of the ultrathin CdTe solar cells under weak light irradiance and at different temperatures were also investigated. This study provides a guideline for the fabrication of ultrathin CdTe solar cells in the future.

Process study about silica anti-reflection coatings prepared by sol–gel method for cadmium telluride solar cells

Silica (SiO2) thin films with different refractive indexes as the anti-reflection coating for cadmium telluride (CdTe) solar cells were prepared by sol–gel method in this paper. In order to obtain the preparation process of SiO2 films with suitable refractive index, the different hydrolysis processes relied on the acid-base environment as well as the ratio of reactants was studied, respectively. It mainly includes that (I) the gel growth model in hydrogen or hydroxide ion environment provided by hydrochloric and ammonia. (II) The ratio of solvent, precursor and catalyst under base catalysis. (III) The weight and amount of polyethylene glycol molecular which is an additive agent to increase the porosity of sol film under acid catalysis. Based on the results above, a monolayer SiO2 coating with optimized refractive index was applied onto the glass substrate of CdTe solar cell, and the quantum efficiency has been enhanced at the band of 400–800 nm. After that, multilayer SiO2 coatings with high–low refractive indexes were designed according to the Fresnel theory to achieve wider spectrum (390–870 nm) enhancement. The broadband anti-reflection coatings (BARCs) were deposited on CdTe solar cell. The results were not consistent with expectations, which required further study about process matching between BARCs and device manufacture.