Predicting Improving the Performance of the n-V2O5/CdTe Thin Film Solar Cell by Adding a Back Surface Field Layer

Vapor transport deposition is an excellent method for preparing large area CdTe thin films with high quality and uniformity. Polycrystalline CdTe thin films were deposited by home-made vapor transport deposition system (VTD). The effects of substrate temperature on the property of CdTe film and the performance of CdTe solar cell were investigated. CdTe thin films were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), UV-Vis spectrometer, and Hall Effect system. The results show that the CdTe thin films deposited by vapor transport deposition are cubic phase with a preferred orientation in (111) direction. The average grain size increases from 2 m to 6 m and the carrier concentration increases from 1.93×10 cm to 2.36×10 cm when the substrate temperature increases from 520 °C to 620 °C. This suggests that high substrate temperature can increase the carrier density significantly due to the suppressed defect recombination. The performance of CdTe thin film solar cells deposited at different substrate temperatures demonstrates that high substrate temperature (610°C) can greatly improve the efficiency, open circuit voltage and fill factor of the solar cells. But the substrate temperature higher than 610°C will reduce the spectral 142 无 机 材 料 学 报 第 31卷 response of the cells in long wavelength region, which results in the degradation of solar cell performance. The small-area CdTe thin film solar cell without back contact layer deposited at substrate temperature of 610°C obtains the best conversion efficiency of 11.2%.

Simulation and performance analysis of CdTe thin film solar cell using different Cd-free zinc chalcogenide-based buffer layers

CdTe thin film (TF) solar cells are most promising photovoltaic (PV) technology in commercial platform. Back contacts and interface defects related opto-electrical losses are still vital to limit its further technological benefit. TF PV cells shallow recombination and parasitic loss lessening purpose carrier selective back contact with band matching window layers are essential. Beside that back and front contact thickness choice is vital for field associated selective carrier collection and generous optical transmission into the active junction of the cell. It can make variation of cell efficiency. Window and front contact layers band edge variation and back contact thickness effect is analyzed by SCAPS-1D simulation software. ZnO and SnO2 front contact for CdS and CdSe window layers effect are numerically studied for 1 μm CdTe thin film PV cell. Significance of materials for front contact and its thickness effect on current density while ZnTe back surface field contact thickness effect on open circuit voltage and efficiency are demonstrated. Finally, ZnO/CdS/CdTe/ZnTe cell of 0.925 V open circuit voltage and 19.06% efficiency has been achieved for 90 nm of ZnTe with Molybdenum (Mo) back contact.

In this work, we report that NiO thin film can be used as a back contact buffer layer in CdTe thin film solar cells. The NiO layer is prepared by electron beam evaporation. To optimize the thickness of the NiO thin film, we fabricate some CdTe solar cells with different NiO thickness values. A NiO/Au back contact CdTe solar cell with an efficiency of 12.17% and an open-circuit voltage Voc of 789 mV is obtained, which are comparable to those of a standard Cu/Au back contact solar cell. The X-ray photoelectron spectroscopy (XPS) is used to quantitatively characterize the band alignment at the CdTe/NiO interface. It can be seen from the band alignment that the valence band offset (EVBO) is 0.52 eV and the conduction band offset (ECBO) is 2.68 eV. The EVBO presents no energy barrier for hole to transport from CdTe to NiO. The value of ECBO indicates that NiO can act as a back surface field layer (BSF) to dramatically reduce carrier recombination in the contact region of a CdTe cell, leading to an improved Voc. The band alignment obtained from XPS measurement shows that the band alignments of NiO and CdTe are perfectly matched. However, the conductivity of NiO film is poor. The insertion of a NiO buffer layer in the back contact increases the series resistance and reduces the fill factor (FF). We propose to use Cu/NiO composite structure as a bi-layer contact to improve the conductivity of the NiO buffer layer, which at the same time can be used to dope the CdTe film surface by Cu to obtain a low resistive contact. We fabricate a cell with a contact structure of 3-nm-Cu/20-nm-NiO/Au and the cell has a Voc of 796 mV, a Jsc (short-circuit currrent) of 24.2 mA/cm2, an FF of 70.2% and an efficiency of 13.5%. In order to study the stability of the solar cell with a Cu/NiO/Au back contact, a thermal stressing test is carried out at a temperature of 80 ℃ in the air atmosphere. For the Cu/NiO/Au back contact structure solar cell, the efficiency decreases from 13.1% to 12.9% after the cell is stressed for 80 h, showing that the stability of the Cu/NiO/Au back contact cell is significantly improved compared with that of the standard Cu/Au contact cell. In summary, the experimental results obtained in this study demonstrate that NiO thin film is a promising buffer layer for manufacturing stable and high efficiency CdTe thin film solar cells.

The effect of grain boundaries (GBs) especially twin boundaries (TBs) on the performance of polycrystalline CdTe thin film (1–2 μm in thickness) solar cells remains largely controversial. Understanding the GB’s microstructure and physical property and their impact on the photovoltaic efficiency is critically important to the optimization of the solar cell performance. In this work, we present a systematic study of GBs in CdTe thin film solar cells of different efficiencies in the range of 1.97–9.68%, aiming to elucidate a quantitative correlation of GBs especially TBs with the solar cell charge transport by integrating several advanced approaches including electron backscattering diffraction, transmission electron microscopy, conductive atomic force microscopy, and scanning Kelvin probe force microscopy. Importantly, we have confirmed {111} Σ3 TBs form in the PLD CdTe solar cells and are beneficial by providing an efficient channel for charge transport. The percentage of {111} Σ3 TBs was found to be corre...

A comparative study on thermally and laser annealed copper and silver doped CdTe thin film solar cells

Thin-film polycrystalline CdTe solar cells have been analyzed using current-voltage, reflection, quantum efficiency, and capacitance measurements. The objective is to quantify the individual current and voltage losses in cells from different sources. Compared to an optimum photocurrent density of 30.5 mA/cm/sup 2/, they typically lose 2 mA/cm/sup 2/ to reflection, 2-3 to uncollected CdTe carriers, and 2-6 to window-layer absorption. Voltage loss at maximum power is of the order of 200 mV because of the polycrystallinity, 100 mV due to light-dark differences in forward current, and 50 mV resulting from series resistance. Individual voltage loss values vary considerably among samples. The capacitance measurement implies that a significant fraction of the CdTe is a highly compensated i-layer and that the extraneous bandgap state density is above 10/sup 11/ eV/sup -1/ cm/sup -2/ under operating conditions. >

The nanoscale electrical and mechanical properties in the CdTe thin films solar cells were investigated using the scanning probe microscopy. The comparative localized electrical and mechanical properties between as-grown and CdCl2 treated CdTe thin films for the grain and grain boundaries were studied using the conductive atomic force microscopy (cAFM) and force modulation microscopy (FMM). An increased electrical behavior and decreased elastic stiffness in the CdCl2 treated thin films were recorded to elucidate the impact from the grain growth of CdTe grains. On applying a simulated working electrical bias into the CdTe thin-film solar cells, the electric field across the CdTe film can increase the softness of CdTe thin film. The results imply the presence of a potential mechanical failure site in the CdTe grain boundary, which may lead to device degradation.

Carbon-nanofibers film as a back-contact buffer layer in CdTe thin film solar cell

Copper doping effect in the back surface field layer of CdTe thin film solar cells

This paper simulates the relationship between the output characteristic curve of CdTe thin film solar cells and the changes of ambient light intensity and ambient temperature in the actual application process. On the basis of the equivalent circuit of CdTe thin film solar cell, the engineering mathematical model is established with the simulation parameters of the solar cell module provided by the CdTe battery manufacturer. According to the engineering mathematical model of the CdTe thin film solar cell, the model is built and simulated in the Matlab/Simulink simulation system to simulate the gradual change of the light intensity when the temperature is constant and the gradual change of the ambient temperature when the light intensity is constant, and observe the I-V P-V output characteristic curve, analyze the relationship between CdTe thin film solar cell and external light intensity and ambient temperature.

pin double-heterojunction thin-film solar cell p-layer assessment

Photovoltaic (PV) energy is one of the significant renewable energies with free and permanent resource. Cadmium Telluride (CdTe) is from group II-VI of compound polycrystalline semiconductors. The CdTe solar cell material can be produced in thinness of film; hence, it is very appropriate for thin film solar cell industry production. The main purpose of this investigation is to model and analyze a prospect structure of thin film CdTe solar cell by AMPS-1D software. In this solar cell structure, Thin Oxide (SnO 2 ) as front contact, Cadmium Sulfide (CdS) as window layer, CdTe as absorber layer and Molybdenum (Mo) as back contact are used respectively. In this paper, the carrier concentration changes, thicknesses effects, temperature stability and effect of two buffer layers of Zinc Stannate (Zn 2 SnO 4 ) and Zinc Oxide (ZnO) on the CdTe solar cell output performance are investigated to obtain an optimum thin film structure of CdTe solar cell. This study aims to obtain the thickness of 1 μm for CdTe absorber layer. The conversion efficiency of the optimized structure of CdTe thin film solar cell (SnO 2 /CdS/CdTe/Mo) is up to 21.313% (V OC = 0.756 V, J SC = 34.282 mA/cm2 and FF = 0.822) with a thickness of 1.2 μm. Obviously, this structure is cost-efficient due to its thickness. Furthermore, temperature coefficient of the final proposed structure (ZnO, Zn 2 SnO 4 /SnO 2 /CdS/CdTe/Mo) is less than 0.031% /°K which shows a well stability at higher operating temperature of the proposed CdTe thin film solar cell. Therefore, this CdS/CdTe structure can be a prospect of thin film solar cell with high efficiency.

A computational study on the energy bandgap engineering in performance enhancement of CdTe thin film solar cells

Interface modification of CdTe thin film solar cells by CdCl 2-activation