John Raguse

Effects of Sodium and Potassium on the Photovoltaic Performance of CIGS Solar Cells

The deliberate introduction of K and Na into Cu(In, Ga)Se2 (CIGS) absorbers was investigated by varying a combination of an SiO2 diffusion barrier, coevaporation of KF with the CIGS absorber, and a KF postdeposition treatment (PDT). Devices made with no diffusion barrier and KF coevaporation treatment exhibited the highest photovoltaic conversion efficiency with the smallest overall distribution in key current density-voltage (J-V) performance metrics. Out-diffusion of Na and K from the substrate, KF coevaporation, and KF PDT all increased carrier concentration, open-circuit voltage, fill factor, and power conversion efficiency. Quantum-efficiency analysis of devices highlighted the greatest loss in the short-circuit current density due to incomplete absorption and collection. Secondary ion mass spectrometry illustrated the efficacy of the SiO2 film as a sodium and potassium diffusion barrier, as well as their relative concentration in the absorber. Introduction of KF appeared to enhance diffusion of Na from the substrate, in agreement with previous studies.

Analysis of electroluminescence images in small-area circular CdTe solar cells

The electroluminescence (EL) imaging process of small area solar cells is investigated in detail to expose optical and electrical effects that influence image acquisition and corrupt the acquired image. An approach to correct the measured EL images and to extract the exact EL radiation as emitted from the photovoltaic device is presented. EL images of circular cadmium telluride (CdTe) solar cells are obtained under different conditions. The power-law relationship between forward injection current and EL emission and a negative temperature coefficient of EL radiation are observed. The distributed Simulation Program with Integrated Circuit Emphasis (SPICE®) model of the circular CdTe solar cell is used to simulate the dark J-V curve and current distribution under the conditions used during EL measurements. Simulation results are presented as circularly averaged EL intensity profiles, which clearly show that the ratio between resistive parameters determines the current distribution in thin-film solar cells. ...

Electroluminescence system for analysis of defects in CdTe cells and modules

A highly versatile electroluminescence (EL) system was constructed to measure various non-uniformities in CdTe photovoltaic devices spanning a wide range of areas. The emitted photons from these devices are detected with a Si-CCD camera. A small-area cell with a thick and thin CdS region was investigated, and a separate set of cells with varying CdCl2 treatment. Several modules were also investigated, demonstrating different defects. Such defects include a shunt, likely due to a defect that existed before the cell layers were deposited, and a conductive bridge across scribe line in a CdTe module. The mean EL signal has been found to vary exponentially with the open-circuit voltage (VOC) of CdTe devices and holds for devices with differing CdS thicknesses and CdCl2 treatment. A GaAs device was also compared to this trend.

Deposition and characterization of Cd1−xMgxTe thin films grown by a novel cosublimation method

Photovoltaic cells utilizing the CdS/CdTe structure have improved substantially in the past few years. Despite the recent advances, the efficiency of CdS/CdTe cells is still significantly below their Shockley–Queisser limit. CdTe based ternary alloy thin films, such as Cd1−xMgxTe (CMT), could be used to improve efficiency of CdS/CdTe photovoltaic cells. Higher band gap Cd1−xMgxTe films can be the absorber in top cells of a tandem structure or an electron reflector layer in CdS/CdTe cells. A novel cosublimation method to deposit CMT thin films has been developed. This method can deposit CMT films of band gaps ranging from 1.5 to 2.3 eV. The cosublimation method is fast, repeatable, and scalable for large areas, making it suitable for implementing into large-scale manufacturing. Characterization of as-deposited CMT films, with x varying from 0 to 0.35, reveals a linear relationship between Mg content measured by energy dispersive x-ray spectroscopy and the optical band gap. Glancing angle x-ray diffraction ...

A physics-based compact model for CIGS and CdTe solar cells: From voltage-dependent carrier collection to light-enhanced reverse breakdown

In this paper, we develop a physics-based compact model for CIGS and CdTe heterojunction solar cells that attributes the failure of superposition to voltage-dependent carrier collection in the absorber layer, and interprets light-enhanced reverse breakdown as a consequence of tunneling-assisted Poole-Frenkel conduction. The temperature dependence of the model is validated against both simulation and experimental data for the entire range of bias conditions. The model can be used to characterize device parameters, optimize new designs, and most importantly, predict performance and reliability of solar panels including the effects of self-heating and reverse breakdown due to partial-shading degradation.

Correlation of Electroluminescence With Open-Circuit Voltage From Thin-Film CdTe Solar Cells

Electroluminescence (EL) measurements of several CdTe solar cells, a GaAs solar cell, and an AlGaAs LED were performed. The mean integrated EL intensity, normalized to exposure time and injection-current density, correlates well with the difference between ideal open-circuit voltage and measured open-circuit voltage, nearly independently of cell details or uniformity. This trend holds for devices of different absorber materials but does not hold for devices which exhibit poor superposition between light and dark current-voltage (J-V) curves.

Effect of varying process parameters on CdTe thin film device performance and its relationship to film microstructure

The performance of CdTe thin film photovoltaic devices are sensitive to process parameters. In this study, efforts are made to further understand the effects of process parameters like process temperature and variation in cadmium chloride passivation treatment on CdTe films deposited using a sublimation based deposition system. The effects on film microstructure are studied using advanced microstructural characterization methods like TEM, SEM, EDS and SIMS while electrical performance is studied using various electrical measurements such as current density vs. voltage and electroluminescence. The aim of this study is to provide new insight into the understanding of relationship between fabrication process, device performance and thin film microstructure.

Progress towards a CdS/CdTe solar cell implementing An Electron Reflector

An Electron Reflector (ER) structure in CdS/CdTe photovoltaic cells has potential to increase the cells open circuit voltage. Progress towards implementing a Cd1-xMgxTe thin film as an electron reflector is presented, including deposition method, film characterization, investigation of passivation, and initial results of ER structures.

Polycrystalline CdSeTe/CdTe Absorber Cells With 28 mA/cm 2 Short-Circuit Current

An 800 nm CdSeTe layer was added to the CdTe absorber used in high-efficiency CdTe cells to increase the current and produce an increase in efficiency. The CdSeTe layer employed had a band-gap near 1.41 eV, compared with 1.5 eV for CdTe. This lower band-gap enabled a current density increase from approximately 26 to over 28 mA/cm2. The open-circuit voltage obtained in the high-efficiency CdTe-only device was maintained and the fill-factor remained close to 80%. Improving the short-circuit current density and maintaining the open-circuit voltage lead to device efficiency over 19%. External quantum efficiency implied that about half the current was generated in the CdSeTe layer and half in the CdTe. Cross-sectional STEM and EDS showed good grain structure throughout. Diffusion of Se into the CdTe layer was observed. This is the highest efficiency polycrystalline CdTe photovoltaic device demonstrated by a university or national laboratory.

Compact accelerated life testing with expanded measurement suite

An accelerated-life-testing (ALT) system has been built at the Colorado State University Photovoltaics Laboratory with an emphasis on versatility and periodically performing a suite of electronic measurements on stressed devices. The setup utilizes a scientific oven with a footprint of 17 × 17 inches as a stress chamber and four commercially available 40 W broad-spectrum LED arrays. A preliminary study has been performed on Cadmium Telluride (CdTe) devices. Devices were held at elevated temperature and were exposed to nominally one-sun illumination. Measurements taken of stressed devices include J-V, QE, C-V, electroluminescence (EL) and light-beam-induced current (LBIC).