Rebekah L. Garris

Thermal and Electrical Effects of Partial Shade in Monolithic Thin-Film Photovoltaic Modules

Photovoltaic cells can be damaged by reverse bias stress, which arises during service when a monolithically integrated thin-film module is partially shaded. We introduce a model for describing a modules internal thermal and electrical state, which cannot normally be measured. Using this model and experimental measurements, we present several results with relevance for reliability testing and module engineering: Modules with a small breakdown voltage experience less stress than those with a large breakdown voltage, with some exceptions for modules having light-enhanced reverse breakdown. Masks leaving a small part of the masked cells illuminated can lead to very high temperature and current density compared to masks covering entire cells.

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.

Low-Cd CIGS Solar Cells Made With a Hybrid CdS/Zn(O,S) Buffer Layer

In Cu(In,Ga)Se2 (CIGS) solar cells, CdS and Zn(O,S) buffer layers were compared with a hybrid buffer layer consisting of thin CdS followed Zn(O,S). We explore the physics of this hybrid layer that combines the standard (Cd) approach with the alternative (Zn) approach in the pursuit to unlock further potential for CIGS technology. CdS buffer development has shown optimal interface properties, whereas Zn(O,S) buffer development has shown increased photocurrent. Although a totally Cd-free solar module is more marketable, the retention of a small amount of Cd can be beneficial to achieve optimum junction properties. As long as the amount of Cd is reduced to less than 0.01% by weight, the presence of Cd does not violate the hazardous substance restrictions of the European Union (EU). We estimate the amount of Cd allowed in the EU for CIGS on both glass and stainless steel substrates, and we show that reducing Cd becomes increasingly important as substrate weights decrease. This hybrid buffer layer had reduced Cd content and a wider space charge region, while achieving equal or better solar cell performance than buffer layers of either CdS or Zn(O,S) alone.

Efficient and stable CIGS solar cells with ZnOS buffer layer

A chemical bath deposition (CBD) ZnOS process has been designed to yield repeatable and robust ZnOS/CIGS solar cells that do not require light soaking or annealing to reach conversion efficiency levels comparable with CdS/CIGS solar cells. In this study, copper content was varied over a wide range to understand its impact on these devices. Capacitance and temperature-dependent performance measurements were used to characterize the electrical performance as a function of Cu composition. The optimum Cu concentration was found to be about the same for both CBD CdS and CBD ZnOS junctions.

Comparison of CIGS Solar Cells Made With Different Structures and Fabrication Techniques

Cu(In, Ga)Se2 (CIGS)-based solar cells from six fabricators were characterized and compared. The devices had differing substrates, absorber deposition processes, buffer materials, and contact materials. The effective bandgaps of devices varied from 1.05 to 1.22 eV, with the lowest optical bandgaps occurring in those with metal-precursor absorber processes. Devices with Zn(O, S) or thin CdS buffers had quantum efficiencies above 90% down to 400 nm. Most voltages were 250–300 mV below the Shockley–Queisser limit for their bandgap. Electroluminescence intensity tracked well with the respective voltage deficits. Fill factor (FF) was as high as 95% of the maximum for each devices respective current and voltage, with higher FF corresponding to lower diode quality factors (∼1.3). An in-depth analysis of FF losses determined that diode quality reflected in the quality factor, voltage-dependent photocurrent, and, to a lesser extent, the parasitic resistances are the limiting factors. Different absorber processes and device structures led to a range of electrical and physical characteristics, yet this investigation showed that multiple fabrication pathways could lead to high-quality and high-efficiency solar cells.

An Illumination- and Temperature-Dependent Analytical Model for Copper Indium Gallium Diselenide (CIGS) Solar Cells

In this paper, we present a physics-based analytical model for copper indium gallium diselenide (CIGS) solar cells that describes the illumination- and temperature-dependent current-voltage (I-V) characteristics and accounts for the statistical shunt variation of each cell. The model is derived by solving the drift-diffusion transport equation so that its parameters are physical and, therefore, can be obtained from independent characterization experiments. The model is validated against CIGS I-V characteristics as a function of temperature and illumination intensity. This physics-based model can be integrated into a large-scale simulation framework to optimize the performance of solar modules, as well as predict the long-term output yields of photovoltaic farms under different environmental conditions.

Photoelectron spectroscopy, and photovoltaic device study of Cu 2 ZnSnSe 4 and ZnO x S 1−x buffer layer interface

Recent research has enabled Cu2ZnSnSe4 (CZTSe) to reach efficiencies close to 10% in photovoltaic devices with CdS as the junction partner and over 12% when the CZTSe is alloyed with sulfur. Little work, however, has been reported on the potential for wide band gap, Cd-free buffer layers in these devices. Reported here are photoelectron spectroscopy measurements (XPS/UPS) of the band energy positions between CZTSe and zinc oxysulfide (ZnOS) with sputter depth profiling. Measurements indicate the formation of a large conduction band offset (CBO) of 1.2 eV with chemical-bath deposition (CBD) of ZnOS on CZTSe (Eg = 0.96 eV). However, Ar ion sputter depth profiling is shown to produce compositional changes of the ZnOS thin film resulting in an apparent increase of the valence band maximum (VBM) for the buffer layer. With this in mind, the valence band edge energy offsets (VBO) are calculated and used to study solar cells made with the configuration glass/Mo/CZTSe/ZnOS/i-ZnO/Al:ZnO/Ni/Al. Variation of the deposition time of the ZnOS buffer layer during the CBD process has led to device efficiencies above 5%. For the thinnest ZnOS buffer layers, the short-circuit current matches that of devices with CdS buffer layers, but suffers from loss of open-circuit voltage. Interpretation of the solar cell measurements are aided by SCAPS thin-film device modeling.

Band alignment of CBD deposited Zn(O,S)/Cu(In 1−x Ga x )Se 2 interface

Chemical bath deposition (CBD) Zn(O,S) buffer layers grown on Cu(In1-xGax)Se2 (CIGS) thin films have recently surpassed CdS in high efficiency cells (20.9%). A critical component of a CIGS device is the buffer layer - the layer that is found between the absorber CIGS layer and the ZnO window layer. Although CBD CdS is an effective buffer layer and traditionally used for devices, it is not entirely effective for high bandgap absorber films. The Zn(O,S)/CIGS interface was studied by X-ray photoelectron spectroscopy to reveal the valence band offset (VBO) and conduction band offset (CBO) as -1.15 eV and 1.17 eV respectively. Band bending that accompanies junction formation is also characterized in both layers.