Shubhra Bansal

Intrinsic stability of thin-film CdS/CdTe modules

Stability of thin-film CdTe module is crucial to the advancement of the technology. Long-term stability studies are carried out under a variety of accelerated test conditions. Accelerated-life-tests (ALT) are performed in the laboratory to accelerate degradation and hence quantify performance change in reasonably short periods of time. ALT studies attempt to simulate the conditions a solar module experiences outdoors during its lifetime, and ALT results are correlated to outdoor test results to estimate module service lifetimes, nameplate parameters, and product warranty. To accelerate degradation, laboratory experiments are often performed at elevated temperature, voltage bias and under continuous illumination. In this study laboratory size cells (0.5 cm2), mini-modules (15 cm × 15 cm) and full-size encapsulated modules (60 cm × 120 cm) that consist of monolithically interconnected cells were subjected to different levels of illumination, elevated temperatures and electrical biases for extended periods of time. Changes in efficiency, fill-factor (FF), open-circuit voltage (Voc) and short-circuit current (Isc) as a function stress time and stress conditions are discussed.

The use of 2 nd and 3 rd level correlation analysis for studying degradation in polycrystalline thin-film solar cells

The correlation of stress-induced changes in the performance of laboratory-made CdTe solar cells with various 2nd and 3rd level metrics is discussed. The overall behavior of aggregated data showing how cell efficiency changes as a function of open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) is explained using a two-diode, PSpice model in which degradation is simulated by systematically changing model parameters. FF shows the highest correlation with performance during stress, and is subsequently shown to be most affected by shunt resistance, recombination and in some cases voltage-dependent collection. Large decreases in Jsc as well as increasing rates of Voc degradation are related to voltage-dependent collection effects and catastrophic shunting respectively. Large decreases in Voc in the absence of catastrophic shunting are attributed to increased recombination. The relevance of capacitance-derived data correlated with both Voc and FF is discussed.

Effects of Microstructure Evolution on High-Temperature Mechanical Deformation of 95Sn-5Sb

Lead-free solders have garnered much attention in recent years due to legislation banning the use of lead in electronics. As use of lead solders is phased out, there is a need for lead-free alternatives for niche applications such as high temperature environments where traditionally high lead solders are used. Electronics and sensors exposed to high-temperature environments such as those associated with deep well drilling require solder interconnects that can withstand high thermal-mechanical stresses. In an effort to characterize solder alloys for such applications, this study focuses on deformation behavior of the Sn95-Sb5 solder under high-temperature exposures (from 298°K to 473°K). As compared to conventional high-temperature Pb-based solder 90Pb–10Sn, Sn95–Sb5 exhibited very high tensile strength and modulus, as well as superior creep properties despite its lower melting temperature. Importantly, high-temperature deformation was shown to be influenced by the presence of the second phase (SnSb) distributed within the Sn-rich matrix. These second phase precipitates appeared to be dissolved into the Sn-rich phase above 453°K, which converted the solder into a single-phase alloy and resulted in a change in its deformation mechanism. Furthermore, as the service temperature is of such high homologous temperature (T > 0.5Tm), creep deformation will contribute significantly toward the life of the solder joint during thermal cycling. In order to characterize the creep behavior and to identify controlling mechanism(s), creep tests were carried out, from which the stress exponent and activation energy were determined. In this study, detailed microstructures under high-temperature are presented in conjunction with the corresponding mechanical behavior to further understand the controlling deformation mechanisms.Copyright

Constitutive Relations of High Temperature Solders

This study focuses on microelectronic package design for the oil and natural gas drilling of wells with depths in excess of 20,000 ft, where package temperatures can exceed 204°C. At these high temperatures, solder interconnect sites are subject to fatigue and creep failures due to the stress generated by the thermal expansion mismatch between various components in the package. Typically this phenomenon is modeled by finite element analysis (FEA) to predict the number of cycles to failure. To ensure meaningful model results, however, accurate time and temperature-dependent mechanical properties are needed. This study examines five solders suitable for high temperature: 90Pb-10Sn, 95Sn-5Sb, 92.5Pb-5Sn-2.5Ag, 95Pb-5In, and 93Pb-3Sn-2Ag-2In. Uniaxial tension tests of the solder wires are carried out on a MTS servohydraulic machine using wedge grips. To evaluate the time-dependence on deformation, a strain rate study was carried out at 0.5%/sec, 1%/sec, and 5%/sec. Nanoindentation of solder wire is performed and compared to the corresponding solder wires tested through uniaxial tension tests. Dynamic nanoindentation through continuous stiffness measurement is performed on the wires to obtain the indentation data less sensitive to creep of the material, as well as to assess the effect of indentation depth on elastic modulus for each solder. One purpose of nanoindentation testing is to determine its suitability for the mechanical testing of soft solders. Mechanical properties obtained from these tests will be used in future modeling studies to estimate the cyclic fatigue life of these solders under thermal loading.Copyright

Interface analysis in CdTe/CdS solar cells

We present results of chemical composition analysis across CdTe/CdS interfaces using depth profiling in Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS). The analysis of these buried interfaces is typically challenging due to significant interface broadening due to CdTe initial roughness and developing roughness during depth profiling. We have developed two alternative methods for sample preparation, namely chemical etching and mechanical polishing, and we will present S conc. profiles obtained using both methods in samples grown with variable CdTe temperature. AES depth profiling near bottom of CdTe solar cells showed widening of S conc. profiles for the hottest CdTe as compared to the coldest CdTe. In addition, the peak S conc. decreases from the coldest to the hottest sample, suggesting that S out-diffusion from CdS is also temperature dependent. Finally we employ focused ion beam cross-sectioning and scanning electron microscopy to measure layer thicknesses, evaluate the success of the sample preparation methods, and to discuss the effects of interface roughness on S conc. profiles and Te-S interdiffusion.

Harsh-Environment Packaging for Downhole Gas and Oil Exploration

This research into new packaging materials and methods for elevated temperatures and harsh environment electronics focused on gaining a basic understanding of current state-of-the-art in electronics packaging used in industry today, formulating the thermal-mechanical models of the material interactions and developing test structures to confirm these models. Discussions were initiated with the major General Electric (GE) businesses that currently sell into markets requiring high temperature electronics and packaging. They related the major modes of failure they encounter routinely and the hurdles needed to be overcome in order to improve the temperature specifications of these products. We consulted with our GE business partners about the reliability specifications and investigated specifications and guidelines that from IPC and the SAE body that is currently developing guidelines for electronics package reliability. Following this, a risk analysis was conducted for the program to identify the critical risks which need to be mitigated in order to demonstrate a flex-based packaging approach under these conditions. This process identified metal/polyimide adhesion, via reliability for flex substrates and high temperature interconnect as important technical areas for reliability improvement.