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Thrust 1: 2016 Projects: Terawatt Silicon Photovoltaics

Co-Leaders: Buonassissi and Bertoni

High-mobility Hydrogen-doped Indium Oxide as a TCO in Silicon Heterojunction and CIGS Solar Cells (Bertoni)

The overarching goal of this project is to develop a high-mobility transparent conductive oxide (TCO) for use in solar cells with resistive emitters, including silicon heterojunction and CIGS solar cells. In these devices, the TCO serves as a lateral carrier transport medium, and its contribution to solar cell series resistance is proportional to its sheet resistance: Rsheet = (eµnt)-1, where e is the electronic charge, µ the mobility, n the carrier density, and t the thickness. For the lumped series resistance of a silicon heterojunction to be < 1.5 Ohms.cm2, Rsheet < 45 Ohms/sq is required. As t is usually fixed (e.g., because the TCO simultaneously serves as an anti-reflection coating), and unwanted free-carrier absorption is proportional to n, high mobility is the only route to higher efficiencies. We estimate that an absolute efficiency gain of 1% is possible when switching from µ = 30 (a typical value for indium tin oxide, ITO) to µ = 100 cm2/Vs.

 

High Efficiency Silicon Solar Cells at 1gram/watt (Bowden)

The broad goal of this project is the demonstration of surface passivation for a variety of aspects in silicon and nanostructured solar cells. The immediate application is the surface passivation of thin silicon wafers for heterostructure solar cells. The preferred method is the use of ordered/disordered interfaces where the passivation layer is amorphous on a crystalline substrate allowing for fast processing and high performance. In the long term, the surface passivation will permit the fabrication of novel, inexpensive tandem structures. The surface passivation layer approach will be also tested on other types of solar cells, including nanostructured and nanowire-based devices. We also plan to gain a better understanding of the effects of localized defects caused by suboptimal processing on overall passivation and collaboration with other efforts on the impact of defects in cell performance.

 

Defect Assessment and Modeling in High-quality c-Si Materials (Buonassissi)

Both low surface recombination velocity and bulk quality are key enablers for efficiency improvements in Si-based solar cells:

Bulk recombination: As device efficiencies continue to increase, so too do the demands on bulk recombination. The purity of many solar-cell materials now exceeds the detection limits of common mass spectroscopy techniques requiring the development of novel bulk-defect detection and modeling techniques. We aim to develop next-generation defect-detection and -modeling tools, to identify the root cause(s) of bulk-lifetime degradation in state-of-the-art silicon materials.

Interface/surface recombination: As the ratio of surface/volume keeps increasing, driven by the need to reduce costs, surface recombination has become an increasingly important factor for accurate bulk lifetime measurements. We aim to develop an understanding of the temperature-dependence of the most common passivation layers, which can provide an insight into the effects that processing temperatures have on the final passivation quality.

 

Characterization and Reliability in Cell Manufacturing (Mani)

Reliability of modules in the field is of key importance for photovoltaic deployment on a large scale. Many of the degradation mechanisms are not fully apparent at the cell level and only become an issue after full scale deployment. The project is connected to other QESST areas as reliability problems are often only apparent on a system scale. For instance, changes in safety regulations can cause a change in the way systems are grounded. Switching from a negative ground to a positive or center ground can adversely affect some cells as has been the case in potential induced degradation. Cell redesign is possible by changing the anti-reflection coating but this in turn affect the surface passivation of cells.

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