October 27, 2011, International Solar Energy Technology Conference, Santa Clara, CA—Victor Moroz from Synopsys discussed the design, modeling, and optimization of silicon solar cells and modules. Optimization must be across all levels of the system, from the silicon, to the cells, and to the modules through virtual prototypes.

The challenges in improving PV efficiency are in lifetime variation and system performance. To address these issues, PV cell developers need to use cell TCAD to model the cells and obtain maximum efficiency. Simulations are necessary for innovation since physical experiments are too costly and time consuming. A move to simulated designs would also help the PV companies who generally only spend about 1-3 percent of revenues for R&D by greatly reducing developmental costs.

A typical design flow would map directly into the process and allow evaluation of parameters like texture and optical and electrical properties. The driver for the development is cost per watt, or production cost over efficiency. A small change in cell efficiency can return much more than any other R&D expenditures.

Here are some examples of the types of possible investigations. Light behavior in silicon is dependent on wavelength. UV is effective under a micron, visible light uses 10's of microns, and IR uses 100's of microns. Texturing the surface can increase reflections and create longer effective internal lengths. The performance is enhanced by adding a nitride anti-reflection layer. Contact optimization is another area. Increasing contact area is good for reducing contact resistance on the bottom of the cell, but has to be balanced with and can adversely affect optical and surface recombination properties.

With the appropriate tools, a user could model all major electrical and optical effects. Some of these effects are optical reflections, surface recombination, contact resistance, bulk recombination, and current crowding. Current crowding requires a full 3-D analysis similar to junction optimization, to model the tradeoff in resistivity versus recombination. Their models show that lightly doped junctions have higher efficiency and lower variability.

Going above the cells, designers need to simulate the modules. This is a systems integration and optimization problem. The major factors are I-V curve and temperature. For full system reliability, the designer has to perform an analysis of string faults and other interconnection issues. With all these knobs and levers for the various parameters, the designer can achieve greater efficiency and lower costs for the PV systems.