Solar cell developers look beyond silicon
Peterborough, New Hampshire, United States February 20, 2006

By Chappell Brown, EE Times

A new slant on compound semiconductors has produced a high-efficiency photovoltaic process that might beat existing technologies in cost of production as well.

HelioVolt Corp. (Austin, Texas) has developed a process based on rapid thermal annealing and anodic bonding that allows high-performance copper-indium-gallium-selinide (CIGS) films to be deposited on just about any substrate. Founder and photovoltaic pioneer Billy Stanbery claims the process can dramatically shorten manufacturing time and reduce the thermal budget by a factor of 10 to 100. The process could allow a new class of materials for building integrated photovoltaics that serve, for example, as a robust coating on external building materials or on interior furnishings like curtains, to turn buildings into self-powered photovoltaic plants.

The breakthrough came out of X-ray diffraction studies of CIGS films under different annealing schedules. Stanbery, who began his photovoltaic research at Boeing in 1978, was trying to understand why those films had such a high efficiency in generating electrons from incident light. What he found - dense, nanostructured domains that act as p-n junctions - is now known as the Stanbery model and has shifted the photovoltaic community away from silicon.

Shell and Honda, both of which have substantial photovoltaic development groups, recently announced that they were focusing their efforts exclusively on copper-indium-selinide (CIS) systems, the parent system for CIGS. Shell currently has an 80 megawatt system based on silicon solar cells, but announced last week that it will be devoting all of its billion-dollar R&D budget to CIS-based thin-film panels.

As a compound semiconductor, these systems have a direct bandgap, unlike silicon, and are therefore inherently more efficient at converting photons to electric current. "In 1983 I began work on CIS thin films," said Stanbery. "At that time it was our group at Boeing and another group at ARCO who were developing the technology."Shell inherited its current photovoltaic effort through a few corporate transfers, and due to the long development period by both the Boeing and ARCO groups, Stanbery is very familiar with Shell's approach. The exact nature of Honda's photovoltaic efforts is less well-known, he said.

In 2001, Stanbery founded HelioVolt to develop his own solar-cell manufacturing process based on CIGS. Last year he received $8 million in venture funding from New Enterprise Associates (Menlo Park, Calif.). "CIS is the most absorbing semiconductor known to man and it allows us to get good results with very thin films," he said. "Thin-film photovoltaics have their roots in IC technology and really resemble large-area display technology."

While CIGS thin films hold the record for efficiency, it is not that advantage alone that is generating the excitement about using them as basic photovoltaic material.

"As I studied CIGS systems, I began to realize that it had big advantages for manufacturing," Stanbery said. "Silicon solar cells are essentially printed-circuit boards. You build the solar cells on silicon wafers and then solder and wire them together in panels. As a thin-film technology, CIGS can use the type of lithography and deposition processes used in integrated circuits."

Stanbery's key discovery was that rapidly annealed CIGS films were actually a mixture of two phases. Past analyses had started with slowly annealed films and the X-ray techniques did not pick up on the second phase, which is not as prominent with slow annealing schedules. Stanbery found that with more careful observations and rapidly annealed films, two phases, an alpha and a beta, were tightly intermixed at the nanoscale. The second phase was not showing up because it did not have long-range order that would be revealed easily with X-ray diffraction. "The structure is actually a lot like the high-critical-temperature superconductors and represents a new class of materials with complex intermixed phases," he said.

The reason a tightly intertwined structure consisting of two phases increases the films' conversion efficiency has to do with the role of domain walls. When photons strike a semiconductor, they generate an electron and hole, which often simply recombine to either generate a quantum of lattice vibration, called a phonon, or another photon. That means the electron is not available to contribute to a current. In the CIGS system, the holes and electrons become separated by the domain walls between the alpha and beta phases and do not recombine easily, making more electrons available.

The rapid annealing of very thin films is the ideal way to create this intermixed phase material, which makes them ideal for high-throughput manufacturing. "There are several levels of advantages that you get out of this," Stanbery said. "On the first level, the films do not use very much material. That is significant because over half the cost of silicon solar cells is in the silicon itself. At the next level, you have a shorter value chain. The silicon cells have to be wired together and many additional packaging costs occur. With CIGS films, we eliminate most of that."

Stable, efficient
Moreover, Stanbery maintains that only CIS-based systems address the two principal disadvantages of all thin-film techniques: long-term stability and efficiency. "The perception among buyers is that thin-film systems do not last long and have low efficiency, and frankly there is a lot of justification for that," he said. CIS, he went on, "is the most efficient of any photovoltaic thin-film technology and it is beginning to overlap with silicon in that area." Moreover, "CIS is the only thin-film technology that has been demonstrated to be intrinsically stable," he said, though "they may corrode due to external factors if not properly packaged.

"All the other thin-film approaches have inherent instabilities, but CIS, like silicon, has no inherent degradation mechanism that has been identified," Stanbery said.

Copyright © 2006 CMP Media LLC.