German company Heliatek GmbH recently announced that its tandem solar cell technology has been verified to be the highest efficiency ever (9.8%) for an organic solar cell. The record-breaking cells were fabricated on glass substrates produced in its laboratory environment, and the breakthrough was achieved by optimizing the absorber layer's material, deposition process, and morphology.
According to Dr. Martin Pfeiffer, Heliatek's CTO and co-founder, "we can routinely reproduce this efficiency level and can get reasonable module efficiency, even though the lab equipment uses point sources for evaporation which leads to significant thickness inhomogeneities. For the production process, we will use linear sources that have been optimized for the OLED industry so that the providers can guarantee excellent homogeneity."
Progress on flexible substrates
Though the record was achieved with a glass substrate, Heliatek's goal is to use this technology on flexible plastic. Flexible organic photovoltaic (OPV) cells are formed from multiple layers of active regions stacked on plastic substrates. Heliatek's semi-transparent OPV cell takes advantage of low temperature processes and "short chain" hydrocarbon molecules and manufacturing processes similar to those widely used in the Organic LED industry.
Currently, Steffanie Rohr, Heliatek's marketing director, says that the highest efficiency they have achieved by cells produced on PET foil substrates are internally measured to be around 8.5%, and external certification of that measurement is being obtained.
The hope is that large scale manufacturing processes that don't require a cleanroom and operate at low temperatures will allow processing on plastic substrates to produce extremely cost-effective customizable OPV solar cells. Heliatek estimates that only a small amount of expensive semiconducting materials are needed, about a gram, to create tandem cells that cover 1 square meter.
Scaling Up Towards Larger Organic PV Module Production
Even more important than the externally verified record-breaking efficiency achieved on cells sized 1.1 square cm is that scaling the same technology to more than one hundred times larger produced pieces at nearly the same efficiency.
Pfeiffer explains that efficiency was measured on "a module on one piece of glass with a number of series-interconnected cells. While the efficiency on the active area only is 9%, the efficiency for the total area is reduced to 7.5% due to area losses. The relative area losses will be reduced in the production line by using laser scribing rather than shadow masks for the structuring."
The timeliness of the announcement demonstrates that as a company, Heliatek is on track and on time for meeting its 5-year roadmap, which includes a clear path to achieving 12% efficiency and a theoretical path to 15% (both as measured on glass substrates). The company has met its predicted milestones to date and expects to reach 10% efficiency in the next year.
In the 5 years Heliatek has been in business, two rounds of outside investor funding have been completed, including partners BASF and Robert Bosch GmbH. In 2012, a third round of funding is expected to be completed, and production is to begin on a roll to roll manufacturing facility.
The efficiency levels demonstrated on glass and PET on a laboratory scale validates Heliatek's OPV approach. This is a step in the right direction and a milestone worth celebrating. It provides hope that this approach could be suitable for roll to roll processing for low cost devices to be produced with efficiencies needed for mainstream adoption. Much work still lies ahead.
About the Author: Debbie Sniderman writes, owns, and consults with VI Ventures, an R&D and manufacturing consulting company for renewable energy products and technologies. She can be contacted at [email protected]