Organic/Polymer Photovoltaics
The recent demand for alternative energy production has propelled considerable interest in next-generation photovoltaics technology. Of the many types of solar cells being developed, the bulk heterojunction polymer devices can be considered one of the most disruptive technologies. This is due to the lure of a low cost liquid solution that can easily be deposited onto either rigid or flexible substrates. The most successful device architecture to date has an active layer containing a composite blend using derivatized fullerenes (e.g. PCBM[60]) and conducting polymers from the polyphenylenevinylene or polythiophene class of structures. Significant progress has been made to optimize the appropriate composite morphology to maximize absorption and carrier transport through solution processing, annealing, and material selection. This work has led to small active area devices (generally <<1 cm2) having confirmed power conversion efficiencies of 4.8% under AM1.5 illumination.
Significant progress has been made to optimize the appropriate composite morphology to maximize absorption and carrier transport through solution processing, annealing, and material selection. This has led to state-of-the-art developments to date on small active area devices (generally <<1 cm2) having confirmed photovoltaic conversion efficiencies as high as of 4.8% under 1 Sun Air-Mass 1.5 or AM1.5 illumination. Unfortunately, the host polymers have bandgaps that are not well suited solar spectrum (i.e., ~2 eV), their carrier mobilities are low, and the resulting solar cells are not very environmentally stable. The prospect of polymer solar cells emerging as a more sustainable technology than silicon relies upon the development of higher efficiency and robust environmentally stable devices. This project is directed towards fundamentally investigating both of these issues with the objective of the development of high efficiency (> 20%) low-cost organic solar cells (< 10 cents/kWhr) and therefore long-lived (> 1 year with less than 1% efficiency degradation).
- Recent Publications:
- Ryne P. Raffaelle, Annick Anctil, Roberta DiLeo, Andrew Merrill, Oxana Petritchenko, and Brian J. Landi, “Dye-Sensitized Bulk Heterojunction Polymer Solar Cells”, Proc. of 33rd IEEE Photovoltaic Specialists Conf. 1, pp. pending (2008).
Polycrystalline III-V Solar Cells
State-of-the-art III-V high efficiency photovoltaic devices (~ 30%) have exclusively been fabricated on single crystal substrates, such as GaAs and germanium (Ge). Ge is the substrate material of choice for commercial multi-junction (MJ) III-V devices because it offers the opportunity to form a bottom photovoltaic junction as well as being more robust than GaAs. This allows the use of a thinner substrate, resulting in a lower mass and thus a higher areal and mass specific power solar cell. Unfortunately, crystalline Ge poses significant hurdles for achieving array specific powers approaching 1000 W/kg. We have been attempting to incorporate the strengths of high efficiency III-V MJ cell technology with substrates that offer significant benefits compared to the Ge typically used today.
The ability to accomplish this integration is based upon two separate laboratory demonstrations. First, >20% AM1.5 efficiency polycrystalline GaAs cells were demonstrated several years ago. The second development involves the recently demonstrated ability to re-crystallize thin amorphous Ge films deposited on ceramic plates and metal foils. Thus, by combining these demonstrated technologies and extending them to include MJ III-V devices, a pathway to high efficiency (>20%) flexible thin film solar arrays is readily apparent. A 20% efficient device could exceed 1000 W/kg on a 25 micron stainless steel foil or another mass equivalent metal substrate.
- Recent Publications:
- Sheila G. Bailey, David M. Wilt, Jeremiah S. McNatt, Les Fritzenmeier, Seth M. Hubbard, Christopher G. Bailey, Ryne P. Raffaelle, “Thin Film Poly III-V Space Solar Cells”, Proc. of 33rd IEEE Photovoltaic Specialists Conf. 1, pp. pending (2008).