CeNS Colloquium

Prof. David Cahen
Optoelectronic Materials Group, Weizmann Institute of Science, Rehovot/Israel

Comparing performance data for all types of solar cells allows gauging in possibilities and limitations for the various cell types. Our analysis suggests that additional limits exist for cells with molecular absorbers/conductor, beyond "classical" ones. Our long-term objective is figuring out these limits. To that end we use organic materials in hybrid, organic / inorganic photovoltaics (PV). Of most interest is hybrid PV with dipolar molecules to control interface energetics, i.e., use the electrostatic molecular properties. A crucial finding is that incomplete partial dipolar monomolecular films can control single, poly- and nano-crystalline cell behavior as long as the molecules passivate the semiconductor. Working with alkyl chains on Si we can make *MIS* cells without a separate *I*(nsulator) layer, suggesting that 'MIS' effects are more "chemical" than often thought, with the molecules also acting as buffer.  With this understanding we can make and modify prototype cells with molecules as short as two carbons, that self-assemble onto the semiconductor surface. Interface passivation, along with strong inversion allows photo-generated minority carriers to move laterally within the inverted semiconductor layer, for collection by a minimal-area grid, deposited on a conducting polymer. Using molecules conveys a real advantage over inorganic passivation or buffers and also serves as test system to explore molecule-imposed limits on PV conversion.