David CahenBy assessing possibilities & limits for solar cells, taking into account known loss mechanisms to gauge in how far significant progress can be expected for the various cell types, we conclude that additional limits exist for cells that include organic materials as absorbers/conductors. Thus, while organics present a promising direction for potentially cheaper solar cells, cells with them also have some inherent limitations, as compared to “classical” cells.  We are primarily interested in figuring out what are these limits. To that end we use organic materials in hybrid, organic / inorganic photovoltaics (PV).

We distinguish two main directions to hybrid PV, and the one with molecules, as electronic carrier transport medium is actually less relevant than the one that relies on dipolar molecules to control interface energetics, i.e., use the electro-static rather than –dynamic molecular properties. This especially so, because we can control solar cell behavior with incomplete partial dipolar monomolecular films, as we showed for /single, poly- and nano-crystalline cells.  From work with alkyl chains on Si we find that we can make *MIS* cells without a separate *I*(nsulator) layer, suggesting that 'MIS' effects are more “chemical’ than is often thought.  We also find that only with good passivation we can express the molecule-induced interface dipole effects. We illustrate how we can use this understanding (at a rudimentary level) with a near-ambient, simple, potentially low-cost approach to make and modify semiconductor solar cells, using molecules, as short as two carbons, that self-assemble onto the semiconductor (absorber) surface, passivating and buffering it. Good, stable interface passivation along with strong inversion allows minority carriers, generated by absorbed light, to move laterally within the semiconductor top layer, for collection by a minimal-area grid, deposited on the conducting polymer and also minimizes photo-current losses, due to sheet resistance.  Thus, using ≤ 1 nm thick organic molecules appears to convey a unique advantage over inorganic passivation or buffer layers and provide a test system to explore molecule-imposed limits on photovoltaic conversion.


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