Photoluminescence and fluorescence as a
probe for chemical potential and free energy

Greg Smestad, Ph.D., Associate Editor
Solar Energy Materials and Solar Cells
Pacific Grove, California

Wednesday, April 22, 1998
3:30 p.m.—Pacific Forum

This talk presents experimental data which supports the idea that room temperature photoluminescence measurements can be used to estimate the maximum thermodynamically allowed chemical potential and solar conversion efficiency for quantum solar converters. Experimental photoluminescence spectra obtained for silicon detectors, chlorophyll-based photosynthesis, and Ruthenium dye sensitized TiO₂ photoelectrochemical solar cells, are compared to predictions made using optical absorption data. The absorption data includes measurements of transmission and reflection, as well as those using the action (or induced photo-product) spectrum. Predictions are made using the measured quantum absorptivity, the Generalized Planck equation, and detailed balance techniques that relate the chemical potential of the excited state to the photoluminescent emission and photocurrent available for performing work.

The photoluminescence spectrum for silicon, the Ruthenium dye cell, and photosynthesis may also be predicted from action spectra. The predicted chemical potentials and voltages are consistent with electrical measurements. Specifically, the maximum allowed chemical potentials for the silicon solar cell are found to be 0.6-0.65 eV , while those for photosynthesis and the dye-sensitized cell are found to be near 1.3 eV. Standard laboratory instrumentation used to obtain the data is discussed, as well as novel low-cost, LED-based, lock–in amplifier electronics that may lend themselves to ocean-based measurements of photosynthesis.

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