Summer 2012 Intern Project- Car Bycraft
QUANTUM EFFICIENCY MEASUREMENT FOR MULTIJUNCTION PHOTOVOLTAICS
UC Santa Barbara
Mentor: Emmett Perl
Faculty Advisor: John Bowers
Department: Electrical and Computer Engineering
The efficiency of Silicon solar cells is reaching the theoretical limits outlined by Schokley & Queisser in 1961. Improvements are necessary to decrease the price-to-performance ratio to the levels required for becoming competitive with fossil fuels. Multijunction photovoltaic devices have already exceeded the Shockley-Queisser limit for single junction solar cells, and have proven themselves to be the highest efficiency photovoltaic devices. However, this increased efficiency has to be quantified through quantum efficiency (QE) measurements, which determine the photon to electron conversion ratio. Traditional methods for measuring QE are inadequate for multijunction devices since the full device is constrained by the current-limiting junction. Our approach allows us to characterize the individual junctions separately by flooding the non-measured junctions with photons at their respective bandgap energies and voltage biasing the remaining junction to short circuit conditions. We then sweep through a range of photon energies, and the output current of the device is proportional to the quantum efficiency of the isolated junction. Our data suggests that current measurements are accurate, but more data must be gathered to confirm the setup´s reliability. Namely, our results must be compared to data from other calibrated QE setups, such as those from the National Renewable Energy Laboratory (NREL), to confirm the accuracy of the setup.