Presentation Details
| Excess Defects: How Illumination Affects the Thermodynamic Stability of Charged Point Defects Michael A.Scarpulla, Mike Kirby, Isaac D.Thomas, Jack Halbersleben. University of Utah, Salt Lake City, UT, USA |
Abstract
Under steady-state illumination non-identical to its blackbody spectrum, a photovoltaic material stores chemical potential energy in the form of excess carriers: this is the origin of Voc. Conventionally, excess carrier dynamics assume that the densities of defects are static, which should instead be modified to state that defects’ generation/recombination kinetics are slow compared to those of carriers. The nonequilibrium steady state formation energies of defects are important to understand, as they provide driving forces that could be exploited to optimize material processing as well as solar cell degradation modes on long time scales. Here we demonstrate that illuminated materials also store energy in the form of defect numbers unequal to their thermal equilibrium values. Under the influence of any types of non-equilibrated energy fluxes, a solar cell material’s carrier and defect subsystems store excess electrochemical energy with the balance governed by its particular kinetics. For impurity defects (as opposed to native defects), this bulk photoionic effect can be expressed as a change in the “equilibrium” solubility limit of the impurity under illumination. This provides a processing control parameter that is perhaps unwittingly utilized in rapid thermal annealing, but with understanding could be intentionally exploited. Taking into account the requirement of charge balance, we demonstrate that above-gap illumination changes the steady-state density of charged point defects coupled to the carrier system by phonon- and or photon- mediated capture/emission processes. While the general solution for a material with many defect types requires numerical treatment, analytical solutions are possible for the case of a single defect type. A defect’s concentration may suppressed or increased depending on the differences in its chargestates’ dark and light occupation functions.
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No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the author.