Passivation of Interfaces in High-Efficiency Photovoltaic Devices Page: 4 of 18
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the emitter layer. Light with energy close to, but above the band edge (referred to hereafter as
"red" light), is weakly absorbed (generates photocarriers) throughout the cell. Sub-band-gap
light is not absorbed and passes through the cell. The photocarriers generated by the super-band-
gap light diffuse inside the cell until they are either collected at the p-n junction or recombine
with a majority carrier, either by bulk or interface recombination. The efficiency of the solar cell
is increased when all the photocarriers are collected at the junction instead of recombining
elsewhere. Thus, effective passivation of the front and back of the cell improves the efficiency of
An ideally passivated interface "reflects" minority carriers, but passes majority carriers.
Examples of two such interfaces are included in Fig. 1. The passivating layer at the front of the
cell is often referred to as the "window" layer because it must be transparent if the solar cell is to
have a high efficiency. The back of the cell is passivated by a structure referred to as a "back-
ligh base assivating
A... depleted rack-surface field
Fig. 1. Schematic of an n-on-p solar cell. Blue light is strongly absorbed close to the front of the cell, generating
photocarriers in the emitter. Red light is absorbed throughout the cell. The photo-generated minority carriers diffuse
within the emitter and base, may be "reflected" by the passivating layers, and are collected at the junction by the field
in the depleted layer. The resulting majority carriers then pass through the passivating layers and are used in an
surface field." This layer does not need to have a higher band gap as long as it provides an
adequate barrier to minority carriers. This can sometimes be provided primarily by increased
doping in the passivating layer (for n-type GaAs and for GaInP), as shown in Fig. 1, but a higher
band gap material may work more reliably, especially when doping causes a reduction in the
band gap, as in the case of p-type GaAs.
The current-voltage (IV) curve for a solar cell (Fig. 2) in the dark is given by the standard
diode equation. If superposition holds (and the photocurrent is independent of the voltage), the
light IV curve has the same shape as the dark IV curve, but is shifted downward by the short-
circuit current (Jsc). The open-circuit voltage (Voc) is the voltage at which the magnitude of the
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Kurtz, S. R.; Olson, J. M.; Friedman, D. J.; Geisz, J. F.; Kibbler, A. E. (National Renewable Energy Laboratory) & Bertness, K. A. (NIST, Boulder, Colorado). Passivation of Interfaces in High-Efficiency Photovoltaic Devices, article, May 13, 1999; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc708227/m1/4/: accessed October 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.