Cathode
Anode
p+ p n+ / wafer
p+
FROM MODEL
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FIGURE 1. The MHGP VMJ cell has the
potential to deliver higher power density than
conventional cells through both application of
greater light intensity and higher PV receiver
efficiency.
positive performance feedback loop) or
that a lower-cost and often lower-weight
heat sink can be used. Another advan-
tage of more-efficient PV receivers is that
the overall system efficiency will be en-
hanced, which is important for some ap-
plications in terms of both cost and system operation.
How the VMJ PV cell achieves
high power density
The MHGP silicon-based VMJ PV cell
(VMJ cell) is a cost-competitive concen-
trating PV cell, which works most ef-
fectively with laser systems with wavelengths in the 900 to 1000 nm range. The
VMJ cell’s material composition and device structure serve as the foundation of
the cell’s competitiveness for laser power transmission applications.
The VMJ cell is an integrally bond-
ed series-connected array of miniature
silicon vertical-junction unit cells. The
VMJ cell is fabricated by bonding a
stack of diffused and metalized sili-
con wafers together and then dicing
the stack into thin slices, resulting
in high-voltage, low-current cells
that each contain approximately 50
unit cells (junctions) per centimeter
of length, resulting in near 30 V/cm.
Electrical leads are attached to the
end contacts, so that current flows
from the ends of the cells (see Fig. 1).
Increasing intensity on conven-
tional PV cells will increase output
to a point. Gallium arsenide (GaAs)
concentrating PV cells designed for
laser light conversion require dense
grid lines on the front and back surface
of the cell to manage higher concentra-
tions of light. This has the effect of reduc-
ing cell performance as intensities go up.
The MHGP VMJ cell has been shown
to maintain constant performance in efficiency (based on flash test results) up
to intensities greater than 100 W/cm2.
The major challenge for PV cells under
highly concentrated light is managing
the waste thermal energy. As intensi-
ties increase, cell temperatures rise, re-
sulting in lower performance, which re-
sults in still proportionally greater waste
energy. In other words, high intensities
have a compound negative impact on
cell performance.
The barrier to efficiently removing
heat from PV arrays is the thermal
