Basic Research Needs for Solar Energy Utilization - Office of ...

Strain Relaxation. Growth of layers on a single- or polycrystalline substrate is affected by the
orientation and lattice constant of the crystalline substrate. If two materials (labeled Egap 1 and
Egap 2 in Figure 25) are not precisely lattice-matched, strain will increase in the growing layer
until relaxation occurs, introducing defects that can propagate throughout the layer. If the
relaxation process is understood and can be controlled, then the relaxation can be forced to occur
within a confined part of the device, allowing the layers of interest to remain pristine.
Experimental and theoretical studies may determine the growth parameters (growth temperature,
growth rate, rate of change of lattice constant, etc.) that are key to controlling the relaxation and
how they are affected by the composition of the epilayers. Ultimately, the goal of these studies
would be to define the limits of the composition range that can be accessed with near-perfect
crystal quality while minimizing the thickness of the graded layer and final strain (wafer bow) of
the sample.
Templating. Methods are needed to enable flexible control of the crystallographic structure and
morphology of active semiconductor absorber layers that are dissimilar from the underlying
support substrate. Synthesis and processing methods that enable template layers to control crystal
structure, phase, and in-plane and out-of-plane orientation of thin films synthesized on
inexpensive substrates are desirable. Such methods include vapor-deposited template films with
controlled microstructures, transferred single crystalline layers, lithographically stamped
patterns, and colloidally assembled materials, among others.
Light-trapping Structures. Increasing the coupling between the incident radiation and the
absorber material is a central component of high-efficiency solar cells. Historically, this has been
done by using simple macroscale design principles, such as minimizing the front surface
reflectivity of a solar cell. Recently, tremendous advances have been made in the understanding
of periodic and non-periodic optical cavity and waveguide structures (e.g., photonic crystals,
plasmonic materials that control optical dispersion). Finding methods for incorporating these
types of structures and materials onto inexpensive substrates, and integrating them with
multilayer heterostructures, constitutes a very significant challenge for fundamental materials
science and engineering.
Egap 2; lattice constant 2
Egap 1; lattice constant 1
96
• Mitigating defects by
relieving strain
• Template Layer for
Interface &
crystallographic
Control
• Light trapping
structure
• Large area, Inexpensive
Substrates
Figure 2.
Figure 25 Integration of dissimilar materials to harness sunlight