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

Control over Nucleation and Growth for Producing High-quality Thin Films
The growth of a thin semiconductor layer on an inexpensive substrate usually results in a
polycrystalline material. The properties of polycrystalline materials are challenging to study
because each individual grain is likely to have a different size, orientation, shape, surface
termination, structural quality, and impurity content. A variation in any of these attributes can
translate into variations in device performance, and thus an associated reduction in
manufacturing yield.
The challenge of characterizing, modeling, and controlling polycrystalline film properties is
formidable. Figure 26 illustrates how control over the nucleation and growth can yield control
over grain size, orientation, and shape, and, therefore, material quality. The remarkable advances
in nanostructure synthesis over the past decade provide scientists with tremendous opportunities
for controlling the structure of thin polycrystalline films. Similar advances in materials
characterization tools provide new opportunities for quantifying and thus eventually controlling
grain boundaries, defect states, etc. Understanding and controlling thin-film nucleation and
growth are key for both achieving high-performance photovoltaics and for achieving practical
success in the manufacturing environment.
Large oriented grains
Misaligned grains
Sparse, oriented
uniform nuclei Non-uniform, nonoriented
nuclei
Film Nucleation
Figure 26 Controlling nucleation and growth
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Growth
Improve Understanding of Carrier Dynamics at Interfaces
Interfaces between different materials are necessary ingredients of every type of solar cell. Their
number increases with the number of different materials and with the complexity of the solar cell
structure. Electronic interface states can fall within the band gap but show up also as resonances
isoenergetic with the electronic bulk states. Recent advances in experimental and theoretical
techniques now give access to real-time measurements and modeling of the underlying
interfacial charge carrier dynamics on the relevant time scales. The latter range from a few
femtoseconds to milliseconds. The actual energy distribution of hot charge carriers in
semiconductors under solar irradiation with up to a thousand-fold concentration can remain nonthermalized
and thus cannot be characterized by the lattice temperature. A realistic description of
interfacial loss processes requires (a) a model for the respective charge carrier dynamics that is
based on the detailed atomic and electronic structure of bulk interface and (b) a sufficiently
detailed time-dependent model calculation that comprises all the relevant electronic levels and all