SCIENTIFIC CHALLENGES
Despite the promise of the new approaches utilizing novel phenomena and materials for energy
conversion, substantial scientific challenges exist in understanding and realizing photovoltaic
devices that produce >50% efficiency in cost-effective device structures. In addition to the
fundamental scientific challenges described above for each new approach, there are additional
scientific opportunities that apply to all approaches arising from a deeper understanding of
interfaces, non-ideal recombination mechanisms, transport processes, and improved light
coupling with the electronic devices.
Control over Interfaces between Dissimilar Materials
Defects within a material or at the interface between two dissimilar materials can cause non-
radiative recombination, and, therefore, degrade the performance of solar cells. Defects within a
material can originate from a number of causes, including, as examples, those that originate from
impurities, or from the defects that can arise from heteroepitaxial growth (Schroder 1997; Aberle
2000).
Interfaces between dissimilar materials also play very important roles in determining the
performance of heterostructures. Not only can they affect the crystallographic structure of the
thin films on either side of the interface, but they can also be the source of interdiffusion and
foreign impurities. As a consequence, interfaces can dominate the transport and recombination of
carriers. A fundamental understanding of how to mitigate non-radiative recombination will
provide the foundation needed to achieve higher performance for all solar cells. This is
especially important for integrated materials because they usually show higher defect densities.
There are four general ways to mitigate non-radiative recombination: (a) produce materials with
few or no defects, (b) utilize naturally passivated materials (e.g., copper indium diselenide),
(c) take advantage of high-quality artificial passivation of materials (e.g., silicon dioxide
passivation of silicon), and (d) design materials for the collection of carriers by drift instead of
by diffusion.
Interfaces also provide opportunities for harnessing the transmission or reflection of light, or they
can be utilized to control the spatial confinement or distribution of photocarriers. For example,
thin-film silicon films need light trapping to increase the absorption path, while multijunction
structures can benefit by guiding light to the appropriate layer. Careful engineering of the
interface shape, composition, and refractive index change can thus improve the properties of a
heterostructure, once theoretical and experimental studies have thoroughly characterized the
interface of interest.
A fundamental understanding of how to mitigate non-radiative recombination will provide the
foundation needed to achieve higher performance for all solar cells, but is especially important
for integrated materials because they usually show higher defect densities. Many fundamental
materials issues related to the integration of dissimilar materials for harnessing of sunlight are
illustrated in Figure 25, including: