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