Intellectual merit: Achieving the predicted efficiency over 60% from the third generation solar cells based on Intermediate Band (IB) has been troublesome due to the defects and low light absorption by Self-Assembled Quantum Dots (SAQD) via the Stranski-Krastanov (S-K) growth mode. The nature of the SAQDs fabricated by the S-K mode is randomness in size and position. This randomness broadens the IB structure formed between p- and n-junctions, which reduces the absorption at a given spectrum, reducing the efficiency. Another problem in applying SAQDs for solar cells is the difficulty in realizing defect-free layers of high concentration of dots when the stacking of SAQDs is more than ~20 layers. In order to overcome the problems related with controls of size and site, and the defects in the SAQD-stacks, this project will develop low cost and direct in-situ patterning processes to guide the self-assembly of uniformly sized, multi-stacked SAQDs for high efficiency solar cells. Interferential irradiation of high power laser pulses will be employed to create thermal modulations on surface in order to produce defect-free, uniform and ordered nanoscale patterns on surfaces. Atomistic understanding will be pursued on the patterning processes and the stacking SAQDs more than 50 layers. The atomistic optimization, using in-vacuum Scanning Tunneling Microscopy (STM), will provide ways to scale-up the arrays of desired sizes and shapes of SAQDs. In particular, the project will study how the size and periodicity influence the electronic structure of IBs formed by SAQDs. In-situ optical characterizations will be used to analyze optical defects, the correlation between IB structure and the properties of QD arrays such as size and density so that the patterning and growth processes maximize the efficiency of QD-based solar cells. Gained insight will be used to fabricate SAQD-based solar cells with target efficiency over 50%.
Broad impact: To generate electricity at $0.02/kWh, solar cells efficiency over 50% are crucial. When used with concentrator designs, the developed high efficiency solar cells will have huge impact on society. Also the project will advance understanding on nanoscale thermal processes to produce defect-free high quality nano lines and dots on various substrates. Students will be trained in design and fabrication processes of solar cells through research based education. This will be accomplished by exposing them to the cutting edge research on atomistic studies on the patterning using STM, the optical properties of IB solar cells, and heteroepitaxial growth techniques such as Molecular Beam Epitaxy and Pulsed Laser Deposition. Demonstrations of the solar cells and quantum size effects will be used to enhance public understanding on solar energy and nanoscale science and engineering through public demonstrations in annual Physics Day in Lagoon.
This project was supported by the Energy for Sustainability Program of the CBET Division of the Engineering Directorate and the Electronic/Photonic Materials Program of the Division of Materials Research (DMR) of the Mathematical and Physical Sciences Directorate.
A new way to guide self-assembly of nano dots has been discovered in this project. Extensive study on direct laser heating/patterning has been carried out by overlapping high power laser pulses on semiconductor surfaces, which heats the surface with a lateral period. The laser exposed surfaces were studied by atomic force microscope and scanning tunneling microscope. The research activities yielded numerous discoveries on nanostructures when semiconductor surfaces heated by a laser; (1) direct laser heating can be a promising technique to fabricate nanoscale patterns on surfaces including nano trenches and nano dots. These can be used to fabricate novel quantum wires and quantum dots as well as templates for other nanostructures; (2) the dimensions of the nano features are sensitive functions of interferential parameters such as laser intensity, period, and material properties; (3) direct laser etching can produce 300 nm deep trenches on semiconductor surfaces. This depth is different from the reported trench depth of 70 nm when a similar laser patterning has been done on other semiconductor surfaces. This suggests that direct laser patterning yield different results depending on material properties; (4) trench width can be smaller than one half of the interference period; (5) nano dots are formed along the interference lines where the laser intensity is lower than those areas with trenches; (6) application of direct laser heating on strained-but-flat epilayer inside the vacuum growth reactor of Molecular Beam Epitaxy machine induces formation of nano dots on the surfaces; (7) the density of dots formed by the process modulates sinusoidally at the same wavelength as that of interferential parameters; (8) size of dots are smaller than self-assembled quantum dots that are formed by conventional heating; (9) optical characterizations indicate that the laser irradiation does provide a way to fabricate high quality quantum dots. Figure 1 shows a picture of actual laser system and the growth reactor for laser heating experiment. This setup is used to fabricate nano dots shown in Figure 2. Figure 2 shows surface images of quantum dots and line profiles. The nano dots are formed when laser pulses overlap on the semiconductor surface inside the growth reactor. This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
