The objective of this collaborative research project between Arizona State University and Notre Dame University is to explore novel multijunction solar cell designs that offer ultra-high efficiencies for both space and terrestrial applications.
The intellectual merit of the approach is to use lattice-matched II/VI (MgZnCd)(SeTe) and III/V (AlGaAsSb) material systems grown on GaSb to demonstrate the designs with an increased number of junctions to realize breakthroughs in conversion efficiency. Before the device demonstration, very substantial research effort will be focused on the study of the doping in the II/VI (MgZnCd)(SeTe) materials. Various doping sources and methods will be explored. After the successful demonstration of both p- and n-types of doping, p-n junctions and tunnel junctions will be carefully studied before the final integration of them into new device designs. Device testing will be carried out to compare the performance with the theoretical predictions.
The broader impacts of this study include the understanding of the properties of the proposed new material systems and their application to ultrahigh efficiency multijunction solar cells, which can not only reduce the spacecraft launch payload but also enable the terrestrial concentrating photovoltaics systems to minimize the overall wafer demand and cost, from currently 75% down to 10% or less. Since the proposed material systems uniquely offer very broad wavelength coverage, from UV to IR, they can also be used for full-color LEDs, multicolor photodetectors, and other optoelectronic devices. The program will train 1.5 Ph.D. students and reach out to domestic multi-junction solar cell companies and local communities.
Intellectual Merit This project proposed a new multi-junction (MJ) solar cell design that uses the recently proposed lattice-matched II/VI (MgZnCd)(SeTe) and III/V (AlGaAsSb) material systems grown on GaSb. The integrated materials and novel design can potentially address all of the challenges in achieving ultra-high efficiency. The proposed material systems uniquely offer very broad wavelength coverage; from UV to IR. The main advantages of these materials and the proposed practical 4-junction cell design are: 1) an increased number of junctions (4 or more) to improve efficiency and to reduce current and Joule heating; 2) ultra-high theoretical (achievable) efficiencies, 49% (40%) under 1 sun and 57% (47%) under 1000 suns; 3) ultra-low series resistance of the tunnel diodes using type-II heterointerfaces; and 4) the use of wide bandgap materials for surface passivation. Using the state-of-the-art and quite unique MBE and solar cell test facilities available at ASU and Notre Dame, this program has accomplished the following: 1) further improvement of the semi-analytical solar cell model developed at ASU; 2) successful demonstration of MBE growth of high-quality II-VI heterostructures and superlattices on many III-V substrates; 3) demonstration of the world’s first 7-pair ZnTe/GaSb DBR for IR VCSEL and solar cell applications; 4) preliminary demonstration of n-doping in ZnTe; 5) demonstration of ultra-thin GaAs single junction solar cells with very high efficiency; 6) discovery of strong electrical and optical coupling in multi-junction solar cells and interpretation of the artifacts in multi-junction solar cell EQE measurement results; 6) invention of two novel test methods for multi-junction solar cells EQE measurement. These accomplishments have attracted strong interest from academia and industry. This program has contributed to the publication of 1 book chapter, 17 peer-reviewed journal papers, 18 conference proceeding papers, 8 invited talks and 43 contributed conference presentations. Broader Impacts Ultra-high efficiency solar cells greatly reduce the spacecraft launch payload and enable many more capabilities that are not possible with low efficiency solar cells. They are also necessary to enable terrestrial concentrating solar cell systems to reduce their overall semiconductor wafer demand and cost from the current 75% down to 10% or less, making it possible to solve the semiconductor materials bottleneck and lower overall electricity costs. It is worth noting that the proposed material systems can also be used for full-color LEDs, offering an important alternative to nitrides. It is expected that the reliability of the devices will be much improved compared to those grown on GaAs a decade ago because the (MgZnCd)(SeTe) materials can be perfectly lattice matched to GaSb or InAs substrates. Arizona is an ideal location for both R&D and manufacturing of solar cells, PV panels, and related systems. This program has collaborated with Sumika Inc., Soitec Phoenix Lab, Emcore and Boeing Spectrolab, which are all key US manufacturers of MJ solar cells and CPV systems. Together with other sponsored research programs related to the research of solar cells, this program has partially supported 6 PhD students (including one NSF graduate research fellow). Two PhD students have graduated during the program. The program has also involved three undergraduate senior design projects with a total of 10 students. Many of the graduate students have also reached out to local communities through seminars, solar cell and system demonstrations, and judging at science fairs.
