Jun
29
The new architecture takes very few processing steps to produce an affordable solar cell with efficiencies comparable to conventional silicon solar cells. This new architecture uses alternative, transparent materials that can be deposited at room temperature, eliminating the need for high temperature chemical doping – the process currently used to increase the electrical conductivity of key surfaces in solar cells.
The cross-section shows key features of a new solar cell architecture. Textured silicon is coated and the required solar cell contacts are deposited at room temperature. A metal oxide layer is deposited on the sun-facing top and a metal fluoride layer is deposited onto the bottom. Additional coatings are added to managing light reflection. The new design produces a solar cell that rivals the performance achieved from cells made using conventional processing that requires heat and an ultra-clean environment. Image Credit: James Bullock/Berkeley Lab, UC Berkeley, ANU. Click image for the largest view.
The team’s research paper has been published in Nature Energy. Scientists from Berkeley Lab, UC Berkeley, ANU and The Swiss Federal Institute of Technology of Lausanne (EPFL) also participated in the study
“The solar cell industry is driven by the need to reduce costs and increase performance,” said James Bullock, the lead author of the study. “If you look at the architecture of the solar cell we made, it is very simple. That simplicity can translate to reduced cost.”
By proving that this simple design can lead to high conversion efficiencies, turning sunlight into electricity, makes it a useful tool to lower costs and improve performance of a wide range of solar cell designs. In addition this simple process could be extended to improve contacts in semiconductor transistors used to speed today’s computers.
Low cost and high efficiency comes from simplicity. In the new architecture, sunlight passes through the top layer (metal oxide) and creates electron-hole pairs in the silicon. The holes are drawn to the molybdenum oxide layer, while the electrons are drawn to the lithium fluoride layer, which can be used to produce electricity.
Boasting an average efficiency above 19 percent comes from the new materials and a simple coating process for layers on the top and bottom of the device.
This design uses a seven-step process and low-temperature processing to produce a device that efficiently separates photo-generated elections and holes.
In this process, the crystalline silicon with a pyramid texture is coated with a passivating layer of amorphous silicon. Then, ultrathin coatings of molybdenum oxide is deposited at room temperature on the top side of the device. Molybdenum oxide’s advantage is transparency, allowing the sunlight to reach the silicon core, and has the appropriate electronic properties to conduct the photo-generated holes.
Next, lithium fluoride is deposited at room temperature onto the bottom side of the solar cell to draw the photo-generated electrons from the silicon core.
The two layers, having thicknesses of tens of nanometers, act as dopant-free contacts for holes and electrons, respectively.
The team used a room-temperature technique called thermal evaporation to deposit the layers of lithium fluoride and moly oxide for the new solar cell. There are many other materials that the research teams hopes to test to see if they can improve the cell’s efficiency.
Ali Javey, program leader of Electronic Materials at Berkeley Lab and a professor of Electrical Engineering and Computer Sciences at UC Berkeley said, “Moly oxide and lithium fluoride have properties that make them ideal for dopant-free electrical contacts.” Both materials are transparent, and they have complementary electronic structures that are well-suited for solar cells. “They were previously explored for other types of devices, but they were not carefully explored by the crystalline silicon solar cell community,” he said.
Javey noted that his group had discovered the utility of moly oxide as an efficient hole contact for crystalline silicon solar cells a couple of years ago. “It has a lot of defects, and these defects are critical and important for the arising properties. These are good defects,” he said.
Stefaan de Wolf, another author who is team leader for crystalline silicon research at EPFL in Neuchâtel, Switzerland, said, “We have adapted the technology in our solar cell manufacturing platform at EPFL and found out that these moly oxide layers work extremely well when optimized and used in combination with thin amorphous layer of silicon on crystalline wafers. They allow amazing variations of our standard approach.”
In the study, the team identified lithium fluoride as a good candidate for electron contacts to crystalline silicon coated with a thin amorphous layer. That layer complements the moly oxide layer for hole contacts.
This simple, processing is less expensive than conventional processing for silicon solar cells that requires chemical doping at high temperatures to create contacts that separate the photo-generated electrons and holes. Impressively, the simplified architecture achieves solar energy conversion comparable to conventional silicon solar cells at a lower cost.
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