Highly Efficient Solar Cells using Hot Electron Transfer

University of Minnesota researchers clear major hurdle in road to high-efficiency solar cells

MINNEAPOLIS / ST. PAUL (06/17/2010) —A team of University of Minnesota-led researchers has cleared a major hurdle in the drive to build solar cells with potential efficiencies up to twice as high as current levels, which rarely exceed 30 percent.

By showing how energy that is now being lost from semiconductors in solar cells can be captured and transferred to electric circuits, the team has opened a new avenue for solar cell researchers seeking to build cheaper, more efficient solar energy devices. The work is published in this week’s Science.

A system built on the research could also slash the cost of manufacturing solar cells by removing the need to process them at very high temperatures.

Quantum Dots Could Minimize Energy Loss in Solar Cells

Researchers from the University of Texas in Austin and the University of Minnesota in Minneapolis have discovered that hot electrons can be transferred from quantum dots to an electron acceptor. This process could, theoretically, be used to increase the conversion efficiency of solar cells to 66%.

In typical semiconductor solar cells, photons with energies above the semiconductor’s bandgap generate hot electrons, and much of the energy from the hot electrons is lost through heat before it can be captured and used for electricity.

Using these hot electrons requires slowing down how fast they cool, which quantum dots can do. Then the electrons must be captured and transferred. As detailed in Science, the researchers were able to observe transfer of hot electrons from colloidal lead selenide (PbSe) quantum dots to a titanium dioxide (TiO2) electron acceptor using time-resolved optical second harmonic generation.

The University of Texas at Austin- What Starts Here Changes the World

Highly Efficient Solar Cells Could Result from Quantum Dot Research

Dr. Xiaoyang Zhu, professor of chemistry, has discovered a method to capture the higher energy sunlight that is lost as heat in conventional solar cells.

"There are a few steps needed to create what I call this 'ultimate solar cell,'" says Zhu, professor of chemistry and director of the Center for Materials Chemistry. "First, the cooling rate of hot electrons needs to be slowed down. Second, we need to be able to grab those hot electrons and use them quickly before they lose all of their energy."

Zhu's team has now figured out the next critical step: how to take those electrons out.
They discovered that hot electrons can be transferred from photo-excited lead selenide nanocrystals to an electron conductor made of widely used titanium dioxide.

"If we take the hot electrons out, we can do work with them," says Zhu. "The demonstration of this hot electron transfer establishes that a highly efficient hot carrier solar cell is not just a theoretical concept, but an experimental possibility."

Center for Materials Chemistry

Electron transfer  Photovoltaics  Interfacial Science

Photovoltaics: quantum dot interfaces
Semiconductor quantum dots (QDs) are emerging as key materials for light harvesting in next generation photovoltaics. QDs allow tunable optical absorption, solution processability, & potentially high efficiency. We are investigating the extraction of charge carriers from photoexcited QDs. In the example shown here, we use monolayer PbSe QD on TiO2(110) to provide the first experimental evidence of hot electron transfer from QDs to TiO2. Read more in publications 152 (just published in Science.)

Solar Novus Today - Quantum Dots Could Minimize Energy Loss in Solar Cells
University of Minnesota researchers clear major hurdle in road to high-efficiency solar cells : UMNews : University of Minnesota
Highly Efficient Solar Cells Could Result from Quantum Dot Research | The University of Texas at Austin
Welcome to the Zhu research group

Hot electrons could double solar cell efficiency - tech - 24 June 2010 - New Scientist
Highly efficient solar cells could result from quantum dot research
Initiative for Renewable Energy and the Environment | Institute on the Environment | University of Minnesota
News : CEMS : University of Minnesota
CEMS : University of Minnesota
Welcome to the Zhu research group
The Ultimate Solar Cell?
Hot-Electron Transfer from Semiconductor Nanocrystals -- Tisdale et al. 328 (5985): 1543 -- Science