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Undoped Si-solar cells yield high efficiency at lower cost

Posted: 29 Jan 2016     Print Version  Bookmark and Share

Keywords:solar cell  EPFL  doping  crystalline silicon  transistor 

An international research team led by Berkeley scientists has developed what they describe as a high-efficiency silicon device in seven steps. They claimed to have simplified the process to create silicon solar cells that convert sunlight to electricity with high efficiency by applying a novel mix of materials to a standard design. The core part of the prototypes have been built at EPFL's Photovoltaics Lab, led by Christophe Ballif, in Neuchatel.

Published in Nature Energy, the work is a result of a collaboration between the Lawrence Berkeley National Laboratory (Berkeley Lab), EPFL, and the Australian National University (ANU).

The special blend of materials that could also prove useful in semiconductor components eliminates the need for a process called "doping," which steers the device's semiconductor properties by introducing foreign atoms, but also makes the device's processing more complicated and causes losses in performance.

"The solar cell industry is driven by the need to reduce costs and increase performance," stated James Bullock, the lead author of the study. Bullock participated in the study as a visiting researcher from the ANU at the U.S. Department of Energy's Berkeley Lab and UC Berkeley, where he will return soon as a postdoc.

"Conventional silicon solar cells use impurity doping, which brings about a number of limitations that make progress increasingly difficult. If you look at the architecture of the solar cell we made, it is very simple. That simplicity can translate to reduced costs."

The majority of today's solar cells use crystalline silicon wafers. The wafer itself, and sometimes layers deposited on the wafer, are doped with atoms that either have electrons to spare when they bond with silicon atoms, or alternatively generate electron deficiencies, so called "holes." In either case, doping enhances electrical conductivity.

These two types of dopant atoms are required at the electrical contacts to regulate the way that electrons and holes travel across a solar cell so that sunlight is efficiently converted to electrical current flowing out of the cell. However, doping itself can degrade its overall performance.

High-efficiency silicon device

A team of researchers has created a high-efficiency silicon device by applying a novel mix of materials to a standard design, eliminates the need for a process called "doping." PVLab-EPFL

Crystalline silicon-based solar cells with doped contacts can exceed 20 per cent power-conversion efficiency, meaning more than 20 per cent of the sun's energy is converted to electricity. Dopant-free silicon cells had never exceeded 14 per cent efficiency before. The study, though, introduces a dopant-free silicon cell, referred to as a dopant-free asymmetric heterocontact (DASH) cell, with an efficiency approaching 20 per cent. The cell is made with novel materials and a simple coating process for layers on the top and bottom of the device. The study also shows that it is possible to create a solar cell in only seven steps.

In this study, the research team used a crystalline silicon core (or wafer) and applied layers of dopant-free amorphous silicon for surface passivation. Then, they applied ultrathin coatings of a material called molybdenum oxide, also known as moly oxide, at the sun-facing side of the solar cell, and lithium fluoride at the bottom surface. The two layers act as dopant-free contacts for holes and electrons, respectively.

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