Liquid-to-Solid material gives new environmental solar cells

Dyesol Cells, using Grätzel cell technology
Dyesol Cells, using Grätzel cell technology. © Dyesol

Researchers at Northwestern University have developed a way to make solar cells that are inexpensive, got good operating efficiency, and also lasts longer than traditional dye-sensitized solar cells. Based on Grätzel cells, that use a molecular dye to absorb sunlight and convert it to electricity, it doesn’t rely on toxic or scarce materials during manufacturing making it very environmentally friendly.

The problem with Grätzel cells is that they typically don’t last more than 18 months. Grätzel cells use dye-sensitized cell’s electrolyte, made of an organic liquid and intended to mimic how chlorophyll work in plants. This organic liquid can leak and corrode the solar cell itself, making it’s life expectancy very short. Researchers have been searching for an alternative for two decades, and a team at Northwestern University have now found a possible solution using a liquid, that after applied ends up as a solid, preventing leaks and greatly improving the life of the solar cells.

At Northwestern, nanotechnology expert Robert P. H. Chang challenged chemist Mercouri G. Kanatzidis with the problem of the Grätzel cell. Kanatzidis solution was to use a new material for the electrolyte that actually starts as a liquid but ends up a solid mass, creating a solid-state solar cell that is inherently more stable.

“The Grätzel cell is like having the concept for the light bulb but not having the tungsten wire or carbon material”, said Kanatzidis. “We created a robust novel material that makes the Grätzel cell concept work better. Our material is solid, not liquid, so it should not leak or corrode.”

Unlike the Grätzel cell, the new solar cell uses both n-type and p-type semiconductors, and a monolayer dye molecule serving as the junction between the two. Each nearly spherical nanoparticle, made of titanium dioxide, is an n-type semiconductor. Kanatzidis CsSnI3 thin-film material acts as a new kind of soluble p-type semiconductor.

“Our inexpensive solar cell uses nanotechnology to the hilt”, Chang said. “We have millions and millions of nanoparticles, which gives us a huge effective surface area, and we coat all the particles with light-absorbing dye.”

One single solar cell measures half a centimeter by half a centimeter, and is about 10 microns thick. The dye-coated nano particles are packed in, and Kanatzidis new material is poured in when in a liquid form. The material flows around the nano particles, and much like paint, the solvent evaporates. This makes the end result a solid mass, and the sunlight-absorbing dye where photons are converted into electricity, lies right between the two semiconductors.

Chang chose to use nano particles approximately 20 nanometers in diameter. This size optimizes the device, he said, increasing the surface area and allowing enough space between the particles for Kanatzidis material to flow through and set.

Technically, this new cell is not really a Grätzel cell, since the hole-conducting material CsSnI3 is itself light absorbing, and in fact the material absorbs more light over a wider range of the visible spectrum than the typical dye used in a Grätzel cell. In the Kanatzidis-Chang cell, the CsSnI3 plays an additional role in the operation of the cell that is not played by the liquid electrolyte couple, and that role is light absorption.

“This is the first demonstration of an all solid-state dye-sensitized solar cell system that promises to exceed the performance of the Grätzel cell. Our work opens up the possibility of these materials becoming state of the art with much higher efficiencies than we’ve seen so far.This is only the beginning. Our concept is applicable to many types of solar cells.There is a lot of room to grow”, said Chang.

The lightweight thin-film structures are compatible with automated manufacturing, the researchers point out. They next plan to build a large array of the solar cells.

In the Northwestern cell, a thin-film compound made up of cesium, tin and iodine, called CsSnI3, replaces the entire liquid electrolyte of the Grätzel cell. Details of the new solar cell was published online May 23 by the journal Nature.

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