Energy Loss In CdTe Solar Cells

The theoretical maximum efficiency of a CdTe cell is 30%, but has yet to be achieved. In fact, the band gap of a CdTe semiconductor is between 1.4-1.5 eV, yet the average cell outputs only 0.85 V; this is only a little more than half the expected electrical output of a CdTe cell. The reason for this? Energy loss. A lot of it.

Energy loss occurs when minority carriers recombine with a semiconductor before reaching an electrical contact on the side of a solar cell (by side I mean the broad front and back, not the thin edges of a cell). When an excited electron fails to make it to a contact, it is unable to pass on its excess energy, so the energy it absorbed from a photon is effectively wasted. There are many reasons why a semiconductor would have a low carrier lifetime and contain minority carriers that fail to reach an electrical contact, but they all revolve around one concept: defects.

A defect in a crystal is an aberration of the crystal structure. If a semiconductor has a defect, one or more of the atoms in its crystal are out of place, making it different from its natural state. Some defects are created on purpose; in the case of solar cells, this is usually done through doping—the process of injecting trace amounts of certain elements into materials—to increase the number of possible minority carriers and consequently heighten electricity generation. Most defects, however, are not engineered by humans and considered undesirable. It is these undesirable defects that cause CdTe cells to have low carrier lifetimes and high levels of energy loss.

Vocab

Electronvolt - eV

A unit of measurement that quantifies the energy gained by an electron.

Volt- V

The unit used to measure voltage that represents electrical potential.

Minority Carrier

An electron that has received energy from a photon, entering an excited state and becoming free from the shell of its atom.

Carrier Lifetime

The amount of time a minority carrier has to reach an electrical contact before it recombines with the semiconductor material it came from.

Majority Carrier

A hole, or absence of an electron, in the shell of an atom that is created when an electron becomes excited and turns into a minority carrier.

Undesirable Defects

An illustrated representation of point defects in a crystalline structure.

Image: Physics Stack Exchange

Let’s explore some of the most common types of undesirable defects that occur in semiconductor crystals like CdTe. One overarching type of defect is known as a point defect. Point defects occur when there is one atom out of place in a crystal lattice - the defect is, just as the name implies, at a singular point. Within this defect classification are various subsets.

An interstitial point defect is when an extra atom is shoved into the crystal lattice where it doesn’t belong.
An extrinsic, or substitutional, point defect, is when an atom of an incorrect element is found in place of a normal atom. For example, a Selenium atom could be taking up the spot in a CdTe crystal where a Tellurium atom is supposed to be.
A vacancy is when an atom is simply missing from it’s natural place in a crystal.

Another overarching type of defect is an extended defect. These occur when multiple atoms in a crystal are out place, and cause a large disruption in the crystal structure of a material.

So why do unwanted defects in a semiconductor’s crystal structure lower the carrier lifetime of a material like CdTe? Put simply, minority carriers try to follow a path along the semiconductor to reach electrical contacts. Any unwanted defects in the crystal serve as roadblocks that prevent excited electrons from reaching contacts before their time is up, their extra energy subsides, and they have to recombine with the semiconductor material they were freed from. The more unwanted defects there are in a semiconductor, the more obstacles there will be to hinder the process of minority carriers, and the more energy will be wasted.

Since unwanted defects are the main cause of energy loss in a CdTe cell, you'd think the main priority of CdTe solar cell manufacturers would be to reduce them. This isn't the case. The main priority of CdTe cell manufacturers is actually reducing costs to make them market-competitive with Silicon cells. CdTe devices are consequently created in a fast, cheap manner that results in a high number of unwanted defects. In a way, the high levels of energy loss and resulting low efficiency of CdTe cells is the price to pay for making them immediately commercially viable.

Some manufacturers try to combat unwanted defects by performing changes to CdTe crystals like Cadmium chloride (CdCl2) treatment. This treatment eliminates extended defects well, as they are large and consequently easier to identify and alter, but struggles to deal with point defects. Processes such as CdCl treatment are still expensive, however, and are often avoided to reduce manufacturing costs. If manufacturers wanted to raise the efficiency of CdTe cells they could change the manufacturing process to be longer and more thorough like that of III-V solar cells, but this would raise costs and lower the production rate of CdTe devices tremendously. So far, such an approach has not been favored by the CdTe industry.

A graph comparing the electrical output of normal CdTe solar cells with CdTe cells that have received CdCl2 treatment.

Image: Semantic Scholar

References

Munshi, Amit et al. “Effect of CdCl2 passivation treatment on microstructure and performance of CdSeTe/CdTe thin-film photovoltaic devices.” Solar Energy Materials and Solar Cells (2018).

Special thanks to the Cadmium Telluride Solar Cells Lab at NREL