While the concept of graded solar cells has been known for some time in conventional, vacuum-deposited photovoltaics, it is only recently that scientists have shown that such devices can be made using simple, solution-based deposition techniques. A team led by Jacek Jasieniak has now used a layer-by-layer method to systematically vary the ratio of selenium to tellurium across the thickness of a cadmium-selenium-telluride solar cell. The technique is reported in ACS Nano.

The researchers made their nanostructured devices by depositing a solution containing CdSe nanocrystals around 5 nm in diameter onto a transparent, conducting substrate to produce a thin film. Next, they subjected the film to chemical and thermal treatments to increase the diameters of the crystals to somewhere between 50 and 100 nm. This process results in optical and electrical properties better suited to solar-cell applications, explains team member Brandon MacDonald, and is repeated a number of times until the desired film thickness is obtained.

The solar cell is completed by depositing a thin film of a separate nanocrystal material, zinc oxide, to ensure that the devices have the right electronic structure. A metal electrode – to collect photogenerated electron and holes – made of aluminum is attached to the back of the cell in the final step.

Alloying is key

The key to the process is alloying, explains MacDonald: “In this work, we mixed together two semiconducting materials, CdTe and CdSe, rather than using solutions containing CdTe crystals alone. We found that when thin films of these CdTe:CdSe mixtures are heated to temperatures of around 350 °C, they do not stay as separate materials but instead form an alloy structure CdSexTe1–x.”

The properties of CdSexTe1–x are different to those of either pure CdSe or CdTe. By varying the relative amounts of Se and Te within the alloyed films, the researchers are able to tune these properties over a broad range of light wavelengths. “For instance, we showed that by incorporating the CdSexTe1–x layers into solar cells, we could extend the spectral response of the devices into the infrared part of the electromagnetic spectrum,” said MacDonald. “This is possible thanks to an effect called ‘optical bowing’ in the CdSexTe1–x alloys, whereby the bandgap of the alloyed material is smaller than that in either CdTe or CdSe alone – something that allows for lower energy, infrared photons to be absorbed.” Light absorption in the infrared is important for solar cells since approximately 40% of the Sun’s radiation lies in the infrared region.

The CdSexTe1–x solar cells made by this approach do indeed absorb a portion of these infrared photons and convert them into useful photocurrent.

According to the team, the simple method for making alloyed structures using solution-based methods may also be applied to other materials. The approach could thus not only be useful for making solar cells but other optoelectronic devices too, such as light-emitting diodes and transistors. “It might even be useful for developing ‘tandem solar cells’, with one cell designed to absorb primarily in the visible region and the other to absorb in the infrared,” said MacDonald. “In this way, the entire solar spectrum could be exploited and better power conversion efficiencies obtained.”

The team now plans to further examine the properties of their CdSexTe1–x solar cells in an effort to improve their efficiencies, which lie below those of more conventional devices. “We shall also continue to examine novel material and device structures with the aim of developing next-generation solar cells,” stated MacDonald.