Unraveling the effectiveness of perovskite solar cells

Dr. Neil Canter, Contributing Editor | TLT Tech Beat April 2017

Diffusion length, a measure of solar cell efficiency, is determined by a new analytical approach. 

KEY CONCEPTS
• Perovskite solar cells show potential for converting sunlight into electricity.
• To further improve their efficiency, a technique known as Scanning Photocurrent Microscopy has been used to measure the diffusion length.
• The diffusion length of electrons and holes is between 10 and 20 microns.

PEROVSKITE SOLAR CELLS HAVE BECOME the focus of research because they have demonstrated the ability to efficiently convert sunlight to electricity. Efficiencies for this type of solar cell have increased to about 22% in only a few years. 

Xuan Gao, associate professor of physics at Case Western Reserve University in Cleveland, says, “Perovskites work in a similar manner to conventional semiconductor solar cells by creating negatively charged electrons and positively charged holes when exposed to sunlight. However, conventional semiconductor solar cell devices have a built-in electric field junction to separate these two species enabling an electric current to be generated. In perovskite solar cells, electrons and holes diffuse around in the perovskite. When they reach the anode and cathode, they are separately collected and a current is generated.”

The use of a perovskite solar cell is described in a previous TLT article that discusses the recharging of lithium-ion batteries with solar energy (1). The researchers installed four perovskite solar cells in series with a lithium-ion battery and determined good photoelectric conversion, good cycling stability and durability when the battery is illuminated with solar light for nearly 18 hours. The purpose of this approach is to develop an alternative way to recharge lithium-ion batteries due to the limited number of charging stations currently available. 

One other advantage in using perovskite solar cells is that they can easily be processed. The most attractive perovskites currently in use are organometal halides. Gao says, “Perovskite solar cells are prepared by reacting the raw materials in an aqueous solution and then dropping the finished material on a glass substrate or silicon wafer. After a thin coating is applied, it is baked at elevated temperature to remove the water. This is an inexpensive, simple approach that can be contrasted with the complexity of preparing such films through the use of deposition techniques under high vacuum conditions.”

The key issue in the solar energy field is how to extend the distance that an electron or a hole travels from the point of generation until the two species recombine or the electrons are extracted as electric field. This distance is known as the diffusion length. One other factor impacting the diffusion length is the presence of defects in the crystalline perovskites that can trap both electrons and holes.

A key element in this process is to find an analytical approach to measure the diffusion length. Gao says, “Previous experiments run measured the diffusion length in an indirect manner most of the time. Most of this work was conducted using photoluminescence-based optical methods that could determine that the diffusion length was greater than a distance of 0.5 micron but could not definitively determine the actual distance traveled by the electron.”

A new approach is needed to measure the diffusion length in order to assist researchers with improving the efficiency of perovskite solar cells. Such an approach has now been developed.

SCANNING PHOTOCURRENT MICROSCOPY 
Gao and his colleagues have used a technique known as Scanning Photocurrent Microscopy (SPCM) to accurately measure the diffusion length in an organometal halide perovskite solar cell based on methyl ammonium lead tri-iodide. 

SPCM involves the application of laser light from a halogen-white light source (633 nanometer wavelength) onto a solar cell prepared by depositing a 300-nanometer thick film of an organometal halide perovskite onto the silicon oxide surface of a silicon wafer. A 1 micron thick layer of parylene is added to encapsulate the perovskite and prevent it from degrading. 

Figure 2 shows the experimental setup. Measurements were taken by focusing tiny laser spots on the perovskite film. Gold or nickel electrodes were positioned approximately 120 microns away from each other and the perovskite film was scanned along two perpendicular directions to measure the diffusion length.


Figure 2. The experimental setup shown was used to determine the diffusion length of electrons and holes in an organometal halide perovskite film on the silicon oxide surface of a silicon wafer. (Figure courtesy of Case Western Reserve University.)

Gao says, “We used SPCM to determine that the diffusion length of electrons and holes is between 10 and 20 microns. This is significant because the diffusion length is greater than the grain sizes in the perovskite crystals. The impact of grain boundaries on electron and hole diffusion has received broad interest, and some past work found negative effect from grain boundaries, but we did not see this in our experiments.”

In preparing the perovskite crystal, the researchers included chloride in the form of lead chloride. Gao says, “Past testing by my colleague Clemens Burda, chemical professor of the College of Arts and Sciences at Case Western Reserve University, found that chloride improved the crystallinity of the organometal halide perovskite.”

One of the reasons that the researchers were able to determine and obtain the long diffusion length was the high quality of the crystal. Gao says, “We developed crystalline films that were oriented along the (110) plane. The success of this work means that perovskite solar cells can be made thicker leading to the absorbance of more light and the potential for generation of higher levels of electricity.”

In the future, Gao would like to evaluate thicker films to determine how much they can improve solar cell efficiency. He says, “We also would like to understand the mechanism for how electrons are formed and what role this plays in the conversion of solar energy into electricity. One additional aspect that we will be studying is whether the spin properties of an electron have any influence on diffusion length.”

Additional information on this research can be found in a recent article (2) or by contacting Gao at xuan.gao@case.edu. 

REFERENCES
1. Canter, N. (2015), “Lithium-ion batteries: Recharge with solar energy,” TLT, 71 (11), pp. 14-15.
2. Liu, S., Wang, L., Lin, W., Sucharitakul, S., Burda, C. and Gao, X. (2016), “Imaging the long transport lengths of photo-generated carriers in oriented perovskite films, “Nano Letters,” 16 (12), pp. 7925-7929.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.