KEY CONCEPTS
• Addition of bromine and indium to cesium lead iodide produced a more stable all-inorganic perovskite.
• Substituting bromine for iodine reduced the number of defects in the perovskite while the use of indium reduced the bandgap to the desired value.
• The new perovskite displays much higher humidity resistance and can be produced without the need of special conditions.

The effort to make solar energy commercially viable has led researchers to focus their efforts on a class of materials known as perovskites, which have readily identifiable crystal structures that contain two cations and one anion. When exposed to sunlight, negatively charged electrons and positively charged holes are produced that diffuse through the perovskite until they are separately collected at the anode and cathode to create an electric current. 

Solar cell efficiency can be increased if the distance that an electron or a hole travels from the point of generation until the two species recombine is increased. This distance is known as the diffusion length. In a previous TLT article (1), researchers used a new analytical technique known as scanning photocurrent microscopy to measure diffusion length in a perovskite solar cell. They found that the diffusion length ranged from 10 to 20 microns. 

The perovskite used in this study was an organometal halide perovskite based on methyl ammonium lead iodide. This type of perovskite is known as a hybrid organic-inorganic halide that have been widely evaluated. Researchers have found they can produce power conversion efficiencies greater than 25%. 

Dr. Jia Liang, postdoctoral researcher at Rice University in Houston, Texas, says, “Hybrid organic-inorganic halide perovskites are quite effective but do not use the photochemical and thermal stability to be commercially viable.”

Concern about combining organic and inorganic components led researchers to replace the organic moiety with an inorganic cation such as cesium. Cesium lead halides based on iodine, bromine or a mixture of the two also show promise in solar cells. Liang says, “All-inorganic perovskites do provide better stability but unfortunately the best candidate, cesium lead iodide itself, does not have a stable crystal phase and quickly degrades to a yellow non-perovskite phase at room temperature.”

The other problem with cesium lead iodide is the presence of defects in the crystal lattice. Liang says, “Defects are a problem because they can reduce the diffusion distance leading to a decrease in voltage and a reduction in solar cell efficiency.”

While all-inorganic perovskites have limitations, they appear to be easier to correct than hybrid organic-inorganic halides. New research has now been published that discusses the synthesis of an all inorganic perovskite with superior stability that also minimizes defects.

Addition of bromine and indium
Liang, Jun Lou, professor of material science and nanoengineering at Rice University, and their colleagues produced a more stable all-inorganic perovskite that contains a minimal number of defects in its crystal structure. This perovskite involves adding bromine and indium to cesium lead iodide and has the structure, CsPbI₃:Br:InI₃.

Liang says, “Bromine was added as a replacement for iodine because cesium lead bromide exhibits better stability. Indium was added because this metal has been used successfully in other perovskite salts and is a non-toxic alternative to lead.”

While the substitution of bromine for iodine led to a reduction in defects, the process also led to an undesirable increase in the bandgap. Utilization of indium iodide for lead iodide not only reduced lead content but also reduced the bandgap to the desired value realized by cesium lead iodide. 

Lou indicated that the researchers estimate indium replaced approximately 15% of the lead. He says, “We know from precursor ratios that the amount of lead used is reduced by 15%. But use of X-ray photoelectron spectroscopy and time-of-flight secondary mass spectrometry did not provide us with a clear understanding of the composition of the new perovskite. These techniques show a nonuniform distribution of indium and lead atoms through the new perovskite’s crystal structure.”

This new perovskite also achieves another objective by decreasing the amount of lead in the perovskite to produce a more environmentally friendly material. The improved stability enabled the researchers to synthesize the new perovskite using simple process techniques. Liang says, “We were able to use a basic solution-phase process that involved dissolving the raw materials in dimethylformamide. The resulting product was dried before use. Due to its stability, no special conditions such as the use of a glove box were required.”

The new perovskite displays much higher humidity resistance than is found with cesium lead iodide. Uniform films can be prepared in environments with greater than 90% relative humidity without showing any evidence of decomposition. 

The researchers fabricated a solar cell by incorporating films of the various components into the perovskite. Figure 3 shows a schematic of a cross-section of the solar cell, which is organized into five layers. Starting at the top with a carbon electrode, then the perovskite, titanium dioxide, fluorine-doped tin oxide and glass. The thickness of the perovskite layer is approximately 350 nanometers.


Figure 3. This schematic shows a cross section of a solar cell that contains an all-inorganic perovskite with greater stability needed for use commercially. (Figure courtesy of Rice University.)

The new perovskite generated a power conversion efficiency above 12% and an open circuit voltage of 1.20 V. Lou says, “In the future, we will try to gain a better understanding of the composition of the new perovskite in order to determine how to boost its power conversion efficiency above 20%. We also intend to adjust the concentration of bromine in an effort to further improve the stability of the perovskite.”

Additional information on this research can be found in a recent article (2) or by contacting Lou at jlou@rice.edu. 

REFERENCES
1. Canter, N. (2017), “Unraveling the effectiveness of perovskite solar cells,” TLT, 73 (4), pp. 12-13.
2. Liang, J., Han, X., Yang, J., Zhang, B., Fang, Q., Zhang, J., Ai, Q., Ogle, M., Terlier, T., Marti, A. and Lou, J. (2019), “Defect-Engineering-Enabled High-Efficiency All-Inorganic Perovskite Solar Cells,” Advanced Materials, 31 (51) 1903448.