UC San Diego scientists devised a temporary salt-melting technique to heal defects in solar cell structure during a laboratory production process.
Crystal formation occurs through the mixing of zinc bromide and methylammonium chloride in a low-temperature liquid state.
The temporary liquid state eliminates the tiny energy traps that show up within the semiconductor material.
Test solar cells demonstrated an efficiency rate of 24 to 26 percent energy conversion efficiency.
Liquid eutectic solutions that fill microscopic crystal interface boundaries
Halide perovskite films formed through the salt solution process feature small crystal grains in the glass substrates.
The area between two crystals has structural defects that inhibit the flow of electrical charges.
Zinc bromide was mixed with methylammonium chloride in a liquid precursor to help in the formation of solar cells.
The heating of the mixture resulted in a temporary molten salt phase that surrounded individual crystal boundaries.
Covering the crystals with molten salt allowed misplaced atoms to move around and create perfect crystalline structures.
As a result of the liquid boundary, crystals developed perfect interfaces during the crystallization stage.
Evaporating salt additives make crystal boundaries permanent
The application of heat led to the continuous evaporation of volatile methylammonium chloride salt from the baking membrane.
This process resulted in instantaneous transformation of the surrounding molten material into an organized crystal form.
Zinc ions were left concentrated at the crystal edges and occupied the empty electronic traps within grain interfaces.
X-ray imaging recorded the instant the boundaries transformed from ordered solids into liquid phases and back again.
The simultaneous processes of melting and evaporation occurred at a constant temperature during the entire process of heating.
Eliminating flaws at the grain boundary level means continuous pathways can be created that transport photo-generated electrical current straight to collection contacts.
New levels of performance: Breaking conversion efficiency records in the lab
Electrons captured in unpassivated perovskite films generate heat from sunlight instead of producing electricity.
The liquid eutectic process ensured that grain boundary traps did not cause premature charge recombination in the active photovoltaic material.
In the lab tests, it was found that the power conversion efficiency was in the range of 24 to 26 percent under standard simulated sunlight.
No defects were detected.
Treated perovskite films showed consistent chemical behavior in line with laboratory performance expectations.
Blocking electrical conduction through grain boundaries boosts the voltage produced by sunlight.
Maintenance of power generation during various light tests
Untreated perovskite solar cells are degraded very quickly because the grain boundaries absorb moisture from the surrounding environment.
Defects present in microscopic traps cause faster chemical decomposition, and hence it is not possible to sustain the efficiency of the solar cells.
In the case of liquid-grown perovskite films, the dense packing of crystals helped prevent the breakdown of internal semiconductor layers.
Dense packing of crystals helped inhibit the chemical process when exposed to extended lighting, according to the University of California San Diego.
The small solar cells maintained the same power output that they had when they were initially generated despite being exposed to simulated sunlight for eight weeks.
According to Princeton University, prevention of structural degradation indicates that the temporary liquid processing method solves perovskite operation stability problems.
Liquid salt synthesis for commercial use in panel production
The laboratory experiments confirm that the small-scale liquid salt treatment can be used on samples.
Uniformity is required for big pieces of glass for industrial manufacturing.
Chemical engineers need to monitor the precise speed of evaporation to prevent crystal imperfections.
Non-uniform salt distribution while heating may result in some performance decline along the edges of industrial panels.
Large-scale manufacturing trials are currently investigating ways to apply liquid salt technology in automated production equipment.
Perfecting large-scale liquid crystal techniques will determine whether high-performance perovskite solar modules are adopted into the market.
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