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A Chinese-Canadian research group has used for the first time a cadmium iodide doping technique to stabilize the film coating process in the manufacture of solar cells based on formamidinium lead iodide perovskite (FAPbI3). The academics built a cell that showed a considerable increase in efficiency compared to an identical device without cadmium doping.
An international team of researchers has manufactured for the first time a perovskite solar cell based on a cadmium-doped formamidinium lead iodide (FAPbI3) absorber.
FAPbI3 is one of the most promising perovskite materials for solar cell applications, offering a narrow energetic bandgap and remarkable stability. “Given the advantages of FAPbI3 perovskite solar cells, it is now important to scale them up, a challenge yet to be solved taking into account the problem of FAPbI3 polymorphism and its hypersensitivity to manufacturing conditions,” the researchers highlighted. “So, our hypothesis was that the introduction of a homovalent Pb additive would solve the problem. One of these alternatives is cadmium.”
The use of cadmium in the form of cadmium iodide (CdI2) aims to stabilize the blade coating process at room temperature, which could otherwise cause the aforementioned problem of polymorphism, which depends on environmental conditions and air humidity. atmosphere.
To investigate the possible effects of CdI2 doping, the research group first used the composition graded film (CGF) optimization method, which provides a continuous combinatorial combination of all potential binary alloys. They created a FAPbI3 film with CdI2 deposited at different concentrations throughout the film, from 0% to 5%. Next, using a compact spectrometer with a reflection probe, photoluminescence (PL) spectra were measured.
“The PL intensity increased throughout the film, reaching the maximum intensity at = 3.6 cm, corresponding to a Cd2+ concentration of around 0.6%, and gradually decreasing thereafter,” the academics explained. “The enhancement of PL indicates the reduction of the non-radiative recombination rate in the perovskite film, which is beneficial for the performance of solar cells.”
Next, the scientists coated FAPbI3 films with different levels of CdI2 on indium tin oxide (ITO)/glass substrates. Using scanning electron microscopy (SEM) images, they found that at 0.6% cadmium, the size and density of the grains reached full coverage, an important attribute for solar cell films.
“We also evaluate the stability of the films,” the group noted. “After aging in ambient air for 30 days at 35% relative humidity (RH), the Cd-free FAPbI3 film predominantly showed the ?? phase, while the optimal Cd-doped samples showed no signs of degradation. Thus, we concluded that Cd doping effectively released the structural stress of the FAPbI3 perovskite and stabilized its desired ??” polymorph.
The team designed the cell with a glass and ITO substrate, a tin oxide (SnO2) electron transport layer (ETL), the cadmium-doped FAPbI3 absorber, a Spiro-OMeTAD-based hole transport layer, and a gold (Au) metallic contact.
Tested under standard lighting conditions, the device achieved an efficiency of 22.7%, an open circuit voltage of 1.10 V, a short circuit current density of 25.9 mA/cm2, and a fill factor of 79. 6%. A reference cell without cadmium doping achieved an efficiency of 20.8%, an open circuit voltage of 1.05 V, a short circuit current density of 25.2 mA/cm2 and a fill factor of 78.5 %.
“The control FAPbI3 showed poor reproducibility due to the presence of pinholes, while more than 80% of the target Cd-FAPbI3 showed an efficiency greater than 20%. Solar cells of 0.9 cm2 active area with and without Cd showed a champion efficiency of 16.41% and 13.90%, respectively,” the scientists stated.
Using transient absorption microscopy (TAM), the researchers also discovered that the target sample has less charge trapping and a longer shelf life. “The trap states of the target sample fill more easily, leading to greater charge accumulation and higher VOCs.”
The novel manufacturing process was presented in the study “ Cadmium-Doping Slows Trap Emptying in Ambient-Air Blade-Coated Formamidinium Lead Iodide Perovskite Solar Cells ” and formamidinium coated with sheets of ambient air), published in Advanced Energy Materials . The research group consisted of scientists from the Canadian University of Victoria, the University of British Columbia and the Chinese University of Henan. |
