The Yang Yang Laboratory at University of California Los Angeles (UCLA) (click here to see our Yang Yang Laboratory review) again made the headlines with a breakthrough research, this time in the form of a novel approach to produce low cost & high efficiency Perovskite solar cells.
In this research, as explained in their paper “Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process” (click here to read the paper) the Yang Yang Laboratory team focused on perovskite materials due to their tolerance for defects, excellent carrier transport properties, optimum band-gap, and high absorption coefficient. By using a planar structure, the team also benefited from enhanced flexibility.
Until today, the two main methods to produce perovskite thin films in a planar structure were vacuum evaporation & solution based. Although very successful, vacuum evaporation technique, where the two materials (organic & inorganic) are co-evaporated into a uniform perovskite film, required too much energy, making it an unfeasible solution for large scale deployments. The solution based technique, on the other hand, would be formed with a mixture of the two materials (organic & inorganic) and, due to lack of proper solvents to dissolve both & their high reaction rate, results in pinholes. The cells with films created in this fashion are low in quality & performance.
The novel approach the Prof.Yang’s team follows in this research is making use of vapor assisted solution process (VASP), in which the organic & inorganic materials are not co-deposited, unlike the two methods mentioned earlier. Simply put, the inorganic material, PbI2 in this example, is deposited on a substrate and the organic material vapor, CH3NH3I in this example, is added to form the perovskite material. The process is demonstrated in the following figure.
This process is executed at low temperatures which makes it feasible for large scale deployments. The resulting perovskite film has full surface coverage, large grain sizes at micrometer levels and a completely consumed PbI2 with no impurity.
The following picture shows the process with 1 hour & 3 hour annealing time. After 2 hours, the process is complete with large grain sizes.
The current density / voltage graph below shows a world record efficiency from a perovskite solar cell created with CH3NH3PbI3 in a planar structure. At the optimum level; where current density is 19.8 mA/cm2, open circuit voltage is 0.924 V, fill factor is 66.3%; the team achieved power conversion efficiency of 12.1%.
For more information, please visit the Yang Yang Laboratory website by clicking on this link.