In a perovskite with the following architecture, how much of an impact does the mobility of the hole transporting material, have on the efficiency of the device? Furthermore is the efficiency of a perovskite device dependent on the mobility of the hole transporting material?
An explanation or equation of the contributions/factors to the overall device efficiency would also be greatly appreciated.
Dear James,
The hole mobility of the HTL has appreciable effect on the performance of the peroveskite solar cells. The basic role of this layer is to extract the holes from the excitons reaching the interface between the perovskite and the HTL while rejecting the electrons backward to the ETL.This function depend on the energy level alignments.
It is so that the HTL must be good hole conductor such that it dispatches swiftly the extracted holes. Therefore, to perform its hole collecting function well. it must not allow the pile up of holes in it near the interface. If this occurs because of too low hole mobility the free holes will recombine with the free electrons on both sides of the interface leading to a decrease in the collected photo current and hence decrease in the efficiency.
Another important effect also comes in the play when the mobility is low. It is so that the resistance of this layer increases causing a larger voltage drop for the same cell current. That is the series resistance of the cell increases leading to a decrease in the fill factor and consequently the conversion efficiency.
So the effect of the mobility is two fold, in one side it causes loss in the photocurrent and on the other side decreases the fill factor.
wish you ssuccess
In addition, although the HTL layer contributes to the cell's performance, there are some studies on HTL-free Perovskite solar cells, such as this one:
http://chem.ch.huji.ac.il/etgar/Publications.html
They demonstrate relatively nice efficiencies, with good Voc. The current are lower, of course, due to the reasons that were elaborated above.
HTL's mobility should always be more or equal to that of the perovskite absorber for good collection efficiency or it may recombine because of high resistance which is highly undesirable.
Also HTL material should have very low surface roughness when made into a thin film that is it should be pinhole free to avoid the short from electrodes to the absorber. This effects its stability.
Other details about the bandgap of HTL's have been clearly explained by Mr. Zekry.
I appreciate everyone answers. They have helped me better understand the mechanistic role of the HTL in the device. However, is there any quantitative relationship that is available between this mobility and the efficiency?
I am trying to use the principal that mobility limits efficiency to create a calibration curve of the efficiency of a device to the mobility of the HTM of hole materials with similar HOMO/LUMO levels. For this to work there must be a direct correlation between the mobility of the HTM and overall efficiency, and I would like to understand how these values are correlated.
I would believe that Efficiency - mobility relationship might be not as direct as you think. As Abdelhalim said it can lead to reduced FF and/or Jsc if not chosen correctly. Further the work function also is very important as it creates the built in electric field for the device and hence its Voc. To gain considerable insight modeling might be required that would include surface recombination rate and resistance at the interface.
You may also need to consider the carrier concentration (doping) as well as the electric field given. The structural quality also contribute to the hole mobility and the efficiency factor.
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