Simulation of Perovskite absorber materials for the application in flexible thin-film devices following HyET's solar cell structure

Abstract

Individual actions cannot solve the current climate crisis. Part of the collective effort is to research solar energy as new ways to produce cleaner, cheaper and more efficient electricity. Among all the current solar cell technologies, a thriving and promising one is perovskite as an absorber material. In the last ten years, the power conversion efficiency evolved with a steep rhythm. In 2019, a maximum of 25.2% in a single junction configuration was reported, while more actual research proves that using them as a tandem can reach up to 30%. Perovskites have the advantage of changing their bandgap by ion-­mixing, from as low as 1.2 eV to even higher than 2.5 eV. Besides, it is flexible, translucent and has similar efficiencies to c­-Si; while being cheaper and easier to produce. It is expected that it will replace c­-Si-­based solar cells in the long run and significantly reduce the price of solar energy.

This project is developed under the Photovoltaic Materials and Devices (PVMD) section of TU Delft, in conjunction with HyET Solar and Flamingo PV. The software ASA and GenPro4 were used to validate single-­junction solar cells and optimize tandem devices. The goal of this thesis is aligned to HyET’s ob­jective: improve its flexible thin-­film solar cell by increasing the power conversion efficiency above 12%.

To carry out the validation and calibration of the single­-junction references, first, a study of all the possible perovskite­-based solar cells was performed. The chosen perovskites have a Methylamonium and Formamidinium combination. The first one has a bandgap of 1.55 eV and works as a high bandgap
absorber material (K0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3), while the low bandgap corresponds to 1.25 eV ((FASnI3)0.6(MAPbI3)0.4) [20]. By adjusting some of the absorber layers’ electrical parame­ters, the references’ electrical properties were matched.

Once validated, the respective p­i­n junctions of the perovskite solar cells were introduced into HyET’s solar cell architecture. By doing this, it is already possible to see the advantages of using per­ovskites as absorber materials: the power conversion efficiency grew from 7.71% to 18% and 19.45% for the high and low bandgap cells. Besides, there is also a better use of the light spectrum. Cali­brating a tandem solar cell is much more complicated. That is why it is done first on the individual single-­junction solar cells, and afterwards, both are stacked together.

The first step to optimize a tandem device is to do an optical simulation (GenPro4). By analyzing the absorptance of the tandem device and the optical current of the top and bottom absorber materials, it is possible to obtain the best combination for the current matching of the tandem solar cell. By modifying
both of the absorber materials’ thicknesses, the optical current mismatch can be reduced. As a result: 300 nm and 600 nm thickness for the a­-Si / LBG perovskite tandem, and 120 nm and 1700 nm for the HBG perovskite / nc-­Si; achieving a mismatch below 0.04 mA/cm2 and 0.2 mA/cm2 correspondingly.

The second step in optimizing a tandem device involves an electrical simulation (ASA). The optimization is related to facilitating tunneling recombination at the n­-p junction by adjusting the doping of these layers. The optimized tandem solar cells have the following electrical properties: 1.56 V, 14.54 mA/cm2, 0.85 and 19.69% for fill factor and PCE for the HBG perovskite / nc-­Si tandem solar cell; while 1.55 V, 11.13 mA/cm2, 0.72 and 12.36% for fill factor and PCE of the other. With these results, it is plausible to confirm that HyET’s objective of having thin-­film flexible solar cells with a PCE above 12% can be met by using perovskite technology in their production line.