Development of Low Band Gap and High Band Gap Perovskite Materials for 2t Monolithic Tandem Solar Devices

Hybrid organic-inorganic lead halide perovskite solar cells (PSCs) have gained a remarkable popularity over the last decade. This technology showed a potential for commercialization by promising highly-efficient
devices with lower costs and shorter energy-payback times. Especially, all-perovskite multi-junction solar cells
are considered as a unique candidate to provide solutions for sustainable energy conversion according to the
calculated power conversion efficiency (PCE) values of over 40% and very low additional cost contribution
over single-junction devices. In order to increase light harvesting over the whole visible spectrum, wide
bandgap (~ 1.7-1.9 eV) and low-bandgap (~ 1.1-1.3 eV) sub-cells are employed together as top and bottom
cells, respectively, in all-perovskite tandem devices. However, the current PSC technology still encounter
challenges that needs to be addressed in order to obtain stable and photo-electrochemically superb wide
bandgap and low-bandgap perovskite thin films.
In this project, the applicant aims to develop high-band gap (Eg>1.70 eV) and low band gap (Eg<1.3 eV)
perovskite thin films for tandem devices. By changing the perovskite composition (anion, cation and metal
halide ratios), the lattice structure will be modified and desired band-gap values will be obtained. In order to
attain high-quality, single-phase and stable thin films of the perovskites, novel multi-step evaporation methods
will be developed. Several interface functionalization and surface/defect passivation techniques with various
additives/solvents will be tested by vapor processing methods.
The valence bands of the perovskite materials are expected to shift as a result of band-gap changes.
Therefore, the selection of the energetically proper hole transport materials (HTMs) becomes important.
During the project the driving force to extract holes across perovskite/HTM interface will be determined by the
choice of copper-complex HTMs. It is known that, with the copper-complexes, different ligand substitutions to
the copper metal center enable the tuning of the HOMO levels and charge transfer kinetics. The applicant has
an extensive experience in the development of copper redox mediators for liquid-state and quasi solid-state
dye-sensitized solar cells and she will use a previously developed copper-complex library or propose new
copper-complexes with alternative ligands in order to optimize the energy alignment across Perovskite/HTM
interface. (She also has preliminary results and basic supporting data for the proposed project). Most
particularly, with an innovative approach, the copper-complex films will be obtained by novel evaporation
techniques and the film morphology will be controlled by deposition rates, precursor types and process
temperature and new procedures will be developed. It is anticipated that the use of copper complexes as
HTMs in PSCs will be advantageous considering their simpler and cheaper synthetic procedures and tunable
properties. For the developed perovskite films and the copper-based HTMs, the analysis will be made by
using many experimental techniques such as photoluminescence, XRD, SEM, Photoemission spectroscopy,
UV-vis absorption spectroscopy, and NMR.
The applicant will use the developed perovskite materials and copper complex HTMs in order to fabricate
single junction PSCs and 2-terminal monolithic tandem devices. Firstly, the single junction devices will be
fabricated as top cells and bottom cells, and will be optimized for current matching. Later, the tandem device
architecture will be developed with the obtained bottom-cell and top-cell structures. The photovoltaic
performance and the stabilities of the devices will be tested according to the existing protocols.