Analysis of the Interaction of Halide Perovskite with Other Materials and its Effect on the Performance of Optoelectronic Devices

Halide perovskite (HP) materials have recently attracted worldwide attention due to its fast progress in photovoltaic community. Photoconversion efficiencies (PCEs) of solar cells based on HP have increased quickly from 3.8% in 20091 to over 25%2 in 2019, for single junction architecture exceeding the maximum efficiencies achieved with CdTe (22.1%) and CIGS (22.9%).2 Moreover, HPs have been also applied not only for the preparation of high efficiency solar cells, but also photodetectors,3 light emitting diodes (LEDs),4-7 light amplifier8-9 and lasers.10-13 Such huge promising range of optoelectronic application is due to the outstanding versatility of HP materials. Structurally all components of HPs, with general chemical formula AMX3, can be easily changed/modified for adapting the specific requirement of a concrete optoelectronic application. For an example different dimensionalities of HPs can be obtained by controlling the size of an organic cation A. In addition to this versatility the success of HPs is based in the low non-radiative recombination, even for polycrystalline samples, due to a benign defect physics.14 Furthermore HPs can be prepared from solution methods at low temperatures and, consequently, using low cost fabrication techniques. Solution processes facilitate HPs to combine easily with other materials.

Combinations of materials with different nature has been a successful strategy in order to develop new materials with enhanced properties. As demonstrated in the past, adobe, stained glass and stainless steel are just some testimonies of great combined materials which have been supported for human life since centuries. This strategy is still useful nowadays when the development of chemistry, quantum mechanics and nanotechnology has created a revolution of material science. In the same strategy, the purpose of this thesis is to investigate the interaction of HP with other materials, in order to obtain properties and/or devices with enhanced functionalities and performance by the synergistic combination of different materials. Within the scope of this thesis, we have studied the interaction of methylammonium lead iodide (MAPI) HP with colloidal quantum dots (QDs), organic molecules and electron transport materials (ETMs).

QDs have been chosen for the combination with HP because QDs offer a huge versatility of optoelectronic properties such as tunable band gap, strong emission with highly pure color which are simply tuned by size or shape control due to the confinement effect. And QDs have been potentially applied in different optoelectronic devices such as LEDs15-16 and solar cells.17-18 Among the different semiconductor colloidal QDs, in this thesis we selected PbS QDs to study their interaction with HP because those materials possess a similar crystal structure with a relatively low lattice mismatch.19 The interaction of HP and QDs has been studied in the situation in which QDs were embedded in MAPI perovskite matrix. The presence of both PbS QDs and their capping ligands have a strong impact on the formation of HP. Small density of QDs intermixed in the precursor solution serves as nucleation centers promoting the growth of HP along a preferred direction. As a result, the morphological, optical and structural properties of HP were significantly improved. Consequently the performance of solar cells based on HP with embedded QDs was enhanced. Interestingly the properties and hybrid HP-QD films and those based devices were also dependent on the QD capping ligands. Moreover, the interaction between MAPI HP and QDs also resulted in a new property which is an emission of the exciplex state at energy lower than that of both HP and QDs.

Within the scope of this thesis we also studied the interaction of HP with organic molecules because organic molecules are very flexible materials. They present an unlimited number of structures that can be potentially synthesized and easily modified to meet the requirements of specific applications. In our study, organic molecules were introduced to HP films through the anti-deposition step. We have found that those organic molecules were preferentially located at the grain boundaries of HP. At the grain boundaries the structural disorders potentially can form defect states that can contribute to the degradation of the optoelectronic quality of the HP films. We show that the presence of organic molecules passivated efficiently those grain boundaries. Consequently, we obtained an improvement in performance of three different optoelectronic devices (solar cells, LEDs, and light amplifiers) based on HP with organic additives, comparing with references (without organic molecules).

Electron transport materials (ETMs) are very important in the performance of optoelectronic devices as they decide how efficient electrons can be extracted or injected. Additionally, in n-i-p configuration ETMs also have a crucial role on the formation of HP as it is directly deposited on ETM. It has been previously demonstrated that many factors of ETMs including nature,20 roughness,21 temperature,22 crystallinity23 and structure24 affecting on the formation and thermal stability of HP films. In this thesis, we studied the interaction of HP with spray-pyrolyzed ZnO ETMs. And we found that different termination of ZnO surfaces, obtained by different gas component used during the spray pyrolysis, not only influences the formation of fresh MAPI HP films but also their evolution during storage and finally impacts in the long-term stability of device performance. As a result, our HP solar cells based on ZnO ETMs showed not only a good stability but also an improvement in performance even after more than one month of preparation, under the storage condition of 35% humidity.

Through the study of the interaction of HP with three different materials and in three different situations where the interaction takes place including underneath, inside and up surface of HP layers, we demonstrate that combining HP with other materials of different nature is a promising strategy in order to create new material composites with enhanced properties which strongly influence, in turn, the optoelectronic development.
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