Ultrahydrophobic 3D/2D fluoroarene bilayer-based water-resistant perovskite solar cells with efficiencies exceeding 22%

Preventing the degradation of metal perovskite solar cells (PSCs) by humid air poses a substantial challenge for their future deployment. We introduce here a two-dimensional (2D) A(2)PbI(4) perovskite layer using pentafluoro-phenylethylammonium (FEA) as a fluoroarene cation inserted between the 3D light-harvesting perovskite film and the hole-transporting material (HTM). The perfluorinated benzene moiety confers an ultrahydrophobic character to the spacer layer, protecting the perovskite light-harvesting material from ambient moisture while mitigating ionic diffusion in the device. Unsealed 3D/2D PSCs retain 90% of their efficiency during photovoltaic operation for 1000 hours in humid air under simulated sunlight. Remarkably, the 2D layer also enhances interfacial hole extraction, suppressing nonradiative carrier recombination and enabling a power conversion efficiency (PCE) > 22%, the highest reported for 3D/2D architectures. Our new approach provides water-and heat-resistant operationally stable PSCs with a record-level PCE.

Molecularly Engineered Hole Transporting Materials for High Performance Perovskite Solar Cells

Kasparas Rakstys

Perovskite solar cells have rapidly revolutionized the photovoltaic research showing an im-pressively dynamic progress on power conversion efficiency from 3.8 to 22% in only several years, a record for a nascent technology. Furthermore, inexpensive precursors and simple fabrication methods of perovskite materials hold a great potential for future low-cost energy generation enabling the global transition to a low-carbon society. The best performing device configuration of perovskite solar cell is composed of an electron transporting material, typi-cally a mesoporous layer of titanium dioxide, which is infiltrated with perovskite material and coated with a hole transporting material. However, although perovskite solar cells have achieved high power conversion efficiency values, there are several challenges limiting the industrial realization of low-cost, stable, and high-efficiency photovoltaic devices. To date, spiro-OMeTAD and PTAA are hole transporting materials of choice in order to main-tain the highest efficiency, however, the prohibitively high price hinders progress towards cheap perovskite solar cell manufacturing and may contribute to more than 30% of the overall module cost. Additionally, such wide bandgap hole transporting materials typically require doping in order to match necessary electrical conductivity and the use of additives is prob-lematic, since hygroscopic nature of doping makes the hole transporting layer highly hydro-philic leading to rapid degradation, negatively influencing the stability of the entire device. In order to overcome these problems, the rational design, synthesis, and characterization of a variety of small molecule-based hole transporting materials have been on a focus of this the-sis. Through judicious molecular engineering four innovative hole transporting materials KR131, KR216, KR374, and DDOF were developed via alternative synthetic schemes with the minimized number of steps and simple workup procedures allowing cost-effective upscale. Employing various characterization methods, the relationship between the molecular struc-ture of the novel hole transporting materials and performance of perovskite solar cells was investigated, leading to a fundamental understanding of the requirements of the hole trans-porting materials and further improvement of the photovoltaic performance. Furthermore, the synthesis of the dopant-free hole transporting materials based on push-pull architecture is presented. Highly ordered characteristic face-on organization of KR321 hole transporting molecules benefits to increased vertical charge carrier transport within a perov-skite solar cell, leading to a power conversion efficiency over 19% with improved durability. The obtained result using pristine hole transporting material is the highest and outperforms most of the other dopant-free hole transporting materials reported to date. Highly hydropho-bic nature of KR321 may serve as a protection of perovskite layer from the moisture and pre-vent the diffusion of external moieties, showing a promising avenue to stabilize perovskite solar cells.

EPFL2018

Compositional Characterization of Organo-Lead tri-Halide Perovskite Solar Cells

Paul Gratia

Perovskite photovoltaics is one of the fastest growing research fields in materials science. Apart from photovoltaics, a wide range of energy-related research fields have gained substan-tial new momentum owing to the fantastic optoelectronic properties of hybrid lead halide per-ovskites. Starting with solar cells showing a mere 3% power conversion efficiency in 2009, researchers have been able to increase this value to over 22% in 2017, comparable to the best monocrystalline silicon solar cells. These improvements are chiefly due to the compositional tuning and mixing of the ABX3 perovskite structure. However, this strategy increases not only the efficiency but also in the same time the complexity of the perovskite formulation. Differ-ent phases, from the nano- to macroscale, can potentially form. In order to rationalize the effi-ciency progress, one needs to understand and characterize the complex perovskite composi-tion and crystal structure in great detail. The main focus of this thesis lies in the in-depth compositional and phase analysis of perovskite thin films as well as full perovskite solar cells, trying to rationalize the consequences of A cation and X anion tuning and mixing. In the first part, it is shown that fabricating solar cells from a non-stoichiometric perovskite precursor composition, namely one with PbI2 excess, leads to increased grain size, enhanced cristallinity, reduced recombination as well as an improved TiO2/perovskite interface. As op-posed to lead iodide phases resulting from film degradation, which is hampering device per-formance, unreacted excess PbI2 phases originating from an excess of PbI2 in the precursor solution are very beneficial for the final solar cell efficiency, reproducibility and stability. The next scientific challenge adressed in this thesis is related to the nano- and microscale composition of the record-breaking mixed cation/mixed anion perovskite composition. Mac-roscale investigations have previously suggested that this formulation results in a single ho-mogenous phase. However, using nanoscale elemental and charge carrier distribution mapping as well as microscale structural and optical film analysis, it is here found that in high efficient solar cells partial phase segregation does take place at the micro- and nanoscale. Moreover, it is shown that mixed cation/anion formulations allow the formation of never re-ported 3D hexagonal lead halide perovskite polytypes, namely 4H and 6H. These polytypes are shown to play a key role during the crystallization process, which is also revealed for the first time here. Indeed, mixed cation/mixed anion perovskite films are fabricated via the crys-tallization sequence 2Hï 4Hï 6Hï 3R(3C). It is demonstrated that the complex crystalliza-tion via these defect-prone hexagonal intermediates can be by-passed by the incorporaton of low amounts of Cs+ cations into the structure. This can explain the improved stability as well as the increased power conversion efficiency and device reproducibility. It gives for the first time a rational explanation as to why the halide perovskite community rapidly adopted the incorporation of Cs+ in mixed perovskites. The last results presented in this thesis are related to the hole-transporting materials (HTM) used in perovskite solar cells. It is shown that carbazole-based small molecule HTMs are a cheap and efficient alternative to the much costlier commercial spiro-OMeTAD.

EPFL2018

Composition and Interface Engineering of Organic-Inorganic Hybrid Perovskites to Improve Photovoltaic Performance and Stability

Kyung Taek Cho

Faced with the growing demands for energy in modern society, organic-inorganic metal halide perovskite materials have recently fascinated the photovoltaics (PV) research community due to a combination of their high quality optoelectronic properties and their ease in the fabrication proceed. As a result, solar cells employing the perovskite materials have been researched exponentially on their way and now the power conversion efficiency (PCE) of perovskite solar cells have been improved over 23% since the first report showing the PCE of 3% in 2009. In this thesis, I investigate compositional modification of perovskite materials and optimization of the charge transporting materials to produce high efficiency, stable and reproducible perovskite solar cells. First of all, after the reasonable performance was achieved, we have innovated a new approach of interface engineering in a perovskite layer to boost efficiency of devices. Engineering a compositional gradient with formamidinium bromide (FABr) at the rear interface between a pristine mixed perovskite ((FAPbI3)0.85(MAPbBr3)0.15) film and a hole transporting material (spiro-OMeTAD) demonstrated that charge collection is improved and charge recombination is reduced at the interface, which leads to a striking enhancement in open-circuit voltage (VOC). This result shed light on the importance of the passivation engineering on the rear surface of perovskite layers. However, beyond the improvement of efficiency, a long-term stability under moisture and continuous illumination is still remained as another challenge for market deployment of the perovskite solar cells. To develop the stability, we have developed the engineering by the surface growth of a two-dimensional (2D) perovskite, the crystal structure of A2BX4, on top of a bulk three-dimensional (3D) perovskite ABX3 film. It is well-known that the 2D perovskite has the superior stability, but suffered because of their low efficiency in the application of photovoltaics. The formation of a distinct 2D perovskite on top of the 3D perovskite (Cs0.1FA0.74MA0.13PbI2.48Br0.39) layer was proved by investigation of structural and optical properties of the stack. This embodying two different type of perovskite layer in one film had never been shown. Finally, this innovative approach led to the PCE of 21% and enhanced stability sustaining 85% of the initial value after 800 hours under full illumination. Thus, my approach of coating 2D perovskite layer make the perovskite solar cells more effective and stable for commercialization. Besides, to avoid the toxic chemical the lead free perovskite (Cs2AgBiBr6) was explored as photovoltaics and the further improvement can be expected. The last results presented in this thesis are related to optimization of the electron transporting layer (ETL) and hole-transporting materials (HTM) for efficient perovskite solar cells. It is shown that the ETL and the HTMs are playing a significant role in realizing efficient and stable perovskite solar cells.

EPFL2018

Molecularly Engineered Hole Transporting Materials for High Performance Perovskite Solar Cells

Kasparas Rakstys

EPFL2018

Compositional Characterization of Organo-Lead tri-Halide Perovskite Solar Cells

Paul Gratia

EPFL2018

Composition and Interface Engineering of Organic-Inorganic Hybrid Perovskites to Improve Photovoltaic Performance and Stability

Kyung Taek Cho

EPFL2018