Atomic-level passivation mechanism of ammonium salts enabling highly efficient perovskite solar cells

The high conversion efficiency has made metal halide perovskite solar cells a real break-through in thin film photovoltaic technology in recent years. Here, we introduce a straight-forward strategy to reduce the level of electronic defects present at the interface between the perovskite film and the hole transport layer by treating the perovskite surface with different types of ammonium salts, namely ethylammonium, imidazolium and guanidinium iodide. We use a triple cation perovskite formulation containing primarily formamidinium and small amounts of cesium and methylammonium. We find that this treatment boosts the power conversion efficiency from 20.5% for the control to 22.3%, 22.1%, and 21.0% for the devices treated with ethylammonium, imidazolium and guanidinium iodide, respectively. Best performing devices showed a loss in efficiency of only 5% under full sunlight intensity with maximum power tracking for 550 h. We apply 2D-solid-state NMR to unravel the atomic-level mechanism of this passivation effect.

Compositional and interfacial optimization for enhancement of perovskite solar cells performance

Essa Awadh R Alharbi

Non-renewable sources are responsible for most of the overall greenhouse gas emissions, the development of sustainable energy sources is of prime importance to mitigate the negative effects of climate change. Among renewable sources, solar energy is one of the most abundant forms of energy available. Perovskite photovoltaics has emerged as one of the promising candidates for harvesting solar energy. However, these perovskite solar cells suffer from instability which is primarily due to the bulk and interface defects. In this thesis, we focus on the passivation of the bulk and interface defects to enable high-performance and stable devices. At the beginning of the thesis, we engineered the composition of mixed-cation and mixed-halide perovskite films by judicious incorporation of guanidinium iodide to suppress the parasitic charge carrier recombination, which enabled the fabrication of >20% efficient and operationally stable PSCs yielding reproducible photovoltage as high as 1.20 V. Thereafter, in our next project, we develop a facile strategy to reduce the level of electronic defects present at the interface between the perovskite film and the hole transport layer by treating the perovskite surface with different types of ammonium salts, namely ethylammonium, imidazolium, and guanidinium iodide. We find that this treatment boosts the power conversion efficiency from 20.5% for the control to 22.3%, 22.1%, and 21.0% for the devices treated with ethylammonium, imidazolium, and guanidinium iodide, respectively. Best performing devices showed a loss in efficiency of only 5% under full sunlight intensity with maximum power tracking for 550 h. We employed 2D-solid-state NMR (1H-1H correlation) spectroscopy to unravel the atomic-level mechanism of this passivation effect. Following that, we applied a facile and very effective method that employs methylammonium triiodide (MAI3) for bulk passivation together with interface passivation developed in the second project for holistic defect mitigation in perovskite solar cells. As a result, the champion device shows a power conversion efficiency (PCE) of 23.46% with a high fill factor (FF) of over 80%. Furthermore, these devices exhibit enhanced operational stability, with the best device retaining ~ 90% of its initial PCE under 1 Sun illumination with maximum power point tracking for 350 h. Finally, in our last project, we reconfigured the design of the alkylammonium halide salts to create a holistic bulk and interface engineering. We show that dimethylammonium head along with octyl chain and iodide counter anion works best in the bulk whereas dimethylammonium head along with octyl chain and fluoride counter anion works best at the interface. With a synergistic combination of both bulk and surface passivation, we achieved a high PCE of 24.91% along with exceptional long-term operational stability. In summary, in this thesis we employed different strategies based on organic ammonium salts to engineer the bulk and interface of the PSCs. We did an in-depth investigation of the optoelectronic properties, local structure, and molecular interactions with a combination of characterization at different length and time scales. Our results show that it is imperative to passivate both bulk and interface defects for obtaining high efficiency and stable solar cells. From our work, one can take inspiration from the design and analysis of new materials for the passivation of the PSCs.

EPFL2022

Formation of High-Performance Multi-Cation Halide Perovskites Photovoltaics by delta-CsPbI3/delta-RbPbI3 Seed-Assisted Heterogeneous Nucleation

Essa Awadh R Alharbi, Masaud Hassan S Almalki, Thomas Paul Baumeler, Felix Thomas Eickemeyer, Ulf Anders Hagfeldt, Anurag Krishna, Mounir Driss Mensi, Olivier Ouellette, Linfeng Pan, Zaiwei Wang, Bowen Yang, Shaik Mohammed Zakeeruddin

The performance of perovskite solar cells is highly dependent on the fabrication method; thus, controlling the growth mechanism of perovskite crystals is a promising way towards increasing their efficiency and stability. Herein, a multi-cation halide composition of perovskite solar cells is engineered via the two-step sequential deposition method. Strikingly, it is found that adding mixtures of 1D polymorphs of orthorhombic delta-RbPbI3 and delta-CsPbI3 to the PbI2 precursor solution induces the formation of porous mesostructured hexagonal films. This porosity greatly facilitates the heterogeneous nucleation and the penetration of FA (formamidinium)/MA (methylammonium) cations within the PbI2 film. Thus, the subsequent conversion of PbI2 into the desired multication cubic alpha-structure by exposing it to a solution of formamidinium methylammonium halides is greatly enhanced. During the conversion step, the delta-CsPbI3 also is fully integrated into the 3D mixed cation perovskite lattice, which exhibits high crystallinity and superior optoelectronic properties. The champion device shows a power conversion efficiency (PCE) over 22%. Furthermore, these devices exhibit enhanced operational stability, with the best device retaining more than 90% of its initial value of PCE under 1 Sun illumination with maximum power point tracking for 400 h.

WILEY-V C H VERLAG GMBH2021

Strategies to Optimizing Dye-Sensitized Solar Cells

Sophie Wenger

Dye-sensitized solar cells (DSCs) constitute a novel class of hybrid organic-inorganic solar cells. At the heart of the device is a mesoporous film of titanium dioxide (TiO2) nanoparticles, which are coated with a monolayer of dye sensitive to the visible region of the solar spectrum. The role of the dye is similar to the role of chlorophyll in plants; it harvests solar light and transfers the energy via electron transfer to a suitable material (here TiO2) to produce electricity — as opposed to chemical energy in plants. DSCs are fabricated of abundant and cheap materials using inexpensive processes (e.g. screen-printing) and are likely to be a significant contributor to the future commercial photovoltaic technology portfolio. The work conducted during this thesis aimed at optimizing the DSC using three different strategies: The use of versatile organic sensitizers for stable and efficient DSCs, the study of tandem device architectures in combination with other solar cells to harvest a larger fraction of the solar spectrum, and the development of a validated optoelectric model of the DSC. Organic donor-π-acceptor dyes are an interesting alternative to the standard metal-organic complexes used in DSCs. Efficient photovoltaic conversion and stable performance could be demonstrated with three classes of donor systems, namely diphenylamine, difluorenylaminophenyl, and π-extended tetrathiafulvalene. The highest conversion efficiencies were obtained with a difluorenylaminophenyl donor system (η = 8.3 % with a volatile electrolyte and η = 7.6 % with a solvent-free ionic liquid, which was a new record for organic dyes at the time of publication). Surprisingly, efficient regeneration of the oxidized dye by the I-/I3- redox mediator was found with the π-extended tetrathiafulvalene system, even though the thermodynamic driving force was as low as 150 mV. So far driving forces of 300-500 mV had been regarded as necessary for efficient regeneration of the dye cation. Also, important structure-property relationships pertaining to the recombination of electrons with the electrolyte and to the stability of the device could be identified (i.e. effect of linear vs. branched structure, linker length, and moieties used). The power conversion efficiency of solar cells can be extended beyond the limit for a single cell (∼ 30 %) by using multiple cells with different optical gaps in a tandem device. DSCs and chalcopyrite Cu(In,Ga)Se2 (CIGS) solar cells have complementary optical gaps and are thus suitable systems for integration in a tandem device. It was shown that a monolithic DSC/CIGS tandem device has the potential for increased efficiency over a mechanically stacked device due to increased light transmission to the bottom cell, and a monolithic DSC/CIGS device with an initial efficiency of η = 12.2 % was demonstrated. The degradation of the devices — induced by the corrosion of the CIGS cell in contact with the I-/I3- redox mediator — could be retarded with a protective thin conformal ZnO/TiO2 oxide layer coated on the CIGS cell by atomic layer deposition. Finally, an experimentally validated optical and electrical model of the DSC has been developed to assist the optimization process, which is predominantly conducted by empirical means in the DSC research community. The optical model allows to accurately calculate the internal quantum efficiency of devices, i.e. the ratio of extracted electrons to absorbed photons by the dye, a crucial and so far difficult to determine characteristic. Intrinsic parameters — like injection efficiency, electron diffusion length, or distribution of trap states in the TiO2 — can be extracted from experimental steady-state and time-dependent data with the electric model. The model allows to make a comprehensive and quantitative loss analysis of the different optical and electric loss channels in the DSC. The model has been implemented with a graphical user interface for straightforward usage. All three optimization strategies — organic dyes, tandem architecture, and device modeling — developed during this thesis make a valuable contribution to the development and commercialization of inexpensive and high efficiency DSCs. They enable a comprehensive view of the system and pave the way for a systematic analysis and reduction of losses, which has been the ultimate route to success for several established photovoltaic technologies.