Investigations on hybrid organic-inorganic perovskites for high performance solar cells

Undoubtedly, perovskite solar cells have become a key player in 3rd generation photovoltaics over the last few years. Although it is only in 2012 that the first solid state perovskite solar cell was reported, power conversion efficiencies have increased so rapidly that PSCs are now serious contenders to the well-established and marketed thin-film and wafer technologies. Over the timespan on this work, over 3000 published articles in peer-review journals have been published. The work that I report in this thesis is merely a small contribution to the gigantic amount of data, models and theories that have been made over the last 4 years. I hope that my modest contribution will be of value to the scientific community. Over the course of this work, I attempted not to focus my attention on a single issue that relates to perovskite photovoltaics, but to approach it from different perspectives. As a result, a wide array of subjects have been treated. Starting with the fabrication of perovskite solar cells, an innovative way to deposit thin film of perovskite from solution is reported, using a two-step deposition technique. Using this approach, the power conversion efficiency of perovskite solar cells has been increased from 12.3% to over 17% in less than a year. Subsequently, I looked into the possible compositional variations of the perovskite layer. This study showed that the cation methylammonium and formamidinium can be used inter- changeably using the deposition technique that we have reported on earlier. We showed that a mixture of the two cations leads to a stabilization of the tetragonal perovskite phase and shrinks the optical band gap of the photoabsorber, allowing more photons to be absorbed and converted into electrons. Following this work, I studied some of the reactivity properties of the phase pure organic inorganic CH3NH3PbX3(X=Cl,Br,I). Notably, it could be shown that the halide component can be easily exchanged by another one from a simple solution based anion exchange reaction. Later, the power conversion characteristics of perovskite solar cells were investigated. By developing a electronic circuit allowing versatile power point tracking of the solar cells, a new algorithm that deals with the issues specifically related to perovskite solar cells is reported. i A major topic of my work was the study of the frequency resolved dynamics that results for the generation carriers by light. Using a novel way to measure the intensity modulated spectral response of perovskite solar cells, I was able to identify some of the key limiting factors that cap the power conversion efficiency of the devices. A numerical model that answers to the laws of carrier dynamics was assembled and used as a tool to better understand perovskite solar cells. Finally, I reported on a way to deal with the end of life of perovskite solar cells. Some of the relevant regulatory framework in the European Union that deals with the recycling of solar panels are highlighted. Using the concept of "upcycling", a way to recycle the valuable components of the solar cell in an efficient way was outlined.

From Single-Crystals to High-Throughput

Jeannette Medina Kadro

Perovskite solar cells have undergone increase in power-conversion efficiencies at an unprecedented pace. Since their emergence in academic research in 2009, thousands of scientific papers have been published on their unique properties, variations cell architectures, material composition optimization and potential applications. In the course of this work, I attempted to cover multiple topics and focussed on applied research aspects. The first chapter describes a novel, rapid process for the growth of hybrid perovskite single crystals. Using minimal equipment and growth times, the solution-based process allows the fabrication of freestanding single crystals within minutes by heating stoichiometric precursor solutions. This efficient, simple process allowed me to produce single crystals in large amounts which, in turn, enabled me to develop a vacuum deposition technique for the preparation of perovskite thin films. Using rapid thermal flash vacuum deposition, thin films could be prepared with short deposition times and minimal manual sample preparation. The use of single crystals as sublimation precursors ensured stoichiometry of the deposit without the need for the challenging, precise control of multiple thermal sources. Another project covered in this thesis aimed to develop methods for high-throughput perovskite material screening or high-throughput research. Combinatorial libraries for the screening of perovskite compositions for optoelectronic applications were prepared by the vacuum deposition process also developed in the course of my PhD. Inkjet printing was also identified as suitable process and preliminary results are included in the last chapter of the thesis. The end-of-life and recycling of perovskite solar cells are covered in two chapters. A simple, highly selective dismantling process using orthogonal solvents is developed to reverse-engineer full solar cell devices and successful re-use of important device components is demonstrated. A detailed discussion of technological, environmental and regulatory challenges for the end-of-life management of perovskite solar panels is included as chapter three.

EPFL2018

Interface engineering in solid-state dye-sensitized solar cells

Jessica Krüger

Dye-sensitised nanocrystalline solar cells are currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. In particular dye-sensitised cells based on spiro-MeOTAD have gained attention as promising approach towards an organic solid-state solar cell. However, the efficiency in such dye-sensitised solid-state solar devices (SSD) was so far only ca. 10 % of the values reported at AM1.5 for the classical dye-sensitised solar cell with an electrolyte hole transporting medium (DSSC). The objective of the present work is to study the limitations that emerge from the exchange of the electrolyte by the solid-state system and that oppose photovoltaic photon-to-electron conversion as high as for the DSSC. Interfacial charge recombination is an important loss mechanism in dye-sensitised solar cells. This is particularly true for SSD, as the solid hole-transporting medium is less efficient in screening of internal fields which assist recombination. A variety of strategies were tested in the SSD to minimise interfacial charge recombination. The most promising approach was the blending of the hole-transporting medium with tert.-butylpyridine (tBP) and lithium ions. Optical and electrochemical techniques, such as nanosecond laser spectroscopy, impedance spectroscopy and photovoltaic characterisation measurements, were used to study the impact of the additives on the SSD. Both lithium ions as well as tBP were found to increase the open circuit potential of the SSD. At the same time tBP was found to considerably lower the current output. The interaction of the additives was studied and their concentration in the spiro-MeOTAD medium optimised. The doping of the spiro-MeOTAD film, which was intended to support the hole transport, was found to enhance interfacial recombination significantly. The morphological properties of the TiO2, in particular layer thickness, particle size and film porosity, play a more important role in the SSD than in the DSSC. Penetration of the hole conductor into the TiO2 pores and electron diffusion length are coupled to these properties. As a result the light harvesting cannot be controlled at will via the TiO2 film thickness and the active surface area for dye adsorption. An enhanced light harvesting for thin TiO2 layers offers advantages for the charge transport and the formation of the interpenetrating network. The dye uptake in presence of silver ions was found to increase the dye loading and to significantly improve the device performance of thin TiO2 layer devices. The mechanism of this simple dye modification technique was studied by a variety of spectroscopic techniques. From spectroscopic evidence it is inferred that the silver is binding to the sensitiser via the ambidentate thiocyanate, allowing the formation of ligand-bridged dye complexes. The beneficial influence of the silver ions on the photovoltaic performance was not limited to the application of the standard N3 dye nor to the spiro-MeOTAD. SSDs were furthermore studied by frequency resolved techniques. Intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) were performed over a wide range of illumination intensities. The IMPS and IMVS responses provide information about charge transport and electron-hole recombination processes respectively. For the range of light intensities investigated, the dynamic photocurrent response appears to be limited by the transport of electrons in the nanocrystalline TiO2 film rather than by the transport of holes in the spiro-MeOTAD. The diffusion length of electrons in the TiO2 was found to be 4.4 μm. This value was almost independent of the light intensity as a consequence of the fact that the electron diffusion coefficient and the rate constant for electron-hole recombination both increase in the same way with light intensity but with opposite sign. The results of this work provide for a substantial improvement of the overall photovoltaic performance compared to earlier results for this type of SSD. However, this study reveals also that high conversion efficiencies as are measured for DSSC are not likely to be reachable with the spiro-MeOTAD system due to the significantly slower charge transport in the spiro-MeOTAD compared to the electrolyte redox mediator.

EPFL2003

Sequential deposition as a route to high-performance perovskite-sensitized solar cells

Julian Burschka, Peng Gao, Michael Graetzel, Robin Humphry-Baker, Soo-Jin Moon, Mohammad Khaja Nazeeruddin, Norman Pellet

Following pioneering work(1), solution-processable organic-inorganic hybrid perovskites-such as CH3NH3PbX3 (X = Cl, Br, I)-have attracted attention as light-harvesting materials for mesoscopic solar cells(2-15). So far, the perovskite pigment has been deposited in a single step onto mesoporous metal oxide films using a mixture of PbX2 and CH3NH3X in a common solvent. However, the uncontrolled precipitation of the perovskite produces large morphological variations, resulting in a wide spread of photovoltaic performance in the resulting devices, which hampers the prospects for practical applications. Here we describe a sequential deposition method for the formation of the perovskite pigment within the porous metal oxide film. PbI2 is first introduced from solution into a nanoporous titanium dioxide film and subsequently transformed into the perovskite by exposing it to a solution of CH3NH3I. We find that the conversion occurs within the nanoporous host as soon as the two components come into contact, permitting much better control over the perovskite morphology than is possible with the previously employed route. Using this technique for the fabrication of solid-state mesoscopic solar cells greatly increases the reproducibility of their performance and allows us to achieve a power conversion efficiency of approximately 15 per cent (measured under standard AM1.5G test conditions on solar zenith angle, solar light intensity and cell temperature). This two-step method should provide new opportunities for the fabrication of solution-processed photovoltaic cells with unprecedented power conversion efficiencies and high stability equal to or even greater than those of today's best thin-film photovoltaic devices.