A dopant free linear acene derivative as a hole transport material for perovskite pigmented solar cells

A solution processable, molecular organic semiconductor, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene), was employed as hole transport material (HTM) in mesoscopic methylammonium lead iodide perovskite solar cells. TIPS-pentacene is potentially cost effective, exhibits a relatively high hole mobility and has a favourable HOMO level with respect to the valence band of perovskite. The photovoltaic performance of perovskite solar cells with TIPS-pentacene as HTM in its pristine form and with a dopant/additive was investigated and compared with classical spiro-OMeTAD based devices. Through solvoneering (solvent engineering) and concentration optimization TIPS-pentacene in its pristine form gave a very competitive power conversion efficiency (PCE) of 11.8% under 1 sun conditions. The open circuit voltage of 0.92 V and a short circuit current density of 20.86 mA cm(-2) for the devices with pristine TIPS-pentacene were higher compared to doped spiro-OMeTAD based devices under similar conditions, thus paving the use of TIPS-pentacene as an alternative to an expensive spiro-OMeTAD for large area integration in perovskite based solar cells.

The effect of illumination on the formation of metal halide perovskite films

Antonio Abate, Taisuke Matsui, Jiyoun Seo, Ludmilla Steier, Wolfgang Richard Tress, Amita Ummadisingu

Optimizing the morphology of metal halide perovskite films is an important way to improve the performance of solar cells(1) when these materials are used as light harvesters(2), because film homogeneity is correlated with photovoltaic performance(3). Many device architectures and processing techniques have been explored with the aim of achieving high-performance devices(4), including single-step deposition(5), sequential deposition(6,7) and anti-solvent methods(1,8). Earlier studies have looked at the influence of reaction conditions on film quality(3), such as the concentration of the reactants9,10 and the reaction temperature(11). However, the precise mechanism of the reaction and the main factors that govern it are poorly understood. The consequent lack of control is the main reason for the large variability observed in perovskite morphology and the related solar-cell performance(2,3). Here we show that light has a strong influence on the rate of perovskite formation and on film morphology in both of the main deposition methods currently used: sequential deposition and the anti-solvent method. We study the reaction of a metal halide (lead iodide) with an organic compound (methylammonium iodide) using confocal laser scanning fluorescence microscopy and scanning electron microscopy. The lead iodide crystallizes before the intercalation of methylammonium iodide commences, producing the methylammonium lead iodide perovskite. We find that the formation of perovskite via such a sequential deposition is much accelerated by light. The influence of light on morphology is reflected in a doubling of solar-cell efficiency. Conversely, using the anti-solvent method to form methyl ammonium lead iodide perovskite in a single step from the same starting materials, we find that the best photovoltaic performance is obtained when films are produced in the dark. The discovery of light-activated crystallization not only identifies a previously unknown source of variability in opto-electronic properties, but also opens up new ways of tuning morphology and structuring perovskites for various applications.

Nature Publishing Group2017

Fundamentals of perovskite formation for photovoltaics

Amita Ummadisingu

Perovskite solar cells have become strong contenders in the arena of photovoltaics due to the stunning rise in their efficiency from 3 % to over 20 % in just seven years. In this time, numerous device architectures and thin film deposition methods have been explored. The sequential deposition and anti-solvent methods are among the most widely used for preparing perovskite solar cells. Optimizing these perovskite deposition processes to tune the perovskite film morphology plays a key role in the race for the highest efficiencies as homogeneity of the film correlates with superior photovoltaic performance. To date, the factors controlling perovskite film formation in various deposition methods and the precise mechanism are little understood. In light of this, the aim of the thesis is to unravel the salient aspects of perovskite film formation. I first study the formation of methylammonium lead iodide in sequential deposition using confocal laser scanning fluorescence microscopy (CLSM) and scanning electron microscopy (SEM). I discover that illumination during film formation is a major factor in the reaction as it greatly accelerates perovskite formation and tunes the final film morphology by controlling nucleation of lead iodide. The nucleation density increases logarithmically with the illumination intensity. I uncover the mechanism behind this effect and demonstrate that it is a quantum phenomenon and not merely a heating effect. I posit that absorbed light lowers the surface tension, thereby facilitating nucleation. Not only is this effect present in the main deposition methods â sequential deposition and the anti-solvent methods, but also in perovskites of more complex compositions that are employed in state-of-the-art solar cells. Second, I scrutinize the individual stages in the formation of methylammonium lead iodide perovskite in sequential deposition. SEM-cathodoluminescence imaging identifies the presence of mixed crystalline aggregates composed of methylammonium lead iodide perovskite and lead iodide in forming perovskite films. Using cross-sectional CLSM for the first time, I identify the directionality of perovskite formation. Kinetic monitoring and model fitting bring forward the Avrami models as the most suitable to quantify and represent the formation of perovskite under different conditions of temperature, film thickness and illumination. Finally, I identify a maturation effect in the perovskite film in solar cells stored in the dark and investigate the dynamic spontaneous coalescence of crystals in the film. During maturation, small crystals merge to form larger ones and this reduces the number of grain boundaries and the associated trap states, supressing non-radiative recombination. The photovoltaic performance of high efficiency solar cells thus increases and it is accompanied by a reduction in the hysteresis. Overall, apart from providing insights into the dynamic nature of deposited films, I present a fundamental understanding of the perovskite formation process in different deposition methods and for various compositions. Such a comprehensive picture aids in the achievement of high photovoltaic performance.

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