Doped but Stable: Spirobisacridine Hole Transporting Materials for Hysteresis-Free and Stable Perovskite Solar Cells

Four spirobisacridine (SBA) hole-transporting materials were synthesized and employed in perovskite solar cells (PSCs). The molecules bear electronically inert alkyl chains of different length and bulkiness, attached to in-plane N atoms of nearly orthogonal spiro-connected acridines. Di-p-methoxyphenylamine (DMPA) substituents tailored to the central SBA-platform define electronic properties of the materials mimicking the structure of the benchmark 2,2',7,7'-tetrakis(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-MeOTAD), while the alkyl pending groups affect molecular packing in thin films and affect the long-term performance of PSCs. Devices with SBA-based hole transporting layers (HTL) attain efficiencies on par with spiro-MeOTAD. More importantly, solar cells with the new HTMs are hysteresis-free and demonstrate good operational stability, despite being doped as spiro-MeOTAD. The best performing MeSBA-DMPA retained 88% of the initial efficiency after a 1000 h aging test under constant illumination. The results clearly demonstrate that SBA-based compounds are potent candidates for a design of new HTMs for PSCs with improved longevity.

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.

EPFL2010

Charge collection and pore filling in solid-state dye-sensitized solar cells

Ilkay Cesar, Michael Graetzel, Robin Humphry-Baker, Henry Snaith, Shaik Mohammed Zakeeruddin

The solar to electrical power conversion efficiency for dye-sensitized solar cells (DSCs) incorporating a solid-state organic hole-transporter can be over 5%. However, this is for devices significantly thinner than the optical depth of the active composites and by comparison to the liquid electrolyte based DSCs, which exhibit efficiencies in excess of 10%, more than doubling of this efficiency is clearly attainable if all the steps in the photovoltaic process can be optimized. Two issues are currently being addressed by the field. The first aims at enhancing the electron diffusion length by either reducing the charge recombination or enhancing the charge transport rates. This should enable a larger fraction of photogenerated charges to be collected. The second, though less actively investigated, aims to improve the physical composite formation, which in this instance is the infiltration of mesoporous TiO2 with the organic hole-transporter 2,2',7,7'-tetrakis(N,N-di-p-methoxypheny-amine)-9,9'-spirobifluorene (spiro-MeOTAD). Here, we perform a broad experimental study to elucidate the limiting factors to the solar cell performance. We first investigate the charge transport and recombination in the solid-state dye-sensitized solar cell under realistic working conditions via small perturbation photovoltage and photocurrent decay measurements. From these measurements we deduce that the electron diffusion length near short-circuit is as long as 20 µm. However, at applied biases approaching open-circuit potential under realistic solar conditions, the diffusion length becomes comparable with the film thickness, ~2 µm, illustrating that real losses to open-circuit voltage, fill factor and hence efficiency are occurring due to ineffective charge collection. The long diffusion length near short-circuit, on the other hand, illustrates that another process, separate from ineffective charge collection, is rendering the solar cell less than ideal. We investigate the process of TiO2 mesopore infiltration with spiro-MeOTAD by examining the cross-sectional images of and performing photo-induced absorption spectroscopy on devices with a range of thickness, infiltrated with spiro-MeOTAD with a range of concentrations. We present our interpretation of the mechanism for material infiltration, and by improving the casting conditions demonstrate efficient charge collection through devices of over 7 µm in thickness. This investigation represents an improvement in our understanding of the limiting factors to the dye-sensitized solar cell. However, much work, focused on composite formation and improved kinetic competition, is required to realize the true potential of this concept.

2008

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