Role of ions and interfaces for efficient and stable perovskite solar cells

Perovskite Solar Cells (PSCs) have grabbed global attention of the researchers due to their outstanding Photovoltaic (PV) performance. PSCs have the potential to be the future of the PV technology as they can generate power with performance being comparable with the leading Silicon solar cells, with the cost being lower than Silicon solar cells. The enormous potential of PSCs is evident from the fact that the efficiency of these cells has risen from 3.8% to 25.5% within a decade, and it is continuously rising to date. In my thesis, I investigate the impact of ions in the bulk of perovskite and also look at the ETL-perovskite interface to create a simpler, more efficient, more reproducible and more stable system for PSCs. At first, I managed to achieve high efficiency devices with the standard triple cation perovskite on both planar and mesoporous n-i-p ETL device architecture. Adding KI to the already established compositions showed a boost in performance of the solar cells while significantly reducing hysteresis. We then performed a systematic study of KI on various perovskite compositions comprised of FA, MA, Cs, Rb, I and Br ions. We demonstrated that KI reacts with the bromide ions in perovskite passivating the grains leading to a red shift hence and improvement in current density as well as efficiency.MAPbBr3 is generally used is small proportions along with FAPbI3 to stabilise the phase of FAPbI3. But the effect of Br- ions on the perovskite performance was relatively unknown. The introduction of Br- ions in the lattice causes a blue shift in the band gap pushing it further away from the ideal Shockley-Quessier limit. On top of this, the effect of MAPbBr3 on phase stability of FAPbI3 is similar to the effect of CsPbI3. This led me to examine the impact of reduced bromine concentration of the device performance. Interestingly, reducing the bromine concentration not only leads to a red shift in band gap, hence increasing the current density but also causes a reduced voltage loss resulting in much superior performance. Moreover, the Br- ions cause increased halide diffusion in the bulk leading to decreased stability with higher the bromide concentration.While perovskite composition plays an important role in defining the device characteristics, it could be argued that the interfaces play an equally important role in doing the same. With the rise in usage of SnO2 as viable substitute for TiO2 as ETL, I explore the effect of using TiO2 particles that exhibit similar properties to the SnO2 particles. These smaller size TiO2 nanoparticles can form a very thin, compact, uniform and pin-hole free layer with a morphology that follows the morphology of FTO. This results in a reduced recombination in the interface of TiO2 and perovskite causing the open circuit voltage and hence the device performance to improve significantly.

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

Development of Highly Efficient Perovskite-on-Silicon Tandem Solar Cells

Florent Sahli

Crystalline Silicon (c-Si) solar cells are dominating the photovoltaic (PV) market. Owing to their large manufacturing capacity, reliability and efficiency, c-Si solar cells are now cost-competitive with other non-renewable electricity sources in many places. The price of c-Si modules has been decreasing drastically for the past decades. They now account for less than half of the cost of a PV system. Other costs come from the balance-of-system and these are rather inflexible. And so increasing efficiency of solar modules is the most efficient approach to lower the cost of PV electricity. One issue is that c-Si cells are approaching their efficiency limit. One strategy to increase efficiency beyond this limit relies on adding another solar cell on c-Si to form a tandem solar cell as thermalisation losses are reduced. Metal lead halide perovskite solar cells are promising top cell candidates for c-Si based on their high optoelectronic properties, band gap tunability, high efficiencies, ease of manufacturing and low material costs. By combining both technologies, efficiencies >30% are realistic, well above best-in-class c-Si cells. This thesis aims to produce high efficiency perovskite solar cells for 2-terminal monolithic tandems on c-Si. First, we develop a perovskite fabrication method, which employs thermal evaporation to produce a lead halide template and then organohalide spin-coating. By varying composition at each step, the perovskite band gap can be tuned in the range 1.6-1.8 eV, ideal values for 2-terminal tandems. The use of the template makes the deposition compatible with various substrate textures. Then, we develop a recombination junction that features nanocrystalline hydrogenated silicon layers (nc-Si:H). When used with a font-side polished c-Si bottom cell, it shows a superior optical performance compared to standard transparent conductive oxides thanks to a better matching of refractive indices. Owing to its low conductivity, the top cell leakage current is reduced, enabling to scale-up the cell area. Then, perovskite/c-Si tandems featuring a double-side textured c-Si wafer are demonstrated, achieving a certified efficiency of 25.2%. This efficiency, the highest at the time of publication, is enabled by low reflection losses and efficient light trapping thanks to the c-Si pyramids present on both sides. More importantly, this double-side texture yields an optically close-to-optimum system that is simpler and more efficient compared to alternatives. Furthermore, this top cell process flow does not require any modification to existing c-Si manufacturing production lines as these use front-side textured c-Si. In the second part, we replace the spin-coating of organohalides by a vapour transport deposition (VTD) using a home-made vapour transport deposition setup that offers large processing flexibility. Using a thermally evaporated lead iodide template and methylammonium iodide vapours, perovskite solar cells with an efficiency > 12% are made. Thanks to the presence of a showerhead, a homogeneous perovskite growth is obtained on 6 inch textured c-Si substrates, the industry standard. The VTD of formaminidium iodide (FAI) is more challenging. A trimerisation of FAI forms sym-triazine, which does not react with the template to form the perovskite. Still, in the presence of ammonium cations, sym-triazine can be cleaved to form formamidine, thus offering an alternative pathway to deposit perovskite layers.

EPFL2020

Material Development for Perovskite/Silicon Tandem Photovoltaics

Peter Joseph Fiala

The developed world is built on the fact that energy is readily available and functionally infinite. The electricity from the wall, the gas at the station, and the heat in our homes are reliable and low-cost. But this comfort is so far only possible through the combustion of fossilized carbon sources. This is not a sustainable system. Not only are these sources finite, but the long-term effects of this much combustion are catastrophic for our planet and species. Transitioning from current energy production to sustainable sources will require the development and deployment of a diverse group of technologies. Chief among these is the generation of electricity from sunlight, or photovoltaics (PV).Most modern PV is based on crystalline silicon. Silicon PV alone could be scaled to provide the necessary energy production, but this could be realized more quickly and efficiently if more advanced technologies are used. The defining metric is the cost per generated electricity. In the case of silicon, the current cost breakdown of materials, manufacturing, installation, and system costs leaves little room for further price reduction; and light-to-electricity efficiencies are near their practical limit. Of the proposed alternatives, the pairing of silicon with a thin film of lead-halide perovskite in a multijunction or "tandem" architecture is one of the most promising. This splits the solar spectrum in two, and raises the theoretical maximum efficiency from 33.3% to 45.1% based on each material better using its segment than any one material could use the full spectrum. The central goal of this thesis was therefore to advance the capability of this type of solar cell while remaining within the constraints of industrially-plausible production methods.Towards this goal, we first isolated perovskite layers to study what limits their performance. This led to improvements in the bulk material quality, the interfaces with adjacent materials, and the layer morphology. We reduced bulk disorder via balancing the annealing temperature, time, and environment. The interfaces were improved by selecting passivating materials which were compatible with our deposition methods. We further observed a complex interplay between the materials used and the environment of the annealing, which illuminated critical dependencies of device performance and gave us a more complete understanding of our material. Finally the morphology was improved via high-temperature annealing. This was enabled by a surface-binding additive which reduced the detrimental effects of such an anneal while preserving the benefits, resulting in a perovskite with less internal resistance. From there, we moved outside of the perovskite layer and developed a light scattering ZnO layer which increased the optical performance and absorption of our cells. Returning to the main goal of the thesis, the lessons learned from work on isolated perovskites were applied in tandems on textured silicon. We additionally increased the perovskite layer thickness and made the bottom-cell more robust to processing damages. Together, the result was a stabilized efficiency of 27.3%. This is better than any silicon cell to date, but work remains to achieve 30% and beyond, extend material stability, scale up the deposition area, and transfer these capabilities to industry. Together, the work done as a part of this thesis represents a step forward in the journey towards carbon-neutral energy production.

EPFL2022