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

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.

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

High-Quality Microcrystalline Silicon for Efficient Thin-Film Solar Cells

Grégory Bugnon

This thesis investigates the link between the plasma deposition conditions and microcrystalline silicon (μc-Si:H) material quality for thin-film silicon photovoltaic applications. The role of interfaces and the μc-Si:H material quality on the device performance are analyzed in detail. The low absorption of μc-Si:H at long wavelengths requires the deposition of absorber layers with thicknesses of typically a few micrometers for use in multi-junction TF Si solar cells. The growth typically takes place on highly textured surfaces, which provide increased light absorption—often called light trapping—but which potentially induce structural defects in the film during its growth. Therefore, to further improve the TF Si technology, one of the main challenges is the identification of the determinant plasma deposition parameters that result in the growth of very high-quality μc-Si:H at an increased deposition rate on textured substrates that guarantee efficient light trapping. As a first approach to better understand the plasma conditions necessary for the growth of high-quality μc-Si:H, the roles of both the silane depletion fraction and the deposition pressure are studied in an industrial-type large-area KAI reactor. With increasing pressure and silane depletion, the μc-Si:H defect density is significantly lowered leading to improved solar cell performance. An estimation of the average energy with which ions impinge on the substrate supports the hypothesis that ion bombardment is mainly responsible for the observed differences. Then, a fundamental aspect of μc-Si:H deposition on highly textured substrates is highlighted: two different phases of μc-Si:H material contribute to the overall solar cell efficiency, both of which can drive cell performance. The first phase relates to the bulk material and dominates the performance of cells on flat substrates. However, on rough morphologies, substrate-induced defective localized nanoporous regions—the second phase—develop and are found to be significantly more sensitive to the plasma process conditions and substrate morphology than the bulk phase. The relative importance of this secondary defective phase is shown through the use of new damp-heat experiments. Silicon oxide doped layers are demonstrated to mitigate the influence of these nanoporous regions on the solar cell performance. Next, a comparative study of the plasma excitation frequencies of 13.56 MHz (RF) and 40.68 MHz (VHF) shows that, while both allow for the growth of very good-quality bulk material, the efficiency of VHF-prepared cells is always poorer compared to that of RF-prepared cells within the range of our study for growth rates below 5 Å s−1. This decrease in solar cell performance is related to a higher density of nanoporous regions in the VHF-prepared cells as evidenced by damp-heat experiments, leading to strong open-circuit voltage instabilities. Still, the use of VHF is shown to be beneficial at increased deposition rates, thanks to reduced ion bombardment and improved bulk material quality. The crucial interplay between μc-Si:H growth rate and substrate morphology with regard to the formation of nanoporous regions is further discussed for regimes with high deposition rates of around 10 Ås−1. It is shown that high-silane-depletion regimes with significantly reduced H2 flow rate or increased pressure lead to a denser μc-Si:H material but are associated with increased secondary gas-phase reactions and powder formation. The use of a reduced interelectrode distance is demonstrated to allow for the growth of μc-Si:H with significantly improved bulk material quality at higher growth rates. Plasma simulations performed in collaboration with the University of Patras are presented and suggest that improvements observed in the μc-Si:H material quality are related mainly to an increased contribution of less reactive silane monoradicals, such as SiH3 and Si2H5, to the growing film, as compared to highly reactive ones such as SiH2 and SiH. Then, in an effort to better understand the formation of these two distinct μc-Si:H phases, the intrinsic stress within μc-Si:H i-layers is studied and correlated with the bulk defect density. Further improvements to both μc-Si:H and a-Si:H solar cells are obtained by introducing a novel intrinsic silicon oxide buffer layer at the p -i interface. For μc -Si:H solar cells, all electrical parameters are improved unless the i -layer is significantly more amorphous-rich and high quality, in which case an improvement only in carrier collection in the blue region is observed. In a-Si:H solar cells the silicon oxide buffer is shown to lower light-induced degradation, which is one of the weak points of TF Si technology. Furthermore, for both a-Si:H and μc-Si:H solar cells, the buffer can also act as an efficient barrier to boron cross-contamination, eliminating the need for additional time-consuming processing steps such as a water flush for single-chamber processes. Overall, this work contributes to a better understanding of the μc-Si:H material requirements for PV applications and how they relate to the plasma deposition conditions. Based on all the aforementioned developments, significant progress has been made in the understanding and the fabrication of thin-film silicon solar cells based on μc-Si:H. An outstanding single-junction μc-Si:H solar cell of 10.9% was attained; to our knowledge this is the highest reported in the literature. This work also contributed to the development of very high-efficiency tandem and triple-junction thin-film silicon solar cells at PV-Lab.

EPFL2013

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

Anand Agarwalla

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.

EPFL2021