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See detailCo-evaporation and Scanning Probe Microscopy Characterizations of Hybrid Halide Perovskite Thin Films for Solar Cells
Gallet, Thibaut UL

Doctoral thesis (2021)

Hybrid organic-inorganic perovskites (HOIPs) are the trending materials when discussing solar cells. Their power conversion efficiency went from 3.8% to 25.5% in twelve years, making them extremely ... [more ▼]

Hybrid organic-inorganic perovskites (HOIPs) are the trending materials when discussing solar cells. Their power conversion efficiency went from 3.8% to 25.5% in twelve years, making them extremely promising, especially when combined with silicon in a tandem configuration. This improvement has been achieved by finding the best candidates for charge extraction and by interface engineering, compositional engineering and surface passivation. However, the surface of the HOIPs is still not well understood, and the role of grains boundaries for example is still highly debated. Determining the intrinsic surface properties of HOIPs is therefore crucial to find the best passivation strategies or fabrication designs to limit the surface and interfacial losses, and further improve solar cell efficiencies. Currently, solution-based processes are the most used techniques for fabrication, even though their upscalability towards commercialization is highly unlikely, and the use of solvents, sometimes toxic, considerably alters the perovskite surface, which makes the interpretation of their characterization challenging and sometimes misleading. The aim of this thesis is to clarify the intrinsic surface properties of HOIPs, and mainly CH3NH3PbI3 (or MAPbI3), by using surface-sensitive techniques such as scanning tunneling microscopy and spectroscopy (STM and STS) and Kelvin probe force microscopy (KPFM). To that end, HOIP thin films are mainly fabricated by thermal co-evaporation to achieve pristine surfaces, and inert-gas transfer is used to avoid contamination before their characterization. The lateral variations of the local density of states of MAPbI3 and mixed halide HOIPs are investigated. The grain-to-grain and facet variations are linked to different density of surface states that pin the Fermi level at the surface, and different workfunctions (WF), which are both attributed for MAPbI3 to different surface terminations, and for the mixed HOIPs to an additional degradation of the perovskites. The effect of varying the methylammonium iodide (MAI) content, via the partial pressure, in co-evaporated MAPbI3 is studied and the excess of MAI proves to be detrimental, as it introduces low-dimensional perovskites and stacked perovskite sheets that considerably reduce its intrinsic stability. Therefore near-stoichiometric conditions are preferred and yield films more stable to light and heat and without photostriction. Nevertheless this intrinsic stability is still not optimal, and the continuous variations of the WF measured by KPFM upon prolonged illumination is investigated. Combined with X-ray photoelectron spectroscopy (XPS), the photo-induced degradation of MAPbI3, and evaporation of I2 are revealed as the causes of these variations. Besides, by combining KPFM and photoluminescence (PL) techniques for different thicknesses and substrates, energy band diagrams can be drawn and unveil a bending of the bands in the bulk. Lastly, the surface sensitivity of HOIPs is investigated when they are intentionally put in contact with extrinsic factors such as oxygen and solvents, and the surface properties are shown to be considerably altered. In addition, passivation strategies are used to demonstrate how surfaces can be improved. [less ▲]

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See detailThe impact of Kelvin probe force microscopy operation modes and environment on grain boundary band bending in perovskite and Cu(In,Ga)Se2 solar cells
Martin Lanzoni, Evandro UL; Gallet, Thibaut UL; Spindler, Conrad UL et al

in Nano Energy (2021), 88

An in-depth understanding of the electronic properties of grain boundaries (GBs) in polycrystalline semiconductor absorbers is of high importance since their charge carrier recombination rates may be very ... [more ▼]

An in-depth understanding of the electronic properties of grain boundaries (GBs) in polycrystalline semiconductor absorbers is of high importance since their charge carrier recombination rates may be very high and hence limit the solar cell device performance. Kelvin Probe Force Microscopy (KPFM) is the method of choice to investigate GB band bending on the nanometer scale and thereby helps to develop passivation strategies. Here, it is shown that the workfunction, measured with amplitude modulation (AM)-KPFM, which is by far the most common KPFM measurement mode, is prone to exhibit measurement artifacts at grain boundaries on typical solar cell absorbers such as Cu(In,Ga)Se2 and CH3NH3PbI3. This is a direct consequence of a change in the cantilever–sample distance that varies on rough samples. Furthermore, we critically discuss the impact of different environments (air versus vacuum) and show that air exposure alters the GB and facet contrast, which leads to erroneous interpretations of the GB physics. Frequency modulation (FM)-KPFM measurements on non-air-exposed CIGSe and perovskite absorbers show that the amount of band bending measured at the GB is negligible and that the electronic landscape of the semiconductor surface is dominated by facet-related contrast due to the polycrystalline nature of the absorbers. [less ▲]

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See detailCo-evaporation of CH3NH3PbI3: How Growth Conditions Impact Phase Purity, Photostriction, and Intrinsic Stability
Gallet, Thibaut UL; Goncalinho Poeira, Ricardo Jorge UL; Martin Lanzoni, Evandro UL et al

in ACS Applied Materials and Interfaces (2021), 13(2), 26422653

Hybrid organic–inorganic perovskites are highly promising candidates for the upcoming generation of single- and multijunction solar cells. Despite their extraordinarily good semiconducting properties ... [more ▼]

Hybrid organic–inorganic perovskites are highly promising candidates for the upcoming generation of single- and multijunction solar cells. Despite their extraordinarily good semiconducting properties, there is a need to increase the intrinsic material stability against heat, moisture, and light exposure. Understanding how variations in synthesis affect the bulk and surface stability is therefore of paramount importance to achieve a rapid commercialization on large scales. In this work, we show for the case of methylammonium lead iodide that a thorough control of the methylammonium iodide (MAI) partial pressure during co-evaporation is essential to limit photostriction and reach phase purity, which dictate the absorber stability. Kelvin probe force microscopy measurements in ultrahigh vacuum corroborate that off-stoichiometric absorbers prepared with an excess of MAI partial pressure exhibit traces of low-dimensional (two-dimensional, 2D) perovskites and stacking faults that have adverse effects on the intrinsic material stability. Under optimized growth conditions, time-resolved photoluminescence and work functions mapping corroborate that the perovskite films are less prone to heat and light degradation. [less ▲]

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See detailNanoscale interfacial engineering enables highly stable and efficient perovskite photovoltaics
Krishna, Anurag; Zhang, Hong; Zhou, Zhiwen et al

in Energy and Environmental Science (2021)

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See detailEffects of Annealing and Light on Co-evaporated Methylammonium Lead Iodide Perovskites using Kelvin Probe Force Microscopy in Ultra-High Vacuum
Gallet, Thibaut UL; Martin Lanzoni, Evandro UL; Redinger, Alex UL

in 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC) (2020, February 06)

Careful surface analysis is essential to understand the electronic and ionic behaviors in perovskite absorbers. In this contribution we discuss Kelvin probe force microscopy performed in ultra-high vacuum ... [more ▼]

Careful surface analysis is essential to understand the electronic and ionic behaviors in perovskite absorbers. In this contribution we discuss Kelvin probe force microscopy performed in ultra-high vacuum on as-grown and annealed co-evaporated methylammonium lead iodide perovskite thin films. By comparing the contact potential difference upon annealing and illumination, we find that annealing increases the average workfunction, indicating a change either in doping or in surface composition. Illumination also increases the average workfunction, indicating a p-type absorber, by reducing band bending as the photo-generated carriers screen the surface states. The effect of light shows a two-step process, with a first fast trend, linked to the surface photovoltage and a second slower trend indicating a possible redistribution of mobile charges. [less ▲]

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See detailFermi-level pinning in methylammonium lead iodide perovskites
Gallet, Thibaut UL; Grabowski, David; Kirchartz, Thomas et al

in Nanoscale (2019)

Hybrid organic inorganic perovskites are ideal candidates for absorber layers in next generation thin film photovoltaics. The polycrystalline nature of these layers imposes substantial complications for ... [more ▼]

Hybrid organic inorganic perovskites are ideal candidates for absorber layers in next generation thin film photovoltaics. The polycrystalline nature of these layers imposes substantial complications for the design of high efficiency devices since the optoelectronic properties can vary on the nanometre scale. Here we show via scanning tunnelling microscopy and spectroscopy that different grains and grain facets exhibit variations in the local density of states. Modeling of the tunneling spectroscopy curves allows us to quantify the density and fluctuations of surface states and estimate the variations in workfunction on the nanometre scale. The simulations corroborate that the high number of surface states leads to Fermi-level pinning of the methylammonium lead iodide surfaces. We do not observe a variation of the local density of states at the grain boundaries compared to the grain interior. These results are in contrast to other reported SPM measurements in literature. Our results show that most of the fluctuations of the electrical properties in these polycrystalline materials arise due to grain to grain variations and not due to distinct electronic properties of the grain boundaries. The measured workfunction changes at the different grains result in local variations of the band alignment with the carrier selective top contact and the varying number of surface states influence the recombination activity in the devices. [less ▲]

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See detailFabrication of Si tunnel diodes for c-Si based tandem solar cells using proximity rapid thermal diffusion
Fave, Alain; Lelièvre, Jean-François; Gallet, Thibaut UL et al

in Energy Procedia (2017), 124

Increasing competitiveness of photovoltaic (PV) devices is currently an important objective in technological research, especially with the development of tandem solar cells based on c-Si as the bottom ... [more ▼]

Increasing competitiveness of photovoltaic (PV) devices is currently an important objective in technological research, especially with the development of tandem solar cells based on c-Si as the bottom cell. For a monolithical structure, a tunnel diode in between the top and bottom cells is necessary. In this work we report on the development of the fabrication of Si tunnel junction using a combination of spin-on doping and proximity rapid thermal diffusion. A desirable attribute of this process is simplicity. Two different structures p++/n++ or n++/p++ were fabricated on (100) Si substrates. Carrier density profiles were measured by ECV to characterize the shallow doping profiles. Vertical tunnel diodes were fabricated and I(V) characteristics are presented. It is shown that device peak current densities up to 270 A/cm² are achieved using this technique, which is the best value reported with such simple technique. [less ▲]

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