128 research outputs found
Grain boundaries and potassium post-deposition treatments in chalcopyrite solar cells
No full textOver the last years, alkali post-deposition treatments (PDT) have been attributed as the main driver for the continuous improvements in the power conversion efficiency (PCE) of Cu(In,Ga)Se2 (CIGSe) solar cells. All the alkali elements have shown beneficial optoelectronic effects, ranging from sodium to cesium, with many reports linking the improvements to grain boundary (GB) passivation. The most common process for alkali incorporation into the CIGS absorber is based on the thermal evaporation of alkali fluorides in a selenium atmosphere. Besides the demonstrated improvements in performance, disentangling the individual contributions of the PDTs on the GB, surface, and bulk is very challenging because of the many concurring chemical reactions and diffusion processes.
This thesis aims to investigate how pure metallic potassium interacts with CIGSe epitaxially grown on GaAs (100) and multi-crystalline GaAs. Surface sensitive Kelvin probe force microscopy (KPFM) and X-ray photoelectron spectroscopy (XPS) measurements are used to, in-situ, analyze changes in workfunction and compositional changes before and after each deposition step. Inert gas transfer systems and ultrahigh vacuum (UHV) are used to keep the pristine surface properties of the CIGSe.
An in-depth understanding of how different KPFM operation modes and environments influence the measured workfunction is discussed in detail in this thesis. It is shown that AM-KPFM, the most common KPFM operation mode, leads to misinterpretations of the measured workfunction at GBs on rough samples. Frequency modulation KPFM (FM-KPFM), on the other hand, turns out to be the most suitable KPFM mode to investigate GB band bending.
Pure metallic potassium evaporation on CIGSe epitaxially grown on GaAs (100) leads to diffusion of K from the surface down to the CIGS/GaAs interface even in the absence of GBs. Evaporation of metallic K is performed using a metallic dispenser, in which the evaporation rate can be controlled to deposit a few monolayers of K. The deposition is done in UHV, and an annealing step is used to diffuse K from the surface to the bulk.
Pure metallic potassium is also evaporated on CIGSe epitaxially grown on multicrystalline GaAs substrate, where well-defined GBs are present. Negligible workfunction changes at the GB were observed. XPS shows a strong Cu depletion after K deposition followed by annealing. Interestingly, the amount of K on the absorber surface after the K-deposition and subsequent annealing is almost equal to the amount of Cu that diffused into the bulk, suggesting a 1:1 exchange mechanism and no KInSe2 secondary phase.GRIS
Degradation Study of Co-evaporated Methylammonium Tin Iodide
No full textHybrid organic-inorganic metal halide perovskites (HOIPs) have been under the spotlight
since they were first used for solar cell applications. Since then, the power conversion efficiency
of HOIP-based solar cells has increased significantly and the current record is 26.7% on the
laboratory scale [1], which is comparable to the record of more mature technologies such as
silicon solar cells. Furthermore, HOIP solar cells are a low-cost alternative that is relatively
easy to produce and can be easily adapted by changing the substrate or composition. The
major drawbacks of HOIP based solar cells are that they tend to degrade when exposed to
external stresses, and record efficiency devices contain lead (Pb). The latter may hinder the
commercialization of this technology, because of the toxicity of Pb, which is of great concern
to human health and the environment. A possible way to avoid Pb in HOIP-based solar
cells is to replace it with tin (Sn). Which is a less toxic substitute for Pb and it is placed
right above Pb in the periodic table, meaning that both elements have similar reactivity.
Furthermore, Sn-based perovskite solar cells were demonstrated and yielded the best results
in terms of stability and efficiency for Pb-free HOIPs. Nevertheless, Sn-based HOIPs are also
prone to degradation under external stimuli and present an additional challenge compared
to Pb-based perovskites: Sn is more prone to oxidation.
In this thesis, Sn-based HOIP, more specifically methylammonium tin iodide (MASnI3)
is synthesized using physical vapor deposition, as it is a solvent-free technique. The MASnI3
films were exposed in a controlled environment to different external stimuli, such as light,
water, and synthetic air in order to study the different degradation pathways. Light, air, and water have been shown to degrade MASnI3, but the degradation pathway is different for
each of them. First, light-induced degradation is shown to result in the formation of majorly
SnI2, while the organic component leaves the surface. Second, water-induced degradation
also mainly forms SnI2, but further degradation into SnO2 was observed. Third, synthetic
air-induced degradation is shown to form SnO and an intermediate phase that was assumed
to combine tin, iodine, and oxygen. Finally, the formation of SnI4 was not detected for any
of the stimuli, contrary to what is described in the literature.7. Affordable and clean energ
Characterization of the surface properties of polycrystalline Cu(In,Ga)Se2 using a combination of scanning probe microscopy and X-ray photoelectron spectroscopy
No full textPolycrystalline Cu(In,Ga)Se2 (CIGSe) exhibit excellent properties for high power conversion efficiency (PCE) thin film solar cells. In recent years, photovoltaic cells made from CIGSe reached a PCE of 23.4\%, surpassing that of multicrystalline silicon photovoltaic cells. Nevertheless, the changes in surface composition and electronic properties of the absorbers after various solution-based surface treatments are still under intensive investigation and are widely discussed in the literature. In this thesis, the front, the rear surface properties as well as the impact of post-deposition treatments (PDT) on CIGSe absorbers with different elemental compositions were analyzed by scanning tunneling microscopy and spectroscopy, Kelvin probe force microscopy, and X-ray photoelectron spectroscopy.
I show that potassium cyanide (KCN) etching reduces the Cu content at the surface of Cu-rich absorbers substantially. The reduction of the Cu-content is accompanied with the formation of a large number of defects at the surface. Scanning tunneling spectroscopy measurements showed that most of these defects could be passivated with Cd ions. A semiconducting surface and no changes in the density of states were measured across the grain boundaries. In addition to the defect passivation an increase in surface band bending due to the substitution of Cu vacancies by Cd ions, which act as shallow donor defects was observed. As in the case of the front surface, the analyses carried out on the back surface of Cu-rich absorbers showed that a detrimental CuxSe secondary phase was also formed at the interface between the MoSe2 layer and CISe absorber after growth. This CuxSe secondary phase at the back contact was not present in Cu-poor absorbers.
Regarding the alkali metal post-treated absorbers, I show that the occurrence of an enlarged surface bandgap, often reported on CIGSe absorbers after PDT treatment is only present after H2O rinsing. After ammonia (NH4OH) washing, which is always applied before buffer layer deposition, all the high bandgap precipitates disappeared and an increased amount of an ordered vacancy compound was observed.
The thesis thereby gives a comprehensive overview of the CIGSe surfaces after various chemical and post deposition treatments
Co-evaporation and Scanning Probe Microscopy Characterizations of Hybrid Halide Perovskite Thin Films for Solar Cells
No full textHybrid 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
Effects of Annealing and Light on Co-evaporated Methylammonium Lead Iodide Perovskites using Kelvin Probe Force Microscopy in Ultra-High Vacuum
No full textpeer reviewedCareful 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
absorbers
No full textpeer reviewedIn-depth understanding and subsequent optimization of the contact layers in thin
film solar cells are of high importance in order to reduce the amount of nonradiative
recombination and thereby improve device performance. In this work, the buried
MoSe2/CuInSe2 interface of stoichiometric absorbers is investigated with scanning
tunneling spectroscopy and Kelvin probe force microscopy combined with compositional
measurements acquired via photo-electron spectroscopy after a mechanical
lift-off process. We find that the local density of states, as measured with scanning
tunneling spectroscopy, is similar to the front-side of ultra-high vacuum annealed
CISe absorbers. The grain boundaries exhibit a weak upward band bending, opposite
to Cu-poor CuGaSe2, and we measure an increased Cu accumulation at the rear CISe
surface compared to the bulk composition and a non-zero concentration of Cu on
the Mo-side. Grazing incidence X-ray diffraction measurements corroborate that a
small amount of a CuxSe secondary phase is present at the MoSe2/CuInSe2 interface
in contrast to reports on Cu-poor material. Our findings shed new light into the complex
interface formation of CuInSe2-based thin film solar cells grown under Cu-rich
conditions.SUNSPO
Impact of metallic potassium post-deposition treatment on epitaxial Cu(In, Ga)Se2
No full textpeer reviewedAlkali post-deposition treatments (PDTs) of Cu(In, Ga)Se2 (CIGSe) absorbers are known to improve the power conversion efficiency of the thin-film solar cell devices. The PDTs are usually carried out via evaporation of alkali fluorides in a selenium atmosphere onto a hot substrate. In this work, an alkali metal dispenser was used to evaporate pure metallic potassium onto epitaxial CIGSe absorbers. Subsequently, the absorber layers were heated in-situ to monitor chemical reactions and diffusion into the bulk. Due to the absence of grain boundaries, fluorine, and selenium, the effect of K on CIGSe absorber properties can be directly monitored. We find that potassium effectively diffuses into the bulk of epitaxial CIGSe absorber layers. The diffusion depends on the Cucontent of the CIGSe absorbers, in which Cu-depleted films present higher diffusion rates of K. Photoluminescence (PL) imaging corroborates that K in the bulk of the CIGSe absorber increases the PL yield, suggesting a passivation of defects or an increase in doping. This work highlights that alkali PDTs are not limited by interface and grain boundary modifications but also changes the absorber bulk properties, which needs to be taken into account
Detecting ZnSe secondary phase in Cu2ZnSnSe4 by room temperature photoluminescence
No full textpeer reviewe
Surface characterization of epitaxial Cu-rich CuInSe2 absorbers
No full textpeer reviewedWe investigated the electrical properties of epitaxial Cu-rich CuInSe 2 by Kelvin probe force microscopy (KPFM) under ambient and ultra-high vacuum conditions. We first measured the sample under ambient conditions before and after potassium cyanide (KCN) etching. In both cases, we do not see any substantial contrast in the surface potential data; furthermore, after the KCN etching we observed outgrowths with a height around 2nm over the sample surface. On the other hand, the KPFM measurements under ultra-high vacuum conditions show a work function dependence according to the surface orientation of the Cu-rich CuInSe 2 crystal. Our results show the possibility to increase the efficiency of epitaxial Cu-rich CuInSe 2 by growing the materials in the appropriated surface orientation where the variations in work function are reduced
Single-crystalline TiO2 nanoparticles for stable and efficient perovskite modules
No full textpeer reviewe
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