Grain boundaries and potassium post-deposition treatments in chalcopyrite solar cells

Doctoral thesis (2022)

Over 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 ... [more ▼]

Over 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. [less ▲]

Impact of metallic potassium post-deposition treatment on epitaxial Cu(In, Ga)Se2

in Thin Solid Films (2021)

Alkali 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 ... [more ▼]

Alkali 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. [less ▲]

Co-evaporation of CH3NH3PbI3: How Growth Conditions Impact Phase Purity, Photostriction, and Intrinsic Stability

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 ▲]

Surface characterization of epitaxial Cu-rich CuInSe2 absorbers

in IEEE Photovoltaic Specialists Conference. Conference Record (2019, July)

We 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 ... [more ▼]

We 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. [less ▲]

Unraveling the Role of Sn Segregation in the Electronic Transport of Polycrystalline Hematite: Raising the Electronic Conductivity by Lowering the Grain-Boundary Blocking Effect

in Advanced Electronic Materials (2019), 0(0), 1900065

This paper describes the role of SnO2 in the electronic transport of polycrystalline hematite (α-Fe2O3). The proper sintering process allows for freezing of a state of electronic defects, in which the ... [more ▼]

This paper describes the role of SnO2 in the electronic transport of polycrystalline hematite (α-Fe2O3). The proper sintering process allows for freezing of a state of electronic defects, in which the electrical properties of hematite are controlled by the grain boundary and Sn segregation. Impedance spectroscopy and dc conductivity measurements show that current flows through preferential pathways associated with Sn segregation that occurs at the grain boundary, leading to a decrease in grain-boundary resistance. Atomic force microscopy and electric force microscopy measurements confirm the results of the impedance analysis. The identification of preferential grain boundaries for electrical conductivity may have a direct influence on the light-induced water-splitting performance of the hematite photoanode. [less ▲]

Covalent functionalization of graphene oxide with D-mannose: evaluating the hemolytic effect and protein corona formation

in Journal of Materials Chemistry B (2018), 6(18), 2803--2812

In this work, graphene oxide (GO) was covalently functionalized with D-mannose (man-GO) using mannosylated ethylenediamine. XPS (C1s and N1s) confirmed the functionalization of GO through the binding ... [more ▼]

In this work, graphene oxide (GO) was covalently functionalized with D-mannose (man-GO) using mannosylated ethylenediamine. XPS (C1s and N1s) confirmed the functionalization of GO through the binding energies at 288.2 eV and 399.8 eV, respectively, which are attributed to the amide bond. ATR-FTIR spectroscopy showed an increase in the amine bond intensity, at 1625 cm−1 (stretching CO), after the functionalization step. Furthermore, the man-GO toxicity to human red blood cells (hemolysis) and its nanobiointeractions with human plasma proteins (hard corona formation) were evaluated. The mannosylation of GO drastically reduced its toxicity to red blood cells. SDS-PAGE analysis showed that the mannosylation process of GO also drastically reduced the amount of the proteins in the hard corona. Additionally, proteomics analysis by LC–MS/MS revealed 109 proteins in the composition of the man-GO hard corona. Finally, this work contributes to future biomedical applications of graphene-based materials functionalized with active biomolecules. [less ▲]

Separating the influence of electric charges in magnetic force microscopy images of inhomogeneous metal samples

in Journal of Magnetism and Magnetic Materials (2018), 446

In this study, we investigate artifacts arising from electric charges present in magnetic force microscopy images. Therefore, we use two austenitic steel samples with different microstructural conditions ... [more ▼]

In this study, we investigate artifacts arising from electric charges present in magnetic force microscopy images. Therefore, we use two austenitic steel samples with different microstructural conditions. Furthermore, we examine the influence of the surface preparation, like etching, in magnetic force images. Using Kelvin probe force microscopy we can quantify the charges present on the surface. Our results show that electrical charges give rise to a signature in the magnetic force microscopy, which is indistinguishable from a magnetic signal. Our results on two differently aged steel samples demonstrate that the magnetic force microscopy images need to be interpreted with care and must be corrected due to the influence of electrical charges present. We discuss three approaches, how to identify these artifacts – parallel acquisition of magnetic force and electric force images on the same position, sample surface preparation to decrease the presence of charges and inversion of the magnetic polarization in two succeeding measurement. [less ▲]

Surface modified cellulose scaffolds for tissue engineering

in Cellulose (2016)

We report the ability of cellulose to support cells without the use of matrix ligands on the surface of the material, thus creating a two-component system for tissue engineering of cells and materials ... [more ▼]

We report the ability of cellulose to support cells without the use of matrix ligands on the surface of the material, thus creating a two-component system for tissue engineering of cells and materials. Sheets of bacterial cellulose, grown from a culture medium containing Acetobacter organism were chemically modified with glycidyltrimethylammonium chloride or by oxidation with sodium hypochlorite in the presence of sodium bromide and 2,2,6,6-tetramethylpipiridine 1-oxyl radical to introduce a positive, or negative, charge, respectively. This modification process did not degrade the mechanical properties of the bulk material, but grafting of a positively charged moiety to the cellulose surface (cationic cellulose) increased cell attachment by 70 compared to unmodified cellulose, while negatively charged, oxidised cellulose films (anionic cellulose), showed low levels of cell attachment comparable to those seen for unmodified cellulose. Only a minimal level of cationic surface derivitisation (ca 3 degree of substitution) was required for increased cell attachment and no mediating proteins were required. Cell adhesion studies exhibited the same trends as the attachment studies, while the mean cell area and aspect ratio was highest on the cationic surfaces. Overall, we demonstrated the utility of positively charged bacterial cellulose in tissue engineering in the absence of proteins for cell attachment. [less ▲]

Determination of High Frequency Dielectric Constant and Surface Potential of Graphene Oxide and Influence of Humidity by KPFM.

in Langmuir (2015)

We use Kelvin probe force microscopy (KPFM) and capacitance coupling (dC/dz) to study the electrical properties of graphene oxide (GO). We propose using the dC/dz signal to probe the high frequency ... [more ▼]

We use Kelvin probe force microscopy (KPFM) and capacitance coupling (dC/dz) to study the electrical properties of graphene oxide (GO). We propose using the dC/dz signal to probe the high frequency dielectric constant of mono- and few-layer GO. Our measurements suggest that the dynamic dielectric constant of GO is on the order of εGO ? 3.0 ε0, in the high frequency limit, and independent of the number of GO layers. The measurements are performed at a humidity controlled environment (5 of humidity). The effects of increasing humidity on both the dC/dz and KPFM measurements are analyzed. [less ▲]

Scratch testing for micro- and nanoscale evaluation of tribocharging in DLC films containing silver nanoparticles using AFM and KPFM techniques

in Surface and Coatings Technology (2014), 260

Scratch testing is a fast and effective method for the measurement of critical loads in order to determine the adhesion properties of coatings and their behavior in tribological applications. Kelvin probe ... [more ▼]

Scratch testing is a fast and effective method for the measurement of critical loads in order to determine the adhesion properties of coatings and their behavior in tribological applications. Kelvin probe force microscopy (KPFM) provides a means of monitoring electrostatic charging on the surfaces of materials. In this paper, we describe the use of a combination of scratch testing and KPFM analysis to evaluate the electrostatic effect induced by silver nanoparticles incorporated as clusters in diamond-like carbon (DLC) films, as well as its correlation with the rubbing process. KPFM was used for mapping of the potentials on the surfaces of DLC–Ag films subjected to nanoscale scratching. The procedure was also conducted at the microscale in order to analyze the way in which silver nanoparticles were spread in the track. After scratching, the track was analyzed using backscattered electrons (BSE) and energy dispersive X-ray diffraction (EDX). The BSE images highlighted bright domains of metallic nanoparticles dispersed in the amorphous coating and EDX confirmed the presence of silver nanoparticles in the scratched track. Micro Raman spectroscopy was used to check the DLC signature. The electric potentials of DLC films with and without silver nanoparticles were also analyzed. The results indicated that the incorporation of silver nanoparticles in amorphous materials could offer new option for electrostatic energy storage on the surfaces of materials. [less ▲]