Modifications of the CZTSe/Mo Back-Contact Interface by Plasma Treatments

Molybdenum (Mo) is the most commonly used back-contact material for copper zinc tin selenide (CZTSe)-based thin-film solar cells. For most fabrication methods, an interfacial molybdenum diselenide (MoSe2) layer with an uncontrolled thickness is formed, ranging from a few tens of nm up to ≈1 μm. In order to improve the control of the back-contact interface in CZTSe solar cells, the formation of a MoSe2 layer with a homogeneous and defined thickness is necessary. In this study, we use plasma treatments on the as-grown Mo surface prior to the CZTSe absorber formation, which consists of the deposition of stacked metallic layers and the annealing in selenium (Se) atmosphere. The plasma treatments include the application of a pure argon (Ar) plasma and a mixed argon–nitrogen (Ar–N2) plasma. We observe a clear impact of the Ar plasma treatment on the MoSe2 thickness and interfacial morphology. With the Ar–N2 plasma treatment, a nitrided Mo surface can be obtained. Furthermore, we combine the Ar plasma treatment with the application of titanium nitride (TiN) as back-contact barrier and discuss the obtained results in terms of MoSe2 formation and solar cell performance, thus showing possible directions of back-contact engineering for CZTSe solar cells

In2_{2}O3_{3}:H-Based Hole-Transport-Layer-Free Tin/Lead Perovskite Solar Cells for Efficient Four-Terminal All-Perovskite Tandem Solar Cells

Narrow-band gap (NBG) Sn–Pb perovskites with band gaps of ∼1.2 eV, which correspond to a broad photon absorption range up to ∼1033 nm, are highly promising candidates for bottom solar cells in all-perovskite tandem photovoltaics. To exploit their potential, avoiding optical losses in the top layer stacks of the tandem configuration is essential. This study addresses this challenge in two ways (1) removing the hole-transport layer (HTL) and (2) implementing highly transparent hydrogen-doped indium oxide In2O3:H (IO:H) electrodes instead of the commonly used indium tin oxide (ITO). Removing HTL reduces parasitic absorption loss in shorter wavelengths without compromising the photovoltaic performance. IO:H, with an ultra-low near-infrared optical loss and a high charge carrier mobility, results in a remarkable increase in the photocurrent of the semitransparent top and (HTL-free) NBG bottom perovskite solar cells when substituted for ITO. As a result, an IO:H-based four-terminal all-perovskite tandem solar cell (4T all-PTSCs) with a power conversion efficiency (PCE) as high as 24.8% is demonstrated, outperforming ITO-based 4T all-PTSCs with PCE up to 23.3%

2D/3D Heterostructure for Semitransparent Perovskite Solar Cells with Engineered Bandgap Enables Efficiencies Exceeding 25% in Four‐Terminal Tandems with Silicon and CIGS

Wide-bandgap perovskite solar cells (PSCs) with optimal bandgap (E$_{g}$) and high power conversion efficiency (PCE) are key to high-performance perovskite-based tandem photovoltaics. A 2D/3D perovskite heterostructure passivation is employed for double-cation wide-bandgap PSCs with engineered bandgap (1.65 eV ≤ E$_{g}$ ≤ 1.85 eV), which results in improved stabilized PCEs and a strong enhancement in open-circuit voltages of around 45 mV compared to reference devices for all investigated bandgaps. Making use of this strategy, semitransparent PSCs with engineered bandgap are developed, which show stabilized PCEs of up to 25.7% and 25.0% in fourterminal perovskite/c-Si and perovskite/CIGS tandem solar cells, respectively. Moreover, comparable tandem PCEs are observed for a broad range of perovskite bandgaps. For the first time, the robustness of the four-terminal tandem configuration with respect to variations in the perovskite bandgap for two state-of-the-art bottom solar cells is experimentally validated

Scalable two-terminal all-perovskite tandem solar modules with a 19.1% efficiency

Monolithic all-perovskite tandem photovoltaics promise to combine low-cost and high-efficiency solar energy harvesting with the advantages of all-thin-film technologies. To date, laboratory-scale all-perovskite tandem solar cells have only been fabricated using non-scalable fabrication techniques. In response, this work reports on laser-scribed all-perovskite tandem modules processed exclusively with scalable fabrication methods (blade coating and vacuum deposition), demonstrating power conversion efficiencies up to 19.1% (aperture area, 12.25 cm2; geometric fill factor, 94.7%) and stable power output. Compared to the performance of our spin-coated reference tandem solar cells (efficiency, 23.5%; area, 0.1 cm2), our prototypes demonstrate substantial advances in the technological readiness of all-perovskite tandem photovoltaics. By means of electroluminescence imaging and laser-beam-induced current mapping, we demonstrate the homogeneous current collection in both subcells over the entire module area, which explains low losses (<5%rel) in open-circuit voltage and fill factor for our scalable modules

Noise Sensitivity in Continuum Percolation

We prove that the Poisson Boolean model, also known as the Gilbert disc model, is noise sensitive at criticality. This is the first such result for a Continuum Percolation model, and the first for which the critical probability p_c \ne 1/2. Our proof uses a version of the Benjamini-Kalai-Schramm Theorem for biased product measures. A quantitative version of this result was recently proved by Keller and Kindler. We give a simple deduction of the non-quantitative result from the unbiased version. We also develop a quite general method of approximating Continuum Percolation models by discrete models with p_c bounded away from zero; this method is based on an extremal result on non-uniform hypergraphs.Comment: 42 page

Potential and current distribution of the quantum Hall effect measured by scanning force microscopy

Obwohl der Quanten-Hall-Effekt seit seiner Entdeckung 1980 eine Vielzahl an Forschungsarbeiten nach sich gezogen hat, ist bis heute nicht geklärt, wo mikroskopisch der externe Strom innerhalb des zweidimensionalen Elektronensystems (2DES) fließt. Unter Quanten-Hall-Bedingungen ist die Existenz sogenannter kompressibler und inkompressibler Streifen innerhalb der Randverarmungszone des 2DES vorhergesagt worden, die sich entweder metallisch oder isolierend verhalten und damit die Hall-Potentialverteilung innerhalb des 2DES beeinflussen. Die Details des Stromtransports werden dabei aber noch immer kontrovers diskutiert. Daher wird in dieser Arbeit ein Tieftemperatur-Rasterkraftmikroskop verwendet, das auf elektrostatische Wechselwirkungen sensitiv ist, um die Potentialverteilung unter Quanten-Hall-Bedingungen mit einer Auflösung im Sub-Mikron-Bereich zu vermessen. Dies erlaubt dann auch Rückschlüsse auf die Stromverteilung. Abhängig vom Landau-Niveau-Füllfaktor (FF) ändern sich die Potentialprofile dramatisch und die Gesamtinterpretation ergibt sich daher folgendermaßen: Für ganzzahlige FF fällt das Potential nichtlinear über das Probeninnere ab und ein dissipationsfreier Strom wird demnach auch im Inneren der Probe getrieben. Aber bei etwas höheren FF fällt das Potential über die innersten inkompressiblen Streifen ab, was für einen nahezu dissipationsfreien Strom innerhalb dieser Randstreifen spricht. Wenn man sich an den nächsthöheren FF annähert, steigt der Potentialabfall über die Probenmitte zunehmend an. Hier fließt nun ein dissipativer Strom im Inneren der Probe, bis sich diese Entwicklung beim nächsten ganzzahligen FF wiederholt. Dieses Verhalten wird für verschiedene Probengeometrien und ebenso unter dem Einfluß eines benachbarten Metallkontakts diskutiert.Although the quantum Hall effect has stimulated a tremendous amount of research activities since its discovery in 1980, it has not been clarified up to now where microscopically the externally biased current is flowing through the two-dimensional electron system (2DES). Under quantum Hall conditions, the existence of so-called compressible and incompressible strips within the depletion region of the edge of the 2DES has been predicted which behave either metal- or insulator-like, influencing the Hall-potential distribution within the 2DES. The details of the current carrying process are however still controversially discussed. Therefore in this work a low-temperature scanning force microscope sensitive to electrostatics is used to investigate the potential distribution of a 2DES under quantum Hall conditions with submicron resolution which also allows further conclusions about the current distribution. Depending on the Landau level filling factor (FF) the potential profiles change dramatically and the final picture is as follows: For integer FF the potential drops rather nonlinearly across the sample and a dissipationless current is driven inside the sample. However, at slightly higher FF the potential drops across the innermost incompressible strips, indicating a nearly dissipationless current inside these strips at the sample edges. When approaching the next higher integer FF the potential drops increasingly across the interior of the sample hence suggesting a dissipative current transport inside the bulk of the sample until this evolution is repeated at the next integer FF. This behaviour is discussed for several sample geometries and also under the influence of nearby metal contacts

Evaluation of different metal salt solutions for the preparation of solar cells with wide-gap Cu2ZnGeSxSe4-x absorbers

In this work, thin-film solar cells with a kesterite-type Cu2ZnGeSxSe4-x (CZGSSe) absorber were prepared from four different metal salt solutions. Their high band gap makes them an interesting material for tandem solar cells. The structural and morphological properties of the absorbers are compared with an additional focus on the electrical properties of the resulting thin-film solar cells. Efficiencies exceeding 5 % could be demonstrated