Sevan Gharabeiki

University of Luxembourg

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Susanne Siebentritt

University of Luxembourg

Michele Melchiorre

University of Luxembourg

Ajay Singh

University of Luxembourg

Taowen Wang

University of Luxembourg

Phillip J. Dale

University of Luxembourg

Alex Redinger

University of Luxembourg

Himanshu Phirke

University of Luxembourg

Ricardo G. Poeira

University of Luxembourg

Jan P. F. Lagerwall

University of Luxembourg

Mohit Sood

University of Luxembourg

Cooperative Institutions

University of Luxembourg

Luxembourg Institute of Science and Technology

Helmholtz-Zentrum Berlin für Materialien und Energie

University of Cambridge

Centre National de la Recherche Scientifique

International Iberian Nanotechnology Laboratory

New York University Press

National Institute of Advanced Industrial Science and Technology

Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute

Clinical Academic Center of Braga

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Impact of Minimal Silver Incorporation on Chalcopyrite Absorbers—Origins for Improved Open‐Circuit Voltages in (Ag,Cu)(In,Ga)Se₂ Solar Cells

Solar RRL (2025)

Sateesh Prathapani Sevan Gharabeiki Jakob Lauche René Schwiddessen P. Reyes-Figueroa

The influence of minimal amounts of Ag (0.5–1.4 at%) on elemental distribution and crystalline quality of (Ag,Cu)(In,Ga)Se 2 (ACIGSe) absorbers grown by the three‐stage coevaporation without added alkali elements is reported. The elemental ratios affect the amount of Ag to be uniformly incorporated into the chalcopyrite absorber and the open‐circuit voltage ( V OC ) of the ACIGSe solar cell devices. Ag‐containing absorbers deposited at 530 °C achieve a best photoconversion efficiency of 18.2%. Due to an increased V OC , ACIGSe absorbers perform better than their Ag‐free variants at low deposition temperatures. The factors contributing to this increased V OC of low‐temperature devices are: 1) enhanced elemental Ga and In interdiffusion and hence their spatial distribution across the absorber thickness, leading to an increase in the minimum bandgap, 2) an improved absorber crystalline quality with larger grains resulting in high quasi‐Fermi‐level splitting and lower nonradiative losses. The photoluminescence data obtained on the ACIGSe absorbers reveal the corresponding variations in their bandgap and photoluminescence quantum yield. These material‐level insights into Ag incorporation in chalcopyrite help to advance the development of chalcopyrite‐based tandem solar cells, which—so far—is limited by the requirement of high deposition temperatures.

Open-circuit voltage

Deposition

10.1002/solr.202400863

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Optoelectrical and structural characterization of Cu2SnS3 thin films grown via spray pyrolysis using stable molecular ink

Solar Energy (2021)

Maryam Heidariramsheh Sevan Gharabeiki Seyed Mohammad Mahdavi Nima Taghavinia

Tetragonal crystal system

10.1016/j.solener.2021.05.088

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Vacuum‐Healing of Grain Boundaries in Sodium‐Doped CuInSe₂ Solar Cell Absorbers

Advanced Energy Materials (2023)

Finn Babbe Nicoleta Nicoara Harvey Guthrey Nathalie Valle Omar Ramírez

Abstract Alkali metal doping and grain boundaries (GB) have been at the center of attention within the Cu(In,Ga)(S,Se) 2 photovoltaics community for years. This study provides the first experimental evidence that the GB of sodium‐doped CuInSe 2 thin films may undertake reversible oxidation even at room temperature, whereas undoped films may not. The findings are corroborated by cathodoluminescence imaging, secondary ion mass spectrometry, and Kelvin probe force microscopy on air‐exposed films subsequently subject to vacuum. A thermochemical assessment identifies the likely solid–gas equilibria involved. These reactions open new research questions with respect to the beneficial role played by alkali metal dopants in chalcopyrite solar cells and may steer the community toward new breakthroughs.

Cathodoluminescence

10.1002/aenm.202204183

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Effect of Band Gap Gradient on the Open Circuit Voltage Limit of the Cu(In,Ga)Se2 Absorbers

Sevan Gharabeiki Tilly Schaaf Francesco Lodola Taowen Wang Michele Melchiorre

Open-circuit voltage

10.1109/pvsc57443.2024.10749239

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Quantifying recombination and charge carrier extraction in halide perovskites via hyperspectral time-resolved photoluminescence imaging

APL Energy (2024)

Himanshu Phirke Sevan Gharabeiki Ajay Singh Anurag Krishna Susanne Siebentritt

Identifying sources of nonradiative recombination and quantifying charge carrier extraction in halide perovskite solar cells are important in further developing this thin-film technology. Steady-state and time-resolved photoluminescence (TRPL), in combination with analytical modeling, have emerged as non-destructive tools to achieve the desired results. However, the exact location of the recombination and charge carrier extraction losses in devices is often obscured by various competing processes when photoluminescence measurements are analyzed. Here, we show via absolute-photon-calibrated hyperspectral photoluminescence and TRPL imaging how surface passivation and inhomogeneities at interfaces impact the photoluminescence quantum yields and minority carrier lifetimes. Laser illumination from the perovskite and glass/TiO2 sides allows us to disentangle changes in surface recombination velocity from the charge carrier extraction at the electron transport layer. We find that charge extraction is spatially modulated due to an inhomogeneous mesoporous (mp)-TiO2 film thickness. Our results show that the mp-TiO2 layer is not fully optimized since the electronic properties are spatially modified, leading to lateral changes in quasi-Fermi-level splitting, minority carrier lifetime and, consequently, a reduction in open-circuit voltage.

Charge carrier

10.1063/5.0188166

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Shifting the Paradigm: A Functional Hole‐Selective Transport Layer for Chalcopyrite Solar Cells

Solar RRL (2024)

Taowen Wang Longfei Song Sevan Gharabeiki Mohit Sood Aubin JC. M. Prot

High‐efficiency Cu(In,Ga)Se 2 solar cells rely on Ga grading to mitigate back surface recombination. However, the inhomogeneous absorber has drawbacks, including increased non‐radiative loss and inadequate absorption. Therefore, literatures demand a paradigm shift of using a hole‐transport layer to passivate the back surface. Herein, a functional hole‐transport layer is demonstrated as an alternative to Ga grading. The novel hole‐transport layer is prepared as a double‐layer: co‐evaporated CuGaSe 2 covered by solution combustion synthesis prepared In 2 O 3 . As demonstrated by micrographs, elemental mapping, and photoluminescence spectroscopy, the oxide layer improves thermal stability and prevents Ga diffusion. However, during the absorber deposition, a complete ion exchange of In and Ga converts CuGaSe 2 /In 2 O 3 into CuInSe 2 /GaO x . Incorporating this hole‐transport layer in co‐evaporated nongraded CuInSe 2 solar cells leads to significantly increased minority carrier lifetime from 5 to 113 ns, yielding an 80 meV improvement in quasi‐Fermi‐level splitting. The devices exhibit improved open‐circuit voltage, as well as a promising fill factor of over 71%, indicating good hole‐transport properties. In these results, the passivation effect and good hole‐transport properties of the hole‐transport layer are experimentally demonstrated. Thus, high‐efficiency solar cells can be achieved by using a functional hole‐transport layer without relying on Ga grading.

Passivation

10.1002/solr.202400212

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Coevaporation Stabilizes Tin-Based Perovskites in a Single Sn-Oxidation State

Nano Letters (2022)

Ajay Singh Jérémy Hieulle Joana Ferreira Machado Sevan Gharabeiki Weiwei Zuo

Chemically processed methylammonium tin-triiodide (CH3NH3SnI3) films include Sn in different oxidation states, leading to poor stability and low power conversion efficiency of the resulting solar cells (PSCs). The development of absorbers with Sn [2+] only has been identified as one of the critical steps to develop all Sn-based devices. Here, we report on coevaporation of CH3NH3I and SnI2 to obtain absorbers with Sn being only in the preferred oxidation state [+2] as confirmed by X-ray photoelectron spectroscopy. The Sn [4+]-free absorbers exhibit smooth highly crystalline surfaces and photoluminescence measurements corroborating their excellent optoelectronic properties. The films show very good stability under heat and light. Photoluminescence quantum yields up to 4 × 10-3 translate in a quasi Fermi-level splittings exceeding 850 meV under one sun equivalent conditions showing high promise in developing lead-free, high efficiency, and stable PSCs.

Triiodide

Quantum Efficiency

Oxidation state

10.1021/acs.nanolett.2c02204

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Sn4+-free, stable tin perovskite films for lead-free perovskite solar cells

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC) (2022)

Ajay Singh Jérémy Hieulle Himanshu Phirke Joana A. F. Machado Sevan Gharabeiki

Organic-inorganic hybrid perovskite solar cells (PSCs) achieved already more than 25% power conversion efficiency making them one of the fastest-growing solar cell technologies. However, most state-of-art PSCs employ lead (Pb) as a "B" cation in the ABX3 type crystalline absorbers. Sn has been considered to be a potential replacement for Pb. However, chemically processed Sn-perovskite films often exhibit a high number of Sn vacancies and a high density of Sn4+ species, leading to poor stability and low power conversion efficiency of the tin-based PSCs. Herein we report co-evaporation of CH3NH3I and SnI2 to obtain Sn4+-free CH3NH3SnI3 perovskite films with excellent optoelectronic properties and improved stability. XRD measurements confirm the polycrystalline films, mainly oriented in the (100) plane of a cubic crystal. XPS analysis confirms that the films consist of Sn only in the 2+ oxidation state. AFM and KPFM analysis reveal smooth topology and uniform surface workfunction. The films show very good stability under heating and photodegradation confirmed by XRD and photoluminescence (PL) degradation measurements. The co-evaporated CH3NH3SnI3 films exhibit PL quantum yields up to 9x10-4 translating in a quasi Fermi-level splitting of 844 meV under one sun equivalent conditions. Extracted low values of Urbach energies (16 meV) suggest that the films exhibit a low number of defects near the band edges showing high promise in developing lead-free, high efficiency, and stable PSCs.

10.1109/pvsc48317.2022.9938891

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Influence of NaF and KF Post-Deposition Treatment on the Sub-Band Gap Absorption of Cu(In,Ga)Se₂ Absorber Layers

Sevan Gharabeiki Michele Melchiorre Susanne Siebentritt

Urbach tails describe a density of states that decays exponentially into the band gap. Non-radiative and radiative recombinations in the Urbach tails contributes to voltage loss in the Cu(In, Ga)Se2 CIGSe solar cells. In the CIGSe semiconductor, the Urbach tails can be suppressed by alkali fluoride post-deposition treatment (PDT). We report the impact of NaF and KF-PDT on the quasi-Fermi level splitting (QFLS) and Urbach energy of the CIGSe absorbers, which are evaluated by absolute photoluminescence measurement. Our data reveal that both NaF and KF PDT have beneficial impacts on the QFLS. Samples with a low Na concentration show high Urbach energies, which are decreased by NaF-PDT. Sufficient KF PDT on the samples deposited on Mo-coated soda lime glass (i.e., with a high concentration of Na) further decreases the Urbach energy. The improvement of the QFLS is not solely due to the reduction of tail states, but it can also be due to surface passivation, grain boundary passivation or increase in the doping concentration.

Passivation

Deposition

Wide-bandgap semiconductor

10.1109/pvsc48320.2023.10360015

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Photoluminescence assessment of materials for solar cell absorbers

Faraday Discussions (2022)

Susanne Siebentritt Uwe Rau Sevan Gharabeiki Thomas Paul Weiss Aubin JC. M. Prot

Absolute photoluminescence measurements present a tool to predict the quality of photovoltaic absorber materials before finishing the solar cells. Quasi Fermi level splitting predicts the maximal open circuit voltage. However, various methods to extract quasi Fermi level splitting are plagued by systematic errors in the range of 10-20 meV. It is important to differentiate between the radiative loss and the shift of the emission maximum. They are not the same and when using the emission maximum as the "radiative" band gap to extract the quasi Fermi level splitting from the radiative efficiency, the quasi Fermi level splitting is 10 to 40 meV too low for a typical broadening of the emission spectrum. However, radiative efficiency presents an ideal tool to compare different materials without determining the quasi Fermi level splitting. For comparison with the open circuit voltage, a fit of the high energy slope to generalised Planck's law gives more reliable results if the fitted temperature, i.e. the slope of the high energy part, is close to the actual measurement temperature. Generalised Planck's law also allows the extraction of a non-absolute absorptance spectrum, which enables a comparison between the emission maximum energy and the absorption edge. We discuss the errors and the indications when they are negligible and when not.

Open-circuit voltage

10.1039/d2fd00057a

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Citations (18)