Graded bandgap concepts have been shown to be a disruptive issue in order to achieve high efficiency devices in thin film chalcogenide photovoltaics. In the case of kesterite (Cu2ZnSn(SxSe(1-x))4) based solar cells, two main strategies are being explored in order to achieve bandgap grading: cationic substitution (Cu by Ag, Zn by Cd, Sn by Ge and Si) and anionic (S-Se) compositional engineering. Both the cationic and anionic substitutions are complementary as they primarily affect either the conduction (cation) or valence (anion) bands. The specific control of the anionic composition along the volume of kesterite absorbers has been found challenging due to the similar formation energy of selenide and sulfide compounds that favors S-Se solid solutions over phase segregation. This work presents the development of a novel and non-toxic reactive annealing process that allows the synthesis of CZTSSe solar cells with S-Se sharp graded compositional profiles and sheds light onto the dynamics of anionic bandgap grading in kesterite solar cells. This by employing different S sources (pure S > CH4N2S > SeS2 > SnS, ordered by expected reactivity), trying to control perfectly the S and Se content of the final CZTSSe layers and to form an S-enriched top surface. After an optimization of the annealing parameters, we demonstrate the feasibility of a graded composition CZTSSe phase with high S content at the top by introducing pure S during the cooling down process of the CZTSe kesterite. Multi-wavelength depth-resolved Raman spectroscopy measurements allowed to characterize the complex graded structure and estimate the composition of the CZTSSe layers. A bandgap > 1.20 eV within the depletion zone and < 1.03 eV towards the back contact interface is obtained this way. These results are correlated with in-depth Auger Electron Spectroscopy (AES), Grazing Incidence X-Ray Diffraction (GIXRD) and X-ray fluorescence (XRF), confirming the possibility of achieving a complex graded anionic compositional, and therefore, graded bandgap profiles. The fabricated solar cell devices show conversion efficiencies > 9 %, with a Voc deficit of 420 mV, which currently represent a promising result for a non-optimized graded absorber.
In summary, preliminary results in which the samples are first selenized and then, in-situ superficially sulfurized, show the possibility of creating an inverted grading profile with higher bandgap towards the surface of the absorber by using pure elemental S source. We will show that obtaining S-Se graded bandgap profiles (both at the front and back interfaces) is only possible when the S is introduced in an out-of-equilibrium state. Strategies for achieving more complex graded bandgap profiles will be discussed including effects on device performance using theoretical analysis by SCAPS modeling.