Controlling the Anionic Ratio and Gradient in Kesterite Technology

Abstract

Accurate anionic control during the formation of chalcogenide solid solutions is fundamental for tuning the physicochemical properties of this class of materials. Compositional grading is the key aspect of band gap engineering and is especially valuable at the device interfaces for an optimum band alignment, for controlling interface defects and recombination and for optimizing the formation of carrier-selective contacts. However, a simple and reliable technique that allows standardizing anionic compositional profiles is currently missing for kesterites and the feasibility of achieving a compositional gradient remains a challenging task. This work aims at addressing these issues by a simple and innovative technique. It basically consists of first preparing a pure sulfide absorber with a specific thickness followed by the synthesis of a pure selenide part of complementary thickness on top of it. Specifically, the technique is applied to the synthesis of Cu 2 ZnSn(S,Se) 4 and Cu 2 ZnGe(S,Se) 4 kesterite absorbers, and a series of characterizations are performed to understand the anionic redistribution within the absorbers. For identical processing conditions, different Se incorporation dynamics is identified for Sn- and Ge-based kesterites, leading to a homogeneous or graded composition in depth. It is first demonstrated that for Sn-based kesterite the anionic composition can be perfectly controlled through the thicknesses ratio of the sulfide and selenide absorber parts. Then, it is demonstrated that for Ge-based kesterite an anionic (Se–S) gradient is obtained and that by adjusting the processing conditions the composition at the back side can be finely tuned. This technique represents an innovative approach that will help to improve the compositional reproducibility and determine a band gap grading strategy pathway for kesterites. Furthermore, due to its simplicity and reliability, the proposed methodology could be extended to other chalcogenide materials.

Characterization of the Stability of Indium Tin Oxide and Functional Layers for Semitransparent Back‐Contact Applications on Cu(in,Ga)Se2Solar Cells

“…To check if the process induced additional damage, selected samples were removed using a developed lift‐off process using a microscope slide and an adhesive epoxy. [ 55 ] 2) The back CIGSe/FL/ITO interface was analyzed by measuring the spectra with the laser spot on the back surface of the devices as the used wavelengths were able to penetrate through the glass substrate. Configuration (2) had the advantage that measurements were not affected by potential damage effects related to the removal of the CIGSe top layer but were compromised by a lower signal‐to‐noise ratio.…”

Gradient Doping in Cu 2 ZnSnSe 4 by Temperature and Potential Induced Defect Steering

“…[11] On the other hand, a gradient in the charge carrier density has been experimentally achieved by only one approach: intrinsic or extrinsic compositional grading. [12][13][14][15][16][17][18][19][20][21] In light of the foregoing, in this work we propose and prove an innovative approach to improve the charge carrier selectivity in PV devices by generating for the first time grading in the shallow doping concentration, taking advantage of the O/D transition in kesterite absorbers. In particular, by controlling the local concentration of ionized Cu Zn -, V Cu -(acceptors, negatively charged), and Zn Cu + (donor, positively charged) defects.…”

Microenvironment Created by SnSe2 Vapor and Pre‐Selenization to Stabilize the Surface and Back Contact in Kesterite Solar Cells

“…[ 38,39 ] Second, most previous studies on selenization techniques have focused on crystal growth accompanied by Sn chemical equilibrium, which is dependent on thermodynamic and kinetic conditions, including cationic content of the precursors, [ 40,41 ] pre‐selenization, [ 12,42 ] and chalcogen vapor partial pressure. [ 43–45 ]…”

“…[38,39] Second, most previous studies on selenization techniques have focused on crystal growth accompanied by Sn chemical equilibrium, which is dependent on thermodynamic and kinetic conditions, including cationic content of the precursors, [40,41] pre-selenization, [12,42] and chalcogen vapor partial pressure. [43][44][45] The ambient air-processed preparation of kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin film is highly promising for the fabrication of low-cost and eco-friendly solar cells. However, the Sn volatilization loss and formation of a thick Mo(S,Se) 2 interfacial layer during the traditional selenization process pose challenges for fabricating high-efficiency CZTSSe solar cells.…”