Real-time observation of Cu2ZnSn(S,Se)4 solar cell absorber layer formation from nanoparticle precursors

Abstract

The selenization of Cu-Zn-Sn-S nanocrystals is a promising route for the fabrication of low-cost thin film solar cells. However, the reaction pathway of this process is not completely understood. Here, the evolution of phase formation, grain size, and elemental distributions is investigated during the selenization of Cu-Zn-Sn-S nanoparticle precursor thin films by synchrotron-based in situ energy-dispersive X-ray diffraction and fluorescence analysis as well as by ex situ electron microscopy. The precursor films are heated in a closed volume inside a vacuum chamber under presence of selenium vapor while diffraction and fluorescence signals are recorded. The presented results reveal that during the selenization the cations diffuse to the surface to form large grains on top of the nanoparticle layer and the selenization of the film takes place in two simultaneous reactions: 1) a direct and fast formation of large grained selenides, starting with copper selenide which is subsequently transformed into Cu 2 ZnSnSe 4 ; 2) a slower selenization of the remaining nanoparticles. As a consequence of the initial formation of copper selenides at the surface, the subsequent formation of CZTSe starts under Cu-rich conditions despite an overall Cu-poor composition of the film. The implications of this process path on the film quality is discussed. Additionally, the proposed growth model provides an explanation of the previously observed accumulation of carbon from the nanoparticle precursor beneath the large grained layer.

The role of interparticle heterogeneities in the selenization pathway of Cu–Zn–Sn–S nanoparticle thin films: a real-time study

“…The real-time EDXRD experiment follows the work reported in Ref. 13. Further description of the nanoparticle synthesis, sizeseparation, and characterization is provided in Ref.…”

“…(The same holds for the corresponding sulfide phases and the resulting peak near 36.5 keV denoted by Σ1. 21 ) However, since the peak near 22.4 keV corresponds to the 101 planes in tetragonal CZTSe, 12,13,22 we attribute Σ2 primarily to CZTSe grains once the CZTSe 101 signal is detected. During the appearance of Cu2-δSe and Σ2, the broad Σ1 peak of the large and mixed particle samples shifts towards the position of Σ2.…”

“…The EDXRD experiments conducted in Ref. 13 identified a transition through intermediate copper selenide phases during the growth of the quaternary CZTSe grains, which was proposed to be beneficial for the formation of a large grained morphology; however, the origin of the copper selenide was unclear, as its diffraction signal appeared abruptly without noticeable change in the signals from the nanoparticles in the precursor film. 13 The differences observed in the large and small particles are hypothesized to influence the various phases of growth observed in real-time EDXRD data of the unseparated (mixed) particles.…”

“…Previous research investigated the selenization mechanism in films of CZTS nanoparticles via real-time energy-dispersive xray diffraction (EDXRD) and identified the growth mechanism through the various phases that grow and recede throughout the process; 13 in this work, it was hypothesized that the samples contained small particles undetected by diffraction in the nanoparticle film that actively participate in the selenization process. This hypothesis is corroborated by a detailed investigation of the particles comprising the nanoparticle ink after size separation, 12 which reveals that the nanoparticle inks in question consist of particles from two populations with different size-correlated compositions: Cuand Sn-rich particles roughly 5 nm in diameter and Zn-rich particles 10 to 20 nm in diameter (herein referred to as "small" and "large" particles, respectively).…”

Synthesis of large CZTSe nanoparticles through a two-step hot-injection method

“…This suggests that with large amounts of organic ligands, annealing does not necessarily result in the sintering of the surfactant-coated NPs but instead it causes an out-diffusion of cations from the NPs in the film and new CZTS grains on top of the carbon-rich film. 12 This carbon-rich layer can increase the series resistance and act as a barrier for charge transport, 12 or it can protect the layer so that it remains conductive. 13 Nonetheless, the presence of the surfactant is necessary for solubility, and the prospects of this approach are brighter if the amount of organics in the film is limited.…”