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Kesterite Thin-Film Solar Cell Absorbers Derived Using Inhomogeneous Cu2ZnSnS4 Nanoparticle Inks

Kesterite is a quaternary/pentenary semiconductor Cu2ZnSn(S,Se)4 (CZTSSe) that consists earth-abundant, low-toxicity elements. It attracts great interest because of its promise as an absorber material for low-cost thin-film solar cells. Solar cells that employ a hydrazine-processed CZTSSe absorber have achieved a world record 12.6% power conversion efficiency (PCE). We develop a competitive nanoparticle ink based process without the use of toxic hydrazine to derive CZTSSe absorbers from Cu2ZnSnS4 (CZTS) nanoparticles by means of a heat treatment in Se vapor (selenization). Band gap tuning of the resultant kesterite thin-films can be accomplished by Ge doping or by tailoring the S/Se ratio. By our method, PCE of 9.3% for CZTSSe and 9.4% for Cu2Zn(Sn,Ge)(S,Se)4 are currently realized.

My doctoral research focuses on understanding the structure-property-processing relationship of the kesterite materials to improve their device performance. It is recognized in both my own work and the recent literature that the structural and compositional integrities of CZTSSe are crucial to derive the solar cell grade kesterite thin-films. Analytical electron microscopy (AEM) allows me to demonstrate the structural and compositional inhomogeneity of the CZTS nanoparticles and CZTSSe thin-films at the nanoscale. For example, the observed forbidden reflections in TED patterns and FFT diffractograms corresponding to HRTEM images indicate that cation disorder leads to stacking faults in CZTS nanoparticles. Probe-corrected STEM and EDS analysis also shows that the quaternary nanoparticles inherit intra- and inter-particle compositional fluctuations due to the kinetic effects during synthesis reactions.

AEM is further applied to FIB-prepared lamellae to study CZTSSe film formation during selenization. It is found that the compositionally inhomogeneous nanoparticles supply the metal cations – namely Cu, Zn, and Sn – to the CZTSSe nuclei that grow at the top surface of the nanoparticle layer. These CZTSSe grains form a continuous thin-film and grow in size suggesting a liquid-assisted sintering mechanism in which molten Se dissolves Cu, Zn, and Sn and expedites cation diffusions to facilitate CZTSSe film growth. Despite the fact that excess Se starts to evaporate from the films toward the end of selenization, residual Se and metal cations are found to form fine grain layers underneath the coarsened CZTSSe grains, which are expected to increase the series resistance in the cells. STEM-EDS line-scans detect compositional fluctuations through the CZTSSe grains. This suggests the cause of the electrostatic potential fluctuations that are believed to limit the Voc for kesterite thin-film solar cells.

In sum, my work at Purdue has integrated advanced analytical materials characterization with novel synthesis and fabrication techniques to probe mechanisms of nanoparticle and thin-film formation enabling high efficiency, solution-processed kesterite solar cells. The protocol I have developed to study these materials is also broadly applicable to other multinary semiconductors of interest for high efficiency optoelectronic devices.


renu.sharma [at] (Renu Sharma), 301-975-2418
Created March 25, 2015, Updated May 13, 2016