Surface modification enabled carrier mobility adjustment in CZTS nanoparticle thin films

https://doi.org/10.1016/j.solmat.2014.04.027Get rights and content

Highlights

  • Present a DFT model to estimate carrier mobility in a thin-film of nanocrystals and ligands array.

  • Showed that carrier mobility has exponential dependence on ligand chain length.

  • CZTS nanocrystal film (low temperature process) obtains a carrier mobility of up to 10.9 cm2/(VS).

Abstract

As the essential building blocks of many electronic devices, solid state thin-films are attracting extensive interest. Soluble nanocrystals (NCs) make it possible to develop robust, low-cost, large-scale fabrication methods for thin-films. However, the organic surface ligands normally used to stabilize the NCs make those thin-films a NC–ligand complex which may possess varied electrical performance compared to a single component system. Previous models could only estimate the charge transportation characteristics in those films quantitatively by considering the capping ligands as a barrier of charges from inter-particle hopping. In this work, we demonstrated that starting from first principles density functional theory, the carrier mobility in a CZTS NC–ligand complex can be determined quantitatively, and guided by this model, we developed a low-cost, low-temperature technique to fabricate CZTS thin films which have a carrier mobility of up to 10.9 cm2/(VS).

Introduction

Semiconductor thin films are the fundamental building blocks for rapidly growing fields involving thin-film-transistors (TFTs), solar cells, and transparent electrodes. While many fabrication methods have been developed, semiconductor nanocrystals (NCs) substantially reduce the cost of thin-film electronic and photovoltaic device fabrication because the soluble NCs ‘ink’ can be applied to large-scale, low-cost fabrication techniques such as drop casting, dip casting, spin coating, spray casting, and inkjet printing. However, to avoid aggregation caused by their high surface area, the NCs are normally stabilized by organic insulating capping ligands, which introduce a major obstacle in improving the electronic performance of these thin films. To get rid of these organic ligands, many thin-film preparations employ high-temperature annealing: during this aggressive procedure, defects occur in the film formation due to high weight losses [1], and the high annealing temperature limits the choices of substrate. Otherwise, removing the surface capping ligands requires a hazardous and toxic procedure involving hydrazine [2], [3].Therefore, searching for a new low-cost method to improve the NCs thin film electrical performance becomes worthwhile.

One solution is to modify the surface of the NCs by exchanging the long chain insulating ligands with new shorter ligands. Past studies have revealed that the charge transport in a PbSe NC–ligand system can be considered as a series of incoherent tunneling transitions between neighboring NCs [2], [4], using the ligand monolayer as the tunneling barrier. In these previous models, carrier mobility is determined by the site energies, exchange coupling energy between two NCs, and average barrier width. However, given that those parameters are difficult to determine experimentally, these models can only analyze them qualitatively.

In this work, we studied Cu2ZnSnS4 (CZTS) NC–ligand systems. Instead of describing the ligand as an individual tunneling barrier for charge hopping, we coupled it with the NCs and studied how the surface modification influenced the electron structure and the effective mass of charges in this NC–ligand complex. This allowed us to determine the carrier mobility quantitatively.

The kesterite material CZTS possesses promising characteristics to be a conventional absorber for thin film solar cells with the added benefits of being low-cost, non-toxic, and comprised of earth-abundant elements. Various techniques have been developed for the preparation of CZTS thin films, such as sulfurization followed co-sputtering [5], chemical vapor deposition (CVD) [6], electrodepostion [7], and hydrazine based solution processing [8], which leads to the current highest performing CZTS solar cells (>11% efficiency) [9]. Nevertheless, the nonvacuum, low-toxic preparation of CZTS inks could produce low-cost thin films with superior homogeneous composition. Also, the traditional CZTS ink technique requires a high-temperature procedure which has the same issues that other solution-based NC devices have confronted, but with additional drawbacks: Sn losses through desorption of SnS from CZTS due to high vapor pressure of SnS [10] leads to impurity and defects, and sulfur diffusion into the molybdenum back contact forms MoS2 and yields secondary phases at the CZTS|Mo interface, lowering the performance [11]. Therefore, the theoretical investigation on CZTS NC–ligand systems would not only help us anticipate the electrical performance but also guide us to develop a low-temperature surface modification process that solves the above issues.

Section snippets

Discussion

To reveal the origin of how the ligands cap the CZTS NCs and further influence carrier transport in NC–ligand systems, a method based on first principles density functional theory (DFT) is proposed below:

Compared to the bulk material, NC thin films are a complex of organic ligands and nanoparticles. This distinct structure causes great differences between the charge transport features of neighboring particles. Instead of considering the NCs and ligands separately, the CZTS nanoparticles are

Conclusion

In summary, we reported a novel theoretical method to quantitatively estimate the carrier mobility in a NC–ligand complex, and we demonstrated both experimentally and theoretically that simple surface modification could remarkably change the carrier transportation characteristics in the NC thin film. In the CZTS NC–ligand system, we showed that the carrier mobility increases exponentially with decreasing ligand length because the carrier׳s effective mass is significantly influenced by the

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    These authors contributed equally.

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