Solution-processing promises to revolutionize the electronic industry by providing cost-effective and rapid fabrication of flexible electronic devices. Whereas traditional electronic devices are processed on hard substrates using slow and costly vacuum-based batch processing, solution-processing enables rapid, roll-to-roll manufacturing on plastic films and foils. Electronic devices on such substrates are desirable for wearable electronics and to apply conformally to any surface. Important applications of solution-processed electronics include photovoltaics, thermoelectrics, thin film transistors, batteries, and sensors.
Ultimately, it is desirable to combine the superior performance of inorganic electronic materials with the facile processing of organic solutions. Unfortunately, most inorganic salts that would be useful precursors are insoluble in many organic solvents compatible with roll-to-roll, solution-based techniques. We have overcome this challenge by developing a mixture of commonly available thiols and amines to dissolve a host of materials that are otherwise insoluble in either solvent by itself. The solvent system not only takes the form as pure amine-thiol mixtures, but also as mixtures such as thioglycolic acid-ethanolamine,1 ethanolamine-thioacetamide,2 and ammonium thioglycolate solutions3 in water. These mixtures have been used with great success for nanoparticle synthesis,4,5 photovoltaic absorber layers,1,6,7 luminescent quantum dot films,1 and other thin films8making it a very general solvent system for processing of inorganic thin films.
Here we present for the first time the fabrication of CdTe thin films via a solution-processed molecular precursor approach. We further elucidate the chemistry of the amine-thiol system using advanced tandem mass spectrometry techniques such as ESI and APCI to identify species in solution and verify their structure using quantum chemical calculations and modeling. In a typical preparation CdCl2and Te are each dissolved in 1-propanethiol and ethylenediamine in concentrations ranging from 0.15 M to 0.22 M. ITO-coated or Mo-coated glass are then spin-coated with 375 µL of the solution. To obtain a film thickness of about 1 µm, eight to twenty-five coatings are applied, depending on the concentration and spin coating speed employed. Between each coating, the film is annealed on a hot plate at 500 °C to grow the CdTe phase. Grazing incidence x-ray diffraction reveals that pure CdTe can be recovered under optimized annealing conditions. A plane view and cross section of a CdTe film are given in Figure 1. However, we have found that very short annealing times (less than 2 mins) have the potential to produce a small amount of a segregated Te phase. Scanning electron microscopy coupled with energy dispersive x-ray spectroscopy (EDS) linescans has confirmed the presence of this phase and found that it is located at the surface of the film.
Modest efficiencies of about 0.2% are currently obtained from these films. One limitation we have found is significant chlorine residue remaining in the film, often in between grain boundaries. Cadmium acetylacetonate can be used to either partially or entirely replace CdCl2 as a precursor to reduce the presence of chlorine, but films prepared in this manner have much smaller grains than films prepared with CdCl2and have more voids. Further insight into the solution chemistry and grain growth will enable the removal of chlorine contamination and improvement of device performance.
(1) Tian, Q.; Wang, G.; Zhao, W.; Chen, Y.; Yang, Y.; Huang, L.; Pan, D. Chem. Mater. 2014, 26(10), 3098.
(2) Sun, Y.; Zhang, Y.; Wang, H.; Xie, M.; Zong, K.; Zheng, H.; Shu, Y.; Liu, J.; Yan, H.; Zhu, M.; Lau, W. J. Mater. Chem. A 2013, 1(23), 6880.
(3) Tian, Q.; Huang, L.; Zhao, W.; Yang, Y.; Wang, G.; Pan, D. Green Chem. 2015, 17(2), 1269.
(4) Yang, W.-C.; Miskin, C. K.; Hages, C. J.; Hanley, E. C.; Handwerker, C.; Stach, E. A.; Agrawal, R. Chem. Mater. 2014, 26(11), 3530.
(5) Walker, B. C.; Agrawal, R. Chem. Commun. (Camb). 2014, 50(61), 8331.
(6) Yang, Y.; Wang, G.; Zhao, W.; Tian, Q.; Huang, L.; Pan, D. ACS Appl. Mater. Interfaces 2014, 7(1), 460.
(7) Zhang, R.; Szczepaniak, S. M.; Carter, N. J.; Handwerker, C. A.; Agrawal, R. Chem. Mater. 2015, 27(6), 2114.
(8) Antunez, P. D.; Torelli, D. A.; Yang, F.; Rabu, F. A.; Lewis, N. S.; Brutchey, R. L. Chem. Mater. 2014, 26 (19), 5444.