Cation-substituted kesterite solar cells from precursor solutions

Engberg, S. L. J. (Guest lecturer)

Activity: Talks and presentationsConference presentations

Description

In recent years, point defect engineering has proven beneficial for many types of thin-film solar cells. Particularly, defect control has been adopted for kesterite absorbers resulting in an increase in open-circuit voltage (VOC). Due to its quaternary nature with three cations of similar coordination environment, the material is very sensitive to synthesis conditions, and the energy barrier to disorder is low. The most common point defects present in Cu2ZnSnS4 (CZTS) are CuZn and VCu, which also account for the p-type doping of the material. However, the number of possible defects and defect complexes is substantial, and most are a cause for undesired recombination inside the absorber layer. One commonly observed phenomenon is Cu/Zn-disorder, which originates from the similar ionic radii of Cu and Zn, and results in band tailing and a decrease in VOC.
By replacing Cu with an element of a larger ionic radius, such as Ag, the degree of Cu/Zn-disorder is decreased because Ag is less likely to exchange positions with Zn. In other words, the formation energy of AgZn antisite defects is high and thus renders the incident unlikely. Until now, several new cations have been studied for different deposition techniques, however, only partly for the solution processing route we apply here.
We fabricate CZTS through spin-coating a precursor ink consisting of DMSO, thiourea, and chloride salts in consecutive layers until the desired thickness is achieved. A power conversion efficiency of 4% has been achieved for an alkali-free absorber, which is on the order of the non-doped record cell by Tiwari et al., [1]. The VOC was 580 mV, which lags behind the record pure-sulfide CZTS device of 683 mV, [2], and suggests that there is room for improvement.
In this work, we screen the effect of incorporating various types of new cations into the kesterite, among them Ag and Ba. They will be included directly into the precursor ink. A comparison will be given in terms of the structure, morphology, and opto-electronic properties.

[1] D. Tiwari et al., Chem. Mater. 2016, 28, 14, 4991-4997.
[2] C. Yan et al., ACS Energy Lett. 2017, 2, 4, 930-936.
Period27 Mar 2019
Event title2019 Spring Meeting of the European Materials Research Society : null
Event typeConference
LocationNice, France
Degree of RecognitionInternational