Spectroscopy of Cu2ZnSnS4 nanoparticle inks and Cu2ZnSn(S,Se)4 solar cells

Campbell, Stephen (2020) Spectroscopy of Cu2ZnSnS4 nanoparticle inks and Cu2ZnSn(S,Se)4 solar cells. Doctoral thesis, Northumbria University.

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Abstract

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is an encouraging candidate for thin film photovoltaic de-vices as it is a direct bandgap semiconductor with a high absorption coefficient. Its constituent ele-ments are Earth-abundant which is advantageous for large scale commercialisation. Despite being adapted from Cu(In,Ga)(S,Se)2 (CIGS) technology, CZTSSe PV performance is lacking in com-parison to CIGS. To identify factors affecting the performance of CZTSSe PV, a comprehensive study using a number of characterisation techniques, such as photoluminescence spectroscopy, was performed on materials and interfaces within CZTSSe solar cells which were fabricated from Cu2ZnSnS4 nanoparticle inks by injection of metallic precursors into a hot surfactant.

To eliminate the carbon-rich CZTSSe fine grain layer usually found at the Mo back contact, the long carbon chain ligand oleylamine used in CZTS nanoparticle synthesis was replaced with low carbon content formamide. The substitution of the solvent removed the fine grain CZTSSe layer but no working device could be fabricated due to the porosity of the absorber film. Working solar cells were made by employing a dual layer of oleylamine/formamide CZTSSe absorbers, which reduced the height of the back contact barrier to ∼16 meV compared to the standard devices and improved device parameters.

The effects of precursor chemical quality on optoelectronic properties of CZTSSe absorber films were investigated. Sn loss was observed in films fabricated using lower grade chemical precursors. Deep level transient spectroscopy revealed the presence of an additional defect level in those films, suggesting the defect is Sn-related. Using higher grade precursor chemicals produced higher quality CZTSSe absorbers. However, improvement in film quality did not translate into increased performance in those devices, implying an issue other than defects in the bulk of the CZTSSe absorbers inhibits device performance.

In2S3 was used to replace buffer CdS commonly utilized in CZTSSe device structure. Pho-toemission spectroscopy demonstrated In2S3 formed a favourable Type I band alignment at the buffer/absorber interface, leading to increased open circuit voltage in the In2S3-based devices. Mott-Schottky analysis indicated increased defects at the buffer/absorber interface when using In2S3. Optimising the In2S3 layer and suppressing interface defects, using a barrier layer or passi-vating the absorber surface, could lead to the In2S3-based CZTSSe device outperforming devices using CdS due to better band alignment and increased current collection.

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