Ecodesign of kesterite nanoparticle synthesis and understanding the back contact of thin-film photovoltaic devices

  • Michael Jones

Abstract

CZTS (Cu2ZnSnS4) offers a low-cost, low-toxicity material with suitability for photovoltaic absorber applications. Comprised of earth-abundant elements, CZTS has a high absorption coefficient (104 cm−1), direct band gap and can be fabricated into thin films with solution processing. This work focuses on the scaling of the solution processing from an environmentally conscious perspective, taking into account the impact of this method. The work also considers the limitations of solution processing by investigating the carbon content of CZTSSe (Cu2ZnSn(Sx,Se1–x)4) thin films following selenisation. This is presented in two studies where firstly a softbake study is performed to reduce the carbon content of the films and secondly, a novel photonic lift-off process provides access to the carbon rich fine grain layer for characterisation to further probe its composition and effects in photovoltaic devices.
CZTS nanoparticle synthesis was successfully scaled from 100 ml to 500 ml at the laboratory scale. Nanoparticle composition was determined to be in the ideal range with EDXS analysis. The study was motivated with an LCA study and a COMSOL finite element simulation dictating the optimum stirrer size and rotational speed for the synthesis. Nanoparticle mean diameter was calculated to be 15.43 nm and 22.26 nm for large and small synthesis, respectively. The reduced size was described in the COMSOL simulation to be a product of the higher shear rate. XRD measurements determined crystallographic consistency with kesterite standards and Raman spectroscopy demonstrated CZTS phase purity was consistent between both synthesis volumes. Device PCE of small and large batches were 3.40% and 5.06%, respectively.
In the following study, TEM measurements demonstrated more effective removal of carbon ligands at 400◦C softbake than 300◦C. This was then demonstrated with SLG substrates slot die coated in CZTS nanoparticle solution and soft baked in an inert N2 environment, then selenised to convert CZTS to CZTSSe. The highest temperature and longest duration softbake (400◦ for 120 seconds) yielded an absorber layer with the least prominent fine grain carbon layer of 150 nm and a CZTSSe large grain layer of 750 nm. Raman spectroscopy showed a reduction of the unwanted CTS phase in the precursor layer soft baked at 400◦C, along with a more substantial reduction in carbon content of the corresponding CZTSSe absorber films. SIMS analysis detailed a uniform distribution of elements in the precursor films but after selenisation a separation of carbon rich and poor films. This was also visible with SEM cross sectional imagery.
A novel photonic lift off method was used to delaminate the absorber films from the back contact using a 4 mWcm−1 light pulse of duration 1 ms. XPS depth profile performed in conjunction with Ar+ ion sputtering showed that 70% of the rear interface of the absorber layer was carbon, this was reinforced with ATR measurements depicting high C H bond signal believed to be from the OLA ligands remaining in the films. After 35 minutes of Ar+ sputtering, there was no detectable Cu signal along with very limited Zn and Sn. This emphasised the importance of cation concentration for CZTSSe crystal growth. The XPS analysis also revealed the composition of the fine grain layer to be formed of 97% C, S and Se. The lack of metallic content is believed to cause a resistive layer blocking the charge transport at the rear interface.
Date of Award19 Dec 2024
Original languageEnglish
Awarding Institution
  • Northumbria University
SupervisorYongtao Qu (Supervisor), Vincent Barrioz (Supervisor) & Lucy Whalley (Supervisor)

Keywords

  • CZTS
  • Kesterite
  • Photovoltaics
  • Thin-film
  • Nanoparticles

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