Low cost synthesis and characterization of CZTS solar cells

Supervisors:  Dr Vin Dhanak and Dr Jon Major

For large scale and sustainable PV the ability to combine cheap abundant materials with low cost and low power deposition techniques is incredibly attractive. The prime candidate is Cu₂ZnSnS₄ (CZTS), an abundant, cheap and non-toxic alternative with an optimal bandgap of 1.4 eV. Whilst CZTS device have been in development for a number of years their progress has been limited by issues related to secondary phases and interfacial recombination, and they are yet to achieve anywhere near their Shockley-Queisser limit for efficiency (around 32%). Currently the highest recorded efficiency for a lab scale device is only 11.1% [1]. A new approach is needed to break the current performance limit of these solar cells. For CZTS deposition via non-vacuum techniques have therefore naturaly seen a large ammount of focus with i)Sol-gel routes ii) nanoparticle inks and iii) hybrid particle-solution routes have all being atempted. Whilst all have been shown to be capable of  producing device quality material, device performances have stagnated and even the highest efficiecny devices, based on the hybrid route, still require the use of hydrazine for processing which is far from ideal. An alternative non-vacuum route is chemical bath deposition (CBD) which is comparatively unexplored for CZTS but affords the oportunity to develop a low cost single step depsoition route for CZTS. What work which has been done on CBD deposition has tended to foucs on Cu₂S, ZnS and SnS precursors followed by an annealing/sulfurization step. This project instead focusses on the devlopement and optimisation of complete CZTS device sturcutres utilitising a fast and low cost “single step” CBD CZTS deposition process that has been developed at Liverpool.  Furthermore,the CBD process redily allows changing precursors and recently it has been shown that incorporation of Ag in the kesterite cell structure improves the performance.

An additional consideration is the comparatively low performance of CZTS cells. CZTS devices remain limited primarily through the low V_OC values in comparison to the semiconductor bandgap, although recent work incorporating Ag in place of Cu has shown some promise [3, and see below]. One of the primary loss mechanisms is via recombination at the PV junction. This has been attributed to the buffer-absorber interface with there being a large conduction band offset between CZTS and CdS [1,2], the standard buffer layer for CZTS. This is a key area for improvement of this technology and the device platform developed in this project will be used as a base to investigate this issue and to identify alternative buffer layers to improve performance.

The project

This project will combine the expertise of the two supervisors to couple in-depth surface physics to complete cell development.Preliminary work has already been undertaken and a CBD process which deposits single-phase CZTS material has been established. The student will lead the development of complete cells from this process whilst undertaking in-depth XPS analysis of layers and film stacks. Initial test cells will use the established CdS buffer layer before moving on to the testing of alternative buffer layers, such as be ZnS, ZnOS, MgO and ZnMgO, and comparative band line-up measurements. In addition to standard JV and EQE analysis further characterisation will be of surface chemistry via XPS and cell carrier transport mechanisms via current-voltage- temperature measurements.Additionally as the CZTS chemical process is via precursors, it would be relatively simple to include a silver precursor to form a mixed (AgCu)₂ZnSnS₄phase. Recent work has reported that the efficiency of CZTS is improved by incorporating a thin Ag layer between the molybdenum back contact and the absorber [3]. The Ag reduces the formation of detrimental secondary phases, copper vacancies and planar defects [4],therefore development of a mixed AgCu zinc chalcogenide is an appealing candidate for furtherdevelopment and characterisation.

Development of a CZTS platform would represent a new capability for SIRE and offer significant scope to develop collaborations on additional characterisation of the produced cells.

References

[1] Tang Jiao Huang, Xuesong Yin.“CZTS-based materials and interfaces and their effects on the performance of thin film solar cells”.Phys. Status Solidi RRL. (2014).

[2] R. Haight et al “Band alignment at the Cu₂ZnSnS₄/ CdS interface”. App. Phys. Lett. (2011).

[3] H. Cui et al, “Improving Efficiency of Evaporated Cu2ZnSnS4Thin Film Solar Cells by a Thin Ag Intermediate Layer between Absorber and Back Contact”, Int. J. Photoenergy (2015).

[4] W. Li et al The role of Ag in (Ag,Cu)2ZnSnS4 thin film for solar cell application, Journal of Alloys and Compounds (2015).

For more details contact Dr Dhanak (V.R.Dhanak@liverpool.ac.uk) and see the Condensed Matter Physics website

To apply, please complete the online application form that is available at https://www.liv.ac.uk/physics/postgraduate/postgraduate-research/physics-mphil-phd/applying/