Kesterite Solar Cells: state of art and perspective

Prof. Simona Binetti introduces that state of the art , open issues and perspectives of kesterite-based optoelectronic devices.

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The fundamental optical and electrical properties of kesterite absorbers determine the performance of the corresponding kesterite based solar cells. The challenges in the development of Cu₂ZnSn(S,Se)₄ kesterite solar cells include the reduction of the recombination of the charge carriers and overcoming the large open circuit voltage deficit. Naturally complex crystal structure introduces variation of point defects and defect complexes to the kesterite absorber. In addition, the presence of Cu-Zn disordering was found to play key role in determining the optical and electrical properties of kesterites. Fortunately, in addition to the elemental composition modification or modification of the defect structure through various thermal treatments, the optoelectronic properties of Cu₂ZnSn(S,Se)₄ kesterites can also be tailored by cation or anion substitution. This webinar aims at giving an overview of the current knowledge on the optoelectronic properties of kesterite absorbers for photovoltaics. Due to the background of the speaker in defect structure and recombination mechanism analysis by luminescence and admittance spectroscopy methods, more attention is given to information obtained by using these techniques.

Alkali treatment of kesterite solar cells is one of the measures to reduce the high V_OC - deficit and most of today’s >10% efficient kesterite devices utilize the beneficial effects of alkali elements on absorber layer morphology and opto-electronic properties. So far, the most research attention has been paid to sodium, resulting in many thorough investigations which revealed grain size enhancement, passivation of grain boundaries and an increase in net hole concentration as the major beneficial effects. Also, lithium addition has shown to improve device performance by boosting the electronic quality of the CZTSSe absorber material and grain boundaries. First studies on the effect of potassium addition confirmed advantageous effects on kesterite absorber growth and optoelectronic properties like Na. Several studies comparing different alkali elements and their effect on solar cell properties and device performance have recently been published, but the results are often contradictory. In this webinar, a detailed analysis of the effects of alkali bulk treatment on solution processed high efficiency kesterite solar cells with various alkali elements, concentrations and bulk metal ratios is presented. The findings are that from Li to Cs the nominal Sn concentration ( Sn/(Cu+Zn+Sn)) required for best device properties should be reduced from 33.3% to 26.5%, i.e. more than 20% (relative). Additionally, the alkali concentration resulting in the best devices for Li (11.1 %), Na (10.0 %) and K (10.0 %) was 100 mM while for Rb (8.8 %) and Cs (9.1 %) only 10 mM were required. The PV parameters correlate with the changes in morphology with best devices exhibiting large grains throughout the whole absorber layer with a low density of grain boundaries. The formation of Sn(S,Se)₂ secondary phase is increased for the heaver alkali elements and for high concentrations. As revealed by EQE and C-V measurements the carrier concentration varies by three orders of magnitude depending on the Sn content, except for high Rb and Cs content where a constant kesterite composition and therefore constant doping concentration is observed. A ranking of best device performances employing alkali treatment resulted in the order of Li > Na > K > Rb > Cs based on the statistics of more than 700 individual cells.

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