Paper: Complex Interplay between Absorber Composition and Alkali Doping in High-Efficiency Kesterite Solar Cells

Title: Complex Interplay between Absorber Composition and Alkali Doping in High-Efficiency Kesterite Solar Cells
Authors: Stefan G. Haass,* Christian Andres, Renato Figi, Claudia Schreiner, Melanie Bürki, Yaroslav E. Romanyuk, and Ayodhya N. Tiwari
Link (Open Acess): Adv. Energy Mater. 2018, 8, 1701760

Abstract:
Sodium treatment of kesterite layers is a widely used and efficient method to boost solar cell efficiency. However, first experiments employing other alkali elements cause confusion as reported results contradict each other. In this comprehensive investigation, the effects of absorber composition, alkali element, and concentration on optoelectronic properties and device performance are investigated. Experimental results show that in the row Li–Na–K–Rb–Cs the nominal Sn content should be reduced by more than 20% (relative) to achieve the highest conversion efficiency. The alkali concentration resulting in highest device efficiencies is lower by an order of magnitude for the heavy alkali elements (Rb, Cs) compared to the lighter ones (Li, Na, K). Utilization of a wide range of characterization techniques helps to unveil the complex interplay between absorber composition and alkali doping. A ranking of alkali for best device performances, when employing alkali treatment, resulted in the order of Li > Na > K > Rb > Cs based on the statistics of more than 700 individual cells. Finally, a champion device with 11.5% efficiency (12.3% active area) is achieved using a high Li concentration with an optimized Sn content.
Highlights:
  • Best published solar cell CZTSe: 12.6 % by IBM and DGIST (0.4 – 0.5 cm2 active area)
  • Solution process deposition technique [14]
  • The secondary phase Sn(S,Se)2 can be identify from XRD when Sn nominal content is > 33.3%
  • The formation of the second phase tin selenide is influenced by the type of concentration of alkali elements
  • Minority carriers trapping, surface effects and energetic relaxation of carriers has been identified to severely affect the PL transition times. Thus the measurement of transition decay times does not represent the real minority carriers lifetime in the kesterite absorber layer.
    • The champion solar cell has high Li content (3.3%) and 33.3% of Sn nominal concentration of 33.3 %.  11.55 % with metal electrodes and 12.3 without a metal grid. Area = 0.29 cm2.
  • 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.
Characterization techniques:
 Material
  • ICP-MS (Inductively coupled plasma-mass spectroscopy), detect alkali content in the absorber layer.
  • SEM (Scanning electron microscopy)
  • XRD (X-ray diffraction): To understand the device performance reduction at high Sn content
  • XRF (X-ray fluorescence)
Solar cell
  • JV (current-voltage)
  • C-V (capacitance-voltage): Apparent carrier concentration and depletion region width.
  • TRPL (Time-resolved photoluminescence): 639 nm pulse diode laser , 90ps pulse width and 10 MHz
  • EQE (External quantum efficiency)
Device fabrication:
  1. Substrate: Soda lime glass
  2. Barrier: SiOx alkali diffusion barrier layer sputtered onto a 1 mm thick glass substrate
  3. Back contact: 1 µm molybdenum
  4. Absorber: CZTSe precursor solution was spin-coated onto the back contact and dried n a hot plate at 320 ºC. (12 times to obtain 1.5µm thickness)
  5. Selenization: RTP (Rapid thermal annealing) inside a close graphite box with selenium pellets (800 mg)
  6. Etching: After selenization, the Mo/CZTSe has been immersed in 10% KCN solution  (30 seconds) in order to remove copper-rich phases and clean the surface from contamination and oxides.
  7. Buffer: 50-70 nm of CdS, (Chemical deposition)
  8. Front contact: 70/250 nm of i-ZnO/Al:ZnO bilayer (Sputtering process)
  9. Grid contact: Ni/Al grid contact (evaporation PVD?)
  10. Anti-reflective: MgF2 anti-reflective coating (e-beam evaporation)
Notes:
  • Alcaly High doping ratio of 3.3% (100mM)  (related to all elements present in the solution of reaction).  Doping is necessary for increased efficiency but it depends on Sn content as seen in the above image.
Disclaimer: The intention of this post is to bring some personal notes of the literature review. Open access, follow the link to fin the PDF. 

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