Thin Films

Tin Sulfide (SnS):

SnS-grains

The ideal absorber material for photovoltaic power generation should combine strong optical absorption (favoring a direct band-gap, as in GaAs) and long minority carrier lifetimes (favoring an indirect band-gap, as in Si).  Existing photovoltaic materials excel in only one of these areas.  We have identified Tin Sulfide (SnS) as a material that could optimize both properties simultaneously, thanks to its staggered band gap structure.  SnS has a direct band gap of 1.3 eV and an indirect band gap of 1.1 eV.  This combination leads carriers excited over the direct band-gap to thermalize down to energy levels where they recombine over the indirect one, increasing carrier lifetimes.

Not only is SnS uniquely suited for photovoltaic energy conversion, it is also abundant enough to be used for terawatts of power generation.  In addition, good quality SnS films can be fabricated at relatively low temperatures, supporting low-cost manufacturing.  Photovoltaic modules using other chalcogenides, including cadmium telluride (CdTe) and copper (indium, gallium) (diselenide, disulfide) — CIGS — represent the vast majority of the commercial thin-film PV market.  This existing scientific and manufacturing infrastructure allows us to leverage insights from CIGS and CdTe research to improve the efficiency SnS-based devices and will reduce the capital expenditures associated to begin producing SnS-based devices industrially.

Our work on SnS focuses in three areas:

  • Developing fast vapor deposition techniques for the fabrication of high quality SnS films [1, 2].
  • Developing annealing procedures to improve the structural quality and electronic properties of SnS films.  For these experiments, we employ a custom-built sulfur and hydrogen sulfide annealing furnace.
  • Together with Prof. Roy Gordon’s group at Harvard, improve interfaces of SnS with various electron-selective contacts [3].

[1] V. Steinmann et al., “3.88% Efficient Tin Sulfide Solar Cells using Congruent Thermal Evaporation,” Advanced Materials 26, 7488 (2014)
[2] K. Hartman et al., “SnS thin-films by RF sputtering at room temperature,” Thin Solid Films 519, 7421 (2011)
[3] P. Sinsermsuksakul et al., “Enhancing the efficiency of SnS solar cells via band-offset engineering with a zinc oxysulfide buffer layer,” Appl. Phys. Lett. 102, 053901 (2013)

 

Cuprous Oxide (Cu2O):

Sputtering

Cuprous or copper oxide (Cu2O) was one of the first semiconductors studied in 1936, and one of the first to be used in photovoltaic devices in 1962. Cu2O has several properties that make it an attractive absorbing layer in a thin-film solar cell devices:

  • Its constituent elements are non-toxic and abundant enough to enable terrawatts of power generation.  Furthermore, as an oxide, it is chemically robust.
  • It absorbs strongly above 1.9-2.1eV, which is appropriate for combination with a lower bandgap material in a tandem cell structure similar to a-Si/μc-Si cells.
  • The ionic nature of its bonds suggest the potential for long minority carrier lifetimes.  This supposition is further borne out by the observation of long exciton lifetimes.

We recently developed a method to deposit high-quality Cu2O material using reactive magnetron direct-current sputtering [4], a relatively cost-effective process that can be used for large-area device fabrication.  Through doping [5] and interface engineering [6,7], we achieved certified conversion efficiency of 3.97%, at the time a record for electrochemically deposited cuprous oxide.

[4] Y.S. Lee et al., “Hall mobility of cuprous oxide thin films deposited by reactive direct-current magnetron sputtering,” Applied Physics Letters 98, 192115 (2011)
[5] Y.S. Lee et al., “Nitrogen-doped cuprous oxide as a p-type hole-transporting layer in thin-film solar cells,” Journal of Materials Chemistry A 1, 15416–15422 (2013)
[6] Y.S. Lee et al., “Atomic layer deposited gallium oxide buffer layer enables 1.2 V open-circuit voltage in cuprous oxide solar cells,” Advanced Materials 26, 4704–4710 (2014)
[7] R.E. Brandt et al., “Band offsets of n-type electron-selective contacts on cuprous oxide (Cu2O) for photovoltaics,” Applied Physics Letters 105, 263901 (2014)

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