| Author(s) |
Nicholas
LiCausi, Sunil Rao, Ishwara B. Bhat, Jim Wang, Nemanja Vockic, Matt
Pfenninger, John Kenney, Zhuo Chen, Kai Shum |
| Abstract
Scope |
In order to
lower cost of solar cells, there is a push to make high-efficient thin
film solar cells which are grown on non-crystalline substrates such as
glass. However, films grown on glass are typically polycrystalline or
amorphous and usually have low efficiency due to defects,
grain-boundaries, etc. Another alternative method to reduce cost is to use
a thin film that can absorb the solar radiation broadly and reemit the
energy at a narrow wavelength band suitable for absorption by silicon
solar cells. For this to be cost effective, the luminescence conversion
efficiency should be very high. We have explored the use of a new class of
materials, namely perovskite semiconductor CsSnI<SUB>3</SUB>
thin films, for this purpose that luminescence at 950nm. This film has
exhibited high photoluminescence conversion efficiency and shows potential
for use as a "spectral down converter" in photovoltaic applications.
CsSnI<SUB>3</SUB> films have been grown on glass substrates in
a low pressure chemical vapor deposition (CVD) reactor using cesium
formate (CF), tetramethyl-tin (TET), and ethyl-iodide (EI) as the cesium,
tin and iodine precursors, respectively. Both TET and EI are liquids at
room temperature with sufficiently high vapor pressures and hence these
could be transported to the reaction zone using standard carrier gas
bubbling method. However, CF is a solid with relatively low vapor pressure
and transport to the reaction zone was challenging. To evaluate whether
growth by this method is possible, we used two primary methods. In the
first method, cesium formate is dissolved in methanol. The glass substrate
is then dipped in this solution and subsequently dried on a hotplate at
80°C. This creates a film varying in thickness from ~20-100 µm. The
substrate is then loaded into the CVD chamber and heated to the growth
temperature. Reaction to get CsSnI<SUB>3</SUB> was
accomplished by passing TET and EI precursors using hydrogen as the
carrier gas. Growth proceeds for 30-40 minutes and the sample is cooled
under H<SUB>2</SUB> flow. The films are characterized by
photoluminescence (PL) as well as scanning electron microscopy, energy
dispersive x-ray analysis and x-ray fluorescence. In the second method, CF
is first transported to the wafer by passing nitrogen over CF kept in a
heated crucible, followed by passing TET and EI using hydrogen as the
carrier gas. The second method resulted in more uniform film compared to
the first method, but the overall thickness of the films is much lower.
Growth temperatures of 300 - 600°C have been explored with the best PL
efficiency obtained at 475°C. Further studies on the growth and film
characterization will be carried out and the results will be reported at
the conference. This is the first report of CsSnI<SUB>3</SUB>
growth by the CVD method. |