The Renaissance of Iron Pyrite Photovoltaics: Progress, Challenges, and Perspectives - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 150

that both of the facets eventually degrade, which makes water a poor choice as a
solvent when used in electrochemical/photoelectrochemial studies or devices
unless these defect sites can be passivated.
3 Iron Pyrite Photovoltaic and Photodetector Technology
3.1 Introduction
Once again the first reports of using pyrite in a photovoltaic device come out of the
Tributsch group. Throughout their exploration of pyrite material, they created a
photoelectrochemical (PCE), Schottky barrier, and thin film-sensitized solar cells.
In the PCE device, it was shown that by coupling synthetic n-type FeS 2 with an
iodine electrolyte (I - /I 3 - ) that a device with 2.8 % efficiency could be obtained
[ 4 ]. This is the champion device of the pyrite system at the time of writing.
Figure 10 shows quantum yield measurements for the device and can be seen to
exceed 90 % at points while maintaining high yield over a wide band of the energy
spectrum. Figure 11 shows the J-V curve of the device, showing high short circuit
photocurrent while showing the typical low V oc that plagues the pyrite system of
0.2 V. It is mentioned that the electrolyte system of 4 M HI, 0.05 M I 2 , and 2 M
CaI 2 is vital to the performance. Previously, it was found that both I - ions and
hydrogen treatment (either molecular hydrogen or acid etching) helps to reduce
dark current. Although it reduces dark current, it still limits this device's efficiency
due to the dark current reducing the photovoltage.
The Schottky device was made from a pyrite/platinum interface [ 57 ]. In these
devices, a 0.1 mm layer of electrochemically etched pyrite surface was covered by
a thin transparent platinum film (50-120 Å) was utilized. The metal film was
deposited by electron beam evaporation under vacuum, though it was thought that
thin oxide layers or sulfide layers could exist, created by the evaporation, deteri-
orating the performance. Figure 12 shows the J-V characteristics. The device
exhibited high short circuit currents of around 30 mA/cm 2 and could achieve
saturation currents of 100 mA/cm 2 under higher illumination power. The quantum
yield of these devices was lower than those of the PCE, only reaching *40 %.
Although the system worked, it still produced low efficiency, prompting a shift to
switch to thin layer-sensitized cells.
Thin layer-sensitized cells were a modification of the now well-known dye-
sensitized solar cells pioneered by Gratzel [ 58 ]. The contrast between the two is
the replacement of the dye in the Gratzel cell with a thin (10-15 nm) layer of
pyrite to act as the absorber [ 59 ]. Figure 13 shows the schematic of such a device
and the energy diagram of the overall system. The pyrite acts as a photoabsorber,
where when it is excited by a photon an exiton is formed. When this exciton
diffuses to the interface between the pyrite and the titanium oxide (TiO 2 ), it splits,
injecting the electron into the TiO 2 and the hole is transported to an electrolyte