Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 122

Fig. 25 Left graph plots capacitance of: flat sample with perovskite (PS); blank NS TiO 2
(0.35 lm thickness) with no PS; NS TiO 2 (0.55 lm thickness) with PS extracted from
measurement under dark and under 1 sun illumination (light) conditions; and all-solid DSSC
(2.2 lm thickness) with N719 as dye and spiro-MeOTAD as HTM. Right graph plots capacitance
of flat sample with perovskite (PS); blank NS ZrO 2 (0.39 lm thickness) with no PS; NS TiO 2
(0.36 lm thickness) with PS extracted from measurement under dark and under 1 sun
illumination (light) conditions; and all-solid DSSC (2.2 lm thickness) with N719 as dye and
spiro-MeOTAD as HTM. Capacitance has been normalized to the electrode volume. Capacitance
for both graphs has been extracted by fitting the impedance spectroscopy spectra from the
intermediate frequency (if) region if nothing else is indicated. In some cases capacitance has been
extracted from the high frequency (hf) region as it is indicated in the legend (Reprinted
permission with Kim et al. [ 182 ])
of 12.3 % [ 180 ]. In addition impressive V oc of 1.3 V has been also reported for
(CH 3 NH 3 )PbBr 3 [ 181 ].
Finally, a recent study [ 182 ] focusing on the analysis of capacitances in
perovskite solar cells using photoanodes with both TiO 2 and ZrO 2 mesoporous
photoanodes, have shown a similar behavior despites the significant differences
between both (electrons cannot be injected from (CH 3 NH 3 )PbI 3 into ZrO 2 as in the
case of Al 2 O 3 ). In both cases a capacitance that can be attributed to the perovskite
has been detected (Blue triangles and green diamonds in Fig. 25 ). This capacitance
indicates that charge accumulation is occurring in the perovskite itself in contrast
with conventional all-solid DSSC where the charge accumulation is produced in
the TiO 2 after photoinjection (red inverted triangles in Fig. 25 ). This is, the first
observation of charge accumulation in the light absorbing material for nano-
structured solar cells, indicating that it constitutes a new kind of photovoltaic
device, halfway between sensitized and thin-film solar cell for NS TiO 2 and a thin-
film solar cell with ZrO 2 scaffold for NS ZrO 2 .
In summary, perovskite (CH 3 NH 3 )PbI 3 -sensitized or mesoscopic solid-state
solar cells using different HTMs (polymeric or molecular materials), scaffolds and
halides are a promising type of solar cells to be converted in real-life manufactured
devices in short time. However, the fast succession of record devices prepared last
year, Fig. 26 , is not being balanced with a full understanding of these cells work,
and more studies in the theoretical plane are needed.