119
MOF remained intact throughout this process. The resulting PANI-ZIF-67-CC
exhibits an extraordinary areal capacitance of 2146 mF cm
−2
at a sweep rate of
10 mV s
−1
, and further, a functioning supercapacitor device was successfully made.
See Fig. 7, top [120].
7 Solar Cells
The first solid-state solar cells were developed in 1954 at Bell Labs and quickly
found application, particularly in satellites. The devices soon advanced and the efficiency of conversion of photons to electricity increased from an initial 6% to 25%
for these first generation silicon solar cells. Although they are based on an abundant
element, it is required that high quality, monocrystalline silicon wafers are formed
for high efficiency, making them costly to produce. Additionally, there are environmental concerns with their manufacturing process. In SiO 2 the silicon must be
reduced and treated with caustic chemicals at high temperatures for asymmetric
doping with negative (i.e. nitrogen) and positive (i.e. boron) additives to create a
charge gradient and allow electrons to travel directionally. Furthermore, flat, brittle
sheets of silicon are inflexible, limiting their utility. Second generation cells consist
of amorphous silicon and semiconductor compounds such as GaAs, and have
reached similar or higher efficiencies (~10% up to 30%). The cost is lowered, but
their production also requires significant energy consumption and is based on less
abundant chemicals in some cases.
The third generation of solar cell design includes dye-sensitized solar cells
(DSSCs), first demonstrated by Grätzel in 1991 [122], and related organic and quantum dot-containing cells [123]. Intense research into this area is ongoing. A review
article in 2015 pointed out that upon searching under the keywords “dye-sensitized
solar cell,” more than five articles are published a day [124]. DSSCs are of interest
because they are amenable to variations in manufacturing and can be adapted to a
wide variety of device geometries. They also might require less intense manufacturing and not be constrained by demanding high purities compared to silicon. However,
the organic components in current generation solar cells are not nearly as stable or
efficient (maximum ~13%) [125] as silicon solar cells, but they continue to be investigated for their other favorable attributes. These solar cells have many parts to them,
as the energy source is direct from the “tap”: A DSSC must absorb light, transfer
electrons, support this with an electrolyte, and utilize a redox couple to replenish the
organic molecule with electrons for continued use. One can rationalize that this
complexity is relatively simple compared to photosynthesis and formation of fossil
fuels, however. Essentially, photosynthesis, followed by the formation of fossil fuels
over billions of years to yield hydrocarbons that power internal combustion engines
is bypassed in DSSCs. See Fig. 8 for a representative schematic of DSSCs.
The first DSSCs utilized a redox couple in a liquid electrolyte, sandwiched
between electrodes to support the movement of electrons and replenish electrons in
the organic dye, where I
−
/I 3
−
is the classic couple system [124]. This is because
Polymer Nanocomposites for Ion Transport
MOF remained intact throughout this process. The resulting PANI-ZIF-67-CC
exhibits an extraordinary areal capacitance of 2146 mF cm
−2
at a sweep rate of
10 mV s
−1
, and further, a functioning supercapacitor device was successfully made.
See Fig. 7, top [120].
7 Solar Cells
The first solid-state solar cells were developed in 1954 at Bell Labs and quickly
found application, particularly in satellites. The devices soon advanced and the efficiency of conversion of photons to electricity increased from an initial 6% to 25%
for these first generation silicon solar cells. Although they are based on an abundant
element, it is required that high quality, monocrystalline silicon wafers are formed
for high efficiency, making them costly to produce. Additionally, there are environmental concerns with their manufacturing process. In SiO 2 the silicon must be
reduced and treated with caustic chemicals at high temperatures for asymmetric
doping with negative (i.e. nitrogen) and positive (i.e. boron) additives to create a
charge gradient and allow electrons to travel directionally. Furthermore, flat, brittle
sheets of silicon are inflexible, limiting their utility. Second generation cells consist
of amorphous silicon and semiconductor compounds such as GaAs, and have
reached similar or higher efficiencies (~10% up to 30%). The cost is lowered, but
their production also requires significant energy consumption and is based on less
abundant chemicals in some cases.
The third generation of solar cell design includes dye-sensitized solar cells
(DSSCs), first demonstrated by Grätzel in 1991 [122], and related organic and quantum dot-containing cells [123]. Intense research into this area is ongoing. A review
article in 2015 pointed out that upon searching under the keywords “dye-sensitized
solar cell,” more than five articles are published a day [124]. DSSCs are of interest
because they are amenable to variations in manufacturing and can be adapted to a
wide variety of device geometries. They also might require less intense manufacturing and not be constrained by demanding high purities compared to silicon. However,
the organic components in current generation solar cells are not nearly as stable or
efficient (maximum ~13%) [125] as silicon solar cells, but they continue to be investigated for their other favorable attributes. These solar cells have many parts to them,
as the energy source is direct from the “tap”: A DSSC must absorb light, transfer
electrons, support this with an electrolyte, and utilize a redox couple to replenish the
organic molecule with electrons for continued use. One can rationalize that this
complexity is relatively simple compared to photosynthesis and formation of fossil
fuels, however. Essentially, photosynthesis, followed by the formation of fossil fuels
over billions of years to yield hydrocarbons that power internal combustion engines
is bypassed in DSSCs. See Fig. 8 for a representative schematic of DSSCs.
The first DSSCs utilized a redox couple in a liquid electrolyte, sandwiched
between electrodes to support the movement of electrons and replenish electrons in
the organic dye, where I
−
/I 3
−
is the classic couple system [124]. This is because
Polymer Nanocomposites for Ion Transport