called AM1.5G (Fig. 2a). This spectrum is equivalent to the sun’s spectrum at a tilt
of 48.2
, which increases the path length through the atmosphere by 150% with
respect to 0
incidence. On a sunny day, the total sun power is approximately
930 W/m
2 and includes scattered light from clouds and pollution. This spectrum is
approximated using a Xe arc lamp and an optical filter. The Xe lamp solarsimulated spectrum is relatively accurate throughout the visible range but does
not have full spectral coverage in the near infrared. Usually solar simulator lamps
are calibrated to 1,000 W/m
2 and direct incidence (0
) is used to simplify the
experiment. A detailed (and worth reading) description of how to properly measure
the mismatch factor for a simulated solar spectrum can be found in a publication by
Shrotriya et al. [5]. The standard PCE measurement and testing protocol has also
been published [6].
A PV device is made from a semiconductor with an optical band gap (E g ). This
E g is the minimum energy at which electromagnetic energy absorbed within the
semiconductor promotes an electron from the valence band to the conduction band.
In the case of an OPV device, the excited states are localized onto one or more
molecular species, so extended energy bands do not exist. Instead, E g is the
minimum energy needed to promote an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the
molecules or polymers in the film. The band gap sets the maximum short-circuit
photocurrent (J sc ) and power conversion efficiency (PCE) that any PV device can
attain. Photons with energy below E g do not contribute to photocurrent production.
The maximum J sc is shown in Fig. 2a. Due to the principle of detailed balance, the
E g also sets the maximum possible potential at which a photocurrent can be
extracted [7]. The combination of these two limitations means that, at best, only
~33% of the total solar power P(E) in the solar spectrum could be collected and
converted to electricity by a single junction PV device. Practical considerations like
the necessity of a p/n junction, finite area, recombination, reflection, imperfect
materials, and series resistance mean that record laboratory scale efficiencies are
~25 and ~28% for single crystalline Si and GaAs, respectively. Figure 2b shows the
relationship between band gap and maximum attainable PV efficiency at one sun
power given several different assumptions.
The PCE is commonly obtained by measuring the current density (J) versus the
applied potential (V ) for a PV device under AM1.5G illumination. Figure 2c shows
a typical J/V curve for an OPV device based on the donor P3HT and the acceptor
phenyl-C61-butyric acid methyl ester (PCBM). The current density measured at
zero applied potential is the short-circuit current density (J sc ). The applied voltage
necessary to drive the current to zero is the open-circuit voltage (V oc ). The PCE is
determined by calculating the maximum power produced, which is the maximum
product of J Â V and is denoted by J max and V max :
⁄
ä
Fig. 2 (continued) (black), detailed balance and a FF of 0.75 (blue), and OPV conditions of 0.6 V
energy loss, 0.65 EQE and 0.65 FF. (c) J/V curve for a P3HT:PCBM OPV device
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
185
of 48.2
, which increases the path length through the atmosphere by 150% with
respect to 0
incidence. On a sunny day, the total sun power is approximately
930 W/m
2 and includes scattered light from clouds and pollution. This spectrum is
approximated using a Xe arc lamp and an optical filter. The Xe lamp solarsimulated spectrum is relatively accurate throughout the visible range but does
not have full spectral coverage in the near infrared. Usually solar simulator lamps
are calibrated to 1,000 W/m
2 and direct incidence (0
) is used to simplify the
experiment. A detailed (and worth reading) description of how to properly measure
the mismatch factor for a simulated solar spectrum can be found in a publication by
Shrotriya et al. [5]. The standard PCE measurement and testing protocol has also
been published [6].
A PV device is made from a semiconductor with an optical band gap (E g ). This
E g is the minimum energy at which electromagnetic energy absorbed within the
semiconductor promotes an electron from the valence band to the conduction band.
In the case of an OPV device, the excited states are localized onto one or more
molecular species, so extended energy bands do not exist. Instead, E g is the
minimum energy needed to promote an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the
molecules or polymers in the film. The band gap sets the maximum short-circuit
photocurrent (J sc ) and power conversion efficiency (PCE) that any PV device can
attain. Photons with energy below E g do not contribute to photocurrent production.
The maximum J sc is shown in Fig. 2a. Due to the principle of detailed balance, the
E g also sets the maximum possible potential at which a photocurrent can be
extracted [7]. The combination of these two limitations means that, at best, only
~33% of the total solar power P(E) in the solar spectrum could be collected and
converted to electricity by a single junction PV device. Practical considerations like
the necessity of a p/n junction, finite area, recombination, reflection, imperfect
materials, and series resistance mean that record laboratory scale efficiencies are
~25 and ~28% for single crystalline Si and GaAs, respectively. Figure 2b shows the
relationship between band gap and maximum attainable PV efficiency at one sun
power given several different assumptions.
The PCE is commonly obtained by measuring the current density (J) versus the
applied potential (V ) for a PV device under AM1.5G illumination. Figure 2c shows
a typical J/V curve for an OPV device based on the donor P3HT and the acceptor
phenyl-C61-butyric acid methyl ester (PCBM). The current density measured at
zero applied potential is the short-circuit current density (J sc ). The applied voltage
necessary to drive the current to zero is the open-circuit voltage (V oc ). The PCE is
determined by calculating the maximum power produced, which is the maximum
product of J Â V and is denoted by J max and V max :
⁄
ä
Fig. 2 (continued) (black), detailed balance and a FF of 0.75 (blue), and OPV conditions of 0.6 V
energy loss, 0.65 EQE and 0.65 FF. (c) J/V curve for a P3HT:PCBM OPV device
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
185