Hybrid Solar Cells with Polymer and Inorganic Nanocrystals - Organic Solar Cells: Materials and Device Physics - page 244

Fig. 9.2 Spectral photon flux
density of the AM 1.5 G solar
irradiation

Then the short circuit current (J sc ) could be obtained as [ 39 ]:

J sc ¼ q Z

k max

U ð k Þ EQE ð k Þ dk ð 9 : 2 Þ

k min

The A (k) is spectral photon flux (see Fig. 9.2 ).
The theoretical maximum open circuit voltage (V oc,max ) is determined by the
energy level difference between the CB of the inorganic acceptor and the HOMO
of the polymer donor, i.e., [ 38 ]

E CB ; A

eV oc ; max ¼ E HOMO ; D

E CB ; A

ð 9 : 3 Þ

¼ E g þ E LUMO ; D

For a given inorganic material with fixed conduction band, the photovoltaic
performance of HSC is mainly determined by the E g and LUMO of the conjugated
polymer and this could be understood from the views of J sc and V oc . On one hand,
the polymer bandgap (E g ) should be as narrow as possible to absorb as much light
as it can to generate more photocurrent and the LUMO of the polymer should lies
at least E b higher than the CB of the inorganic acceptor for efficient exciton
dissociation. On the other hand, the energy difference between the CB of the
inorganic crystal and the HOMO of the polymer should be as large as possible for
high V oc as described in Eq. 9.3 . Due to the two contradictory requirements for
high J sc and V oc , the E g and the LUMO level of the conjugated polymer should be
balanced in a real HSC for optimized device performance.
Usually, the HSCs show a similar structure with OSCs, i.e., an active layer
consisting of conjugated polymers and inorganic nanocrystals sandwiched by two
electrodes with different work functions. According to the different morphologies
of the active layer, the architectures of HSCs can be classified as the following
three types: (1) bilayer heterojunction, (2) bulk heterojunction, and (3) ordered
heterojunction, as shown in Fig. 9.3 .