spectra provides a simple way to determine the percentage crystallinity in the
P3HT:PCBM blend films.
In conclusion, optical spectroscopy is capable of quantifying some important
morphological parameters of P3HT:PCBM blends. Annealing clearly enlarges the
P3HT crystallites in the composites, but also improves intra- and interchain order
within the polymer domains. It is documented that this improvement assists charge
extraction via a higher hole mobility. Structural disorder was recently proposed to
be one origin of gap states in semicrystalline polymer domains [158]. Although
as-prepared chloroform-cast layers are less crystalline, they contain a noticeable
number of aggregated chains. It has been postulated that free carrier generation in
P3HT:PCBM cells is assisted by the delocalization of holes on fully conjugated
chains [159]. It is, therefore, concluded that the number of aggregated P3HT chains
in as-prepared blends is sufficiently high for the efficient photogeneration of free
carriers (as documented in the next section), while the poor structural order in these
layers prevents the efficient extraction of free carriers to the external circuit [146].
4 Geminate and Non-geminate Recombination
As described in the previous section, optimized samples of P3HT:PCBM exhibit
high fill factors, meaning that the photogenerated current is independent of bias
over a wide range. On the other hand, samples with non-optimum morphology
suffer from low fill factors. In this case, the photocurrent becomes continuously
smaller with increasing bias (decreasing internal electric field), which points to
photocurrent losses that are most prominent at low internal electric field. Clearly,
the identification of these loss processes in relation to morphology is of interest, not
only with regard to an overall understanding of these complex devices but particularly when targeting the knowledge-based optimization of BHJ devices.
The elementary steps that lead to charge generation and extraction in BHJ solar
cells are shown in Fig. 14. Free carrier formation from photogenerated excitons
(created in either the donor or acceptor phase) involves formation and split-up of
interfacial electron hole pairs (often called geminate pairs because they originate
from the same photoexcited exciton). As these pairs can be generated directly via
optical sub-bandgap excitation, they are commonly denoted as charge transfer
states. Geminate pairs may either split up into free carriers or recombine geminately
to the ground state. Clearly, the competition between these two processes sets the
upper limit for the efficiency of the internal photon-to-charge conversion. The
second important step is the extraction of photogenerated charge to the electrodes.
The efficiency of this process is, in general, not unity because these charges might
recombine with carriers of opposite sign (either free or trapped, photogenerated or
injected) on their way to the electrodes. This recombination is called “non-geminate” or “free carrier recombination”. Therefore, the efficiency of an organic donor/
acceptor blend is determined by the fate of three elementary states (see Fig. 15): the
photogenerated exciton (mostly singlet excitons) with energy E S1 , the charge
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
207
P3HT:PCBM blend films.
In conclusion, optical spectroscopy is capable of quantifying some important
morphological parameters of P3HT:PCBM blends. Annealing clearly enlarges the
P3HT crystallites in the composites, but also improves intra- and interchain order
within the polymer domains. It is documented that this improvement assists charge
extraction via a higher hole mobility. Structural disorder was recently proposed to
be one origin of gap states in semicrystalline polymer domains [158]. Although
as-prepared chloroform-cast layers are less crystalline, they contain a noticeable
number of aggregated chains. It has been postulated that free carrier generation in
P3HT:PCBM cells is assisted by the delocalization of holes on fully conjugated
chains [159]. It is, therefore, concluded that the number of aggregated P3HT chains
in as-prepared blends is sufficiently high for the efficient photogeneration of free
carriers (as documented in the next section), while the poor structural order in these
layers prevents the efficient extraction of free carriers to the external circuit [146].
4 Geminate and Non-geminate Recombination
As described in the previous section, optimized samples of P3HT:PCBM exhibit
high fill factors, meaning that the photogenerated current is independent of bias
over a wide range. On the other hand, samples with non-optimum morphology
suffer from low fill factors. In this case, the photocurrent becomes continuously
smaller with increasing bias (decreasing internal electric field), which points to
photocurrent losses that are most prominent at low internal electric field. Clearly,
the identification of these loss processes in relation to morphology is of interest, not
only with regard to an overall understanding of these complex devices but particularly when targeting the knowledge-based optimization of BHJ devices.
The elementary steps that lead to charge generation and extraction in BHJ solar
cells are shown in Fig. 14. Free carrier formation from photogenerated excitons
(created in either the donor or acceptor phase) involves formation and split-up of
interfacial electron hole pairs (often called geminate pairs because they originate
from the same photoexcited exciton). As these pairs can be generated directly via
optical sub-bandgap excitation, they are commonly denoted as charge transfer
states. Geminate pairs may either split up into free carriers or recombine geminately
to the ground state. Clearly, the competition between these two processes sets the
upper limit for the efficiency of the internal photon-to-charge conversion. The
second important step is the extraction of photogenerated charge to the electrodes.
The efficiency of this process is, in general, not unity because these charges might
recombine with carriers of opposite sign (either free or trapped, photogenerated or
injected) on their way to the electrodes. This recombination is called “non-geminate” or “free carrier recombination”. Therefore, the efficiency of an organic donor/
acceptor blend is determined by the fate of three elementary states (see Fig. 15): the
photogenerated exciton (mostly singlet excitons) with energy E S1 , the charge
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
207