one component more than the other can lead to a dizzying variety of optical,
electrical, and morphological features that are not particularly easy to measure
and are difficult to interpret. Further complicating the literature are polymer
samples with a variety of molecular weights and regioregularities. Mixtures have
different mixing ratios, and are cast from different solvents.
Regarding charge carrier dynamics, charge generation in P3HT:PCBM was
shown to be ultrafast and activationless, even when exciting low energy CT states.
Therefore, formation of free charge must be driven by particular energetics at the
mesoscale, which counterbalance the Coulomb attraction of the geminate pair
forming the CT state. Recent quantum dynamic simulations suggest that delocalization of electrons and holes on well-ordered domains assists free charge generation by reducing the Coulomb binding of interfacial CT states [211]. Because most
blends of regioregular P3HT with PCBM studied so far exhibit a significant fraction
of crystallized P3HT chains, charge delocalization might explain why free charge
generation is field-independent, irrespective of the exact preparation scheme. A
second particularity of well-performing P3HT:PCBM devices is highly reduced
non-geminate recombination. In comparison to the Langevin limit of electron–hole
recombination in an isotropic homogeneous medium, free charge recombination in
P3HT:PCBM is slowed down by a factor of 100–1,000. This vast reduction in
recombination speed allows the use of thick and well-absorbing blend layers,
without risking inefficient charge extraction and low fill factors.
In total, it took thousands of people 10 years, and more than 10
3 published
articles, to reach this level of understanding about how P3HT:PCBM BHJ OPV
devices really function, how the materials self-assemble, and how to alter the selfassembly process via fabrication conditions to achieve a desired morphology for
efficient charge generation and extraction. The lessons learned from P3HT:PCBM
are being applied to a variety of new polymers and fullerenes with the goal of
making better OPV devices. Mixed solvents, a high level of synthesis control, and
highly specialized methods to measure the morphology are now necessary to
produce new insights into BHJ function. Thus, the low hanging fruit have been
eaten.
Also, advanced electronic and optical measurements are needed to describe
common features of BHJ OPV devices. Clear (although complicated) optical and
electrical models have been developed that do a good job of predicting the efficiency and other I/V characteristics of a given donor–acceptor mixture.
So is OPV all figured out? If not, what is next? Now that the science of
determining how to synthesize high efficiency materials, how to process the
mixtures, how to fabricate the devices, how to characterize each step of the film
formation, and how to measure the optical and electrical features of the device have
been established, we still need to reliably engineer good devices. In particular, it is
necessary to determine the following:
– How to coat BHJ layers quickly and without defects
– How to protect films against O 2 and UV light, which destroy device function
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
225
electrical, and morphological features that are not particularly easy to measure
and are difficult to interpret. Further complicating the literature are polymer
samples with a variety of molecular weights and regioregularities. Mixtures have
different mixing ratios, and are cast from different solvents.
Regarding charge carrier dynamics, charge generation in P3HT:PCBM was
shown to be ultrafast and activationless, even when exciting low energy CT states.
Therefore, formation of free charge must be driven by particular energetics at the
mesoscale, which counterbalance the Coulomb attraction of the geminate pair
forming the CT state. Recent quantum dynamic simulations suggest that delocalization of electrons and holes on well-ordered domains assists free charge generation by reducing the Coulomb binding of interfacial CT states [211]. Because most
blends of regioregular P3HT with PCBM studied so far exhibit a significant fraction
of crystallized P3HT chains, charge delocalization might explain why free charge
generation is field-independent, irrespective of the exact preparation scheme. A
second particularity of well-performing P3HT:PCBM devices is highly reduced
non-geminate recombination. In comparison to the Langevin limit of electron–hole
recombination in an isotropic homogeneous medium, free charge recombination in
P3HT:PCBM is slowed down by a factor of 100–1,000. This vast reduction in
recombination speed allows the use of thick and well-absorbing blend layers,
without risking inefficient charge extraction and low fill factors.
In total, it took thousands of people 10 years, and more than 10
3 published
articles, to reach this level of understanding about how P3HT:PCBM BHJ OPV
devices really function, how the materials self-assemble, and how to alter the selfassembly process via fabrication conditions to achieve a desired morphology for
efficient charge generation and extraction. The lessons learned from P3HT:PCBM
are being applied to a variety of new polymers and fullerenes with the goal of
making better OPV devices. Mixed solvents, a high level of synthesis control, and
highly specialized methods to measure the morphology are now necessary to
produce new insights into BHJ function. Thus, the low hanging fruit have been
eaten.
Also, advanced electronic and optical measurements are needed to describe
common features of BHJ OPV devices. Clear (although complicated) optical and
electrical models have been developed that do a good job of predicting the efficiency and other I/V characteristics of a given donor–acceptor mixture.
So is OPV all figured out? If not, what is next? Now that the science of
determining how to synthesize high efficiency materials, how to process the
mixtures, how to fabricate the devices, how to characterize each step of the film
formation, and how to measure the optical and electrical features of the device have
been established, we still need to reliably engineer good devices. In particular, it is
necessary to determine the following:
– How to coat BHJ layers quickly and without defects
– How to protect films against O 2 and UV light, which destroy device function
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
225