1.2 Scale-Up and Other Challenges
One exciting aspect of OPV research is the knowledge that there is a direct link
between increasing basic science knowledge and progress towards a marketable
product. But what does this product look like? What makes it marketable? The goal
is to fabricate a PV module with 10–15% PCE that is printed from solution onto a
flexible support in a continuous manner, similarly to printing newspapers. This
OPV device needs to be inexpensive, lightweight, flexible, and must last for more
than 5 years.
The most thorough publication to address the scale-up of OPV is a book by
Frederick Krebs [13]. In this book and in follow-up articles, Krebs explores the use
of several reel-to-reel printing methods with OPV materials [14–19]. Several other
groups are working on continuous coating methods for OPV [20–29]. One of the
more pressing problems for the OPV field is that most laboratory work is carried out
using spin-coaters and rigid substrates, whereas any expected application of the
research will use reel-to-reel coating and flexible substrates [30]. For this reason,
future research work should focus on blends prepared by blade coating, which can
more easily be compared to a reel-to-reel coated device. Also, blade coaters are
more efficient in the use of material, so new polymers can be investigated more
efficiently. Another issue is that flexible substrates such as PET–ITO (polyethylene
terephthalate coated with indium tin oxide) tend to crack and degrade when bent,
which defeats the advantage of flexibility. Flexible substrates are also not as well
sealed to prevent O 2 and H 2 O penetration of the device. All of these issues show
that more studies on OPV device longevity for more device geometries, and the
causes of degradation for each geometry, are needed.
1.3 A (Very) Brief History of P3HT:PCBM Solar Cells
Unlike traditional inorganic semiconductors, conjugated organic materials have
tightly bound excited states. As a result, light excitation does not result in separated
charges, but instead tightly bound exciton states are formed with photoexcitation
above the band gap. These excitons recombine quickly (picoseconds to milliseconds),
making single-component conjugated organics very inefficient PV materials. In 1986,
Tang demonstrated that excitons could be effectively split into separated holes and
electrons at a bilayer interface between electron-rich (donor) and electron-poor
(acceptor) materials [31]. It was later shown that fullerenes make ideal electron
acceptors, but the device efficiencies that could be reached were limited by the short
exciton diffusion length [32]. The real breakthrough for OPV applications was the
discovery that fullerenes and conjugated polymers could be mixed together to form a
mixed bulk-heterojunction (BHJ) layer in which donors and acceptors are in intimate
contact and separated charges must navigate through a disordered (mixed) material to
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
187
One exciting aspect of OPV research is the knowledge that there is a direct link
between increasing basic science knowledge and progress towards a marketable
product. But what does this product look like? What makes it marketable? The goal
is to fabricate a PV module with 10–15% PCE that is printed from solution onto a
flexible support in a continuous manner, similarly to printing newspapers. This
OPV device needs to be inexpensive, lightweight, flexible, and must last for more
than 5 years.
The most thorough publication to address the scale-up of OPV is a book by
Frederick Krebs [13]. In this book and in follow-up articles, Krebs explores the use
of several reel-to-reel printing methods with OPV materials [14–19]. Several other
groups are working on continuous coating methods for OPV [20–29]. One of the
more pressing problems for the OPV field is that most laboratory work is carried out
using spin-coaters and rigid substrates, whereas any expected application of the
research will use reel-to-reel coating and flexible substrates [30]. For this reason,
future research work should focus on blends prepared by blade coating, which can
more easily be compared to a reel-to-reel coated device. Also, blade coaters are
more efficient in the use of material, so new polymers can be investigated more
efficiently. Another issue is that flexible substrates such as PET–ITO (polyethylene
terephthalate coated with indium tin oxide) tend to crack and degrade when bent,
which defeats the advantage of flexibility. Flexible substrates are also not as well
sealed to prevent O 2 and H 2 O penetration of the device. All of these issues show
that more studies on OPV device longevity for more device geometries, and the
causes of degradation for each geometry, are needed.
1.3 A (Very) Brief History of P3HT:PCBM Solar Cells
Unlike traditional inorganic semiconductors, conjugated organic materials have
tightly bound excited states. As a result, light excitation does not result in separated
charges, but instead tightly bound exciton states are formed with photoexcitation
above the band gap. These excitons recombine quickly (picoseconds to milliseconds),
making single-component conjugated organics very inefficient PV materials. In 1986,
Tang demonstrated that excitons could be effectively split into separated holes and
electrons at a bilayer interface between electron-rich (donor) and electron-poor
(acceptor) materials [31]. It was later shown that fullerenes make ideal electron
acceptors, but the device efficiencies that could be reached were limited by the short
exciton diffusion length [32]. The real breakthrough for OPV applications was the
discovery that fullerenes and conjugated polymers could be mixed together to form a
mixed bulk-heterojunction (BHJ) layer in which donors and acceptors are in intimate
contact and separated charges must navigate through a disordered (mixed) material to
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
187