Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
1.1 Device Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
1.2 Scale-Up and Other Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
1.3 A (Very) Brief History of P3HT:PCBM Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
2 Relating Processing Conditions to Bulk-Heterojunction Morphology . . . . . . . . . . . . . . . . . . . . 190
2.1 The Fabrication Toolkit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
2.2 The Post-deposition Toolkit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
3 Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
4 Geminate and Non-geminate Recombination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
4.1 Free Carrier Generation Versus Geminate Recombination . . . . . . . . . . . . . . . . . . . . . . . . . . 209
4.2 Charge Extraction Versus Non-geminate Recombination . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
5 Summary, Conclusions, and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
1 Introduction
Increasing signs of global warming such as recent atmospheric CO 2 levels measured above 400 ppm, polar ice sheet loss, and super-storms, have recently focused
attention on short- and long-term solutions to combat climate change [1–4]. Similar
to the gold rush in the 1800s and the oil boom in the 1900s, intellectual property on
new technologies is now the boom industry for innovative people to become rich
and influential fast. In the twenty-first century, scientists and engineers are the
pioneers and our ideas are the prize. One of the most alluring energy technologies of
the past decade has been organic photovoltaics (OPV). This technology is alluring
because it could potentially reduce the cost of producing photovoltaic
(PV) modules and thereby make solar energy cost-competitive with fossil fuels.
As can be seen in Fig. 1, the allure of OPV brought thousands of scientists and
engineers into this new field, generating an exponential increase in scientific
knowledge (as measured by the number of scientific articles) in this area. The
sharp focus on OPV technology has led to an explosion of interest in enabling
technologies such as polymer synthesis, polymer physics, microstructural measurement techniques, multiscale modeling, photophysics, organic electronics, organic–
inorganic hybrid materials, etc. All of this intense focus into one research area has
also created intense competition between research groups. With so many new
scientific articles published yearly, it is impossible to read them all, and repeat or
redundant articles have become unfortunately and unavoidably common. Even
review articles and books about OPV have proliferated, making production of an
original perspective difficult and a complete literature review impractical. We
apologize in advance if any important work is not cited here.
Under this backdrop, we have decided to produce an article that is designed to be
helpful to students and postdocs who are entering this field. Rather than focusing on
the efficiency of devices or the morphology of materials (subjects that are covered
very well elsewhere), we instead focus some attention on how to approach OPV
research from a more practical (laboratory-based) perspective. Section 1 introduces
182
A.J. Moule ´ et al.
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
1.1 Device Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
1.2 Scale-Up and Other Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
1.3 A (Very) Brief History of P3HT:PCBM Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
2 Relating Processing Conditions to Bulk-Heterojunction Morphology . . . . . . . . . . . . . . . . . . . . 190
2.1 The Fabrication Toolkit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
2.2 The Post-deposition Toolkit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
3 Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
4 Geminate and Non-geminate Recombination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
4.1 Free Carrier Generation Versus Geminate Recombination . . . . . . . . . . . . . . . . . . . . . . . . . . 209
4.2 Charge Extraction Versus Non-geminate Recombination . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
5 Summary, Conclusions, and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
1 Introduction
Increasing signs of global warming such as recent atmospheric CO 2 levels measured above 400 ppm, polar ice sheet loss, and super-storms, have recently focused
attention on short- and long-term solutions to combat climate change [1–4]. Similar
to the gold rush in the 1800s and the oil boom in the 1900s, intellectual property on
new technologies is now the boom industry for innovative people to become rich
and influential fast. In the twenty-first century, scientists and engineers are the
pioneers and our ideas are the prize. One of the most alluring energy technologies of
the past decade has been organic photovoltaics (OPV). This technology is alluring
because it could potentially reduce the cost of producing photovoltaic
(PV) modules and thereby make solar energy cost-competitive with fossil fuels.
As can be seen in Fig. 1, the allure of OPV brought thousands of scientists and
engineers into this new field, generating an exponential increase in scientific
knowledge (as measured by the number of scientific articles) in this area. The
sharp focus on OPV technology has led to an explosion of interest in enabling
technologies such as polymer synthesis, polymer physics, microstructural measurement techniques, multiscale modeling, photophysics, organic electronics, organic–
inorganic hybrid materials, etc. All of this intense focus into one research area has
also created intense competition between research groups. With so many new
scientific articles published yearly, it is impossible to read them all, and repeat or
redundant articles have become unfortunately and unavoidably common. Even
review articles and books about OPV have proliferated, making production of an
original perspective difficult and a complete literature review impractical. We
apologize in advance if any important work is not cited here.
Under this backdrop, we have decided to produce an article that is designed to be
helpful to students and postdocs who are entering this field. Rather than focusing on
the efficiency of devices or the morphology of materials (subjects that are covered
very well elsewhere), we instead focus some attention on how to approach OPV
research from a more practical (laboratory-based) perspective. Section 1 introduces
182
A.J. Moule ´ et al.