news & views ORGANIC ELECTRONICS
Addressing challenges
Although promising, the use of organic semiconductors has not yet revolutionized consumer electronics. Synthesis of high-performance materials, enhanced control of morphology and smart exploitation of unique photophysical phenomena are the way forward to overcome the technological hurdles of this field.
John E. Anthony
A
lmost since its inception, the notion of organic electronics (the replacement of traditional inorganic conductors and semiconductors with carbon-based counterparts) has ignited the imagination with promises of unique form factors, ultra-lightweight electronics and low-cost fabrication of ‘printed electronics’. Translating imagination to reality with such complex materials has taken significant time and required impressive effort, but recent commercial offerings of organic lightemitting-diode displays suggest that organic electronics remains a viable technology 1. However, wider adoption of carbon-based materials will require demonstration of more competitive properties in the areas of power generation, displays, logic and lighting. The symposium ‘The Grand Challenges in Organic Electronics’ at the 2014 Materials Research Society Spring meeting in San Francisco brought together hundreds of researchers in the field to outline some of the more challenging aspects of translating unique materials observations in the laboratory to exploitable technologies for commercialization. The diverse viewpoints, from industrial perspectives on issues such as shelf-life and scalability to academic presentations of unusual phenomena discovered in organic semiconductors, confirm the impressive potential of organic electronics for advancing technology. A mainstream research area in the field certainly involves the development of new materials to enhance the performance of organic devices. For example, in photovoltaics, Iain McCulloch (Imperial College, London, UK) and Luping Yu (University of Chicago, USA) described their latest advances in the synthesis of low-bandgap polymers, highlighting the impressive leap in power-conversion efficiencies in organic solar cells, with several different material systems converging on the 10% efficiency often touted as the threshold for commercialization. All-small-molecule systems are not far behind, with recently developed molecular donors closely approaching the performance
a
b
Applied shear
c
d
Micropillar-patterned shearing blade
Film
Ink trate
Heated subs
3.33 Å
3.08 Å
Figure 1 | Using process techniques to alter the solid-state order of a small-molecule organic semiconductor. a, Structure of the pentacene derivative employed in the works by Bao et al.7,8 (blue and black spheres represent Si and C, respectively; the alkyl substituent groups have been omitted for clarity). b, Equilibrium crystal packing of the semiconductor in a, viewed normal to the coated substrate, showing typical interplanar separation between adjacent chromophores. c, Deposition method leading to solution shearing in the crystallographic direction represented by the blue arrow in b. This arrow also represents the charge-transport axis. d, Crystal packing achieved by careful control of deposition conditions under shear, showing a dramatically reduced intermolecular spacing that leads to significantly improved chargetransport properties. Panel c reproduced from ref. 7, Nature Publishing Group.
seen in polymeric systems2. From an industrial perspective, high performance is not the only parameter that must be taken into account in materials design, as Antonio Facchetti (Polyera, Chicago, USA) pointed out. Indeed, organic semiconductors should also be compatible with a wide array of device processing methods; yet developing compounds that can survive typical photolithographic patterning steps is not a trivial task. Moreover, although low-temperature solution-processing of semiconductors partially fulfils the ‘low cost’ promise of organic electronics, the field must also focus on the development of inexpensive and scalable polymer synthesis methods, such as the continuous-flow techniques discussed by Andrew Holmes (University of
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
Melbourne, Australia) and Martin Heeney (Imperial College, London, UK). Several talks in the symposium demonstrated that the ability to control crystallization is an efficient way to improve device performance without the need to synthesize new materials. Solid-state intermolecular contacts are at the heart of performance in organic semiconductors3,4, thus the ability to induce and control crystal growth can yield impressive improvements in both stability and device performance. In a number of instances, inducing crystallization through the use of nucleating agents showed particular promise. For example, crystallization of fullerenes in organic bulk-heterojunction solar cells yielded devices with improved thermal stability 5, 1
news & views whereas the inclusion of minute amounts of common polymer-nucleating agents has been used with small-molecule organic semiconductors to facilitate crystallization and enhance the uniformity of device performance6. External stimuli can also be exploited to improve crystal uniformity, as demonstrated by Oana Jurchescu (Wake Forest University, Winston-Salem, North Carolina, USA) and her use of low-frequency vibrations to eliminate low-energy defects during semiconductor crystallization. A dramatic example of the benefits of controlling crystallization in small-molecule semiconductors was presented by the group of Zhenan Bao (Stanford University, California, USA). Using carefully tailored surfaces and well-tuned process parameters, these researchers were able to induce previously unknown polymorphs of a smallmolecule organic semiconductor (Fig. 1). This structural arrangement raised the fieldeffect mobility of organic transistors based on these molecules to 11 cm2 V–1 s–1 — a nearly order-of-magnitude improvement brought on by the subtle changes in intermolecular ordering 7,8. These talks strongly suggest that exploring the many reported crystalline semiconductors for the possibility of new polymorphs may yield similarly dramatic improvement in performance from simple, low-cost materials. Understanding of the physical mechanisms behind materials performance also continues to advance. The mechanism of charge-carrier generation in organic
2
solar cells was a very common topic, addressed by numerous speakers with experimental and spectroscopic studies. A practical application of these studies was compellingly presented by Michael McGehee (Stanford University, California, USA), who suggested that the best performance is seen in photovoltaic systems where there are both crystalline and amorphous domains of the blended semiconductors. Indeed, the simultaneous presence of different morphologies leads to a gradient of energetic offsets that enhances charge separation. Both fundamental and applied studies are also focusing on electron spin in organic semiconductors — for instance, investigating the role of triplet states in common device structures9, and devising unprecedented device structures to exploit the potential presented by the singlet fission process10. Along with a detailed description of the device metrics necessary for commercial use of organic semiconductors in displays11, Henning Sirringhaus (University of Cambridge, UK) discussed how exploitation of electron spin could yield a further performance advantage for organic semiconductors. Research on fundamental phenomena that, until now, have been less explored in organics applications can therefore provide solutions to some of the problems currently faced in the realization of efficient organic devices. The field of organic electronics continues to grow, and as researchers with new expertise enter the field, our understanding
of organic semiconductors increases in parallel. Linking the simple interactions between individual molecules to eventual film microstructure, and further correlating that microstructure with electronic device performance, has led to reliable design rules for the creation of high-performance materials. At this stage, exploration of effective ways to force the present materials to adopt different intermolecular motifs, allowing tuning of properties by simple processing modifications, is poised to further accelerate the demonstration of highperformance devices. These results could yield substantial returns in the creation of low-cost, high-performance solar cells, displays and solid-state lighting elements. ❐ John E. Anthony is in the Department of Chemistry, The University of Kentucky, Lexington, Kentucky 40506, USA. e-mail: [email protected] References 1. Organic Electronics: Web focus (Nature Mater., 2013); http://www.nature.com/nmat/focus/organic-electronics/index.html 2. Coughlin, J. E. et al. Acc. Chem. Res. 47, 257–270 (2014). 3. Noriega, R. et al. Nature Mater. 12, 1038–1044 (2013). 4. Brédas, J. L. et al. Proc. Natl Acad. Sci. USA 99, 5804–5809 (2002). 5. Lindqvist, C., Wang, E., Andersson, M. R. & Muller, C. Macromol. Chem. Phys. 215, 530–535 (2014). 6. Treat, N. et al. Nature Mater. 12, 628–633 (2013). 7. Diao, Y. et al. Nature Mater. 12, 665–671 (2013). 8. Giri, G. et al. Nature 480, 504–408 (2011). 9. Soon, Y. W. et al. Adv. Funct. Mater. 24, 1474–1482 (2014). 10. Tabachnyk, M. et al. Appl. Phys. Lett. 103, 153302 (2013). 11. Sirringhaus, H. Adv. Mater. 26, 1319–1335 (2014).
Published online: 6 July 2014
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
Addressing challenges
Although promising, the use of organic semiconductors has not yet revolutionized consumer electronics. Synthesis of high-performance materials, enhanced control of morphology and smart exploitation of unique photophysical phenomena are the way forward to overcome the technological hurdles of this field.
John E. Anthony
A
lmost since its inception, the notion of organic electronics (the replacement of traditional inorganic conductors and semiconductors with carbon-based counterparts) has ignited the imagination with promises of unique form factors, ultra-lightweight electronics and low-cost fabrication of ‘printed electronics’. Translating imagination to reality with such complex materials has taken significant time and required impressive effort, but recent commercial offerings of organic lightemitting-diode displays suggest that organic electronics remains a viable technology 1. However, wider adoption of carbon-based materials will require demonstration of more competitive properties in the areas of power generation, displays, logic and lighting. The symposium ‘The Grand Challenges in Organic Electronics’ at the 2014 Materials Research Society Spring meeting in San Francisco brought together hundreds of researchers in the field to outline some of the more challenging aspects of translating unique materials observations in the laboratory to exploitable technologies for commercialization. The diverse viewpoints, from industrial perspectives on issues such as shelf-life and scalability to academic presentations of unusual phenomena discovered in organic semiconductors, confirm the impressive potential of organic electronics for advancing technology. A mainstream research area in the field certainly involves the development of new materials to enhance the performance of organic devices. For example, in photovoltaics, Iain McCulloch (Imperial College, London, UK) and Luping Yu (University of Chicago, USA) described their latest advances in the synthesis of low-bandgap polymers, highlighting the impressive leap in power-conversion efficiencies in organic solar cells, with several different material systems converging on the 10% efficiency often touted as the threshold for commercialization. All-small-molecule systems are not far behind, with recently developed molecular donors closely approaching the performance
a
b
Applied shear
c
d
Micropillar-patterned shearing blade
Film
Ink trate
Heated subs
3.33 Å
3.08 Å
Figure 1 | Using process techniques to alter the solid-state order of a small-molecule organic semiconductor. a, Structure of the pentacene derivative employed in the works by Bao et al.7,8 (blue and black spheres represent Si and C, respectively; the alkyl substituent groups have been omitted for clarity). b, Equilibrium crystal packing of the semiconductor in a, viewed normal to the coated substrate, showing typical interplanar separation between adjacent chromophores. c, Deposition method leading to solution shearing in the crystallographic direction represented by the blue arrow in b. This arrow also represents the charge-transport axis. d, Crystal packing achieved by careful control of deposition conditions under shear, showing a dramatically reduced intermolecular spacing that leads to significantly improved chargetransport properties. Panel c reproduced from ref. 7, Nature Publishing Group.
seen in polymeric systems2. From an industrial perspective, high performance is not the only parameter that must be taken into account in materials design, as Antonio Facchetti (Polyera, Chicago, USA) pointed out. Indeed, organic semiconductors should also be compatible with a wide array of device processing methods; yet developing compounds that can survive typical photolithographic patterning steps is not a trivial task. Moreover, although low-temperature solution-processing of semiconductors partially fulfils the ‘low cost’ promise of organic electronics, the field must also focus on the development of inexpensive and scalable polymer synthesis methods, such as the continuous-flow techniques discussed by Andrew Holmes (University of
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
Melbourne, Australia) and Martin Heeney (Imperial College, London, UK). Several talks in the symposium demonstrated that the ability to control crystallization is an efficient way to improve device performance without the need to synthesize new materials. Solid-state intermolecular contacts are at the heart of performance in organic semiconductors3,4, thus the ability to induce and control crystal growth can yield impressive improvements in both stability and device performance. In a number of instances, inducing crystallization through the use of nucleating agents showed particular promise. For example, crystallization of fullerenes in organic bulk-heterojunction solar cells yielded devices with improved thermal stability 5, 1
news & views whereas the inclusion of minute amounts of common polymer-nucleating agents has been used with small-molecule organic semiconductors to facilitate crystallization and enhance the uniformity of device performance6. External stimuli can also be exploited to improve crystal uniformity, as demonstrated by Oana Jurchescu (Wake Forest University, Winston-Salem, North Carolina, USA) and her use of low-frequency vibrations to eliminate low-energy defects during semiconductor crystallization. A dramatic example of the benefits of controlling crystallization in small-molecule semiconductors was presented by the group of Zhenan Bao (Stanford University, California, USA). Using carefully tailored surfaces and well-tuned process parameters, these researchers were able to induce previously unknown polymorphs of a smallmolecule organic semiconductor (Fig. 1). This structural arrangement raised the fieldeffect mobility of organic transistors based on these molecules to 11 cm2 V–1 s–1 — a nearly order-of-magnitude improvement brought on by the subtle changes in intermolecular ordering 7,8. These talks strongly suggest that exploring the many reported crystalline semiconductors for the possibility of new polymorphs may yield similarly dramatic improvement in performance from simple, low-cost materials. Understanding of the physical mechanisms behind materials performance also continues to advance. The mechanism of charge-carrier generation in organic
2
solar cells was a very common topic, addressed by numerous speakers with experimental and spectroscopic studies. A practical application of these studies was compellingly presented by Michael McGehee (Stanford University, California, USA), who suggested that the best performance is seen in photovoltaic systems where there are both crystalline and amorphous domains of the blended semiconductors. Indeed, the simultaneous presence of different morphologies leads to a gradient of energetic offsets that enhances charge separation. Both fundamental and applied studies are also focusing on electron spin in organic semiconductors — for instance, investigating the role of triplet states in common device structures9, and devising unprecedented device structures to exploit the potential presented by the singlet fission process10. Along with a detailed description of the device metrics necessary for commercial use of organic semiconductors in displays11, Henning Sirringhaus (University of Cambridge, UK) discussed how exploitation of electron spin could yield a further performance advantage for organic semiconductors. Research on fundamental phenomena that, until now, have been less explored in organics applications can therefore provide solutions to some of the problems currently faced in the realization of efficient organic devices. The field of organic electronics continues to grow, and as researchers with new expertise enter the field, our understanding
of organic semiconductors increases in parallel. Linking the simple interactions between individual molecules to eventual film microstructure, and further correlating that microstructure with electronic device performance, has led to reliable design rules for the creation of high-performance materials. At this stage, exploration of effective ways to force the present materials to adopt different intermolecular motifs, allowing tuning of properties by simple processing modifications, is poised to further accelerate the demonstration of highperformance devices. These results could yield substantial returns in the creation of low-cost, high-performance solar cells, displays and solid-state lighting elements. ❐ John E. Anthony is in the Department of Chemistry, The University of Kentucky, Lexington, Kentucky 40506, USA. e-mail: [email protected] References 1. Organic Electronics: Web focus (Nature Mater., 2013); http://www.nature.com/nmat/focus/organic-electronics/index.html 2. Coughlin, J. E. et al. Acc. Chem. Res. 47, 257–270 (2014). 3. Noriega, R. et al. Nature Mater. 12, 1038–1044 (2013). 4. Brédas, J. L. et al. Proc. Natl Acad. Sci. USA 99, 5804–5809 (2002). 5. Lindqvist, C., Wang, E., Andersson, M. R. & Muller, C. Macromol. Chem. Phys. 215, 530–535 (2014). 6. Treat, N. et al. Nature Mater. 12, 628–633 (2013). 7. Diao, Y. et al. Nature Mater. 12, 665–671 (2013). 8. Giri, G. et al. Nature 480, 504–408 (2011). 9. Soon, Y. W. et al. Adv. Funct. Mater. 24, 1474–1482 (2014). 10. Tabachnyk, M. et al. Appl. Phys. Lett. 103, 153302 (2013). 11. Sirringhaus, H. Adv. Mater. 26, 1319–1335 (2014).
Published online: 6 July 2014
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
