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Nanotechnology

Using plasmonic-electrical effects for high-performance organic solar cells

Plasmonically induced carrier generation and enhanced carrier extraction improve photovoltaic performance.
13 November 2013, SPIE Newsroom. DOI: 10.1117/2.1201311.005125

Fulfilling ever-growing demand for energy has been and will be a major concern for many countries around the world. Solar cells, which harness abundant sunlight and convert solar energy into electricity, provide a sustainable and clean way to meet future energy needs. The flexibility and low-cost processing requirements of organic materials make organic solar cells (OSCs) stand out among various photovoltaic designs as offering a much wider range of applications at lower cost than their inorganic counterparts.

In OSCs, light is absorbed by a layer of organic material (the active layer), where mobile charge carriers (electrons and holes) are generated from the dissociation of photoexcited excitons. The efficient extraction of carriers from the active layer to their respective electrodes, however, usually requires a highly conductive buffer layer inserted in between. At present, relatively low light absorption by OSCs puts them at a significant disadvantage to inorganic solar cells. OSC light harvesting needs to be improved, and various light-trapping techniques have been developed recently.

Researchers have introduced plasmonic metal nanostructures into the active layer and highly conductive buffer layer. These have multiple effects on the structure and operation of OSCs. There are optical effects,1–5 material effects on the morphology and electrical conduction of the devices,1, 2,6–8 and also electrical effects directly induced by the optical plasmonic resonances, called plasmonic-electrical effects. These last include plasmon-induced (electrical) carrier generation and a change of carrier extraction properties on charge transport layers. They affect the operation and performance of OSCs but have not yet been studied in depth.9

The best-performing OSCs have an inverted single junction with a plasmonic-electrical titanium dioxide (TiO2) electron transport layer and reach a power conversion efficiency (PCE) of 8.74%: see Figure 1. Interestingly, these highly efficient OSCs absorb light at a plasmonic excitation wavelength far longer than the 400nm UV light absorbed by pristine TiO2, and their performance is also better.9 It is well known that electron states in TiO2 need to be filled by electrons from carrier excitation (bandgap about 3.0–3.3eV),10 typically by UV irradiation, to provide favorable electron transport.


Figure 1. Device structure of inverted organic solar cells (OSCs) using titanium dioxide (TiO2) nanoparticle (NP) composite electron transport layer. ITO: Indium tin oxide. Au: Gold. MoO3: Molybdenum trioxide. Ag: Silver.

We explored the contribution of plasmonic-electrical effects by incorporating gold (Au) nanoparticles (NPs) into the TiO2-based electron transport layer and measuring the behavior of devices illuminated by light of different wavelengths. We found that the increased and extended operability of NP-doped TiO2 OSCs at visible wavelengths well below the UV region is closely related to the injection of plasmonically excited hot carriers (electrons) from metal NPs into TiO2.

As well as experimental studies, we considered an integrated optical and electrical model,9, 11,12 which takes into account the hot carrier tunneling probability and the extraction barrier between the TiO2 and active layers. Both the theoretical and experimental results show that the contributions of metal NPs in the TiO2 of OSCs are the plasmonically generated hot carriers and enhanced charge extraction of TiO2, which activate effective device operation at plasmonic wavelengths far below the bandgap of TiO2 in the UV region.9

Although there have been studies on the effects of plasmonically excited metal NPs in the metal-semiconductor nanostructures, the explanation is still inconclusive. Some studies indicate that the photocatalytic or photocurrent enhancement is induced by hot carrier injection from the plasmonically excited metal NPs into the semiconductor.13–16 However, some other studies report that the improvement of photocatalytic activity in the visible range is caused by the local electric field enhancement near the TiO2 surface, rather than by the direct transfer of charges between the NPs and TiO2.17

Based on our work, we propose that it is the charge transfer process of plasmonically excited electrons from metal NPs to TiO2, rather than the induced field effect near the TiO2 surface, that is responsible for these improvements: see Figure 2. Under plasmonic (wavelength) light irradiation, the transferred plasmonically excited electrons from Au NPs to TiO2 can achieve trap filling in the TiO2 layer, reducing the effective extraction barrier, resistance, and charge recombination, which enhances electron extraction.


Figure 2. Plasmonic-induced charge injection processes. CB and VB denote the conduction band and valence band, respectively. VL and WF denote vacuum level and work function, respectively. ψB is the Schottky barrier height. PCBM: [6,6]-Phenyl C61-butyric acid methyl ester, a common electron acceptor.

This physical understanding points to a new approach of using plasmonic nanostructures in OSCs to enhance device performance. TiO2 can be used as an electron transport layer in organic LEDs and OSCs, so our work also contributes to enabling efficient organic optoelectronic devices with practical applications. In the future, we plan to explore other electrical and optical effects of plasmonic nanostructures on a variety of photovoltaic devices.


Wallace C. H. Choy
Department of Electrical and Electronic Engineering
The University of Hong Kong
Hong Kong, China

Wallace C. H. Choy received his PhD in electronic engineering from the University of Surrey, UK. He subsequently worked for the National Research Council of Canada before joining Fujitsu at San Jose, CA. He is now an associate professor.


References:
1. D. H. Wang, D. Y. Kim, K. W. Choi, J. H. Seo, S. H. Im, J. H. Park, O. O. Park, A. J. Heeger, Enhancement of donor-acceptor polymer bulk heterojunction solar cell power conversion efficiencies by addition of Au nanoparticles, Angew. Chem. Int'l Ed. 50, p. 5519, 2011.
2. D. H. Wang, K. H. Park, J. H. Seo, J. Seifter, J. H. Jeon, J. K. Kim, J. H. Park, O. O. Park, A. J. Heeger, Enhanced power conversion efficiency in PCDTBT/PC70BM bulk heterojunction photovoltaic devices with embedded silver nanoparticle clusters, Adv. Energy Mater. 1, p. 766, 2011.
3. F. X. Xie, W. C. H. Choy, C. C. D. Wang, W. E. I. Sha, D. D. S. Fung, Improving the efficiency of polymer solar cells by incorporating gold nanoparticles into all polymer layers, Appl. Phys. Lett. 99, p. 153304, 2011.
4. J. Yang, J. You, C.-C. Chen, W.-C. Hsu, H.-R. Tan, X. W. Zhang, Z. Hong, Y. Yang, Plasmonic polymer tandem solar cell, ACS Nano 5, p. 6210, 2011.
5. J.-L. Wu, F.-C. Chen, Y.-S. Hsiao, F.-C. Chien, P. Chen, C.-H. Kuo, M. H. Huang, C.-S. Hsu, Surface plasmonic effects of metallic nanoparticles on the performance of polymer bulk heterojunction solar cells, ACS Nano 5, p. 959, 2011.
6. C.-H. Kim, S.-H. Cha, S. C. Kim, M. Song, J. Lee, W. S. Shin, S.-J. Moon, J. H. Bahng, N. A. Kotov, S.-H. Jin, Silver nanowire embedded in P3HT:PCBM for high-efficiency hybrid photovoltaic device applications, ACS Nano 5, p. 3319, 2011.
7. K. Kim, D. L. Carroll, Roles of Au and Ag nanoparticles in efficiency enhancement of poly(3-octylthiophene)/C60 bulk heterojunction photovoltaic devices, Appl. Phys. Lett. 87, p. 203113, 2005.
8. D. D. S. Fung, L. Qiao, W. C. H. Choy, C. C. D. Wang, W. E. I. Sha, F. Xie, S. He, Optical and electrical properties of efficiency enhanced polymer solar cells with Au nanoparticles in PEDOT-PSS layer, J. Mater. Chem. 21, p. 16349, 2011.
9. D. Zhang, W. C. H. Choy, F. Xie, W. E. I. Sha, X. Li, B. Ding, K. Zhang, F. Huang, Y. Cao, Plasmonic-electrically functionalized TiO2 for high performance organic solar cells, Adv. Funct. Mater. 23, p. 4255-4261, 2013. doi:10.1002/adfm.201203776
10. H. Luo, T. Takata, Y. Lee, J. Zhao, K. Domen, Y. Yan, Photocatalytic activity enhancing for titanium dioxide by co-doping with bromine and chlorine, Chem. Mater. 16, p. 846, 2004.
11. W. E. I. Sha, W. C. H. Choy, W. C. Chew, A comprehensive study for the plasmonic thin-film solar cell with periodic structure, Opt. Express 18, p. 5993, 2010.
12. W. E. I. Sha, W. C. H. Choy, Y. Wu, W. C. Chew, Optical and electrical study of organic solar cells with a 2D grating anode, Opt. Express 20, p. 2572, 2012.
13. A. Furube, L. Du, K. Hara, R. Katoh, M. Tachiya, Ultrafast plasmon-induced electron transfer from gold nanodots into TiO2 nanoparticles, J. Am. Chem. Soc. 129, p. 14852, 2007.
14. Y. Tian, T. Tatsuma, Mechanisms and applications of plasmon-induced charge separation at TiO2 films loaded with gold nanoparticles, J. Am. Chem. Soc. 127, p. 7632, 2005.
15. Y. Nishijima, K. Ueno, Y. Yokota, K. Murakoshi, H. Misawa, Plasmon-assisted photocurrent generation from visible to near-infrared wavelength using a Au-nanorods/TiO2 electrode, J. Phys. Chem. Lett. 1, p. 2031, 2010.
16. M. W. Knight, H. Sobhani, P. Nordlander, N. J. Halas, Photodetection with active optical antennas, Science 332, p. 702, 2011.
17. Z. Liu, W. Hou, P. Pavaskar, M. Aykol, S. B. Cronin, Plasmon resonant enhancement of photocatalytic water splitting under visible illumination, Nano Lett. 11, p. 1111, 2011.