news & views optical fibre. In contrast with micropillars, the goal of the nanowire design is to avoid reflection at the top facet, and thereby achieve transmission close to 100%. To achieve this, Claudon et al. adapted a conical taper design typically used in classical optoelectronics, in which efficient laser mode conversion into optical fibres or waveguides is required. The tapering at the top decreases the effective refractive index such that the mode is predominantly supported in air, which strongly reduces back-scattering. At taper angles of around 5°, an outcoupling efficiency of 72% was demonstrated6. Furthermore, large transmission through the top avoids the multiple reflections typical for Fabry–Pérot-type cavities, eliminating the frequency selectivity of the nanowire and thus providing outcoupling over a spectrally broad range of up to 70 nm. These results were obtained under optical pumping at cryogenic temperatures (5 K) and at an emission wavelength of 950 nm. An exciting observation is that the single-photon stream is very pure, with a multiphoton emission probability of less than 1% even if the quantum emitter is pumped to saturation. This is in strong contrast with previous results based on cavity modes3–5, where effective suppression of multiphoton emission was only achieved under low pump powers or by resonant excitation schemes. In the context of quantum dot nanolasers, it is known that high-Q cavity modes can effectively harvest quantum dot emission even if the spectral resonance condition between the fundamental radiative transition and the cavity mode is not fulfilled11.
Although several microscopic mechanisms are responsible for this counter-intuitive effect 12, the absence of a high-Q cavity mode inside the nanowire design seems to suppress such effects, leading to singlephoton emission with a much lower multiphoton emission probability. There are, however, certain drawbacks to the quantum photonic nanowire design such as its brittleness, vertical emission direction (which causes difficulty in interfacing with planar photonics) and the lack of the Purcell enhancement of the spontaneous emission rate. This lack of Purcell enhancement is of particular importance for achieving higher clock rates for single-photon sources or creating bright sources of indistinguishable photons for advanced on-chip quantum processing schemes relying on two-photon interference. This problem could, at least in principle, be overcome by using quantum dots or impurity-bound excitons with intrinsically larger oscillator strengths — and thus shorter radiative lifetimes — than the quantum dots of Claudon et al. The results of Claudon et al. open exciting new possibilities for semiconductor quantum optics. If the extraction efficiency can be further improved to approach the theoretical limit of around 96%, either by optimization of the device design or the fabrication process, then a deterministic indistinguishable photon source with high fidelity can be created, which is important for the development of large-scale on-chip linear optical quantum computation. Perhaps the most appealing opportunity of the nanowire approach is the incorporation of
emitters that are lacking a mature cavity technology, such as quantum dots made from wide-bandgap ii–vi semiconductors, nitrides and oxides. Furthermore, these nanowires are able to extract the spectrally broadband emission of nitrogen-vacancy centres and quantum dots operated at room temperature, which is of particular interest for realizing a commercial single-photon source. In all of these cases, the quantum photonic nanowire design would not only create quantum light states efficiently, but may also allow optical initialization, manipulation and read-out of individual charge and spin states with high fidelity. These prospects are of importance for future advances in semiconductor quantum optics, and can now be implemented in a more versatile and simpler fabrication process compared with traditional microand nanocavities. ❐ Stefan Strauf is at the Physics and Engineering Physics Department at the Stevens Institute of Technology, Hoboken, New Jersey 07030, USA. e-mail:
[email protected] References 1. Klingshirn, C. F. Semiconductor Optics (Springer, 2007). 2. Bouwmeester, D., Ekert, A. & Zeilinger, A. The Physics of Quantum Information (Springer, 2000). 3. Michler, P. Single Semiconductor Quantum Dots (Springer, 2009). 4. Pelton, M. et al. Phys. Rev. Lett. 89, 233602 (2002). 5. Strauf, S. et al. Nature Photon. 1, 704–708 (2007). 6. Claudon, J. et al. Nature Photon. 4, 174–177 (2010). 7. http://qcvictoria.com 8. Vahala, K. J. Nature 424, 839–846 (2003). 9. Yan, R., Gargas, D. & Yang, P. Nature Photon. 3, 569–576 (2009). 10. Friedlern, I. Opt. Express 17, 2095–2110 (2009). 11. Strauf, S. et al. Phys. Rev. Lett. 96, 127404 (2006). 12. Winger, M. et al. Phys. Rev. Lett. 103, 207403 (2009).
SOLAR ENERGY
Ferroelectric photovoltaics Ferroelectrics may have a bright future for solar-energy generation, following the report that the domain walls of such materials can be engineered to exhibit a photovoltaic effect with an impressively high voltage output.
Haitao Huang
F
erroelectric materials are characterized by a spontaneous polarization that occurs below the Curie temperature and can be switched using an external electric field. It is well-known that this switchable polarization is sensitive to external stimuli such as stress, strain, temperature, electric field or chemical substances, and for decades ferroelectrics have been used in sensors and actuators. 134
Now, writing in Nature Nanotechnology 1, S. Y. Yang and co-workers report that a ferroelectric material can also respond to light quanta to generate a steady-state photocurrent, a phenomenon called the photovoltaic effect. Although the photovoltaic effect in ferroelectrics has been known for more than half a century 2, it has been overlooked by many researchers and only investigated in
a small number of studies. Accompanied by the ever-increasing global concern for clean and sustainable energy, there is now a renaissance in research into ferroelectric photovoltaics3–6. So far, research activities have mainly focused on the ‘bulk’ photovoltaic effect in ferroelectric materials, and the underlying mechanism for such an effect was not very clear. It was generally believed that the effect was associated
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news & views with the loss of inversion symmetry in the distribution of defects, impurities, space charges and polarizations in ferroelectrics. The work of Yang et al. is a big step forward because it provides important insight into the photovoltaic effect in ferroelectrics. The researchers observed a very high photovoltaic voltage and found that it originated from the domain walls of the ferroelectric material, instead of from the bulk1. In their study, Yang et al. used BiFeO3 thin films of varying thicknesses and domain types. Through the careful design of the experiments and the exclusion of spurious signals such as the pyroelectric current (a thermal effect), they showed unambiguously that the photovoltaic effect in BiFeO3 originated from the 71° and/or 109° domain walls, owing to the asymmetry in polarization. The bulk photovoltaic effect was shown to be negligibly small. The biggest difference between the photovoltaic effect in a ferroelectric material and that in a conventional semiconductor p–n junction is the magnitude of the electric field that separates the photogenerated electron– hole pair; the effective electric field in a ferroelectric material is around one order of magnitude higher than in a p–n junction. The working principle of the photovoltaic effect in a conventional p–n junction solar cell is shown in Fig. 1a. The photogenerated electron–hole pair is separated by the electric field in the depletion region; the electron and hole move out of this region under the acceleration induced by the electric field7. The open-circuit voltage (Voc) in this case is limited by the bandgap (Eg) of the semiconductor, with eVoc (e is the elementary electric charge) being normally less than Eg. In BiFeO3, the domain wall functions in a similar manner to a classical p–n junction. Owing to the abrupt change of polarization at the 71° and/or 109° domain walls, the divergence of the polarization is non-zero (that is, . P ≠ 0), and hence an imbalance of charge develops at the domain walls. According to first-principles calculations8,9, the charges induce an electrostatic potential drop across the domain wall — this varies from 0.02 eV to 0.20 eV for different types of domain walls in various materials. Such a potential offset causes a band bending across the domain wall, similar to the band bending in the depletion layer of a p–n junction. Because the domain wall can be as thin as 2 nm — much thinner than the depletion layer in a p–n junction — the electric field is much stronger and so efficient charge separation occurs. In contrast, in the bulk of a domain
a
b
Domain
DW
Domain
DW
Domain
CB CB
eVoc
Eg hν
Recombination hν
VB VB
P n-type
Depletion layer
P
P
p-type
Figure 1 | Principles of operation for a conventional solar cell and the ferroelectric alternative. a, Band diagram of a conventional semiconductor p–n junction, showing the conduction band (CB), valence band (VB), bandgap (Eg) and open-circuit voltage (Voc). Here, hv is the energy of the incident photon and e is the elementary electric charge. b, In the ferroelectric domain, band bending occurs across the domain wall (DW). The directions of electric polarization (P) are shown by arrows.
in BiFeO3 the photogenerated charge carriers are localized and tightly bound, allowing them to quickly recombine before drifting or diffusing to either side of a domain wall — this is why the bulk photovoltaic effect in BiFeO3 is negligibly small. The open-circuit voltage increases linearly with the number of domain walls1, and this is strong evidence to support the domain wall photovoltaic effect. Indeed, a high voltage output (eVoc much higher than, for example, that of BiFeO3) can be obtained from multiple domain walls arranged in series. In a single-domain sample, however, there is no appreciable change in open-circuit voltage with increasing sample thickness. Unfortunately, at present the power conversion efficiency of such a ferroelectric solar cell is still poor, compared with that of a commercially available siliconbased solar cell. One major obstacle in achieving high conversion efficiency is the intrinsically low bulk conductivity of ferroelectric domains; indeed, any effort to increase the bulk conductivity of ferroelectric domains has proved to be ineffective, as leaky ferroelectric domains cannot withstand strong electric polarization. The charge density associated with . P across a domain wall will therefore be reduced for a leaky ferroelectric material, resulting in a low open-circuit voltage. Ferroelectric photovoltaic materials do not, however, necessarily need to compete with other types of solar cells in terms of their conversion efficiencies, as the most attractive aspect of these materials is their tunable voltage output. Ferroelectric domain structures can be easily controlled by an external electric field, and this allows control of the photovoltaic output. Yang et al. have already done some preliminary work in
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this area by applying an electric field higher than the coercive field of the ferroelectric material1. Interestingly, they found that the photovoltaic voltage could be turned on and off, as well as be flipped in polarity, after the manipulation of ferroelectric domain structures by an external field. Much more can be expected in the future. It is important to remember that BiFeO3 is a multiferroic material, in which the ferroelectric, ferromagnetic and ferroelastic orderings interact with each other. The coupling among these ferroic orderings provides us with multiple degrees of freedom for controlling the photovoltaic effect, and this can be used to endow future solar cells with much new functionality. Furthermore, the output signal of a solar cell made of ferroelectric materials can be tuned by various mechanical, electrical and magnetic means. Through the use of ferromagnetic electrodes and clever design of the junction structure10, it may also be possible to control the spin polarization of the output photocurrent. ❐ Haitao Huang is in the Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. e-mail:
[email protected] References 1. Yang, S. Y. et al. Nature Nanotech. 5, 143–147 (2010). 2. Lines, M. E. & Glass, A. M. Principles and Applications of Ferroelectrics and Related Materials (Clarendon Press, 1977). 3. Ichiki, M. et al. Appl. Phys. Lett. 87, 222903 (2005). 4. Ichiki, M. et al. Appl. Phys. Lett. 84, 395–397 (2004). 5. Pintilie, L., Vrejoiu, I., Le Rhun, G. & Alexe, M. J. Appl. Phys. 101, 064109 (2007). 6. Choi, T., Lee, S., Choi, Y. J., Kiryukhin, V. & Cheong, S.-W. Science 324, 63–66 (2009). 7. Sze, S. M. Physics of Semiconductor Devices 2nd edn (John Willey & Sons, 1981). 8. Meyer, B. & Vanderbilt, D. Phys. Rev. B 65, 104111 (2002). 9. Seidel, J. et al. Nature Mater. 8, 229–234 (2009). 10. Garcia, V. et al. Science doi:10.1126/science.1184028 (2010).
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