Resonant light trapping in ultrathin films for water splitting
Summary (2 min read)
4 mA cm−2 was achieved using a V-shaped cell comprising∼26-nm-thick Ti-doped α-Fe2O3 films on back-reflector substrates coated with silver–gold alloy.
- The efficient conversion of solar energy to hydrogen bymeans of water photoelectrolysis is a long-standing challengewith promise for solar energy conversion and storage in the form of synthetic fuels (so-called solar fuels)1,2.
- Important advances in studying water photoelectrolysis by semiconductor photoelectrodes3 have been achieved in the past four decades, since the seminal report4 on photoinduced water splitting using TiO2 photoanodes.
- This tradeoff limits the solar energy conversion efficiency of semiconductors with poor transport properties wherein the minority chargecarrier collection length is smaller than the light penetration depth16.
- Recent studies on thick layers18,19 as well as ultrathin films20 confirm that the efficiency of α-Fe2O3 photoanodes is mainly limited by the collection of photogenerated holes at the surface.
- This enables maximization of the absorption in regions where the 158 NATURE MATERIALS | VOL 12 | FEBRUARY 2013 | www.nature.com/naturematerials © 2013 Macmillan Publishers Limited.
Ultrathin film α-Fe2O3 photoanodes
- The rationale for using ultrathin (<50 nm) film α-Fe2O3 photoanodes stems from their high IQE compared with their thick-layer counterparts22.
- Furthermore, the photon flux close to the film/substrate interface is suppressed in films on ideally reflective substrates because of the π phase shift on reflection from an ideal reflector27.
- These values were found to fit quite well the photocurrent densities obtained with their Ti-doped α-Fe2O3 films on platinized substrates, and they are within range of the expected values13,17,32.
- These profiles show that mainly the front region down to ∼20 nm from the surface contributes to the photocurrent.
- All the other parameters were obtained from optical measurements of the specimens (Supplementary Fig. S11).
Ultrathinα-Fe2O3 films on silver-based back reflectors
- To improve the results of their light trapping structures the authors replaced the lossy platinum reflective coatings by silver-based back reflectors and reduced the backward-injection loss by adding selective electron transport layers between the Ti-doped α-Fe2O3 films and the substrates.
- With these structures the authors obtained water photo-oxidation current densities as high as 2.90±0.01mA cm−2 (at 1.63VRHE, see Supplementary Fig. S22) for a Ti-doped α-Fe2O3 film deposited using 2,000 laser pulses, yielding a film thickness of 22±3 nm (see Supplementary Fig. S6-B).
- The escaped back-reflected photons can be harvested by photon retrapping schemes such as using V-shaped cells, thereby further boosting the light-harvesting NATURE MATERIALS | VOL 12 | FEBRUARY 2013 | www.nature.com/naturematerials.
- The challenge that their light trapping strategy comes to solve and the underlying physical principles are common to other photoelectrode materials, besides α-Fe2O3, and other types of solar cell employing semiconductors with poor transport properties.
Author contributions
- H.D. and O.K. developed the optical simulation model, and H.D. and A.R. added to it the charge transport model.
- H.D. designed and fabricated the photoelectrodes and carried out most of the optical and photoelectrochemical measurements.
- H.D. and M.G. developed the selective electron transport hole blocking under layers.
- I.D. carried out the cross-section transmission electron microscope measurements.
- A.R. supervised the project and wrote the manuscript.
Additional information
- Supplementary information is available in the online version of the paper.
- Reprints and permissions information is available online at www.nature.com/reprints.
- Correspondence and requests for materials should be addressed to A.R.
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Frequently Asked Questions (15)
Q2. Why is the photon flux suppressed in films on ideally reflective substrates?
the photon flux close to the film/substrate interface is suppressed in films on ideally reflective substrates because of the π phase shift on reflection from an ideal reflector27.
Q3. What is the maximum gain for a 42-nm-thick film?
Optical cavities comprising ultrathin α-Fe2O3 films are predicted to exhibit considerable gains reaching 3.6, 2.8, 2.3 and 2.0 for 14-, 28-, 18- and 24-nm-thick films on silver-, aluminium-, gold- or platinum-coated substrates, respectively, whereas the gain for films on ideally reflective substrates reaches 2.9 for a 42-nm-thick film.
Q4. What is the photocurrent density of -Fe2O3 films on transparent substrate?
Photocurrent densities of 4.6, 4.3, 3.1 and 2.9mA cm−2 are predicted for 22-, 31-, 24- and 29-nm-thick α-Fe2O3 films on silver-, aluminium-, gold- and platinum-coated substrates, respectively.
Q5. What is the photocurrent density for films on reflective substrates?
Films on reflective substrates are predicted to yield maximum photocurrents close to the first resonance modes of the respective optical cavities.
Q6. How can the authors measure the absorption of back-reflected photons?
The back-reflected photons that escape can be collected using photon retrapping schemes such as usingV-shaped cells30, thereby further boosting the absorption.
Q7. How do the authors calculate the photon flux profiles of a quarter-wave film?
By integrating Iλ0(λ0, x) the authors obtain the photon flux profiles,I (x)= ∫ λ max 0λ min 0Iλ0(λ0,x) dλ0, as a function of depth (x) inside the film.
Q8. What is the effect of the first resonance mode on the film thickness?
Partially reflective metallized substrates give rise to smaller photon fluxes, due to absorption in the metal coating, and the resonance modes are shifted to smaller film thicknesses due to phase shifts larger than π at the film/substrate interface (see Supplementary Fig. S25).
Q9. What is the effect of bulk recombination on the backside of the film?
The rest of the film is inactive, for the most part, owing to bulk recombination, whereas the backside of the film has a negative contribution due to backward injection to the substrate.
Q10. What is the absorption in -Fe2O3 films on platinized?
3.The absorption in α-Fe2O3 films on platinized substrates is predicted to reach the first maximum at a film thickness of 36 nm with a photogenerated current density of 5.1mA cm−2, that is 40% of the ultimate limit forα-Fe2O3 (ref. 10).
Q11. What is the way to reduce the backward injection to the substrate?
The highest photocurrent densities are predicted for the ideal cavity, which is also effective in reducing the backward injection to the substrate by suppressing the light intensity close to the film/substrate interface (see Fig. 2a).
Q12. What is the optical gain of a silver-coated substrate?
aluminium- and gold-coated substrates are predicted to yield high optical gains (see Fig. 3, inset), with a maximum gain of 4.2 for a 16-nm-thick film on a silver-coated substrate (green curve).
Q13. How do the authors calculate the light intensity distribution in a quarter-wave film?
To calculate the light intensity distribution inside the film the authors take the plane-wave solution of Maxwell’s electromagnetic wave equation and tailor it to fit the boundary conditions of their system with incident solar radiation, at AM1.5G conditions, striking the optical stack illustrated in Fig.
Q14. What is the importance of separating the photogenerated holes from the electrons?
This loss highlights the importance of separating the photogenerated holes from the electrons by imposing asymmetric charge transport conditions and blocking the backward injection of holes to the substrate, which is especially critical for ultrathin films wherein a sizeable portion of the photogeneration occurs at the backside of the film28,33.
Q15. What is the main reason for the low photovoltage?
Such films can be easily produced using thin-film deposition methods, and they do not have a high surface area that enhances surface recombination and may lower the surface photovoltage—the driving force for photoelectrochemical processes on semiconductor photoelectrodes21.