Biophotonics: Neural Optoelectronic Interface

Home , Biophotonics

news & views electrons (to the right). The surface layer The QFL splits are not illustrated in layer. This resulted in a lower current, prevents front surface recombination. the original figures of López et al. The which clearly loses the advantage of high The layer to the right of the IB region amount of QFL splitting, which ultimately current in IB solar cells. The fact that the is a blocking layer that prevents the determines the output voltage, can be external quantum efficiency is low for passage of electrons from the IB to the CB visualized by qualitatively modifying both the VB→CB and VB→IB transitions by tunnelling. the energy bands of the original figure in the cell with a blocking layer suggests Although the need for this blocking to illustrate operation of the cell under that the problem is not low photon layer is well-understood and already illumination, as shown in Fig. 1b. In absorption but difficulty in extracting the used in QD-IB solar cells9, its application fact, one of the key features revealing the current, possibly because of the blocking to alloy-based IB materials — one QFL splitting between the CB and the IB layer itself. This important issue requires determining factor for producing a working was that radiative recombination occurs further investigation. IB solar cell in a bulk material — has so far between the CB and the IB, allowing Although the efficiency of this cell is been difficult to achieve. The cell developed the cell to behave like a light-emitting still far from promised levels, the work of by López et al. has an open-circuit voltage diode. Although the IB→CB threshold López et al. represents a significant step of 0.92 V at room temperature and an was not visible from measurements of forwards in both the efficiency and our illumination of 20 Suns, which far exceeds the external quantum efficiency, the understanding of IB solar cells. ❐ the reference cell’s open-circuit voltage researchers observed the light-emitting of 0.42 V. diode effect by directly measuring, for the Antonio Luque and Antonio Martí are at the Instituto López and co-workers complete their first time in an IB solar cell, the CB→IB de Energía Solar, Universidad Politécnica de Madrid, demonstration by discussing how the electroluminescence. They observed an Avda. Complutense 30, 28040 Madrid, Spain. two thresholds in the external quantum energy peak at 0.9 eV originating from e‑mail: [email protected] efficiency of their cell — situated at 1.1 eV radiative recombination between the CB and 2 eV — reveal the VB→IB and VB→CB and the partially occupied IB, which exists References transitions. The IB→CB threshold is not only when there is QFL splitting between 1. Shockley, W. & Queisser, H. J. J. Appl. Phys. 32, 510–519 (1961). 2. Luque, A. & Martí, A. Phys. Rev. Lett. 78, 5014–5017 (1997). visible, but this is to be expected from the CB and the IB. This key result is 3. López, N., Reichertz, L. A., Yu, K. M., Campman, K. & the way in which the experiment was evidence of the optical transitions between Walukiewic, W. Phys. Rev. Lett. 106, 028701 (2011). performed. The only threshold visible the CB and the IB, giving the missing 4. Antolin, E. et al. 35th IEEE Photovoltaic Spec. Conf. in the test cell is the VB→IB transition, link towards developing a working IB 000065–000070 (2010). proving that both the IB and the CB are photovoltaic device. 5. Yu, K. M. et al. Phys. Rev. Lett. 91, 246403 (2003). short-circuited through the GaAs substrate. The external quantum efficiency of 6. Yu, K. M. et al. Appl. Phys. Lett. 88, 092110 (2006). 9 7. Walukiewicz, W. et al. Phys. Rev. Lett. 85, 1552–1555 (2000). Note that a two-photon experiment the cell containing a blocking layer was 8. Wang, W., Lin, A. S. & Phillips, J. D. Appl. Phys. Lett. 95, must be performed to give additional around an order of magnitude lower 011103 (2009). confirmation regarding the IB operation. than that of the cell without a blocking 9. Martí, A. et al. Phys. Rev. Lett. 97, 247701 (2006).

Biophotonics Neural optoelectronic interface The realization of an organic biocompatible showed that a train of short visible light artificial retina for restoring lost vision Photoexcitation pulses (20 ms duration, 1 Hz repetition could now be one stage closer to reality, rate) reliably triggered action potentials. thanks to a hybrid device that successfully The triggering was shown to be spatially integrates nerve cells from a rat’s brain with confined to the region fully illuminated by polymer optoelectronics. Bath the light. Writing in Nature Communications solution The most obvious application of this Polymer Ag/AgCl (doi:10.1038/ncomms1164), Guglielmo ITO research is for the creation of a new Lanzani and co-workers from Istituto form of organic artificial retina. Although Italiano di Tecnologia and Politecnico di silicon photodetectors have already been Milano in Italy show that it is possible to Neuron proposed and tested for use as an artificial grow hippocampal neurons on top of an retina, Lanzani and co-workers say that an organic photodetector to create a functional polymeric light-sensitive film made from rr- organic approach brings several benefits. interface. A visible light pulse striking the P3HT:PCMB, a fullerene–polythiophene blend First, it works without any externally photodetector directly beneath a neuron commonly used in organic solar cells. The applied electric field and with minimal heat generates an electrical charge that changes researchers cover one side of the film with dissipation. Second, organic optoelectronics the neuron’s ionic local environment, indium tin oxide (ITO) to form a transparent can be fabricated cheaply by methods such causing it to fire. In essence, this conversion electrode, and the other with neurons as ink-jet deposition, thereby allowing the of light into a neural signal exactly mimics surrounded by an aqueous salt solution, creation of thin, flexible devices that are what happens in a healthy retina. which keeps them alive for up to 28 days. well-suited to implantation in the eye. The photodetector developed by According to the researchers, patch- Lanzani and co-workers consists of a clamp experiments recording neuron activity Oliver Graydon