Aiming at building such a tandem device that can split water without an external bias, our effort has been focused on developing efficient photoanodes for water oxidation. We are working on several Ta-based (oxy-)nitride semiconductors that energetically favor water splitting [1-2]. Combining our experiences in materials synthesis and the expertise of semiconductor defects physics of our collaborators (Sharp Group @ TU Munich), we are trying to understand how defects in these semiconductors affect their photon-to-electron conversion efficiency [3], which would eventually allow us to improve their PEC water splitting performance by tailoring their defect properties [4]. Coupling the semiconductor light absorbers with efficient oxygen-evolving co-catalysts is of equal importance. We are mostly interested in methods that can effectively couple OER co-catalysts on the active sites of the photoanode, including in-situ (photo-)electrochemical deposition and atomic layer deposition.

[1] Y. Li et al., Adv. Mater. 2013, 25, 125.

[2] Y. Li et al., Nat. Commun. 2013, 4, 2566.

[3] J. Fu et al., ACS Cata. 2020,10, 10316.

[4] Y. Xiao et al., Nat. Cata. 2020, doi: 10.1038/s41929-020-00522-9.

Complimenting to our PEC water splitting work, we are also working on perovskite solar cells that potentially can be used to develop a PV-PEC tandem water splitting device. We are trying to make our contribution to this hot research field by developing new inorganic electron and hole transport layers that can be produced on a large scale and with low cost, yet maintaining high charge extraction efficiency. Again, controlling defects in these inorganic charge transport layers to achieve specific functionalities is our main goal. An example (see figure) of this is the hole traps in the electron beam deposited TiO2 electron transport layer that improves the electron extraction efficiency due to high photoconductive gain and stability due to suppressed photocatalytic activity [5-7].

[5] Y. Li et al., J. Phys. Chem. Lett. 2015, 6, 493.

[6] Y. Li et al., Nat. Commun. 2016, 7, 12446.

[7] X. Liu et al., J. Phys. Chem. Lett. 2019, 10, 6382.

We have worked on several bridged nanowire-based UV photodetectors for "visible-blind" and "solar-blind" photodetection [8, 9]. The experiences we have learned from these works, especially the influence of bulk and surface defects on the photoconductive [10], have helped us to improve the efficiencies of the solar energy conversion devices (PEC & PV) that we are currently working on. We also use photoconductive measurement as an approach to investigate the defect properties of the semiconductors for PV and PEC applications [3, 6]. More advanced techniques, such as sub-bandgap photoconductivity spectroscopy, are being developed.

[8] Y. Li et al., Nanotechnology 2010, 21, 295502.

[9] Y. Li et al., Adv. Funct. Mater. 2010, 20, 3972.

[10] Y. Li et al., Appl. Phys. Lett. 2009, 94, 023110.

We have accumulated over time quite a few nanomaterials fabrication techniques in our toolbox, such as synthesis of anodic aluminum oxide (AAO) by hard anodization [11], nanowires (ZnO, Ga₂O₃, SnO₂) by chemical vapor deposition (CVD) [12], and vertically-aligned nanorod arrays by through-mask anodization [1, 13]. Although we are currently more interested in thin-film structures deposited by "conventional" techniques (e.g., electron beam evaporation for thicker films and atomic layer deposition (ALD) for thinner films), these nanofabrication techniques are readily available if we do find it necessary to use certain nanostructures in our devices.

[11] Y. Li et al., Nanotechnology 2006, 17, 5101.

[12] Y. Li et al., Nanotechnology 2008, 20, 045501.

[13] Y. Li et al., Nanotechnology 2013, 25, 014013.