Effect of nanostructured TiO2 morphology on electronic structure studied by the photoacoustic, photoluminescence, and photoelectron yield spectroscopy

Witoon Yindeesuk^a, Tsuyoshi Okuno^a, Qing Shen^a^,^b, Taro Toyoda^a^,^b, Shuzi Hayase^b^,^d, Keita Kamiyama^c

^aThe University of Electro-Communications, Japan, Japan

^bCREST, Japan Science and Technology (jST), Kawaguchi, Saitama 332-0012

^cBunkoukeiki, Co., Ltd., Hachioji, Tokyo 192-0033

^dKyushu Institute of Technology, Japan, 204 Hibikino Wakamatsu-ku, Kitakyushu - Fukuoka, 808, Japan

Proceedings of International Conference of Fundamental Processes in Semiconductor Nanocrystals (FQDots14)

Oxford, United Kingdom, 2014 September 8th - 10th

Oral, Taro Toyoda, presentation 015
Publication date: 27th June 2014

TiO₂ is used in the numerous applications sensitized-solar-cells. A main factor for the photovoltaic performance is the morphology of TiO₂ electrode for sensitizer assembly [1]. Nanoparticulate (NP)-TiO₂ electrodes have been used for sensitized-solar-cells. NP-electrode has high surface area to increase the loading of sensitizers. However, the recombination process is proportional to the electrode surface area. A balance between recombination and light harvesting is needed to maximize solar cell performance. TiO₂ electrodes with a higher degree of order than NP-electrodes are desirable for higher conversion efficiency. An approach has been proposed using an inverse opal (IO)-TiO₂. There are several reports showing higher conversion efficiency in IO-case (mainly increase of open circuit voltage, V_oc) than that in NP-case [2.3]. However, the key factors that determine the enhancement of conversion efficiency of IO-case still remain unclear. The fundamental studies of comparison of electronic structures in IO- and NP-TiO₂ is lacking. Photoacoustic (PA) spectroscopy is used for the optical absorption. Photoluminescence (PL) spectroscopy is used for defect state characterization. In addition to those measurements, photoelectron yield (PY) spectroscopy was applied to characterize the valence band maximum (VBM) position which is correlated with the conversion efficiency. Band gap value of IO-TiO₂ agrees with that of NP-TiO₂ (~ 3.2 eV) by PA measurements in good agreement with that of anatase TiO₂. Above the band gap region, the slope of exponential absorption in IO-TiO₂ is larger than that in NP-TiO₂, indicating the larger state density of IO-case than that of NP-case in the conduction band. The surface oxygen vacancies and bulk oxygen vacancies in IO-TiO₂ are lower than those in NP-TiO₂ from the PA measurements. PL measurements show that the self-trapped-exciton emission is dominant in IO-TiO₂, contrary to dominant emission from oxygen vacancy states in NP-TiO₂. The positions of VBM measured for IO- and NP-TiO₂ were -7.38 and -7.52 eV, respectively. The position of VBM of IO-TiO₂ is higher than that of NP-TiO₂, indicating the possibility of increase in V_oc in sensitized-solar-cells [1].

[1] T. Toyoda and Q. Shen, J. Phys. Chem. Lett. 3, 1885 (2012).

[2] L. Diguna, Q. Shen, J. Kobayashi, and T. Toyoda, Appl. Phys. Lett. 91, 023116 (2007).

[3] H.-N. Kim and J.-H. Moon, Curr. Appl. Phys. 13, 841 (2013).

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