In the last few years, solar cells based on organic-inorganic hybrid halide perovskites (e.g., methylammonium lead halides CH₃NH₃PbX₃, where X = halogen) have witnessed tremendous growth and continued to attract a huge interest. A typical cell architecture consists of a perovskite absorbing layer sandwiched between a hole-transporting material (HTM) and a compact TiO₂ (cTiO₂) as an electron-transporting layer. Mesoporous TiO₂ (mTiO₂) scaffold is frequently integrated as the electron conduction pathway, because the carrier diffusion length of a typical perovskite CH₃NH₃PbI₃ is too short (~100 nm) to collect the carrier effectively at the electrode. Nevertheless, the relatively slow electron diffusion through TiO₂ layers limits the charge conduction in the solar cells when combined with HTM with high hole conductivity such as doped 2,2’-,7,7’-tetrakis(N,N-bis(p-methoxyphenyl)amino)-9,9’-spirobifluorene (spiro-OMeTAD). For the improvement of device performances, maximizing the charge transport properties in TiO₂ layers is one of the leading challenges. In this talk I will present two examples of chemical aproach for improving perovskite solar cell performance.

The effects of the inclusion of reduced graphene oxide (RGO) in TiO₂ layers on performance of perovskite solar cells were systematically investigated. For this purpose, a wet chemical approach was examined to embed graphene oxide (GO) across the thickness of compact TiO₂ (cTiO₂) and mesoporous TiO₂ (mTiO₂) layers, which was followed by a thermally driven in-situ conversion from GO to RGO. The presence of RGO at loadings of 0.15 wt% in the cTiO₂ layer and of 0.015 wt% in the mTiO₂ layer led to a power conversion efficiency (PCE) from 6.6% to 9.3% by 40% increase. All the photovoltaic parameters are favorably improved with the presence of the RGO in the systems due to the decrease of the series resistance and increase of the recombination resistance. This work offers the promising option to cost effectively integrate RGO as a critical component to achieve high PCE in perovskite-based solar cells.

A general polymer, poly(methyl methacrylate) (PMMA) was utilized as a unique templating agent for forming crack-free mesoporous TiO₂ films by a sol-gel method. The pore morphologies were found to be controllable by varying the amount of PMMA. The PMMA-mediated mesoporous TiO₂ layer has been applied for the first time to perovskite solar cells exhibiting a best power conversion efficiency of ≥14%, which is ca. three times higher than that using a TiO₂ layer prepared by the same sol-gel method without the polymer addition (5.28%). Remarkably, it was superior to the reference device with mesoporous TiO₂ layer prepared with conventional nanoparticle paste (13.1%). Such mesostructure-tuned TiO₂ layers made by the facile sol-gel technique with a commercially available polymer additive has the great potential to contribute significantly toward the development of low-cost, highly efficient perovskite solar cells as well as other functional hybrid devices.

 References

[1] G. Murugadoss, S. Tanaka, G. Mizuta, S. Kanaya, H. Nishio, T. Umeyama, H. Imahori, and S. Ito, Jpn. J. Appl. Phys., 54, 08KF08 (2015).

[2] T. Umeyama, D. Matano, J. Baek, S. Gupta, S. Ito, V. Bubramanian, and H. Imahori, Chem. Lett., 44, 1410 (2015).

[3] Y. Yue, T. Umeyama, Y. Kohara, H. Kashio, M. Itoh, S. Ito, E. Sivaniah, and H. Imahori, J. Phys. Chem. C, 119, 22847 (2015).