A film of single-walled carbon nanotubes (SWNTs) can be a dual-functional layer as electron-blocking-layer and transparent-conductive electrode in various solar cells. We have demonstrated efficient SWNT/Si solar cells [1,2] and organic solar cells [3,4] using dry-deposited high-quality SWNTs and honeycomb-structured SWNTs. For organic solar cells, the SWNT doped by MoO_xand PEDOT:PSS demonstrated a dual function, which can replace both ITO and electron-blocking-layer [3]. Using this advantage of the SWNT film, flexible organic solar cells were fabricated with ease [3]. Also, it was possible to replace electron-blocking-layer and metal electrode for inverted-type organic solar cells [4]. The removal of metal electrode enabled flexible, the semi-transparent window-like solar cells, which was inexpensive in their production process [4].

Here, this dual functional application of SWNT film is also demonstrated for organic-inorganic hybrid perovskite solar cells. Replacement of ITO in inverted-type perovskite solar cells, SWNTs/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Al, is demonstrated by adjusting the surface energy of PEDOT:PSS [5]. The flexible application on polyethylene terephthalate (PET) is also demonstrated [5]. Furthermore, replacement of electron-blocking-layer and metal electrode in normal-type perovskite solar cells is demonstrated as well. They show high power conversion efficiency (PCE), cost-efficiency, and higher stability. Those devices can have comparable PCE as the conventional design with organic electron-blocking layer and top metal electrode. In addition to the expected lower cost and improved stability, These solar cells can be potentially semi-transparent when transparent SWNT films are used. This means that they can be illuminated from either cathode or anode side. The normal-type perovskite solar cell, composed of ITO/TiO₂/CH₃NH₃PbI₃/SWNTs, can achieve a PCE of 10 % without doping of SWNTs [6]. The PCE can be as high as 17 % with the preliminary doping of the film of SWNTs using Spiro-MeOTAD, which is the typical electron-blocking-layer used for the normal type perovskite solar cells. The PMMA layer on top of the film of SWNTs can also contribute as doping and protection layer. Furthermore, the modified structure with a perovskite layer sandwiched by C₆₀ and SWNTs, i.e. ITO/C₆₀/CH₃NH₃PbI₃/SWNTs, can lead to the solar cells without hysteresis and with much improved air-stability [7]. The effective passivation of the degradation of perovskite material by moisture can be achieved with C₆₀and SWNTs [7]. This device show about 13 % PCE without hole-transporting layer and 17% with spiro-MeOTAD this far [7]. This can be a good candidate for scale-up demonstration of practical perovskite solar cells.

This work was supported by JSPS KAKENHI Grant Numbers JP25107002, JP15H05760, and IRENA Project by JST-EC DG RTD, Strategic International Collaborative Research Program, SICORP. Part of this work is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References

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[6] T. Sakaguchi, I. Jeon, T. Chiba, A. Shawky, R. Xiang, S. Chiashi, E. I. Kauppinen, N.-G. Park, Y. Matsuo, S. Maruyama, to be submitted.

[7] N. Ahn, I. Jeon, J. Yoon, E. I. Kauppinen, Y. Matsuo, S. Maruyama, M. Choi, to be submitted.