N-Type Polythiophene Synthesized By Electro-Polymerization for Perovskite Solar Cell
Authors: Inoue Kohei , Shirasaka Tomoya , Kogo Atsushi , Koura Setsuko 
Published: December 2020

1. Introduction
Recently, organic solar cells exhibit highly attractive features in realizing large areas, being light-weight, being low-cost and possessing flexible photovoltaic panels compared to traditional silicon-based solar cell. There are mainly two types of organic solar cells, one is dye sensitized solar cell (DSC) and the other one is thin film type. In 2009, new type DSC, Perovskite solar cell (PSC) which consists of lead halide and organic amine, has been developed by Miyasaka1). Initially, photoelectric conversion efficiency was 3.9 %, it has been improving. PSC is composed of electrodes and perovskite, hole transport layer (HTL), electron transport layer (ETL). Generally, spiro-OMeTAD, PEDOT:PSS, or PTAA are used as HTL and mesoporous titanium oxide or mesoporous tin oxide are used as ETL.
By the way, conductive polymer, such as PEDOT has electrical characteristic and low cost, is used as new material for energy field. Although, most of them are p-type semiconductor, it can make n-type by electrical or chemical methods that has been clarified from the research until now2). But organic solar cell with n-type conductive polymer used as ETL has not been reported.
In this paper, we tried to make n-type conductive polymer by electrochemical method, that is very low cost and simple method. And the performance of PSC with n-type polymer as ETL was evaluated to confirm the property of the polymer.
2. Experimental
2.1 Electro-polymerization and n-type doping by electrochemical method
The electrochemical polymerization of polythiophene film was carried out under galvanostatic conditions onto the ITO coating PET film (2.4 cm × 2.4 cm, 10 Ω) by applying a constant current density of 1 mA/cm2 for 8 s or 16 s in acetonitrile solution containing 0.5 M thiophene and 0.2 M tetramethylammonium tetrafluoroborate (TMATFB). After polymerization, thiophene film as cathode and Pt as anode in 0.2 M TMATFB acetonitrile solution, n-type doping was carried out same conditions.
2.2 Device fabrication
Prepared n-type polymer films were cleaned up for 1 or 10 min with UV-O3. After the cleaning, 80 µL perovskite solution (Cs0.05(FA0.83MA0.17)Pb(I0.83Br0.17)3 in DMF/DMSO) was spin-coated onto the thiophene layer at 1000 rpm for 10 s to 4000 rpm for 30 s in N2 atmosphere. When at 10 s of 4000 rpm, 500 µL chlorobenzene dropped onto the film and then the films were annealed at 100℃ for 1 hour in a glovebox with N2. Then, 50 µL spiro-OMeTAD solution (in acetonitrile) was spin-coated onto the film. After these processes, the cells were dried in dry air overnight. Finally, Au electrodes were evaporated on the surface of the cells with vacuum deposition.
2.3 Measurement of cell performance
Photoelectric conversion efficiency for PSC with n-type polymer was measured and J-V curves of the sample cells were recorded.
3. Results and Discussion
3.1 Characteristics of thiophene polymers
Electrically conducting thiophene-based polymers were synthesized by electro-polymerization.
Both of conditions, the films were brown only edge and the center was clear. It compared shorter time with longer one, the latter was deeper in color and rougher.
3.2 Cell performance
Cell performance of the PSC with n-type thiophene polymer was evaluated. Photoelectric conversion efficiency for the cell of 8 s polymerization time with n-doping, 10 min UV-O3, was 0.6 %. On the other hand, the performance with 1 min UV-O3 was 4.4 %. It seems that long UV-O3 treatment damaged thiophene structure. Similar results were obtained for solar cells of 16 s polymerization time.
Then, we compared the thiophene with different polymerization time with UV-O3 for 1 min. Solar cell performance of thiophene formed by 8 s polymerization (4.4 %) is about three times better than that of 16 s polymerization (1.5 %). As polymerization time decreases, the thickness and resistance of the film decreases. As a result, the electrical characteristic of film was improved.
References
[1] T. Miyasaka, Organometal halide perovskite as visible-light sensitizers for photovoltaics cells, Journal of American Chemical Society, 131. 6050-6051 (2009)
[2] K. Yoshino, Conductivity and absorption spectrum of n-type polythiophene, Journal of Autumn Chemical Society, 319 (1983)