Electron transport material modification for high-efficiency and stable flexible P3HT:O-IDTBR polymer solar cells blade coated in air from non-halogenated solvents
Luiza de Queiroz Correa a, Juliana Luiza da Silva Martins a, Gabriela de Amorim Soares a, Barbara Hellen de Souza Miranda a, Diego Bagnis a
a CSEM Brasil, Avenida José Cândido da Silveira, 2000, Belo Horizonte, Brazil
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, Luiza de Queiroz Correa, presentation 125
DOI: https://doi.org/10.29363/nanoge.hopv.2019.125
Publication date: 11th February 2019

The recent rise of non-fullerene (NF) acceptor molecules has taken P3HT polymer donor (poly(3-hexylthiophene)) to a next level in the field of organic solar cells. Despite of its competitive price compared to other semiconductor polymers, the bulk heterojunction of P3HT combined with the standard fullerene acceptor [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) has shown low power conversion efficiencies and burn-in under light exposure. For these reasons, P3HT-based devices had limited commercial application for years. However, the rapid development of the NF molecules and the launch of O-IDTBR (rhodanine-benzothiadiazole-coupled indacenodithiophene) [1] a few years ago enabled to think of P3HT as a feasible material to be used in production once this system can combine reasonable efficiency, stability and low cost.

In this work, we present results of 6.0% efficiency in a lab-scale of all solution-processed devices on flexible substrates (AA 0.55 cm2) from environmentally friendly solvents. The cells were fabricated in air by the scalable method of blade coating. The interlayers used are zinc oxide (ZnO) as electron transport material (ETM) and PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) as hole transport material (HTM). The bottom electrode is the IMI, a triple layer of a silver alloy sandwiched by Indium tin oxide (ITO) layers coated on top of the PET-based substrate. The top electrode is the Silver (Ag) deposited by thermal vacuum evaporation.  The inverted device structure is as follows: PET/IMI/ZnO/P3HT:O-IDTBR/PEDOT:PSS/Ag. [2]

We show as well how an ETM modification made by the introduction of a polymer additive in the inorganic ZnO layer played a key role on the stability of devices. This solution shows to be necessary to reduce in a drastic way the drop in performance after standard encapsulation with flexible barriers and enhance the photo-stability. The usual loss in performance that has been observed with the pure ZnO-based devices is connected to the large increase in series resistance (Rs) after encapsulation. The additive concentration was tested from 1 up to 5% in weight and the impact in efficiency and stability were investigated in order to find the optimized recipe and the right additive content. Both solutions of the modificated ETM and the active layer based in green solvents were successfully applied on the roll-to-roll (R2R) R&D pilot machine leading to stable encapsulated large-area modules (AA 21.6cm2) of over 4.0% efficiencies.

This work was supported by the department of Organic Electronics of CSEM Brasil located in Belo Horizonte. CSEM Brasil is a private and non-profit center for applied research, which has by institutional mission to perform activities of basic and applied research of scientific or technological character, as well as technological development.

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