On the stability of inverted polymer solar cells: the role of the surface modifiers

Francesca Brunetti^a, Luigi Salamandra^a, Luca La Notte^a, Aldo Di Carlo^a, Giuseppina Polino^a, Enrica Villari^a, Thomas M. Brown^a, Andrea Reale^a

^aUniversity of Rome (Tor Vergata), Via del Politecnico, 1, Roma, Italy

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Luca La Notte, 251
Publication date: 5th February 2015

The severe limitation to the development of the polymer solar cells (PSCs) is the durability of the devices, still too low for practical commercial uses. The lifetime of PSCs is indeed affected by a variety of degradation mechanisms of the layers upon the concurrent exposure to light, moisture and oxygen [1-3]. The approach to improve this aspect should be addressed to develop the suitable barrier material and technique for packaging and to develop device structures with reduced sensitivity. Many studies are dedicated to the first topic with a plethora of proposed solutions for the protection of the devices but a few are the works about the influence of the single layers on the lifetime, and in particular, of the interface layers. Recently, the so-called surface modifiers have been introduced as electron transport layers (ETL) in the inverted structure with the aim to lower the work function of the transparent conductor oxide (TCO) and to facilitate the electron collection at the cathode [4-6]. We report a comparative study on three common surface modifiers: a metal salt as cesium carbonate (Cs₂CO₃), a conjugated polyelectrolyte as poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene) (PFN) and a non-conjugated polymer as the polyethylenimine ethoxylated (PEIE). Plastic inverted PSCs (ITO/ETL/P3HT:PCBM/MoO₃/Al) have been fabricated by varying the ETL and the encapsulation material (commercial barrier foils) and stored by following ISOS-D1 shelf protocol [7]; it has been found that the degradation was independent from the packaging and that devices comprising PEIE resulted to be more stable (t80>500h) than ones with PFN(t80~300h) and Cs₂CO₃ (t80<100h). Interestingly, it has been noted that the stability also depends from the solvent in which the surface modifier is dissolved. Solutions of Cs₂CO₃ in 2-ethoxyethanol and water, PFN in methanol and ethanol, and PEIE in 2-methoxyethanol and water were prepared and processed in the fabrication of inverted PSCs on glass substrate. They have been shown different efficiency decay trends; ETL based on PEIE in water resulted to be the most stable (t80 up to 7000h). Moreover, it has been observed that the water and ethanol used as solvent presented better performances in terms of stability compared to the toxic counterpart: this represents a very positive finding towards PSC processing for large area.
Normalized conversion efficiency of inverted PSCs comprising different ETLs, each processed with two types of solvent, as a function of the time.
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