Sulfonate-Conjugated Polyelectrolytes as Anode Interfacial Layers in Inverted Organic Solar Cells

Abstract

Conjugated polymers with ionic pendant groups (CPEs) are receiving increasing attention as solution-processed interfacial materials for organic solar cells (OSCs). Various anionic CPEs have been successfully used, on top of ITO (Indium Tin Oxide) electrodes, as solution-processed anode interlayers (AILs) for conventional devices with direct geometry. However, the development of CPE AILs for OSC devices with inverted geometry is an important topic that still needs to be addressed. Here, we have designed three anionic CPEs bearing alkyl-potassium-sulfonate side chains. Their functional behavior as anode interlayers has been investigated in P3HT:PC₆₁BM (poly(3-hexylthiophene): [6,6]-phenyl C61 butyric acid methyl ester) devices with an inverted geometry, using a hole collecting silver electrode evaporated on top. Our results reveal that to obtain effective anode modification, the CPEs' conjugated backbone has to be tailored to grant self-doping and to have a good energy-level match with the photoactive layer. Furthermore, the sulfonate moieties not only ensure the solubility in polar orthogonal solvents, induce self-doping via a right choice of the conjugated backbone, but also play a role in the gaining of hole selectivity of the top silver electrode.

Keywords: anode interfacial layers; conjugated polyelectrolytes; inverted organic solar cells.

Figure 1

Chemical structure of the synthesized CPEs.

Figure 2

(a) Energy level of the materials used to fabricate inverted solar cells in this contribution; and (b) electron paramagnetic resonance (EPR) spectra of P1, P2, and P3 in aqueous solution.

Figure 3

(a) UV/Vis-NIR absorption spectra of P1, P2, and P3 films and (b) IR absorption spectra of P2 and P3 films with a pristine PCPDTBT film during its p-doping through I2 vapor exposure.

Figure 4

(a) Inverted devices architecture; (b,c) current density–voltage (J–V) curves under AM1.5 G irradiation at 100 mW/cm². (b) before air exposure; and (c) after 15 min of air exposure.

Figure 5

Surface topographic AFM images of (a) active layer; (b) active layer/P1; (c) active layer/P2; and (d) active layer/P3 deposition. RMS values: 8.5 nm, 8.2 nm, 10.9 nm, 10.6 nm for (a–d), respectively. The active layer and CPE depositions were carried out in the devices assembly conditions.

Figure 6

Evolution of the J–V curves and PCEs of devices with inverted geometry as a function of time of storage under ambient atmosphere under AM1.5G irradiation at 100 mW/cm². Top electrode: (a) pristine Ag; (b) P2/Ag; (c) P3/Ag; and (d) PCEs of pristine Ag, MoOx/Ag, P2/Ag, and P3/Ag; before air exposure: pink background; after air exposure: white background. The data at day 0 refers to the devices exposed to air for 15 min, similar to Figure 4c.

Figure 7

J–V curves of inverted devices stored for 15 min in different atmosphere conditions; (a) moisture exposure in air or in N₂ atmosphere with a similar moisture content to air (50 % RH): Air and wet N₂; first measure and dry N₂ correspond respectively to a first and a second curve recorded after 15 min, for a device kept inside the glove box. (b) Drying of a device exposed to air for 15 min upon storage in a glovebox overnight prior and after 1 h under vacuum (10–7 atm.): Air 15 min, dry N₂ overnight, 1 h vacuum; first measure is the J–V curve prior to air exposure.

Figure 8

Proposed mechanism for moisture annealing.