Fast-Growth Polymer: Fullerene Bulk-Heterojunction Thin Films for Efficient Organic Photovoltaics

Abstract

The bulk-heterojunction (BHJ) system that uses a π-conjugated polymer as an electron donor, and a fullerene derivative as an electron acceptor, is widely used in organic solar cells (OSCs) to facilitate efficient charge separation and extraction. However, the conventional BHJ system still suffers from unwanted phase segregation caused by the existence of significant differences in surface energy between the two BHJ components and the charge extraction layer during film formation. In the present work, we demonstrate a sophisticated control of fast film-growth kinetics that can be used to achieve a uniform distribution of donor and acceptor materials in the BHJ layer of OSCs without undesirable phase separation. Our approach involves depositing the BHJ solution onto a spinning substrate, thus inducing rapid evaporation of the solvent during BHJ film formation. The fast-growth process prevents the fullerene derivative from migrating toward the charge extraction layer, thereby enabling a homogeneous distribution of the fullerene derivative within the BHJ film. The OSCs based on the fast-growth BHJ thin film are found to exhibit substantial increases in J_SC, fill factor, and a PCE up to 11.27 mA/cm², 66%, and 4.68%, respectively; this last value represents a remarkable 17% increase in PCE compared to that of conventional OSCs.

Keywords: P3HT (Poly-3-hexylthiophene); PCBM ([6,6]-phenyl-C61-butyric acid methyl ester); bulk heterojunction; homogeneous morphology; organic solar cells.

Figure 1

(a) Energy band diagram of conventional cell. (b) Schematic procedure of two different methods, conventional and fast-growth.

Figure 2

Defocused TEM (∆Z = −25 um) cross-sectional images and image profiling of the two different cells: (a) conventional and (b) fast-growth.

Figure 3

(a) 3-D AFM image of the conventional cell and the fast-growth cell, which are annealed at 150 °C for 10 min. (b) Curves showing the number of peaks in the surface of each device with height.

Figure 4

Comparison of various properties of the fast-growth and conventional cell. (a) J–V characteristics, (b) optical properties via reflectance geometry, (c) IPCE, and (d) IQE of the conventional and fast-growth cells.

Figure 5

(a) Device structure of OSC using PM6:Y6 photoactive material and (b) J–V characteristics of the OSCs.