Engineering the vertical concentration distribution within the polymer:fullerene blends for high performance inverted polymer solar cells†
For bulk-heterojunction polymer solar cells (BHJ-PSCs), appropriate concentration distribution and phase separation of the donor and acceptor molecules within the polymer:fullerene blends are crucial for effective exciton dissociation and charge transportation. Herein, we introduce a selectively dissolvable solvent 2-chlorophenol to finely tune the vertical concentration distribution of fullerene within the photoactive layer. The morphology of the photoactive layer is investigated by atomic force microscopy (AFM), Scanning Kelvin Probe Microscopy (SKPM), transmission electron microscopy (TEM) and water contact angle (WCA) measurements. The optimized three-dimensional concentration distribution is further investigated by time-of-flight secondary-ion mass spectroscopy (TOF SIMS). To investigate the effect of vertical concentration distribution on the photovoltaic performance, conventional and inverted structured PSCs are fabricated. Compared with the control device without modulation of the concentration distribution, the short circuit current density (Jsc) increased from 13.19 to 18.50 mA cm−2 and the power conversion efficiency (PCE) increased from 6.18% to 10.15% for the inverted device with 2-chlorophenol flush treatment due to the greatly enhanced charge transportation and reduced exciton recombination. Our findings indicate that simple 2-chlorophenol flush treatment is a feasible and effective way to acquire an optimal vertical composition profile of photoactive blends for high performance inverted PSCs.