The mixing behavior of the hole and electron transporting materials in bulk heterojunction (BHJ) organic photovoltaic (OPV) blends plays a key role in determining the nanoscale morphology, which is believed to be a decisive factor in determining device performance. We present a systematic investigation of the mixing behavior between poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in model multilayer structures. The bilayer structures are composed of amorphous PCBM mechanically laminated to different P3HT layers with varying degrees of crystallinity. We find that amorphous P3HT and amorphous PCBM are highly miscible at 140 ºC. However, this miscibility is significantly decreased as the crystallinity of the P3HT increases. The mixing behavior can be understood in terms of 1) the existence of crystalline P3HT domains that are impermeable to PCBM and therefore reduce uptake and 2) the existence of tie chains between crystalline P3HT domains that inhibit the swelling of the P3HT layer by the PCBM. We also introduce a unique PCBM-P3HT-PCBM trilayer structure where one of the PCBM layers is highly crystalline. The crystalline PCBM dramatically alters the mixing. There is initially mixing of the amorphous PCBM into the P3HT, but it is followed by rapid cold crystallization at the crystalline PCBM layer that depletes the PCBM in the P3HT layer. These simple bilayer and trilayer samples illustrate that mixing of P3HT and PCBM is influenced by multiple factors, such as the semicrystalline nature of P3HT (overall crystallinity, characteristics of amorphous chains) and phase (amorphous or crystalline) of the PCBM.
Pub Type: Journals
OPV, P3HT, PCBM, diffusion, miscibility, NR, XR, GIXD, bilayer, trilayer