Synthesis and characterization of novel fulleropyrrolidine in P3HT blended bulk heterojunction solar cells

Synthesis of novel fullerene derived electron acceptors and characterization of their organic photovoltaic (OPV) properties is important for advancing fundamental knowledge towards developing next generation organic solar cells. We report the synthesis of a novel fulleropyrrolidine derivative C60-fused N-(3-methoxypropyl)-2-(carboxyethyl)-5-(4-cyanophenyl)fulleropyrrolidine (NCPF) by 1,3-dipolar cycloaddition reaction and characterization of NCPF by 1H NMR, 13C NMR, MALDI-TOFMS, FT-IR, UV-Vis and CV. The synthesized NCPF fullerene derivative showed good solubility in common organic solvents such as chlorobenzene and 1,2 dichlorobenzene important for film formation, with optical absorbance and electronic properties comparable to PCBM. Optical micrographs of P3HT:PCBM thin films reveal formation of sparse, phase segregated needle shape PCBM micro-crystalline aggregates after 1 h of annealing at 150 °C whose length follows nucleation and growth kinetics over 24 h. In contrast, the P3HT:NCPF thin films exhibit homogeneity over 24 h, possibly due to weaker interparticle vanderWaals forces and/or stronger interactions with P3HT. This long term morphological stability of P3HT:NCPF is important for extended use in OPV applications. At an order of magnitude smaller scale, AFM of as cast and 10 min annealed at 150 °C P3HT:PCBM and P3HT:NCPF films reveal mostly smooth surfaces, with some NCPF cluster formation. Grazing incidence wide angle X-ray scattering (GIWAXS) measurements of P3HT:NCPF films indicate an increase of P3HT crystallinity with thermal annealing, leading to improvement in device performance. Photovoltaic devices fabricated with the active layer of P3HT:NCPF and P3HT:PCBM sandwiched between ITO/PEDOT:PSS and Al layer showed comparable performance upon short term annealing.