Investigation of Bulk Morphology and Photovoltaic Performance of Synthesized Novel Fulleropyrrolidine Derivatives / P3HT Blend System

The objective of this study is to synthesize and characterize novel fulleropyrrolidine derivatives and investigate their optical, electrochemical, and thermal properties as well as evaluate their morphology and photovoltaic properties when blended with homopolymer or block copolymer. This study is divided into three parts: (1) Synthesis of 1,5 substituted fulleropyrrolidine derivatives (FP-1) and study the dispersion behavior of FP-1 in Poly (styrene- block-methyl methacrylate) [PS-PMMA] block copolymer, PS and PMMA homo-polymer, via the application of cold zone annealing (CZA) and vacuum oven annealing conditions; (2) Synthesis of 1,2,5 substituted fulleropyrrolidine derivatives (FP-2) and study the morphology and photovoltaic performance of solvent casted FP-2/P3HT thin film systems; and (3) Synthesis of polystyrene -b- poly(3-hexylthiophene) [PS-b-P3HT] block copolymer and evaluate the role of PS-b-P3HT as compatibilizer on the photovoltaic performance and morphology of FP-2/P3HT blend system.;A class of 1,5 and 1,2,5 substituted fulleropyrrolidines (FP-1 & FP-2) was synthesized by using Prato reaction. Characteristic peaks corresponding to FP-1 and FP-2 were observed by 1H NMR, 13C NMR, FT-IR, and MALDI-TOF MS which suggest the successful synthesis of desired compounds. The UV-Vis absorption spectra of synthesized fulleropyrrolidine derivatives exhibit a typical fulleropyrrolidine absorption band at or around 430 nm which confirms the formation of desired compounds. The solubility of FP-2 (40-55 mg/ml) in 1,2-dichlorobenzene was significantly higher than FP-1 (2-9 mg/ml). The higher solubility of FP-2 compared to FP-1 was attributed to the introduction of longer alkyl chain functionality on to the fulleropyrrolidine core. The substitution of electron withdrawing groups on fulleropyrrolidine core resulted in an improvement in the electronic properties of synthesized fulleropyrrolidine derivatives. For example, the introduction of strong electron withdrawing groups in compound 6 resulted in increase in LUMO level (-3.86 eV) as compared to compound 5 (-3.90 eV). AFM images revealed that the dispersion state of 1,5 substituted fulleropyrrolidine derivatives (FP-1) in PS-PMMA polymer matrix was found to be highly sensitive to the processing conditions and chemical structure of fullerene derivatives. In addition, the fullerene crystal appearance in the fullerene/P3HT system was found to be strongly influenced by the thermal annealing conditions. [42] For example, PCBM/P3HT thin films showed formation of PCBM crystals upon prolonged thermal annealing while compound 5 (FP-2)/P3HT thin film showed featureless texture. Additionally, a strong correlation between PCBM crystal size and annealing time was established. Grazing incidence wide angle X-ray scattering (GIWAXS) measurements of compound 5 (FP-2)/P3HT films showed an increase in P3HT crystallinity upon thermal annealing. Photovoltaic devices fabricated with compound 5 (FP-2)/P3HT, compound 6 (FP-2)/P3HT showed higher power conversion efficiency than that of PCBM/P3HT under similar processing conditions because of the possible vertical distribution of PCBM and P3HT in the blend films.;We used block copolymer (PS-b-P3HT) as compatibilizer to suppress vertical distribution of active layer components (PCBM and P3HT) in the formulation of blend films. PS-b-P3HT was synthesized by click chemistry using alkyne terminated P3HT and azide terminated polystyrene. The block copolymer was characterized by 1H NMR, FTIR, TGA and GPC. Additionally, AFM was used to study the morphology of block copolymer. The AFM images of annealed PS-b-P3HT BCP thin film showed lamellar stacking of phase separated P3HT fibers. GIWAXS showed that 5 wt% PS-b-P3HT BCP addition to annealed PCBM/P3HT led to a significant enhancement in polymer crystallinity as compared to the blend films without 5 wt% BCP. It appears that vertical distribution in PCBM/P3HT blend films may have been inhibited by the addition of BCP compatibilizer as evidenced by XPS results. The increase in polymer crystallinity and suppression of vertical distribution in PCBM/P3HT may have resulted in significant improvement in PCE (15%) of BCP added system. However, in compound 5/P3HT films with 5 wt% BCP addition only a small increase in polymer crystallinity and no clear evidence of alteration of vertical distribution was noticed, therefore, only marginal improvement in PCE (5%) was observed. Future studies should explore the role of vertical distribution of donor/acceptor components and polymer crystallinity and orientation on the PV properties of OPV devices.