Solution phase n-doping of C₆₀and PCBM using tetrabutylammonium fluoride

Abstract

The solution phase n-doping of C 60 and PCBM with tetrabutylammonium fluoride is shown to occur via an initial chemical reaction followed by electron transfer to a second fullerene molecule. The formation of ionic and radical intermediate species has significant implications for the use of ionically functionalized materials as electron-selective interface layers in OPVs.The use of ionically functionalized polymers and molecules as electron-selective interfacial layers in organic photovoltaic cells (OPVs) has attracted signicant research interest in recent years. 1-5 Devices fabricated using these materials, such as ionically functionalized polyuorenes and fullerenes, have displayed power conversion efficiencies up to $9%, largely due to an increase in the cell open-circuit voltage (V oc ). 1-5 Despite these advances, the mechanism by which these materials affect device performance remains uncertain. 1-8 The observed increase in V oc is typically attributed to electrode work function reduction from preferential orientation of ionic pendant groups and associated counterions, resulting in a net dipole. 1-7 While it is likely that interfacial dipoles are present, the role of potential chemical reactions between the ionically functionalized material and active layer has received relatively little attention. 7,9,10 Recently, Li et al. reported unusually high conductivities (up to 3.2 S m À1 ) in PCBM/fulleropyrrolidinium iodide blend thin lms. 11 It was shown subsequently that lms deposited from solutions of tetrabutylammonium salts (TBAX: X ¼ F, Br, I, OH, AcO) and PCBM also resulted in highly conducting lms. 12 Based on solution phase studies using C 60 , it was postulated that direct electron transfer from anion (OH À or F À ) to fullerene results in the n-doped C 60 radical anion (C 60 _ À ) and donor radical (OH_ or F_). These studies suggest that redox reactions between the interfacial material anions and PCBM in the active layer results in interfacial n-doping which would have signi-cant effects on interfacial energetics and charge extraction. However, given the electrophilicity of the fullerene molecule and nucleophilicity of anions studied, particularly F À and OH À , 13 the possibility of chemical reactions is signicant in both solution and solid state. Based on this, as well as the large mismatch in standard reduction potentials between OH À /F À and C 60 , 14 we hypothesized that the reported observation of radical species resulted from an initial chemical reaction between the nucleophile and C 60 followed by electron transfer to a second C 60 molecule (Scheme 1). Similar reactions have been demonstrated previously in both solution and the solid state 15-18 Such reaction chemistry would result in the generation of numerous potential products. The understanding of such chemistry is essential for elucidation of a mechanistic description of interface energetics and charge extraction processes towards the rational design of highly efficient OPVs.UV-vis-NIR spectroscopy was used to monitor the reaction be...

Limits on anion reduction in an ionically functionalized fullerene by cyclic voltammetry with in situ conductivity and absorbance spectroscopy

“…The suggested mechanism was referred to as, "anion--induced electron transfer." 21 In this solution phase investigation, reduction of C 60 in the presence of TBAF was observed, like the Li et al study. 15 In the PETMA study, unusually high conductivities were measured when films were heated in vacuum.…”

Synergistic Doping of Fullerene Electron Transport Layer and Colloidal Quantum Dot Solids Enhances Solar Cell Performance

“…[ 24 ] Effective electron transfer between the halide X and PCBM induces effective doping. [ 24,25 ] Meanwhile, in this work, the halide is also known to improve passivation of CQDs, leading to the stable n-type doping CQDs, which renders the approach highly compatible with the intended optimized device fabrication. We used tetrabutylammonium iodide (TBAI) salt and followed established procedures.…”

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

“…Specifically, iodide ions form a strong bond with PCBM molecules by a direct electron transfer from the anions (iodide ions here) to PCBM or C 60 , as shown in Figure 12c. [198] The reaction between PCBM and I 3 trimers associated with the Pb I antisite defects leads to the creation of anionic fullerene derivatives. [168] In this respect, the iodide ions/defects are immobilized by combining with the PCBM molecules in the bulk of the perovskite, thus impeding their further motion towards the perovskite/TL interfaces.…”