Negative polarity of phenyl-C61 butyric acid methyl ester adjacent to donor macromolecule domains

Abstract

Interfacial fields within organic photovoltaics influence the movement of free charge carriers, including exciton dissociation and recombination. Open circuit voltage (Voc) can also be dependent on the interfacial fields, in the event that they modulate the energy gap between donor HOMO and acceptor LUMO. A rise in the vacuum level of the acceptor will increase the gap and the Voc, which can be beneficial for device efficiency. Here, we measure the interfacial potential differences at donor-acceptor junctions using Scanning Kelvin Probe Microscopy, and quantify how much of the potential difference originates from physical contact between the donor and acceptor. We see a statistically significant and pervasive negative polarity on the phenyl-C61 butyric acid methyl ester (PCBM) side of PCBM/donor junctions, which should also be present at the complex interfaces in bulk heterojunctions. This potential difference may originate from molecular dipoles, interfacial interactions with donor materials, and/or equilibrium charge transfer due to the higher work function and electron affinity of PCBM. We show that the contact between PCBM and poly(3-hexylthiophene) doubles the interfacial potential difference, a statistically significant difference. Control experiments determined that this potential difference was not due to charges trapped in the underlying substrate. The direction of the observed potential difference would lead to increased Voc, but would also pose a barrier to electrons being injected into the PCBM and make recombination more favorable. Our method may allow unique information to be obtained in new donor-acceptor junctions.

Promising Strategy To Improve Charge Separation in Organic Photovoltaics: Installing Permanent Dipoles in PCBM Analogues

“…Computational support for the model was given by Marchiori et al , who concluded for isolated P3HT/C 60 and P3HT/[60]­PCBM-like complexes that an interfacial dipole exists. Several experimental results − suggest that a dipolar layer can be formed at donor/acceptor interfaces.…”

“…[13][14] Recently, an experimental study performed by Vandewal et al 8 questions this proposal by revealing that the best materials systems show an internal quantum efficiency higher than 90% without the need for excess electronic or vibrational energy. [20][21][22] suggest that a dipolar layer can be formed at donor/acceptor interfaces.…”

Characterization of the interaction between P3ATs with PCBM on ITO using in situ Raman spectroscopy and electrochemical impedance spectroscopy

“…In addition, P3ATs are p-type semiconductors [16][17][18] and have positively charged structures along their chains, these being the radical cations (polarons) and dications (bipolaron) responsible for the transfer of positive charge. Phenyl-C 61 -butyric acid methyl ester (PCBM) can be used as an n-type conductor, as it has the capacity to generate negative polarons and bipolarons, the radical anions and dianions responsible for electron transfer [1,2,[4][5][6][7][8][9][10][19][20][21][22].…”