The origin of high PCE in PTB7 based photovoltaics: proper charge neutrality level and free energy of charge separation at PTB7/PC 71 BM interface.

The energy level alignments at donor/acceptor interfaces in organic photovoltaics (OPVs) play a decisive role in device performance. However, little is known about the interfacial energetics in polymer OPVs due to technical issues of the solution process. Here, the frontier ortbial line-ups at the donor/acceptor interface in high performance polymer OPVs, PTB7/PC71 BM, were investigated using in situ UPS, XPS and IPES. The evolution of energy levels during PTB7/PC71 BM interface formation was investigated using vacuum electrospray deposition, and was compared with that of P3HT/PC61 BM. At the PTB7/PC71 BM interface, the interface dipole and the band bending were absent due to their identical charge neutrality levels. In contrast, a large interfacial dipole was observed at the P3HT/PC61 BM interface. The measured photovoltaic energy gap (EPVG ) was 1.10 eV for PTB7/PC71 BM and 0.90 eV for P3HT/PC61 BM. This difference in the EPVG leads to a larger open-circuit voltage of PTB7/PC71 BM than that of P3HT/PC61 BM.