Bulk heterojunction solar cells based on a low band gap soluble bisazopyrrole and the corresponding BF2-azopyrrole complex

The diazonium salt derived from 4-(hexyloxy)benzamine reacted with half an equivalent of pyrrole to afford 2,5-bis(4-hexyloxyphenylazo)-1H-pyrrole (A). The reaction of the latter with BF3 center dot Et2O gave the corresponding BF2-azopyrrole complex (B). The thin film absorption spectra of A and B were broad and extended into the near infrared region with optical band gaps of 1.54 and 1.49 eV, respectively. The photovoltaic properties have been investigated using blends of A or B with PCBM, and it was found that the device based on B:PCBM showed higher power conversion efficiency (PCE) in comparison to that for the device with A:PCBM blend. This has been attributed to the higher hole mobility and lower band gap of B relative to A. The effect of different thermal annealing on the photovoltaic response of the devices has been investigated and it was found that the contact annealed device displayed PCE of approximately 2.7% and 3.15% for A:PCBM and B:PCBM blends, respectively. The increase in the PCE for the contact annealed device has been interpreted in terms of more balanced charge transport.