Effect of functional groups of acceptor material on photovoltaic response of bulk hetero-junction organic devices based on tin phthalocyanine (SnPc)

We have fabricated and characterized the bulk hetero-junction photovoltaic devices based on the donor and acceptor blends. For the present investigation, we have used SnPc as donor material, whereas a series of acceptor materials with identical backbone but different acceptor strength were used to make the blend. The role of functional groups, attached to backbone of acceptor material on the optical, electrical and photovoltaic properties of the devices was investigated. The origin of open-circuit voltage (Voc) and short-circuit photocurrent (Jsc) has been discussed considering the ionization potential of donor and electron affinity of the acceptor material. The photocurrent of the device depends on choice of acceptor materials, i.e. number of acceptor group attached to the acceptor molecules, showing the importance of energy mismatch between lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels of donor and acceptor molecules. Here, we also investigated the effect of thermal annealing on the optical, electrical and photovoltaic properties of the bulk hetero-junction device employing tin phthalocyanine (SnPc) and Rose Bengal (RB) used as donor and acceptor, respectively. From the impedance spectroscopy, we conclude that the change in bulk resistance and dielectric constant of the active material due to the illumination has a direct relevance to the photocurrent generated by the device.