Renewed interest in metal phthalocyanine donors for small molecule organic solar cells

Vacuum deposited ternary mixture organic solar cells

“…Furthermore, the reduction of ClInPc content serves to limit the number of hole-conductive pathways for hole transport, thereby hindering sweep-out. Coupling this information with the knowledge that ClInPc has generally poorer hole transport properties compared to fullerene's electron transport properties [44,47], it is observed that the slowest devices are the ternary ClInPc:SubPc:C 60 OSCs with high donor content. The results thus support our earlier conclusions, which suggest that the energy levels of ternary OSCs must be well matched in order to retain reasonable charge transport properties, and thus to allow for rapid sweep out of charges.…”

“…SubPc is reasonably well studied in literature, providing high V oc values (owed to its deep HOMO) and high performance vacuum deposited small molecule OSCs [39][40][41][42]. ClInPc is somewhat less studied in literature, but it has also been highlighted for its high open circuit voltage OSCs, while granting near-IR absorption [43][44][45]. According to [46], the corresponding ternary OSCs are classified as 'parallellinkage' OSCs due to the strong inter-mixing of the active layer.…”

“…The OSC device structure employed in this research is a 40 nm BHJ sandwiched between a 5 nm MoO 3 hole extraction layer (HEL) and an 8 nm bathocuproine (BCP) electron extraction layer (EEL), with an indium tin oxide (ITO) anode and an aluminum cathode. The thickness of the BHJ was chosen to be 40 nm since it provides optimal performance for binary metal phthalocyanine:C 60 BHJ OSCs, as established in our previous work [44]. It should also be noted that a neat layer of C 60 in between the BHJ and the BCP layer could be employed in these devices to further bolster the power conversion efficiency (PCE) of the OSCs [47].…”

“…Finally, we observe that at very high donor content device performance decreases due to reductions in J sc and FF, largely associated with the poor electron transport properties of the donor-rich BHJ layer (and in consideration of the associated charge recombination effects) [44,47]. It is worth noting that SubPc:C 60 OSCs are generally more sensitive to varying donor content (with poorer performance at >50% donor content), suggesting that there may be some slight differences in the charge transport properties in the two materials systems.…”

“…In the ternary OSC, the 25% donor content must be split between two different absorbing species, so the individual donor components cannot generate the same magnitude of photocurrent. Unfortunately, increasing the total donor content beyond 25% serves to reduce the V oc (due to Schottky band bending effects) [44], hinder charge transport [45,47], and reduce C 60 aggregate photocurrent [43], thus degrading OSC performance. These observations highlight one of the more critical aspects of the ternary OSC: one must carefully tune the mixed layer composition to balance photocurrent contributions from all three species.…”