The power conversion efficiency of solar cells based on conjugated polymer:fullerene derivative donor:acceptor
bulk heterojunctions is not yet sufficient for commercialization. The two most common techniques used to enhance
cell performances are thermal treatments and utilization of interlayers. In this work we investigated the effect of the
sequence of thermal annealing and the metal evaporation on interlayer formation induced by additives migration
toward the metal/organic interface. For this purpose we chose to study P3HT:PCBM:PEG blends, on which we
performed thermal annealing before or after the Al cathode deposition. We further characterized the device
performances and determined, by XPS, the blend/Al interfacial compositions. We conclude that thermal annealing
before Al deposition inhibits the migration of PEG to the organic/metal interface in the P3HT:PCBM:PEG system, while
annealing after the Al deposition enhances it. Thus, our study reveals that there is a great significance in the sequence of
which the thermal annealing and the cathode deposition are performed in additive-containing organic blends, on the
interlayer formation, and as a result, on the device performance.
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