The role of molecular architecture and layer composition on the properties and performance of CuPc-C60 photovoltaic devices

We have studied the effects of molecular architecture, co-deposition and annealing on the properties and performance of photovoltaic cells based on copper phthalocyanine (CuPc)–fullerene (C60) heterojunctions. Significant improvements in performance are achieved when mixed CuPc:C60 layers are incorporated into the device structure due to the creation of an intermolecularly mixed donor (D)–acceptor (A) blend that favours efficient exciton dissociation. We utilise the control afforded by organic molecular beam deposition to show that the mixed-layer composition plays an important role in determining device performance and correlate device efficiency to the morphological and spectroscopic properties of the organic layers. A maximum power conversion efficiency of ηp = 1.17% is achieved for devices containing a mixed layer of ratio 75:25 CuPc:C60 surrounded by thin continuous layers of pure organic material at the electrode interfaces. A structure containing a compositional gradient where the CuPc:C60 composition is varied from purely D to purely A via three mixed layers of increasing A composition leads to a further improvements in efficiency (ηp = 1.36%). Finally, we use thermal annealing to show how structural defects and morphological templating of organic thin films reduces the interfacial area for exciton separation and yields poor device performance.