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.