Effect of sublimation on performance of CuPc: PTCDA bilayer organic solar cell

Silicon photovoltaic is the most commonly used commercial technology. However it has limitation in terms of efficiency and cost effectiveness. As such there is a need to search for alternate technologies for conversion of solar energy into electricity. Organic photovoltaic (OPV) is one of the emerging R&D areas in this direction. In OPV two approaches are being used for fabrication of solar cells i.e. using small molecule and polymers, respectively. In both the cases donor:acceptor (D/A) concept is being used for the fabrication of the device. For example in small molecule approach bilayer devices having D/A layer is used whereas in polymer solar cells D/A interpenetrating bulk hetrojunction is being used. Both the type of devices need improvement in their performance in terms of efficiency, stability, life time etc. Global efforts are being made to improve the performance of these devices by using new hetrojunction materials, interface layers, thermal treatments, purification/sublimation etc. The present work is also an attempt in this direction wherein we have improved the performance of small molecules based solar cell by the process of sublimation of the material used in device fabrication. The configuration of the device used in the present work was prepared as follows. Thin film of PEDOT: PSS was deposited on ITO substrate by spin casting. Subsequently copper(ll)phthalocyanine (CuPc) film (~45nm) (both without sublimation as well as with sublimation) was vacuum deposited in a vacuum ~ 10^-6. Then on top of CuPc layer Perylen-3, 4,9,10-tetracarbonsaure-dianhydrid 3,4,9,10- Perylenetetracarboxylic dianhydride dianidride perilen-3,4:9,10-tetracarbossilico (PTCDA) film (~40 nm) was also vacuum deposited. Finally LiF and Al electrodes were vacuum deposited resulting in to a bilayer device having the configuration ITO/PEDOT: PSS/CuPc: PTCDA/LiF/AI. The device without sublimation of CuPc shows the solar cell parameters i.e. open circuit voltage (V_{- oc}),short circuit current density (J_sc), fill factor ( FF) and power conversion efficiency( eta) as 0.20 V, 2.8 times10^{- 6}A/cm², 0.32, and 2.2 times 10^{- 4%} .respectively. However when the device is made in same configuration by sublimation of CuPc prior to its deposition a considerable improvement in the J_sc and eta of the device has been observed. In fact new electrical parameter obtained in the second case (sublimated/purified CuPc) being 0.12V, 52.6times 10^{- 6}A/cm², 0.25 and 13.2 times 10^{- 4%} respectively. The improvement in the J_sc and overall eta of CuPc: PTCDA bilayer device has been attributed to the reduction in the defects/impurities in CuPc on sublimation.