The Studies On Property Optimization Of BT/BDT Based Polymer Solar Cells With Submicron-thick Active Layers

Polymer solar cells（PSCs）with a bulk-heterojunction（BHJ）structure are considered to be one of the most promising candidates for the next generation energy sources because of their lightweight,flexibility,and low cost in recent years.A great number of research efforts have been focused in power conversion efficiencies（PCEs）exceeding 12%in single-junction and multi-junction devices based on conjugated polymers electron donor and fullerene derivatives electron acceptor.Its band-gap（E _g）as well as the molecular energy levels and optical properties of the copolymer can be effectively tuned by rational selection and optimization of the electron-rich donor unit and the electron-deficient acceptor unit.Random terpolymers are usually based on the copolymerization of two different electron-rich units and one electron-deficient unit or one electron-rich unit and two different electron-deficient units.The combination of multiple different conjugated units into one polymer backbone and the easy adjustment of their composition ratios would be a low-cost and fast synthetic route to develop high performance polymers.We use two electron-deficient benzothiadiazole（BT）units with different substituent groups（2FBT with two fluorine and 2ORBT with two alkoxy chains）as complementary acceptor units in a copolymer with thienyl-substituted benzo[1,2-b:4,5-b′]dithiophene（BDT）donor units.The ratio between 2FBT and2ORBT was varied from 1:3 to 1:1 to 3:1 in the polymer backbones.As is well-known,a D-A-type copolymer of BDT and 2FBT has exhibited high photovoltaic properties in PSCs.Random copolymers containing BDT and difluorinated BT for use in high performance PSC devices with reported efficiencies of about 8.0%.The introduction of two alkoxy chains on BT not only enhances solubility but also contributes to film formation.Therefore,the combination of these two attractive units and 2ORBT is expected to enhance interchain interaction and thus obtain high device performance.Three new conjugated random copolymers P2FBT-25,P2FBT-50 and P2FBT-75 have complementary and broad light absorption ranging from 300 to 750 nm.The best PCE value of 5.71%was obtained from the blend device of P2FBT-75:PC₇₁BM using CHCl₃ solvent vapor annealing（SVA）for 30 s,with high open circuit voltage(V_oc)of 0.89 V and short-circuit current(J_sc)of 11.38 mA cm^–2,which was attributed to the broad absorption and the balanced hole and electron mobilities.Noticeably,the three random copolymers,P2FBT-25,P2FBT-50 and P2FBT-75,which differ considerably in composition,all afford high PCEs,suggesting that high performance materials can be developed within a reasonable composition range via random copolymerization.Moreover,the PCE of P2FBT-75:PC₇₁BM blend PSCs over 4.5%was observed even though the BHJ thickness over 250 nm,suggesting that this material is potentially suitable for the manufacture of large area PSC devices.Then,based on P2FBT-75:PC₇₁BM blend system,we construct respectively conventional and inverted PSCs with submicron-thick active layers.So,the PCEs of corresponding devices retain at almost the same level when the thickness of active layer is submicron（200³00 nm）.When the thickness of active layer is over 270 nm,the PCE≥4.5%in the conventional devices.However,when the thickness of active layer is over 270 nm,the PCE≥2.5%in the inverted devices.We can find the conventional devices have more property and higher stability.