Amorphous/crystalline blend effects on the performance of polymer-based photovoltaic cells

Abstract

Bilayer TiO2/polymer photovoltaic cells are prepared by using regioregular poly（3-hexylthiophene） （P3HT） and branched poly（ethyleneimine） （BPEI） as an additive to P3HT. Influences of BPEI, an amorphous polymer, on the performance of bilayer TiO2/P3HT cells are investigated by means of steady-state current-voltage measurements and dynamic intensity-modulated photovoltage spectroscopy, in combination with differential thermal analysis, UV-vis absorption and photoluminescence spectroscopy. Due to the high crystallinity of P3HT, bilayer TiO2/P3HT cell exhibits a poor performance. As P3HT is blended with BPEI in the weight ratio of WBPEI/P3HT=1％—5％, the performance of the devices is greatly improved; in particular, as WBPEI/P3HT= 1%, the cell exhibits an open-circuit voltage of 0.8V and a short-circuit current of 20μA/cm2. Results show that the influence of BPEI on the cell performance originates from the changed interfacial contact at TiO2/P3HT interface rather than the varied optical properties of P3HT in P3HT-BPEI blend. BPEI imposes two competitive effects on interfacial contact between TiO2 and P3HT, depending on the compositional structure of the blend. On one hand, BPEI improves the interfacial contact by reducing the crystallinity of P3HT and enhancing the interaction between P3HT and TiO2. On the other hand, the interfacial contact will become worse with more BPEI chains accumulating on TiO2 surface. A good contact at TiO2/P3HT interface facilitates the access to a high exciton dissociation efficiency, a long lifetime of photogenerated electrons and further a high device efficiency. The results presented in this paper are expected to provide new insights and a novel strategy for improving the performance of polymer-based photovoltaic cells.