Annealing temperature effect of hole-collecting polymeric nanolayer in polymer solar cells

Abstract

Recent pronounced global warming has attracted keen interest in solar cells due to their renewable feature exploiting tremendous solar energy (89 PW) during sun’s lifetime of ~5 billion years. Of the solar cells invented so far, inorganic solar cells are now in market but they have a demerit such as high fabrication cost and hard characteristics of inorganic materials which restrict the spreading of solar cells. In this regard organic solar cells have been studied as a contingency because of their potential for cheaper manufacturing cost and variety of applications owing to flexible and semitransparent features. To date, the most widely used polymer as a light-absorbing material is regioregular poly(3-hexylthiophene) (P3HT) which does also act as an electron-donating (i.e., holeaccepting) component in a bulk heterojunction layer inside corresponding solar cell geometry. Here we note that the bulk heterojunction film is, broadly speaking, a mixture of electron-donating and electron-accepting materials, whilst in more specific terms it has integrated and randomly distributed p-n junctions in the bulk polymeric film. As an electron-accepting material, soluble fullerenes such as 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) are widely used. Considering the principle of electron flows in the bulk heterojunction polymer solar cells, it is not necessary to insert a low work function metal in between the bulk heterojunction layer and the electron-collecting electrode, which has been proven in our previous report. However, the direct hole collection from the bulk heterojunction layer to transparent conducting oxide, mostly indium tin-oxide (ITO), was not efficient owing to the insufficient interfacial contact between the bulk heterojunction layer and the ITO surface. Hence most of efficient organic solar cells employ a buffer layer, in most cases poly(3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS), between the bulk heterojunction layer and the ITO layer. However, the influence of thickness and thermal annealing conditions of PEDOT:PSS layer was not much studied. In this work, we have briefly investigated the effect of thermal annealing temperature on the performance of polymer solar cells with the PEDOT:PSS nanolayer of which thickness is 20 nm. The result showed that about 40% power conversion efficiency was improved by thermal annealing of the PEDOT:PSS layer at 120 C.