Enhanced efficiency of dye-sensitized solar cells via controlled thickness of the WO3 Langmuir–Blodgett blocking layer in the Debye length regime

Abstract

Generally, a thin (>50 nm) transition metal oxide (TMO) film (such as TiO₂) is employed as an interfacial layer (also called a blocking layer, BL) between FTO and mesoporous TiO₂ in a dye-sensitized solar cell (DSC). The function of this layer is to provide full coverage of FTO so as to reduce back electron recombination without compromising transparency and charge transport properties. Recent research has demonstrated that WO₃ has better charge transport and stability than TiO₂ and therefore has the potential to be employed as a hole blocking layer. Furthermore, ultra-thin TMO films having thickness less than the Debye length (characteristic length of the space charge layer in the WO₃, L_D) are expected to provide better interfacial and optical properties. However, reducing the thickness in such thin films has been challenging owing to available preparation methods. In this work, the Langmuir–Blodgett (LB) technique has been adopted to prepare high quality crystalline BL of WO₃ of controlled thickness in the range of L_D (4–20 nm). The interfacial properties such as charge transport and blocking behaviour of WO₃ LB films improved significantly when the thickness was reduced to ≤14 nm, which is comparable to L_D. As a result, DSCs fabricated with thinner WO₃ BL (thickness ≤14 nm) showed significant efficiency (η) improvement (22%) leading to η up to 9.79% vis à vis DSCs with WO₃ BL having thickness 20 nm.