Effect of layer-by-layer assembled SnO ₂ interfacial layers in photovoltaic properties of dye-sensitized solar cells

Ultrathin SnO ₂ layers were deposited on FTO substrate by the layer-by-layer (LbL) self-assembly technique utilizing negatively charged 2.5 nm sized SnO ₂ nanoparticles (NPs) and cationic poly(allylamine hydrochloride) (PAH). For the construction of dye-sensitized solar cells (DSC), the bulk TiO ₂ layer was deposited over the (PAH/SnO ₂) _n (n = 1-10) and subsequently calcined at 500 °C to remove organic components. With introducing four layers of self-assembled SnO ₂ interfacial layer (IL), the short circuit current density (J _sc) of DSCs was increased from 8.96 to 10.97 mA/cm ², whereas the open circuit voltage (V _oc) and fill factor (FF) were not appreciably changed. Consequently, photovoltaic conversion efficiency (η) was enhanced from 5.43 to 6.57%. Transient photoelectron spectroscopic analyses revealed that the ultrathin SnO ₂ layer considerably increased the electron diffusion coefficient (D _e) in TiO ₂ layer, but the electron lifetime (Δ _e) was decreased unexpectedly. The observed unusual photovoltaic properties would be caused by the unique conduction band (CB) location of the SnO ₂, inducing the cascadal energy band matching among the CBs of TiO ₂, SnO ₂, and FTO.