Correlation of morphology and device performance in inorganic-organic TiO2-polythiophene hybrid solid-state solar cells

Flat solid-state polymer photovoltaic (PV) cells were constructed using undoped poly(3-undecyl-2,2'-bithiophene) (P3UBT) and flat titanium dioxide (TiO2) films prepared using a sol-gel technique. Layers and interfaces were studied using AFM, SEM, EDX, XPS, and optical microscopy to determine the impact of the interfacial boundary morphology on the PV device performance. Friction mapping and surface roughness measurements of the fluorinated tin oxide (SnO2:F) conductive glass, TiO2, and P3UBT surfaces show a distinct difference in surface contours enabling a better understanding of light absorption as well as charge separation and injection by the polymer. It was found that each working layer in the photovoltaic device exhibited a planarizing effect, where each successive film was slightly smoother than the underlying layers. TiO2 films greater than 70 nm in thickness fractured during sintering and all films failed following sintering times exceeding 1 h at 450 degreesC. Fracture of the TiO2 film resulted in exposure of the underlying SnO2:F glass and short-circuited solar cells. The film thickness varied with viscosity of the sol-gel and could be controlled by the synthesis conditions and age of sol-gel used. Additionally, the hydrophilic TiO2 surfaces readily absorbed water when exposed to ambient conditions thereby altering the interfacial boundary and degrading PV cell performance. Spin-coated P3UBT deposited at high concentrations and low spin speeds was found to deposit a layer a few nanometers thick containing polymer aggregates between 5 and 30 mum in diameter, which resulted in a solar cell with J(sc) = 2 muA/cm(2) and V-oc = 0.2 V. The optimized spin- and cast-coated P3UBT layers were used with the best TiO2 films to create flat solid-state photovoltaic inorganic-organic hybrid cells with J(sc) = 55 muA/cm(2) and V-oc = 0.5-0.7 V. Films with fewer defects and aggregates yielded better PV device performance. (C) 2004 Elsevier B.V. All rights reserved.