Dresden 2011 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 15: Joint Session: Organic Semiconductors I: Solar Cells A

HL 15.6: Talk

Monday, March 14, 2011, 15:45–16:00, ZEU 222

Characterization of trap states in small-molecule organic solar cells by using Impedance Spectrocopy. — •Lorenzo Burtone, Debdutta Ray, Karl Leo, and Moritz Riede — Institut für Angewandte Photophysik, Technische Universität Dresden, 01062 Dresden Germany

In this work, we focus on the characterization of trap levels in the electronic gap of organic semiconductors used in solar cells. These states can modify the electric field inside the device, considerably affecting charge generation and transport and consequently the solar cell efficiency. A method to characterize traps by using impedance spectroscopy is presented. We propose a new equivalent circuit based on previously reported theoretical models and we apply it in the fitting procedure of the measured data. In particular, we measure the impedance spectra of two flat heterojunction small molecule solar cells, where one of them has doped electron and hole transport layers. We observe the contribution of trap states at low frequencies in the capacitance spectra for the devices with doped transport layers. The experimental results can be understood with the equivalent circuit proposed and the trap state contribution can be evaluated. From a fitting procedure, we estimate the density of trap states to be around 1x10¹⁷cm⁻³eV⁻¹. Moreover we observe from the impedance analysis the presence of dopants in the intrisic layer with an estimated effective concentration of about 8.4x10¹⁷cm⁻³. This value is in agreement with very low doping concentration and suggests the formation of a non-abrupt junction between intrinsic and doped layers.