Electroabsorption studies of organic p-i-n solar cells: evaluating the built-in voltage
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Electroabsorption studies of organic p-i-n solar cells: evaluating the built-in voltage Ellen Siebert-Henze, Vadim G. Lyssenko, Robert Brückner, Moritz Riede1, and Karl Leo Institut für Angewandte Photophysik, TU Dresden, 01062 Dresden, Germany ABSTRACT We investigate the built-in voltage in organic bulk heterojunction solar cells using electroabsorption spectroscopy based on the Stark effect, i.e. the variation of the absorption energies of a material caused by an electric field. Due to spectral contributions of permanent dipoles, a novel approach for evaluating the EA spectra is required. We use a fitting routine analyzing a broad spectral range instead of using only a single wavelength. A reliable quantitative determination of the built-in voltage is achieved. INTRODUCTION The built-in voltage (VBI) is an important property of solar cells and light emitting diodes. For organic devices it is usually defined as the difference of the contact work function. Studies on VBI are often conducted using the knowledge of the contact work functions that were measured separately. [1,2] However, dipoles and other interface effects may occur [3,4] resulting in significantly altered built-in voltages. As a result, direct measurements are mandatory. Several methods can be applied to measure the built-in voltage. One technique is the ultraviolet photoelectron spectroscopy which reliably determines the energy levels of the material layers, but a measurement of a complete device is not possible. In contrast, complete devices might be analyzed using capacitance-voltage spectroscopy [5] or photocurrent measurements [6], but for both techniques the built-in electric potential is superimposed by the Dember potential. Another frequently applied technique is electroabsorption (EA) spectroscopy. It was often utilized to measure the built-in electric voltage of complete devices. [7,8,9] Still, several studies showed spectral contributions leading to wavelength-dependent measurement results. [10,11] To quantify the built-in voltage, these contributions have to be analyzed and considered for the evaluation. We developed a method of analyzing several EA spectra simultaneously and evaluate the progression of several first harmonic frequency (1f) spectra for different bias voltages in combination with a second harmonic frequency (2f) spectrum. Therefore, influences due to permanent dipoles are considered, allowing reliable results for the built-in voltage. THEORY Electroabsorption spectroscopy is based on a field-induced change in absorption Δα . An electric field can modify the charge distribution of a molecule changing its transition energies E 0 (Stark effect). The effect is quadratically dependent on the electric field F for non-polar molecules ( ΔE 0 ∝ F ² ), while it is linearly dependent on F for polar molecules ( ΔE 0 ∝ F ). [12] For small energetic shifts, the evaluation in a Taylor series is possible. 1
Now at: Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom
∂α ∂ ²α ΔE0 + ΔE02 (1) 2 ∂E0 ∂E0 Polar molecules of isotr
