Michael Niggemann, Markus Glatthaar, Andreas Gombert, Andreas Hinsch, Volker Wittwer, Birger Zimmermann, Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstr.2, 79110 Freiburg, Germany

The common architecture for organic solar cells is planar with the organic light-absorbing layer sandwiched between a semitransparent ITO electrode and an aluminum counter electrode. The presented work focuses on the development of alternative solar cell archi­tectures wherein substitution of cost-intensive ITO is envisioned with respect to an optimi­sation of the device performance. The pursued approach is to built up organic solar cells on micro structured substrates. Two types of structures — micro prisms and buried nano electrodes — are under investigation. The micro prism structure with a period of 100^m can be regarded as a folded planar cell. The benefits are an increased absorbance due to a twofold reflection at the photoactive layer and the substitution of the ITO-electrode by a metal-grid supported polymer electrode. Besides experimental results, optical simula­tions and the calculation of optimum dimensions are presented. The substrate for buried nano electrodes is made by replication of holograpical originated structures. At least one planar electrode — preferably the ITO-electrode is substituted by a comb-like array of verti­cal electrodes embedded in the active polymer. The substitution of both planar electrodes by buried nano-electrodes results in a interdigital electrode set-up. The period of the mi­cro structure is in the range of the light-wavelength (720nm) of the incident light. First de­vices have been built and the results are presented.

Introduction

The bulk heterojunction concept for organic solar cells (OSC) based on a donor — type conjugated polymer blended with methanofullerenes forming an interpenetrat­ing donor/acceptor network is very promising [1]. A power conversion efficiency of 5.0% under AM 1.5 illumination has been reported for these cells [2]. Poly — 3(hexylhiophene)(P3HT) and 3,7-dimethyloctyloxy methyloxy polyparaphenylene vinylene (MDMO-PPV) are examples for donor-type conjugated polymers in combination with the electron acceptor [6,6]-Phenyl C61 — butyric acid methyl ester (PCBM, a C60-derivative). The most commonly investigated planar OSC is built up as follows: An ITO (indium tin ox­ide) coated glass or plastic substrate is coated with a p-conducting conjugated polymer poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) named PEDOT/PSS forming the p-contact. Subsequently the photoactive layer is spin-coated. The n-contact is repre­sented by an evaporated aluminum electrode. The efficiency is limited in this case by the small absorbance of the two photoactive components in the range of the solar spectrum combined with the necessity of a small film thickness of the absorbing layer. The thick­ness of the absorbing layer (100-200nm) is restricted by the limited transport properties of at least on type of charge carriers. Increasing the film thickness significantly will lead to a reduction of the fill factor and the overall device efficiency. New cell architectures should contribute to a cost reduction by substituting ITO with the challenge of improving the device efficiency. Microstructured plastic substrates which can be made in a replica­tion process offer a wide range of design possibilities. The two presented concepts, micro prisms and buried nano-electrodes, differ in their dimensions of the structure by two or­ders of magnitude (figure 1a/b). The micro prism structure can be regarded as a folded planar cell (figure 1a). The dimensions of the micro prisms (lattice distance=100(um) are three orders of magnitude larger than the thickness of the applied organic films. Light from normal incidence on the substrate is reflected twice and contributes to a gain in ab-

sorbance. The ITO-electrode is substituted by a highly doped p-conducting polymer layer (PEDOT CPP105d) with a supporting metal grid which is located in the grooves of the prism structure. Optical simulations were performed with supporting experiments. The thin film system is treated with wave optics and the prism structure can be well described with geometrical optics. A significant increase of the overall light absorbance in contrast to a planar device was calculated. It has to be emphazised that the prismatic structure has to be regarded as an example for a linear microstructure. Other types of structures with different shapes like paraboloids are under investigation. In contrast to the concept of the micro prism solar cell, the dimensions of the buried nano electrodes are compara­ble to the thickness of the organic films. The period of the structure is in the range of the light- wavelength (720nm) of the incident light. Two concepts — asymmetric and interdig­ital buried electrodes — are presented (figure 1b). In the case of buried nano-electrodes at least one planar electrode — preferably the ITO-electrode is substituted by a comb-like array of vertical electrodes embedded in the active polymer. The influence of asymmet­ric electrodes on the charge extraction is an evident question and has not been answered yet. In this case, the description with wave optics becomes a necessity.