V oc from a Morphology Point of View : the In fl uence of Molecular Orientation on the Open Circuit Voltage of Organic Planar Heterojunction Solar Cells

The film morphology and device performance of planar heterojunction solar cells based on the molecular donor material α-sexithiophene (6T) are investigated. Planar heterojunctions of 6T with two different acceptor molecules, the C60 fullerene and diindenoperylene (DIP), have been prepared. The growth temperature of the 6T bottom layer has been varied between room temperature and 100 °C for each acceptor. By means of X-ray diffraction and X-ray absorption, we show that the crystallinity and the molecular orientation of 6T is influenced by the preparation conditions and that the 6T film templates the growth of the subsequent acceptor layer. These structural changes are accompanied by changes in the characteristic parameters of the corresponding photovoltaic cells. This is most prominently observed as a shift of the open circuit voltage (Voc): In the case of 6T/C60 heterojunctions, Voc decreases from 0.4 to 0.3 V, approximately, if the growth temperature of 6T is increased from room temperature to 100 °C. By contrast, Voc increases from about 1.2 V to almost 1.4 V in the case of 6T/DIP solar cells under the same conditions. We attribute these changes upon substrate heating to increased recombination in the C60 case while an orientation dependent intermolecular coupling seems to change the origin of the photovoltaic gap in the DIP case. ■ INTRODUCTION A high open circuit voltage (Voc) is one of the key factors governing the performance of a photovoltaic cell. Consequently, the origin of Voc in organic donor/acceptor heterojunction solar cells has been the subject of intense research in recent years. In analogy to inorganic semiconductors, this has led to the general understanding that the open circuit voltage is vastly determined by a photovoltaic energy gap ΔE. In a broad range of material combinations this energy has been identified as the donor/acceptor intermolecular energy gap. This is the energy difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor and is denoted as ED/A in the scope of this article. (Note that several terms exist in the literature all referring to this energy, under slightly different conditions.) To determine ED/A, it is most common to measure the HOMO levels of the donor and acceptor material separately and then derive ED/A by calculating the acceptor LUMO from the HOMO and the transport gap. In this case it is crucial that the transport gap is precisely known and that the HOMO levels are determined in a configuration as close to the device as possible since vacuum level alignment cannot generally be assumed in an organic multilayer device. In particular, intermolecular energy gaps calculated from literature values of different sources have to be treated with caution. Additionally, ΔE may be estimated by various optical and electrical techniques, some of which allow a more or less direct access to ΔE. For Received: June 21, 2014 Revised: September 30, 2014 Published: October 27, 2014 Article