Tandem stacking structure for polymer solar cells by using semitransparent electrodes

To increase the absorption of sunlight in polymer solar cells a large active layer thickness is desired. This, however, is limited by the short charge carrier diffusion lengths in the active organic materials. Efficient light harvesting can be achieved in organic solar cells by using a tandem structure. However, fabricating a tandem structure for polymer solar cells presents its own difficulties. Since the polymer film is solution processed, spin-coating multiple layers in tandem can result in significant damage to the underlying layers. This problem can be overcome by fabricating separate PV cells and stacking them in tandem. Here, we report a multiple-device stacked structure where two polymer photovoltaic cells are stacked together with the help of a multi-layer semi-transparent electrode, made of lithium fluoride (LiF) / aluminum (Al) / gold (Au) metal layers. The semi-transparent electrode is used as the top contact in the bottom cell to efficiently transmit the unabsorbed photons to the upper cell. Maximum transparency of up to 80% is achieved for the semi-transparent cathode. In the stacked structure, the open circuit voltage or the short circuit current are twice those of a subcell depending on the serial or parallel connection between the subcells. As a result, power conversion efficiency of up to 2.6% is achieved, which is double than that of a single cell.