Large area organic solar cells

Abstract

In a time where the energy consumption is increasing year by year the need for alternative energy sources is becoming vitally important. One way to satisfy this demand is to harvest solar radiation with photovoltaic cells. In recent years new classes of solar cells beside the prevalent silicon solar cells have emerged. One class of these new 3rd generation solar cells are organic solar cells, which have the advantages of adjustable optical gaps with the active layers being solution processable. Organic solar cells have made significant steps forward in recent times with efficiencies now the same as cells made from amorphous silicon. One aspect that has not received enough attention yet is how to translate the good results for small laboratory cells to large area cells or sub-modules. In this thesis different methods for upscaling organic solar cells are discussed.In the first part of the thesis the working principle of organic bulk heterojunction solar cells is explained and important points regarding the characterization of organic solar cells are discussed. We show the influence the use of an aperture mask has on the measured solar cell parameters depending on the design of the cells. Another important point involves the configuration of the measurement setup for current-voltage characteristics, i.e. the difference in using a 2wire or 4wire setup. Depending on the setup and the fact of using or not using an aperture mask the power conversion efficiency of 0.2 cm2 solar cells varies by up to 20%. This should emphasize how important it is to state clearly the measurement conditions when publishing solar cell data so that the results can be put in relation to already reported dataMost of the organic solar cells reported in literature use spin coating as the deposition technique for the photoactive layer. This works well for small substrates, but for larger substrates it becomes difficult to get a uniform film over the whole substrate. Spin coating also wastes a lot of the material since most of the solution is just spun off the substrate. One solution deposition technique that is easy to implement even in smaller research labs and is not too different from common roll-to-roll coating processes is blade coating. Large area devices (active area 25 cm2) with spin and blade coated layers of a poly[N-9” heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester blend (PCDTBT:PC70BM) using a conventional architecture with an indium tin oxide/poly(3,4ethylenedioxythiophene)polystyrene sulfonic acid (ITO/PEDOT:PSS) electrode were fabricated and compared in terms of the layer quality and the solar cell performance. The results showed similar behavior with efficiencies of 1.8% and 1.9% for both types of cells demonstrating the viability of blade coating for the fabrication of large area solar cells.The main issue when it comes to upscaling organic solar cells is the shortage of highly conductive transparent electrodes. The widely used indium tin oxide typically has a sheet resistance of 10 - 50 Ω/ depending on the substrate and processing conditions. This limits the charge collection efficiency and therefore the overall power conversion efficiency of large area organic solar cells. Promising alternative electrodes recently developed are insulator/metal/insulator stacks to replace the ITO electrode. The metal layer, for example silver, needs to be thin enough to still be semi-transparent, but thick enough to have good conductivity. By using a stack comprised of molybdenum(VI) oxide/silver/molybdenum(VI) oxide (MoOx/Ag/MoOx) it is possible to reduce the sheet resistance of the electrode to around 5 Ω/ with a similar peak transmission (≈ 80%) as ITO. Due to the higher conductivity of this electrode the power conversion efficiency of 25 cm2 cells with a PCDTBT:PC70BM active layer was improved from 1.9% to 3.1%. We also show stack electrodes with different top layers and compare their stability in air by neutron reflectometry. The results show that the structure of the MoOx layer at the surface changes resulting in a swelling of the film. Zinc sulfide on the other hand does not swell and therefore is a more stable alternative for solar cells. The best 25 cm2 devices with the molybdenum(VI) oxide/silver/zinc sulfide (MoOx/Ag/ZnS) stack electrode achieved efficiencies of 2.7% and 2.8% on glass and polyethylene terephthalate foil, respectively. This easy transition without any loss in performance from glass substrates to foil is based on the top illumination architecture, where the optical properties of the substrates have no influence on the performance of the device.Finally, highly efficient large area, monolithic organic solar cells with a silver grid are presented. By using silver lines in combination with an indium tin oxide electrode the conductivity of this electrode was improved by more than an order of magnitude (R <1.5 Ω/) with losing around 7% of the active area due to the opaque nature of the grid. These silver grid based electrodes in combination with a new high molecular weight donor copolymer based on diketopyrrolo-pyrrole and dithienylthieno[3,2-b]thiophene resulted in 25 cm2 solar cells on glass with a maximum power conversion efficiency of 4.7%. One advantage of the new donor polymer is its high mobility, which allows for thicker (≈ 250 nm) active layers reducing the probability of short circuits and defects in large area devices using thick grid electrodes. We also fabricated 25 cm2 cells with the silver grid on foil and achieved a maximum efficiency of 2.8%.The work presented in this thesis shows ways of increasing the efficiency of monolithic large area organic solar cells. Depending on the properties of the photoactive layer different approaches can be used. For active blends with the peak absorption between 500 - 600 nm the presented stack electrodes are an superior alternative to the standard ITO electrode. For devices with photoactive blends with an optical gap below 1.7 eV and balanced mobilities, which allow for thicker junctions, the use of a metallic grid in combination with ITO helps to achieve efficient large area devices. This knowledge will help in the future to choose the most promising architecture for large area organic solar cells with new active materials.