Hybrid polymer solar cells based on ZnO

With the increasing demand for energy, and the limited availability of fossil fuels, there is a need for a renewable, sustainable energy source. One source that always will be available is the energy of the sun. Sunlight energy may be captured in roughly three ways: as heat, electricity, or chemical energy. Heat may be used directly or used to power for example an electricity generator. Sunlight may be used to drive chemical reactions, where energy is stored in chemical bonds. Finally, sunlight can be converted directly into electricity. Since electricity is possibly the most valuable and versatile form of energy, the direct conversion deserves attention. By increasing the efficiency and lowering device cost, a more sustainable global energy society may be achieved. At present, the photovoltaic solar energy market is dominated by solar cells based on high-purity silicon. Producing this high-purity material takes considerable effort and cost, and therefore cheaper, more environmentally friendly alternatives are being explored. Polymer photovoltaics is an option, but the morphological stability of this type of devices is currently one of the issues that hinders commercialization. Hybrid polymer solar cells, using an inorganic material combined with an organic polymer, overcome this issue due to the morphological stability of the inorganic component, while combining this stability with the advantages of solution processing of organic materials. This thesis focuses on characterizing the morphology of hybrid ZnO-polymer solar cells, and altering the morphology to enhance the power conversion efficiency of these devices. Chapter 2 describes the use of an in-situ sol-gel precursor method to make hybrid ZnO-polymer solar cells and strategies to obtain maximum power conversion efficiency. Organic ZnO precursors with different reactivity have been tested in combination with poly(3-hexylthiophene) (P3HT) to create the photoactive layers. Power conversion efficiency (PCE) and external quantum efficiency (EQE) were measured on the devices. The morphology and surface roughness of the active layers was investigated using atomic force microscopy (AFM). From these studies diethylzinc emerged as the best working ZnO precursor in combination with P3HT. The efficiency of polymer solar cells critically depends on the intimacy of mixing of the donor and acceptor semiconductors used in these devices to create charges and on the presence of unhindered percolation pathways in the individual components to transport holes and electrons. Therefore, the e ect of three-dimensional morphology of the active layer on the efficiency was investigated in chapter 3. For ZnO:P3HT cells the internal quantum efficiency (IQE) improves with increasing layer thickness. Photoinduced absorption measurements indicated less triplet state formation in thick layers, meaning that excitons created in these thick layers are better separated into free charges compared to thin films. Three-dimensional transmission electron microscopy (3D-TEM) tomography revealed the 3D morphology in great detail and allowed to determine the probability to find P3HT domains at a certain distance from a ZnO domain with nanometer resolution. From these data the exciton dissociation efficiency of the layers was calculated by solving the 3D exciton diffusion equation in three dimensions, and the percolation pathways for electrons in the ZnO phase were analyzed. Exciton dissociation was higher in thicker blends, explaining the higher IQE for these layers, while the available percolation pathways for electrons in the ZnO were reduced with increasing layer thickness. In chapter 4 it is shown that the morphology of the active layer can be controlled by introducing polar side groups in the polymer. Carboxylic acid esters and alcohols introduce hydrophilicity into the polymer, making it more compatible with the hydrophilic ZnO. 3D TEM analyses have shown that the intimacy of mixing and the extent of exciton dissociation are indeed improved significantly for thin active layers. Device performance was still limited due to the lower amount of ZnO connected to the respective electrode hindering charge collection, and lower hole mobility of the polymer, which is not able to form a crystalline phase as observed in the optical absorption due to the very intimate mixing with ZnO. To absorb more light, in chapter 5 poly(3-hexylselenophene) (P3HS) was tested with ZnO in solar cells. The advantage of P3HS is a smaller optical band gap than P3HT, such that more photons from the solar spectrum can be absorbed, leading to higher current and efficiency. Device optimization was performed and active layers were analyzed using TEM. Eventually, the device performance was less compared to ZnO:P3HT blends. This could be ascribed to a lower exciton dissociation from excitons in the crystalline part of P3HS, judging from the difference in shape of absorption compared to the shape of the EQE. Probably, the lifetime and/or exciton diffusion length in crystalline P3HS is shorter than in amorphous P3HS. Chapter 6 describes the use of a ZnO nanorod template layer, infiltrated with a ZnO:P3HT blend to create a hybrid ZnO-polymer solar cell that features a hierarchically built inorganic phase, with 300 nm ZnO nanorods for charge collection connected to a fine ZnO network for charge generation throughout the bulk. These hierarchical ZnO:P3HT layers were prepared by casting P3HT and diethylzinc (as reactive ZnO precursor) from a common solvent mixture in moist atmosphere on a ZnO nanorod carpet grown from a compact ZnO nucleation layer, followed by thermal conversion. AFM and scanning electron microscopy confirmed the infiltration of the ZnO:P3HT blend in between the ZnO nanorods. Photoinduced absorption spectroscopy revealed that the relative efficiency of charge generation in the ZnO:P3HT blend layers is not influenced by connecting to the ZnO nanorod carpet. We find that the performance of the combined ZnO nanorod ZnO:P3HT blend solar cells is not improved compared to ZnO:P3HT blend cells without nanorods and, actually, somewhat less. This is interpreted as being due to a poor contact between the ZnO network in the ZnO:P3HT blend and the ZnO nanorods. In conclusion, the research described in this thesis provides a first, detailed insight in the relation between the active layer morphology and the performance of hybrid metal oxide - polymer bulk heterojunction solar cells. The morphology can be controlled by changing the deposition conditions (e.g. spin rate), the nature of the polymer (e.g. presence of polar side chains), and using metal oxide template structures (e.g. nanorod carpet). In the best devices power conversion efficiencies of 2% in simulated solar light have been obtained. The thesis demonstrates that the main challenge toward more efficient solar cells is creating a morphology that is fine enough for efficient exciton dissociation and coarse enough to maintain a high mobility of charge carriers and short percolation pathways to the electrodes. A first attempt to achieving such a morphology has been described in the last chapter.