Optoelectronic processes at polymer-fullerene heterojunctions : charge transfer states in organic solar cells

Abstract

Polymer photovoltaic cells currently achieve power conversion efficiencies (PCE) above 10% on lab scale. To compete with the efficiencies above 20% of inorganic solar cells, understanding and elimination of all the loss channels is necessary. This thesis investigates charge generation and recombination processes in polymer-fullerene solar cells with the aim of elucidating the nature of such losses. Chapter 1 provides a general introduction to the subject of the thesis and an overview of the most relevant literature. In Chapter 2 the effect of the thermodynamic driving force on the efficiency of photoinduced charge transfer between semiconducting polymers and fullerene is investigated. A polyfluorene copolymer is mixed with a variety of fullerene mono- and bisadducts having different LUMO energy levels. The difference in LUMO energy of the acceptors results in different energies of the charge transfer (CT) states in the blends. By spectroscopic characterization of bulk heterojunction thin films and photovoltaic devices it is found that the CT state needs to be at least 0.1 eV lower in energy than the singlet excited state of the fullerene to be formed efficiently. The central finding of Chapters 3 to 5 is that the nanoscale morphology of the polymer-fullerene heterojunction strongly influences the CT state dissociation and recombination. Blends composed of a small optical bandgap semiconducting polymer (PCPDTBT) mixed with fullerene are studied with various techniques. The nanoscale phase separation of these blends is controlled by adding high-boiling point co-solvents to the solutions used to process the thin films. Without co-solvent the blends are very finely mixed and the addition of co-solvents increases the phase separation. In blends that are too finely mixed we observe recombination of the CT state to the triplet state of the polymer (Chapter 3). We monitor the formation of triplets and charges in the thin films by means of photo-induced absorption (PIA).The recombination to the triplet state is a loss mechanism, in competition with the dissociation of the CT state into free charges. This process is reduced in favor of free charge formation when the phase separation is increased, correlating with the increased device performance. Specifically, increased fill factor and short circuit current are observed in the devices with optimized morphology. These results show that photophysical processes are controlled by the nanoscale morphology. In Chapter 4 we perform charge extraction experiments on solar cells having different nanoscale phase separation and we support the results with PIA experiments on corresponding thin films. In the charge extraction and PIA experiments we study the same set of long lived charges. We confirm that CT recombination (to the triplet and to the ground state) is largely responsible for the poor device performances of not-optimized blends. Finally, in Chapter 5 we show a correlation between population of triplet excited states and decreased photo-stability of the blends. We suggest that triplet states can lead to the formation of singlet oxygen, which in turn can initiate destructive chemical reactions in the polymer chains. In Chapters 6 and 7 the influence of excess photon energy (i.e. the difference between the energy of the absorbed photon and that of the CT state) in the CT state dissociation is investigated by means of charge extraction and spectral response measurements in different polymer:fullerene blends. We find that excess energy is not necessary for CT dissociation in blends with coarse phase separation. A measurable influence of the excess photon energy on CT dissociation is observed only in PCPDTBT:PCBM finely dispersed blends, which were shown in the previous chapters to be strongly affected by CT state recombination. Chapter 8 investigates what is limiting the photo-current of solar cells close to the open-circuit voltage. It is found that under these conditions space-charge can build up, so that the photocurrent becomes space-charge limited. This condition is reached only in blends where CT dissociation is efficient at low electric fields. Impedance spectroscopy is the technique used here. The experimental results are supported by an analytical model and by drift diffusion simulations. Summarizing, in this thesis it is shown that the CT state is a critical intermediate state in the charge generation process in polymer solar cells. The energy of the CT state needs to be low enough to convert neutral excitations into charges. The blend morphology is a crucial factor determining the branching ratio between dissociation into free carriers and recombination to the ground state or to the triplet state of the polymer.