RESUMO
The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C60 and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state.
RESUMO
Indolent B-cell lymphomas are characterized by repeated remissions and relapses with most patients eventually dying of the disease. Although combination treatments with chemotherapy and the anti-CD20 antibody rituximab improved duration of remissions and overall survival, the disease is essentially incurable. Thus, novel therapeutic approaches are needed. One such approach is active immunization with dendritic cells (DCs). Given that rituximab depletes patients of normal B cells, optimal vaccination strategies for rituximab-treated patients require induction of effector T cells. We have previously demonstrated in a murine model that idiotype (Id)-keyhole limpet hemocyanin-pulsed DCs induced Id-reactive CD8 T cells and protection against tumor challenge in the absence of anti-Id antibodies. On the basis of these results, we investigated vaccination in a therapeutic model, in which mice carrying advanced tumors of the highly aggressive 38C-13 lymphoma were treated with chemotherapy and anti-CD20 antibodies combined with a DC-based vaccine. As a rule, cytoreduction by cyclophosphamide was required in each regimen of combination treatment, and vaccination with tumor cell-loaded DCs was more effective than vaccination with Id-keyhole limpet hemocyanin-loaded DCs. We demonstrated that under conditions of large primary tumors that had already spread to lymph nodes, when anti-CD20 antibody treatment showed minimal effect and DC vaccination had no effect, synergism between anti-CD20 antibodies and DC vaccines resulted in significant long-term survival that did not involve active antitumor antibody production. Combination treatments including tumor cell-loaded DC vaccines may therefore provide a strategy for enhancing the potency of therapy in rituximab-treated patients.
