| Abstract [eng] |
The work presented in this thesis focuses on the extraction of charges photogenerated in organic bulk-heterojunction solar cells. Charge migration dynamics were observed experimentally on a vast time range – from sub-ps to ms – as well as in a variety of devices in order to form a comprehensive view on the entire process. Role of coherence and delocalization on charge transfer at donor-acceptor interface and the initial electron migration was analyzed by measuring the electric field drop in the solar cell after the ultrafast photoexcitation. Time-resolved electric field induced second harmonic generation (TREFISH) method was used to obtain sub-ps time resolution. Experimental data was then used as a basis for the Stochastic Schrödinger Equation (SSE) simulations. Obtained results indicate that even a relatively weak coupling between PCBM molecules is sufficient to facilitate electron delocalization and efficient charge separation at organic interfaces. Subsequent electron and hole migration was investigated using TREFISH method combined with integrated photocurrent (IPC) measurements on a vast pool of devices. Experimental results over different donor-acceptor ratio devices in combination with time dependent mobility modelling and Monte-Carlo calculations enabled the separation of extraction of electrons and holes. It was found that charge extraction strongly depends on the fraction of the corresponding material in the blend. Balanced carrier mobility did not ensure the most efficient extraction. Rather, fast motion of electrons was found to be essential for efficient charge carrier separation helping to avoid geminate recombination. Mobility of the photogenerated charges was observed to decrease over orders of magnitude in the time span from their generation to extraction. This drop in mobility was found to originate from carrier relaxation within their respective density of states (DOS). Furthermore, a remarkable distribution of the photocurrent over energy was found, in which the most relaxed charge carriers counteract the net photocurrent. Morphology optimization using the solvent additive 1,8-diiodooctane (DIO) was found to double the charge pair separation efficiency and the short-circuit current. Carrier extraction at low internal electric field was slightly faster from the cells prepared with DIO, which can reduce recombination losses and enhance fill factor. |
