Research - Solar Energy Conversion Group

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Research

Our research topics include dye-sensitized solar cells (DSSC), perovskite solar cells (PSC) and, recently, water splitting systems. In DSSC, our key interest is the interaction of organic dyes with the nanostructures of metal-oxide semiconductors. In PSC, the properties of novel of organometal-halide materials and the processes at their interfaces with electron and hole transporting layers are investigated. We construct the prototype solar cells, measure their photovoltaic parameters, and use them as the samples for spectroscopic characterization, including time-resolved laser spectroscopy studies.

Our photo-electrodes are made by the preparation of metal-oxide nanostructure layers (on the right) on conducting glass using doctor-blade, screen printing or spin coating techniques, followed by thermal annealing. The active material is introduced by sensitization in solution (DSSC) or spin-coating (PSC). Liquid electrolytes are used in the configuration with polymer sealing and conducting glass as counter electrode (DSSC, left up), while solid-state charge transporting layers are formed by spin coating with further sputtering of gold as counter electrode (DSSC, PSC, left down).

The prepared cells are characterized by basic photovoltaic techniques: current-voltage measurements (under simulated sunlight illumination) and incident photon to current efficiency (IPCE) spectra (Instytut Fotonowy, Poland, on the left). Charge dynamics on millisecond time scale is investigated by electrochemical impedance spectroscopy, photocurrent decay and photovoltage decay studies (Autolab potentiostat). The active materials are measured by stationary absorption (Jasco V-770, 190 - 2700 nm with 150 mm integrating sphere). Scanning electron microscopy (SEM) pictures are taken at Nanobiomedical Centre, UAM.

Our main specialization are the studies of the ultrafast and fast elementary charge separation processes taking place in the solar cells. After absorption of photon, an electron in active material is moved to its higher energetic state. For efficient photovoltaic operation, the electron must undergo a number of partial charge separation process (e.g. movie on the right), and many of them occur on the time scales from femtoseconds to microseconds, unavailable for typical opto-electrical techniques used in the photovoltaics. Therefore, we use laser pulses (on the left) to track the dynamics of charge separation in complete solar cells as the samples, and correlate it with global photovoltaic parameters of the studied cells.

The main tools employed are: femtosecond transient absorption, picosecond time-resolved fluorescence and nanosecond flash photolysis. These setups are located at Faculty of Physics, UAM. In femtosecond transient absorption the train of pump pulses overlaps in time and space with the broadband probe pulses (first movie on the right). By varying time delay between pump and probe, the changes in the absorbance as a function of time and wavelength are obtained (second movie).

The movie on the left show the measured transient absorption data for varying pump-probe delay of examplary solar cell. Note that the duration of this movie (20 s) is 10 trillion times slower than the actual duration of the visualized absorption changes (2 picoseconds). On the right, the effect of global analysis of the data is shown (kinetics at different probe wavelengths together with the fitted model).