Investigation of Ultrafast Dynamics in Dye-Sensitized Solar Cells by Two-Dimensional Electronic Spectroscopy

K. Giannaris¹, S. Rinconcellis², S. Haacke², D. Brida¹

1. Department of Physics and Materials Science, University of Luxembourg, 162A avenue de la Faïencerie, L-1511 Luxembourg, Luxembourg

2. Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS) UMR 7504, CNRS, Université de Strasbourg, 23 rue du Loess, Strasbourg 67083, France

In this work we are investigating the ultrafast dynamics that occur upon impulsive photoexcitation in flexible and transparent dye-sensitized solar cells. The cell design consists of a semiconducting layer (TiO₂/Al₂O₃), an electrolyte solute for the charge transfer and the dye molecules (pyrrolopyrrole cyanine TB207) for the carrier injection into the semiconducting layers. Our experiments aim at observing the carriers dynamics, from the dye molecules to the semiconductors energy bands, with the goal of fully interpreting the fundamental mechanisms that follow the absorption of a photon, in order to  optimize the geometry of the cells. Importantly, the devices under study introduce Chenodeoxycholic Acid (CDCA) to separate the dye molecules and increase their carrier transfer while avoiding the creation of aggregates that affect negatively the cell performance. Samples with different concentrations of CDCA were fabricated and studied to correlate its presence to the cells efficiency. To disentangle these complex phenomena, we use an advanced implementation of Two-Dimensional Electronic Spectroscopy (2DES).  This technique exploits the interference of two identical pump pulses in order to get a complete insight into the ultrafast dynamics, and the corresponding excitation energies, occurring in complex media. The pulse pair is generated by a system employing birefringent optical elements thus ensuring maximum stability. The pump pulse pairs are derived by a non-collinear OPA driven by a Yb:KWG laser with fundamental at 1030 nm. The resulting pulses are sub7-fs and span the visible range from 500-700 nm. The probe is a white light supercontinuum that ranges from 400 to 900 nm covering the relevant spectral range for dynamics in the solar cells. By varying the time delay between the pump and the probe (i.e., 400-900 nm) and resolving the differential transmission signal between the two pulses, we obtain both the detection and excitation frequencies information. In this fashion, cells cations signal was studied, i.e. the charge transfer dynamics between the dye compounds and the semiconductors layers. Their significant signal appears at two different energy regions. An intense broadband absorbance peak at 600-675 nm and a narrower one at 850-900 nm. By time resolving those spectral features in a detailed two-dimensional map we conclude that both of the signatures are excited by the same photon frequencies, from 730-820 nm ending up in a ground state bleaching intense signal for the early picosecond time delays. This intense signal indicates that there is sufficient charge transfer to the semiconducting layers, confirming the substantial cells performance. Finally, taking into account the CDCA compound, there is a general correlation between its concentration and the cell performance owing to the separation between dye molecules thus improving the overall photon absorption. This leads to the enhanced performance of the fabricated devices. Further investigations are ongoing to fully disentangle the complex dynamics that follow light absorption in the solar cells to achieve optimum efficiency.