Achieving quantum coherence in organic solar cells

In the last decades, modern society has focused its attention on the production of energy in a green and eco-sustainable way. Among the investigated carbon-neutral energy sources, solar energy is one of the most available in nature. In photovoltaic cells, the power conversion efficiency is limited by Shockley-Queisser thermal losses. The quantum-coherent dissipation-free dynamics in an organic cell can ideally overcome this limitation. Quantum coherence, which results from the superposition of states, is predicted to play a role in all the steps of the photo-conversion process. When the solar light is absorbed by the solar cell, an electron-hole pair (exciton) is formed. At this stage, the coherent superposition of excitonic states can allow for a multiple exciton generation from each absorbed photon. Then, once excitons are formed, they propagate through the material towards the interface where charge separation occurs. During the exciton transfer, losses can be prevented if coherence is preserved, i.e. excitons do not interact (scatter) with their environment (charges, phonons, defects). Thus, if the positive and negative charges are collected on a time scale lower than the decoherence time, thermalization is suppressed or strongly reduced. In such quantum-coherent regime, power conversion efficiency can largely exceed the the Shockley-Queisser limit. Motivated by these outstanding assets, my PhD project aims to directly study the coherence and the dephasing dynamics of excitons in layered materials and organic-inorganic heterostructures for which great potential in photovoltaics has been reported in literature. To achieve these goals, experimental techniques sensitive to both decoherence and depopulation dynamics of photoexcited states are needed. Here, we choose a combination of transient optical spectroscopy and interferometric time-resolved multi-photon photoemission spectroscopy (inter-tr-mPPE) to achieve a complementary view on the ultrafast quasi-particle relaxation dynamics, the coherence of optical excitation and its dephasing in the time and energy domain. In the first technique, two delayed pulses, the pump and the probe, follow the quasi-particle excitation and the ultrafast relaxation dynamics photoinduced in the selected material. Inter-tr-mPPE exploits two phase-locked delayed laser pulses created by a Mach-Zender interferometer to record one photoemission spectrum per time delay and to directly access the coherence time of the excited states. The first part of my doctoral project will be devoted to the development of the inter-tr-2PPE set-up. Subsequently, both the techniques will be used on layered materials, such as Bismuth tri-iodide (BiI3), and organic-inorganic interfaces, like Tetracene-Silicon interface (TET-Si interface), where coherence is expected to act both at the excitation time and during the excitation transport. The layered material BiI3 is ideal for photovoltaic applications due to its strong absorption peak in the visible range of the solar light and the ns-long electron-hole recombination time. On the other hand, the organic material TET is suitable for high-efficiency solar cells due to the singlet fission that creates multiple exciton generation upon photoexcitation. Besides the multiple exciton generation, in the interface TET-Si, a charge transfer between the two materials is expected, leading to a more efficient transport in the solar cells.