Modeling of internal conversion in photoexcited conjugated molecular donors used in organic photovoltaics

Abstract

Using the Non-Adiabatic Excited States Molecular Dynamics (NA-ESMD) approach, we investigate the ultrafast electronic relaxation in a recently synthesized small molecule donor, p-DTS(PTTh 2 ) 2 , which belongs to the dithienosilole-pyridylthiadiazole family of chromophores. In combination with the PC 70 BM acceptor, p-DTS(PTTh 2 ) 2 can be used to fabricate high efficiency bulk heterojunction organic solar cells. After photoexcitation to its broad high-energy peak in the 3-4 eV range, associated with multiple excited states, p-DTS(PTTh 2 ) 2 undergoes efficient ultrafast internal conversion to its lowest excited state. During this process, about 1-2 eV electronic energy transfers to the vibrational degrees of freedom leading to rapid heating of the molecule. Nevertheless, our simulations do not detect possible bond-breaking or decomposition of the system. This suggests minimal intra-molecular photodamage after photoexcitation to high-energy states in the 3-4 eV region. Calculated radiationless deactivation mainly consists of a sequential mechanism that involves electronic transitions between the current transient state and the corresponding state directly below in energy. Changes in the density of states along the relaxation process lead to pronounced variations and time-dependence of the accumulated populations of the different intermediate electronic excited states. Visualization of the electronic transition density during internal conversion reveals spatial intramolecular delocalization of electronic excitation from the thiophene moieties to the entire chromophore. Finally, our analysis of non-adiabatic coupling vectors suggests characteristic vibrational degrees of freedom coupled to the electronic system during various stages of non-radiative relaxation. Broader contextConsiderable world-wide effort in both industry and academia has been focused on developing organic photovoltaic (OPV) cell and module technologies, which can potentially meet multiple requirements for solar energy conversion. Here, power conversion efficiencies (PCEs) exceeding 10% have been achieved in the case of bulk-heterojuntion (BHJ) solar cell structures. These values of PCE are still below those corresponding to inorganic solar cells but steady improvements have been made. In recent years, small molecules have emerged as favorable novel organic compounds owing to their exceptional light-harvesting capacity and efficient charge transport. For example, solar cells fabricated using a recently synthesized molecular donor p-DTS(PTTh 2 ) 2 exhibited the highest record value of 6.7% for the PCE, this revealing the potential of small donor molecular materials. The performance of such OPV devices depends on a complex interplay of charge and energy transfer timescales competing with radiative and non-radiative relaxations, and various energy-loss mechanisms. Subsequently, design of new molecules involves optimization of many important parameters including detailed understanding of excited state dynamics relevant to charge transfer reactions in ...

Photoinduced Intra- and Intermolecular Energy Transfer in Chlorophyll a Dimer

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Modeling ultrafast exciton deactivation in oligothiophenes via nonadiabatic dynamics

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Photoinduced Energy Transfer in Linear Guest–Host Chromophores: A Computational Study

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