Open-Circuit Voltage of Organic Photovoltaics: A Time-Dependent and Unrestricted DFT Study in a P3HT/PCBM Complex

Revealing the Role of Dynamic and Static Disorder on Charge-Transfer-State Absorption in Polymer Solar Cells

In polymer solar cells (PSCs), charge-transfer (CT) state absorption plays an important role in evaluating the CT-state energy and energy loss. However, due to the disordered nature of polymers, a comprehensive understanding of CT absorption properties remains elusive. Especially, the dominant role of dynamic and static disorder in determining CT absorption is frequently debated. Herein, we theoretically constructed an organic donor-acceptor model to investigate the impact of these two types of disorders on CT absorption properties. It is demonstrated that the CT absorption properties depend significantly on the type of disorder. Specifically, it is found that dynamic disorder has a more significant impact on the peak and position of CT absorption as well as the broadening properties, compared to static disorder. The study indicates that minimizing dynamic disorder can lead to a reduction in overall disorder, which is beneficial for improving the performance of PSCs.

An extension of electron acceptor sites around Thiazolothiazole unit for evaluation of large power conversion efficiency: A theoretical insight

Small organic solar cells containing thiazolothiazole unit as an electron acceptor for solution processed bulk heterojunction (BHJ) small donor-acceptor-donor (D-A-D) type materials have been designed and studied theoretically with state-of-the-art density functional theory and time-dependent density functional theory (TD-DFT) for reliable estimation of their excited state and charge transfer photophysical characteristics for estimating their power conversion efficiencies. The suggested possible synthetic routes with complete reaction information have been also provided for synthesis. The electron acceptor sites around the thiazolothiazole unit have been enlarged by introducing different strong electron withdrawing groups and checked their effects on the voltages (V_OC) and fill factor (FF) which are the two main parameters directly influences on power conversion efficiencies. Out of five theoretically studied molecules, the experimental reported data of TT-TTPA (Thiazolothiazole-thiaophene triphenyl amine) has been compared with four designed molecules and concluded that extension of acceptor sites significantly contributed towards the better charge transport properties of electron and hole.

Solvent effects on excited-state relaxation dynamics of paddle-wheel BODIPY-Hexaoxatriphenylene conjugates: Insights from non-adiabatic dynamics simulations

Understanding the excited state dynamics of donor-acceptor (D-A) complexes is of fundamental importance both experimentally and theoretically. Herein, we have first explored the photoinduced dynamics of a recently synthesized paddle-wheel BODIPY-hexaoxatriphenylene (BODIPY is the abbreviation for BF2-chelated dipyrromethenes) conjugates D-A complexes with the combination of both electronic structure calculations and non-adiabatic dynamics simulations. On the basis of computational results, we concluded that the BODIPY-hexaoxatriphenylene (BH) conjugates will be promoted to the local excited (LE) states of the BODIPY fragments upon excitation, which is followed by the ultrafast exciton transfer from LE state to charge transfer (CT). Instead of the photoinduced electron transfer process proposed in previous experimental work, such a exciton transfer process is accompanied with the photoinduced hole transfer from BODIPY to hexaoxatriphenylene. Additionally, solvent effects are found to play an important role in the photoinduced dynamics. Specifically, the hole transfer dynamics is accelerated by the acetonitrile solvent, which can be ascribed to significant influences of the solvents on the charge transfer states, i.e. the energy gaps between LE and CT excitons are reduced greatly and the non-adiabatic couplings are increased in the meantime. Our present work not only provides valuable insights into the underlying photoinduced mechanism of BH, but also can be helpful for the future design of novel donor-acceptor conjugates with better optoelectronic performance.

Effect of static external electric field on bulk and interfaces in organic solar cell systems: A density-functional-theory-based study

In this work, a detailed comparison of optical and electronic properties in bulk and interfaces of well-known organic semiconductor systems in presence of an external electric field is reported. We have used density functional theory (DFT) to model organic solar cell systems. The study promotes a deeper understanding of the connection between the chemical structures and the optical and electronic properties in the well-known organic solar cell systems based on thiophene and fullerene polymers. We have performed a vibration-mode analysis by simulating Raman spectra in presence of external electric fields. Time-dependent DFT has been used to investigate the effect of an external electric field on excited state properties. The charge-transfer rate controlled by the external electric field has been quantitatively extracted using the simulated excited state dipole moment, Gibbs free energy, and Marcus theory. Our results provide a detailed characterization of the effect of the external electric field on the neat polymers (bulk) and on the donor-acceptor heterojunctions (interfaces) in organic solar cell systems. This theoretical approach not only helps to understand the effect of an external field on bulk and interfaces in organic semiconductors, but it also supports the design of novel devices.