The advantage in low cost makes P3HT one of the most attractive electron donors for photovoltaic applications, but the power conversion efficiency (PCE) of the P3HT-based organic solar cells (OSCs)...
Decreasing the energy loss is one of the most feasible ways to improve the efficiencies of organic photovoltaic (OPV) cells. Recent studies have suggested that non‐radiative energy loss (Enon-radloss
) is the dominant factor that hinders further improvements in state‐of‐the‐art OPV cells. However, there is no rational molecular design strategy for OPV materials with suppressed Enon-radloss
. Herein, taking molecular surface electrostatic potential (ESP) as a quantitative parameter, we establish a general relationship between chemical structure and intermolecular interactions. The results reveal that increasing the ESP difference between donor and acceptor will enhance the intermolecular interaction. In the OPV cells, the enhanced intermolecular interaction will increase the charge‐transfer (CT) state ratio in its hybridization with the local exciton state to facilitate charge generation, but simultaneously result in a larger Enon-radloss
. These results suggest that finely tuning the ESP of OPV materials is a feasible method to further improve the efficiencies of OPV cells.
Despite significant improvement in power conversion efficiencies
of bulk-heterojunction solar cells, the mechanism of mobile charge
carrier generation is still under debate. The time-resolved microwave
conductivity technique is used to investigate the mobile charge carrier
generation in blends of P3HT with monoPCBM and bisPCBM by varying
the excitation wavelength from the visible to NIR and the temperature
from 88 to 300 K. NIR excitation corresponds to the transition of
an electron from the HOMO of the P3HT directly to the LUMO of the
fullerene forming the charge transfer band (CT). From the results
it is inferred that the binding energy between the electron and hole
in the CT state is smaller than thermal energy at 88 K (7.8 meV) that
is in large contrast to previously reported values of 0.3 eV. This
is ascribed to efficient charge delocalization, which increases the
mean distance between the electron and hole at the interface. For
P3HT:bisPCBM, the yield of charge carries decreases by a factor of
3 on changing the wavelength from the visible to the NIR. This is
attributed to recombination of CT states to triplet level of P3HT.
However, as the yields for P3HT:PCBM and P3HT:bisPCBM are comparable
on visible excitation, we conclude that for the latter blend formation
of mobile charge carrier occurs primarily via a thermally nonrelaxed,
hot CT state. This observation indicates that the excess energy involved
in the exciton dissociation process is indeed important to avoid recombination
to the triplet level and to achieve higher yields of charge carrier
generation. On the basis of these findings, we suggest that the excess
energy can be small as long as the triplet level of the polymer is
located energetically higher than the CT state. This insight is of
particular interest for the rational design of novel polymer/fullerene
systems to achieve higher power conversion efficiencies.
Developing novel solid additives has been regarded as a promising strategy to achieve highly efficient organic solar cells with good stability and reproducibility. Herein, a small molecule, 2,2′-(perfluoro-1,4-phenylene)dithiophene (DTBF), designed with high volatility and a strong quadrupole moment, is applied as a solid additive to implement active layer morphology control in organic solar cells. Systematic theory simulations have revealed the charge distribution of DTBF and its analog and their non-covalent interaction with the active layer materials. Benefitting from the more vital charge-quadrupole interaction, the introduction, and volatilization of DTBF effectively induced more regular and condensed molecular packing in the active layer, leading to enhanced photoelectric properties. Thus, high efficiency of over 17% is obtained in the DTBF-processed devices, which is higher than that of the control devices. Further application of DTBF in different active layer systems contributed to a deeper comprehension of this type of additive. This study highlights a facile approach to optimizing the active layer morphology by finely manipulating the quadrupole moment of volatile solid additives.
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