A hybridization-induced charge-transfer state energy arrangement reduces nonradiative energy loss in organic solar cells.

Here, we explain why the Energy Gap Law and the energy inversion related to the charge-transfer state have opposite effects on the trend of nonradiative energy loss of organic solar cells. The root is the existing condition of energy inversion. There is indeed a certain probability of energy inversion, but it will eventually be implicit or explicit as determined by the hybridization, which depends on the electron-withdrawing unit of the donor, giving rise to different stacking sites. The triplet-state hybridization leads to an explicit characteristic, while singlet-state hybridization leads to an implicit characteristic.

Energy differences as descriptors for the correlation between JSC and VOC in nonfullerene organic photovoltaics.

ITIC-series nonfullerene organic photovoltaics (NF OPVs) have realized the simultaneous increases of the short-circuit current density (JSC) and open-circuit voltage (VOC), called the positive correlation between JSC and VOC, which could improve the power conversion efficiency (PCE). However, it is complicated to predict the formation of positive correlation in devices through simple calculations of single molecules due to their dimensional differences. Here, a series of symmetrical NF acceptors blended with the PBDB-T donor were chosen to establish an association framework between the molecular modification strategy and positive correlation. It can be found that the positive correlation is modification site-dependent following the energy variation at the different levels. Furthermore, to illustrate a positive correlation, the energy gap differences (ΔEg) and the energy level differences of the lowest unoccupied molecular orbitals (ΔELUMO) between the two changed acceptors were proposed as two molecular descriptors. Combined with the machine learning model, the accuracy of the proposed descriptor is more than 70% for predicting the correlation, which verifies the reliability of the prediction model. This work establishes the relative relationship between two molecular descriptors with different molecular modification sites and realizes the prediction of the trend of efficiency. Therefore, future research should focus on the simultaneous enhancement of photovoltaic parameters for high-performance NF OPVs.

Design of single-porphyrin donors toward high open-circuit voltage for organic solar cells via an energy level gradient-distribution screening strategy of fragments: a theoretical study.

Open-circuit voltage (VOC) is a key factor for improving the power conversion efficiency (PCE) of bulk heterojunction (BHJ) organic solar cells (OSCs). At present, increasing attention has been devoted towards modifying π bridges in single-porphyrin small molecule donors with an A-π-D-π-A configuration to reduce the highest occupied molecular orbital (HOMO) levels and improve the VOC of devices. However, how to screen the π bridges is a key issue. In this work, nine π bridges were screened by the HOMO level gradient-distribution strategy of fragments (electron-donating donor (D), π bridges, and electron-withdrawing acceptor (A)), where fragments meeting the requirements were combined into five novel small molecule donors. Meanwhile, in order to test whether the strategy is beneficial to increasing VOC, [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) was selected as the acceptor material. The energy levels of all molecules were compared and the photoelectric properties (i.e., energy gap, energy driving force, reorganization energy, intermolecular charge transfer rate, charge recombination rate, and VOC) of the five small molecules were studied. The results showed that the HOMO levels of porphyrin donors could be significantly lowered via this strategy, and VOC was raised without losing the short-circuit current (JSC) and fill factor (FF) of the devices. Meanwhile, the designed five small molecules could be used as donor candidates to improve the performance of OSCs.