Probing molecular orientation by polarization-selective transient absorption spectroscopy

The natural solar cell (OSC) heterojunction interface is the place key photophysical processes resembling cost separation and recombination happen. The molecular orientation on the interface is without doubt one of the key elements that decide the effectivity of solar cells. Due to the advanced three-dimensional molecular association on the interface and the shortage of know-how to measure native molecular orientation, it’s difficult to find out the molecular orientation on the interface.

X-ray scattering is extensively used to check the molecular packing morphology of natural donor and acceptor supplies within the lively layer. Grazing incidence wide-angle X-ray scattering (GIWAXS) exhibits the facet or face stacking course of the donor and acceptor phases relative to the substrate (Figure 1a), but it surely can not distinguish the relative course between the 2 phases on the interface.

Although the relative orientation between anisotropic donor and isotropic fullerene acceptor phases might be measured based mostly on the polarization resonance tender X-ray scattering technique (P-SoXS) (Figure 1b), the rise of non-fullerene solar cells presents new challenges to explain the native molecular orientation on the interface of two anisotropic phases.

The analysis group of Professor Wang Cheng at Xiamen University and his collaborators made use of linear polarization-selective transient absorption (LP-TA, Figure 1c) to check the molecular orientation at heterojunctions of all-small-molecule natural solar cells. By selectively recording the polarization element of the probe sign aligned parallel or perpendicular to the linear polarization of the pump pulse, the anisotropy calculated from the transient absorption sign is used to estimate the angle between the donor and acceptor molecules at their interfaces.

They used LP-TA spectroscopy to detect the molecular orientation of bulk heterojunctions in three all-small-molecule natural solar cells. The angle between interfacial donor and acceptor molecules is calculated by the preliminary anisotropy worth of the transient absorption sign similar to cost separation course of. On the opposite hand, the dynamics of anisotropy give a touch of the dissociation and separation technique of the electron/gap pair away from the interface. For the ZR1:Y6 (PCE: 14.3%) mix, the evaluation confirmed that the angle between ZR1 and Y6 molecules on the interface was near 90°. In distinction, comparable experiments on the B1:BO-4Cl (PCE: 15.3%) mix confirmed that the B1 and BO-4Cl molecules are parallel to one another on the interface. BTR: BO-4Cl (PCE: 11.3%) interface is extra disordered and has a random relative orientation. By analyzing the cost separation kinetics, the charge separation effectivity of the B1:BO-4Cl interface the place the molecules are oriented in parallel is increased than that on the ZR1:Y6 interface the place the molecules are oriented vertically (70%>63%).

These observations present complementary info for X-ray scattering measurements and spotlight polarization-selective transient absorption spectroscopy as a device to probe the interface construction and dynamics of key photophysical steps in vitality conversion.