The Role of Device Mobility on the Charge Generation Process in Polymerized Small-Molecule Acceptor Based Organic Solar Cells

Abstract

We investigate the role of backbone tuning of contemporary polymerized small-molecule acceptors (PSMAs), namely PJ2, PJTVT, and PJTET, on the charge generation process when blended with JD40 polymers to explain the difference between their device short circuit current, JSC. We utilize transient absorption (TA) spectroscopy and grazing incidence wide-angle X-ray scattering (GIWAXS) measurements to demonstrate the face-on π–π and lamellar stackings impact on the diffusion of excitons in pristine films. Comparing the charge generation process to data obtained from space-charge-limited current (SCLC) measurements, we show that the generation of free charge carriers, although limited by geminate recombination of charge transfer (CT) pairs, is improved with a higher charge-carrier device mobility. A higher mobility is found to also drive charge generation limited by the same recombination more efficiently than blend systems where the charges recombine nongeminately. We show that an increment of ∼0.1 eV corresponds to an ∼5% increase in the device EQE for the same mobility. Based on the data compiled from the literature and the study herein, a mobility of at least 10–3 cm2 V–1 s–1 is postulated for an EQE of at least 90%. Finally, we observe an apparent logarithmic relationship between the charge-carrier mobility and the generation of photocurrent, in lieu of a linear relationship introduced by the Braun–Onsager model. The same trend remains for surveyed literature data of contemporary Y-series acceptor-based OPV devices. This work highlights that to improve the JSC of current state-of-art OPV devices, the mobility of a device can be improved through material designs and device engineering for a higher charge generation efficiency.