Temperature and composition effects on charge transport in pbdb-t-2cl: ieico-4f solar blends for next-generation organic photovoltaics

Abstract

Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved ~18 % power-conversion efficiency, benefitting from superior thermal stability and broader absorption compared to fullerene acceptors (FAs). Despite these advances, the performance and stability of OPVs remain strongly influenced by blend composition and temperature, which affect charge transport and recombination. The main objective of this study is to investigate the influence of blend composition on optical, morphological, electrical properties and the effect of temperature variation on photoluminescence properties of PBDB-T-2CL: IEICO-4F for organic photovoltaics application and the specific objectives are; To investigate the optical and morphological of pristine PBDB-T-2CL and IEICO-4F thin films, to analyze the effect of IEICO-4F dopant on optical, morphological, and electrical properties of PBDB-T-2CL: IEICO-4F thin films and to evaluate the effect of temperature variation on photoluminescence properties of PBDB-T-2CL: IEICO-4F thin films. Specifically, the bulk heterojunction (BHJ) structure formed by donor-acceptor blend was analyzed using temperature-dependent PL to evaluate charge transfer dynamics across different blend ratios. Spin coated thin films were characterized using ultraviolet-visible (UV-Vis) spectroscopy, atomic force microscopy (AFM), PL spectroscopy and four-point probe measurements. Data analysis was performed using Origin software. The UV-Vis spectra revealed that PBDB-T-2CL exhibited a primary absorption peak near 620 nm, while IEICO-4F showed complementary absorption at ~ 860 nm and the blends forms four characteristic peaks at 568 nm, 620 nm, 817 nm and 833 nm effectively broadening the spectra and enhance photo harvesting. Temperature – dependent PL analysis showed suppressed radiative recombination and enhanced charge transfer with increasing annealing temperature, particularly in the blends. AFM measurements indicated strong morphology dependence: pristine PBDB-T-2CL film were smooth (root mean square ≈2.28 nm), IEICO-4F film were rougher (root mean square ≈5.18 nm), while the optimized 70:30 blend exhibited the finest morphology (root mean square ≈1.398 nm ). These morphology features translated into distinct device characteristics. The 50% IEICO-4F composition achieved the highest PCE of 2.12 %, with a short circuit current density (Jsc) of 9.6 mA/cm2, open circuit voltage (Voc) of 0.60 V, and fill factor (FF) of 36 %. In contrast donor-acceptor blends showed reduced efficiencies (≤1.70 %), correlating with roughness and recombination losses. In conclusion, the study demonstrates that broad optical absorption, efficient charge transfer, favorable nanoscale morphology, and stable photovoltaic response are the key thin film properties supporting high efficiency bulk heterojunction and flexible thin film OPV devices. These finding proved experimental evidence that fine donor-acceptor morphology is crucial to balance charge generation and transport, offering insights for the design of next generation OPVs.