Spin-Dependent Recombination Pathways Revealed by Magnetoconductance in Annealed Polymer–Fullerene Bulk Heterojunction Solar Cells

Organic bulk heterojunction (BHJ) solar cells are strongly influenced by spin-dependent processes that govern charge separation and recombination. In this work, we investigate the current density–voltage (J–V) characteristics and magnetic field-dependent conductance (magnetoconductance, MC) of indium tin oxide (ITO)/poly(3,4 ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/AnE-PVstat:phenyl-C₆₁ butyric acid methyl ester (PCBM)/LiF/Al polymer solar cells subjected to thermal annealing between 25°C and 120°C under AM 1.5G illumination (100 \({mW cm}- 2\) ). A positive MC response is observed at 25°C, 100°C, 105°C, and 110°C, with a maximum amplitude of 3.6%, while a clear sign reversal to negative MC emerges at 120°C. These MC variations correlate with moderate changes in power conversion efficiency (PCE), whereas the open-circuit voltage ( \({V}_{{{OC}}}\) ) remains essentially unchanged, indicating that the magnetic field primarily affects spin-dependent recombination kinetics rather than the interfacial energy level alignment. To rationalize these findings, we employ a spin-dependent recombination model based on the interaction between triplet excitons and spin-1/2 charge carriers (triplet–charge interaction). Using a density matrix formalism within the stochastic Liouville framework, we demonstrate that the MC line shapes originate from the magnetic-field-induced evolution of the doublet population, energy levels, and density matrix elements of triplet–charge pairs. Our analysis shows that thermal annealing modulates the dissociation and recombination rates of these spin-dependent states, thereby linking annealing-induced morphological changes in the active layer to spin-governed recombination pathways and overall photovoltaic performance. These results establish MC as a sensitive probe of efficiency-limiting spin dynamics in organic solar cells.