PM6/L8-BO Thin Films through Layer-by-Layer Engineering: Formation Mechanism, Energetic Disorder, and Carrier Mobility

Layer-by-layer (LBL) process has emerged as a promising method in the advancement of organic photovoltaics, emphasizing scalability and reproducibility. More importantly, it provides enhanced morphological control for boosting carrier mobility (μ) and power conversion efficiency. By employing a multiscale approach that combined first-principles calculations, molecular dynamics simulations, and kinetic Monte Carlo methods, the relationship between LBL morphology engineering and carrier mobility in donor/acceptor (PM6/L8-BO) thin films is elucidated. During solvent evaporation, the order of solid-phase formation in LBL films was top surface, bottom region, and then the middle region. The early solid precipitation from precursor solutions was acceptor, resulting in a well-ordered molecular arrangement and reducing energy disorder of acceptor LUMO levels. Furthermore, the difference in energy disorders between the A/D blend region and the pure A or D domains enabled LBL morphology engineering to balance electron and hole mobilities, thereby mitigating charge accumulation and recombination. LBL-manufactured films presented higher carrier mobility (${\mu }_e^{\mathrm{LBL}}$ = ${\mu }_h^{\mathrm{LBL}}$ = 1.9 × 10⁻³ cm² V⁻¹ s⁻¹) compared to bulk heterojunction (BHJ) films (${\mu }_e^{\mathrm{BHJ}}$ > ${\mu }_h^{\mathrm{BHJ}}$ = 0.1 × 10⁻³ cm²·V⁻¹ s⁻¹). These mechanisms provided insights into strategies for enhancing charge extraction of photo-generated charge carriers through LBL engineering, driving the development of efficient organic photovoltaic materials.