Abstract
Organic photovoltaics has emerged as a promising alternative to conventional inorganic solar cells due to its unique advantages, including light weight, mechanical flexibility, solution processability, and potential for low-cost manufacturing. One of the key challenges remains the precise control over the nanoscale morphology of the bulk heterojunction active layer, which directly governs exciton dissociation, charge transport, and recombination losses. Additionally, the quality of charge-extracting interfaces, particularly the hole-transporting layer and its contact with the active layer, plays a critical role in determining device performance and stability. Among various strategies explored to address these issues, the incorporation of carbon based nanomaterials such as carbon nanotubes, graphene derivatives, and nanodiamonds - has attracted considerable interest. These materials can modify the active layer morphology, enhance charge carrier mobility, and passivate trap states. In this work, two independent approaches to improve the performance of organic solar cells based on the P3HT:PC₇₀BM donor-acceptor blend were investigated using carbon nanomaterials. In the first approach, detonation nanodiamonds (NDs) were introduced directly into the active layer at various concentrations (1, 2.5, and 5 wt.%). A pronounced non-monotonic concentration dependence was observed: at 5 wt.%, severe aggregation of NDs disrupts the donor-acceptor network, leading to drop in photocurrent, while at 2.5 wt.%, the PCE increases from 1.01% to 1.80% and the fill factor nearly doubles (from 16.6% to 31.4%). The optimal concentration improves phase separation and reduces recombination losses. In the second approach, an interpenetrating electrode (IE) consisting of hybrids of carboxylated multi-walled carbon nanotubes with PEDOT:PSS was deposited onto the conventional PEDOT:PSS hole-transporting layer. Deposition of this electrode from water solution with concentration of 0.5 mg/ml leads to a noticeable improvement in all photovoltaic parameters. The addition of 0.05% cetyltrimethylammonium bromide (CTAB) to the hybrid dispersion further enhances dispersion quality and coating uniformity, increasing the PCE nearly fourfold (to 3.43% vs. 0.87% for reference devices) and doubling the fill factor. It is important to note that previous work has shown that adding MWCNTs directly into the active layer can lead to shunting, which deteriorates OSC performance. However, insulating the MWCNTs with a polymer can prevent shunting and even improve the photovoltaic parameters. Cells incorporating IE exhibit no drop in open-circuit voltage, confirming the successful insulation of the metallic nanotube shells and the suppression of shunting paths through the PEDOT:PSS coating. Drift-diffusion numerical modelling revealed that both approaches lead to a reduction in the bimolecular recombination constant and changes in effective carrier mobility, with suggested mechanisms: NDs act as passivators of recombination centers, while IE electrode provides percolation pathways for rapid charge collection, locally alters the morphology of the donor-acceptor blend, potentially creating P3HT domains which envelope MWCNTs.