Percolative Charge Transport in Organic Semiconductor Devices with Host-Guest Layers

In organic semiconductor devices with a host-guest active layer, guest molecules can transport charges by forming conductive filamentary paths once their concentration exceeds a critical threshold. However, distinguishing this percolative transport─or percolation─from overall charge movements presents a formidable challenge. Here, we introduce an organic unipolar host-guest device wherein charge transport occurs exclusively through guest molecules, i.e., percolation, while the hosts remain electrically inactive. This device configuration allows for the independent identification and modeling of percolation as a function of guest concentration. Moreover, in standard unipolar devices where charges can move across all components (host-to-host, guest-to-guest, host-to-guest and guest-to-host), we found that charge transport predominantly occurs through guest-to-guest percolation. We define the critical concentration threshold for percolation and demonstrate its significant dependence on the trap depth of guest molecules. Our study establishes a foundational understanding of collective charge dynamics, particularly emphasizing the role of percolation in organic host-guest systems.