Ternary Organic Photovoltaics at a Turning Point: Mechanistic Perspectives on Their Constraints

Abstract

Ternary organic photovoltaics (OPVs) are considered as the next step beyond binary systems, aiming to achieve synergistic improvements in absorption, energetic alignment, and charge transport. However, despite their conceptual appeal, most ternary blends do not outperform binary counterparts, particularly under indoor illumination where photon flux and carrier dynamics impose strict limitations. To comprehensively understand this discrepancy, multiple ternary systems were systematically examined to ensure that the observed behaviors are representative rather than case specific. In this study, we systematically investigate this discrepancy by comparing representative donor-donor-acceptor (D-D-A) and donor-acceptor-acceptor (D-A-A) systems under both AM 1.5G and TL84 lighting. In all cases, the broadened absorption fails to yield effective photocurrent; instead, redundant excitations, reduced driving forces for charge separation, and disrupted percolation networks collectively diminish device performance. Recombination and transient analyses reveal that the third component often introduces energetic disorder and trap-assisted recombination instead of facilitating beneficial cascade pathways. Although the film morphology remains smooth, interfacial instability under low-light conditions further intensifies performance losses. The inclusion of several systems allows the identification of consistent mechanistic trends across different ternary architectures, reinforcing the generality of the conclusions. This work establishes a mechanistic framework linking molecular miscibility, energetic alignment, and percolation continuity to device-level behavior, clarifying why ternary strategies rarely deliver consistent efficiency improvements. Ultimately, indoor OPV performance is determined not by spectral breadth but by maintaining balanced charge transport and stable energetic landscapes, which represents an essential paradigm for advancing ternary OPVs from concept to practical application.

Keywords: carrier mobility; indoor photovoltaic; recombination; spectral overlap; ternary organic photovoltaics.

Figure 1

(a) Energy level diagram and (b) absorption spectra of the active layer materials PBDB-T, PM6, and ITIC-4F. J–V characteristics of devices with different PM6:PBDB-T:ITIC-4F ratios under (c) solar illumination and (d) indoor lighting, and (e) the corresponding EQE spectra.

Figure 2

Light-intensity dependence of (a) V_OC and (b) J_SC under solar illumination, and (c) V_OC and (d) J_SC under indoor illumination for devices with different PM6:PBDB-T:ITIC-4F ratios.

Figure 3

AFM images of PM6:PBDB-T:IT-4F active layers with different ratios: (a) 1:0:1, (b) 1:0.2:1, and (c) 0:1:1.

Figure 4

PM6:PBDB-T:IT-4F devices under solar illumination: (a) TPC, (b) TPV, and (c) Photo-CELIV; and under indoor illumination: (d) TPC, (e) TPV, and (f) Photo-CELIV.

Figure 5

(a) Energy-level diagram of the active-layer materials PM6, BT-CIC, and ITIC-4F. (b) Absorption spectra of PM6:BT-CIC:ITIC-4F blend films at different composition ratios. (c) J–V characteristics under AM 1.5G illumination (100 mW cm⁻²). (d) J–V characteristics under TL84 indoor lighting (1000 lux). (e) External quantum efficiency (EQE) spectra for the corresponding devices.

Figure 6

Light-intensity dependence of (a) V_OC and (b) J_SC under AM 1.5G, and (c) V_OC and (d) J_SC under TL84 indoor illumination for devices with different PM6:BT-CIC:ITIC-4F composition ratios.

Figure 7

AFM height images of active layers with PM6:BT-CIC:ITIC-4F compositions: (a) 1:0:1.2, (b) 1:0.2:1, and (c) 1:1.2:0.

Figure 8

Transient responses for PM6:BT-CIC:ITIC-4F devices: (a) TPC, (b) TPV, and (c) Photo-CELIV under AM 1.5G illumination; (d) TPC, (e) TPV, and (f) Photo-CELIV under TL84 indoor lighting.