Improving NIR Detection in All-Polymer OPDs with Planar Heterojunction and Waveguide Structures

Organic photodetectors (OPDs) have attracted widespread attention due to several favorable properties, such as narrow spectral band detection, low power consumption, compact size, flexibility, cost-effective fabrication, and tunability through molecular design. As the name suggests, these devices detect light by absorbing photons and generating an electric current. OPDs with sensitivity beyond 800 nm are particularly useful in applications like health monitoring, wavelength sensing, and night vision. While silicon-based photodetectors are commonly used for their high photon-to-free-carrier external quantum yields, they face several limitations. Traditionally, narrow-band OPDs have been fabricated using a Bulk Heterojunction (BHJ) structure, with techniques such as charge injection narrowing (CIN) and charge collection narrowing (CCN) employed to suppress broad-band absorption and avoid the need for optical filters. In this work, we explore a new configuration, the Planar Heterojunction (PHJ), which enables a narrow peak in the red/near IR region without requiring optical filters, resulting in an all-polymer, high-performance, selective OPD.
Typically, an OPD consists of an active layer containing an electron donor (D) and an electron acceptor (A) material. In a Planar Heterojunction (PHJ), these layers are stacked without polymer mixing. Upon light absorption, Frenkel excitons are formed. High-energy excitons recombine in the p-type layer, while lower-energy excitons diffuse to the donor-acceptor interface due to the energy difference between the p-type and n-type polymers. As a result, the PHJ structure strongly detects red and near-infrared (NIR) wavelengths. In our OPD devices, a thin layer of Poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (PNDI-T2), an n-type polymer, is solution-deposited on top of a thick layer of poly(3-hexylthiophene-2,5-diyl) (P3HT), a p-type polymer, forming a planar heterojunction. This structure is achieved by doping the P3HT layer to make it insoluble, then coating PNDI-T2 on top, followed by dedoping the entire stack with 1-Butylamine. By adjusting the thickness of the optical cavity (P3HT and PNDI-T2 layers), we achieved a peak response at 721 nm, with a responsivity of 0.06 A/W, an external quantum efficiency (EQE) of 10%, and a full width at half maximum (FWHM) of 85 nm, without the need for optical filters, CIN, or CCN. The EQE can be tuned by varying the thickness of the p-type layer, enhancing the peak in the near-IR while suppressing ultraviolet and blue photons.
In addition to achieving narrowband absorption, we have patterned the p-type layer to create a waveguide structure at the D-A interface. This surface modification is a critical result, as it generates a cavity effect that significantly enhances the device's responsivity at longer wavelengths. As a result, we achieved a second detectivity peak at ~884 nm, with a detectivity of 0.012 A/W and an EQE of 2%. The process was successfully replicated with a different n-type polymer (WQ295), demonstrating the versatility of this technique and advancing the development of all-polymer devices.