Noise and detectivity limits in organic shortwave infrared photodiodes with low disorder

Abstract

To achieve high detectivity in infrared detectors, it is critical to reduce the device noise. However, for non-crystalline semiconductors, an essential framework is missing to understand and predict the effects of disorder on the dark current. This report presents experimental and modeling studies on the noise current in exemplar organic bulk heterojunction photodiodes, with 10 donor-acceptor combinations spanning wavelength between 800 and 1600 nm. A significant reduction of the noise and higher detectivity were found in devices using non-fullerene acceptors (NFAs) in comparison to those using fullerene derivatives. The low noise in NFA blends was attributed to a sharp drop off in the distribution of bandtail states, as revealed by variable-temperature density-of-states measurements. Taking disorder into account, we developed a general physical model to explain the dependence of thermal noise on the effective bandgap and bandtail spread. The model provides theoretical targets for the maximum detectivity that can be obtained at different detection wavelengths in inherently disordered infrared photodiodes.

Intrinsic Detectivity Limits of Organic Near‐Infrared Photodetectors

“…This dark mechanism has been shown to be present in both non-fullerene acceptor (NFA) containing and NFA free BHJs, and results, as compared to Ref. [48], in a stronger increase in the noise current toward longer wavelengths. [40] To increase D* to the theoretical maximum for NIR organic photodiodes, as indicated by the red curve in Figure 4b, one should find a way to decrease ΔV OC,NR while maintaining the NIR absorption.…”

“…This dark mechanism has been shown to be present in both non‐fullerene acceptor (NFA) containing and NFA free BHJs, and results, as compared to Ref. [ 48 ] , in a stronger increase in the noise current toward longer wavelengths. [ 40 ]…”

“…reported on an upper limit for D * that is around a 100 times higher at 2 µm than calculated here. [ 48 ] Their model considers as a limit to D * the effect of tail‐state broadening on noise related to thermally activated charge generation and recombination over the effective bandgap. Here, we follow a different approach and considered the non‐radiative dark saturation current as a more important limiting factor.…”

Reverse dark current in organic photodetectors and the major role of traps as source of noise

“…Several publications address the limitations on charge transport 36 – 38 , increase of recombination rates 39 and change of recombination dynamics 40 caused by these states. However, only very few studies investigated the influence of trap states on J D 41 . Drift-diffusion models with band-tail 42 and mid-gap 43 trap states were employed to reproduce the experimental JV characteristics of organic solar cells and OPDs, respectively.…”

Organic Upconversion Imager with Dual Electronic and Optical Readouts for Shortwave Infrared Light Detection

“…At an applied bias of 3 V, the imager EQE increased to 35%, slightly lower than the OPD (EQE < 40%). For the dark current measured at equivalent electric fields, [ 38–40 ] the imager exhibited less noise than the OPD (Figure 3b). The EQE and noise characteristics taken together determined the specific detectivities D * shown in Figure 3c.…”