Low dark leakage current in organic planar heterojunction photodiodes

Abstract

It is often suggested that the dark leakage current of organic photodiodes is due to extrinsic leakage paths that do not involve the electronic junction. By studying a series of devices, where the acceptor is kept constant (C70) and the donor material is varied, we find a direct correlation between the strength of the sub-gap signature of the charge-transfer states and the leakage current. Attributing the differences in the sub-gap absorption to the donor's sub-gap states suggests that the donor's side of the junction should be made longer, to push the Fermi level at V = 0 towards the acceptor's LUMO, and thus, an optimized value of 800 Pacm−2 at V = −1 V is reported.

Organic Photodetectors and their Application in Large Area and Flexible Image Sensors: The Role of Dark Current

“…In addition, the photoactive layer stratification can be adjusted specifically with the aim of reducing the dark current density (see Section ). Planar heterojunction (PHJ) OPDs made via sequential deposition of individual donor and acceptor layers are widely investigated . Alternatively, sequentially solution‐processed (SSP) OPDs can lead to a partial donor‐acceptor interpenetration, resulting in a well‐mixed interdiffusion phase in the middle of the bulk.…”

“…Recently, Yoon et al achieved J d = 6 × 10 −7 mA cm −2 at F = 6 × 10 6 V m −1 for a color‐selective inverted‐polarity PHJ OPD using a blue‐sensitive polymer donor in combination with non‐absorbing ZnO acceptor. Shekhar et al reached J d in the order of 10 −6 mA cm −2 in combination with EQE well above 10% in the spectral range between 400 and 800 nm. This promising result was obtained using very thin donor and acceptor layers [20 nm of boron subphthalocyanine chloride (SubPc) and 40 nm of C 70 , respectively] to ensure fast migration of the photogenerated exciton to the D‐A interface and efficient charge separation.…”

“…Shekhar et al studied the effect of deep subgap states on the dark current density of PHJ OPDs. Measuring EQE spectra extended up to 1600 nm, the presence of subgap states and/or defect‐induced states in the proximity of the D‐A interface was investigated.…”

On the Origin of Dark Current in Organic Photodiodes

“…[2,4] Since minimizing J d while maintaining a high responsivity is a prerequisite for high detectivity OPDs, an enhanced understanding of the mechanism that determines J d can thus direct further detectivity improvement strategies.The intrinsic dark current of OPDs is typically attributed to either charge carrier injection from the metal contacts into the organic semiconductor [4,5,8] or to bulk thermal generation within the active layer. [9,10] Whereas thermal generation typically makes a limited contribution to J d as organic materials have a relatively large bandgap (>1 eV), [2] charge injection may not be negligible under an applied reverse bias voltage. As such, modifying the metal-semiconductor interfaces by introducing electron and hole blocking layers (EBLs and HBLs) to suppress charge injection is a common strategy to reduce J d , [5,11,12] along with increasing the active layer thickness.…”

“…Under forward bias, no appreciable barriers for charge injection exist and then J d is dominated by the electron current, because the electron mobility is higher than the hole mobility. Further reduction of the dark current, and thus improvement of OPD detectivity at a given photon energy, may thus be achieved by reducing the energetic disorder of the organic semiconductors, [10] or by otherwise reducing injection of carriers under reverse bias. [5,11,12] The open-circuit voltage of each OPD analyzed in this work was recorded in a N 2 -filled glove box under simulated solar light using a tungsten-halogen lamp coupled to a UV filter and daylight filter (Hoya LB120).…”