On the Origin of Dark Current in Organic Photodiodes

Abstract

Minimizing the reverse bias dark current while retaining external quantum efficiency is crucial if the light detection sensitivity of organic photodiodes (OPDs) is to compete with inorganic photodetectors. However, a quantitative relationship between the magnitude of the dark current density under reverse bias ( Jd) and the properties of the bulk heterojunction (BHJ) active layer has so far not been established. Here, a systematic analysis of Jd in state‐of‐the‐art BHJ OPDs using five polymers with a range of energy levels and charge transport characteristics is presented. The magnitude and activation energy of Jd are explained using a model that assumes charge injection from the metal contacts into an energetically disordered semiconductor. By relating Jd to material parameters, insights into the origin of Jd are obtained that enable the future selection of successful OPD materials.

Color Determination from a Single Broadband Organic Photodiode

“…At the 900 nm optical mode close to the optical band edge R ≈ 0.08 A W −1 and D * ≈ 6.0 × 10 12 Jones (Figure S4 in the Supporting Information shows full spectra for R and D *). Relative to other reported OPDs,12,15,28,56,68 this device has a lower than average EQE (and thus R ) but a shot noise limited D * commensurate with the state‐of‐the‐art due to the low J d .…”

“…Thus, η col (λ, V ) must be predominantly responsible for the observed wavelength and voltage dependence of photocurrent (even if there is or can be different for absorption by the donor or by the acceptor in the blend. We assert that trap states within the metal oxide EEL causes this wavelength variation, since it is not apparent in PDPP3T:PCBM OPDs with a LiF/Al EEL (see Figure S7 in the Supporting Information) 28. As such, we propose in Figure a mechanism for the influence of photoinduced IGZO trap states38 on charge extraction and hence J–V characteristics.…”

“…Various approaches have been employed to induce spectral sensitivity in OPDs without external color filtering 27. These include changing the chemical structure of the component species28 and using optical cavities (particularly near the optical band edge) 5–9. Other approaches include charge‐collection narrowing (CCN), in which only colors with sufficiently low absorption coefficients produce carriers close enough to the far electrode for efficient extraction,29,30 and charge injection narrowing (CIN), which similarly relies on different absorption coefficients but in this case employs photomultiplication at the far electrode due to charge tunneling injection induced by interfacial trap filling 31–33.…”

“…Combining these conjugated species in the ubiquitous bulk‐heterojunction (BHJ) architecture allows this energy level offset to facilitate charge separation at the donor–acceptor (D–A) interface, with free charges extracted via a bicontinuous networks 12,56,57. PDPP3T is chosen here due to its low optical bandgap of ≈930 nm (≈1.3 eV)28 that enables NIR detectivity. Furthermore, PDPP3T:PC 60 BM blends have been widely used in solar cells,58–61 with the blend ratio and processing conditions used here previously optimized for maximum power conversion efficiency 59.…”

“…This is achieved by minimizing extrinsic leakage pathways such as pinholes65 by using an SU‐8 edge cover layer and having a comparatively thick (≈260 nm) active layer 25. Using the EQE spectrum (Figure 1e) together with J d (both at −2 V) to calculate the common photodetector figures‐of‐merit28 responsivity R = q EQE/ E and shot noise limited (i.e., without considering frequency dependence)66,67 specific detectivity gives maximum values of ≈0.16 A W −1 and ≈1.1 × 10 13 Jones at 550 nm, respectively ( q is the electronic charge, E is energy). At the 900 nm optical mode close to the optical band edge R ≈ 0.08 A W −1 and D * ≈ 6.0 × 10 12 Jones (Figure S4 in the Supporting Information shows full spectra for R and D *).…”