The origin of dark current and trap states in organic photodiodes

Despite significant progress in developing donor-acceptor (D−A) bulk-heterojunction (BHJ) organic photodiodes (OPDs) with near 100% external quantum efficiency (EQE), the specific detectivity (D*) of OPDs is still limited by the high noise current spectral density that primarily stems from the reverse bias dark current density (J_d). Thus, understanding the factors that determine J_d is an important prerequisite to further reduce the J_d and thereby improve D*.

History first

To address this question, Xiao Ma compiled an overview of the basic development history of organic photodiodes, which explains how the organic active layer evolved from the single-component layer to the BHJ layer for state-of-the-art photovoltaic devices. Due to the disordered nature of organic semiconductors, the presence of energetic disorder i.e., trap states, is commonly observed in many organic electronic devices.

Thus, various techniques or methods that can be used to characterize the trap states were illustrated and compiled by Ma, including sub-bandgap EQE measurement, thermal admittance spectroscopy, light intensity dependent EQE measurement, and temperature dependent current density – voltage characteristics (J–V).

Investigation

To investigate the origin of J_d, Ma and his collaborators in the department of Applied Physics employed temperature dependent J–V measurements in OPDs with J_d down to 10⁻⁹ mA cm⁻², where the shunt current was substantially eliminated by the edge cover layer. The activation barrier of J_d was determined by the activation energy (E_a) extracted from the slope in the Arrhenius plot.

Ma discovered that the activation energy of OPDs with the same BHJ active layer did not change over different charge transport layers or contacts. E_a remained the same even for OPDs with an extra hole blocking layer (C60), which has a much deeper HOMO level. However, for diode-based on polymer-only active layer, its E_a decreased dramatically with the addition of C60 leading to the formation bi-layer diode. These indicates that the activation barrier of J_d is formed within the active layer and generation of J_d happens at the D–A interface. In the end, for a variety of donor-acceptor BHJs, Ma found that the activation energy of J_d is lower than the effective bandgap of the blends, by ca. 0.3 to 0.5 eV, suggesting there are intra-gap states in various BHJs.

Intra-gap states in more detail

To characterize the intra-gap states in more detail, Ma employed ultra-sensitive sub-bandgap photocurrent measurements for the same group of BHJs in both semi-transparent and opaque stack.

A distinct low-energy band was observed below the exponential band edge of the BHJ for each D-A combination. These low-energy transitions correspond to charge generation by absorption via the intra-gap states. The low-energy onsets (E_onset) in these spectra represents the lowest-energy photo-excitations that generate photocurrent in the BHJs. The correspondence between E_a and E_onset proves that thermal and photo-induced charge carrier generation have the similar minimal energy for charge generation and use the same intra-gap states in BHJ layers, and lie close to the band edges.

The origin of trap states?

However, it is still unclear where trap states in the BHJs come from. To explore their origin, Ma compared the the activation energy of BHJs with the same polymer donor and three non-fullerene acceptors (NFAs). It was found that J_d and E_a strongly depend on the hole transport level of acceptors. Sub-bandgap EQE spectra of polymer donor-only and NFA-only photodiodes suggest that trap states in BHJs are derived from polymer donors rather than NFAs.

Combined with the activation energy of each OPD based on different NFAs, Ma concluded that the activation barrier of J_d is formed between NFA LUMO and trap states above the polymer HOMO of  approximately 0.3−0.4 eV. Moreover, the light intensity-dependent photocurrent (J_ph) measurement under ultra-low light intensity, down to 10⁻⁷ mW cm⁻², revealed an unreported first-order loss, which is attributed to trap-induce photocurrent recombination.

The presence of this first-order photocurrent loss in BHJs and polymer-only diodes was not observed in NFA-only diodes suggesting that the trap states in BHJs are derived from polymers rather than NFAs. Ma’s results emphasize the critical and detrimental role of trap states in polymers in limiting the J_d of NIR OPDs, and shed new light on future optimization strategies, which may target polymer material properties rather than device engineering.

Title of PhD thesis: The Origin of Dark Current and Trap States in Organic Photodiodes. Supervisors: René Janssen, Gerwin Gelinck, and Stefan Meskers.