Reversible Formation of Gold Halides in Single‐Crystal Hybrid‐Perovskite/Au Interface upon Biasing and Effect on Electronic Carrier Injection

Abstract

Solar cells, light emitting diodes, and X-ray detectors based on perovskite materials often incorporate gold electrodes, either in direct or indirect contact with the perovskite compound. Chemical interactions between active layers and contacts deteriorate the operation and induce degradation, being the identification of the chemical nature of such interfacial structures an open question. Chemical reactivity of gold in contact with the perovskite semiconductor leads to reversible formation of oxidized gold halide species and explains the generation of halide vacancies in the vicinity of the interface.Electrical biasing induces contact reaction and produces modifications of the current level by favoring the ability of perovskite/Au interfaces to inject electronic carriers. The current injection increment does not depend on the halogen source used, either extrinsically by iodine vapor sublimation of Au electrodes, or intrinsically by bias-driven migration of bromide ions. In addition, the formation of a dipole-like structure at the reacted electrode that lowers the potential barrier for electronic carriers is confirmed. These findings highlight adequate selection of the external contacts and suggest the need for a deeper understanding of contact reactivity as it dominates the operation characteristics, rather than being governed by the bulk transport properties of the charge carriers, either electronic or ionic.conversion in the near future. In addition to photovoltaic applications, perovskitebased materials are being explored as sensitive layers for high-energy radiation detectors and imaging devices for medical diagnostics. [2] These detection technologies rely upon the ability of perovskite compounds of absorbing high-energy photons and convert their energy into electronic carriers that are finally collected at the outer contacts. For soft X-ray photons (<10 keV), absorbing layer thickness of tens of micrometers suffices to stop the radiation. But hard X-ray radiation (10-50 keV) possesses much longer penetration length (≈100 µm) in compounds as methylammonium lead iodide or bromide (MAPbI 3 and MAPbBr 3 ). [3] Hence, relatively thick layers (0.1-1 mm) need to be employed to achieve sufficient electrical signal. Despite the large electronic carrier mobility and mobility-lifetime product, [4,5] such thick absorbing layers should be externally biased in order to increase the detector responsivity. Electrical fields as high as ≈0.1-1 V µm −1 are commonly needed for poly-crystalline perovskite deposits incorporated into X-ray detectors. [3,5] Large field strength requirements are in some amount lowered when devising single-crystal approaches for X-ray detection using perovskite materials. [4,6] In any case, it is widely recognized that the application of an external bias promotes the displacement and interfacial built-up of intrinsic mobile ions. [7,8] Ion accumulation finally shields the electrical field within the absorbing layer bulk, reducing as a consequence the detector sensitivity. The applicability of perovs...

Ferroelectric Poling of Methylammonium Lead Iodide Thin Films

“…Birkhold et al previously reported ionic charging and PL quenching near electrodes on comparable timescales which indicates that slow poling fields are likely to influence the defect densities in MAPbI 3 due to ionic movement . Other reports described similar ionic charge build‐up or even chemical interaction of ionic species with the electrode interface at high voltages …”

Methylammonium Lead Tribromide Single Crystal Detectors towards Robust Gamma‐Ray Photon Sensing

“…In order to solve this problem in single crystalline hybrid perovskites, studies have shown that certain interfaces can cause hysteretic behavior due to interfacial recombination, low interfacial resistance, and chemical reactions at the interface forming metal‐halide compounds between the semiconductor and electrode. [ 31,44 ] In the single crystal devices, similar issue may also exist. Therefore, we characterize the current–voltage curves and instability through monitoring hysteresis as a function of long‐term, high voltage biasing on our detector in dark as summarized in Figure .…”

“…In previous reports, it has been shown that Br − atoms can form complexes with metals such as Au. [ 44 ] However, this was found to be a reversible formation at low biases, important for optoelectronic processes, where the Cr/MAPbBr 3 interface did not show signs of hysteresis at lower voltages. [ 31 ] For high‐energy γ‐ray detection, where single photon induced pulses need to be collected, high electric fields are required for optimized charge collection efficiency and reduction of trapping to ensure high resolution signal.…”

Stabilizing Perovskite Light‐Emitting Diodes by Incorporation of Binary Alkali Cations

“…[ 14,18,23 ] In particular, the accumulation and reaction of the ions with the Au electrode could further result in deterioration of the perovskite layer [ 16,29,31 ] under long‐term operation and aggravate the decay of EL efficiency and thus EQE. We also note that the driving voltage for keeping the constant current density is decreased during operation (Figure 1c), this might be related to the modification of carrier injection barrier [ 32 ] or formation of leakage paths after I − migration and metal‐halide reaction.…”