Role of Intrinsic Ion Accumulation in the Photocurrent and Photocapacitive Responses of MAPbBr₃ Photodetectors

We studied steady state and transient photocurrents in thin film and single-crystal devices of MAPbBr₃, a prototype organic–inorganic hybrid perovskite. We found that the devices’ capacitance is abnormally large, which originates from accumulation of large densities of Pb²⁺ and Br^– in the active perovskite layer. Under applied bias, these ions are driven toward the opposite electrodes leading to space-charge fields close to the metal/perovskite interfaces. The ion accumulation, in turn, causes photocurrent reversal polarity that depends on the history of the applied bias and excitation photon energy with respect to the optical gap. Furthermore, the large capacitive response dominates the transient photocurrent and, therefore, obscures the weaker contribution from the photocarriers’ drift. We show that these properties depend on the ambient conditions in which the measurements are performed. Understanding these phenomena may lead to better control over the stability of perovskite photodetectors for visible light

Large Photocurrent Response and External Quantum Efficiency in Biophotoelectrochemical Cells Incorporating Reaction Center Plus Light Harvesting Complexes

Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy harvesting, due to their high ratio of photogenerated electrons to absorbed photons and long recombination time of generated charges. In this work, photoactive electrodes were prepared from a bacterial RC-light-harvesting 1 (LH1) core complex, where the RC is encircled by the LH1 antenna, to increase light capture. A simple immobilization method was used to prepare RC-LH1 photoactive layer. Herein, we demonstrate that the combination of pretreatment of the RC-LH1 protein complexes with quinone and the immobilization method results in biophotoelectrochemical cells with a large peak transient photocurrent density and photocurrent response of 7.1 and 3.5 μA cm^–2, respectively. The current study with monochromatic excitation showed maximum external quantum efficiency (EQE) and photocurrent density of 0.21% and 2 μA cm^–2, respectively, with illumination power of ∼6 mW cm^–2 at ∼875 nm, under ambient conditions. This work provides new directions to higher performance biophotoelectrochemical cells as well as possibly other applications of this broadly functional photoactive material

Electroabsorption Spectroscopy Studies of (C₄H₉NH₃)₂PbI₄ Organic–Inorganic Hybrid Perovskite Multiple Quantum Wells

Two-dimensional (2D) organic–inorganic hybrid perovskite multiple quantum wells that consist of multilayers of alternate organic and inorganic layers exhibit large exciton binding energies of order of 0.3 eV due to the dielectric confinement between the inorganic and organic layers. We have investigated the exciton characteristics of 2D butylammonium lead iodide, (C₄H₉NH₃)₂PbI₄ using photoluminescence and UV–vis absorption in the temperature range of 10 K to 300 K, and electroabsorption spectroscopy. The evolution of an additional absorption/emission at low temperature indicates that this compound undergoes a phase transition at ≈250 K. We found that the electroabsorption spectrum of each structural phase contains contributions from both quantum confined exciton Stark effect and Franz–Keldysh oscillation of the continuum band, from which we could determine more accurately the 1s exciton, continuum band edge, and the exciton binding energy

Hybrid Wiring of the Rhodobacter sphaeroides Reaction Center for Applications in Bio-photoelectrochemical Solar Cells

The growing demand for nonfossil fuel-based energy production has drawn attention to the utilization of natural proteins such as photosynthetic reaction center (RC) protein complexes to harvest solar energy. The current study reports on an immobilization method to bind the wild type Rhodobacter sphaeroides RC from the primary donor side onto a Au electrode using an immobilized cytochrome <i>c</i> (cyt <i>c</i>) protein via a docking mechanism. The new structure has been assembled on a Au electrode by layer-by-layer deposition of a carboxylic acid-terminated alkanethiol (HOOC (CH₂)₅S) self-assembled monolayer (SAM), and layers of cyt <i>c</i> and RC. The Au|SAM|cyt <i>c</i>|RC working electrode was applied in a three-probe electrochemical cell where a peak cathodic photocurrent density of 0.5 μA cm^–2 was achieved. Further electrochemical study of the Au|SAM|cyt <i>c</i>|RC structure demonstrated ∼70% RC surface coverage. To understand the limitations in the electron transfer through the linker structure, a detailed energy study of the SAM and cyt <i>c</i> was performed using photochronoamperometry, ellipsometry, photoemission spectroscopy, and cyclic voltammetry (CV). Using a simple rectangle energy barrier model, it was found that the electrode work function and the large barrier of the SAM are accountable for the low conductance in the devised linker structure

Core/Alloyed-Shell Quantum Dot Robust Solid Films with High Optical Gains

We report high optical gain from freestanding, optically stable, and mechanically robust films that are loaded with cross-linked CdSe/Cd_1–<i>x</i>Zn_<i>x</i>Se_1–<i>y</i>S_<i>y</i> core/alloyed shell quantum dots (QD). These solid films display very high net optical gain as high as 650 cm^–1 combined with a low pump excitation gain threshold of 44 μJ/cm². The functionalization of the QDs using short-chain bifunctional cross-linkers not only significantly improves the net optical gain by allowing for a nearly 2-fold increase in QD loading but also provides stable passivation of the QDs which imparts excellent thermal stability, mechanical robustness, and stability under harsh chemical environments. The gain achieved here is up to 3-fold higher than that typically reported for traditional drop-cast QD films. Moreover, stable photoluminescence over long shelf storage time is a distinguished characteristic of the films. The QD films fabricated here span large areas (several cm²), can be readily micropatterned and sustain multiple harsh chemical treatment. Furthermore, they can be readily transferred onto different substrates without compromising their structural integrity and without diminishing optical activity that opens the paths to design complex and robust gain–loss optical structures