Hugh L. Zhu

Room-Temperature Solution-Processed NiOx: PbI2 Nanocomposite Structures for Realizing High-Performance Perovskite Photodetectors

While methylammonium lead iodide (MAPbI3) with interesting properties, such as a direct band gap, high and well-balanced electron/hole mobilities, as well as long electron/hole diffusion length, is a potential candidate to become the light absorbers in photodetectors, the challenges for realizing efficient perovskite photodetectors are to suppress dark current, to increase linear dynamic range, and to achieve high specific detectivity and fast response speed. Here, we demonstrate NiOx:PbI2 nanocomposite structures, which can offer dual roles of functioning as an efficient hole extraction layer and favoring the formation of high-quality MAPbI3 to address these challenges. We introduce a room-temperature solution process to form the NiOx:PbI2 nanocomposite structures. The nanocomposite structures facilitate the growth of the compact and ordered MAPbI3 crystalline films, which is essential for efficient photodetectors. Furthermore, the nanocomposite structures work as an effective hole extraction layer, which provides a large electron injection barrier and favorable hole extraction as well as passivates the surface of the perovskite, leading to suppressed dark current and enhanced photocurrent. By optimizing the NiOx:PbI2 nanocomposite structures, a low dark current density of 2 × 10(-10) A/cm(2) at -200 mV and a large linear dynamic range of 112 dB are achieved. Meanwhile, a high responsivity in the visible spectral range of 450-750 nm, a large measured specific detectivity approaching 10(13) Jones, and a fast fall time of 168 ns are demonstrated. The high-performance perovskite photodetectors demonstrated here offer a promising candidate for low-cost and high-performance near-ultraviolet-visible photodetection.

A general design rule to manipulate photocarrier transport path in solar cells and its realization by the plasmonic-electrical effect.

It is well known that transport paths of photocarriers (electrons and holes) before collected by electrodes strongly affect bulk recombination and thus electrical properties of solar cells, including open-circuit voltage and fill factor. For boosting device performance, a general design rule, tailored to arbitrary electron to hole mobility ratio, is proposed to decide the transport paths of photocarriers. Due to a unique ability to localize and concentrate light, plasmonics is explored to manipulate photocarrier transport through spatially redistributing light absorption at the active layer of devices. Without changing the active materials, we conceive a plasmonic-electrical concept, which tunes electrical properties of solar cells via the plasmon-modified optical field distribution, to realize the design rule. Incorporating spectrally and spatially configurable metallic nanostructures, thin-film solar cells are theoretically modelled and experimentally fabricated to validate the design rule and verify the plasmonic-tunable electrical properties. The general design rule, together with the plasmonic-electrical effect, contributes to the evolution of emerging photovoltaics.

A low temperature gradual annealing scheme for achieving high performance perovskite solar cells with no hysteresis

CH3NH3PbI3 is commonly used in perovskite solar cells due to its long diffusion length and good crystallinity. In this paper, in the one-step approach using CH3NH3I and PbCl2 for forming the perovskite, we present a new low temperature annealing approach of gradually increasing the temperature to fabricate perovskite films. Various temperatures and temperature ranges for the formation of perovskite films have been studied. Using the gradual annealing process, we can tune the amount of chlorine in the atomic ratio of chlorine/iodine from 1.2 to 4.0%. Meanwhile, the gradual annealing process influences the quality of the perovskite film and importantly the device performance. The results show that through the optimized process, the film quality is improved with high surface coverage and good photoluminescence and reproducibility. We find that a higher amount of chlorine in the perovskite film plays a positive role in the device performance in the approach for achieving a power conversion efficiency of 14.9% with no obvious hysteresis.

High efficient planar-heterojunction solar cells achieved by using a smooth CH 3 NH 3 PbI 3 film via a new approach of forming the PbI 2 nanostructure together with strategically high CH 3 NH 3 I concentration

Summary form only given: In a typical two-step sequential deposition of perovskites such as MAPbI₃ (MA= CH₃NH₃⁺, PbI₂ is first deposited on the substrate (mesoporous or planar scaffold) by spin-coating or vacuum evaporation, subsequently transformed into the perovskite (MAPbI₃) by exposing it to an anhydrous isopropanol (IPA) solution of MAI. For the two-step sequential deposition, the conversion and film morphology of the final perovskite film strongly depend on the initial PbI₂ film during the first step of the process. Conventionally, PbI₂ from dimethyl formamide (DMF) solution tends to form a layered and dense crystalline film on a flat substrate However, the complete conversion of PbI₂ to perovskite on exposure to the MAI solution usually requires several hours. However, this long reaction time in MAI solution could lead to the dissolution of perovskites. These drawbacks make it difficult to fabricate planar-structured PSCs by sequential deposition method. To our knowledge, a simple and effective method for fast conversion of PbI₂ film into perovskite on flat substrate via two-step sequential deposition process at room temperature has not been reported. Although many groups have demonstrated the PbI₂ residue in perovskite film has a positive effect on the efficiency of PSCs, the effect of PbI₂ residue on the long-term stability of PSCs is unclear. In this work, we demonstrate a new approach for forming the PbI₂ nanostructure and the use of high CH₃NH₃I concentration which are adopted to form high quality (large crystal size and smooth) perovskite film with better photovoltaic performances. On one hand, self-assembled porous PbI₂ is formed by incorporating small amount of rationally chosen additives into the PbI₂ precursor solutions, which significantly facilitate the conversion of perovskite without any PbI₂ residue. On the other hand, through employing a relatively high CH₃NH₃I concentration, a firmly crystallized and uniform CH₃NH₃PbI₃ film is formed. As a result, a promising power conversion efficiency (PCE) of 16.21% is achieved in planar-heterojunction PSCs. Furthermore, we experimentally demonstrate that the PbI₂ residue in perovskite film has a negative effect on the long-term stability of devices.