Engineered optical and electrical performance of rf-sputtered undoped nickel oxide thin films for inverted perovskite solar cells.
ABSTRACT: Inverted perovskite solar cells incorporating RF sputtered NiO thin films as a hole transport layer and window layer are demonstrated. The electrical and optical properties of the NiO thin films are engineered using varied sputtering conditions. The localized states within bandgap owing to its crystal disorder and nonstoichiometric features affect the transmittance and the optical bandgap of the NiO thin films which in turn influences the Jsc of the perovskite solar cells. In addition, the electrical properties of the NiO thin films can be also varied during sputtering condition affecting the concentration of nickel vacancies and the resulting hole concentration. The conductivity largely originates from the hole concentration relating to the density of states in the NiO thin films which influence the fill factor (FF) of the solar cells. The solar cells fabricated with the NiO thin films made at 4?Pa of deposition pressure show highest performance owing to excellent transmittance and wider bandgap along with moderate conductivity. With further optimization, the perovskite solar cells exhibit ~20?mA/cm2 of Jsc and a 12.4% PCE (11.3% of averaged PCE).
Project description:Cu/Cu2O films grown by ion beam sputtering were used as p-type modified layers to improve the efficiency and stability of perovskite solar cells (PSCs) with an n-i-p heterojunction structure. The ratio of Cu to Cu2O in the films can be tuned by the oxygen flow ratio (O2/(O2?+?Ar)) during the sputtering of copper. Auger electron spectroscopy was performed to determine the elemental composition and chemical state of Cu in the films. Ultraviolet photoelectron spectroscopy and photoluminescence spectroscopy revealed that the valence band maximum of the p-type Cu/Cu2O matches well with the perovskite. The Cu/Cu2O film not only acts as a p-type modified layer but also plays the role of an electron blocking buffer layer. By introducing the p-type Cu/Cu2O films between the low-mobility hole transport material, spiro-OMeTAD, and the Ag electrode in the PSCs, the device durability and power conversion efficiency (PCE) were effectively improved as compared to the reference devices without the Cu/Cu2O interlayer. The enhanced PCE is mainly attributed to the high hole mobility of the p-type Cu/Cu2O film. Additionally, the Cu/Cu2O film serves as a protective layer against the penetration of humidity and Ag into the perovskite active layer.
Project description:In this work, we report a physical deposition based, compact (cp) layer synthesis for planar heterojunction perovskite solar cells. Typical solution-based synthesis of cp layer for perovskite solar cells involves low-quality of thin films, high-temperature annealing, non-flexible devices, limitation of large-scale production and that the effects of the cp layer on carrier transport have not been fully understood. In this research, using radio frequency magnetron sputtering (RFMS), TiO2 cp layers were fabricated and the thickness could be controlled by deposition time; CH3NH3PbI3 films were prepared by evaporation &immersion (E &I) method, in which PbI2 films made by thermal evaporation technique were immersed in CH3NH3I solution. The devices exhibit power conversion efficiency (PCE) of 12.1% and the photovoltaic performance can maintain 77% of its initial PCE after 1440 h. The method developed in this study has the capability of fabricating large active area devices (40 × 40 mm(2)) showing a promising PCE of 4.8%. Low temperature and flexible devices were realized and a PCE of 8.9% was obtained on the PET/ITO substrates. These approaches could be used in thin film based solar cells which require high-quality films leading to reduced fabrication cost and improved device performance.
Project description:Perovskite solar cells (PSCs) based on a NiO x hole transport layer (HTL) with an inverted p-i-n configuration have yielded highly efficient and relatively stable devices. Here, we develop a simple electrochemical deposition method for quickly and evenly preparing a mesoporous NiO x film. It is demonstrated that the increasing thickness and decreasing surface roughness of the NiO x film are beneficial for light transmission. The optimal condition for preparing NiO x films is achieved by adjusting the deposition time at a certain applied current density, which exhibits excellent optical transmittance and suitable thickness and band gap, thus reducing optical loss and enhancing hole extraction at the interface between HTL and the perovskite layer and therefore improving photovoltaic performances. The finite-difference time-domain simulation confirms the optimal thickness of the NiO x layer and coincides with our experiment results. An optimal power conversion efficiency (PCE) of 17.77% with an active area of 0.25 cm2 is achieved. The prepared device shows negligible hysteresis, high reproducibility, and high uniformity with a PCE difference of 2% for measuring the different sites from edge to center. This simple fabrication process paves a novel way to the evolution of PSCs based on NiO x and rapid commercialization.
Project description:Three new spirofluorene-based hole transport materials, Spiro-S, Spiro-N, and Spiro-E, are synthesized by replacing the para-methoxy substituent in 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-MeOTAD) with methylsulfanyl, N,N-dimethylamino and ethyl groups. Their properties as hole transport materials in perovskite solar cells are investigated. The impact of replacing the para-methoxy substituent on bulk properties, such as the photophysical properties, HOMO/LUMO energy level, hole extraction properties and morphologies of perovskite thin films are investigated. Their optoelectronic and charge-transport properties and performance in perovskite solar cells are compared with the current benchmarked and structurally-related hole transport material (HTM) Spiro-MeOTAD. Surprisingly, the methylsulfanyl substituted spirofluorene shows the highest power conversion efficiency of 15.92% among the investigated spirofluorenes, which is an over 38% increase in PCE compared with that of Spiro-MeOTAD under similar device fabrication conditions.
Project description:In this work, niobium oxide films were deposited by reactive magnetron sputtering under different oxygen flow rate and applied as electron transport layer in perovskite solar cells. It was found that the deposition made using 3.5 sccm of oxygen flow resulted in films with better electrical properties which helped the extraction of the photogenerated electrons to the external circuit, improving the Jsc and consequently the device efficiency. In addition, by photoluminescence measurements, we found a better charge transfer from perovskite to TiO2/niobium oxide film deposited at 3.5 sccm of oxygen flow.
Project description:Stability concerns of organic solar cell devices have led to the development of alternative hole transporting layers such as NiO which lead to superior device life times over conventional Poly(3,4-ethylenedioxythiophene) Polystyrene sulfonate (PEDOT:PSS) buffered solar cells. From the printability of such devices, it is imperative to be able to print NiO layers in the organic solar cell devices with normal architecture which has so far remained unreported. In this manuscript, we report on the successful ink-jet printing of very thin NiO thin films with controlled thickness and morphology and their integration in organic solar cell devices. The parameters that were found to strongly affect the formation of a thin yet continuous NiO film were substrate surface treatment, drop spacing, and substrate temperature during printing. The effect of these parameters was investigated through detailed morphological characterization using optical and atomic force microscopy and the results suggested that one can achieve a transmittance of ~89% for a ~18?nm thin NiO film with uniform structure and morphology, fabricated using a drop spacing of 50??m and a heat treatment temperature of 400?°C. The devices fabricated with printed NiO hole transporting layers exhibit power conversion efficiencies comparable to the devices with spin coated NiO films.
Project description:Developing multijunction perovskite solar cells (PSCs) is an attractive route to boost PSC efficiencies to above the single-junction Shockley-Queisser limit. However, commonly used tin-based narrow-bandgap perovskites have shorter carrier diffusion lengths and lower absorption coefficient than lead-based perovskites, limiting the efficiency of perovskite-perovskite tandem solar cells. In this work, we discover that the charge collection efficiency in tin-based PSCs is limited by a short diffusion length of electrons. Adding 0.03 molar percent of cadmium ions into tin-perovskite precursors reduce the background free hole concentration and electron trap density, yielding a long electron diffusion length of 2.72 ± 0.15 µm. It increases the optimized thickness of narrow-bandgap perovskite films to 1000 nm, yielding exceptional stabilized efficiencies of 20.2 and 22.7% for single junction narrow-bandgap PSCs and monolithic perovskite-perovskite tandem cells, respectively. This work provides a promising method to enhance the optoelectronic properties of narrow-bandgap perovskites and unleash the potential of perovskite-perovskite tandem solar cells.
Project description:Surface states of mesoporous NiO semiconductor films have particular properties differing from the bulk and are able to dramatically influence the interfacial electron transfer and adsorption of chemical species. To achieve a better performance of NiO-based p-type dye-sensitized solar cells (p-DSCs), the function of the surface states has to be understood. In this paper, we applied a modified atomic layer deposition procedure that is able to passivate 72% of the surface states on NiO by depositing a monolayer of Al2O3. This provides us with representative control samples to study the functions of the surface states on NiO films. A main conclusion is that surface states, rather than the bulk, are mainly responsible for the conductivity in mesoporous NiO films. Furthermore, surface states significantly affect dye regeneration (with I-/I3- as redox couple) and hole transport in NiO-based p-DSCs. A new dye regeneration mechanism is proposed in which electrons are transferred from reduced dye molecules to intra-bandgap states, and then to I3- species. The intra-bandgap states here act as catalysts to assist I3- reduction. A more complete mechanism is suggested to understand the particular hole transport behavior in p-DSCs, in which the hole transport time is independent of light intensity. This is ascribed to the percolation hole hopping on the surface states. When the concentration of surface states was significantly reduced, the light-independent charge transport behavior in pristine NiO-based p-DSCs transformed into having an exponential dependence on light intensity, similar to that observed in TiO2-based n-type DSCs. These conclusions on the function of surface states provide new insight into the electronic properties of mesoporous NiO films.
Project description:Despite organometal halide perovskite solar cells have recently exhibited a significant leap in efficiency, the Sn-based perovskite solar cells still suffer from low efficiency. Here, a series homogeneous CH3NH3Pb(1-x)SnxI3 (0???x???1) perovskite thin films with full coverage were obtained via solvent engineering. In particular, the intermediate complexes of PbI2/(SnI2)?(DMSO)x were proved to retard the crystallization of CH3NH3SnI3, thus allowing the realization of high quality Sn-introduced perovskite thin films. The external quantum efficiency (EQE) of as-prepared solar cells were demonstrated to extend a broad absorption minimum over 50% in the wavelength range from 350 to 950 nm accompanied by a noteworthy absorption onset up to 1050?nm. The CH3NH3Pb0.75Sn0.25I3 perovskite solar cells with inverted structure were consequently realized with maximum power conversion efficiency (PCE) of 14.12%.
Project description:In this article, we present a new paradigm for organometallic hybrid perovskite solar cell using NiO inorganic metal oxide nanocrystalline as p-type electrode material and realized the first mesoscopic NiO/perovskite/[6,6]-phenyl C61-butyric acid methyl ester (PC61BM) heterojunction photovoltaic device. The photo-induced transient absorption spectroscopy results verified that the architecture is an effective p-type sensitized junction, which is the first inorganic p-type, metal oxide contact material for perovskite-based solar cell. Power conversion efficiency of 9.51% was achieved under AM 1.5 G illumination, which significantly surpassed the reported conventional p-type dye-sensitized solar cells. The replacement of the organic hole transport materials by a p-type metal oxide has the advantages to provide robust device architecture for further development of all-inorganic perovskite-based thin-film solar cells and tandem photovoltaics.