Xingang Ren

Vacuum-Assisted Thermal Annealing of CH3NH3PbI3 for Highly Stable and Efficient Perovskite Solar Cells

Solar cells incorporating lead halide-based perovskite absorbers can exhibit impressive power conversion efficiencies (PCEs), recently surpassing 15%. Despite rapid developments, achieving precise control over the morphologies of the perovskite films (minimizing pore formation) and enhanced stability and reproducibility of the devices remain challenging, both of which are necessary for further advancements. Here we demonstrate vacuum-assisted thermal annealing as an effective means for controlling the composition and morphology of the CH(3)NH(3)PbI(3) films formed from the precursors of PbCl(2) and CH(3)NH(3)I. We identify the critical role played by the byproduct of CH(3)NH(3)Cl on the formation and the photovoltaic performance of the perovskite film. By completely removing the byproduct through our vacuum-assisted thermal annealing approach, we are able to produce pure, pore-free planar CH(3)NH(3)PbI(3) films with high PCE reaching 14.5% in solar cell device. Importantly, the removal of CH(3)NH(3)Cl significantly improves the device stability and reproducibility with a standard deviation of only 0.92% in PCE as well as strongly reducing the photocurrent hysteresis.

The efficiency limit of CH3NH3PbI3 perovskite solar cells

With the consideration of photon recycling effect, the efficiency limit of methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells is predicted by a detailed balance model. To obtain convincing predictions, both AM 1.5 spectrum of Sun and experimentally measured complex refractive index of perovskite material are employed in the detailed balance model. The roles of light trapping and angular restriction in improving the maximal output power of thin-film perovskite solar cells are also clarified. The efficiency limit of perovskite cells (without the angular restriction) is about 31%, which approaches to Shockley-Queisser limit (33%) achievable by gallium arsenide (GaAs) cells. Moreover, the Shockley-Queisser limit could be reached with a 200 nm-thick perovskite solar cell, through integrating a wavelength-dependent angular-restriction design with a textured light-trapping structure. Additionally, the influence of the trap-assisted nonradiative recombination on the device efficiency is investigated. The work is fundamentally important to high-performance perovskite photovoltaics.

Selective Growth and Integration of Silver Nanoparticles on Silver Nanowires at Room Conditions for Transparent Nano-Network Electrode

Recently, metal nanowires have received great research interests due to their potential as next-generation flexible transparent electrodes. While great efforts have been devoted to develop enabling nanowire electrodes, reduced contact resistance of the metal nanowires and improved electrical stability under continuous bias operation are key issues for practical applications. Here, we propose and demonstrate an approach through a low-cost, robust, room temperature and room atmosphere process to fabricate a conductive silver nano-network comprising silver nanowires and silver nanoparticles. To be more specific, silver nanoparticles are selectively grown and chemically integrated in situ at the junction where silver nanowires meet. The site-selective growth of silver nanoparticles is achieved by a plasmon-induced chemical reaction using a simple light source at very low optical power density. Compared to silver nanowire electrodes without chemical treatment, we observe tremendous conductivity improvement in our silver nano-networks, while the loss in optical transmission is negligible. Furthermore, the silver nano-networks exhibit superior electrical stability under continuous bias operation compared to silver nanowire electrodes formed by thermal annealing. Interestingly, our silver nano-network is readily peeled off in water, which can be easily transferred to other substrates and devices for versatile applications. We demonstrate the feasibly transferrable silver conductive nano-network as the top electrode in organic solar cells. Consequently, the transparent and conductive silver nano-networks formed by our approach would be an excellent candidate for various applications in optoelectronics and electronics.

Polarization-independent efficiency enhancement of organic solar cells by using 3-dimensional plasmonic electrode

Plasmonic back reflectors have recently become a promising strategy for realizing efficient organic solar cell (OSCs). Since plasmonic effects are strongly sensitive to light polarization, it is highly desirable to simultaneously achieve polarization-independent response and enhanced power conversion efficiency (PCE) by designing the nanostructured geometry of plasmonic reflector electrode. Here, through a strategic analysis of 2-dimensional grating (2D) and 3-dimensional patterns (3D), with similar periodicity as a plasmonic back reflector, we find that the OSCs with 3D pattern achieve the best PCE enhancement by 24.6%, while the OSCs with 2D pattern can offer 17.5% PCE enhancement compared to the optimized control OSCs. Importantly, compared with the 2D pattern, the 3D pattern shows a polarization independent plasmonic response, which will greatly extend its uses in photovoltaic applications. This work shows the significances of carefully selecting and designing geometry of plasmonic nanostructures in a...

Recent Advances in Organic Photovoltaics: Device Structure and Optical Engineering Optimization on the Nanoscale.

Organic photovoltaic (OPV) devices, which can directly convert absorbed sunlight to electricity, are stacked thin films of tens to hundreds of nanometers. They have emerged as a promising candidate for affordable, clean, and renewable energy. In the past few years, a rapid increase has been seen in the power conversion efficiency of OPV devices toward 10% and above, through comprehensive optimizations via novel photoactive donor and acceptor materials, control of thin-film morphology on the nanoscale, device structure developments, and interfacial and optical engineering. The intrinsic problems of short exciton diffusion length and low carrier mobility in organic semiconductors creates a challenge for OPV designs for achieving optically thick and electrically thin device structures to achieve sufficient light absorption and efficient electron/hole extraction. Recent advances in the field of OPV devices are reviewed, with a focus on the progress in device architecture and optical engineering approaches that lead to improved electrical and optical characteristics in OPV devices. Successful strategies are highlighted for light wave distribution, modulation, and absorption promotion inside the active layer of OPV devices by incorporating periodic nanopatterns/nanostructures or incorporating metallic nanomaterials and nanostructures.

High Efficiency Organic Solar Cells Achieved by the Simultaneous Plasmon-Optical and Plasmon-Electrical Effects from Plasmonic Asymmetric Modes of Gold Nanostars

The plasmon-optical effects have been utilized to optically enhance active layer absorption in organic solar cells (OSCs). The exploited plasmonic resonances of metal nanomaterials are typically from the fundamental dipole/high-order modes with narrow spectral widths for regional OSC absorption improvement. The conventional broadband absorption enhancement (using plasmonic effects) needs linear-superposition of plasmonic resonances. In this work, through strategic incorporation of gold nanostars (Au NSs) in between hole transport layer (HTL) and active layer, the excited plasmonic asymmetric modes offer a new approach toward broadband enhancement. Remarkably, the improvement is explained by energy transfer of plasmonic asymmetric modes of Au NS. In more detail, after incorporation of Au NSs, the optical power in electron transport layer transfers to active layer for improving OSC absorption, which otherwise will become dissipation or leakage as the role of carrier transport layer is not for photon-absorption induced carrier generation. Moreover, Au NSs simultaneously deliver plasmon-electrical effects which shorten transport path length of the typically low-mobility holes and lengthen that of high-mobility electrons for better balanced carrier collection. Meanwhile, the resistance of HTL is reduced by Au NSs. Consequently, power conversion efficiency of 10.5% has been achieved through cooperatively plasmon-optical and plasmon-electrical effects of Au NSs.

Tuning optical responses of metallic dipole nanoantenna using graphene.

Nanoantennas play a fundamental role in the nanotechnology due to their capabilities to confine and enhance the light through converting the localized fields to propagating ones, and vice versa. Here, we theoretically propose a novel nanoantenna with the metal-insulator-graphene configuration where a graphene sheet dynamically controls the characteristics of a metallic dipole antenna in terms of near-field distribution, resonance frequency, bandwidth, radiation pattern, etc. Our results show that by modifying dispersion relation of the graphene sheet attached to the insulator through tuning chemical potentials, we can achieve strong mode couplings between the graphene sheet and the metallic nanoantenna on the top of the insulator. Interestingly, the in-phase and out-of-phase couplings between metallic plasmonics and graphene plasmonics not only split the single resonance frequency of the conventional metallic dipole antenna but also modify the near-field and far-field responses of the metal-graphene nanoantenna. This work is of a great help to design high-performance electrically-tunable nanoantennas applicable both in nano-optics and nano-electronics fields.

Plasmon-Electrical Effects on Organic Solar Cells by Incorporation of Metal Nanostructures

Recently, plasmonic effects of metal nanostructures have been intensively exploited to enhance the performance of organic solar cells (OSCs). The plasmonic-optical effects of localized surface plasmon resonances, scattering effect and propagating surface plasmon resonances, etc., are well recognized in considerably enhancing the active layer absorption. Besides, the improved electrical properties of OSCs by plasmonic-electrical effects have recently been reported to be of equal importance in enhancing the performance of OSCs through the introduction of plasmonic metal nanostructures into different layers of OSCs. In this review, the plasmonic-electrical effects of metal nanostructures-induced space-charge limit elimination, transfer of plasmonically excited hot carriers, and charge-accumulation effects would be briefly discussed. The mechanisms of each plasmonic-electrical effect including tailored transport path of charge carriers (electron and hole), reduced extraction barrier of carrier transport layer, and redistributed exciton generation in active layer for enhancing the performance of OSCs would also be studied simultaneously with the experimental and theoretical results. The promoted electrical properties of OSCs by the plasmonic-electrical effects and the future combination of plasmonic-optical and electrical effects would promisingly offer an alternative way to further improve the performance of OSCs.

Exploring the Way To Approach the Efficiency Limit of Perovskite Solar Cells by Drift-Diffusion Model

Drift-diffusion model is an indispensable modeling tool to understand the carrier dynamics (transport, recombination, and collection) and simulate practical-efficiency of solar cells (SCs) through taking into account various carrier recombination losses existing in multilayered device structures. Exploring the way to predict and approach the SC efficiency limit by using the drift-diffusion model will enable us to gain more physical insights and design guidelines for emerging photovoltaics, particularly perovskite solar cells. Our work finds out that two procedures are the prerequisites for predicting and approaching the SC efficiency limit. First, the intrinsic radiative recombination needs to be corrected after adopting optical designs which will significantly affect the open-circuit voltage at its Shockley–Queisser limit. Through considering a detailed balance between emission and absorption of semiconductor materials at the thermal equilibrium and the Boltzmann statistics at the nonequilibrium, we offe...

Improving the stability and performance of perovskite solar cells via off-the-shelf post-device ligand treatment

While metal halide perovskite solar cells (PVSCs) have drawn intense attention due to their high solar-to-power conversion efficiency (PCE), their practical application is hampered by their poor long-term stability against moisture. Although strategies have been reported to solve this issue, these methods are introduced during core-device fabrication processes which will increase the risk of introducing unexpected impurities during the fabrication. Herein, we introduce the first kind of simple post-device ligand (PDL) treatment to significantly improve the PCE of completely fabricated PVSCs from 18.7% to 20.13%. Meanwhile, the stability of the treated devices without any encapsulation remarkably improves, with 70% PCE maintained under ambient conditions after a 500-hour maximum-power-point tracking test, while the control unencapsulated device will completely break down within 100 hours. Equally important is that this post-device treatment shows a special ‘stitching effect’, namely repairing the as-fabricated ‘poor devices’ by healing the defects of the perovskite active region, and can improve the PCE by over 900%. We also experimentally and theoretically study the fundamental mechanism of the improvement. Consequently, our approach greatly improves the production yield of high-quality PVSCs and their module performances as well as the reduction of lead-waste. Additionally, the treatment is an off-the-shelf post-device approach that can be integrated into any existing perovskite-device fabrication, offering a general strategy to improve the stability and performance of perovskite optoelectronic devices.