Nripan Mathews

Low-temperature solution-processed wavelength-tunable perovskites for lasing

Low-temperature solution-processed materials that show optical gain and can be embedded into a wide range of cavity resonators are attractive for the realization of on-chip coherent light sources. Organic semiconductors and colloidal quantum dots are considered the main candidates for this application. However, stumbling blocks in organic lasing include intrinsic losses from bimolecular annihilation and the conflicting requirements of high charge carrier mobility and large stimulated emission; whereas challenges pertaining to Auger losses and charge transport in quantum dots still remain. Herein, we reveal that solution-processed organic-inorganic halide perovskites (CH3NH3PbX3 where X = Cl, Br, I), which demonstrated huge potential in photovoltaics, also have promising optical gain. Their ultra-stable amplified spontaneous emission at strikingly low thresholds stems from their large absorption coefficients, ultralow bulk defect densities and slow Auger recombination. Straightforward visible spectral tunability (390-790 nm) is demonstrated. Importantly, in view of their balanced ambipolar charge transport characteristics, these materials may show electrically driven lasing.

The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells

This work reports a study into the origin of the high efficiency in solution-processable bilayer solar cells based on methylammonium lead iodide (CH3NH3PbI3) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Our cell has a power conversion efficiency (PCE) of 5.2% under simulated AM 1.5G irradiation (100 mW cm−2) and an internal quantum efficiency of close to 100%, which means that nearly all the absorbed photons are converted to electrons and are efficiently collected at the electrodes. This implies that the exciton diffusion, charge transfer and charge collection are highly efficient. The high exciton diffusion efficiency is enabled by the long diffusion length of CH3NH3PbI3 relative to its thickness. Furthermore, the low exciton binding energy of CH3NH3PbI3 implies that exciton splitting at the CH3NH3PbI3/PC61BM interface is very efficient. With further increase in CH3NH3PbI3 thickness, a higher PCE of 7.4% could be obtained. This is the highest efficiency attained for low temperature solution-processable bilayer solar cells to date.

Advancements in perovskite solar cells : photophysics behind the photovoltaics

Solution-processed organic–inorganic perovskite solar cells are hailed as the recent major breakthrough in low-cost photovoltaics. Power conversion efficiencies approaching those of crystalline Si solar cells (exceeding 15%) have been reported. Remarkably, such phenomenal performances were achieved in a matter of 5 years – up from ∼3.8% back in 2009. Since then, the field has expanded exponentially. In this perspective, we review the basic working mechanisms of perovskite solar cells in relation to their intrinsic properties and fundamental photophysics. The current state-of-the-art and the open questions in this maturing field are also highlighted.

Flexible, low-temperature, solution processed ZnO-based perovskite solid state solar cells

A ZnO compact layer formed by electrodeposition and ZnO nanorods grown by chemical bath deposition (CBD) allow the processing of low-temperature, solution based and flexible solid state perovskite CH3NH3PbI3 solar cells. Conversion efficiencies of 8.90% were achieved on rigid substrates while the flexible ones yielded 2.62%.

Perovskite Materials for Light-Emitting Diodes and Lasers

Organic-inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light-emitting diodes, lasers, and light-emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light-emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A(-1) have been achieved. Although perovskite light-emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light-emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower-dimensionality layered and three-dimensional perovskites, nanostructures, charge-transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light-emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light-emitting devices.

Lead-Free Halide Perovskite Solar Cells with High Photocurrents Realized Through Vacancy Modulation

Lead free perovskite solar cells based on a CsSnI3 light absorber with a spectral response from 950 nm is demonstrated. The high photocurrents noted in the system are a consequence of SnF2 addition which reduces defect concentrations and hence the background charge carrier density.

Laminated Carbon Nanotube Networks for Metal Electrode-Free Efficient Perovskite Solar Cells

Organic-inorganic metal halide perovskite solar cells were fabricated by laminating films of a carbon nanotube (CNT) network onto a CH3NH3PbI3 substrate as a hole collector, bypassing the energy-consuming vacuum process of metal deposition. In the absence of an organic hole-transporting material and metal contact, CH3NH3PbI3 and CNTs formed a solar cell with an efficiency of up to 6.87%. The CH3NH3PbI3/CNTs solar cells were semitransparent and showed photovoltaic output with dual side illuminations due to the transparency of the CNT electrode. Adding spiro-OMeTAD to the CNT network forms a composite electrode that improved the efficiency to 9.90% due to the enhanced hole extraction and reduced recombination in solar cells. The interfacial charge transfer and transport in solar cells were investigated through photoluminescence and impedance measurements. The flexible and transparent CNT network film shows great potential for realizing flexible and semitransparent perovskite solar cells.

Enhancement in the Performance of Ultrathin Hematite Photoanode for Water Splitting by an Oxide Underlayer

A 2-nm thick Nb(2)O(5) underlayer deposited by atomic layer deposition increases the charge separation efficiency and the photovoltage of ultrathin hematite films by suppressing electron back injection. Absorbed photon-to-current efficiencies (APCE) as high as 40%, which are one of the highest ever reported with hematite photoanodes, are obtained at 400 nm at +1.43 V vs. RHE.

Band-gap tuning of lead halide perovskites using a sequential deposition process

Band-gap tuning of mixed anion lead halide perovskites (MAPb(I1−xBrx)2 (0 ≤ x ≤ 1)) has been demonstrated by means of a sequential deposition process. The optical properties of perovskite hybrids can be flexibly modified by changing (mixing) the concentration of halogen precursors. The concentrations of precursor solution as well as the conversion time play an important role in determining the band-gap of perovskites. A systematic shift of the absorption band edge to shorter wavelengths is observed with increasing Br content in the perovskite films, which results in the decrement of the photocurrent. Nanorod like morphological features are also observed for perovskite films with an iodide to bromide molar ratio of <0.7.

Inorganic Halide Perovskites for Efficient Light-Emitting Diodes

Lead-halide perovskites have transcended photovoltaics. Perovskite light-emitting diodes (PeLEDs) emerge as a new field to leverage on these fascinating semiconductors. Here, we report the first use of completely inorganic CsPbBr3 thin films for enhanced light emission through controlled modulation of the trap density by varying the CsBr-PbBr2 precursor concentration. Although pure CsPbBr3 films can be deposited from equimolar CsBr-PbBr2 and CsBr-rich solutions, strikingly narrow emission line (17 nm), accompanied by elongated radiative lifetimes (3.9 ns) and increased photoluminescence quantum yield (16%), was achieved with the latter. This is translated into the enhanced performance of the resulting PeLED devices, with lower turn-on voltage (3 V), narrow electroluminescence spectra (18 nm) and higher electroluminescence intensity (407 Cd/m(2)) achieved from the CsBr-rich solutions.