Recombination Layer Research Articles

Imbedded Nanocrystals of CsPbBr3 in Cs4 PbBr6 : Kinetics, Enhanced Oscillator Strength, and Application in Light-Emitting Diodes.

Solution-grown films of CsPbBr3 nanocrystals imbedded in Cs4 PbBr6 are incorporated as the recombination layer in light-emitting diode (LED) structures. The kinetics at high carrier density of pure (extended) CsPbBr3 and the nanoinclusion composite are measured and analyzed, indicating second-order kinetics in extended and mainly first-order kinetics in the confined CsPbBr3 , respectively. Analysis of absorption strength of this all-perovskite, all-inorganic imbedded nanocrystal composite relative to pure CsPbBr3 indicates enhanced oscillator strength consistent with earlier published attribution of the sub-nanosecond exciton radiative lifetime in nanoprecipitates of CsPbBr3 in melt-grown CsBr host crystals and CsPbBr3 evaporated films.

Improved Optics in Monolithic Perovskite/Silicon Tandem Solar Cells with a Nanocrystalline Silicon Recombination Junction

AbstractPerovskite/silicon tandem solar cells are increasingly recognized as promi­sing candidates for next‐generation photovoltaics with performance beyond the single‐junction limit at potentially low production costs. Current designs for monolithic tandems rely on transparent conductive oxides as an intermediate recombination layer, which lead to optical losses and reduced shunt resistance. An improved recombination junction based on nanocrystalline silicon layers to mitigate these losses is demonstrated. When employed in monolithic perovskite/silicon heterojunction tandem cells with a planar front side, this junction is found to increase the bottom cell photocurrent by more than 1 mA cm−2. In combination with a cesium‐based perovskite top cell, this leads to tandem cell power‐conversion efficiencies of up to 22.7% obtained from J–V measurements and steady‐state efficiencies of up to 22.0% during maximum power point tracking. Thanks to its low lateral conductivity, the nanocrystalline silicon recombination junction enables upscaling of monolithic perovskite/silicon heterojunction tandem cells, resulting in a 12.96 cm2 monolithic tandem cell with a steady‐state efficiency of 18%.

Critical Interface States Controlling Rectification of Ultrathin NiO-ZnO p-n Heterojunctions.

Herein, we consider the heterojunction formation of two prototypical metal oxides: p-type NiO and n-type ZnO. Elementally abundant, low-cost metal oxide/oxide' heterojunctions are of interest for UV optical sensing, gas sensing, photocatalysis, charge confinement layers, piezoelectric nanogenerators, and flash memory devices. These heterojunctions can also be used as current rectifiers and potentially as recombination layers in tandem photovoltaic stacks by making the two oxide layers ultrathin. In the ultrathin geometry, understanding and control of interface electronic structure and chemical reactions at the oxide/oxide' interface are critical to functionality, as oxygen atoms are shared at the interface of the dissimilar materials. In the studies presented here the extent of chemical reactions and interface band bending is monitored using X-ray and ultraviolet photoelectron spectroscopies. Interface reactivity is controlled by varying the near surface composition of nickel oxide, nickel hydroxide, and nickel oxyhydroxide using standard surface-treatment procedures. A direct correlation between relative percentage of interface hydroxyl chemistry (and hence surface Lewis basicity) and the local band edge alignment for ultrathin p-n junctions (6 nm NiO/30 nm ZnO) is observed. We propose an acid-base formulism to explain these results: the stronger the acid-base reaction, the greater the fraction of interfacial electronic states which lower the band offset between the ZnO conduction band and the NiO valence band. Increased interfacial gap states result in larger reverse bias current of the p-n junction and lower rectification ratios. The acid-base formulism could serve as a future design principle for oxide/oxide' and other heterojunctions based on dissimilar materials.

Effect of nonequilibrium near-electrode layers on the structure of EHD flows in the three-ions model of a dielectric liquid

An electrohydrodynamic (EHD) flow is a spontaneous flow of a liquid in the electrode gap under the action of a strong electric field. Most experimental data from an investigation of the velocity field of EHD flows were obtained in the wire-over-plane electrode configuration. For this system, the flow can be treated as a 2D flow. We report on the results of a computer simulation of the complete system of electrohydrodynamics equations in the three-ion model of a dielectric liquid. The structure of nonequilibrium dissociation–recombination layers and their effect on the structure of EHD flows have been analyzed based on the results of the computer simulation of EHD flows in liquids with different low-voltage conductivities for the wireover- plane electrode system.

Stacking Sequence and Acceptor Dependence of Photocurrent Spectra and Photovoltage in Organic Two-Junction Devices.

Both single-junction and tandem organic photovoltaic cells have been well developed. A tandem cell contains two junctions with a charge recombination layer (CRL) inserted between the two junctions. So far, there is no detailed report on how the device will perform that contains two junctions but without a CRL in between. In this work, we report the photocurrent spectra and photovoltage output of the devices that contains two bulk-heterojunctions (BHJ) stacked directly on top of each other without a CRL. The top active layer is prepared by transfer printing. The photocurrent response spectra and photovoltage are found to be sensitive to stacking sequence and the selection of electron acceptors. The open-circuit voltage of the devices (up to 1.09 V) can be higher than the devices containing either junction layer. The new phenomenon in the new device architecture increases the versatility of the optoelectronic devices based on organic semiconductors.

Perovskite Tandem Solar Cells

The meteoric rise of perovskite single‐junction solar cells has been accompanied by similar stunning developments in perovskite tandem solar cells. Debuting with efficiencies less than 14% in 2014, silicon–perovskite solar cells are now above 25% and will soon surpass record silicon single‐junction efficiencies. Unconstrained by the Shockley–Quiesser single‐junction limit, perovskite tandems suggest a real possibility of true third‐generation thin‐film photovoltaics; monolithic all‐perovskite tandems have reached 18% efficiency and will likely pass perovskite single‐junction efficiencies within the next 5 years. Inorganic–organic metal–halide perovskites are ideal candidates for inclusion in tandem solar cells due to their high radiative recombination efficiencies, excellent absorption, long‐range charge‐transport, and broad ability to tune the bandgap. In this progress report, the development of perovskite tandem cells is reviewed, with presentation of their key motivations and challenges. In detail, it presents an overview of recombination layer materials, bandgap‐tuneability, transparent contact architectures, and perovskite compounds for use in tandems. Theoretical estimates of efficiency for future tandem and triple‐junction perovskite cells are presented, outlining roadmaps for future focused research.

Stable and Highly Efficient PbS Quantum Dot Tandem Solar Cells Employing a Rationally Designed Recombination Layer

This study reports the fabrication of stable, high‐performance, simple structured tandem solar cells based on PbS colloidal quantum dots (CQDs) under ambient air. This study also reveals detailed device engineering to deposit each functional layer in the subcells at low temperature to avoid damage to the PbS CQDs and meanwhile makes the fabrication process compatible to flexible plastic substrate. Two efficient recombination layers (RLs) are rationally designed to connect the two subcells in series. The use of solution‐processed RL with an organic PEDOT:PSS (poly(3,4‐ethylenedioxythiophene): polystyrene sulfonate) interlayer leads to the fabrication of the tandem devices in solution process. The use of robust inorganic RL containing an ultrathin Au interlayer results in more efficient device performance and remarkably improved device lifetime. The optimal PbS CQDs tandem cells based on inorganic RL demonstrate a high power conversion efficiency approaching 9%. This efficiency is more than two times higher than the previous record of 4.2%, which has been kept for more than five years. The remarkable stability, high performance, and low‐temperature processing of these tandem devices may provide insight into the commercialization of flexible and large‐area CQDs tandem solar cells in the near future.

Basal Plane Dislocation Free Recombination Layers on Low-Doped Buffer Layer for Power Devices

We report a novel approach to grow BPD-free 4H-SiC device-ready epilayers, where we start by growing a thin low-doped buffer layer (5 × 1015 to 1 × 1016 cm–3, N-type) to achieve 100% BPD conversion, followed by a moderately thick (∼10 μm) higher-doped recombination layer (5 × 1016 to 1.6 × 1017 cm–3, N-type) to ensure that all recombination occurs within a BPD-free region. High doping of the BPD-free recombination layer ensures fast carrier recombination under forward bias, preventing any stacking fault nucleation in the active layer during bipolar device operation. All the individual BPDs in the buffer epilayer are converted to benign threading edge dislocations (TEDs) over a wide range of C/Si ratios (1 to 1.8), introducing a minimal on-resistance of <0.5 mΩ-cm2. 100% BPD conversion occurs due to the controlled and highly anisotropic eutectic etching of the buffer layer which produces narrow sector angle (5°) for the BPD etch pits to enable conversion of the BPDs within ∼1.5 μm of epilayer growth into T...

Solution-Processed Conducting Polymer/Metal Oxide Charge Generation Layer: Preparation, Electrical Properties, and Charge Generation Mechanism

Conducting polymer/metal oxide heterostructures have been commonly used in solution-processed tandem organic optoelectronic devices as charge generation layer (CGL) or charge recombination layer. However, the underlying working mechanism remains unexplored. We optimize the preparation of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/zinc oxide (ZnO) CGL and report that the configurations and compositions of the interconnects significantly affect the performance of light-emitting devices. Two-unit and in particular the first reported three-unit solution-processed tandem polymer light-emitting devices show the luminance efficiency matching the total luminance efficiency of the constituent two and three light-emitting units, respectively. Current–voltage and capacitance–voltage measurements on the devices with various interconnects indicate that charges are generated at the PEDOT:PSS/ZnO interface. CGL-generated current can be described with the Richardson–Schottky thermal emission mode...

An All-Solution Processed Recombination Layer with Mild Post-Treatment Enabling Efficient Homo-Tandem Non-fullerene Organic Solar Cells.

The first homo-tandem non-fullerene organic solar cell enabled by a novel recombination layer which only requires a very mild thermal annealing treatment is reported. The best efficiency achieved is 10.8% with a Voc over 2.1 V, which is the highest Voc for double-junction organic solar cells reported to date.

Zinc tin oxide as high-temperature stable recombination layer for mesoscopic perovskite/silicon monolithic tandem solar cells

Perovskite/crystalline silicon tandem solar cells have the potential to reach efficiencies beyond those of silicon single-junction record devices. However, the high-temperature process of 500 °C needed for state-of-the-art mesoscopic perovskite cells has, so far, been limiting their implementation in monolithic tandem devices. Here, we demonstrate the applicability of zinc tin oxide as a recombination layer and show its electrical and optical stability at temperatures up to 500 °C. To prove the concept, we fabricate monolithic tandem cells with mesoscopic top cell with up to 16% efficiency. We then investigate the effect of zinc tin oxide layer thickness variation, showing a strong influence on the optical interference pattern within the tandem device. Finally, we discuss the perspective of mesoscopic perovskite cells for high-efficiency monolithic tandem solar cells.

Solution-processed colloidal quantum dot/organic hybrid tandem photovoltaic devices with 8.3% efficiency

High-efficiency colloidal quantum dot (CQD)/organic hybrid tandem photovoltaic devices (HT-PVs) are developed through a low-temperature solution process. PbS-CQDs and blends of PTB7-Th/PC71BM are used as the active materials for front- and back-cells, respectively. The effective series connection of the two heterogeneous sub-cells is successfully achieved using MoOx/Au/ZnO-based intermediate recombination layers (IMLs). By fine-tuning the current densities in the sub-cells and optimizing the IMLs by balancing the optical and electrical properties, HT-PVs with power conversion efficiency (PCE) of 8.27% (VOC of 1.27V, JSC of 10.36mAcm−2, and FF of 0.63) are achieved. Furthermore, the fabrication of HT-PVs is achievable in ambient atmospheric conditions without thermal annealing that have comparable PCE (7.64%). The PCEs exhibited by these HT-PVs are thus far the highest values reported for CQD-based tandem photovoltaic devices.

Nonequilibrium mechanisms of weak electrolyte electrification under the action of constant voltage

The formation of space charge in weak electrolytes, specifically in liquid dielectrics, has been considered. An analytical solution is given to a simplified set of Nernst–Planck equations that describe the formation of nonequilibrium recombination layers in weak electrolytes. This approximate analytical solution is compared with computer simulation data for a complete set of Poisson–Nernst–Planck equations. It has been shown that the current passage in weak electrolytes can be described by a single dimensionless parameter that equals the length of a near-electrode recombination layer divided by the width of the interelectrode gap. The formation mechanism and the structure of charged nonequilibrium near-electrode layers in the nonstationary regime have been analyzed for different injection-to-conduction current ratios. It has been found that almost all charge structures encountered in weak dielectrics can be accounted for by the nonequilibrium dissociation–recombination mechanism of space charge formation.

A Hybrid Tandem Solar Cell Combining a Dye-Sensitized and a Polymer Solar Cell.

A hybrid tandem solar cell was assambled by connecting a dye sensitized solar cell and a polymer solar cell in series. A N719 sensitized TiO2 was used as photocathode in dye-sensitized subcell, and a MEH-PPV/PCBM composite was used as active layer in the polymer subcell. The polymer subcell fabricated on the counter electrode of the dye sensitized solar cell. A solution processed TiO(x) layer was used as electron collection layer of the polymer sub cell and the charge recombination layer. The effects of the TiO(x) interlayer and the spectral overlap between the two sub cells have been studied and optimized. The results shows that a proper thickness of the TiO(x) layer is needed for tandem solar cells. Thick TiO(x) will enhance the series resistance, but too thin TiO(x), layer will damage the hole blocking effect and its hydrophilic. The resulting optimized tandem solar cells exhibited a power conversion efficiency of 1.28% with a V(oc) of 0.95 V under simulated 100 mW cm(-2) AM 1.5 illumination.

Substrate configuration, bifacial CdTe solar cells grown directly on transparent single wall carbon nanotube back contacts

Substrate configuration CdTe/CdS solar cells were fabricated directly on 25nm thick single wall carbon nanotube (SWCNT) back contacts using a high temperature, commercial deposition process. No copper was introduced to facilitate the formation of an ohmic back contact at the back of the device. The champion device had an efficiency of 6.5%. The thin SWCNT layer allowed the device to be illuminated through the back contact, and a comparison of the device performance with front and backside illumination revealed several novel features of the SWCNT back contact. Our findings indicate that SWCNT films may be useful as a recombination layer contact for bifacial solar cells, photovoltaic windows, or two-terminal thin-film tandem solar cells.

Over 11% Efficiency in Tandem Polymer Solar Cells Featured by a Low-Band-Gap Polymer with Fine-Tuned Properties.

Highly efficient polymer solar cells with tandem structure are fabricated by using two excellent photovoltaic polymers and a highly transparent intermediate recombination layer. Power conversion efficiencies over 11% can be realized featured by a low-band-gap polymer with fine-tuned properties.

High efficiency all-polymer tandem solar cells.

In this work, we have reported for the first time an efficient all-polymer tandem cell using identical sub-cells based on P2F-DO:N2200. A high power conversion efficiency (PCE) of 6.70% was achieved, which is among the highest efficiencies for all polymer solar cells and 43% larger than the PCE of single junction cell. The largely improved device performance can be mainly attributed to the enhanced absorption of tandem cell. Meanwhile, the carrier collection in device remains efficient by optimizing the recombination layer and sub-cell film thickness. Thus tandem structure can become an easy approach to effectively boost the performance of current all polymer solar cells.

High Efficiency Tandem Thin-Perovskite/Polymer Solar Cells with a Graded Recombination Layer.

Perovskite-containing tandem solar cells are attracting attention for their potential to achieve high efficiencies. We demonstrate a series connection of a ∼ 90 nm thick perovskite front subcell and a ∼ 100 nm thick polymer:fullerene blend back subcell that benefits from an efficient graded recombination layer containing a zwitterionic fullerene, silver (Ag), and molybdenum trioxide (MoO3). This methodology eliminates the adverse effects of thermal annealing or chemical treatment that occurs during perovskite fabrication on polymer-based front subcells. The record tandem perovskite/polymer solar cell efficiency of 16.0%, with low hysteresis, is 75% greater than that of the corresponding ∼ 90 nm thick perovskite single-junction device and 65% greater than that of the polymer single-junction device. The high efficiency of this hybrid tandem device, achieved using only a ∼ 90 nm thick perovskite layer, provides an opportunity to substantially reduce the lead content in the device, while maintaining the high performance derived from perovskites.

Long-Term Stable Recombination Layer for Tandem Polymer Solar Cells Using Self-Doped Conducting Polymers.

PSS) as a p-type component of recombination layer (RL). However, its undesirable acidic nature, originating from insulating PSS, of PSS drastically reduces the lifetime of PSCs. Here, we demonstrate the efficient and stable tandem PSCs by introducing acid-free self-doped conducting polymer (SCP), combined with zinc oxide nanoparticles (ZnO NPs), as RL for PSS-free tandem PSCs. Moreover, we introduce an innovative and versatile nanocomposite system containing photoactive and p-type conjugated polyelectrolyte (p-CPE) into the tandem fabrication of an ideal self-organized recombination layer. In our new RL, highly conductive SCP facilitates charge transport and recombination process, and p-CPE helps to achieve nearly loss-free charge collection by increasing effective work function of indium tin oxide (ITO) and SCP. Because of the synergistic effect of extremely low electrical resistance, ohmic contact, and pH neutrality, tandem devices with our novel RL performed well, exhibiting a high power conversion efficiency of 10.2% and a prolonged lifetime. These findings provide a new insight for strategic design of RLs using SCPs to achieve efficient and stable tandem PSCs and enable us to review and extend the usefulness of SCPs in various electronics research fields.

Charge transport dependent high open circuit voltage tandem organic photovoltaic cells with low temperature deposited HATCN-based charge recombination layers.

Mechanisms of charge transport between the interconnector and its neighboring layers in tandem organic photovoltaic cells have been systematically investigated by studying electronic properties of the involving interfaces with photoelectron spectroscopies and performance of the corresponding devices. The results show that charge recombination occurs at HATCN and its neighboring hole transport layers which can be deposited at low temperature. The hole transport layer plays an equal role to the interconnector itself. These insights provide guidance for the identification of new materials and the device architecture for high performance devices.