Graded Heterojunction Research Articles

Performance analysis of gallium nitride-based DH-HEMT with polarization-graded AlGaN back-barrier layer

In this paper, polarization-graded AlGaN back-barrier nanolayer has been introduced to improve the DC and RF parameters of gallium nitride-based high electron mobility transistors (HEMT). To explore the characteristics, both graded and non-graded double heterojunction high electron mobility transistor (DH-HEMT) structures are optimized using SILVACO-ATLAS physical simulator. Enhanced DC and RF parameters have been observed in the optimized graded DH-HEMT. In this paper, we have also studied the development of the quantum wells at the AlGaN/GaN interfaces due to the conduction band discontinuity in both structures.

Graded Heterojunction Improves Wide-Bandgap Perovskite for Highly Efficient 4-Terminal Perovskite/Silicon Tandem Solar Cells.

Wide-bandgap (WBG) perovskite solar cells (PSCs) are essential for highly efficient and stable silicon/perovskite tandem solar cells. In this study, we adopted a synthetic strategy with lead thiocyanate (Pb(SCN)2) additive and methylammonium chloride (MACl) posttreatment to enhance the crystallinity and improve the interface of WBG perovskite films with a bandgap of 1.68 eV. The excessive PbI2 was formed at grain boundaries and converted into MAPbI3-xClx perovskites, which are utilized to form the graded heterojunction (GHJ) and compressive strain. This is beneficial for passivating nonradiative recombination defects, suppressing halide phase segregation, and facilitating carrier extraction. Subsequently, the device with GHJ delivered a champion efficiency of 20.30% and superior stability in ambient air and under 85 °C. Finally, we achieved a recorded efficiency of 30.91% for 4-terminal WBG perovskite/TOPCon tandem silicon solar cells. Our findings demonstrate a promising approach for fabricating efficient and stable WBG PSCs through the formation of GHJ.

Enhanced photo-response performance of Cu2O-based graded heterojunction optoelectronic devices with a Ga2O3 buffer layer

Heterojunctions with gradient energy bands confirmed via Kelvin probe force microscopy (KPFM) are confirmed to be effective in accelerating charge transport and suppressing carrier recombination in Cu2O-based-film photodetector and solar cell devices.

Numerical study of organic graded bulk heterojunction solar cell using SCAPS simulation

The graded bulk heterojunction (GBHJ) organic solar cell (OSC), with an active layer of donor-blend-acceptor structure, has recently paid much attention in developing the performance of OSCs. In this work, a numerical simulation has been performed for a GBHJ solar cell using SCAPS simulator to provide some design guidelines for this type of OSC. Firstly, the validation of the simulation is done by comparing the simulation with experimental results. Then, a comparative study is performed between the presented GBHJ with conventional bulk heterojunction (BHJ) and bi-layer solar cells. Simulation results indicate that GBHJ has the highest power conversion efficiency (PCE) of 10.15%, while BHJ has the highest collection efficiency (ζcoll) of 96.71%. The GBHJ performance is optimized by inspecting each layer individually and the interfacial trap states. It has been found that the main layer, having the most crucial impact on the performance, is the blend layer and its optimum thickness is 70 nm. Further, an improvement of the performance of GBHJ is achieved by using other candidates of non-fullerene acceptors (NFAs) with higher reported blend mobilities. A short circuit current density (Jsc) of 19.88 mA/cm2, open circuit voltage (Voc) of 0.798 V, fill factor (FF) of 78.68%, PCE of 12.49% and ζcoll of 95.12% is obtained.

Graded heterojunction of perovskite/dopant-free polymeric hole-transport layer for efficient and stable metal halide perovskite devices

Abstract Solution-processed polycrystalline perovskite films possess numerous imperfections in their surface and grain-boundary, limiting their solar cell performance and stability. To attain a full thermodynamic potential from the device along with high stability, an efficient passivation strategy that can suppress those imperfections, inducing a trap-assisted charge recombination and a defect-initiated crystal decomposition, is needed. Herein, we demonstrate a perovskite/dopant-free polymer hole-transport material (HTM) graded heterojunction (GHJ), maximizing their intermolecular interactions that can passivate under-coordinated lead cations in perovskite and immobilize its volatile organic cations by forming Lewis-adducts and hydrogen bonds. For this purpose, a series of polymer HTMs, containing defect-healable and cross-linkable functional units, are newly designed. By composing a GHJ structure, it is confirmed the perovskite crystallinity increases with reduced trap-density, enhancing built-in potential of the solar cell device and thus decreasing carrier recombination, and its heat-, water-, and light-resistibility are enhanced. Consequently, superior optoelectronic properties, providing efficiencies of 22.1% (0.096 cm2) and 20.0% (1 cm2) with a Voc of 1.22 V having only 0.37 V Voc loss, and stability, preserving 92% of the initial efficiency after 500 h of light-illumination (AM 1.5G 100 mWcm−2 without UV-cut) in ambient air without encapsulation, are attained with the GHJ n-i-p devices.

Transport properties at a sigmoidal graded heterojunction

A sigmoidal grading profile is proposed for the efficient study of the influence of material parameters and effective mass mismatch on the properties of graded heterojunctions. For the interest of accessing bound and scattering states, we have shown the exact solvability of the model on the entire real line. Focussing on transport across a compositionally graded heterojunction, we have discussed the effect of interface width and effective mass mismatch on the transmission probability.

Fusing Nanowires into Thin Films: Fabrication of Graded‐Heterojunction Perovskite Solar Cells with Enhanced Performance

AbstractPerovskite solar cells (PSCs) have recently experienced a rapid rise in power conversion efficiency (PCE), but the prevailing PSCs with conventional mesoscopic or planar device architectures still contain nonideal perovskite/hole‐transporting‐layer (HTL) interfaces, limiting further enhancement in PCE and device stability. In this work, CsPbBr3 perovskite nanowires are employed for modifying the surface electronic states of bulk perovskite thin films, forming compositionally‐graded heterojunction at the perovskite/HTL interface of PSCs. The nanowire morphology is found to be key to achieving lateral homogeneity in the perovskite film surface states resulting in a near‐ideal graded heterojunction. The hidden role of such lateral homogeneity on the performance of graded‐heterojunction PSCs is revealed for the first time. The resulting PSCs show high PCE up to 21.4%, as well as high operational stability, which is superior to control PSCs fabricated without CsPbBr3‐nanocrystals modification and with CsPbBr3‐nanocubes modification. This study demonstrates the promise of controlled hybridization of perovskite nanowires and bulk thin films for more efficient and stable PSCs.

High Efficient Hole Extraction and Stable All‐Bromide Inorganic Perovskite Solar Cells via Derivative‐Phase Gradient Bandgap Architecture

Inorganic perovskite materials have demonstrated outstanding performance in the field of photovoltaic devices due to their superior charge carrier transport properties and excellent thermal stability. In particular, the inorganic perovskite derivative phases show special properties in terms of phase stability and optoelectronic application, especially in the phase transition investigation. However, their commercial applications still face challenges due to the large recombination at the interface, resulting in poor efficiency and metastable phases such as iodide perovskite existing in the film. Herein, an all‐bromide inorganic perovskite solar cell has been developed by introducing the derivative phases (CsPb2Br5 and Cs4PbBr6) to construct gradient bandgap architecture. This graded heterojunction device is realized with a controllable sequential vapor deposition procedure. The valance band maximum elevates gradually with the presence of derivative phases and effectively blocks electrons and boosts the hole extraction efficiency at the counter electrode, which promotes charge separation and reduces the interface recombination. Ultimately, an impressive power conversion efficiency of 10.17% is achieved through a CsPbBr3/CsPbBr3‐CsPb2Br5/CsPbBr3‐Cs4PbBr6 architecture strategy with excellent stability above 3000 h (85% of initial performance) in a humid environment (@RH ≈45%) and 700 h (83% of initial efficiency) under thermal conditions (@ 100 °C).

Facile Fabrication of SnO2 Nanorod Arrays Films as Electron Transporting Layer for Perovskite Solar Cells

As the vital competent in high performance perovskite solar cells (PSCs), the inorganic metal oxide electron transporting materials (ETMs) with tailored architectures are responsible for extracting and transporting photogenerated electrons and preventing the recombination effectively. Here, a facile hydrothermal method is demonstrated for the growth of highly crystalline and uniform 1D SnO2 nanorod arrays (SR) with excellent optical property, which are further successfully exploited as ETMs in PSCs. Both length and diameter of 1D SR are tuned carefully by optimizing the hydrothermal process. Power conversion efficiency (PCE) as high as 16.7% can be obtained for PSCs based on these SR. Furthermore, a graded heterojunction (GHJ) configuration was built by intercalating a TiO2 thin interlayer to improve the band alignment between perovskite and SR, which pushes the PCE to 18.7% with better ambient stability. This work demonstrates that the SR can act as a promising candidate for ETM in PSCs, and provides a workable stategy to improve the performance of PSCs.

Highly efficient perovskite solar cells fabricated by simplified one-step deposition method with non-halogenated anti-solvents

Abstract Organic-inorganic hybrid halide perovskite solar cells (PVSCs) are promising low-cost photovoltaic technologies with their impressive power conversion efficiency (PCE) and low temperature solution processability. A large amount of toxic solvents has been used in the fabrication of PVSCs, especially the halogenated solvent chlorobenzene (CB) which is heavily applied as the anti-solvent of the perovskite layer and the solvent of electron transport layer (ETL) in p-i-n type PVSCs. Herein, we replace CB with non-halogenated solvents, o-xylene (OXY) and 1,2,4-trimethylbenzene (TMB), and introduce perovskite-fullerene graded heterojunction (GHJ) to engineer efficient perovskite layer and ETL. By merging two deposition processes of perovskite layer and ETL, high efficiencies of 16.82% and 16.88% are obtained for PVSCs with OXY and TMB, respectively. The PVSCs prepared from non-halogenated solvents engineering demonstrate superior performance over their CB counterparts of which the PCE is 14.45%, mainly due to much larger grains, improved electron collection and reduced recombination loss in the former PVSCs. These results indicate both OXY and TMB are potential alternative non-halogenated solvents for PVSCs manufacturing. Moreover, since two separate deposition processes are merged, the process becomes easier and simpler, leading to reduction of fabrication costs.

Heterojunction Engineering for High Efficiency Cesium Formamidinium Double-Cation Lead Halide Perovskite Solar Cells.

It is essential to minimize the interfacial trap states and improve the carrier collection for high efficiency perovskite solar cells (PSCs). Herein, we present a facile method to construct a p-type graded heterojunction (GHJ) in normal PSCs by deploying a gradient distribution of hole-transporting materials (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], PTAA, in this case) in the shallow perovskite layer. The formation of the GHJ structure facilitates charge transfer and collection, and passivates interfacial trap states, thus delivering a power conversion efficiency (PCE) of 20.05 % along with steady output efficiency of 19.3 %, which is among the highest efficiencies for cesium formamidinium (Cs-FA) lead halide PSCs. Moreover, the unencapsulated devices based on these (Cs-FA) lead halide perovskites show excellent long-term stability; more than 95 % of their initial PCE can be retained after 1440 h storage under ambient conditions. This study may provide an effective strategy to fabricate high-efficiency PSCs with great stability.

Defect Passivation via a Graded Fullerene Heterojunction in Low-Bandgap Pb–Sn Binary Perovskite Photovoltaics

Development of low-bandgap (∼1.2 eV) Pb–Sn binary perovskites is exciting and has recently gained immense attention because of their high photovoltages, lowered Pb toxicity, and pivotal role in realizing perovskite tandem solar cells. Defect passivation in this class of perovskite alloys has immense potential to further reduce the photovoltage deficit but is relatively unexplored. Here, we investigate and report the passivation of defect sites in low-bandgap CH3NH3Pb0.5Sn0.5I3 perovskite through the incorporation of fluoroalkyl-substituted fullerene (DF-C60) via a graded heterojunction (GHJ) structure. Graded distribution of DF-C60 successfully reduced the number of trap sites, and the resultant films had characteristically lower Urbach energy, dominant bimolecular recombination, and higher surface/bulk recombination resistance. The improved optoelectronic quality of films with GHJ structure was reflected in improved performance for corresponding photovoltaic devices, with the best PCE up to 15.61% and a ...

Drive current enhancement in tunnel field-effect transistors by graded heterojunction approach

The heterostructure technique has recently demonstrated an excellent solution to resolve the trade-off between on- and off-state currents in tunnel field-effect transistors (TFETs). This paper shows the weakness of abrupt heterojunctions and explores the physics of drive current enhancement as well as generalizes the proposed graded heterojunction approach in both n-type and p-type TFETs. It is shown that the presence of thermal emission barriers formed by abrupt band offsets is the physical reason of the on-current lowering observed in abrupt heterojunction TFETs. By employing graded heterojunctions in TFETs, the thermal emission barriers for electrons and holes are completely eliminated to narrow the tunnel widths in n-type and p-type TFETs, respectively. With the significant improvement in on-current, this novel approach of graded heterojunctions provides an effective technique for enhancing the drive current in heterostructure-based TFET devices.

Exploiting piezoelectric charge for high performance graded InGaN nanowire solar cells

The effect of piezoelectric charge on the performance of p-GaN/n-InGaN abrupt and graded heterojunction nanowire solar cells is investigated by numerical simulation. In abrupt junctions, piezoelectric charge increases the energy barrier height for hole transport into GaN, resulting in poor overall efficiency. Incorporation of a linearly graded junction improves the performance by removing the valence band barrier. Grading distributes the strain field over a wider region than in an abrupt junction. Both spontaneous and piezoelectric charge boost the efficiency by generating polarization-induced p-type conductivity. A maximum overall efficiency of ∼21% is predicted considering the effect of polarization charge.

Efficient, bulk heterojunction organic photovoltaic cells based on boron subphthalocyanine chloride-C70

In this work, we describe the performance of organic photovoltaic cells (OPVs) based on the electron donor-acceptor pairing of boron subphthalocyanine chloride (SubPc) and C70. As an acceptor, C70 is a promising alternative to C60 due to its red-shifted absorption edge and large extinction coefficient, permitting increased photocurrent generation. In uniformly mixed OPVs based on SubPc:C70, peak power conversion efficiency is realized in devices containing 80 wt. % C70. Further enhancement in device performance can be realized through the use of a continuously graded donor-acceptor heterojunction. The optimized graded heterojunction OPV shows a power conversion efficiency of ηP = (5.4 ± 0.2)% under simulated AM1.5 G solar illumination at 100 mW/cm2. This efficiency is significantly larger than the value of ηP = (4.5 ± 0.1)% realized using C60 due to a substantial increase in the short-circuit current density.

Design of flat-band AlGaAs heterojunction Bragg reflectors

Bragg reflectors using periodic AlGaAs heterojunction layers can be built into a light emitting diode to reduce the absorption in the substrate, or into a laser diode to form a cavity. In this paper, a numerical model for the design of low resistance flat-band p-type or n-type graded heterojunction reflectors is presented. It computes the required doping profile to achieve a flat conduction band or valence band across a graded junction, such that the reflector will add minimum series resistance to the devices.

Effect of recombination current on current gain of HBTs

In order to predict more accurately the current gain behaviour of HBTs at low collector current region, surface recombination and interface recombination have to be considered. Recombination current has been obtained by the combination of bulk recombination in the neutral base, interface recombination at the heterointerface, space charge recombination, and surface recombination components. Results from the present model have been compared with experimental results, and the most recent analytical models for abrupt and graded heterojunction bipolar transistors (HBTs). The study shows that this model can predict the device gain more accurately.

A Gummel-Poon model for abrupt and graded heterojunction bipolar transistors (HBTs)

A Gummel-Poon model for abrupt and graded GaAlAs/GaAs/GaAs heterojunction bipolar transistors (HBTs) is developed. The effect of carrier recombination at the emitter-base heterojunction, space charge region (SCR) width modulation effect, and base-widening effect at large collector currents have been considered. Results from this model are compared with numerical results, experimental results, and results from the most recent analytical models. The results show that the common-emitter current gain behavior in the low collector current region can be predicted more accurately by this model, and that interface and surface recombination affect the current gain more dominantly than the other recombination processes. Dependence of cutoff frequency on collector current obtained from the present model agrees well with the experimental results. This model can also predict both current gain and cutoff frequency falloffs at large collector current. This model can be easily implemented in the SPICE program.

AlGaAs/GaAs bipolar transistors with a modulation-doped superlattice emitter

A new emitter-base concept for heterojunction bipolar transistors is proposed and demonstrated. The abrupt or graded emitter-base heterojunction drawbacks are circumvented by means of a modulation-doped stack of ternary compound alternating with binary compound layers. This stack provides efficient barriers to the hole current while preserving good diode characteristics. These concepts are demonstrated by experimental results on small-area devices with a common-emitter gain of ~40 and a collector-emitter voltage offset of ~80mV.

Negative differential conductivity of a reverse-biased graded heterojunction

The reverse current of a P +- n junction with nonmonotonic variation of the band-gap has been theoretically investigated. As a result of the influence of the “quasielectric” fields and spatial variation of the minority carrier density, the voltage-current characteristic (VCC) can be substantially modified. In particular, for reasonable parameters, when the reverse current is dominated by the diffusion component, the structure can exhibit negative differential conductivity.