Device simulation of lead-free CH_3NH_3SnI_3 perovskite solar cells with high efficiency

The lead-free perovskite solar cells(PSCs) have drawn a great deal of research interest due to the Pb toxicity of the lead halide perovskite.CHNHSnIis a viable alternative to CHNHPbX,because it has a narrower band gap of 1.3 eV and a wider visible absorption spectrum than the lead halide perovskite.The progress of fabricating tin iodide PSCs with good stability has stimulated the studies of these CHNHSnIbased cells greatly.In the paper,we study the influences of various parameters on the solar cell performance through theoretical analysis and device simulation.It is found in the simulation that the solar cell performance can be improved to some extent by adjusting the doping concentration of the perovskite absorption layer and the electron affinity of the buffer and HTM,while the reduction of the defect density of the perovskite absorption layer significantly improves the cell performance.By further optimizing the parameters of the doping concentration(1.3 × 10cm~3) and the defect density(1 × 10cm~3) of perovskite absorption layer,and the electron affinity of buffer(4.0 eV) and HTM(2.6 eV),we finally obtain some encouraging results of the Jof 31.59 mA/cm~2,Vof 0.92 V,FF of 79.99%,and PCE of 23.36%.The results show that the lead-free CHNHSnIPSC is a potential environmentally friendly solar cell with high efficiency.Improving the Snstability and reducing the defect density of CHNHSnIare key issues for the future research,which can be solved by improving the fabrication and encapsulation process of the cell.

Device simulation of quasi-two-dimensional perovskite/silicon tandem solar cells towards 30%-efficiency

Perovskite/silicon(Si) tandem solar cells have been recognized as the next-generation photovoltaic technology with efficiency over 30% and low cost. However, the intrinsic instability of traditional three-dimensional(3D) hybrid perovskite seriously hinders the lifetimes of tandem devices. In this work, the quasi-two-dimensional(2D)(BA)_(2)(MA)_(n-1)Pbn I_(3n+1)(n = 1, 2, 3, 4, 5)(where MA denotes methylammonium and BA represents butylammonium), with senior stability and wider bandgap, are first used as an absorber of semitransparent top perovskite solar cells(PSCs) to construct a fourterminal(4T) tandem devices with a bottom Si-heterojunction cell. The device model is established by Silvaco Atlas based on experimental parameters. Simulation results show that in the optimized tandem device, the top cell(n = 4) obtains a power conversion efficiency(PCE) of 17.39% and the Si bottom cell shows a PCE of 11.44%, thus an overall PCE of 28.83%. Furthermore, by introducing a 90-nm lithium fluoride(LiF) anti-reflection layer to reduce the surface reflection loss, the current density(J_(sc)) of the top cell is enhanced from 15.56 m A/cm^(2) to 17.09 m A/cm^(2), the corresponding PCE reaches 19.05%, and the tandem PCE increases to 30.58%. Simultaneously, in the cases of n = 3, 4, and 5, all the tandem PCEs exceed the limiting theoretical efficiency of Si cells. Therefore, the 4T quasi-2D perovskite/Si devices provide a more cost-effective tandem strategy and long-term stability solutions.

Study on device simulation and performance optimization of the epitaxial crystalline silicon thin film solar cell

Because crystalline silicon thin film （CSiTF） solar cells possess the advantages of crystalline silicon solar cells such as high ef- ficiency and stable performance and those of thin film solar cells such as low cost and so on, it is regarded as the next genera- tion solar cell technology, which is most likely to replace the existing crystalline silicon solar cell technology. In this paper, we performed device simulation on the epitaxial CSiTF solar cell by using PCI D software. In order to make simulation results closer to the actual situation, we adopted a more realistic device structure and parameters. On this basis, we comprehensively and systematically investigated the effect of physical parameters of back surface field （BSF） layer, base and emitter, electrical quality of crystalline silicon active layer, situation of surface passivation, internal recombination and p-n junction leakage on the optoelectronic performance of the epitaxial CSiTF solar cell. Among various factors affecting the efficiency of the epitaxial CSiTF solar cell, we identified the three largest efficiency-affecting parameters. They are the base minority carrier diffusion length, the diode dark saturation current and the front surface recombination velocity in order. Through simulations, we found that the base is not the thicker the better, and the base minority carrier diffusion length must be taken into account when deter- mining the optimal base thickness. When the base minority carrier diffusion length is smaller, the optimal base thickness should be less than or equal to the base minority carrier diffusion length; when the base minority carrier diffusion length is larger, the base minority carrier diffusion length should be at least twice the optimal base thickness. In addition, this paper not only illustrates the simulation results but also explains their changes from the aspect of physical mechanisms. Because epitaxi- al CSiTF solar cells possess a device structure that is similar to crystalline silicon solar cells, the conclusions drawn in this pa- per are also applied to crystalline silicon solar cells to a certain extent, particularly to thin silicon solar cells which are the hot- test research topic at present.

Fabrication and Simulation of an AlGaAs/GaAs Ultra-Thin Base NDR HBT

A novel mesa ultra-thin base AlGaAs/GaAs HBT is designed and fabricated with wet chemical selective etch technique and monitor electrode technique. It has a particular and obvious voltage-controlled NDR whose PVCR is larger than 120. By use of device simulation,the cause of NDR is that increasing collector voltage makes the ultrathin base reach through and the device transforms from a bipolar state to a bulk barrier state. In addition, the simulated cutoff frequency is about 60-80GHz.

The System Design of Semiconductor Device Simulator

This paper studies the numerical simulation for semiconductor devices and discusses the software design of the simulation system. Our focus is on the deep submicron device simulation for BJT, MOSFET, heterojunction bipolar transistors (HBT), etc. So the object oriented technology for software design and its realization is used to make the system easy to implement, maintain and extend. Besides the discussion of simulation and software system design, this paper introduces a device simulator SMDS and its parallel extension under local network environment using CORBA technology.

Simulation and Experimental Research on a Schottky Gate Resonant Tunneling Transistor

A Schottky gate resonant tunneling transistor （SGRTT） is fabricated. Relying on simulation by ATLAS software,we find that the gate voltages can be used to control the current of SGRTT when the emitter terminal is grounded and a positive bias voltage is applied to the collector terminal. When the collector terminal is grounded, the gate voltages can control the peak voltage. As revealed by measurement results, the reason is that the gate voltages and the electric field distribution on emitter and collector terminal change the distribution of the depletion region.

Using 3D TCAD Simulation to Study Charge Collection of a p-n Junction in a 0.18μm Bulk Process

Single event transient of a real p-n junction in a 0.18μm bulk process is studied by 3D TCAD simulation. The impact of voltage, temperature, substrate concentration, and LET on SET is studied. Our simulation results demonstrate that biases in the range 1.62 to 1.98V influence DSET current shape greatly and total collected charge weakly. Peak current and charge collection within 2ns decreases as temperature increases,and temperature has a stronger influence on SET currents than on total charge. Typical variation of substrate concentration in modern VDSM processes has a negligible effect on SEEs. Both peak current and total collection charge increases as LET increases.

Simulation study of the losses and influences of geminate and bimolecular recombination on the performances of bulk heterojunction organic solar cells

We use the method of device simulation to study the losses and influences of geminate and bimolecular recombinations on the performances and properties of the bulk heterojunction organic solar cells. We find that a fraction of electrons（holes）in the device are collected by anode（cathode）. The direction of the corresponding current is opposite to the direction of photocurrent. And the current density increases with the bias increasing but decreases as bimolecular recombination（BR）or geminate recombination（GR） intensity increases. The maximum power, short circuit current, and fill factor display a stronger dependence on GR than on BR. While the influences of GR and BR on open circuit voltage are about the same.Our studies shed a new light on the loss mechanism and may provide a new way of improving the efficiency of bulk heterojunction organic solar cells.

Simulation design of P–I–N-type all-perovskite solar cells with high efficiency

According to the good charge transporting property of perovskite, we design and simulate a p–i–n-type all-perovskite solar cell by using one-dimensional device simulator. The perovskite charge transporting layers and the perovskite absorber constitute the all-perovskite cell. By modulating the cell parameters, such as layer thickness values, doping concentrations and energy bands of n-, i-, and p-type perovskite layers, the all-perovskite solar cell obtains a high power conversion efficiency of 25.84%. The band matched cell shows appreciably improved performance with widen absorption spectrum and lowered recombination rate, so weobtain a high J_（sc） of 32.47 m A/cm^2. The small series resistance of the all-perovskite solar cell also benefits the high J_（sc）. The simulation provides a novel thought of designing perovskite solar cells with simple producing process, low production cost and high efficient structure to solve the energy problem.

Numerical simulation study of organic nonvolatile memory with polysilicon floating gate

A polysilicon-based organic nonvolatile floating-gate memory device with a bottom-gate top-contact configuration is investigated,in which polysilicon is sandwiched between oxide layers as a floating gate.Simulations for the electrical characteristics of the polysilicon floating gate-based memory device are performed.The shifted transfer characteristics and corresponding charge trapping mechanisms during programing and erasing（P/E） operations at various P/E voltages are discussed.The simulated results show that present memory exhibits a large memory window of 57.5 V,and a high read current on/off ratio of ≈ 10~3.Compared with the reported experimental results,these simulated results indicate that the polysilicon floating gate based memory device demonstrates remarkable memory effects,which shows great promise in device designing and practical application.

Combination of Loosely and Compactly Coupled Integrations of CAD Tools and Its Applications In SAW Devices Design and Fabrication

A combination of the loosely and compactly coupled integrations of CAD tools and its applications in the design and fabrication of surface acoustic wave (SAW) devices are proposed in this paper. Three core modules are developed as design and mask pattern verification module,database module, and device characteristics simulation module. All the operations are controlled under Microsoft Windows GUI interface. This leads to a reduced design and fabrication cycle, workload and cost. With the knowledge-based library for intelligent design and Lotus Notes database for distributed and networked engineering database management (EDM) and effective control of documents, a new enterprise-leveled, computer-integrated design and manufacturing system (CIDMS)can be established for monolithic and hybrid device design (not only confined to SAW device).

P-type cold-source field-effect transistors with TcX_(2) and ReX_(2)(X=S,Se)cold source electrodes:A computational study

Cold-source field-effect transistors(CS-FETs)have been developed to overcome the major challenge of power dissipation in modern integrated circuits.Cold metals suitable for n-type CS-FETs have been proposed as the ideal electrode to filter the high-energy electrons and break the thermal limit on subthreshold swing(SS).In this work,regarding the p-type CS-FETs,we propose TcX_(2) and ReX_(2)(X=S,Se)as the injection source to realize the sub-thermal switching for holes.First-principles calculations unveils the cold-metal characteristics of monolayer TcX_(2) and ReX_(2),possessing a sub-gap below the Fermi level and a decreasing DOS with energy.Quantum device simulations demonstrate that TcX_(2) and ReX_(2) can enable the cold source effects in WSe_(2) p-type FETs,achieving steep SS of 29-38 mV/dec and high on/off ratios of(2.3-5.6)×10^(7).Moreover,multilayer Re S2retains the cold metal characteristic,thus ensuring similar CS-FET performances to that of the monolayer source.This work underlines the significance of cold metals for the design of p-type CS-FETs.

Direct Tunneling Effect in Metal-Semiconductor Contacts Simulated with Monte Carlo Method

Considering the tunneling effect and the Schottky effect,the metal semiconductor contact is simulated by using self consistent ensemble Monte Carlo method.Under different biases or at different barrier heights,the investigation into the tunneling current indicates that the tunneling effect is of great importance under reverse biases.The Schottky barrier diode current due to Schottky effect is in agreement with the theoretical one.The barrier lowering is found a profound effect on the current transport at the metal semiconductor interface.

Monolithically Fabricated OEICs Using RTD and MSM

Two kinds of monolithically fabricated circuits are demonstrated in GaAs-based material systems using resonant tunneling diodes（RTD） and metal-semiconductor-metal photo detectors（MSM PD）. The electronic characteristics of these fabricated RTD devices,MSM devices,and integrated circuits are tested at room temperature. The results show that the current peak-to-valley ratio is 4,and the photocurrent at 5V is enhanced by a factor of nearly 9,from 2 to about 18μA by use of recessed electrodes. The working theory and logical functions of the circuits are validated.

Progress of Microbial Enhanced Oil Recovery in Laboratory Investigation

This paper describes a simple, easy process for screening microorganisms, and introduces a laboratory simulation device and process of microbial enhanced oil recovery (MEOR) , which is a necessary research step for trial in oilfields. The MEOR mechanism and the influence of adsorption, diffusion, metabolism, nutrition, porosity, and permeability are analyzed. The research indicates that different microbes have different efficiencies in EOR and that different culture types play different roles in EOR. The effect of syrup is better than that of glucose, and larger porosity is favorable to the reproduction and growth of microbes, thereby improving the oil recovery. Using crude oil as a single carbon source is more appreciable because of the decrease in cost of oil recovery. At the end of this paper, the development of polymerase chain reaction (PCR) for the future is discussed.

A New Method for Fitting Current–Voltage Curves of Planar Heterojunction Perovskite Solar Cells

Herein we propose a new equivalent circuit including double heterojunctions in series to simulate the current–voltage characteristic of P–I–N planar structure perovskite solar cells. This new method can theoretically solve the dilemma of the parameter diode ideal factor being larger than2 from an ideal single heterojunction equivalent circuit,which usually is in the range from 1 to 2. The diode ideal factor reflects PN junction quality, which influences the recombination at electron transport layer/perovskite and perovskite/hole transport layer interface. Based on the double PN junction equivalent circuit, we can also simulate the dark current–voltage curve for analyzing recombination current(Shockley–Read–Hall recombination) and diffusion current(including direct recombination), and thus carrier recombination and transportation characteristics. This new model offers an efficacious and simple method to investigate interfaces condition, film quality of perovskite absorbing layer and performance of transport layer, helping us furtherimprove the device efficiency and analyze the working mechanism.

Small Signal Circuit Model of Double Photodiodes

The transmission delay of photogenerated carriers in a CMOS-process-compatible double photodiode (DPD) is analyzed by using device simulation.The DPD small signal equivalent circuit model which includes transmission delay of photogenerated carriers is given.From analysis on the frequency domain of the circuit model the device has two poles.One has the relationship with junction capacitance and the DPD’s load,the other with the depth and the doping concentration of the N-well in the DPD.Different depth of the N-well and different area of the DPDs with bandwidth were compared.The analysis results are important to design the high speed DPDs.

Modeling,fabrication and measurement of a novel CMOS UV/blue-extended photodiode

A new complementary metal oxide semiconductor UV/blue-extended photodiode was presented for light detection in the UV/blue spectral range. Photoelectric characteristics of this presented photodiode were studied by numerical modeling and device simulation. Technology computer aided design simulation was done first to analyze its photoelectric characteristics. The structure characteristic and depletion situation of space between two adjacent P+ anodes were discussed. The reverse characteristic, spectral response characteristic and DC characteristic were discussed in detail. For the numerical modeling, dead layer effect is considered in the building of analytical mode. Dead layer is a space in which the boron doping profile decreases towards the surface due to high doping effects and boron redistribution, which affects the sensitivity of photodiode in the UV range seriously. Reverse characteristics and spectral response characteristics were modeled and analyzed typically. At last, silicon test results were given and compared with the simulated result, which shows reasonable match for each.

Reveal the large open-circuit voltage deficit of all-inorganic CsPbIBr_(2) perovskite solar cells

Due to excellent thermal stability and optoelectronic properties, all-inorganic perovskite is one of the promising candidates to solve the thermal decomposition problem of conventional organic–inorganic hybrid perovskite solar cells(PSCs),but the larger voltage loss(V_(loss)) cannot be ignored, especially CsPbIBr_(2), which limits the improvement of efficiency. To reduce V_(loss), one promising solution is the modification of the energy level alignment between the perovskite layer and adjacent charge transport layer(CTL), which can facilitate charge extraction and reduce carrier recombination rate at the perovskite/CTL interface. Therefore, the key issues of minimum V_(loss) and high efficiency of CsPbIBr_(2)-based PSCs were studied in terms of the perovskite layer thickness, the effects of band offset of the CTL/perovskite layer, the doping concentration of the CTL, and the electrode work function in this study based on device simulations. The open-circuit voltage(V_(oc)) is increased from 1.37 V to 1.52 V by replacing SnO_(2) with ZnO as the electron transport layer(ETL) due to more matching conduction band with the CsPbIBr;layer.

Study on a novel vertical enhancement-mode Ga_(2)O_(3) MOSFET with FINFET structure

A novel enhanced mode(E-mode)Ga_(2)O_(3) metal-oxide-semiconductor field-effect transistor(MOSFET)with vertical FINFET structure is proposed and the characteristics of that device are numerically investigated.It is found that the concentration of the source region and the width coupled with the height of the channel mainly effect the on-state characteristics.The metal material of the gate,the oxide material,the oxide thickness,and the epitaxial layer concentration strongly affect the threshold voltage and the output currents.Enabling an E-mode MOSFET device requires a large work function gate metal and an oxide with large dielectric constant.When the output current density of the device increases,the source concentration,the thickness of the epitaxial layer,and the total width of the device need to be expanded.The threshold voltage decreases with the increase of the width of the channel area under the same gate voltage.It is indicated that a set of optimal parameters of a practical vertical enhancement-mode Ga_(2)O_(3) MOSFET requires the epitaxial layer concentration,the channel height of the device,the thickness of the source region,and the oxide thickness of the device should be less than 5×10^(16) cm^(-3),less than 1.5μm,between 0.1μm-0.3μm and less than 0.08μm,respectively.