Jingya Hou

Computer analysis of the role of p -layer quality, thickness, transport mechanisms, and contact barrier height in the performance of hydrogenated amorphous silicon p - i - n solar cells

The transport simulations provided by the computer program AMPS have been used to give an in‐depth analysis of the role of the p‐layer contact barrier height, contact transport mechanism, p‐layer thickness, and p‐layer quality on the performance of hydrogenated amorphous silicon p‐i‐n solar cells. We demonstrate for the first time that, if the contact barrier height to the p‐layer is below a critical value and if tunneling through the p‐layer is not important, then the performance of cells with either active or dead p‐layers varies with contact barrier height regardless of p‐layer thickness. We show that, even for an optimistic p‐layer active doping density of 1019 cm−3, this critical barrier height is high (∼1.2 eV). Our analysis implies that one of two situations must occur in an actual a‐Si:H p‐i‐n structure: the p‐layer contact plays an important role in determining cell efficiency, or the tunneling of holes through the front contact/p‐layer interface must be important. Comparison of simulated results...

Applications of AMPS-1D for solar cell simulation

The AMPS-1D PC computer program is now used by over 70 groups world-wide for detector and solar cell analysis. It has proved to be a very powerful tool in understanding device operation and physics for single crystal, poly-crystalline and amorphous structures. For example, AMPS-1D has been successful in explaining the “red kink” [1] and the “transient effect” in CdS/CIGS poly-crystalline solar cells. It has been used to show that thin film poly-Si structures, with reasonable light trapping, are capable of competitive solar cell conversion efficiencies. In the case of a-Si:H structures, it has been used, for example, to settle the discrepancies in bandgap measurement, to predict the effective QE>1 phenomenon later seen in these materials [2], to determine the relative roles of interface and bulk properties, and to point the direction toward 16% triple junction structures. In general AMPS-1D is used for cell and detector design, material parameter sensitivity studies, and parameter extraction. Recently we h...

Bias‐voltage‐ and bias‐light‐dependent high photocurrent gains in amorphous silicon Schottky barriers

Experimental results of very large, long‐wavelength photocurrent gains in amorphous silicon‐based Schottky barrier structures are reported. It is shown that these occur for devices in the space‐charge current regime operated at forward bias voltages past the flatband condition. The analysis of microelectronic and photonic structures computer program is used to show that these high gains are due to hole trapping and the resulting modulation of the virtual cathode barrier potential at the ohmic contact. As demonstrated, these gains are bias voltage, bias light, monochromatic light intensity, material property, and device thickness dependent. This type of photocurrent gain is another photogating effect and is much different than the usual bulk photoconductive gain.

An examination of the 'tunnel junctions' in triple junction a-Si:H based solar cells: modeling and effects on performance

The first-principles computer model AMPS is used to model multijunction solar cell performance. The specific cell modeled uses two a-Si:H absorber layers and an a-SiGe:H absorber layer with all a-SiC:H p/sup +/ layers and all a-Si:H n/sup +/ layers, the modeling shows the contacts in these cells depend critically on recombination in x-layers which may be independently present or, in a well-designed cell, purposefully present. These x-layers must be supplied with carriers. This process may be optimized by appropriate bandgap grading. The modeling shows that multijunction contact layers-unless fully optimized-are not simply resistor-like or diode-like in their behavior.<<ETX>>

Photodetection enhancement in two-terminal amorphous silicon-based devices: an experimental and computer simulation study

Although amorphous silicon (a-Si:H) is very photosensitive and can be doped n and p type, it does not give effective phototransistors because of the extremely poor diffusion lengths. Hence enhanced photodetection in two-terminal a-Si:H devices is of considerable interest. Using the Analysis of Microelectronic and Photonic Structures (AMPS) computer model, we explore enhanced photodetection possibilities in two-terminal a-Si:H structures and show situations where it can occur. These situations, which we then experimentally verify, are of two types: one can yield quantum efficiencies greater than unity and the other can yield gains of 103. Both of these enhanced photodetection situations occur because of what we term photogating.

First principles computer model showing the effect of p-layer thickness and front contact barrier height on the performance of a-Si:H p-i-n solar cells

The performance of a-Si:H p-i-n solar cells with different p-layer thicknesses, front contact barrier heights, and front contact/p-layer hole transport mechanisms were modeled. The authors examined how p-layer material and contact quality influenced the results obtained, and they also explored the role that tunneling of holes across the barrier near the front contact can play in determining cell performance. It was found that, for active p-layer cells with the optimistic p-layer activation energy of 0.27 eV, cell performance is maximum for a p-layer thickness of about 100 AA. However, this maximum depends on the quality of the p-layer. Surprisingly, it was found that, for front contact barrier height values less than some critical value, cell performance, in the absence of tunneling, depends on the front contact barrier height regardless of the thickness or quality of the p-layer.<<ETX>>

A payback assessment for materials development for the low-gap unit of a multi-junction cell: a numerical modeling study

The objective of this paper is to demonstrate how computer simulations can be used to determine the best payback when the improvement of solar cell efficiency is sought. For this demonstration, modeling work has been undertaken to examine how the material quality of the absorber and p/i interface layers of the a-SiGe:H heterojunction of a triple junction can affect device performance. According to simulation results, which are based on assuming a simplified a-SiGe:H sub-cell structure with only doped layers, interfacial layers, and a homogeneous absorber, the authors find that the scheme with the best payback for enhancing cell efficiency is different for the absorber layer and the p/i interface layer. For the absorber, the density of mid-gap states is the most important factor, but for the p/i interface layer the mobility gap is the most dominant factor. Their numerical analysis approach allows the determination of where material improvement efforts will have the most effect. Put succinctly, the relative importance analysis, which can be achieved with simulation, shows where material development efforts will have their best payoff chances.<<ETX>>

Computer modeling of a-SiC:H/a-Si:H heterojunction solar cell performance as a function of the apportionment of the bandgap discontinuity

A computer-model study is used to investigate the effects of the distribution of the bandgap discontinuity in a-SiC:H/a-Si:H heterojunction p-i-n solar cells. It is found that the influence of the energy location of the p-layer/i-layer bandgap discontinuity on cell performance is strongly dependent on the acceptor doping density in the p-layer and the gap state densities in both the p-layer and the p/i interface region. If all the bandgap discontinuity appears in valence band, then achieving the best performance requires high values of the acceptor doping density. If the discontinuity is equally shared between the bands, then only moderate doping in the p-region is required. If the bandgap discontinuity is entirely in the conduction band, then lower doping can be tolerated.<<ETX>>

A new tool for multijunction cell diagnostics-QE measurements under AM1 bias light

The authors attempt to obtain a relatively simple diagnostic tool that can be used to optimize the efficiency of multijunction solar cells. This measurement should be able to pinpoint which sub-cell most needs improvement. In this report, they explore such a tool: the quantum efficiency at the maximum power point under AM1 light (AM1QE). They demonstrate the application of AM1QE-based guidelines with AMPS computer program simulations and show that the most substantial improvement from sub-cell thickness adjustments comes from the sub-cell which causes a peak in the AM1QE. They also show that this proposed AM1QE quantum efficiency measurement for multijunctions is a much more sensitive monitor of changes in a multijunction than the so-called color light bias quantum efficiency measurement.<<ETX>>

Performance and degradation of the (p)a-SiC:H/(i)a-Si:H/(n)a-Si:H solar cell: a computer modeling study

The computer code AMPS (analysis of microelectronic and photonic structures) has been used to study the performance of the (p)a-SiC:H/(i)a-Si:H/(n)a-Si:H heterojunction solar cell. The authors found initially that AMPS predicts for an ideal (p)a-Si:H/(i)a-Si:H/(n)a-Si:H heterojunction an open-circuit voltage V/sub oc/ value of about 1 V. However, experimental values of V/sub oc/ are around 0.8 V without (i)a-SiC:H buffer layers at the p/i interface and around 0.85 V with these buffer layers. The authors studied the possible origins of these lower experimentally observed values and have found that although higher concentrations of defect states anywhere in a cell can reduce V/sub oc/, the impact of these defect states on V/sub oc/ and fill factor (FF) is different according to their location.<<ETX>>