Caspar Leendertz

Interplay of amorphous silicon disorder and hydrogen content with interface defects in amorphous/crystalline silicon heterojunctions

We analyze the dependence of the interface defect density Dit in amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions on the microscopic properties of ultrathin (10 nm) undoped a-Si:H passivation layers. It is shown that the hydrogen bonding and network disorder, probed by infrared- and photoelectron spectroscopy, govern the initial Dit and its behavior upon a short thermal treatment at 200 °C. While the initial Dit is determined by the local and nonequilibrated interface structure, the annealed Dit is defined by the bulk a-Si:H network strain. Thus it appears that the equilibrated a-Si:H/c-Si interface does not possess unique electronic properties but is governed by the a-Si:H bulk defects.

Discerning passivation mechanisms at a-Si:H/c-Si interfaces by means of photoconductance measurements

The photoconductance decay (PCD) measurement is a fast and simple method to characterize amorphous/crystalline (a-Si:H/c-Si) silicon interfaces for high-efficiency solar cells. However, PCD only yields information concerning the overall recombination rate in the structure. To overcome this limitation, we have developed and validated a computer-aided PCD (CA-PCD) analysis method to determine the defect density of recombination-active dangling bonds at the interface and the potential drop in the crystalline absorber adjacent to the interface. As a practical example, we investigate a-Si:H(p)/a-Si:H(i)/c-Si(n) layer stacks and show that the CA-PCD method is capable of discerning the influence of field-effect and defect passivation.

Numerical Simulation of Solar Cells and Solar Cell Characterization Methods: the Open-Source on Demand Program AFORS-HET

Within this chapter, the principles of numerical solar cell simulation are described, using AFORS-HET (automat for simulation of heterostructures). AFORS-HET is a one dimensional numerical computer program for modelling multi layer homoor heterojunction solar cells as well as some common solar cell characterization methods. Solar cell simulation subdivides into two parts: optical and electrical simulation. By optical simulation the local generation rate ) , ( t G x within the solar cell is calculated, that is the number of excess carriers (electrons and holes) that are created per second and per unit volume at the time t at the position x within the solar cell due to light absorption. Depending on the optical model chosen for the simulation, effects like external or internal reflections, coherent superposition of the propagating light or light scattering at internal surfaces can be considered. By electrical simulation the local electron and hole particle densities ) , ( ), , ( t p t n x x and the local electric potential ) , ( t x φ within the solar cell are calculated, while the solar cell is operated under a specified condition (for example operated under open-circuit conditions or at a specified external cell voltage). From that, all other internal cell quantities, such like band diagrams, local recombination rates, local cell currents and local phase shifts can be calculated. In order to perform an electrical simulation, (1) the local generation rate ) , ( t G x has to be specified, that is, an optical simulation has to be done, (2) the local recombination rate ) , ( t R x has to be explicitly stated in terms of the unknown variables φ , , p n , ( ) φ , , ) , ( p n f t R = x . This is a recombination model has to be chosen. Depending on the recombination model chosen for the simulation, effects like direct band to band recombination (radiative recombination), indirect band to band recombination (Auger recombination) or recombination via defects (Shockley-Read-Hall recombination, dangling-bond recombination) can be considered. In order to simulate a real measurement, the optical and electrical simulations are repeatedly calculated while changing a boundary condition of the problem, which is specific to the measurement. For example, the simulation of a i-V characteristic of a solar cell is done by calculating the internal electron and hole current (the sum of which is the total current) as a function of the externally applied voltage. Source: Solar Energy, Book edited by: Radu D. Rugescu, ISBN 978-953-307-052-0, pp. 432, February 2010, INTECH, Croatia, downloaded from SCIYO.COM

Impact of the transparent conductive oxide work function on injection-dependent a-Si:H/c-Si band bending and solar cell parameters

An analysis of the contact formation between degenerated n-type transparent conductive oxide (TCO) and p-type amorphous silicon (a-Si:H) as it is used for front side contacts in high efficiency a-Si:H/crystalline silicon (c-Si) heterojunction solar cells is presented. It is shown that the deposition of a TCO on a (p)a-Si:H emitter layer causes a reduction of charge carrier lifetime in low injection levels which leads to a lowering of the implied fill factor. Simulation based analysis of charge carrier lifetime and direct measurements by surface photovoltage reveals that TCO deposition induces a change of the c-Si band bending. The magnitude of this change depends on the (p)a-Si:H doping level. Both observations are explained by the impact of the TCO/a-Si:H work function difference on the c-Si band bending. Based on numerical simulations, the reduced injection-dependent band bending is identified as the reason for the reduced fill factor of final solar cells.

The influence of surface topography on Kelvin probe force microscopy

Long-range electrostatic forces govern the imaging mechanism in electrostatic force microscopy as well as in Kelvin probe force microscopy. To improve the analysis of such images, simulations of the electrostatic field distribution have been performed in the past using a flat surface and a cone-shaped tip. However, the electrostatic field distribution between a tip and a sample depends strongly on the surface topography, which has been neglected in previous studies. It is therefore of general importance to study the influence of sample topography features on Kelvin probe force microscopy images, which we address here by performing finite element simulations. We show how the surface potential measurement is influenced by surface steps and surface grooves, considering potential variations in the form of a potential peak and a potential step. The influence of the topography on the measurement of the surface potential is found to be rather small compared to a typical experimental resolution. Surprisingly, in the case of a coinciding topography and potential step an improvement of the potential profile due to the inclusion of the topography is observed. Finally, based on the obtained results, suggestions for the realization of KPFM measurement are given.

Evaluation of Kelvin probe force microscopy for imaging grain boundaries in chalcopyrite thin films

In view of the outstanding performance of polycrystalline thin film solar cells on the basis of Cu(In,Ga)Se2, the electrical activity at grain boundaries currently receives considerable attention. Recently, Kelvin probe force microscopy (KPFM) has been applied to characterize the properties of individual grain boundaries, observing a drop in the work function in many cases. We present finite element simulations of the electrostatic forces to assess the experimental resolution of KPFM. Depending on the tip-sample distance, the observed drop in the work function amounts to only a fraction of the real potential drop. The simulations are considered for different grain boundary models and consequences for the quantitative evaluation of experimental results are discussed.

General Principles of Solar Cell Simulation and Introduction to AFORS-HET

The principles of numerical solar cell simulation are described, using AFORS-HET (automat for simulation of heterostructures) which is a device simulator program for modelling multi layer homo- or heterojunction solar cells and typical characterization methods in one dimension. The basic equations for the optical and electrical calculations used in AFORS-HET are explained including a detailed description of the equations needed to calculate the recombination via defects in the semiconductor layers.

Field-effect passivation and degradation analyzed with photoconductance decay measurements

In this article, an expression for the surface passivation has been derived in terms of the surface recombination velocity and the field-effect exponential. The analytical solutions provide a comprehensive understanding of the injection dependency of minority charge carrier lifetime as measured by photoconductance decay. The model has been utilized to analyze the field-effect passivation of silicon exerted by the fixed dielectric charge in an overlying dielectric film. Possible limitations and restrictions of the technique are also addressed.

The influence of space charge regions on effective charge carrier lifetime in thin films and resulting opportunities for materials characterization

The analysis of injection-dependent charge carrier lifetimes is a well-established method to determine material and interface quality in crystalline silicon wafer-based device structures such as solar cells. However, for thin films, this method has rarely been used. One reason is that the physical interpretation of experimental data must rely on advanced theoretical models. In this study, we show by numerical simulations and analytical approximations that the effective charge carrier lifetime in thin films is heavily affected by space charge regions (SCR) over a wide range of injection levels. By analysis of the characteristic features in the injection-dependent effective charge carrier lifetime curves, qualitative information about SCRs that occur at grain boundaries or interfaces can be obtained. In contrast, information about the defect density can only be extracted in a very limited range of injection levels and the relationship between effective charge carrier lifetime and the quasi-Fermi level split...

Modeling an a-Si:H/c-Si Solar Cell with AFORS-HET

The physics models and material parameters needed to simulate an a-Si:H/c-Si solar cell with AFORS-HET are discussed and a simulation study showing solar cell characteristics subject to emitter doping, i-layer thickness and interface quality is presented. The AFORS-HET user interface is introduced so that the interested reader can repeat the simulation study. It is explained in detail how to define a structure and how to simulate a solar cell under different external conditions such as external current, voltage and illumination and how to calculate I-V curves to obtain solar cell characteristics.