Concept

ASA Basics

General description

The ASA (Advanced Semiconductor Analysis) program is designed for the simulation of devices based on amorphous and crystalline semiconductors. The ASA program solves the basic semiconductor equations in one dimension (the Poisson equation and two continuity equations for electrons and holes) and uses the free electron concentration, n, the hole concentration, p, and the electrostatic potential, ψ, as variables. Further it uses several advanced physical models which describe specific device operation and material optoelectronic properties.

Modeling of thin-film silicon devices requires one to take the electronic structure of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) into account. The spatial disorder in the atomic structure of a-Si:H results in a continuous density of states (DOS) in the energy band gap with no well defined conduction-band (CB) and valence-band (VB) edges. When considering the transport properties of charge carriers in a-Si:H one has to distinguish between the extended states and the localized states in the DOS distribution. The localized states within the mobility gap strongly influence the trapping and recombination processes and therefore the trapped charge in the localized states cannot be ignored as is often the case in modeling of crystalline semiconductor devices. The localized states in the mobility gap of a-Si:H can be of different nature which requires different approaches for the calculation of recombination-generation (R-G) statistics through these states. The models that are commonly used to describe the localized states in a-Si:H and their corresponding R-G statistics are described in detail in Chapter 2. The Shockley-Read-Hall (SRH) recombination through the states introduced by dopants and/or impurities is negligible in a-Si:H compared to the recombination via the tail states or dangling bond states and therefore is not used for amorphous films in device structures. However, the ASA program allows the SRH recombination based on the carrier lifetimes to be used for crystalline materials.

From the optical point of view, both the efficient use of the solar spectrum and the light management inside solar cells are important to obtain high conversion efficiencies. In today’s thin-film solar cells light management is accomplished by implementing light trapping techniques. The light trapping techniques are based on the introduction of surface-textured substrates and the use of special (back-) reflector layers. The surface-textured substrates introduce rough interfaces into the solar cell. The incident light is scattered at rough interfaces and modeling of solar cells must take into account the scattering processes at rough interfaces in order to determine accurately the generation profile of charge carriers inside the solar cell. This requires the development of optical models that take both coherent non-scattered (specular) and incoherent scattered (diffused) light propagation through a device into consideration. The different optical models that are implemented in the ASA program are described in Chapter 3. The efficient use of solar spectrum requires a multi-junction approach to thin-film silicon solar cells. The tunneling assisted recombination at an interface between two adjoining junctions is responsible for charge carrier transfer through a multi-junction solar cell. This interface is described as the tunnel-recombination junction (TRJ). There are two approaches that can be used in the ASA program to model TRJ. The Delft approach is based on the trapassisted tunneling model and enhanced carrier transport in the high-field region of the TRJ and is described in Chapter 2. The Pennsylvania approach is based on the introduction of a highly defective layer with strongly reduced band gap at the n/p interface and grading of the mobility gap of the n-layer and p-layer in the regions adjacent to the defective layer.

Main features

  • Modeling of multilayer amorphous and/or crystalline semiconductor devices
  • Models describing a complete DOS as function of energy, which include both the extended and localized (tail and defect) states
  • Calculation of the defect-states distribution in a-Si:H using the defect-pool models
  • Correct recombination-generation statistics for the acceptor- and donor-like states and for ambipolar states
  • Optical models for calculation of the absorption profile in devices with flat and/or rough interfaces
  • Continuous change (grading) of almost all input parameters as a function of position in the device or energy level in the gap
  • Model for the tunnel-recombination junction
  • Modeling of degradation of a-Si:H solar cells

The ASA one dimensional (1-D) device simulator is highly suitable for the simulation of (thinfilm) silicon solar cells. The program meets the standard requirements for a thin-film solar cell simulation program [Zeman 2007], [Burgelman 2004].

Model requirements for a thin film solar cell simulation program

  • Multiple layers
  • Band discontinuities in the conduction and valence bands
  • Large band gap materials: Eg > 2.0 – 3.7 eV
  • Grading of material parameters
  • Recombination and charge in the localized states within the band gap
  • Simulation of non-routine measurements: J(V), QE, C(V), etc., all as a function of T
  • Fast and easy to use

For a complete description, please refer to the ASA manual.