NCPV preprints for the 2. world conference on photovoltaic solar energy conversion Page: 96 of 144
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APPLICATIONS OF "PV OPTICS" FOR SOLAR CELL AND MODULE DESIGN
Bhushan L Sopori, Jamal Madjdpour, and Wei Chen
National Renewable Energy Laboratory
1617 Cole Boulevard
Golden, CO 80401, U. S. A.
ABSTRACT: This paper describes some applications of a new optics software.package, PV Optics, developed for the optical
design of solar cells and modules. PV Optics is suitable for the analysis and design of both thick and thin solar cells. It also
includes a feature for calculation of metallic losses related to contacts and back reflectors.
Keywords: Solar Cell Design -1: Optical Properties -2
High-efficiency solar cells involve a number of features
that are difficult to handle by simple optics. An optical
design and analysis software package for solar cells and
modules must include capabilities to handle: (i) nonplanar
interfaces such as those required for optimizing light-
trapping; (ii) thick devices such as crystalline silicon solar
cells as well as thin cells based on a-Si, CdTe, and CIS; (iii)
antireflection and dielectric coatings, (iv) metallic absorption
arising from contacts and back reflectors; and (v) thicker
materials such as glass and encapsulants used in modules.
We have developed a new, commercial, computer
software package, PV Optics, for the optical design and
analysis of solar cells and modules. This paper describes
some applications of this software package.
2, FEATURES OF PV OPTICS
PV Optics is an easy-to-use software that accurately
models the optics of any solar cell or module, and provides
information needed to design a device with maximum-
effective light-trapping and optimum photocurrent. PV
Optics' sophisticated model uses the coherence length of
light as a criterion to categorize various regions of a cell as
"thin" or "thick" - the former have thicknesses less than
the coherence length of light and include interference and
polarization effects; the latter are much thicker than the
coherence length and are treated on the basis of ray optics.
The model separates a multilayer structure into several
composite layers each as a "thin" or "thick" group. Each
group of layers is analyzed and the entire structure is
reassembled. Regions such as glass superstrates or
encapsulation layers having thicknesses greater than a few
microns, and textured structures, are treated in a
noncoherent regime. Thin and specula layers such as those
used for antireflection (AR) coatings in a-Si devices are
treated as coherent regions.
* Accommodates device design for single and
multijunction cells, with as many as three active
semiconductor layers plus cover glass, encapsulation,
AR coating, buffer, and metal backing.
" Calculates the light-trapping impact of nonplanar
(textured or intentionally rough) interfaces of any of
the device layers.
. Accurately calculates interference effects (caused by
coherence of light) of very thin layers such as AR
coatings or thin-film semiconductor materials.
. Includes default refractive index and extinction
coefficient values for crystalline silicon, amorphous
silicon, glass used for encapsulation, encapsulation
materials such as EVA, and buffer layer.
+ Calculates maximum achievable current density
(MACD in mA/cm2) for each semiconductor layer,
providing a benchmark for cell performance.
. For each device, automatically and clearly plots:
- Reflection, transmission, semiconductor absorbance,
and metal absorbance, each as a function of wavelength
- (For multijunction devices) absorbance for each
separate layer plus total absorption
- Absorbance by wavelength for typical sunlight
(AM 1.5), predicting actual cell performance
* Photon absorbance as a function of depth within each
semiconductor layer-facilitating selection of optimum
thickness(es) (this data can be used in an electronic
model like AMPS or PCID for complete cell
PV Optics can be used for optimizing a variety of cell
parameters, such as cell absorber thicknesses, the structure of
the texture, AR coating parameters, and the back-reflector
design. Here, we will demonstrate the capabilities of the
package by specific examples. The model is a user-friendly
tool using a "Windows" environment. It requires as input the
optical constants of each layer as a function of wavelength
and the layer thicknesses. Texture is simulated by allowing
the user to select appropriate geometric features in addition
to co-planar layers. Computing times depend significantly
on the conditions chosen and can vary from minutes to hours.
The examples we present are intentionally kept simple to
demonstrate the capabilities of the package, and show that
results are often obtained that would not have been expected
3. OPERATING PV OPTICS
PV Optics (Version 1) for the PC comes on a single
3.5" floppy disk. The software requires an IBM
compatible PC/60 MHz (or a higher speed), a minimum of
8 MB RAM, windows, a VGA monitor, and a color printer
(can also operate with a black & white printer).
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NCPV preprints for the 2. world conference on photovoltaic solar energy conversion, article, September 1, 1998; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc707815/m1/96/: accessed December 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.