Angle of incidence effects on the energy conversion behavior of
polycrystalline silicon photovoltaic cells. R.J. Beal 1, B.G.
Potter, Jr. 1,2 and J.H. Simmons 1,2,3 1 Materials Science and
Engineering, 2 Optical Sciences, University of Arizona, Tucson, AZ
85721 3 Florida Gulf Coast University, Ft. Myers, FL 33965
Introduction and Motivation Accurate prediction of solar power
production is critical to effective grid integration. The energy
conversion efficiency of fielded photovoltaic systems is dependent
upon cell design, installation and environmentally mediated factors
contributing to module response. ConclusionsAcknowledgements
Experimental Details ML Solar polycrystalline Si cells (SiN x AR
coating, cell thickness = 250 m) Specimen size - 2.5cm x 5cm Bare
cells and cells with encapsulant overlayer laminate (ethylene-vinyl
acetate (EVA)/Na-Ca silicate glass) to provide optical consistency
with typical module structure. Incident-angle-dependent EQE:
Commercial EQE measurement system (PV Measurements, Inc.) modified
to provide angle- dependent measurement Collimated, tunable,
spectral source Coupled rotation of electronic contacts and cell.
Isolation of geometric (Lambert cosine law) and optical effects
intrinsic to materials and cell construction via integrated shadow
mask Empirically defined parameter sets are often used to model
total integrated irradiance to electrical energy transduction,
including the impact of local variation in irradiance and
environmental conditions, to predict cell irradiance-to- power
(ITP) behavior. Nonempirical, physics-based models would provide an
opportunity to predict module behavior under arbitrary optical and
environmental conditions. GOAL: evaluate a nonempirical, intrinsic
spectral response function (EQE) that could be integrated into
established ITP models for improved PV power conversion prediction.
Results: EQE( ,, specimen configuration) Short circuit current
(i.e. indicator of cell energy conversion efficiency) computed
assuming AM1.5 irradiance spectrum and experimentally determined
EQE( results. No mask data (blue diamonds) isolates -related
optical phenomena associated with material/cell architecture.
Enhancement of 3-4% in J sc over normal incidence value observed in
encapsulated cells. Associated with increased incoherent scattering
within laminate structure (multiple interfaces, volumetric
scattering) increased irradiance at PV junction. Masked measurement
- cosine law reduction in irradiance at the cell results in
significant reduction in EQE values and associated J sc. No mask -
the cosine related effects on irradiance are suppressed, isolating
incidence angle effects now associated with the intrinsic materials
and design of the cell. Bare cell exhibits increased EQE
variability with wavelength that is enhanced with increased .
Contrasts that observed with encapsulated cell structure, showing
more flat EQE dispersion from 500 800 nm. Consistent with specular
reflectance data showing increased reflectivity dispersion in bare
cell. Physically derived, cell-specific spectral response functions
(EQE( )) are identified as a potential refinement over empirical
parameters sets used in standard ITP models that can enhance module
output prediction under arbitrary irradiance and incidence
conditions for a given cell/module design. The novel integration of
a removable mask into EQE measurement allows the examination of
intrinsic, angle- dependent variability in cell/module response
associated with materials of construction and architecture, free
from extrinsic, cosine effects on irradiance. New opportunity for
direct, experimental validation of enhanced light management
strategies for emerging third generation PV technologies. External
Quantum Efficiency (EQE) Specimens Sample configuration and
measurement conditions shown with encapsulant overlayer at
arbitrary incidence angle ( ). With mask Mask maintains consistent
illumination area on cell. Projected area of illumination (i.e.
irradiance magnitude) scales with cos( ) (Lambert cosine law). No
mask Projected area of illumination constant. Lambert cosine effect
offset by cos( )-scaled increase in illuminated area of cell with
increase. Lambert Cosine Law: ( , ) = 0 (0, ) cos ( ) J sc ( ,,
specimen configuration) Cosine law observed with change Cosine law
not exhibited with change EQE system with modified optical path and
stage Masked No Mask Reflectance This material is based upon work
supported by the Department of Energy/EERE under DE-EE0006017. The
authors also thank C. Hansen (Sandia National Laboratories) for
informative and insightful discussions. Representative PV cells
tested, with (right) and without (left) encapulsant layers