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Computational analysis of thin film InGaAs/GaAs quantum well solar cells with back side light trapping structures

Claiborne O. McPheeters and Edward T. Yu

Open Access Open Access

Abstract

Simulations of thin film (~2.5 µm thick) InGaAs/GaAs quantum well solar cells with various back side reflective and planar, symmetric scattering structures used for light trapping have been performed using rigorous coupled-wave analysis. Two-dimensional periodic metal/dielectric scattering structures were numerically optimized for Airmass 0 photocurrent generation for each device structure. The simulation results indicate that the absorption spectra of devices with both reflective and scattering structures are largely determined by the Fabry-Perot resonance characteristics of the thin film device structure. The scattering structures substantially increase absorption in the quantum wells at wavelengths longer than the GaAs absorption edge through a combination of coupling to modes of the thin film device structures and by reducing parasitic metal absorption compared to planar metal reflectors. For Airmass 0 illumination and 100% carrier collection, the estimated short-circuit current density of devices with In0.3Ga0.7As/GaAs quantum wells improves by up to 4.6 mA/cm2 (15%) relative to a GaAs homojunction device, with the improvement resulting approximately equally from scattering of light into thin film modes and reduction of metal absorption compared to a planar reflective layer.

©2012 Optical Society of America

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Figures (6)
Fig. 1
Fig. 1 The polarization-dependent absorption coefficient of an 8 nm In0.3Ga0.7As/GaAs QW for (‘x-, y-polarized’) x- and y-polarized light and for (‘z-polarized’) z-polarized light, according to the coordinates shown in the illustration at right, at wavelengths corresponding to absorption in QW sub-band states (λ > 850 nm) and in the continuum of states (λ ≤ 850 nm). Peaks corresponding to resonant absorption by heavy-hole (‘HH’) and light hole (‘LH’) excitons are labeled, where the number in the exciton designation indicates the QW sub-band in which the exciton is generated. The absorption coefficient was calculated using the semi-empirical approach of Ref [26]. The calculated absorption coefficient spans the wavelength range 300 nm to 1200 nm, of which a subset is shown here to improve the clarity of features at λ > 850 nm.

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Fig. 2
Fig. 2 (a) A representative diagram of the simulated device structures, which has a two-dimensional diffractive structure located on its back side with the following parameters defining it: the period in the x and y directions (D1 and D2, respectively); the pitch in those directions, which is collectively determined by W1, W2, L1, and L2; and the height of the grating, H2. The device has an SiO2 anti-reflection coating on the top surface, the thickness of which, H1, is variable in optimizations. (b) Jsc of thin film QWSCs, computed as a function of the GaAs base and emitter thickness and assuming AM 0 illumination. The maximum Jsc over the range of simulated values of 31.7 mA/cm2 is produced for a base thickness of 2.1 µm and an emitter thickness of 0.1 µm.

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Fig. 3
Fig. 3 Simulated optical response spectra for a ~2.5 μm QWSC, as illustrated in Fig. 2(a), with QWs composed of 8 nm In0.3Ga0.7As/20 nm GaAs, where the back side geometry consists of (‘Pd reflector’) semi-infinite Pd, (‘SiO2 interlayer-Pd’) 165 nm SiO2 followed by semi-infinite Pd, and (‘Pd-SiO2 diff. struct.’) a Pd-SiO2 diffractive structure or (‘Ag-SiO2 diff. struct.’) a Ag-SiO2 diffractive structure, which were optimized for this QWSC and which have the geometry shown in Fig. 2(a), where W1 = 268 nm, L1 = 800 nm, W2 = 686 nm, L2 = 205 nm, D1 = 1355 nm, D2 = 1070 nm, and H2 = 165 nm. (a) illustrates the complete absorption spectrum of the ‘Pd-SiO2 diff. struct.’ device and the wavelengths at which resonant absorption by heavy hole (‘HH’) and light hole (‘LH’) excitons occurs are marked. (b) illustrates the absorption spectra of QWSCs with each of the Pd-based back side geometries at wavelengths longer than the nominal GaAs absorption edge (850 nm). (c) illustrates the reflectivity (‘R’) and the net absorption (‘1-R’), which includes loss due to metal absorption, when the Pd-SiO2 diffractive structure was used. (d) illustrates the absorption spectrum of a QWSC with a diffractive structure made of Ag and SiO2.

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Fig. 4
Fig. 4 (a) Cross-sectional plots at x = 0 [i.e., through the center of the unit cell of the diffractive structure shown in Fig. 2(a)] of the Ez component of the electric field in the ~2.5 μm QWSC structure, at incident wavelengths of 860 nm and (b) 1105 nm. The incident field is polarized in the x-y plane, so Ez corresponds to a component of the diffracted field, which is seen to couple to a high order waveguide mode for λ = 860 nm, and to a lower order mode for λ = 1105 nm. (b) An illustration of different device structures that were used in RCWA simulations to determine how Jsc of a QWSC with six periods of {λ nm In0.3Ga0.7As / t nm GaAs} varies according to the position of QWs in the device, dQW, where λ is the thickness of the QW and t is the thickness of the QW barrier. The total thickness of the device is the same for each simulated structure (~2.5 µm). All of the illustrations in this figure correspond to λ = 8 nm and t = 20 nm. The device is integrated with an optimized Pd-SiO2 diffractive structure similar to the one illustrated in Fig. 2(a).

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Fig. 5
Fig. 5 Variation of the AM 0 Jsc of devices with optimized back side diffractive structures as a function of the thickness of the depth of the multi-QW structure from the GaAs emitter, dQW, for selected values of the InGaAs QW thickness, λ, and the GaAs barrier thickness, t, as illustrated in Fig. 4(b). (a,c) illustrate the full range of dQW that was simulated for t = 20 nm and t = 40 nm, respectively, while (b,d) focus on device structures where the multi-QW layer is located near the top of the active layers, also for t = 20 nm and t = 40 nm, respectively.

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Fig. 6
Fig. 6 The simulated AM 0 Jsc as a function of QW composition (x in InxGa1-xAs) for ~2.5 μm thick QWSCs, for x = 0.12, 0.20, and 0.30. (a) InGaAs/GaAs QWSCs were simulated with different back side geometries: a planar Pd reflector/contact (‘Pd reflector’); a planar Ag reflector/contact (‘Ag reflector’); and optimized diffractive structures that consist of Pd and SiO2 (‘Pd-SiO2 diff. struct.’) or Ag and SiO2 (‘Ag-SiO2 diff. struct.’). (b) The data designated by the prefix “GaAs w/” is the same as that shown in (a) for the specified back side geometries. The data designated by the prefix “AlGaAs w/” corresponds to simulations of device structures equivalent to the “GaAs w/” devices, but where all GaAs in the device was replaced by Al0.29Ga0.71As. Devices were simulated with two of the same back side geometries as in (a): a planar Ag reflector/contact (‘Ag reflector’) or with optimized diffractive structures that consist of Ag and SiO2 (‘Ag-SiO2 diff. struct.’).

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Tables (2)

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Table 1 AM 0 Jsc (mA/cm2) of InGaAs/GaAs devicesa

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Table 2 AM 0 Jsc (mA/cm2) of InGaAs/AlGaAs and InGaAs/GaAs devicesb

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Equations (3)

Equations on this page are rendered with MathJax. Learn more.

A s (λ)= ω 2 ∫ V S ε 2 (ω,r) | E(r) | 2 dr,
j(λ)= Φ ph × A S (λ)× η C (λ).
J sc = ∫ j(λ)dλ.
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