JM5A.11.pdf Renewable Energy and the Environment © 2012 OSA
3-D Modeling of Triple Junction Solar Cells on 2-D Gratings
with Optimized Intermediate and Back Reflectors
O. Isabella, M. El-Shinawy, S. Solntsev, M. Zeman
Photovoltaic Materials and Devices/Dimes, Delft University of Technology, Feldmannweg 17, 2628 CT Delft, e-mail: olindo.isabella@gmail.com
Abstract: Superstrate thin-film silicon triple-junction solar cells on 2-D gratings were optimized
using opto-electrical modeling. Tuning the thickness of intermediate and back reflectors and the band gap of the middle cell resulted in 17% initial efficiency.
OCIS codes: 050.1755, 050.1950, 310.6845
Light trapping in thin-film silicon solar cells is accomplished by light scattering at internal interfaces and reflection at the rear side. We introduced glass-based 2-D pyramidal gratings (P = 1500 nm) for light scattering at the front side of a
superstrate triple-junction solar cell structure and a ZnO/Ag stack for reflection at the back side. We optimized both i) a thin design for a high stability and throughput (target: JPH-TOT = 27.9 mA/cm2) and ii) thick design for a high efficiency
(target: JPH-TOT = 30 mA/cm2). The investigated triple junction solar cells had the following structure: glass-based
textured substrate coated with a thin film of In2O3:H (IOH) [2] on which three p-i-n junctions were stacked on top of
each other with a-Si:H, a-SiGe:H, and nc-Si:H absorber layers, respectively. The rear side was completed with ZnO/Ag reflector/electrode. SiOx films (nSiOx = 2.1 at 770 nm) were used as n-type layer for the top and middle component cells
and as intermediate reflectors (IR1 and IR2, respectively) at the same time. Optical simulations were carried out with 3-D finite element method package (HFSS) [1] . The calculated optical generation rate was used as input for opto-electrical simulations carried out by the ASA software.
We carried out optimization of the current-matched solar cell structure in order to achieve a higher JPH-TOT with
respect to the specified target. The results of the optimization are reported in Fig. 1. We tuned (i) the thickness of intermediate and back reflectors (series 1-3), (ii) the thickness of front IOH and top p-layer (series 4), and (iii) the pyramid shape (series 5-8). In order to obtain current-matching after changing the band gap of a-SiGe:H absorber layer, we varied the thickness of absorber layers (series 9 and 10). We found two structures (denoted #2 and #3) fulfilling our requirements and their details are reported in Fig. 1. We show in Fig. 2 that using our light trapping scheme in a single junction nc-Si:H solar cell (inc-Si:H = 2.8 μm) the absorption limit predicted by 4n2 enhancement using ideal scattering is
approached. In Fig. 3 the spectral absorption rate profile in the triple-junction cell is shown. From these absorption rate profiles the optical generation rates were calculated and used in electrical simulations. Modeling enables us to visualize the spatial distribution of the magnitude of electric field inside the complete cell at different wavelengths that helps us to study the contribution of the textured back contact to scattering (see Fig. 4). Analysis of the total reflectance and absorptance in absorber and supporting layers guides us in further optimization of triple junction devices (see Fig. 5).
To evaluate external parameters from electrical simulations we used structures #2 and #3 in both current-matched and current-mismatched conditions. The latter was achieved by using not-optimized thicknesses for IR’s and back ZnO. The fill factor was found to be higher for thinner and mismatched structures, while the open circuit voltage resulted slightly higher only for thinner structures. An interplay between short circuit current density and fill factor occurred in case of structures #3, for which the current-mismatched structure presented slightly higher efficiency. Current matched thin (thick) design resulted in 16.3% (17.1%) potential initial efficiency.
Fig. 1 Total photo-current density in simulated triple junction cells. From left to right results of the subsequent optimization series are reported. Circled dots indicate current-matched triple junction devices above at least one of the two targets.
Number of simulations 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 To ta l JPH [mA /cm 2] 25.5 26.5 27.5 28.5 29.5 30.5 31.5 Intial structure 1 - IR1 thickness 2 - IR2 thickness 3 - BAZO thickness 4 - Supp. layers tuning 5 - Height series 6 - Angle series () 7 - Skewed pyramid 8 - Truncated pyramid 9 - For thin design 10 - For thick design Matching target #1 Matching target #2 Egi-a-SiGe:H = 1.55 eV Egi-a-SiGe:H = 1.5 eV #1 #2 #3 1: P = 1500, h = 450, 120 / 150 / 900 nm ( = 30.9°) 2: P = 1500, h = 450, 130 / 105 / 1100 nm ( = 30.9°) 3: P = 1500, h = 450, 175 / 175 / 2800 nm ( = 30.9°) 20 nm 70 nm 30 nm 100 nm 60 nm 150 nm 562 nm 750 nm 900 nm 50.2° 45° 36.8° 24.4° 17.75° Best IR1: 30 nm Best IR2: 80 nm Best BAZO: 70 nm #1 Symmetric pyramid Skewed pyramid #1 h = 250 nm h = 350 nm JPH-TOT = 30 mA/cm 2 JPH-TOT = 27.9 mA/cm 2
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