2016 International Conference on Power Engineering & Energy, Environment (PEEE 2016) ISBN: 978-1-60595-376-2 2-simulation of Inverted Metamorphic GaInP/Gas/In 0.3 Ga 0.7 s/in 0.58 Ga 0.42 s Four-unction Solar Cell an Xue 1,a, Jielei Tu 1,b, Wangfu Lai, Wei Zhang,Yuqing Sheng and Tengteng Guo 3,c 1 Yunnan Provincial Renewable Energy Engineering Key Laboratory, Kunming, China a 907862235@qq.com, b km-tl@263.net, corresponding author Keywords: PSYS; GaInP/Gas/InGas/InGas four-unction solar cell; theoretical simulation bstract. In the paper, based on the Crosslight PSYS, two-dimensional simulation has been performed on the GaInP/Gas/In 0.3 Ga 0.7 s/in 0.58 Ga 0.42 s four-unction solar cell. The optimized performance was shown as following: Voc=3540mV,Jsc=20.08 m/cm 2,FF=0.88,Eff=47.8%. nd the comparison with the exhibition ( Voc=3301mV, Jsc=17.33 m/cm 2, FF=0.834, Eff=34.87%)of a practical cell with the optimized structure was carried out as well. The reason could be come from anti-reflect film, pattern of front contact, voltage decrease of tunnel-unction and preparation proceeding. 1 Introduction Multiple-unction tandem solar cells have been shown as an attractive alternative for many terrestrial and space applications [1-2]. Multi-unction solar cells based on Gas are most competitive mong them, which are the urgent main electric source with good performance and long life-time applied on space. While the traditional TJ Gas solar cells have difficulty on improving further. nd the novel four-unction solar cells with better efficiency should be put forward by dividing sunlight spectrum. Recently, GaInNs(1.0eV) and InGas (1.0eV) are considered to be ideal third-unction subcell material for the four-unction solar cells [3-4]. But the efficiency of four-unction solar cells with GaInNs subcell is very low because of small short-circuit current. Therefore, in this work, based on Crosslight PSYS, GaInP(1.90eV)/Gas(1.42eV)/InGas(1.0eV)/InGas(0.7eV)4- solar cell with tunnel unctions are modeled. 2 Theoretical background and Introduction of PSYS PSYS, dvanced Physical Models of Semiconductor evices, is based on 2/3 finite element analysis of electrical, optical and thermal properties of compound and silicon semiconductor devices. Emphasis has been placed on band structure engineering and quantum mechanical effects [5]. Inclusion of various optical modules also makes this simulation package attractive for applications involving photosensitive or light emitting devices. PSYS simulator solves the Poisson s equation, and the current continuity equation for electrons and holes. These equations govern the electrical behavior (e.g., I-V characteristics) of a semiconductor device. 0 dc V n p N (1 f ) N f N t( ft) q (1) t n f J n Rn Rsp Rst Rau Gopt ( t) N (2)
t p J p Rp Rsp Rst Rau Gopt ( t) N f (3) Here the last term Eq(1) describes the deep trap density effect, V is electrical potential, ε 0 is vacuum dielectric constant, ε dc is relative C or low frequency dielectric constant, n is electron concentration, p is hole concentration, N is shallow donor density, N is shallow acceptor density, f is occupancy of the donor level, and f is occupancy of the acceptor level. In Eqs. (2) and (3), J n and J p are electron and hole current flux density respectively. R t n and R t p are electron and hole recombination rates per unit volume through the th deep trap respectively. G opt is the optic generation rate, R sp, R st, and R au are the spontaneous recombination rate, the stimulated recombination rate and the uger recombination rate per unit volume respectively. 3 evice structure and simulation details Based on the previous studies on the triple-unction(tj) Gas solar cell, the original device structure has been proposed and presented in Fig.1. s seen from the figure, the four-unction solar cell is constructed with four subcells, namely GaInP(1.90eV), Gas(1.41eV), In 0.3 Ga 0.7 s(1.00ev) and In 0.58 Ga 0.42 s(0.70ev) unctions stacked in series. Every subcell has a window layer at the top and a back surface field (BSF) layer at the bottom. Three tunnel unctions are implemented and placed between each pair of three subcells. The solar spectra used for irradiation is the standard 1 sun extra-terrestrial (M0) and the irradiance of M0 is around 1367 W/m 2 In the device simulation. Besides this, index file including wavelength dependent refractive index n and extinction coefficient k for Ga 0.5 In 0.5 P,Gas,In 0.3 Ga 0.7 s and In 0.58 Ga 0.42 s materials are also compiled and provided for the simulation.
Figure 1. Schematic of GaInP/Gas/InGas/InGas four-uncton solar cell device structure. 4 Results and discussion For a four-unction solar cell, the short-circuit current (I sc ) of the overall device is limited by the lowest I sc in the four sub-cells (here, Ga 0.5 In 0.5 P, Gas, In 0.3 Ga 0.7 s and In 0.58 Ga 0.42 s ), the open-circuit voltage (V oc ) of the four-unction cell is approximately the sum of their V oc [6]. So in this paper, the structure of every subcell and their performance should be developed thoroughly by PSYS at first. We get the simulation results: (1) In order to obtain good performance for the Ga 0.5 In 0.5 P top cell, the thicknesses and doping concentration of base layer should be controlled at 0.7μm and 4e17/cm 3 respectively; the thicknesses and doping concentration of emitter layer should be controlled at 0.05μm and 2e18/cm 3 respectively, the efficiency can achieve 19.46%. (2) Similarly, when the thicknesses and doping concentration of base layer were controlled at about 2.9μm and 7e17/cm 3 respectively, the efficiency of Gas subcell can achieve 21.05%. (3) For the In 0.3 Ga 0.7 s subcell and In 0.58 Ga 0.42 s subcell, when the minority carrier lifetime kept at 3ns and 8ns, it can obtain maximum short-circuit current. fter calculation of buffer layer for In 0.30 Ga 0.70 s/ In 0.58 Ga 0.42 s and current match for In 0.30 Ga 0.70 s/in 0.58 Ga 0.42 s and Ga 0.5 In 0.5 P/Gas dual-unction solar cells, the optimized structure of InGaP/Gas/In 0.30 Ga 0.70 s/ In 0.58 Ga 0.42 s was revealed. The band diagram in equilibrium is displayed in Fig.2, Cell 1,Cell 2,Cell 3 and Cell 4 are GaInP subcell (1.90eV), Gas subcell (1.41eV), In 0.3 Ga 0.7 s subcell (1.00eV)and In 0.58 Ga 0.42 s subcell (0.70eV) respectively. From the band diagram, We could see the Fermi level is unified clearly. One
could also notice the band diagram near the tunnel unctions where the conduction band electrons could be aligned with the valence empty states [7]. Figure 2. Band diagram of the Ga 0.5 In 0.5 P/Gas/In 0.3 Ga 0.7 s/in 0.58 Ga 0.42 s four-unction solar cell in equilibrium without solar irradiation ( In 0.58 Ga 0.42 s (left), Ga 0.5 In 0.5 P) (right). The optical power distribution is also displayed in Fig.3(a),where we could see clearly how the optical power varies as the subcell goes from Ga 0.5 In 0.5 P (at the right in the figure) to In 0.58 Ga 0.42 s (at left in the figure), and the high energy irradiation is mainly absorbed by the wide band gap Ga 0.5 In 0.5 P subcell at the top. This is also reflected in the profile of optical generation rate, which is presented in Fig.3(b). (a) (b) Figure 3. Optical power distribution (a) and optical generation rate (b) for four-unction solar cell. The comparison of J-V curves for the optimized and original structures under M0 solar spectrum is presented in Fig.4. s can be seen, V oc and J sc of optimized four-unction solar cell has been improved significantly. The short-circuit current density, open circuit voltage and conversion efficiency are
increased from 16.92m/cm 2 to 20.08m/cm 2, from 3.174V to 3.54V and from 28.07% to 47.8%, respectively. (a) (b) Figure 4. IV characteristics. (a) Experimental result from original device structure, and (b) simulation result for the optimized GaInP/Gas/InGas/InGas four-unction solar cell from Crosslight PSYS. ccording to the optimized solar cell structure, another group of team produced high-efficiency triple-unction and four-unction solar cells with inverted metamorphic device structures, the conversion efficiency achieved 32.64% and 34.87% under the standard 1 sun space spectrum(m0), respectively. We have listed the results for the simulation and experiment in table 1.The results are different, the reason could be come from anti-reflect film, pattern of front contact, voltage decrease of tunnel-unction and preparation proceeding. Table 1. Comparison of J-V performances under M0 solar spectrum. V oc (mv) J sc (m/cm 2 ) FF Eff(%) Simulation value 3540 20.08 0.88 47.8 Experimental value 3301 17.33 0.834 34.87 5 Summary In the work, the simulation and optimization have been performed on the GaInP/Gas/In 0.3 Ga 0.7 s/in 0.58 Ga 0.42 s four-unction solar cell by Crosslight PSYS. The excellent performance was shown as following, Voc=3540mV,Jsc=20.08 m/cm 2,FF=0.88,Eff=47.8%. nd the comparison with the exhibition ( Voc=3301mV, Jsc=17.33 m/cm 2, FF=0.834, Eff=34.87%)of a practical cell with the optimized structure was carried out as well. The reason could be come from anti-reflect film, pattern of front contact, voltage decrease of tunnel-unction and preparation proceeding. cknowledgement This work is supported by Collaborative Innovation Center of Research and evelopment of Renewable Energy in the Southwest rea (0530020516009).
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