Simulation and optimization of n-type interdigitated back contact silicon heterojunction (IBC-SiHJ) solar cell structure using Silvaco Tcad Atlas

Available online at www.sciencedirect.com

ScienceDirect Solar Energy 127 (2016) 206–215 www.elsevier.com/locate/solener

Simulation and optimization of n-type interdigitated back contact silicon heterojunction (IBC-SiHJ) solar cell structure using Silvaco Tcad Atlas M. Belarbi a,⇑, M. Beghdad b, A. Mekemeche b a

Laboratory ‘‘Physico-Mecanical and Metallurgical Elaboration and Control of Materials” and Department of Technics Sciences, Faculty of Engineering Sciences, University Abdelhamid Ibn Badis, Mostaganem, Algeria b Laboratory ‘‘Signal-Systems” and Department of Physics, Faculty of Exact Sciences and informatics, University Abdelhamid Ibn Badis, Mostaganem, Algeria Received 30 November 2015; received in revised form 3 January 2016; accepted 12 January 2016

Communicated by: Associate Editor Takhir M. Razykov

Abstract Simulation models of interdigitated back contact silicon heterojunction (IBC-SiHJ) solar cells, not only help in understanding the cell behavior in line with the experimental results but also help further in predicting the cell performance, adding to the cost effectiveness in the cell processing. IBC-SiHJ solar cells that combine the hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction and interdigitated back contact (IBC) concepts are very promising in order to reach the highest one-junction efficiency (g). In this paper, we have studied these solar cells by two dimensional modeling using Silvaco Tcad Atlas software which has recently extended its capability to simulate these devices and given accurate bulk and interface complex defect models and allowed special specification of transport physics for the hetero-interface. The study has been done on the IBC-SiHJ structure based on n-type crystalline silicon (c-Si) by introducing a very thin intrinsic hydrogenated amorphous silicon (i-a-Si:H) layer between the c-Si base and the doped a-Si:H layers and varying the values of the following parameters: c-Si substrate and back-surface field (BSF) doping concentration, thickness of i-a-Si:H layer (Thi-a-Si) and rear side geometry. The impact of these parameters has been tested by generating the current–voltage characteristics under illumination. It is shown that the open circuit voltage (VOC) and g of IBC-SiHJ solar cells increase with decreasing i-a-Si:H thickness. The g improves further with the increase of p-type emitter width (2Wp), the decrease of n-type BSF width (2Wn) and gap width (Wg) which are explained by the simulation. The S-shaped J–V curves with low fill factor (FF) observed previously in experiments are confirmed by simulation. To improve FF, Thi-a-Si and Wg should decrease. Results indicate that to achieve high g, c-Si substrate and BSF doping concentration must be optimized. The Wg (spacing between BSF and the emitter) must be kept as small as possible to avoid recombination of minority carriers in the base. The optimum geometry corresponds to a minimum size BSF region and a maximum size emitter region. With these optimizations, an enhanced g 23.20% is demonstrated by the simulation. Ó 2016 Elsevier Ltd. All rights reserved.

Keywords: Silicon solar cells; Interdigitated back contact; Amorphous-silicon; IBC-SiHJ; Atlas software

⇑ Corresponding author.

E-mail address: [email protected] (M. Belarbi). http://dx.doi.org/10.1016/j.solener.2016.01.020 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.

M. Belarbi et al. / Solar Energy 127 (2016) 206–215

1. Introduction Physically-based simulation for solar cells has become very important for the reasons that, it is almost always much quicker and cheaper than performing experiments and it provides information that is difficult or impossible to measure. The IBC-SiHJ solar cell using thin layers of a-Si:H deposited at low temperature on a c-Si substrate is one of the most interesting technological solutions for the photovoltaic market, basically due to the excellent performance and the simple low-temperature process (Taira et al., 2007). Recently, Panasonic Corporation has achieved a world conversion efficiency of 25.6% on a commercial sized (143.7 cm2) monocrystalline-based ‘HIT’ solar cell (Masuko et al., 2014). The reduction improvements in recombination loss, optical loss and resistance loss were contributors to the efficiency record. The IBC-SiHJ solar cell combines the advantages of the IBC which has all the contacts at the back of the cell eliminating contact shading, leading to a higher short-circuit current (JSC) and silicon heterojunction solar cells with high VOC due to the better surface passivation of the deposited i-a-Si:H layer (Lammert and Schwartz, 1977). As result of this, IBC-SiHJ has the potential of higher VOC and JSC (Smith et al., 2010). However, it is found experimentally that, this deposited i-a-Si:H layer leads to a low FF, and ‘‘S” shape J–V curve is observed (Lu, 2008; Lu et al., 2007). But experimental results show that lowering its band gap improves the fill factor (Lu et al., 2007). To confirm these experimental results, there were several publications about 2-d modeling approaches using numerical simulator such as: Sentaurus and Atlas devices. Their simulations studies were performed to optimize the cell parameters and help further in predicting its performance. They reported high performance (>20%) for small area cells demonstrating the potential of IBC-SiHJ approach (Diouf et al., 2009, 2011; Lu et al., 2009). Even though they have achieved some improvements, IBC-SiHJ cell is still far from its expected efficiency potential. The main bottleneck for higher efficiency is up to now mainly limited by low FF values and high series resistance (Rs), compared with the conventional Si solar cell with diffused emitter homojunction and metal–Si direct contacts. The FF is low due to the re´sistance of i-a-Si:H layer, rear emitter (Diouf et al., 2009; Tucci et al., 2008) and contact (Lu et al., 2007; Ji et al., 2012). Improving the efficiency further requires the following: (1) Optimized intrinsic buffer layer for surface passivation to provide lowest surface recombination. (2) The geometry of back side (pitch, p-type emitter, n-type BSF and gap width) should be optimized to minimize series resistance. In this work, methods to optimize IBC-SiHJ solar cell with improved FF, less resistive losses and passivation quality are discussed

207

and guided by Atlas device simulation software operating in two dimensions and including a wide variety semiconductor physics models for drift-diffusion transport, SRH recombination, Auger recombination, surface recombination, carrier generation, Fermi–Dirac statistics, doping effects, band gap narrowing, tunneling, etc. We first present the geometrical structure of the solar cell, then explore different ways of the simulation by specifying the different physical models, and finally examine the simulation results in order to determine the important parameters to solve these problems and reach high g values.

2. Device structure 2.1. Geometry of the structure Due to the structure periodicity of the IBC-SiHJ solar cell, an elementary cell is used, that will serve as a basis for optimizing the performance of this type of cell and its schematic illustration is shown in Fig. 1. The width of this elementary structure (pitch) is equal to half the distance between two electrodes with the same polarity. The geometrical and material parameters of the simulated device were chosen in agreement with a realistic fabrication process. The substrate is a 250 lm thick and 1180 lm wide ntype Cz-Silicon wafer which contains fewer defects than float-zone (FZ) or multicrystalline material and therefore higher efficiencies are obtained. The sheet resistance is 2.25 X cm (Basore et al., 2014). The base is chosen n-type leading to distinctly higher VOC and hence higher g compared to n-type a-Si:H emitters on p-type c-Si substrates (Sawada et al., 1994; Jensen et al., 2002). For the reduction in the interfacial recombination state density to a minimum, it is necessary to introduce a very thin i-a-Si:H layer between the c-Si base and the heavily doped a-Si:H layers (Taguchi et al., 2000). On the front of the c-Si substrate an optical layer is placed that plays the role of anti-reflective coating layer like silicon nitride (SiNx). This front surface is subject to the illumination. On the rear side, the contacts of the p-type emitter and n-type BSF have an interdigitated pattern (that are formed using multiple lithography/masking, alignment, etching and/or deposition steps) of which the pitch was determined by optimization toward so called pseudo shadow losses. The thickness of Al contacts is 0.2 lm (Lu et al., 2009). The emitter (p-strip) and the BSF (n-strip) are completely covered by aluminum (Al) metal contacts, their depth is 0.02 lm (Lu et al., 2009), theirs half default widths are respectively 950 and 180 lm and are interdigitated as mentioned above, and the gap between them is set to be 50 lm. Some dimensions will be tuned in the simulation.

208

M. Belarbi et al. / Solar Energy 127 (2016) 206–215

Fig. 1. Schematic picture of IBC-SHJ solar cell: (a) cross sectional view and (b) bottom view.

2.2. Physical models Modeling of IBC-SiHJ solar cell is a matter of some difficulty caused by the necessity of treating 2-dimensional currents, the presence of amorphous materials with ambiguous parameters some of which are difficult to obtain from experiment, complex transport mechanisms, modified light generation profile and so on. Neither of conventional programs used in photovoltaic (e.g. PC1D, AFORS-HET, PC2D, . . .) are suitable. However, Silvaco ATLAS supports all these requirements. In this paper, the work presented is a step toward understanding the IBC-SiHJ cells with simulation performed using Silvaco ATLAS, which has recently extended its capability to support heterojunctions, midgap and interface traps for these types of solar cells (Atlas, 2013). The paper focuses on the behavioral trends in the cell electrical parameters viz. JSC, VOC, FF and g, with variations in IBC-SiHJ geometrical parameters and c-Si substrate and back-surface field (BSF) doping concentration. Atlas is a physically-based two and three dimensional device simulator that predicts the electrical behavior of semiconductor devices at specified bias conditions. The physical structures simulated with Athena are used as input by Atlas. The combination of Athena and Atlas makes possible to determine the impact of process parameters on device characteristics. The simulation is based on the solution of the three governing semiconductor equations: Poisson’s equation, electrons and holes continuity equations. Fermi statistic was used for carriers with drift-diffusion combined with Bohm

Quantum Potential for quantum correction. Fermi model and recombination models (Shockley–Read–Hall (SRH), Auger and surface recombination) were also included into the simulation. The radiative recombination is not included, because in silicon, band to band generation/ recombination is insignificant. The carrier mobility was taken dependent on the doping concentration. Recombination of carriers through SRH and Auger mechanisms were also modeled as a function of doping concentration (Lu et al., 2009). 2.3. Recombination models applied to c-Si 2.3.1. SRH recombination The SRH recombination is modeled as follows: RSRH ¼

pn  n2ie Ei h D Ei Ei Et i sp n þ nie exp Ekt E p þ n exp þ s n ie T k T B B h

D

ð1Þ sn and sp are the electron and hole lifetimes that depend on the c-Si doping concentration (Fossum and Lee, 1982) and nie is the effective intrinsic carrier concentration. Et and Ei are the trap energy level and the intrinsic Fermi level respectively, T is the lattice temperature in degrees Kelvin. n and p are the electron and hole concentrations. This model only presumes one trap level in which, Et = Ei and it corresponds to the most efficient recombination center. The lifetime associated to SRH recombination is given by Law et al. (1991):

M. Belarbi et al. / Solar Energy 127 (2016) 206–215

sn ¼

sn0 sp0 sp ¼ 1 þ N =N SRHN 1 þ N =N SRHP

here N is the doping concentration NSRHN = NSRHP = 5  1016 cm3.

of

c-Si,

2.3.2. Auger recombination Auger recombination is commonly modeled using the expression: RAuger ¼ AUGN ðpn2  n0 n2ie Þ þ AUGp ðnp2  p0 n2ie Þ

ð2Þ

where Auger recombination parameters are taken as: AUGN = 8.3  1032 cm6/s and AUGP = 1.8  1031 cm6/s. n (p) the concentration of electrons (holes), n0(p0) the corresponding values at equilibrium and nie is the effective intrinsic concentration. For low-injection conditions, the Auger lifetime for n-type c-Si is: sAug ¼

1 1:8  1024 N 1:65 D

2.3.3. Surface recombination The standard method is to model surface recombination in a similar manner as the bulk generation–recombination rate. The calculation of this rate is an extension of the SRH theory by introducing the surface recombination velocities for electrons and holes respectively (Sn0 or Sp0). The recombination rate is calculated as follows: Rsurf ¼

pn  n2i h D Ei h D Ei i t seff n þ ni exp Ekt E p þ ni exp Eki E þ seff n p BT BT ð3Þ

Here: 1 1 di ¼ i þ S n0 s Ai seff n n

and

1 1 di ¼ i þ S p0 s Ai seff p p

sin is the bulk lifetime calculated at node i along the interface. The di and Ai parameters are the length and area of the interface for node i. For a-Si layers, critical parameters like band gap, doping and defect distribution are defined in the input deck. The critical parameters for accurate simulation are energy distribution of the exponential band tails, and the Gaussian distribution of the mid-gap trap states. They were chosen according to reference (Lu et al., 2009; Munos et al., 2011) and shown in Fig. 2. The input parameters used in simulation and the parameters characterizing the mid-gap and band tail defect densities are shown in Table 1 and similar to Lu et al. (2009) and Munos et al. (2011). For c-Si/a-Si interfaces at the back surface we have used a thermionic emission model in which the distribution function of the interface defect is modeled by two capture cross-sections, one for the holes and one for the electrons. To have realistic modeling of this interface defect states, it

209

has been introduced at the hetero-interface a very thin defective layer of c-Si (Diouf et al., 2011). An AM1.5G solar spectrum is used for the optical generation to simulate the J–V curve under standard one-sun illumination conditions at an intensity of 0.1 W cm2. A Sopra database is used for a-Si index of refraction (Atlas, 2013). 2.4. Meshing of the structure The correct specification of a mesh is critical in process simulation. The number of nodes in the mesh has a direct influence on simulation accuracy and time. A mesh as thin as possible applied to the whole structure ensures good accuracy of calculations but requires greater computation time to simulate the behavior of this structure. It is therefore necessary to find a compromise between computational time and accuracy of the calculation. A finer mesh should exist in those areas of the simulation structure where ion implantation will occur, where p-n junction will be formed, or where optical illumination will change photoactive component concentration and a coarse mesh in areas where these quantities are quasi static. In the middle of the c-Si substrate, a coarse mesh is used as the physical quantities do not vary significantly. Fig. 3 represents the mesh used to simulate IBC-SiHJ solar cell. 3. Results and discussion The efficiency is a major parameter in order to reduce the overall costs of a given PV silicon technology. This means that every effort to improve the efficiency at the cell and module level has a direct impact on the whole system cost. On IBC-SiHJ structure on n-type crystalline silicon substrate, previous simulations studies (Lu et al., 2007, 2009; Diouf et al., 2009, 2011) reported limiting factor in the improvement of JSC, VOC, FF and g. Our work presented is a step toward improving further this factors specially the g. The simulation of IBC-SiHJ solar cell was performed, and Figs. 4 and 5 show the current density–voltage (J–V) curves for the variation of parameters i-a-Si thicknesses and n-BSF dopings respectively. The influence of the i-a-Si:H thickness variation is illustrated in Fig. 4, where the J–V characteristic is depicted. It helps us determine the electrical performance of an IBC-SiHJ solar cell. Increasing the i-a-Si:H thickness from 2 to 15 nm has no influence on JSC and on VOC, also the FF is not affected, but beyond the value 15 nm the ‘‘S” shape with a low FF is demonstrated and it confirmed the experiment results given in Lu et al. (2009). The variation of n-BSF doping (we do not consider the possible increase in interface recombination velocity with increased doping level of a-Si:H (n) (Jeyakumar et al., 2014)) and its influence on the J–V characteristic is illustrated in Fig. 5. As it can be seen from the J–V curves, the S-shape is totally removed when n-BSF doping is P5  1018 cm3. Increasing the n-BSF doping from 5  1016 to

210

M. Belarbi et al. / Solar Energy 127 (2016) 206–215 1E21

1E21

Acceptor tail state density Donor tail state density Acceptor mid-gap defects Donor mid-gap defects

Acceptor tail state density Donor tail state density Acceptor mid-gap defects Donor mid-gap defects

1E20

) -1

1E19

1E19

Defect levels (cm-3 .eV

Defect levels (cm -3 .eV

-1

)

1E20

1E18

1E17

1E16

1E15

1E18

1E17

1E16

1E15 0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

0,0

0,2

0,4

Energy (eV)

1,2

1,4

1,6

1,8

1E17

1E16

Defect levels (cm- 3.eV - 1 )

Defect levels (cm -3.eV -1)

1,0

Defect density levels in n-type amorphous layer

Acceptor tail state density Donor tail state density Acceptor mid-gap defects Donor mid-gap defects

1E17

0,8

Energy (eV)

Defect density levels in p-type amorphous layer

1E18

0,6

1E15 1E14 1E13 1E12

1E16

1E15

Acceptor mid-gap defects Donor-midgap defects 1E14

1E11 1E10

1E13 0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

Energy (eV)

Defect density levels in intrinsic amorphous layer

0,0

0,2

0,4

0,6

0,8

1,0

Energy ( eV )

Defect density levels in c -Si defective layer

Fig. 2. Density of states (DOS) in a-Si:H p-type, a-Si:H n-type, i-a-Si:H and defect region layer respectively.

5  1018 cm3 has a significant influence on the JSC, but not on the VOC. 3.1. Sensitivity on c-Si substrate doping A set of simulation is performed in order to estimate the sensitivity of cell performance to different bulk dopings, which is varied within the range 2  1016–3.5 1016 cm3. This sensitivity was evaluated while keeping the total device parameters fixed, with front surface recombination velocity (SRV) 10 cm/s derived from lifetime measurements (Lu et al., 2009), which gives good passivation at the front and rear surfaces and low recombining a-Si:H/c-Si interfaces. The bulk lifetime is defined as: 1=sbulk ¼ 1=sSRH þ 1=sAug It is determined by SRH and Auger recombination process and these processes depend on the doping concentrations. sbulk is depicted in Fig. 6.

For doping concentrations below 1016 cm3 (which corresponds to NSRH), the SRH recombination mechanism is the predominant one. The lifetime is mainly determined by the SRH process as seen in Fig. 6. The Auger recombination becomes significant when concentrations are above 1016 cm3 and even becomes the dominant recombination process for Nd > 5  1016 cm3. When the doping concentration increases, the JSC decreases as we observe in Fig. 7. This is due to the lifetime of carriers and their mobility which decrease with higher doping concentration and the Auger recombinations becomes stronger. The impact of c-Si substrate doping on VOC is negligible. By increasing the c-Si doping concentration, the FF is improved, see Fig. 7. This improvement is related to the decrease of the resistivity of the c-Si substrate. The parameters JSC and FF do not evolve in the same direction when the doping concentration varies. These various changes result in the existence of an optimum c-Si

M. Belarbi et al. / Solar Energy 127 (2016) 206–215

211

Table 1 List of modeling parameters and defect distributions used in the simulations. Material

Interface defects

Bulk lifetime (s) Band gap (eV) Electron affinity (eV) Effective conduction band DOS (cm3) Effective valence band DOS (cm3)

1.17

c-Si

n-Type a-Si:H

Buffer a-Si:H

p-Type a-Si:H

2  103 1.17 4.05 2.89  1019 3.14  1019

2  1012 1.70 3.9 2.5  1020 2.5  1020

2  1012 1.70 3.9 2.5  1020 2.5  1020

2  1012 1.65 3.9 2.5  1020 2.5  1020

Conduction tail states

Nctail A (cm3) Ectail A (eV)

0 0.07

n/a n/a

1021 0.12

1018 0.09

1021 0.07

Valence tail states

NVtail D (cm3) EVtail D (eV)

0

n/a

1021

1018

1021

3

0.12

n/a

0.12

0.09

0.12

16

19

16

Acceptor-like (A) dangling bond states

N A (cm ) Edb A (eV) rdb A (eV)

2.4  10 0.5 0.2

n/a n/a n/a

10 0.7 0.2

10 1.1 0.15

1019 1.3 0.2

Donor-like (D) dangling bond states

Ndb D (cm3) Edb D (eV) rdb D (eV)

2.4  1016 0.5 0.2

n/a n/a n/a

1019 0.45 0.2

1016 0.9 0.15

1019 1.1 0.2

1417 470

1417 470

2.03

2.03

2.03

db

Electron mobility (cm2 V/s) Hole mobility (cm2 V/s)

Current density (mA.cm-2 )

40

30

0.002 μm 20

0.006 μm 0.010 μm 0.015 μm 0.020 μm

10

0 0,0

Fig. 3. Meshing of elementary IBC-SiHJ solar cell structure.

doping concentration corresponding to the maximum g of IBC-SiHJ solar cells, as shown in Fig. 7. This optimal doping concentration is around ND = 2.4  1016 cm3 (q = 2.25 O cm) for s0,SRH = 1 ms and tends to decrease due to the degradation of the carrier lifetime and mobility. Then, it is necessary to use c-Si substrates resistive (q = 2.25 O cm) for better performance. 3.2. Sensitivity on n-BSF doping The a-Si:H applied to the rear side of IBC-SiHJ devices should have an enhanced rd no matter the rph, since there is no light-activated conduction at the back side of the solar

0,1

0,2

0,3

0,4

0,5

0,6

0,7

Voltage ( V ) Fig. 4. J–V curves as function of i-a-Si:H thicknesses.

cell. Besides, low defect state density is also required to enhance passivation properties. Then, since dark conductivity of good quality (low defective) intrinsic layers is very low, the only way to reduce series resistance is by doping them, which thus also introduces defects. The effect of n-BSF doping was evaluated while keeping the total device parameters fixed as above. The JSC increases slightly when the doping concentration increases from 4.5  1018 to 5.4  1018 cm3, beyond this value, the JSC becomes saturated as we observe in Fig. 8. When increasing active doping concentration of (n) a-Si:H, band bending at the a-Si:H/c-Si hetero interface increases, which

212

M. Belarbi et al. / Solar Energy 127 (2016) 206–215

the maximum g of IBC-SiHJ solar cells, as shown in Fig. 8. This optimal doping concentration is around ND = 4.8  1018 cm3 which corresponds to the maximum of g.

40

Current density (mA.cm-2 )

30

3.3. Effect of IBC-SiHJ solar cell device dimensions The results of the data of experiments (DoE) are shown in Table 2 below.

20

15

-3

5x10 ( cm ) 10

16

( cm )

17

( cm )

18

( cm )

5x10

18

( cm )

0,1

0,2

5x10 5x10 1x10

0 0,0

-3 -3 -3

-3

0,3

0,4

0,5

0,6

0,7

Voltage ( V ) Fig. 5. J–V curves as function of n-BSF doping.

3.3.1. Effect of Thi-a-Si The aim of including an intrinsic a-Si:H layer is to passivate the dangling bonds on the c-Si surface. As a result, the a-Si:H/c-Si interface defect-state density is significantly reduced. Applying an emitter and BSF buffer layer, 726 mV is reached (Table 2) due to the relatively low recombination rate at the a-Si:H/c-Si interface. Table 2 shows a slight increase of VOC from 720 to 726 mV when the Thi-a-Si increases from 3 to 17 nm. This enhancement of the VOC is due to low defective i-a-Si:H layers. However, the FF reduces drastically from 83.44% to 74.60% with increasing i-layer thickness. The impact of Thi-a-Si on JSC is negligible, as we observe in Table 2. These predicted trends are qualitatively consistent with the results of other authors e.g. in references Lu et al. (2009) and Diouf et al. (2011). We found an optimum Thi-a-Si = 6 nm, which leads to a highest g. 3.3.2. Effect of the rear side geometry The geometry of the rear side is also one of the research areas to optimize the performance of IBC-SiHJ solar cells. The Wp, Wn and Wg, may have an impact on IBC-SiHJ solar cells. Note that the varied widths in the simulation, Wp and Wn, correspond to half of the entire emitter and BSF stripe width respectively.

Fig. 6. Effect of SRH and Auger recombination mechanisms on the bulk lifetime sbulk, depending on the doping concentrations ND.

is favorable for better charge carrier collection. The doping reduces series resistance, but also introduces defects which thus limit the JSC. As it can be seen from the curve VOC vs n-BSF doping (Fig. 8), the VOC decreases slightly when n-BSF doping increases, because defect density (number of gap states in the a-Si:H layers and interface states) is increased. By increasing the n-BSF doping concentration, the FF is improved, see Fig. 8. This improvement is related to enhanced conductivity and to the strong electrical field which collects carriers and provokes enough band bending. The parameters VOC and JSC or FF do not evolve in the same direction when the doping concentration varies. These various changes result in the existence of an optimum n-BSF doping concentration corresponding to

3.3.2.1. Effect of Wp. The study of the Wp influence is important to optimize the geometry of the rear side. We therefore varied Wp by fixing Wn and Wg and maintainig the pitch constant. Increasing Wp, from 400 to 1050 lm, results in a large increase of JSC from 35 to 38.5 mA cm2, also increasing efficiencies at first, followed by a decrease, as it can be seen in Table 2. Minority carriers in the c-Si generated over the emitter region do not need to travel laterally, resulting in higher efficiencies for wider emitters. Wider emitters also increase the length that the majority carriers have to travel to reach the BSF increasing the series resistance. This explains the decrease of the FF observed in Table 2 when Wp increases. Once series resistance is large enough a drop in efficiency occurs. The different evolution of JSC, VOC, FF and g leads to an optimum of the Wp of 950 lm. 3.3.2.2. Effect of Wn and Wg. Results show that large Wn and Wg result in decreased efficiencies. This is due to increased average lateral distance that minority carriers have to travel to reach the emitter, which increases their

M. Belarbi et al. / Solar Energy 127 (2016) 206–215 0,724

38,5

213

84,2 23,20 84,0

38,4

23,18

Jsc 83,8

38,2

FF (%)

23,16

η

83,6

Voc

0,722 83,4

η (%)

38,3

Voc ( V )

Jsc (mA.cm -2)

0,723

23,14

FF

23,12

38,1 83,2 38,0 2,0E16

23,10

0,721 2,4E16

2,8E16

3,2E16

2,0E16

2,4E16

2,8E16

3,2E16

c-Si doping concentration (cm-3)

-3

c-Si doping concentration (cm )

Fig. 7. IBC-SiHJ solar cell key figure of merits as a function of c-Si doping concentration.

38,378 83,65 83,60

23,19

83,55

0,722

Voc

23,18

η

83,50

FF

23,17

83,45 23,16

83,40 0,720 5,6E18

38,376 4,8E18

5,2E18 -3

n-BSF doping concentration ( cm )

η(%)

38,377

Voc (V)

Jsc

FF (%)

-2

Jsc (mA.cm )

0,724

23,20

83,35 4,8E18

5,2E18

23,15 5,6E18

n-BSF doping concentration ( cm -3 )

Fig. 8. IBC-SiHJ solar cell key figure of merits as a function of n-BSF doping concentration.

chances to recombine. Increasing the Wn from 80 to 450 lm results in a slightly decrease of JSC as we can see in Table 2. Wn variation has no influence on VOC. The BSF should be taken as narrow as possible to reach best JSC values, but narrow n-strip can cause electrical shading effects. From the other side wide n-strip can increase series resistance to majority carriers current. Thus Wn should be optimized. The optimum value which corresponds to the highest g is 180 lm. In Table 2, the smaller the Wg, the higher the efficiency is, because the recombination of minority carriers on the surface between emitter and BSF is smaller. As for the BSF region, increased Wg corresponds to an additional lateral distance to travel for minority carriers photogenerated in c-Si above the BSF region. This additional distance increases their chances to recombine before reaching the emitter. In addition, the interface with the c-Si substrate and the gap region can also be recombining. The impact of the Wg on JSC, on FF and g is shown in Table 2.

Increasing Wg results in a decrease of JSC, FF and g, but the VOC remains constant. The Wg will be taken as short as possible with good surface passivation to avoid recombination of minority carriers in the bulk c-Si and damage of the cell performance. To remove this gap region would be an attractive solution but this would lead to internal cell short circuit, thus making this solution is unsuited. Smaller gap (between emitter and BSF region) was effective to increase g and thus, we assumed a suitable value of 50 lm in order to eliminate the possibility of shunting. With these optimizations, a conversion g 23.20% is reached. 4. Conclusion The simulation has provided an initiative to help understand and obtain a detailed analysis of IBC-SiHJ cell characteristics. 2D numerical simulations for IBC-SiHJ solar cells have been studied by using the software SILVACO

214

M. Belarbi et al. / Solar Energy 127 (2016) 206–215

Table 2 Summary of device performance of IBC-SiHJ solar cell. JSC (mA cm2)

VOC (mV)

FF (%)

g (%)

Wp (lm)

JSC (mA cm2)

VOC (mV)

FF (%)

g (%)

38.4646 38.4647 38.4646 38.4645 38.4643 38.4641 38.4636 38.4632 38.4625

720 721 723 724 725 725 726 726 724

83.44 83.40 83.25 83.06 82.81 82.47 81.75 80.60 74.60

23.14 23.17 23.20 23.16 23.12 23.05 22.87 22.54 20.80

400 500 600 700 800 900 950 1000 1050

35.09 36.08 36.83 37.48 37.93 38.27 38.38 38.46 38.52

722 722 722 722 723 723 723 723 723

84.05 84.02 83.97 83.91 83.81 83.64 83.50 83.23 82.69

21.34 21.94 22.38 22.76 23.01 23.16 23.20 23.17 23.06

Wn (lm)

JSC (mA cm2)

VOC (mV)

FF (%)

g (%)

Wg (lm)

JSC (mA cm2)

VOC (mV)

FF (%)

g (%)

080 110 130 180 250 300 350 400 450

38.52 38.48 38.46 38.37 38.21 38.03 37.84 37.62 37.32

723 723 723 723 723 723 723 723 723

82.71 83.12 83.26 83.51 83.68 83.77 83.83 83.88 83.92

23.07 23.16 23.18 23.20 23.14 23.06 22.96 22.83 22.67

38.41 38.39 38.37 38.35 38.33 38.31 38.29 38.27 35.25

723 723 723 723 723 723 723 723 723

83.52 83.51 83.50 83.49 83.48 83.47 83.46 83.45 83.44

23.22 23.21 23.20 23.18 23.16 23.15 23.13 23.12 23.11

Thi-a-Si (lm) 0.003 0.004 0.006 0.008 0.010 0.012 0.015 0.017 0.020

30 40 50 60 70 80 90 100 110

Values in bold correspond to the maximum efficiency.

ATLAS Device, and several geometrical parameters were studied and their impact was shown on the IBC-SiHJ solar cell output characteristics. The general results of 2D distribution and current density–voltage curves were generated, and the dependence of cell performance on the c-Si substrate and n-BSF dopings and rear side geometry are evaluated. The simulations suggest that c-Si substrate and n-BSF dopings should be optimized with the values 2.4  1016 cm3 and 4.8  1018 cm3 respectively. The optimum geometry corresponds to a minimum size Wn equals to 180 lm and a maximum size Wp region equals to 950 lm. The width of the gap region (spacing between BSF and the emitter) must be kept as small as possible to avoid recombination of minority carriers in the bulk c-Si and its value is 50 lm. These specific parameters values have increased the cell performance, and the g is enhanced and permitted a conversion equals 23.20%, thus demonstrating the great potential of these optimum parameters. References Atlas User’s Manual: Device Simulation from Silvaco International, Version 5.19.20, 2 October 2013 (in ). Basore, P.A., Clugston, D.A., Cabanas-Holmen, K., 2014. Aid PC2D V2.4 and PC1D V5.9 (in , on 10 April 2014). Diouf, D., Kleider, J.P., Desrues, T., Ribeyron, P.J., 2009. Study of interdigitated back contact silicon heterojunctions solar cells by twodimensional numerical simulations. Mater. Sci. Eng. B 159–160, 291– 294. Diouf, D., Kleider, J.P., Longeaud, C., 2011. Two-Dimensional simulations of interdigitated back contact silicon heterojunctions solar cells. Book Physics and Technology of Amorphous-Crystalline Silicon Heterostructure Solar Cells. Edit. Springer, pp. 483–519 (Chapter 15). Fossum, J.G., Lee, D.S., 1982. A physical model for the dependence of carrier lifetime on doping density in nondegenerate silicon. Solid State Electron. 25, 741–747.

Jensen, N., Hausner, R.M., Bergmann, R.B., Werner, J.H., Rau, U., 2002. Optimization and characterization of amorphous/crystalline silicon heterojunction solar cells. Prog. Photovolt. Res. Appl. 10, 1–13. Jeyakumar, R., Maiti, T.K., Verma, Amit, 2014. Two-dimensional simulation studies on high-efficiency point contact back heterojunction (a-Si:H/c-Si) solar cells. Sol. Energy 105, 109–115. http://dx.doi.org/ 10.1016/j.solener.2014.03.030. Ji, K., Syn, H., Choi, J., Lee, H.M., Kim, D., 2012. The emitter having microcrystalline surface in silicon heterojunction interdigitated back contact solar cells. Jpn. J. Appl. Phys. 51, 10NA05. Lammert, M.D., Schwartz, R.J., 1977. The interdigitated back contact solar cell: a silicon solar cell for use in concentrated sunlight. IEEE Trans. Electron Devices 24, 337–342. Law, M.E. et al., 1991. Self-consistent model of minority-carrier lifetime, diffusion length, and mobility. IEEE Electron Device Lett. 12 (Nov 8). Lu, Meijun, Das, U., Bowden, S., Birkmire, Robert, 2008. Rear surface passivation of interdigitated back contact silicon heterojunction solar cell and 2D simulation study. In: 33rd IEEE Photovoltaic Specialists Conference, PVSC ’08, pp. 1–5. http://dx.doi.org/10.1109/PVSC.2008. 4922757. Lu, M., Bowden, S., Das, U., Birkmire, R., 2007. Interdigitated back contact silicon heterojunction solar cell and the effect of front surface passivation. In: Proceedings of the 22nd European Photovoltaic Solar Energy Conference, Milano, Italy, pp. 924-927, or Appl. Phys. Lett. 91 (2007) 063507. http://dx.doi.org/10.1063/1.2768635, URL: . Lu, M. et al., 2009. Optimization of interdigitated back contact silicon heterojunction solar cells by two-dimensional numerical simulation. In: Proceedings of 34th IEEE PVSC, Philadelphia, USA. Masuko, Keiichiro, Shigematsu, Masato, Hashiguchi, Taiki, Fujishima, Daisuke, Kai, Motohide, Yoshimura, Naoki, Yamaguchi, Tsutomu, Ichihashi, Yoshinari, Mishima, Takahiro, Matsubara, Naoteru, Yamanishi, Tsutomu, Takahama, Tsuyoshi, Taguchi, Mikio, Maruyama, Eiji, Okamoto, Shingo, 2014. Achievement of more than 25% conversion efficiency with crystalline silicon heterojunction solar cell. IEEE J. Photovolt. 4 (6). Munos D. et al., 2011. Key features of highly efficient a-Si:Hc-Si heterojunction solar cells. 7th Workshop on the Future Direction of Photovoltaics (by JSPS 175th committee), March.

M. Belarbi et al. / Solar Energy 127 (2016) 206–215 Sawada, T., Terada, N., Tsuge, S., Baba, T., Takahama, T., Wakisaka, K., Tsuda, S., Nakano, S., 1994. High-efficiency a-Si/c-Si heterojunction solar cell. In: Proceedings of 1994 IEEE 1st World Conference on Photovoltaic Energy Conversion, Waikoloa, USA, pp. 1219–1226. Smith, D.D., Luan, H.C., Manning, J., Dennis, T.D., Waldhauser, A., Wilson, K.E., Harley, G., Mulligan, W.P., 2010. Generation 3, improved performance at lower cost. In: Proceedings of 35th IEEE PV SC, pp. 275–278. Taguchi, M., Kawamoto, K., Tsuge, S., Baba, T., Sakata, H., Morizane, M., Uchihashi, K., Nakamura, N., Kiyama, S., Oota, O., 2000.

215

HITTM cells-high efficiency crystalline Si cells with novel structure. Prog. Photovolt. Res. Appl. 8, 503–505. Taira, S., Yoshimine, Y., Baba, T., Taguchi, M., Kanno, H., Kinoshita, T., Sakata, H., Maruyama, E., Tanaka, M., 2007. Our approaches for achieving HIT solar cells with more than 23% efficiencies. In: Proceedings of the 22nd EPVSEC, Milan, Italy, pp. 932–935 (ISBN:3-936338-22-1). Tucci et al., 2008. Back enhanced heterostructure with interdigitated contact behind solar cell. In: Proceedings of IEEE Conference on Optoelectronic and Microelectronic Materials and Devices COMMAD, pp. 242–245.