Computer modeling of the front surface field layer on the performance of the rear-emitter silicon heterojunction solar cell with 25 % efficiency

Journal Pre-proof Computer modeling of the front surface field layer on the performance of the rear-emitter silicon heterojunction solar cell with 25 % efficiency Hyeongsik Park, Muhammad Quddamah Khokhar, Eun-Chel Cho, Minkyu Ju, Youngkuk Kim, Sangho Kim, Junsin Yi

PII:

S0030-4026(19)31910-2

DOI:

https://doi.org/10.1016/j.ijleo.2019.164011

Reference:

IJLEO 164011

To appear in:

Optik

Received Date:

27 October 2019

Accepted Date:

6 December 2019

Please cite this article as: Park H, Quddamah Khokhar M, Cho E-Chel, Ju M, Kim Y, Kim S, Yi J, Computer modeling of the front surface field layer on the performance of the rear-emitter silicon heterojunction solar cell with 25 % efficiency, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.164011

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Computer modeling of the front surface field layer on the performance of the rear-emitter silicon heterojunction solar cell with 25 % efficiency

Hyeongsik Parka, Muhammad Quddamah Khokhara, Eun-Chel Choa, Minkyu Jua, Youngkuk

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Kima, Sangho Kimb and Junsin Yia,* College of Information and Communication Engineering, Sungkyunkwan University, Suwon,

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea

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16419, Republic of Korea

First author: Hyeongsik Park ([email protected])

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Corresponding author: Professor Junsin Yi ([email protected])

Abstract

Highly conductive materials with wide band gaps are used as front surface field (FSF) layer to achieve a prominent efficiency in silicon heterojunction (SHJ) solar cells. In this study, we demonstrate an n-type hydrogenated microcrystalline silicon oxide (µc-SiO:H) layer with high conductivity and beneficial optical properties for its application in SHJ solar cells. To develop a

substitute to a-Si:H (n), we started our research in the synthesis of HIT-type solar cells with three different layers, namely, a-Si:H (n), micro-crystalline silicon (µc-Si: H(n)), and µc-SiO:H (n). Owing to its better surface passivation, wide optical gap, and high conductivity, the µc-SiO:H (n) layer was employed as a substitute to a-Si:H (n). It is difficult to thoroughly investigate the effects of every parameter, such as the thickness, the electron affinity, and the doping density on the device performance experimentally. We, therefore, used a program based on the automat for simulation

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of heterostructures (AFORS-HET), to evaluate the limitation of the conversion efficiency, which

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provides a convenient way to accurately evaluate the role of various parameters. We obtained a high efficiency with open circuit voltage, (VOC) of 755.3 mV and a fill factor (FF) of 79.82% are

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essential factors owing to a favorable bending of the conduction band in the μc-SiO:H (n) next to

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the a-Si:H (i). We achieved a high efficiency of 25.35 % using a μc-SiO:H film with both an

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appropriate electron affinity of 4.1 eV and the doping density of 1019 cm-3.

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Keywords: High efficiency, Front surface field, Silicon heterojunction solar cell, AFORS-HET 1. Introduction

Silicon heterojunction (SHJ) solar cells applied at the interfaces of n-type/Si and p-type/Si (n-

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and p-layers) through an intrinsic thin amorphous silicon layer (i-layer) has appealed significant consciousness owing to their high energy conversion efficiency and inferior thermal budget in comparison with conventional crystalline silicon (c-Si) solar cells [1-8]. SHJ solar cells have become the cutting-edge research over the last 20 years because of certain advantages, namely, a higher performance stability, low degradation, cost efficiency, a short production process, a high open-circuit voltage (VOC) compared with conventional homojunction c-Si cells, and a low

processing temperature [9, 10]. To date, the highest efficiency achieved in an SHJ solar cell has been reported to be approximately 26.63% [11]. Various attempts at further improving the efficiency of SHJ solar cells have been made by emphasizing certain aspects, namely, reductions in the material cost and carrier collection loss by employing a thinner wafer, a reduction in the parasitic absorption loss by optimizing the silver grid electrode at the front side through a higher

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conductivity and aspect ratio, and the use of a superior quality surface passivation for a reduction of the carrier recombination losses.

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We studied a rear emitter SHJ (RE-SHJ) structure, and achieved greater superiority than that of

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a front emitter SHJ structure [12]. In the case of a front emitter structure, the p-layer thickness should be resolute while considering the electrical properties and absorption losses of the p-layer.

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A p-type SHJ emitter is luminous at the front emitter, which is powerfully achieved through the parasitic absorption within the SHJ layers. Optimization of the transparent conductive oxide (TCO)

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and a-Si:H at the front stack is tricky because it occurs at the cost of the fill factor (FF) and VOC [13]. In addition, the doping of the a-Si:H layers is strongly related to the performance of the

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device. A thick layer and sufficient doping has to be preferred for higher built-in voltage [14, 15]. Nevertheless, with high doping, the junction recombination can be enhanced [16], and above a certain level of doping concentration, VOC will decrease. A key problem is the establishment of a

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Schottky barrier among the interfaces of the n-type TCO with p-type a-Si:H. To describe the Schottky barrier, a thick layer and the maximum a-Si:H doping are needed, which can result in a trade-off among FF, the current density (JSC), and VOC [13, 17]. Thus, to overcome the consequences of some of the issues described above, we chose the application of a RE-SHJ, which has a low parasitic absorption at the front [18-20]. Thus, optimization of the RE-SHJ can be mostly confined to the interface, carrier transport and

recombination. A broad range of contact layers can be utilized, and an extremely conductive TCO for a contact layer, essential for the lateral conductivity of an SHJ solar cell at the front side. Thus, assuming the interface properties, the best possible doped a-Si:H layer is clearly simplified [18, 21, 22]. Still, based on the conditions of JSC, the TCO is excluded at the rear, which behaves chiefly as an optical layer through a decline of the parasitic absorption into the rear metal. The choice is

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limited for the rear metal as a candidate to maintain the minimum optical losses. The maximum efficiency of a practical RE-SHJ solar cell is 23.43% at present [23]. It is

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difficult to investigate each parameter thoroughly with regard to its effect on the device

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performance experimentally because of the large number of processing variables required, such as the work function, the optical band gap (Eg), doping density of the a-Si:H, and the band alignment

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of a-Si:H/c-Si heterojunction, etc. Nevertheless, a systematic optimization of these fundamental parameters is still required to improve the efficiency of RE-SHJ solar cells. A simulation program

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based on the automat for simulation of heterostructures (AFORS-HET), applied to evaluate the limitation of the conversion efficiency, provides a convenient way to accurately evaluate the roles

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of various parameters [24-26]. In particular, we investigated the influences of the parameters on an FSF layer in RE-SHJ solar cells, in which the TCO layer was fixed to optimize an n-type μcSi:H. After running the simulation for the optimization of the n-type μc-Si:H, we also carried out

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a comparison of the other band gap materials having different Eg, such as a-Si:H and μc-SiO:H, for a high-efficiency solar cell. Accordingly, the design optimization of an RE-SHJ solar cell on an n-type substrate for further improvement of the efficiency is described.

2. Cell structure for the fitting in AFORS-HET simulation Before fitting in the simulation, we fabricated a RE-SHJ solar cell. Figure 1 shows the process flow and a schematic structure of the RE-SHJ solar cell. A commercial n-type solar-graded Si wafer (1.5 Ω cm, 150 ~ 160 μm thick, (100) oriented) was used as the base material. The wafer was cleaned by RCA processes (RCA-1: H2O2-NH4OH-H2O) and RCA-2: H2O2-HCl-H2O) after

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ultrasonic treatment, and dipped into diluted HF 1% solution before they were introduced to the vacuum chamber. The SHJ solar cell was deposited at a temperature of 200 °C by the plasma

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enhanced chemical vapor deposition (PECVD) system from a gas source consisting of SiH4, H2,

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B2H6, and PH3 in an experimental condition already used in a previous work [27]. ITO films were deposited by RF magnetron sputtering. A sintered ceramic ITO (indium oxide (In2O3) doped with

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10 wt.% tin oxide (SnO2), diameter of 15.24 cm) target was used as source materials. A turbomolecular pump coupled with a rotary pump was used to achieve the ultimate background pressure

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of 13.3×10-4 Pa before introducing the Ar gas. The pre-sputtering was applied for 5 min to clean

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the target surface and remove any possible contamination. During the deposition, the working pressure was maintained at 2.67 Pa and the substrate was rotated with the angular speed of 5 rpm. The distance between the target and the substrate was fixed at 13.6 cm. We deposited ITO films on the SHJ solar cell for the front TCO electrode (thickness of ~110 nm) and for the rear TCO

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electrode (thickness of ~130 nm). The front side of the cell was prepared by forming Ag electrodes on the ITO film, whereas the reverse side consisted of a TCO film that was obtained using screen printing to create good ohmic contacts and was subsequently cured at 160 °C for 30 min.

3. Modeling approaches The AFORS-HET simulation was modeled for a defect state distribution using Shockley– Read–Hall recombination statistics [24-26]. In this modeling, we represented the RE-SHJ solar cell structure as shown in Fig. 2. The basic parameters and defect-state distributions for different types, such as a-Si:H, μc-Si:H, μc-SiO:H as an n-type, and a c-Si (n) substrate were taken from the

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references and are listed in Tables 1 and 2 [28-36]. The thickness of each layer was decided from an optimized thickness in our laboratory. The interface defect density (Dit) of the a-Si:H/c-Si

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hetero-interface at the front side was fixed at 1  1010 cm-2/eV owing to the decrease in the negative

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impact on the solar cell performance [37].

For the numerical modeling, we also set up the simulation parameters for the interface

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(thermionic emission), activated the tunneling at the hetero-interface, and calculated the

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Richardson constant from the effective mass. For the Richardson constant of a thermionic emission, we used the default value (Ae, 9.56186 As/cm2sK2; Ah, 9.56186 As/cm2sK2) and

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activated the tunneling through an insulator by setting a thickness of 1.5  10-6 cm, an electron affinity of 4.05 eV, and an optical band gap of 1.12 eV; in addition, the pinhole density through the oxide (Dph) and the relative dielectric constant (dk) were fixed at 0 and 3.9, respectively. The work function of the front and rear TCO contact was assumed as 4.5 eV, and 5.7 eV, respectively.

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The performance regarding illuminated current-voltage characteristics for the global spectrum of 1 sun of AM 1.5 was studied with a power density of 100 mW/cm2 during the simulation. The light reflections of the front and back contacts were set to 0.05 and 1, respectively. During the simulation, all parameters were fixed to the above values except for the specific variables considered for the n-type FSF layer.

4. Results and Discussion Figure 3 shows the fitted current density-voltage (J-V) curves using the experimental result

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for the simulation. Here, we found that the error rate of the fitted efficiency between the experiment and simulation results was within 1.3%. Although the simulated result was different from the

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realized result closely to match and more importantly theoretical calculations were considered to

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further investigate the influence of parameters on the device performance.

Figure 4 plots the simulated results as a function of the front ITO work functions on the FSF

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layer of a-Si:H (n). An increasing the work function shows a negative influence on solar cell performance. By increasing the work function to greater than 4.4 eV, VOC and FF decrease

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significantly and thus solar cell efficiency decreases. The obtained efficiencies were as low as 21.27 % for a fixed ITO work function of 4.5 eV, which can arise owing to the inappropriate work

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function of ITO. However, it was increased to the efficiency of 23.9 % at the ITO work function of 3.9 eV. As decreasing the work function of ITO, the reduction in the Schottky barrier approaches a near ohmic contact [28]. As taking into account the effect of the Schottky barrier contact, we

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could explain through the two-diode model in RE-SHJ solar cells and the equivalent circuit model of RE-SHJ solar cells defined by two-diodes in series [38]. Here, we focused on the Schottky barrier between TCO/a-Si:H (n) interface.

ϕb = ITOWF − (χ − k B ln (

NC )) ND,act

χ is the electron affinity of amorphous silicon. NC and ND,act are the effective state density in the conduction band edge and the active electron density in the a-Si:H (n), respectively. When work function difference is sufficiently low, the photo-generated carriers are collected by the front TCO and metal contacts to be considered for carrier transport, which is related to the reduction of a parasitic Schottky barrier at the TCO/a-Si:H (n) interface. If Anderson model is valid when the

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work function of ITO equals the work function of a-Si:H (n) [38]. We can be calculated the band offset has become zero. However, the Schottky barrier has increased by a high ITO work function

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which is confirmed by the simulated band diagrams as shown in Fig. 5 (a) and (b) under the assumption of a constant electron affinity of ITO (χ: 3.9 eV). As a theoretical calculation, we can

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be obtained efficiency improvement from the lower work function of ITO, however, it is not

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realized to get from the experiment result. Generally, the n-type ITO work function is approximately 4.2–4.8 eV [38-41]. Our result also showed the 4.2 eV of ITO work function

function of 4.2 eV.

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through the experiment [42]. Therefore, we concluded to use the parameter value of the ITO work

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The doping density (Nd), thickness and the electron affinity (χ) to determine the efficiency, FF, JSC, and VOC, respectively at the different FSF layers, are shown in Fig. 6-9 from the next section. In the case of Fig. 6 (a), there is no change in VOC when disregarding the thickness of the

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a-Si:H, whereas JSC is increased with a decrease in the thickness, as shown in Fig. 6 (b), owing to a reduction of the optical loss. Figure 6 (b) shows that the FF also reaches approximately 80.33% without regard to the increase in the thickness of a-Si:H. However, VOC for the thickness of 30 nm decreases around 4 mV compared to the thickness of 5 nm. This might be for lower VOC the etching of a-Si:H (i) passivation layer during the deposition [43]. As shown in Fig. 6 (c), it is increased

from 753 mV to 755 mV for VOC, the FF and JSC are decreased and increased to 74.08 % and 40.2 mA/cm2 respectively with regards to the increase in the χ of a-Si:H. We plotted a full band diagram (conduction band (CB) and valence band (VB)) as a function of the χ in the a-Si:H layer, as shown in Fig. 7 (a), while the Fig. 7 (b and c) is drawn to see clearly the change in CB and VB, respectively. As the χ increased, the CB in the a-Si:H (n) layer shifted

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downward and CB in a-Si:H (i) obviously shifted upward [44, 45]. Forming an interfacial barrier between a-Si:H (n) and a-Si:H (i), the electron transport from the absorber layer towards the FSF

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layer can get trapped, leading to the lower FF. However, it seems to be favorable to collect the

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hole in VB as increasing the barrier as a function of the χ in a-Si:H (n), allowed hole-carriers to easily move into the TCO/a-Si:H (p). We found the optimization of χ=4.1 eV in a-Si:H (n) for the

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efficiency of 24 %. When the χ was increased to 4.3 eV, the bending of the CB became unfavorable for electron transport towards the FSF layer, therefore, FF and the efficiency sharply decreased to

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74.1 % and 22.5%, respectively.

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It is obviously known that higher Nd of the FSF layer results in a better performance of the device. We carried out the Nd, thickness, χ in the μc-Si:H (n) for the higher efficiency from the parameter values. As a theoretical calculation for the Nd, it was observed the difference in the JSC between lower and higher values is approximately 0.27 mA/cm2. However, it was shown the Fig.

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8 (a) for a little bit of improved the FF and VOC as increasing the Nd. We found an appropriate optimization for 1020 cm-3 of Nd in μc-Si:H (n) and achieved the efficiency of 22.86 %. We also investigated the effect of χ on photovoltaic performance and results show a similar trend. We varied thickness from 3 nm to 30 nm and the resultant I-V curves of these solar cells are shown in Fig. 8 (b). The I-V plot indicates that VOC and FF are similar while JSC increases with a decrease in the thickness of μc-Si:H (n). JSC reaches approximately 39.58 mA/cm2 and is achieved a

maximum efficiency of around 23.91% until the thickness of 3 nm. The better optical properties of μc-Si:H contributed to the lower thickness of μc-Si:H with high doping concentration due to the high void fraction reported by H. Wernerus et al [46]. Thus, lowering the parasitic absorption in μc-Si:H film itself the refractive index with an increase in the high doping concentration, which allows for the a better coupling effect due to the reduction of the reflection [13, 47]. Therefore, the

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reduced thickness of the μc-Si:H (n) has the positive influence on the JSC due to the reduction of the optical loss.

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We also carried out the device performance on the χ = 3.85 ~ 4.3 eV for the better efficiency

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of solar cells. There is not a change the parameters for VOC, JSC while the FF is reduced from lower to higher χ of μc-Si:H (n). It is might be that the interfacial barrier as explained earlier section in

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the χ of the a-Si:H (n). We have optimized efficiency of 23.96 % for χ = 4.2 eV of μc-Si:H (n). Compared to [23], we achieved an improved efficiency of 23.96 % from a VOC of 754.1 mV, JSC

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of 39.71 mA/cm2, and FF of 80.03 % using the simulated parameters of μc-Si:H (n). The simulation results show that the thickness in the FSF layer contributes significantly to the solar cell

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performance and it also has done a little bit of the solar cell performance in terms of the Nd and χ. Figure 9 shows μc-SiO:H (n) FSF layer optimization of such parameters as the Nd, thickness, and χ for a comparison of the optimized results of other FSF layers. The doping density is related

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to the PH3 flow rate, leading to enhanced passivation properties [48]. The aim of an optimal doping density for use at the FSF layer was determined through a simulation. As shown in Fig. 9 (a), FF increases to 79.98 % when increasing the Nd to 1021 cm-3, whereas JSC and VOC are decreased to 41.15 mA/cm2 and 75.9% for the Nd to 1013 cm-3, and then decreased to 40.6 mA/cm2 and VOC of 754.7 mV for the Nd of 1021 cm-3, respectively. As shown in Fig. 9 (a), we achieved an improved efficiency of 24.64 % from a doping density of 1019 cm-3 owing to the better contact properties.

Therefore, we obtained a doping density of 1019~1020 cm-3, which is in agreement with the results of Nicolas et al. [48] from the simulation. When the thickness decreases by 3 nm, a higher VOC of 755.3 mV occurs, which may be attributed to the passivation damage during the TCO deposition process and a lack of an electrical field in the front side [31]. We carried out a simulation to optimize the thickness from 3 to 30 nm based on the Nd of 1019 cm-3, as shown in Fig. 9 (b). A VOC of 755.3 mV was reached at a thickness of 3 nm and similar to 79.9 % in the FF. When the

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thickness is further increased to 30 nm, showing a decreased efficiency of 24.26 % with a decrease

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in the FF of 79.99 %.

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A simulation of μc-SiO:H as a wide band gap material was conducted for a little bit of the improvement of efficiency, as shown in Fig. 9 (c). The χ in the μc-SiO:H (n) of an RE-SHJ solar

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cell shows an improved of JSC in the solar cell performance. We carried out a variation of the work function (3.85~4.3 eV) and achieved a high efficiency of 25.34% (VOC, 755.3 mV; JSC, 42.04

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mA/cm2; FF, 79.82%) for the χ of 4.1 eV. However, FF decreases to 76.29 % for χ of 4.3 eV. The improvement in the conversion efficiency depends on an optimal χ in the μc-SiO:H (n) with a Nd

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and thickness, which indicates a better separation of the generated carriers owing to the favorable band-bending [49]. Hence, an optimal χ for the reduction in the recombination leads to an improvement in the FF. Here, a χ in the μc-SiO:H film contributed to an improvement of the FF because the conduction band shifted to the Fermi-level, and band bending then occurred in the

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conduction band of the c-SiO:H layer adjacent to the a-Si:H (i). Therefore, an increase in the χ was shown to have an influence on the performance of the solar cell for FF, achieving a high efficiency.

5. Conclusions In this study, we investigated the use of an AFORS-HET simulation of the FSF layers with the different properties for high efficiency in an RE-SHJ solar cell. We achieved the efficiency of 24.01 % for the optimization of a-Si:H (n) layer from the parameter values (Nd=1016 cm-3, t=3 nm,

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and χ=4.1 eV). At the μc-Si:H film, the JSC was increased to 39.71 mA/cm2 together with the

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optimization of an electron affinity of 4.2 eV and a thickness of 3 nm, while maintaining a Nd of above 1020 cm-3. Therefore, we achieved a higher efficiency of 23.96 % as compared to the rear-

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emitter structure developed by T. Watahiki et al. The μc-SiO:H yields an improvement in JSC of 1.34 mA/cm2, which will contribute to a wide band gap with high transparency for the future

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application of high-efficiency RE-SHJ solar cells (JSC of the simulated reference: 40.7 mA/cm2

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and optimized JSC result: 42.04 mA/cm2). However, a high efficiency has also an influence of a VOC of 755.3 mV and an FF of 79.82 % owing to a favorable bending of the conduction band in

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the μc-SiO:H (n) adjacent to the a-Si:H (i). We achieved a high efficiency of 25.35 % with a μcSiO:H film under an electron affinity of 4.1 eV as well as the doping density of 1019 cm-3.

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Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Acknowledgments

This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and granted financial resources from the Ministry of Trade, Industry & Energy, Republic of

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Korea (No. 20163010012230).

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[26] A. Froitzheim, R. Stangl, L. Elstner, M. Schmidt, and W. Fuhs, in Conference Record of the 29th IEEE Photovoltaic Specialists Conference, New Orleans, USA, (2002), p. 1238. [27] S. B. Kim, S. M. Iftiquar, D. H. Lee, H. J. Lee, J. H. Kim, J. H. Jung, D. H. Oh, V. A. Dao, and J. Yi, IEEE J. Photovolt. 6 (4), (2016) 837-845. [28] L. Zhao, C. L. Zhou, H. L. Li, H. W. Diao, and W. J. Wang, Phys. Stat. Sol. (a) 205 (5) (2008)

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[29] https://www.el-cat.com/silicon-properties.htm

[31] A. H. T. Le, V. A. Dao, D. P. Pham, S. Kim, S. Dutta, C. P. T. Nguyen, Y. Lee, Y. Kim, and

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[35] Y. Zhang, C. Yu, M. Yang, H. Yan, J. Zhang, and X. Xu, IEEE 42nd Photovoltaic Specialist Conference (PVSC), New Orleans, Louisiana (2015), p. 1 [36] X. Yang, J. Chen, W. Liu, F. Li, and Y. Sun, Phys. Status Solidi A 214 (2017) 1700193 [37] F. Wang, Y. Gao, Z. Pang, L. Yang, and J. Yang, RSC Adv. 7 (2017) 26776.

[38] C. Aliani, M. Krichen, and A. Zouari, J. Comput. Electron. 18 (2019) 576 [39] L. Zhao, C.L. Zhou, H.L. Li, H.W. Diao, and W.J. Wang, Sol. Energy Mater. Sol. Cells 92 (2008) 673. [40] S. Kim, J. Jung, Y. -J. Lee, S. Ahn, S. Q. Hussain, J. Park, B. -S. Song, S. Han, V. A. Dao, J.

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Velumani, and J. Yi, J. Nanosci. Nanotechnol. 14 (2014) 9237.

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[43] O. Sergeev, A. Neumüller, I. Shutsko, M. Vehse, C. Agert, Energy Procedia 124 (2017) 371.

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[44] M. Fang, C. Zhang, and Q. Chen, Appl. Surf. Sci. 385 (2016) 28. [45] D. -K. Hwang, M. Misra, Y. -E. Lee, S. -D. Baek, J.-M. Myoung, and T. I. Lee, Appl. Surf.

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Sci. 405 (2017) 344.

[46] H. Wernerus, M. Bivour, L. Kroely, M. Hermle and W. Wolke, Energy Procedia 55 (2014) 310.

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[47] M. Bivour, S. Schröer, and M. Hermle, 38 (2013) 658. [48] S.Martín de Nicolas. Doctor Thesis (2012) a-Si:H/c-Si heterojunction solar cells: backside assessment and improvement. [49] P. Sethi, S. Agarwal, and I. Saha, Energy Procedia 25 (2012) 50.

Figure captions Figure 1 Process flow of a rear-emitter silicon heterojunction (RE-SHJ) solar cell. Figure 2 Schematic diagram of rear-emitter silicon heterojunction (RE-SHJ) solar cell for optimization of the front surface field (FSF) layer using AFORS-HET simulation. Here, n-type

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doped layers were used as FSF layers such as the different optical bandgap (a-Si:H, μc-Si:H and

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μc-SiO:H).

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Figure 3 Current density-voltage (J-V) curves fitting with an experimental result for the AFORSHET simulation.

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Figure 4 Simulated performance of RE-SHJ solar cell as a function of the indium tin oxide (ITO)

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work function on the FSF layer (n-type a-Si:H): (a) open circuit voltage (VOC), (b) current density (JSC), (c) fill factor (FF) and (d) PCE

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Figure 5 Simulated energy band diagram of different ITO work function in rear-emitter silicon heterojunction solar cell (a) WFITO: 3.9 eV, (b) WFITO: 4.5 eV. Figure 6 Estimated solar cell parameters for varying the (a) doping concentration (Nd) from 1013

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to 1021 cm-3, (b) thickness from 3 to 30 nm, and (c) the electron affinity (χ) from 3.85 to 4.3 eV for the optimization of a-Si:H (n) as FSF layer. Figure 7 Energy band diagram of solar cells according to the variation of the electron affinity (χ) in the a-Si (n) layer (a) both conduction band (CB) and valence band (VB), (b) CB, (c) VB.

Figure 8 Estimated solar cell parameters on the (a) optimization of μc-Si:H (n) for varying the doping concentration (Nd) from 1013 ~ 1021 cm-3, (b) I-V curve for varying the thickness from 3 to 30 nm, and (c) the electron affinity (χ) from 3.85 to 4.3 eV Figure 9 Device parameters for the solar cells for various (a) doping concentration (Nd= 1013 ~

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1021 cm-3), (b) thickness (t=3~ 30 nm) and (c) electron affinity (χ=3.85~4.3 eV)

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Jo Figure 2

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30

0

0

Simulation VOC= 733.4 mV

JSC= 37.655 mA/cm2

JSC= 37.59 mA/cm2

FF= 77.86% PCE= 21.55%

FF= 77.14% PCE= 21.27%

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Experimental VOC= 734.98 mV

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10

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20

100 200 300 400 500 600 700 800

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Current density (mA/cm2)

40

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Voltage (mV)

Figure 3

39

740

38

730

3.9

4.0

(b) 4.1

4.2

4.3

4.4

4.5

3.9

4.0

4.3

4.4

4.5

(c) 3.9

23

22

(d)

4.0

4.1

4.2

4.3

4.4

4.5

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ITO work function (eV)

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37 24

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78

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FF (%)

79

77

4.2

-p

80

4.1

Figure 4

3.9

4.0

4.1

4.2

4.3

4.4

ITO work function (eV)

4.5

21

PCE (%)

(a)

JSC (mA/cm2)

750

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40

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VOC (mV)

760

-3.5 EC Ef c-Si (n)

-4.5 a-Si:H (n)

EV

-5.5

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-5.0

TCO: 3.9 eV

Energy (eV)

-4.0

-6.0 10

-10

10

-8

10

-6

-4

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(a)

10

-p

Position (cm)

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a-Si:H (i)

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-3.5 EC

-4.0

-5.0

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-5.5

TCO: 4.5 eV

Energy (eV)

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Ef

-4.5

c-Si (n)

a-Si:H (n)

EV

-6.0 10

-11

10

-10

10

a-Si:H (i) -9

-8

-7

-6

10 10 10 10 Position (cm)

(b) Figure 5

-5

10

-4

10

-3

24

20 80 78 76 74 42 40 38 36 34 756 754 752 750 748 1012

1014

1016

1018

1020 -3

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(a)

26 24 22

20 80 78 76 74 42 40 38 36 34 756 754 752 750 748

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VOC (mV)

JSC (mA/cm2)

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FF (%)

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PCE (%)

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a-Si:H (n) Nd (cm )

1022

0

5

10

15

20

25

a-Si:H (n) thickness (nm)

(b)

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PCE (%) FF (%) JSC (mA/cm2) VOC (mV)

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30

24 22

3.8

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20 80 78 76 74 42 40 38 36 34 756 754 752 750 748 3.9

4.0

4.1

4.2

4.3

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(c)

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Figure 6

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4.4

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a-Si:H (n)  (eV)

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VOC (mV)

JSC (mA/cm2)

FF (%)

PCE (%)

26

-3.5 -4.0

Energy (eV)

Conduction band

3.85 eV 4 eV 4.2 eV

-4.5 -5.0

3.9 eV 4.1 eV 4.3 eV

Valence band

-5.5 -6.0 -7 10

10

-6

10

-5

-2

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Position (cm )

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(a)

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-4.0

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Energy (eV)

Conduction band

-4.5 -7 10

10

3.85 eV 4 eV 4.2 eV

-6

3.9 eV 4.1 eV 4.3 eV

10

-5

-2

(b)

-5.0

Energy (eV)

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Position (cm )

3.85 eV 4 eV 4.2 eV

-5.5

3.9 eV 4.1 eV 4.3 eV

Valence band

-6.0 -7 10

10

-6

10 -2

Position (cm )

-5

(c)

26 24

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22

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20 80 78 76 74 42 40 38 36 34 756 754 752 750 748 1012

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VOC (mV)

JSC (mA/cm2)

FF (%)

PCE (%)

Figure 7

1014

1016

1018

1022

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c-Si:H (n) Nd (cm

1020

-3

40

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2

Cuurent density (mA/cm )

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(a)

30

20

10

0

0

3 nm 5 nm 10 nm 20 nm 30 nm

200

400

Voltage (mV)

(b)

600

800

24 22

3.8

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20 80 78 76 74 42 40 38 36 34 756 754 752 750 748 3.9

4.0

4.1

4.2

4.3

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(c)

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Figure 8

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4.4

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c-Si:H (n)  (eV)

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VOC (mV)

JSC (mA/cm2)

FF (%)

PCE (%)

26

24

20 80 78 76 74 42 40 38 36 34 756 754 752 750 748 1012

1014

1016

1018

1020

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(a)

26 24 22

20 80 78 76 74 42 40 38 36 34 756 754 752 750 748

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VOC (mV)

JSC (mA/cm2)

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FF (%)

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PCE (%)

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1022

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c-SiO:H (n) Nd (cm

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0

5

10

15

20

25

c-SiO:H (n) thickness (nm)

(b)

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PCE (%) FF (%) JSC (mA/cm2) VOC (mV)

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30

24 22 20 80 78 76 74 42 40 38 36 34

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800 750 700 3.8

3.9

4.0

4.1

4.2

4.3

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Figure 9

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4.4

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c-SiO:H (n)  (eV)

(c)

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VOC (mV)

JSC (mA/cm2)

FF (%)

PCE (%)

26

Table 1. Input parameter values for the rear-emitter SHJ solar cells in AFORS-HET simulations. Further details of numerical values are given below.

constant(g) Electron affinity (eV)(h) Band gap /Optical band gap (eV)(i)

a-Si:H

μc-Si:H

μc-SiO:H

(n)

(i)

(p)

(n)

(n)

(n)

150000(a)

5(b)

12(c)

3-30(d)

3-30(e)

3-30(f)

11.9

11.9

11.9

11.9

11.9

11.9

4.05

3.9

3.9

3.85-4.3

3.85-4.3

3.75~4.3

1.12

1.61

1.71

1.71

1.6

2.05

(1.12)

(1.61)

(1.71)

(1.71)

(1.6)

(2.05)

19

Effective CB (VB) 2.84310 -3

19

(110 )

1263

Electron (Hole) mobility 2

(444)

Doping

concentration of acceptors

6

210

20

15

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7

of electron (hole) (m)

(cm/s)

7

20

5

5

5

50

50

(1)

(1)

(1)

(5)

(5)

0

0

0

18

3.7810

(0)

(0)

7

110

7

13

(10

20

~10 )

110

7

7

7

110

7

13

(10

21

~10 ) 7

110

7

13

(10

21

~10 ) 7

110

7

(1.6510 )

(110 )

(110 )

(110 )

(110 )

(110 )

2.328

2.285

2.285

2.285

2.328

2.328

Layer density 3

19

20

110

(110 )

(donators) (cm )

2.310

21

19

110

(110 )

-3 (l)

Thermal velocity

21

110

(110 )

0

(510 )

20

(110 )

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(cm /Vs)(k)

20

(1.0410 )

110

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density (cm )(j)

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110

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Dielectric

a-Si:H

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(nm)

a-Si:H

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Layer thickness

c-Si

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Parameters

(g/cm )(n) ※ (a)~(c): Experimental values, (d)~(f): Thickness range for the fitting (g)~(n): literature values,

Table 2. Defect state distributions of a-Si:H (i), a-Si:H (p), a-Si:H (n), μc-Si:H (n), μc-SiO:H (n) layers in the simulations.

a-Si:H

a-Si:H

a-Si:H

μc-Si:H

μc-SiO:H

(n)

(i)

(p)

(n)

(n)

0.025 (0.03)

0.025 (0.025)

0.035 (0.03)

0.1

0.045

(0.07)

(0.037)

e (h) for CB tail (cm2)

710-16 (710-16)

710-16 (710-16)

710-16 (710-16)

110-17 (110-15)

e (h) for VB tail (cm2)

710-16 (710-16)

710-16 (710-16)

710-16 (710-16)

Maximum A-like Gaussian state density (cm-3 eV-1)

2.511017

1.61017

Maximum D-like Gaussian state density (cm-3 eV-1)

2.511017

Specific energy of Gaussian peak for donor (eV)

ro 110-15 (110-17)

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CB (VB) tail Urbach energy (eV)

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Parameters

110-17 (110-15) 110-15 (110-17)

11020

11018

1.131017

11020

11018

0.45

0.7

1.02

1.16

1.35

Specific energy of Gaussian peak for acceptor (eV)

0.65

1

1.2

1.36

1.1

Energy of distribution (eV)

0.21

0.1

0.21

0.2

0.2

e (h) for A-like Gaussian state (cm2)

310-15 (310-14)

310-15 (310-14)

310-15 (310-14)

110-15 (110-14)

310-15 (310-14)

e (h) for D-like Gaussian state (cm2)

310-14 (310-15)

310-14 (310-15)

310-14 (310-15)

110-14 (110-15)

310-14 (310-15)

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1.61017