Numerical simulation for optimization of ultra-thin n-type AZO and TiO2 based textured p-type c-Si Heterojunction Solar Cells

doi:10.21203/rs.3.rs-225454/v1

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Numerical simulation for optimization of ultra-thin n-type AZO and TiO2 based textured p-type c-Si Heterojunction Solar Cells

https://doi.org/10.21203/rs.3.rs-225454/v1

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A maximum efficiency of 17% for ultra-thin n-type AZO layer and 17.5% for ultra-thin n-type TiO₂layer based silicon heterojunction solar cell is reported by optimizing its properties which is much higher than practically obtained efficiency signifying a lot of improvements can be performed to improve efficiency of TiO₂/Si and AZO/Si heterojunction solar cell. AZO layer and TiO₂ layer is used as n-type emitter layer and crystalline silicon wafer is used as p-type (p-cSi) layer for modelling AZO/Si and TiO₂/Si heterojunctions solar cell respectively using AFORS HET automat simulation software. Various parameters like thickness of AZO, TiO₂ layer, p-cSi layer, doping concentration of donors (Nd) and effective conduction band density (Nc) are optimized. Finally, texturing at different angle is studied and maximum efficiency is reported at 70 µm thick p-type crystalline Silicon (p-cSi) wafer, that can be very helpful for manufacturing low cost HJ solar cells at industrial scale because of thin wafer and removal of additional processing setup required for deposition of amorphous silicon i-layer. Utilization of TiO₂ and Aluminium doped Zinc Oxide as n-type layer and p-cSi as p-type layer can help in producing low cost and efficient heterojunction (HJ) than compared to HJ with intrinsic thin layer HIT solar cells.

Electronic Materials and Devices

Heterojunction

HIT solar cells

Texturing angle

Numerical simulation

AFORS-HET

Demand of energy consumption is tremendously increasing day by day in today’s world and to meet the same, contribution from renewable energy sources has to be increased. Conventional non-renewable energy sources are limited and have many environmental concerns. Renewable energy sources are the solution to overcome the problems related to non-renewable energy sources like pollution, limited stock, environmental problems, etc. [1]. In recent times, solar energy is one of most widely commercially used renewable energy source. Photovoltaic solar cell technology is currently dominated by silicon wafer based solar cells by almost 90% [2]. In recent past, silicon heterojunction solar cells developed by sanyo company based on heterojunction with intrinsic thin layer (HIT) concept is widely used to manufacture solar cells at industrial scale due to their good efficiency, cost effectiveness and economic viability [3, 4]. Further, numerous research on thin film based silicon heterojunction solar cell is ongoing using metal oxides as n-type layer with crystalline silicon (p-cSi) wafer as p-type as well as metal oxides as p-type layer with n-type cSi (n-cSi) wafer due to their easy availability and environment friendly [5–9]. Aluminium doped zinc oxide (AZO) and zinc oxide can be used as window layers, antireflection coating (ARC) layer & as TCO layer in solar cells [10, 11]. They also find applications in other devices like transducers, ultrasonic oscillators and gas sensors. ZnO can be used as a substitute for indium tin oxide (ITO) as it is cheaper than ITO and also enhances the efficiency of solar cells [12]. ZnO can be doped with elements like Al, B, F, Ga, In etc. to make it n-type ZnO. Titanium dioxide (TiO₂) can also be used as n-type layer which has significant electrical conductivity and high visible transparency [13]. TiO₂ can be doped to make further n-type or p-type material, where it can be doped with Cr³⁺, Fe³⁺, Ni⁺² and Co⁺² to make p-type and elements like Sn, N, Al, F etc. to make n-type TiO₂. TiO₂ can be fabricated using physical and chemical deposition techniques. Some vaccum based techniques like sputtering, pulsed laser deposition (PLD), chemical vapour deposition (CVD) and some solution based techniques like sol-gel, dip/spin coating, and spray pyrolysis can be used for deposition of TiO₂ films. Vaccum based technology is used for fabrication of solid state device, although this technology is more expensive than solution based technology. Sputtering method is suitable for uniform coating on substrate with large area. It has been established as standard technique for deposition of transparent semiconducting oxide films. Pulsed laser deposition (PLD) technique is used for deposition of robust and nanostructured thin films. This technique is very efficient technique which is capable to assist laser desorption and ionization of low molecular weight. Chemical vapour deposition (CVD) offer various advantages over other techniques like purity of thin film, low cost than compared with other vaccum based technology, thin film with different morphology and good adhesion with substrate[13]. Although, sol-gel technique is not suitable to produce TiO₂thin film for solar cell application, they offer other advantages like compositional control, homogeneity and low crystallization temperature [13]. Spray pyrolysis offers advantages like low cost, scalable and uniform deposition on large area substrate. TiO₂thin film in its pure form, without doping has low conductivity that makes them unsuitable for solar cell application where doped TiO₂layer can be used to make them suitable for photovoltaic [14, 15]. However, practical efficiency is very low for such solar cells due to various defects and improper optimization of parameters. Their various parameters can be optimized using various numerical simulation softwares like PC1D, AFORS-HET [16], silvaco ATLAS [17, 18], TCAD [19, 20], AMPS [21], SCAPS 1D [22–24] etc. AFORS-HET is an automat simulation software based on Shockley-read-hall recombination statistics which solves one-dimension semiconductor equation [7, 25, 26]. This software is used for modelling homojunction and heterojunction devices [25, 27, 28]. This program solves 1-D semiconductor equation which is based on physical differential equation like transport & continuity equation and poisson’s equation [29]. Lambert beer law is used which is based on optical model for estimation of optical parameter [7]. In this work, AFORS-HET automat simulation software is used to simulate AZO/Si and TiO₂/Si heterojunction solar cell and its various parameters like thickness of silicon wafer, thickness of AZO layer, thickness of TiO₂ layer, doping concentration of donors (N_d), effective conduction band density (N_c), and texturing at different angle is optimized [30]. Texturing of silicon wafer at different angle is performed to study the change in behaviour and performance of solar cell with respect to plane heterojunction solar cell [31, 32]. Texturing plays important role for improvement in performance of thin film based silicon heterojunction solar cells due to increment in light trapping of incoming light and multiple internal reflection, which helps to increase charge carrier transport and hence, enhances the overall efficiency of solar cell [33, 34]. Various literatures are reported for plane and textured (pyramidal and inverted pyramidal) solar cells [35, 36]. Efficiency is also dependent at various texturing angle and hence, desired texturing angle can be applied to produce maximum power conversion efficiency from solar cell [37]. Using AZO as n-type material, an efficiency of 6.8% is reported for Al:ZnO/CdS/CuInSe₂ polycrystalline solar cell in [10]. 6.8% efficiency is reported for Al:ZnO/Si heterojunction solar cell in [38]. However, maximum theoretical efficiency of 29.43% can be achieved for silicon heterojunction in [39–41] respectively. A maximum efficiency of 17% for ultra-thin n-type AZO layer and 17.5% for ultra-thin TiO₂ as n-type layer is achieved by optimizing its properties which is much higher than practically obtained efficiency signifying a lot of improvements can be performed to improve efficiency of TiO₂/Si and AZO/Si heterojunction solar cell.

AFORS-HET automat simulation software is used in this work for modelling the AZO/Si and TiO₂/Si heterojunction solar cell where ultra-thin AZO layer & TiO₂ layer acts as n-type layer and crystalline p-type silicon (p-cSi) wafer acts as p-type absorber layer. Device structure used for modelling in this work is described in figure 1. All default values present in AFORS-HET software are considered for the modelling TiO₂& AZO layer based silicon heterojunction (SHJ) solar cells except the parameters used to be optimized. Illuminance of radiation AM 1.5 is used with power density of 100 mW/cm²for simulation in present study. Standard values of Al:ZnO, TiO₂ and p-cSi are taken from references [7, 15, 25]. Flatband schottky front interface and flatband schottky back interface is chosen as default in present study. No interface effect is studied in this article, hence ‘No Interface’ is considered as default. Front contact boundary and back contact boundary are chosen as constant (zero) i.e. no absorption loss is considered. Carrier lifetime in silicon is considered as 1 msec; Diffusion length of carriers in silicon wafer is considered to be around 100 µm. Resistivity and order of series resistance and shunt resistance of layer has been taken as default values i.e. minimum for series resistance (R_s: 0 ohm) and max for shunt resistance (R_shunt: 10³⁰ ohm). In this work, following studies are carried out: (i) thickness optimization of p-cSi layer, (ii) thickness optimization of ultra-thin AZO and TiO₂ layer, (iii) optimization of doping concentration of donors (N_d), (iv) optimization of effective conduction band density (N_c) and (v) optimization of texturing at various angles.

Table 1 Input parameters used for AFORS-HET numerical simulation with p-type crystalline silicon wafer as p-layer and AZO &TiO₂ as n-layer.

Parameter		p-cSi	AZO	TiO₂
Thickness	d	varied (µm)	varied (nm)	varied (nm)
Dielectric Constant	dk	11.9	9	8.6
Electron Affinity	eV	4.05	4.4	3.9
band gap	eV	1.12	3.37	3
Opt. BG	eV	1.12	3.3	3
Effective CB density	cm^-3	varied	varied	varied
Effective VB density	cm^-3	2.5×10¹⁹	1×10¹⁹	1×10¹⁹
Electron mobility	cm²v^-1s^-1	1041	100	20
hole mobility	cm²v^-1s^-1	412.9	25	10
Acceptor concentration	cm^-3	1.5×10¹⁶	0	0
donor concentration	cm^-3	0	varied	varied
Thermal vel. of e	cms^-1	1×10⁷	1×10⁷	2.99×10⁶
Thermal vel. of h	cms^-1	1×10⁷	1×10⁷	6.76×10⁶
layer density	gcm^-3	2.328	2.33	2.328

Texturing plays important role in enhancing the efficiency of solar cells [33]. Texturing of silicon wafer surface can be performed through chemical etching. Due to textured morphology of layers in solar cells there is re-absorption of reflected rays which helps in minimizing the lost reflected light [12]. Texturing increases excess charge carriers due to light trapping and multiple internal reflection which helps in enhancing overall performance & conversion efficiency of solar cell. Moreover, efficiency also depends on angle at which texturing is performed to produce an efficient solar cell [42]. There are several reports on pyramidal and inverted pyramidal texturing for silicon solar cells [12, 33]. In this article, texturing is performed at various texturing angle and performance of modelled AZO/Si heterojunction solar cell is optimized.

1 Thickness optimization of p-cSi silicon wafer:

In this section, thickness of p-type crystalline silicon wafer is optimized. Manufacturing of solar cells using thin p-cSi wafer can be cost effective up to certain extent at industrial scale. Though nowadays n-cSi ingots are also reasonable in cost compared to p-cSi and wafer of around 200 µm thickness are in use at industrial scale. However, through our simulation we have obtained that its thickness can further be reduced to upto 70 µm without affecting its performance. All other parameters were in accordance with the values reported in Table-1. Efficiency rapidly increases with thickness initially, begins to saturate after 70 µm and remains constant upto 500 µm. This can be well explained as initially as thickness increases, more number of charge carriers are generated and contribute to flow of charge across p-n junction. However, after certain thickness, charge carriers generated does not travel across junction due to thickening of wafer and less diffusion length of charge carriers in silicon wafer which is around 100 µm. When thickness of silicon wafer gets thicker than its diffusion length, charger carriers generated does not travel upto p-n junction and gets recombined and hence charge carriers generated having diffusion length more than 100 µm does not participate in charge collection. Thus, efficiency begins to saturate after 70 µm. Also, film gets flexible when its thickness reduces beyond 50 µm. Experimentally, an ultrathin flexible film of thickness 45 µm has been fabricated by [43] using Cu-assisted chemical etching of bulk c-Si producing conversion efficiency of over 17%. Also, mechanical stress, multi flexure, fracture and static test has been conducted by [44] for flexible thin films based crystalline silicon solar cells where thickness of flexible thin film varying from 20 µm to 50 µm. Thus, an ultrathin stable film can be utilized for silicon based HJ solar cells. Hence, the thickness of p-type silicon wafer at 70 µm is optimized in this work. Similar pattern is also observed for short circuit current (J_sc) curve. This behaviour is well illustrated in V_oc vs thickness curve figure: 2(a), J_sc vs thickness curve figure: 2(b) and fill factor (FF) vs thickness curve figure: 2(c). V_oc, J_sc and FF values remains nearly constant from 70 µm to 500 µm; V_oc and J_sc rapidly decreases on further reducing the thickness of p-cSi layer which accounts for the decrease in efficiency. Fill factor firstly increases slightly and then decreases but this increase is less significant than rapid decrease in V_oc and J_sc which results in decrease in efficiency. Hence, the thickness of p-cSi layer is optimized at 70 µm to obtain an efficiency of 12.84% for AZO layer and 12.16% for TiO₂layer based p-cSi HJ solar cells.

2 Thickness optimization of ultra-thin n-type AZO and TiO₂ layer:

Thickness of AZO and TiO₂layer is varied from 0.5 nm to 10 nm and open circuit voltage (V_oc), short circuit current (J_sc), fill factor (FF) and efficiency is recorded with respect to thickness of AZO & TiO₂layer, Figure 3 (a) to (d) represents curve for each respectively. All other parameters were in accordance with the values reported in Table-1. There is no effect of thickness variation of TiO₂layer on V_ocand it remains constant at 546.1 mV. Same pattern is observed for AZO layer as well, However, there is slight enhancement in V_oc at 3 - 4 nm, giving maximum V_oc of 549.2 mV below 3 nm. Short circuit current linearly decreases with increase in its thickness giving maximum J_scat 0.5 nm. However, due to practical limitations, deposition of layers using modern techniques is limited upto 3 nm for stable film and hence J_scat 3 nm giving short circuit current of 29.12 mA for ultra-thin AZO layer and 29.02 mA for ultra-thin TiO₂layer based solar cell is considered as optimized short circuit current [7]. This behaviour is expected, since thinning of emitter layer will contribute to flow more number of charge carriers generated in absorber layer and easy transport across layer due to decrease in resistance, thus recombination rate also decreases; hence resulting in enhancement of short circuit current with decrease in thickness of emitter layer. Fill factor also decreases linearly with increasing thickness. This can be attributed due to decrease of series resistance as we decrease its thickness. Hence, overall efficiency increases linearly with decreasing thickness and gives maximum efficiency of 12.84% for ultra-thin n-type AZO layer and 12.59% for ultra-thin n-type TiO₂layer based HJ solar cell.

3 Doping concentration of donors (N_d)

Doping of donor concentration plays an important role in enhancing the charge carriers which resulted in improved efficiency. Figure 4 (a) to (d) represents the variation of V_oc, J_sc, FF and efficiency curve with respect to doping concentration of donors respectively, where N_d is varied from 10¹⁴cm^-3to 10¹⁸ cm^-3. Doping can be achieved in order of 10²⁰cm^-3 using various techniques like ion implantation, mixed molecular monolayer doping technique etc. [45]. Introduction of doping concentration of donors doesn’t affect open circuit voltage (V_oc) much and it nearly remains constant for both, AZO and TiO₂layer excluding for TiO₂ at lower concentration. Short circuit current (J_sc) effectively decreases with increase of doping concentration of donors, giving maximum short circuit current of 30.1 mA and 30.3 mA for AZO and TiO₂ layer respectively. Maximum fill factor at 10¹⁵cm^-3 for TiO₂layer and at 10¹⁶cm^-3 for AZO layer is observed and it nearly remains unaffected on further increasing doping of donors. Similar pattern is followed in efficiency curve as well, where maximum efficiency of 13.13% for AZO layer and 13.21% for TiO₂ is reported at doping concentration of 1×10¹⁵cm^-3and 1×10¹⁴cm^-3 respectively. Performance of device degrades as doping concentration increases beyond order of 10¹⁵ which can be due to introduction of defect states in AZO and TiO₂ layer at higher doping concentration limiting its performance. With increase in defect states in space charge region, it can create conduction channel across it and leakage current starts flowing which degrades the performance of device. Hence, efficiency decreases with further increase of doping concentration for AZO layer beyond 10¹⁵cm^-3. Similar pattern is followed for TiO₂layer as well, where efficiency remains nearly constant upto 10¹⁵cm^-3 and decreases thereafter due to introduction of defect states at higher concentration.

4 Effective conduction band density

During carrier charge transport, effective conduction band density and effective valence band density are the major density of states that plays important role during charge transport. Figure 5.1 to 5.4 represents the variation of V_oc, J_sc, FF and efficiency curve with respect to effective conduction band density respectively. All other parameters were in accordance with the values reported in Table-1. Effective conduction band density is varied from 10¹⁵cm^-3to 10¹⁹ cm^-3as shown in figure 5(a) to 5(d). Similar variation in tail density of states from 10¹³ cm^-3to 10²¹ cm^-3 has been reported in [46]. Open circuit voltage remains constant on varying conduction band density for both, AZO & TiO₂ layer. Short circuit current linearly decreases slightly on increasing N_c giving maximum J_scof 30.4 mA at 10¹⁵cm^-3for TiO₂layer and 30.08 mA at 10¹⁹cm^-3 for AZO layer based solar cell. Fill factor is significantly influenced with N_cand thus contributes more to efficiency. Hence, efficiency follows the pattern like fill factor. Efficiency increases initially exhibiting best efficiency at around 10¹⁶ cm^-3 and remains nearly constant thereafter which can be attributed to increase in shunt resistance and decrease in series resistance initially thus affecting fill factor of device accordingly. After certain level, there is no significant change in shunt resistance and series resistance with change in effective conduction band density resulting in nearly constant fill factor, thus efficiency also gets saturated. However, initial increase in efficiency is more significant in case of TiO₂ based solar cell than compare to AZO based silicon solar cell and further study needs to be carried out for more fundamental reason behind this. Maximum efficiency of 13.24% for AZO layer and 13.21% for TiO₂ layer at 5 × 10¹⁵cm^-3 and 1 × 10¹⁷cm^-3respectively is obtained for AZO/Si and TiO₂/Si based HJ solar cells respectively.

5 Texturing angle

Efficiency of solar cell can be significantly enhanced through texturing [35]. Many reports are there on pyramidal and inverted pyramid textured silicon wafer solar cell. Texturing angle is varied from 0° to 89° as shown in figure 6. V_ocincreases linearly with texturing angle giving maximum open circuit voltage 555.5 mV at 89° for both, AZO layer based and TiO₂ layer based silicon HJ solar cells. Similar trend is followed in J_sccurve giving maximum short circuit current of 37.81 mA and 38.92 mA at 89° for AZO layer based and TiO₂ layer based silicon HJ solar cells. Fill factor nearly remains constant and varies from 80.25% to 80.96% for entire range of texturing angle. Efficiency follows the same curve, giving maximum efficiency of 17% for AZO layer and 17.5% for TiO₂ layer at texturing angle of 89°. Efficiency increases linearly with texturing angle due to increment in close packing density of textured surface which increases the internal multiple reflection of illuminated light and helps to enhance absorption loss, thus contributes in collecting more charge carriers [42]. Thus efficiency of modelled AZO/Si HJ solar cell is significantly increased. Hence all the parameters of TiO₂, AZO layer and p-cSi wafer layer is optimized to attain maximum efficiency of 17% for AZO/Si and 17.5% for TiO₂/p-cSi heterojunction solar cells.

Numerical simulation is performed using AFORS-HET software program to achieve a maximum efficiency of 17% and 17.5% for ultra-thin n-type AZO and TiO₂ layer based silicon HJ solar cell by optimizing its various parameters. In this work, thickness of p-cSi wafer is optimized at 70 µm to obtain an efficiency of 12.84% for ultra-thin AZO layer and 12.16% for ultra-thin TiO₂layer based p-cSi HJ solar cells respectively. It is followed by optimizing thickness of ultra-thin TiO₂and AZO layer at 3 nm to obtain an efficiency of 12.84% for AZO layer and 12.59% for TiO₂layer based HJ solar cell respectively. Doping concentration of donors (N_d) is optimized at 1 × 10¹⁵cm^-3for ultra-thin AZO layer and 1 × 10¹⁴cm^-3 for ultra-thin TiO₂ layer to obtain efficiency of 13.13% and 13.21% for AZO/Si and TiO₂/Si HJ solar cells respectively. Effective conduction band density is optimized at 5 × 10¹⁵cm^-3for AZO layer and at 1 × 10¹⁷cm^-3for TiO₂ layer based solar cell and obtained an efficiency of 13.24% and 13.21% respectively. Finally, the role of surface texturing at various angle is studied and reported the maximum efficiency of 17% and 17.5% for AZO and TiO₂ layer based silicon HJ solar cells respectively. Linear increase in efficiency with respect to texturing angle is observed, where efficiency of 14.21% for AZO layer and 14.45% for TiO₂layer based silicon HJ solar cells for pyramidal surface texturing is obtained. Thickness optimization of p-cSi at 70 µm and removal of intrinsic amorphous silicon layer can be very cost effective for manufacturing AZO/Si heterojunction solar cells at industrial scale for commercial production; as deposition of i-layer a-Si and other similar based HJ solar cells with intrinsic layer involves additional processing setup. Processing of such layers also has some serious environmental & safety issues. Optimization of texturing angle plays a significant role in enhancing the efficiency above 17% for the above modelled HJ solar cell. Hence, efficiency reported using AZO and TiO₂ layer based silicon heterojunction solar cell in this article can be cost effective and efficient solar cell. Such results are encouraging and exciting for practical applications. However, practically it is challenging to texture the silicon wafer at such angle with high accuracy. Lasers can be used to texture silicon surface at such specific angles though such process may cost more. Further investigation on above modelled cell can be performed by introducing intrinsic thin layer, p+ layer, n+ layer and BSF layer to simulate HIT, HITBSF and HIT-BIFACIAL solar cells to further improve the efficiency of solar cell.

Acknowledgement:

Authors are grateful to Director, National Physical Laboratory, New Delhi India for his kind support. Author, Chandan Yadav is thankful to Council of Scientific and Industrial Research (CSIR), Govt. of India for providing Junior Research Fellowship (JRF, 31/001(0627)/2020-EMR-1). Authors also acknowledge Helmholtz-Zentrum Berlin for providing AFORS-HET simulation software.

Funding Statement: The research leading to these results received funding from Council of Scientific and Industrial Research (CSIR), Govt. of India, for providing JRF under grant no. 31/001(0627)/2020-EMR-1 to author Chandan Yadav.

Authors Contribution:

Chandan Yadav: Writing original draft, simulation work. Sushil Kumar: Conceptualization, review & editing, supervision.

Compliance with Ethical Standards:

Conflict of Interest: Authors declare no conflict of interest that are directly or indirectly related to the work submitted for publication.

Research Involving human participants or animals: Not applicable.

Informed Consent: Not applicable.

Consent to participate: Not applicable.

Consent for publication: Not applicable.

Data availability statement: This paper does not use any raw or processed data. It includes all the data necessary to perform the calculations and supports the findings of this study.

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Table 1 Input parameters used for AFORS-HET numerical simulation with p-type crystalline silicon wafer as p-layer and AZO &TiO₂ as n-layer.

Parameter		p-cSi	AZO	TiO₂
Thickness	d	varied (µm)	varied (nm)	varied (nm)
Dielectric Constant	dk	11.9	9	8.6
Electron Affinity	eV	4.05	4.4	3.9
band gap	eV	1.12	3.37	3
Opt. BG	eV	1.12	3.3	3
Effective CB density	cm^-3	varied	varied	varied
Effective VB density	cm^-3	2.5×10¹⁹	1×10¹⁹	1×10¹⁹
Electron mobility	cm²v^-1s^-1	1041	100	20
hole mobility	cm²v^-1s^-1	412.9	25	10
Acceptor concentration	cm^-3	1.5×10¹⁶	0	0
donor concentration	cm^-3	0	varied	varied
Thermal vel. of e	cms^-1	1×10⁷	1×10⁷	2.99×10⁶
Thermal vel. of h	cms^-1	1×10⁷	1×10⁷	6.76×10⁶
layer density	gcm^-3	2.328	2.33	2.328

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11 Feb, 2021
First submitted to journal
08 Feb, 2021

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Numerical simulation for optimization of ultra-thin n-type AZO and TiO2 based textured p-type c-Si Heterojunction Solar Cells

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