Elsevier

Applied Surface Science

Volume 476, 15 May 2019, Pages 49-60
Applied Surface Science

Full Length Article
Sulfur-hyperdoped silicon nanocrystalline layer prepared on polycrystalline silicon solar cell substrate by thin film deposition and nanosecond-pulsed laser irradiation

https://doi.org/10.1016/j.apsusc.2019.01.074Get rights and content

Highlights

  • S-hyperdoped Si nanocrystalline layer was prepared on Si solar cell substrates.

  • Rectangle shaped ns-pulsed laser irradiated multilayered films to produce this layer.

  • S-hyperdoped layers have good near-IR absorption and electrical transport properties.

  • Mechanism of laser interaction with films to achieve hyperdoping was proposed.

  • Preparation method is applicable to other hyperdoped impurities as well.

Abstract

Silicon (Si)-based solar cells are the main products in current photovoltaics market; therefore, any development in their conversion efficiency and cost effectiveness provides an extremely important contribution to the photovoltaic industry. Use of hyperdoping process for the achievement of Si sub-bandgap light absorption to improve conversion efficiency has attracted significant attention of researchers. However, several problems are encountered during this process such as the poor crystallinity, severe carrier recombination, and high series resistance loss. In this study, a sulfur (S)-hyperdoped Si nanocrystalline layer is prepared on a commercial polycrystalline Si solar cell substrate to efficiently utilize near-infrared (NIR) and visible lights, respectively. An inexpensive, rectangle shaped nanosecond-pulsed laser beam is used to rapidly irradiate Sisingle bondSsingle bondSi multilayered films to produce this hyperdoped layer via melting, vaporization, resolidification, and crystallization processes. The hyperdoped samples which have S impurity concentration in the range of 0.15 ± 0.07–0.78 ± 0.03 at.%, exhibit high NIR light absorptance (75–90%), high bulk carrier concentration (≥1019 electrons·cm−3) and mobility (∼102 cm2 V−1 s−1), and low sheet resistance (<100 Ω·square−1) and resistivity (∼10−3 Ω cm). The results confirm that this method not only improves the laser processing efficiency and saves costs, but also realizes a stable liquid S-hyperdoping process to prepare a nanocrystalline layer with strong IR properties.

Introduction

Clean and abundant solar energy is the focus of energy utilization and development. Among the many types of solar cells, silicon (Si)-based solar cells occupy the core of photovoltaics market (occupies ∼90%) due to the abundant Si element reserves (27.7% of the earth’s abundance), mature crystal growth and device fabrication process, and high conversion efficiency. Notably, when the conversion efficiency increases by 1%, the cost of solar cells can be reduced by ∼6%. Therefore, studies to increase the conversion efficiency of Si-based solar cells are extensive and mainly focused on improving the cell structure and manufacturing process [1], [2]. However, this increase in conversion efficiency has become slow in recent years due to approaching limit of the current manufacturing processes. For example, the highest efficiency values for single crystalline and polycrystalline Si single-junction solar cells have only increased from 25.6 and 20.8% in 2014 [3] to current 26.7 and 22.3%, respectively [4]. Nonetheless, the cost of power is still high (more than two times that of hydropower), thus hindering the enlargement of photovoltaic power.

In order to increase the conversion efficiency, another promising approach includes the exploration of sub-bandgap light absorption to achieve full light spectrum (wavelength λ = 250–2500 nm) photoelectric conversion [5], [6], [7], which has been found since 1998 and the semiconductor material is usually named as black Si [5]. The schematic illustration of sub-bandgap photoconductivity through the deep-level impurity band (intermediate band) is shown in Fig. 1 [8], [9]. The intrinsic Si bandgap is Eg = 1.12 eV; therefore, only photons with energy greater than Eg (λ ≤ 1100 nm) can be absorbed to form pairs of electron (e) and hole (h) in the conduction band and valence band, respectively. However, with the increase of deep-level impurity concentration in Si, the formation of deep-level impurity band in the Si bandgap, i.e. generation of sub-bandgaps, by hyperdoping (the concentration of impurities far exceeding their equilibrium solid solubility in Si), can get rid of the Eg limit. Thus two photons with energy lower than Eg can be absorbed through the sub-bandgap light absorption to form an electronhole pair. Charge carriers can then be transported to the N or P region of neighboring PN junction. In this case, sub-bandgap absorption of infrared (IR) photons with λ = 1100–2500 nm, which accounts for ∼30% of the total sunlight energy, can be successfully achieved. As a result, the theoretical efficiency of single-junction Si-based solar cells has increased from 40.7% (when taking into account photon reflection, carrier recombination, and series resistance loss, the efficiency is ∼30% [2]), i.e. the Shockley–Queisser limit, to 54.8% and even ∼70% (depending on Auger generation) [10], [11].

However, over the past 20 years, the conversion efficiency of solar cells manufactured using the hyperdoped Si is far lower than the expected value. In 2007, the conversion efficiency of sulfur (S)-hyperdoped Si photovoltaic device reached 2.2% [12]. In 2014, the conversion efficiency was only 8.2% [13]. In 2018, the highest sub-bandgap internal quantum efficiency was still 2.3 × 10−4 [14]. On the other hand, solar cells fabricated using the nanostructured “black Si”, although achieved a conversion efficiency of 18.2% [15] through good light trapping which increased to 22.1% after surface passivation [16], are in fact not better than the mature chemically textured solar cells because the former does not utilize the full-spectrum light absorption characteristics. This unexpected solar cell performance can be attributed to the fact that the hyperdoped Si material has high sheet resistance (several kΩ·square−1) [13], low mobility (<102 cm2 V−1 s−1) [6], short carrier lifetime (picoseconds) [17], severe Auger recombination at the surface layer, and low open circuit voltage (Voc) due to sub-bandgap.

Currently, Si is hyperdoped with deep-level impurity such as S, mainly by ultra-short femtosecond (fs) pulsed laser irradiation under impurity background atmosphere (such as SF6) [13], [18] and ion implantation followed by pulsed laser melting [9], [17], [19]. For the former, the depth of hyperdoping (S concentration ≥1019 cm−3) is usually limited within the surface layer [18]. Furthermore, damage of Si lattice due to radiation is serious, usually forming an amorphous surface structure [20], many charge carrier recombination centers, and leading to high sheet resistance (several kΩ·square−1) [13]. Obviously, these characteristics seriously affect the conversion of absorbed IR photons to charge carriers and carriers transport, thus resulting in device performance far from practical use [12], [13], [14]. For the latter, the impurity segregation and cellular breakdown hinder the diffusion of impurities [9]. Furthermore, the resultant hyperdoped Si is typically flat, thus it does not show good light trapping effects. Therefore, the main challenges for the fabrication of hyperdoped Si material involve the achievement of (1) high full-spectrum, in particular near-IR (NIR) optical absorptance, which needs high impurity concentrations to produce an intermediate impurity band, (2) high crystallinity for achieving low sheet resistance and high carrier mobility, and (3) controlled hyperdoped layer thickness so that charge carriers with short carrier lifetime of the order of picoseconds [17], can be extracted efficiently [21] without allowing enhanced recombination and/or thermalization.

Besides the development of novel approach for hyperdoped Si material, design of new solar cell structures is also needed. Fig. 2a shows S-hyperdoped Si directly grown on the back surface by laser irradiation, instead of the front surface, for the Si-based solar cell. The main objective is to avoid the annihilation of charge carriers generated by the absorbed photons with λ ≤ 1100 nm, due to the additional surface Auger recombination effect, and thus increase the photocurrent [22], [23]. However, a conflict is still observed between the long transport distance from PN junction of IR photogenerated charge carriers in this cell geometry and the short carrier lifetime. Therefore, hyperdoped Si/Si tandem geometry composed of a S-hyperdoped Si thin layer (N+ type) on the front surface of cell and a commercial Si-based solar cell substrate [N (P) structure] was designed and fabricated, as shown in Fig. 2b [24]. The S-hyperdoped Si layer is not directly grown on the solar cell surface (as shown in Fig. 2a); therefore, this cell geometry retains the PN junction of the commercial solar cell substrate, avoids the reduction of Voc, and increases the photocurrents of near IR and even ultraviolet (UV) light. Furthermore, the S-hyperdoped Si layer is on the front surface of the solar cell, thus the transport distance from PN junction of IR photogenerated charge carriers is only a few microns, which significantly decreases the carrier recombination loss.

More importantly, the S-hyperdoped Si thin layer can also be fabricated by nanosecond (ns)-pulsed laser irradiation of thin films [24]. The use of ns-pulsed laser is based on the fact that the ns-laser irradiated hyperdoped Si exhibited a high degree of crystallinity, strong visible (VIS) and broadband IR absorption [25], and good thermal annealing performance [26]. Although the ns-laser hyperdoping method has been studied [25], [27], [28] in parallel with the discovery of fs laser method [5], studies on the latter have continued since then, while the emphasis on ns-laser hyperdoping has switched to ion implantation followed by pulsed laser melting [9], [17], [19], leaving it largely unexplored. However, ns-lasers are inexpensive and can rapidly prepare large areas of hyperdoped Si by using rectangle shaped laser beam scanning [29]. Moreover, thin film hyperdoping process occurs in much less time and at much lower expense than hyperdoping by a gas [30] or ion implantation process, thus making the ns-laser based irradiation suitable for industrial applications. For example, several tens of hours of machine time is required to ion implant a full 15-cm wafer for hyperdoping [31]. Different from pulsed laser mixing technique [31], [32], which uses laser irradiation of a very thin film (<10 nm) to incorporate dopants in the underlying Si wafer, herein, the ns-pulsed laser irradiation of Sisingle bondSsingle bondSi multilayered films grown on an inexpensive commercial polycrystalline Si solar cell substrate was studied. Furthermore, the macroscopic and microscopic characteristics were systematically investigated to comprehensively understand the S-hyperdoped Si layer in the N+/N (P) tandem geometry.

Section snippets

Thin film deposition

Fig. 3 shows sample geometry of ns-pulsed laser irradiated Sisingle bondSsingle bondSi multilayered films for the preparation of S-hyperdoped Si layer, which is similar to that shown in Fig. 2b. An inexpensive commercial polycrystalline Si solar cell with a thickness of ∼200 μm was used as the substrate. A ∼80 nm SiNx anti-reflection and passivation film (without Ag gridlines) and an Al paste were prepared on front and back surfaces of the substrate, respectively. Prior to the multilayered films deposition, the

Optical reflection and absorption

Fig. 5a shows the optical reflectance profiles, at λ = 250–2500 nm, of the S-hyperdoped Si samples fabricated by ns-pulsed laser with different fluences (120, 175, and 220 mJ·cm−2), compared to the front surface of polycrystalline Si solar cell substrate (without the SiNx film and Al paste). The reflectance of hyperdoped samples is lower than that of the substrate for the whole wavelength region. For the wavelength region of λ < 1100 nm, the reflectance of hyperdoped samples decreases with the

Conclusion

S-hyperdoped Si (usually named as black Si), due to its broad-spectrum light absorption characteristics, has been considered to be a very promising full-spectrum (λ = 250–2500 nm) solar cell material since last 20 years. However, its development is far below the expectation. At present, it is usually prepared directly on a single crystalline Si surface by using fs-pulsed laser irradiation in SF6 background atmosphere or ion implantation followed by pulsed-laser melting. Several problems are

Acknowledgements

This work was supported by NSAF of China (Nos. U1530102 and U1730114), the Project of State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology (No. 17FKSY0116), and the Longshan Academic Talent Research Supporting Program of Southwest University of Science and Technology (Nos. 17LZX624 and 18LZX623).

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