# Boron doped nanocrystalline silicon: understanding a tricky material for high performance heterojunction solar cells

Amanda Merino, Patricia Martinez, Maria Saavedra, Patricio Häberle, Tristan Gageot, José Alvarez, Jean-Paul Kleider, Valeria del Campo, Delfina Muñoz

<figure><img src="image_1.png" /></figure>

## ► To cite this version:

Amanda Merino, Patricia Martinez, Maria Saavedra, Patricio Häberle, Tristan Gageot, et al., Boron doped nanocrystalline silicon: understanding a tricky material for high performance heterojunction solar cells. PVSC 2025 - IEEE 53rd Photovoltaic Specialists Conference, Jun 2025, Montreal, Canada. pp.0180-0182, 10.1109/PVSC59419.2025.11132544. hal-05383352

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Submitted on 26 Nov 2025

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# Boron doped nanocrystalline silicon: understanding a tricky material for high performance heterojunction solar cells

Amanda Merino¹,², Patricia Martinez³, Maria Saavedra³, Patricio Häberle³, Tristan Gageot¹, José Alvarez², Jean-Paul Kleider², Valeria del Campo³ and Delfina Muñoz¹

¹Université Grenoble Alpes CEA, Liten, Campus Ines, Le Bourget du Lac, 73375, France

²Laboratoire de Génie Electrique et Electronique de Paris (GeePs), CNRS, CentraleSupélec, Université Paris-Saclay, Sorbonne Université, F-91190 Gif sur Yvette, France

³Department of Physics, Universidad Técnica Federico Santa María, Valparaíso, Chile.

**Abstract** — The best silicon heterojunction (SHJ) solar cells show the potential of nanocrystalline silicon (nc-Si:H) to improve their performance on both the emitter and BSF (back surface field) sides. However, in production, nc-Si:H presents several challenges, particularly in terms of deposition rate, reproducibility and stability of layers and interfaces. This work presents a comprehensive study using advanced characterization of the properties of (p) nc-Si:H films grown in an industrial scale equipment. We have focused on studying different thicknesses and the impact of annealing on our layers, although we need very thin nc-Si:H layers to avoid parasitic absorption and reduce deposition time. Above 40 nm nc-Si:H seems to crystallize and stabilize its properties, while below this thickness we deal with a transition phase with evolution in crystalline fraction and properties giving a more unstable material. On the other hand, blisters appeared on layers with thicknesses over 60 nm due to hydrogen accumulation and material stress. Understanding the material properties can serve not only for SHJ but also for TCO-free recombination junction in tandems using nc-Si:H layers.

## I. INTRODUCTION

Among single-junction photovoltaic technologies, silicon heterojunction (SHJ) solar cells stand out for their ability to achieve high power conversion efficiencies (PCE). In this technology, the buffer and carrier collector layers are typically based on amorphous silicon (a-Si:H) due to their optoelectrical properties and passivating layers. However, the solar cell performance is limited by the low doping efficiency of a-Si:H, which leads to high series resistance and poor contact with the adjacent TCO [1]. Several works [2]-[4] have shown that nanocrystalline silicon (nc-Si:H) is a promising alternative due to its superior electronic and optical properties, enhanced by crystallinity, allowing higher PCE, with the world record using this technology being at 27.3% [5]. The potential of this material is not restricted to SHJ technologies. In an effort to avoid critical raw material consumption, researchers have replaced the indium-based recombination junction of perovskite/silicon tandem solar cells with a heavily doped nc-Si:H tunnel junction [6], [7]. However, nc-Si:H layer deposition poses several challenges, particularly in terms of deposition rate, reproducibility and layer stability. In addition,

it is necessary to achieve good properties while maintaining a thin layer (~20 nm) and the growth of boron doped nc-Si:H layers presents an additional challenge, due to the amorphising nature of boron [8]. In order to optimize the layer properties it is necessary to have a better understanding of their evolution under different conditions. This work presents a comprehensive study using advanced characterization on the properties of p-doped nanocrystalline silicon layers grown in an industrial scale equipment with industrially relevant conditions.

## II. MATERIALS AND METHODS

Nanocrystalline silicon films with thicknesses between 10 and 120 nm were deposited on both n-type crystalline silicon (c-Si) and Corning glass using a Meyerburger Helia plasma enhanced chemical vapour deposition (PECVD) system with hydrogen (H₂) and silane (SiH₄) as precursor gases (H₂/SiH₄ = 10000/30). For p-doped samples, diborane (B₂H₆) was added to the mixture (45 sccm). Deposition was performed at 175 °C with an RF power of 13.56 MHz and chamber pressure of 9.5 mbar. All nc-Si:H films were grown on top of a 9 nm (i) a-Si:H buffer layer to replicate the backside SHJ conditions. Some samples were annealed for 10-30 min at 205 °C to simulate the real annealing by TCO PVD deposition and silver screen-printing in the device production.

Film thickness, crystallinity fraction, and optical bandgap were obtained from spectroscopic ellipsometry (SE) measurements. The optical bandgap was obtained from Tauc plot analysis by extrapolating the linear portion of the (αhv)¹/² versus hv plot to the x-axis, α being the absorption coefficient and hv the photon energy [9]. Additional values for the crystallinity fraction (φc) were obtained from Raman spectroscopy of samples deposited on glass. This factor was obtained from the ratio of the integrated intensities of the crystalline (510 and 520 cm⁻¹) and amorphous (480 cm⁻¹) peaks [10]. The sheet resistance of the films on glass was measured using the 4-point probe (4PP) technique.
In order to study the evolution of the layer with increasing thickness, atomic force microscopy (AFM) was performed on (p) nc-Si:H samples deposited on glass for surface characterization and work function were obtained from stopping voltage measurements performed with the electron gun from an inverse photoemission spectroscopy (IPES) instrument [11] on layers deposited on c-Si. Both of these measurements were performed at the Universidad Tecnica Federico Santa Maria.

### III. RESULTS

#### A. Annealing

Samples of ~13 nm thickness were annealed at 205 °C for 10, 20 and 30 min immediately after PECVD deposition. The evolution of the crystallinity obtained from SE measurements of intrinsic and p-doped samples is shown in Fig. 1. As expected, the annealing seems to increase the crystallinity of undoped layers. For doped samples, the crystallinity improvement is not clear enough. The crystallinity improvement was confirmed by the Raman crystallinity fractions (not shown here): 20 min annealing of the layers resulted in a $φ_c$ increase of 2-5%$_{abs}$. In addition, the sheet resistance of the (p) nc-Si:H layers is reduced by more than 50%, reaching its minimum value at either 20 or 30 min of annealing (not shown here). The increase in conductivity, even though the crystallinity is not significantly improved, could be explained by changes in hydrogen bonding configuration [12], although it requires further characterization.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Crystallinity versus annealing time for intrinsic (0 sccm) and p-doped (45 sccm) 13 nm thick nc-Si:H samples obtained from SE measurements.</figcaption></figure>

#### B. Thickness

Fig. 2 shows the Raman crystallinity fraction and the sheet resistance of the (p) nc-Si:H samples with increasing thickness. The crystallinity increases from 14 ± 2% to

53 ± 2% when the film thickness is increased from 12 to 120 nm, in agreement with reports in the literature [13]. The increase in crystallinity leads to a decrease in sheet resistance from 3.8 ± 0.6 MΩ/sq to 15 ± 10 kΩ/sq.

The work functions of the (p) nc-Si:H samples with thicknesses of 12-120 nm are shown in Fig. 3. Up to a thickness of 60 nm, we have a clear trend of decreasing work function with increasing thickness. From 45 to 120 nm the work function values seem to remain unchanged. More discussion on WF results will be presented in the final paper. The optical bandgap decreases from 1.8 to 1.0 eV with increasing thickness, which is related to the increase in crystallinity [14].

<figure><img src="image_3.png" /><figcaption>Fig. 2. Raman crystallinity fraction and sheet resistance of (p) nc-Si:H layers versus layer thickness.</figcaption></figure>

Starting from a thickness of 60 nm, samples deposited on 2.5x2.5 cm² glass show blisters on the surface, visible to the naked eye. Fig. 4 (left) and Fig. 4 (right) show the height distribution obtained by AFM of a 45 nm and 60 nm thick sample, respectively. The surface of the 45 nm sample is homogenous over the whole area. There are blisters, craters and voids on the surface of the 60 nm sample as well as on thicker samples, where craters and voids are found even more numerous. The size of these blisters ranges from 20 to 400 µm, which is related to their implantation depth [15]. Table 1 shows the diameter and height of the measured blisters on 60 nm and 75 nm thick samples. The blisters labeled “dome” are of the type shown in Fig. 4 (right); “crater” is completely deflated and “half” is only half deflated. Note that these dimensions are measured from the surface of the layer and blisters could extend through the entire thickness depending on the depth of formation. It is important to note that the height of the blisters is significantly larger than the film thickness.

<table><caption>TABLE I
AFM BLISTERS DIMENSIONS</caption><thead><tr><td>Thickness [nm]</td><td>Blister</td><td>Diameter [µm]</td><td>Height [µm]</td></tr></thead></table>
<table><tr><td rowspan="4">75</td><td>Dome</td><td>45</td><td>0.7</td></tr><tr><td>Dome</td><td>40</td><td>1.0</td></tr><tr><td>Half</td><td>48</td><td>1.3</td></tr><tr><td>Crater</td><td>70</td><td>1.1</td></tr></table>

<figure><img src="image_4.png" /><figcaption>Fig. 3. Work function versus thickness of (p) nc-Si:H layers.</figcaption></figure>

The crystallinity fraction measured on blisters is <5%<sub>abs</sub> lower than that measured on flat areas. In addition, samples deposited on small glass substrates (1x1 cm<sup>2</sup>) do not show blisters on their surface and their crystallinity is ~5%<sub>abs</sub> higher than for samples deposited on larger glass substrates (2.5x2.5 cm<sup>2</sup>). Blistering formation occurs with hydrogen accumulation and is facilitated by boron doping [16]. The reason why it only occurs at certain sample sizes and thicknesses may be related to stress. Further film studies will be performed in order to understand this phenomenon.

<figure><img src="image_5.png" /><figcaption>Fig. 4. AFM measurements of (p) nc-Si:H layers. (left): surface of a 45 nm thick layer; and (right): small blister on a 60 nm thick layer.</figcaption></figure>

ACKNOWLEDGEMENTS

The authors would like to thank the ECOS-ANID cooperation program n° C21E08 between France and Chile for co-funding the research that led to this article.

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