**2. Configurations and fabrication of HJT devices**

In this section, we describe basic configurations of solar cells based on crystalline materials. **Figure 1** shows cross-section diagrams for crystalline silicon solar cell (a) fabricated by standard diffusion processes with typical efficiency of 17–19%, PECVD thin silicon film "tandem" structure (c) comprising two p-i-n junctions with efficiency of 9–11% and HJT silicon-based solar cell incorporating some PECVD films.

**1. Introduction**

62 Solar Panels and Photovoltaic Materials

reported for such cells [2].

performance demonstrated.

Market of PV devices shows continuous increase, for example, for only 1 year, that is, 2016–2017, it has grown from 76 GW to about 100 GW (by more than 30%) [1]. PV devices based on silicon dominate in the market (>90%). Interdigitated back contact (IBC) cells on monocrystalline n-type silicon demonstrate mass production efficiency, PCE = 23% (2016) with prognosis to rise to PCE = 27% by 2027 [1]. Fabrication of these devices is complicated because of multiple deposition and etching steps required to form both p- and n-doped contact areas on rear surface of the cells. Moreover this fabrication is based on conventional crystalline silicon technology including high temperature processes. Alternative and relatively simple approach to get high efficiency defined as heterojunction technology (HJT) includes deposition of thin layers by plasma-enhanced chemical vapor deposition (PECVD) conducted at low temperature. PECVD technique provides a wide range of possibilities for material engineering with variation of structure, electronic properties and doping of the films. These films can be used for surface passivation and for creation of additional built in electric field at interfaces with silicon. HJT solar cells exhibit PCE = 22% (2016) with prognosis for rise to PCE = 24% in 2027 [1]. Furthermore, a combination of HJT and back contact technology will allow to overcome PCE values predicted for conventional IBC cells made with diffusion approach as it is confirmed by a world record PCE values above 26%

PECVD is a rather mature industrial technology exploited to fabricate both PV modules on

substrates. The best developed PECVD PV structures provide efficiency, that is, PCE = 11% for "micromorph" two junction tandem [3] and PCE = 13% for triple tandem on stainless steel foil [4]. These values are less than those theoretically predicted PCE = 24%. Therefore, PECVD PV solar cell modules on glass are not able to compete with those based on crystalline silicon technology for terrestrial applications, though they occupy a segment of flexible solar cells in PV market. Advantages of PECVD technology for material engineering together with compatibility of this technique with c-Si technology made promising implementation of PECVD materials in c-Si PV technology resulting in development of HJT solar cells. The latter is attractive because of PECVD is a low-temperature process and also because of its

This chapter describes our experience in research and development of HJT solar cells and modules based on our previous background in fabrication of "micromorph" modules; implementation of HJT modules consisted of 60 cells in industrial production is also discussed.

In this section, we describe basic configurations of solar cells based on crystalline materials. **Figure 1** shows cross-section diagrams for crystalline silicon solar cell (a) fabricated by standard

and flexible plastic or metal foil

both glass substrate with dimensions up to 2200 × 2600 mm2

**2. Configurations and fabrication of HJT devices**

n-Type c-Si is a conventional material for HJT cells nowadays, although HJT solar cells based on p-type silicon with efficiency above 20% have been also reported, for example, see Ref. [5].

Despite using floating zone (FZ), c-Si for record cells and reasonable parameters obtained on high-quality multicrystalline wafers manufactured with direct solidification technique [6], crystalline silicon made by Czochralski (CZ) technique is conventionally used for HJT cells' mass production. In this study, 6" CZ Si pseudo square n-doped <100> wafers with typical resistivity in the range from 1 to 5 Ohm⋅cm were used. The wafers were sliced with diamond wire technology from ingots with low impurity level providing bulk lifetime of minority carriers τ > 1 ms measured by transient photoconductance technique on ingots.

It is worthy of some comments in terms of different HJT configurations. Frontal side of solar cell is determined as that for penetration of incident light, and opposite side is determined as rare (or back) side. There is also not well-justified term "emitter" which nevertheless is widely used in the literature, it is referred to the position of p(or p<sup>+</sup> ) layer. Two configurations of HJT cells are possible: with frontal emitter meaning p-layer position on frontal side and rare (back) emitter meaning p-layer on rear side. For industrial production, to our mind rear emitter is preferable because of higher contribution of the wafer in lateral conductivity resulting in lower requirements for contact grid (lines may be narrower and separated by longer distance) and consequently, reducing shadow losses. In addition, employing n-layer made of nanocrystalline silicon (PECVD nc-Si) on frontal side results in reducing absorption losses from frontal side. However, lower holes diffusion length and nonuniform absorption of the incident light inside of c-Si wafer resulting in much higher carrier generation rate at

**Figure 1.** Configurations of different solar cells: (a) crystalline silicon c-Si device fabricated with diffusion processes, (b) HJT cell comprising c-Si and PECVD materials and (c) thin film a-Si:H/mk-Si:H solar cell (two junction tandem).

front interface lead to record efficiencies on laboratory cells with frontal emitter configuration [7]. As seen in **Figure 1(b)** and **(c)**, the structures comprise different PECVD films such as undoped (e.g., amorphous silicon a-Si:H, microcrystalline mk-Si:H), p-doped (e.g. p-a-SiC:H, p-mk-Si:H) and n-doped (e.g. n-a-Si:H, n-mk-Si:H). Electronic properties of these films are discussed in Section 3 and film deposition in Section 5.

germane (GeH4

), which are often supplied as a mixture with hydrogen. Thus, glow discharge

From 11% Thin Film to 23% Heterojunction Technology (HJT) PV Cell: Research…

http://dx.doi.org/10.5772/intechopen.75013

65

during film growth contains significant amount of hydrogen in the form of molecules, atoms and ions. The latter two are very active chemically promoting passivation of substrate surface,

There are various techniques to grow thin device quality films, for example, atomic layer deposition (ALD), hot wire (HW) deposition, inductively coupled plasma (ICP), direct current (DC), low frequency (LF), radio frequency (RF), very high frequency (VHF), microwave plasma in capacitance type reactors, and so on. Comparison of these techniques is out of the scope of the chapter; therefore, we only notice that the industry is mostly employed with RF and VHF PECVD systems. The latter type is used for fabrication of HJT solar cells in this chapter.

Nanometer-scale thicknesses of the PECVD films in HJT structures are really a challenge in material engineering and electronic characterization of such films. Conventionally, at initial stage, material of each film and its electronic properties are optimized by preparing the samples on appropriate substrate such as glass or silicon. Thickness of the film at this stage is more than 100 nm, while in HJT structures, we need 5–20 nm thickness. Therefore, questions arise: is it possible to characterize such thin films? Is it possible to apply electronic characteristics measured in thicker films in device design? Up to now, there are contradictory data reported. Some researchers have observed changes in electronic properties with thickness [10], while others have revealed such behavior. Here, we present some data obtained in thin films by attenuated total reflection infrared spectroscopy (ATR IR) spectroscopy technique allowing characterization of rather thin films. To our mind, both spectral ellipsometry and ATR IR are widely used technique for thin film characterization. ATR IR spectroscopy allows measurements of the films with thickness less than 20 nm, that is, in the range of thickness of the films in HJT device structures. **Figure 2** shows IR spectra (measured by transmission on silicon substrate and ATR IR technique) of the films deposited on different substrate (glass and silicon) in the same run. The peaks for both curves are located at the value of k = 2000 cm−1suggesting

**Figure 3** presents ATR IR spectra around k = 2000 cm−1 (Si-H stretching mode) for the intrinsic and doped samples with different thicknesses. One can see in the figure that using ATR IR absorption spectra of a-Si:H film, it is possible to observe Si-H stretching mode in the films with thickness less than 20 nm; therefore, this technique can be effectively used for optimization of the films with thickness required for HJT cells. It is clearly seen that all the curves are

This is revealed even more clearly in **Figure 4**, where the spectra are normalized at maximum value for both intrinsic and p-doped layer. Thus, we have demonstrated some evidence for negligible effect of thickness on microstructure and consequently on electronic properties.

In other words, some basic material optimization can be performed by optical measurements with ellipsometry or ATR IR spectroscopy in the films deposited on some acceptable substrates (e.g., on glass), which is of principal importance for optimization of uniformity and

electronic properties in ultrathin films deposited in mass production PECVD systems.

. No detectable changes have been observed with thickness.

which is of principle importance during fabrication of HJT solar cells.

practically the same hydrogen bonding structure in the samples.

well centered at k = 2000 cm1

Here, we would like to provide some comments on functions of these films in device structures. Historically, first c-Si solar cells contained p- and/or n-doped layers prepared with diffusion, and they contacted to metals. They are characterized by significant losses at interfaces due to several processes, for example, free carrier absorption, surface recombination, and so on, and efficiency achieved is 17–19%. Further progress is related to the development of significantly more complex structures such as passivated emitter rear locally diffused (PERL) [8] and passivated emitter and rear cell (PERC) solar cells [9]. Then PV structures prepared by heterojunction technology called HJT solar cells\_ have been developed [9]. In these HJT structures crystalline silicon surfaces is passivated by PECVD films which also create heterojunctions providing additional built-in electric fields, reduce surface recombination and back diffusion of photocarriers and serve as anti-epitaxial buffer.
