**3. Electronic properties of PECVD materials used in HJT solar cells**

Let us consider some principal electronic properties of PECVD films used in HJT solar cells. They are listed in **Table 1** that contains some electrical and optical characteristics of the films. It is seen that properties of PECVD films differ significantly from those in crystalline materials comprising the same atoms. For example, optical gap for c-Si Eg = 1.1 eV and for amorphous silicon a-Si:H Eg = 1.62–1.65 eV, there is also a difference in activation energy of conductivity. These films can be also doped in n- and p-type though with less efficiency of doping compared to crystalline one. Difference of these characteristics provides possibility creation of heterojunctions with crystalline silicon resulting in local built-in electric fields. Conventionally, PECVD films are deposited from hydride gases such as silane (SiH4 ), methane (CH4 ), and


**Table 1.** Electronic properties of PECVD films incorporated in HJT solar cells.

germane (GeH4 ), which are often supplied as a mixture with hydrogen. Thus, glow discharge 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, which is of principle importance during fabrication of HJT solar cells.

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

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

**3. Electronic properties of PECVD materials used in HJT solar cells**

PECVD films are deposited from hydride gases such as silane (SiH4

**Table 1.** Electronic properties of PECVD films incorporated in HJT solar cells.

i-layer(a-Si:H)\* 1.65 4.79 0.61 0.75–0.82 p-layer(a-Si:H) 1.62 4.2 0.47 0.3–0.4 n-layer(a-Si:H) 1.63 4.72 0.6 0.25–0.35 p-layer (nc-Si:H) 1.87\* 4.22 0.18 0.08–0.12 n-layer (nc-Si:H) 1.83\* 4.16 0.20 0.02–0.04 p-layer (a-SiC:H) 2.02 3.49 0,24 0.45–0.50

**Layer type** *E***<sup>g</sup>**

\*

E04 values are presented.

Let us consider some principal electronic properties of PECVD films used in HJT solar cells. They are listed in **Table 1** that contains some electrical and optical characteristics of the films. It is seen that properties of PECVD films differ significantly from those in crystalline materials comprising the same atoms. For example, optical gap for c-Si Eg = 1.1 eV and for amorphous silicon a-Si:H Eg = 1.62–1.65 eV, there is also a difference in activation energy of conductivity. These films can be also doped in n- and p-type though with less efficiency of doping compared to crystalline one. Difference of these characteristics provides possibility creation of heterojunctions with crystalline silicon resulting in local built-in electric fields. Conventionally,

 **(eV) n@500 nm k@500 nm** *E<sup>a</sup>*

), methane (CH4

 **(eV)**

), and

discussed in Section 3 and film deposition in Section 5.

64 Solar Panels and Photovoltaic Materials

diffusion of photocarriers and serve as anti-epitaxial buffer.

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 practically the same hydrogen bonding structure in the samples.

**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 well centered at k = 2000 cm1 . No detectable changes have been observed with thickness.

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.

**Figure 2.** IR absorption spectra measured by transmission and ATR IR spectroscopy.

After the basic optimization, the characteristics obtained can be extrapolated to the thicknesses of the films in device structures. However, such optimization is only initial stage because always growth conditions for the films even in the same run (with fixed operator deposition parameters) are different for the film deposited on glass from those when the film is deposited on stack of previously deposited films. Therefore, it should be noted that final optimization of the films is performed in concrete device structures for concrete film inserted between other materials. Figures of merit for such optimization are performance characteristics of the device.

**4. Performance characteristics of HJT solar cells (single wafer** 

**Figure 4.** Normalized IR absorbance spectra for intrinsic i-a-Si:H (a) and p-doped p-Si:H layers (b).

tion, we describe and discuss these characteristics for single wafer HJT solar cells.

with illumination of solar simulator providing incident light intensity Iinc = 1000 W/m2

determined at J = 0, Iinc is the incident light intensity and FF is the fill factor.

function of R&D time in RDC TF TE are presented in **Figure 5**.

base for large area modules with dimensions 1600 × 1000 mm2

of this module are presented and discussed in Section 5.

*PCE* = *η* = (*J*

cell with 156 × 156 mm2

Performance characteristics of both solar cells and modules allow obtaining finally power conversion efficiency (PCE) of solar energy into electric energy, to see harvesting photons of different energy of the Sun spectrum and to get insight into technological issues. In this sec-

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

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

67

PCE values are conventionally determined from current–voltage J(U) characteristics measured

1.5 conditions. An example of J(U) characteristic is shown in **Figure 7**, and the characteristics calculated from this J(U) curve are given in the insert. PCE is defined by the following equation:

where Jsc is the short circuit current density determined at U = 0, Uoc is the open circuit voltage

Progress in efficiency for single wafer HJT solar cells and some images of the samples as a

Starting in 2014, with small area prototypes RDC TF TE in 2017 achieved PCE = 22.8% for solar

dimensions (corresponding area S = 244 cm2

*sc Uoc*) *FF*/*I*

and AM

*inc* (1)

) in 2017. This cell is the

. Fabrication and characteristics

**devices)**

**Figure 3.** ATR IR spectra of intrinsic (a) and doped (b) a-Si:H layers of different thickness.

From 11% Thin Film to 23% Heterojunction Technology (HJT) PV Cell: Research… http://dx.doi.org/10.5772/intechopen.75013 67

**Figure 4.** Normalized IR absorbance spectra for intrinsic i-a-Si:H (a) and p-doped p-Si:H layers (b).

After the basic optimization, the characteristics obtained can be extrapolated to the thicknesses of the films in device structures. However, such optimization is only initial stage because always growth conditions for the films even in the same run (with fixed operator deposition parameters) are different for the film deposited on glass from those when the film is deposited on stack of previously deposited films. Therefore, it should be noted that final optimization of the films is performed in concrete device structures for concrete film inserted between other materials. Figures of merit for such optimization are performance characteristics of the device.

**Figure 2.** IR absorption spectra measured by transmission and ATR IR spectroscopy.

66 Solar Panels and Photovoltaic Materials

**Figure 3.** ATR IR spectra of intrinsic (a) and doped (b) a-Si:H layers of different thickness.
