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

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

PCE values are conventionally determined from current–voltage J(U) characteristics measured with illumination of solar simulator providing incident light intensity Iinc = 1000 W/m2 and AM 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:

$$PCE = \eta = \left(\mathbb{J}\_{sc}\mathbb{U}\_{ac}\right)FF/\mathbb{I}\_{inc} \tag{1}$$

where Jsc is the short circuit current density determined at U = 0, Uoc is the open circuit voltage determined at J = 0, Iinc is the incident light intensity and FF is the fill factor.

Progress in efficiency for single wafer HJT solar cells and some images of the samples as a function of R&D time in RDC TF TE are presented in **Figure 5**.

Starting in 2014, with small area prototypes RDC TF TE in 2017 achieved PCE = 22.8% for solar cell with 156 × 156 mm2 dimensions (corresponding area S = 244 cm2 ) in 2017. This cell is the base for large area modules with dimensions 1600 × 1000 mm2 . Fabrication and characteristics of this module are presented and discussed in Section 5.

As a consequence, textured wafers have pyramidal surface topology with size and distribution of pyramids controlled by parameters of etching process. An example of AFM image of textured wafer is shown in **Figure 6**. One can see in the figure that surface of textured wafer is

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

Special attention has been paid to final cleaning of wafers surface from organic and metal impurities. We used several cleaning steps followed by final HF-dip and hot nitrogen drying procedure. Current-voltage characteristic of our best cell measured under standard test conditions (STC)

Spectral characteristics for different c-Si solar cells, including single wafer HJT solar cell, are presented in **Figure 8**. Comparing spectral curves in **Figure 8**, we can see that PERC solar cell has higher response in short wavelength range (λ < 500 nm) and lower response in long wavelength region (λ > 900 nm) than HJT device resulting in a small difference about 2% in integral current. Multicrystalline solar cell made by the BSF technology currently dominating in PV market has a little bit better response for λ < 350 nm and worse response for λ > 900 nm when compared to that for HJT device resulting in lower value of integral current. Both PERC and HJT silicon solar cells provide high and close values of integral short

Dispersion values for the measured characteristics indicate rather good reproducibility of electronic properties observed in the device structures. For comparison, record data on HJT single wafer solar cell reported by "Kaneka Corp," Japan in 2017 [2]. This company increased its previous record in 2015 from PCE =25.1% to PCE = 26.6% in 2017 and the predicted efficiency exceeded 27% soon. It is worth to note that cells with record characteristics require

**Figure 7.** Current-voltage I(U) graph and characteristics for single wafer HJT cell (156 × 156 mm2

). This cell exhibits such

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69

).

uniformly covered by pyramids with average height about 1.5 mkm.

is shown in **Figure 7** for single wafer HJT solar cell (156 × 156 mm2

Average values of characteristics are presented in **Table 2**.

circuit currents.

parameters as efficiency PCE = 23.04%, Uoc = 735 mV, Isc = 9.45 A and FF = 81.0%.

**Figure 5.** PCE increase of HJT single wafer solar cell during the development in RDC TF TE.

In order to improve light trapping in the device structures, Si wafers were textured using isopropanol alcohol (IPA) free alkaline process. Despite diamond wire, sliced wafer can be successfully textured from as cut state without surface damage etch (SDE) step, we used SDE because it facilitated uniform texturing, improved process stability and reduced production costs by lower consumption of surfactants providing anisotropic Si etching along <111> direction.

**Figure 6.** AFM image of textured Si wafer surface (a) and distribution function of heights (b).

As a consequence, textured wafers have pyramidal surface topology with size and distribution of pyramids controlled by parameters of etching process. An example of AFM image of textured wafer is shown in **Figure 6**. One can see in the figure that surface of textured wafer is uniformly covered by pyramids with average height about 1.5 mkm.

Special attention has been paid to final cleaning of wafers surface from organic and metal impurities. We used several cleaning steps followed by final HF-dip and hot nitrogen drying procedure.

Current-voltage characteristic of our best cell measured under standard test conditions (STC) is shown in **Figure 7** for single wafer HJT solar cell (156 × 156 mm2 ). This cell exhibits such parameters as efficiency PCE = 23.04%, Uoc = 735 mV, Isc = 9.45 A and FF = 81.0%.

Spectral characteristics for different c-Si solar cells, including single wafer HJT solar cell, are presented in **Figure 8**. Comparing spectral curves in **Figure 8**, we can see that PERC solar cell has higher response in short wavelength range (λ < 500 nm) and lower response in long wavelength region (λ > 900 nm) than HJT device resulting in a small difference about 2% in integral current. Multicrystalline solar cell made by the BSF technology currently dominating in PV market has a little bit better response for λ < 350 nm and worse response for λ > 900 nm when compared to that for HJT device resulting in lower value of integral current. Both PERC and HJT silicon solar cells provide high and close values of integral short circuit currents.

Average values of characteristics are presented in **Table 2**.

In order to improve light trapping in the device structures, Si wafers were textured using isopropanol alcohol (IPA) free alkaline process. Despite diamond wire, sliced wafer can be successfully textured from as cut state without surface damage etch (SDE) step, we used SDE because it facilitated uniform texturing, improved process stability and reduced production costs by lower consumption of surfactants providing anisotropic Si etching along <111> direction.

**Figure 5.** PCE increase of HJT single wafer solar cell during the development in RDC TF TE.

68 Solar Panels and Photovoltaic Materials

**Figure 6.** AFM image of textured Si wafer surface (a) and distribution function of heights (b).

Dispersion values for the measured characteristics indicate rather good reproducibility of electronic properties observed in the device structures. For comparison, record data on HJT single wafer solar cell reported by "Kaneka Corp," Japan in 2017 [2]. This company increased its previous record in 2015 from PCE =25.1% to PCE = 26.6% in 2017 and the predicted efficiency exceeded 27% soon. It is worth to note that cells with record characteristics require

**Figure 7.** Current-voltage I(U) graph and characteristics for single wafer HJT cell (156 × 156 mm2 ).

**Figure 8.** Spectral response of PV cells fabricated with crystalline silicon, PECVD silicon films and combination in HJT structures.

We shall skip description of technological process details and discuss only some specific issues related to HJT with crystalline silicon wafers large area module fabrication. Initially, these systems were employed for fabrication of thin film double junction tandem module on glass substrate (see diagram in **Figure 1(c)**). For HJT solar cell module, silicon wafers are placed on a special wafer carrier developed by RDC TF TE and then loaded into reactor. More details (process sequence, equipment, project milestones, etc.) on process of conversion silicon thin film solar module to high efficiency HJT module production can be found in ref. [11]. An important issue for large area modules is uniformity of electronic properties of the films

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

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71

Map of thicknesses for a-Si:H films deposited on large area glass substrate is shown in **Figure 10** with mean thickness value <d > = 48.5 nm and deviation of thickness <Δd> =7.1%. In other words,

**Figure 11** shows five-point average thicknesses and five-point bandgap values for the array of 5 × 5 silicon wafers, with mean values of thickness < d > =94.7 and deviation 3.4% and with mean value of optical gap <E<sup>g</sup> > =1.72 eV and deviation <ΔE<sup>g</sup> > =HJT 1.1%. These characteristics meet uniformity requirement for fabrication of HJT cells on wafer distributed over entire reac-

Performance characteristic of "Hevel" HJT solar cell module is shown in **Figure 12**. Comparison of this characteristic with that for HJT module reported by "Meyer-Burger" is presented in **Table 3**. One can see in **Table 3** that the values reported by "Hevel Solar" are still less than those of "Meyer Burger." However, this difference partially comes from using full square wafers in MB modules, which results in reduced dead space area and corresponding gain in current value. It should be noted that it is difficult to perform correct comparison because of the difference in the form of silicon wafers (not always reported), in normalization over area taking

) is better than 10%.

deposited on large area substrate (or the carrier with 60 wafers).

**Figure 9.** Laboratory installation (a) and industrial PECVD system (b).

thickness of the films over entire reactor active area (1300 × 1000 mm2

tor area with thin films incorporated in the device structure.


**Table 2.** Performance characteristics of HJT single wafer (156 × 156 mm2 ) HJT solar cells.

usually special design and materials which may not be compatible with mass production conditions and/or facilities; however, such high efficiency level of laboratory cells demonstrates definitely potential for further improvement of HJT technology.

#### **5. Implementation of the developed single wafer HJT structures in 1600 × 1000 mm<sup>2</sup> modules in industrial production**

PECVD films for previous optimization were deposited in RDC TF TE laboratory system from "Oerlikon Solar," Switzerland, model Gen 5 KAI, photo is presented in **Figure 9(a)**. Reactor of this system is similar to the industrial system "KAI MT R1.0 Modular PECVD System" installed for industrial fabrication of the large modules S = 1600 × 1000 mm2 . Photo of this system is shown in **Figure 9(b)**. In both PECVD systems, capacitive glow discharge at frequency f = 40.68 MHz, is used, deposition temperature is about T<sup>d</sup> = 200 C, technological gases of semiconductor purity.

From 11% Thin Film to 23% Heterojunction Technology (HJT) PV Cell: Research… http://dx.doi.org/10.5772/intechopen.75013 71

**Figure 9.** Laboratory installation (a) and industrial PECVD system (b).

usually special design and materials which may not be compatible with mass production conditions and/or facilities; however, such high efficiency level of laboratory cells demon-

Avg. value 9.45 ± 0.01 0.73 ± 0.01 5.52 ± 0.01 0.63 ± 0.01 8.80 ± 0.01 79.9 ± 0.1 22.6 ± 0.1 "Kaneka" 2017 [2] 10.37\* 0.740 Not reported Not reported Not reported 84.7 26.6

**Figure 8.** Spectral response of PV cells fabricated with crystalline silicon, PECVD silicon films and combination in HJT

**Isc (A) Voc (V) Pmax (W) Vpmax (V) Ipmax (A) FF (%) Eff (%)**

, density of short circuit current Jsc = 42.5 mA/cm2

) HJT solar cells.

PECVD films for previous optimization were deposited in RDC TF TE laboratory system from "Oerlikon Solar," Switzerland, model Gen 5 KAI, photo is presented in **Figure 9(a)**. Reactor of this system is similar to the industrial system "KAI MT R1.0 Modular PECVD System"

tem is shown in **Figure 9(b)**. In both PECVD systems, capacitive glow discharge at frequency f = 40.68 MHz, is used, deposition temperature is about T<sup>d</sup> = 200 C, technological gases of

. Photo of this sys-

.

**5. Implementation of the developed single wafer HJT structures in** 

 **modules in industrial production**

installed for industrial fabrication of the large modules S = 1600 × 1000 mm2

strates definitely potential for further improvement of HJT technology.

**1600 × 1000 mm<sup>2</sup>**

Calculation based on data in [2]: area S = 180 cm2

**Table 2.** Performance characteristics of HJT single wafer (156 × 156 mm2

\*

structures.

70 Solar Panels and Photovoltaic Materials

semiconductor purity.

We shall skip description of technological process details and discuss only some specific issues related to HJT with crystalline silicon wafers large area module fabrication. Initially, these systems were employed for fabrication of thin film double junction tandem module on glass substrate (see diagram in **Figure 1(c)**). For HJT solar cell module, silicon wafers are placed on a special wafer carrier developed by RDC TF TE and then loaded into reactor. More details (process sequence, equipment, project milestones, etc.) on process of conversion silicon thin film solar module to high efficiency HJT module production can be found in ref. [11].

An important issue for large area modules is uniformity of electronic properties of the films deposited on large area substrate (or the carrier with 60 wafers).

Map of thicknesses for a-Si:H films deposited on large area glass substrate is shown in **Figure 10** with mean thickness value <d > = 48.5 nm and deviation of thickness <Δd> =7.1%. In other words, thickness of the films over entire reactor active area (1300 × 1000 mm2 ) is better than 10%.

**Figure 11** shows five-point average thicknesses and five-point bandgap values for the array of 5 × 5 silicon wafers, with mean values of thickness < d > =94.7 and deviation 3.4% and with mean value of optical gap <E<sup>g</sup> > =1.72 eV and deviation <ΔE<sup>g</sup> > =HJT 1.1%. These characteristics meet uniformity requirement for fabrication of HJT cells on wafer distributed over entire reactor area with thin films incorporated in the device structure.

Performance characteristic of "Hevel" HJT solar cell module is shown in **Figure 12**. Comparison of this characteristic with that for HJT module reported by "Meyer-Burger" is presented in **Table 3**.

One can see in **Table 3** that the values reported by "Hevel Solar" are still less than those of "Meyer Burger." However, this difference partially comes from using full square wafers in MB modules, which results in reduced dead space area and corresponding gain in current value. It should be noted that it is difficult to perform correct comparison because of the difference in the form of silicon wafers (not always reported), in normalization over area taking

**Figure 10.** Thickness mapping for a-Si:H films deposited on glass substrate. Data are obtained by spectral ellipsometry.

into account substrate area occupied by contact grid or not, and so on resulting in some uncertainties when comparing the devices.

Interesting data on outdoor 1-year testing of "Hevel Solar" HJT modules can be found in Ref. [13] (**Figure 12**).

**6. Summary and outlook**

RDC TF TE and fabricated by "Hevel solar".

modules (1000 × 1300 mm2

engineering.

solar cell modules (1000 × 1600 mm2

cially produced by "Hevel Solar" (Russia) [14].

**Figure 12.** Current-voltage I(U) characteristics of HJT module 1600 × 1000 mm2

We have briefly described a successful transformation of technology for thin film solar cell

**Module Pmax Vmp (V) Imp (A) Voc (V) Jsc (A) FF (%) PCE (%)** "Hevel Solar" 317 35.78 8.85 43.88 9.44 76.4 19.1 "Meyer Burger" 329 37.04 8.89 44.28 9.66 77.0 19.8

by electrode stripes and elements of hermetization and assembling.

made by "Hevel Solar" and "Meyer Burger" [12].

**Table 3.** Comparison of performance characteristics of 1600 × 1000 mm2

Note: PCE indicated in table are effective values calculated with integral module area without subtraction area occupied

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

essential equipment for PECVD materials. Now, the developed HJT modules are commer-

PECVD technique being principal in HJT module fabrication for both passivation and growth of semiconductor films is very versatile technology with high potential for further material

Well-known theoretical estimation of efficiency for one bandgap material c-Si gives value around PCE ≈ 30–34%, while record value achieved in 2017 by "Kaneka Corp." (Japan) is about PCE ≈ 27%. Thus some potential still exists for PCE increase for one gap c-Si HJT solar

) with efficiency 11% to heterojunction technology (HJT) for c-Si

) with efficiency around 20% with employing the same

(60 cells 156 × 156 mm2

), developed by

HJT modules consisted of 60 single wafer cells

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73

**Figure 11.** Thickness and optical gap mapping for the films deposited on silicon substrate (wafer dimensions 156 × 156 mm2 ). Data are obtained by spectral ellipsometry.


Note: PCE indicated in table are effective values calculated with integral module area without subtraction area occupied by electrode stripes and elements of hermetization and assembling.

**Table 3.** Comparison of performance characteristics of 1600 × 1000 mm2 HJT modules consisted of 60 single wafer cells made by "Hevel Solar" and "Meyer Burger" [12].

**Figure 12.** Current-voltage I(U) characteristics of HJT module 1600 × 1000 mm2 (60 cells 156 × 156 mm2 ), developed by RDC TF TE and fabricated by "Hevel solar".

#### **6. Summary and outlook**

into account substrate area occupied by contact grid or not, and so on resulting in some uncer-

**Figure 10.** Thickness mapping for a-Si:H films deposited on glass substrate. Data are obtained by spectral ellipsometry.

Interesting data on outdoor 1-year testing of "Hevel Solar" HJT modules can be found in Ref.

**Figure 11.** Thickness and optical gap mapping for the films deposited on silicon substrate (wafer dimensions 156 ×

tainties when comparing the devices.

72 Solar Panels and Photovoltaic Materials

). Data are obtained by spectral ellipsometry.

[13] (**Figure 12**).

156 mm2

We have briefly described a successful transformation of technology for thin film solar cell modules (1000 × 1300 mm2 ) with efficiency 11% to heterojunction technology (HJT) for c-Si solar cell modules (1000 × 1600 mm2 ) with efficiency around 20% with employing the same essential equipment for PECVD materials. Now, the developed HJT modules are commercially produced by "Hevel Solar" (Russia) [14].

PECVD technique being principal in HJT module fabrication for both passivation and growth of semiconductor films is very versatile technology with high potential for further material engineering.

Well-known theoretical estimation of efficiency for one bandgap material c-Si gives value around PCE ≈ 30–34%, while record value achieved in 2017 by "Kaneka Corp." (Japan) is about PCE ≈ 27%. Thus some potential still exists for PCE increase for one gap c-Si HJT solar cells, which can be realized by improving passivation, electrodes, improving short wavelength collection by frontal interface, and so on.

BSF back surface field (solar cell)

CZ silicon silicon fabricated by Czochralski technique

Jsc short circuit current density, mA/cm2

Isc short circuit current, A

TMB trimethyl boron

**Author details**

Eugenii Terukov1

**References**

Uoc open circuit voltage, V, mV

Rsh effective shunt resistance, Ohm Rs effective series resistance, Ohm

, Andrey Kosarev2

\*Address all correspondence to: akosarev@inaoep.mx

3 Ioffe Institute, Polytechnicheskaya, St-Petersburg, Russia

Applications. 2004;**12**:113-142. DOI: 10.1002/pip.533

1 R and D Center TFTE, Polytechnicheskaya, St-Petersburg, Russia

2 National Institute for Astrophysics, Optics and Electronics, Puebla, Mexico

PERC passivated emitter (usually p-Si) and rear cell (silicon solar cell).

STC standard test conditions, output performance conditions used by most

\*, Alexey Abramov1

[1] Int. Technology Roadmap for Photovoltaics, 2016 Results, 8th ed. September 8, 2017

and Nanocrystalline Semiconductors, August 21-25, 2017, Seoul, Korea. p. 92

[2] Yamamoto K, Yoshikawa K, Yoshida W, Irie T, Kawasaki H, Konishi K, Asatani T, Kanematsu M, Mishima R, Nakano K, Uzu H, Adachi D. High efficiency a-Si/c-Si heterojunction solar cells. In: Program Book, 27th International Conferece on Amorphous

[3] Shah AV, Schade H, Vanecek M, Meier J, Vallat-Sauvain E, Wyrsch N, Kroll U, Droz C, Bailat J. Thin-film silicon solar cell technology. Progress in Photovoltaics: Research and

[4] Yang J, Banerjee A, Guha S. Triple junction amorphous silicon alloy solar cell with 14.6 initial and 13.0% stable conversion efficiencies. Applied Physics Letters. 1997;**70**:2975-2977

.

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75

, T = 25°C

and Eugenia Malchukova3

,

PERL passivated emitter rear locally diffused (silicon solar cell).

PERT passivated emitter rear totally diffused (solar cells).

manufactures AM 1.5, I = 1000 W/m2

General road to increase conversion efficiency is related to multijunction (MJ) design and fabrication of PV structures comprising materials with different bandgaps adjusted for harvesting maximum of solar energy spectrum. This has been demonstrated by MJ solar cells with A3 B5 semiconductors provided the highest reported values of PCE = 46% [15].

Therefore, MJ approach should be taken into account considering further development of HJT c-Si solar cells with efficiency above 34%.
