**1. Introduction**

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% reported for such cells [2].

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

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

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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

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 car-

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

tions 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).

) layer. Two configura-

riers τ > 1 ms measured by transient photoconductance technique on ingots.

widely used in the literature, it is referred to the position of p(or p<sup>+</sup>

incorporating some PECVD films.

Ref. [5].

PECVD is a rather mature industrial technology exploited to fabricate both PV modules on both glass substrate with dimensions up to 2200 × 2600 mm2 and flexible plastic or metal foil 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 performance demonstrated.

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.
