**1.1 CdTe cells and modules**

Cadmium telluride based solar cells are one of the most promising in thin film photovoltaics. With a bandgap of 1.45 eV the material well suited to match the AM 1.5 solar spectrum. Furthermore, its high absorption coefficient causes that only a few microns absorber film is required for solar cell operation. The typical thin film CdTe/CdS structure is shown on Figure 2. Figure 3 presents the total life-cycle Cd emissions to prove that CdTe based PV cell are environment friendly and health safe.

Fig. 2. Typical structure of CdTe thin film solar cell.

Fig. 1. Best laboratory PV cell efficiencies for thin films [source: www.nrel.gov]

potential to reach the needed performance, reliability and cost goals [1].

based PV cell are environment friendly and health safe.

Fig. 2. Typical structure of CdTe thin film solar cell.

**1.1 CdTe cells and modules** 

To comprehend the developmental issues of thin films, it is important to examine each individually. Each has a unique set of advantages and shortcomings in terms of their

Cadmium telluride based solar cells are one of the most promising in thin film photovoltaics. With a bandgap of 1.45 eV the material well suited to match the AM 1.5 solar spectrum. Furthermore, its high absorption coefficient causes that only a few microns absorber film is required for solar cell operation. The typical thin film CdTe/CdS structure is shown on Figure 2. Figure 3 presents the total life-cycle Cd emissions to prove that CdTe

Fig. 3. Total life-cycle Cd emissions by Brookhaven National Laboratory [9].

Low-cost soda-lime glass, foil or polymer film can be used as the substrate of CdTe/CdS solar cell. The best results of 16.5% efficiency are achieved with glass substrate (Table 1). However, Laboratory for Thin Films and Photovoltaics at EMPA, Switzerland obtained 12.7% efficiency of single CdTe solar cell on polymer foil and 7.5% of monolithically interconnected flexible CdTe solar module of 32 cm2 total area [10]. Transparent conductive layers (TCL) are usually thin conductive metal oxides, such as ITO (*Indium Tin Oxide*), Zn2O4, Cd2SnO4, ZnO:Al, CdO, ZnO, In2O3, SnO2 or RuSiO4. However, lastly in flexible solar cells, transparent conductive oxides (TCO) are being replaced by carbon nanotube (CNT) composites [11] or graphene. The CdS film is grown either by chemical bath deposition (CBD), close space sublimation (CSS), chemical vapor deposition (CVD), sputtering or vapor transport deposition (VTD). For the growth of CdTe, three leading methods are used for module fabrication: CSS, electro-deposition (ED) and VTD. Wide variety of metals can be used as back contact for thin film CdTe solar cells, e.g. Cu, Au, Cu/Au, Ni, Ni/Al, Sb/Al, Sb/Au [12].

Several thin-film PV companies are actively involved in commercializing thin-film PV technologies. In the area of CdTe technology major players are or were in the past: First Solar (USA), Primestar Solar (USA), BP Solar (USA), Antec Solar (Germany), Calyxo (Germany), CTF Solar (Germany), Arendi (Italy), Abound Solar (USA), Matsushita Battery (Japan) [8, 12, 13]. This effort lead to 18% share of CdTe cells in global PV marked in 2009 [14].

#### **1.2 CIS/ CIGS/ CIGSS structures**

Other promising material for thin film solar cell absorber layer is copper indium diselenide CuInSe2. CIS has a direct bandgap of 0.95 eV which can be increased by the addition of gallium to the absorber film. About 30% of Ga added to CIS layer (CIGS cell), changes the bandgap from 0.95 eV to almost 1.2 eV, which improves its match with the AM 1.5 solar spectrum. Higher gallium content (of 40%) has a detrimental effect on the device performance, because of its negative impact on the charge transport properties. The best

Innovative Elastic Thin-Film Solar Cell Structures 257

film CIGS PV technologies. These companies are using different deposition methods for

**Company Substrate Back contact Process Front contact** 

The absorber layer for commercial products uses either co-evaporation or the two-stage process such as the deposition of the precursors (Cu, Ga, In) by sputtering followed by selenization. Absorber layer can be made out of three chalcopyrite chemical compounds:

As a back, metal contact Mo deposited by sputtering is most commonly used. Majority of companies (Table 2) use ZnO as the front transparent conductive layer. Zinc oxide is deposited either by sputtering or chemical vapor deposition [13]. Window layer of CdS (or alternatives, such as ZnS [8]) can be grown by analogous methods as in CdTe solar structure

As a substrate of thin film CIGS solar cell, either glass, metal (steel) sheet or polymer might be chosen. Highest efficiencies, as noted in Table 1, were achieved for modules on glass substrate. However, such solution have several inconveniences, which are for example: bulkiness, fragility and heaviness. Flexible substrates, on the other hand, offer both manufacturing and application related advantages, such as: large active area, roll-to-roll high speed (throughput) deposition, high material utilization, low thermal budget, monolithic interconnection, lower costs, light-weight and flexibility. Table 3 presents the

To conclude this subsection, we should ask a question: "why thin film CIGS and CdTe solar cells are worth attention?". The answer was given and it can be summarized in several

Cu(In,Ga)Se2, Cu(In,Ga)(S,Se)2, CuInS2 which are respectively CIGS, CIGSS and CIS.

comparison of thin film CIGS solar cell on steel and polymer substrate.

2. active layers can be deposited on various substrates including flexible ones,

6. these cells are attractive for both terrestrial and space applications [8].

Shell Solar Glass Mo Sputter/ Selenization ZnO Global Solar Steel Mo Co-evaporation ITO Miasole Glass Mo Sputter ZnO Wurt Solar Glass Mo Co-evaporation ZnO Avancis Glass Mo Sputter/ RTP ZnO Daystar Tech. Glass Mo Sputter ZnO EPV Glass Mo Sputter/ Evaporation ZnO Ascent Solar Polymer Mo Co-evaporation ZnO ISET Glass/Flexible Mo Ink/ Selenization ZnO Nanosolar Flexible Mo Print/ RTP ZnO Solo Power Steel Mo ED/ RTP ZnO

growing the thin CIGS absorber layers, as is shown in Table 2 [13].

Table 2. Thin film CIGS technology.

(CBD, CSS, CVD etc.).

1. they are highly efficient,

5. they are low cost,

3. they have extremely stable performance, 4. they cause no environmental or health hazards,

points:

gallium to indium ratio is 3:7 for high efficiency PV devices. The role of sulfur in CIGSS is to increase the bandgap of the absorber film [12], which can boost the AM 1.5 spectrum fitting even more. The typical thin film CIGS solar cell structure is shown on Figure 4. Figure 5 presents an example CIGS cell structure manufactured in the Laboratory for Thin Films and Photovoltaics at EMPA, Switzerland.

Fig. 4. Typical structure of CIGS thin film solar cell.

Fig. 5. CIGS cell structure manufactured at EMPA, Switzerland [8].

Worldwide, several companies are presently offering commercial thin-film PV CIGS products: Würth Solar, (Germany), Global Solar, (USA), Honda, (Japan), Showa Shell, (Japan), Sulfurcell, (Germany), Solibro (Germany), Avancis (Germany), Solyndra (USA), Centrotherm (Germany). Also, worldwide, about 34 companies are actively developing thin-

gallium to indium ratio is 3:7 for high efficiency PV devices. The role of sulfur in CIGSS is to increase the bandgap of the absorber film [12], which can boost the AM 1.5 spectrum fitting even more. The typical thin film CIGS solar cell structure is shown on Figure 4. Figure 5 presents an example CIGS cell structure manufactured in the Laboratory for Thin Films and

Photovoltaics at EMPA, Switzerland.

Fig. 4. Typical structure of CIGS thin film solar cell.

Fig. 5. CIGS cell structure manufactured at EMPA, Switzerland [8].

Worldwide, several companies are presently offering commercial thin-film PV CIGS products: Würth Solar, (Germany), Global Solar, (USA), Honda, (Japan), Showa Shell, (Japan), Sulfurcell, (Germany), Solibro (Germany), Avancis (Germany), Solyndra (USA), Centrotherm (Germany). Also, worldwide, about 34 companies are actively developing thin-


film CIGS PV technologies. These companies are using different deposition methods for growing the thin CIGS absorber layers, as is shown in Table 2 [13].

Table 2. Thin film CIGS technology.

The absorber layer for commercial products uses either co-evaporation or the two-stage process such as the deposition of the precursors (Cu, Ga, In) by sputtering followed by selenization. Absorber layer can be made out of three chalcopyrite chemical compounds: Cu(In,Ga)Se2, Cu(In,Ga)(S,Se)2, CuInS2 which are respectively CIGS, CIGSS and CIS.

As a back, metal contact Mo deposited by sputtering is most commonly used. Majority of companies (Table 2) use ZnO as the front transparent conductive layer. Zinc oxide is deposited either by sputtering or chemical vapor deposition [13]. Window layer of CdS (or alternatives, such as ZnS [8]) can be grown by analogous methods as in CdTe solar structure (CBD, CSS, CVD etc.).

As a substrate of thin film CIGS solar cell, either glass, metal (steel) sheet or polymer might be chosen. Highest efficiencies, as noted in Table 1, were achieved for modules on glass substrate. However, such solution have several inconveniences, which are for example: bulkiness, fragility and heaviness. Flexible substrates, on the other hand, offer both manufacturing and application related advantages, such as: large active area, roll-to-roll high speed (throughput) deposition, high material utilization, low thermal budget, monolithic interconnection, lower costs, light-weight and flexibility. Table 3 presents the comparison of thin film CIGS solar cell on steel and polymer substrate.

To conclude this subsection, we should ask a question: "why thin film CIGS and CdTe solar cells are worth attention?". The answer was given and it can be summarized in several points:


Innovative Elastic Thin-Film Solar Cell Structures 259

Fig. 6. SEM picture and the diagram of CdS wurtzite grains with vertical growth orientation

to achieving of this structure under some technology circumstances [17] and may be matched with crystal constant differences not higher than 9,7% [18]. Structure of model CdS

The most popular manufacturing technologies of CdS/CdTe solar cells are nowadays CVD *(Chemical Vapour Deposition)* and the variants like PECVD (*ang: Plasma Enhanced CVD, or*  MOCVD ( *Metall Organic Chemical Vapour Deposition*) [19],CBD (*Chemical Bath Deposition) and physical methods like* PVD (*Physical Vapour Deposition*), CSS (*Close Space Sublimation*) [20], and variants of CSVT (*Close Space Vapour Transport*) [21, 22]. Alternatively screen-printing technology was also successfully employed for production of relatively thick CdTe base [23]. Morphology of the last mentioned layers was verified by authors with the help of SEM

Fig. 7. SEM picture of dense, compact CdTe grain layer up to 8µm, manufactured by ICSVT

As the additional experiments AFM profile of this layer, presented in Figure 8 was

layer, obtained by authors, organized in wurtzite phase is presented in Figure 6.

analysis indicating dense compact structure of hexagonal grains (Figure 7).

and CdS hexagonal grain model.

technology on glass substrate.

prepared.


Table 3. Substrates for flexible CIGS solar modules [8].

Thin-film photovoltaic cells and modules are already widely popularized, mainly because of their small production costs and relatively high efficiency [15]. Moreover, some other, significant advantages, such as small weight and flexibility may be offered. That is the reason why large number of applications is being pursued using thin-film PV technologies, including building-integrated photovoltaics (BIPV), roof-top applications and utility-scale applications [13].
