**2. Manufacturing technologies of elastic thin – film CdS/CdTe solar cells**

Cadmium telluride solar cells are placed among thin-film polycrystalline structures of high optical absorption and relatively low material consumption, with long development history [16]. Numerous virtues of each semiconductor compound as well as good cooperation within p-n heterojunction give the opportunity of efficient and cheap monolithic solar modules construction. Additionally, elastic structure of polycrystalline layers enables the flexibility of the manufactured modules and gives possibility of new implementations. The experiments leading to this goal were undertaken by various manufacturing technologies, however to maintain high optoelectronic parameters of the manufactured cells, independently on employed technology, proper polycrystalline structure must be preserved.

Standard technology of CdTe solar cells manufacturing is based of column grain structure.

This specific material organization occurs owing to vertical growth of hexagonal CdTe and CdS grains and gives the opportunity of high optical generation, smooth vertical charge carrier transport and thus high conversion efficiency. Both semiconductors are predestined

No need of barrier coatings for monolithic

interconnection

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

**2. Manufacturing technologies of elastic thin – film CdS/CdTe solar cells** 

Cadmium telluride solar cells are placed among thin-film polycrystalline structures of high optical absorption and relatively low material consumption, with long development history [16]. Numerous virtues of each semiconductor compound as well as good cooperation within p-n heterojunction give the opportunity of efficient and cheap monolithic solar modules construction. Additionally, elastic structure of polycrystalline layers enables the flexibility of the manufactured modules and gives possibility of new implementations. The experiments leading to this goal were undertaken by various manufacturing technologies, however to maintain high optoelectronic parameters of the manufactured cells, independently on employed technology, proper polycrystalline structure must be

Standard technology of CdTe solar cells manufacturing is based of column grain structure. This specific material organization occurs owing to vertical growth of hexagonal CdTe and CdS grains and gives the opportunity of high optical generation, smooth vertical charge carrier transport and thus high conversion efficiency. Both semiconductors are predestined

Metal (steel) Polymer

Rough surface, kinks Smooth surface Conducting surface Insulating surface Metal impurities No metal impurities

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

High temperature: 550- 650 °C Low temperature: 450 °C Highest efficiency: 18% on Ti foil Highest efficiency: 16%

Barrier coatings needed for monolithic

interconnection

applications [13].

preserved.

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

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

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

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

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

Innovative Elastic Thin-Film Solar Cell Structures 261

Fig. 9. Morphology of CdTe solar cell base absorber manufactured by ICSVT technology on

The investigation of CdTe base manufactured by evaporation and subsequent

Fig. 10. Morphology of CdTe solar cell base absorber manufactured by evaporation and

glass substrate.

recrystallization is presented in Figure 10.

subsequent high-temp. recrystallization.

Fig. 8. AFM profiling of CdTe polycrystalline layer, made by ICSVT technology*.* 

This measurement gives some important information about grains structure and intercrystal surfaces. By means of polycrystalline CdS/CdTe layers profiling one may easily detect the diameter and grain shape but also the inter-grain valleys depth and possible structure fluctuations and layer discontinuities. These structure disorders may result in serious parameter losses by producing of shunt interconnections or other charge flow parasitic effects. Taking into account cadmium dichloride dissolvent presence, frequently caused by recrystallization demands morphology defects may cause a real and serious threat for CdS/CdTe structure functioning. No such phenomenon was confirmed by presented results.

All new production techniques of thin –film polycrystalline solar cells and particularly CdS/CdTe structure, designed for new application field, should be verified according to obtained layers profile to eliminate structure disorder. Since AFM spectroscopy gives the profiling results for some strictly limited area for wider statistical examination mechanical profiling of high accuracy may be applied. These experiments were also conducted for test CdS/CdTe structure. The experiment for each sample was conducted by the help of mechanical profilometer Dektak 3 VEECO Instruments. The measurements were performed for representative 100 µm scan range with 50nm resolution.

As the first analysis ICSVT CdTe layer morphology was checked (Figure 9). By this investigation the average grain diameter of 6-8µm was confirmed and the typical roughness of 1200 A. Series of measurements confirmed dense, compact structure of absorber grains with no interlayer shunts. Obtained average roughness and the CdTe grain surface profile suggests that inter-grain trenches are insufficient for significant degradation of shunt resistance value. The total layer level fluctuations are smaller than 4 µm, which taking into consideration typical glass thickness accuracy confirms homogenous thickness of the whole base layer.

Alternative technology, used for CdS/CdTe cell layers deposition is a standard PVD method. Under some circumstances it offers a possibility of semiconductor material deposition even on profiled, elastic and untypical substrates. Evaporation of examined materials caused serious technology problems connected with proper thickness of obtained layers and homogeneous structure of deposited material. Some serious parasitic effect like boiling, splitting and granulation of the material were solved by proper temperature profile and optimized one-directional tantalum evaporation source adopted by authors [24].

Fig. 8. AFM profiling of CdTe polycrystalline layer, made by ICSVT technology*.* 

for representative 100 µm scan range with 50nm resolution.

presented results.

base layer.

This measurement gives some important information about grains structure and intercrystal surfaces. By means of polycrystalline CdS/CdTe layers profiling one may easily detect the diameter and grain shape but also the inter-grain valleys depth and possible structure fluctuations and layer discontinuities. These structure disorders may result in serious parameter losses by producing of shunt interconnections or other charge flow parasitic effects. Taking into account cadmium dichloride dissolvent presence, frequently caused by recrystallization demands morphology defects may cause a real and serious threat for CdS/CdTe structure functioning. No such phenomenon was confirmed by

All new production techniques of thin –film polycrystalline solar cells and particularly CdS/CdTe structure, designed for new application field, should be verified according to obtained layers profile to eliminate structure disorder. Since AFM spectroscopy gives the profiling results for some strictly limited area for wider statistical examination mechanical profiling of high accuracy may be applied. These experiments were also conducted for test CdS/CdTe structure. The experiment for each sample was conducted by the help of mechanical profilometer Dektak 3 VEECO Instruments. The measurements were performed

As the first analysis ICSVT CdTe layer morphology was checked (Figure 9). By this investigation the average grain diameter of 6-8µm was confirmed and the typical roughness of 1200 A. Series of measurements confirmed dense, compact structure of absorber grains with no interlayer shunts. Obtained average roughness and the CdTe grain surface profile suggests that inter-grain trenches are insufficient for significant degradation of shunt resistance value. The total layer level fluctuations are smaller than 4 µm, which taking into consideration typical glass thickness accuracy confirms homogenous thickness of the whole

Alternative technology, used for CdS/CdTe cell layers deposition is a standard PVD method. Under some circumstances it offers a possibility of semiconductor material deposition even on profiled, elastic and untypical substrates. Evaporation of examined materials caused serious technology problems connected with proper thickness of obtained layers and homogeneous structure of deposited material. Some serious parasitic effect like boiling, splitting and granulation of the material were solved by proper temperature profile

and optimized one-directional tantalum evaporation source adopted by authors [24].

Fig. 9. Morphology of CdTe solar cell base absorber manufactured by ICSVT technology on glass substrate.

The investigation of CdTe base manufactured by evaporation and subsequent recrystallization is presented in Figure 10.

Fig. 10. Morphology of CdTe solar cell base absorber manufactured by evaporation and subsequent high-temp. recrystallization.

Innovative Elastic Thin-Film Solar Cell Structures 263

development of this technique is desired in terms of non-flat architectonic elements as the

Evaporation and post annealing of CdTe material in order to formation of proper base structure appeared to be less efficient. However simple and effective for various substrates technique leads to thin film base production only. Moreover the diameters of single grains

Screen – printing and sintering as the last investigated method appeared to be insufficient for the stabile cell-base production. The advantage of high layer thickness is seriously diminished by poor homogeneity, thickness instability and high surface porosity. Nevertheless screen printed layers can be effectively used as the in-production material for

**3. Innovative polycrystalline elastic structures, based on polymer substrates**  Although CdS/CdTe cells have now entered the mass - production phase, but still there are many possibilities of their new applications fields. Basing on this idea, authors proposed the implementation of modified CdS/CdTe cell structure in universal, attractive application called BIPV (Building Integrated Photovoltaics) and also elaborated elastic cell structure [25]. The CdTe cell construction gives the opportunity of achieving these goals, under the conditions of the proper technology modifications, as well as proper substrate and contacts implementation. Due to successful application of CSS variants of CdS/CdTe manufacturing technology for effective solar cells production, further experiments towards new cell structure and properties became possible. Considering cell composition, two opposite configuration of CdTe cell became possible. Historically first one is a classical substrate configuration (Figure 12 a), whereas based on glass + ITO, emitter-based configuration is called superstrate (Figure 12 b).

Fig. 12. Substrate a) and superstrate b) configuration of CdS/CdTe solar cell. A- glass cover, B- CdS emitter, C-CdTe base, D – base P+ sub layer, E-back contact, F-TCO layer, emitter

A

B C D E

b

A F B C D E

Both of them possess some important advantages and technology drawbacks. Substrate configuration offers more mature manufacturing technology and lower substrate demands, while superstrate configuration ensures higher efficiencies (smaller surface shadowing) and better encapsulation. Adaptation of the described technology for new application and cell

construction, demands deep consideration of all possible solutions.

solar cell substrate. First steps towards this goal have been undertaken.

obtained by the author are significantly lower than by ICSVT technology.

ICSVT or similar processes.

metal contacts not visible.

a)

The measurements indicated homogenous grains of the average dimension 2-4µm. Typical roughness of 1230 µm is similar layers obtained by ICSVT, however one may observe higher peak values of grain top-trench profile. This may result in some interlayer parasitic connections. Additionally in this case, total layer thickness fluctuation is similar to the layer thickness (±2 µm), what may cause some absorber discontinuities.

Third investigated technology (Figure 11) is based on adaptation of screen-printing technique for semiconductor layers manufacturing. Printing paste is produced by milling of cadmium and tellurium in stochiometric proportion and mixing with special binder. After printing, leveling and drying process CdTe layer is recrystallized in high-temperature process similarly to the previously described way.

Fig. 11. Morphology of CdTe solar cell base absorber manufactured by screen-printing and subsequent high-temp. recrystalization.

Profilometry of screen-printed base (Figure 11) indicated very high thickness of investigated layer (even up to 20 µm), but also serious morphology defects. Typical roughness of obtained layers is beyond 3000µm, which exceeds 240% of average value for ICSVT and PVD techniques. Moreover obtained grains present different diameters from 1-2 µm up to 10 µm. Total layer thickness also varies strongly (locally up to 50%), which results in various optoelectronic parameters. Layers produced by screen-printing technology presents additionally high porosity, which prevents cadmium dichloride from volatilization and causes fast base oxidation and thus cell parameters degradation.

Considering the results obtained for three technologies aimed at manufacturing of CdS/CdTe solar structure on novel substrate material conclusion of their applicability and further development may be drawn.

ICSVT being the most complicated and so-far not commercialized method appeared to be the most efficient in creation of the proper polycrystalline base structure. The morphology of obtained layers confirms proper column grain structure of hexagonal CdTe crystals and thus high electrical parameters of final solar cell. Taking this into consideration further

The measurements indicated homogenous grains of the average dimension 2-4µm. Typical roughness of 1230 µm is similar layers obtained by ICSVT, however one may observe higher peak values of grain top-trench profile. This may result in some interlayer parasitic connections. Additionally in this case, total layer thickness fluctuation is similar to the layer

Third investigated technology (Figure 11) is based on adaptation of screen-printing technique for semiconductor layers manufacturing. Printing paste is produced by milling of cadmium and tellurium in stochiometric proportion and mixing with special binder. After printing, leveling and drying process CdTe layer is recrystallized in high-temperature

Fig. 11. Morphology of CdTe solar cell base absorber manufactured by screen-printing and

Profilometry of screen-printed base (Figure 11) indicated very high thickness of investigated layer (even up to 20 µm), but also serious morphology defects. Typical roughness of obtained layers is beyond 3000µm, which exceeds 240% of average value for ICSVT and PVD techniques. Moreover obtained grains present different diameters from 1-2 µm up to 10 µm. Total layer thickness also varies strongly (locally up to 50%), which results in various optoelectronic parameters. Layers produced by screen-printing technology presents additionally high porosity, which prevents cadmium dichloride from volatilization and

Considering the results obtained for three technologies aimed at manufacturing of CdS/CdTe solar structure on novel substrate material conclusion of their applicability and

ICSVT being the most complicated and so-far not commercialized method appeared to be the most efficient in creation of the proper polycrystalline base structure. The morphology of obtained layers confirms proper column grain structure of hexagonal CdTe crystals and thus high electrical parameters of final solar cell. Taking this into consideration further

thickness (±2 µm), what may cause some absorber discontinuities.

process similarly to the previously described way.

subsequent high-temp. recrystalization.

further development may be drawn.

causes fast base oxidation and thus cell parameters degradation.

development of this technique is desired in terms of non-flat architectonic elements as the solar cell substrate. First steps towards this goal have been undertaken.

Evaporation and post annealing of CdTe material in order to formation of proper base structure appeared to be less efficient. However simple and effective for various substrates technique leads to thin film base production only. Moreover the diameters of single grains obtained by the author are significantly lower than by ICSVT technology.

Screen – printing and sintering as the last investigated method appeared to be insufficient for the stabile cell-base production. The advantage of high layer thickness is seriously diminished by poor homogeneity, thickness instability and high surface porosity. Nevertheless screen printed layers can be effectively used as the in-production material for ICSVT or similar processes.

#### **3. Innovative polycrystalline elastic structures, based on polymer substrates**

Although CdS/CdTe cells have now entered the mass - production phase, but still there are many possibilities of their new applications fields. Basing on this idea, authors proposed the implementation of modified CdS/CdTe cell structure in universal, attractive application called BIPV (Building Integrated Photovoltaics) and also elaborated elastic cell structure [25]. The CdTe cell construction gives the opportunity of achieving these goals, under the conditions of the proper technology modifications, as well as proper substrate and contacts implementation. Due to successful application of CSS variants of CdS/CdTe manufacturing technology for effective solar cells production, further experiments towards new cell structure and properties became possible. Considering cell composition, two opposite configuration of CdTe cell became possible. Historically first one is a classical substrate configuration (Figure 12 a), whereas based on glass + ITO, emitter-based configuration is called superstrate (Figure 12 b).

Fig. 12. Substrate a) and superstrate b) configuration of CdS/CdTe solar cell. A- glass cover, B- CdS emitter, C-CdTe base, D – base P+ sub layer, E-back contact, F-TCO layer, emitter metal contacts not visible.

Both of them possess some important advantages and technology drawbacks. Substrate configuration offers more mature manufacturing technology and lower substrate demands, while superstrate configuration ensures higher efficiencies (smaller surface shadowing) and better encapsulation. Adaptation of the described technology for new application and cell construction, demands deep consideration of all possible solutions.

Innovative Elastic Thin-Film Solar Cell Structures 265

comprehensive evaluation of polymer foils physical parameters, potentially efficient as the

As the test group of polymer foils wide set of materials, including standard commercial solutions as well as high – temperature polyester and polyamide, was accepted. Among polyamide foils of high thermal durability, two materials - KAPTON® and UPILEX® foils were chosen. Both of them are commercially available high-technology materials implemented in specific applications (eg: space shuttles wings and nose cover, high power loudspeakers membranes). They are characterized by high mechanical and thermal durability, high dielectric constant and UV durability. Among the polyester materials high – temperature MYLAR® material was adopted. As the reference material, popular PET foil in standard and high - temperature production version was applied. First evaluation step of material properties is a verification of their mechanical parameters. Comparison of these

[μm] 25.0 30.0 30.0 25.4

[g/m2 ] 30.0 44.1 41.7 35.0

[m2/kg] 31.2 22.7 23.98 27.9

[%/ 1oC] 0.025 0.018 0.007 0.005

(25 oC) [%] 600.0 54.0 103.5 40.0

Obtained parameters suggest similar properties of all investigated materials. However, some important differences are evident. The most important is the value of the thermal expansion coefficient (TEC). In general, one may say that in the case of high-temperature materials the value of thermal expansion is lower. Exceptionally, in the case of UPILEX® the value of this parameter is close to standard PET foil. According to considered configuration, thermal expansion coefficient of substrate foil should be adjusted to the value of the semiconductor base or emitter and contact layer. In both cases of semiconductor materials (CdS, CdTe), the value of TEC is very low (at the level of 5·10-4 [% / 1ºC]), but the most

The critical parameter in the standard re-crystallization process, as well as in the ICSVT, is a thermal durability of layer material. The maximum values of declared operational temperature for each investigated foil are: 130ºC for Standard PET, 185ºC for High-temp PET, 254ºC for Polyester MYLAR®, 380ºC and 430ºC for Polyamide KAPTON® foil. Basing on the declared temperatures and considering the ICSVT temperature demands, two most durable foils were accepted for further investigations. As the subsequent step the weight loss of KAPTON® and UPILEX® in higher temperatures was measured. The measurements of thermal durability were performed in the temperature range of a standard re-

temp PET UPILEX® MYLAR® KAPTON® HN

100

CdS/CdTe cell substrate materials.

results is presented in Table 2.

Thickness

Weight

Surface mass coefficient

Thermal expansion

Standard elongation

Parameter\Foil PET/High

Table 2. Main mechanical parameters of tested polymer foils.

typical metal contacts present TEC value higher by the order of magnitude.

Every introduced concept posses some value according to different aspects of BIPV applications and each is subsequently investigated by Technical University of Łódz research group. Ceramic substrates could be recognized as the best platform for the complete integration of the photovoltaic element with the architectonic component. One may find the reports on practical investigation of this construction for other thin –film solar cells e.g. CIS devices [26]. However, for CdS/CdTe construction, there is still research and technology adaptation needed. Additionally this kind of application is strictly connected with one particular architectonic element type such as roof-tile or brick. Moreover it has to provide the complete modular interconnection and regulation system, since the whole installation is made of hundreds of elements, working in different conditions. Furthermore, different interconnection systems (series, parallel and series-parallel) are necessary for optimum power and load polarization. Finally, standard ICSVT/CSS technology needs some fundamental modifications, in case of implementation in profiled architectonic elements (roof tiles or ornaments), since the material transport occurs only between very closely positioned source and target.

Taking into account cadmium telluride solar cells, possessing flexible construction two base materials may be considered. One is thin metal foil, while the second is the polymer material. Implementation of metal foils, for example Mo substrates, for CdTe construction has been already investigated and reported [27]. In this work we focus on polymer foil implementation as the elastic solar cell substrate. Flexibility of this material, combined with policrystalline thin-film structure properties, gives a promise that manufacturing of elastic solar panel, ready for integration with any shape architectonic substrate is possible. Moreover, it offers the opportunity of constructing both substrate and superstrate configuration of CdS/CdTe cell. Additionally, polymer foils are lightweight, high-durable materials, which enhances the possible application field of cells. Depending on the configuration, production technology and desired application different properties of the substrate foils will be demanded. Finding proper foil material and appropriate technology adaptation are the key to obtain efficient elastic PV cells.

To define the properties of polymer base foils, one may consider the specific of each configuration. So far, in the superstrate configuration highest conversion efficiencies were obtained However, in this case, polymer substrates must meet several conditions. One can mention as the most important: high optical transparency in the full conversion range of CdS/CdTe cell, ability of TCO surface electrode covering, high thermal durability, high chemical and water resistance. Apart from these specific demands, substrate foil of any configuration is expected to be light-weight, have high elongation coefficient, thermal expansion similar to semiconductor polycrystalline layers (CdS and CdTe) and be low cost. In both cases elastic cells can be easily attached to different shape architectonic elements.

Taking this into account, also substrate configuration of elastic cadmium telluride cell was investigated. As the preliminary step possible polymer material options were verified. Polymers, as the materials, are constructed on a base of multi-modular chains of single, repetitive units called monomers. In the manmade polymers, even the number of a few thousand monomer types is being achieved. The properties of manufactured polymer material depend strongly, not only on its chemical content and even monomer construction, but also on the monomers interconnecting system. Due to complexity of the typical polymer construction, it is impossible to evaluate the physical properties of these materials using theoretical analysis. This gave the prompt to the series of experiments, aimed at

Every introduced concept posses some value according to different aspects of BIPV applications and each is subsequently investigated by Technical University of Łódz research group. Ceramic substrates could be recognized as the best platform for the complete integration of the photovoltaic element with the architectonic component. One may find the reports on practical investigation of this construction for other thin –film solar cells e.g. CIS devices [26]. However, for CdS/CdTe construction, there is still research and technology adaptation needed. Additionally this kind of application is strictly connected with one particular architectonic element type such as roof-tile or brick. Moreover it has to provide the complete modular interconnection and regulation system, since the whole installation is made of hundreds of elements, working in different conditions. Furthermore, different interconnection systems (series, parallel and series-parallel) are necessary for optimum power and load polarization. Finally, standard ICSVT/CSS technology needs some fundamental modifications, in case of implementation in profiled architectonic elements (roof tiles or ornaments), since the material transport occurs only between very closely

Taking into account cadmium telluride solar cells, possessing flexible construction two base materials may be considered. One is thin metal foil, while the second is the polymer material. Implementation of metal foils, for example Mo substrates, for CdTe construction has been already investigated and reported [27]. In this work we focus on polymer foil implementation as the elastic solar cell substrate. Flexibility of this material, combined with policrystalline thin-film structure properties, gives a promise that manufacturing of elastic solar panel, ready for integration with any shape architectonic substrate is possible. Moreover, it offers the opportunity of constructing both substrate and superstrate configuration of CdS/CdTe cell. Additionally, polymer foils are lightweight, high-durable materials, which enhances the possible application field of cells. Depending on the configuration, production technology and desired application different properties of the substrate foils will be demanded. Finding proper foil material and appropriate technology

To define the properties of polymer base foils, one may consider the specific of each configuration. So far, in the superstrate configuration highest conversion efficiencies were obtained However, in this case, polymer substrates must meet several conditions. One can mention as the most important: high optical transparency in the full conversion range of CdS/CdTe cell, ability of TCO surface electrode covering, high thermal durability, high chemical and water resistance. Apart from these specific demands, substrate foil of any configuration is expected to be light-weight, have high elongation coefficient, thermal expansion similar to semiconductor polycrystalline layers (CdS and CdTe) and be low cost. In both cases elastic cells can be easily attached to different shape architectonic

Taking this into account, also substrate configuration of elastic cadmium telluride cell was investigated. As the preliminary step possible polymer material options were verified. Polymers, as the materials, are constructed on a base of multi-modular chains of single, repetitive units called monomers. In the manmade polymers, even the number of a few thousand monomer types is being achieved. The properties of manufactured polymer material depend strongly, not only on its chemical content and even monomer construction, but also on the monomers interconnecting system. Due to complexity of the typical polymer construction, it is impossible to evaluate the physical properties of these materials using theoretical analysis. This gave the prompt to the series of experiments, aimed at

positioned source and target.

elements.

adaptation are the key to obtain efficient elastic PV cells.

comprehensive evaluation of polymer foils physical parameters, potentially efficient as the CdS/CdTe cell substrate materials.

As the test group of polymer foils wide set of materials, including standard commercial solutions as well as high – temperature polyester and polyamide, was accepted. Among polyamide foils of high thermal durability, two materials - KAPTON® and UPILEX® foils were chosen. Both of them are commercially available high-technology materials implemented in specific applications (eg: space shuttles wings and nose cover, high power loudspeakers membranes). They are characterized by high mechanical and thermal durability, high dielectric constant and UV durability. Among the polyester materials high – temperature MYLAR® material was adopted. As the reference material, popular PET foil in standard and high - temperature production version was applied. First evaluation step of material properties is a verification of their mechanical parameters. Comparison of these results is presented in Table 2.



Obtained parameters suggest similar properties of all investigated materials. However, some important differences are evident. The most important is the value of the thermal expansion coefficient (TEC). In general, one may say that in the case of high-temperature materials the value of thermal expansion is lower. Exceptionally, in the case of UPILEX® the value of this parameter is close to standard PET foil. According to considered configuration, thermal expansion coefficient of substrate foil should be adjusted to the value of the semiconductor base or emitter and contact layer. In both cases of semiconductor materials (CdS, CdTe), the value of TEC is very low (at the level of 5·10-4 [% / 1ºC]), but the most typical metal contacts present TEC value higher by the order of magnitude.

The critical parameter in the standard re-crystallization process, as well as in the ICSVT, is a thermal durability of layer material. The maximum values of declared operational temperature for each investigated foil are: 130ºC for Standard PET, 185ºC for High-temp PET, 254ºC for Polyester MYLAR®, 380ºC and 430ºC for Polyamide KAPTON® foil. Basing on the declared temperatures and considering the ICSVT temperature demands, two most durable foils were accepted for further investigations. As the subsequent step the weight loss of KAPTON® and UPILEX® in higher temperatures was measured. The measurements of thermal durability were performed in the temperature range of a standard re-

Innovative Elastic Thin-Film Solar Cell Structures 267

400 450 500 550 600 650 700 750 800 850

Fig. 13. Optical transparency of KAPTON® and UPILEX® foils in the wavelength range of

Fig. 14. Test structure of elastic CdTe layer based on UPILEX® foil and contacted by 2 μm

mechanical properties of some high-temperature polymer foils give the possibility to construct complete solar cell with some technological modifications (particularly during the re-crystallization process). Another important factor is a proper, flexible and durable

Wavelenght [nm]

UPILEX

CdS/CdTe cell effective photoconversion.

Transmission [%]

Cu layer.

contacting system of such cell.

crystallization process (450ºC - 650ºC). During the experiment, the percentage loss of the foil weight was measured. Additionally, plastic properties were tested as the indicator of usefulness for the elastic substrate application. For higher accuracy of obtained outcomes, as the additional test, the plastic properties of the materials for each temperature were estimated. Complete results of this test are presented in Table 3. Grey color of the table cell marks a permanent deformation or loss of elastic properties.


Table 3. Temperature durability of examined foils. Dark-grey color indicates the loss of elastic properties or permanent deformation.

Analyzing obtained results, one may state that in the opposite to the manufacturer suggestions, the biggest weight loss in temperatures above 500ºC, is observed in polyamide KAPTON®. Additionally, the loss of its elastic parameters occurs very rapidly. Contrary, UPILEX®, which melting point is declared below 400ºC proved to be fairly resistant to temperatures until 550ºC. In both cases thicker foils reacted slower for the temperature rise, which was expected due to their relatively high thermal resistance. It is worth mentioning that the experiment was conducted in conditions (time, equipment) similar to the manufacturing process. However, identified maximal allowable temperature is relatively lower than standard demanded temperature for ICSVT process. There were reasonable presumptions suggesting the possibility of re-crystallization temperature decreasing, in favor of longer process duration. Thus, examined foils were conditionally positively evaluated. Taking this into account, UPILEX® foil was accepted for further experiments, leading to manufacturing of the CdS/CdTe elastic layers. Considering possible configuration of designed cell, the light transparency characteristic of investigated foil was measured. The light transmission in the conversion range of CdS/CdTe cell both of KAPTON® and UPILEX® foils is presented in Figure 13.

Due to low transmission (below 60%) in the range 400 nm – 700 nm, which would decrease largely the total cell efficiency, for UPILEX®, substrate cell configuration was chosen. Basing on presented results, experimental sample of CdTe absorber, manufactured on 25 μm UPILEX® foil was prepared. Obtained semiconductor layer is based on Cu contact of 2 µm thick, made by PVD in pressure 5·105 Torr. The total area of the sample is 30 cm2 and elastic properties of all manufactured layers are preserved (Figure 14). After the investigation, the average thickness of 2 µm and good uniformity of manufactured layer was confirmed. This makes proper base for CdS layer manufacturing and completing of the elastic CdS/CdTe construction.

Obtained results confirm the assumption that flexibility of polycrystalline cadmium compound layers may be employed in alternative applications, such as elastic cell structure. Finding the proper material for substrate of these devices is a key to manufacturing of efficient cell, however it demands considering of many technological aspects. Thermal and

crystallization process (450ºC - 650ºC). During the experiment, the percentage loss of the foil weight was measured. Additionally, plastic properties were tested as the indicator of usefulness for the elastic substrate application. For higher accuracy of obtained outcomes, as the additional test, the plastic properties of the materials for each temperature were estimated. Complete results of this test are presented in Table 3. Grey color of the table cell

> **480ºC** 91.82% 95.16% 96.70% 95.30% **500ºC** 91.36% 94.84% 96.00% 94.60% **550ºC** 89.55% 92.26% 74.70% 81.12% **600ºC** 70.00% 78.38% Burnt Burnt

Table 3. Temperature durability of examined foils. Dark-grey color indicates the loss of

Analyzing obtained results, one may state that in the opposite to the manufacturer suggestions, the biggest weight loss in temperatures above 500ºC, is observed in polyamide KAPTON®. Additionally, the loss of its elastic parameters occurs very rapidly. Contrary, UPILEX®, which melting point is declared below 400ºC proved to be fairly resistant to temperatures until 550ºC. In both cases thicker foils reacted slower for the temperature rise, which was expected due to their relatively high thermal resistance. It is worth mentioning that the experiment was conducted in conditions (time, equipment) similar to the manufacturing process. However, identified maximal allowable temperature is relatively lower than standard demanded temperature for ICSVT process. There were reasonable presumptions suggesting the possibility of re-crystallization temperature decreasing, in favor of longer process duration. Thus, examined foils were conditionally positively evaluated. Taking this into account, UPILEX® foil was accepted for further experiments, leading to manufacturing of the CdS/CdTe elastic layers. Considering possible configuration of designed cell, the light transparency characteristic of investigated foil was measured. The light transmission in the conversion range of CdS/CdTe cell both of

Due to low transmission (below 60%) in the range 400 nm – 700 nm, which would decrease largely the total cell efficiency, for UPILEX®, substrate cell configuration was chosen. Basing on presented results, experimental sample of CdTe absorber, manufactured on 25 μm UPILEX® foil was prepared. Obtained semiconductor layer is based on Cu contact of 2 µm thick, made by PVD in pressure 5·105 Torr. The total area of the sample is 30 cm2 and elastic properties of all manufactured layers are preserved (Figure 14). After the investigation, the average thickness of 2 µm and good uniformity of manufactured layer was confirmed. This makes proper base for CdS layer manufacturing and completing of the elastic CdS/CdTe

Obtained results confirm the assumption that flexibility of polycrystalline cadmium compound layers may be employed in alternative applications, such as elastic cell structure. Finding the proper material for substrate of these devices is a key to manufacturing of efficient cell, however it demands considering of many technological aspects. Thermal and

**UPILEX® KAPTON® 12.5µm 25.0µm 12.5µm 25.0µm** 

marks a permanent deformation or loss of elastic properties.

**Weight in temperature:** 

elastic properties or permanent deformation.

KAPTON® and UPILEX® foils is presented in Figure 13.

construction.

Fig. 13. Optical transparency of KAPTON® and UPILEX® foils in the wavelength range of CdS/CdTe cell effective photoconversion.

Fig. 14. Test structure of elastic CdTe layer based on UPILEX® foil and contacted by 2 μm Cu layer.

mechanical properties of some high-temperature polymer foils give the possibility to construct complete solar cell with some technological modifications (particularly during the re-crystallization process). Another important factor is a proper, flexible and durable contacting system of such cell.

Innovative Elastic Thin-Film Solar Cell Structures 269

Paste No CNT content in the composition [%] Sheet resistance [Ω/square]

Transparent conductive layers were prepared using four composites with various CNT content (Table 1). As a substrate borosilicate glass was used. In order to compare CNT and ITO layer parameters, an identical Bo Si glass sample, covered by 160 nm sputtered ITO, was taken. As a first step of carbon nanotubes TCL application in solar cell structure,

Fig. 16. Transmittance comparison of 0,25%, 1,5 µm CNT layer and 160 nm ITO on

borosilicate glass, for standard solar cell absorption spectrum

CNT.0.1 0.10 613 k CNT.0.25 0.25 28 k CNT.0.5 0.50 3.3 k CNT.1.0 1.00 870 Table 4. Sheet resistance values for samples with different CNT amount [11].

transmittance of printed layers have been measured (Figure 16).

Fig. 15. HRSEM image of applied carbon nanotubes
