**4. Solar cell performance**

The n-CdS/p-CdTe HJ was fabricated and their electrical and photoelectric properties were investigated. The CdS thin films with 100 nm thickness were deposited by CSD using CdCl2

Chemical Surface Deposition of CdS Ultra Thin Films from Aqueous Solutions 401

1,5 2,1

1,5 2,0 2,5 3,0

*h*, eV

Fig. 15. Spectral distribution of the quantum efficiency of *n*-СdS/*p*-CdTe heterojunction at 300 K. Ilumination form СdS film side. The 1 and 2 curves corespond to samples with diferet

The II-VI binary CdS compound semiconductor thin films (30–100 nm) has been successfully deposited from aqueous solutions of CdCl2, CdSO4, CdI2 salts using Chemical Surface

The linear dependence increase of film thickness on the deposition time was experimentally demonstrated for CSD CdCl2 based films. For the two other salts of the film thickness

Established that for growth rate <15 nm/min. chemical deposition method allows to growth

It is proved that CdI2 based CdS film composition was close to stoichiometric, compared to

The possibility of n-СdS/p-CdTe high quality solar cell fabrication by CSD of СdS thin film is demonstrated. High value of СdS/CdTe heterojunction photoconversion, in region limited by

Adams, W.G. (1877). The action of light on selenium. *Proceedings of the Royal Society,* No.25,

Adirovich, E.I., Yuabov, Y.M., & Yagudaev, G.R. (1969). CdTe thin film deposition on transparent substrates. *Fiz. Tekh. Poluprovodnikov,* Vol.3, No.1, pp. 81–85 Aguilar-Hernández, J., Sastre-Hernández, J., Ximello-Quiebras, N., Mendoza-Pérez, R.,

Vigil-Galán, O., Contreras-Puente, G., & Cárdenas-García, M. (2006). Influence of the S/Cd ratio on the luminescent properties of chemical bath deposited CdS films.

dependence on deposition time is more complex, but has a character close to linear.

solid polycrystalline CdS films with 106-107 cm-2 surface macrodefects concentration.

СdS and CdTe band gaps, in our opinion was provided by СdS deposition method.

films deposited from solutions of two ather salts under identical conditions.

*Solar Energy Materials & Solar Cells*, Vol.90, pp. 2305–2311

2,3

2

1

2,3

0,01

Deposition and employing the direct heating of the substrate.

0,1

arb. unit

thickness of СdS film.

**5. Conclusions** 

**6. References** 

pp. 113–117

1,5

1

cadmium chloride solution. Thin polycrystalline CdS films completely covered the substrate across the sample area, hade stoichiometric composition, was solid with a small surface macrodefects concentration (107sm-2). Typical spectral dependence of transmission of CSD CdS film is shown on Fig. 13. Resistance of is fabricated n-CdS/p-CdTe SC R0≈104-105 Ω at T= 300 K and was determined by the electrical properties of the p-CdTe substrates. This is coused by the resistivity of used substrates which is 2-3 orders of magnitude greater than the similar parameter for n-CdS films (RCdS≈ 103 Ω). Voltage cutoff in n-CdS/p-CdTe structures, as seen in Fig. 14 is U0 ≈ 1,4 V and its value is close to CdTe bandgap (Landolt-Börnstein, 1999). Inverse branches of curent-voltage characteristic for anisotropic structures are well described by power dependence IR~Um, where the m ≈ 1 to U> 2, which is typical for charge carriers tunneling or inherent space charge limited currents in velocity saturation mode (Hernandez, 1998, Lamperg & Mark, 1973). Reverse current increase observed in the investigated anisotropic heterojunction with increasing voltage bias can also be caused by imperfections in their periphery.

Fig. 15 shows relative quantum efficiency of photoconversion (ratio of short circuit current to number of incident photons) η(hν) spectra of CdS/CdTe heterojunction fabricated by CSD of CdS film on CdTe wafer. The η(hν) spectra find out to be similar for structures fabricated on different substrates what indicate high local homogeneity of substrates and reproducibility of the CSD films properties. The sharp long wave increase of η(hν) in narrow spectral range 1.4–1.5 eV for CdS/CdTe structure illumination from CdS film side is observed. Its value reach maximum in region hνm≈1.5 еV what correspond to energy of direct band transitions in CdTe (Landolt-Börnstein, 1999, Aven & Prener, 1967).

Fig. 14. Curent-voltage characteristic of *n*-СdS/*p*-CdTe HJ at 300 K

It should be notice that photosensitivity of the fabricated СdS/CdTe heterostructures maintain on high level (fig. 15, curve 1, 2) in vide region of incident photons energy. The table like part of η(hν) curve confirm fabrication of the СdS/CdTe high quality heterojunction. The observed η(hν) curve decrease at hν≥2.3 eV is similar to specular transmission spectra of CdS film used for СdS/CdTe heterostructure fabrication. The full wide on half of the maximum (FWHM) of η(hν) spectra δ≈1.1–1.2 eV in our structures is more bigger then FWHM of Ох/CdTe heterostructure (Il'chuk at al., 2000) and indicate higher quality of fabricated structures compared to known.

Fig. 15. Spectral distribution of the quantum efficiency of *n*-СdS/*p*-CdTe heterojunction at 300 K. Ilumination form СdS film side. The 1 and 2 curves corespond to samples with diferet thickness of СdS film.

#### **5. Conclusions**

400 Solar Cells – Thin-Film Technologies

cadmium chloride solution. Thin polycrystalline CdS films completely covered the substrate across the sample area, hade stoichiometric composition, was solid with a small surface macrodefects concentration (107sm-2). Typical spectral dependence of transmission of CSD CdS film is shown on Fig. 13. Resistance of is fabricated n-CdS/p-CdTe SC R0≈104-105 Ω at T= 300 K and was determined by the electrical properties of the p-CdTe substrates. This is coused by the resistivity of used substrates which is 2-3 orders of magnitude greater than the similar parameter for n-CdS films (RCdS≈ 103 Ω). Voltage cutoff in n-CdS/p-CdTe structures, as seen in Fig. 14 is U0 ≈ 1,4 V and its value is close to CdTe bandgap (Landolt-Börnstein, 1999). Inverse branches of curent-voltage characteristic for anisotropic structures are well described by power dependence IR~Um, where the m ≈ 1 to U> 2, which is typical for charge carriers tunneling or inherent space charge limited currents in velocity saturation mode (Hernandez, 1998, Lamperg & Mark, 1973). Reverse current increase observed in the investigated anisotropic heterojunction with increasing voltage bias can also be caused by

Fig. 15 shows relative quantum efficiency of photoconversion (ratio of short circuit current to number of incident photons) η(hν) spectra of CdS/CdTe heterojunction fabricated by CSD of CdS film on CdTe wafer. The η(hν) spectra find out to be similar for structures fabricated on different substrates what indicate high local homogeneity of substrates and reproducibility of the CSD films properties. The sharp long wave increase of η(hν) in narrow spectral range 1.4–1.5 eV for CdS/CdTe structure illumination from CdS film side is observed. Its value reach maximum in region hνm≈1.5 еV what correspond to energy of


It should be notice that photosensitivity of the fabricated СdS/CdTe heterostructures maintain on high level (fig. 15, curve 1, 2) in vide region of incident photons energy. The table like part of η(hν) curve confirm fabrication of the СdS/CdTe high quality heterojunction. The observed η(hν) curve decrease at hν≥2.3 eV is similar to specular transmission spectra of CdS film used for СdS/CdTe heterostructure fabrication. The full wide on half of the maximum (FWHM) of η(hν) spectra δ≈1.1–1.2 eV in our structures is more bigger then FWHM of Ох/CdTe heterostructure (Il'chuk at al., 2000) and indicate

U, V

0


1x10-5

2x10-5

I, A

Fig. 14. Curent-voltage characteristic of *n*-СdS/*p*-CdTe HJ at 300 K

higher quality of fabricated structures compared to known.

3x10-5

direct band transitions in CdTe (Landolt-Börnstein, 1999, Aven & Prener, 1967).

imperfections in their periphery.

The II-VI binary CdS compound semiconductor thin films (30–100 nm) has been successfully deposited from aqueous solutions of CdCl2, CdSO4, CdI2 salts using Chemical Surface Deposition and employing the direct heating of the substrate.

The linear dependence increase of film thickness on the deposition time was experimentally demonstrated for CSD CdCl2 based films. For the two other salts of the film thickness dependence on deposition time is more complex, but has a character close to linear.

Established that for growth rate <15 nm/min. chemical deposition method allows to growth solid polycrystalline CdS films with 106-107 cm-2 surface macrodefects concentration.

It is proved that CdI2 based CdS film composition was close to stoichiometric, compared to films deposited from solutions of two ather salts under identical conditions.

The possibility of n-СdS/p-CdTe high quality solar cell fabrication by CSD of СdS thin film is demonstrated. High value of СdS/CdTe heterojunction photoconversion, in region limited by СdS and CdTe band gaps, in our opinion was provided by СdS deposition method.

### **6. References**


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**19** 

*Japan* 

*Ritsumeikan University* 

**Development of Flexible Cu(In,Ga)Se2** 

Yasuhiro Abe, Takashi Minemoto and Hideyuki Takakura

**Thin Film Solar Cell by Lift-Off Process** 

Clean energy resources as an alternative to fossil fuels has been required. Photovoltaics is the most promising among renewable energy technologies. On the other hand, the cost of the electrical energy generated by the solar cells was higher than that generated by fossil

Since high-conversion efficiencies have been demonstrated for solar cells using GaAs substrates in 1977 (Kamath et al., 1977; Woodall et al., 1977), a critical problem is how to reduce power generation cost. The characters required to solar cells strongly depend on its applications. In particular, thin film solar cells are promising for terrestrial applications, because thin film solar cells are more advantageous than bulk type solar cells in terms of consumption of raw materials. Konagai et al. fabricated the thin film solar cells on a single crystalline GaAs substrate by the liquid phase epitaxy mehtod, and focused on the reuse of GaAs substrates by detaching these thin film solar cells from the GaAs substrates (Konagai et al., 1978). Konagai et al. named this separation technique the Peeled Film Technology (PFT). This is the invention of the lift-off method in solar cell development. A specific explanation of the PFT is as follows. An Al1-xGaxAs layer was introduced between the thin film solar cell and the GaAs substrate as a release layer. The thin film solar cell was separated from the GaAs substrate by etching the Al1-xGaxAs layer by the HF solution, because Al1-xGaxAs was readily dissolved by the HF solution compared to GaAs. Since a chemical technique was mainly used for the peeling, this method is defined as a chemical lift-off process. Recently, this has been researched as the epitaxial lift-off (ELO) method (Geelen et al., 1997; Schemer et al., 2000, 2005a; Voncken et al., 2002; Yablonovitch et al.,

On the other hand, the cleavage of lateral epitaxial films for transfer (CLEFT) process, where the thin film was mechanically peeled, was developed as a transfer method of a single crystalline GaAs thin film (McClelland et al., 1980). A specific explanation of the CLEFT process is as follows. A photoresist was applied to a surface of a GaAs substrate. The photoresist was patterned with equally-spaced stripe openings by standard photolithographic techniques. Next, a GaAs layer was grown on this patterned substrate surface. In this case, a GaAs layer was grown on only the openings of the photoresist. The lateral growth of a GaAs layer occurs during the GaAs deposition. A single crystalline GaAs layer is therefore formed on the photoresist. Alternative substrate was bonded onto this

fuels. The cost reduction of the solar cell is therefore required.

**1. Introduction**

1987).

