WIDE GAP CPV: Wide gap chalcopyrites for advanced photovoltaic devices

• Forschungsstelle: EU FRP
• Projektnummer: 96.0254
• Projekttitel Englisch: WIDE GAP CPV: Wide gap chalcopyrites for advanced photovoltaic devices

Schlüsselwörter (Englisch)

CIGS; CuGaSe2; ZnO; solar cells; solar energy; thin films; transparent conduc

Alternative Projektnummern (Englisch)

EU project number: JOR3-CT97-0135

Forschungsprogramme (Englisch)

EU-programme: 4. Frame Research Programme - 5.1 Nonnuclear energies

Kurzbeschreibung (Englisch)

See abstract

Partner und Internationale Organisationen (Englisch)

HMI (D), EPICHEM (GB), AIXTRON (D), Uni. Montpellier (F), Uni. Stuttgart (D), ETHZ (CH)

Abstract (Englisch)

Development of chalcopyrite based tandem solar cells requires the growth of CuGaSe2 (called CGS) Cu(In,Ga)xSe2 (called CIGS) solar cells in a 'superstrate configuration'. Investigations on epitaxial and polycrystalline layers were carried out to understand the structural and optoelectronic properties of compounds with different stoichiometry. Heteroepitaxial CuGaxSey layers on Si and GaAs substrates were grown by molecular beam epitaxy. Crack-free layers with good crystal quality (RBS ?min. ~12% and XRD rocking curve width of 250 arcs) were obtained. The microstructural defects in epitaxial layers and interfaces were investigated with high resolution transmission electron microscopy (HRTEM).

CuGaSe2 solar cells in a 'substrate configuration' (ZnO:Al/CdS/CGS/Mo/glass) were developed with a conversion efficiency of 6.5%. However, higher efficiency cells with efficiency of more than 15% were obtained with CIGS absorber layers. Solar cells made with Cu-rich CGS absorber layers have shown the CuxSe precipitates in the CuGaSe2 matrix. These precipitates may limit the photovoltaic performance of CGS solar cells. The crystallographic structure and microstructural defects in CBD grown CdS depend on the deposition temperature.

The choice of a suitable buffer layer of n-type conductivity and a large energy gap (> 2.4 eV) is important for the growth of efficient superstrate solar cells. Because inter-diffusion of elements across the heterojunction may decrease the efficiency due to shunting effects. We have investigated the properties of intrinsic ZnO as a buffer layer and ZnO:Al as a transparent conducting front layer for superstrate solar cells. The electrical properties depend on the RF sputtering power and substrate temperature. The sheet resistance of ZnO:Al increases from 10 to 60 W/sq after the deposition of absorber layers at 500 °C. However this change was minimized by optimization of the sputtering conditions. Layers of good thermal stability were grown with a high rate of deposition. The micro-probe composition measurements HRTEM indicate insignificant interdiffusion, suggesting that ZnO is a suitable buffer layer. However, accumulation of Ga near the ZnO interface and formation of a thin interface layer was observed. Superstrate solar cells with 8.2% efficiency were developed using CIGS absorber layers. CIGS cells in the substrate configuration have efficiency of more than 15%.

Datenbankreferenzen (Englisch)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 96.0254