Current Transport in Cu(In,Ga)S 2 Based Solar Cells with High Open Circuit Voltage -Bulk vs. Interface
- PDF / 105,672 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 42 Downloads / 224 Views
1165-M05-15
Current Transport in Cu(In,Ga)S2 Based Solar Cells with High Open Circuit Voltage Bulk vs. Interface Saoussen Merdes1, Benjamin Johnson1, Rodrigo Sáez-Araoz1, Ahmed Ennaoui1, Joachim Klaer1, Iver Lauermann1, Roland Mainz1, Alexander Meeder2 and Reiner Klenk1 1 Helmholtz-Zentrum-Berlin, Glienickerstr. 100, D14109 Berlin, Germany 2 Sulfurcell Solartechnik GmbH, Barbara-McClintock-Str. 11, D12489 Berlin, Germany ABSTRACT In a previous work, Cu(In,Ga)S2 thin films prepared by rapid thermal sulfurization of metallic precursors yielded solar cells with efficiencies reaching 12.9%, a short circuit current density of 22.3 mA/cm2 and open circuit voltages up to 850 mV. However, the fill factor was close to, but typically did not exceed 70%. In this contribution we report on the role of junction formation by chemical bath deposition on these parameters. Concentrations in the bath and deposition times were varied. A comparison is made between CdS and Zn(S,O) buffer layers. The influence of the incorporated gallium on surface properties was investigated by ultraviolet photoelectron spectroscopy (UPS) for the valence band edge and near edge X-ray absorption fine structure (NEXAFS) for the conduction band edge. Even in our best cell (13.1%) the activation energy of the saturation current is found to be still smaller than the band gap. High diode ideality factors and voltage dependent current collection prevent higher fill factors. INTRODUCTION In the last decades, chalcopyrite semiconductors have proven to be efficient materials for photovoltaic applications. In addition to the low band gap Cu(In,Ga)Se2 based solar cells, CuInS2 has attracted much attention due to its high gap of 1.5 eV, which is close to ideal. CuInS2 based solar cells, prepared from sputtered metals subsequently sulfurized using rapid thermal processing (RTP) in sulfur vapor, have reached 11.4% total area efficiency (area = 0.5 cm2) [1]. However, significant improvement could not be achieved and open circuit voltage values remained below the predicted theoretical values for this band gap. The introduction of Ga to the absorber has led to higher open circuit voltages. A small band gap widening was observed and Cu(In,Ga)S2 based solar cells with efficiencies of 12.9% could be prepared [2]. The excellent photocurrent collection under short circuit conditions could be sustained when adding the Ga (Jsc = 22.3 mA/cm2 for Cu(In,Ga)S2 and Jsc = 21.8 mA/cm2 for CuInS2). The increase in open circuit voltage exceeded that of the effective band gap. However, the fill factor was typically found to be reduced while one would have expected higher values following the open circuit voltage. The fill factor problem had been observed previously in wide band gap CuGaSe2 based solar cells. Kassis at al. [3] have investigated this issue through numerical calculations of current-voltage characteristics. High fill factor losses were attributed to the increase of saturation
current densities and diode ideality factor under illumination. In chalcopyrite solar cells, the h
Data Loading...
