Measurement of Built-in Electrical Potential in Cu(In, Ga)Se 2 Solar Cells
- PDF / 1,036,609 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 18 Downloads / 159 Views
B8.3.1
Measurement of Built-in Electrical Potential in Cu(In,Ga)Se2 Solar Cells C.-S. Jiang, F.S. Hasoon, H.R. Moutinho, H.A. Al-Thani, M.J. Romero, and M.M. Al-Jassim National Renewable Energy Laboratory, 1617 Cole Blvd., Golden CO 80401
ABSTRACT The built-in electrical potential on cross sections of Cu(In,Ga)Se2 (CIGS) solar cells was measured quantitatively and resolved spatially using scanning Kelvin probe microscopy. In the conditions of open and short circuits, no significant potential variation on the p-n junction was probed due to the surface Fermi-level pinning. With an external reverse-bias voltage applied to the device, we were able to probe the potential on the junction; the potential profiles demonstrate that the p-n junction is a buried homojunction, located 30–80 nm from the CIGS/CdS interface in the CIGS film. The potential measurement over the CdS and ZnO layers, which is consistent with the band diagram calculations, indicates that the CdS and ZnO layers are inactive for the collection of photoexcited carriers. INTRODUCTION Built-in electrical potential plays a major role in photovoltaic devices, because it collects photoexcited carriers and is a key factor in determining the open-circuit voltage (Voc) of the device. However, characterization of built-in potentials has been limited to indirect ways, such as current-voltage (I-V) and capacitance-voltage (C-V) measurements. For Cu(In,Ga)Se2 (CIGS) thin-film solar cells, the location of the p-n junction is not yet determined, in contrast to the great development in the energy conversion efficiency [1]. Based on measurements of photoluminescence and electron-beam induced current [2,3], it has been postulated that the p-n junction is located on the CIGS side, close to the CIGS/CdS interface. However, many researchers involved in investigating this material system explain their experimental data and build theoretical models based on the natural assumption that the junction is abrupt on the pGIGS/n-CdS interface, and that the n-region extends to the intrinsic and slightly doped ZnO layers [4]. Therefore, a direct measurement with spatially resolved capabilities is highly desirable for allocating the p-n junction position and understanding solar cell performances. In the last decade, nano-electrical property measurements based on the atomic force microscopy (AFM) technique, such as scanning Kelvin probe microscopy (SKPM) and scanning capacitance microscopy (SCM), have been developed and applied to the characterization of semiconductor devices. Recently, we have applied SKPM and SCM to the characterization of photovoltaic devices [5-8]. We measured the potential distributions on cross sections of thin-film solar cells of CIGS and amorphous-Si, and epitaxial III-V cells of GaInP2, GaNPAs, and GaInP2/GaAs. In this paper, we will briefly introduce the measurement technique and report on the potential measurements of CIGS solar cells. MEASUREMENT TECHNIQUE Using the SKPM technique (Figure 1), we quantitatively measured the distribution of electrochemical pote
Data Loading...
