MoO3 back contact for CuInSe2-based thin film solar cells

ABSTRACTMoO3 films with a high work function (5.5 eV), high transparency, and a wide bandgap (3.0 - 3.4 eV) are a potential candidate for the primary back contact of Cu(InGa)Se2 thin film solar cells. This may be advantageous to form ohmic contact in superstrate devices where the back contact will be deposited after the Cu(InGa)Se2 layer and MoSe2 layer doesn’t form during Cu(InGa)Se2 deposition. In addition, the MoO3 may be incorporated in a transparent back contact in tandem or bifacial cells. In this study, MoO3 films for use as a back contact for Cu(In,Ga)Se2 thin film solar cells were prepared by reactive rf sputtering with O2/(O2+Ar) = 35%. The effect of post processing on the structural properties of the deposited films were investigated using x-ray diffraction and scanning electron microscopy. Annealing resulted in crystallization of the films to the α-MoO3 phases at 400°C. Increasing the oxygen partial pressure had no significant effect on optical transmittance of the films, and bandgaps in the range of 2.6-2.9 eV and 3.1-3.4 eV were obtained for the as deposited and annealed films, respectively. Cu(In,Ga)Se2 thin film solar cells prepared using an as-deposited Mo-MoO3 back contact yielded an efficiency of >14% with VOC = 647 (mV), JSC = 28.4 (mA), and FF. = 78.1%. Cells with ITO-MoO3 back contact showed an efficiency of ∼12% with VOC = 642 (mV), JSC = 26.8 (mA), and FF. = 69.2%. The efficiency of cells with an annealed MoO3 back contact was limited to 4%, showing a blocking diode behavior in the forward bias J-V curve. This may be caused by the presence of a barrier between the valence bands of the Cu(In,Ga)Se2 and MoO3, due to the higher bandgap of the annealed MoO3 films. SEM cross section studies showed uniform coverage of the as-deposited MoO3 layer and formation of voids for the annealed MoO3 film. Structural orientation of the Cu(In,Ga)Se2 absorber layer was also altered by the MoO3 film and less-oriented films were observed for either cases.

Download Full-text

CIGS2 Thin Film Solar Cells on Stainless Steel Foil

MRS Proceedings ◽

10.1557/proc-668-h3.4 ◽

2001 ◽

Vol 668 ◽

Cited By ~ 4

Author(s):

Neelkanth G. Dhere ◽

Shantinath R. Ghongadi

Keyword(s):

Thin Film ◽

Stainless Steel ◽

Solar Cells ◽

Rf Sputtering ◽

Contact Layer ◽

Low Earth Orbit ◽

Thin Film Solar Cells ◽

Maximum Temperature ◽

Specific Power ◽

Back Contact

ABSTRACTCuIn1−xGaxS2 (CIGS2) thin-film solar cells are of interest for space power applications because of the near optimum bandgap for AM0 solar radiation in space. CuIn1−xGaxSe2−ySy (CIGS) and CIGS2 solar cells are expected to be superior to Si and GaAs solar cells for the space missions especially in terms of the performance at the end of low earth orbit (LEO) mission. Ultra-lightweight thin-film solar cells deposited on flexible stainless steel (SS) foils have a potential for achieving high specific power.Magnetron-sputter-deposition parameters of molybdenum back-contact layer were optimized so as to minimize residual stress. Cu-rich Cu-Ga/In layers were sputter-deposited on unheated Mo-coated SS foils from CuGa(22%) and In targets. Well-adherent, large (3 μm), compact-grain Cu-rich CIGS2 films were obtained by sulfurization in a Ar:H2S 1:0.04 mixture and argon flow rate of 650 sccm, at the maximum temperature of 475° C for 60 minutes with intermediate 30 minute annealing step at 120° C. p-type CIGS2 thin films were obtained by etching away the Cu- rich layer segregated at the surface in a dilute KCN solution. XRD analysis of a CIGS2 film on SS foil revealed growth of chalcopyrite CIGS2 phase having ao= 5.519 Å and co= 11.125 Å and {112} preferred orientation. Positive SIMS depth profile of CIGS2 film showed gallium concentration increasing toward the back contact.Solar cells were completed by deposition of CdS heterojunction partner layer by chemical bath deposition, transparent-conducting ZnO/ZnO:Al window bilayer by RF sputtering, and vacuum deposition of Ni/Al contact fingers through metal mask. PV parameters of a CIGS2 solar cell on SS flexible foil measured under AM 0 conditions at the NASA GRC were: Voc = 802.9 mV, Jsc = 25.07 mA/cm2, FF = 60.06%, and η = 8.84%. For this cell, AM 1.5 PV parameters measured at NREL were: Voc = 788 mV, Jsc = 19.78 mA/cm2, FF = 59.44%, η = 9.26%. Quantum efficiency curve showed a sharp QE cutoff equivalent to CIGS2 bandgap of ∼1.50 eV, fairly close to the optimum value for efficient AM0 PV conversion in the space.

Download Full-text

Phototransistor Behavior in CIGS Solar Cells and the Effect of the Back Contact Barrier

Energies ◽

10.3390/en13184753 ◽

2020 ◽

Vol 13 (18) ◽

pp. 4753

Author(s):

Ricardo Vidal Lorbada ◽

Thomas Walter ◽

David Fuertes Marrón ◽

Dennis Muecke ◽

Tetiana Lavrenko ◽

...

Keyword(s):

Thin Film ◽

Solar Cells ◽

Numerical Simulations ◽

Forward Bias ◽

Thin Film Solar Cells ◽

Normal Operation ◽

Back Contact ◽

Operation Conditions ◽

Contact Barrier ◽

The Impact

In this paper, the impact of the back contact barrier on the performance of Cu (In, Ga) Se2 solar cells is addressed. This effect is clearly visible at lower temperatures, but it also influences the fundamental parameters of a solar cell, such as open-circuit voltage, fill factor and the efficiency at normal operation conditions. A phototransistor model was proposed in previous works and could satisfactorily explain specific effects associated with the back contact barrier, such as the dependence of the saturated current in the forward bias on the illumination level. The effect of this contribution is also studied in this research in the context of metastable parameter drift, typical for Cu (In, Ga) Se2 thin-film solar cells, as a consequence of different bias or light soaking treatments under high-temperature conditions. The impact of the back contact barrier on Cu (In, Ga) Se2 thin-film solar cells is analyzed based on experimental measurements as well as numerical simulations with Technology Computer-Aided Design (TCAD). A barrier-lowering model for the molybdenum/Cu (In, Ga) Se2 Schottky interface was proposed to reach a better agreement between the simulations and the experimental results. Thus, in this work, the phototransistor behavior is discussed further in the context of metastabilities supported by numerical simulations.

Download Full-text