MoO₃ back contact for CuInSe₂-based thin film solar cells

Abstract

MoO3 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.

Improved Performance of Ultrathin Cu(InGa)Se<inline-formula><tex-math>$_{\bf 2}$</tex-math> </inline-formula> Solar Cells With a Backwall Superstrate Configuration

“…The backwall cells above use a MoO 3 transparent back contact. MoO 3 has a bandgap in the range 3.1-3.4 eV when annealed at 500°C [5] and forms an ohmic contact with Cu(In,Ga)Se 2 [6], [7], making it well suited for the backwall devices.…”

“…The backwall structure of glass/ITO/MoO 3 /Cu(In,Ga)Se 2 /CdS/i-ZnO/Ag utilizes a MoO 3 transparent back contact [5]- [7] to allow illumination of the device from the back. This structure provides opportunities to improve the device performance by optical enhancement and optimization of deposition conditions.…”

Flexible and Semi‐Transparent Ultra‐Thin CIGSe Solar Cells Prepared on Ultra‐Thin Glass Substrate: A Key to Flexible Bifacial Photovoltaic Applications

“…[ 23 ] In general, TCO BC, which is indispensable for STUT CIGSe solar cells, is known to have a higher resistance compared to metal BC and to form a BC barrier with a high BC recombination rate. [ 24,25 ] The BC hole barrier at CIGSe/TCO BC, which is known as an interfacial GaO x layer, is formed spontaneously owing to the Ga diffusion from CIGSe into the TCO layer. The barrier height (Φ b ) is given by Φ b = χ + E g − Φ, [ 26 ] where χ and E g of CIGSe are expected to be 4.5 and 1.15 eV, respectively, and Φ of indium‐doped tin oxide (ITO) is 4.7–4.8 eV.…”

Significant Passivation Effect of Cu(In, Ga)Se2 Solar Cells via Back Contact Surface Modification Using Oxygen Plasma

“…This indicates that the molybdenum oxide inhibited selenium accumulation on the molybdenum surface to form molybdenum selenide to a certain extent. Thermodynamic calculations [ 21,29,32 ] were conducted to prove that oxygen has better oxidative activity to molybdenum than selenium, so it can react with molybdenum more easily to generate molybdenum oxide. Thus, the molybdenum oxide effectively inhibited the reaction between the molybdenum and selenium at the Mo/CIGS interface.…”

“…Various methods have been used to prepare molybdenum oxide, such as thermal evaporation, [ 21,23,24 ] solution processing, [ 14,25 ] air annealing, [ 20 ] sputtering in oxygen, [ 26–29 ] atomic layer deposition, [ 30,31 ] and so on. The morphology, composition, and work function of molybdenum oxide vary dramatically with different preparation methods.…”