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

Abstract

Molybdenum back contact plays a significant role in chalcopyrite Cu(In, Ga)Se2 (CIGS) solar cells. A molybdenum oxide intermediate layer is applied to optimize the back contact from the aspect of absorber crystal growth regulation and back surface field reconstruction. A novel molybdenum oxide preparation method is introduced through oxygen plasma treatment of the molybdenum layer. CIGS film is obtained by sputtering from a quaternary target with post‐selenization. The shortage of Cu and Se in the middle of the absorption layer causes crystallization stratification. A bidirectional regulation method for controlling intermediate fine grains is proposed. Superior crystallinity is produced to facilitate carriers’ transport and reduce recombination at grain boundaries. Back passivation is achieved by forming a reverse p–n junction on the back surface. Despite the formation of this reverse junction, the efficiency of the device is still improved because the transport of holes is assisted by the molybdenum oxide's gap states. Thus, back surface recombination is effectively reduced by back passivation, leading to the increase in open‐circuit voltage (Voc) and fill factor (FF). The power conversion efficiency of treated solar cells increases by 34.1%. An innovative direction is proposed to optimize the back contact of CIGS solar cells in this study.

Back Contact Plasma Treatment Enables 14.5% Efficient Solution‐Processed CuIn(S,Se)2 Solar Cells

“…[ 27 ] The presence of MoO 2 in Mo‐Ref comes from the natural oxidation of Mo in air. [ 19 ] For Mo‐PT, except for metallic Mo and MoO 2 , binding energies at 232.2 eV and 235.4 eV can be assigned to the 3d 5/2 and 3d 3/2 peaks of MoO 3‐x (0< x <1) and that at 232.6 eV and 235.8 eV can be assigned to the 3d 5/2 and 3d 3/2 peaks of MoO 3 . [ 27 ] The results reveal Mo can only be oxidized to low oxidation (+4) in ambient air whereas plasma treatment oxidizes Mo and MoO 2 to higher oxidation states (+5 and +6).…”

“…For example, Hong et al. [ 19 ] found plasma treatment led to better band alignment and absorber crystallization, improving the efficiency of CIGSSe solar cell from 10.01% to 13.42%. Chen et al.…”

8.0% Efficient Submicron CuIn(S,Se)2 Solar Cells on Sn:In2O3 Back Contact via a Facile Solution Process

“…A large amount of liquid Se penetrates the whole film under high Se vapor pressure, and Se bonds with Cu to form Cu 2– x Se, which facilitates the formation of large grains and denser films. , However, S tends to stronger volatilize when the precursor films are selenized at 560 °C. Additionally, the as-formed dense large grains will inhibit the S outflow (Figure d) . Sulfur can only evaporate in the area containing abundant grain boundaries, leading to the formation of S-rich CISSe grains with small grain size after selenization.…”

“…Additionally, the as-formed dense large grains will inhibit the S outflow (Figure 2d). 23 Sulfur can only evaporate in the area containing abundant grain boundaries, leading to the formation of S-rich CISSe grains with small grain size after selenization. Even though large grains are located at both the bottom and top of the absorber, the grain size of the bottom layer is more homogeneous than that of the top one.…”

Growth-Promoting Mechanism of Bismuth-Doped Cu(In,Ga)Se2 Solar Cells Fabricated at 400 °C

“…It is worth mentioning that this reduction in E F – E V is generally considered to be caused by the increase in carrier concentration, which is consistent with the results of the Hall effect test mentioned above. Because the work function of CdS is smaller than that of CIGS, based on Anderson’s model, , when CdS/CIGS heterojunction is building, the energy bands of CdS should move downward and the energy bands of CIGS should move upward and finally reach the alignment of E F . If the work function of CIGS is increased, this band bending will be strengthened, thereby increasing the V oc of solar cells.…”