(Invited, Digital Presentation) Application of Transition Metal Dichalcogenides for Chalcopyrite Solar Cells
Authors: Nishimura Takahito 
Published: November 2022

Introduction
Chalcopyrite compound materials, such as Cu(In,Ga)Se2 (CIGSe), Cu(In,Ga)S2 (CIGS), and Cu(In,Ga)(S,Se)2, are expected as promising photovoltaic materials since they can be applied for flexible light-weight solar cells, which realize a high-speed and low-cost production utilizing a roll-to-roll process. In this work, transition metal dichalcogenides, i.e., MoSe2 and MoS2 are applied to develop new functions in the chalcopyrite solar cells. First, a device peeling technique for CIGSe solar cells are developed by utilizing two-dimensional MoSe2 atomic layers. To boost an electric power generation via collecting of the ground albedo radiation, bifacial-type structures for the flexible CIGSe solar cells are constructed by depositing a TCO layer on CIGS rear side after applying device-peeling technique to traditional substrate-type structure. Second, an interfacial control technique to modify CIGS absorber / Mo back contact interfaces for the wide-gap CIGS solar cells are developed by introducing a p+-type MoS2 layer, where the carrier concentration is controlled by Nb-doping.
Experimental methods
1. Peeling techniques for flexible-bifacial CIGSe solar cells
2-μm-thick CIGSe absorber was prepared on a Mo-covered glass substrate using a three-stage process, where the MoSe2 atomic layers were intentionally formed to control the adhesion at the Mo/CIGSe interface. CIGSe solar cells with a structure of glass/Mo/CIGSe/CdS/i-ZnO/Al:ZnO/Ni-Al grids were fabricated (Figure 1 (a)). For the peeling-off procedure, fluorinated ethylene propylene (FEP) films as alternative flexible substrates were attached to the front side with thermosetting epoxy glue under a temperature of 100 °C. While cooling down to room temperature, residual stress remained in the epoxy glue and FEP films due to their higher thermal expansion coefficients. Therefore, the CIGSe/CdS/i-ZnO/Al:ZnO/epoxy/FEP layers spontaneously detached from the Mo-covered glass. Finally, flexible-bifacial CIGSe solar cells were completed via deposition of 300-nm-thick ITO films on the rear side of the CIGSe by sputtering methods.
2. Fabrication of Nb-doped MoS2 and wide-gap CIGS solar cells
20-nm-thick Nb-Mo metal precursors were deposited on glass substrates via co-sputtering methods utilizing Mo and Nb targets. The sulfurization process for 30 min at 600 °C under H2S/Ar atmosphere was performed on the Nb-Mo metal precursors to form the p+-type Nb-doped MoS2 (Nb:MoS2) films. Hall effect measurement was performed for the Nb:MoS2 films to evaluate their electrical properties.
The wide-gap CIGS solar cells with a structure of glass/Mo/Nb:MoS2/CIGS/CdS/i-ZnO/Al:ZnO/Ni-Al grids were fabricated (Figure 1 (b)). The Nb:MoS2 thin films with the [Nb] / ([Nb] + [Mo]) ratios of 0 and 0.02 were deposited on a Mo-covered glass substrate. Then, 2-μm-thick CIGS absorber were prepared. Stacked layers of Cu–Ga and Cu–In precursors were deposited using the evaporation method, and followed by a sulfurization process at the substrate temperature of 600 °C under H2S/Ar atmosphere.
Results and discussions
1. Conversion efficiency of flexible-bifacial CIGSe solar cells
The CIGSe solar cells were successfully peeled from Mo back contacts, when the layered-grown MoSe2 (c-axis orientation) was formed at the Mo/CIGSe interface, suggesting the controllability of interfacial adhesion via cleavage by weak chemical bonding due to van der Waals force in the MoSe2 atomic layers. A high-performance ratio of 95.0% was achieved in the 11.5%-efficient lift-off cells (with alternative Au back contact) compared with 12.1%-efficient substrate cells on Mo back contact. Furthermore, the results demonstrated the device operation as a bifacial solar cell with conversion efficiency, V
OC, J
SC, and FF of 10.1% 0.487 V, 33.9 mA/cm2, and 0.609 under front illumination and 2.8%, 0.435 V, 9.8 mA/cm2, and 0.658 under rear illumination.
2. Conductivity control of MoS2 interface layer for wide-gap CIGS solar cells
Carrier density of Nb:MoS2 thin films was monotonically increased from 9.9 × 1015 to 1.9 × 1020 cm-3 and the conductivity type was inverted from n- to p-type with increasing the [Nb] / ([Nb] + [Mo]) compositional ratio from 0 to 0.06. This result suggests that Nb element acts as an acceptor in MoS2.
For the wide-gap CIGS solar cells the, the roll-over in the current density‒voltage curves was observed in the samples with the [Nb] / ([Nb] + [Mo]) ratio of 0 (without Nb-doping) in MoS2, whereas the roll-over was disappeared in the [Nb] / ([Nb] + [Mo]) ratio of 0.02. This demonstrated that a highly doped p-type Nb:MoS2 introduced in the CIGS/Mo back junction improved the performance of wide-gap CIGS solar cells.
Conclusions
We demonstrated a usefulness of the transition metal dichalcogenides, MoSe2 and MoS2, on the device peeling technique to fabricate the flexible-bifacial CIGSe solar cells and the interfacial modification technique for the wide-gap CIGS solar cells.
Acknowledgements
We gratefully acknowledge the support of JSPS KAKENHI (20K14780) and Kato Foundation for Promotion of Science (KJ–3020) in Japan.

Figure 1