https://doi.org/10.1007/s13391-021-00294-9
ORIGINAL ARTICLE - ENERGY AND SUSTAINABILITY
Quantification of the Contact Resistance of ZnO/MoSe
2/Mo Contact Formed in a Monolithic CIGS Photovoltaic Module
Sung‑Wook Cho1 · A.‑Hyun Kim1 · Gyeong‑A. Lee1 · Chan‑Wook Jeon1
Received: 1 February 2021 / Accepted: 28 April 2021 / Published online: 7 May 2021
© The Korean Institute of Metals and Materials 2021
Abstract
This study investigated the effect of MoSe2 on the contact resistance (RC) of the transparent conducting oxide (TCO) and Mo contact in the P2 region of the CIGS photovoltaic module. MoSe2 formed in the process of making the Cu(In,Ga)Se2 (CIGS) absorber layer imparts ohmic contact properties to the CIGS/Mo contact. In the process of connecting cells in series to fabricate a CIGS photovoltaic module, TCO/MoSe2/Mo contact was formed, and it was confirmed that MoSe2 increased the RC of this contact using the transmission line method. It is estimated that the reason MoSe2 increases the RC is due to conduction band offset (CBO). When ZnO used as TCO forms a contact with MoSe2, 0.6 eV CBO is formed due to the dif- ference in electron affinity. This CBO can act as a resistor that impedes the flow of current. Therefore, in order to reduce the contact resistance of the CIGS solar module and increase the power conversion efficiency, it is necessary to make the MoSe2 thin enough to facilitate carrier tunneling.
Graphic abstract
Keywords Cu(In,Ga)Se2 · MoSe2 · Transparent conducting oxide · Contact resistance
1 Introduction
Since Cu(In,Ga)Se2 (CIGS) has a high absorption coeffi- cient (> 105 cm−1), it is a suitable material as an absorber layer for thin-film solar cells [1]. The bandgap energy
(Eg) of CIGS absorber can be adjusted by changing the composition of each element, and it is generally accepted that the power conversion efficiency is optimized when the Eg is about 1.2 eV [2, 3]. The open-circuit voltage (VOC) of a solar cell is limited by the Eg of the absorber, and the VOC that can be achieved by a single p–n junc- tion of the absorber with an Eg of 1.2 eV is limited to 0.7 V [4]. Therefore, in order to increase the output voltage performance, a module in which a plurality of cells are connected in series is manufactured. To make a CIGS
* Chan-Wook Jeon cwjeon@ynu.ac.kr
1 School of Chemical Engineering, Yeungnam University, Gyeongsan, Korea
photovoltaic module, Mo, which is a back contact, is deposited on a large-area glass substrate, and the 1st pat- terning (P1) is formed by laser scribing to separate the cells. Then, after depositing a CIGS and a buffer layer such as CdS or ZnOS, a 2nd patterning (P2) is performed.
Finally, after depositing a transparent conducting oxide (TCO), a 3rd patterning (P3) is performed. In general, mechanical scribing is used to make P2 and P3 [5, 6]. P1 and P3 are for electrical insulation between cells, and P2 is necessary for electrical connection between the rear electrode Mo of the nth cell and the front electrode TCO of the (n + 1)th cell. [7, 8].
Mo, which is mainly used as a back contact, is known to form a schottky barrier when a contact with CIGS is formed [9]. When a Cu–In–Ga (CIG) precursor in sub- jected to 2-step selenization to make a CIGS absorber layer, Mo is selenized to form MoSe2, which makes the CIGS/Mo an ohmic contact. It is known that the forma- tion of a MoSe2 layer is essential for manufacturing a CIGS solar cell [10, 11].
When MoSe2 is formed, TCO/MoSe2/Mo contact is formed in the P2 region of the CIGS module instead of TCO/Mo contact, but it is difficult to remove the MoSe2 layer by mechanical scribing. In this study, the effect of MoSe2 on the contact resistance of the BZO/Mo contact in the P2 region was investigated. Although current–volt- age (I–V) measurements provide performance parameters such as VOC, short-circuit current (ISC), fill factor (FF), and series resistance (RS), but each contribution of the factors affecting RS cannot be determined. In order to find out how MoSe2 affects the contact resistance (RC) in the P2 region, the modified transmission line method (M-TLM) suggested by J.H. Jeong, was used [12].
2 Experiments
Figure 1 is a schematic diagram of a sample prepared to cal- culate the RC of the TCO/Mo contact by the M-TLM. In this study, CIG/Mo/Glass precursor manufactured by Avancis Korea, in which P1 is spaced by 6 mm, was used to manu- facture the M-TLM test module. Samples having different thicknesses of MoSe2 were prepared by performing at 470
℃ or 580 ℃ by rapid thermal process (RTP) [13]. The CIGS absorber layer produced by selenization was subjected to surface etching with 0.15 M KCN solution for 1 min, and then CdS was deposited by the chemical bath deposition process. Then, P2 was formed at the same interval as P1 using a mechanical scriber. The window layer, Boron doped ZnO (BZO) of 1.9 μm thickness, measured by a surface pro- filer (DektakXT-A, at Core Research Support Center For Natural Products and Medical Materials), was deposited by metal organic chemical vapor deposition (MOCVD). In the P2 region, BZO/MoSe2/Mo or BZO/Mo contact was formed depending on the selenization process. P3 patterning was not applied. In order to make a mesa structure, The depos- ited BZO left only 10 mm wide (W) in the center, and the rest was mechanically removed, and In contact pads were formed on the exposed Mo to complete M-TLM test mod- ule as shown in Fig. 1. In order to evaluate the Rc of the P2 region, the total resistance (RT) according to the finger spacing was obtained from the current–voltage (I–V) curves measured by sequentially probing the Mo finger separated by a multiple of 6 mm from the 1st Mo finger. The current was measured by applying a voltage from − 0.5 to 0.5 V using a source meter (Model 2400, Keithley) in the dark. During the measurement, the temperature of the M-TLM module was maintained at 25 ℃ using a thermostat (K901T, McScience).
In addition, to confirm the formation of the MoSe2 layer, a cross section of the P2 region of the M-TLM test
Fig. 1 Schematic diagram of M-TLM test module
module was observed using a scanning electron microscope (SEM, S-4800, HITACHI), and MoSe2 peak detection was confirmed by X-ray Diffraction (XRD, MPD for bulk, DIATOME).
3 Results and discussion
Figure 2 shows the cross-sectional SEM images of the P2 region of the M-TLM test module made of the absorber lay- ers made by selenization at 470 ℃ (Sample A) and at 580 ℃ (Sample B). Sample B prepared at higher temperature had a MoSe2 layer thickness of about 255 nm, but Sample A sub- jected to selenization at a relatively low temperature, unlike Sample B, the MoSe2 layer could not be clearly defined. This means that MoSe2 is absent or very thin at least in Sample A.
These results agree well with the XRD results in Fig. 3. While
(100) and (110) MoSe2 peaks were clearly resolved in Sample B, but no signal of the MoSe2 peak was detected in Sample A.
The two M-TLM test modules were compared to investi- gate how MoSe2 affects the RC of the BZO/Mo contact. When applying voltage by connecting M1 and M2 of the M-TLM module, the RT is the sum of each resistance including the sheet resistance (Rsheet) of Mo, the Rsheet of BZO, the RC of the P2 region and the RC of the In/Mo contact. Typically, the resistivity of metal is very low, about 10–6–10–8 Ω cm, so it makes a negligible contribution to RT. Therefore, RT is gov- erned by the Rsheet of BZO and RC of the P2 region Based on these considerations, RT can be expressed by Eq. (1) [12].
Here, D is the electrode distance between two probed Mo fingers, which is a multiple of 6 mm.
(1) RT= Rsheet
W D+2RC[Ω]
Fig. 2 Cross-sectional SEM image of P2 region of the M-TLM test module fabricated with CIGS absorber selenized at different temperatures of a 470 ℃ and b 580 ℃
Fig. 3 XRD patterns of CIGS layers with different CIGS absorber formation selenization temperature. Not indexed peaks correspond to chalcopyrite CIGS
Figure 4a, c show I–V graphs of Sample A and Sample B, respectively. Here, M1M2 means an IV curve obtained by probing M1 and M2. All curves show linear behavior, so the ohmic resistance RT can be obtained from the slope at 0 V.
The RT values obtained at different electrode distance D, are summarized in Table 1, and its linear relationship is shown in Fig. 4b, d. The RT intercept value obtained by extrapolating each graph corresponds to 2RC according to Eq. (1). The RC of Sample A, which has negligibly thin interfacial MoSe2, was calculated to be 0.73 Ω, and the RC of Sample B with MoSe2 of 255 nm thickness was 1.12 Ω. As described above, MoSe2 has a desirable function of imparting ohmic contact properties to the CIGS/Mo interface, and corresponds to a layer that must be formed.
However, it was confirmed that the RC of P2 was rather increased due to the presence of MoSe2. The reason why the RC is higher when MoSe2 is present can be estimated from the conduction band offset (CBO) occurring in the
ZnO/MoSe2 contact. The electron affinity of ZnO and MoSe2 is 4.5 eV and 3.9 eV, respectively, and the work function of Mo is 4.6 eV [14, 15]. In Fig. 5b, the difference in electron affinity between ZnO and Mo is small, but In Fig. 5a, 0.6 eV CBO is formed at the ZnO/MoSe2 contact interface due to the difference in electron affinity. CBO
Fig. 4 a Change of slope of I–V graph according to electrode distance of sample A, b RT-Distance graph of sample A, c Change of slope of I–V graph according to electrode distance of Sample B and d RT-Distance graph of Sample B. The linear fit equations are added in b and d
Table 1 RT according to electrode distance calculated from the slope of the I–V graph
Electrode distance (mm) RT of sample A (Ω) RT of sample B (Ω)
6 9.28 10.12
12 16.53 17.38
18 23.85 25.02
24 31.81 32.73
30 39.55 40.69
of 0.5 eV or more is known to act as an energy barrier preventing electron transfer [16].
4 Conclusion
This study reports the effect of MoSe2 on the RC of TCO/
Mo contact in the P2 region of the CIGS photovoltaic mod- ule. Unlike a small-area solar cell, since TCO and Mo form a contact in a monolithic module, their contact resistance should be lowered to suppress the loss of module fill factor.
Since the MoSe2 layer formed in the process of selenization of the Cu–In–Ga precursor is hardly removed by mechani- cal scribing, a TCO/MoSe2/Mo contact is inevitably intro- duced in the P2 region. We experimentally confirmed that the contact resistance increased by more than 50% due to the presence of MoSe2 in the P2 contact. Although MoSe2 of sufficient thickness should be formed to make the CIGS/Mo interface into ohmic contact, but in a module having TCO/
MoSe2 interface, it is necessary to make MoSe2 as thin as possible to prevent resistance increase due to CBO.
Acknowledgements This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010012980). This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted finan- cial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20204010600100).
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