Hybrid Ag Nanowire-ITO as Transparent Conductive Electrode for

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Hybrid Ag Nanowire-ITO as Transparent Conductive Electrode for Pure Sulphide Kesterite CuZnSnS Solar Cells 2

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Jongsung Park, Zi Ouyang, Chang Yan, Kaiwen Sun, Heng Sun, Fangyang Liu, Martin A. Green, and Xiaojing Hao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05776 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017

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The Journal of Physical Chemistry

Hybrid Ag Nanowire-ITO as Transparent Conductive Electrode for Pure Sulphide Kesterite Cu2ZnSnS4 Solar Cells

Jongsung Park, Zi Ouyang, Chang Yan, Kaiwen Sun, Heng Sun, Fangyang Liu, Martin Green and Xiaojing Hao* School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia

Author to whom correspondence should be addressed; E-Mail: [email protected]

Abstract We present the hybrid silver nanowire network-ultra thin ITO as the transparent conductive electrode for earth-abundant and environmentally-friendly pure sulphide Cu2ZnSnS4 solar cells, replacing the scarce and expensive ITO. For this new hybrid transparent electrode, sputtered intrinsic zinc oxide is introduced as an anti-reflective coating replacing MgF2 in traditional Cu2ZnSnS4 solar cells. Random arrays of silver nanowire network with different densities hybridized with 20nm ITO layer are employed in Cu2ZnSnS4 devices and their device performance is compared with the reference device with conventional ITO electrode. The devices with the hybrid electrodes with similar sheet resistance to ITO show generally improved current density due to their better optical transparency than ITO while maintaining similar fill factor. Cu2ZnSnS4 devices with low density Ag NW hybrid electrode give the highest efficiency of 6.4 % on average mainly due to improved current density, among which the highest efficiency value is 6.72 %. 20nm

ACS Paragon Plus Environment


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intrinsic zinc oxide is selected as an anti-reflective coating based on optical FDTD simulations, which gives additional 5 % increase in current density and efficiency. Cu2ZnSnS4 device with the low density hybrid electrode is improved from 6.72 to 6.84 % after introducing intrinsic zinc oxide anti-reflective coating layer.

1. Introduction Kesterite Pure sulphide Cu2ZnSnS4 (CZTS) solar cell is one of the most promising thin film solar cells as it is composed of earth abundant, environmentally-friendly constituent 1, and has optimal bandgap [1.4 – 1.5 eV] for high efficiency solar cells and high absorption coefficient over 104cm-1 2-4. Due to its high bandgap and high theoretical power conversion efficiency (PCE) of 32.2% according to Shockley-Queisser (S-Q) limits 1, 5, pure sulphide CZTS is suitable for both single junction solar cells or multi-junction solar cells 6. Despite these advantages, PCE of pure sulphide CZTS solar cells is still below 10% 7-8 and also lower than other inorganic thin film solar cells such as CIGS and CdTe solar cells. It has been revealed that the current limitations of CZTS device performance mainly lie in its high open circuit voltage (Voc) deficit which is believed to be due to the band tailing effects 9, secondary phases in the absorber and/or interfaces

10

, non-optimal band alignment between the

absorber and buffer layer 11. While many efforts have been made to reduce the Voc deficit, there are few studies on the optical improvement. Based on S-Q limit 5, the calculated theoretical maximum current density (Jsc) at bandgap (Eg) of 1.5 eV is close to 30 mA/cm2. However, the Jsc of the highest efficiency sulphide CZTS solar cells (Eg = 1.5 eV) currently reported is 21.25 mA/cm2 (PCE = 9.5 %) 12. Even in this champion pure sulphide CZTS solar cell, the Jsc is about 30 % lower than its theoretical limit, meaning that PCE of the cells can be further improved if better optical management scheme is employed. There are multiple


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factors which contribute to the Jsc loss such as reflection from CZTS/CdS/ZnO, parasitic absorption in window layer including low resistance transparent conductive oxide (TCO) and high resistance ZnO layer and buffer layer, uncollected light in bulk and shadowing by front contact

13

. Therefore, it is known that optical loss due to TCOs is one of the bottle-necks

limiting Jsc of the cells. M. Wnkler et al., reported 14 that the optical loss at TCO accounts for 35% of the total optical loss in CZTSSe solar cells (Eg = 1.15 eV). This figure would be slightly lower in the case of CZTS with a bandgap of 1.5 eV for single-junction solar cells as the effect of free carrier absorption of TCO in the infrared (IR) range can be neglected, but maintains the similar level for its application in multi-junction case where photons in the IR wavelength range is expected to be absorbed by the bottom cell. Indium Tin Oxide (ITO) is a well-known TCO material as its properties are suitable for TCO and currently employed in many different types of thin film solar cells including CZTS solar cells. Despite its advantages, there are also drawbacks. Due to scarcity of indium (In) 15, it is foreseen that the price of ITO will be increased. Also, the nature of ITO is brittle, so that it limits its application to flexible devices 16. ITO gives both high transparency (~ 90 %) and low sheet resistance (Rsheet, under 30 Ω/□) when sputtered at 200 – 350°C 17. It is challenging to apply ITO with such processing conditions to substrate-configuration like kesterite solar cells where ITO is used as the front electrode. This is because the performance of CZTS / CdS heterojunction can get worse subject to high temperature in the sputtering process. For this reason, ITO as a top electrode is generally deposited at room temperature for kesterite solar cells 18-19. However, room temperature deposition of the ITO layer results in relatively higher Rsheet of the film and hence high series resistance (Rs) of final devices, compared with the high temperature deposition process. In order to compensate high Rsheet, thick ITO layer ~210 nm is usually required in CZTS devices. Such a thick ITO layer causes optical loss due to increased parasitic



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absorption accounting for 10 % of Jsc loss based on our calculation. Therefore, CZTS solar cells require a new type of transparent conductive electrode (TCE) processed at room temperature with superior transmittance and conductance to further enhance Jsc. Silver nanowire (AgNW) network is well-known TCE as it has both outstanding conductivity and transparency so that it is widely used in display 20-22, touch screen 23 as well as in photovoltaic applications

24-29

. However, it has been found that AgNW network

required post-annealing in order to minimize junction resistance between AgNWs, which affected Rsheet of the network. The AgNW network with thermal annealing process has been widely employed in thin film solar cells which are less sensitive to post-heat treatment such as CIGS solar cells and CdTe in which ITO or ZnO:Al can be deposited around 150 °C

30-31

.

However there is still debate on the impact of post-heat treatment on the performance of CZTS and CIGS heterojunction cells particularly when different buffer materials are used. It is necessary to minimise the impact of AgNWs process on the underlying hetero-junction so that the application of Ag NWs will not be limited by the buffer options and their associated post-annealing treatment and the cell performance can be more robust and reproducible. One viable solution is to apply AgNWs electrode processed at room-temperature onto the heterojunction. For this reason, we diversely investigated different types of commercially available AgNWs and carefully controlled deposition conditions. AgNW network processed at room temperature was developed as a TCE which have not yet been reported in pure sulphide CZTS solar cells. As there is a large lateral resistance for the charge generated at the submicron gaps while traveling between adjacent NWs

32

. We also compared CZTS

devices with standalone AgNW electrode to those with the hybrid Ag NWs / ultrathin TCO. An ultrathin ITO space layer will not cause significant optical loss in TCE, but is necessary for charge collection at the front contact. Another advantage of this electrode (so called hybrid


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TCE) is that it can be flexible. Even though flexible CZTS solar cells are not discussed in this research, it can be applied to flexible CZTS solar cells such as CZTS on PET, flexible glass or stainless steel foil in future research. This designed hybrid Ag NW electrode, with merits of high transmittance and conductivity, would be applicable for both CZTS related single junction and multi-junction solar cells. Even though hybrid electrodes containing AgNW network has been applied in other solar cells as a top electrode in substrate configuration 25, 30, 33-35

, both total replacement of ITO by AgNW network and AgNW / thin ITO hybrid

electrode have not been studied in pure sulphide CZTS solar cells. In this paper, we fabricated the hybrid AgNW network / ultrathin ITO layer for CZTS solar cells for the first time. The electrical and optical properties of the hybrid structure were first investigated, followed by studies on its application on CZTS devices as the top electrode. With the same conductivity compared with our conventional 210 nm thick ITO layer, we found that the hybrid AgNW network / thin ITO electrode shows better optical transmission, improved photo-current density of CZTS devices, and hence PCE. To minimize the reflection of the hybrid AgNW / ITO electrode, both computational simulations (Finite Domain Time Difference, FDTD) and experiments were performed to identify the suitable anti-reflection coating (ARC) materials and the optimal ARC thickness. We found that intrinsic Zinc Oxide (ZnO) yields outstanding anti-reflection performance.

2. Experimental AgNW networks were spin-coated on both quartz (2.5 cm x 2.5 cm) and 20 nm ITO film (2.5cm x 2.5cm) using Ag nanowire solution (Cambrios, USA). In the spin-coating process, 500 ul of AgNW solution was dropped on the substrate, followed by spinning. The density of AgNW network was controlled by varying the rotation rate (1000 – 6000 rpm).



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Note that the networks were dried in room temperature without any post-heat treatments after spin-coating. A thin ITO film of 20 nm was sputtered on both quartz and our CZTS devices by using ATC-2200 (AJA international) at room temperature for optical / structural characterisation and device performance characterisation, respectively. In order to compare the performance of AgNW network and hybrid electrode (AgNW network + 20 nm ITO), we use the CZTS devices with normal sputtered 210 nm ITO window layer as the reference, representing the conventional cell structure. CZTS precursor films were deposited by co-sputtering of Cu, ZnS and SnS targets on Mo coated soda lime glass (SLG). After deposition, precursor films underwent a sulfurization process in a combined S and SnS atmosphere in a rapid thermal process (RTP) tool. A 50 nm CdS buffer layer was deposited on CZTS absorber by chemical bath deposition (CBD). In case of the reference device, a 50 nm intrinsic zinc oxide (ZnO) layer and a 210 nm ITO layer were deposited on CdS layer sequentially by sputtering method. For AgNW network only samples, 50 nm ZnO layer was deposited on CdS directly followed by AgNW spin coating (i.e. no ITO layer was processed between ZnO and Ag NW). For hybrid electrode sample, 50nm ZnO layer and 20nm ITO layer were deposited sequentially on CdS layer, followed by AgNW spin coating. Scanning Electron Microscopy (SEM, FEI, USA) was used to examine morphology of AgNW network. Spectrophotometer (Perkin Elmer, USA) was employed to measure optical properties of the electrodes. Four-point probe was used to measure Rsheet of AgNW networks and the electrodes. The J-V characterisation (PV measurement, USA) was performed to measure the performance of CZTS devices (AM 1.5, 1000 W/m2).


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3. Results and discussion 3.1 Hybrid AgNW-ITO High transmittance and low sheet resistance are two key properties required for the transparent conductive electrode. Here we compare three electrode systems: (i) the 210 nm ITO layer on quartz as the conventional front electrode design, (ii) the AgNW network on quartz, and (iii) AgNW network on a 20 nm ITO spacer coated on quartz. The reason we employed 20 nm ITO as a supporting layer is that it corresponds to the highest optical gain in CZTS absorber when placed underneath of AgNW network in FDTD simulations. For valid estimation of the performance of electrode systems, the Rsheet was maintained the same by controlling the density of AgNW, and then the optical properties were compared.

Before AgNW

Quartz

20nm ITO

210nm ITO

Beyond detection

335 Ω/□

23 Ω/□

23.1 Ω/□

N/A

limit After AgNW

24.2 Ω/□

Table 1. Rsheet of different types of substrate before and after AgNW network formation.

As shown in Table 1, the densities of AgNW in (ii) and (iii) were tuned to result in the same overall Rsheet of 23 – 24 Ω/□ with that of (i). With the formation of AgNW network, Rsheet of 20 nm ITO layer was decreased dramatically from 335 to 23.1 Ω/□, meaning that AgNW network plays a crucial role in determining electrical conductivity of the hybrid TCE. On the other hand, although the thin ITO layer is not a major contributor to the conductance, it is a critical component to allow sufficient lateral transport of the charges. This function will be discussed below.



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The transmission spectra of the three electrode systems discussed in Table 1 is presented in Figure 1. It shows transmission spectral of quartz, 20 nm ITO / quartz, 210 nm ITO / quartz, and the hybrid TCE. As can be seen in the figure, transmission of quartz is ~ 93 % throughout the wavelength range of 300 – 1000 nm. After coating the 20 nm ITO layer on quartz, transmission is decreased particularly at the shorter wavelengths with a sharp drop in 300 – 400 nm and average transmission (500 – 1000 nm) is 88%. Compared to the transmission of quartz which is a baseline here, the amount of transmission deceased is about 5 %. The reason for decreased transmission is mainly due to increased reflection with a small portion of parasitic absorption. As the ITO thickness increases from 20 nm to 210 nm, which is the conventional TCO employed in our CZTS devices, the transmission is reduced in the entire spectrum. This is mainly due to increased surface scattering with the increased thickness 36 and increased free carrier absorption 37. The transmission spectra vary significantly with the coated AgNW. Although transmission of quartz after network formation (green dash line) is decreased in the entire wavelength range, the optical loss caused by AgNW is only 3 % overall, and it is much less than the optical loss in ITO with increased thickness. Similar to this, spectral transmission of AgNW network / 20 nm ITO (pink dash line) sample is also decreased and the optical loss is similar to that of AgNW network / quartz sample. As the spin-coating condition of AgNW network is the same, the optical loss is expected to be the same in two samples even though the surface condition of the substrates is different. Importantly, spectral transmission of AgNW network / 20 nm ITO is better than that of 210 nm ITO layer even though their Rsheet is nearly the same. That means, in optical point of view, hybrid AgNW network / 20 nm ITO


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electrode can improve the optical gain and thereby current density of CZTS device, when 210 nm ITO layer is replaced by the hybrid TCEs.

Figure 2 shows schematic structure of CZTS device with the hybrid TCE and morphology of AgNW network spin-coated on 20 nm ITO. For fabrication of CZTS devices with the hybrid TCE, two different density of AgNW networks were employed: (i) high density Ag NW network and (ii) low density Ag NW network. The purpose to employ (i) high density Ag NW network is to compare with the CZTS device with 210nm ITO as their Rsheet is similar, and (ii) low density Ag NW network is to investigate whether more optical gain is beneficial for PCE boost even the NW network is less conductive. As mentioned in the experimental section, AgNW networks were deposited without any thermal annealing, but dried at room temperature in air ambient. The mean diameter and length of AgNW is 40 nm and 20 um, respectably. As can be seen, Figure 2(a) indicates a high density AgNW network / 20 nm ITO and its Rsheet is 20 Ω/□, Figure 2(b) shows a low density AgNW network / 20 nm ITO and its Rsheet is 36 Ω/□.

Figure 3 (a) shows the Jsc of the reference CZTS device and the devices with the high and low density of AgNW network / 20 nm ITO layer (denoted as 20nm ITO_20 ohm/sq and 20nm ITO_36ohm/sq in the Figure, respectively). The average Jsc of the reference, high density AgNW network / 20 nm ITO and low density of AgNW network / 20 nm ITO are 18.5, 19.2 and 19.7 mA/cm2, respectively. CZTS devices with hybrid TCEs (AgNW / 20 nm ITO) show higher Jsc than the reference device. It is believed to result from the better spectral transmission of hybrid TCE than that of traditional ITO reference. In addition, it is obvious



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that CZTS device with low density hybrid TCE (20nm ITO_36 ohm/sq) demonstrates higher Jsc than that of CZTS device with high density hybrid TCE (20nm ITO_20ohm/sq) It is notable that the CZTS devices with TCE of AgNW/ ZnO (i.e. without supporting thin ITO underneath of AgNW network) show fill factor (FF) of 22 – 35 % and PCE of 0.5 – 1.5 %, indicating that although with achievable low sheet resistance, the AgNWs alone, without the thin TCO underneath, were not effective in collecting the charge generated at the surface between AgNWs. In terms of Voc as shown in Figure 3 (b), it is expected that their Voc are nearly the same as they were fabricated from the same batch. The CZTS device with reference ITO and the device with the low density hybrid TCE show similar Voc, in the range of 625 – 630 mV, but the device with high density hybrid TCE indicates around 20 mV lower Voc than that of the reference and the device with the low density hybrid TCE. We believe that this difference was not due to the differences in electrodes, but rather from the fluctuation of cell fabrication process. Even the samples studied in this paper were fabricated from the same batch, several factors such as position in RTP tool during sulfurization process and CdS deposition process can cause this subtle Voc variation. Figure 3 (c) shows the FF of the samples. The CZTS devices with reference ITO and CZTS device with high density hybrid TCE show similar FF value of 55 %, which is due to the fact that Rsheet of the electrode of two devices are similar. However, CZTS device with the low density hybrid TCE shows lower FF, which is 53.5 % on average. It is because the low density hybrid TCE is less conductive than that of others. The higher Rsheet of the low density hybrid TCE would lead to increased Rs of the device, hence lower FF. Further investigation on this matter is discussed below.


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Overall, in Figure 3 (d), CZTS devices with the low density hybrid TCE show the highest PCE of 6.4 % on average and the highest PCE value of 6.72 %. The CZTS device with the highest PCE of 6.72 % is mainly due to better Jsc improvement than other samples. Even though this device showed slightly lower FF than the CZTS device with reference ITO and CZTS devices with the high density hybrid TCE, the improvement in Jsc compensated the FF loss. Interestingly, CZTS devices with the high density hybrid TCE showed even lower average PCE than that with reference ITO although it demonstrated better Jsc and nearly the same FF. This is because its Voc is ~20 mV lower than that with reference ITO. When comparing the devices with low density and high density AgNW hybrid TCEs, it can be concluded that optical loss by AgNW itself is a more critical factor for PCE of the cell rather than the lateral resistance in the electrode. Jsc difference between two CZTS devices with the hybrid TCEs is 0.5 mA/cm2 on average, but FF difference is less than 1 %.

In order to better understand the underlying mechanism for the device performance difference between high-density and low-density hybrid transparent electrode based cells, spectral transmission and external quantum efficiency (EQE) measurements were carried out on the hybrid TCEs on quartz and on devices respectively. As afore-mentioned in Figure 3, Jsc of the CZTS device with low-density hybrid electrode is mainly improved because of the better transparency. As can be seen in Figure 4 (a), the low-density TCE on quartz shows better spectral transmission in entire wavelength range than the high-density TCE on quartz. It is consistent with their EQE trends in Figure 4 (b), showing that the device with lowdensity hybrid TCE exhibits improved EQE in the wavelength range of (550 – 900 nm) compared to that of the device with high-density hybrid TCE. In addition, compared with the reference, the EQE spectra of the CZTS devices with hybrid electrodes (both low- and high-



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density) is also improved at short wavelength range (300 – 380 nm) due to higher transmission. The EQE of the reference device at 300 nm is only 3 %, whilst that of devices with the hybrid TCEs is beyond 10 %. Figure 4(c) shows J-V curves of the CZTS devices with reference ITO, with the low and high density hybrid TCEs. The device with the low density hybrid TCE yields the highest efficiency of 6.72 % with Jsc of 19.31 mA/cm2, Voc of 634.2 mV and FF of 54.8 %. The Rs of the device with reference ITO is close to that with the high density electrode, 1.72 and 1.74 Ω·cm2 respectively. In contrast, Rs of the device with low density hybrid TCE is approximately 1.5 times higher than that of the devices with the high density hybrid TCE and with the reference ITO as it has higher value of Rsheet than that of other two devices.

3.2 ARC- ZnO layer for AgNW network As we discussed in the introduction, further improvement in Jsc can be achieved by reducing reflection of the hybrid TCE. Antireflection coating (ARC) is one of the known way minimizing the reflection of incident light

38-39

, and it was successfully employed in AgNW

networks as well 30. MgF2 is widely used as AR coating for thin films solar cells

40

including

CZTS solar cells. For AgNW network electrode, ZnO shows good ability as AR coating

41-42

.

Therefore, we applied these two candidates in optical simulations to identify the suitable AR coating and optimum thickness for AgNW network. The structure used for optical simulation is shown in Figure 1.

Figure 5 (a) shows simulated reflection of CZTS device with the hybrid AgNW / ITO electrode, the hybrid TCE with 50 nm MgF2 and with 20 nm ZnO, respectively. The thickness of MgF2 and ZnO layer was varied between 10 – 150 nm with a step size of 20 nm. Figure 5


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(a) only shows those reflection spectra giving the lowest reflection for each case. The reflection pattern of the hybrid TCE without ARC is like “U” shape. High reflection is observed at both short (300 – 350 nm) and long wavelength (900 – 1000 nm) regions and low reflection at mid-range wavelengths (350 – 900 nm). The lowest reflection was found tin 600 – 700 nm, but its minimum is beyond 10 %. In terms of the hybrid TCE with MgF2, 50 nm MgF2 layer showed the lowest reflection, which is 3 – 4 % less than the hybrid TCE without ARC at the entire wavelength range. In the simulations, 20 nm ZnO gives the best performance as the ARC for the hybrid TCE applied in the CZTS solar cells. The deposition of 20 nm ZnO layer decreased reflection by 7 – 8 % in mid-range wavelengths and more than 15 % in short and long wavelength range. Figure 5 (b) shows measured reflection of CZTS device with the hybrid TCE with and without sputtered ZnO ARC on the hybrid TCE. As can be seen, reflection of the electrode is decreased after ZnO deposition and the decrement of reflection is around 2%. These results indicate that 20 nm ZnO can act as ARC for the hybrid AgNW network / 20 nm ITO electrode. However, the amount of reflection reduction is different between the simulated and the experimental result. There are two reasons which might be responsible for the slightly difference in spectral reflection between experimental and simulations. The first reason is the uniformity/roughness of ZnO (ARC layer). In Lumerical FDTD simulation, ZnO is designed to uniformly cover both ITO exposed area and AgNWs, which can be different from that

deposited by sputtering. The variation in

refractive index of ZnO can also contribute to the difference between the simulated and the experimental spectra.

The effectiveness of the ZnO ARC on CZTS devices with the hybrid TCE was further studied in terms of the device performance. Figure 6 (a) shows EQE of the CZTS devices with



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high and low density AgNW / 20 nm ITO before and after 20 nm ARC-ZnO deposition. In all devices, the improved EQE mostly comes from 400 – 800 nm wavelength range. This EQE improvement well agrees with the reflection results shown in Figure 5 (b) where the reflection of the hybrid TCE is improved at the similar wavelength range. Interestingly, EQE of the CZTS device with the high density electrodes is improved more than that of the device with the low density electrode after ZnO deposition. As ZnO layer is more effective as ARC when it deposited on AgNWs / 20 nm ITO, in the case of the high density electrodes where there are more AgNWs on the surface (less ITO exposed area), it can be deduced that improvement in EQE is better than that of the low density hybrid TCE. Figure 6 (b) shows J-V curves of the CZTS devices, showing the highest efficiency before and after ZnO deposition. Enhancement in both Jsc and Voc was observed. The best PCE of the CZTS device after ZnO deposition is improved from 6.72 % to 6.84 %. More details of the device performance are described in Table 2.

Jsc (mA/cm2)

Voc (mV)

FF (%)

Eff (%)

20nm_20ohm/sq w/o 18.23 590.5 56.6 6.10 ZnO 20nm_20ohm/sq with 19.01 598.2 56.3 6.40 ZnO 20nm_36ohm/sq w/o 19.31 634.2 54.8 6.72 ZnO 20nm_36ohm/sq with 19.64 637.5 54.6 6.84 ZnO Table 2. The device performances of CZTS devices with hybrid TCEs before and after 20 nm ZnO layer deposition.


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After ZnO deposition, Jsc of the CZTS with the high density hybrid TCE is improved from 18.23 to 19.01 mA/cm2, and the average enhancement is 0.7 mA/cm2. In case of the CZTS device with the low density hybrid TCE, Jsc is improved from 19.31 to 19.64 mA/cm2, and the average enhancement is 0.4mA/cm2. It is also notable that Voc was increased by 3 – 10 mV after ZnO deposition. Although it is not clear why Voc was enhanced after ZnO deposition, the trend is quite obvious. We assumed that Voc enhancement is mainly due to the passivation of edge area (scribed area) by ZnO layer as it also covers the edge of CZTS devices, but it is required to investigate further. FF of both devices with the hybrid TCEs is slightly deceased after ZnO deposition, which is believed due to the high resistivity of intrinsic ZnO. Finally, PCE of the CZTS device with the low density hybrid TCE is improved from 6.72 to 6.84 % after ZnO deposition.

4. Conclusion In this research, we proposed a hybrid AgNW network – ultra thin ITO as a transparent conductive electrode for pure sulphide Cu2ZnSnS4 solar cells as it demonstrates outstanding high transparency and conductivity. The hybrid AgNW network – ultra thin ITO electrode showed improved transmission in the entire wavelength range with similar Rsheet compared with the conventional electrode, 210 nm ITO. AgNW networks with two different densities ((i) high density (20 Ω/□) and (ii) low density (36 Ω/□)) on ITO layers were employed in CZTS devices, and their electrical and optical properties were investigated. We found that CZTS devices with the hybrid TCEs demonstrated better Jsc. The average Jsc of the devices with the reference ITO, the high density and the low density hybrid TCE are 18.5, 19.2 and 19.7 mA/cm2, respectively. It is clear that improved Jsc in devices with low density hybrid TCEs is mainly due to the higher spectral transmission than that with traditional ITO



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reference. In case of FF, the CZTS devices with reference ITO is similar to those with high density hybrid TCEs, showing similar FF value of 55 % as Rsheet of the electrodes are similar. In contrast, CZTS device with the low density hybrid TCE shows slight lower FF, which is 53.5 % on average, which is mainly due to less conductivee low density hybrid TCE. As a result, CZTS device with the low density hybrid TCE gives the highest PCE of 6.4 % on average and the highest PCE value of 6.72 %. The CZTS device with the highest PCE of 6.72 % is believed to be mainly due to Jsc improvement. In order to minimize the reflection of the the hydrid TCE, both computational simulations and experiments were performed. We found that 20nm ZnO shows outstanding ARC performance in the simulations. With applied 20 nm ZnO layer on the hybrid TCEs, the reflection of the electrodes was decreased by about 2 %. After ZnO deposition, the Jsc of the CZTS devices with both the high and low density hybrid TCE is improved from 18.23 to 19.01 mA/cm2, and 19.31 to 19.64 mA/cm2, respectably. The highest efficient CZTS device with the low density hybrid TCE is improved from 6.72 to 6.84 % upon the introduction of ZnO antireflection coating.

Acknowledgement This research has been financially supported by the Australian Government through the Australian Renewable Energy Agency (ARENA) and Australian Research Council (ARC). Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. We acknowledge the facilities, and the scientific and technical assistance of the Electron Microscope Unit (EMU) and Mark Wainwright Analytical Centre, the University of New South Wales (UNSW). References


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Fig. 1 Transmission spectra of Quartz, 20 nm ITO / Quartz, 210 nm ITO / Quartz and (solid lines), AgNW network on quartz and AgNW network on 20 nm ITO / Quartz (dash lines).



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Fig. 2 (a) Schematic structure of CZTS device with the hybrid electrode and morphology of AgNW networks (b) high density (20 Ω/□) Ag NW network / 20nm ITO (c) low density (36 Ω/□) Ag NW network / 20nm ITO.


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Fig. 3 J-V characteristics of CZTS devices with different types of electrodes. (The numbers of samples measured were between 10 – 15 for each device, sample labelling with IZO and 20nm ITO is the type of substrates, and numbered ohm/sq is Rsheet of the hybrid electrodes)



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Fig. 4 (a) Spectral transmission of the reference (from Figure 1) and the hybrid TCEs deposited on Quartz substrate (b) EQEs and (b) J-V curves of the reference, CZTS device with the high density (20nm ITO _20 ohm/sq) and the low density hybrid electrode (20nm ITO_36ohm/sq).


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Fig. 5 (a) Simulated reflection (Lumerical FDTD) of the hybrid TCE (AgNW network / 20nm ITO as a reference), the hybrid TCE with 50 nm MgF2 and the hybrid TCE with 20 nm ZnO; (b) Experimentally measured reflection of CZTS device with the hybrid TCE before and after ZnO deposition on top of AgNW network.



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Fig. 6 (a) EQEs of the CZTS devices with hybrid electrodes before and after ZnO deposition, (b) and J-V curves of the CZTS device which shows the highest efficiency before and after ZnO deposition.


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“TOC graph”

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Hybrid Ag Nanowire-ITO as Transparent Conductive Electrode for

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