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Self-assembled nanometer-scale ZnS structure at the CZTS/ZnCdS hetero-interface for high efficiency wide bandgap Cu2ZnSnS4 solar cells Kaiwen Sun, Jialiang Huang, Chang Yan, Aobo Pu, Fangyang Liu, Heng Sun, Xu Liu, Zhao Fang, John A. Stride, Martin Green, and Xiaojing Hao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00009 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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Chemistry of Materials

Self-assembled nanometer-scale ZnS structure at the CZTS/ZnCdS hetero-interface for high efficiency wide bandgap Cu2ZnSnS4 solar cells Kaiwen Sun+ a, Jialiang Huang+ a, Chang Yan a, Aobo Pu a, Fangyang Liu a, Heng Sun a , Xu Liu a, Zhao Fang b, John A. Stride c, Martin Green a and Xiaojing Hao* a + These authors contribute equally to this work. E-mail: [email protected] a

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia b School of Metallurgical Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China c School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia Abstract Despite remarkable progress in the performance of kesterite Cu2ZnSnS4 (CZTS) based photovoltaic technology has been achieved, the interface recombination and associated open-circuit voltage (Voc) deficit still dominate the loss mechanism in this technology. To alleviate heterojunction interface recombination in pure sulfide thin film solar cells, passivation structure at the interface is required. In this work, we developed an ultrathin nanometer-scale ZnS dielectric passivation layer which is readily formed in situ at the CZTS/ZnCdS hetero-interface during the ZnCdS buffer deposition process via Zn diffusion from the ZnCdS bulk to the interface. With this nanoscale structure, a remarkable open circuit voltage and fill factor improvement is illustrated, and a total area efficiency of 9.25% is obtained. The formation and features of the nanoscale ZnS layer is investigated by high resolution scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS). This self-assembled ZnS layer with dielectric properties passivates defects at the interface while still enabling the electrons to transport across the buffer layer

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because of the ultrathin thickness, which satisfies the requirement of dielectric passivation layer but requiring no complicated regular patterning. The correlation between the effects of passivation and device performance is investigated by device simulation, presenting a reasonable understanding of the experimental results. The results open a new aspect to passivate the interface recombination and expand the potential of upscaling CZTS technology. Introduction Thin-film absorber materials consisting of earth abundant elements having low toxicity are a promising approach to fulfilling the increasing global demand for clean energy. Absorbers made of kesterite, Cu2ZnSn(S,Se)4 (CZTSSe) have been considered as an attractive class of photovoltaic (PV) materials for low-cost and scalable next generation PVs, having excellent optical and electronic properties. Considerable progress has been achieved during the last few years1-6 and the record efficiencies of 11% for pure sulfide CZTS7 and 12.6% for Se-containing CZTSSe8 solar cells have been obtained, further reinforcing the substantial promise of kesterite photovoltaic devices in future commercial applications. However, these efficiencies remain far below their theoretical limits and the well-developed Cu(In,Ga)Se2 (CIGS) counterparts. In contrast to CIGS solar cells, CZTS devices suffer from a low open circuit voltage when compared to the band gap of the absorber (i.e., a large Voc deficit). Defects and disorder in the bulk CZTS absorber result in local fluctuations of the band gap and/or the band gap edges are believed to be significant contributions to the Voc deficit.9-11 In addition, interfacial recombination especially at the


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absorber/buffer interface is another important factor that limits the Voc.12, 13 Alternative buffer layers have been proposed to alleviate the interface recombination and reduce the Voc deficit, believed to be due to more favorable band alignment between the absorber and buffer.12, 14-16 Besides, Eliminating the recombination defect centres clustered at the hetero-interface is another way to mitigate interface recombination. Based on previous simulation studies, the Voc of CZTS devices with a defect-free interface could reach beyond 900 mV, which will lead to remarkable improvement of the device performance.17, 18 Therefore searching for an effective approach to integrate band-alignment amendments and elimination of interface defects is essential to completely eradicate interface-related Voc bottleneck.

One effective route to controlling interface recombination is the introduction of a passivating layer at the heterojunction interfaces. Wide-bandgap dielectric material is a good option for passivation because they not only passivate the charged point defects, but suppress recombination through surface states. Previous studies have reported the passivation effect of dielectric layers either formed during processing, or purposely deposited. For example, the role of sodium incorporation into CZTS films is reported to relate to the formation of wide-bandgap oxides with insulating properties at the interface; this can passivate the CZTS surface and grain boundaries.19, 20

Improvement in device performance has been found to arise from heat treatment

(air annealing) of the obtained bare absorber prior to buffer deposition; this is also proposed to correlate to the formation of wide-bandgap tin oxide upon annealing, which effectively suppresses the electron-hole pair recombination at the surface and



grain boundaries.21-24 Additionally, ideal dielectric materials like Al2O3, TiO2 and SiO2, which are widely used in the rear surface of Si solar cell technologies and commonly known as passivated emitter, rear locally diffused (PERL) cells, have also been applied to the top and bottom surface passivation in CZTS devices.24-28 It has been shown that a thin Al2O3 or TiO2 layer at the top hetero-interface, or a patterned thin dielectric layer at the bottom interface, is able to suppress the non-radiative recombination, thereby enhancing the Voc and Jsc. However, in order to avoid the high resistivity and associated high series resistance in the final devices, the dielectric layer should be precisely controlled rather thin (less than 5 nm) so that the photo-excited electrons can tunnel through. Alternatively patterned nanosized openings, with optimal diameters of usually less than half of the carrier diffusion length, can enable lateral transportation of electrons to the buffer layer. Considering the much shorter diffusion length of CZTS compared to CIGS and thin film deposition techniques, methods of generating well controlled ultra-thin layers or nanosized openings appropriate for upscaling are difficult to realize. Developing an in-situ growth technology of the interface passivation layer is desirable for the upscaling of CZTS to industrial production. ZnS is a good dielectric material, with a resistivity of 1010-1012 Ωcm 29 and similar crystal structure and lattice parameters to CZTS. It has been reported that precipitation of ZnS as a secondary phase along CZTS grain boundaries in the CZTS matrix can passivate the grain boundary and reduce the recombination velocities.30 Fu et al. showed the effective passivation role of ZnS at the heterojunction of a CIGS


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device.31, 32 In their work, a thin ZnS nanodot layer of around 10 nm thickness and 5 nm average distance was placed between CIGS and the In2S3 buffer layer, not only serving as the passivation layer, but also enabling the lateral diffusion of charge carriers to the In2S3 contact bridge, which forms the well-known point-contact-like structure at the interface. However, the deposition technology used to obtain the ZnS nanodots was spray-ion layer gas reaction (spray-ILGAR), not a common technique in industrial production methods, whilst control of the nanodot size and spacing would lead to additional costs in any future applications. In this contribution, we show that a self-assembled nanometer-scale ZnS layer can form at the CZTS/ZnCdS hetero-interface during the ZnCdS deposition process via the Zn diffusion from the ZnCdS bulk to the interface. The desirable features of this dielectric layer were studied by high resolution scanning transmission electron microscopy (STEM) and the passivation effect supported by photoluminescence (PL) measurements. A remarkable Voc and fill factor (FF) enhancement was demonstrated due to the passivation layer and the correlation between them investigated by device simulation. Experiment Film synthesis: A magnetron sputtering system (AJA International, Inc., model ATC-2200) was utilized to deposit CZTS precursors (co-sputtering) on Mo-coated soda lime glass substrate and then the precursor were subjected to sulfurization using Rapid Thermal Processor (AS-One 100) within a combined sulfur and SnS atmosphere at 560℃ for several minutes. The composition of the precursor was



controlled to be different in Zn content i.e., Cu/Sn ratio is kept at around 1.8, while Zn/Sn ratio is controlled to be 1.3, 1.2 and 1.1. Device fabrication: The conventional cadmium sulfide buffer layer was deposited by CBD method as reported elsewhere.33, 34 The fZnCdS buffer was prepared via SILAR method using ZnSO4 (0.05 mol L-1) and CdSO4 (0.01 mol L-1) mixed solution as the cation precursor solution and Na2S (0.1 mol L-1) as anion precursor solution. The obtained absorber was immersed into separate cation and anion precursor solutions for adsorption and reaction for 15 seconds, respectively, and then rinsed with DI water after each immersion to remove excess ion and avoid homogeneous precipitation. By repeating the SILAR cycle for 30 times, 70 nm thick ZnCdS layer can be obtained. Then, a heat treatment at 260 ℃ for 10 mins on hot plate in air atmosphere is applied to increase crystallinity of ZnCdS layer. After that, a thin i-ZnO (~50 nm) and ITO (~220 nm) layers were deposited using the AJA RF power sputtering. Evaporated Al patterns were applied as the top contact for the device. Finally, a ~100 nm MgF2 was evaporated as the Anti-reflection coating (ARC). The total area of the final cells is ~0.22 cm2 defined by mechanical scribing. Characterizations: The microstructure and elemental distribution across the CZTS and buffer interface were carefully examined using JEOL JEM-ARM200F (200 kV) aberration-corrected scanning transmission electron microscope (STEM) equipped with energy dispersive X-ray spectroscopy (EDAX) system. The TEM specimens were prepared using a FEI xT Nova NanoLab 200 FIB system, which combines a high resolution focused ion beam (FIB) and a high resolution field emission scanning


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electron microscope (FESEM). A thick Pt layer was deposited on top of the sampling area to protect the specimen from possible beam damage as well as provide good conductivity to avoid charging and overheating of the specimen during the specimen preparation process. The surface of the specimen was finished with a very low energy gallium ion beam of 5keV and ~20pA to ensure the best polishing of the finish specimen. Photoluminescence (PL) spectra were measured using a 1/4 m monochromator (CornerstoneTM 260) equipped with a silicon charge-coupled device (CCD) camera. The continuous wave (CW) laser (405 nm, 50 mW) was used as the excitation source and the luminescence was detected by the CCD. The J-V curves were performed using a solar simulator (Newport) with AM1.5G illumination (100 mW/cm2) calibrated with a standard Si reference. External quantum efﬁciency (EQE) measurements were conducted by utilizing a QEX10 spectral response system (PV measurements, Inc.) calibrated by the National Institute of Standards and Technology (NIST)-certified reference Si and Ge photodiodes. Device Simulation: Sentaurus software package of version L-2016.03 was used to simulate the CZTS device light current-voltage curves. A stack of ITO (200 nm), i-ZnO (50 nm), ZnCdS (50 nm) and CZTS (900 nm) was composed as the device structure. Electronic and optical parameters in all layers except buffer were extracted from our baseline CZTS model.18 For the new buffer layer, electron affinity of 3.79 eV was used to generate a spike of 0.11 eV relative to CZTS. Bandgap was set to 2.51 eV which was estimated from UV-Vis measurement.. A relative dielectric constant of 10 was used. Moreover the conduction & valence band density of states was set to be



the same as CdS, 2.2×1018 and 1.8×1019 cm-3 respectively. Electron & hole mobility were based on CdS. The varying parameters in our model are hetero-interface recombination velocity (105-103 cm/s) and buffer carrier concentration (1015-1013 cm-3). The carrier concentration range was chosen so simulated Voc fit within measured range. Results and Discussion Because the formation of the ZnS layer is highly likely dependent on the Zn content of the CZTS absorber, the composition of the absorber needs to be controlled at a reasonable range and Zn ratio is of specific focus in this work. Generally, Cu/Sn = 1.8 and Zn/Sn=1.2 is the standard absorber composition in our baseline process, therefore the Cu/Sn ratio was fixed at Cu/Sn ~ 1.8 for different groups of absorbers with Zn/Sn ratios at 3 different compositions, i.e. Zn/Sn = 1.3, 1.2, 1.1, respectively. The ZnCdS buffer layers were prepared by successive ionic layer absorption and reaction (SILAR) method and its composition is as per our optimized Zn0.35Cd0.65S recipe.15 In order to investigate the features of the CZTS/ZnCdS hetero-interface with and without the nanoscale ZnS dielectric passivation layer and their relationship with the absorber composition, high resolution scanning transmission electron microscopy (STEM) was used to examine the interfacial microstructure and chemistry.


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a)

b)



c)

Figure 1. Energy dispersive X-ray spectroscopy (EDS) mapping of the interface region (marked by red rectangle in the TEM images at the top left of each figure) from CZTS devices buffered with ZnCdS by using absorbers with different composition (a) Zn/Sn = 1.3, (b) Zn/Sn = 1.2, (c) Zn/Sn = 1.1. The distribution of Cu and Sn in all the three samples shows no distinct difference while the Zn and Cd distribution are obviously different.

The TEM images (top left figures of Figure 1a, 1b and 1c) illustrate that the interface of device with Zn/Sn = 1.1 absorber has denser and more compact morphology than those of the higher Zn loading absorbers. Following careful analysis of the elemental distribution at the interface of three different absorbers by EDS mapping (note that the mapping regions are only the areas labelled with red rectangle in the TEM images at the left of each figure) shows the element diffusion behaviour between the absorber and buffer. Distribution of Zn and Cd at the interface are very different from each other. For the interfaces of the Zn-rich absorbers, shown in Figures 1a and 1b, the Zn signals distribute homogeneously at the interface region and the bulk ZnCdS buffer region. No Zn precipitate is observed at these two localised regions. However, at the


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interface of absorber with lower Zn content, Zn signal shows a distinct sufficient content of width less than 5 nm (Figure 1c) at the immediate vicinity interface. Detailed enlarged mapping (see Supporting Information Figure S1a) of the interface region of Zn/Sn = 1.1 sample shows more obvious Zn superfluous layer at the interface. This means an ultrathin ZnS layer forms at the CZTS/ZnCdS hetero-interface, which can also be confirmed by the EDS line scan of the three different interfaces as shown in Figure S1 of Supporting Information.

Figure 2. a) Atomic resolution HAADF image taken at the interface region. b) The histogram of a raw of cations columns at the interface marked by the red rectangular in Fig. 2a. c) HAADF image of selected area in a), after removing the background by low pass filter mask and different colour is used to demonstrate different element columns according to their intensities.

In order to further verify the formation of the ZnS layer, atomic resolution HAADF-STEM image was taken at the CZTS/ZnCdS interface from lower Zn content absorber (Zn/Sn=1.1). The intensity of atomic columns in HAADF–STEM images is approximately proportional to the mean square atomic number (Z2) of the



constituent atoms.35, 36 So the intensity profile (Figure 2b) of the cations from top to bottom in the area as marked by the red rectangular in Figure 2a can clearly present the distribution of different elements. In the region corresponding to the CZTS bulk, ଶ Sn and Cu/Zn (Cu or Zn) columns are easy to be picked up since the value of ܼ(ௌ௡) is ଶ ଶ more than twice of the values of ܼ(஼௨) and ܼ(௓௡) . In the interface area direct contact

with the CZTS bulk, column of almost similar intensity (with slightly higher background compared to the CZTS bulk due to the interface effect) with Cu/Zn is observed. No column such as Cd whose intensity should be double that of Zn is detected in the immediate vicinity, confirming the formation of the ZnS layer. To give a more clear illustration, Figure 2c presents another HAADF image of selected area in Figure 2a after removing the background by a low pass filter mask and using different colours to demonstrate different cation columns according to their intensity in Z-contrast HAADF image. The formation of this ZnS layer is supposed to originate from the ZnCdS deposition process because such ZnS intermediate layer cannot be detected either at the CdS/CZTS interface of a finished device or CZTS surface of a bare absorber whether being heated or not (as evidenced in Supporting Information Figure S2) when employing CZTS with reasonable compositions as we discussed in this work. The fact that ZnS layer is more likely to form at the interface of absorber with lower Zn content might because the surface chemistry of Zn poorer sites change the initial growth of ZnCdS where the formation of ZnS is easier than ZnCdS, or Zn in the ZnCdS layer is more likely to diffuse to the poorer Zn sites which is the most Zn poor


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sample (i.e. “Zn/Sn = 1.1”) in this work especially when applying heat treatment after the ZnCdS deposition. Another evidence supporting this assumption is that the Cd distribution is dependent on the Zn content (of CZTS absorber). As shown in Figure 1, Cd gradient region can be observed in the upper region of the ZnCdS layer of all the three samples and it gets more evident when the Zn ratio of CZTS absorber becomes lower. This observation explains indirectly how this ZnS layer is originated from the ZnCdS formation process via Zn diffusion from ZnCdS bulk to the interface, leaving a Cd gradient region behind in the upper part of ZnCdS layers. The diffusion becomes more likely when the Zn content in the CZTS absorber decreases, thereby an obvious ZnS nanometer-scale layer formed at the interface of the “Zn/Sn = 1.1” sample. Although an epitaxial growth of CdS on the CZTS absorber has been observed and reported in our previous work,37 the lattice mismatch between the two heterojunction materials is quite large (~7%).38 In contrast, ZnS has similar crystal structure and lattice parameters with CZTS, showing only ~0.33% mismatch.30, 39 The rather small lattice mismatch will improve the epitaxial growth from CZTS to ZnCdS, which is evident as presented in Figure 2a. Better epitaxial growth will help decrease interface defect density and associated interface recombination. In addition, this ZnS layer can also act as a passivation layer because of its wide bandgap insulating property. This self-assembled nanometer-scale ZnS layer observed in our present study shows similar structure with the nanodot ZnS layer used for passivating the CIGS hetero-interface reported by Fu et al.32 , but with much thinner scale. Moreover, this nanoscale ZnS layer is formed in situ during the ZnCdS buffer deposition process just



by controlling the composition of the absorber, showing the scalable potential in case such passivation layers would become tempting for industrial application. The obtained layer is sufficiently thin (less than 5 nm) to permit the tunneling of photo-excited electrons, enabling the transport of electrons into the buffer layer. All these features satisfy the requirement for a dielectric passivation layer while requiring no regular patterning which would add additional complexity to processing and result in higher cost and difficulty. Hence, this ZnS layer formed at the interface can not only reduce the interface defect density because of its coherent crystal structure and lattice parameter with CZTS, but also passivate the charge carrier recombination due to its dielectric property.

Figure 3. Photoluminescence (PL) spectra of CZTS devices applying ZnCdS buffer with and without ZnS nanoscale layer.

Steady state photoluminescence (PL) measurements were performed on the finished devices to further study the passivation effect of the ZnS layer. The PL spectra of the devices are overlaid on the plot in Figure 3. Interestingly, we observed a shoulder to


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the peak, or wider peaks in those spectra in relation to the extent of structural disorder or bulk defects in the absorber. Here we mainly focus on the trend of PL intensity, as it is more related to the radiative versus non-radiative recombination difference. It is indicative that device with the self-assembled ZnS passivation layer has the highest overall PL intensity, approximately by a factor of 2, implying the significant passivation effect with respect to non-radiative recombination from the ZnS layer at the CZTS surface. Moreover, in order to exclude the influence of the CZTS composition (Zn/Sn ratio in this case) on the passivation effect, we compared the PL spectra of CZTS samples of different Zn/Sn ratios with the standard CdS buffer layer (see Figure S3 in Supporting Information). It is obvious that sample with composition of Zn/Sn=1.1 demonstrates lower PL intensity than that of sample with composition of Zn/Sn=1.3, which, however, are both lower than those with ZnCdS buffer. Additionally, the most distinguished difference in PL intensity between CdS and ZnCdS buffer is in the case of CZTS with Zn/Sn=1.1. This indicates that the passivation effect mainly results from the ZnS nanoscale layer. The formation of the self-assembled nanoscale ZnS interface layer greatly improves the associated device performance. The current density-voltage (J-V) curves of the typical CZTS solar cells applying ZnCdS buffer layer with or without this ZnS passivation layers are shown in Figure 4a, accompanied with device electrical parameters presented in Table 1 (statistical device results are presented in Supporting Information Table S1). It is observed that devices from absorbers of medium-range Zn/Sn ratios (i.e. Zn/Sn = 1.2) demonstrates a similar Voc with that of device from



higher Zn content absorbers, but shows higher current density (Jsc) and fill factor (FF), so that the efficiency increases substantially from 7.58% to 8.28%. This is in agreement with the improved interface morphology of the lower Zn content absorber as we discussed in Figure 1. The porous and loose structure of the interface from higher Zn content hinders the carrier transportation of the device to some extent, thereby lowering the FF and Jsc. The improved Jsc can be further confirmed from the external quantum efficiency (EQE) in Figure 4b, showing overall enhancement and demonstrating better carrier transportation. a)


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Figure 4. a) Current density-voltage (J-V), b) external quantum efficiency (EQE) curves of devices applying ZnCdS buffer with and without ZnS nanoscale passivation layer.

Table 1. Device characteristics of the CZTS solar cells applying ZnCdS buffer with and without ZnS nanoscale passivation layer. The series resistance under light (RS,L), ideality factor (A) and reverse saturation current (J0) are determined by the sites’ method.40

Zn/Sn ratio

ZnS layer

Voc (mV)

Jsc (mA cm-2)

FF (%)

Efficiency (%)

RS,L (Ω cm2)

1.3 1.2 1.1

NO NO YES

711.89 711.87 735.25

18.42 19.56 20.11

57.77 59.47 62.53

7.58 8.28 9.25

0.76 0.75 0.51

A

J0 (mA cm-2)

2.96 2.88 2.31

1.42×10-3 1.14×10-3 3.68×10-4

Devices fabricated with ZnS passivation layer at the hetero-interface (Zn/Sn = 1.1), show higher Voc and FF, reaching 735.25 mV and 62.53%, respectively. The obvious enhancement in Voc is mainly attributed to the reduced ideality factor and saturation current in the device as shown in Table 1. This is a strong indication that the recombination at the interface has been greatly reduced due to the aforementioned self-assembled nanoscale ZnS passivation layer. With this ZnS layer, excess charge carriers are kept away from contact between ZnCdS and CZTS at which recombination rate should be theoretically high if no passivation. Therefore, the absorber surface is passivated by this dielectric layer, which leads to the reduction of the detrimental charge carrier recombination at the p-n junction – one of the most important positions for performance loss in CZTS solar cells. Consequently, the Voc and the FF increase as well as the cell efficiency. Moreover, the Jsc of 20.11 mA cm-2 is also the highest among all the three devices, which can be further analysed by EQE. As shown in Figure 4b, first of all, the band gap of absorber with Zn/Sn = 1.1 composition is about 0.03 eV smaller than that of the other two samples, which will contribute to the long wavelength improvement and around 0.2 mA/cm2 Jsc increase ACS Paragon Plus Environment


according to our calculation. Moreover, the EQE of device with ZnS nanoscale passivation layer is improved at all wavelengths. The EQE improvement in the region around the peak response from 500 nm to 650 nm indicates an improvement in the collection length of photo-generated carriers, likely due to passivation of the CZTS surface by the ZnS layer. Finally, 9.25% total area efficiency is obtained, surpassing our previous 8.56% efficiency (9.2% active area efficiency). 15 To further corroborate the relationship between the ZnS nanoscale passivation layer and observed device electronic performance, we conducted a simulation study using Sentaurus TCAD with detail specified in experimental section. A standard set of parameter data, based on our previous baseline CZTS simulation,18 was used to construct the computational model, except parameters input for the buffer layer. A positive spike band-alignment and an enlarged bandgap were implemented in the new ZnCdS layer to conform to XPS and optical measurements results in our reported ZnCdS based device.18 (The simulation results will be detailed elsewhere). The passivation effect of ZnS layer was simulated by tuning the interface recombination velocity. Another variable parameter is the buffer carrier concentration (doping). According to literature,41-43 incorporation of Zn in the buffer layer leads to a degraded conductivity and reduction of carrier concentration. Therefore a range of smaller carrier concentration values, compared to 1×1018 cm-3 in CdS layer, was used to adapt for literature results.


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Figure 5. a) simulated Voc, b) simulated Jsc as a function of hetero-interface recombination velocity and buffer doping from Zn0.35Cd0.65S buffer device model. At a selected buffer concentration, Voc of devices with different passivation levels were marked as different-coloured stars.

The simulated device Voc contour is shown in Figure 5a as a function of interface recombination velocity and buffer carrier concentration. It is apparent that introduction of interface passivation (reducing interface velocity) could help boost device Voc, even in the condition of a benign band alignment (small spike). The Voc range of 710-736 mV measured from light J-V is marked in contour as thick-lines with labels. For devices without ZnS nanoscale passivation layer i.e. absorber recipes of medium to high Zn/Sn ratio, 1.2-1.3, the measured Voc could be matched with a peak interface recombination velocity of 105 cm/s assuming largest possible carrier concentration in buffer. Such a high interface recombination velocity can be attributed to the fact that ZnS layers is barely assembled in the hetero-interface hence defects and recombination sites in its proximity are left intact. By reducing the interface recombination velocity by about 1 order of magnitude we can find a matched Voc for device with low Zn/Sn (1.1) composition ratio. This observation is in good agreement with aforementioned experimental results as formation of ZnS passivation layer is consolidated at low Zn/Sn ratio. The simulated device Jsc contour is exhibited in



Figure 5b. It is notable that substantial improvement in Jsc can be observed for lower interface velocity and better passivation mainly due to higher collection probability of carriers generated from photons absorbed in close proximity to hetero-interface region. The fact that improvement of simulated electronic performances correlating well with reduction of interface recombination further confirms the existence of the ZnS passivation layer and the positive role it plays. Apart from interface recombination, simulation data indicated that doping of the buffer can also enhance device performance. We will pay more efforts in this direction later to realize the doping of ZnCdS buffer for high efficiency CZTS solar cells. Conclusion In conclusion, we develop a new strategy to generate a nanometer-scale ZnS passivation layer at the CZTS/ZnCdS hetero-interface. By simply controlling the CZTS absorber composition with lower Zn content and utilizing ZnCdS buffer, a desirable ultrathin self-assembled ZnS dielectric layer formed in situ at the interface during the buffer deposition process via element Zn diffusion from ZnCdS. This nanoscale dielectric layer can passivate the charge carrier at the interface, thereby reducing the non-radiative interface recombination. The passivated device shows lower ideality factor and saturation current, which directly correlates to the improved Voc and FF, demonstrating total area efficiency of 9.25%. PL measurement and device simulation are conducted to further confirm the passivation effect of the ZnS layer and understand the correlation between the passivation layer and device performance. Supporting Information


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EDS mapping and line scan of the interface region, Photoluminescence (PL) spectra and statistical device characteristic. Acknowledgements This contribution has been financially supported by the Australian Government through the Australian Renewable Energy Agency (ARENA), Australian Research Council (ARC), Baosteel-Australia Joint Research and Development Centre (Project BAJC 13051) and Guodian New Energy Technology Research Institute. 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). The authors appreciate the use of facilities and the assistance of David Mitchell and Gilberto Casillas Garcia at the University of Wollongong (UOW) Electron Microscopy Centre.

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Table of Contents Self-assembled nanometer-scale ZnS structure at the CZTS/ZnCdS hetero-interface for high efficiency wide bandgap Cu2ZnSnS4 solar cells Kaiwen Sun+ a, Jialiang Huang+ a, Chang Yan a, Aobo Pu a, Fangyang Liu a, Heng Sun a , Xu Liu a, Zhao Fang b, John A. Stride c, Martin Green a and Xiaojing Hao* a + These authors contribute equally to this work. E-mail: [email protected] a

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia b School of Metallurgical Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China c School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia


Self-assembled nanometer-scale ZnS structure at ... - ACS Publications

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