Electrical characterization of grain boundaries of ...

Materials Research Bulletin 70 (2015) 373–378

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Electrical characterization of grain boundaries of CZTS thin films using conductive atomic force microscopy techniques N. Muhunthan a , Om Pal Singh a , Vijaykumar Toutam b, * , V.N. Singh a, * a b

Compound Semiconductor Solar Cell, Physics of Energy Harvesting Division, New Delhi 110012, India Quantum Phenomena and Applications Division, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 20 September 2014 Accepted 2 May 2015 Available online 5 May 2015

Electrical characterization of grain boundaries (GB) of Cu-deficient CZTS (Copper Zinc Tin Sulfide) thin films was done using atomic force microscopic (AFM) techniques like Conductive atomic force microscopy (CAFM), Kelvin probe force microscopy (KPFM) and scanning capacitance microscopy (SCM). Absorbance spectroscopy was done for optical band gap calculations and Raman, XRD and EDS for structural and compositional characterization. Hall measurements were done for estimation of carrier mobility. CAFM and KPFM measurements showed that the currents flow mainly through grain boundaries (GB) rather than grain interiors. SCM results showed that charge separation mainly occurs at the interface of grain and grain boundaries and not all along the grain boundaries. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Thin films Sputtering Atomic force microscopy Surface properties

1. Introduction CZTS is a potential candidate to replace Cu(In,Ga)Se2 (CIGS) and CdTe for large scale production of solar cells due to its abundance and non toxicity. Hence, CZTS thin film based solar cells have received great attention in recent years [1–5]. A band gap between 1.4 and 1.5 eV [1–3] and a band edge absorption coefficient of above 104 cm1 makes it highly attractive as a single junction solar cell material. Efficiencies up to 8.4% and 12.6% have been achieved for CZTS [4] and Cu2ZnSn(S,Se)4 (CZTS,Se) [5] solar cells, respectively. Electronic transport through interfaces in a device structure has always been a challenge for thin film technology to minimize the loss and enhance the efficiency. Experimentally it was proven that defects and impurities which segregate at the grain boundaries (GBs) in a polycrystalline electronic materials dictates electrical behavior of the device by creating localized states and serve as charge carrier traps [6]. This increases the recombination of photo generated e–h pairs during electronic transport in solar cells. But Jiang et al. have proposed that the GBs in CIGS enhance minority carrier collection and provide a current pathway for these charge carriers to reach the n-type CdS and get collected in CIGS/CdS based solar cell structure [7]. Cu vacancies which are mostly present across GBs favors band bending suitable for e–h pair separation and creates a barrier for hole transport between

* Corresponding author. Tel.: +91 1145608562; fax: +91 1145609310. E-mail addresses: [email protected] (V. Toutam), [email protected], [email protected] (V.N. Singh). http://dx.doi.org/10.1016/j.materresbull.2015.05.002 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.

adjacent grains [8]. As a result, the minority carriers in the bulk of the grains are transported to the GBs and reach n-type material with decreased recombination [9–11]. Also it is found that soda lime glass (SLG) substrate extrinsically passivates the GBs due to the presence of Na+ and O2 ions [12–14]. In the case of CIGS and CdTe thin film solar cells, the best efficiency reported is 21.7% [15] and 20.4% [16], respectively. It has been established that local electronic properties of CIGS and CdTe in the vicinity of GBs affect the device performance and thereby efficiency of solar cells [6,17]. Since the efficiency of CZTS based solar cell is much less compared to the CIGS and CdTe based solar cells, it is very important to understand electron–hole transport characteristics for improving the solar cell performance. In this paper, we report our results on electrical characterization of GBs of CZTS films using AFM along with optical, structural and transport studies. Through microscopic imaging we show that the interface region in the vicinity of GBs of CZTS films gets electronically modified leading to charge collection through minority carriers. 2. Experiments 2.1. Film deposition To deposit CZTS thin films, a reactive co-sputtering process is adapted over two step process to overcome the problems of roughness, loss of Sn due to high temperatures of sulfurisation and formation of secondary or tertiary phases during sulfurization [18]. For CZTS film deposition, Cu was sputtered at a DC power of 38.5 W, simultaneously Sn and Zn were sputtered using RF power of 65 W,

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Fig. 1. (a) SEM image of CZTS thin film on SLG, (b) cross-sectional SEM image of ZnO/CZTS/Mo film on SLG, (c) GI-XRD pattern and (d) Raman spectrum of CZTS thin film deposited on SLG substrate.

and 100 W, respectively. All the three targets were reactively cosputtered in the presence of [H2S (15%) + Ar (85%)] gas. The deposited thickness of individual elements can be controlled by controlling the power. All the targets were either 99.99% or more pure. Substrate was kept at 120 C and was rotated at 10 rpm for uniform deposition. CZTS films of thickness 780 nm were deposited at a pressure of 5.2 103 mbar using above conditions for 1 h and was further confirmed by stylus profiling and cross sectional SEM imaging. Post deposition annealing in Ar atmosphere was carried out in a sliding horizontal tube furnace for 3 min at 500 C temperature. Prior to CZTS deposition, Mo was deposited as bottom electrode on SLG substrate at 100 C, using DC magnetron sputtering. The deposition was carried out in two-steps in Ar atmosphere to increase adhesion with SLG substrate and decrease the resistivity

of the film. During first step, deposition was carried out at high pressure and low power (a working pressure of 2.8 102 mbar and DC power at 30 W), while second deposition was carried out at low pressure and high power (working pressure of 1.8 102 mbar and DC power at 50 W). The thickness of Mo film was 1.2 mm. For SCM studies, ZnO film of 150 nm was deposited on CZTS/Mo/SLG system using RF power of 50 W and substrate temperature of 200 C. 2.2. Characterization Morphological study of thin film sample was carried out using scanning electron microscope (Zeiss EVO MA10, SEM). For compositional analysis, energy dispersive spectroscopic study was carried out using EDAX attached with the SEM. Glancing

Fig. 2. (a) The optical absorbance curve of CZTS film on SLG substrate and (b) Tauc’s plot of hn versus (ahn)2 for CZTS thin film. The estimated band gap is 1.47 eV.


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Fig. 3. (a), (b) and (c) C-AFM current mapping with bias voltage +100 mV, 100 mV and 0 mV, respectively, (d) a line profile of the topography and C-AFM signal corresponding to the marking A–B of Fig. 3(a), (b) and (c), respectively. The line scans display the current at 100 mV (green solid line), the current at 100 mV (red solid line) and the current at 0 mV (black solid line) of the Cu-deficient CZTS thin film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

incidence X-ray diffraction (GI-XRD) study was carried out using Rigaku Miniflex X-ray diffractometer with an incident angle of 1. Optical properties were studied using UV 1800 Shimadzu spectrophotometer. Raman study was carried out using Jobin Yvon T64000, triple monochromator Raman spectrometer with an excitation wavelength of 514.5 nm. Multimode AFM with Nanoscope V controller, (Veeco Ltd., USA) was used for all AFM studies. For current mapping, an extended Tunneling Atomic Force Microscopy (TUNA) module, Bruker AXS was used along with Pt coated AFM tips. For conductivity mapping, the current sensitivity of the amplifier was set to 1 nA/V and scanned at 100 mV and 100 mV of sample bias. For scanning Kelvin probe microscopy, Pt coated tip biased at ac-amplitude of 1 V and a frequency of 80 kHz was used in interleave potential mode scan with a lift height of 50 nm. Scanning capacitance microscopy was carried out on ZnO/ CZTS/Mo films using an SCM module with capacitor sensor frequency of 1 GHz. Samples are scanned in contact mode with an ac bias of amplitude 1 V at 90 kHz.

3. Results and discussion The SEM micrograph of the surface of the CZTS film is shown in Fig. 1(a). The film consists of homogeneous grains with uniform sizes having dense morphology without any voids. Fig. 1(b) shows a cross-sectional image of the film. The grains look very compact and tightly adhered to the Mo layer, which are beneficial for decreasing the minority carrier recombination. Compositional analysis of CZTS film detected by EDS shows the atomic ratio of [Zn]/[Sn] = 1.14, [Cu]/([Zn] + [Sn]) = 0.90, and [S]/[Cu + Zn + Sn] = 0.85. Thus, the CZTS thin films are almost stoichiometric but are Zn-rich and copper deficient, which are reported to have good optoelectronic properties suitable for solar cell applications [19–21]. Fig. 1(c) shows the GI-XRD pattern of CZTS thin film. It is clear from the XRD pattern that the CZTS thin film is having good crystallinity (low background and sharp peaks). The high intensity of (11 2) peak revealed that the growth is preferential. Other observed peaks correspond to (1 0 3), (2 2 0),

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Fig. 4. (a) AFM-topography, (b) KPFM image of the CZTS thin film, (c) a line profile of the topography and KPFM signal corresponding to the marking C–D of Fig. 4(a) and (b). Line profile analysis shows high potential across grain boundaries.

(3 1 2) and (2 2 4) planes of kesterite CZTS (JCPDS 26-0575). [22] Fig. 1(d) shows the Raman spectrum of CZTS film. Absence of Raman peaks due to Cu2SnS3, ZnS, and Cu2xS phases and prominence of peaks at 251, 288, 338 and 374 cm 1 corresponding to kesterite phase [19,21,22] confirms good quality of CZTS films desired for solar cell applications. The optical absorbance curve is shown in Fig. 2(a) CZTS film is having an optical absorption coefficient (a) higher than 3 104 cm1. The Tauc’s plot graph is shown in Fig. 2(b). The direct optical band gap of the CZTS film is 1.47 eV which is estimated from the intercept of the linear region of the plot (ahn)2 versus hn on x-axis at ahn = 0. This value is quite close to the theoretical optimal value of 1.50 eV for a single-junction solar cell [21,23]. Electrical transport measurements were carried out in four probe configuration using Hall effect measurement setup. Hall effect measurement was carried out at room temperature under a magnetic field intensity of 0.5 T and current of 20 mA. Hall effect measurements confirmed that the material is p-type with hole mobility of 12.91 cm2 V1 s1. The resistivity (s ) of the CZTS thin film was 1.89 101 V cm and the hole carrier concentration was 2.55 1018 cm3, which is consistent with literature values [20,24,25]. Thus, above optical and electrical characterizations indicate that

the CZTS film grown by reactive co-sputtering can be used as an absorber layer in thin film solar cells. C-AFM measurements were carried out on Cu-deficient CZTS thin films with Pt tip grounded and the bottom electrode biased at 100 mV (and 100 mV). Fig. 3(a) represents the conductivity mapping of CZTS under 100 mV bias. All along the grain boundaries the current is maximum forming network. When the bias is changed from +ve to –ve of 100 mV, the polarity of current flips as shown in Fig 3(b). This current being maximum at grain boundaries is a clear indication that there is no recombination of charge carriers at the GBs and the current is favored mostly due to minority charge carriers i.e., electrons, as there is no current observed through grain interiors. When no bias is applied then also the current is observed along the GBs as shown in Fig. 3(c) and this is due to photo induced current from the laser light. This polarity switching of current along the GBs and photocurrent under no bias unequivocally demonstrates that the minority charge carriers are contributing for the conduction and most importantly they are not undergoing recombination at the GBs [26,27], which is the most common with most of the materials. Fig 3(d) shows the current profile for both +ve sample bias (green) and –ve sample bias (red) along with zero bias, along the region


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Fig. 5. (a) AFM topography, and (b) dC/dV phase image under SCM mode of ZnO/CZTS thin film on Mo–coated SLG. (c) Profile analysis corresponding to the marking E–F across GBs from the dC/dV phase image. Presence of two different phases in the line profile analysis indicates the presence of different charge species across GBs and charge separation along it.

across the grain boundary marked as A ! B in the current mapping images. A very small current of