Communication pubs.acs.org/crystal
Solution Processed Cu2CoSnS4 Thin Films for Photovoltaic Applications Banavoth Murali, M. Madhuri, and S. B. Krupanidhi* Materials Research Center, Indian Institute of Science, Bangalore, India, 560012 S Supporting Information *
ABSTRACT: Earth abundant alternative chalcopyrite Cu2CoSnS4 (CCTS) thin films were deposited by a facile sol−gel process onto larger substrates. Temperature dependence of the process control of deposition and desired phase formations was studied in detail. Films were analyzed for complete transformation from amorphous to polycrystalline, with textured structures for stannite phase, as reflected from the X-ray diffraction and with nearly stoichiometric compositions of Cu:Co:Sn:S = 2:0:1:0:1:0:4:0 from EDAX analysis. Morphological investigations revealed that the CCTS films with larger grains, on the order of its thickness, were synthesized at higher temperature of 500 °C. The optimal band gap for application in photovoltaics was estimated to be 1.4 eV. Devices with SLG/CCTS/Al geometry were fabricated for real time demonstration of photoconductivity under A.M 1.5 G solar and 1064 nm infrared laser illuminations. A photodetector showed one order current amplification from ∼1.9 × 10−6 A in the dark to 2.2 × 10−5 A and 9.8 × 10−6 A under A.M 1.5 G illumination and 50 mW cm−2 IR laser, respectively. Detector sensitivity, responsivity, external quantum efficiency, and gain were estimated as 4.2, 0.12 A/W, 14.74% and 14.77%, respectively, at 50 mW cm−2 laser illuminations. An ON and OFF ratio of 2.5 proved that CCTS can be considered as a potential absorber in low cost photovoltaics applications. films would suffer from the higher film roughness and uniformity over larger areas. Hence the thin films with low root-meansquare (RMS) roughness are desirable for good heterojunction formation, and therefore, we for the first time have deposited CCTS thin films by a simple sol−gel route with smooth surfaces and have obtained photocurrent amplification of one order, under both A.M 1.5 G and 1064 laser illuminations.
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nnual global energy consumption and its growing demand in the coming days prompts investigation into alternative sources of energy. Renewable solar energy seems to be the need of the hour in meeting the future 27 terawatt level power consumption by 2050. The utilization of eco-friendly and inexpensive materials are the current challenges for harvesting solar energy. Despite current technologies like CdTe (CT) and Cu(In, Ga)Se2 (CIGS) comparable in current to electricity production to silicon technology, the toxicity of cadmium and scarcity of indium and tellurium restricts widespread utilization. Chalcogenide thin film technology would therefore provide exceptional utilization in large-area module monolithic integrations benefiting from the low material consumption, owing to the direct band gap of the absorbers. Recent advancements in solar cell absorber material alternatives were focused on binary, ternary, and quaternary absorber materials like SnS,1 FeS2,2 Cu3BiS3,3,4 Cu2SnS3,5 and Cu2ZnSnS4.6,7 Hence the quest for alternative absorbers is currently a hot research area. In spite of alternative material availability, vacuum based thin film deposition would involve multinary components and incorporate secondary phases as impurities; hence the all-solution process for thin film solar cells is a major concern.8−10 Cu2CoSnS4, being an analogous alternative absorber11−14 with an optimal band gap in the range13 of 1.2 to 1.5 eV, is well suited for application in inorganic thin film photovoltaics. Most of the CCTS research has been focused on the nanostructures/quantum dots.15−17 Recently, Xiaoyan Zhang et al.18 have demonstrated the photoresponse behavior of spray coated CCTS thin films, which involves the synthesis of nano crystals and preparation of colloidal ink. The nano crystal based © XXXX American Chemical Society
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RESULTS AND DISCUSSION Figure 1 showed the X-ray diffraction pattern of CCTS thin films recorded on an X-pert Pro PANalytical X-ray diffractometer. All the Bragg reflections (Figure 1b) can be indexed to the standard high quality ICSD file 01-074-4111 of Cu2CoSnS4 (CCTS), with the lattice constants as a (Å) 5.4020 and b (Å) 5.4020. The obtained CCTS XRD pattern was compared with the standard ICSD 41-4115 of the SnO2 phase, to account for the oxide based impurities of SnO2 (Figure 1b). Figure 1c showed the unit cell of stannite CCTS. Samples annealed at temperatures 200, 300, 400, 450, and 500 °C are labeled as CCTS-A, CCTS-B, CCTS-C, CCTS-D, and CCTS-E, respectively. The broad hump and absence of peaks below annealing temperature 300 °C, in the case of CCTS-A and CCTS-B films, showed that the films were of amorphous nature. CCTS-C films showed the broad (112), (204), and (312) Bragg reflections, indicating the onset of crystallization in these films. Received: May 1, 2014 Revised: June 23, 2014
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Figure 1. X-ray diffraction pattern of CCTS thin films: (a) effect of annealing temperatures of the phase formation, (b) CCTS phase formation compared to standard ICSD19,20 data of Cu2CoSnS4 and SnO2 to account for impurities.
CCTS-D films showed well resolved (004), (204) and (006), (312) and (215) peaks indicating complete phase transformation occurring at 450 °C. The intensity of peaks was increased with annealing temperature at 500 °C, indicating an ordering in the CCTS-E films. Hence it can be concluded from the X-ray diffraction pattern (Figure 1) that the phase formation started at a temperature greater than 300 °C and complete stannite phase transformation of Cu2CoSnS4 was possible at 500 °C. Crystallinity was found to incorporate into the spin coated films at 400 °C and higher at 500 °C, which was also supported by the SEM morphological investigations that the grains became euhedral/faceted (Figure 2d) upon increasing annealing temperature.
Average crystallite size was calculated to be 18, 8, and 10 nm, respectively, for CCTS-C, CCTS-D, and CCTS-E films, using the Scherer relation.21 Texture coefficients22 (TChkl) were calculated to quantify the orientation of a particular plane. Bragg reflection (112) showed higher TChkl value than all other reflections, indicating the (112) orientation of the CCTS thin films. Moreover, the deviations of TC values for various planes could be attributed to the increase in the structural factor or induced lattice deformation.23 Strain and dislocation densities were estimated24 as 0.01 and 0.0025, respectively, indicating that the obtained films were devoid of strain. The samples annealed at temperatures 200, 300, 400, 450, and 500 °C are labeled as CCTS-A, CCTS-B, CCTS-C, CCTS-D, and CCTS-E, respectively, henceforth. Variation of TChkl and strain with the different Bragg reflections for CCTS-E are shown in SI Figure S1. Morphological investigations were carried out on the Karl Zeiss Gemini column FESEM. Morphology of as deposited CCTS films was examined at various temperatures. Figure 2a,b showed the columnar grains at lower temperatures up to 300 °C. Moreover the grains (CCTS-A to C) were loosely packed with porosity. Upon increasing the temperature to 450 °C, a drastic change in the morphology with densely packed grains (Figure 2c) was observed. At higher annealing temperatures of 500 °C, where phase pure CCTS films were synthesized, the columnar grains coalesced turning to nearly spherical grains (Figure 2d) with higher density than obtained at 450 °C. It is to be noted that the grains were the same size as the film thickness, which helped in better carrier transport; this will be explained later. This observation suggests that stannite CCTS sol−gel based films with compact densely packed grains would be formed only at 500 °C. CCTS-E films were characterized by transmission
Figure 2. Scanning electron micrographs of CCTS films annealed at (a) 300 °C, (b) 400 °C, (c) 450 °C, and (d) 500 °C. All the scale bars are 200 nm. B
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Figure 3. (a) HRTEM and (b) SAED patterns of CCTS-E thin films.
Figure 4. (a) Absorbance spectra and (b) plot of (αhυ)2 vs hυ showing the band gap of CCTS-E thin film.
photovoltaic energy conversion, the photodetection under A.M. 1.5 G solar simulator was demonstrated, the report a first of its kind in solution processed CCTS-E thin films. Moreover, the possibility of light detection in the infrared region was explored, employing a 1064 nm laser.
electron microscopy, further confirming the crystallinity of annealed films. Figure 3a,b showed HRTEM and selected area electron diffraction (SAED) pattern of CCTS-E films; the diffraction fringes indexed to the stannite phase of CCTS. The crystallite sizes obtained from the XRD were correlated to those obtained from the low magnification TEM images (SI Figure S3). Low magnification TEM images showed that the crystallite sizes were in accordance with the XRD results. SI Figure S4 showed the high resolution TEM images of CCTS-E, CCTS-D, and CCTS-C particles, respectively. All the crystallites of CCTS-E films showed lattice fringes characteristic d spacing of stannite phase. Comparatively few and very few lattice fringes of crystallites were observed in the case of CCTS-D and CCTS-C films, respectively, indicating lower crystallinity films at lower annealing temperatures, further supporting the XRD results. The optical absorbance recorded over the Hitachi 2900 U spectrophotometer is shown in Figure 4a. Band gap of CCTS-E film was estimated to be 1.4 eV from the linear portion of (αE)2 versus photon energy plot (Figure 4b), using the relation25 α = A(hυ − Eg)n/hυ), where A and h are the constants, hυ is the energy of the photon, n is considered to be 2 for directly allowed transitions, and Eg is the optical band gap. The obtained band gaps of CCTS-E thin films were in accordance with those reported in the literature.26 Since the band gap of CCTS is optimal for
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PHOTODETECTION Steps involved in the photodetector device fabrication are shown in SI Figure S5. Current−voltage (I−V) characteristics of the device in SLG/CCTS/Al (Figure 5a) configuration were measured at room temperature, in air, using a Keithley-2420 source measurement unit. An IR source (IR laser with 1064 nm wavelength) and solar A.M 1.5 lamp, with KG5 filtered lamp source, NREL calibrated Si reference cell (oriel P/N-91150 V) to 100 mW/cm2 (one sun calibration) was used to study the photoresponsivity of the 500 nm thick CCTS thin films. Figure 5b,c shows the photoconductivity measurements of CCTS-C and CCTS-D in the dark and A.M. 1.5 G sun illuminations, respectively. Photocurrents under solar illumination were 2 × 10−6 A and 6 × 10−6 A for CCTS-C and CCTS-D films, respectively. CCTS-E films showed lower dark currents and higher photocurrent of one order of magnitude, i.e., 2 × 10−5 A compared to CCTS-C and CCTS-D films. Hence, emphasis was laid to study the detailed transport properties of CCTS-E films. C
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Figure 5. (a) Schematic device geometry employed to study photodetection. Current−voltage characteristics of (b) CCTS-C thin films under A.M. 1.5 G solar lamp, (c) CCTS-D thin films under A.M. 1.5 G solar lamp, (d) CCTS-E thin films under A.M. 1.5 G solar lamp and 50 mW cm−2 intensity 1064 nm infrared laser.
Reversibility and stability are the characteristic properties to prove the detector performance. Three on and off cycles for CCTS-C (Figure 6a,b) and CCTS-D (Figure 6c,d) films were demonstrated under A.M. 1.5 G illuminations. The photoresponsive single cycle was fitted to the second order exponential growth and decay with the relations I(t) = Idark + A[exp (t/τ1)] + B[exp(t/τ2)] and I(t) = Idark + A[exp(−t/τ1)] + B[exp(−t/τ2)], respectively, where τ is the time constant, Idark the dark current, A,B the scaling constants, and t the time when the laser was switched on and off. The rise and decay curves followed second order exponential behavior with first order growth and decay constants higher, further confirming the poor crystallinity of CCTS-C and CCTS-D films. Few ON and OFF cycles were measured to analyze the detector steady state photoresponse of CCTS-E films at 300 K under A.M 1.5 G solar lamp (Figure 7a) and 1064 nm IR laser (Figure 7c) illuminations. Figure 7b,d shows the single responsive cycle fitted, where the solid curve represents the exponential fitting and open circles represent the experimental data obtained. Fitted exponential curves followed a second order growth and decay, respectively. The growth cycle lasted for 40 s upon saturation (Figure 5b), whereas upon turning off the solar lamp, the current decayed rapidly over 16 s in the case of faster first order constants, followed by slower second order constants which lasted for few tens of seconds. IR laser photodetection showed a similar trend as observed for solar lamp illuminations but with faster first order growth (1.97 s) and decay (1.56 s) constants followed by slower second order constants which lasted for few seconds. Hence higher crystallinity CCTS-E films with faster growth and decay constants and higher carrier mobility compared to CCTS-C and CCTS-D films are found suitable for optoelectronic devices. The possibility of photoexcitation, employing a high intensity infrared laser, with slightly lower energy than the band gap of the semiconductor was previously reported elsewhere.32
Dark and photocurrent voltage characteristics (Figure 5d) of CCTS thin films exhibited symmetric behavior on illuminations with a laser of 50 m Wcm−2 optical power and solar lamp, in the forward and reverse bias conditions. The current increased by an order of magnitude from 1.9 × 10−6 A in the dark at fixed bias of 2 V to 2.22 × 10−5 A upon solar lamp illumination. CCTS-E films showed lower dark currents and higher photocurrent by one order of magnitude compared to CCTS-C and CCTS-D films. A more or less similar result was demonstrated employing an IR laser source, in which the photocurrent was amplified to 9.87 × 10−6 A at 2 V with 50 mW cm−2 power illumination. Sensitivity estimated by using the relation27 Iλ/Idark, where Iλ = Ilight − Idark, was 4.19 and 10.84 for laser and lamp illuminations, respectively. Responsivity, which in general is the photocurrent generated per unit power of the incident light used on the photodetector effective area, was calculated from the relation28 Rλ = Iλ/Pλ × S, where Pλ is the intensity of the illumination, S is the effective area of the illumination, and Iλ is the photocurrent where Iλ = Ilight − Idark. Responsivity was 0.126 A W−1 at 50 mW cm−2 and 1.25 mA W−1 for solar illumination, respectively. External quantum efficiency,29 the number of electrons collected by the detector per incident photon, was estimated to be 14.74% from the relation Q.E= hcRλ/qλ where h is Planck’s constant, q the electron charge, c the velocity of the light, and λ the wavelength of the laser (1.64 μm IR) used for excitation. Internal gain30 G = R × 1.24/λη, was found to be 14.14.8%, where η is the external quantum efficiency (taken as 1 for calculations) and λ the excitation wavelength taken in micrometers. The grain boundary recombinations were anticipated to limit the high photocurrent in the CCTS films. Moreover, it is to be noted in MSM device geometry that, due to the Na diffusion from the grain boundaries,31 location along the path of the carrier collection would also limit the photocurrent. D
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Figure 6. Photoresponsive ON and OFF cycles of (a,b) CCTS-D and (c,d) CCTS-C films, respectively, under A.M. 1.5 G Solar illuminations. Open circles represent the experimental data and solid lines represent the exponentially fitted curves.
Figure 7. (a,c) Photoresponsive ON and OFF cycles. (b,d) Growth and decay constants fitting to one response cycle under solar A.M. 1.5 G lamp and IR laser illuminations, respectively. Solid lines represent the exponential fits and open circles represent the response cycle data.
knowledge in the case of CCTS sol−gel processed thin films, and several strategies are to be implemented in order to improve the efficiency of the photodetector. The ON and OFF ratio of 1.26 and 2.5 in the case of 50 mW cm−2 IR laser and A.M 1.5 G solar illuminations proves CCTS as a potential candidate in the forthcoming technological photovoltaic applications.
Moreover, the multiple photon excitations were also an expected phenomenon, to support the lower decay constants to growth constants, since the ratio of carriers entering the conduction band to those decaying from the intermediate defect states would be a minimum. The photodetection for SLG/CCTS/Al fabricated devices was reported here for the first time to the best of our E
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CONCLUSIONS CCTS thin films were synthesized by a facile spin-casting solution route. Effects of annealing temperatures showed that 500 °C temperature was desired for stannite phase formation. Morphological investigations showed the evolution from porous columnar grains at lower annealing temperatures to densely packed spherical grains at higher temperatures. High absorption coefficient and optimal band gap of 1.4 eV were used to study the photodetection. The photodetector was demonstrated in SLG/CCTS/Al configuration by both A.M. 1.5 G solar lamp and 1064 nm IR laser illuminations. Photocurrent amplification and ON and OFF ratio of 2.5 demonstrate the utilization of CCTS as a potential absorber in photovoltaics and infrared photodetectors.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
* Supporting Information S
Experimental details: synthesis, characterization, thin film deposition, device fabrication. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are thankful to the Council of Scientific and Industrial Research (CSIR), Department of Science and Technology (DST), Centre for Nano Science and Engineering (CENSE) India for the financial support. The author Dr. Murali is thankful for IISc-Research Associate Fellowship.
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