Annealing effects on the solution processed CdTe nanocrystals solar cells

Physica E 60 (2014) 17–22

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Physica E journal homepage: www.elsevier.com/locate/physe

Annealing effects on the solution processed CdTe nanocrystals solar cells Yuping Gao, Jiaoyan Zhu, Yiyao Tian, Donghuan Qin n Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, China

H I G H L I G H T S

G R A P H I C A L


CdTe NCs solar cells with ITO/CdTe/ Al structure were prepared by a layer-by-layer solution process.
The PCE was found strongly related to the annealing (with CdCl2) temperature.
It was found that promising PCE could be obtained at around 3501 annealing temperature.

I-V curve of ITO/CdTe NCs/Al device with different annealing temperature.

art ic l e i nf o

a b s t r a c t

Article history: Received 13 December 2013 Received in revised form 16 January 2014 Accepted 21 January 2014 Available online 12 February 2014

CdTe nanocrystal (NC) solar cells with Schottky diode configuration of ITO/CdTe/Al structure were prepared by using a layer by layer solution process. The annealing effects on the performance of device was investigated and discussed. It was found that the device performance was strongly related to CdCl2 treatment and annealing temperature. UV–vis, AFM, SEM, EDS etc. were used to characterize the optical and structural properties of CdTe NCs active layer, while the J–V curve of CdTe NCs solar cells was measured using a Keithley 240 source measure unit. It was found that the open circuit voltage (Voc) of devices decreased almost linearly with annealing temperature while short circuit current (Jsc) was kept under a very stable value of about 11 mA/cm2. Device exhibited promising power conversion efficiency (PCE) of 4.09% in the case of 350 1C CdCl2 annealing for 40 s, which was a good efficiency for solution processed CdTe NCs solar cells. & 2014 Elsevier B.V. All rights reserved.

Keywords: Nanocrystals Solution process CdTe Solar cells

A B S T R A C T

1. Introduction Solution processed NCs solar cells had attracted more attention in the past several years as low cost solution processed fabrication combined with band-gap tunability of semiconductor NCs, which enabled optimal matching of the absorbance of solar cells to the solar spectrum and permits further efficiency improvements for multiple-junction devices [1–4]. In the case of PbS and PbSe NCs, the bandgap of NCs could be changed effective by altering their

n

Corresponding author. E-mail address: [email protected] (D. Qin).

http://dx.doi.org/10.1016/j.physe.2014.01.027 1386-9477 & 2014 Elsevier B.V. All rights reserved.

size, which provided efficient conversion of the sun's full energy including visible, near–IR, and short wavelength. To date, solar cells based on PbS and CIS NCs with promising power conversion efficiencies (PCEs) [5,6] had been reported and around 7% [7,8] PCEs had been obtained in the case of PbS–TiO2 heterojunction structure. However, device stability was still a big problem in such case as NCs suffered from oxidation when exposure to air [9]. Furthermore, the carrier mobility was low (below 10  2 cm2/V s) due to the high trap state densities under low processing temperature [10]. Comparing with these NCs, CdTe NCs was a low bandgap (Eg  1.45 eV) semiconductor materials. In order to achieve high efficiency devices, sintering strategies were often used to obtain excellent electronic properties of bulk inorganic semiconductors. During the sintering process, the

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size of CdTe NCs increased rapidly and more compact thin film would be obtained in such case, which promised low trap state and long time stability [11]. Solution processed CdTe NCs solar cells with promising PCEs were first time reported by Gur et al. in 2005, who fabricated CdTe/CdSe NCs hetero-junction with PCEs as high as 2.9% by employing a solution processing method and sintering strategy [12]. Yang et al. [13–15] also reported high performance solar cells based on CdTe NCs and polymer hybrid active layer. More recently, to decrease the stress developed during the thermal treatment step within a film of NCs, layer by layer sintering process was adapted and up to 5% PCEs was realized by Olson [16] and our research group [17], while CdTe– CdS heterojunction NCs solar cells was reported by our research group [18] and Sahoo group [19]. In our previous work, it was found that NCs morphology had great effect on the performance of as-prepared products [17]. In this paper, the annealing effects on the performance of CdTe NCs solar cell with Schottky architecture was further investigated and discussed in detail. It was found that CdTe NCs thin film showed more compact with large particle size after CdCl2 annealing. Relative high mobility was found in the CdTe NCs thin film after high temperature annealing with CdCl2 treatment. Suitable annealing strategy could largely improve device performance and promising PCEs were obtained in the case of 350 1C annealing for 40 s. Too high annealing temperature will make device performance dropdown, which may be due to the breakdown in some place of NCs thin film. A reasonable explanation on the performance changed in the CdTe NCs solar cells was given based on the experiment results. As low cost fabrication process could be developed for ink-jet printing technics, such kind CdTe NCs solar had potential application in low cost thin film solar cells.

2. Experimental Cadmium myristate (Cd(C14H27O2)2) was selected as cadmium precursor for the fabrication of CdTe NCs. High quality CdTe NCs with diameter about 5 nm and arm length about 12 nm was fabricated by using solvent hydrothermal method, as reported before [17]. The CdTe NCs were dispersed into a mixture of pyridine and 1-propanol with a volume ratio of 1:1 at about 40–50 mg mL  1 concentration. The resulting CdTe NCs solution was then filtered from a Whatman filter (PTFE, 0.45 μm) to eliminate any impurity and NCs aggregation. The CdTe NCs solar cells were fabricated by a layer-by-layer solution process. Firstly, one layer of the CdTe NCs was deposited onto the ITO substrate by spin-coating the CdTe NCs solution at 1000 rpm for 30 s. The ITO/CdTe was placed on a hot plate at 150 1C for 3 min to eliminate any organic solvent. Finally, the substrate was dipped into a saturated CdCl2 methanol solution for 1–2 s and taken out, rinsed with 1-PrOH and dried under a nitrogen stream. The sample was then sintered on a hot plate at different temperature ranging from 300 1C to 400 1C for different time (0–1 min). This process was repeated 4 times with a final CdTe thickness of about 400 nm. The aluminum contact with 100 nm thickness was deposited via thermal evaporation through a shadow mask, resulting to active area of 0.16 cm2.

3. Result and discussion In the case of CdTe/CdS heterojunction thin film solar cells, it had been found that the CdCl2 treatment at around 400 1C could offer many benefits to the solar cells. For example, the size of CdTe

Fig. 1. SEM image and EDS of CdTe NCs thin film (5 layers) prepared by solution processed, annealing at 350 1C for 40 s without CdCl2 treatment (a) SEM images and corresponding (b) EDS results; annealing at 350 1C for 40 s with CdCl2 treatment (c) SEM images and corresponding (d) EDS results.

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after CdCl2 treatment. The red-shift of absorption edge implied that the band gap of CdTe NCs thin film was decreased and showed good response in the red light region, which was promising for sun light harvesting. After annealing and CdCl2 treatment, the size of CdTe particle increased, which would eliminate quantum confined effect in NCs and the absorption properties of NCs showed similar to its bulk materials, so the absorption peak was red-shifted after annealing. On the contrary, blue-shift was found in the case of NCs size decreasing, which had been confirmed by many research groups [5]. The surface and cross-section morphology of CdTe NCs solar cells were further characterized by a high resolution SEM. The surface of CdTe NCs thin film showed compact and large particle

0

Current Density(mA/cm2)

particle would increase and result in trap state decrease, the active CdTe/CdS junction, stress release and so on. Shown in Fig. 1 are the SEM images and EDS of CdTe NCs thin film treated with or without CdCl2 coupled with 350 1C annealing temperature for 40 s. It was evident that the size of CdTe particle was small and the boundary was not identifiable in the case of without CdCl2 treatment, as shown in Fig. 1a. On the other hand, the size of CdTe particle increased rapidly after CdCl2 treatment (Fig. 1c). The average size of CdTe particle is up to 100 nm, almost 20 times increased comparing to its original size (about 5 nm). The energy dispersive spectroscopy (EDS) (shown in Fig. 1b and d taken from a large area of Fig. 1a and c) revealed strong cadmium, tellurium signals along with the Cl signal resulting from CdCl2 treatment. By comparing the relative areas under the peaks for Cd, Te and Cl, it was concluded that the atomic ratio of Cd to Te was 1.02:1, close to the stoichiometric ratio of CdTe NCs, which implied that almost no impurity existed after NCs washing process. The atom ratio of Cd to Te increased to 1.08:1 in the case of CdCl2 treatment samples and element Cl was also found which may be due to the diffusion of Cd and Cl element during the CdCl2 annealing process. In order to investigate CdCl2 annealing effect on the absorbance of CdTe NCs thin film, UV absorption was used to characterize several samples prepared by using different annealing strategies. As shown in Fig. 2, without CdCl2 treatment, samples with different annealing temperature had similar absorption edge of about 750 nm. On the other hand, the absorption edge was found red-shift to about 800 nm

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Fig. 2. Absorption spectrum of the CdTe NCs thin film prepared under different annealing temperature with or without (w/o) CdCl2 treatment.

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Temperature (oC) Fig. 4. J–V characteristics of the ITO/CdTe/Al solar cells (a) with different annealing time, (b) Voc and Jsc of device with different annealing temperatures.

Fig. 3. (a) High resolution SEM image of CdTe NCs thin film and (b) cross-section SEM image of ITO/CdTe/Al device implied CdTe active layer with 405 nm thickness (4 layers).

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size after CdCl2 treatment (Fig. 3a). The cross-section SEM view of device is shown in Fig. 3b. The active layer consisted of four layered CdTe NCs film prepared by layer by layer solution sintering process. The thickness of CdTe NCs layer was about 405 nm in the whole device, which implied that high quality and homogeneous CdTe NCs thin film could be prepared by this method. Each spincoating CdTe NCs layer was about 100 nm and one could control the thickness of active layer easily by controlling the number of the spin-coating layers. The ITO/CdTe/Al device with active layers of 500 nm thickness (five layers) were prepared on top of ITO by using a layer-by-layer

Table 1 Photovoltaic performances of solar cells fabricated under different conditions (under irradiation of AM1.5G at 667.4 W/m2). Photovoltaic performances of ITO/CdTe/Al solar cells prepared by using CdTe NCs under different treatment temperatures Heat treatment temperature ( 1C)

Voc (V)

Jsc (mA cm  2)

FF (%)

PCE (%)

300 350 360 370 380 400

0.50 0.46 0.46 0.42 0.40 0.38

10.43 12.19 11.46 12.62 10.88 12.89

51.40 49.41 48.01 47.20 47.11 46.73

3.96 4.09 3.74 3.69 3.03 3.38

sintering process. The as prepared CdTe NCs layers were then subjected to CdCl2 treatment at 300–400 1C for optimal time of 40 s in order to increase the particle size and improve carrier mobility. It should be pointed out that the CdCl2 treatment had many benefits to the solar cell devices, such as increased grain size, stress release and eliminating any organic impurity, which will improve the electrical properties of the CdTe solar cells and result in high PCE of the as-prepared devices. It was found that devices showed very low performance (PCE less than 1% in all devices) in the case of without CdCl2 treatment, so all the devices were carried out under CdCl2 treatment to increase the carrier mobility of CdTe NCs film. Fig. 4a presents the current density versus voltage (J–V) characteristics of the ITO/CdTe/Al solar cells with CdCl2 treatment at different temperature for 40 s, under 677.4 W m  2 (AM 1.5 G) illumination. Device parameters such as Jsc, Voc, FF and PCE were deduced from the J–V characteristics (summarized in Table 1). The Jsc was calibrated with EQE and we found that less than 5% deviation in our case. Moreover, inorganic solar cells were different from organic solar cells, there were no bimolecular in our case since high annealing temperature was adopted in our case, so no bimolecular recombination problem occurred in our case. We can see from Table 1 that all the devices had promising PCE up to 3% for different annealing temperature. A little increased in PCE was found when the annealing temperature increased from 300 1C to 350 1C, after that, the PCE decreased almost linearly with annealing temperature. The highest PCE of 4.09% was obtained in the case of 350 1C annealing

Fig. 5. AFM images of CdTe NCs thin film prepared at different annealing temperatures coupled with CdCl2 treatment: (a) 300 1C; (b) 350 1C; (c) 360 1C; (d) 370 1C; (e) 380 1C; and (f) 400 1C.

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Bias Voltage(V) Fig.6. The J–V characteristics of hole-only devices where CdTe NCs thin film were treated with/without CdCl2 at 350 1C with the thickness of the CdTe NCs layer reaching about 400 nm.

samples, which was conformed to our previous report [17]. Promising PCE could be obtained in the case of annealing at around 350 1C with CdCl2 treatment. The Jsc and Voc with different annealing temperatures are shown in Fig. 4b. A little increase in Jsc of device with annealing temperature was observed in this case and was kept at a high value  11.0 mA/cm2. On the other hand, the Voc of device decreased linearly with increasing annealing temperature. As the Fermi energy pinning, the Voc of solar cells device with Schottky diode configuration is about half bandgap of the semiconductor [20]. The bandgap of CdTe is about 1.45 V, so the Voc is around 0.7 V for CdTe Schottky diode solar cells. However, the highest Voc was only 0.5, only 70% of the idea value. We speculated that NCs defects and breakdown in some place of thin film during annealing process resulted in low Voc in this case. For solar cells, the power conversion efficiency was given by: PCE ¼ ðJ sc V oc FF=P in Þ  100%, where Pin is the incident light intensity (AM 1.5 G, 1000 W/m2) and FF is the fill factor. The main reason of PCE dropdown with annealing temperature up to 350 1C was the dropdown in Voc caused by over treatment of the samples. To characterize the morphology changes in the CdTe NCs thin film, samples annealing at different temperature coupled with CdCl2 treatment were investigated by AFM. As shown in Fig. 5, the particles size was found to be increased when the annealing temperature increased from 300 1C to 360 1C. The surface of samples was smooth and compact, which was important to obtain high carrier mobility. The RMS values for sample with different annealing temperatures (300–400 1C, Fig. 5, AFM image) were 7.277 nm, 6.678 nm, 7.699 nm, 9.016 nm, 9.551 nm, 9.431 nm. On the other hand, when annealing temperature was up to 370 1C, more particle aggregation was found and the surface of samples showed rougher. It was well known that the CdCl2 treatment at moderate temperature resulted in particle size increase and finally improves the electrical properties of the CdTe solar cells. Fig. 6 shows the J–V characteristics of the hole-only devices (device structure ITO/CdTe/Au) where CdTe NCs thin film were treated with/without CdCl2 at 350 1C with the thickness about 400 nm. We analyzed this data with simple Space Charge Limited Currents (SCLC) model where the mobility of CdTe NCs thin film treated with CdCl2 at 350 1C reached 3.65  10  5 cm2 V  1 s  1 and there is no exact mobility results in the case of CdTe thin film treated without CdCl2 at 350 1C. CdTe NCs in the thin film would aggregate and grow into large particle after CdCl2 annealing, which would eliminate surface defects and result in high carrier mobility. However, too high annealing temperature should be avoided as the particle aggregates large size and resulting inner stress increases, which would decrease the smoothness of the CdTe NCs thin film and more defects may be formed between CdTe

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Fig. 7. EQE spectrum for ITO/CdTe/Al device prepared at different conditions.

and Al electrode. Leakage current was likely to increase in this case resulting in low PCE, so PCE dropdown was found in the case of high annealing temperature (up to 370 1C). Fig. 7 shows the external quantum efficiency (EQE) spectrum of devices with different annealing temperature. Except for the 300 1C annealing sample, the maximum EQE value for the solar cell was up to 60% at around 600 nm. The EQE value for the solar cells could be higher than 40% in the wavelength range between 400 and 790 nm. The Jsc calculated from EQE were 14.65 mA/cm2, 17.09 mA/cm2, 16.12 mA/cm2, 17.68 mA/cm2, 15.34 mA/cm2 and 18.08 mA/cm2 respectively for CdTe NCs solar cells with different annealing temperatures (300 1C, 350 1C, 360 1C, 370 1C, 380 1C, 400 1C). The spectral response could reach 830 nm, corresponding to the band edge absorption of the CdTe NCs.

4. Conclusion In conclusion, CdTe NCs solar cells with Schottky diode configuration had been fabricated successfully by a simple solution sintering process. The morphology, absorption and J–V characterization of the as prepared samples with different annealing temperature were investigated and discussed. It was found that high efficiency  4% could be obtained in the case of  350 1C annealing temperature. The dropdown in Voc when annealing temperature increased was due to the particle aggregation and breakdown in the NCs thin film. For the low cost fabrication process, such devices had potential application in low cost solar cells.

Acknowledgments We gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 51073056 and 91333206), National Science Foundation for Distinguished Young Scholars of China (Grant no. 51225301) and SCUT Grant (No. 2013ZZ0016). References [1] C.C. Vidyasagar, Y. Arthoba Naik, T.G. Venkatesha, R. Viswanatha, Nano–Micro Lett. 4 (2) (2012) 73. [2] D.A.R. Barkhouse, A.G. Pattantyus-Abraham, L.L. evina, E.H. Sargent, ACS Nano 2 (11) (2008) 2356. [3] A.J. Nozik, Physica E 14 (2002) 115. [4] J.M. Luther, M. Law, M.C. Beard, Q. Song, M.O. Reese, R.J. Ellingson, A.J. Nozik, Nano Lett. 8 (10) (2008) 488. [5] W. Ma, J.M. Luther, H. Zheng, Y. Wu, A.P. Alivisatos, Nano Lett. 9 (4) (2009) 1699. [6] Z.H. Li, S.J. Kwon, Appl. Surf. Sci. 284 (2013) 379.

22

Y. Gao et al. / Physica E 60 (2014) 17–22

[7] A.G. Pattantyus-Abraham, I. Kramer, A.R. Barkhouse, X. Wang, G. Konstantatos, E.H. Sargent, ACS Nano 4 (6) (2010) 3374. [8] A.H. Ip, S.M. Thon, S. Hoogland, O. Voznyy, D. Zhitomirsky, R. Debnath, L. Levina, A. Amassian, E.H. Sargent, Nat. Nanotechnol. 7 (9) (2012) 577. [9] K.W. Johnston, A.G. Pattantyus-Abraham, J.P. Clifford, S.H. Myrskog, D.D. MacNeil, L. Levina, E.H. Sargent, Appl. Phys. Lett. 92 (15) (2008) 151115. [10] J.P. Clifford, G. Konstantatos, K.W. Johnston, S. Hoogland, L. Levina, E.H. Sargent, Nat. Nanotechnol. 4 (1) (2009) 40. [11] Brandon I. MacDonald, Alessandro Martucci, Sergey Rubanov, Scott E. Watkins, Paul Mulvaney, Acek J. Jasieniak, ACS Nano 6 (7) (2012) 5995. [12] I. Gur, N.A. Fromer, M.L. Geier, A.P. Alivisatos, Science 310 (5747) (2005) 462. [13] Z. Chen, H. Zhang, W. Yu, Z. Li, J. Hou, H. Wei, B. Yang, Adv. Energy Mater. 3 (2013) 433.

[14] Z. Chen, H. Zhang, X. Du, X. Cheng, X. Chen, Y. Jiang, B. Yang, Energy Environ. Sci. 6 (2013) 1597. [15] Z. Chen, H. Zhang, Z. Xing, J. Hou, J. Li, H. Wei, W. Tian, Bai Yang, Sol. Energy Mater. Sol. Cells 109 (2013) 254. [16] J.D. Olson, Y.W. Rodriguez, L.D. Yang, G.B. Alers, S.A. Carter, Appl. Phys. Lett. 96 (24) (2010) 242103. [17] S. Sun, H. Liu, Y. Gao, D. Qin, J. Chen, J. Mater. Chem. 22 (2012) 19207. [18] Y. Tian, Y. Zhang, Y. Lin, K. Gao, et al., J. Nanopart. Res. 15 (2013) 2053. [19] R.K. Katiyar, S. Sahoo, A.P.S. Gaur, A. Singh, G. Morell, R.S. Katiyar, J. Alloys Compd. 509 (2011) 10003. [20] J. Tang, E.H. Sargent, Adv. Mater. 23 (2011) 12.