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Modified synthesis of FeS2 quantum dots for hybrid bulk-heterojunction solar cells
Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Modified synthesis of FeS2 quantum dots for hybrid bulk-heterojunction solar cells Ping Yu a, Shengchun Qu b, Caihong Jia a, Kong Liu b, Furui Tan a,n a b
Key Laboratory of Photovoltaic Materials, Department of Physics and Electronics, Henan University, Henan 475004, PR China Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 April 2015 Received in revised form 9 May 2015 Accepted 11 May 2015
Phase pure and well-dispersed FeS2 quantum dots (QDs) have been synthesized with a method combining hot-injection and solvothermal reaction at a relatively low temperature. Hybrid composites composed of organic semiconductor and FeS2 QDs effectively dissociates excitons at the donor–acceptor interface, which qualifies efficient bulk-heterojunction solar cells. It has been found that the photovoltaic performance depends greatly on different ways of ligands exchange. It shows that mercaptopropanoic acid (MPA) treated hybrid thin film performs better in solar cells efficiency than pyridine exchange treatment to as-synthesized FeS2 nanoparticles due to a suppressed current leakage. & 2015 Published by Elsevier B.V.
Keywords: Quantum dots Hybrid Solar cells Thin films
1. Introduction Pyrite iron disulfide (FeS2) has been researched extensively as a promising optic-electric semiconductor with strong light absorption and abundant elements storage in the earth. Pyrite FeS2 has a band gap value of 0.95 eV that is suitable for light absorption as an active material in solar cells [1,2]. For solar cells where FeS2 plays either as photosensitive active layer or electron acceptor in p-type organic matrix, an excellent dispersion of FeS2 nanoparticles in organic solvent is needed for thin film uniformity or organic– inorganic compatibility. Thus, synthesis of FeS2 nanocrystals or QDs with hot injection method has been commonly researched [3,4]. Usually, high temperature is needed to accelerate the nuclei and growth process after hot injection. Even though, several hours are still needed to obtain crystallized and well-dispersed FeS2 nanocrystals. Besides the hot-injection, Solvothermal synthesis of FeS2 nanocrystals has also been extensively researched because of the comparatively moderate reaction process [5,6]. However, the FeS2 particles obtained is usually too large to serve as electron acceptor in efficient hybrid solar cells requiring a large enough heterojunction area. Besides, large-sized nanocrystal usually dispersed poorly in organic solvent such as chlorobenzene or chloroform. To obtain well dispersed FeS2 QDs that is applicable in hybrid solar cells, here in this work we have modified the fabrication
n
Corresponding author. Tel.: þ 86 371 23880659. E-mail address: [email protected] (F. Tan).
process where a hot-injection precursor was first obtained and then solvothermal reaction was carried out at a low temperature. Compared to the traditional hot-injection method, our modified synthesis process is relatively moderate and much more easily controllable. The obtained phase pure FeS2 QDs show good dispersion in organic solvent and does well in accepting electrons from organic photovoltaic semiconductor, poly-(3-hexylthiophene) (P3HT). The notable photovoltaic performance demonstrates effectiveness of our method in preparing FeS2 quantum dots. Besides, the modified reaction process here is promising in the synthesis of other semiconductor nanocrystals with good dispersion that is required for thin film fabrication.
2. Experimental Specific synthesis of FeS2 QDs is as follows. 100 mg iron chloride was dissolved in 20 ml octadecylamine (ODA) at 150 1C under N2 flow. Then, a sulfur solution (containing 96 mg S, 5 ml ODA and 10 ml diphenyl ther) was injected after which the hot Fe–S precursor was transferred into a stainless steel with teflon liner and underwent the solvothermal reaction at 150 1C for 5 h. The precipitate was collected and purified by chlorobenzene/ethanol solvent/antisolvent for at least 3 times. Pyridine exchange to FeS2 QDs were performed by dissolving the QDs into 5 ml pyridine and stirring at 70 1C for 10 h and then precipitated by adding extra amount of hexane. Organic/inorganic hybrid solar cells were fabricated as follows. Poly(thiophene) (3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) was spin coated at 4000 rpm onto the precleaned
http://dx.doi.org/10.1016/j.matlet.2015.05.033 0167-577X/& 2015 Published by Elsevier B.V.
Please cite this article as: Yu P, et al. Modified synthesis of FeS2 quantum dots for hybrid bulk-heterojunction solar cells. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.033i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
P. Yu et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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ITO substrates (optimized thickness of about 40 nm). The organic/ inorganic hybrid thin film composed of P3HT and FeS2 QDs (optimized mass ratio of 1:2) was prepared by spin-coating the hybrid solution in a glove box in N2 atmosphere. MPA treated solar cells were fabricated by spin-coating MPA:methanol solvent (1:9 in volume ratio) on top and then washing with methanol for 2 times. Afterward, onto the hybrid film, a ZnO buffer layer of about 20 nm was prepared by spin-coating a ZnO methanol solution [7]. Finally a 100 nm Ag cathode was thermally deposited on top. The crystalline phase of FeS2 nanoparticles was characterized by X-ray diffraction (XRD) on a Rigaku D/max-gA X-ray Diffractometer. The morphology was given by a high-resolution transmission electron microscope (HR-TEM, JEM-2100). The absorption and photoluminescence (PL) spectrum were recorded from ultraviolet– visible spectrophotometer (Varian Cary 5000) and Varian Cary Eclipse fluorescence spectrophotometer, respectively. Raman scattering was collected on a Renishaw RW1000 confocal microscope. The current–voltage (I–V) measurements were performed on Keithley 2440 Source Meter unite.
3. Results and discussion The morphology of synthesized FeS2 QDs is shown in Fig. 1. Well crystallization and mono-dispersion of QDs are clearly demonstrated (Fig. 1(a) and (b)). The strong XRD signals also indicate the formation of well crystallized quantum dots with a pyrite FeS2 phase (JCPDS card no. 060710) (Fig. 1(c)). The Raman peaks at 340 cm 1 and
380 cm 1 further confirms the pure FeS2 QDs. (Fig. 1(d)). The above morphology and phase characterization indicates the success in obtaining FeS2 QDs with our synthesis method. The synthesized FeS2 QDs exhibits a broad absorption of light extending to the red and infrared region (Fig. 2(a)). The optical energy band gap is estimated to be about 1.1 eV, showing the quantum size effect of our FeS2 QDs compared to the bulk value of 0.95 eV. Compared to the pure P3HT, the P3HT:FeS2 hybrid exhibits enhanced absorption intensity, attributing to the introduction of FeS2 QDs (Fig. 2(b)). Photoluminescence (PL) measurement demonstrates a strong quenching of PL intensity in the hybrid compared to the P3HT sample (Fig. 2(c)), revealing efficient excitons splitting and electrons transfer at the heterojunction interface of P3HT and FeS2 QDs. Based on the dissociation of photogenerated excitons, the hybrid solar cells were fabricated and the device skeleton is shown in Fig. 3(a). A type-II energy level alignment is formed at the donor–acceptor (P3HT-FeS2) interface (Fig. 3(b)), insuring efficient charges transfer. Noticing that the surface ligand (ODA) should be removed to obtain an efficient solar cell, we deal with this problem in two different ways. One is that the ODA ligand is exchanged by pyridine before mixing FeS2 with P3HT, a method that is commonly used in organic/inorganic hybrid solar cells [8,9]. Another way is to wash the premixed P3HT:FeS2 hybrid thin film with MPA: methanol solvent and the long ODA molecule can be replaced with a short ligand [10,11]. In order to more accurately compare the influence of solvent treatment on photovoltaic performance, we have fabricated three batches of solar cells. The same two pieces of
Fig. 1. TEM images (a) (b), XRD phase (c) and Raman signals of FeS2 QDs.
Please cite this article as: Yu P, et al. Modified synthesis of FeS2 quantum dots for hybrid bulk-heterojunction solar cells. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.033i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
P. Yu et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Q5 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
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Fig. 2. Light absorption (a–b) and PL properties (c) of FeS2 QDs and the hybrids. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
solar cells were fabricated for each solvent treatment device in a batch. Thus, the performance parameters those are the most close to the average values were chosen and used here for each solar cell. Fig. 3(c) shows the I–V performance of the two solar cells under 100 mW cm 2 light illumination. The optimized thickness of P3HT:FeS2 hybrid film is the same 120 nm for the pyridine exchange and MPA treated solar cells. It can be seen from the I– V curves that the solar cell with pyridine exchange generates an average short current density (Jsc) of 2.4 mA cm 2, an open circuit voltage (Voc) of 0.59 V, a fill factor (FF) of 32% and a conversion efficiency of 0.45%. In comparison, the solar cell with MPA treatment show a Jsc value of 3.1 mA cm 2, a Voc value of 0.58 V,
an FF value of 34% and an efficiency of 0.61%. The obviously enhanced Jsc value in MPA treated device contributes to the efficiency improvement. The I–V properties in dark were measured to evaluate the performance difference. Compared to the pyridine exchange device, the MPA treated hybrid solar cell generates a smaller reverse saturation current (Fig. 3(d)). An increased diode characteristic is demonstrated in the MPA treated solar cell, showing that the charge collection property in this cell is better than that in the pyridine exchange device. It is necessary to point out that while the FeS2 nanocrystal itself has the same convenience in transporting charge carriers determined by its energy level [12], the improved charge collection property in MPA treated device is thus
Please cite this article as: Yu P, et al. Modified synthesis of FeS2 quantum dots for hybrid bulk-heterojunction solar cells. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.033i
67 68 69 70 71 72 73 74 75 76 77 78 79
P. Yu et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Fig. 3. Hybrid solar cell skeleton (a) and its energy level alignment (b), photovoltaic performance of solar cells with pyridine exchange and MPA treatment (c) and I–V properties in dark (d).
attributed to the MPA treatment. It is speculated that the MPA treatment to the P3HT:FeS2 hybrid thin film could enhance better heterojunction contact and phase separation. This is reasonable considering that an in-situ treatment to film will not cause nanoparticles' aggregation that usually happens in surfactant exchange, ensuring a more efficient hybrid solar cell.
4. Conclusion In conclusion, we have synthesized well-dispersed FeS2 QDs with a combined injection and solvothermal method in which well-crystallized and phase pure FeS2 nanoparticles were obtained at a relatively low temperature. The reaction process is comparatively moderate and sassily controllable. Photogenerated excitons are dissociated efficiently at the bulk-heterojunction interface of P3HT:FeS2 hybrid, showing the promising role of electrons acceptor of FeS2 QDs in organic/inorganic hybrid solar cells. It has been found that MPA treatment to hybrid thin film enables further improvement in solar cell performance compared to pyridine exchange because of suppressed reverse dark current as well as increased charge collection. The success in obtaining FeS2 QDs in our work suggests the promising application of the synthesis method in other semiconductor nanocrystals.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant no. 61306019), the National Basic Research Program of China (Grant no. 2014CB643503) and the Postdoctoral Science Foundation of China (Grant no. 2013M541972). This work is also supported by the Natural Science Foundation of Henan Provincial Education Department (Grant no. 13B430912) and the Scientific Research Found of Henan Provincial Department of Science and Technology (Grant no. 132300413210). References [1] Ennaoui A, Fiechter S, Pettenkofer C, Alonsovante N, Buker K, Bronold M, et al. Sol Energy Mater Sol Cells 1993;29(4):289–370. [2] Dasbach R, Willeke G, Blenk O. MRS Bull 1993;18(10):56–60. [3] Puthussery J, Seefeld S, Berry N, Gibbs M, Law M. J Am Chem Soc 2011;133 (4):716–9. [4] Macpherson HA, Stoldt CR. ACS Nano 2012;6:8940–9. [5] Wang D, Wu M, Wang Q, Wang T, Chen J. Ionics 2011;17:163–7. [6] Chen X, Fan R. Chem Mater 2001;13:802–5. [7] Pacholski C, Kornowski A, Weller H. Angew Chem Int Ed 2002;41:1188. [8] Dayal S, Kopidakis N, Olson DC, Ginley DS, Rumbles G. Nano Lett 2010;10:239–42. [9] Tan F, Qu S, Wu J, Liu K, Zhou S, Wang Z. Nanoscale Res Lett 2011;6:298. [10] Seo J, Cho MJ, Lee D, Cartwright AN, Prasad PN. Adv Mater 2011;23:3984–8. [11] Tan F, Qu S, Wang L, Jiang Q, Zhang W, Wang Z. J Mater Chem A 2014;2:14502. [12] Matusiewicz M, Czerwiński M, Kasperczyk J, Kityk IV. J Chem Phys 1999;111:6446.
Please cite this article as: Yu P, et al. Modified synthesis of FeS2 quantum dots for hybrid bulk-heterojunction solar cells. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.033i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 Q3 Q4 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Modified synthesis of FeS2 quantum dots for hybrid bulk-heterojunction solar cells Ping Yu a, Shengchun Qu b, Caihong Jia a, Kong Liu b, Furui Tan a,n a b
Key Laboratory of Photovoltaic Materials, Department of Physics and Electronics, Henan University, Henan 475004, PR China Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 April 2015 Received in revised form 9 May 2015 Accepted 11 May 2015
Phase pure and well-dispersed FeS2 quantum dots (QDs) have been synthesized with a method combining hot-injection and solvothermal reaction at a relatively low temperature. Hybrid composites composed of organic semiconductor and FeS2 QDs effectively dissociates excitons at the donor–acceptor interface, which qualifies efficient bulk-heterojunction solar cells. It has been found that the photovoltaic performance depends greatly on different ways of ligands exchange. It shows that mercaptopropanoic acid (MPA) treated hybrid thin film performs better in solar cells efficiency than pyridine exchange treatment to as-synthesized FeS2 nanoparticles due to a suppressed current leakage. & 2015 Published by Elsevier B.V.
Keywords: Quantum dots Hybrid Solar cells Thin films
1. Introduction Pyrite iron disulfide (FeS2) has been researched extensively as a promising optic-electric semiconductor with strong light absorption and abundant elements storage in the earth. Pyrite FeS2 has a band gap value of 0.95 eV that is suitable for light absorption as an active material in solar cells [1,2]. For solar cells where FeS2 plays either as photosensitive active layer or electron acceptor in p-type organic matrix, an excellent dispersion of FeS2 nanoparticles in organic solvent is needed for thin film uniformity or organic– inorganic compatibility. Thus, synthesis of FeS2 nanocrystals or QDs with hot injection method has been commonly researched [3,4]. Usually, high temperature is needed to accelerate the nuclei and growth process after hot injection. Even though, several hours are still needed to obtain crystallized and well-dispersed FeS2 nanocrystals. Besides the hot-injection, Solvothermal synthesis of FeS2 nanocrystals has also been extensively researched because of the comparatively moderate reaction process [5,6]. However, the FeS2 particles obtained is usually too large to serve as electron acceptor in efficient hybrid solar cells requiring a large enough heterojunction area. Besides, large-sized nanocrystal usually dispersed poorly in organic solvent such as chlorobenzene or chloroform. To obtain well dispersed FeS2 QDs that is applicable in hybrid solar cells, here in this work we have modified the fabrication
n
Corresponding author. Tel.: þ 86 371 23880659. E-mail address: [email protected] (F. Tan).
process where a hot-injection precursor was first obtained and then solvothermal reaction was carried out at a low temperature. Compared to the traditional hot-injection method, our modified synthesis process is relatively moderate and much more easily controllable. The obtained phase pure FeS2 QDs show good dispersion in organic solvent and does well in accepting electrons from organic photovoltaic semiconductor, poly-(3-hexylthiophene) (P3HT). The notable photovoltaic performance demonstrates effectiveness of our method in preparing FeS2 quantum dots. Besides, the modified reaction process here is promising in the synthesis of other semiconductor nanocrystals with good dispersion that is required for thin film fabrication.
2. Experimental Specific synthesis of FeS2 QDs is as follows. 100 mg iron chloride was dissolved in 20 ml octadecylamine (ODA) at 150 1C under N2 flow. Then, a sulfur solution (containing 96 mg S, 5 ml ODA and 10 ml diphenyl ther) was injected after which the hot Fe–S precursor was transferred into a stainless steel with teflon liner and underwent the solvothermal reaction at 150 1C for 5 h. The precipitate was collected and purified by chlorobenzene/ethanol solvent/antisolvent for at least 3 times. Pyridine exchange to FeS2 QDs were performed by dissolving the QDs into 5 ml pyridine and stirring at 70 1C for 10 h and then precipitated by adding extra amount of hexane. Organic/inorganic hybrid solar cells were fabricated as follows. Poly(thiophene) (3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) was spin coated at 4000 rpm onto the precleaned
http://dx.doi.org/10.1016/j.matlet.2015.05.033 0167-577X/& 2015 Published by Elsevier B.V.
Please cite this article as: Yu P, et al. Modified synthesis of FeS2 quantum dots for hybrid bulk-heterojunction solar cells. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.033i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
P. Yu et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
ITO substrates (optimized thickness of about 40 nm). The organic/ inorganic hybrid thin film composed of P3HT and FeS2 QDs (optimized mass ratio of 1:2) was prepared by spin-coating the hybrid solution in a glove box in N2 atmosphere. MPA treated solar cells were fabricated by spin-coating MPA:methanol solvent (1:9 in volume ratio) on top and then washing with methanol for 2 times. Afterward, onto the hybrid film, a ZnO buffer layer of about 20 nm was prepared by spin-coating a ZnO methanol solution [7]. Finally a 100 nm Ag cathode was thermally deposited on top. The crystalline phase of FeS2 nanoparticles was characterized by X-ray diffraction (XRD) on a Rigaku D/max-gA X-ray Diffractometer. The morphology was given by a high-resolution transmission electron microscope (HR-TEM, JEM-2100). The absorption and photoluminescence (PL) spectrum were recorded from ultraviolet– visible spectrophotometer (Varian Cary 5000) and Varian Cary Eclipse fluorescence spectrophotometer, respectively. Raman scattering was collected on a Renishaw RW1000 confocal microscope. The current–voltage (I–V) measurements were performed on Keithley 2440 Source Meter unite.
3. Results and discussion The morphology of synthesized FeS2 QDs is shown in Fig. 1. Well crystallization and mono-dispersion of QDs are clearly demonstrated (Fig. 1(a) and (b)). The strong XRD signals also indicate the formation of well crystallized quantum dots with a pyrite FeS2 phase (JCPDS card no. 060710) (Fig. 1(c)). The Raman peaks at 340 cm 1 and
380 cm 1 further confirms the pure FeS2 QDs. (Fig. 1(d)). The above morphology and phase characterization indicates the success in obtaining FeS2 QDs with our synthesis method. The synthesized FeS2 QDs exhibits a broad absorption of light extending to the red and infrared region (Fig. 2(a)). The optical energy band gap is estimated to be about 1.1 eV, showing the quantum size effect of our FeS2 QDs compared to the bulk value of 0.95 eV. Compared to the pure P3HT, the P3HT:FeS2 hybrid exhibits enhanced absorption intensity, attributing to the introduction of FeS2 QDs (Fig. 2(b)). Photoluminescence (PL) measurement demonstrates a strong quenching of PL intensity in the hybrid compared to the P3HT sample (Fig. 2(c)), revealing efficient excitons splitting and electrons transfer at the heterojunction interface of P3HT and FeS2 QDs. Based on the dissociation of photogenerated excitons, the hybrid solar cells were fabricated and the device skeleton is shown in Fig. 3(a). A type-II energy level alignment is formed at the donor–acceptor (P3HT-FeS2) interface (Fig. 3(b)), insuring efficient charges transfer. Noticing that the surface ligand (ODA) should be removed to obtain an efficient solar cell, we deal with this problem in two different ways. One is that the ODA ligand is exchanged by pyridine before mixing FeS2 with P3HT, a method that is commonly used in organic/inorganic hybrid solar cells [8,9]. Another way is to wash the premixed P3HT:FeS2 hybrid thin film with MPA: methanol solvent and the long ODA molecule can be replaced with a short ligand [10,11]. In order to more accurately compare the influence of solvent treatment on photovoltaic performance, we have fabricated three batches of solar cells. The same two pieces of
Fig. 1. TEM images (a) (b), XRD phase (c) and Raman signals of FeS2 QDs.
Please cite this article as: Yu P, et al. Modified synthesis of FeS2 quantum dots for hybrid bulk-heterojunction solar cells. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.033i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
P. Yu et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Q5 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
3
Fig. 2. Light absorption (a–b) and PL properties (c) of FeS2 QDs and the hybrids. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
solar cells were fabricated for each solvent treatment device in a batch. Thus, the performance parameters those are the most close to the average values were chosen and used here for each solar cell. Fig. 3(c) shows the I–V performance of the two solar cells under 100 mW cm 2 light illumination. The optimized thickness of P3HT:FeS2 hybrid film is the same 120 nm for the pyridine exchange and MPA treated solar cells. It can be seen from the I– V curves that the solar cell with pyridine exchange generates an average short current density (Jsc) of 2.4 mA cm 2, an open circuit voltage (Voc) of 0.59 V, a fill factor (FF) of 32% and a conversion efficiency of 0.45%. In comparison, the solar cell with MPA treatment show a Jsc value of 3.1 mA cm 2, a Voc value of 0.58 V,
an FF value of 34% and an efficiency of 0.61%. The obviously enhanced Jsc value in MPA treated device contributes to the efficiency improvement. The I–V properties in dark were measured to evaluate the performance difference. Compared to the pyridine exchange device, the MPA treated hybrid solar cell generates a smaller reverse saturation current (Fig. 3(d)). An increased diode characteristic is demonstrated in the MPA treated solar cell, showing that the charge collection property in this cell is better than that in the pyridine exchange device. It is necessary to point out that while the FeS2 nanocrystal itself has the same convenience in transporting charge carriers determined by its energy level [12], the improved charge collection property in MPA treated device is thus
Please cite this article as: Yu P, et al. Modified synthesis of FeS2 quantum dots for hybrid bulk-heterojunction solar cells. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.033i
67 68 69 70 71 72 73 74 75 76 77 78 79
P. Yu et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Fig. 3. Hybrid solar cell skeleton (a) and its energy level alignment (b), photovoltaic performance of solar cells with pyridine exchange and MPA treatment (c) and I–V properties in dark (d).
attributed to the MPA treatment. It is speculated that the MPA treatment to the P3HT:FeS2 hybrid thin film could enhance better heterojunction contact and phase separation. This is reasonable considering that an in-situ treatment to film will not cause nanoparticles' aggregation that usually happens in surfactant exchange, ensuring a more efficient hybrid solar cell.
4. Conclusion In conclusion, we have synthesized well-dispersed FeS2 QDs with a combined injection and solvothermal method in which well-crystallized and phase pure FeS2 nanoparticles were obtained at a relatively low temperature. The reaction process is comparatively moderate and sassily controllable. Photogenerated excitons are dissociated efficiently at the bulk-heterojunction interface of P3HT:FeS2 hybrid, showing the promising role of electrons acceptor of FeS2 QDs in organic/inorganic hybrid solar cells. It has been found that MPA treatment to hybrid thin film enables further improvement in solar cell performance compared to pyridine exchange because of suppressed reverse dark current as well as increased charge collection. The success in obtaining FeS2 QDs in our work suggests the promising application of the synthesis method in other semiconductor nanocrystals.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant no. 61306019), the National Basic Research Program of China (Grant no. 2014CB643503) and the Postdoctoral Science Foundation of China (Grant no. 2013M541972). This work is also supported by the Natural Science Foundation of Henan Provincial Education Department (Grant no. 13B430912) and the Scientific Research Found of Henan Provincial Department of Science and Technology (Grant no. 132300413210). References [1] Ennaoui A, Fiechter S, Pettenkofer C, Alonsovante N, Buker K, Bronold M, et al. Sol Energy Mater Sol Cells 1993;29(4):289–370. [2] Dasbach R, Willeke G, Blenk O. MRS Bull 1993;18(10):56–60. [3] Puthussery J, Seefeld S, Berry N, Gibbs M, Law M. J Am Chem Soc 2011;133 (4):716–9. [4] Macpherson HA, Stoldt CR. ACS Nano 2012;6:8940–9. [5] Wang D, Wu M, Wang Q, Wang T, Chen J. Ionics 2011;17:163–7. [6] Chen X, Fan R. Chem Mater 2001;13:802–5. [7] Pacholski C, Kornowski A, Weller H. Angew Chem Int Ed 2002;41:1188. [8] Dayal S, Kopidakis N, Olson DC, Ginley DS, Rumbles G. Nano Lett 2010;10:239–42. [9] Tan F, Qu S, Wu J, Liu K, Zhou S, Wang Z. Nanoscale Res Lett 2011;6:298. [10] Seo J, Cho MJ, Lee D, Cartwright AN, Prasad PN. Adv Mater 2011;23:3984–8. [11] Tan F, Qu S, Wang L, Jiang Q, Zhang W, Wang Z. J Mater Chem A 2014;2:14502. [12] Matusiewicz M, Czerwiński M, Kasperczyk J, Kityk IV. J Chem Phys 1999;111:6446.
Please cite this article as: Yu P, et al. Modified synthesis of FeS2 quantum dots for hybrid bulk-heterojunction solar cells. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.033i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 Q3 Q4 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132