- Email: [email protected]
Surface Engineering of Pbs Colloidal Quantum Dots Using Atomic Passivation for Photovoltaic Applications
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 139 (2016) 117 – 122
MRS Singapore – ICMAT Symposia Proceedings 8th International Conference on Materials for Advanced Technologies
Surface Engineering of Pbs Colloidal Quantum Dots Using Atomic Passivation for Photovoltaic Applications Mohammad Mahdi Tavakolia,* a
Department of Materials Science and Engineering, Sharif University of Technology, 14588 Tehran, Iran
Abstract Solution-processed quantum dots (QDs) have attracted significant attention for the low-cost fabrication of optoelectronic devices. Here, we synthesized PbS QDs via hot injection method and passivated the trap states by using short thiols and dopant elements for photovoltaic application. In order to study the effect of dopants on photovoltaic application, PbS QDs were doped by using three different cations: Cadmium, Calcium, and Zinc. We utilized Time resolvel Photoluminescence measurement to study the carriers lifetime for different samples and found that the carriers life time increases ~80% by using Cd as a dopant compared with undoped sample. In addition, the results of J-V measurement showed that Cd dopant has a better improvement than Ca and Zn dopants due to the enhancement of the efficiency of PbS QDs solar cell. Finally, we achieved solar power conversion efficiencies of 5.81% using Cd therapy. TheAuthors. Authors. Published Elsevier ©2016 2015The © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT Keywords: Colloidal quantum dots; Solar cell; PbS; Atomic passivation; ligand exchange
1. Introduction PbS quantum dots (QDs) are attracting increasing attention because of their optoelectronic properties promising for photovoltaic (PV) applications because of facile and cheap synthesis process, large Bohr radius (~20 nm), and its tunable bandgap [1-6]. Despite the many advantages of PbS QDs, the presence of trap states and weak charge transfer between the dots degrades the PV performance of solar cells [7-10]. In order to tackle theses problems,
* Corresponding author. Tel.: +98 (21) 6600 5717; fax: +98 (21) 6600 5717. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT
doi:10.1016/j.proeng.2015.08.1122
118
Mohammad Mahdi Tavakoli / Procedia Engineering 139 (2016) 117 – 122
many studies have recently been performed [11,12]. The main approaches include ligand exchange by short molecules to decrease the interparticle distance, obtain a more compact structure, and passivation of surface traps by transition metals [13,14]. Further improvements have been achieved by atomic-ligand passivation [15] and construction of quantum junction devices [16-18]. Recently, a record solar conversion performance of 8.55% has been reported for PbS QD solar cells using a short ligand (1,2-ethanedithiol) [19-27]. In the present work, PbS QDs were synthesized via a hot injection method. Then, the QDs were surface-passivated with oleic acid followed by Cd, Ca, and Zn therapy in three different processes. In order to study the effect of doping, the optical properties of QDs and their nanostructured films were investigated. The prepared QDs were used to fabricate bulk heterojunction solar cells by solid-state ligand exchange through spin-coating. Finally, the PV performance was measured and the highest efficiency was achieved for the passivation of mid-gap trap states of QDs by Cd. 2. Experimental In order to prepare the PbS QDs with a band-gap of about 1.34 eV, a solution-based approach, hot injection method, was utilized. 0.45 g of PbO, 1.5 ml of oleic acid, and 3 ml of 1-octadecene (ODE) were mixed in a three-necked flask at 110 °C for 16 h to produce a lead oleate solution. Then 15 ml of ODE was added to the flask and a solution containing 210 ȝl of Bis (trimethylsilyl) sulfide (TMS) dissolved in 10 ml of ODE was swiftly injected into the flask. After washing, PbS QDs were dispersed in octane (50 mg/ml). In order to do doping, different cations M2+ (Cd, Ca, and Zn) were used. In this regard, 2 mmol MCl2, 0.1 g tetradecylphosphonic acid (TDPA), and 10 ml oleylamine (OLA) were mixed in a flask and heated at 100°C for 30 min. 12 ml of PbS QDs were mixed with 4 ml of the M-containing solution. After washing and preparing 50 mg/ml PbS QDs in octane, the PV devices were prepared through a layer-by layer spin-coating process on TiO2-coated FTO substrates. The methods used for the preparation of these substrates are explained elsewhere [8]. 3. Results and Discussion A high-resolution TEM image of the synthesized PbS QDs is shown in Fig. 1a. The nanocrystals have an average size of ~3 nm with narrow size distribution. Fig. 1b shows the magnified TEM image of the PbS QDs which are highly crystalline. The EDS chemical analysis (Fig. 1c) illustrates the quality of the as-synthesized PbS QDs. The XRD pattern of the nanoparticles (Fig. 1d) also indicates their high crystallinity and shows that it has a good agreement with the standard diffraction pattern of PbS (JCPDS 02-0699).
Fig. 1. (a,b) HRTEM images, (c) EDS analysis, and (d) X-ray diffraction patterns of PbS QDs synthesized by using hot injection method.
Mohammad Mahdi Tavakoli / Procedia Engineering 139 (2016) 117 – 122
The effect of doping was studied by using the optical characterizations of PbS QDs. UV-vis-near infrared absorption and photoluminescence (PL) spectra of PbS QDs before and after doping are shown in Fig. 2. Referring to Fig. 2a, a well-defined excitonic peak at about 923 nm can be seen for PbS QDs without doping. The sharp peak indicates that the dots have a narrow size distribution with an optical band gap of about 1.34 eV.
Fig. 2. (a) Absorbtion and (b) photoluminescence spectra of PbS QDs before and after elemental doping.
This peak becomes a little bit broader and shifts to the red after doping using Zn, Ca, and Cd, however, the amount of red-shift is different for these cations. For Cd-doped nanocrystals, the excitonic peak further shifts to the red, which is beneficial in terms of the improved near-infrared absorption of the solar spectrum. Similar to the absorption spectra, the PL spectrum of M-doped PbS QDs exhibits a red shift compared with the PbS QDs, while the PL spectrum of M-doped nanocrystals has nearly the same spectral width as for PbS QDs but further shifts to the red. The largest red-shift PL is for Cd-doped QDs (Fig. 2b). The photoluminescence quantum efficiencies (PLQE) of undoped PbS, Zn, Ca, and Cd doped PbS QDs were 27, 28, 28.7, and 32 percent, respectively, and as a result, Cd atoms showed the best improvement of PLQE. To further study the effect of different cation doping on PbS QDs film, X-ray photoelectron spectroscopy (XPS) and Time-resolved PL (TRPL) were employed. As shown in Fig. 3a, the corresponding peaks to M elements (Zn, Ca, and Cd) in the PbS QDs were determined. Furthermore, in Cd-doped PbS QDs, there was a trace of chlorine doping at the surface of the PbS QDs, as shown in Fig. 3a. In addition, TRPL results demonstrated a lifetime enhancement of PbS QDs using different cation doping, of which the Cd-doped nanocrystals showed the best carrier lifetime compared with the others (Ca and Zn).
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Fig. 3. (a) XPS spectra, and (b) time-resolved photoluminescence lifetime of PbS QDs spectra before and after elemental doping using different cations.
The maximum carrier lifetime was for the Cd doped sample with 1.41 µs which has almost an 80% improvement compared with the undoped sample. Adsorption of M cations on the surface of the PbS QDs reduces the trap formation due to the change of charge balance. In order to investigate the effect of cation doping on the photovoltaic application, bulk heterojunction PbS QDs solar cells were fabricated. In this regard, a layer-by layer spin-coating process on TiO2-coated FTO substrates was afforded. The methods used for the preparation of these substrates are explained elsewhere [5]. Spin coating was performed at 2500 rpm and solid-state ligand exchange was applied by 1% v/v (Mercapto propionic acid) MPA in methanol. The thickness of the PbS film was about 300 nm. The top contacts were deposited by thermal evaporation of MoO3 and Au. Fig. 4 shows the characteristics of the devices. The device architecture and schematic of the PbS QD after doping are shown in Fig. 4a and 4b, respectively. Fig. 4c andd show the current density-voltage (J-V) curves under simulated (AM 1.5 G) solar irradiation, and the external quantum efficiency (EQE) spectrum of the cells, respectively. The figures of merit for these cells are summarized in Table 1. The highest power conversion efficiency (PCE) was for the Cd doped PbS QDs device with a 5.81% efficiency. The high current density (Jsc) and open circuit voltage (Voc) of this device is attributed to the longer lifetime of carriers due to the surface doping of PbS QDs by Cd atoms [20]. Compared with the undoped device with a 3.77% PCE, we improved the device performance by about 54% using Cd doping. In addition, Ca and Zn dopants enhanced the PCE of the devices up to 5.3% and 4.85%, respectively. As a result, cation doping presents an
Mohammad Mahdi Tavakoli / Procedia Engineering 139 (2016) 117 – 122
effective approach to improve the PCE of QD solar cells due to the enhancement of Jsc and Voc. As can be seen in fig 4d, the EQEs of these devices have a good agreement with J-V measurements.
Fig. 4. Device characterizations: (a) Cross-sectional SEM and schematic images of device, (b) Schematic of PbS QD doped with different cations, (c) Photovoltaic measurement under 1.5 AMG, and (d) External quantum efficiency vs. wavelength of bulk heterojunction PbS QDs device. Table 1. The figures of merit for doped and undoped PbS QDs solar cells. Device
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
PbS
0.53
13.2
54
3.77
PbS-Cd
0.59
20.1
49
5.81
PbS-Ca
0.59
17.4
52
5.3
PbS-Zn
0.57
16.7
51
4.85
4. Conclusions We introduced a hot-injection procedure to synthesize monodispersed, highly crystalline PbS QDs with an average size of 3 nm. Optical studies and TRPL measurements further revealed that Cd doped PbS QDs have a longer PL lifetime (1.41 µs) compared with undoped PbS QDs (0.78 µs). These favorable characteristics of PbS QDs allowed us to use them as active layers of solar cells with an efficiency of 5.8%. Our study demonstrates the importance of proper heterostructure design of hybrid materials for photovoltaic applications. When passivation with M2+ cations was performed, the current density increased as well as the open circuit voltage due to passivation of the mid-gap trap states. As a result, the power conversion efficiency (PCE) was improved by Cd doping by up to 54%. References [1] E.H. Sargent, Colloidal quantum dot solar cells, Nat. photon. 6 (2012) 133-135. [2] M. Graetzel, R.A.J. Janssen, D.B. Mitzi, E.H. Sargent, Materials interface engineering for solution-processed photovoltaics, Nat. 488 (2012) 304-312. [3] E.H. Sargent, Infrared photovoltaics made by solution processing, Nat. Photon. 3 (2009) 325-331.
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[4] M.A. Hines, G.D. Scholes, Colloidal PbS nanocrystals with size-tunable near-infrared emission: observation of post-synthesis self-narrowing of the particle size distribution, Adv. Mater. 15 (2003) 1844-1849. [5] A.H. Ip, S.M. Thon, S. Hoogland, O. Voznyy, D. Zhitomirsky, R. Debnath, L. Levina, L.R. Rollny, G.H. Carey, A. Fischer, K.W. Kemp, I.J. Kramer, Z. Ning, A.J. Labelle, K.W. Chou, A. Amassian, E.H. Sargent, Hybrid passivated colloidal quantum dot solids, Nat. nanotech. 7 (2012) 577-582. [6] O. Voznyy, D. Zhitomirsky, P. Stadler, Z. Ning, S. Hoogland, E.H. Sargent, A charge-orbital balance picture of doping in colloidal quantum dot solids, ACS Nano 6 (2012) 8448-8455. [7] A.G. Pattantyus-Abraham, I.J. Kramer, I.J. Barkhouse, A.R. Wang, X. Konstantatos, G. Debnath, R. Debnath, L. Levina, I. Raabe, M.K. Nazeeruddin, M. Gratzel, E.H. Sargent, Depleted-heterojunction colloidal quantum dot solar cells, ACS Nano 4 (2010) 3374-3380. [8] M.M. Tavakoli, A. Tayyebi, A. Simchi, H. Aashuri, M. Outokesh, Z. Fan, Physicochemical properties of hybrid graphene–lead sulfide quantum dots prepared by supercritical ethanol, J. Nanoparticle Research 17 (2015) 1-13. [9] M.M. Tavakoli, R. Tavakoli, P. Davami, H. Aashuri, A quantitative approach to study solid state phase coarsening in solder alloys using combined phase-field modeling and experimental observation, J. Comput. Electron. 13 (2014) 425-431. [10] M.M. Tavakoli, S.M. Abbasi, Effect of molybdenum on grain boundary segregation in Incoloy 901 superalloy, Mater. Design. 46 (2013) 573-578. [11] M.M. Tavakoli, A. Simchi, H. Aashuri, Supercritical synthesis and in situ deposition of PbS nanocrystals with oleic acid passivation for quantum dot solar cells, Mater. Chem. Phys. 156 (2015) 163-169. [12] A. Tayyebi, M. M. Tavakoli, M. Outokesh, A. Shafiekhani, A. Simchi, Supercritical Synthesis and Characterization of “Graphene-PbS Quantum Dots” Composite with Enhanced Photovoltaic Properties, Industrial & Engineering Chemistry Research 54 (2015) 7382í7392. [13] S.A. McDonald, G. Konstantatos, S. Zhang, P.W. Cyr, E.J.D. Klem, L. Levina, E.H. Sargent, Solution-processed PbS quantum dot infrared photodetectors and photovoltaics, Nat. Mater. 4 (2005) 138-142. [14] D. Aaron, R. Barkhouse, R. Debnath, I.J. Kramer, D. Zhitomirsky, A.G. Pattantyus-Abraham, L. Levina, L. Etgar, M. Grätzel, E.H. Sargent, Depleted bulk heterojunction colloidal quantum dot photovoltaics, Adv. Mater. 23 (2011) 3134-3138. [15] D. Zhitomirsky, Q. Voznyy, S. Hoogland, E.H. Sargent, Measuring charge carrier diffusion in coupled colloidal quantum dot solids, ACS Nano 7(2013) 5282-5290. [16] S. Xiong, B. Xi, D. Xu, C. Wang, X. Feng, H. Zhou, Y. Qian, L-cysteine-assisted tunable synthesis of PbS of various morphologies, J. Phys. Chem. C 45 (2007) 16761-16767. [17] S.M. Thon, A.H. Ip, O. Voznyy, L. Levina, K.W. Kemp, G.H. Carey, S. Masala, E.H. Sargent, Role of Bond Adaptability in the Passivation of Colloidal Quantum Dot Solids, ACS nano 7 (2013) 7680-7688. [18] M.J. Speirs, D.M. Balazs, H.H. Fang, L.H. Lai, L. Protesescu, M.V. Kovalenko, M.A. Loi, Origin of the increased open circuit voltage in PbS–CdS core–shell quantum dot solar cells, J. Mater. Chem. A 3 (2015) 1450-1457. [19] Z. Ning, O. Voznyy, J. Pan, S. Hoogland, V. Adinolfi, J. Xu, M. Li, A.R. Kirmani, J. Sun, J. Minor, K.W. Kemp, H. Dong, L. Rollny, A. Labelle, G. Carey, B. Sutherland, I. Hill, A. Amassian, H. Liu, J. Tang, O.M. Bakr, E.H. Sargent, Air-stable n-type colloidal quantum dot solids, Nat. mater. 13 (2014) 822-828. [20] O. Voznyy, S.M. Thon, A.H. Ip, E.H. Sargent, Dynamic trap formation and elimination in colloidal quantum dots, J. Phys. Chem. Lett. 4 (2013) 987-992. [21] M.M. Tavakoli, S.G. Shabestari, M. Ghanbari, Effect of thixoforming on morphological changes in iron-bearing intermetallics and mechanical properties of Al–Si–Cu alloys, International Journal of Materials Research 105 (2014) 1210-1217. [22] Y. Gao, H. Jin, Q. Lin, X. Li, M.M. Tavakoli, S. Leung, W.M. Tang, L. Zhou, H.L. Wa Chan, Z. Fan, Highly flexible and transferable supercapacitors with ordered three-dimensional MnO 2/Au/MnO2 nanospike arrays, Journal of Materials Chemistry A 3, (2015) 10199-10204. [23] S.G. Shabestari, M.M. Tavakoli, M. Ghanbari, Effect of Semi-Solid Processing on Iron-Bearing Intermetallic Compounds in A380 Aluminum Alloy, Solid State Phenomena, 217 (2015) 397-404. [24] M.M. Tavakoli, K. H. Tsui, S. F. Leung, Q. Zhang, J. He, Y. Yao, D. Li, Z. Fan, Highly Efficient Flexible Perovskite Solar Cell with AntiReflection and Self-Cleaning Nanostructures, ACS Nano, (2015) DOI: 10.1021/acsnano.5b04284. [25] M. M. Tavakoli, H. Aashuri, A. Simchi, S. Kalytchuk, Z. Fan, Quasi Core/Shell Lead Sulfide/Graphene Quantum Dots for Bulk Heterojunction Solar Cells, J. Phys. Chem. C 119 (2015) 18886–18895. [26] M.M. Tavakoli, H. Aashuri, A. Simchi, Z. Fan, Hybrid zinc oxide/graphene electrodes for depleted heterojunction colloidal quantum-dot solar cells, Phys. Chem. Chem. Phys., (2015) DOI: 10.1039/C5CP03571F. [27] M. M. Tavakoli, L. Gu, Y. Gao, C. Reckmeier, J. He, A. L. Rogach, Y. Yao, Z. Fan, Fabrication of efficient planar perovskite solar cells using a one-step chemical vapor deposition method, Sci. Rep. (2015) DOI: 10.1038/srep14083.
ScienceDirect Procedia Engineering 139 (2016) 117 – 122
MRS Singapore – ICMAT Symposia Proceedings 8th International Conference on Materials for Advanced Technologies
Surface Engineering of Pbs Colloidal Quantum Dots Using Atomic Passivation for Photovoltaic Applications Mohammad Mahdi Tavakolia,* a
Department of Materials Science and Engineering, Sharif University of Technology, 14588 Tehran, Iran
Abstract Solution-processed quantum dots (QDs) have attracted significant attention for the low-cost fabrication of optoelectronic devices. Here, we synthesized PbS QDs via hot injection method and passivated the trap states by using short thiols and dopant elements for photovoltaic application. In order to study the effect of dopants on photovoltaic application, PbS QDs were doped by using three different cations: Cadmium, Calcium, and Zinc. We utilized Time resolvel Photoluminescence measurement to study the carriers lifetime for different samples and found that the carriers life time increases ~80% by using Cd as a dopant compared with undoped sample. In addition, the results of J-V measurement showed that Cd dopant has a better improvement than Ca and Zn dopants due to the enhancement of the efficiency of PbS QDs solar cell. Finally, we achieved solar power conversion efficiencies of 5.81% using Cd therapy. TheAuthors. Authors. Published Elsevier ©2016 2015The © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT Keywords: Colloidal quantum dots; Solar cell; PbS; Atomic passivation; ligand exchange
1. Introduction PbS quantum dots (QDs) are attracting increasing attention because of their optoelectronic properties promising for photovoltaic (PV) applications because of facile and cheap synthesis process, large Bohr radius (~20 nm), and its tunable bandgap [1-6]. Despite the many advantages of PbS QDs, the presence of trap states and weak charge transfer between the dots degrades the PV performance of solar cells [7-10]. In order to tackle theses problems,
* Corresponding author. Tel.: +98 (21) 6600 5717; fax: +98 (21) 6600 5717. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT
doi:10.1016/j.proeng.2015.08.1122
118
Mohammad Mahdi Tavakoli / Procedia Engineering 139 (2016) 117 – 122
many studies have recently been performed [11,12]. The main approaches include ligand exchange by short molecules to decrease the interparticle distance, obtain a more compact structure, and passivation of surface traps by transition metals [13,14]. Further improvements have been achieved by atomic-ligand passivation [15] and construction of quantum junction devices [16-18]. Recently, a record solar conversion performance of 8.55% has been reported for PbS QD solar cells using a short ligand (1,2-ethanedithiol) [19-27]. In the present work, PbS QDs were synthesized via a hot injection method. Then, the QDs were surface-passivated with oleic acid followed by Cd, Ca, and Zn therapy in three different processes. In order to study the effect of doping, the optical properties of QDs and their nanostructured films were investigated. The prepared QDs were used to fabricate bulk heterojunction solar cells by solid-state ligand exchange through spin-coating. Finally, the PV performance was measured and the highest efficiency was achieved for the passivation of mid-gap trap states of QDs by Cd. 2. Experimental In order to prepare the PbS QDs with a band-gap of about 1.34 eV, a solution-based approach, hot injection method, was utilized. 0.45 g of PbO, 1.5 ml of oleic acid, and 3 ml of 1-octadecene (ODE) were mixed in a three-necked flask at 110 °C for 16 h to produce a lead oleate solution. Then 15 ml of ODE was added to the flask and a solution containing 210 ȝl of Bis (trimethylsilyl) sulfide (TMS) dissolved in 10 ml of ODE was swiftly injected into the flask. After washing, PbS QDs were dispersed in octane (50 mg/ml). In order to do doping, different cations M2+ (Cd, Ca, and Zn) were used. In this regard, 2 mmol MCl2, 0.1 g tetradecylphosphonic acid (TDPA), and 10 ml oleylamine (OLA) were mixed in a flask and heated at 100°C for 30 min. 12 ml of PbS QDs were mixed with 4 ml of the M-containing solution. After washing and preparing 50 mg/ml PbS QDs in octane, the PV devices were prepared through a layer-by layer spin-coating process on TiO2-coated FTO substrates. The methods used for the preparation of these substrates are explained elsewhere [8]. 3. Results and Discussion A high-resolution TEM image of the synthesized PbS QDs is shown in Fig. 1a. The nanocrystals have an average size of ~3 nm with narrow size distribution. Fig. 1b shows the magnified TEM image of the PbS QDs which are highly crystalline. The EDS chemical analysis (Fig. 1c) illustrates the quality of the as-synthesized PbS QDs. The XRD pattern of the nanoparticles (Fig. 1d) also indicates their high crystallinity and shows that it has a good agreement with the standard diffraction pattern of PbS (JCPDS 02-0699).
Fig. 1. (a,b) HRTEM images, (c) EDS analysis, and (d) X-ray diffraction patterns of PbS QDs synthesized by using hot injection method.
Mohammad Mahdi Tavakoli / Procedia Engineering 139 (2016) 117 – 122
The effect of doping was studied by using the optical characterizations of PbS QDs. UV-vis-near infrared absorption and photoluminescence (PL) spectra of PbS QDs before and after doping are shown in Fig. 2. Referring to Fig. 2a, a well-defined excitonic peak at about 923 nm can be seen for PbS QDs without doping. The sharp peak indicates that the dots have a narrow size distribution with an optical band gap of about 1.34 eV.
Fig. 2. (a) Absorbtion and (b) photoluminescence spectra of PbS QDs before and after elemental doping.
This peak becomes a little bit broader and shifts to the red after doping using Zn, Ca, and Cd, however, the amount of red-shift is different for these cations. For Cd-doped nanocrystals, the excitonic peak further shifts to the red, which is beneficial in terms of the improved near-infrared absorption of the solar spectrum. Similar to the absorption spectra, the PL spectrum of M-doped PbS QDs exhibits a red shift compared with the PbS QDs, while the PL spectrum of M-doped nanocrystals has nearly the same spectral width as for PbS QDs but further shifts to the red. The largest red-shift PL is for Cd-doped QDs (Fig. 2b). The photoluminescence quantum efficiencies (PLQE) of undoped PbS, Zn, Ca, and Cd doped PbS QDs were 27, 28, 28.7, and 32 percent, respectively, and as a result, Cd atoms showed the best improvement of PLQE. To further study the effect of different cation doping on PbS QDs film, X-ray photoelectron spectroscopy (XPS) and Time-resolved PL (TRPL) were employed. As shown in Fig. 3a, the corresponding peaks to M elements (Zn, Ca, and Cd) in the PbS QDs were determined. Furthermore, in Cd-doped PbS QDs, there was a trace of chlorine doping at the surface of the PbS QDs, as shown in Fig. 3a. In addition, TRPL results demonstrated a lifetime enhancement of PbS QDs using different cation doping, of which the Cd-doped nanocrystals showed the best carrier lifetime compared with the others (Ca and Zn).
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Fig. 3. (a) XPS spectra, and (b) time-resolved photoluminescence lifetime of PbS QDs spectra before and after elemental doping using different cations.
The maximum carrier lifetime was for the Cd doped sample with 1.41 µs which has almost an 80% improvement compared with the undoped sample. Adsorption of M cations on the surface of the PbS QDs reduces the trap formation due to the change of charge balance. In order to investigate the effect of cation doping on the photovoltaic application, bulk heterojunction PbS QDs solar cells were fabricated. In this regard, a layer-by layer spin-coating process on TiO2-coated FTO substrates was afforded. The methods used for the preparation of these substrates are explained elsewhere [5]. Spin coating was performed at 2500 rpm and solid-state ligand exchange was applied by 1% v/v (Mercapto propionic acid) MPA in methanol. The thickness of the PbS film was about 300 nm. The top contacts were deposited by thermal evaporation of MoO3 and Au. Fig. 4 shows the characteristics of the devices. The device architecture and schematic of the PbS QD after doping are shown in Fig. 4a and 4b, respectively. Fig. 4c andd show the current density-voltage (J-V) curves under simulated (AM 1.5 G) solar irradiation, and the external quantum efficiency (EQE) spectrum of the cells, respectively. The figures of merit for these cells are summarized in Table 1. The highest power conversion efficiency (PCE) was for the Cd doped PbS QDs device with a 5.81% efficiency. The high current density (Jsc) and open circuit voltage (Voc) of this device is attributed to the longer lifetime of carriers due to the surface doping of PbS QDs by Cd atoms [20]. Compared with the undoped device with a 3.77% PCE, we improved the device performance by about 54% using Cd doping. In addition, Ca and Zn dopants enhanced the PCE of the devices up to 5.3% and 4.85%, respectively. As a result, cation doping presents an
Mohammad Mahdi Tavakoli / Procedia Engineering 139 (2016) 117 – 122
effective approach to improve the PCE of QD solar cells due to the enhancement of Jsc and Voc. As can be seen in fig 4d, the EQEs of these devices have a good agreement with J-V measurements.
Fig. 4. Device characterizations: (a) Cross-sectional SEM and schematic images of device, (b) Schematic of PbS QD doped with different cations, (c) Photovoltaic measurement under 1.5 AMG, and (d) External quantum efficiency vs. wavelength of bulk heterojunction PbS QDs device. Table 1. The figures of merit for doped and undoped PbS QDs solar cells. Device
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
PbS
0.53
13.2
54
3.77
PbS-Cd
0.59
20.1
49
5.81
PbS-Ca
0.59
17.4
52
5.3
PbS-Zn
0.57
16.7
51
4.85
4. Conclusions We introduced a hot-injection procedure to synthesize monodispersed, highly crystalline PbS QDs with an average size of 3 nm. Optical studies and TRPL measurements further revealed that Cd doped PbS QDs have a longer PL lifetime (1.41 µs) compared with undoped PbS QDs (0.78 µs). These favorable characteristics of PbS QDs allowed us to use them as active layers of solar cells with an efficiency of 5.8%. Our study demonstrates the importance of proper heterostructure design of hybrid materials for photovoltaic applications. When passivation with M2+ cations was performed, the current density increased as well as the open circuit voltage due to passivation of the mid-gap trap states. As a result, the power conversion efficiency (PCE) was improved by Cd doping by up to 54%. References [1] E.H. Sargent, Colloidal quantum dot solar cells, Nat. photon. 6 (2012) 133-135. [2] M. Graetzel, R.A.J. Janssen, D.B. Mitzi, E.H. Sargent, Materials interface engineering for solution-processed photovoltaics, Nat. 488 (2012) 304-312. [3] E.H. Sargent, Infrared photovoltaics made by solution processing, Nat. Photon. 3 (2009) 325-331.
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