We investigate time-dependent inorganic ligand exchanging effect and photovoltaic performance of lead sulfide (PbS) nanocrystal films. With optimal processing time, volume shrinkage induced by residual oleic acid of the PbS colloidal quantum dot (CQD) was minimized and a crack-free film was obtained with improved flatness. Furthermore, sufficient surface passivation significantly increased the packing density by replacing from long oleic acid to a short iodide molecule. It thus facilities exciton dissociation via enhanced charge carrier transport in PbS CQD films, resulting in the improved power conversion efficiency from 3.39% to 6.62%. We also found that excess iodine ions on the PbS surface rather hinder high photovoltaic performance of the CQD solar cell

Cha, S

Cho, Y

Hong, J

Hou, B

Kim, BS

Kim, JM

Sohn, JI

Applied Physics Letters

English

Kim, Byung-Sung

Hong, John

Hou, Bo

Cho, Yuljae

Sohn, Jung Inn

Cha, Seung

Kim, Jong Min

Oxford University Research Archive

Inorganic-ligand exchanging time effect in PbS quantum dot solar cell

ORA - Oxford University Research Archive

Apollo (Cambridge)

Reuse of AIP Publishing content is subject to the terms at: <a href="https://publishing.aip.org/authors/rights-and-permissions">https://publishing.aip.org/authors/rights-and-permissions</a>. Downloaded to: 129.67.116.159 on 07 November 2018, At: 15:18Inorganic-ligand exchanging time effect in PbS quantum dot solar cellByung-Sung Kim, John Hong, Bo Hou, Yuljae Cho, Jung Inn Sohn, SeungNam Cha, and Jong Min KimCitation: Appl. Phys. Lett. 109, 063901 (2016); doi: 10.1063/1.4960645View online: https://doi.org/10.1063/1.4960645View Table of Contents: http://aip.scitation.org/toc/apl/109/6Published by the American Institute of PhysicsArticles you may be interested inImpact of dithiol treatment and air annealing on the conductivity, mobility, and hole density in PbS colloidalquantum dot solidsApplied Physics Letters 92, 212105 (2008); 10.1063/1.2917800Increasing photon absorption and stability of PbS quantum dot solar cells using a ZnO interlayerApplied Physics Letters 107, 183901 (2015); 10.1063/1.4934946Single-step colloidal quantum dot films for infrared solar harvestingApplied Physics Letters 109, 183105 (2016); 10.1063/1.4966217Free carrier generation and recombination in PbS quantum dot solar cellsApplied Physics Letters 108, 103102 (2016); 10.1063/1.4943379Detailed Balance Limit of Efficiency of p-n Junction Solar CellsJournal of Applied Physics 32, 510 (1961); 10.1063/1.1736034 Increased efficiency in pn-junction PbS QD solar cells via NaHS treatment of the p-type layerApplied Physics Letters 110, 103904 (2017); 10.1063/1.4978444Inorganic-ligand exchanging time effect in PbS quantum dot solar cellByung-Sung Kim,1,a) John Hong,1,a) Bo Hou,1 Yuljae Cho,1 Jung Inn Sohn,1,b)SeungNam Cha,1,b) and Jong Min Kim21Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, United Kingdom2Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA,United Kingdom(Received 23 May 2016; accepted 28 July 2016; published online 9 August 2016)We investigate time-dependent inorganic ligand exchanging effect and photovoltaic performanceof lead sulfide (PbS) nanocrystal films. With optimal processing time, volume shrinkage inducedby residual oleic acid of the PbS colloidal quantum dot (CQD) was minimized and a crack-freefilm was obtained with improved flatness. Furthermore, sufficient surface passivation significantlyincreased the packing density by replacing from long oleic acid to a short iodide molecule. It thusfacilities exciton dissociation via enhanced charge carrier transport in PbS CQD films, resulting inthe improved power conversion efficiency from 3.39% to 6.62%. We also found that excess iodineions on the PbS surface rather hinder high photovoltaic performance of the CQD solar cell.VC 2016Author(s). All article content, except where otherwise noted, is licensed under a CreativeCommons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4960645]Colloidal quantum dots (CQDs) are promising candi-dates for next-generation photovoltaics owing to their uniqueproperties such as high absorption coefficient, tunable bandgap, and multiple exciton generation effect.1–4 The solution-based process also has provided high feasibility of realizingflexible and large scale photovoltaic devices with cost-effective fabrication.5–8 Despite these favorable characteris-tics, poor charge transport and imperfect surface status ofCQDs hinder high photovoltaic performance compared tothe theoretical conversion efficiency. During the syntheticprocess, CQDs are capped by long and bulky organic ligandswhich enable to control the size of CQDs by preventingover-size aggregation and enhancing chemical stability.9However, the insulating properties of long hydrocarbonligands limited photoexcited carrier transport, eventuallyresulting in low power conversion efficiency (PCE).Therefore, control of the ligand exchange process which effi-ciently substitutes the surface from long ligands to short mol-ecules is necessary for CQD-based device applications.10,11Enhanced performances of CQD solar cells using the inor-ganic halide ligands (e.g., Cl, Br, I) have been recentlyreported,12,13 but the detailed characterization of the CQDfilm during the ligand exchange process has not been wellinvestigated. Those parameters such as packing density orhalide ion density on the surface should be considered for aneffective way to control the surface status and furtherimprove photovoltaic performance. Here, we investigatetime-dependent influence of inorganic ligand exchange inlead sulfide (PbS) quantum dot films with systematic analysisof photovoltaic performance. Within the same concentrationof tetrabutylammonium iodide (TBAI) which induces thesame driving force of ligand exchange, we studied the chargetransfer mechanism on quantum dot (QD) films based on thequantitative interaction between surface halide ions and QDsurface. Moreover, our study focused on the detailed statusof the PbS surface for understanding the relationshipbetween the packing density and charge carrier transport ofPbS CQD films by adjusting ligand exchanging time. Thesefindings will provide a useful guideline for achieving thehigh efficiency CQD-based solar cells.Inorganic molecule halide treatment was carried outusing TBAI solution in ambient conditions, which is one ofthe most commonly used ligand materials for PbS QD solarcells due to air-stability and low density of trapped carriers byhighly effective passivation.12,13 During the ligand exchange,it introduces negatively charged iodine ions which substitutepristine oleic acid (OA). The halide anions subsequently bindto Pb cations on the PbS surface. If the surface is not perfectlyupgraded, the remaining oleic acids will play a role as insulat-ing barriers which decrease the carrier transport as well asexciton dissociation in PbS CQD films (Fig. 1(a)). Therefore,it is important to minimize the degree of the remaining oleicacids to improve the performance of photovoltaic devices bythe controlled ligand exchange process, exposing the OA-capped CQD films to the solution containing the iodine ions.To systematically investigate the ligand exchanging effect, weprepared PbS CQD films with different ligand exchangingtimes (TL): 15, 30, 60, and 90 s. 3 layers of PbS were depos-ited on glass substrates with TBAI solution (10mg/ml inmethanol). Evidence for chemical composition changes of theCQD surface can be determined using Fourier transform-infrared spectroscopy (FT-IR) and X-ray photoelectron spec-troscopy (XPS) spectra of the iodide-treated PbS CQD filmswith the different ligand exchanging time of TL. The presenceof the oleic acids can be denoted by the four characteristicpeaks including vibrations at 2920 cm1 (asymmetric C-H),2850 cm1 (symmetric C-H), 1545 cm1 (asymmetricCOO–), and 1403 cm1 (symmetric COO–), respectively.(Fig. 1(b)) Significant reduction of the OA-related vibrationswas observed according to the treatment time.14 Togethera)B.-S. Kim and J. Hong contributed equally to this work.b)Electronic addresses: junginn.sohn@eng.ox.ac.uk and seungnam.cha@eng.ox.ac.uk0003-6951/2016/109(6)/063901/4 VC Author(s) 2016.109, 063901-1APPLIED PHYSICS LETTERS 109, 063901 (2016)with the XPS energy spectra, the atomic ratios of lead andiodine on the surface of PbS CQD films can be evaluated withrespect to the ligand exchange time. Note that the short ligandexchanging time (TL< 30 s) does not provide enough surfacepassivation and considerable oleic acids still remain on thePbS surface. With an increasing of TL, the oleic acids werealmost eliminated and surface iodine concentration is quanti-tatively increased. (Figs. 1(c) and 1(d)).A degree of surface passivation could affect the morpho-logical property of the PbS CQD film. Micro- and nano-scalecracks have been frequently observed in ligand-exchangedCQD devices due to the considerable volume shrink of thefilms when long oleic acids were substituted by short organicand/or inorganic ligands.15–17 Furthermore, the crack forma-tion results in severe problems such as highly inducing shortcircuit or leakage current in CQD-based device applications.18Therefore, careful control of the ligand exchange process isessential for producing crack-free CQD films. The morpho-logical changes of the PbS CQD film were shown with the dif-ferent ligand exchanging times of TL (Fig. 2, Figs. S1 and S2in supplementary material). A very rough surface with thenumerous cracks was obtained at the TL of 15 s while longerligand exchanging time leads to a crack-free surface with theimproved film flatness. This result implies that the presence ofthe residual oleic acids observed at insufficient ligandexchanging time induces the large physical strain with thecoexistent short iodide ligands in PbS CQD films. It thus leadsto the non-uniform volume shrink and accelerating crack for-mation during the solvent evaporation.A highly densely packed CQD film is a critical factor todetermine the photovoltaic performance since the charge car-rier transport is enhanced with minimizing the CQD inter-particle distance.10,11 Figure 3 shows that the packing densi-ties of the CQD film could be effectively tuned by adjustingthe ligand exchanging time. Compared to OA-capped PbSCQDs, the packing densities were gradually increased withproceeding ligand exchange from OA to iodide molecules.(Figs. 3(a)–3(c)) Longer treatment makes the neighboringQDs closer, resulting in partially connection between eachother. Furthermore, exciton absorption peaks were certainlyFIG. 1. A schematic illustrating theligand exchange process on PbS CQDfilms. (b) FTIR spectra and (c) bindingenergy of I 3d and Pb 4f in TBAI-PbSCQD films with an increasing ligandexchanging time of TL. They showsuccessful exchange of iodine on theCQD surface. (d) Variation of theatomic ratio of lead to iodine as a func-tion of the corresponding TL. Thesquares and solid lines of black,orange, blue, and red are TL¼ 15, 30,60, and 90 s, respectively. The dottedlines in Fig. 1(b) are pristine PbS CQDfilm capping with oleic acid.FIG. 2. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of surface morphologies of TBAI-PbS films with increasing TL: ((a)and (e)) 15 s, ((b) and (f)) 30 s, ((c) and (g)) 60 s, and ((d) and (h)) 90 s. These samples are prepared by spin coating the 3 layers of PbS onto the glass substrates.It was shown that the root mean square (RMS) roughness of PbS CQD films was gradually decreased, dependent on TL.063901-2 Kim et al. Appl. Phys. Lett. 109, 063901 (2016)observed to be broadened and red-shifted as seen in Fig.3(d). These phenomena can be explained by electron cou-pling or sintering/necking between QDs as the inter-particledistance becomes closer.19–21 This correlates well with theincreased packing densities with TL observed by carefulTransmission electron microscopy (TEM) analysis.To investigate the relationship between photovoltaicperformance of the PbS film with the different packing den-sities, we obtained photocurrent density and voltage (J–V)curves of the PbS CQD films with the different ligandexchanging times of TL under AM 1.5G illustration (100mW cm2). (Fig. 4(a)) The devices were prepared by 10cycles spin coating of TBAI-PbS CQD films on ZnO/ITOsubstrates (Fig. S3 in supplementary material) and the photo-voltaic performance of the devices is summarized in Figs.4(c) and 4(d) (also see Fig. S4 in supplementary material).Incomplete passivation on the PbS surface caused by shortligand exchanging time of TL hinders efficient charge trans-port in the film (Fig. 4(b)), resulting in relatively low short-circuit current (Jsc) and PCE with high series resistance (Rs).The device with a short TL of 15 s gives a low PCE of 3.39%with a Jsc of 13.75mA/cm2. Moreover, in Fig. S5, Jsc from theexternal quantum efficiency (EQE) measurement also showsthe same trend which correlates with the Jsc enhancement ofthe J–V measurement by the different ligand exchangingtimes. With an increase of the ligand exchanging time, thepacking densities of PbS QDs as well as surface coverage ofthe iodide molecule were significantly increased. (Figs. 1 and3) It thus facilities exciton dissociation via enhanced chargetransfer in PbS films, leading to substantial improvement of77.58% and 74.98% in PCE, and Jsc at TL of 60 s, respectively(Fig. 4(c) and Table S1 in supplementary material). The valuesof Rs were also decreased from 10.14 X to 3.51 X, dependenton the TL. Interestingly, we found that the photovoltaic perfor-mance was declined after longer TL of 90 s in spite of the highsurface coverage of iodide ligands on the PbS CQDs. It can beassumed that excess iodide molecules which are not bound toPb cations may act as the insulating barrier between PbSCQDs. These results clearly indicate that the controllingligand exchange on the PbS CQD film should be highlyrequired to manipulate the photovoltaic performance.FIG. 3. Transmission electron microscopy (TEM) and selected area diffrac-tion (SAED) pattern images of (a) pristine PbS CQD film capping with oleicacids and (b) TBAI-PbS CQD films at TL of 30 s. With a longer time of 30 s,the packing density of the TBAI-PbS CQD film was slightly increased.Insets are low-magnification TEM images of the corresponding OA- andTBAI-PbS CQD films. (c) Variation of the packing density of PbS QDs as afunction of TL. (d) Absorption spectra of the corresponding PbS CQD films.The closed squares and solid lines of black, orange, blue, and red areTL¼ 15, 30, 60, and 90 s, respectively. The open square and dotted line arepristine PbS CQD film capping with oleic acid.FIG. 4. (a) Representative J–V charac-teristics of the TBAI-PbS CQD devicesby varying TL. The devices were mea-sured in the dark (open square) andunder AM 1.5 illumination (closesquare) at an incident intensity of 100mW/cm2. (b) Schematic of iodine- andOA-rich in PbS CQD. Plots of (c)PCE, Jsc, and Rs, (d) Voc, Rsh, and fillfactor (FF) of the devices as a functionof TL. The closed squares of black,orange, blue, and red indicate differentTL of 15, 30, 60, and 90 s, respectively.063901-3 Kim et al. Appl. Phys. Lett. 109, 063901 (2016)We have demonstrated time-dependent inorganic ligandexchanging effect in PbS CQD films. Controlled surface pas-sivation increases the packing density with a crack-free PbSsurface by minimizing residual oleic acid. It also enables tofacilitate charge transport and significantly improve PbSsolar cell efficiency from 3.39% to 6.62%. Ligand exchangeover a longer TL of a critical value further decreases interpar-ticle spacing but rather slightly deteriorates the photovoltaicperformance by excess iodide molecules acting as a chargetransport barrier on the PbS surface. These results distinc-tively prove that appropriate ligand passivation is highlyrequired for high performance of the CQD solar cell.See supplementary material for detailed growth proce-dures, supporting characterization, and analysis ofnanomaterials.The research leading to these results has receivedfunding from the European Research Council under ERCGrant Agreement No. 340538. The authors would like tothank the financial support from the National ResearchFoundation (NRF) of Korea (2015M2A2A6A02045252).This work was conducted under the framework of Researchand Development Program of the Korea Institute of EnergyResearch (KIER) (B6-2498).1A. P. Alivisatos, Science 271(5251), 933 (1996).2Z. Kang, Y. Liu, C. H. A. Tsang, D. D. D. Ma, X. Fan, N. B. Wong, and S.T. Lee, Adv. Mater. 21(6), 661 (2009).3D. A. Ruddy, J. C. Johnson, E. R. Smith, and N. R. Neale, ACS Nano4(12), 7459 (2010).4L. Lu, J. Chen, and W. Wang, Appl. Phys. Lett. 103, 123902 (2013).5S. A. McDonald, G. Konstantatos, S. Zhang, P. W. Cyr, E. J. D. Klem, L.Levina, and E. H. Sargent, Nat. Mater. 4(2), 138 (2005).6X. Geng, L. Niu, Z. Xing, R. Song, G. Liu, M. Sun, G. Cheng, H. Zhong, Z.Liu, Z. Zhang, L. Sun, H. Xu, L. Lu, and L. Liu, Adv. Mater. 22(5), 638(2010).7I. J. Kramer, G. Moreno-Bautista, J. C. Minor, D. Kopilovic, and E. H.Sargent, Appl. Phys. Lett. 105(16), 163902 (2014).8Z. Tan, J. Xu, C. Zhang, T. Zhu, F. Zhang, B. Hedrick, S. Pickering, J.Wu, H. Su, S. Gao, A. Y. Wang, B. Kimball, J. Ruzyllo, N. S. Dellas, andS. E. Mohney, J. Appl. Phys. 105(3), 034312 (2009).9C. B. Murray, C. R. Kagan, and M. G. Bawendi, Annu. Rev. Mater. Sci.30, 545 (2000).10J. J. Choi, J. Luria, B. R. Hyun, A. C. Bartnik, L. Sun, Y. F. Lim, J. A.Marohn, F. W. Wise, and T. Hanrath, Nano Lett. 10(5), 1805 (2010).11Y. Liu, M. Gibbs, J. Puthussery, S. Gaik, R. Ihly, H. W. Hillhouse, and M.Law, Nano Lett. 10(5), 1960 (2010).12C. H. M. Chuang, P. R. Brown, V. Bulovic´, and M. G. Bawendi, Nat.Mater. 13(8), 796 (2014).13Z. Ning, O. Voznyy, J. Pan, S. Hoogland, V. Adinolfi, J. Xu, M. Li, A. R.Kirmani, J. P. 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, and E. H. Sargent, Nat. Mater. 13(8), 822 (2014).14J. Tang, K. W. Kemp, S. Hoogland, K. S. Jeong, H. Liu, L. Levina, M.Furukawa, X. Wang, R. Debnath, D. Cha, K. W. Chou, A. Fischer, A.Amassian, J. B. Asbury, and E. H. Sargent, Nat. Mater. 10(10), 765 (2011).15M. H. Zarghami, Y. Liu, M. Gibbs, E. Gebremichael, C. Webster, and M.Law, ACS Nano 4(4), 2475 (2010).16J. M. Luther, M. Law, Q. Song, C. L. Perkins, M. C. Beard, and A. J.Nozik, ACS Nano 2(2), 271 (2008).17E. J. D. Klem, H. Shukla, S. Hinds, D. D. MacNeil, L. Levina, and E. H.Sargent, Appl. Phys. Lett. 9221), 212105 (2008).18F. Hetsch, N. Zhao, S. V. Kershaw, and A. L. Rogach, Mater. Today16(9), 312 (2013).19R. Koole, P. Liljeroth, C. De Mello Donega, D. Vanmaekelbergh, and A.Meijerink, J. Am. Chem. Soc. 128(32), 10436 (2006).20K. J. Williams, W. A. Tisdale, K. S. Leschkies, G. Haugstad, D. J. Norris,E. S. Aydil, and X. Y. Zhu, ACS Nano 3(6), 1532 (2009).21W. Ma, J. M. Luther, H. Zheng, Y. Wu, and A. P. Alivisatos, Nano Lett.9(4), 1699 (2009).063901-4 Kim et al. Appl. Phys. Lett. 109, 063901 (2016)

Byung-Sung Kim

John Hong

Bo Hou

Yuljae Cho

Jung Inn Sohn

SeungNam Cha

Jong Min Kim

Crossref

Cha, SeungNam

Online Research @ Cardiff

Appl. Phys. Lett. 109, 063901 (2016); https://doi.org/10.1063/1.4960645 109, 063901© 2016 Author(s).Inorganic-ligand exchanging time effect inPbS quantum dot solar cellCite as: Appl. Phys. Lett. 109, 063901 (2016); https://doi.org/10.1063/1.4960645Submitted: 23 May 2016 . Accepted: 28 July 2016 . Published Online: 09 August 2016Byung-Sung Kim, John Hong , Bo Hou, Yuljae Cho, Jung Inn Sohn, SeungNam Cha, and Jong Min KimARTICLES YOU MAY BE INTERESTED INImpact of dithiol treatment and air annealing on the conductivity, mobility, and hole density inPbS colloidal quantum dot solidsApplied Physics Letters 92, 212105 (2008); https://doi.org/10.1063/1.2917800PbS colloidal quantum dot photoconductive photodetectors: Transport, traps, and gainApplied Physics Letters 91, 173505 (2007); https://doi.org/10.1063/1.2800805 Increased efficiency in pn-junction PbS QD solar cells via NaHS treatment of the p-type layerApplied Physics Letters 110, 103904 (2017); https://doi.org/10.1063/1.4978444Inorganic-ligand exchanging time effect in PbS quantum dot solar cellByung-Sung Kim,1,a) John Hong,1,a) Bo Hou,1 Yuljae Cho,1 Jung Inn Sohn,1,b)SeungNam Cha,1,b) and Jong Min Kim21Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, United Kingdom2Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA,United Kingdom(Received 23 May 2016; accepted 28 July 2016; published online 9 August 2016)We investigate time-dependent inorganic ligand exchanging effect and photovoltaic performanceof lead sulfide (PbS) nanocrystal films. With optimal processing time, volume shrinkage inducedby residual oleic acid of the PbS colloidal quantum dot (CQD) was minimized and a crack-freefilm was obtained with improved flatness. Furthermore, sufficient surface passivation significantlyincreased the packing density by replacing from long oleic acid to a short iodide molecule. It thusfacilities exciton dissociation via enhanced charge carrier transport in PbS CQD films, resulting inthe improved power conversion efficiency from 3.39% to 6.62%. We also found that excess iodineions on the PbS surface rather hinder high photovoltaic performance of the CQD solar cell. VC 2016Author(s). All article content, except where otherwise noted, is licensed under a CreativeCommons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4960645]Colloidal quantum dots (CQDs) are promising candi-dates for next-generation photovoltaics owing to their uniqueproperties such as high absorption coefficient, tunable bandgap, and multiple exciton generation effect.1–4 The solution-based process also has provided high feasibility of realizingflexible and large scale photovoltaic devices with cost-effective fabrication.5–8 Despite these favorable characteris-tics, poor charge transport and imperfect surface status ofCQDs hinder high photovoltaic performance compared tothe theoretical conversion efficiency. During the syntheticprocess, CQDs are capped by long and bulky organic ligandswhich enable to control the size of CQDs by preventingover-size aggregation and enhancing chemical stability.9However, the insulating properties of long hydrocarbonligands limited photoexcited carrier transport, eventuallyresulting in low power conversion efficiency (PCE).Therefore, control of the ligand exchange process which effi-ciently substitutes the surface from long ligands to short mol-ecules is necessary for CQD-based device applications.10,11Enhanced performances of CQD solar cells using the inor-ganic halide ligands (e.g., Cl, Br, I) have been recentlyreported,12,13 but the detailed characterization of the CQDfilm during the ligand exchange process has not been wellinvestigated. Those parameters such as packing density orhalide ion density on the surface should be considered for aneffective way to control the surface status and furtherimprove photovoltaic performance. Here, we investigatetime-dependent influence of inorganic ligand exchange inlead sulfide (PbS) quantum dot films with systematic analysisof photovoltaic performance. Within the same concentrationof tetrabutylammonium iodide (TBAI) which induces thesame driving force of ligand exchange, we studied the chargetransfer mechanism on quantum dot (QD) films based on thequantitative interaction between surface halide ions and QDsurface. Moreover, our study focused on the detailed statusof the PbS surface for understanding the relationshipbetween the packing density and charge carrier transport ofPbS CQD films by adjusting ligand exchanging time. Thesefindings will provide a useful guideline for achieving thehigh efficiency CQD-based solar cells.Inorganic molecule halide treatment was carried outusing TBAI solution in ambient conditions, which is one ofthe most commonly used ligand materials for PbS QD solarcells due to air-stability and low density of trapped carriers byhighly effective passivation.12,13 During the ligand exchange,it introduces negatively charged iodine ions which substitutepristine oleic acid (OA). The halide anions subsequently bindto Pb cations on the PbS surface. If the surface is not perfectlyupgraded, the remaining oleic acids will play a role as insulat-ing barriers which decrease the carrier transport as well asexciton dissociation in PbS CQD films (Fig. 1(a)). Therefore,it is important to minimize the degree of the remaining oleicacids to improve the performance of photovoltaic devices bythe controlled ligand exchange process, exposing the OA-capped CQD films to the solution containing the iodine ions.To systematically investigate the ligand exchanging effect, weprepared PbS CQD films with different ligand exchangingtimes (TL): 15, 30, 60, and 90 s. 3 layers of PbS were depos-ited on glass substrates with TBAI solution (10 mg/ml inmethanol). Evidence for chemical composition changes of theCQD surface can be determined using Fourier transform-infrared spectroscopy (FT-IR) and X-ray photoelectron spec-troscopy (XPS) spectra of the iodide-treated PbS CQD filmswith the different ligand exchanging time of TL. The presenceof the oleic acids can be denoted by the four characteristicpeaks including vibrations at 2920 cm1 (asymmetric C-H),2850 cm1 (symmetric C-H), 1545 cm1 (asymmetricCOO–), and 1403 cm1 (symmetric COO–), respectively.(Fig. 1(b)) Significant reduction of the OA-related vibrationswas observed according to the treatment time.14 Togethera)B.-S. Kim and J. Hong contributed equally to this work.b)Electronic addresses: junginn.sohn@eng.ox.ac.uk and seungnam.cha@eng.ox.ac.uk0003-6951/2016/109(6)/063901/4 VC Author(s) 2016.109, 063901-1APPLIED PHYSICS LETTERS 109, 063901 (2016)with the XPS energy spectra, the atomic ratios of lead andiodine on the surface of PbS CQD films can be evaluated withrespect to the ligand exchange time. Note that the short ligandexchanging time (TL< 30 s) does not provide enough surfacepassivation and considerable oleic acids still remain on thePbS surface. With an increasing of TL, the oleic acids werealmost eliminated and surface iodine concentration is quanti-tatively increased. (Figs. 1(c) and 1(d)).A degree of surface passivation could affect the morpho-logical property of the PbS CQD film. Micro- and nano-scalecracks have been frequently observed in ligand-exchangedCQD devices due to the considerable volume shrink of thefilms when long oleic acids were substituted by short organicand/or inorganic ligands.15–17 Furthermore, the crack forma-tion results in severe problems such as highly inducing shortcircuit or leakage current in CQD-based device applications.18Therefore, careful control of the ligand exchange process isessential for producing crack-free CQD films. The morpho-logical changes of the PbS CQD film were shown with the dif-ferent ligand exchanging times of TL (Fig. 2, Figs. S1 and S2in supplementary material). A very rough surface with thenumerous cracks was obtained at the TL of 15 s while longerligand exchanging time leads to a crack-free surface with theimproved film flatness. This result implies that the presence ofthe residual oleic acids observed at insufficient ligandexchanging time induces the large physical strain with thecoexistent short iodide ligands in PbS CQD films. It thus leadsto the non-uniform volume shrink and accelerating crack for-mation during the solvent evaporation.A highly densely packed CQD film is a critical factor todetermine the photovoltaic performance since the charge car-rier transport is enhanced with minimizing the CQD inter-particle distance.10,11 Figure 3 shows that the packing densi-ties of the CQD film could be effectively tuned by adjustingthe ligand exchanging time. Compared to OA-capped PbSCQDs, the packing densities were gradually increased withproceeding ligand exchange from OA to iodide molecules.(Figs. 3(a)–3(c)) Longer treatment makes the neighboringQDs closer, resulting in partially connection between eachother. Furthermore, exciton absorption peaks were certainlyFIG. 1. A schematic illustrating theligand exchange process on PbS CQDfilms. (b) FTIR spectra and (c) bindingenergy of I 3d and Pb 4f in TBAI-PbSCQD films with an increasing ligandexchanging time of TL. They showsuccessful exchange of iodine on theCQD surface. (d) Variation of theatomic ratio of lead to iodine as a func-tion of the corresponding TL. Thesquares and solid lines of black,orange, blue, and red are TL¼ 15, 30,60, and 90 s, respectively. The dottedlines in Fig. 1(b) are pristine PbS CQDfilm capping with oleic acid.FIG. 2. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of surface morphologies of TBAI-PbS films with increasing TL: ((a)and (e)) 15 s, ((b) and (f)) 30 s, ((c) and (g)) 60 s, and ((d) and (h)) 90 s. These samples are prepared by spin coating the 3 layers of PbS onto the glass substrates.It was shown that the root mean square (RMS) roughness of PbS CQD films was gradually decreased, dependent on TL.063901-2 Kim et al. Appl. Phys. Lett. 109, 063901 (2016)observed to be broadened and red-shifted as seen in Fig.3(d). These phenomena can be explained by electron cou-pling or sintering/necking between QDs as the inter-particledistance becomes closer.19–21 This correlates well with theincreased packing densities with TL observed by carefulTransmission electron microscopy (TEM) analysis.To investigate the relationship between photovoltaicperformance of the PbS film with the different packing den-sities, we obtained photocurrent density and voltage (J–V)curves of the PbS CQD films with the different ligandexchanging times of TL under AM 1.5G illustration (100mW cm2). (Fig. 4(a)) The devices were prepared by 10cycles spin coating of TBAI-PbS CQD films on ZnO/ITOsubstrates (Fig. S3 in supplementary material) and the photo-voltaic performance of the devices is summarized in Figs.4(c) and 4(d) (also see Fig. S4 in supplementary material).Incomplete passivation on the PbS surface caused by shortligand exchanging time of TL hinders efficient charge trans-port in the film (Fig. 4(b)), resulting in relatively low short-circuit current (Jsc) and PCE with high series resistance (Rs).The device with a short TL of 15 s gives a low PCE of 3.39%with a Jsc of 13.75 mA/cm2. Moreover, in Fig. S5, Jsc from theexternal quantum efficiency (EQE) measurement also showsthe same trend which correlates with the Jsc enhancement ofthe J–V measurement by the different ligand exchangingtimes. With an increase of the ligand exchanging time, thepacking densities of PbS QDs as well as surface coverage ofthe iodide molecule were significantly increased. (Figs. 1 and3) It thus facilities exciton dissociation via enhanced chargetransfer in PbS films, leading to substantial improvement of77.58% and 74.98% in PCE, and Jsc at TL of 60 s, respectively(Fig. 4(c) and Table S1 in supplementary material). The valuesof Rs were also decreased from 10.14 X to 3.51 X, dependenton the TL. Interestingly, we found that the photovoltaic perfor-mance was declined after longer TL of 90 s in spite of the highsurface coverage of iodide ligands on the PbS CQDs. It can beassumed that excess iodide molecules which are not bound toPb cations may act as the insulating barrier between PbSCQDs. These results clearly indicate that the controllingligand exchange on the PbS CQD film should be highlyrequired to manipulate the photovoltaic performance.FIG. 3. Transmission electron microscopy (TEM) and selected area diffrac-tion (SAED) pattern images of (a) pristine PbS CQD film capping with oleicacids and (b) TBAI-PbS CQD films at TL of 30 s. With a longer time of 30 s,the packing density of the TBAI-PbS CQD film was slightly increased.Insets are low-magnification TEM images of the corresponding OA- andTBAI-PbS CQD films. (c) Variation of the packing density of PbS QDs as afunction of TL. (d) Absorption spectra of the corresponding PbS CQD films.The closed squares and solid lines of black, orange, blue, and red areTL¼ 15, 30, 60, and 90 s, respectively. The open square and dotted line arepristine PbS CQD film capping with oleic acid.FIG. 4. (a) Representative J–V charac-teristics of the TBAI-PbS CQD devicesby varying TL. The devices were mea-sured in the dark (open square) andunder AM 1.5 illumination (closesquare) at an incident intensity of 100mW/cm2. (b) Schematic of iodine- andOA-rich in PbS CQD. Plots of (c)PCE, Jsc, and Rs, (d) Voc, Rsh, and fillfactor (FF) of the devices as a functionof TL. The closed squares of black,orange, blue, and red indicate differentTL of 15, 30, 60, and 90 s, respectively.063901-3 Kim et al. Appl. Phys. Lett. 109, 063901 (2016)We have demonstrated time-dependent inorganic ligandexchanging effect in PbS CQD films. Controlled surface pas-sivation increases the packing density with a crack-free PbSsurface by minimizing residual oleic acid. It also enables tofacilitate charge transport and significantly improve PbSsolar cell efficiency from 3.39% to 6.62%. Ligand exchangeover a longer TL of a critical value further decreases interpar-ticle spacing but rather slightly deteriorates the photovoltaicperformance by excess iodide molecules acting as a chargetransport barrier on the PbS surface. 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Inorganic-ligand exchanging time effect in PbS quantum dot solar cell

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