High performance solid-state PbS quantum dot-sensitized solar cells

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High performance solid-state PbS quantum dot-sensitized solar cells prepared by introduction of hybrid perovskite inter-layer Jin Hyuck Heo, Min Hyuk Jang, Min Ho Lee, Dong Hee Shin, Do Hun Kim, Sang Hwa Moon, Sang-Wook Kim, Bum Jun Park, and Sang Hyuk Im ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12046 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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ACS Applied Materials & Interfaces

High performance solid-state PbS quantum dotsensitized solar cells prepared by introduction of hybrid perovskite inter-layer Jin Hyuck Heoa,b, Min Hyuk Jangc, Min Ho Leec, Dong Hee Shina, Do Hun Kima, Sang Hwa Moona, Sang Wook Kimb, Bum Jun Parkc,*, Sang Hyuk Ima,* a

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro,

Seongbuk-gu, Seoul 136-713, Republic of Korea. b

Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Republic

of Korea. c

Functional Crystallization Center (ERC), Department of Chemical Engineering, Kyung Hee

University, 1732 Deogyoung-daero, Yongin-si, Gyeonggi-do, Republic of Korea * Corresponding authors. E-mail: [email protected] (B. J. Park), [email protected] (S. H. Im) KEYWORDS. Lead sulfide, quantum dots, sensitized solar cells, perovskite, passivation.

ACS Paragon Plus Environment

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ABSTRACT

High performance solid-state PbS quantum dot-sensitized solar cells (QD-SSCs) with stable 9.2 % power conversion efficiency at 1 Sun condition are demonstrated by introduction of hybrid perovskite inter-layer. The PbS QDs formed on mesoscopic TiO2 (mp-TiO2) by spin-assisted successive precipitation and anionic exchange reaction method do not exhibit PbSO4 but have PbSO3 oxidation species. By introducing perovskite inter-layer in between mp-TiO2/PbS QDs and poly-3-hexylthiophene, the PbSO3 oxidation species are fully removed in the PbS QDs and thereby the efficiency of PbS QD-SSCs is enhanced over 90 % compared to the pristine PbS QDSSCs.


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Recently, metal chalcogenide such as CdS, CdSe, CdTe, PbS, PbSe, CuInS, Sb2S3, Sb2Se3, and Bi2S3 nanoparticles or quantum dots (QDs) have been received great attention as light absorber because of their unique properties such as strong absorptivity, convenient band gap controllability, large dipole moment, multiple exciton generation and solution processability.1-12 Among them, the PbS QDs seems an ideal light absorber because they have large Bohr radius of 18 nm and low bulk energy band gap of ~0.41 eV. Generally, the PbS QD-sensitized solar cells (QD-SSCs) are fabricated by depositing the PbS QDs on a mesoporous TiO2 (mp-TiO2) electron conductor. The conventional colloidal PbS QDs are capped with long alkyl ligand such as oleic acid. Therefore, the capping ligands should be replaced to short ligands such as 1, 2-ethandithiol (EDT) and 3-mercaptopropinic acid (MPA) in order to improve the charge injection and transport.6 On the other hand, the successive ionic layer adsorption and reaction (SILAR) method allows the PbS QDs to grow directly on the mpTiO2 surface so it is more desirable in terms of charge transfer/transport. However, the conventional SILAR method requires long tedious repeated process cycle which is consisted to dipping in anionic (cationic) solution, washing, drying, dipping in cationic (anionic) solution, washing, and drying. Recently, spin-assisted SILAR (spin-SILAR) process was developed to shorten the SILAR process.13 We (Im et al.) also fabricated the PbS QD-SSCs via the spinSILAR method.14,15 Very recently, we (Im et al.) developed the spin-assisted successive precipitation and anion exchange reaction (spin-SPAER) method, which produces high purity PbS QDs with better size uniformity and demonstrated 4.96 % solid-state PbS QD-SSCs.16 However, the solid-state PbS QD-SSCs still exhibit relatively low power conversion efficiency. In PbS QD-SSCs, the bare TiO2 surface and PbS QDs are directly adhered to hole transporting material (HTM). Accordingly, the electrons in mp-TiO2 and PbS QDs can be recombined with


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the holes in HTM if the interface is not properly passivated. For examples, Lee et al. reported that the performance of CdSe-SSCs can be improved by 4-methoxybenzenethiol molecular dipole interface modification and ZnS surface passivation at mp-TiO2/CdSe/P3HT (poly-3hexylthiophene) interface.17 Tang et al. reported that the monovalent halide atomic passivation of PbS colloidal QDs can enhance the electronic transport and the passivation of surface defects of PbS colloidal QDs, thereby exhibiting ~6 % power conversion efficiency (PCE).18 Recently, Yang et al. reported that the ITO (indium tin oxide)/ZnO/PbS colloidal QDs shelled by CH3NH3PbI3 (MAPbI3)/PbS-EDT/Au devices have high PCE of 8.95 % because the MAPbI3 thin-shell makes the PbS colloidal QDs to be more intrinsic QDs, thereby forming deeper depletion region.19 Unlike to the depleted hetero-junction PbS colloidal QDs solar cells, the PbS QD-SSCs expose bare TiO2 surface. It is well known that the charge carriers generated in the MAPbI3 perovskite are effectively transferred into TiO2.20-22 Accordingly, here we deposited thin MAPbI3 inter-layer on the bare TiO2 and PbS QDs in PbS QD-SSCs because it is expected that the MAPbI3 interlayer plays an important role as passivation and trap-healing layer at mp-TiO2/MAPbI3/P3HT and mp-TiO2/PbS QDs/MAPbI3/P3HT interface, respectively. By introduction of thin MAPbI3 interlayer, we could greatly enhance the device efficiency from 4.8 % to 9.2 % at standard 1 Sun condition. Figure1 is a schematic illustration of the solid-state PbS QD-SSC with MAPbI3 inter-layer. The device is constructed to glass/FTO (F-doped tin oxide)/bl-TiO2 (blocking TiO2)/mp-TiO2/PbS QDs/MAPbI3 inter-layer/P3HT/Au. The energy band diagram of solid-state PbS QD-SSC with MAPbI3 inter-layer was shown in Figure S1. Upon illumination, the PbS QD-sensitizer generates electron-hole pairs. The generated electrons are promptly transferred/transported into


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the mp-TiO2 electron conductor and at the same time the generated holes are transferred/transported into P3HT hole conductor through the MAPbI3 inter-layer. In addition, the thin MAPbI3 inter-layer formed on the bare mp-TiO2 surface can also generate loosely bonded electron-hole pairs or free charge carriers due to small exciton binding energy. The electrons (holes) generated in the MAPbI3 inter-layer can be additionally transferred/transported into mp-TiO2 (P3HT). Accordingly, we speculate that it acts as additional panchromatic sensitizer and passivation layer which prevents the backward recombination between the electrons in mp-TiO2 and the hole in P3HT. On the other hand, we speculate that the thin MAPbI3 inter-layer positioned on the PbS QDs can heal the surface traps of PbS QDs by donation of halide source, thereby improving device performance. Here, we deposited PbS QDs on mp-TiO2 by spin-SPAER method.16 In a typical one cycle process, 5 mM PbBr2/DMF (N, N-dimethylformamide) solution was spin-coated at 3000 rpm for 60 s. 5 mM Na2S/H2O/methanol solution was then spin-coated at 3000 rpm for 60 s in order to convert PbBr2 nanocrystals into PbS QDs. Then EDT/ethanol solution was spin-coated at 3000 rpm for 60 s. We repeated above coating cycle to 15 times (PbS QDs-15) and produced PbS QDs with 3-5 nm in diameter on mp-TiO2.16 2.5 wt% MAPbI3/DMF solution was then spin-coated on the PbS QDs-15/mp-TiO2/bl-TiO2/FTO/glass at 5000 rpm for 60 s (PbS QDs-15-2.5MA). Figure 2(a) and (e) is a photograph of PbS QDs-15/mp-TiO2/bl-TiO2/FTO/glass and PbS QDs-152.5MA/mp-TiO2/bl-TiO2/FTO/glass, respectively. The SEM (scanning electron microscopy) surface (Fig. 2(b) and (f)) and cross-sectional image (Fig. 2(c) and (g)) of PbS QDs-15/mpTiO2/bl-TiO2/FTO/glass

(Fig.

2(b)

and

(c))

and

PbS

QDs-15-2.5MA/mp-TiO2/bl-

TiO2/FTO/glass (Fig. 2(f) and (g)), respectively. These images clearly show that the meso-pores are still remained in the mp-TiO2 layer after deposition of PbS QDs-15 and PbS QDs-15-2.5MA


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on the mp-TiO2 film and consequently it is expected that the P3HT hole transporting layer will be well infiltrated within the meso-pores. Figure 2(d) and (h) is TEM (transmission electron microscopy) image of PbS QDs-15/mp-TiO2 and PbS QDs-15-2.5MA/mp-TiO2, respectively. These images show that the PbS nanocrystals are successfully formed by the repeated spinSPAER process (PbS QDs-15) and the MAPbI3 inter-layer is formed on the PbS nanocrystals and the TiO2 nanoparticles (see the red arrows in Fig. 2(h)). Here we controlled the concentration of MAPbI3/DMF solution to 1 (PbS QDs-15-1MA), 2.5 (PbS QDs-15-2.5MA), and 5 wt% (PbS QDs-15-5MA) and formed the MAPbI3 inter-layer. The TEM images of PbS QDs-15-1M and PbS QDs-15-5MA were shown in Figure S2. As the concentration of MAPbI3/DMF solution, apparently, the bare surface of mp-TiO2 seems to be gradually more covered by MAPbI3. At the same time, the surface of PbS QDs is also covered by MAPbI3 but its crystalline image is faded under exposure of e-beam, which is often observed in organicinorganic hybrid perovskite under TEM measurement. The SEM surface and cross-sectional images of P3HT (~50 nm in thickness)/PbS QDs-15 with (1, 2.5, and 5 MA) and without MAPbI3 inter-layers/mp-TiO2/bl-TiO2/FTO/glass were shown in Figure S3. To check the crystalline structure of formed product on mp-TiO2, we measured X-ray diffraction (XRD) patterns as shown in Figure 3(a). The PbS QDs-15 sample displays FTO, TiO2 and PbS diffraction peaks. On the other hand, the MAPbI3 coated films exhibited additional MAPbI3 peaks and its peak intensity is increased as the solution concentration of MAPbI3 increases. The PbS QDs-15-1MA sample showed the (110) diffraction peak of MAPbI3 at 14.1° and the PbS QDs-2.5MA samples exhibited the (220) diffraction peak at 28.4° and the slightly increased peak intensity of (110) facet. However, the PbS crystal peaks disappeared and the MAPbI3 crystal peaks were intensified in the PbS QDs-5MA sample. This implies that the PbS


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can be transformed into MAPbI3 or washed away under spin-coating of excess concentration of MAPbI3/DMF solution. The possible reactions are as follow: CH3NH3PbI3 ↔ CH3NH3I + PbI2 ↔ CH3NH2 + HI + PbI2 (1) PbS + 2HI ↔ PbI2 + H2S

(2)

Here, the PbI2 is soluble in DMF or can be transformed into MAPbI3 by reacting with MAI. Accordingly, above XRD results are well matched with the TEM images. The intensity ratio of MAPbI3 and PbS in PbS QDs-15-2.5MA sample in Fig. 3a is 1:71 so we expect that the contribution of MAPbI3 as co-sensitizer is ~1.4 % at best. Figure 3(b-d) is the XPS (X-ray photoelectron spectroscopy) spectra of PbS QDs-15, PbS QDs-15-1MA, PbS QDs-15-2.5MA, and PbS QDs-15-5MA. The Pb4f peaks of all samples in Fig. 3(b) indicate that metallic Pb or PbI2 does not remained, thereby forming pure PbS and MAPbI3. The slight chemical shift of two Pb4f peaks in PbS QDs-15-5MA is attributed to the formation of MAPbI3. The I3d peak spectra in Fig. 3(c) indicate that the concentration of I is gradually increased in the surface of mp-TiO2/PbS QDs/MAPbI3 layer. Figure 3(d) is the peak spectra of S2p indicating that the PbS QDs are disappeared in PbS QDs-15-5MA sample, which is caused by the chemical reaction. In addition, the SO3 peaks around 162-164 eV are diminished/disappeared by introduction of MAPbI3 inter-layer. This implies that the surface defects of PbS QDs can be recovered by introduction MAPbI3 inter-layer. Figure 4(a) is absorption spectra of PbS QDs-15, PbS QDs-15-1MA, PbS QDs-15-2.5MA, and PbS QDs-15-5MA sample. The absorption spectra clearly indicate that the deposited PbS


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nanoparticles on mp-TiO2 is QDs because the size of PbS QDs can be calculated by the following equation.16,23 Eg = 0.41 + 1/(0.025d2 + 0.283d)

(3)

Where Eg is the electronic bandgap of PbS QDs and d is a diameter of PbS QDs. From the above equation, the calculated average diameter of PbS QDs was ~3.7 nm, which is well matched with our previous report.16 Apparently, the absorption spectra of PbS QDs-15 and PbS QDs-15-1MA are almost same but the absorptivity of PbS QDs-15-2.5MA is increased. From the XRD and XPS results, we can understand that the increased absorptivity below 800 nmwavelength is attributed to the deposited MAPbI3 layer but it cannot explain the increased absorptivity over 800 nm-wavelength. For a model experiment, we formed PbS film on a glass by spin-SPAER method and spin-coated 2.5 wt% MAPbI3/DMF solution on it as shown in Figure S4. This indicates that the PbS can be formed by the chemical reaction of residual Na2S remained during spin-SPAER process and MAPbI3 as follow: MAPbI3 + Na2S ↔ MAI + PbI2 + Na2S ↔ MAI +PbS

(4)

Basically, above chemical reaction is similar with the spin-SPAER process which sequentially reacts PbI2 with Na2S. So, additional absorptivity over 800 nm-wavelength can be appeared by the formed PbS. This implies that if we consider the absorption spectrum pattern of PbS, the enhanced absorptivity in PbS QDs-15-2.5MA sample is mainly originated by the formed PbS. In PbS QDs-15-5MA sample, its absorptivity is abruptly shortened to 800 nm-wavelength. This is well matched with the results of XRD and XPS because PbS QDs can be transformed into MAPbI3 or washed away by spin-coating of 5 wt% MAPbI3/DMF solution.


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Figure 4(b) is EQE (external quantum efficiency) spectra of PbS QDs-15, PbS QDs-15-1MA, PbS QDs-15-2.5MA, and PbS QDs-15-5MA based solar cells. The on-set values of EQE spectra as shown in the magnified spectra are blue-shifted by spin-coating of MAPbI3/DMF solution due to the transformation of surface of PbS QDs into MAPbI3 or etching. The EQE is a product of light harvesting efficiency (ηlhe), charge separation/chare injection efficiency (ηcs), and charge collection efficiency (ηcc). The PbS QDs-15 and PbS QDs-15-1MA devices have similar absorption spectra but the EQE values of PbS QDs-15-1MA device was greatly improved. Therefore, the enhanced EQE values are attributed to the improved ηcs and/or ηcs by introduction of MAPbI3 inter-layer. The PbS QDs-15-2.5MA solar cell exhibited more improved EQE values than the PbS QDs-15-1MA device due to the enhanced ηlhe by the formed PbS and MAPbI3 interlayer. Similar to the absorption spectrum, the on-set EQE spectrum of PbS QDs-15-5MA device was greatly blue-shifted to ~825 nm. Figure 4(c) is current density-voltage (J-V) curves of PbS QDs-15, PbS QDs-15-1MA, PbS QDs-15-2.5MA, and PbS QDs-15-5MA based solar cells with the highest performance. The standard deviation of photovoltaic parameters of 30 samples for all devices and the best device’s photovoltaic parameters are summarized in Table 1. The box plots of photovoltaic parameters of 30 samples for all devices are shown in Figure S5. The pristine PbS QDs-15 device had 0.60 V open-circuit-voltage (Voc), 13.8 mA/cm2 short-circuit-current density (Jsc), 57.9 % fill factor (FF), and 4.5 % power conversion efficiency (PCE) at 1 Sun condition. The PbS QDs-15-1MA device had improved performance of 0.60 V Voc, 18.0 mA/cm2 Jsc, 60.4 % FF, and 6.5 % PCE. So the device improvement is mainly attributed to the improved Jsc due to the enhanced ηcs and/or ηcc by defects/traps healing. Eventually, the PbS QDs-15-2.5MA device exhibited the best performance of 0.62 V Voc, 22.5 mA/cm2 Jsc, 66.3 % FF, and 9.2 % PCE because the absorptivity is enhanced


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by the formed PbS and MAPbI3 and the defects/traps are healed by the MAPbI3 inter-layer formed on the PbS QDs. The PbS QDs-15-5MA device had a reduced performance of 0.74 V Voc, 16.3 mA/cm2 Jsc, 68.5 % FF, and 8.3 % PCE because its Jsc is reduced by the chemical reaction of PbS QDs and MAPbI3. The calculated Jsc values from EQE spectra in Fig. 4(b) were well matched with the Jsc values of J-V curves. In order to understand the contribution of MAPbI3 passivation layer in the PbS QD-SSCs with MAPbI3 passivation layer, we measured the photovoltaic properties of MAPbI3 perovskite model devices prepared by the MAPbI3 passivation solution with 1, 2.5, and 5 wt% concentration as shown in Figure S6. The PCE was 0.7 and 1.8 % in 1 and 2.5 wt% sample, respectively. Therefore, the contribution of MAPbI3 perovskite in the PCE is small in 1 and 2.5 wt% samples. In addition, it should be noted that most surface of the mp-TiO2 is covered by PbS QDs in the real devices and consequently the occupation volume of MAPbI3 layer in mp-TiO2/PbS QDs will be much smaller than the model devices. Accordingly, we can think that the real contribution of MAPbI3 layer in PbS QD-SSCs with MAPbI3 passivation layer (1 and 2.5 wt% samples) is much smaller than the model cases. The device stability was shown in Figure S7, of which the efficiency of unencapsulated PbS QDs-15-2.5MA device was recorded with continuous light soaking of 1 Sun. This indicates that the PbS QD-SSCs with MAPbI3 inter-layer has good stability. Finally, we checked the static PL (photo-luminescent) and dynamic PL spectra of PbS QDs-15, PbS QDs-15-1MA, PbS QDs-15-2.5MA, and PbS QDs-15-5MA samples as shown in Figure 5. To understand the PL characteristics of PbS QDs and PbS QDs with MAPbI3 passivation layer, we deposited the PbS QDs and PbS QDs with MAPbI3 inter-layer on mp-Al2O3 as shown in Fig. 5(a) and (b). The static PL spectra in Fig. 5(a) indicate that the maximum positions of PL spectra are gradually blue-shifted as the concentration of MAPbI3/DMF spin-coating solution:


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PbS QDs-15 = ~ 1025 nm, PbS QDs-15-1MA = ~1035 nm, PbS QDs-15-2.5 MA = ~1055 nm, and PbS QDs-15-5MA = ~780 nm. The increased PL intensity of PbS QDs-15-1MA sample than the pristine PbS QDs-15 sample indicates that the defects/traps are recovered by spin-coating of 1 wt% MAPbI3/DMF solution. However, the PbS QDs-15-2.5MA sample had a decreased PL intensity at ~1055 nm and a small PL peak at ~780 nm. The decreased PL intensity at ~1055 nm might be attributed to the charge separation of PbS QDs/MAPbI3 and the small PL peak at ~780 nm is originated from the MAPbI3 formed on bare mp-TiO2. The PL peak of PbS QDs-15-5MA sample at ~780 nm confirms the disappearance of PbS QDs by chemical reaction. The transient PL decays in Fig. 5(b) indicate that the pristine PbS QDs-15 sample has many traps (see the magnified plots) so that its PL is significantly quenched at early time. By introduction of MAPbI3 inter-layer, the traps were significantly removed. As the concentration of MAPbI3/DMF spin-coating solution increases, the fast PL quenching (trap-assisted PL quenching) species are reduced so that we can think that the defects/traps in PbS QDs are reduced by introduction of MAPbI3 inter-layer. To check the charge separation/charge injection characteristics from PbS QDs and PbS QDs with MAPbI3 inter-layer, we deposited the PbS QDs-15, PbS QDs-15-1MA, PbS QDs-15-2.5MA, and PbS QDs-15-5MA on mp-TiO2 and compared their transient PL decays as shown in Figure 5(c). Once the n-p junction is made, the charge carriers produced in PbS QDs and PbS QDs with MAPbI3 inter-layer are effectively separated/injected into mp-TiO2. The degree of overall PL quenching by charge separation/injection was in the order to PbS QDs-15 < PbS QDs-15-1MA < PbS QDs-15-5MA < PbS QDs-15-2.5MA. Accordingly, the PbS QDs-15-2.5MA devices have the best efficiency in the current experimental condition because they have greatly reduced defects/traps due to the MAPbI3 inter-layer. The fitting parameters for the dynamic PL decay curves


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of PbS QDs-15, PbS QDs-15-1MA, PbS QDs-15-2.5MA, and PbS QDs-15-5MA deposited on mp-Al2O3 and on mp-TiO2 was summarized in Table S1.

We successfully demonstrated high performance PbS QD-SSCs with stable 9.2 % PCE at 1 Sun condition by introduction of MAPbI3 inter-layer. Although the PbS QDs deposited by spinSPAER method using PbBr2 and Na2S precursors do not have PbSO4 species which are known to cause deep traps, they still have oxidized PbSO3 species. By spin-coating of MAPbI3/DMF dilute solution, we formed MAPbI3 inter-layer on PbS QDs and bare surface of mp-TiO2. During the spin-coating of MAPbI3/DMF solution on the PbS QDs/mp-TiO2/bl-TiO2/FTO substrate, we found that the surface of PbS QDs can be etched by the chemical reaction between MAPbI3/DMF and PbS QDs and at the same time additional PbS QDs are also created by the reaction of residual Na2S and MAPbI3. Accordingly, the bandgap of pristine PbS QDs are gradually blue-shifted as the concentration of MAPbI3/DMF spin-coating solution increases from 1 wt% to 5 wt%. From the XRD and TEM analysis, we found that small quantity of MAPbI3 inter-layer is formed on PbS QDs on mp-TiO2 film by spin-coating of 1 and 2.5 wt% MAPbI3/DMF solution, but by 5 wt% solution the MAPbI3 nanoparticles are only remained on mp-TiO2 film. Therefore, the on-set wavelength of absorption and EQE spectra are gradually blue-shifted.

From the XPS analysis, we found that the PbSO3 oxidized species are fully

eliminated by introduction MAPbI3 inter-layer, thereby greatly improving the device performance. From the static and transient PL analysis, we found that the defects/traps in the pristine PbS QDs can be very significantly removed by introduction of MAPbI3 inter-layer and the charge extraction is also improved. Due to the defects/traps healing, improved charge extraction, and passivation of MAPbI3 inter-layer on PbS QDs on mp-TiO2, we could fabricate high performance and stable PbS QD-SSCs.


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ASSOCIATED CONTENT Supporting Information. Experimental, SEM, TEM, UV-vis absorption spectra, device performance, and device stability are included. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

ACKNOWLEDGMENT This study was supported by the National Research Foundation of Korea (NRF) under the Ministry of

Science,

ICT

(No.2014R1A5A1009799),

&

Future Planning

Nano-Material

(Basic Science Research

Technology

Development

Program

Program

(No.

2017M3A7B4041696), the Priority Research Program (2009-0093826)) and by grant from Kyung Hee University (KHU-20141582).

REFERENCES (1) Lee, Y. H.; Im. S. H.; Lee, J. H.; Lee, J. H.; Seok, S. I. Performance Enhancement Through Post-treatments Of CdS-sensitized Solar Cells Fabricated By Spray Pyrolysis Deposition. ACS Appl. Mater. Interface 2010, 2, 1648-1652.


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(2) Lee, H. J.; Wang, M. K.; Chen, P.; Gamelin, D. R.; Zakeeruddin, S. M.; Grätzel, M.; Nazeeruddin, M. K. Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by an Improved Successive Ionic Layer Adsorption and Reaction Process. Nano lett. 2009, 9, 4221-4227. (3) Im, S. H.; Lee, Y. H.; Seok, S. I. Photoelectrochemical Solar Cells Fabricated From Porous CdSe And CdS Layers. Electrochim. Acta 2010, 55, 5665-5669. (4) Lan, G. Y.; Yang, Z.; Lin, Y. W.; Lin, Z. H.; Liao, H. Y.; Chang, H. T. A Simple Strategy For Improving The Energy Conversion Of Multilayered CdTe Quantum Dot-Sensitized Solar Cells, J. Mater. Chem. 2009, 19, 2349-2355. (5) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Highly Efficient Multiple Exciton Generation In Colloidal PbSe And PbS Quantum Dots. Nano Lett. 2005, 5, 865-871. (6) Im, S. H.; Kim, H.-j; Kim, S. W.; Kim, S.-W.; Seok, S. I. All Solid State Multiply Layered PbS Colloidal Quantum-dot-sensitized Photovoltaic Cells. Energy Environ. Sci. 2011, 4, 4181-4186. (7) Zhang, J. B.; Gao, J. B.; Church, C. P.; Miller, E. M.; Luther, J. M.; Klimov, V. I.; Beard, M. C. PbSe Quantum Dot Solar Cells With More Than 6% Efficiency Fabricated In Ambient Atmosphere. Nano Lett. 2014, 14, 6010-6015. (8) Yang, J.; Kim, J.-Y.; Yu, J. H.; Ahn, T.-Y.; Lee, H.; Choi, T.-S.; Kim, Y.-W.; Joo, J.; Ko, M. J.; Hyeon, T. Copper–indium–selenide Quantum Dot-sensitized Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 20517-20525. (9) Kim, D. H.; Lee, S. J.; Park, M. S.; Kang, J. K.; Heo, J. H.; Im, S. H.; Sung, S. J. Highly Reproducible Planar Sb2S3-sensitized Solar Cells Based On Atomic Layer Deposition. Nanoscale 2014, 6, 14549-14554. (10) Choi, Y. C.; Mandal, T. N.; Yang, W. S.; Lee, Y. H.; Im, S. H.; Noh, J. H.; Seok, S. I. Sb2Se3‐Sensitized Inorganic–Organic Heterojunction Solar Cells Fabricated Using a Single‐Source Precursor. Angew. Chem. Int. Ed. 2014, 53, 1329-1333.


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(11) Martinez, L.; Bernechea, M.; García de Arquer, F. P.; Konstantatos, G. Near IR-Sensitive, Non-toxic, Polymer/Nanocrystal Solar Cells Employing Bi2S3 as the Electron Acceptor. Adv. Energy Mater. 2011, 1, 1029-1035. (12) Konstantatos, G.; Howard, L.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H. Sensitive Solution-processed Bi2S3 Nanocrystalline Photodetectors. Nature 2006, 442, 180-183. (13) Joo, J.; Kim, D.; Yun, D. -J.; Jun, H.; Rhee, S. -W.; Lee, J. S.; Yong, K.; Kim, S.; Jeon, S. The Fabrication Of Highly Uniform ZnO/CdS Core/shell Structures Using A Spin-coatingbased Successive Ion Layer Adsorption And Reaction Method. Nanotechnology 2010, 21, 325604. (14) Im, S. H.; Kim, H. -J.; Kim, S.; Kim, S. -W.; Seok, S. I. Improved Air Stability Of PbSsensitized Solar Cell By Incorporating Ethanedithiol During Spin-assisted Successive Ionic Layer Adsorption and Reaction. Org. Electron. 2012, 13, 2352-2357. (15) Im, S. H.; Kim, H.-j.; Seok, S. I. Near-infrared Responsive PbS-sensitized Photovoltaic Photodetectors Fabricated By The Spin-assisted Successive Ionic Layer Adsorption And Reaction Method. Nanotechnology 2011, 22, 395502. (16) Heo, J. H.; Jang, M. H.; Lee, M. H.; You, M. S.; Kim, S. -W.; Lee, J. -J.; Im, S. H. Formation Of Uniform PbS Quantum Dots By Spin-assisted Successive Precipitation And Anion Exchange Reaction Process Using PbX2 (X = Br, I) And Na2S Precursor. RSC Adv. 2017, 7, 3072-3077. (17) Lee, Y. H.; Im, S. H.; Chang, J. A.; Lee, J. -H.; Seok, S. I. CdSe-sensitized Inorganic– Organic Heterojunction Solar Cells: The Effect Of Molecular Dipole Interface Modification And Surface Passivation. Org. Electron. 2012, 13, 975-979. (18) Tang, J.; Kemp, K .W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X.; Debnath, R.; Cha, D.; Chou, K. W.; Fischer, A.; Amassian, A.; Asbury, J. B.; Sargent, E. H. Colloidal-quantum-dot Photovoltaics Using Atomic-ligand Passivation. Nat. Mater. 2011, 10, 765-711.


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(19) Yang, Z.; Janmohamed, A.; Fan, X.; Garcia de Arquer, F. P.; Voznyy, O.; Yassitepe, E.; Kim, G. -H.; Ning, Z.; Gong, X.; Comin, R.; Sargent, E. D. Colloidal Quantum Dot Photovoltaics Enhanced By Perovskite Shelling. Nano Lett. 2015, 15, 7539-7543. (20) Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C. -S.; Chang, J. A.; Lee, Y. H.; Kim, H. -J.; Sarkar, A.; Nazeeruddin, M. K.; Grätzel, M.; Soek, S. I. Efficient Inorganic-organic Hybrid Heterojunction Solar Cells Containing Perovskite Compound And Polymeric Hole Conductors. Nat. Photonics 2013, 7, 486-491. (21) Heo, J. H.; Im, S. H. CH3NH3PbI3/poly‐3‐hexylthiophen Perovskite Mesoscopic Solar Cells: Performance Enhancement By Li‐assisted Hole Conduction. Phys. Status Solidi RRL 2014, 8, 816-821. (22) Heo, J. H.; You, M. S.; Chang, M. H.; Yin, W.; Ahn, T. K.; Lee, S.-J.; Sung, S.-J.; Kim, D. H.; Im, S. H. Hysteresis-less Mesoscopic CH3NH3PbI3 Perovskite Hybrid Solar Cells By Introduction Of Li-treated TiO2 Electrode. Nano Energy, 2015, 15, 530-539. (23) Hyun, B. R.; Zhong, Y. W.; Bartnik, A. C.; Sun, L.; Abruña, H. D.; Wise, F. W.; Goodreau, J. D.; Matthews, J. R.; Leslie, T. M.; Borrelli, N. F. Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles. ACS Nano, 2008, 2, 2206–2212.


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LIST OF TABLE AND FIGURE CAPTIONS Table 1. Summary of photovoltaic properties of PbS QD-15, PbS QDs-15-1MA, PbS QDs-152.5MA, and PbS QDs-15-5MA based solar cells.

Figure 1. Schematic illustration of the solid-state PbS QD-SSC with MAPbI3 inter-layer. Figure 2. Photographs (a, e) of PbS QDs-15/mp-TiO2/bl-TiO2/FTO/glass (a) and PbS QDs-152.5MA/mp-TiO2/bl-TiO2/FTO/glass; SEM (scanning electron microscopy) surface (b, f) and cross-sectional image (c, g) of PbS QDs-15/mp-TiO2/bl-TiO2/FTO/glass (b, c) and PbS QDs-152.5MA/mp-TiO2/bl-TiO2/FTO/glass (f, g); and TEM (transmission electron microscopy) image (d, h) of PbS QDs-15/mp-TiO2 (d) and PbS QDs-15-2.5MA/mp-TiO2 (h). The red arrows indicate the MAPbI3 layer. Figure 3. (a) X-ray diffraction (XRD) patterns and XPS (X-ray photoelectron spectroscopy) spectra (b-d) of (b) Pb2p, (c) I3d, and (d) S2p for PbS QDs-15, PbS QDs-15-1MA, PbS QDs-152.5MA, and PbS QDs-15-5MA on mp-TiO2/bl-TiO2/FTO. Figure 4. (a) Absorption spectra, (b) EQE (external quantum efficiency), and (c) current densityvoltage (J-V) curves of PbS QDs-15, PbS QDs-15-1MA, PbS QDs-15-2.5MA, and PbS QDs-155MA based solar cells.


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Figure 5. (a) Static PL (photo-luminescent) and (b, c) transient PL spectra of PbS QDs-15, PbS QDs-15-1MA, PbS QDs-15-2.5MA, and PbS QDs-15-5MA deposited on mp-Al2O3 (a, b) and on mp-TiO2 (c).

Table 1. Summary of photovoltaic properties of PbS QD-15, PbS QDs-15-1MA, PbS QDs-152.5MA, and PbS QDs-15-5MA based solar cells (average = standard deviation of 30 samples). Devices PbS QDs-15 (average) PbS QDs-15 (best) PbS QDs-15-1MA (average) PbS QDs-15-1MA (best) PbS QDs-15-2.5MA (average) PbS QDs-15-2.5MA (best) PbS QDs-15-5MA (average) PbS QDs-15-5MA (best)

Voc (V)

Jsc (mA/cm2)

FF (%)

0.516 ± 0.058 11.49 ± 1.745 0.507 ± 0.051 0.60

13.8

57.9

0.549 ± 0.040 16.93 ± 1.033 0.557 ± 0.043 0.60

18.0

60.4

0.592 ± 0.020 20.94 ± 1.092 0.626 ± 0.032 0.62

22.5

66.3

0.702 ± 0.022 15.70 ± 0.854 0.656 ± 0.019 0.74

16.3

68.5

PCE (%) 3.115 ± 1.021 4.8 5.228 ± 0.983 6.5 7.795 ± 0.975 9.2 7.251 ± 0.708 8.3


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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


Au P3HT

h

PbS

h MAPbI3 e

h

h

h

Bl-TiO2 FTO

e

Glass

TiO2

e

e

e

Figure 1. Schematic illustration of the solid-state PbS QD-SSC with MAPbI3 inter-layer.


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Figure 2. Photographs (a, e) of PbS QDs-15/mp-TiO2/bl-TiO2/FTO/glass (a) and PbS QDs-152.5MA/mp-TiO2/bl-TiO2/FTO/glass; SEM (scanning electron microscopy) surface (b, f) and cross-sectional image (c, g) of PbS QDs-15/mp-TiO2/bl-TiO2/FTO/glass (b, c) and PbS QDs-152.5MA/mp-TiO2/bl-TiO2/FTO/glass (f, g); and TEM (transmission electron microscopy) image (d, h) of PbS QDs-15/mp-TiO2 (d) and PbS QDs-15-2.5MA/mp-TiO2 (h). The red arrows indicate the MAPbI3 layer.


20

(a) Normalized intensity (a.u.) 10

20

30

(b)

PbS MAPbI3

FTO TiO2

40

50

148

60

146

144

(c)

(d) Normalized intensity (a.u.) 622

620

618

140

138

136

134

616

614

PbSO3

168

Binding energy (eV)

PbS QDs-15

142

Binding energy (eV)

2θ (degree)

Normalized intensity (a.u)

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


Normalized intensity (a.u.)

Page 21 of 24

PbS QDs-15-1MA

166

164

162

160

158

Binding energy (eV)

PbS QDs-15-2.5MA

PbS QDs-15-5MA


21


Figure 3. (a) X-ray diffraction (XRD) patterns and XPS (X-ray photoelectron spectroscopy) spectra (b-d) of (b) Pb2p, (c) I3d, and (d) S2p for PbS QDs-15, PbS QDs-15-1MA, PbS QDs-152.5MA, and PbS QDs-15-5MA on mp-TiO2/bl-TiO2/FTO.

(b)

25 2

80

20 EQE (%)

Absorbance (a.u.)

PbS QDs-15 PbS QDs-15-1MA PbS QDs-15-2.5MA PbS QDs-15-5MA

30


60

15 40

10 20

5 0

0 400

600

800

1000

1200

400

Wavelength (nm)

600

800

1000

1200

Wavelength (nm)

25

60

(c)


2

20

10 5


40

EQE (%)

15

20

0 -5 0.0

0 0.2

0.4

0.6

0.8

1.0

Calculated Jsc (mA/cm )

100

(a)

Current density (mA/cm )

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

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1.2

800

Voltage (V)

900

1000

1100

Wavelength (nm)


22

Page 23 of 24

Figure 4. (a) Absorption spectra, (b) EQE (external quantum efficiency), and (c) current densityvoltage (J-V) curves of PbS QDs-15, PbS QDs-15-1MA, PbS QDs-15-2.5MA, and PbS QDs-155MA based solar cells.

(a)

(b)

16.0k

Normalized PL intensity (a.u.)

PL intensity (a.u.)

PbS PbS+1 wt% MAPbI3 PbS+2.5 wt% MAPbI3

12.0k

PbS+5 wt% MAPbI3

8.0k

4.0k

1 PbS PbS+1 wt% MAPbI3 PbS+2.5 wt% MAPbI3 PbS+5 % wtMAPbI3

0.1

IRF

0.01

0.0 800

900

1000

1100

0

1200

20

40

1 PbS PbS + 1 wt% MAPbI3


(c)

PbS + 2.5 wt% MAPbI3 PbS + 5 wt% MAPbI3

0.1

IRF

0.01

0

10

20

60

80

Time (ns)

Wavelength (nm)


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


30

40

1

traps PbS PbS+1 wt% MAPbI3 PbS+2.5 wt% MAPbI3 PbS+5 % wtMAPbI3

0.1

IRF

6

Time (ns)

8

10

12

Time (ns)


23


Figure 5. (a) Static PL (photo-luminescent) and (b, c) transient PL spectra of PbS QDs-15, PbS QDs-15-1MA, PbS QDs-15-2.5MA, and PbS QDs-15-5MA deposited on mp-Al2O3 (a, b) and on mp-TiO2 (c).

TOC Au P3HT

h

PbS

h MAPbI3 e e

Glass

TiO2

25

e

e

e

100

W/ MAPbI3 W/O MAPbI3

2

20

80

EQE (%)

15 10 5

60

40

20

0 -5 0.0

h

h

h

Bl-TiO2 FTO

Current density (mA/cm )

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

Page 24 of 24

0 0.2

0.4

Voltage (V)

0.6

0.8

400

600

800

1000

1200

Wavelength (nm)


24

High performance solid-state PbS quantum dot-sensitized solar cells

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