Cascaded band alignments of PbS heterojunction layers for improved performance of PbS quantum dot solar cells

Cascaded band alignments of PbS heterojunction layers for improved performance of PbS quantum dot solar cells

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Solar Energy Materials & Solar Cells 208 (2020) 110363

Contents lists available at ScienceDirect

Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat

Cascaded band alignments of PbS heterojunction layers for improved performance of PbS quantum dot solar cells Dasom Park, Sanggyu Yim * Department of Chemistry, Kookmin University, Seoul, 02707, South Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Lead sulfide Colloidal quantum dots Solar cells Ligand exchange Alkylammonium iodides Cascaded band alignment

Efficient energy band alignment of heterojunction layers in colloidal quantum dot (CQD)-based solar cells is a crucial factor to govern the charge transport characteristics and device performance. In this work, we develop novel cascaded-junctions of lead sulfide (PbS) CQD triple layers consisting of an alkylammonium iodide (AMI)treated PbS bilayer and a 1,3-propanedithiol (PDT)-treated single layer. The two AMIs, i.e. triethylamine hydroiodide (tri-EAHI) and tetraethylammonium iodide (TEAI), are less hindered and have superior passivation performance compared to tetrabutylammonium iodide (TBAI), the most commonly used AMI. In addition, the band positions of the PbS-TEAI and PbS-tri-EAHI layers are deeper by 0.26 and 0.46 eV, respectively, than those of the PbS-PDT layer, and hence the sequential stacking of these three layers enable an effective cascaded band alignment. The various benefits of the improved band alignment such as increased built-in potential, reduced trap states, widened depletion region, enhanced charge transport and suppressed charge recombination lead to a significant improvement in the device parameters, and the best power conversion efficiency of 10.46% is ob­ tained for the cascaded PbS-tri-EAHI/PbS-TEAI/PbS-PDT-based device.

1. Introduction Colloidal quantum dots (CQDs) have attracted increasing attention as a promising photoactive material for next-generation solar cells due to their high absorption coefficient, large exciton Bohr radius, sizedependent bandgap tunability, solution processability and multiple exciton generation [1–5]. The lead sulfide (PbS)-based CQD solar cell (CQDSC) is one of the most widely studied devices, and its power con­ version efficiency reached over 12% [6,7]. For the enhanced charge transport in the CQD layer, the long-chain oleate ligands on the CQD surface providing dispersibility should be replaced to shorter ones in solid-state or in solution [8–10]. The properties of the CQD films such as the energy level, trap density and charge transport are strongly depen­ dent on the types of ligands and the degree of passivation [10–14]. Especially, a bilayer structure consisting of iodide-treated and dithiol-treated CQDs led to a remarkable enhancement of the device performance [9,15]. The tetrabutylammonium iodide (TBAI) and 1, 2-ethanedithiol (EDT) are the most commonly used ligands for that purpose. In PbS CQD bilayer-based devices with an architecture of ITO/n-type semiconductor/PbS-TBAI/PbS-EDT/anode, the energy levels of the PbS-EDT layer are positioned higher than those of the

PbS-TBAI layer, which suppresses the flow of photogenerated electron and promotes the flow of holes toward the anode. Based on these achievements, various studies on further band alignment engineering for more effective charge transport have been reported [16–19]. The main strategy has been a cascaded or gradient alignment of the energy bands for a smoother flow of the photo­ generated charges using multi-junction-layers with different energy band positions. The different band positions could be prepared using different doping levels [16] or different concentrations [17] of ligands. However, the best power conversion efficiency (PCE) of these cascaded-junction devices were 7.4% and 6.29%, respectively, implying further optimization is necessary. The enhanced charge carrier transport of the cascaded-junction layers with three different sizes of PbS QDs has also been reported [18,19]. However, the size change of the QDs alters their band gap as well as the band positions, which is not desirable for photovoltaic applications. Overall, the development of more effective band alignment engineering that can optimize the benefits of the CQDs is a critical step for the further enhancement of the device performance. Recently, we have reported that the solid-state exchange (SSE) effi­ ciency of alkylammonium iodide (AMI) ligands are compromised in the steric hindrance of the alkyl-substituents and ion dissociation property

* Corresponding author. E-mail address: [email protected] (S. Yim). https://doi.org/10.1016/j.solmat.2019.110363 Received 18 June 2019; Received in revised form 21 October 2019; Accepted 13 December 2019 Available online 24 December 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.

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of the AMIs [20]. As a result, the efficiency of new ligands, triethylamine hydroiodide (tri-EAHI) and tetraethylammonium iodide (TEAI) were superior to conventionally used TBAI. It was also observed that the band positions of the PbS CQD layers treated with different ligands altered without changing their band gaps. In this paper, we propose a new cascaded-junction CQDSC consisting of these two PbS-AMI layers and one PbS-1,3-propanedithiol (PDT) layer with an architecture of ITO/ZnO/PbS CQD-tri-EAHI/PbS CQD-TEAI/ PbS CQD-PDT/Au. This band alignment-engineered cascaded-junction led to a significantly improved charge transport capability of the device. The PbS-TEAI layer increased the electric field near the back side of the cell and hence promoted the extraction of the charges. All the cell pa­ rameters exhibited by the cascaded cell were higher than those of the single PbS-AMI layer cells, and the best PCE reached 10.46%. The inverse-cascaded-junction cell was also fabricated intentionally and investigated to more deeply understand the effect of the cascadedjunction structure on the charge transport property and device performance.

the Au layer with a thickness of 80–100 nm was then thermally depos­ ited in a high-vacuum chamber with a base pressure below 10 6 torr. The active area of the cell was 0.0707 cm2. 2.3. Characterization The morphology of the cells were imaged using a field emission scanning electron microscope (FE-SEM, JSM-7610F, JEOL). The elec­ tronic absorption of the CQD solutions and vibrational absorption of the CQD films were characterized using a UV–vis spectrophotometer (S3000, Scinco) and a Fourier-transform infrared (FT-IR) spectropho­ tometer (Nicolet iS50, Thermo Fisher Scientific), respectively. The XPS measurements were carried out using a K-alpha system (Thermo Sci­ entific Inc., UK). The intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) measurements were carried out using an impedance analyzer (IVIUM Tech., IviumStat). Both the DC and AC components of the illumination were provided by a green-emission LED (λ ¼ 535 nm). The mean transport time (τct) and recombination time (τrec) of the photogenerated charges were calculated from the IMPS and IMVS plots measured under short-circuit and open-circuit conditions, respectively. The carrier den­ sity of the CQD layer was determined by analyzing the Mott-Schottky plots obtained by the same analyzer. The transient photovoltage (TPV) and transient photocurrent (TPC) analyses were performed using a nanosecond laser (10 Hz, NT342A-10, EKSPLA) and a Xe lamp (150 W, Zolix) equipped with a digital oscilloscope (DSO-X 3054A, Agilent). The current density-voltage (J-V) characteristics of the cells were measured using a Keithley 2400 source unit at AM 1.5G 1 sun illumination. A classA solar simulator with a 150 W Xenon lamp (Newport) was used, and the light intensity was adjusted using a KG-5 filter-covered monosilicon detector calibrated by the National Renewable Energy Laboratory. The space-charge-limited-current (SCLC) measurements were carried out using the same instrument under the dark condition. The incident photon-to-current efficiency (IPCE) spectra were recorded using the Newport 818-UV and 818-IR power meters (K3100 IQX, McScience) calibrated with a standard silicon photodiode.

2. Experimental section 2.1. Synthesis of PbS CQDs Lead(II) acetate trihydrate (Pb(CH3CO2)2⋅3H2O, � 99.9% trace metals basis), 1-octadecene (ODE, 90%), oleic acid (technical grade, 90%) and hexamethyldisilathiane (TMS2S, synthesis grade) were pur­ chased from Sigma-Aldrich. The oleate-capped PbS CQDs were synthe­ sized by a rapid hot injection method. Briefly, a solution containing lead (II) acetate trihydrate (8 mmol), oleic acid (22 mmol) and ODE (124 mmol) was heated to 90 � C for 2 h under vacuum. After additional heating to 115 � C under N2 atmosphere, 10 ml of the ODE solution containing 720 μl TMS2S was rapidly injected into the flask for thermal quenching. After stirring for 5 s, toluene (20 ml) was poured, followed by cooling to room temperature. The resulting oleate-capped PbS QDs were purified four times by precipitation in acetone and toluene/ methanol, followed by drying in a vacuum desiccator to remove any residual solvents. The electronic absorption spectra of the synthesized CQDs dispersed in octane are shown in Fig. S1a. The diameter, d (in nm), of the PbS CQDs is well known to correlated with the bandgap, E0 (in eV), with the following equation [21]. E0 ¼ 0:41 þ

1 0:0252d2 þ 0:283d

3. Results and discussion The strategy for the band alignment engineering in this work is illustrated in Fig. 1. The three types of PbS CQD layers, i.e. bilayer (a), cascaded triple layer (b) and inverse-cascaded triple layer (c), were fabricated and investigated. The bilayer consisted of the PbS-PDT layer and PbS-AMI layer for which the tri-EAHI or TEAI was used as an AMI material (Fig. 1a). Recently, we have reported that these two AMI li­ gands exhibited more effective iodide passivation for PbS CQDs than that of TBAI [20]. In addition, the valence and conduction energy levels of the PbS-tri-EAHI layer were positioned 0.20 and 0.46 eV deeper than those of the PbS-TEAI and PbS-PDT layer, respectively [20], and hence the combination of the PbS-tri-EAHI, PbS-TEAI and PbS-PDT layers would enable the cascaded band alignment. The band edge energy levels based on the ultraviolet photoelectron spectroscopy (UPS) measure­ ments are summarized in Fig. S2. Both AMIs were highly effective for exchanging ligands on the surface of the PbS CQDs, which was confirmed by the dramatic reduction of the vibrational bands of pristine oleate ligands in FT-IR spectra (Fig. S3). The cascaded triple layer was fabricated by depositing the PbS-triEAHI, PbS-TEAI and PbS-PDT layers sequentially on the ZnO layer (Fig. 1b). The nine cycles of the PbS deposition and subsequent ligand exchange were repeated. The ligands used were tri-EAHI for five cycles, TEAI for two cycles and PDT for two cycles. The entire thickness of this PbS CQD triple layer was approximately 360 nm which is equivalent to that of the bilayer for which the seven cycles of AMI and two cycles of PDT treatment were applied (Fig. 2). It was also supported by the fact that the UV–vis absorption spectra of all these CQD films were identical (Fig. S4). The depth profile XPS measurements at the binding energy

(1)

The CQDs with an absorption at 890 nm were used in this study, the diameter of which is estimated to be approximately 2.86 nm. The highresolution transmission electron microscopy (HRTEM) also confirmed the size and crystallinity of the CQDs (Fig. S1b). 2.2. Fabrication of PbS CQDSCs An indium-tin-oxide (ITO)-coated glass substrate (sheet resistance < 20 Ω □ 1) was thoroughly cleaned with acetone and isopropyl alcohol in an ultrasonic bath for 15 min each and dried at 120 � C for 12 h under vacuum. The zinc oxide (ZnO) film was fabricated using the sol-gel process by spin-coating of the ZnO sol and subsequent dynamic annealing to 200 � C. The PbS CQD solution (50 mg of PbS in 1 ml octane) was spin-coated on the glass/ITO/ZnO substrate at 2000 rpm for 10 s in a layer-by-layer manner. At each spin-coating, the deposited PbS film was rinsed with an AMI solution (10 mM in methanol) or a PDT solution (2 mM in acetonitrile). For the fabrication of the bilayer device with an architecture of ITO/ZnO/PbS-AMI/PbS-PDT/Au, the ligand exchange process was repeated nine times, seven times for the PbS-AMI layer plus two times for the PbS-PDT layer. For the cascaded-junction device, the tri-EAHI-treatment and TEAI-treatment were repeated five and two times, respectively, followed by the PDT-treatment two times. After each ligand exchange, the film was washed with the solvent just used. Finally, 2

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Fig. 1. Schematic illustration of the cell structure and energy band alignment of ITO/ZnO/PbS-AMI/PbS-PDT/Au devices. The PbS CQD layer structures are (a) bilayer, (b) cascaded triple layer and (c) inverse-cascaded triple layer.

Fig. 2. Cross-sectional FE-SEM images of PbS CQDSCs. The PbS CQD layer structures are (a) PbS-tri-EAHI/PbS-PDT bilayer, (b) PbS-TEAI/PbS-PDT bilayer, (c) PbStri-EAHI/PbS-TEAI/PbS-PDT cascaded triple layer and (d) PbS-TEAI/PbS- tri-EAHI/PbS-PDT inverse-cascaded triple layer.

regions of C1s (Fig. 3a) and I3d (Fig. 3b) electrons indicated that the PbS-TEAI layer was successfully formed on the PbS-tri-EAHI layer, although it was difficult to identify each layer in the FE-SEM images. The atomic percentages of the C and I atoms at the depth of 20 and 250 nm from the surface of the PbS-AMI layer are plotted in Fig. 3c. At a deeper position, the content of the C atoms was smaller while that of the I atoms was larger. Because the lead and sulfur contents are nearly equal at both depths (Fig. S5), the smaller carbon and larger iodine content at the deeper position are indicative of more effective iodide passivation. This indicates that the tri-EAHI-treated PbS QDs are predominant in the

lower part of the PbS-AMI layer because the tri-EAHI was reported to be superior for iodide passivation compared to TEAI [20]. SCLC measurements for the device with the ITO/CQD-AMI/LiF/Al architecture were carried out to confirm the improvement of electron mobility (μe) by the band alignment engineering (Fig. 4a). The calcu­ lated μe value of the cascaded-junction device was 7.09 � 10 2 cm2v 1s 1 which was 1.6 and 3.2 times faster than those of the PbS-triEAHI and PbS-TEAI single layer-based device, respectively. This enhancement is probably attributed to the easier transport of photo­ generated electrons due to the cascaded band energy alignment. The 3

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Fig. 3. XPS spectra for the (a) C1s and (b) I3d electrons of the PbS-tri-EAHI/PbS-TEAI heterojunction film measured at a depth of 20 and 250 nm from the film surface. The C/Pb and I/Pb atomic ratios are plotted in (c).

Fig. 4. Results of (a) SCLC measurements using an electron-only device (ITO/CQD-AMI/LiF/Al) and calculated μe values, (b) Mott-Schottky plots and calculated Na values, (c) WD versus applied voltage and WD values at V ¼ 0 and (d) PL spectra of the PbS CQDSCs.

obtained μe values of the devices are summarized in Table 1. To further investigate the effects of the cascaded band alignment on the built-in potential (Vbi) and depletion width (WD) of devices, capacitancevoltage (Cp-V) analysis was carried out (Fig. 4b). From the MottSchottky plot of the devices, the Vbi of the cascaded-junction cell was determined to be 0.615 V whereas that of the PbS-tri-EAHI and PbSTEAI single layer device was 0.585 and 0.558 V, respectively. The enhanced Vbi of the cascaded-junction cell was probably attributed to

the escalated electric field toward the back side of the device [16]. The number of carriers (Na) of the device was determined by the slope of the C 2–V curve. As shown in the inset of Fig. 4b, the Na of the cascaded-junction cell was smaller than that of the single cells, which led to an increase in WD of the cell. Under the short-circuit condition, the WD of the cascaded-junction cell was 387 nm which was approximately 17 and 29% wider than that of the PbS-tri-EAHI and PbS-TEAI single cell, respectively (Fig. 4c). The Vbi, and WD values of the devices are 4

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respectively. The larger diameter of the impedance circle corresponded to a smaller time constant, indicating that the cascaded cell had longer τrec and shorter τct, consistent with the TPV/TPC results. The τrec and τct values of the cells determined from the TPV/TPC and IMVS/IMPS measurements are summarized in Table 2. In addition, from the TPC results, the density of states (DOS) for each open circuit voltage (Voc) could be obtained by integrating the capacitance over the voltage [24, 25]; Z VOC 1 DOS ¼ CdV (2) Aed 0

Table 1 Summary of the CQD layer characterization results obtained from the SCLC and Cp-V analyses. PbS-AMI layer structure

μe

tri-EAHI single TEAI single Cascaded Inverse-cascaded

4.35 2.24 7.09 0.01

(10 2cm2v

1

s

1

)

Na (1015 cm 3)

Vbi (V)

WD at V0/ VMPP (nm)

4.93 6.02 4.53 77.1

0.585 0.558 0.615 0.305

332/127 299/96 387/168 88/23

summarized in Table 1. The photoluminescence (PL) measurements for the devices with the ITO/ZnO/PbS-AMI/PbS-PDT architecture also supported the enhanced charge transport of the cascaded-junction cell (Fig. 4d). The cascaded-junction cell showed the lowest PL intensity, indicating that the charge extraction of this cell was the fastest since the increased PL quenching is indicative of the successful exciton dissocia­ tion via improved interfacial charge transfer and rapid charge extraction [22,23]. To investigate the charge transport and recombination char­ acteristics more quantitatively, the charge recombination time (τrec) and charge transport time (τct) of the cells were evaluated by TPV (Fig. 5a) and TPC (Fig. 5b) measurements, respectively. The longer τrec and shorter τct of the cascaded cell compared to those of the single AMI cells revealed that charge transport was promoted and recombination was suppressed by the cascaded band alignment. The τrec and τct values were also extracted by IMVS (Fig. 5c) and IMPS (Fig. 5d) measurements,

where A is the device area, e is the elementary charge of an electron and d is the film thickness. The calculated DOS value of the cascadedTable 2 Summary of the CQD layer characterization results obtained from the TPV/TPC analyses. PbS-AMI layer structure tri-EAHI single TEAI single Cascaded Inverse-cascaded

τrec (μs)

τct (μs)

from TPV

from IMVS

from TPC

from IMPS

1.19 0.55 1.35 0.51

2.34 2.10 2.53 0.21

0.52 0.89 0.25 1.15

0.69 0.82 0.60 1.04

Fig. 5. Results of charge transport and recombination property analyses; (a) TPV spectra and calculated τrec values, (b) TPC spectra and calculated τct values, (c) IMVS and (d) IMPS plots. 5

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junction cell was 3.72 � 1017 cm 3 which was significantly lower than the values of the tri-EAHI and PbS-TEAI single layer-based device, 4.02 � 1017 cm 3 and 4.21 � 1017 cm 3, respectively. The widened WD and increased μe of the cascaded-junction cell are probably correlated with the reduced trap states [20,26]. The current density-voltage (J-V) characteristics and IPCE spectra of the PbS CQDSCs are shown in Fig. 6a and b, respectively. A slightly higher VOC value of 0.630 V was obtained for the cascaded-junction cell. The VOC values of the tri-EAHI single cell and TEAI single cell were 0.626 V and 0.621 V, respectively. This VOC increase was probably attributed to the increased Vbi of the cascaded-junction cell, confirmed by the Mott-Schottky plot analyses. The short-circuit current density (JSC) of the cascaded-junction cell was 25.17 mA cm 2, which was 0.24 and 0.89 mA cm 2 larger than those of the tri-EAHI single cell (24.93 mA cm 2) and TEAI single cell (24.28 mA cm 2), respectively. This enhancement was apparently attributed to the improved extraction of the photogenerated charges due to their enhanced transport and sup­ pressed recombination. The PCE of the cascaded-junction cell reached 10.46%, whereas those of the tri-EAHI single and TEAI single cells were 10.13% and 9.57%, respectively. The obtained device parameters are summarized in Table 3. The average values and standard deviations are also presented in Fig. S8a. To further investigate the effect of the band alignment, we also fabricated an inverse-cascaded cell (Fig. 1c). In the fabrication of the inverse-cascaded cell, only the deposition order of the PbS-tri-EAHI and PbS-TEAI layer was opposite to the cascaded cell, resulting in an ar­ chitecture of ITO/ZnO/PbS CQD-TEAI/PbS CQD-tri-EAHI/PbS CQDPDT/Au. While the layer thickness (Fig. 2d) and electronic absorption characteristics (Fig. S4) of the inverse-cascaded cell were identical to the other single and cascaded cells, the charge transport and recombination characteristics were significantly different. Fig. 7a and b show the MottSchottky curve and extracted WD plot of the inverse-cascaded cell, respectively. For the cascaded cell (Fig. 1b), the conduction band min­ imum (Ec) gradually decreased toward the ZnO layer and the valence band maximum (Ev) gradually increased toward the Au electrode, which promotes the transport of the photogenerated charges. In contrast, for the inverse-cascaded cell (Fig. 1c), the Ec increased and the Ev decreased abruptly at the junction of the PbS-tri-EAHI and PbS-TEAI layers. This energy barrier creates an electric field in the opposite direction against the main electric field between the PbS-TEAI and PbS-PDT layers, which lowers the Vbi significantly to 0.305 V shown in Fig. 7a. The WD of the inverse-cascaded cell under the short-circuit condition was 88 nm which was also significantly shorter than that of the cascaded cell (387 nm)

Table 3 Parameters of the best PCE cells with the architecture of ITO/ZnO/PbS-AMI/ PbS-PDT/Au. PbS-AMI layer structure

VOC (V)

JSC (mA cm 2)

FF

PCE (%)

JSC from EQE (mA cm 2)

tri-EAHI single TEAI single Cascaded Inverse-cascaded

0.626 0.621 0.630 0.526

24.93 24.28 25.17 18.45

0.65 0.63 0.66 0.53

10.13 9.57 10.46 5.17

25.0 24.6 25.8 18.3

(Fig. 7b). The calculated Na, Vbi, and WD values of the cells are sum­ marized in Table 1. The τrec and τct were also significantly inferior to those of the cascaded cell (Fig. S6). While the τrec of the inversecascaded cell obtained from the TPV measurements was 0.51 μs, significantly smaller than that of the cascaded cell (1.35 μs), the τct value of 1.15 μs obtained from the TPC measurements was larger than that of the cascaded cell (0.25 μs). The τrec and τct values obtained from the IMVS and IMPS measurements showed the same tendency. The obtained τrec and τct values are summarized in Table 2. Interestingly, the carrier density of the inverse-cascaded cell calcu­ lated from the Mott-Schottky plot analyses, 77.1 � 1015 cm 3, was more than one order larger compared to the cascaded cell (4.53 � 1015 cm 3) despite that the same ligand exchange process was applied except for the order of the layers (inset of Fig. 7a). As shown in Fig. 1c, the generated electrons and holes have difficulty in drifting due to the energy barrier at the interface between the PbS-TEAI and PbS-tri-EAHI layer of the inverse-cascaded cell, which presumably acted as traps. The accumu­ lated electrons and holes at the interface would recombine easily, leading to a reduction in the depletion width. The adverse effects of the inversely aligned energy levels were also confirmed by the SCLC and PL measurements (Fig. S7). The μe of the inverse-cascaded cell was extremely low, 0.01 � 10 2 cm2v 1s 1, compared to the cascaded cell (7.09 � 10 2 cm2v 1s 1) (Fig. S7a). The significantly increased PL in­ tensity also indicated the reduced charge extraction of the inversecascaded cell (Fig. S7b). The suppressed charge transport and pro­ moted charge recombination of the inverse-cascaded cell led to a considerable decrease in charge harvesting and consequently a low PCE of 5.14%, only approximately half of the cascaded cell (10.47%) (Fig. 7c). The inverse-cascaded cell had a lower external quantum effi­ ciency (EQE) over the entire wavelength range (Fig. 7d), indicating its suppressed charge extraction.

Fig. 6. Performance of the cells with an architecture of ITO/ZnO/PbS-AMI/PbS-PDT/Au; (a) J-V characteristics under AM1.5G 1sun illumination and (b) IPCE spectra. 6

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Fig. 7. PbS CQD layer property analyses and device performance for the cascaded- and inverse-cascaded devices; (a) Mott-Schottky plots and calculated Na values, (b) WD versus applied voltage, (c) J-V characteristics under AM1.5G 1sun illumination and (d) IPCE spectra.

4. Conclusion

Acknowledgments

In summary, by engineering an effective band alignment of the PbS CQD layers, we achieved a significant improvement in transport and extraction of photogenerated charges in the PbS CQDSCs. The PbS CQD layers passivated with tri-EAHI and TEAI ligands had a difference in the band edge positions of 0.20 eV without changing the band gap. The sequential deposition of the PbS-tri-EAHI, PbS-TEAI and PbS-PDT layers therefore led to a cascaded band alignment between the n-type ZnO layer and the Au electrode. The increased Vbi due to the escalated electric field toward the back side of the device enhanced the VOC of the cascaded cell up to 0.615 V. The current density of the cascaded cell was also improved due to the enhanced transport and suppressed recombi­ nation of the photogenerated charges. As a result, the cascaded-junctionbased cell achieved approximately 3.3 and 9.3% higher PCE (10.46%) compared to the tri-EAHI single cell (10.13%) and TEAI single cell (9.57%). The significantly reduced PCE of the inversely cascaded cell (5.14%) also demonstrated the importance of the effective band align­ ment in the PbS CQDSCs.

This work was supported by National Research Foundation of Korea (NRF) Grant (No. 2016R1A5A1012966 and 2017R1A2B4012375) fun­ ded by the Korean Government. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110363. References [1] E.M. Miller, D.M. Kroupa, J. Zhang, P. Schulz, A.R. Marshall, A. Kahn, S. Lany, J. M. Luther, M.C. Beard, C.L. Perkins, J. Lagemaat, ACS Nano 10 (2016) 3302–3311. [2] M.R. Kin, D. Ma, J. Phys. Chem. Lett. 6 (2015) 85–99. [3] L. Cademartiri, E. Montanari, G. Calestani, A. Migliori, A. Guagliardi, G.A. Ozin, J. Am. Chem. Soc. 128 (2006) 10337–10346. [4] C. Wadia, A.P. Alivisatos, D.M. Kammen, Environ. Sci. Technol. 43 (2009) 2072–2077. [5] I. Mora-Ser� o, S. Gim�enez, F. Fabregat-Santiago, R. G� omez, Q. Shen, T. Toyoda, J. Bisquert, Acc. Chem. Res. 43 (2009) 1848–1857. [6] J. Kim, O. Ouellette, O. Voznyy, M. Wei, J. Choi, M.-J. Choi, J.W. Jo, S.-W. Baek, J. Fan, M.I. Saidaminov, B. Sun, P. Li, D.-H. Nam, S. Hoogland, Z.-H. Lu, F.P.G. de Arquer, E.H. Sargent, Adv. Mater. 30 (2018) 1803830. [7] J. Xu, O. Voznyy, M. Liu, A.R. Kirmani, G. Walters, R. Munir, M. Abbelsamie, A. H. Proppe, A. Sarkar, F.P.G. de Arquer, M. Wei, B. Sun, M. Liu, O. Ouellette, R. Quintero-Bermudez, J. Li, J. Fan, L. Quan, P. Todorovic, H. Tan, S. Hoogland, S. O. Kelley, M. Stefik, A. Amassian, E.H. Sargent, Nat. Nanotechnol. 13 (2018) 456–462.

Declaration of competing interest We, the authors, declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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