Improving Charge Collection from Colloidal Quantum Dot

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Improving Charge Collection from Colloidal Quantum Dot Photovoltaics by Single-Walled Carbon Nanotube Incorporation Jonghee Yang, Jongtaek Lee, Junyoung Lee, and Whikun Yi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07089 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Applied Materials & Interfaces

Improving Charge Collection from Colloidal Quantum Dot Photovoltaics by Single-Walled Carbon Nanotube Incorporation Jonghee Yang, Jongtaek Lee, Junyoung Lee, and Whikun Yi*

Research Institute for Natural Sciences and Department of Chemistry, Hanyang University, Seoul 04763, Republic of Korea

* Corresponding author. Tel: +82-2-2220-0931, E-mail: [email protected]

ABSTRACT

Improving charge collection is one of the key issues toward high-performance PbS colloidal quantum dot photovoltaics (CQDPVs) due to considerable charge loss resulting from the low mobility and large defect densities of 1,2-ethanedithiol-treated PbS quantum

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dot hole-transporting layer (HTL). To overcome the limitations, single-walled carbon nanotubes (SWNTs) and C60-encapsulated SWNTs (C60@SWNTs) are incorporated into the HTL in CQDPVs. The SWNT-incorporated CQDPV demonstrates a significantly improved short-circuit current density (JSC) and the C60@SWNT-incorporated CQDPV exhibits an even higher JSC than that of pristine SWNT, both resulting in improved power conversion efficiencies. Hole-selective, photoinduced charge extraction with linearly increasing voltage measurements demonstrate that SWNT or C60@SWNT incorporation improves hole-transporting behavior, rendering suppressed charge recombination are enhanced mobility of the HTL. The enhanced p-type characteristics and the improved hole diffusion length of SWNT- or C60@SWNT-incorporated HTL bring the improvement of entire hole-transporting length and enable the lossless hole collection, which resulting in the JSC enhancement of the CQDPVs.

Keywords: quantum-dot, solar cell, single-walled carbon nanotube, charge collection, photo-CELIV, diffusion length


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Introduction The fascinating features of colloidal quantum dots (CQDs), including their tunable bandgaps, controllable size and the ease of solution processing have led to intensive applications in optoelectronic devices. CQD-based photovoltaics (CQDPVs) with PbS CQD have been spotlighted as a promising alternative for the next-generation photovoltaics owing to its accessibility to the near-infrared region of the solar spectrum, low cost of fabrication and superior air stability.1–4 Compared to the light absorption range of Si solar cells (< 1100 nm) and perovskite solar cells (< 800 nm), PbS CQDPVs can cover the wavelength range up to ~ 1400 nm by modulating the size of the CQD.2,5,6 This unique characteristic of PbS CQDPV also provides a promising possibility for application to a tandem solar cell combined with Si or perovskite solar cells.7,8 In addition, the remarkable stability of PbS CQDPVs under ambient air (maintaining 90% of initial performance over 1 year), which is hard to be achieved in case of perovskite or organic photovoltaics, renders a promising possibility for the realization of its use.3 This is a distinctive advantage of PbS CQDPVs compared to other CQDPVs based on PbSe or CsPbX3, which exhibit a power conversion efficiency (PCE) over 10%. In the case of PbSe


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CQDPV for example, although the exciton Bohr radius of PbSe is larger (46 nm) than that of PbS (23 nm), which renders stronger electronic coupling and subsequently results in enhanced charge transport and photocurrent, the poor air stability attributed to the oxidation of PbSe CQDs hinders the improvement of their performance.9,10 In the case of CsPbX3 perovskite CQDPV, the PCE value exceeded 14% via facile surface passivation with cesium salts.11 However, the moisture-induced phase transition of CsPbX3, which leads to fast degradation (PCE reach to 70% of initial performance within 54 h under ambient condition) has been a critical problem since the first demonstration of the perovskite CQDPVs.11,12 Recently, the power conversion efficiency (PCE) of PbS CQDPV exceeded 12% by enabling homogeneous CQD coupling and other approaches have been used to improve the CQDPV performance by passivating the surface of CQD.13–18 These efforts are mainly stemmed from the fact that the PbS CQD has a large number of surface defects, which results in considerable charge recombination and open-circuit voltage (VOC) deficits of PbS CQDPVs.19,20 Improving the collection of photogenerated charge is another strategy towards the realization of high-performance CQDPVs. In this regard, numerous attempts have been


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expended to improve the collection of electrons by doping the electron-transporting layer (ETL, usually ZnO) with ions.21–25 Appropriate doping of ions renders a notable improvement of the ETLs such as defect passivation, favorable energy level alignment and mobility enhancement, consequently resulting in a performance enhancement of CQDPV. By contrast, few investigations regarding hole collection has been carried out since the first application of the 1,2-ethanedithiol (EDT)-treated PbS (EDT-PbS) CQD layer as an additional hole-transporting layer (HTL).26 An EDT-PbS CQD layer exhibits a deeper Fermi level (EF) with respect to the position of its valence band maximum (EV) (i.e., enhanced p-type characteristic in terms of the energy level) compared to the tetrabutylammonium iodide (TBAI)-treated PbS (TBAI-PbS) CQD layer, which has been commonly used as a p-type light absorber in CQDPV. Chuang et al. demonstrated an improved hole extraction through band alignment engineering between the TBAI-PbS CQD layer and EDT-PbS CQD layer.26 In terms of charge transport during photovoltaic operation, a EDT-PbS HTL easily accepts holes while blocking electrons since the aligned energy levels (i.e., EV and conduction band minimum, (EC)) of the HTL are shallower than that of TBAI-PbS CQD absorber. This feature of the EDT-PbS CQD HTL results in a


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remarkably improved JSC and air stability of PbS CQDPVs compared to the device performance with MoO3 HTL-based counterparts, which previously exhibited the highest performance.27–29 The discovery switched the use of HTL material from vacuumprocessed MoO3 to solution-processed EDT-PbS CQD, excluding a vacuum process while maintaining ohmic contact with Au.26 In terms of air stability, Kirmani et al. demonstrated that the oxidation by ambient oxygen induced extra p-doping of the EDTPbS layer, resulting in the initial PCE improvement and superior air stability of PbS CQDPV.30 This phenomenon has been exclusively observed to be due to the EDT-PbS. To date, the EDT-PbS CQD layer has been generally used as an HTL even for state-ofthe-art PbS CQDPVs using a conformal PbI2 matrix-passivated absorber.5 Though the improved charge extraction by the solution-processed EDT-PbS CQD layer, instead of MoO3, provides notable performance enhancement of CQDPVs, the considerably low intrinsic hole mobility of EDT-PbS CQD, possibly stemming from the lower inter-dot coupling compared to halide-passivated CQD, and the large amount of defect sites due to the poor surface passivation, still limit the efficient hole collection and performance of CQDPVs.31 Several reports have demonstrated enhanced hole collection by doping Ag


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or a metal–organic hybrid molecule on the EDT-PbS CQD HTL.32,33 However, these strategies are mainly focused on the establishment of pronounced energy level alignment by reducing the difference between the EF and EV of an HTL, which induces both an enhanced driving force to extract the charge and a suppressed electron back transfer. Related to this, an attempt to solve the intrinsic limits (i.e. low carrier mobility and large amount of surface defects) of EDT-PbS CQDs has not been suggested yet. From this perspective, a more powerful strategy to overcome the limit of EDT-PbS CQD HTL is still necessary. Single-walled carbon nanotube (SWNT) is well known for its ultrafast chargetransporting properties,34,35 and hence, several endeavors to improve charge transporting behavior within the photovoltaics have been carried out by incorporating the SWNT into the devices. For example, Ihly et al. demonstrated a remarkable performance enhancement of perovskite solar cell by introducing SWNT interlayer between the perovskite and HTL, which was attributed to the rapid hole extraction from the absorber layer with extremely slow back-transfer/recombination.36 In addition, Tazawa et al. applied poly(3-hexylthiophene-2,5-diyl)-wrapped SWNT to the CQDPV by embedding the


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composites into the poly(methyl methacrylate) matrix, serving as a HTL, and observed significantly improved JSC owing to the fast carrier transfer.37 Both reports suggest that the SWNT incorporation into a solar cell can render lossless charge-transporting behavior within the device, resulting in improved charge carrier collection and consequently, enhanced solar cell performance. Also, another report suggested that improved air stability could be achieved by introducing a semiconducting SWNT interlayer below the Au electrode.38 Such endeavors commonly suggest that the application of SWNT to solar cell devices indeed led to a performance enhancement owing to the superior properties of SWNT. However, considering that the superior charge transport properties of SWNT is mainly originated from its elongated, π-conjugated wall, these methods still hinder the actual potential of SWNT. The methods mentioned above use SWNT as a composite with polymers, or interlayer, which screens the considerable portion of the SWNT wall from the active materials. If the SWNTs are dispersed inside the active materials and the close contact between the SWNT and the materials is established along the overall SWNT length, the hybrid system can exhibit notable performance enhancement of devices. Thus, a more powerful approach to establish the structure for performance improvement


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of solar cells using SWNT, such as a structure in which the active materials directly contact the SWNT wall, should be suggested. In this study, a strategy for the SWNT incorporation was applied to conventional PbS CQDPV to overcome the intrinsic limits of the EDT-PbS HTL. By spin-coating of a mixture of both PbS CQD and SWNT dispersions, SWNT-incorporated EDT-PbS CQD layers, in which the CQDs directly contacted the SWNT wall, were successfully constructed, as described in Figure 1a. In addition, fullerene (C60)-encapsulated SWNT (C60@SWNT), a novel carbon-based material exhibiting enhanced p-type transporting properties without compromising the dimensionality of the pristine SWNT,39–43 was also incorporated into the HTL in the same manner. Evidences for extra p-doping of the HTL and the existence of charge transfer process from the HTL to the incorporated SWNT (or C60@SWNT) were observed. Additionally, PbS CQDPVs with the SWNT-incorporated HTL exhibited significantly enhanced PCEs, which were mainly originated from the improved shortcircuit current density (JSC). The JSC and PCE of CQDPVs were further improved when the C60@SWNT-incorporated HTL was used, achieving the highest PCE value of 11.04 % in this study. To unravel the JSC enhancement of SWNT- and C60@SWNT-incorporated


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CQDPVs, a technique of photoinduced charge extraction by linearly increasing the voltage (photo-CELIV) was applied to the CQDPVs. A combination of photo-CELIV measurement and the insertion of an insulator within the CQDPVs elucidated that the SWNT or C60@SWNT incorporation induced improved hole transport with reduced charge recombination compared to the bare EDT-PbS CQD film (denoted as control film), which consequently led to the enhanced JSC and PCE of the CQDPVs.

Results and Discussion Prior to performing the main study, the successful synthesis of C60@SWNTs was confirmed using transmission electron microscopy (TEM), as shown in Figures S1 and S2. Compared to the TEM image of pristine SWNT bundles (Figure S1), the TEM images of C60@SWNTs obtained at 5 individual spots commonly reveal that all SWNTs are completely filled with circular C60 molecules with a diameter of ~0.7 nm while the outer walls of the SWNTs remain intact, demonstrating the successful production of C60@SWNTs without surface contamination. Based on statistical evaluation of the TEM images, the average diameters of the pristine SWNT s and C60@SWNTs were found to


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be 1.35 (±0.08) nm and 1.40 (±0.07) nm, respectively. The Raman scattering spectra of both species also suggest that the C60 encapsulation established extra p-doping of the SWNT, as exhibited in Figure S3. In the low-frequency region of the Raman spectra in Figure S3a, distinctive peaks associated with the radial-breathing modes (RBMs) are commonly observed from both species, indicating the nanotubes used in this study were indeed single-walled. As stated in the experimental section, the SWNTs used in this study were indeed a mixture of both metallic and semiconducting SWNT based on the Kataura plot.44 After C60 encapsulation, 2-4 cm−1 downshifts of the RBM peaks, possibly originated from the enlarged diameter and/or the charge transfer from SWNT to C60, were observed, which is in agreement with previous reports.43,45,46 In the high-frequency spectra in Figure S3b, a distinctive, shouldered peak above 1500 cm−1 corresponding to the G band of SWNT with a negligible D band around 1300 cm−1 are commonly observed for both cases. In contrast to the RBM downshifts, the G modes were upshifted to 3-4 cm−1 upon C60 encapsulation. This signature seemed to originate from the charge (i.e., electron) transfer from SWNT to C60, which resulted in the enhanced p-type characteristics of [email protected],48


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Using the SWNTs (or C60@SWNTs), a stock dispersion for the formation of a CQD film incorporating SWNT species was made by simple mixing of both the SWNT (or C60@SWNT) and CQD dispersion with an appropriate concentration. Here, PbS CQDs with a size distribution of 3.51 (±0.25) nm were used (Figure S4). At high concentrations of SWNT species (> 0.020 wt.%), an aggregation of the prepared dispersion (mixture of SWNT (or C60@SWNT) and PbS CQD) occurs due to the π–π stacking nature between SWNTs (or C60@SWNTs) and therefore, continuous sonication of the dispersions is necessary prior to loading the dispersion for spin coating. Nevertheless, poor surface morphology of the films was observed when excessive amounts of SWNT (or C60@SWNT) were mixed, as confirmed by optical microscopy (Figure S5). Compared to the nearly intact surface of the control film (i.e., the EDT-PbS CQD film without any SWNT species), the incorporation of SWNT (or C60@SWNT) with appropriate concentrations (< 0.005 wt.%) was not observed to cause a significant change on the film surface. However, an excessively high concentration of 0.040 wt.% of SWNT species notably deformed the film surface and a large number of macroscopic pinholes—which usually adversely affect the performance of optoelectronic devices—was observed. Similar trends of surface


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morphology were also observed from the microscopic dimensions, which was probed by atomic force microscopy (AFM, Figure S6). While the surface morphologies of the control and EDT-PbS CQD films incorporated with an adequate amount of SWNT (or C60@SWNT) exhibited flat surfaces with root-mean-square (RMS) surface roughness values of ~ 1.2–1.4 nm, the surface morphologies of the films with excessively high SWNT (or C60@SWNT) amounts exhibited degraded surfaces with RMS roughness values > 4 nm. To check the quality of the CQD layer surface in detail with respect to the concentration of incorporated SWNT species, surface scanning electron microscopy (SEM) images of the EDT-PbS films were taken, as exhibited in Figure S7. Compared to the surface of both the control EDT-PbS CQD film and CQD film incorporating 0.005 wt.% SWNTs at a defective spot, the CQD film with 0.040 wt.% SWNTs exhibited a quite damaged surface (Figure S7c). Several root-shaped aggregates, which seem to be SWNT bundles with widths ranging from 40 to 100 nm were positioned at the crack of the CQD film, which was not observed from the control film and CQD film with a low concentration of SWNTs. This indicates that the high concentration of SWNT species could not be completely dispersed in a solvent even though continuous sonication was


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applied to the dispersion. Additionally, given that cracks were formed along the SWNT bundles, it seems that the large SWNT bundles induced large surface cracks, even revealing the ITO surface underneath the CQD film. Furthermore, the CQD film with a high concentration of SWNTs exhibited a poor surface even at a position away from the SWNT bundles compared to the other surfaces of the films, which is consistent with the AFM results. The surface analysis results suggest the necessity for optimizing the concentration of SWNTs (or C60@SWNTs). Therefore, optimization of the SWNT (or C60@SWNT) concentration was carried out based on the photovoltaic performance of the CQDPVs, which will be discussed later. An optimal concentration of 0.005 wt.% with respect to the PbS CQDs was determined and fixed in both cases. To gain insight into the interface between CQDs and SWNT species with the optimal concentration, TEM images of EDT-PbS CQD matrix incorporating 0.005 wt.% of SWNT (or C60@SWNT) were acquired, as exhibited in Figure 1b and 1c. Clearly, one could identify that ~20 nm of SWNT (or C60@SWNT) bundle was embedded inside the CQD matrix. In case of C60@SWNT-incorporated CQD film, the encapsulated C60 molecules were hard to recognize, possibly attributed to the thin


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Figure 1. (a) Schematic of EDT–PbS CQD film incorporating SWNT (or C60@SWNT). TEM images of the CQD matrix incorporating (b) SWNT and (c) C60@SWNT bundle (scale bar: 20 nm). The red and blue lines roughly indicate the border between the SWNT (or C60@SWNT) bundle and EDT-PbS CQDs. (d) UPS spectra and (e) corresponding energy level diagram of EDT–PbS CQD films. (f) Steady-state PL spectra of the EDT-PbS CQD films. The relative PL intensities of the CQD films with respect to the PL intensity of control film were labeled.


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carbon film of TEM grid. According to the additional TEM images obtained from other spots (Figure S8), the width of the bundle embedded into the CQD matrix was ranged from 10 to 40 nm without any cracks, suggesting that a defect-less SWNT- (or C60@SWNT-) incorporated EDT-PbS CQD film could be established when the optimal concentration was used. At a high concentration (0.040 wt.%), the TEM images in Figure S9 exhibit that a void space appeared between the CQDs and SWNT species, seemed to be a crack, was also observed at the end of the bundle, while the CQDs in the vicinity of the main SWNT bundle are contacted to the SWNT wall. It is worth noting that even at the low SWNT (or C60@SWNT) concentration, almost all of the bundles exist not as individuals but as aggregates inside the CQD matrix. The different SWNT (or C60@SWNT)/CQD interface depending on the concentration of SWNT species may be attributed to an imbalance between the interaction at the SWNT (or C60@SWNT)/CQD interface and the cohesive force among the SWNT or CQD species (particularly, at the end site of the bundle) during film formation and the EDT treatment process. In-depth investigation associated with the concentration-dependent surface morphology and interfacial crack of the CQD matrix incorporating SWNT species, which can contribute to


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improve the performance of the hybrid system, is now ongoing. To confirm the impact of SWNT- and C60@SWNT-incorporation on the energy level of EDT-PbS CQD films, ultraviolet photoemission spectra (UPS) of the films were measured, as shown in Figure 1d. Assuming that the bandgap of the EDT-PbS CQD film corresponds to the 1st excitonic peak of the absorption spectra (1.29 eV) in Figure S10, the EF, EV, and EC can be estimated, as summarized in Table S1 and depicted in Figure 1e. The energy level analysis suggests that the SWNT (or C60@SWNT) induces a downshift of the EF toward the EV with a slight downshift of the overall energy levels. This strongly indicates that the incorporation of SWNT indeed leads to the extra p-doping of the films and the degree of p-doping was more pronounced in the case of the C60@SWNT-incorporated EDT-PbS film though the SWNT species were not evenly dispersed inside of the film. Moreover, the reduction of photoluminescence (PL) was observed in the cases of the SWNT-and C60@SWNT-incorporated films, while the PL peak positions (~1080 nm) were not shifted (Figure 1f). Since the density of CQD could be reduced by the incorporation of SWNT species, a confirmation of how many CQDs were reduced by the incorporated SWNT species should be determined first. However, direct estimation of the CQD density of films


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from SEM or TEM images is quite difficult, since the SWNT species at a low concentration (0.005 wt.%) were well-embedded into the CQD matrix (although they existed as aggregates), as previously demonstrated in Figures S7 and S8. This induces partial exposure of the SWNT species, which hinders the precise estimation of volume portion of them inside the CQD matrix. In the case of the CQD film with a high concentration of SWNT species (0.040 wt.%), however, the SWNT (or C60@SWNT) aggregates are exposed in the CQD matrix even after the deposition of CQD films (Figure S7c). At this concentration, although the revealed SWNT bundles do not represent the entire portion of the SWNT, a rough and exaggerated estimation (in order to overestimate the SWNT portion to compensate the embedded, undiscovered SWNTs) can be valid since the SWNT species mainly existed as aggregates even at the low concentration, as already verified by TEM images in Figure 1b and 1c. Therefore, a rough portion of the SWNT species to the CQD matrix was estimated by assuming that the SWNT species possesses the marked area (or volume) in the low magnified surface SEM image in Figure S11 and the similar density of SWNT aggregates was evenly distributed over the entire films. As shown in Figure S11, the portion of SWNT species (incorporation concentration: 0.040


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wt.%) with respect to EDT-PbS CQDs was estimated to be 1.46 %. Given that the concentration of SWNT species incorporated into CQD films for the PL measurements was 0.005 wt.% and the actual portion of them should be proportional to the incorporated concentration, the estimated portion of the incorporated SWNT species into the EDT-PbS CQD film (i.e., reduced portion of EDT-PbS CQD) is roughly 0.18 %. Considering the degree of PL reduction by SWNT species incorporation (31.3 % and 39.2 % for 0.005 wt.% of SWNT and C60@SWNT, respectively), the portion of PL reduction of the CQD film due to the reduced amount of CQDs is negligible. The recorded absorption spectra of the EDT-PbS films exhibited similar absorbance in spite of SWNT or C60@SWNT incorporation (Figure S10), suggesting the actual amounts of CQDs in the films were also similar to each other, which is in line with the rough estimation. Since the PL spectra were collected from the same films, the PL reduction implies that the SWNT species considerably quenched the excited charges from the PbS CQDs rather than CQDs were lost from the film by SWNT (or C60@SWNT) incorporation. Time-resolved PL (TRPL) decay curves in Figure S12 also show similar trends as the steady state PL results. The decay curves were fitted to a tri-exponential decay model which showed that the average


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charge lifetime was reduced from 1.7 to 1.1 ns when the SWNT species was incorporated with PbS CQDs, confirming that the charge transfer from the PbS CQDs to the SWNT species occurs (Table S2).15 To investigate the effect of the SWNT- or C60@SWNT-incorporated EDT-PbS CQD layer on the performance of a CQDPV, CQDPVs were fabricated via a conventional layerby-layer ligand exchange protocol.26 Herein, ZnO nanoparticles were used as the ETL, and the tetrabutylammonium iodide-treated PbS (TBAI-PbS) CQD layer was deposited onto the ETL. A bare EDT-PbS CQD HTL (control), and HTL with SWNTs and C60@SWNTs were deposited onto the TBAI-PbS CQD layer, as shown in Figure 2a. The thickness of each layer in a CQDPV was estimated from the cross- sectional SEM images, as representatively shown in Figure S13. The SEM images of the CQDPVs showed that there was no significant deviation of the thickness of the EDT-PbS CQD layers. As already mentioned, the concentration of SWNT (or C60@SWNT) incorporated with the EDT-PbS CQD layer was fixed at the optimized concentration level of 0.005 wt.% based on the results of the repetitive experiments presented in Figure S14 and Table S3 (see Supporting Information). The existence of such an optimum concentration of SWNT


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species can be explained by the surface morphology degradation of the EDT-PbS CQD layer observed at excessively high concentrations, as previously exhibited in Figures S4, S5, S6, and S7. The current density–voltage (J–V) curves of CQDPVs in Figure 2b show that the JSC is significantly enhanced

Figure 2. (a) A schematic of the CQDPV architecture. (b) J–V curves and (c) IPCE spectra of the champion CQDPVs. (f) Average PCE values of the CQDPVs with respect to air-


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storage time. The error bars indicate the standard deviation of PCE values from the average values.

by the SWNT or C60@SWNT incorporation, whereas no significant change is observed in the case of other parameters, such as the VOC and fill factor (FF), as summarized in Table 1. In particular, the C60@SWNT-incorporated CQDPVs exhibited higher JSC values compared to those of the SWNT-incorporated CQDPVs. The highest PCE (11.04%) observed for the C60@SWNT-incorporated CQDPVs in this study was ~16% higher compared to the PCE of the control CQDPVs (9.54%). Similar JSC values were estimated from the integration of incident photon-to-current efficiency (IPCE) spectra in all cases, as shown in Figure 2c. Previous reports demonstrated that Table 1. Champion and average values of the photovoltaic performance of the CQDPVs. The values in the parenthesis indicate the average values and the standard deviation obtained from 30 individual devices.

Integra CQDPV

VOC

JSC

FF

PCE

ted JSC

configuration

(V)

(mA/cm2)

(%)

(%)

(mA/c m2)


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Control

+ SWNT

+ C60@SWNT

0.5908

25.867

62.29

(0.5969±0.005

(25.603±0.52

(60.30±1.42

)

)

)

0.6011

27.559

63.31

10.49

(0.6002±0.005

(27.421±0.51

(61.65±1.37

(10.14±0.17

)

)

)

)

0.6085

28.507

63.54

11.04

(0.6013±0.007

(28.503±0.51

(62.10±1.32

(10.64±0.19

)

)

)

)

9.52 (9.21±0.15)

26.09

27.76

28.86

the quantum efficiency of CQDPV above a wavelength of 600 nm could be enhanced when the p-type characteristics of the EDT-PbS CQD layer were pronounced, attributing to the improved charge collection properties at the back electrode.32,33 Both J–V results and IPCE spectra suggested that the incorporation of SWNT and C60@SWNT led to significant JSC enhancements. It was also evident from the statistics of the CQDPV parameters estimated from 30 CQDPVs that the PCE enhancement was mainly attributed to the JSC enhancement, while there were negligible changes of VOC and FF, as shown in Figure S15. In conjunction with the observed PL quenching and reduced carrier lifetime from SWNT- and C60@SWNT-incorporated EDT-PbS CQD films, the JSC enhancement could be mainly attributed to the additional charge collection by the incorporated SWNT


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species during CQDPV operation. Compared to other reports to improve the charge collection properties of EDT-PbS CQD HTL,32,33,49,50 SWNT- (or C60@SWNT-) incorporation provided comparable (or superior) PCE value in spite of the use of TBAIPbS CQDs, which generally exhibits an inferior surface passivation and results in lower performance than the devices Table 2. Comparison of the CQDPV performance with other reported methods for the improvement of the EDT-PbS HTL. Absorber

HTL

Application

Highest

ref

PCE (%) MAPbI3-

EDT-PbS

Mo-complex (p-dopant) doping

9.0

32

EDT-PbS

Mo-complex (p-dopant) doping

9.5

32

EDT-PbS

Ag doping

10.6

33

PbS PbX2-PbS (X=Br, I) PbI2-PbS


24

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PbX2-PbS

Ligand

cleavage-based

EDT

EDT-PbS (X=Br, I)

11.01

49

9.34

50

passivation EDT-PbS

Oxygen plasma-induced p-doping

This EDT-PbS

SWNT-incorporated EDT-PbS CQD

10.49

TBAI-PbS

work C60@SWNT-incorporated EDT-PbS

EDT-PbS

This 11.04

CQD

work

in which the absorber layers are treated with PbX2.11,15 The comparisons are summarized in Table 2. From the perspective of air stability, all types of CQDPVs maintained their PCE values for almost three months in ambient air after the initial PCE improvement— which occured within a week period—thereby indicating that the SWNT or C60@SWNT incorporation did not compromise the air stability of the CQDPVs (Figure 2d). As explained by Kirmani et al., the initial PCE improvement, which is commonly observed in PbS-based CQDPVs, is attributed to the oxidation of EDT-PbS by ambient oxygen, and


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enhances the p-type characteristics of the EDT-PbS CQD layer.30 To elucidate the charge-transporting behavior in the CQDPVs, photo-CELIV measurements were performed. Using the photo-CELIV technique, the charges generated by the irradiation of a 5 ns-width laser pulse can be extracted by applying the voltage pulse to the sample with an appropriate delay time and be monitored with a digital oscilloscope. This technique, which is generally employed to measure the chargetransporting behavior of an organic or polymer solar cell, is advantageous in that the mobility can be measured without compromising the device structure, providing reliable information about charge-transporting behavior in solar cells. Furthermore, the photoCELIV measurement simultaneously renders information about carrier dynamics from the time-dependent charge concentration.51–53 A disadvantage of the photo-CELIV measurement is the fact that the charge carrier type is indistinguishable.54 However, given that the SWNT- and C60@SWNT-incorporated PbS CQD layers also play a role of an HTL and the CQDPV configuration is not changed except to the HTL, it is essential that the hole-transporting behavior, rather than that of electron, should be selectively investigated. To solve this issue, an insulating layer of Al2O3 (20 nm) was inserted between the TBAI-


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PbS CQD and ZnO layers. By blocking the electron extraction with an insulating layer, only holes are selectively extracted from the CQDPVs during photo-CELIV measurements.55 Figure 3a, b, and c show the hole-selective photo-CELIV transient signals of the CQDPVs at various delay times. In all cases, holes were observed to be fully extracted within 15 μs after the application of the ramp voltage, and recovered to the displacement current. Compared to the dispersed photocurrent transients of control CQDPV, those of SWNT- and C60@SWNT-incorporated CQDPV exhibited more compact transients and peaked at the earlier time, which indicates that the hole collection characteristics from the CQDPVs were improved by SWNT and C60@SWNT incorporation. From the photo-CELIV


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Figure 3. Hole-selective photo-CELIV transient profiles of (a) control CQDPV, and CQDPV incorporating (b) SWNT and (c) C60@SWNT. The yellow arrows indicate the direction of increasing delay time. Estimated (d) μh and (e) hole concentration of the CQDPVs with respect to the delay time obtained from hole-selective photo-CELIV measurement. (f) Averaged carrier lifetimes in CQDPVs at various VOC obtained from the TPV measurements.

Table 3. Averaged dynamic parameters obtained from hole-selective photo-CELIV measurements. The values in the parenthesis indicate the standard deviation.


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Configuration

Control

p0

τB

LD

(cm−2 V−1 s−1)

(cm−3)

(μs)

(nm)

1.77 (±0.13)×1016

3.17 (±0.57)

69.9 (±9.2)

1.75(±0.11)×1016

4.03 (±0.53)

83.6 (±8.4)

1.54(±0.13)×1016

5.88 (±0.55)

107.4 (±8.9)

6.00 (±0.50)×10−4

+ SWNT

6.75 (±0.47)×10−4

+ C60@SWNT

7.64 (±0.54)×10−4

transient, the hole mobility (μh) value was estimated at each delay time based on the following equation,

μh =

2d

2 3Atmax

2

(1)

(1 + 0.36 ) ΔJ

J0

where ΔJ, J0, A, d, and tmax correspond to the maximum extraction current, displacement current, applied ramp rate, device thickness, and the time corresponding to the peak of the current transient, respectively.51,56 The μh values were plotted with respect to the delay time and the average value of μh (denoted as ) for the CQDPVs are marked with horizontal lines in Figure 3d. Notably, the enhanced μh values compared to the control CQDPV were observed from


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the SWNT- and C60@SWNT-incorporated CQDPVs within the applied delay time range. The estimated of the control CQDPV was 6.00×10−4 cm2 V−1 s−1, while those of the CQDPVs incorporating SWNT and C60@SWNT were 6.75×10−4 and 7.64×10−4 cm2 V−1 s−1, corresponding to enhanced mobilities of ~12 and ~27%, respectively. Furthermore, the carrier (in this case, the hole) dynamics in the CQDPVs was evaluated based on the calculated hole concentrations, which were estimated from the integration of photocurrent transient within the duration of voltage ramp and plotted in Figure 3e. Assuming a nondispersive recombination, the extracted hole concentration p(t) is expressed by the Langevin recombination with following equation,

p(t) =

p0 1+

(2)

( ) t

𝜏

B

where p0 and τB indicate the initial hole concentration and bimolecular recombination lifetime, respectively.51,57 Although the initial hole concentration was slightly reduced, the

τB of CQDPVs was increased by the SWNT or C60@SWNT incorporation, respectively. This implied that the SWNT and C60@SWNT incorporation effectively suppressed bimolecular recombination, enabling extra hole collection during CQDPV operation.


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Based on the photo-CELIV results, the diffusion length (LD) of the hole was calculated with the following equation based on the product of mobility and lifetime, LD = (kTμτ/e)1/2.58 Herein, and τB were used for the calculation of LD. The parameters obtained from the hole-selective photo-CELIV experiments, including , p0, τB, and LD, are summarized in Table 3. The calculated LD of the control CQDPVs was 69.9 (±9.2) nm, whereas those of the CQDPVs incorporating SWNT and C60@SWNT were 83.6 (±8.4) and 107.4 (±8.9) nm, respectively. The same experiments were carried out with the devices consisted of solely EDT-PbS CQD films. In these cases, similar but more pronounced current transients were observed, which were exhibited in Figure S16 and the dynamic parameters were summarized in Table S4. It is worth to note that the obtained LD values were increased from 60.3 (±7.6) nm (control HTL) to 95.0 (±8.2) nm (SWNT-incorporated HTL) and 128.3 (±10.5) nm (C60@SWNT-incorporated HTL), which to our knowledge is the highest LD in EDT-PbS CQD reported to date. The photo-CELIV results confirm that the overall hole-transporting properties was significantly enhanced by the SWNT or C60@SWNT incorporation.


31


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The suppressed recombination within the CQDPV devices was also confirmed with transient photovoltage (TPV) measurements.22,59,60 The TPV decay curves, which directly correspond to the actual charge decay profiles, were fitted to a bi-exponential decay model. The averaged recombination lifetime (τrec) and recombination rate (krec=1/τrec) were obtained from each decay curve as a function of VOC, as exhibited in Figure 3f and Figure S17. In accordance with the photo-CELIV results, the recombination lifetime was increased, and the recombination rate was reduced by the incorporation of SWNT species. Both the TPV and photo-CELIV results strongly suggest that the presence of the SWNT species in the EDT-PbS CQD film indeed suppresses charge recombination and facilitates hole collection, thus resulting in enhanced JSC and PCE of CQDPVs. Similar strategies to improve the lateral carrier-transporting properties of a perovskite film by

Figure 4. (a) The estimated depletion width (WD), diffusion length (LD), and combined


32

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charge transporting length (WD + LD) of the CQDPVs. The error bars indicate the standard deviation from the averaged values. Schematics describing enhanced hole collection from the (b) control and (c) SWNT- or C60@SWNT-incorporated CQDPVs.

incorporation of SWNT have been reported, indicating our strategy can also be applied to other optoelectronic devices.61,62 The maximum length a charge can transport in CQDPVs is determined by the combination of LD, estimated from the photo-CELIV results in this study, and the depletion width (WD), a factor representing the length that a charge drifts out to the electrode owing to the internal electric field generated by the built-in potential (Vbi).63,64 To estimate WD, the capacitance– voltage (C-V) characteristics of the CQDPVs were quantified (Figure S18). The estimated

Vbi of the CQDPVs was slightly enhanced upon the incorporation of SWNT species, which is consistent with our expectation based on the UPS analysis in Figures 1d and 1e. The estimated values of WD from the C–V plots were around 150 nm for all cases with slight improvements upon SWNT or C60@SWNT incorporation.63 As previously mentioned


33


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above, the charge transport in the CQDPV is determined by both diffusion and drift. Thus, the sum of the values of the LD of holes and the WD in the CQDPVs directly indicates the maximum hole-transporting length, as summarized in Figure 4a.15,63 Compared to the combined hole-transporting lengths of control CQDPVs (218.8±13.3 nm), those of the SWNT- and C60@SWNT-incorporated CQDPVs were notably increased (235.5±12.9 nm and 263.0±13.4 nm, respectively). Note that the maximum required length for a hole to travel out from the CQDPVs is equal to the actual thick thickness of the CQD layer (~260 nm, Figure S13) since the photogenerated charges are mainly generated at the CQD layer. Thus, it is plausible to suggest that the loss of hole is reduced as the combined hole-transporting length (i.e., WD + LD) approaches to actual CQD layer thickness and when the WD + LD exceeds the thickness, lossless hole collection can be achieved during CQDPVs operation. The combined hole-transporting lengths were increased by incorporating SWNT or C60@SWNT with the HTL as described in Figure 4b and 4c, which is in line with the JSC enhancement of the CQDPVs previously exhibited in Figure 2b. Particularly in the case of C60@SWNT-incorporated CQDPV, the required holetransporting length was fully covered, resulting in the highest JSC and PCE in this study.


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These suggest that the incorporation of SWNT species indeed facilitated the charge collection from the CQDPVs. Recently, enhanced charge collection from a PbS CQD film has been demonstrated with intercalated graphene layers inside the film, strongly suggesting that our result is an effective, facile and reliable strategy toward state-of-theart CQDPVs.64

Conclusion In summary, SWNTs and C60@SWNTs were incorporated into the EDT-PbS CQD matrix. The signatures for extra p-doping from the perspective of the energy level and the existence of charge transfer process were observed from the SWNT or C60@SWNTincorporated CQD films. Furthermore, there was a significant improvement of the JSC and PCE of CQDPVs with pristine SWNTs and C60@SWNTs. In particular, the C60@SWNTincorporated CQDPV exhibited the highest PCE of 11.04 % in this study. The holeselective photo-CELIV measurements and the TPV results demonstrate that the SWNT or C60@SWNT incorporation effectively augmented the hole-transporting behavior in the EDT-PbS CQD films, resulting in the enhancement of μh, suppression of bimolecular


35


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recombination, and consequently, improved LD of the HTL. Our findings collectively suggest that incorporation of SWNT species into the HTL led to the JSC enhancement of the CQDPVs, which can be attributed to the improved charge collection by the enhanced

LD of HTL. Particularly, C60@SWNT-incorporated HTL could fully cover the transporting length required for holes to travel out from the CQDPVs, rendering the lossless hole collection and resulting in the highest PCEs. This implies that the improved holetransporting behaviors of the SWNT species-incorporated EDT-PbS HTL can further render a new path for the development of high-performance PbS CQDPVs. Our findings can also provide the novel routes for the realization of high-performance in other nextgeneration PV technologies.

ASSOCIATED CONTENT

Supporting Information. Experimental details, TEM images of SWNT and C60@SWNT bundles, Diameter distribution of the C60@SWNTs, Raman spectra of SWNT and C60@SWNT bundles, TEM image and size distribution of EDT-PbS CQDs, Optical


36

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microscopy and AFM images of the EDT-PbS CQD film surfaces, Surface SEM images of the EDT-PbS CQD film surfaces, TEM images of SWNT- (or C60@SWNT-) incorporated EDT-PbS CQD films, UV/Vis absorption spectra of EDT-PbS CQD films, TRPL decay profiles of the EDT-PbS CQD films, cross-sectional SEM image of a CQDPV, Statistical distribution of solar cell parameters of the CQDPVs, photo-CELIV results of the EDT-PbS films, TPV-estimated recombination rates and Mott-Schottky plots of the CQDPVs, summarized energy levels, fitted TRPL results of the PbS CQD films, and the dynamic parameters of EDT-PbS films obtained from photo-CELIV measurements.

AUTHOR INFORMATION

Corresponding Author * Whikun Yi, E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. J. Yang designed the investigations and performed the overall experiments. J. Yang wrote and revised the manuscript. J. Lee and J. Lee purified the SWNTs and helped develop the procedure for


37


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C60@SWNT synthesis. W. Yi supervised the work. All authors have given approval of the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2019R1F1A1045506).

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TOC Graphic


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Figure 1. (a) Schematic of EDT–PbS CQD film incorporating SWNT (or C60@SWNT). TEM images of the CQD matrix incorporating (b) SWNT and (c) C60@SWNT bundle (scale bar: 20 nm). The red and blue lines roughly indicate the border between the SWNT (or C60@SWNT) bundle and EDT-PbS CQDs. (d) UPS spectra and (e) corresponding energy level diagram of EDT–PbS CQD films. (f) Steady-state PL spectra of the EDTPbS CQD films. The relative PL intensities of the CQD films with respect to the PL intensity of control film were labeled. 427x243mm (300 x 300 DPI)


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Figure 2. (a) A schematic of the CQDPV architecture. (b) J–V curves and (c) IPCE spectra of the champion CQDPVs. (f) Average PCE values of the CQDPVs with respect to air-storage time. The error bars indicate the standard deviation of PCE values from the average values. 286x220mm (300 x 300 DPI)



Figure 3. Hole-selective photo-CELIV transient profiles of (a) control CQDPV, and CQDPV incorporating (b) SWNT and (c) C60@SWNT. The yellow arrows indicate the direction of increasing delay time. Estimated (d) μh and (e) hole concentration of the CQDPVs with respect to the delay time obtained from hole-selective photo-CELIV measurement. (f) Averaged carrier lifetimes in CQDPVs at various VOC obtained from the TPV measurements. 421x224mm (300 x 300 DPI)


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Figure 4. (a) The estimated depletion width (WD), diffusion length (LD), and combined charge transporting length (WD + LD) of the CQDPVs. The error bars indicate the standard deviation from the averaged values. Schematics describing enhanced hole collection from the (b) control and (c) SWNT- or C60@SWNTincorporated CQDPVs. 480x138mm (300 x 300 DPI)



Table of Contents 237x135mm (300 x 300 DPI)


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Improving Charge Collection from Colloidal Quantum Dot

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