Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 26047−26052
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Oxygen Plasma-Induced p‑Type Doping Improves Performance and Stability of PbS Quantum Dot Solar Cells Hadi Tavakoli Dastjerdi,*,† Rouhollah Tavakoli,‡ Pankaj Yadav,§ Daniel Prochowicz,∥ Michael Saliba,⊥ and Mohammad Mahdi Tavakoli*,‡,#
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†
Department of Materials Science and Engineering and #Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ‡ Department of Materials Science and Engineering, Sharif University of Technology, 14588 Tehran, Iran § Department of Solar Energy, School of Technology, Pandit Deendayal Petroleum University, 382 007 Gandhinagar, Gujarat, India ∥ Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland ⊥ Institute of Materials Science, Technical University of Darmstadt, Alarich-Weiss-Strasse 2, D-64287 Darmstadt, Germany S Supporting Information *
ABSTRACT: PbS quantum dots (QDs) have been extensively studied for photovoltaic applications, thanks to their facile and low-cost fabrication processing and interesting physical properties such as size dependent and tunable band gap. However, the performance of PbS QD-based solar cells is highly sensitive to the humidity level in the ambient air, which is a serious obstacle toward its practical applications. Although it has been previously revealed that oxygen doping of the hole transporting layer can mitigate the cause of this issue, the suggested methods to recover the device performance are timeconsuming and relatively costly. Here, we report a low-power oxygen plasma treatment as a rapid and low-cost method to effectively recover the device performance and stability. Our optimization results show that a 10 min treatment is the best condition, resulting in an enhanced power conversion efficiency from 6.9% for the as-prepared device to 9% for the plasma treated one. Moreover, our modified device shows long-term shelf-life stability. KEYWORDS: colloidal quantum dots, lead sulfide, photovoltaics, plasma, stability
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INTRODUCTION Colloidal quantum dots (CQDs) have been studied extensively as promising candidates for third generation photovoltaic (PV) devices due to their direct and tunable band gap, stability, and solution processability.1−9 In particular, lead sulfide (PbS) QD devices are interesting thanks to their air stability and compatibility with roll-to-roll manufacturing.10−16 To be considered as a competitive candidate for industry, efficiency, scalability, and robustness of the QD PV need to be addressed. Over the past few years, many works in QD PV have been focused on developing various strategies to enhance the overall power conversion efficiency (PCE), leading to a certified PCE of 12% for PbS QD PV.5 In this regard, substantial advances have been accomplished to optimize the device architecture and improve surface passivation of the QDs through new ligand exchange processes. These findings can address wellknown issues associated with PbS QD PV such as an opencircuit voltage (VOC) deficit.4,10,17−23 Moreover, significant steps have been taken to overcome the stability issues24 and scalability issues through replacing the conventional multiple layer-by-layer (LbL) depositions using in situ solid-state ligand exchange processes with blade or spray-coating techniques.11,25−27 © 2019 American Chemical Society
Although these strategies have resulted in substantial improvements in the efficiency and scalability of the QD PV, its robustness issue remains to be addressed.28 It is well-known that the humidity level has an adverse impact on the performance of PbS QD PV. The relative humidity (RH) is lower (higher) during the colder (warmer) seasons of the year, resulting in inconsistency in the efficiency of the devices fabricated in different seasons of a year. The devices made under low RH conditions significantly outperform those made under high RH. This discrepancy in PV metrics contradicts with the scale-up requirements. Recently, the impact of RH on PbS QD PV has been studied by Kirmani, et al.,25 showing that the PV metrics of the devices fabricated under high RH can be greatly improved by storing them under very low RH (dry air) conditions. It was found that the air moisture and ambient oxygen compete to functionalize the surface of the PbS/EDT hole transporting layer (HTL). The storing of the as-prepared devices under dry air conditions shifts the equilibrium in favor of oxygen, which diffuses through the top metal electrode and effectively p-dopes the HTL, resulting in enhanced device Received: May 15, 2019 Accepted: July 1, 2019 Published: July 1, 2019 26047
DOI: 10.1021/acsami.9b08466 ACS Appl. Mater. Interfaces 2019, 11, 26047−26052
Research Article
ACS Applied Materials & Interfaces
added in a dry air glovebox and restored there for 24 h for comparison. The last set contains devices without using plasma treatment and was measured right after fabrication (asprepared). The current density−voltage (J−V) characteristics of all four device sets with the same thickness of QDs (fabricated in the same batch) were measured under 100 mW/cm2 simulated AM1.5MG illumination and are shown in Figure 2a. The PV metrics of these devices are listed in Table 1. We note that compared to the as-prepared device, the PV performance of the plasma-treated devices is noticeably enhanced. After only 1 min of plasma treatment, the VOC, JSC, and FF of the devices improve by 20 mV, 1.1 mA/cm2, and 1%, respectively, leading to an enhancement in PCE by ∼0.7% compared to the asprepared device, as shown in Figure 2a (red curve). We find that the prolonged oxygen plasma treatment for 5 min further enhances the PV metrics of the devices leading to a maximum PCE of 8.7%. It is worth nothing that the PV parameters are increased slightly by further prolonging the duration time up to 10 min reaching a maximum PCE of ∼9%. In turn, the oxygen plasma treatment for 15 min resulted in a slightly increased JSC but negatively impacts the VOC and FF (see Table S2, Supporting Information). Thus, our results indicate that plasma treatment for 10 min is the best optimized condition for improving QD PV performance. Interestingly, the devices partially fabricated in a dry air glovebox and stored in these conditions for 24 h show the best PV performance with a VOC of 620 mV, a JSC of 26.45 mA/cm2, a FF of 0.57, and a PCE of 9.34%. The 10 min plasma-treated device already reaches almost 96% of the PCE of the devices partially fabricated in a dry air glovebox and stored there for 24 h. These results suggest that it is possible to effectively accelerate the performance enhancement of the as-prepared devices via oxygen plasma treatment without the need for storing in a dry air glovebox, which would greatly reduce the costs and duration of developing efficient QD PV. To investigate the validity of JSC of the corresponding devices, we performed the external quantum efficiency (EQE) measurement, as shown in Figure 2b. The JSC of the devices were calculated by integrating the AM1.5G spectrum with the EQE spectra. These results reveal that the JSC values from the J−V curves are roughly 1 mA/cm2 higher than those estimated from the EQE results, due to the spectral mismatch between the spectra of the utilized solar simulator and that of AMG1.5. Figure 2c shows the long-term stability result of as-prepared and 10 min plasma-treated devices 4 months after fabrication. The long-term stability of the plasma-treated devices are comparable to that of the devices fabricated and stored in a dry air glovebox. Also, despite the initial improvement of the PCE after 1 month for the as-prepared device, its performance decreases faster over time compared to the 10 min plasmatreated device. In addition, the overall PCE is more than 2% lower than the average PCE of the plasma-treated device. We find that the long-term stability results are consistent with the previous study on the impact of oxygen doping,25 in which the storage of devices at 0% RH was performed to promote oxygen over air moisture to functionalize the surface of QDs and resulted in doping of the HTL and ultimately p-doping the EDT-capped PbS QDs. In our study, however, we believe that the oxygen plasma treatment shifts this equilibrium in favor of oxygen. The plasma chamber provides an environment with 0% RH, similar to a dry air-box, and oxygen can diffuse through the Au electrode and edges.29−31 Oxygen doping of
performance. However, one drawback of the dry air storage for the as-prepared devices is the relatively long storage time of 24 h to achieve this improvement. This procedure could be a disadvantage for the fast-paced and high-throughput industrial level fabrication. Also, the need for dry air storage equipment can potentially increase the cost of device fabrication. Therefore, in this work, we introduce a novel approach to substantially accelerate the device recovery and improve the device performance in a much shorter time without the need to use dry air storage. We employ relatively low-power oxygen plasma as an effective tool to provide a dry air environment to promote oxygen doping of the HTL within only a few minutes. We find that a 10 min oxygen plasma treatment significantly improves the PCE up to ∼9% compared to the as-prepared device with only 6.9% PCE because of rapid and effective pdoping of the HTL. Interestingly, the reference device stored under dry air conditions for 24 h shows a slightly better performance with a PCE of 9.3%. Moreover, our plasmatreated devices exhibit improved long-term stability.
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RESULTS AND DISCUSSION Figure 1a shows the schematic of the device architecture of the PbS QD PV. To assess the impact of oxygen plasma on
Figure 1. (a) Schematic of the device layer architecture (b) FIBmilled cross-sectional SEM of a device with the layers delineated by false color. (c) Band structure inferred from UPS measurement for asprepared and plasma-treated devices qualitatively showing reduced (EF − EV) following p-doping of the HTL via plasma treatment.
accelerating the oxidation process of PbS/1,2-ethanedithiol (EDT) QDs, five sets of PbS QD devices were fabricated at relatively humid ambient conditions (RH of 50%, 23 °C). A 40 nm thick textured polycrystalline seed film of ZnO was initially deposited on prepatterned indium tin oxide (ITO) substrates by a sol−gel process (see the Experimental Section for details). Then, PbS QDs with the first absorption peak at 980 nm in solution (Figure S1, Supporting Information) were deposited using a LbL deposition technique. To replace the native oleic acid ligands in the first 10 layers a solution of tetrabutylammonium iodide (TBAI) in methanol was used. The ligand exchange for the last 2 layers was performed by using a solution of EDT in acetonitrile. Finally, Au electrodes were deposited by thermal evaporation through a shadow mask. The cross-sectional scanning electron microscopy (SEM) image of the device with the layers delineated by false colors is shown in Figure 1b. Three of the fabricated device sets were exposed to oxygen plasma for durations of 1, 5, and 10 min, respectively (see Experimental Section). The fourth set was fabricated in the ambient air but its HTL (2 top PbS/EDT layers) were 26048
DOI: 10.1021/acsami.9b08466 ACS Appl. Mater. Interfaces 2019, 11, 26047−26052
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) PV characteristics of PbS QD devices exposed to different conditions. The measurements were done under 100 mW/cm2 AM1.5G illumination. (b) EQE spectra of each device with corresponding integrated current density calculated for all devices. (c) Long-term stability measurements: evolution of PV performance metrics of as-prepared and 10 min plasma-treated device sets stored at ambient conditions (RH ≈ 50%). Although the as-prepared devices show improved PCEs after 1 month, likely because of partial air oxidation, their performance and longterm stability are clearly lower than plasma-treated devices.
Table 1. PV Performance Metrics of Solar Cells Exposed to Different Treatment Conditionsa device as-prepared after 1 min O2 plasma after 5 min O2 plasma after 10 min O2 plasma dry air
VOC (mV) 550 570 600 610 620
± ± ± ± ±
20 25 20 10 10
JSC (mA/cm2) 24.2 25.3 25.8 26.0 26.3
± ± ± ± ±
0.2 0.3 0.2 0.2 0.1
FF 0.52 0.53 0.55 0.56 0.57
± ± ± ± ±
PCE (%) 0.01 0.02 0.02 0.01 0.01
6.9 7.6 8.6 8.9 9.3
± ± ± ± ±
0.2 0.4 0.1 0.1 0.1
Rsh (shunt resistance) (kΩ/cm2) 0.31 5.81 4.95 5.32 5.63
± ± ± ± ±
0.05 0.09 0.04 0.05 0.09
RS (series resistance) (Ω/cm2) 12.1 4.5 3.1 2.9 2.5
± ± ± ± ±
0.2 0.1 0.2 0.1 0.3
a
The mean and standard deviations are reported for six devices on each set.
the HTL can likely lead to the partial removal of EDT ligands and increase unbound thiolates, SO3, and SO4.25 To study the effect of plasma treatment on the energy levels of the PbS/ EDT layer, ultraviolet photoelectron spectroscopy (UPS) measurements were performed on a set of PbS/EDT-coated glass samples, as shown in Figure 3a. The difference between the Fermi level (Ef) and valence band maximum is extracted by the intersection of the linear portion of the spectranear the low binding energy regionwith the baseline. Increasing the duration of plasma treatment monotonically lowers the EF and therefore results in p-doping of the HTL. Compared to the as-prepared QD film, the EF is reduced by ∼0.05 eV after treatment for 1 min, as seen in Figure 3a. A further increase of the time of plasma treatment to 5 and 10 min reduces the EF by 0.1 and 0.02 eV, respectively. The EF of the sample stored for 24 h in a dry air-box is about 0.18 eV lower than that of the as-prepared sample. For the 10 min plasma-treated film, the resulting EF is shifted down by ∼0.17 eV, which is very close to the EF shift found in the sample fabricated and stored in a dry air-box (∼0.18 eV). These results suggest that oxygen plasma treatment of the QD PV effectively p-dopes the HTL of the device. The secondary electron cut-off for the corresponding samples was shifted in a similar manner to their associated shifts in the valance band edge (see Figure S3 in the Supporting Information for the extracted Fermi level energies). The shift in EF towards the valence band upon oxygen plasma treatment (as depicted in Figure 1c) tunes the band alignment in favor of hole transport leading to reduced carrier recombination.25 Photolumines-
Figure 3. (a) Magnified UPS spectra near the Fermi edge of QD samples exposed to different conditions. The Fermi-level shift towards the valence band increases with plasma treatment duration, suggestive of p-type doping the HTL. (b) PL spectra for PbS QD samples treated under different plasma conditions. (c) Absorption spectra for PbS QD samples treated under different plasma conditions. (d) Electroluminescence spectra for PbS QD PV exposed to different plasma conditions.
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DOI: 10.1021/acsami.9b08466 ACS Appl. Mater. Interfaces 2019, 11, 26047−26052
Research Article
ACS Applied Materials & Interfaces cence (PL) and absorption spectra of the PbS/EDT-coated samples exposed to different conditions are shown in Figure 3b,c. It is observed that increasing the plasma treatment duration leads to slightly narrower spectra with increased intensity, suggesting the enhanced passivation as a result of oxygen-doping of the QD surface. Moreover, the spectra of samples exposed to longer plasma treatment show a red-shift which is likely the result of the decreased interdot spacing between adjacent QDs because of the removal of some of the EDT ligands by plasma treatment which brings the QDs closer together.32 These results are consistent with the measured electroluminescence spectra of the devices as shown in Figure 3d. Devices subjected to longer plasma treatment show increased EL intensity as a result of the reduced carrier recombination. PL spectra (from bare PbS/EDT) and EL spectra from devices exposed to the plasmawith Au electrodeshow similar evolution in terms of peak red-shift and intensity. This supports the idea of oxygen penetration to the PbS/EDT layer. To confirm the impact of oxygen plasma treatment on the number of short EDT ligands, we performed X-ray photoelectron spectroscopy (XPS) on PbS/EDT samples before and after exposing to oxygen plasma. Figure 4 shows the S 2p core
Figure 5. (a) Semi-log J−V characteristics of representative QD PV devices under dark conditions. (b) IS measurements under 0 V bias at dark conditions for devices treated under different conditions. Cole− Cole plot of impedance measurements for devices show increased diameter upon plasma treatment suggesting reduced carrier recombination.
(IS)35 measurements were employed. The impedance spectra were recorded by applying varied ac signals in the range of 1 Hz to 1 MHz. The ac oscillating amplitudes were set as low as 20 mV (rms) to maintain the linearity of the response. The Cole−Cole plots in Figure 5b show the impedance spectra of the devices measured under dark conditions and at 0 V dc bias. The diameters of the semicircles are associated with the recombination resistance, which indicates the difficulty of exciton recombination.36,37 We derived the recombination resistances of the devices by fitting the data, as shown in Figure 5b. The simple equivalent circuit of the PbS QD device is shown in the inset of Figure 5b. The capacitor element in the equivalent circuit represents the energy storing capability of the photogenerated carriers and RS represents the device series resistance. The recombination resistance of the device stored under dry air conditions and plasma-treated devices are larger than that of the as-prepared devices, suggesting the reduced carrier recombination in the active layer. In addition, the prolonged duration of plasma treatment leads to the improved recombination resistance. This is consistent with the associated increase of shunt resistance levels for the devices exposed to longer oxygen plasma treatment, which is another indication of the reduced carrier recombination. We attribute these changes to the lower EF for the plasma-treated devices, as a result of oxygen doping in the PbS/EDT layer leading to effective improvement of hole transport in the device structure.
Figure 4. S 2p core level peaks of (a) as-prepared, and (b) sample treated with 10 min oxygen plasma, showing reduced levels of EDT ligands upon oxygen plasma treatment (blue peaks) resulting in increased levels of the unbound thiolate (green peaks). Also, the measurements suggest an increase in SO3 and SO4 as a result of plasma treatment (pink and orange peaks).
level peaks for the as-prepared sample and for the sample exposed to 10 min oxygen plasma. Comparing the XPS data of both films reveals that the peak intensity of EDT ligands is decreased, and the unbound thiolates, SO3, and SO4 are increased upon oxygen plasma treatment. These results confirm the impact of oxygen plasma treatment on removal of EDT ligands and QD oxidation. Dark J−V measurement of the fabricated devices (10 min plasma treated and dry air stored devices) is depicted in Figure 5a (see Figure S4, Supporting Information for the dark J−V data of other devices). The reverse bias current of the plasmatreated device is significantly lower than that of the as-prepared device indicating the improved diode characteristics. Moreover, we observe that the reduction in the reverse bias current is increased by prolonging the plasma treatment duration. Compared to the as-prepared devices, plasma-treated devices show a lower series resistance (Table 1), which is likely due to the enhanced charge transport.33 In addition, the shunt resistance of the plasma-treated devices is noticeably higher likely due to the reduced trap-assisted recombination (Table 1).34 To further confirm the impact of oxygen plasma treatment on the carrier transport, impedance spectroscopy
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CONCLUSIONS In summary, we reported on a low-power oxygen plasma treatment as a rapid and low-cost approach to effectively pdope the HTL of as-prepared PbS QD PVs, and thereby rapidly recover their PCEs to meet the high-speed requirement of industrial-level fabrication. We demonstrated that this treatment improves the PCE of as-prepared devices from ∼7 to 9%. The significantly faster performance enhancement step increases the PCE almost to the same level as the previously reported recovery method at a lower cost. Moreover, our plasma-treated devices show long-term shelf-life stability. The proposed approach indicates that the low-power oxygen plasma doping of the HTL can be potentially considered for high-throughput industrial-level fabrication of PbS QD PV as a fast and effective performance recovery tool.
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EXPERIMENTAL SECTION
ZnO Seed Layer Preparation. Prepatterned ITO/glass substrates of 150 nm thickness were initially cleaned in deionized water, acetone, and oxygen plasma (PDC 32G, Harrick Plasma). Then, ZnO 26050
DOI: 10.1021/acsami.9b08466 ACS Appl. Mater. Interfaces 2019, 11, 26047−26052
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ACS Applied Materials & Interfaces ORCID
seed layers were deposited on prepatterned ITO substrates by spincoating a solution of zinc acetate dihydrate (0.5 M) and ethanolamine (0.5 M) in 2-methoxyethanol inside a nitrogen-filled glovebox followed by annealing at 230 °C for 15 min in dry air. PbS QD Device Fabrication. PbS QDsfirst exciton absorption peak in solution at ∼980 nm and capped with oleic acidwere synthesized as previously reported in the literature.23,38,39 PbS QDs were then spin-coated onto the ZnO-coated ITO/glass substrates. A solution of PbS QDs (50 mg/mL) in octane (Sigma-Aldrich) was spin-coated at 2000 rpm for 15 s. A TBAI solution (10 mg/mL) in methanol was used for the exchange of TBAI (Sigma-Aldrich) for the native oleic acid ligands. QD layer was exposed to TBAI solution for 30 s before spin-drying, followed by rinsing two times with methanol. The QD deposition steps with the TBAI ligand exchange were repeated for 10 layers. Two layers of 1,2-EDT (Sigma-Aldrich) in acetonitrile with an acetonitrile rinse and spin-coating steps at 2500 rpm were used for the top 2 layers. For the reference device, Au top electrodes (Kurt J. Lesker, 99.999%) were thermally evaporated using a shadow mask at a rate of 1 Å/s. The solar cell area of the device with an Au electrode is 0.054 cm2. Oxygen Plasma Treatment. Oxygen plasma treatment was done using a Plasma CleanerPDC 32G (Harrick Plasma). The plasma conditions were set at 6 W RF power, 100 mTorr, 50 sccm. Device Characterization. Current−voltage (J−V) characteristics were measured using a semiconductor device analyzer (Agilent Technologies, B1500A). A 150 W xenon arc-lamp (Newport 96000)with an AM1.5G filterwas employed for solar simulator illumination of 100 mW/cm2. Prior to obtaining J−V characteristics, the devices were initially light soaked under these illumination conditions for 5 min. Optical transmittance and absorbance were recorded by a UV−vis/NIR spectrometer (LAMBDA 1050). EQE data were measured by a 250 (W) tungsten halogen lamp source with an Oriel Cornerstone 130 monochromator. To obtain integrated photocurrent density values, the product of the EQE and the AM1.5G spectra over the wavelength range of 370−1100 nm was integrated. UV−vis was measured by a Varian Cary 5. A FEI Helios NanoLab 600 at 5 kV SEM was used for SEM imaging. The device cross section for SEM was prepared by a Ga-focused ion beam (FIB) milling at 30 kV using the same tool. UPS was performed in an Omicron ultrahigh vacuum system using the He I line (21.2 eV) of a helium discharge lamp to study the Fermi level and valence bands of different layers. PL spectra were measured with a SpectraPro 300i spectrometer employing liquid nitrogen-cooled InGaAs detector (Princeton Instruments). For all PL measurements, a 532 nm laser with 1 mW power was used as the excitation source. For EL measurements the devices were connected to a sourcemeter (Keithley 2636A) and all EL measurements were done at the forward bias of 1 V and on the same setup as for the PL measurements. An X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific Inc., USA) with Al Kα radiation (hν = 1486.6 eV) as a source was used to measure XPS.
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Hadi Tavakoli Dastjerdi: 0000-0002-8925-5493 Mohammad Mahdi Tavakoli: 0000-0002-8393-6028 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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REFERENCES
This work was in part supported by Natural Sciences and Engineering Research Council (NSERC) of Canada (award number: PDF-487850-2016). M.M.T. would like to acknowledge the research laboratory of electronics (RLE) at the Massachusetts institute of technology.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b08466. Additional experimental details and results including PL and absorption spectra for PbS QDs, PV metrics for devices exposed to 15 min plasma treatment, and UPS and dark current measurements (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected],
[email protected] (H.T.D.). *E-mail:
[email protected] (M.M.T.). 26051
DOI: 10.1021/acsami.9b08466 ACS Appl. Mater. Interfaces 2019, 11, 26047−26052
Research Article
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DOI: 10.1021/acsami.9b08466 ACS Appl. Mater. Interfaces 2019, 11, 26047−26052
