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Oxygen annealing of the ZnO nanoparticle layer for the high-performance PbS colloidal quantum-dot photovoltaics
Journal of Power Sources 421 (2019) 124–131
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Oxygen annealing of the ZnO nanoparticle layer for the high-performance PbS colloidal quantum-dot photovoltaics
T
Jonghee Yang, Jongtaek Lee, Junyoung Lee, Whikun Yi∗ Research Institute for Natural Sciences and Department of Chemistry, Hanyang University, Seoul, 04763, Republic of Korea
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
of annealing atmosphere on the • Effects surface defects of ZnO were elucidated.
PbS quantum dot solar cell with an O • annealed ZnO showed enhanced open2
circuit voltage.
As increasing O concentration during • annealing, the defects of ZnO were 2
passivated.
passivation suppressed inter• Defect facial charge recombination in the solar cell.
suppression of charge re• The combination induced the improved VOC of the solar cell.
A R T I C LE I N FO
A B S T R A C T
Keywords: Quantum-dot Solar cell ZnO nanoparticle Surface defect Interfacial recombination
Though numerous researches regarding the influence of annealing atmospheric condition of ZnO have been carried out, the impact of annealing atmosphere on the carrier transporting properties and the performance of the ZnO-based optoelectronics has not been well-established. Here, the effects of annealing atmosphere (i.e., N2, ambient air, and O2) used to generate ZnO nanoparticle (NP) layers are elucidated. The chemical nature of ZnO layers, especially the amount of oxygen vacancies in ZnO NPs, is modulated by the annealing atmosphere. As the composition of O2 gas increases in the annealing atmosphere, a notable reduction of oxygen vacancies of ZnO NPs and electron mobility enhancement are observed, indicating that O2 gas contributes to a reduction of surface defects on ZnO NPs during the annealing process. In addition, trap-filling by reduced oxygen vacancies of airand O2-annealed ZnO layers, induces the enhanced built-in potential in colloidal quantum-dot photovoltaic (CQDPV) devices. As expected, PbS CQDPVs with an air- and O2-annealed ZnO layer demonstrate significantly improved power conversion efficiencies than CQDPVs with an N2-annealed ZnO layer. Further analysis shows that the interfacial recombination is reduced for CQDPVs with an air- and O2-annealed ZnO layer due to the reduced trap states of ZnO NPs.
1. Introduction Colloidal quantum-dot (QD) Photovoltaics (CQDPVs), especially with lead sulfide (PbS) QDs, have been alluring to many researchers
∗
due to their several distinctive advantages; (1) a broad absorption spectrum covering the NIR range over 1100 nm, which is an upper limit for commercial Si PVs, (2) applicability with solution processes for CQDPV fabrication, and (3) superior air stability compared with
Corresponding author. E-mail address: [email protected] (W. Yi).
https://doi.org/10.1016/j.jpowsour.2019.03.013 Received 20 October 2018; Received in revised form 22 February 2019; Accepted 5 March 2019 Available online 14 March 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
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organometallic halide perovskite PVs [1]. From the first report of airstability and high-performance (> 8%) PbS CQDPV, multiple studies have been reported in this field to improve CQDPV performance [2–16]. Recently, a certified power conversion efficiency (PCE) of 12.01% (champion PCE: 12.48%) by 2D matrix engineering of quantum dot solid in CQDPV was reported [17]. Also, Baek et al. reported an 11.7% CQPDV maintaining over 90% of the initial performance by introducing a hydro/oxo-phobic hole selective layer [18]. High-performance and highly stable PbS CQDPVs usually employ zinc oxide (ZnO) as an electron transport layer. However, ZnO possesses considerable surface defects that originate from the oxygen vacancies in the ZnO lattice. Although the oxygen vacancies have been well-known to contribute n-type properties of ZnO, these defects also act as trap states within the band gap of ZnO and can induce poor solar cell performance via interfacial charge recombination of photogenerated electrons in CQDPVs. Therefore, appropriate degree of ZnO surface defects passivation is an important step to achieve high performance CQDPVs. Several attempts to reduce the surface defects of ZnO have been conducted by doping Mg or Cl during ZnO nanoparticle (NP) synthesis [5,19,20]. In spite of these advances, a more efficient and simple path for reducing the surface defects of ZnO is needed. Though the modulation of annealing atmosphere of ZnO layer has been widely investigated [21–24], the investigation that which phenomenon occurs and which effect, particularly in the perspective of charge transport, can be the observed from the heterojunction-based optoelectronic devices when annealing atmospheric condition of ZnO layer is modulated has not been well established. Here, we found that the annealing atmosphere of the ZnO layer can effectively modulate the surface defects of ZnO NPs and change the performance of CQDPVs. To compare the effect of annealing atmosphere on the ZnO layers, three atmospheric conditions (N2, ambient air, and O2) were tested during annealing, and notable chemical changes were observed. As the portion of O2 gas increases (i.e., N2 to air, and O2), surface oxygen vacancies of ZnO NPs were reduced, resulting in enhanced electron mobility within ZnO layer and reduced interfacial recombination at the ZnO/PbS interface in CQDPVs. In addition, an increased built-in potential (Vbi) due to the trap-filling via annealing ZnO in an O2 atmosphere, significant open-circuit voltage (VOC) enhancement, and a PCE of 9.44%, a ∼33% enhancement were achieved. Furthermore, by controlling the annealing temperature and time of ZnO layer under O2 atmosphere, CQDPV performance reached to 10.00%.
removed, and the precipitated QDs were re-dispersed in heptane. This QD dispersion was precipitated with acetone and centrifuged at 8000 rpm for 10 min. The supernatant was again removed, and the product was dried under a flow of N2. The resulting QD precipitates were re-dispersed in heptane (50 mg mL−1) for thin film fabrication. A previously reported method to prepare ZnO NPs was followed with slight modification [26]. Zinc acetate (0.5504 g) solution in dimethyl sulfoxide (30 mL) and TMAH (0.9995 g) solution in ethanol (10 mL) were mixed dropwise and stirred for 24 h under ambient conditions. Ethyl acetate was then added, and the resultant opaque dispersion was centrifuged at 8000 rpm for 10 min. After removing the supernatant, ethanolamine (200 μL) and ethanol (5 mL) were added to stabilize the NPs. The NPs were again washed with ethyl acetate and centrifuged at 8000 rpm for 10 min. The supernatant was again removed. The resulting ZnO NPs were dried under N2 flow and re-dispersed in ethanol. The dispersion was filtered through a membrane filter (pore size: 100 nm), and the concentration was adjusted to 20 mg mL−1 for ZnO layer fabrication. 2.3. Fabrication of CQDPVs ZnO layers were prepared by spin coating at 1500 rpm for 20 s on pre-patterned ITO substrates, followed by 250 °C annealing for 20 min in a N2 (ZnO-N2), ambient air (ZnO-Air), or O2 atmosphere (ZnO-O2). The PbS QD layers were fabricated via layer-by-layer spin coating in ambient air. TBAI-treated QD layers were prepared as follows: the presynthesized PbS QDs were deposited by spin coating at 2000 rpm for 15 s. Next, TBAI solution (10 mg mL−1 in methanol) was added to fully cover the substrate for 30 s, followed by spinning the substrate at 2000 rpm for 15 s. Two washing steps were performed via spin coating pure methanol onto TBAI-treated QD substrates. The same processes were repeated 7 times to achieve the appropriate thickness of the TBAIPbS QD layer. EDT-treated PbS QD layers were added to the TBAI-PbS QD layer as follows: the pre-synthesized PbS QDs were deposited by spin coating at 2000 rpm for 15 s. Next, 0.01 vol% EDT solution (in acetonitrile) was fully loaded onto the substrate for 30 s, followed by spinning at 2000 rpm for 15s. The QDs were washed 3 times with pure acetonitrile by spin coating under the same conditions. The same procedures were repeated to obtain complete QD layers (2 layers of EDTPbS layers onto 7 layers of TBAI-PbS layer). For both processes, the QD dispersions were loaded through a syringe filter (pore size: 450 nm). Patterned Au electrodes were deposited onto the substrate with a shadow mask to achieve an active area of 0.075 cm2 by a radio-frequency magnetron sputtering system under a base pressure of 2.0× 10−6 Torr.
2. Experimental 2.1. Materials Lead(II) oxide (PbO) (trace metal basis, 99.999%), oleic acid (OA) (technical grade, 90%), 1-octadecene (ODE) (technical grade, 90%), hexamethyldisilathane (TMS2S) (synthesis grade), zinc acetate (trace metal basis, 99.99%), tetramethylammonium hydroxide pentahydrate (TMAH) (97%), ethanolamine (99.0%), tetramethylammonium iodide (TBAI) (99.0%), and 1,2-ethanedithiol (EDT) (98.0%), dimethyl sulfoxide (99.9%) were purchased from Sigma-Aldrich. Acetonitrile (+99.8%) was purchased from Alfa Aesar.
2.4. Characterization of ZnO layers The surface morphology images of ZnO layers on ITO substrates were obtained using a Park Systems XE-100 atomic force microscope. XRD patterns of ZnO layers were recorded by a high-resolution X-ray diffractometer (Rigaku, SmartLab). UV/Vis absorption and transmission spectra were measured with a Mecasys Optizen 2120 UV spectrophotometer. XPS and UPS spectra of ZnO layers on ITO glass were collected using a K-alpha and Theta Probe photoelectron spectrometer (Thermo Fisher Scientific). XPS spectra were collected with the Al Kα line. Signals were collected under a base pressure of 3.0 × 10−8 Torr and were calibrated with the carbon 1s core-level peak (284.6 eV). UPS measurements were performed with a He I (21.22 eV) source under a base pressure of 8 × 10−7 Torr while applying a −9 V bias on samples. PL spectra were obtained with a Perkin Elmer LS 55 luminescence spectrometer. For the FET measurement, ∼40 nm ZnO Layers were fabricated onto the SiO2 (300 nm, oxide capacitance ∼11.7 nF cm−2)/ p++ Si substrate by spin coating and annealed for 20 min in N2-, ambient air-, and O2-filled glove box. Aluminum electrodes (100 nm) were fabricated with a patterned shadow mask (channel length: 1000 μm,
2.2. Synthesis of PbS QDs and ZnO NPs PbS quantum dots were synthesized by slight modification of the method reported by Hines et al. [25] PbO (0.45 g), OA (1.3 mL), and ODE (18 mL) were mixed in a 3-neck flask connected to a Schlenk line and heated to 100 °C under vacuum for 2 h. The flask was then cooled to 90 °C and maintained at that temperature under an Ar atmosphere for 2 h TMS2S (210 μL) dissolved in ODE (10 mL) was quickly injected into the flask. After 1 min, the flask was quickly cooled to room temperature. The products were precipitated with acetone and centrifuged at 8000 rpm for 10 min. After centrifugation, the supernatant was 125
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channel width: 100 μm) via thermal evaporator under a base pressure of 2 × 10−6 Torr. The electrical properties of ZnO FETs were measured using a semiconductor parameter analyzer (Keithley, 4200A-SCS). The cross-sectional image of ZnO FET was captured by FE-SEM (FEI Nova NanoSEM 450).
Fig. 1c shows that the defect-induced PL around 550 nm was decreased, strongly supporting that the annealing ZnO layer in an O2 atmosphere can effectively passivate surface defects and consequently reduce trapstates of ZnO NPs [2,19,20,28]. Surface defect passivation by O2 annealing also altered carrier transport property of ZnO layers. Fig. 1d shows traditional n-type transfer curves of field-effect transistor (FET) with ZnO layers as an active channel. An remarkably increased drainsource current (IDS) was observed as the annealing atmosphere became O2-dominant. The saturation mobility (μsat) values for each ZnO layer were calculated from the linear regime of (IDS)1/2 vs gate voltage (VG) with a following equation,
2.5. Characterization of CQDPVs The cross-sectional images of CQDPVs were produced by FE-SEM (FEI Nova NanoSEM 450) All electrical characterizations were performed under ambient conditions. The J-V curves of CQDPVs were measured using a Keithley 2400 sourcemeter under 1 sun AM 1.5 illumination with a class AAA solar simulator (McScience K201 LAB 55) calibrated with a standard Si reference solar cell (McScience K801SB041). A black shadow mask (0.06 cm2) was used to exclude the possibility of overestimation. The IPCEs of the CQDPVs were measured using a McScience K3100 IQX IPCE measurement system. Light intensity-dependent JSC and VOC characteristics were measured using a Keithley 2400 sourcemeter and by modulating the intensity of light output from a Xe lamp reaching the CQDPV, simultaneously tracking the intensity of light output by broadband power meter (Melles-Griot power/energy meter 13PEM001). TPV measurements were conducted using the light output from a Xe lamp as a continuous illumination source for VOC generation. A small perturbation from a 500 nm wavelength laser pulse was applied on the CQDPVs with a repetition rate of 10 Hz and 5 ns pulse width. An oscilloscope (Agilent DSO3202A) with 1 MΩ input terminal was used to record the TPV signals, which were averaged > 200 times. EIS spectra of the CQDPVs were obtained using an impedance analyzer (IviumTech., Iviumstat). An AC signal with an amplitude of 50 mV and frequency ranging from 100 Hz to 1 MHz was applied. The same apparatus was used to acquire capacitance-voltage (C-V) characteristics and drive-level capacitance profiling (DLCP) measurements of the CQDPVs. An AC signal with an amplitude of 20 mV and frequency of 1 kHz was applied. A DC voltage was applied from −1 V to 1 V. For DLCP measurement, various AC amplitude from 20 mV to 100 mV and frequency ranging from 100 Hz to 1 MHz were applied. All measurements were performed with 16 individual CQDPVs for each type, and the results were averaged.
2
μsat =
2L ⎛ ∂ IDS ⎞ ⎜ ⎟ WC ⎝ ∂VG ⎠
(1)
where L, W and C is the channel length, width and oxide capacitance, respectively. The μsat of ZnO-N2, ZnO-Air and ZnO- O2 were estimated to 8.13 × 10−4, 6.24 × 10−3 and 8.30 × 10−3 cm2 V−1 s−1, strongly suggesting that the defect passivation by annealing in O2 atmosphere can also significantly enhance the charge (i.e., electron) transport through the ZnO layer [29]. The XPS N 1s spectra of ZnO layers were also collected to check the contribution of nitrogen on the ZnO (Fig. S5). The XPS N 1s spectrum of as-prepared, not annealed ZnO layer was also checked since the ethanolamine was added during ZnO NP synthesis so that to monitor the species after various annealing condition. In case of as-prepared ZnO layer, an asymmetric peak was pronounced. The peak was deconvoluted to 3 individual peaks positioned at 398.1, 399.4 and 401.8 eV, which have been assigned to the N incorporated with Zn in the ZnO lattice (Zn-N), molecularly adsorbed N compounds (i.e., C-NH2 of ethanolamine) and N coordinated with O of ZnO (O-N), respectively [30,31]. After annealing, N 1s peak was significantly reduced regardless of the atmospheric conditions. The lattice-incorporated N components were thoroughly removed and tiny molecularly adsorbed N component was observed. Furthermore, as the O2 concentration increases during thermal annealing, the N components were more suppressed. This strongly indicates that the thermal annealing could not induce N doping of ZnO and only the O2 could actively contribute to the defect site (i.e., vacancy) passivation. The reason for this gas-selective surface passivation may be due to the relatively low energy required to dissociate O2 gas compared to that of N2. Furthermore, the annealing atmosphere imparted a significant difference on the energy level of ZnO layers. The ultraviolet photoelectron spectroscopy (UPS) spectrum in Fig. 2a showed that the Fermi level (EF) of the ZnO layer was downshifted, while the difference between EF and the valence band maximum (EV) became smaller as the annealing atmosphere was changed from N2 to ambient air and O2. The trend of EFEV values of the ZnO layers (2.98, 2.89 and 2.85 eV for ZnO-N2, ZnO-Air and ZnO-O2, respectively) obtained from the valence band region of UPS spectrum was considerably in accordance with the reduced PL intensity, both of which were originated from the reduced concentration of oxygen vacancies (and consequent in-gap states) of ZnO [32]. The Tauc plot in Fig. 2b, derived from the absorption spectrum, showed that the band gaps of ZnO layers were almost the same (3.41–3.42 eV). The overall energy levels of ZnO layers are depicted in Fig. 2c. The deeper EF of ZnO layer achieved by the reduction of oxygen vacancies of ZnO surface is in agreement with the previous reports [22,33]. The effect of ZnO annealing atmosphere led to a notable performance enhancement of CQDPV, as shown in Fig. 3. The CQDPVs with an ITO/ZnO NPs/TBAI-PbS/EDT-PbS/Au architecture, depicted in Fig. 3a, were fabricated by layer-by-layer spin coating. Here, PbS QDs with an excitonic peak positioning at 927 nm (diameter ∼ 3 nm) were used (Fig. S6). Considering the UPS spectrum of TBAI-PbS layer (Fig. S7), favorable photocarrier transport could be achieved for all types of CQDPVs in this study. The thicknesses of the ZnO and PbS layers in the CQDPVs were fixed at ∼40 and ∼260 nm, respectively (Fig. S8 and
3. Results and discussion To monitor the surface morphology of ZnO layers depending on the annealing atmosphere, atomic force microscopy (AFM) measurements were carried out, as displayed in Fig. S1. Here, ∼4 nm size of polycrystalline ZnO NPs were synthesized and used in this study (Fig. S2). Root-mean-square surface roughness values were 1.60, 1.53, and 1.39 nm for ZnO layers annealed in N2 (ZnO-N2), ambient air (ZnO-Air), and O2 (ZnO-O2), respectively, indicating that the annealing atmosphere does not induce the morphological change. Also, X-ray diffraction (XRD) patterns of ZnO layers (Fig. S3) suggest that the crystallinity of ZnO does not notably change depending on the annealing atmosphere. The transmittance of ZnO layers (Fig. S4) also showed no significant difference in the Vis-NIR range. However, X-ray photoelectron spectroscopy (XPS) results showed that the chemical nature of ZnO layers changed considerably, as displayed in Fig. 1. In Fig. 1a, the binding energy of the Zn 2p peaks were increased as the portion of oxygen in the annealing atmosphere increased, which indicates that oxygen-deficient Zn species form Zn-O bonding in the presence of O2 gas during annealing process [21]. In the O 1s core-level spectrum (Fig. 1b), an asymmetric peak was observed for each case and was deconvoluted to two peaks; one originated from oxygen of the ZnO lattice at ∼530 eV (IZnO) and the other from the oxygen vacancy of the ZnO surface at ∼532 eV (ID) [2,27]. The values of ID/IZnO (i.e., relative density of oxygen vacancy) were notably reduced as the annealing atmosphere became O2-dominant, again implying reduction of oxygen vacancies in ZnO layers. The photoluminescence (PL) spectrum in 126
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Fig. 1. XPS Zn 2p (a) and O 1s (b) core-level and PL spectrum (c) of ZnO layers. (d) transfer curves of FET devices with ZnO layers.
Both J-V curve and IPCE result demonstrated a slightly increased JSC, implying that the main factors contributing to the increased PCE are fill-factor (FF) and VOC enhancement. After initial PCE improvement originated from the p+ doping of EDT-PbS layer and consequent enhanced hole extraction by ambient oxidation, all CQDPVs maintained their PCE values over 4 months when stored in ambient air, indicating high stability CQDPVs (Fig. 4d) [34]. Since the PbS layers were fabricated with the same protocol for each CQDPV, the performance enhancements of CQDPVs with ZnO-O2 and ZnO-Air mainly originated from the ZnO NP layers, especially the interface between the ZnO layers and PbS layer. To access the information occurring at the ZnO/PbS interface, light intensity-dependent JSC and VOC values were obtained by modulating the light intensity irradiated to
S9). Fig. 3b shows J-V curves of CQDPVs at varying ZnO annealing atmospheres. The VOC values of the CQPDVs were dramatically increased, while the short-circuit current density (JSC) also slightly increased as the annealing atmosphere changed from N2 to air and O2. The overall CQDPV performances are listed in Table 1. The PCEs of the champion devices with ZnO-Air and ZnO-O2 were 8.20 and 9.44%, which is ∼15 and ∼33% enhanced, respectively, compared with that with ZnO-N2. Although this value is lower than the highest performance of CQDPV ever reported, the ZnO annealing atmosphere induced a significant influence on the CQDPV performance. The integrated JSC values of the CQDPVs, derived from the incident-photon to current efficiency (IPCE) spectrum in Fig. 3c and summarized in Table 1, also showed similar values and trend to those obtained from the J-V curve.
Fig. 2. UPS spectrum (a) and Tauc-plot obtained from the UV/Vis absorption spectrum (b) of ZnO layers. (c) Corresponding energy level diagram of ZnO layers. 127
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Fig. 3. (a) A schematic of the structure of a CQDPV. J-V (b) and IPCE (c) of champion CQDPVs. (d) Time-evolution of the PCE value of CQDPVs. Table 1 Summarized CQDPV performances. ZnO annealing atmosphere
VOC (V)
JSC (mA/cm2)
FF (%)
PCE (%)
Integrated JSC (mA/cm2)
N2 Ambient air O2
0.5229 (0.5287 ± 0.010) 0.5564 (0.5587 ± 0.011) 0.5831 (0.5899 ± 0.010)
24.14 (23.83 ± 1.21) 25.05 (24.88 ± 1.30) 25.53 (25.00 ± 1.17)
56.29 (54.77 ± 1.52) 58.84 (57.41 ± 1.43) 63.40 (61.36 ± 2.03)
7.10 (6.90 ± 0.20) 8.20 (7.98 ± 0.36) 9.44 (9.05 ± 0.39)
23.20 24.08 24.67
ambient air and O2 atmospheres. To gain more insight into the interfacial recombination of CQDPVs, transient photovoltage (TPV) measurements with various VOC conditions were conducted [3,12,27,28,36,38,39]. Under constant white light for generating appropriate VOC of CQDPV, small perturbation with a nanosecond laser pulse was applied to the sample and the extra photovoltage transients were recorded by digital oscilloscope. Since the photovoltage is proportional to the photogenerated charge in CQDPV, the direct monitoring of the charge recombination process in CQDPV was enabled by fitting the photovoltage decay curve [39]. The representative TPV decay profiles of the CQDPVs at the fixed VOC of 0.35 V were also exhibited in Fig. S10. By fitting transient decay curves to a bi-exponential decay model, carrier lifetime and recombination rate constant (reciprocal to the carrier lifetime) were obtained for each CQDPV, as displayed in Fig. 4c and d. The interfacial recombination process is suppressed as the annealing atmosphere changes from N2 to ambient air and O2, which is in accordance with the intensity-dependent VOC features and CQDPV performances. Note that the annealing ZnO layer in air and O2 atmospheres induced trap-passivation of ZnO NPs, resulting in a reduction of the surface trap states, as demonstrated in Fig. 1. Due to the reduced trap states of the ZnO layer by annealing in air (or O2), photogenerated electrons are prone to extraction through the ZnO layer to the ITO electrode without severe interfacial recombination, resulting in enhanced performance of the CQDPVs. The electrochemical impedance spectroscopy (EIS) spectrum under dark conditions, shown in Fig. 4e, further supports that the interfacial
the CQDPVs, as is demonstrated in Fig. 4a and b. Generally, the JSC of CQDPV is proportional to Φα, where Φ and α are the light intensity and exponential factor, respectively [2,35,37,38]. When α is close to unity, the photocurrent is predominantly determined by the generation rate of electron-hole pairs upon photon absorption [37]. By fitting the JSC results, the α values were estimated to have similar values around 0.90–0.97, indicating that the CQDPVs generated photocurrent in a similar manner. However, the intensity-dependent VOC results in Fig. 4b reveal that a notable difference exists depending on the annealing atmosphere of ZnO layers for CQDPV fabrication. Usually the relation between VOC and light intensity is expressed by the following equation,
VOC =
nkT αln (Φ ) + C q
(2)
where k, T, and q are the Boltzmann constant, temperature, and elementary charge, respectively [2,12]. By fitting the VOC plot, the ideality factor (n) could be estimated, which showed a gradual reduction as the annealing atmosphere changed from N2 (1.85) to ambient air (1.79) and O2 (1.62). It is well known that n, usually between 1 and 2, is an indicator of the influence of the in-gap states (here, trap states of ZnO NPs induced by surface-defects) on the charge recombination dynamics of CQDPVs; as n approaches unity, trap-assisted recombination in the device is reduced [2,12,36]. Considering that the estimated n values of CQDPVs reflect the process associated with the interfacial recombination dynamics at the ZnO/PbS heterojunction, we can roughly assume that interfacial recombination is reduced by annealing the ZnO layer in 128
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Fig. 4. Intensity-dependent JSC (a) and VOC (b) values of CQDPVs. Carrier lifetimes (c) and corresponding recombination rates (d) depending on the VOC of CQDPVs, derived from the TPV measurements. EIS spectrum (e) and Mott-Schottky plot (f) of CQDPVs.
intrinsic states are associated with the capacitive response [45]. By comparing the estimated DOS values at both regions, trap states in CQDPVs can be deduced. Since the only difference of CQDPVs in this study was the annealing atmosphere of ZnO layer, the difference of DOS values obtained from DLCP measurements was solely originated from the ZnO layer. Fig. 5 shows the DOS profiles of CQDPVs with various ZnO layers. At low frequency (< 10 kHz), the notable reduction of DOS values (from 3.83 to 2.37 and 0.94 × 1017 cm−3 at 100 Hz) were observed, while those values were converged around ∼1015 cm−3 at high frequency region (> 100 kHz). Given that the PbS layers were deposited through the same procedure and the only difference was the ZnO annealing atmosphere, the significant reduction of DOS indicated that surface defects of ZnO layers were indeed passivated via controlling the annealing atmosphere from N2 to O2. The passivation of ZnO NP surface defects by the annealing atmosphere has been elucidated so far and proved that reduced surface
recombination is reduced. An equivalent circuit with contact resistance, recombination resistance (RRe), and corresponding capacitance elements was defined and fitted to obtain the EIS spectrum (inset in Fig. 4e). Since the contact resistance was low compared to the RRe, which was 2–3 orders of magnitudes greater, we excluded the contact resistance; consequently, the diameter of the semi-circle directly implies RRe, a difficulty to occur charge recombination in CQDPVs [40,41]. Estimated RRe values were 12574, 15084, and 16505 Ω for CQDPVs with ZnO-N2, ZnO-Air, and ZnO-O2, respectively, in agreement with the TPV results. These results strongly suggesting that the surface defect passivation by O2 annealing could suppress the interfacial charge recombination between ZnO/PbS surface and facilitate efficient charge extraction within CQDPV under operation condition, resulting in JSC and FF enhancements. Since annealing in air (and O2) downshifted the EF of the ZnO layer, the Vbi in the device was expected to be decreased, which can result in VOC reduction and PCE suppression. To confirm this, C-V measurements were performed with the CQDPVs. The Mott-Schottky plot shown in Fig. 4f was derived from the C-V results and indicated that the Vbi of the CQDPVs was increased by annealing the ZnO layer in air (and O2), which was contradict to the expectation. Kim et al. previously demonstrated enhanced built-in potential when EF of ZnO layer was downshifted by sulfur doping [42]. They speculated this circumstance that is attributed to the reduction of mid-gap states of ZnO, which can induce Fermi level pinning between the interface. Since the mid-gap states were reduced by surface defect passivation by O2 annealing in this study, actual effective built-in potential became higher, which strongly supports the VOC enhancement of CQDPV in this study [9,43]. A quantitative analysis of the defect densities of the CQDPVs were elucidated using DLCP measurement, which is originally invented to measure the density of states (DOS) of the materials with a large number of defects, such as polycrystalline CuIn1-xGaxSe and PbS CQDs [44–46]. The DOS in the CQDPVs were carefully calculated from the various capacitive responses at various AC voltage amplitudes and frequencies based on the previous reports [44–46]. Under high frequency region, only the DOS associated with the intrinsic free carriers contribute to the capacitive response due to the short time to interact with trap states. By contrast, under low frequency region, both trap and
Fig. 5. DOS profiles of the CQDPVs with various ZnO layers obtained by DLCP measurements. The red and blue box in the plot indicate the low and high frequency region. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 129
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Fig. 6. ID/IZnO Ratio at various annealing temperature (a) and annealing time (b) obtained from XPS. J-V curve (c) and IPCE spectrum (d) of champion device with a ZnO layer annealed under O2 (at 300 °C for an hour).
feature observed commonly from the XPS and PL analyses was that the surface defects originated from the oxygen vacancy of ZnO NPs were notably reduced for ZnO-Air and ZnO-O2 compared to that of ZnO-N2. This difference indicated that O2 gas can contribute to defect passivation during the annealing process. Significant electron mobility enhancements induced from the defect passivation of ZnO NPs were also observed. Furthermore, UPS analysis demonstrated that the Fermi level downshift induced by passivation of oxygen vacancies can occur when the ZnO layers are annealed in the presence of O2 gas. The PbS CQDPVs fabricated with ZnO-Air and ZnO-O2 demonstrated ∼15% and ∼33% improved PCE values, respectively, compared to CQDPVs with ZnO-N2, mainly due to the enhanced VOC of the devices. The TPV and EIS results showed that the interfacial recombination at the ZnO/PbS interface was dramatically reduced for CQDPVs in ZnO-Air and ZnO-O2 compared to those in ZnO-N2, which is due to the reduced surface trap states of ZnO NPs by annealing in the presence of O2. In addition to the reduced interfacial recombination, enhanced Vbi in CQDPV devices in ZnO-Air or ZnO-O2 contributed to significant VOC enhancement and, consequently, solar cell performance enhancement. Further ZnO defect passivation by controlling the annealing temperature and time under O2, which was confirmed by XPS analysis, led to an additional performance enhancement of CQDPV with a highest PCE value of 10.00% in this study.
defects of ZnO NP can significantly improve CQDPV device performance. Based on this, further XPS investigations were conducted by controlling the annealing temperature and time of ZnO layer under O2 atmosphere. XPS O 1s spectra of ZnO layers annealed with various temperatures and times were deconvoluted to two peaks, in the same manner discussed above in Fig. 1b (Fig. S11). The calculated ID/IZnO ratios deduced by XPS O1s peak deconvolution were summarized in Fig. 6a and b. As increasing annealing temperature from 25 °C (without annealing) to 300 °C, the ID/IZnO values were linearly decreased, while those were exponentially saturated to ∼0.56 as increasing annealing time up to an hour. Since excess annealing temperature could induce detrimental effect to ITO glass substrate, ZnO annealing condition was fixed to 300 °C for an hour and CQDPVs were fabricated in this condition. As expected, all parameters were improved (average VOC = 0.5973 ± 0.002, JSC = 25.42 ± 1.11, FF = 62.85 ± 1.98, PCE = 9.54 ± 0.45) and a best PCE of 10.00% were recorded, which was displayed in Fig. 6c. The integrated JSC value from IPCE spectrum in Fig. 6d also corresponds to the JSC values obtained from the J-V curve. The DLCP DOS profiles of the CQDPV (Fig. S12) further justified that the significant performance improvements could be achieved by surface defect passivation of ZnO layer by annealing with O2. 4. Conclusion
Conflicts of interest
The effects of annealing atmosphere of ZnO NPs layers were elucidated. The chemical nature of the ZnO layers was remarkably modulated by the annealing atmosphere, while their surface morphology, crystallinity, and optical transparency were conserved. A distinctive
There are no conflicts to declare.
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Acknowledgement [21]
This work was supported by the Korean Science and Engineering Foundation (NRF-2015R1D1A1A01057622).
[22]
Appendix A. Supplementary data Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.03.013.
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Oxygen annealing of the ZnO nanoparticle layer for the high-performance PbS colloidal quantum-dot photovoltaics
T
Jonghee Yang, Jongtaek Lee, Junyoung Lee, Whikun Yi∗ Research Institute for Natural Sciences and Department of Chemistry, Hanyang University, Seoul, 04763, Republic of Korea
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
of annealing atmosphere on the • Effects surface defects of ZnO were elucidated.
PbS quantum dot solar cell with an O • annealed ZnO showed enhanced open2
circuit voltage.
As increasing O concentration during • annealing, the defects of ZnO were 2
passivated.
passivation suppressed inter• Defect facial charge recombination in the solar cell.
suppression of charge re• The combination induced the improved VOC of the solar cell.
A R T I C LE I N FO
A B S T R A C T
Keywords: Quantum-dot Solar cell ZnO nanoparticle Surface defect Interfacial recombination
Though numerous researches regarding the influence of annealing atmospheric condition of ZnO have been carried out, the impact of annealing atmosphere on the carrier transporting properties and the performance of the ZnO-based optoelectronics has not been well-established. Here, the effects of annealing atmosphere (i.e., N2, ambient air, and O2) used to generate ZnO nanoparticle (NP) layers are elucidated. The chemical nature of ZnO layers, especially the amount of oxygen vacancies in ZnO NPs, is modulated by the annealing atmosphere. As the composition of O2 gas increases in the annealing atmosphere, a notable reduction of oxygen vacancies of ZnO NPs and electron mobility enhancement are observed, indicating that O2 gas contributes to a reduction of surface defects on ZnO NPs during the annealing process. In addition, trap-filling by reduced oxygen vacancies of airand O2-annealed ZnO layers, induces the enhanced built-in potential in colloidal quantum-dot photovoltaic (CQDPV) devices. As expected, PbS CQDPVs with an air- and O2-annealed ZnO layer demonstrate significantly improved power conversion efficiencies than CQDPVs with an N2-annealed ZnO layer. Further analysis shows that the interfacial recombination is reduced for CQDPVs with an air- and O2-annealed ZnO layer due to the reduced trap states of ZnO NPs.
1. Introduction Colloidal quantum-dot (QD) Photovoltaics (CQDPVs), especially with lead sulfide (PbS) QDs, have been alluring to many researchers
∗
due to their several distinctive advantages; (1) a broad absorption spectrum covering the NIR range over 1100 nm, which is an upper limit for commercial Si PVs, (2) applicability with solution processes for CQDPV fabrication, and (3) superior air stability compared with
Corresponding author. E-mail address: [email protected] (W. Yi).
https://doi.org/10.1016/j.jpowsour.2019.03.013 Received 20 October 2018; Received in revised form 22 February 2019; Accepted 5 March 2019 Available online 14 March 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
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organometallic halide perovskite PVs [1]. From the first report of airstability and high-performance (> 8%) PbS CQDPV, multiple studies have been reported in this field to improve CQDPV performance [2–16]. Recently, a certified power conversion efficiency (PCE) of 12.01% (champion PCE: 12.48%) by 2D matrix engineering of quantum dot solid in CQDPV was reported [17]. Also, Baek et al. reported an 11.7% CQPDV maintaining over 90% of the initial performance by introducing a hydro/oxo-phobic hole selective layer [18]. High-performance and highly stable PbS CQDPVs usually employ zinc oxide (ZnO) as an electron transport layer. However, ZnO possesses considerable surface defects that originate from the oxygen vacancies in the ZnO lattice. Although the oxygen vacancies have been well-known to contribute n-type properties of ZnO, these defects also act as trap states within the band gap of ZnO and can induce poor solar cell performance via interfacial charge recombination of photogenerated electrons in CQDPVs. Therefore, appropriate degree of ZnO surface defects passivation is an important step to achieve high performance CQDPVs. Several attempts to reduce the surface defects of ZnO have been conducted by doping Mg or Cl during ZnO nanoparticle (NP) synthesis [5,19,20]. In spite of these advances, a more efficient and simple path for reducing the surface defects of ZnO is needed. Though the modulation of annealing atmosphere of ZnO layer has been widely investigated [21–24], the investigation that which phenomenon occurs and which effect, particularly in the perspective of charge transport, can be the observed from the heterojunction-based optoelectronic devices when annealing atmospheric condition of ZnO layer is modulated has not been well established. Here, we found that the annealing atmosphere of the ZnO layer can effectively modulate the surface defects of ZnO NPs and change the performance of CQDPVs. To compare the effect of annealing atmosphere on the ZnO layers, three atmospheric conditions (N2, ambient air, and O2) were tested during annealing, and notable chemical changes were observed. As the portion of O2 gas increases (i.e., N2 to air, and O2), surface oxygen vacancies of ZnO NPs were reduced, resulting in enhanced electron mobility within ZnO layer and reduced interfacial recombination at the ZnO/PbS interface in CQDPVs. In addition, an increased built-in potential (Vbi) due to the trap-filling via annealing ZnO in an O2 atmosphere, significant open-circuit voltage (VOC) enhancement, and a PCE of 9.44%, a ∼33% enhancement were achieved. Furthermore, by controlling the annealing temperature and time of ZnO layer under O2 atmosphere, CQDPV performance reached to 10.00%.
removed, and the precipitated QDs were re-dispersed in heptane. This QD dispersion was precipitated with acetone and centrifuged at 8000 rpm for 10 min. The supernatant was again removed, and the product was dried under a flow of N2. The resulting QD precipitates were re-dispersed in heptane (50 mg mL−1) for thin film fabrication. A previously reported method to prepare ZnO NPs was followed with slight modification [26]. Zinc acetate (0.5504 g) solution in dimethyl sulfoxide (30 mL) and TMAH (0.9995 g) solution in ethanol (10 mL) were mixed dropwise and stirred for 24 h under ambient conditions. Ethyl acetate was then added, and the resultant opaque dispersion was centrifuged at 8000 rpm for 10 min. After removing the supernatant, ethanolamine (200 μL) and ethanol (5 mL) were added to stabilize the NPs. The NPs were again washed with ethyl acetate and centrifuged at 8000 rpm for 10 min. The supernatant was again removed. The resulting ZnO NPs were dried under N2 flow and re-dispersed in ethanol. The dispersion was filtered through a membrane filter (pore size: 100 nm), and the concentration was adjusted to 20 mg mL−1 for ZnO layer fabrication. 2.3. Fabrication of CQDPVs ZnO layers were prepared by spin coating at 1500 rpm for 20 s on pre-patterned ITO substrates, followed by 250 °C annealing for 20 min in a N2 (ZnO-N2), ambient air (ZnO-Air), or O2 atmosphere (ZnO-O2). The PbS QD layers were fabricated via layer-by-layer spin coating in ambient air. TBAI-treated QD layers were prepared as follows: the presynthesized PbS QDs were deposited by spin coating at 2000 rpm for 15 s. Next, TBAI solution (10 mg mL−1 in methanol) was added to fully cover the substrate for 30 s, followed by spinning the substrate at 2000 rpm for 15 s. Two washing steps were performed via spin coating pure methanol onto TBAI-treated QD substrates. The same processes were repeated 7 times to achieve the appropriate thickness of the TBAIPbS QD layer. EDT-treated PbS QD layers were added to the TBAI-PbS QD layer as follows: the pre-synthesized PbS QDs were deposited by spin coating at 2000 rpm for 15 s. Next, 0.01 vol% EDT solution (in acetonitrile) was fully loaded onto the substrate for 30 s, followed by spinning at 2000 rpm for 15s. The QDs were washed 3 times with pure acetonitrile by spin coating under the same conditions. The same procedures were repeated to obtain complete QD layers (2 layers of EDTPbS layers onto 7 layers of TBAI-PbS layer). For both processes, the QD dispersions were loaded through a syringe filter (pore size: 450 nm). Patterned Au electrodes were deposited onto the substrate with a shadow mask to achieve an active area of 0.075 cm2 by a radio-frequency magnetron sputtering system under a base pressure of 2.0× 10−6 Torr.
2. Experimental 2.1. Materials Lead(II) oxide (PbO) (trace metal basis, 99.999%), oleic acid (OA) (technical grade, 90%), 1-octadecene (ODE) (technical grade, 90%), hexamethyldisilathane (TMS2S) (synthesis grade), zinc acetate (trace metal basis, 99.99%), tetramethylammonium hydroxide pentahydrate (TMAH) (97%), ethanolamine (99.0%), tetramethylammonium iodide (TBAI) (99.0%), and 1,2-ethanedithiol (EDT) (98.0%), dimethyl sulfoxide (99.9%) were purchased from Sigma-Aldrich. Acetonitrile (+99.8%) was purchased from Alfa Aesar.
2.4. Characterization of ZnO layers The surface morphology images of ZnO layers on ITO substrates were obtained using a Park Systems XE-100 atomic force microscope. XRD patterns of ZnO layers were recorded by a high-resolution X-ray diffractometer (Rigaku, SmartLab). UV/Vis absorption and transmission spectra were measured with a Mecasys Optizen 2120 UV spectrophotometer. XPS and UPS spectra of ZnO layers on ITO glass were collected using a K-alpha and Theta Probe photoelectron spectrometer (Thermo Fisher Scientific). XPS spectra were collected with the Al Kα line. Signals were collected under a base pressure of 3.0 × 10−8 Torr and were calibrated with the carbon 1s core-level peak (284.6 eV). UPS measurements were performed with a He I (21.22 eV) source under a base pressure of 8 × 10−7 Torr while applying a −9 V bias on samples. PL spectra were obtained with a Perkin Elmer LS 55 luminescence spectrometer. For the FET measurement, ∼40 nm ZnO Layers were fabricated onto the SiO2 (300 nm, oxide capacitance ∼11.7 nF cm−2)/ p++ Si substrate by spin coating and annealed for 20 min in N2-, ambient air-, and O2-filled glove box. Aluminum electrodes (100 nm) were fabricated with a patterned shadow mask (channel length: 1000 μm,
2.2. Synthesis of PbS QDs and ZnO NPs PbS quantum dots were synthesized by slight modification of the method reported by Hines et al. [25] PbO (0.45 g), OA (1.3 mL), and ODE (18 mL) were mixed in a 3-neck flask connected to a Schlenk line and heated to 100 °C under vacuum for 2 h. The flask was then cooled to 90 °C and maintained at that temperature under an Ar atmosphere for 2 h TMS2S (210 μL) dissolved in ODE (10 mL) was quickly injected into the flask. After 1 min, the flask was quickly cooled to room temperature. The products were precipitated with acetone and centrifuged at 8000 rpm for 10 min. After centrifugation, the supernatant was 125
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channel width: 100 μm) via thermal evaporator under a base pressure of 2 × 10−6 Torr. The electrical properties of ZnO FETs were measured using a semiconductor parameter analyzer (Keithley, 4200A-SCS). The cross-sectional image of ZnO FET was captured by FE-SEM (FEI Nova NanoSEM 450).
Fig. 1c shows that the defect-induced PL around 550 nm was decreased, strongly supporting that the annealing ZnO layer in an O2 atmosphere can effectively passivate surface defects and consequently reduce trapstates of ZnO NPs [2,19,20,28]. Surface defect passivation by O2 annealing also altered carrier transport property of ZnO layers. Fig. 1d shows traditional n-type transfer curves of field-effect transistor (FET) with ZnO layers as an active channel. An remarkably increased drainsource current (IDS) was observed as the annealing atmosphere became O2-dominant. The saturation mobility (μsat) values for each ZnO layer were calculated from the linear regime of (IDS)1/2 vs gate voltage (VG) with a following equation,
2.5. Characterization of CQDPVs The cross-sectional images of CQDPVs were produced by FE-SEM (FEI Nova NanoSEM 450) All electrical characterizations were performed under ambient conditions. The J-V curves of CQDPVs were measured using a Keithley 2400 sourcemeter under 1 sun AM 1.5 illumination with a class AAA solar simulator (McScience K201 LAB 55) calibrated with a standard Si reference solar cell (McScience K801SB041). A black shadow mask (0.06 cm2) was used to exclude the possibility of overestimation. The IPCEs of the CQDPVs were measured using a McScience K3100 IQX IPCE measurement system. Light intensity-dependent JSC and VOC characteristics were measured using a Keithley 2400 sourcemeter and by modulating the intensity of light output from a Xe lamp reaching the CQDPV, simultaneously tracking the intensity of light output by broadband power meter (Melles-Griot power/energy meter 13PEM001). TPV measurements were conducted using the light output from a Xe lamp as a continuous illumination source for VOC generation. A small perturbation from a 500 nm wavelength laser pulse was applied on the CQDPVs with a repetition rate of 10 Hz and 5 ns pulse width. An oscilloscope (Agilent DSO3202A) with 1 MΩ input terminal was used to record the TPV signals, which were averaged > 200 times. EIS spectra of the CQDPVs were obtained using an impedance analyzer (IviumTech., Iviumstat). An AC signal with an amplitude of 50 mV and frequency ranging from 100 Hz to 1 MHz was applied. The same apparatus was used to acquire capacitance-voltage (C-V) characteristics and drive-level capacitance profiling (DLCP) measurements of the CQDPVs. An AC signal with an amplitude of 20 mV and frequency of 1 kHz was applied. A DC voltage was applied from −1 V to 1 V. For DLCP measurement, various AC amplitude from 20 mV to 100 mV and frequency ranging from 100 Hz to 1 MHz were applied. All measurements were performed with 16 individual CQDPVs for each type, and the results were averaged.
2
μsat =
2L ⎛ ∂ IDS ⎞ ⎜ ⎟ WC ⎝ ∂VG ⎠
(1)
where L, W and C is the channel length, width and oxide capacitance, respectively. The μsat of ZnO-N2, ZnO-Air and ZnO- O2 were estimated to 8.13 × 10−4, 6.24 × 10−3 and 8.30 × 10−3 cm2 V−1 s−1, strongly suggesting that the defect passivation by annealing in O2 atmosphere can also significantly enhance the charge (i.e., electron) transport through the ZnO layer [29]. The XPS N 1s spectra of ZnO layers were also collected to check the contribution of nitrogen on the ZnO (Fig. S5). The XPS N 1s spectrum of as-prepared, not annealed ZnO layer was also checked since the ethanolamine was added during ZnO NP synthesis so that to monitor the species after various annealing condition. In case of as-prepared ZnO layer, an asymmetric peak was pronounced. The peak was deconvoluted to 3 individual peaks positioned at 398.1, 399.4 and 401.8 eV, which have been assigned to the N incorporated with Zn in the ZnO lattice (Zn-N), molecularly adsorbed N compounds (i.e., C-NH2 of ethanolamine) and N coordinated with O of ZnO (O-N), respectively [30,31]. After annealing, N 1s peak was significantly reduced regardless of the atmospheric conditions. The lattice-incorporated N components were thoroughly removed and tiny molecularly adsorbed N component was observed. Furthermore, as the O2 concentration increases during thermal annealing, the N components were more suppressed. This strongly indicates that the thermal annealing could not induce N doping of ZnO and only the O2 could actively contribute to the defect site (i.e., vacancy) passivation. The reason for this gas-selective surface passivation may be due to the relatively low energy required to dissociate O2 gas compared to that of N2. Furthermore, the annealing atmosphere imparted a significant difference on the energy level of ZnO layers. The ultraviolet photoelectron spectroscopy (UPS) spectrum in Fig. 2a showed that the Fermi level (EF) of the ZnO layer was downshifted, while the difference between EF and the valence band maximum (EV) became smaller as the annealing atmosphere was changed from N2 to ambient air and O2. The trend of EFEV values of the ZnO layers (2.98, 2.89 and 2.85 eV for ZnO-N2, ZnO-Air and ZnO-O2, respectively) obtained from the valence band region of UPS spectrum was considerably in accordance with the reduced PL intensity, both of which were originated from the reduced concentration of oxygen vacancies (and consequent in-gap states) of ZnO [32]. The Tauc plot in Fig. 2b, derived from the absorption spectrum, showed that the band gaps of ZnO layers were almost the same (3.41–3.42 eV). The overall energy levels of ZnO layers are depicted in Fig. 2c. The deeper EF of ZnO layer achieved by the reduction of oxygen vacancies of ZnO surface is in agreement with the previous reports [22,33]. The effect of ZnO annealing atmosphere led to a notable performance enhancement of CQDPV, as shown in Fig. 3. The CQDPVs with an ITO/ZnO NPs/TBAI-PbS/EDT-PbS/Au architecture, depicted in Fig. 3a, were fabricated by layer-by-layer spin coating. Here, PbS QDs with an excitonic peak positioning at 927 nm (diameter ∼ 3 nm) were used (Fig. S6). Considering the UPS spectrum of TBAI-PbS layer (Fig. S7), favorable photocarrier transport could be achieved for all types of CQDPVs in this study. The thicknesses of the ZnO and PbS layers in the CQDPVs were fixed at ∼40 and ∼260 nm, respectively (Fig. S8 and
3. Results and discussion To monitor the surface morphology of ZnO layers depending on the annealing atmosphere, atomic force microscopy (AFM) measurements were carried out, as displayed in Fig. S1. Here, ∼4 nm size of polycrystalline ZnO NPs were synthesized and used in this study (Fig. S2). Root-mean-square surface roughness values were 1.60, 1.53, and 1.39 nm for ZnO layers annealed in N2 (ZnO-N2), ambient air (ZnO-Air), and O2 (ZnO-O2), respectively, indicating that the annealing atmosphere does not induce the morphological change. Also, X-ray diffraction (XRD) patterns of ZnO layers (Fig. S3) suggest that the crystallinity of ZnO does not notably change depending on the annealing atmosphere. The transmittance of ZnO layers (Fig. S4) also showed no significant difference in the Vis-NIR range. However, X-ray photoelectron spectroscopy (XPS) results showed that the chemical nature of ZnO layers changed considerably, as displayed in Fig. 1. In Fig. 1a, the binding energy of the Zn 2p peaks were increased as the portion of oxygen in the annealing atmosphere increased, which indicates that oxygen-deficient Zn species form Zn-O bonding in the presence of O2 gas during annealing process [21]. In the O 1s core-level spectrum (Fig. 1b), an asymmetric peak was observed for each case and was deconvoluted to two peaks; one originated from oxygen of the ZnO lattice at ∼530 eV (IZnO) and the other from the oxygen vacancy of the ZnO surface at ∼532 eV (ID) [2,27]. The values of ID/IZnO (i.e., relative density of oxygen vacancy) were notably reduced as the annealing atmosphere became O2-dominant, again implying reduction of oxygen vacancies in ZnO layers. The photoluminescence (PL) spectrum in 126
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Fig. 1. XPS Zn 2p (a) and O 1s (b) core-level and PL spectrum (c) of ZnO layers. (d) transfer curves of FET devices with ZnO layers.
Both J-V curve and IPCE result demonstrated a slightly increased JSC, implying that the main factors contributing to the increased PCE are fill-factor (FF) and VOC enhancement. After initial PCE improvement originated from the p+ doping of EDT-PbS layer and consequent enhanced hole extraction by ambient oxidation, all CQDPVs maintained their PCE values over 4 months when stored in ambient air, indicating high stability CQDPVs (Fig. 4d) [34]. Since the PbS layers were fabricated with the same protocol for each CQDPV, the performance enhancements of CQDPVs with ZnO-O2 and ZnO-Air mainly originated from the ZnO NP layers, especially the interface between the ZnO layers and PbS layer. To access the information occurring at the ZnO/PbS interface, light intensity-dependent JSC and VOC values were obtained by modulating the light intensity irradiated to
S9). Fig. 3b shows J-V curves of CQDPVs at varying ZnO annealing atmospheres. The VOC values of the CQPDVs were dramatically increased, while the short-circuit current density (JSC) also slightly increased as the annealing atmosphere changed from N2 to air and O2. The overall CQDPV performances are listed in Table 1. The PCEs of the champion devices with ZnO-Air and ZnO-O2 were 8.20 and 9.44%, which is ∼15 and ∼33% enhanced, respectively, compared with that with ZnO-N2. Although this value is lower than the highest performance of CQDPV ever reported, the ZnO annealing atmosphere induced a significant influence on the CQDPV performance. The integrated JSC values of the CQDPVs, derived from the incident-photon to current efficiency (IPCE) spectrum in Fig. 3c and summarized in Table 1, also showed similar values and trend to those obtained from the J-V curve.
Fig. 2. UPS spectrum (a) and Tauc-plot obtained from the UV/Vis absorption spectrum (b) of ZnO layers. (c) Corresponding energy level diagram of ZnO layers. 127
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Fig. 3. (a) A schematic of the structure of a CQDPV. J-V (b) and IPCE (c) of champion CQDPVs. (d) Time-evolution of the PCE value of CQDPVs. Table 1 Summarized CQDPV performances. ZnO annealing atmosphere
VOC (V)
JSC (mA/cm2)
FF (%)
PCE (%)
Integrated JSC (mA/cm2)
N2 Ambient air O2
0.5229 (0.5287 ± 0.010) 0.5564 (0.5587 ± 0.011) 0.5831 (0.5899 ± 0.010)
24.14 (23.83 ± 1.21) 25.05 (24.88 ± 1.30) 25.53 (25.00 ± 1.17)
56.29 (54.77 ± 1.52) 58.84 (57.41 ± 1.43) 63.40 (61.36 ± 2.03)
7.10 (6.90 ± 0.20) 8.20 (7.98 ± 0.36) 9.44 (9.05 ± 0.39)
23.20 24.08 24.67
ambient air and O2 atmospheres. To gain more insight into the interfacial recombination of CQDPVs, transient photovoltage (TPV) measurements with various VOC conditions were conducted [3,12,27,28,36,38,39]. Under constant white light for generating appropriate VOC of CQDPV, small perturbation with a nanosecond laser pulse was applied to the sample and the extra photovoltage transients were recorded by digital oscilloscope. Since the photovoltage is proportional to the photogenerated charge in CQDPV, the direct monitoring of the charge recombination process in CQDPV was enabled by fitting the photovoltage decay curve [39]. The representative TPV decay profiles of the CQDPVs at the fixed VOC of 0.35 V were also exhibited in Fig. S10. By fitting transient decay curves to a bi-exponential decay model, carrier lifetime and recombination rate constant (reciprocal to the carrier lifetime) were obtained for each CQDPV, as displayed in Fig. 4c and d. The interfacial recombination process is suppressed as the annealing atmosphere changes from N2 to ambient air and O2, which is in accordance with the intensity-dependent VOC features and CQDPV performances. Note that the annealing ZnO layer in air and O2 atmospheres induced trap-passivation of ZnO NPs, resulting in a reduction of the surface trap states, as demonstrated in Fig. 1. Due to the reduced trap states of the ZnO layer by annealing in air (or O2), photogenerated electrons are prone to extraction through the ZnO layer to the ITO electrode without severe interfacial recombination, resulting in enhanced performance of the CQDPVs. The electrochemical impedance spectroscopy (EIS) spectrum under dark conditions, shown in Fig. 4e, further supports that the interfacial
the CQDPVs, as is demonstrated in Fig. 4a and b. Generally, the JSC of CQDPV is proportional to Φα, where Φ and α are the light intensity and exponential factor, respectively [2,35,37,38]. When α is close to unity, the photocurrent is predominantly determined by the generation rate of electron-hole pairs upon photon absorption [37]. By fitting the JSC results, the α values were estimated to have similar values around 0.90–0.97, indicating that the CQDPVs generated photocurrent in a similar manner. However, the intensity-dependent VOC results in Fig. 4b reveal that a notable difference exists depending on the annealing atmosphere of ZnO layers for CQDPV fabrication. Usually the relation between VOC and light intensity is expressed by the following equation,
VOC =
nkT αln (Φ ) + C q
(2)
where k, T, and q are the Boltzmann constant, temperature, and elementary charge, respectively [2,12]. By fitting the VOC plot, the ideality factor (n) could be estimated, which showed a gradual reduction as the annealing atmosphere changed from N2 (1.85) to ambient air (1.79) and O2 (1.62). It is well known that n, usually between 1 and 2, is an indicator of the influence of the in-gap states (here, trap states of ZnO NPs induced by surface-defects) on the charge recombination dynamics of CQDPVs; as n approaches unity, trap-assisted recombination in the device is reduced [2,12,36]. Considering that the estimated n values of CQDPVs reflect the process associated with the interfacial recombination dynamics at the ZnO/PbS heterojunction, we can roughly assume that interfacial recombination is reduced by annealing the ZnO layer in 128
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Fig. 4. Intensity-dependent JSC (a) and VOC (b) values of CQDPVs. Carrier lifetimes (c) and corresponding recombination rates (d) depending on the VOC of CQDPVs, derived from the TPV measurements. EIS spectrum (e) and Mott-Schottky plot (f) of CQDPVs.
intrinsic states are associated with the capacitive response [45]. By comparing the estimated DOS values at both regions, trap states in CQDPVs can be deduced. Since the only difference of CQDPVs in this study was the annealing atmosphere of ZnO layer, the difference of DOS values obtained from DLCP measurements was solely originated from the ZnO layer. Fig. 5 shows the DOS profiles of CQDPVs with various ZnO layers. At low frequency (< 10 kHz), the notable reduction of DOS values (from 3.83 to 2.37 and 0.94 × 1017 cm−3 at 100 Hz) were observed, while those values were converged around ∼1015 cm−3 at high frequency region (> 100 kHz). Given that the PbS layers were deposited through the same procedure and the only difference was the ZnO annealing atmosphere, the significant reduction of DOS indicated that surface defects of ZnO layers were indeed passivated via controlling the annealing atmosphere from N2 to O2. The passivation of ZnO NP surface defects by the annealing atmosphere has been elucidated so far and proved that reduced surface
recombination is reduced. An equivalent circuit with contact resistance, recombination resistance (RRe), and corresponding capacitance elements was defined and fitted to obtain the EIS spectrum (inset in Fig. 4e). Since the contact resistance was low compared to the RRe, which was 2–3 orders of magnitudes greater, we excluded the contact resistance; consequently, the diameter of the semi-circle directly implies RRe, a difficulty to occur charge recombination in CQDPVs [40,41]. Estimated RRe values were 12574, 15084, and 16505 Ω for CQDPVs with ZnO-N2, ZnO-Air, and ZnO-O2, respectively, in agreement with the TPV results. These results strongly suggesting that the surface defect passivation by O2 annealing could suppress the interfacial charge recombination between ZnO/PbS surface and facilitate efficient charge extraction within CQDPV under operation condition, resulting in JSC and FF enhancements. Since annealing in air (and O2) downshifted the EF of the ZnO layer, the Vbi in the device was expected to be decreased, which can result in VOC reduction and PCE suppression. To confirm this, C-V measurements were performed with the CQDPVs. The Mott-Schottky plot shown in Fig. 4f was derived from the C-V results and indicated that the Vbi of the CQDPVs was increased by annealing the ZnO layer in air (and O2), which was contradict to the expectation. Kim et al. previously demonstrated enhanced built-in potential when EF of ZnO layer was downshifted by sulfur doping [42]. They speculated this circumstance that is attributed to the reduction of mid-gap states of ZnO, which can induce Fermi level pinning between the interface. Since the mid-gap states were reduced by surface defect passivation by O2 annealing in this study, actual effective built-in potential became higher, which strongly supports the VOC enhancement of CQDPV in this study [9,43]. A quantitative analysis of the defect densities of the CQDPVs were elucidated using DLCP measurement, which is originally invented to measure the density of states (DOS) of the materials with a large number of defects, such as polycrystalline CuIn1-xGaxSe and PbS CQDs [44–46]. The DOS in the CQDPVs were carefully calculated from the various capacitive responses at various AC voltage amplitudes and frequencies based on the previous reports [44–46]. Under high frequency region, only the DOS associated with the intrinsic free carriers contribute to the capacitive response due to the short time to interact with trap states. By contrast, under low frequency region, both trap and
Fig. 5. DOS profiles of the CQDPVs with various ZnO layers obtained by DLCP measurements. The red and blue box in the plot indicate the low and high frequency region. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 129
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Fig. 6. ID/IZnO Ratio at various annealing temperature (a) and annealing time (b) obtained from XPS. J-V curve (c) and IPCE spectrum (d) of champion device with a ZnO layer annealed under O2 (at 300 °C for an hour).
feature observed commonly from the XPS and PL analyses was that the surface defects originated from the oxygen vacancy of ZnO NPs were notably reduced for ZnO-Air and ZnO-O2 compared to that of ZnO-N2. This difference indicated that O2 gas can contribute to defect passivation during the annealing process. Significant electron mobility enhancements induced from the defect passivation of ZnO NPs were also observed. Furthermore, UPS analysis demonstrated that the Fermi level downshift induced by passivation of oxygen vacancies can occur when the ZnO layers are annealed in the presence of O2 gas. The PbS CQDPVs fabricated with ZnO-Air and ZnO-O2 demonstrated ∼15% and ∼33% improved PCE values, respectively, compared to CQDPVs with ZnO-N2, mainly due to the enhanced VOC of the devices. The TPV and EIS results showed that the interfacial recombination at the ZnO/PbS interface was dramatically reduced for CQDPVs in ZnO-Air and ZnO-O2 compared to those in ZnO-N2, which is due to the reduced surface trap states of ZnO NPs by annealing in the presence of O2. In addition to the reduced interfacial recombination, enhanced Vbi in CQDPV devices in ZnO-Air or ZnO-O2 contributed to significant VOC enhancement and, consequently, solar cell performance enhancement. Further ZnO defect passivation by controlling the annealing temperature and time under O2, which was confirmed by XPS analysis, led to an additional performance enhancement of CQDPV with a highest PCE value of 10.00% in this study.
defects of ZnO NP can significantly improve CQDPV device performance. Based on this, further XPS investigations were conducted by controlling the annealing temperature and time of ZnO layer under O2 atmosphere. XPS O 1s spectra of ZnO layers annealed with various temperatures and times were deconvoluted to two peaks, in the same manner discussed above in Fig. 1b (Fig. S11). The calculated ID/IZnO ratios deduced by XPS O1s peak deconvolution were summarized in Fig. 6a and b. As increasing annealing temperature from 25 °C (without annealing) to 300 °C, the ID/IZnO values were linearly decreased, while those were exponentially saturated to ∼0.56 as increasing annealing time up to an hour. Since excess annealing temperature could induce detrimental effect to ITO glass substrate, ZnO annealing condition was fixed to 300 °C for an hour and CQDPVs were fabricated in this condition. As expected, all parameters were improved (average VOC = 0.5973 ± 0.002, JSC = 25.42 ± 1.11, FF = 62.85 ± 1.98, PCE = 9.54 ± 0.45) and a best PCE of 10.00% were recorded, which was displayed in Fig. 6c. The integrated JSC value from IPCE spectrum in Fig. 6d also corresponds to the JSC values obtained from the J-V curve. The DLCP DOS profiles of the CQDPV (Fig. S12) further justified that the significant performance improvements could be achieved by surface defect passivation of ZnO layer by annealing with O2. 4. Conclusion
Conflicts of interest
The effects of annealing atmosphere of ZnO NPs layers were elucidated. The chemical nature of the ZnO layers was remarkably modulated by the annealing atmosphere, while their surface morphology, crystallinity, and optical transparency were conserved. A distinctive
There are no conflicts to declare.
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Acknowledgement [21]
This work was supported by the Korean Science and Engineering Foundation (NRF-2015R1D1A1A01057622).
[22]
Appendix A. Supplementary data Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.03.013.
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