A solution-processed cobalt-doped nickel oxide for high efficiency inverted type perovskite solar cells

Journal of Power Sources 412 (2019) 425–432

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A solution-processed cobalt-doped nickel oxide for high efficiency inverted type perovskite solar cells

T

Ju Ho Lee, Young Wook Noh, In Su Jin, Sang Hyun Park, Jae Woong Jung∗ Department of Advanced Materials Engineering for Information & Electronics, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do, 446701, 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

Co-doping of NiO has been studied • systematically to be used in perovskite X

solar cells.

Co-doped NiO improved the optoe• lectronic properties of NiO for hole X

X

•

transport layer. Co-doped NiOX improved the efficiency of the solar cells from 13.96% to 17.52%.

A R T I C LE I N FO

A B S T R A C T

Keywords: Metal doping NiOX Cobalt Perovskite solar cells Planar heterojunction

Optimal interfaces play an important role in determining the efficiency of perovskite solar cells. Despite nickel oxide is a promising hole transport material in perovskite solar cells, electrical conductivity and surface properties of pristine nickel oxide are not satisfactory for achieving high efficiency perovskite solar cells. Here, we demonstrate that cobalt doping significantly improves not only the electrical conductivity but also the interfaces of nickel oxide hole transport layer. As a result, the hole extraction, transport and collection properties are significantly improved in the device using cobalt-doped nickel oxide as a hole transport layer. Also, it reveals that cobalt-doped nickel oxide reduces the trap-sites of interfaces of perovskite/hole transport layer, leading to suppressed charge recombination in the devices. As a result, the devices using cobalt-doped nickel oxide achieve as high as 17.52% without severe hysteresis, which is attributed to better band alignment, superior hole extraction and decreased resistance, as compared to pristine nickel oxide-based devices.

1. Introduction Over the past decade, perovskite semiconductors have quickly emerged as promising photovoltaic materials with high power conversion efficiencies (PCEs) for solar cells [1–4]. Innovation of perovskite-based solar cells (PSCs) is due to their attractive properties such as an appropriate optical band gap range in 1.2–2.3 eV, an excellent light absorption coefficient, long charge carrier lifetime and excellent

∗

charge transport properties [5]. The state-of-the-art device performance of PSCs has achieved certified PCEs of 23.2% in single junction device through integrated efforts in material engineering and device optimization [6–8]. As compared to conventional mesoscopic device architecture, planar heterojunction devices with p-i-n structure has received much attention due to their simple device structure, low temperature solution processing, and variable materials design for a variety of interfacial layer [9–16]. More importantly, p-i-n devices have been

Corresponding author. E-mail address: [email protected] (J.W. Jung).

https://doi.org/10.1016/j.jpowsour.2018.11.081 Received 30 September 2018; Received in revised form 19 November 2018; Accepted 27 November 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.

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J.H. Lee et al.

with a PTFE-D filter having a pore size of 0.2 μm. The perovskite precursor solution was prepared as reported elsewhere [37]. PbI2 (461 mg of), CH3NH3I (159 mg) and DMSO and (78 mg) (molar ratio to be 1:1:1) were mixed at room temperature in 600 mg of DMF solution under vigorous stirring for 1 h, followed by filtering using PTFE-D filter with a pore size of 2 μm.

reported with a less hysteresis along with scan direction as compared ni-p devices [17]. In addition, they are suitable for plastic substrate, which allows efficient flexible and stretchable solar cells [18–21]. For achieving high PCE of planar heterojunction PSCs, interfacial optimization is a key strategy because charge transport and collection are mainly promoted by the interfaces between perovskite semiconductors and electrodes of device. In particular, hole transport layer (HTL) for planar heterojunction PSCs requires high optical transmittance, good charge transporting properties, well-matched energy bands to the electrode and perovskite semiconductors for maximize the short circuit current density (JSC), open circuit voltage (VOC), and fill factor (FF). In addition, the HTL should be resistant to moisture for high ambient stability of PSCs. Several types of p-type semiconductors such as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), V2Ox, WO3, Cu2O, CuSCN, CuI, have been used in planar heterojunction PSCs to improve the device performance [22–26]. Nickel oxide (NiOX) is one of the well-studied hole transport layer not only for the planar heterojunction PSCs but also in the thin film optoelectronic devices [27,28]. The large band gap, p-type charge transport characteristic and deep valence band of NiOX are appropriate for the use of HTL in PSCs, which deliver appropriate optoelectronic properties for efficient PSCs. In addition, the valence band maximum (VBM) of NiOx matches well with that of perovskite semiconductors while its conduction band minimum (CBM) is much higher than that of the perovskite materials, which ensures the maximum photocurrent generation with a minimized voltage loss of PSCs [29]. Although intrinsic NiOx shows a p-type semiconductor, its major carrier (hole) concentration is far lower than other p-type semiconductors [30,31]. In addition, Fermi level of NiOx is far from its VBM, which results in weak built-in field strength in a limited film thickness and large energy level offset at the NiOx–perovskite interface. Thus, low holes transfer efficiency of NiOx would induce hole accumulation near the perovskite interface, which leads to charge recombination at the interfaces of PSCs. One of viable way to improve the electronic properties of NiOx is a metal doping. Jen and co-workers have reported that Cu-doping of NiOX significantly improved the electrical properties, leading to enhanced PCE of the planar heterojunction PSCs [32,33]. Most recently, Cs or Ag doping of NiOX effectively enhances the electron conductivity and higher work function, which demonstrates the remarkable improvement in PCEs [34,35]. Thus, it is interesting to optimize the metal doping of NiOx for high efficiency and stability of planar heterojunction PSCs [36]. In this work, we demonstrated that Cobalt is a promising metal dopant of NiOX for solution-processed HTL in planar heterojunction PSCs. Cobalt is a promising metal dopant for the NiOx because the atomic radii of Cobalt and Nickel are almost the same (152 p.m. and 149 p.m. for Cobalt and Nickel, respectively) while their valence electrons are only one different. We systematically optimized the doping condition of Cobalt in the NiOx such as concentration and annealing temperature. In the optimal doping condition, the cobalt-doped NiOx (denoted as Co-NiOx hereafter) achieved a PCE up to 17.52%, which is an improvement by 25.5% as compared to the pristine NiOx-based devices. Optoelectronic properties, charge transport behavior, recombination kinetics of Co-NiOx revealed that cobalt is an effective metal dopant for NiOX HTL of planar heterojunction PSCs, achieving high photovoltaic performance.

2.2. Fabrication of thin-film perovskite solar cells The ITO-coated glass substrates (20 Ω sq−1) were cleaned sequentially with water, acetone, and isopropanol under sonication for 30 min, and treated with UV-O3. Co-NiOX layer was formed by spin-coating on ITO coated glass (3000 RPM, 60 s) and the substrate was then annealed at 340 °C for 1 h in ambient air. After cooling the substrate to room temperature, the precursor solution was spin-coated at 4000 rpm for 25 s to form a perovskite layer. After 7 s, 400 μL of chlorobenzene was poured onto the substrate during spin coating. After thermal annealing at 100 °C for 10 min, the PC61BM (15 mg mL−1 in chlorobenzene) and bis-C60 surfactant (2 mg mL−1 in isopropyl alcohol) were then sequentially deposited by spin-coating at 1500 rpm for 60 s and 3000 rpm for 60 s, respectively [38]. Finally, 100 nm of Ag was evaporated under high vacuum (< 5 × 10−6 Torr). The device area is defined as 10 mm2 by the shadow mask. 2.3. Characterizations Elemental investigation was carried out using X-ray Photoelectron Spectroscopy (XPS) (K Alpha, Thermo Science) using a MgKα/AlKα dual anode X-ray source. Absorption and transmittance measurements were carried out using a spectrometer (Caryr 100, Agilent). The fluorescence life time was measured by time-correlated single photon counting system using the picosecond laser with an excitation at 405 nm. Surface morphologies of thin films were observed using an atomic force microscope (AFM) (CoreAFM, Nanosurf) in tapping mode and field-emission scanning electron microscopy (FESEM) (S-4800, Hitach). Work function of NiOx thin films were measured by Kelvin Probe (SKP5050, KP Technology) by using Au film as a reference. The JV curves of the devices were obtained using a parameter analyzer (4200-SCS, Keithley) under 100 mW/cm2 illumination (AM 1.5G), which was calibrated using a NREL-certified photodiode. The incident photon-to-current density efficiency (IPCE) of the device was measured using lock-in amplifier system which records the short-circuit current density under chopped monochromatic light. Electrochemical impedance spectroscopy (EIS) of the devices was investigated using The electrochemical impedance spectra were studied using a potentiostat (Compactstat, IVIUM). Intensity-modulated photocurrent and photovoltage were measured by potentiostat with a light emitting diode (LED) (IVIUM, IM1225). 3. Results and discussion X-ray photoelectron spectroscopy (XPS) analysis was investigated to compare the chemical states of the pristine and the Co-NiOx thin films. Fig. 1a and b shows the XPS spectrum of the Ni 2p3/2 peak in un-doped and Co-NiOX (see Fig. S1 for full XPS spectra). The peaks at 853.9 eV, 855.8 eV, and 861.3 eV denote Ni2+ of the standard NieO octahedral junction structure of cubic rock salt, pore-induced Ni3+ ions and wide peak due to the development process (satellite peak), respectively. The XPS results of Ni 2p3/2 states suggest that the mixed state of divalent and trivalent Ni ions are presented in the sol-gel driven NiOx thin film regardless of Co doping. The obtained ratio between Ni3+/Ni2+ was calculated to be 1.85 and 2.04 for NiOx and Co-NiOx films, respectively, indicating the oxygen deficiency in NiO and thus the presence of Ni vacancy. Because Ni3+ state (present as Ni2O3) induces oxygen deficiency in NiO crystal, higher density of Ni3+ in Co-NiOx would contribute to the increase of hole conductivity [39]. The O 1s states can be

2. Experimental section 2.1. Materials 122.22 mg of Ni(OCOCH3)2 · 4H2O (0.49 mmol) and Co(OCOCH3)2 · 4H2O in various ratios (0, 0.5, 1.0, 2.5 and 5.0 mol%) were mixed in 2methoxyethanol (6.3 mL) containing a small amount of monoethanolamine (30 μl). After the solution was stirred at room temperature for 2 h, a uniform precursor solution was formed and then filtered 426

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Fig. 1. XPS spectra of (a, b) Ni 2p3/2, (c, d) O 1s, (e, f) Co 2p3/2 of the (a, c, e) un-doped NiOX layer, (b, d, f) Co-doped NiOX layer and (c) Co 2p3/2 of the Co-doped NiOX.

The influence of cobalt doping on the surface morphology of CoNiOx film was investigated by AFM. Fig. 2a–e displays the Co-NiOX films with different amount of cobalt (0.5–5 mol%). All the Co-NiOX thin films possessed uniform and smooth surface comparable with pristine NiOx. As the doping concentration was increased up to 5 mol%, the film roughness was still comparable to the pristine NiOX film, which suggests that the Co was effectively embedded in the NiOX without notable deformation of the rock salt crystal structure. The flat surface of the HTL not only is advantageous for efficient hole transporting but also facilitates the formation and full coverage of the perovskite layer in device. Fig. 3a–e shows scanning electron microscope (SEM) images of the perovskite (CH3NH3PbI3) thin films formed on NiOX films with different

decomposed into three components at 529.5 eV, 531.2 eV and 532.5 eV which are attributed to oxygen atoms belonging to the NieO framework, oxygen from OH group and the water molecules adsorbed on the sample surface (Fig. 1c and d). The Co-NiOx films exhibited almost the same O 1s spectra to the pristine NiOx, indicating that the electronic structure of oxygen for NiOx did not change significantly after Co doping. Despite the exact reason for inconsistency in Ni 2p and O 1s is unclear, the oxygen sources for the thin film are not only nickel oxide but also adsorbed oxygen-containing molecules such as surface hydroxyl, ether, water, and so on. Thus, the electronic state for the oxygen cannot be simply concluded in this work. The Co 2p3/2 states for CoNiOx also indicate that cobalt atom is present as a trivalent state predominantly rather than divalent state in Co-NiOx film.

Fig. 2. AFM topographies of (a) pristine NiOX and Co-NiOX with Cobalt concentration of (b) 0.5%, (c) 1%, (d) 2.5%, and (e) 5%. 427

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Fig. 3. (a) SEM images of perovskite layer on (a) pristine NiOx, (b) 0.5% Co-NiOx, (c) 1.0% Co-NiOx, (d) 2.5% Co-NiOx, (e) 5.0% Co-NiOx. (f) A photograph image of the perovskite films formed on different HTLs.

amount of cobalt. It is noted here that mirror-like smooth perovskite films were obtained through antisolvent washing treatment method, followed by thermal annealing (Fig. 3f). All the perovskite films possessed uniform surface with densely-packed grains on Co-NiOx, as referred in low magnification SEM images in Fig. S2. It is interesting that the grain size of perovskite was much larger on the Co-NiOX (ca. 300–500 nm) as compared to the pristine NiOX (ca. 100–200 nm). Although the exact reasons for the morphology discrepancy of the perovskite films are not fully understood, NiOX surface chemistry may depend on the metal dopant, that affects nucleation behavior and promotes the continued growth of a more widely dispersed nuclei [11]. Because the garin boundaries are major trap sites for charge carriers in the PSCs, charge recombination from the perovskite layer will be suppressed on the Co-NiOX. The optical properties of the Co-NiOX and the perovskite layers formed on different NiOx layers were investigated as shown in Fig. 4a. All the transmittance spectra of NiOX with different cobalt concentrations showed high optical transmittance > 90% at the wavelengths greater than 400 nm. Because photons penetrate into the perovskite layer through the HTLs in the planar heterojunction PSCs having a p-i-n structure, the high optical transparency of a broad range of HTLs is a prerequisite to promote the absorbance from the perovskite layers. For the perovskite layers formed on the HTLs, all the films showed similar absorption spectra regardless of Co-doping. In addition, the X-ray diffraction results also indicate that all perovskite films on different HTLs are highly crystalline, regardless of doping concentration, as displayed in Fig. S3. The influence of Co-doping on the photovoltaic performance of planar heterojunction PSCs was studied in an inverted structure (ITO/ Co-NiOX/CH3NH3PbI3/PC61BM/bis-C60/Ag) [40]. (Fig. 5a) It is noted here that better charge transport and low energy loss is expected of the devices with Co-NiOx because of the well-matched energy levels to CH3NH3PbI3 than the pristine NiOx. (see the work function variation of Co-NiOX thin films in Fig. S4) The PCE of the devices were optimized by varying the Cobalt doping concentration from 0.5 to 5.0 mol%. Fig. 6a shows current density voltage (J-V) curves of PSCs using pristine NiOX and Co-NiOX as HTL and the corresponding device parameters are summarized in Table 1. The device with pristine NiOx (un-doped) delivers a PCE of 14.04%, with an open-circuit voltage (VOC) of 1.01 V, a JSC of 21.38 mA/cm2, and an FF of 0.65. It is clear that Cobalt doping effectively enhances the device performance. The devices using CoNiOx as a HTL layer exhibited an improved PCE of 15.48, 16.20 and 16.82% for a Cobalt concentration of 0.5, 1.0. 2.5 mol%, respectively.

Fig. 4. (a) Transmittance of different NiOx layers and (b) absorbance of perovskite layer on different NiOx layers.

As cobalt concentration was further increased to 5.0 mol%, the device showed 17.52% with a VOC of 1.04 V, a JSC of 22.46 mA/cm2 and a FF of 0.75. The device with a pristine NiOx and Co-NiOx exhibited a reduced series resistance (Rs) with an enlarged shunt resistance (Rsh), suggesting 428

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Fig. 5. (a) Cross-sectional SEM image of representative PSCs with 5 mol% CoNiOx. (b) J-V curves of PSCs with different NiOx. (c) EQE spectra of corresponding devices.

that Co-NiOx reduced internal resistance and charge leakage of the PSCs, leading to the significant improvement of JSC and FF. The EQE spectra of the devices show that Co-NiOx HTL promotes the photon-toelectron conversion in entire range, leading to more efficient hole extraction and transport as compared to pristine NiOx. Device results for Co doping higher than 5.0 mol% was listed in Table S1. The hysteresis of J-V measurement along with scan directions is displayed in Fig. S5a. As summarized in Table 2, both devices did not showed notable hysteresis. The steady-state photocurrent generations of the devices with different HTLs at maximum power point (MPP) are also shown in Fig. S5b. After continuous illumination up to 400 s, both devices exhibited comparable PCEs to the initial PCEs, indicating stability and reliability of device performance of PSCs using Co-NiOx HTL. It is also noted that integrated JSC values from EQE spectra are consistent with the

Fig. 6. (a) IMPS, (b) IMVS and (c) EIS spectra of PSCs with different HTLs.

measured JSC under illumination, indicating that the PCE measurement of this work is well calibrated. To understand the charge transport and recombination behavior at the interface of perovskite/HTL, the charge carrier life times of the devices with pristine NiOx and Co-NiOx were compared by using IMPS and IMVS (Fig. 6a and b). Because IMPS measures the transfer lifetime of charge carriers under short-circuit conditions, lower lifetime for the device with Co-NiOx indicates fast and efficient hole transfer/collection from the perovskite layer to anode [41]. Meanwhile, IMVS provides information on the charge carrier lifetime under open-circuit 429

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Table 1 Photovoltaic parameters for the planar heterojunction PSCs based on NiOX with various Co-doping concentrations. Values in parenthesis denotes the average values. Co-doping con. (mol%)

VOC (V)

Un-doped 0.5 1.0 2.5 5.0

1.01 1.01 1.00 1.02 1.03

± ± ± ± ±

JSC (mA/cm2) 0.01 0.01 0.01 0.01 0.01

(1.01) (1.02) (1.00) (1.03) (1.04)

21.10 21.14 21.22 21.45 21.79

± ± ± ± ±

0.30 0.35 0.26 0.51 0.61

FF (21.38) (21.37) (21.47) (22.07) (22.46)

0.64 0.68 0.71 0.71 0.72

PCE (%) ± ± ± ± ±

0.02 0.03 0.02 0.02 0.03

(0.65) (0.71) (0.74) (0.74) (0.75)

13.62 14.91 15.74 16.54 17.24

± ± ± ± ±

0.40 0.38 0.29 0.31 0.26

(14.04) (15.48) (16.20) (16.82) (17.52)

Integrated JSC (mA/cm2)

Rs (Ω·cm2)

Rsh (103 Ω cm2)

21.69 21.42 21.62 21.94 22.35

10.81 10.25 9.68 8.32 6.7

1.56 1.85 2.21 2.58 3.83

Table 2 Hysteresis, stabilized efficiency, EIS results of the planar heterojunction PSCs based on NiOX with or without Co-doping. Co-doping con. (mol%)

Un-doped 5

JSC (mA/cm2)

VOC (V)

FF

PCE (%)

forward

reverse

forward

reverse

forward

reverse

forward

reverse

1.01 1.04

1.01 1.04

21.26 22.97

21.23 22.90

0.63 0.74

0.63 0.74

13.55 17.51

13.52 17.59

conditions, which depicts ionic diffusion or charge accumulation at the interface of the device. Hence, longer life time for Co-NiOx-based devices than that of pristine NiOx suggests the prolonged life time of carrier life time and thus suppressed charge recombination in the device. Thus, IMPS and IMVS results conclude that Cobalt doping of NiOx reduces the concentration of interface traps which in turn impairs nonradiative recombination, leading to enhancement in VOC, JSC, FF and the overall PCE, as shown above. To further confirm our hypothesis, EIS were conducted to investigate the internal electrical properties of PSCs with different HTLs. As shown in Fig. 6c, the Nyquist plots in the frequency range from 1 MHz to 10 mHz for the devices was studied over the frequency range from 0.1 Hz to 10 MHz, and their fitting results of the equivalent circuit are listed in Table 2. The EIS result indicates that the overall resistance of the devices and interfaces for Co-NiOx-based device were significantly reduced as compared to that of pristine NiOx, which contributes to increased recombination resistance. Therefore the impedance analysis also proves that the device using Co-NiOx HTL results in reduced charge recombination in device compared to the pristine NiOx to further enhance the photovoltaic performance. As displayed in Fig. 7a, the photoluminescence (PL) of perovskite layer was more quenched on Co-NiOx as compared to that on pristine NiOx, which means the fast charge transfer and transport at the interface of perovskite and Co-NiOx. This is also evident from the time-resolved photoluminescence (TRPL) measurement, as seen in Fig. 7b. TRPL is an effective tool to monitor the trap-state density in the perovskite films and their interfaces. The perovskite layer without any HTL showed the average lifetime (τave) of photo-generated excitons of98 ns, while the perovskite layer on pristine NiOx and Co-NiOx exhibited τave were 76 ns and 46 ns, respectively, demonstrating the much better hole transfer capability of Co-NiOx than that of pristine NiOx. The quick charge transfer of Co-NiOx is mainly originated to improved quality of interface and more favorable band alignment, as verified above. Furthermore, Co-NiOx exhibited much higher electrical conductivity than pristine NiOx, as measured by C-AFM (Fig. 8) The higher electrical conductivity of HTL facilitates quick hole transport from the perovskite layer to anode, as demonstrated previously [11]. It should be noted here that λmax of PL spectrum of the perovskite were blue-shifted on CoNiOx from 773 to 768 nm, while λmax of perovskite on pristine NiOx was almost the same (772 nm). Thus, it is apparent that the tail-states near band edges of perovskite layer was eliminated on Co-NiOx, which is another evidence for improved interface of perovskite/Co-NiOx.

Rs (Ω)

RCT (Ω)

RCR (Ω)

63 49

267 91

705 1061

Fig. 7. (a) Steady-state and (b) dynamic photoluminescence quenching of perovskite layer on NiOX hole transport layer with or without Cobalt doping.

dopant for NiOx for the use in HTL for planar heterojunction PSCs. A systematic analyses of optical, morphological and crystallographic investigations showed that the cobalt doping effectively improves the optoelectronic properties of NiOX, which promotes efficient hole extraction and better energy level alignment between perovskite layer and HTL. The improved interface of Co-NiOx/perovskite also increased the grain size of perovskite layer up to ∼500 nm to provide a high

4. Conclusions In conclusion, we have demonstrated that cobalt is an effective 430

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[2] [3]

[4]

[5]

[6] [7]

[8]

[9]

[10]

[11]

[12] [13]

[14]

[15] [16]

[17] [18]

Fig. 8. (a) C-AFM images for pristine NiOx (a) and Co-doped NiOx (b).

[19]

quality perovskite layer to reduce trap site density at grain boundaries. The affirmative interface between perovskite and Cu-NiOx raised the PCE from 13.96% to 17.52%. These results suggest that Co-doped NiOX as transport layers are promising material for forming efficient and stable perovskite solar cells for practical use.

[20]

[21]

Conflicts of interest [22]

There are no conflicts of interest to declare. [23]

Acknowledgement

[24]

This research was supported by Korea Electric Power Corporation. (Grant number: R17XA05-11) This work was also supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (Grant number: 2017R1C1B2009691).

[25]

[26]

Appendix A. Supplementary data [27]

Chemical composition result from XPS, full spectrum of XPS measurements and work function comparison for NiOx with different Zn doping. Supplementary data to this article can be found online at https://doi.org/10.1016/j.jpowsour.2018.11.081.

[28]

[29]

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