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Rare earth ions doped NiOx hole transport layer for efficient and stable inverted perovskite solar cells
Journal of Power Sources 444 (2019) 227267
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Rare earth ions doped NiOx hole transport layer for efficient and stable inverted perovskite solar cells Xinfu Chen, Lin Xu **, Cong Chen, Yanjie Wu, Wenbo Bi, Zonglong Song, Xinmeng Zhuang, Shuo Yang, Shidong Zhu, Hongwei Song * State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� REs (Ce, Nd, Eu, Tb, Yb) ions were doped into NiOx HTLs via solution method. � PSC using 3% Eu:NiOx exhibited the best PCE of 15.6%, with 23.4% improvement. � The improvement was attributed to the improved conductivity and perovskite layer.
A R T I C L E I N F O
A B S T R A C T
Keywords: Rare earth ions Doped NiOx film Perovskite solar cells Inverted planar structure
Hole transport layer plays a critical role in achieving high performance and stable inverted perovskite solar cells (PSCs). Doping has been proved to be an effective strategy to modify the electrical and optical properties of semiconductor oxides. Herein, rare earths (REs: Ce, Nd, Eu, Tb, and Yb) elements are systemically doped into the NiOx hole transport layer (HTL) via a simple solution-based method. The results demonstrate that the REs doping could considerably modify the compactness, conductivity, and band alignment of the NiOx HTL, leading to the highly improved permanence of the inverted PSCs. The PSCs using 3% Eu:NiOx HTL yielded the optimum power conversion efficiency of 15.06%, relatively improved 23.4% compared with the PSC using pristine NiOx HTL (12.20%). It also demonstrated much better long time stability. The improved photovoltaic properties of the device can be attributed to the more efficient charge extraction and suppressed interfacial recombination rate by the introduction of appropriate REs in the NiOx HTL. This work indicates that RE doping is a very effective and promising strategy to achieve adjustable hole extraction material for high and stable inverted PSCs.
1. Introduction During the past ten years, PSCs have been emerging as a powerful
photovoltaic device with record efficiency exceeding 24.2% [1]. The widely adopted absorber layer, organic-inorganic halide perovskite, enables the PSC possess strong broad-band absorption, high carrier
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Xu), [email protected] (H. Song). https://doi.org/10.1016/j.jpowsour.2019.227267 Received 12 July 2019; Received in revised form 20 September 2019; Accepted 3 October 2019 Available online 14 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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mobility, long carrier diffusion length as well as low-cost and simple solution procedure [2–8]. Among different device structures of PSC, inverted planar device structure is attracting more attentions thanks to its simple fabrication, low hysteresis, possibility for low temperature, flexible, and large areas prospect compared to the conventional TiO2 based devices [9–12]. Normally, the hole transport layers (HTLs) are considered to be a crucial layer to achieve high performance PSCs with inverted planar structure. NiOx, as the most adopted HTL material, plays an important role in stable and low-cost inverted PSCs due to its superior transmission, environmental stability, and simple processes [13–15]. Moreover, it has large bandgap (3.4–4.3 eV) and deep valence band (VB, 5.2 eV) [16] as well as favorable work function alignment with perov skite photoactive layers, which are proper features for high performance PSCs [17,18]. However, NiOx has intrinsically low conductivity, which caused the increase of recombination rate and the reduction of hole extraction efficiency in the PSCs [19]. Effectively improving the con ductivity of NiOx is still a challenge to further improve the photovoltaic properties of inverted device. Typically, the stoichiometric NiO is an insulator, while the charac teristic of p-type semiconductor appeared by experimental NiOx can be assigned to the presence of Ni vacancies [20,21]. However, the hole density in the pristine NiOx is strongly constrained due to the large ionization energy of the Ni vacancies [20]. Extrinsic doping is consid ered to be an effectively and flexible strategy to improve the conduc tivity by changing the density of the native vacancies or adjusting the surface adsorbates [15]. Meanwhile, through doping or co-doping, the mismatching problem of Fermi levels between NiOx and perovskite light absorber layer can be well mitigated, and even the transmission and stability of the NiOx HTLs can be improved [14,16,22,23]. Among the various dopants, the rare earth (RE) elements are widely employed to improve the optical and electrical properties of various semiconductor oxides [24,25]. Their partially filled 4f orbitals and vacant 5d orbitals enable the lanthanide elements have abundant energy level structures. More importantly, their physical properties, such as electronic structure and ionic radius, regularly change with increasing of the atomic number of the trivalent RE ions [26,27]. These unique properties make RE doping become an important method for investigating the change law regarding to structure and electrical characteristics of the semi conductor oxides. In the field of PSCs, the RE doping has made many noticeable pro gresses. For example, Hu et al. investigated the influence of Y doping on the performance of PSC using NiOx HTL, and the results showed that Y3þ doped NiOx could enhance the hole mobility and improve the charge extraction from perovskite absorber compared to that of pristine NiOx [28]. Teo et al. showed that the defects of NiOx HTL could be effectively passivated through La3þ doping, and thereby achieved 21% improve ment in efficiency and an excellent stability [29]. In addition, our pre vious work on traditional PSC using TiO2 electron transport layer (ETL) demonstrated that the RE dopants, from Y3þ to Lu3þ, could adjust the electronic conductivity and matching energy level, finally the power conversion efficiency (PCE) was increased from 19.0% to 20.53% [30]. Up to now, as far as we know, the study focused on the systematic RE doping into the NiOx HTL and its influence on the photovoltaic perfor mance of the inverted PSC is still rare. In this work, a series of REs (Ce, Nd, Eu, Tb, and Yb) doped NiOx HTLs were prepared by a simple solution process for efficient inverted PSCs building. We systematically investigated the changing of structure, electrical, and photoelectric characteristics of the NiOx HTL regarding to the REs doping types and concentrations. PSC using 3% Eu: NiOx HTL exhibited a best PCE of 15.06% which was at most 23.4% enhanced compared to other RE ions doped and pristine NiOx devices. The enhanced photovoltaic performance of the suitable REs modified de vices could be attributed to the improved electronic conductivity, higher perovskite quality, more efficient hole extraction, and lower interfacial recombination rate. The REs doped NiOx film is expected to be one of the potential candidates for developing flexible and efficient inverted PSCs.
2. Experimental section 2.1. Materials Lead (II) iodide (PbI2, 99.9985%, metals basis) was purchased from Alfa Aesar. Methyfammonium iodide (MAI) (�99.5%) was purchased from Xi’an Polymer Light Technology Crop [6,6]-Phenyl-C61-butyric Acid Methyl Ester (PCBM [60], 99.5%) was obtained from Solenne.b.v. Crop, nickel acetate tetrahydrate (AR, 99.0%) was purchased from Macklin. All the used rare earth (RE) salts were nitrate hexahydrate, which were purchased from Ruike Rare Earth Centre. Solvents including N, N-Dimethylformamide (DMF, 99%, biotechnology level), dimethyl sulfoxide (DMSO, anhydrous grade), ethanolamine (EA, anhydrous grade, 99%), and anhydrous ethanol were purchased from Aladdin. All the chemicals were used as received without purification. 2.2. Preparation of NiOx and RE:NiOx precursor solutions The precursor solution of NiOx HTL was prepared by using a solution process method with some modification [23]. Typically, 0.1 mmol of nickel acetate tetrahydrate was dissolved in 1 mL of anhydrous ethanol containing EA. The molar ratio of Ni2þ/EA in the precursor solution was kept as 1:1. The mixture was then stirred overnight under 70 � C until the light green solution obtained. For the preparation of precursor solutions of RE:NiOx HTLs, the Europium (Eu), Ytterbium (Yb), Terbium (Tb), Cerium (Ce), and Neodymium (Nd) nitrate hexahydrates with different molar ratios (1%, 3%, 5%) were mixed into the above mentioned solu tion with nickel acetate tetrahydrate and EA. The other parts were the same as the preparing process of the precursor solution of NiOx HTL. 2.3. Devices fabrication First, the fluorine-doped tin oxide (FTO) substrates were washed ultrasonically by deionized water, cleaning agent, acetone and ethanol for 10 min, respectively. The HTLs were obtained by spin-coating the prepared NiOx or RE:NiOx precursor solutions at 1500 rpm for 40 s on the cleaned FTO at ambient temperature, followed by an annealing procedure at 300 � C for 1 h. The MAPbI3 perovskite precursor was pre pared via one-step method: 507 mg of PbI2 and 175 mg of MAI with concentration of 1.1 M were dissolved into 1 mL of mixed solution of DMF and DMSO (7:3, v/v). The MAPbI3 perovskite precursor was coated onto the HTLs with a speed of 600 rpm for 10 s and followed by 4000 rpm for 40 s. After 20 s of the second process starting, 400 μL of chlorobenzene anti-solvent was added. Then the obtained substrates were annealed at 60 � C for 5 min and 100 � C for 10 min to form smooth and compact perovskite light absorber layers. Then, 20 mg mL 1 of PCBM [60] solution (in chlorobenzene) was further deposited on the MAPbI3 layer with a speed of 1000 rpm for 30 s and annealed at 100 � C for 30 min. At last, 100 nm of Ag layer was deposited via vacuum evaporation. The active areas of the PSCs were determined to be 0.1 cm2. 3. Results and discussion The adopted device configuration of the inverted planar PSC in this work was depicted in Fig. 1a. It has the structure of glass/FTO/RE:NiOx/ MAPbI3/PCBM/Ag, where MAPbI3 performs as the light absorbed layer, RE:NiOx and PCBM are selected as the HTL and ETL, respectively. Fig. 1b shows the cross-sectional SEM image of the corresponding inverted PSC device. The thicknesses of each layer are determined to be about 30, 300, 80, and 100 nm, respectively corresponding to RE:NiOx HTL, MAPbI3 active layer, PCBM ETL, and Ag electrode, respectively. The thin and continuous RE:NiOx HTL not only ensures the effective block of electron but also reduces the incident light loss. Taking Eu:NiOx HTL for example, the corresponding band energy diagram of each layer are shown in Fig. 1c. The band energy values of FTO, MAPbI3, PCBM, Ag 2
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Journal of Power Sources 444 (2019) 227267
Fig. 1. (a) The structural schematic diagram of the inverted planar PSC. (b) The cross-sectional SEM image and (c) the band energy of the PSC using Eu:NiOx HTL. (d) The XRD patterns, (e) the current-voltage curves, and (f) the carrier concentration and mobility of NiOx and REs:NiOx films. (g) The transmittance spectra of Eu:NiOx and NiOx films deposited on the glass substrates. (h) The UPS spectra of Eu:NiOx and NiOx films deposited on silicon substrates, and (i) the high-resolution XPS spectroscopy of Ni 2p and Eu 3d of Eu:NiOx and NiOx samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
were cited from literatures [31] and the work functions of Eu:NiOx HTL were measured by ultraviolet photoelectron spectroscopy (UPS), which would be discussed later. After Eu doping, the energy level of Eu:NiOx HTL can match better with that of the perovskite absorbed layer in contract with the pristine NiOx HTL, indicating the accelerated hole extraction capacity and reduced carrier recombination between ab sorption layer and HTL [16,29,32]. In order to investigate various 3% REs doping (Ce3þ, Nd3þ, Eu3þ, Tb3þ, and Yb3þ) on the structural and electrical properties of the NiOx film, the X-ray diffraction spectra (XRD), the current-voltage conduc tivity, and the Hall coefficient test are performed. First, the XRD of the REs:NiOx as well as the pristine NiOx samples are carried out to study the influence of REs doping on the crystal structure of NiOx. As shown in Fig. 1d, the peaks of NiOx located at 37.2� , 43.3� , 62.9� correspond to the (111), (200), and (220) faces of NiO (JPCDS No.:47-1049), respec tively. After REs doping, the corresponding XRD peaks doesn’t show obvious shift and no peaks regarding RE2O3 can be observed, while the XRD peaks are obviously broadened indicating decreased crystal size with REs doping. According to the Debye-Scherrer formula, the average crystal size of REs:NiOx and NiOx samples are calculated and compared in Table S1. [33,34]. All the crystal sizes of the REs:NiOx samples are effectively decreased after introducing the REs ions. The reason of the sizes reduction after REs doping may attribute to the presence of REs-O-Ni, which can slow or inhibit the growth of crystal grains [35–37]. The smaller crystal size is beneficial to form a compactness and uniform film. This result is consistent with many previous reports of REs doping metal oxides [38–40]. However, the lattice substitution can
hardly happen due to the large difference of ions radius between REs (99-107 pm, Table S1) and Ni2þ (74 pm). The doping of RE in semi conductor oxide is usually considered to occupy the interstitial site or disperse on the surface, and similar results have been observed in many previous studies [29,41]. Since extrinsic dopants can normally increase the conductivity of the semiconductor oxide substrates [28,42] the conductivity test for the NiOx and REs:NiOx films are conducted (Fig. 1e), with the structure of FTO/NiOx or REs:NiOx/Ag as shown in the inset of Fig. 1e. The con ductivity (σ) can be calculated by the formula: σ ¼ d/(AR), where d is the thickness of NiOx or REs:NiOx layer, A is the active area of testing cell (0.1 cm2) and R is the resistance obtained from the J-V curves. As compared in Table S1, it is obvious that the doped RE ions can effec tively improve electronic conductivity of the pristine NiOx film, espe cially for that of the 3% Eu:NiOx layers. To get further information about the conductivity modulation by Eu3þ doping, the conductivity of NiOx layers with different doping concentration are studied in Fig. 1e and Table S1. The conductivity increases with the doping concentration when the concentration is relatively low (in the case of 1% and 3% Eu: NiOx), which may beneficial to improve the short circuit current density (JSC) values in the corresponding devices. However, when further increasing the doping concentration to 5%, more surface defects may be produced [28] leading to the deterioration of the conductivity. More over, the carrier concentration and mobility of REs:NiOx films are ob tained from Hall test, as shown in Fig. 1f. Compared to the pristine NiOx film, the carrier concentration and mobility of all the selected 3% REs: NiOx films are all enhanced, especially for the carrier mobility capacity. 3
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Journal of Power Sources 444 (2019) 227267
Except the 3% Nd:NiOx film, the 3% Eu:NiOx film shows the largest improvement both in the carrier concentration and mobility, which is 5.1 and 2.5 times higher than that of the pristine NiOx film, respectively. Noted that for 3% Nd:NiOx film, it has the largest carrier concentration, however, the carrier mobility capacity is much lower than that of the 3% Eu:NiOx film. In addition, the corresponding carrier concentration and mobility of the Eu:NiOx films with different doping concentration are also studied in Fig. 1f, which further proves that the superiority of 3% Eu:NiOx film in improving the electrical properties of the NiOx film. On the basis of the above results, the detailed studies are focused on the 3% Eu:NiOx layer to further investigate the improvement of the photovoltaic properties introduced by RE doping. The transmission spectra of Eu:NiOx and NiOx layers deposited on glass substrates are measured by UV–Vis spectroscopy, as displayed in Fig. 1g. As seen, Eu: NiOx layer shows similar transmission behavior compared with that of pristine NiOx layer, with over 90% transmittance from 300 to 800 nm, demonstrating a very low light loss. The UPS spectra are performed to analyze the work functions of NiOx and Eu:NiOx layers. As shown in Fig. 1h, the work function can be determined using the formula of Φ ¼ hv ðE0 EF Þ where ℎ is the Planck constant,vis frequency of mono chromatic H I light source, and EF is banding energy of the fermi edge, E0 is banding energy of the secondary cutoff edge. As calculated, the work functions of NiOx and Eu:NiOx are estimated to be 4.89 and 5.01 eV, respectively. Compared to the HOMO value of MAPbI3 (5.4 eV), the Eu: NiOx is more alignment with the adopted light absorbed layer, which can decrease the barrier level and accelerate the extraction of holes from perovskite active layer. Normally, the conductivity of non-stoichiometric NiOx is mainly decided by the Ni vacancy concentration which closely connected to the content of Ni3þ state [20]. Herein, the high-resolution X-ray photo electron spectroscopy (XPS) of Ni 2p for the two layers are investigated to obtain further information about the enhanced conductivity after Eu3þ doping. From the XPS spectra (Fig. 1i), it is obvious that both NiOx and Eu:NiOx are non-stoichiometric. Two main peaks located around 853.6 and 855.3 eV are in good agreement with the typical XPS peaks of NiOx. The peak at 853.6 eV is from Ni2þ associated with the Ni–O octahedral bonding of cubic NiO, and the peak at 855.3 eV results from metal deficiency Ni3þ or NiOOH. [43–46]. In addition, the ratio of Ni3þ/Ni2þ in NiOx and Eu:NiOx samples calculated from the corre sponding peak area are determined to be 3.47 and 4.12, respectively (summarized in Table S2). The higher ratio of Ni3þ in the Eu:NiOx
sample may lead to an increase of hole concentration [21] and thus increase the corresponding conductivity. The XPS spectrum of Eu 3d of Eu:NiOx sample is displayed in the right part of Fig. 2d. Two obvious peaks appear at 1134.9 and 1164.8 eV, which can be assigned to the XPS peaks of Eu 3d3/2 and Eu 3d5/2, respectively, suggesting tervalence Eu ions were successfully incorporated into the sample [24,39]. As known, the surface morphology of HTL plays an important role in forming high quality perovskite active layer in the inverted planar PSCs. The scanning electron microscope (SEM) images of NiOx and Eu:NiOx layers are studied in Fig. 2a and d, respectively. High-quality NiOx and Eu:NiOx HTLs with uniform, compact, and pin-hole free surface are obtained, which can be used to effectively transport holes and block electrons. The surface roughness of NiOx and Eu:NiOx HTLs are further estimated using atomic force microscope (AFM). As displayed in Fig. 2b and e, the root mean square (RMS) reduced from 17.6 nm of pristine NiOx HTL to 14.9 nm of Eu:NiOx HTL. It means that the RE doping is beneficial to form the HTL with higher quality, which is consistent with the previous XRD result. Meanwhile, energy dispersive spectrometer (EDS) is used to analysis the surface elements distribution in the Eu:NiOx HTL. As shown in Fig. S1, O, Ni, and Eu elements are uniformly distributed, which further proves the successful doping of Eu3þ in the Eu:NiOx sample. In addition, the morphology of the MAPbI3 absorber layers on the top of different HTLs is investigated. Fig. 2c and f shows the top-view of the MAPbI3 layers deposited by one-step crystallization procedure on the top of NiOx and Eu:NiOx HTLs, respectively. It can be found that the MAPbI3 active layer deposited on the Eu:NiOx HTL pos sesses larger grain size (178.3 nm) than that of NiOx (137.4 nm), which can be attributed to the improved conductivity of the lower HTL [14,29, 47,48]. The larger grain size means the relatively lower grain bound aries, suggesting a lower trap-state density. The compact HTL and the improved conductivity after Eu doping can effectively promote the performance of inverted PSCs. Based on the above discussion, the photovoltaic properties of the inverted PSCs are measured and compared in order to investigate various REs doping (3%) on the NiOx HTL. Fig. 3a shows corresponding current density-voltage (J-V) curves, which were measured by standard solar simulator under 1 sun (AM 1.5G, 100 mW cm 2). The summary of the photovoltaic parameters, such as PCE, JSC, open circuit voltage (VOC), and fill factor (FF), that are obtained by measuring 20 devices are shown in Fig. 3b and Table 1. As compared, the PCE of all the REs doping devices exhibit improved PCEs, which are enhanced by 5.4–23.4%
Fig. 2. (a) and (d) The top-view SEM images of NiOx and Eu:NiOx layers on the FTO substrates, respectively. (b) and (e) The AFM images of NiOx and Eu:NiOx layers on the FTO substrates, respectively. The top-view SEM image of MAPbI3 deposited on (c) NiOx HTL and (f) Eu:NiOx HTL, respectively. The inset pictures are the corresponding particle size distribution. 4
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Journal of Power Sources 444 (2019) 227267
Fig. 3. (a) J-V curves of PSCs using 3% RE:NiOx HTLs, which are measured under standard 1 sun (AM 1.5, 100 mW cm 2). (b) Dependence of PCE, FF, VOC, JSC of NiOx based devices on the doping ions types. (c) J-V curves of different doping molar rates (0, 1%, 3%, and 5%) of the PSC using Eu:NiOx HTL. The test conditions are the same.
compared to that using the pristine NiOx HTL. The increased PCE values mainly originate from the increased JSC values due to the smaller Rs values (Table 1) and the improved conductivity (Fig. 1e and f). Better reproducibility of the same devices are also obtained, as proved in Fig. 3b. Note that Rs reflects the internal resistance including each layer and the interfaces of perovskite/carrier transport layer and it can be calculated by fitting formula for P–I–N model [49,50]. It is obvious that the inverted PSC using 3% Eu:NiOx HTL has the best PCE compared to
other REs doped devices, which has the lowest Rs value regarding to the effectively improved JSC values, thereby the higher PCE compared to the other PSCs. Since the inverted PSC using 3% Eu:NiOx HTL displays the best device performance among the studied devices, the photovoltaic properties are further studied by adjusting the Eu3þ doping concentra tion in Eu:NiOx HTLs. Fig. 3c shows the J-V curves of inverted PSCs using Eu:NiOx HTL with Eu3þ doping concentration of 1%, 3%, and 5%, respectively, As compared, inverted PSCs using 3% Eu:NiOx HTL ex hibits the best devices performance, which is consistent with the results in Fig. 1e and f. In addition, the hysteresis effect of inverted PSC using 3% Eu:NiOx HTL is investigated, which is compared with that of the PSC using pristine NiOx HTL. As displayed in Fig. S2, it is obvious that the PCE difference is 6.6% between reverse and forward scan of the PSC using 3% Eu:NiOx HTL, which is smaller than that of PSC using pristine NiOx HTL (16.6%). This can thank to the increased conductivity of NiOx HTL after 3% Eu doping, which make a better balance between holes and electrons transportation inside the inverted PSC. As compared above, the inverted PSC using 3% Eu:NiOx HTL exhibits the highest PCE of 15.06% in our case with a JSC of 21.96 mA cm 2, VOC of 1.03 V and FF of 66.6%. This PCE value is improved by 23.4% compared to that of the device using pristine NiOx HTL (12.20%). To investigate the RE doping on the photovoltaic characteristics of inverted PSC, further studies focused on the device performance of PSCs using 3% Eu:NiOx as well as the pristine NiOx HTLs are performed. Photo luminescence (PL) and time-resolved PL spectra are provided to test the capability of hole extraction ability from perovskite active layer to the NiOx HTLs. The testing structure is FTO/Eu:NiOx(or NiOx)/MAPbI3. As shown in Fig. 4a, an obvious decrease of PL intensity in the Eu:NiOx based structure can be observed, suggesting a more efficient hole extraction [51]. Fig. 4b displays the time-resolved PL spectra of the two testing structures. The faster decay of Eu:NiOx indicates that Eu doping could improve the hole extraction [52] and this result was in accordance with the PL and XPS analyzation. The incident photon to current conversion efficiency (IPCE) test is carried out to characterize the actually photo-to-electron conversion efficiency of the PSCs using NiOx and Eu:NiOx HTLs, respectively. As shown in Fig. 4c, the IPCE value of PSCs using Eu:NiOx HTL is higher than that of pristine NiOx in the whole studied range from 350 to
Table 1 Photovoltaic parameters of different inverted PSCs using RE:NiOx HTL. Doping types
JSC (mA. cm 2)
VOC (V)
FF (%)
PCE (%)
Rs (Ω. cm2)
Pristine
18.13b) 18.49 � 0.75a)
1.00 0.964 � 0.026
66.6 63.9 � 2.9
12.20 11.38 � 0.51
7.68
3% Eu
21.96 21.74 � 0.20
1.03 1.025 � 0.005
66.6 64.6 � 1.3
15.06 14.40 � 0.40
1.80
3% Yb
21.58 20.78 � 0.55
0.99 0.979 � 0.006
64.1 64.3 � 0.9
13.70 13.09 � 0.49
2.55
3% Tb
20.08 20.17 � 0.15
0.98 0.977 � 0.008
69.0 66.3 � 1.2
13.58 13.08 � 0.21
2.81
3% Ce
21.60 21.49 � 0.14
1.03 1.020 � 0.005
58.6 57.9 � 0.9
12.97 12.69 � 0.19
5.37
3% Nd
19.37 19.17 � 0.15
0.94 0.946 � 0.005
70.6 68.9 � 1.4
12.86 12.50 � 0.29
4.57
1% Eu
21.23 20.96 � 0.31
1.03 1.022 � 0.008
64.7 63.7 � 1.2
14.15 13.76 � 0.32
3.24
5% Eu
19.92 19.65 � 0.27
1.03 1.020 � 0.010
65.7 64.7 � 1.7
13.49 13.10 � 0.31
2.51
a)
The averaged values were calculated from 20 devices. The photovoltaic parameter of the best device around all the measured devices. b)
5
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Journal of Power Sources 444 (2019) 227267
Fig. 4. (a) PL spectra (the excitation wavelength is 520 nm), (b) time-resolved PL spectra of FTO/NiOx/MAPbI3 (blue) and FTO/Eu:NiOx/MAPbI3 (red). (c) The IPCE spectra and integrated JSC curves of the NiOx and Eu:NiOx devices. (d) EIS curves fitting with RCR model of NiOx and Eu:NiOx devices. The fitting model of EIS spectra is inserted in Fig. 4d. (e) The VOC as a function of incident light power in logarithmic coordinate. (f) The dark current curves versus bias voltage in a voltage region of 0.2-1.5 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
750 nm. The enhanced IPCE can be assigned to the more efficient car riers transport resulting from the reduced energy barrier between HTL and perovskite absorb layer as well as the higher conductivity of the Eu: NiOx HTL. [16,48,53]. In addition, the integrated JSC values from the corresponding IPCE spectra of the PSCs using NiOx and Eu:NiOx HTLs are determined to be 17.29 and 20.53 mA cm 2, respectively. These values are well consistent with the JSC values obtained from the corre sponding J-V curves (less than 10% error). Electrochemical impedance spectra (EIS) technology is used to analyze the charge transfer resistance and interfacial properties in the electrical view. In Fig. 4d, the EIS spectra of the two devices can be fitted by the RCR model (as shown in the inset), where Rrec is the charge transfer resistance, and Crec is the parallel capacitor. In our case, an increase of Rrec from 13.4 kΩ (NiOx) to 14.26 kΩ (Eu:NiOx) indicates that more efficient carrier separation, transport, and injection process at the light absorbed layer and less carrier recombination at the interface of perovskite/HTL in the Eu:NiOx HTL devices, which can lead to the improvement of FF and JSC of the final device [30,54,55]. This result is consistent with the PL and time-resolved PL characterizations. In order to get more details about the carrier recombination process of the studied devices, the incident light power dependent VOC curves are studied in Fig. 4e. The carrier recombination can be evaluated ac cording to the following formula: � � mKB T JSC ln V¼ þ constant; (1) e J0
the two devices shown in Fig. 4f also verify the reduced trap contents. The leakage current of Eu:NiOx based device is obviously smaller compared to that of pristine NiOx in the work bias region, which may provide an explanation for the higher JSC of corresponding PSCs [59]. The stability of PSCs is an important evaluation indicator for commercialization. Fig. 5a is the steady-state current density test for the best NiOx and Eu:NiOx devices which are encapsulated by the glass under the maximum power point bias voltage (Vmpp) for 40 min. As exhibited, the PSCs all show steady and smooth JSC curves, and the calculated steady-state average PCE are 12.13% and 14.95% for NiOx and Eu:NiOx based devices, respectively. In addition, the normalized long-term stability of the two devices (unencapsulated) are also carried out, as shown in Fig. 5b. The devices are stored in an ambient condition (25-55% relative humidity and 25-35 � C) when they were not using. The PSC using Eu:NiOx HTL can maintain 97% of the initial PEC value after 10 days, however, an obvious decrease is found in the case of pristine NiOx (87%). This can be attributed that the perovskite absorber layer with higher quality is obtained after depositing on the more compact and conductive Eu:NiOx HTL (as proved in Fig. 2d and f). Note that the previous study has revealed that the glass sealed inverted PSC with NiOx as HTL exhibited the long-term stability of 4000 h [60]. In our case, the used devices are not encapsulated when measured the long-term sta bility, thus a better long-term stability can be expected in the sealed conditions, especially for the PSC with 3% Eu:NiOx as HTL. 4. Conclusion
where KB is the Boltzmann constant, T is the absolute temperature, e is the elementary charge, and m is the ideality factor. In Fig. 4e, all the measured plots of Eu:NiOx based device exhibit a higher VOC level compared to that of NiOx, which is consistent with the previous J-V results. Besides, the corresponding plots can be linearly fitted in the whole studied range, and the slopes are calculated to be 1.69 KBT/e and 1.24 KBT/e for NiOx and Eu:NiOx based devices, respectively. Usually, closer of m value to 1, better ideal diode can be achieved. The decreased m value in Eu:NiOx based device demonstrates that trap-associated carrier recombination is effectively reduced by Eu doping [56–58]. Furthermore, the dark-current curves verse bias voltage ( 0.2-1.5 V) of
In summary, we investigated the impact of systematical doping of REs on the structural and optoelectronic characteristics of NiOx HTL in the inverted PSCs and their photovoltaic performances. The results revealed that the conductivity of the NiOx HTL and Rs of the devices could be modified in a large extent through adjusting the RE doping types and concentrations. In the optimal doping condition in our case, 3% Eu:NiOx HTL exhibited more smooth, compact, and conductive film, leading to larger perovskite grain size, and more important, promoted hole extraction ability and suppressed charge recombination rate. Finally, the PSC using Eu:NiOx HTL achieved an best PCE of 15.06% with significantly improved JSC, this PCE showed a 23.4% improvement 6
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Fig. 5. (a) Steady-state JSC of PSCs using NiOx and Eu:NiOx HTLs under the maximum power points for 40 min. The inset is the picture of actual device encapsulation with glass. (b) Normalized PCE of NiOx and Eu:NiOx devices under ambient condition (25-55% relative humidity and 25-35 � C) for 10 days.
compared to that of the pristine NiOx HTL. The optimal device also exhibited good stability when stored under the ambient condition. Our research illustrates that RE: NiOx can be an excellent and controllable hole extraction material for realizing inverted planar PSCs with highperformance.
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Acknowledgments This work was supported by the Key Program of NSFC-Guangdong Joint Funds of China (U1801253), the National Natural Science Foun dation of China (Grant Nos. 61874049, 11874181, 61822506, 61775080), the National Key Research and Development Program (2016YFC0207101), the Special Project of the Province-University Coconstructing Program of Jilin Province (SXGJXX2017-3), the Jilin Province Natural Science Foundation of China (No. 20180101210JC and 20170101170JC) and 13th Five-year Plan on Science and Technology Project of the Education Department of Jilin Province (No. JJKH20190115KJ). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227267. References [1] Q. Jiang, Y. Zhao, X. Zhang, X. Yang, Y. Chen, Z. Chu, Q. Ye, X. Li, Z. Yin, J. You, Nat. Photonics 13 (2019) 460–466. [2] T. Leijtens, S.D. Stranks, G.E. Eperon, R. Lindblad, E.M.J. Johansson, I. J. McPherson, H. Rensmo, J.M. Ball, M.M. Lee, H.J. Snaith, ACS Nano 8 (2014) 7147–7155. [3] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Gr€ atzel, S. Mhaisalkar, T. C. Sum, Science 342 (2013) 344. [4] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J.P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H.J. Snaith, Science 342 (2013) 341–344. [5] M. Liu, M.B. Johnston, H.J. Snaith, Nature 501 (2013) 395–398. [6] J. Burschka, N. Pellet, S.J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Gratzel, Nature 499 (2013) 316–319. [7] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Science 338 (2012) 643. [8] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050–6051. [9] H. Azimi, T. Ameri, H. Zhang, Y. Hou, C.O.R. Quiroz, J. Min, M. Hu, Z.-G. Zhang, T. Przybilla, G.J. Matt, E. Spiecker, Y. Li, C.J. Brabec, Adv. Energy.Mater. 5 (2015), 1401692. [10] G. Kakavelakis, T. Maksudov, D. Konios, I. Paradisanos, G. Kioseoglou, E. Stratakis, E. Kymakis, Adv. Energy.Mater. 7 (2017), 1602120. [11] A.E. Labban, H. Chen, M. Kirkus, J. Barbe, S. Del Gobbo, M. Neophytou, I. McCulloch, J. Eid, Adv. Energy.Mater. 6 (2016) 1502101. [12] Q. Xue, Y. Bai, M. Liu, R. Xia, Z. Hu, Z. Chen, X.-F. Jiang, F. Huang, S. Yang, Y. Matsuo, H.-L. Yip, Y. Cao, Adv. Energy.Mater. 7 (2017), 1602333. [13] Q. He, K. Yao, X. Wang, X. Xia, S. Leng, F. Li, ACS Appl. Mater. Interfaces 9 (2017) 41887–41897. [14] J.W. Jung, C.C. Chueh, A.K. Jen, Adv. Mater. 27 (2015) 7874–7880. [15] W. Chen, Y. Wu, J. Fan, A.B. Djuri�si�c, F. Liu, H.W. Tam, A. Ng, C. Surya, W.K. Chan, D. Wang, Z.-B. He, Adv. Energy.Mater. 8 (2018), 1703519.
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Rare earth ions doped NiOx hole transport layer for efficient and stable inverted perovskite solar cells Xinfu Chen, Lin Xu **, Cong Chen, Yanjie Wu, Wenbo Bi, Zonglong Song, Xinmeng Zhuang, Shuo Yang, Shidong Zhu, Hongwei Song * State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� REs (Ce, Nd, Eu, Tb, Yb) ions were doped into NiOx HTLs via solution method. � PSC using 3% Eu:NiOx exhibited the best PCE of 15.6%, with 23.4% improvement. � The improvement was attributed to the improved conductivity and perovskite layer.
A R T I C L E I N F O
A B S T R A C T
Keywords: Rare earth ions Doped NiOx film Perovskite solar cells Inverted planar structure
Hole transport layer plays a critical role in achieving high performance and stable inverted perovskite solar cells (PSCs). Doping has been proved to be an effective strategy to modify the electrical and optical properties of semiconductor oxides. Herein, rare earths (REs: Ce, Nd, Eu, Tb, and Yb) elements are systemically doped into the NiOx hole transport layer (HTL) via a simple solution-based method. The results demonstrate that the REs doping could considerably modify the compactness, conductivity, and band alignment of the NiOx HTL, leading to the highly improved permanence of the inverted PSCs. The PSCs using 3% Eu:NiOx HTL yielded the optimum power conversion efficiency of 15.06%, relatively improved 23.4% compared with the PSC using pristine NiOx HTL (12.20%). It also demonstrated much better long time stability. The improved photovoltaic properties of the device can be attributed to the more efficient charge extraction and suppressed interfacial recombination rate by the introduction of appropriate REs in the NiOx HTL. This work indicates that RE doping is a very effective and promising strategy to achieve adjustable hole extraction material for high and stable inverted PSCs.
1. Introduction During the past ten years, PSCs have been emerging as a powerful
photovoltaic device with record efficiency exceeding 24.2% [1]. The widely adopted absorber layer, organic-inorganic halide perovskite, enables the PSC possess strong broad-band absorption, high carrier
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Xu), [email protected] (H. Song). https://doi.org/10.1016/j.jpowsour.2019.227267 Received 12 July 2019; Received in revised form 20 September 2019; Accepted 3 October 2019 Available online 14 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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mobility, long carrier diffusion length as well as low-cost and simple solution procedure [2–8]. Among different device structures of PSC, inverted planar device structure is attracting more attentions thanks to its simple fabrication, low hysteresis, possibility for low temperature, flexible, and large areas prospect compared to the conventional TiO2 based devices [9–12]. Normally, the hole transport layers (HTLs) are considered to be a crucial layer to achieve high performance PSCs with inverted planar structure. NiOx, as the most adopted HTL material, plays an important role in stable and low-cost inverted PSCs due to its superior transmission, environmental stability, and simple processes [13–15]. Moreover, it has large bandgap (3.4–4.3 eV) and deep valence band (VB, 5.2 eV) [16] as well as favorable work function alignment with perov skite photoactive layers, which are proper features for high performance PSCs [17,18]. However, NiOx has intrinsically low conductivity, which caused the increase of recombination rate and the reduction of hole extraction efficiency in the PSCs [19]. Effectively improving the con ductivity of NiOx is still a challenge to further improve the photovoltaic properties of inverted device. Typically, the stoichiometric NiO is an insulator, while the charac teristic of p-type semiconductor appeared by experimental NiOx can be assigned to the presence of Ni vacancies [20,21]. However, the hole density in the pristine NiOx is strongly constrained due to the large ionization energy of the Ni vacancies [20]. Extrinsic doping is consid ered to be an effectively and flexible strategy to improve the conduc tivity by changing the density of the native vacancies or adjusting the surface adsorbates [15]. Meanwhile, through doping or co-doping, the mismatching problem of Fermi levels between NiOx and perovskite light absorber layer can be well mitigated, and even the transmission and stability of the NiOx HTLs can be improved [14,16,22,23]. Among the various dopants, the rare earth (RE) elements are widely employed to improve the optical and electrical properties of various semiconductor oxides [24,25]. Their partially filled 4f orbitals and vacant 5d orbitals enable the lanthanide elements have abundant energy level structures. More importantly, their physical properties, such as electronic structure and ionic radius, regularly change with increasing of the atomic number of the trivalent RE ions [26,27]. These unique properties make RE doping become an important method for investigating the change law regarding to structure and electrical characteristics of the semi conductor oxides. In the field of PSCs, the RE doping has made many noticeable pro gresses. For example, Hu et al. investigated the influence of Y doping on the performance of PSC using NiOx HTL, and the results showed that Y3þ doped NiOx could enhance the hole mobility and improve the charge extraction from perovskite absorber compared to that of pristine NiOx [28]. Teo et al. showed that the defects of NiOx HTL could be effectively passivated through La3þ doping, and thereby achieved 21% improve ment in efficiency and an excellent stability [29]. In addition, our pre vious work on traditional PSC using TiO2 electron transport layer (ETL) demonstrated that the RE dopants, from Y3þ to Lu3þ, could adjust the electronic conductivity and matching energy level, finally the power conversion efficiency (PCE) was increased from 19.0% to 20.53% [30]. Up to now, as far as we know, the study focused on the systematic RE doping into the NiOx HTL and its influence on the photovoltaic perfor mance of the inverted PSC is still rare. In this work, a series of REs (Ce, Nd, Eu, Tb, and Yb) doped NiOx HTLs were prepared by a simple solution process for efficient inverted PSCs building. We systematically investigated the changing of structure, electrical, and photoelectric characteristics of the NiOx HTL regarding to the REs doping types and concentrations. PSC using 3% Eu: NiOx HTL exhibited a best PCE of 15.06% which was at most 23.4% enhanced compared to other RE ions doped and pristine NiOx devices. The enhanced photovoltaic performance of the suitable REs modified de vices could be attributed to the improved electronic conductivity, higher perovskite quality, more efficient hole extraction, and lower interfacial recombination rate. The REs doped NiOx film is expected to be one of the potential candidates for developing flexible and efficient inverted PSCs.
2. Experimental section 2.1. Materials Lead (II) iodide (PbI2, 99.9985%, metals basis) was purchased from Alfa Aesar. Methyfammonium iodide (MAI) (�99.5%) was purchased from Xi’an Polymer Light Technology Crop [6,6]-Phenyl-C61-butyric Acid Methyl Ester (PCBM [60], 99.5%) was obtained from Solenne.b.v. Crop, nickel acetate tetrahydrate (AR, 99.0%) was purchased from Macklin. All the used rare earth (RE) salts were nitrate hexahydrate, which were purchased from Ruike Rare Earth Centre. Solvents including N, N-Dimethylformamide (DMF, 99%, biotechnology level), dimethyl sulfoxide (DMSO, anhydrous grade), ethanolamine (EA, anhydrous grade, 99%), and anhydrous ethanol were purchased from Aladdin. All the chemicals were used as received without purification. 2.2. Preparation of NiOx and RE:NiOx precursor solutions The precursor solution of NiOx HTL was prepared by using a solution process method with some modification [23]. Typically, 0.1 mmol of nickel acetate tetrahydrate was dissolved in 1 mL of anhydrous ethanol containing EA. The molar ratio of Ni2þ/EA in the precursor solution was kept as 1:1. The mixture was then stirred overnight under 70 � C until the light green solution obtained. For the preparation of precursor solutions of RE:NiOx HTLs, the Europium (Eu), Ytterbium (Yb), Terbium (Tb), Cerium (Ce), and Neodymium (Nd) nitrate hexahydrates with different molar ratios (1%, 3%, 5%) were mixed into the above mentioned solu tion with nickel acetate tetrahydrate and EA. The other parts were the same as the preparing process of the precursor solution of NiOx HTL. 2.3. Devices fabrication First, the fluorine-doped tin oxide (FTO) substrates were washed ultrasonically by deionized water, cleaning agent, acetone and ethanol for 10 min, respectively. The HTLs were obtained by spin-coating the prepared NiOx or RE:NiOx precursor solutions at 1500 rpm for 40 s on the cleaned FTO at ambient temperature, followed by an annealing procedure at 300 � C for 1 h. The MAPbI3 perovskite precursor was pre pared via one-step method: 507 mg of PbI2 and 175 mg of MAI with concentration of 1.1 M were dissolved into 1 mL of mixed solution of DMF and DMSO (7:3, v/v). The MAPbI3 perovskite precursor was coated onto the HTLs with a speed of 600 rpm for 10 s and followed by 4000 rpm for 40 s. After 20 s of the second process starting, 400 μL of chlorobenzene anti-solvent was added. Then the obtained substrates were annealed at 60 � C for 5 min and 100 � C for 10 min to form smooth and compact perovskite light absorber layers. Then, 20 mg mL 1 of PCBM [60] solution (in chlorobenzene) was further deposited on the MAPbI3 layer with a speed of 1000 rpm for 30 s and annealed at 100 � C for 30 min. At last, 100 nm of Ag layer was deposited via vacuum evaporation. The active areas of the PSCs were determined to be 0.1 cm2. 3. Results and discussion The adopted device configuration of the inverted planar PSC in this work was depicted in Fig. 1a. It has the structure of glass/FTO/RE:NiOx/ MAPbI3/PCBM/Ag, where MAPbI3 performs as the light absorbed layer, RE:NiOx and PCBM are selected as the HTL and ETL, respectively. Fig. 1b shows the cross-sectional SEM image of the corresponding inverted PSC device. The thicknesses of each layer are determined to be about 30, 300, 80, and 100 nm, respectively corresponding to RE:NiOx HTL, MAPbI3 active layer, PCBM ETL, and Ag electrode, respectively. The thin and continuous RE:NiOx HTL not only ensures the effective block of electron but also reduces the incident light loss. Taking Eu:NiOx HTL for example, the corresponding band energy diagram of each layer are shown in Fig. 1c. The band energy values of FTO, MAPbI3, PCBM, Ag 2
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Fig. 1. (a) The structural schematic diagram of the inverted planar PSC. (b) The cross-sectional SEM image and (c) the band energy of the PSC using Eu:NiOx HTL. (d) The XRD patterns, (e) the current-voltage curves, and (f) the carrier concentration and mobility of NiOx and REs:NiOx films. (g) The transmittance spectra of Eu:NiOx and NiOx films deposited on the glass substrates. (h) The UPS spectra of Eu:NiOx and NiOx films deposited on silicon substrates, and (i) the high-resolution XPS spectroscopy of Ni 2p and Eu 3d of Eu:NiOx and NiOx samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
were cited from literatures [31] and the work functions of Eu:NiOx HTL were measured by ultraviolet photoelectron spectroscopy (UPS), which would be discussed later. After Eu doping, the energy level of Eu:NiOx HTL can match better with that of the perovskite absorbed layer in contract with the pristine NiOx HTL, indicating the accelerated hole extraction capacity and reduced carrier recombination between ab sorption layer and HTL [16,29,32]. In order to investigate various 3% REs doping (Ce3þ, Nd3þ, Eu3þ, Tb3þ, and Yb3þ) on the structural and electrical properties of the NiOx film, the X-ray diffraction spectra (XRD), the current-voltage conduc tivity, and the Hall coefficient test are performed. First, the XRD of the REs:NiOx as well as the pristine NiOx samples are carried out to study the influence of REs doping on the crystal structure of NiOx. As shown in Fig. 1d, the peaks of NiOx located at 37.2� , 43.3� , 62.9� correspond to the (111), (200), and (220) faces of NiO (JPCDS No.:47-1049), respec tively. After REs doping, the corresponding XRD peaks doesn’t show obvious shift and no peaks regarding RE2O3 can be observed, while the XRD peaks are obviously broadened indicating decreased crystal size with REs doping. According to the Debye-Scherrer formula, the average crystal size of REs:NiOx and NiOx samples are calculated and compared in Table S1. [33,34]. All the crystal sizes of the REs:NiOx samples are effectively decreased after introducing the REs ions. The reason of the sizes reduction after REs doping may attribute to the presence of REs-O-Ni, which can slow or inhibit the growth of crystal grains [35–37]. The smaller crystal size is beneficial to form a compactness and uniform film. This result is consistent with many previous reports of REs doping metal oxides [38–40]. However, the lattice substitution can
hardly happen due to the large difference of ions radius between REs (99-107 pm, Table S1) and Ni2þ (74 pm). The doping of RE in semi conductor oxide is usually considered to occupy the interstitial site or disperse on the surface, and similar results have been observed in many previous studies [29,41]. Since extrinsic dopants can normally increase the conductivity of the semiconductor oxide substrates [28,42] the conductivity test for the NiOx and REs:NiOx films are conducted (Fig. 1e), with the structure of FTO/NiOx or REs:NiOx/Ag as shown in the inset of Fig. 1e. The con ductivity (σ) can be calculated by the formula: σ ¼ d/(AR), where d is the thickness of NiOx or REs:NiOx layer, A is the active area of testing cell (0.1 cm2) and R is the resistance obtained from the J-V curves. As compared in Table S1, it is obvious that the doped RE ions can effec tively improve electronic conductivity of the pristine NiOx film, espe cially for that of the 3% Eu:NiOx layers. To get further information about the conductivity modulation by Eu3þ doping, the conductivity of NiOx layers with different doping concentration are studied in Fig. 1e and Table S1. The conductivity increases with the doping concentration when the concentration is relatively low (in the case of 1% and 3% Eu: NiOx), which may beneficial to improve the short circuit current density (JSC) values in the corresponding devices. However, when further increasing the doping concentration to 5%, more surface defects may be produced [28] leading to the deterioration of the conductivity. More over, the carrier concentration and mobility of REs:NiOx films are ob tained from Hall test, as shown in Fig. 1f. Compared to the pristine NiOx film, the carrier concentration and mobility of all the selected 3% REs: NiOx films are all enhanced, especially for the carrier mobility capacity. 3
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Except the 3% Nd:NiOx film, the 3% Eu:NiOx film shows the largest improvement both in the carrier concentration and mobility, which is 5.1 and 2.5 times higher than that of the pristine NiOx film, respectively. Noted that for 3% Nd:NiOx film, it has the largest carrier concentration, however, the carrier mobility capacity is much lower than that of the 3% Eu:NiOx film. In addition, the corresponding carrier concentration and mobility of the Eu:NiOx films with different doping concentration are also studied in Fig. 1f, which further proves that the superiority of 3% Eu:NiOx film in improving the electrical properties of the NiOx film. On the basis of the above results, the detailed studies are focused on the 3% Eu:NiOx layer to further investigate the improvement of the photovoltaic properties introduced by RE doping. The transmission spectra of Eu:NiOx and NiOx layers deposited on glass substrates are measured by UV–Vis spectroscopy, as displayed in Fig. 1g. As seen, Eu: NiOx layer shows similar transmission behavior compared with that of pristine NiOx layer, with over 90% transmittance from 300 to 800 nm, demonstrating a very low light loss. The UPS spectra are performed to analyze the work functions of NiOx and Eu:NiOx layers. As shown in Fig. 1h, the work function can be determined using the formula of Φ ¼ hv ðE0 EF Þ where ℎ is the Planck constant,vis frequency of mono chromatic H I light source, and EF is banding energy of the fermi edge, E0 is banding energy of the secondary cutoff edge. As calculated, the work functions of NiOx and Eu:NiOx are estimated to be 4.89 and 5.01 eV, respectively. Compared to the HOMO value of MAPbI3 (5.4 eV), the Eu: NiOx is more alignment with the adopted light absorbed layer, which can decrease the barrier level and accelerate the extraction of holes from perovskite active layer. Normally, the conductivity of non-stoichiometric NiOx is mainly decided by the Ni vacancy concentration which closely connected to the content of Ni3þ state [20]. Herein, the high-resolution X-ray photo electron spectroscopy (XPS) of Ni 2p for the two layers are investigated to obtain further information about the enhanced conductivity after Eu3þ doping. From the XPS spectra (Fig. 1i), it is obvious that both NiOx and Eu:NiOx are non-stoichiometric. Two main peaks located around 853.6 and 855.3 eV are in good agreement with the typical XPS peaks of NiOx. The peak at 853.6 eV is from Ni2þ associated with the Ni–O octahedral bonding of cubic NiO, and the peak at 855.3 eV results from metal deficiency Ni3þ or NiOOH. [43–46]. In addition, the ratio of Ni3þ/Ni2þ in NiOx and Eu:NiOx samples calculated from the corre sponding peak area are determined to be 3.47 and 4.12, respectively (summarized in Table S2). The higher ratio of Ni3þ in the Eu:NiOx
sample may lead to an increase of hole concentration [21] and thus increase the corresponding conductivity. The XPS spectrum of Eu 3d of Eu:NiOx sample is displayed in the right part of Fig. 2d. Two obvious peaks appear at 1134.9 and 1164.8 eV, which can be assigned to the XPS peaks of Eu 3d3/2 and Eu 3d5/2, respectively, suggesting tervalence Eu ions were successfully incorporated into the sample [24,39]. As known, the surface morphology of HTL plays an important role in forming high quality perovskite active layer in the inverted planar PSCs. The scanning electron microscope (SEM) images of NiOx and Eu:NiOx layers are studied in Fig. 2a and d, respectively. High-quality NiOx and Eu:NiOx HTLs with uniform, compact, and pin-hole free surface are obtained, which can be used to effectively transport holes and block electrons. The surface roughness of NiOx and Eu:NiOx HTLs are further estimated using atomic force microscope (AFM). As displayed in Fig. 2b and e, the root mean square (RMS) reduced from 17.6 nm of pristine NiOx HTL to 14.9 nm of Eu:NiOx HTL. It means that the RE doping is beneficial to form the HTL with higher quality, which is consistent with the previous XRD result. Meanwhile, energy dispersive spectrometer (EDS) is used to analysis the surface elements distribution in the Eu:NiOx HTL. As shown in Fig. S1, O, Ni, and Eu elements are uniformly distributed, which further proves the successful doping of Eu3þ in the Eu:NiOx sample. In addition, the morphology of the MAPbI3 absorber layers on the top of different HTLs is investigated. Fig. 2c and f shows the top-view of the MAPbI3 layers deposited by one-step crystallization procedure on the top of NiOx and Eu:NiOx HTLs, respectively. It can be found that the MAPbI3 active layer deposited on the Eu:NiOx HTL pos sesses larger grain size (178.3 nm) than that of NiOx (137.4 nm), which can be attributed to the improved conductivity of the lower HTL [14,29, 47,48]. The larger grain size means the relatively lower grain bound aries, suggesting a lower trap-state density. The compact HTL and the improved conductivity after Eu doping can effectively promote the performance of inverted PSCs. Based on the above discussion, the photovoltaic properties of the inverted PSCs are measured and compared in order to investigate various REs doping (3%) on the NiOx HTL. Fig. 3a shows corresponding current density-voltage (J-V) curves, which were measured by standard solar simulator under 1 sun (AM 1.5G, 100 mW cm 2). The summary of the photovoltaic parameters, such as PCE, JSC, open circuit voltage (VOC), and fill factor (FF), that are obtained by measuring 20 devices are shown in Fig. 3b and Table 1. As compared, the PCE of all the REs doping devices exhibit improved PCEs, which are enhanced by 5.4–23.4%
Fig. 2. (a) and (d) The top-view SEM images of NiOx and Eu:NiOx layers on the FTO substrates, respectively. (b) and (e) The AFM images of NiOx and Eu:NiOx layers on the FTO substrates, respectively. The top-view SEM image of MAPbI3 deposited on (c) NiOx HTL and (f) Eu:NiOx HTL, respectively. The inset pictures are the corresponding particle size distribution. 4
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Journal of Power Sources 444 (2019) 227267
Fig. 3. (a) J-V curves of PSCs using 3% RE:NiOx HTLs, which are measured under standard 1 sun (AM 1.5, 100 mW cm 2). (b) Dependence of PCE, FF, VOC, JSC of NiOx based devices on the doping ions types. (c) J-V curves of different doping molar rates (0, 1%, 3%, and 5%) of the PSC using Eu:NiOx HTL. The test conditions are the same.
compared to that using the pristine NiOx HTL. The increased PCE values mainly originate from the increased JSC values due to the smaller Rs values (Table 1) and the improved conductivity (Fig. 1e and f). Better reproducibility of the same devices are also obtained, as proved in Fig. 3b. Note that Rs reflects the internal resistance including each layer and the interfaces of perovskite/carrier transport layer and it can be calculated by fitting formula for P–I–N model [49,50]. It is obvious that the inverted PSC using 3% Eu:NiOx HTL has the best PCE compared to
other REs doped devices, which has the lowest Rs value regarding to the effectively improved JSC values, thereby the higher PCE compared to the other PSCs. Since the inverted PSC using 3% Eu:NiOx HTL displays the best device performance among the studied devices, the photovoltaic properties are further studied by adjusting the Eu3þ doping concentra tion in Eu:NiOx HTLs. Fig. 3c shows the J-V curves of inverted PSCs using Eu:NiOx HTL with Eu3þ doping concentration of 1%, 3%, and 5%, respectively, As compared, inverted PSCs using 3% Eu:NiOx HTL ex hibits the best devices performance, which is consistent with the results in Fig. 1e and f. In addition, the hysteresis effect of inverted PSC using 3% Eu:NiOx HTL is investigated, which is compared with that of the PSC using pristine NiOx HTL. As displayed in Fig. S2, it is obvious that the PCE difference is 6.6% between reverse and forward scan of the PSC using 3% Eu:NiOx HTL, which is smaller than that of PSC using pristine NiOx HTL (16.6%). This can thank to the increased conductivity of NiOx HTL after 3% Eu doping, which make a better balance between holes and electrons transportation inside the inverted PSC. As compared above, the inverted PSC using 3% Eu:NiOx HTL exhibits the highest PCE of 15.06% in our case with a JSC of 21.96 mA cm 2, VOC of 1.03 V and FF of 66.6%. This PCE value is improved by 23.4% compared to that of the device using pristine NiOx HTL (12.20%). To investigate the RE doping on the photovoltaic characteristics of inverted PSC, further studies focused on the device performance of PSCs using 3% Eu:NiOx as well as the pristine NiOx HTLs are performed. Photo luminescence (PL) and time-resolved PL spectra are provided to test the capability of hole extraction ability from perovskite active layer to the NiOx HTLs. The testing structure is FTO/Eu:NiOx(or NiOx)/MAPbI3. As shown in Fig. 4a, an obvious decrease of PL intensity in the Eu:NiOx based structure can be observed, suggesting a more efficient hole extraction [51]. Fig. 4b displays the time-resolved PL spectra of the two testing structures. The faster decay of Eu:NiOx indicates that Eu doping could improve the hole extraction [52] and this result was in accordance with the PL and XPS analyzation. The incident photon to current conversion efficiency (IPCE) test is carried out to characterize the actually photo-to-electron conversion efficiency of the PSCs using NiOx and Eu:NiOx HTLs, respectively. As shown in Fig. 4c, the IPCE value of PSCs using Eu:NiOx HTL is higher than that of pristine NiOx in the whole studied range from 350 to
Table 1 Photovoltaic parameters of different inverted PSCs using RE:NiOx HTL. Doping types
JSC (mA. cm 2)
VOC (V)
FF (%)
PCE (%)
Rs (Ω. cm2)
Pristine
18.13b) 18.49 � 0.75a)
1.00 0.964 � 0.026
66.6 63.9 � 2.9
12.20 11.38 � 0.51
7.68
3% Eu
21.96 21.74 � 0.20
1.03 1.025 � 0.005
66.6 64.6 � 1.3
15.06 14.40 � 0.40
1.80
3% Yb
21.58 20.78 � 0.55
0.99 0.979 � 0.006
64.1 64.3 � 0.9
13.70 13.09 � 0.49
2.55
3% Tb
20.08 20.17 � 0.15
0.98 0.977 � 0.008
69.0 66.3 � 1.2
13.58 13.08 � 0.21
2.81
3% Ce
21.60 21.49 � 0.14
1.03 1.020 � 0.005
58.6 57.9 � 0.9
12.97 12.69 � 0.19
5.37
3% Nd
19.37 19.17 � 0.15
0.94 0.946 � 0.005
70.6 68.9 � 1.4
12.86 12.50 � 0.29
4.57
1% Eu
21.23 20.96 � 0.31
1.03 1.022 � 0.008
64.7 63.7 � 1.2
14.15 13.76 � 0.32
3.24
5% Eu
19.92 19.65 � 0.27
1.03 1.020 � 0.010
65.7 64.7 � 1.7
13.49 13.10 � 0.31
2.51
a)
The averaged values were calculated from 20 devices. The photovoltaic parameter of the best device around all the measured devices. b)
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Journal of Power Sources 444 (2019) 227267
Fig. 4. (a) PL spectra (the excitation wavelength is 520 nm), (b) time-resolved PL spectra of FTO/NiOx/MAPbI3 (blue) and FTO/Eu:NiOx/MAPbI3 (red). (c) The IPCE spectra and integrated JSC curves of the NiOx and Eu:NiOx devices. (d) EIS curves fitting with RCR model of NiOx and Eu:NiOx devices. The fitting model of EIS spectra is inserted in Fig. 4d. (e) The VOC as a function of incident light power in logarithmic coordinate. (f) The dark current curves versus bias voltage in a voltage region of 0.2-1.5 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
750 nm. The enhanced IPCE can be assigned to the more efficient car riers transport resulting from the reduced energy barrier between HTL and perovskite absorb layer as well as the higher conductivity of the Eu: NiOx HTL. [16,48,53]. In addition, the integrated JSC values from the corresponding IPCE spectra of the PSCs using NiOx and Eu:NiOx HTLs are determined to be 17.29 and 20.53 mA cm 2, respectively. These values are well consistent with the JSC values obtained from the corre sponding J-V curves (less than 10% error). Electrochemical impedance spectra (EIS) technology is used to analyze the charge transfer resistance and interfacial properties in the electrical view. In Fig. 4d, the EIS spectra of the two devices can be fitted by the RCR model (as shown in the inset), where Rrec is the charge transfer resistance, and Crec is the parallel capacitor. In our case, an increase of Rrec from 13.4 kΩ (NiOx) to 14.26 kΩ (Eu:NiOx) indicates that more efficient carrier separation, transport, and injection process at the light absorbed layer and less carrier recombination at the interface of perovskite/HTL in the Eu:NiOx HTL devices, which can lead to the improvement of FF and JSC of the final device [30,54,55]. This result is consistent with the PL and time-resolved PL characterizations. In order to get more details about the carrier recombination process of the studied devices, the incident light power dependent VOC curves are studied in Fig. 4e. The carrier recombination can be evaluated ac cording to the following formula: � � mKB T JSC ln V¼ þ constant; (1) e J0
the two devices shown in Fig. 4f also verify the reduced trap contents. The leakage current of Eu:NiOx based device is obviously smaller compared to that of pristine NiOx in the work bias region, which may provide an explanation for the higher JSC of corresponding PSCs [59]. The stability of PSCs is an important evaluation indicator for commercialization. Fig. 5a is the steady-state current density test for the best NiOx and Eu:NiOx devices which are encapsulated by the glass under the maximum power point bias voltage (Vmpp) for 40 min. As exhibited, the PSCs all show steady and smooth JSC curves, and the calculated steady-state average PCE are 12.13% and 14.95% for NiOx and Eu:NiOx based devices, respectively. In addition, the normalized long-term stability of the two devices (unencapsulated) are also carried out, as shown in Fig. 5b. The devices are stored in an ambient condition (25-55% relative humidity and 25-35 � C) when they were not using. The PSC using Eu:NiOx HTL can maintain 97% of the initial PEC value after 10 days, however, an obvious decrease is found in the case of pristine NiOx (87%). This can be attributed that the perovskite absorber layer with higher quality is obtained after depositing on the more compact and conductive Eu:NiOx HTL (as proved in Fig. 2d and f). Note that the previous study has revealed that the glass sealed inverted PSC with NiOx as HTL exhibited the long-term stability of 4000 h [60]. In our case, the used devices are not encapsulated when measured the long-term sta bility, thus a better long-term stability can be expected in the sealed conditions, especially for the PSC with 3% Eu:NiOx as HTL. 4. Conclusion
where KB is the Boltzmann constant, T is the absolute temperature, e is the elementary charge, and m is the ideality factor. In Fig. 4e, all the measured plots of Eu:NiOx based device exhibit a higher VOC level compared to that of NiOx, which is consistent with the previous J-V results. Besides, the corresponding plots can be linearly fitted in the whole studied range, and the slopes are calculated to be 1.69 KBT/e and 1.24 KBT/e for NiOx and Eu:NiOx based devices, respectively. Usually, closer of m value to 1, better ideal diode can be achieved. The decreased m value in Eu:NiOx based device demonstrates that trap-associated carrier recombination is effectively reduced by Eu doping [56–58]. Furthermore, the dark-current curves verse bias voltage ( 0.2-1.5 V) of
In summary, we investigated the impact of systematical doping of REs on the structural and optoelectronic characteristics of NiOx HTL in the inverted PSCs and their photovoltaic performances. The results revealed that the conductivity of the NiOx HTL and Rs of the devices could be modified in a large extent through adjusting the RE doping types and concentrations. In the optimal doping condition in our case, 3% Eu:NiOx HTL exhibited more smooth, compact, and conductive film, leading to larger perovskite grain size, and more important, promoted hole extraction ability and suppressed charge recombination rate. Finally, the PSC using Eu:NiOx HTL achieved an best PCE of 15.06% with significantly improved JSC, this PCE showed a 23.4% improvement 6
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Fig. 5. (a) Steady-state JSC of PSCs using NiOx and Eu:NiOx HTLs under the maximum power points for 40 min. The inset is the picture of actual device encapsulation with glass. (b) Normalized PCE of NiOx and Eu:NiOx devices under ambient condition (25-55% relative humidity and 25-35 � C) for 10 days.
compared to that of the pristine NiOx HTL. The optimal device also exhibited good stability when stored under the ambient condition. Our research illustrates that RE: NiOx can be an excellent and controllable hole extraction material for realizing inverted planar PSCs with highperformance.
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Acknowledgments This work was supported by the Key Program of NSFC-Guangdong Joint Funds of China (U1801253), the National Natural Science Foun dation of China (Grant Nos. 61874049, 11874181, 61822506, 61775080), the National Key Research and Development Program (2016YFC0207101), the Special Project of the Province-University Coconstructing Program of Jilin Province (SXGJXX2017-3), the Jilin Province Natural Science Foundation of China (No. 20180101210JC and 20170101170JC) and 13th Five-year Plan on Science and Technology Project of the Education Department of Jilin Province (No. JJKH20190115KJ). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227267. References [1] Q. Jiang, Y. Zhao, X. Zhang, X. Yang, Y. Chen, Z. Chu, Q. Ye, X. Li, Z. Yin, J. You, Nat. Photonics 13 (2019) 460–466. [2] T. Leijtens, S.D. Stranks, G.E. Eperon, R. Lindblad, E.M.J. Johansson, I. J. McPherson, H. Rensmo, J.M. Ball, M.M. Lee, H.J. Snaith, ACS Nano 8 (2014) 7147–7155. [3] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Gr€ atzel, S. Mhaisalkar, T. C. Sum, Science 342 (2013) 344. [4] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J.P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H.J. Snaith, Science 342 (2013) 341–344. [5] M. Liu, M.B. Johnston, H.J. Snaith, Nature 501 (2013) 395–398. [6] J. Burschka, N. Pellet, S.J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Gratzel, Nature 499 (2013) 316–319. [7] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Science 338 (2012) 643. [8] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050–6051. [9] H. Azimi, T. Ameri, H. Zhang, Y. Hou, C.O.R. Quiroz, J. Min, M. Hu, Z.-G. Zhang, T. Przybilla, G.J. Matt, E. Spiecker, Y. Li, C.J. Brabec, Adv. Energy.Mater. 5 (2015), 1401692. [10] G. Kakavelakis, T. Maksudov, D. Konios, I. Paradisanos, G. Kioseoglou, E. Stratakis, E. Kymakis, Adv. Energy.Mater. 7 (2017), 1602120. [11] A.E. Labban, H. Chen, M. Kirkus, J. Barbe, S. Del Gobbo, M. Neophytou, I. McCulloch, J. Eid, Adv. Energy.Mater. 6 (2016) 1502101. [12] Q. Xue, Y. Bai, M. Liu, R. Xia, Z. Hu, Z. Chen, X.-F. Jiang, F. Huang, S. Yang, Y. Matsuo, H.-L. Yip, Y. Cao, Adv. Energy.Mater. 7 (2017), 1602333. [13] Q. He, K. Yao, X. Wang, X. Xia, S. Leng, F. Li, ACS Appl. Mater. Interfaces 9 (2017) 41887–41897. [14] J.W. Jung, C.C. Chueh, A.K. Jen, Adv. Mater. 27 (2015) 7874–7880. [15] W. Chen, Y. Wu, J. Fan, A.B. Djuri�si�c, F. Liu, H.W. Tam, A. Ng, C. Surya, W.K. Chan, D. Wang, Z.-B. He, Adv. Energy.Mater. 8 (2018), 1703519.
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