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Performance improvement of planar perovskite solar cells with cobalt-doped interface layer
Journal Pre-proofs Full Length Article Performance improvement of planar perovskite solar cells with cobalt-doped interface layer Guanhua Ren, Zhuowei Li, Wei Wu, Shuo Han, Chunyu Liu, Zhiqi Li, Minnan Dong, Wenbin Guo PII: DOI: Reference:
S0169-4332(19)33898-X https://doi.org/10.1016/j.apsusc.2019.145081 APSUSC 145081
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Applied Surface Science
Received Date: Revised Date: Accepted Date:
7 November 2019 1 December 2019 13 December 2019
Please cite this article as: G. Ren, Z. Li, W. Wu, S. Han, C. Liu, Z. Li, M. Dong, W. Guo, Performance improvement of planar perovskite solar cells with cobalt-doped interface layer, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145081
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Performance improvement of planar perovskite solar cells with cobalt-doped interface layer Guanhua Ren, Zhuowei Li, Wei Wu, Shuo Han, Chunyu Liu*, Zhiqi Li, Minnan Dong, and Wenbin Guo* 1.
State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
2.
College of Materials Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
Abstract
Metal element doping can effectively overcome the drawbacks of transition metal oxide electron transport layer (ETL), such as point defects and unmatched band structure, thereby improving the charge transport and extraction capacity of perovskite solar cells (PSCs). Herein, the application of Co (Ϫ) doped compact TiO2 layer for planar heterojunction PSCs is demonstrated. The Co ions replace the Ti ions, which can reduce the oxygen vacancy defects in pristine TiO2, thus decreasing the carrier recombination in electronic trap states and enabling faster electron transport and collection. Co (Ϫ) doping can also reduce the work function of TiO2 ETL and adjust its band structure to form a better level alignment, which is beneficial to improving the electron extraction capacity by decreasing the barrier against the electron transport. Meanwhile, the TiO2 ETL with smoother surface leads to a good interface contact. Correspondingly, the quality of perovskite film is also improved. An overall rise of the open-circuit voltage, short-circuit current density and the fill factor are realized, yielding an increased PCE from 16.86% to 19.16%. Our work provides a facile and effective method to solve the problems of non-stoichiometry defects and energy level mismatching for TiO2 or other
metal oxide semiconductors transport layer. Keywords: perovskite solar cells, electron transport, charge recombination, doping, defects
1. Introduction
Since the first report by Miyasaka and co-workers in 2009, perovskite solar cells (PSCs) have made an amazing achievement and turned a new chapter for the development of photovoltaic technology [1, 2]. Lots of researchers are devoted to improving the performance of the device through a variety of strategies, including designing new processes and optimizing each functional layer, and so on [3-7]. Recently, the certified power conversion efficiency (PCE) of PSCs has reached 25.2% [8], which further demonstrates that PSCs is a promising candidate in the photovoltaic field. The electron transport layer (ETL) plays a critical role in PSCs with a classical layered structure. To date, many inorganic or organic materials have been used to transport electrons and block holes, but TiO2 is still the most widely used ETL material in the device [9-11]. Owing to the good physical stability, large bandgap and low costs, TiO2 is able to gain an advantage in the competition [12, 13]. However, the defects caused by non-stoichiometry in TiO2 is an obvious short board for the application in solar cells. These defect states as the electron traps between the CB and the valence band (VB) will cause carrier recombination and hinder charge transport, inevitably decreasing the open-circuit voltage (Voc) and short-circuit current density (Jsc) of the device [14-20]. Consequently, PSCs with defect-rich TiO2 suffer from the low efficiency and poor photostability. Doping ETL with metal elements is a common and effective strategy to reduce defects and enhance charge
transfer ability. In addition, the introduction of some metal ions can adjust the work function (WF), the band gap and the band position of transport layer, thus increasing the charge extraction and the Voc of the device [21-26]. Liu et al. adopted Li-doped compact TiO2 layer as ETL in the planar heterojunction PSCs and explored the effect of Li element. They found that Li-doping reduces the electron traps density and increases the mobility of TiO2, resulting in a higher efficiency of PSCs [27]. Recently, Liu et al. appliedNi-doped rutile TiO2 in carbon-based planar PSCs. Ni-doping influences the properties of TiO2 from several aspects, including conductivity, carrier mobility, film morphology and the shift in Fermi level (Ef), contributing to the enhancement of the typical performance parameters [28]. Sidhik et al. and Kim et al. have carried out a series of studies on the doping mechanism of Co (Ϫ) and Co (ϩ) in mesoporous TiO2, respectively. These two types of Co ions effectively reduce the trap states in mesoporous TiO2, thereby alleviating the hysteresis of PSCs while achieving high PCEs [29-32]. However, the process of mesoporous structure is relatively complicated compared to planar structure, which will lead to higher fabrication cost. In addition, there are no detailed reports that focus on Co (Ϫ) doped compact TiO2 layer in the planar PSCs, it is necessary to explore the possibility of improving the performance of planar PSCs by employing Co (Ϫ) doped TiO2 ETL. In this work, Co (Ϫ) doped anatase TiO2 was developed as ETL for planar heterojunction PSCs. In order to eliminate the interference of anions, the material we chose was cobalt acetylacetonate (Ϫ). We attribute the photoelectric performance enhancement of device to three fold. First, the electron trap states arising from the oxygen vacancy defects are reduced. Second, an upshifted Fermi level and a better level alignment form due to Co (Ϫ) doping, resulting in the more efficient charge extraction and transport. Third, the film quality of ETL is improved, which also improves the crystallization of the perovskite film to some extent. As a result, the optimized PSCs based on 0.75 mol% Co-doped TiO2 obtains a maximum PCE of 19.16%, 13.6% higher
than that of a similar device with non-doped TiO2 (16.86%).
2. Results and Discussion
In this study, the TiO2 precursor solution was prepared by sol-gel method [33]. During the sintering crystallization process of TiO2 film, Co ions will embed into TiO2 lattice [34]. The existence of the Co element in the compact TiO2 film is investigated by X-ray photoelectron spectroscopy (XPS) measurements. Fig. 1a-d depict the XPS images of survey, Co 2p, Ti 2p, and O 1s for pristine TiO2 and Co-doped TiO2, respectively. The Co 2p peak appears in the survey image of Co-doped TiO2, indicating that the Co ion was successfully doped into the TiO2. Fig. 1b is the spectra images of Co 2p. Generally speaking, the Co3+-2p core level exhibits weaker shakeup satellite peaks compared with Co2+-2p, and the splitting of the Co 2p1/2Co 2p3/2 orbital components ∆ for Co3+ and Co2+ are different [35, 36]. Hence, the Co atoms are in the mixed form of 3+ and 2+ oxidation state in TiO2. Two major Co3+ peaks can be observed at 780.25 eV for 2p3/2 and 795.74 eV for 2p1/2, and Co2+ peaks are observed at 781.48 eV for 2p3/2 and 797.40 eV for 2p1/2. The corresponding satellite peaks are located at 786.52 eV and 802.57 eV. It is noticed that the intensity of the satellite peak centered at 802.57 eV is high or even exceed the satellite peak for 2p3/2. Some impure peaks appearing in the measurement (the peaks indicated by the dotted lines in the Fig. 1b) should be responsible for the result. The Ti 2p XPS spectraindicates that both Ti 2p3/2 and Ti 2p1/2 peaks have a slight shift to the direction of high binding energy after Co ions doping. The Ti 2p3/2 peak shifts from 458.53 eV to 458.71 eV and the Ti 2p1/2 peak shifts from 464.18 eV to 464.46 eV. Ti and Co have the electronegativity values of 1.54 and 1.88, as a result of which a transfer of negative charge towards Co will happen [37]. The shifting implies Co atoms have been intercalated into TiO2 lattice and affect the electronic properties of Ti atoms [38]. The O
1s core level spectra of TiO2 is showed in Fig. 1d, which can be divided into two peaks: one peak at low binding energy (about 530.00 eV) correspond to the lattice oxygen (O-Ti) and the other peak at high binging energy (about 532.00 eV) correspond to the defective oxygen ions on the surface, such as oxygen vacancy or hydroxyl O-H groups caused by contaminations [39, 40]. Similar to Ti 2p peaks, O 1s peaks shifts by 0.1 eV towards the direction of high binding energy after doping. Furthermore, the intensity ratio of the high-energy peak/low-energy peak (IA/IB) decreased from 0.50 to 0.32, demonstrating the oxygen-deficient defects is reduced due to the incorporation of Co ions [40].
Fig. 1. (a) XPS survey of the pristine TiO2 and the Co-doped TiO2. High-resolution XPS spectra of (b) Co 2p, (c) Ti 2p and (d) O 1s peaks.
Fig. 2. UPS spectra exhibiting (a) the secondary-electron cut-off, and (b) the VB region. (c) Absorption spectra of the pristine TiO2 and the Co-doped TiO2 film. The inset displays the partial magnification of the spectra.
The research for electronic structure of the pristine TiO2 and 0.75 mol% Co-doped TiO2 by means of ultraviolet photoelectron spectroscopy (UPS) is displayed in Fig. 2a and b. The inset in Fig. 2a reveals that the WF of two types of TiO2 are 5.44 and 5.36 eV with the equation of WF=21.2-Ecutoff. It has been reported that the reduced WF can enhance the charge transport ability of TiO2 [41] Fig. 2b shows the difference between Ef and the VB maximum, the value of which for the pristine TiO2 and Co-doped TiO2 are 2.19 and 2.13 eV. Meanwhile, the VB maximum positions can be calculated to be 7.63 and 7.49 eV. In the absorption spectra of TiO2 of Fig. 2c, the absorption edge of 0.75 mol% Co-doped TiO2 has a slight blue shift. Therefore, the optical absorption bandgap increases from 3.51 eV to 3.53 eV. It should be noted that TiO2, a type of indirect transition semiconductor, has a narrower semiconductor bandgap (Eg) than its own optical absorption bandgap according to the previous research [42]. The Eg of the TiO2 film is 3.2 (±0.06) eV and the position of the CB minimum can be obtained from 4.43 eV to 4.29 eV. The rise of the CB position allows the band structure of Co-doped TiO2 better matched with that of the perovskite layer, which is beneficial to reduce
energy loss and promote charge extraction, thus improving Voc and Jsc of the device [43-45].
Fig. 3. (a) Chemical structure of cobalt acetylacetonate (Ϫ) and device structure of PSCs, (b) cross-sectional SEM image of PSCs and (c) energy levels diagram of the device.
Fig. 3a displays the chemical structure of cobalt acetylacetonate (Ϫ) and the device structure of the typical planar PSCs based on Co-doped TiO2. The cross-sectional scanning electron microscope (SEM) image is illustrated in Fig. 3b, in which the thickness of the Co-doped TiO2 and perovskite film are about 30 and 220 nm. Fig. 3c shows the energy level diagram of all materials used in the research based on the UPS analysis above. There is a positive shift in band structure of TiO2 layer, and the CB and VB of other layers also match well at the interface for easy transport of charge carriers.
Fig. 4. (a) J-V characteristics of the devices based on varied TiO2 films. (b) Histogram of efficiencies among 16 devices. (c) IPCE spectrum of the device based on the 0 mol% and 0.75 mol% Co-doped TiO2 films. (d) J-V characteristics of the PSCs measured under reverse and forward scanning direction. Steady-state photocurrent and PCE output of the devices based on the (e) 0 mol% Co-doped TiO2 and (f) 0.75 mol% Codoped TiO2 films measured under one sun (100 mW/cm2) illumination for 55 s. Table 1. The photovoltaic parameters of PSCs with different concentrations of Co-doped TiO2 films Concentration (mol%)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
0
1.08
21.54
72.46
16.86
0.1
1.08
21.85
72.96
17.22
0.25
1.09
22.22
73.57
17.82
0.5
1.09
22.68
74.01
18.30
0.75
1.11
22.86
75.49
19.16
1
1.10
22.49
73.23
18.12
2.5
1.09
20.80
70.21
15.92
Table 2. The photovoltaic parameters of PSCs based on 0 mol% and 0.75 mol% Co-doped TiO2 measured under different scanning directions. Concentration (mol%)
Scanning direction
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
0
reverse
1.09
21.10
71.97
16.55
forward
1.05
18.71
63.96
12.57
reverse
1.10
22.46
74.95
18.52
forward
1.07
20.66
70.68
15.62
0.75
Fig. 4a is the current density-voltage (J-V) characteristics of the devices fabricated with compact TiO2 layer
doped by different concentrations of Co ions (0, 0.1, 0.25, 0.5, 0.75, 1 and 2.5 mol%). The detailed photovoltaic parameters including Voc, Jsc, FF and PCE are summarized in Table 1. The device based on 0 mol% Co-doped TiO2 indicates a PCE of 16.86%, companying with a Voc of 1.08 V, a Jsc of 21.54 mA/cm2, and a FF of 72.46%. The optimal performance of 19.16% is achieved based on 0.75 mol% Co-doped TiO2, obtaining a Voc of 1.11 V, a Jsc of 22.86 mA/cm2, and a FF of 75.49%. A significant enhancement in Jsc and FF is obtained, simultaneously realizing a slight increment in Voc. The histogram of the PCE of 16 individual cells using the pristine TiO2 and 0.75 mol% Co-doped TiO2 are given in Fig. 4b. The PCE of the pristine TiO2 based-devices is distributed in the range of 14-17%, while the PCE of the 0.75 mol% Co-doped TiO2based devices is in the range of 17.5-19.5%. It appears that the PCE of devices exhibits narrower distribution after Co doping, demonstrating a higher repeatability of Co-doped devices compared to the non-doped devices. The incident photon-electron conversion efficiency (IPCE) spectrum is displayed in Fig. 4c to convey the enhancement of Jsc. Within the wavelength range from 350 nm to 750 nm, the device based on the 0.75 mol% Co-doped TiO2 shows an obvious increased IPCE. The hysteresis of the devices was studied and shown in Fig. 4d. The PSC based on 0.75 mol% Co-doped TiO2 exhibits decreased J-V hysteresis and the corresponding parameters are summarized in Table 2. The hysteresis index (HI) was calculated using the following equation: ൌ
ா ିாూ
(1),
ா
where PCERS and PCEFS represent PCE for the reverse scan and forward scan, respectively. The 0.75 mol% Co-doped TiO2-based device shows a smaller HI value of 15.66% compared to 24.05% for that of the pristine TiO2-based device, which can be attributed to the reduced defects and traps after Co (Ϫ) doping. The steadystate photocurrent and PCE measured at the maximum power point of the device without optimization and
optimal device is given in Fig. 4e and f. There is an obvious attenuation on the current density and PCE for the device with the pristine TiO2, and a stabilized efficiency of 16.00% with a photocurrent density of 18.60 mA/cm2 is obtained at a voltage of 0.86 V. By contrast, the 0.75 mol% Co-doped TiO2-based device yields a stabilized efficiency of 18.81% with a photocurrent density of 20.90 mA/cm2 at a voltage of 0.90 V, following a smaller degree of attenuation.
Fig. 5. (a) Dark J-V characteristics of PSCs based on the 0 mol% and 0.75 mol% Co-doped TiO2 films. (b) Dependence of Voc on the light intensity of PSCs with the 0 mol% and 0.75 mol% Co-doped TiO2. (c) CurrentVoltage curves of the device with the structure: ITO/compact TiO2 layer/Ag with the 0 mol% and 0.75 mol% Co (III) doping.
The dark J-V characteristics of device was measured and shown in Fig. 5a, the optimal PSCs exhibits a smaller leakage current in a working bias region (about 0-1.1 V), proving that carrier transport is improved and recombination loss is reduced in the device. The light intensity dependence of Voc was investigated according to the equation [46] ்
Ɂܸ୭ୡ ൌ ݊ ቀ ቁ ሺܫሻ ܿݐ݊ܽݐݏ݊
(2),
in which n is the ideality factor, k is the Boltzmann constant, T is absolute temperature, e is the elementary charge, and I is the light intensity. The Voc-logarithmic light intensity and the corresponding fitting results are
presented in Fig. 5b. The slope values of 1.69kT/e and 1.29kT/e are obtained for the pristine TiO2-based device and 0.75 mol% Co-doped TiO2-based device, meaning that trap-assisted carrier recombination is suppressed after Co doping [37]. In order to probe the difference of electrical conductivity of TiO2 ETL with and without doping. The device with the configuration of ITO/compact TiO2 layer/Ag was fabricated. The current-voltage linear curves in Fig. 5c follow the formula [47]
ܫൌ ߪ ቀ ቁ ܸ ௗ
(3), where σ0, A and d
stand for the sample electrical conductivity, area (0.044 cm2), and thickness (30 nm). The σ0 of the 0.75 mol% Co-doped TiO2 sample can be calculated to be 1.3410-4 ms/cm, higher than the non-doped TiO2 sample (8.7910-5 ms/cm), which may result from upward shift of Ef for Co-doped TiO2. The enhanced electrical conductivity is conducive to transport and extract electrons.
Fig. 6. Contact angle measurement of perovskite precursor in DMF/DMSO (V:V=9:1) on the (a) 0 mol%, (b)
0.5 mol%, (c) 0.75 mol% and (d) 2.5 mol% Co-doped TiO2 films. AFM images of the (e) 0 mol%, (f) 0.5 mol%, (g) 0.75 mol% and (h) 2.5 mol% Co-doped TiO2 films. AFM images of the perovskite films deposited on the (i) 0 mol%, (j) 0.5 mol%, (k) 0.75 mol% and (l) 2.5 mol% Co-doped TiO2 films.
Whereas the physical properties of the ETL and perovskite layer have a direct impact on the performance of perovskite devices, the contact angle and the atomic force microscopy (AFM) measurements were applied. Here, samples with doping concentrations of 0 mol%, 0.5 mol%, 0.75 mol% and 2.5 mol% are selected for a better illustration. Fig. 6a-d display the contact angle of perovskite precursor in DMF/DMSO (V:V=9:1) on these TiO2 films. There is no significant difference in the contact angle of these four TiO2 films, which indicates that the introduction of Co ions does not change the wettability of TiO2 films to a large extent. AFM images presented in Fig. 6e-l reflect the film qualities of the TiO2 and corresponding perovskite layers changes with the increase in the doping amount of Co ions. Compared with the pristine TiO2 film, the rootmean-square roughness (RMS) of 0.75 mol% Co-doped TiO2 film is reduced from 0.64 nm to 0.54 nm, while 2.5 mol% Co-doped TiO2 film exhibits a rougher surface with more pinholes (Fig. 6e-h). The reason for this change is that excess Co ions will occupy substitution and interstitial sites in the TiO2 lattice, resulting in lattice distortion and film instability. The ETL film with a smoother surface would act as a better foundation for the crystallization of perovskites. Consequently, the perovskite films show the uniformity of grains distribution and reduction of pinholes at a low doping degree, accompanying with a decreased RMS from 9.29 nm to 6.91 nm (Fig. 6i-k). Fig. 6l reveals a dramatic increase in the amount of pinholes and surface roughness of the perovskite film owing to the influence of the 2.5 mol% Co-doped TiO2 film. The pinholes in the perovskite film tend to serve as the contact path between the ETL material and hole transport layer
(HTL) material and induce current leakage, yielding a decreased Voc and Jsc [48]. Note that a dense, smooth perovskite film is more conducive to form a better contact with the HTL and achieve high PCE.
Fig. 7. (a) Transmittance spectra of the 0, 0.5, 0.75 and 2.5 mol% Co-doped TiO2 films. (b) Absorption spectra of the perovskite films deposited on the 0, 0.5, 0.75 and 2.5 mol% Co-doped TiO2 films. (c) XRD patterns of the perovskite films deposited on the 0, 0.5, 0.75 and 2.5 mol% Co-doped TiO2 films. (d) Variation of diffraction peak intensity of (110), (220) and (310) planes in the XRD patterns. Top-view SEM images of the perovskite films deposited on the (e) 0 mol% and (f) 0.75 mol% Co-doped TiO2 films.
To further investigate the quality of the perovskite film, the transmittance spectra, absorption spectra, Xray diffraction (XRD) and top-view SEM measurements were performed. Fig. 7a displays the transmittance spectra for TiO2 and Co-doped TiO2 coated on the glass/ITO substrates. Even if the doping concentration of Co increases to 2.5 mol%, the shape of the spectra remains basically the same, demonstrating the optical absorption of perovskites will not be affected by the transmittance of ETL. Fig. 7b indicates a weakened
absorption in the range from 450 nm to 700 nm for 2.5 mol% Co-doped TiO2/perovskite compared with the other three samples. This could be explained by the decline in crystallinity of the perovskite film [49]. The XRD characterization focused on the crystallinity of the perovskite films is shown in Fig. 7c. The CH3NH3PbI3 films covered on the pristine and doped TiO2 exhibits strong diffraction peaks at 14.12°, 28.44°and 31.85°, which are assigned to the (110), (220) and (310) crystal planes of tetragonal perovskite, respectively. These peak positions agree well with the XRD patterns of CH3NH3PbI3 perovskites reported previously [50]. The variation of detailed value of the diffraction peaks intensity is depicted in Fig. 7d. As expected, the perovskite sample with 0.75 mol% Co-doped TiO2 has the highest peak intensity, while the peak intensity drop to the lowest with the doping concentration of 2.5 mol%. It suggests the difference in the crystallinity of the films. Fig. 7e and f shows the top-view SEM images of perovskite film on 0 mol% and 0.75 mol% Co-doped TiO2. Consistent with the AFM images, the grain size of perovskite films is approximately 50-300 nm. The pristine TiO2 shows perovskite film with many pinholes and aggregations of small grain compared to the uniform perovskite film coated on the 0.75 mol% Co-doped TiO2 having less pinholes and boundaries. Based on the above discussion, it can be concluded that the perovskite film quality is enhanced after doping appropriate Co into TiO2 ETL, helping to improve Jsc and Voc.
Fig. 8. Steady-state PL spectra of the TiO2 film doped with 0, 0.5, 0.75 and 2.5 mol% Co at an excitation wavelength of (a) 310 nm and (b) 365 nm. The inset is the partial magnification of the spectra. (c) Steadystate PL spectra of the perovskite films deposited on different TiO2 films with incident light from perovskite side (left) and from the ITO side (right). (d) Oxygen defects give rise to Ti3+ defects that form electronic traps. (e) Co substitution at the Ti3+ sites passivates these defects. (f) Recombination mechanism due to the deep traps levels and (g) energy diagram after traps have been passivated.
The charge transport and carrier recombination in the devices can be analyzed more directly by using steady-state photoluminescence (PL). The PL measurements with an excitation wavelength of 310 nm and 365 nm were carried out and shown in Fig. 8a and b. As can be seen from Fig. 8a, the PL peaks locate at 360 nm for all TiO2 films, which originates from the emission of band-to-band transition corresponding to the optical absorption bandgap energy of TiO2. Since the optical light absorption of TiO2 changes little after doping (Fig. 2c), the intensive PL quenching can be attributed to the more effective charge extraction. The luminescence of TiO2 on Eg excitation helps to further explain the effects of Co ions. The PL spectrum is similar with four visible PL peaks at 391 nm (3.17 eV), 458 nm (2.71 eV), 476 nm (2.61 eV) and 562 nm (2.21 eV). Obviously, the first one coincides with the emission of transition from the VB minimum to the CB maximum. The peak at 458 nm is attributed to surrface defects of TiO2 [51]. Defects caused by oxygen vacancies commonly induce some energy levels (about 0.7-1 eV) below the CB [52, 53]. Naturally, the other two peaks mainly result from these oxygen vacancies. Oxygen vacancies are able to trap zero, one or two electrons, increasing the recombination probability of transported electrons [54]. A phenomenon that the Co ions at low doping degree remarkably quenches the PL intensity of the defects levels is observed. In contrast,
the PL intensity of 2.5 mol% Co-doped TiO2 film increases and exceeds that of the pristine TiO2 at both excitation wavelength. Consistent with the analysis about the AFM images, excessive Co ions could occupy substitution sites in the TiO2 lattice and cause charge imbalance [49]. As a result, new oxygen vacancies are generated. Moreover, the PL spectra for the perovskite films fabricated on the pristine TiO2 and Co-doped TiO2 were given in Fig. 8c. Considering that the penetration depth of excitation light is less than the thickness of the perovskite film [55], we measured from perovskite side and indium tin oxide (ITO) side. The samples under different measurement conditions all have an emission peaks at 769 nm, PL intensity of which presents a trend of decreasing first and then increasing with the grow in number of Co molar ratio. The PL analysis above clearly verified that moderate Co ions reduce oxygen vacancy defects and passivate the electron traps in the TiO2 layer. It can also be inferred that charge transport from perovskites to ITO is accelerated and carrier recombination at interface is suppressed [56]. Hence, the Jsc and FF of PSCs is improved. The schematic diagram depicting the doping mechanism of Co element and the process of traps passivation in anatase TiO2 is also presented in Fig. 8. When an oxygen atom is detached from the crystal lattice, a point defect and unstable Ti with dangling bonds (Ti3+) appear at the corresponding position. The Ti3+ acts as an electron trap to form Ti4++e- (Fig. 8d) [57]. These trap states are more likely to form at subsurface and bulk site rather than surface of TiO2 [58]. Consequently, the deep energy levels below the CB of TiO2 are introduced, serving as a cascading path for the carrier recombination (Fig. 8f). Co3+ has a similar ionic radius as Ti4+ (0.61 nm) and Ti3+ (0.64 nm) [59], the substitution of the Ti3+ adjacent to an oxygen vacancy with Co3+ is reasonable, which is also the best description of our research results. Therefore, the Co3+ substitution effectively remove oxygen defects within the lattice and on the subsurface. As for the Co2+ present in the TiO2 film,we make the following hypothesis based on our study and previous literatures: Due to the charge
transfer effect between Co and Ti, the oxidation state of Ti shifts from Ti3+ to Ti4+. Co2+ are located near the oxygen vacancies and form Co-oxygen vacancy-Ti complex, which plays a role in reducing unstable states associated with Ti3+ (Fig. 8e) [60]. Fig. 8g shows the elimination of the trap levels, so that electrons and holes can be transported to both ends without hindrance, leading to an enhanced photovoltaic performance. Noticeably, Co substitution is likely to decrease the sensitivity of the TiO2 to the presence of oxygen, which is beneficial for the device stability [57].
3. Conclusions
In summary, we have successfully fabricated Co-doped compact TiO2 layer with various doping levels and applied to planar heterojunction PSCs as ETL. It is found that optimized Co (Ϫ) doping leads to a reduced number density of oxygen defects, a positive shift in Ef and better film quality for TiO2 layer. Meanwhile, the quality of CH3NH3PbI3 perovskite film is improved, which has been proved in the research. An accelerated charge transport as well as suppressed charge recombination in PSCs is realized. A low level of Co (Ϫ) doping (0.75mol %) in TiO2 contributes to the increased PCE from 16.91% to 19.06%. Moreover, excessive Co ions have a negative impact on the devices, especially in inducing the emergence of unknown oxygen vacancies and causing lattice distortion. It is believed that this work further explores the potential of planar heterojunction PSCs and provides a feasible method to reach high efficiency of solar cells.
4. Experimental Section
4.1 Materials
The cobalt acetylacetonate (Ϫ) (98%) was purchased from aladdin. Lead iodide (PbI2, >99.99%) ,
methylammonium iodide (MAI, ≥99.5%), 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD, ≥99.8%), bis(trifluoromethane) sulfonamide lithium (Li-TFSI, >99%) and 4-tertbutylpyridine (TBP, >96%) were obtained from Xi’an P-OLED Corp. (China). Acetonitrile (ACN, 99.8%) was purchased from Sigma-Aldrich. N,N-dimethylformamide (DMF, 99.9%), dimethyl sulfoxide (DMSO, 99%) and chlorobenzene (CB, 99%) were obtained from J&K Scientific. All of the purchased materials were used without any further purification.
4.2 Synthesis of TiO2 Precursor Solution
The TiO2 precursor solution was prepared by the sol-gel method.25 For the Co-doped TiO2, the cobalt acetylacetonate (Ϫ) was dissolved in TiO2 precursor solution and stirred until clarified, the molar ratio of which to Ti atoms is 0%, 0.1%, 0.25%, 0.5%, 0.75%, 1% and 2.5%.
4.3 Device Fabrication
The ITO-coated glass substrates were cleaned by ultrasonic treatment in acetone, ethanol, and deionized water for 15 min in sequence. Then they were dried under nitrogen flow. The pristine TiO2 and different concentrations of Co-doped TiO2 precursorsolution was spin-coated onto the ITO substrate at 800 rpm for 3 s and 3000 rpm for 20 s in air. Subsequently, the substrates were annealed in a muffle furnace at 450 °C for 2 h. After cooling down to room temperature, the perovskite layer was prepared by spin coating a solution consisting of 470 mg PbI2 and 162 mg MAI in a solvent of DMF/DMSO (V:V=9:1) at 4000 rpm for 20 s, 400 μL ether was dripped in this process at the 5 s before the end of spin-coating step. Then, they were annealed at 100 °C for 10 min on a hotplate, forming a brownish black perovskite film. The hole transport
layer material was prepared by dissolving 72.3 mg of spiro-OMeTAD in 1 mL of CB, a solution of 17.6 μL of Li-TFSI solution (520 mg/mL in ACN) and 28.8 μL of TBP were added in the spiro-OMeTAD solution. The spiro-OMeTAD solution was spin coated onto the perovskite layer at 4000 rpm for 30 s.Finally, 100 nm-thick of Ag electrode was deposited on top of the HTL in a vacuum condition. The devices were completed and the area is 0.044 cm2.
4.4 Device Characterization
The XPS and UPS measurements were performed on a photoelectron spectrometer. The cross-sectional and Top-view SEM images were characterized by a JEOL JSM-7500F field-emission SEM. The J-V characteristics were measured in an illumination condition under AM 1.5G solar illuminations and dark condition using a Keithley 2601 source meter. The IPCE spectra of PSCs were conducted by Crowntech QTest Station 1000 AD. The contact angle of perovskite precursor solution on the TiO2 films were evaluated using FTA 200 First Ten Angstroms dynamic contact angle analyzer. The AFM images of the TiO2 ETLs and perovskite films were obtained with an ICON-PT in tapping mode. The transmittance spectra and UV-Vis absorption spectra were performed with UV 1700 photometer, Shimadzu. The XRD patterns of the perovskite films were recorded on a Shimadzu XRD-6000 diffractometer. The PL spectrum was measured using a Shimadzu RF 5301 fluorescence spectrophotometer at room temperature.
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected] and [email protected]
Notes
The authors declare no competing financial interest
ACKNOWLEDGMENTS The authors are grateful to National Natural Science Foundation of China (61875072), the Special Project of the Province-University Co-constructing Program of Jilin Province (SXGJXX2017-3), the Science and Technology Innovation Leading Talent and Team Project of Jilin Province (20170519010JH), the National Postdoctoral Program for Innovative Talents (BX20190135), and International Cooperation and Exchange Project of Jilin Province (20170414002GH, 20180414001GH) for the support to the work.
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Statement The authors declare that there is no conflict of interest.
The Co doped ETL is simplistic, low cost, and suitable for mass production. The Co doped ETL could reduce the interfacial defects and improve contact. Doped ETL can enhance the electron extraction by decreasing the injection barrier.
Author contributions Wenbin Guo, Chunyu Liu and Guanhua Ren conceived the idea and designed the experiments. Guanhua Ren carried out the device fabrication, optical and electrical characterizations. Guanhua Ren, Zhuowei Li, Wei Wu and Shuo Han did SEM and AFM measurements. All the authors analyzed and interpreted the data and prepared the manuscript.
S0169-4332(19)33898-X https://doi.org/10.1016/j.apsusc.2019.145081 APSUSC 145081
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Applied Surface Science
Received Date: Revised Date: Accepted Date:
7 November 2019 1 December 2019 13 December 2019
Please cite this article as: G. Ren, Z. Li, W. Wu, S. Han, C. Liu, Z. Li, M. Dong, W. Guo, Performance improvement of planar perovskite solar cells with cobalt-doped interface layer, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145081
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Performance improvement of planar perovskite solar cells with cobalt-doped interface layer Guanhua Ren, Zhuowei Li, Wei Wu, Shuo Han, Chunyu Liu*, Zhiqi Li, Minnan Dong, and Wenbin Guo* 1.
State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
2.
College of Materials Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
Abstract
Metal element doping can effectively overcome the drawbacks of transition metal oxide electron transport layer (ETL), such as point defects and unmatched band structure, thereby improving the charge transport and extraction capacity of perovskite solar cells (PSCs). Herein, the application of Co (Ϫ) doped compact TiO2 layer for planar heterojunction PSCs is demonstrated. The Co ions replace the Ti ions, which can reduce the oxygen vacancy defects in pristine TiO2, thus decreasing the carrier recombination in electronic trap states and enabling faster electron transport and collection. Co (Ϫ) doping can also reduce the work function of TiO2 ETL and adjust its band structure to form a better level alignment, which is beneficial to improving the electron extraction capacity by decreasing the barrier against the electron transport. Meanwhile, the TiO2 ETL with smoother surface leads to a good interface contact. Correspondingly, the quality of perovskite film is also improved. An overall rise of the open-circuit voltage, short-circuit current density and the fill factor are realized, yielding an increased PCE from 16.86% to 19.16%. Our work provides a facile and effective method to solve the problems of non-stoichiometry defects and energy level mismatching for TiO2 or other
metal oxide semiconductors transport layer. Keywords: perovskite solar cells, electron transport, charge recombination, doping, defects
1. Introduction
Since the first report by Miyasaka and co-workers in 2009, perovskite solar cells (PSCs) have made an amazing achievement and turned a new chapter for the development of photovoltaic technology [1, 2]. Lots of researchers are devoted to improving the performance of the device through a variety of strategies, including designing new processes and optimizing each functional layer, and so on [3-7]. Recently, the certified power conversion efficiency (PCE) of PSCs has reached 25.2% [8], which further demonstrates that PSCs is a promising candidate in the photovoltaic field. The electron transport layer (ETL) plays a critical role in PSCs with a classical layered structure. To date, many inorganic or organic materials have been used to transport electrons and block holes, but TiO2 is still the most widely used ETL material in the device [9-11]. Owing to the good physical stability, large bandgap and low costs, TiO2 is able to gain an advantage in the competition [12, 13]. However, the defects caused by non-stoichiometry in TiO2 is an obvious short board for the application in solar cells. These defect states as the electron traps between the CB and the valence band (VB) will cause carrier recombination and hinder charge transport, inevitably decreasing the open-circuit voltage (Voc) and short-circuit current density (Jsc) of the device [14-20]. Consequently, PSCs with defect-rich TiO2 suffer from the low efficiency and poor photostability. Doping ETL with metal elements is a common and effective strategy to reduce defects and enhance charge
transfer ability. In addition, the introduction of some metal ions can adjust the work function (WF), the band gap and the band position of transport layer, thus increasing the charge extraction and the Voc of the device [21-26]. Liu et al. adopted Li-doped compact TiO2 layer as ETL in the planar heterojunction PSCs and explored the effect of Li element. They found that Li-doping reduces the electron traps density and increases the mobility of TiO2, resulting in a higher efficiency of PSCs [27]. Recently, Liu et al. appliedNi-doped rutile TiO2 in carbon-based planar PSCs. Ni-doping influences the properties of TiO2 from several aspects, including conductivity, carrier mobility, film morphology and the shift in Fermi level (Ef), contributing to the enhancement of the typical performance parameters [28]. Sidhik et al. and Kim et al. have carried out a series of studies on the doping mechanism of Co (Ϫ) and Co (ϩ) in mesoporous TiO2, respectively. These two types of Co ions effectively reduce the trap states in mesoporous TiO2, thereby alleviating the hysteresis of PSCs while achieving high PCEs [29-32]. However, the process of mesoporous structure is relatively complicated compared to planar structure, which will lead to higher fabrication cost. In addition, there are no detailed reports that focus on Co (Ϫ) doped compact TiO2 layer in the planar PSCs, it is necessary to explore the possibility of improving the performance of planar PSCs by employing Co (Ϫ) doped TiO2 ETL. In this work, Co (Ϫ) doped anatase TiO2 was developed as ETL for planar heterojunction PSCs. In order to eliminate the interference of anions, the material we chose was cobalt acetylacetonate (Ϫ). We attribute the photoelectric performance enhancement of device to three fold. First, the electron trap states arising from the oxygen vacancy defects are reduced. Second, an upshifted Fermi level and a better level alignment form due to Co (Ϫ) doping, resulting in the more efficient charge extraction and transport. Third, the film quality of ETL is improved, which also improves the crystallization of the perovskite film to some extent. As a result, the optimized PSCs based on 0.75 mol% Co-doped TiO2 obtains a maximum PCE of 19.16%, 13.6% higher
than that of a similar device with non-doped TiO2 (16.86%).
2. Results and Discussion
In this study, the TiO2 precursor solution was prepared by sol-gel method [33]. During the sintering crystallization process of TiO2 film, Co ions will embed into TiO2 lattice [34]. The existence of the Co element in the compact TiO2 film is investigated by X-ray photoelectron spectroscopy (XPS) measurements. Fig. 1a-d depict the XPS images of survey, Co 2p, Ti 2p, and O 1s for pristine TiO2 and Co-doped TiO2, respectively. The Co 2p peak appears in the survey image of Co-doped TiO2, indicating that the Co ion was successfully doped into the TiO2. Fig. 1b is the spectra images of Co 2p. Generally speaking, the Co3+-2p core level exhibits weaker shakeup satellite peaks compared with Co2+-2p, and the splitting of the Co 2p1/2Co 2p3/2 orbital components ∆ for Co3+ and Co2+ are different [35, 36]. Hence, the Co atoms are in the mixed form of 3+ and 2+ oxidation state in TiO2. Two major Co3+ peaks can be observed at 780.25 eV for 2p3/2 and 795.74 eV for 2p1/2, and Co2+ peaks are observed at 781.48 eV for 2p3/2 and 797.40 eV for 2p1/2. The corresponding satellite peaks are located at 786.52 eV and 802.57 eV. It is noticed that the intensity of the satellite peak centered at 802.57 eV is high or even exceed the satellite peak for 2p3/2. Some impure peaks appearing in the measurement (the peaks indicated by the dotted lines in the Fig. 1b) should be responsible for the result. The Ti 2p XPS spectraindicates that both Ti 2p3/2 and Ti 2p1/2 peaks have a slight shift to the direction of high binding energy after Co ions doping. The Ti 2p3/2 peak shifts from 458.53 eV to 458.71 eV and the Ti 2p1/2 peak shifts from 464.18 eV to 464.46 eV. Ti and Co have the electronegativity values of 1.54 and 1.88, as a result of which a transfer of negative charge towards Co will happen [37]. The shifting implies Co atoms have been intercalated into TiO2 lattice and affect the electronic properties of Ti atoms [38]. The O
1s core level spectra of TiO2 is showed in Fig. 1d, which can be divided into two peaks: one peak at low binding energy (about 530.00 eV) correspond to the lattice oxygen (O-Ti) and the other peak at high binging energy (about 532.00 eV) correspond to the defective oxygen ions on the surface, such as oxygen vacancy or hydroxyl O-H groups caused by contaminations [39, 40]. Similar to Ti 2p peaks, O 1s peaks shifts by 0.1 eV towards the direction of high binding energy after doping. Furthermore, the intensity ratio of the high-energy peak/low-energy peak (IA/IB) decreased from 0.50 to 0.32, demonstrating the oxygen-deficient defects is reduced due to the incorporation of Co ions [40].
Fig. 1. (a) XPS survey of the pristine TiO2 and the Co-doped TiO2. High-resolution XPS spectra of (b) Co 2p, (c) Ti 2p and (d) O 1s peaks.
Fig. 2. UPS spectra exhibiting (a) the secondary-electron cut-off, and (b) the VB region. (c) Absorption spectra of the pristine TiO2 and the Co-doped TiO2 film. The inset displays the partial magnification of the spectra.
The research for electronic structure of the pristine TiO2 and 0.75 mol% Co-doped TiO2 by means of ultraviolet photoelectron spectroscopy (UPS) is displayed in Fig. 2a and b. The inset in Fig. 2a reveals that the WF of two types of TiO2 are 5.44 and 5.36 eV with the equation of WF=21.2-Ecutoff. It has been reported that the reduced WF can enhance the charge transport ability of TiO2 [41] Fig. 2b shows the difference between Ef and the VB maximum, the value of which for the pristine TiO2 and Co-doped TiO2 are 2.19 and 2.13 eV. Meanwhile, the VB maximum positions can be calculated to be 7.63 and 7.49 eV. In the absorption spectra of TiO2 of Fig. 2c, the absorption edge of 0.75 mol% Co-doped TiO2 has a slight blue shift. Therefore, the optical absorption bandgap increases from 3.51 eV to 3.53 eV. It should be noted that TiO2, a type of indirect transition semiconductor, has a narrower semiconductor bandgap (Eg) than its own optical absorption bandgap according to the previous research [42]. The Eg of the TiO2 film is 3.2 (±0.06) eV and the position of the CB minimum can be obtained from 4.43 eV to 4.29 eV. The rise of the CB position allows the band structure of Co-doped TiO2 better matched with that of the perovskite layer, which is beneficial to reduce
energy loss and promote charge extraction, thus improving Voc and Jsc of the device [43-45].
Fig. 3. (a) Chemical structure of cobalt acetylacetonate (Ϫ) and device structure of PSCs, (b) cross-sectional SEM image of PSCs and (c) energy levels diagram of the device.
Fig. 3a displays the chemical structure of cobalt acetylacetonate (Ϫ) and the device structure of the typical planar PSCs based on Co-doped TiO2. The cross-sectional scanning electron microscope (SEM) image is illustrated in Fig. 3b, in which the thickness of the Co-doped TiO2 and perovskite film are about 30 and 220 nm. Fig. 3c shows the energy level diagram of all materials used in the research based on the UPS analysis above. There is a positive shift in band structure of TiO2 layer, and the CB and VB of other layers also match well at the interface for easy transport of charge carriers.
Fig. 4. (a) J-V characteristics of the devices based on varied TiO2 films. (b) Histogram of efficiencies among 16 devices. (c) IPCE spectrum of the device based on the 0 mol% and 0.75 mol% Co-doped TiO2 films. (d) J-V characteristics of the PSCs measured under reverse and forward scanning direction. Steady-state photocurrent and PCE output of the devices based on the (e) 0 mol% Co-doped TiO2 and (f) 0.75 mol% Codoped TiO2 films measured under one sun (100 mW/cm2) illumination for 55 s. Table 1. The photovoltaic parameters of PSCs with different concentrations of Co-doped TiO2 films Concentration (mol%)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
0
1.08
21.54
72.46
16.86
0.1
1.08
21.85
72.96
17.22
0.25
1.09
22.22
73.57
17.82
0.5
1.09
22.68
74.01
18.30
0.75
1.11
22.86
75.49
19.16
1
1.10
22.49
73.23
18.12
2.5
1.09
20.80
70.21
15.92
Table 2. The photovoltaic parameters of PSCs based on 0 mol% and 0.75 mol% Co-doped TiO2 measured under different scanning directions. Concentration (mol%)
Scanning direction
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
0
reverse
1.09
21.10
71.97
16.55
forward
1.05
18.71
63.96
12.57
reverse
1.10
22.46
74.95
18.52
forward
1.07
20.66
70.68
15.62
0.75
Fig. 4a is the current density-voltage (J-V) characteristics of the devices fabricated with compact TiO2 layer
doped by different concentrations of Co ions (0, 0.1, 0.25, 0.5, 0.75, 1 and 2.5 mol%). The detailed photovoltaic parameters including Voc, Jsc, FF and PCE are summarized in Table 1. The device based on 0 mol% Co-doped TiO2 indicates a PCE of 16.86%, companying with a Voc of 1.08 V, a Jsc of 21.54 mA/cm2, and a FF of 72.46%. The optimal performance of 19.16% is achieved based on 0.75 mol% Co-doped TiO2, obtaining a Voc of 1.11 V, a Jsc of 22.86 mA/cm2, and a FF of 75.49%. A significant enhancement in Jsc and FF is obtained, simultaneously realizing a slight increment in Voc. The histogram of the PCE of 16 individual cells using the pristine TiO2 and 0.75 mol% Co-doped TiO2 are given in Fig. 4b. The PCE of the pristine TiO2 based-devices is distributed in the range of 14-17%, while the PCE of the 0.75 mol% Co-doped TiO2based devices is in the range of 17.5-19.5%. It appears that the PCE of devices exhibits narrower distribution after Co doping, demonstrating a higher repeatability of Co-doped devices compared to the non-doped devices. The incident photon-electron conversion efficiency (IPCE) spectrum is displayed in Fig. 4c to convey the enhancement of Jsc. Within the wavelength range from 350 nm to 750 nm, the device based on the 0.75 mol% Co-doped TiO2 shows an obvious increased IPCE. The hysteresis of the devices was studied and shown in Fig. 4d. The PSC based on 0.75 mol% Co-doped TiO2 exhibits decreased J-V hysteresis and the corresponding parameters are summarized in Table 2. The hysteresis index (HI) was calculated using the following equation: ൌ
ா ିாూ
(1),
ா
where PCERS and PCEFS represent PCE for the reverse scan and forward scan, respectively. The 0.75 mol% Co-doped TiO2-based device shows a smaller HI value of 15.66% compared to 24.05% for that of the pristine TiO2-based device, which can be attributed to the reduced defects and traps after Co (Ϫ) doping. The steadystate photocurrent and PCE measured at the maximum power point of the device without optimization and
optimal device is given in Fig. 4e and f. There is an obvious attenuation on the current density and PCE for the device with the pristine TiO2, and a stabilized efficiency of 16.00% with a photocurrent density of 18.60 mA/cm2 is obtained at a voltage of 0.86 V. By contrast, the 0.75 mol% Co-doped TiO2-based device yields a stabilized efficiency of 18.81% with a photocurrent density of 20.90 mA/cm2 at a voltage of 0.90 V, following a smaller degree of attenuation.
Fig. 5. (a) Dark J-V characteristics of PSCs based on the 0 mol% and 0.75 mol% Co-doped TiO2 films. (b) Dependence of Voc on the light intensity of PSCs with the 0 mol% and 0.75 mol% Co-doped TiO2. (c) CurrentVoltage curves of the device with the structure: ITO/compact TiO2 layer/Ag with the 0 mol% and 0.75 mol% Co (III) doping.
The dark J-V characteristics of device was measured and shown in Fig. 5a, the optimal PSCs exhibits a smaller leakage current in a working bias region (about 0-1.1 V), proving that carrier transport is improved and recombination loss is reduced in the device. The light intensity dependence of Voc was investigated according to the equation [46] ்
Ɂܸ୭ୡ ൌ ݊ ቀ ቁ ሺܫሻ ܿݐ݊ܽݐݏ݊
(2),
in which n is the ideality factor, k is the Boltzmann constant, T is absolute temperature, e is the elementary charge, and I is the light intensity. The Voc-logarithmic light intensity and the corresponding fitting results are
presented in Fig. 5b. The slope values of 1.69kT/e and 1.29kT/e are obtained for the pristine TiO2-based device and 0.75 mol% Co-doped TiO2-based device, meaning that trap-assisted carrier recombination is suppressed after Co doping [37]. In order to probe the difference of electrical conductivity of TiO2 ETL with and without doping. The device with the configuration of ITO/compact TiO2 layer/Ag was fabricated. The current-voltage linear curves in Fig. 5c follow the formula [47]
ܫൌ ߪ ቀ ቁ ܸ ௗ
(3), where σ0, A and d
stand for the sample electrical conductivity, area (0.044 cm2), and thickness (30 nm). The σ0 of the 0.75 mol% Co-doped TiO2 sample can be calculated to be 1.3410-4 ms/cm, higher than the non-doped TiO2 sample (8.7910-5 ms/cm), which may result from upward shift of Ef for Co-doped TiO2. The enhanced electrical conductivity is conducive to transport and extract electrons.
Fig. 6. Contact angle measurement of perovskite precursor in DMF/DMSO (V:V=9:1) on the (a) 0 mol%, (b)
0.5 mol%, (c) 0.75 mol% and (d) 2.5 mol% Co-doped TiO2 films. AFM images of the (e) 0 mol%, (f) 0.5 mol%, (g) 0.75 mol% and (h) 2.5 mol% Co-doped TiO2 films. AFM images of the perovskite films deposited on the (i) 0 mol%, (j) 0.5 mol%, (k) 0.75 mol% and (l) 2.5 mol% Co-doped TiO2 films.
Whereas the physical properties of the ETL and perovskite layer have a direct impact on the performance of perovskite devices, the contact angle and the atomic force microscopy (AFM) measurements were applied. Here, samples with doping concentrations of 0 mol%, 0.5 mol%, 0.75 mol% and 2.5 mol% are selected for a better illustration. Fig. 6a-d display the contact angle of perovskite precursor in DMF/DMSO (V:V=9:1) on these TiO2 films. There is no significant difference in the contact angle of these four TiO2 films, which indicates that the introduction of Co ions does not change the wettability of TiO2 films to a large extent. AFM images presented in Fig. 6e-l reflect the film qualities of the TiO2 and corresponding perovskite layers changes with the increase in the doping amount of Co ions. Compared with the pristine TiO2 film, the rootmean-square roughness (RMS) of 0.75 mol% Co-doped TiO2 film is reduced from 0.64 nm to 0.54 nm, while 2.5 mol% Co-doped TiO2 film exhibits a rougher surface with more pinholes (Fig. 6e-h). The reason for this change is that excess Co ions will occupy substitution and interstitial sites in the TiO2 lattice, resulting in lattice distortion and film instability. The ETL film with a smoother surface would act as a better foundation for the crystallization of perovskites. Consequently, the perovskite films show the uniformity of grains distribution and reduction of pinholes at a low doping degree, accompanying with a decreased RMS from 9.29 nm to 6.91 nm (Fig. 6i-k). Fig. 6l reveals a dramatic increase in the amount of pinholes and surface roughness of the perovskite film owing to the influence of the 2.5 mol% Co-doped TiO2 film. The pinholes in the perovskite film tend to serve as the contact path between the ETL material and hole transport layer
(HTL) material and induce current leakage, yielding a decreased Voc and Jsc [48]. Note that a dense, smooth perovskite film is more conducive to form a better contact with the HTL and achieve high PCE.
Fig. 7. (a) Transmittance spectra of the 0, 0.5, 0.75 and 2.5 mol% Co-doped TiO2 films. (b) Absorption spectra of the perovskite films deposited on the 0, 0.5, 0.75 and 2.5 mol% Co-doped TiO2 films. (c) XRD patterns of the perovskite films deposited on the 0, 0.5, 0.75 and 2.5 mol% Co-doped TiO2 films. (d) Variation of diffraction peak intensity of (110), (220) and (310) planes in the XRD patterns. Top-view SEM images of the perovskite films deposited on the (e) 0 mol% and (f) 0.75 mol% Co-doped TiO2 films.
To further investigate the quality of the perovskite film, the transmittance spectra, absorption spectra, Xray diffraction (XRD) and top-view SEM measurements were performed. Fig. 7a displays the transmittance spectra for TiO2 and Co-doped TiO2 coated on the glass/ITO substrates. Even if the doping concentration of Co increases to 2.5 mol%, the shape of the spectra remains basically the same, demonstrating the optical absorption of perovskites will not be affected by the transmittance of ETL. Fig. 7b indicates a weakened
absorption in the range from 450 nm to 700 nm for 2.5 mol% Co-doped TiO2/perovskite compared with the other three samples. This could be explained by the decline in crystallinity of the perovskite film [49]. The XRD characterization focused on the crystallinity of the perovskite films is shown in Fig. 7c. The CH3NH3PbI3 films covered on the pristine and doped TiO2 exhibits strong diffraction peaks at 14.12°, 28.44°and 31.85°, which are assigned to the (110), (220) and (310) crystal planes of tetragonal perovskite, respectively. These peak positions agree well with the XRD patterns of CH3NH3PbI3 perovskites reported previously [50]. The variation of detailed value of the diffraction peaks intensity is depicted in Fig. 7d. As expected, the perovskite sample with 0.75 mol% Co-doped TiO2 has the highest peak intensity, while the peak intensity drop to the lowest with the doping concentration of 2.5 mol%. It suggests the difference in the crystallinity of the films. Fig. 7e and f shows the top-view SEM images of perovskite film on 0 mol% and 0.75 mol% Co-doped TiO2. Consistent with the AFM images, the grain size of perovskite films is approximately 50-300 nm. The pristine TiO2 shows perovskite film with many pinholes and aggregations of small grain compared to the uniform perovskite film coated on the 0.75 mol% Co-doped TiO2 having less pinholes and boundaries. Based on the above discussion, it can be concluded that the perovskite film quality is enhanced after doping appropriate Co into TiO2 ETL, helping to improve Jsc and Voc.
Fig. 8. Steady-state PL spectra of the TiO2 film doped with 0, 0.5, 0.75 and 2.5 mol% Co at an excitation wavelength of (a) 310 nm and (b) 365 nm. The inset is the partial magnification of the spectra. (c) Steadystate PL spectra of the perovskite films deposited on different TiO2 films with incident light from perovskite side (left) and from the ITO side (right). (d) Oxygen defects give rise to Ti3+ defects that form electronic traps. (e) Co substitution at the Ti3+ sites passivates these defects. (f) Recombination mechanism due to the deep traps levels and (g) energy diagram after traps have been passivated.
The charge transport and carrier recombination in the devices can be analyzed more directly by using steady-state photoluminescence (PL). The PL measurements with an excitation wavelength of 310 nm and 365 nm were carried out and shown in Fig. 8a and b. As can be seen from Fig. 8a, the PL peaks locate at 360 nm for all TiO2 films, which originates from the emission of band-to-band transition corresponding to the optical absorption bandgap energy of TiO2. Since the optical light absorption of TiO2 changes little after doping (Fig. 2c), the intensive PL quenching can be attributed to the more effective charge extraction. The luminescence of TiO2 on Eg excitation helps to further explain the effects of Co ions. The PL spectrum is similar with four visible PL peaks at 391 nm (3.17 eV), 458 nm (2.71 eV), 476 nm (2.61 eV) and 562 nm (2.21 eV). Obviously, the first one coincides with the emission of transition from the VB minimum to the CB maximum. The peak at 458 nm is attributed to surrface defects of TiO2 [51]. Defects caused by oxygen vacancies commonly induce some energy levels (about 0.7-1 eV) below the CB [52, 53]. Naturally, the other two peaks mainly result from these oxygen vacancies. Oxygen vacancies are able to trap zero, one or two electrons, increasing the recombination probability of transported electrons [54]. A phenomenon that the Co ions at low doping degree remarkably quenches the PL intensity of the defects levels is observed. In contrast,
the PL intensity of 2.5 mol% Co-doped TiO2 film increases and exceeds that of the pristine TiO2 at both excitation wavelength. Consistent with the analysis about the AFM images, excessive Co ions could occupy substitution sites in the TiO2 lattice and cause charge imbalance [49]. As a result, new oxygen vacancies are generated. Moreover, the PL spectra for the perovskite films fabricated on the pristine TiO2 and Co-doped TiO2 were given in Fig. 8c. Considering that the penetration depth of excitation light is less than the thickness of the perovskite film [55], we measured from perovskite side and indium tin oxide (ITO) side. The samples under different measurement conditions all have an emission peaks at 769 nm, PL intensity of which presents a trend of decreasing first and then increasing with the grow in number of Co molar ratio. The PL analysis above clearly verified that moderate Co ions reduce oxygen vacancy defects and passivate the electron traps in the TiO2 layer. It can also be inferred that charge transport from perovskites to ITO is accelerated and carrier recombination at interface is suppressed [56]. Hence, the Jsc and FF of PSCs is improved. The schematic diagram depicting the doping mechanism of Co element and the process of traps passivation in anatase TiO2 is also presented in Fig. 8. When an oxygen atom is detached from the crystal lattice, a point defect and unstable Ti with dangling bonds (Ti3+) appear at the corresponding position. The Ti3+ acts as an electron trap to form Ti4++e- (Fig. 8d) [57]. These trap states are more likely to form at subsurface and bulk site rather than surface of TiO2 [58]. Consequently, the deep energy levels below the CB of TiO2 are introduced, serving as a cascading path for the carrier recombination (Fig. 8f). Co3+ has a similar ionic radius as Ti4+ (0.61 nm) and Ti3+ (0.64 nm) [59], the substitution of the Ti3+ adjacent to an oxygen vacancy with Co3+ is reasonable, which is also the best description of our research results. Therefore, the Co3+ substitution effectively remove oxygen defects within the lattice and on the subsurface. As for the Co2+ present in the TiO2 film,we make the following hypothesis based on our study and previous literatures: Due to the charge
transfer effect between Co and Ti, the oxidation state of Ti shifts from Ti3+ to Ti4+. Co2+ are located near the oxygen vacancies and form Co-oxygen vacancy-Ti complex, which plays a role in reducing unstable states associated with Ti3+ (Fig. 8e) [60]. Fig. 8g shows the elimination of the trap levels, so that electrons and holes can be transported to both ends without hindrance, leading to an enhanced photovoltaic performance. Noticeably, Co substitution is likely to decrease the sensitivity of the TiO2 to the presence of oxygen, which is beneficial for the device stability [57].
3. Conclusions
In summary, we have successfully fabricated Co-doped compact TiO2 layer with various doping levels and applied to planar heterojunction PSCs as ETL. It is found that optimized Co (Ϫ) doping leads to a reduced number density of oxygen defects, a positive shift in Ef and better film quality for TiO2 layer. Meanwhile, the quality of CH3NH3PbI3 perovskite film is improved, which has been proved in the research. An accelerated charge transport as well as suppressed charge recombination in PSCs is realized. A low level of Co (Ϫ) doping (0.75mol %) in TiO2 contributes to the increased PCE from 16.91% to 19.06%. Moreover, excessive Co ions have a negative impact on the devices, especially in inducing the emergence of unknown oxygen vacancies and causing lattice distortion. It is believed that this work further explores the potential of planar heterojunction PSCs and provides a feasible method to reach high efficiency of solar cells.
4. Experimental Section
4.1 Materials
The cobalt acetylacetonate (Ϫ) (98%) was purchased from aladdin. Lead iodide (PbI2, >99.99%) ,
methylammonium iodide (MAI, ≥99.5%), 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD, ≥99.8%), bis(trifluoromethane) sulfonamide lithium (Li-TFSI, >99%) and 4-tertbutylpyridine (TBP, >96%) were obtained from Xi’an P-OLED Corp. (China). Acetonitrile (ACN, 99.8%) was purchased from Sigma-Aldrich. N,N-dimethylformamide (DMF, 99.9%), dimethyl sulfoxide (DMSO, 99%) and chlorobenzene (CB, 99%) were obtained from J&K Scientific. All of the purchased materials were used without any further purification.
4.2 Synthesis of TiO2 Precursor Solution
The TiO2 precursor solution was prepared by the sol-gel method.25 For the Co-doped TiO2, the cobalt acetylacetonate (Ϫ) was dissolved in TiO2 precursor solution and stirred until clarified, the molar ratio of which to Ti atoms is 0%, 0.1%, 0.25%, 0.5%, 0.75%, 1% and 2.5%.
4.3 Device Fabrication
The ITO-coated glass substrates were cleaned by ultrasonic treatment in acetone, ethanol, and deionized water for 15 min in sequence. Then they were dried under nitrogen flow. The pristine TiO2 and different concentrations of Co-doped TiO2 precursorsolution was spin-coated onto the ITO substrate at 800 rpm for 3 s and 3000 rpm for 20 s in air. Subsequently, the substrates were annealed in a muffle furnace at 450 °C for 2 h. After cooling down to room temperature, the perovskite layer was prepared by spin coating a solution consisting of 470 mg PbI2 and 162 mg MAI in a solvent of DMF/DMSO (V:V=9:1) at 4000 rpm for 20 s, 400 μL ether was dripped in this process at the 5 s before the end of spin-coating step. Then, they were annealed at 100 °C for 10 min on a hotplate, forming a brownish black perovskite film. The hole transport
layer material was prepared by dissolving 72.3 mg of spiro-OMeTAD in 1 mL of CB, a solution of 17.6 μL of Li-TFSI solution (520 mg/mL in ACN) and 28.8 μL of TBP were added in the spiro-OMeTAD solution. The spiro-OMeTAD solution was spin coated onto the perovskite layer at 4000 rpm for 30 s.Finally, 100 nm-thick of Ag electrode was deposited on top of the HTL in a vacuum condition. The devices were completed and the area is 0.044 cm2.
4.4 Device Characterization
The XPS and UPS measurements were performed on a photoelectron spectrometer. The cross-sectional and Top-view SEM images were characterized by a JEOL JSM-7500F field-emission SEM. The J-V characteristics were measured in an illumination condition under AM 1.5G solar illuminations and dark condition using a Keithley 2601 source meter. The IPCE spectra of PSCs were conducted by Crowntech QTest Station 1000 AD. The contact angle of perovskite precursor solution on the TiO2 films were evaluated using FTA 200 First Ten Angstroms dynamic contact angle analyzer. The AFM images of the TiO2 ETLs and perovskite films were obtained with an ICON-PT in tapping mode. The transmittance spectra and UV-Vis absorption spectra were performed with UV 1700 photometer, Shimadzu. The XRD patterns of the perovskite films were recorded on a Shimadzu XRD-6000 diffractometer. The PL spectrum was measured using a Shimadzu RF 5301 fluorescence spectrophotometer at room temperature.
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected] and [email protected]
Notes
The authors declare no competing financial interest
ACKNOWLEDGMENTS The authors are grateful to National Natural Science Foundation of China (61875072), the Special Project of the Province-University Co-constructing Program of Jilin Province (SXGJXX2017-3), the Science and Technology Innovation Leading Talent and Team Project of Jilin Province (20170519010JH), the National Postdoctoral Program for Innovative Talents (BX20190135), and International Cooperation and Exchange Project of Jilin Province (20170414002GH, 20180414001GH) for the support to the work.
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Statement The authors declare that there is no conflict of interest.
The Co doped ETL is simplistic, low cost, and suitable for mass production. The Co doped ETL could reduce the interfacial defects and improve contact. Doped ETL can enhance the electron extraction by decreasing the injection barrier.
Author contributions Wenbin Guo, Chunyu Liu and Guanhua Ren conceived the idea and designed the experiments. Guanhua Ren carried out the device fabrication, optical and electrical characterizations. Guanhua Ren, Zhuowei Li, Wei Wu and Shuo Han did SEM and AFM measurements. All the authors analyzed and interpreted the data and prepared the manuscript.