Performance enhancement of perovskite solar cells by doping TiO2 blocking layer with group VB elements

Accepted Manuscript Performance enhancement of perovskite solar cells by doping TiO2 blocking layer with group VB elements J. Song, S.P. Li, Y.L. Zhao, J. Yuan, Y. Zhu, Y. Fang, L. Zhu, X.Q. Gu, Y.H. Qiang PII:

S0925-8388(16)33215-7

DOI:

10.1016/j.jallcom.2016.10.106

Reference:

JALCOM 39273

To appear in:

Journal of Alloys and Compounds

Received Date: 25 July 2016 Revised Date:

1 October 2016

Accepted Date: 12 October 2016

Please cite this article as: J. Song, S.P. Li, Y.L. Zhao, J. Yuan, Y. Zhu, Y. Fang, L. Zhu, X.Q. Gu, Y.H. Qiang, Performance enhancement of perovskite solar cells by doping TiO2 blocking layer with group VB elements, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.106. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT Performance Enhancement of Perovskite Solar Cells by Doping TiO2 Blocking Layer with Group VB Elements J. Song, S. P. Li, Y. L. Zhao*, J. Yuan, Y. Zhu, Y. Fang, L. Zhu, X. Q. Gu, Y. H. Qiang* School of Materials Science and Engineering, China University of Mining and Technology,

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Xuzhou 221116, China

Abstract

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Hole blocking layer with pinhole-free and excellent electron conductivity is critical in high-efficient perovskite solar cells. In this paper, we investigated thoroughly the effect of tantalum (Ta) or niobium (Nb) dopant in TiO2 compact layer. Both Ta and Nb dopants could enhance the electron conductivity of TiO2 compact film without declining their light transmittance property. The PSCs assembled with Ta or Nb-doped TiO2 blocking layer exhibited an evident promotion in short-circuit current density, and the best-performing PSCs had conversion efficiency enhancement from 13.66% (pure TiO2) to 14.41% (Ta-doped TiO2) and 14.29% (Nb-doped TiO2). Moreover, the reproducibility of PSCs was preferable with 3% Ta and 3% Nb-doped TiO2 blocking layer. Besides, the increased conductivity of doped TiO2 compact layer efficiently suppressed the J-V hysteresis of PSCs under different scan directions. PL and EIS results further revealed that the doped-TiO2 compact layer could accelerate electron transfer rate and decrease the recombination probability at TiO2/perovskite interface.

* Corresponding authors

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Key words: Perovskite solar cells; TiO2 blocking layer; Tantalum /Niobium doping

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E-mail addresses: [email protected] (Y. L. Zhao), [email protected] (Y. H. Qiang)

ACCEPTED MANUSCRIPT 1. Introduction

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Perovskite solar cell has emerged as a research hotspot since 2009 as its amazing photovoltaic performance development and low-cost fabrication procedure [1-7]. In a very short period of time, the power conversion efficiency of PSCs has achieved to more than 20% [8], comparable to amorphous Si, CIGS, and other thin film solar cells which have been researched for decades. Such unprecedentedly high photovoltaic performance mainly originate from the excellent intrinsic optical and electrical property of organic-inorganic hybrid lead halide perovskite materials, such as suitable band gap [9, 10], high light absorption coefficient [11], low exciton binding energies [12], and long-range ambipolar charge transport properties [13, 14]. After illuminated with photons with energy higher than the bandgap of perovskite materials, electron-hole pairs are generated which subsequently separate at electron transport layer (ETL)/ perovskite and hole transport layer (HTL)/perovskite interfaces [15]. A dense ETL is indispensable for most configurations of high-efficient PSCs, such as mesoscopic [2, 4, 16], planar [17-19], and meso-superstructured PSCs [3]. The compact ETL layer is commonly named as hole-blocking layer which plays an essential role in minimizing recombination. In a poor compact ETL, the HTL material will permeate through the scaffold layer and even contact with the conductive substrate (for example, FTO). A heavy recombination of electrons, transporting from perovskite layer, and holes which generate in perovskite layer and spread by HTL materials will be inevitable [20]. In general, the role of the hole blocking layer is to effectively prevent direct electrical contact between conductive substrate and the hole transporting material (HTM), and transport the photo-electrons separated at ETL/perovskite interface [21, 22]. Therefore, an ideal hole blocking layer is expected to be ultrathin and pinhole-free which exhibits appropriate conduction band and superior electron conductivity. In recent years, some researchers have attempted to develop efficient techniques to fabricated high quality hole blocking layers, such as spin coating [2], spray pyrolysis [4], atomic layer deposition [23], electrodeposition [24], quantum dots-assisted method [25], thermal oxidation [26], etc. And now, a series of pinhole-free compact blocking layers have been successfully prepared. As a typical material using as hole blocking layer, TiO2 is widely investigated. This material has advantages in stability, suitable conduction band, and intrinsic n type semiconductor. However, TiO2 is vulnerable in electron conductivity as its low carrier density. In fact, mixing TiO2 with some functional components or doping TiO2 with other elements are effectively strategy in modifying electronic property of this material. By using the high charge mobility of graphene, graphene nanoflakes modified TiO2 hole blocking layer provided superior charge-collection capability and improved the photovoltaic performance of PSCs [27]. Yttrium-doped TiO2 compact layer had been used in interface engineering for planar perovskite solar cells, and obtained a conversion efficiency of 19.3% [6]. Niobium dopant could elevate carrier density and electron conductivity of TiO2 blocking layer which facilitated photo-generated electron injection and charge extraction from perovskite [28]. Apart from planar structured perovskite solar cells, the mesoscopic structured PSCs could also be improved by controlling the hole blocking layer, although perovskite and hole blocking layer do not directly contact. Mg-doped TiO2 compact layer could improve the photovoltaic performance of PSCs with scaffold layer by the modification of optical transmission, hole-blocking effect and energy band of TiO2 film [29]. In addition, PSCs assembled by Nb-doped TiO2 compact layer and TiO2 mesoporous film yielded a PCE of 10.26%, which was higher than that of devices using pure TiO2 compact

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layer [30]. Tantalum and niobium, elements in Group VB, have some similar properties as dopants in semiconductors. Additionally, Ta-doped TiO2 compact film has not been employed as blocking layer for perovskite solar cells. In this work, we fabricated a series of different contents of Ta and Nb-doped TiO2 compact layer by spray pyrolysis method. We found that both Ta and Nb could enhance the performance of perovskite solar cell, especially in current density and fill factors. The doping process could facilitate the charge transfer at ETL/perovskite interface.

2. Experimental Section

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2.1 TiO2 compact layer preparation After cleaning and etching of F-doped SnO2 (FTO, 15 Ω per square, Nippon Sheet Glass) substrate, a dense TiO2 layer was deposited by spray pyrolysis at 450 oC using an automatic moving equipment (OPV-Tech). The spray solution was composed of 20 mmol/L titanium diisopropoxide bis(acetylacetonate) (75% in 2-propanol, J&K) in 2-propanol. For Ta-doped TiO2 dense layer, we added a certain amount of TaCl5 (99.9%, J&K) in the spray solution with the atomic ratio of Ta/Ti=1%, 3%, and 5%. Nb-doped TiO2 dense layers were prepared by the similar process except that the niobium source was NbCl5 (99.9%, J&K). The TiO2 films with or without doping were further calcined at 450 oC for 1 hour in a muffle furnace.

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2.2 Solar cell fabrication A mesoporous TiO2 (mp-TiO2) layer was fabricated by spin-coating a dilute TiO2 paste (18NRT, Dyesol, 1:5.5 to ethanol by weight) on the prepared dense TiO2/FTO substrate in 5000 rpm for 30 seconds and then annealed at 500 °C for 1 h in a muffle furnace. PbI2(DMSO) powder was synthesized according to an early report [8]. A clear 0.7 g/mL PbI2(DMSO) solution in N,N-dimethylformamide (DMF) was prepared at 70 oC under stirring in glove box. The mp-TiO2 film was fully covered by the filtered solution and then spin-coated at 3000 rpm for 30 s, and a transparent orange-yellow film was obtained. Then 200 µL CH3NH3I solution in 2-propanol (15 mg/mL) was dripped on the transparent PbI2(DMSO) film, and the film changed to dark brown gradually. Two minutes later, the film was spin-coated at 5000 rpm for 30 s, and then dried on a hot plate at 150 oC for 20 min. After cooling to room temperature, a hole transport material solution was spin-coated on the prepared perovskite film at 3000 rpm for 30 s. Then the film was placed in a box filled with dry and clean air over night. The HTM solution was prepared by dissolving 72.3 mg (2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobi-fluorene) (spiro-MeOTAD), 28.8µL 4-tert-butylpyridine (TBP), 17.5 µL of bis(trifluoromethylsulphonyl)imide (LiTFSI) in acetonitrile (520 mg/mL) in 1 mL chlorobenzene. Finally, 80nm of silver was deposited by thermal evaporation on HTM layer to form a back contact. 2.3 Characterization The X-ray photoelectron spectra (XPS) of the prepared TiO2 compact films with or without doping were measured by X-ray Photoelectron Spectrometer (ESCALAB 250Xi, Thermo Fisher). The microstructure of TiO2 dense layer and perovskite film were observed by a field-emission scanning electron microscope (FEI, Sirion200). The light transmittance of TiO2 films were

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3. Results and discussion

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characterized by a UV-Vis spectrophotometer (CARY 300 Conc). The I-V characteristics of FTO/TiO2 films/Ag devices were tested using electrochemical work station (IM6ex, Zahner) from 1.0 V to -1.0 V at the scan rate of 200 mV/s. Photocurrent–voltage (J–V) characteristic curves of PSCs were performed by an electrochemical workstation (Keithley, 2420 Source Meter) under solar illumination (100 mW·cm-2, Oriel Sol 3A, Newport) which was calibrated by a standard silicon solar cell (Oriel Instrument). The devices were measured between 0V and 1.2 V under reverse or forward scan with the step voltage of 10 mV and the delay time 40 ms, respectively. A non-reflective metal aperture of 0.1 cm2 was used to avoid light scattering through the sides of the cell and defined the active area of the device. The incident photon to current conversion efficiency (IPCE) was measured using a power source (Newport 300W Xenon lamp, 66902) with a monochromator (Newport Cornerstone 260) and a power meter (Newport 2936-C). Steady-state photoluminescence (PL) spectra were measured by a fluorescence spectrophotometer (Eclipse, Cary). Electrochemical impedance spectroscopy (EIS) was also recorded using IM6ex electrochemical workstation under dark. The frequency range was from 100 kHz to 100 mHz with an AC modulation signal of 10 mV and bias DC voltage of 0.60 V.

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As the tiny content of Ta and Nb in TiO2 film, XPS measurement was applied to demonstrate the presence and status of the dopants (Fig. 1). The XPS spectra clearly showed the Ti4+ states with a binding energy of 464.5 eV for the Ti 2p1/2 peak and 458.7 eV for the Ti 2p 3/2 peak in pure TiO2 film. The peaks shifted to 464.1 eV and 458.3 eV respectively after Ta doping, while it barely moved for Nb-doped TiO2 film. Moreover, the Ta5+ and Nb5+ states has also been detected, as shown in Fig. 1b and c, which proved the successfully doping of Ta and Nb in the TiO2 film [28, 31, 32]. A compact TiO2 layer plays a critical role in perovskite solar cell. As shown in Fig. 2a, the TiO2 film presented a dense image without any pinholes. Due to the small thickness (about 50 nm, as shown in Fig. 2b) of the TiO2 film, the surface morphology basically replicated the underlying FTO morphology. Additionally, the perovskite film fabricated in this work was uniform and dense with crystal size of 200-500 nm. The cross-section SEM image of the perovskite solar cell was also presented in Fig. 2 d, and the stack architecture was composed of glass/FTO/compact TiO2 layer/ mesoporous TiO2 and perovskite layer/perovskite layer/spiro-OMeTAD/Ag. To investigate the influence of tantalum and niobium dopants on electrical properties of TiO2 film, devices composed of FTO/TiO2 films (with or without doping)/Ag were fabricated. Current-voltage (I-V) linear scanning measurements were carried out on the device, and the patterns were shown in Fig. 3. The slope of the line is in direct proportion to the conductivity of TiO2 film, and the large slope value indicates high film conductivity. Evidently, as the introduction of pure TiO2 film between FTO and Ag, the slope decreases greatly because of the semiconductor property of TiO2. However, after doping TiO2 with Ta or Nb, the slope values turn large gradually as the increasing amount of dopants, indicating that the conductivity of TiO2 film was improved as Ta or Nb introduction. This result is reasonable as Ta and Nb could enhance the carrier density in TiO2 film [28]. Optical property of TiO2 films after doping is also important as a high transparency allows a large photon flux to reach the perovskite layer for photocurrent generation. In Fig. 3b, we could find that doping of Ta or Nb have little influence on light transmittance of TiO2 film, and all

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these films present a transmittance higher than 90%. The statistical photovoltaic performance parameters of PSC devices based on various compact layers with or without doping were shown in Fig. 4. The average values of Voc changed from 0.99 V for PSCs using pure TiO2 film to 1.01 V and 1.00 V for that using 3% Ta and 3% Nb-doped TiO2 films. The PSCs based on TiO2 dense layers with other doping amounts of Ta or Nb had nearly the same Voc values to the pure TiO2 except the 1% Ta-doped one which presented an average Voc of 0.97 V. The average FF exhibited similar trend with Voc which improved from 0.69 to 0.72 and 0.73 for 3% Ta and 3% Nb-doped TiO2 films. However, a sustained growth of Jsc happened as the doping amount increased for both Ta and Nb in TiO2 film. The values rose up to 19.01 mA cm-2 (5%Ta) and 19.10 mA cm-2 (5% Nb) from 18.55 mA cm-2 (pristine TiO2), respectively, which could be attributed to the improved film conductivity after doping. Finally, the average PCE increased from 12.59% (pure TiO2) to 13.76% (3% Ta) and 13.81% (3% Nb). We could also find from the statistical PCE values that the distributions of PSCs using 3% Ta and 3% Nb doped TiO2 compact layers were relatively concentrated than other ones. This was benefit to fabricate PSCs with good reproducibility. J-V curves of the best-performing PSC devices with different compact layer were presented in Fig. 5 a, b and the corresponding photovoltaic parameters were listed in Table 1. The PSCs based on un-doped TiO2 compact layer exhibited a Voc of 1.02 V, short-circuit current density (Jsc) of 18.76 mA cm-2, fill factor (FF) of 0.71, and conversion efficiency (η) of 13.66%. The photovoltaic performance increased dramatically as the Ta dopant is 3% with the Voc of 1.03 V, short-circuit current density (Jsc) of 19.21 mA cm-2, fill factor (FF) of 0.73, and conversion efficiency (η) of 14.41%. This performance decreased a little as the Ta content increase further to 5%, which originated mainly from the reduction of Voc. For Nb doping, the photovoltaic performance of PSC with 1% Nb-doped TiO2 compact layer increased greatly with Voc of 1.03 V, short-circuit current density (Jsc) of 19.04 mA cm-2, fill factor (FF) of 0.72, and conversion efficiency (η) of 14.10%. Interestingly, dopant contents of Nb affected the photovoltaic performance less obviously than Ta. The conversion efficiency enlarged only from 14.10% to 14.29%, although the Nb dopant contents increased by five-folds. It is apparent that the significant enhancement of PCE relied more on improvement of short-current density after doping. Such variation of Jsc should stemed from the modification of compact layer as no other components changed compared to the pristine device. The augment of electron conductivity after doping TiO2 film with Ta or Nb could be the main reason for Jsc enhancement in PSCs. J-V hysteresis under different scan direction is a common phenomenon in perovskite solar cell which may prevent the correct evaluation of their photovoltaic performance [33, 34]. Fig. 5 c showed the J-V curves of PSCs based on pure TiO2, 3% Ta-doped TiO2, and 5% Nb-doped TiO2 compact layer under reverse and forward scan. According to the specific photovoltaic parameters recorded in Table 1, the scan directions had much larger influence on PSC based on pure TiO2 compact layer than that based on Ta and Nb-doped TiO2. The PCE under forward scan were 82% of the reverse scan for device fabricated with pure TiO2 compact layer while this value was 92% and 91% after employing compact layer doped with 3% Ta and 5% Nb. This result could be attributed to the enhanced charge extraction at TiO2/perovskite interface after doping which could further reduce the charge accumulation, one of the main reason for J-V hysteresis. The incident photo-to-current conversion efficiency (IPCE) of perovskite solar cells employing TiO2 compact layer with or without doping were shown in Fig. 6. The onset of photocurrent at 800

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nm was consistent with the band gap of CH3NH3PbI3 (~1.5 eV). All perovskite solar cells presented high quantum yields from 350 nm to 750 nm. The external quantum efficiency was consistent with the photocurrent output of corresponding PSCs. We found that the quantum yields enlarged in the whole region as the dopant contents increase both for Ta and Nb, which agreed with Jsc variation in PSCs. To illustrate the charge transfer process between perovskite layer and TiO2 layer in the device, steady-state photoluminescence (PL) were conducted by employing samples with configurations of glass/compact TiO2 (with or without doping)/mesoporous TiO2/perovskite. The PL spectra were effective in exploring the recombination properties of light-excited electrons and holes in perovskite film [35, 36]. Figure 7a showed the PL spectra of the above devices with different compact layers, the emission peaks were located at 773 nm (excited at 450 nm), and the peak position of the emission was almost consistent among all of the samples. As the electron transfer potential barrier between CH3NH3PbI3 and TiO2, the corresponding PL density was strong [35]. However, the PL intensities varied and exhibited a decreasing trend when the TiO2 compact layer was doped with Ta and Nb. The devices using 3% Ta and 3% Nb dopants had the largest PL quenching, indicating a low recombination of light-excited electrons and holes at TiO2/perovskite interface. This was the main reason for photovoltaic performance enhancement in PSCs. In fact, as the small contact area between compact TiO2 and perovskite, the reduction of recombination using doped TiO2 compact film might be mainly induced by the faster electron extraction from mesoporous TiO2 layer to compact TiO2 layer compared to the pure TiO2 as the improved conductivity property. Furthermore, electrochemical impedance spectrometry (EIS) was employed to investigate the charge transfer process at perovskite/TiO2 interface with different TiO2 compact layers. Fig. 7b showed the Nyquist plots of PSCs using varied compact TiO2 layers. The EIS spectra were composed of two irregular semicircles. The small semicircle in the high frequency was attributed to the resistance (R1) of hole diffusion on the HTM/perovskite interface, while the second semicircle in the low frequency range was assigned to the recombination resistance (R2) at the TiO2/perovskite interface, a large R2 value indicated a low charge recombination [37-40]. In regard to Ta-doped TiO2 film, the semicircle in the low frequency changed a little as Ta dopant was 1%, but the semicircle enlarged evidently when the dopant increased to 3% and 5%. Differently, the semicircle changed obviously when the dopant was only 1%, and the values turned larger when the dopants were 3% and 5%. It was reasonable for PSCs based on Ta or Nb-doped TiO2 compact layer to gain an enhanced photovoltaic performance, as the reduced charge recombination at TiO2/perovskite interface.

4. Conclusion

We successfully fabricated Ta and Nb-doped TiO2 dense layers for perovskite solar cells in this work. Ta and Nb as dopants in TiO2 film performed similarly. Perovskite solar cells using 3% Ta and 3% Nb-doped TiO2 blocking layer presented the relatively high photovoltaic performance with the maximum PCE of 14.41% and 14.21%, and average PCE of 13.76% and 13.81%, compared to 13.66% (maximum) and 12.59% (average) for PSC based on pure TiO2 compact layer. The modification of charge extraction process from TiO2 scaffold layer to TiO2 dense layer after doping was the main motivation for photovoltaic performance enhancement of PSCs as the doped

ACCEPTED MANUSCRIPT TiO2 films had better electron conductivity. This work expanded the doping elements for TiO2 dense layer in perovskite solar cell which was meaningful for high-efficient perovskite devices development.

Acknowledge

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We appreciate the Fundamental Research Funds for the Central Universities (2015QNA06), the Natural Science Foundation of Jiangsu Province (BK20160262), and the China Postdoctoral Science Foundation (2016M591952).

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ACCEPTED MANUSCRIPT Figure Caption Fig. 1 XPS spectra of (a) Ti, (b) Ta, and (c) Nb in different compact thin films. Fig. 2 SEM images of (a) surface and (b) cross-section of TiO2 compact layer, (c) perovskite surface film. (d) Cross-section SEM image of the perovskite solar cell.

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Fig. 3 (a) I-V characteristics of FTO/Ag and FTO/TiO2 compact films/Ag devices. (b) light transmittance of TiO2 and doped TiO2 films using FTO glass as baseline. Fig. 4 Statistical values of (a) Voc, (b) Jsc, (c) fill factor, and (d) PCE of perovskite solar cells using different compact layers in 10 independent devices.

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Fig. 5 (a), (b) J-V curves of the best-performing perovskite solar cells with different compact layers. (c) J-V curves under different scan direction of PSCs using pure TiO2, 3% Ta, or 5% Nb-doped TiO2 compact layers.

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Fig. 6 IPCE spectra of perovskite solar cells based on compact layers of pure TiO2 and Ta/Nb doped TiO2 with different contents.

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Fig. 7 (a) Steady-state photoluminescence (PL) spectra (excitation at 450 nm) of perovskite films fabricated on different compact layers. (b) Electrochemical impedance spectra (EIS) of perovskite solar cells based on different compact layers.

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Table 1 Photovoltaic parameters of perovskite solar cells under reverse and forward scan. Jsc, Voc, FF, and η represent short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency, respectively. Rs is the series resistance calculated from J-V curves.

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Scan

Voc / V

Jsc / mA cm-2

FF

η /%

Reverse

1.02

18.76

0.71

13.66

Forward

0.99

18.61

0.61

11.15

Reverse

1.01

19.01

0.71

13.59

Reverse

1.03

19.21

0.73

14.41

Forward

1.00

19.05

0.70

13.29

1.01

19.40

0.73

14.31

1.03

19.04

0.72

14.10

1.02

19.07

Reverse

1.02

19.26

Forward

1.00

5% Ta-doped TiO2 1% Nb-doped TiO2

Reverse

3% Nb-doped TiO2

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5% Nb-doped TiO2

0.73

14.21

0.73

14.29

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3% Ta-doped TiO2

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1% Ta-doped TiO2

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TiO2

19.12

0.68

12.96

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Fig. 1

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Fig. 2

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Fig. 3

Fig. 4

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Fig. 5

Fig. 6

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Fig. 7

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ACCEPTED MANUSCRIPT Highlights 1. TiO2 dense layer was doped with tantalum or niobium in perovskite solar cells. 2. Ta or Nb-doped TiO2 blocking layer could effectively promote PSCs performance.

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3. The doping strategy could enhance charge transfer at TiO2/perovskite interface.