Interface Modification for Planar Perovskite Solar Cell Using Room

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Interface Modification for Planar Perovskite Solar Cell Using Room Temperature Deposited Nb2O5 as Electron Transportation Layer Yixin Guo, Jiahua Tao, Fuwen Shi, Xiaobo Hu, Zhigao Hu, Kezhi Zhang, Wenjuan Cheng, Shaohua Zuo, Jinchun Jiang, and Junhao Chu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00094 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Energy Materials

Interface Modification for Planar Perovskite Solar Cell Using Room Temperature Deposited Nb2O5 as Electron Transportation Layer Yixin Guoa, Jiahua Taoad , Fuwen Shiab, Xiaobo Hua, Zhigao Hua , Kezhi Zhangc, Wenjuan Chengd, Shaohua Zuoab*, Jinchun Jiangab*, Junhao Chuabe a

Department of Electronic Engineering, East China Normal University, Shanghai 200241, China b

c

Shanghai Center for Photovoltaics, Shanghai 201201, China

School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovolatic Science and Engineering, Changzhou University, Jiangsu 213164 , China

d

School of Physical and Material Science, East China Normal University, Shanghai 200241, China

e

Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083 , China

Abstract Compare with crystallized TiO2, amorphous Nb2O5 has been applied in planar perovskite solar cell as electron transportation layer due to its excellent optical transmittance, low temperature preparation process and similar Femi level with TiO2. However, the electron transfer rate is still limited by its low electron mobility and surface defect via room-temperature deposition process. Herein, a novel double buffer layer of [6,6]-phenyl-C61- butyric acid methyl ester(PCBM)/ionic liquid([EMIM]PF6) has been inserted between perovskite and Nb2O5 film. The PCBM could passive the surface of Nb2O5 and improve electron extraction ability. The insert of [EMIM]PF6 could improve the hydrophilic of PCBM and decrease the dissolution of PCBM in DMF during spin-coat perovskite precursor solution. A relatively high open voltage (over 1.09V) and conversion efficiency of 18.8% have been achieved by using a double buffer layer which is the highest PCE of Nb2O5 based perovskite solar cell to our best knowledge. The results indicate room temperature deposited Nb2O5 can be a suitable candidate for replacing crystallized TiO2 film and proposed modification strategy could facilitate the future development of interface modified layer for high efficient planar perovskite solar cell. Keywords: Perovskite solar cell; Nb2O5; Surface Passivation; Ionic liquid; PCBM *Corresponding author. E-mail addresses: [email protected] [email protected]

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1. Introduction Recently, solar cell using perovskite as light-absorber layer has obtained high power conversion efficiency (PCE) over 22%. Comparing with perovskite solar cell (PSC) with mesoscopic construction, planar PSC not only simplifies the construction of solar cell device, but also decreases coat of fabrication process.1,

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The existence of electron transportation layer (ETL) is important for carriers

separation in PSC and its quality is crucial to final photovotaic device performance.3 Crystallized TiO2 is the most widely used ETL material for PSC owing to its wide band gap and appropriate electron mobility.4 However, crystallized TiO2 film often required a high temperature thermal treatment (above 500°C) condition,5 which would lead to increased time and energy cost and damage the prospect of PSC for commercialization application. Recently, low temperature solution prepared ZnO,6,7 SnO2,8, 9 In2O3 10,11 and CeOx 12,13 films have been applied as ETL for PSCs and high efficiency over 15% have been achieved. However, extra thermal treatment processes were still required in those ETLs and the efficiency of PSCs based on those ETLs is still lower than the high value of recorded PSCs based on TiO2. Kelly et al.14 used room temperature coated ZnO nanoparticles as ETL and achieved an efficiency of 15.7%.Qiu et al.15 used crosslinked PCBM for replacing low temperature annealed metal oxide ETL and achieved an efficiency over 16%. Nevertheless, research of room temperature prepared ETL is still urgent, which could simplify the fabrication process and benefit the flexible device application especially on cheap plastic substrates. Compared to TiO2, Nb2O5 is found to have higher carrier mobility, excellent optical transmittance and similar conduction band edge.16 High temperature crystallized Nb2O5 (c-Nb2O5) has been successfully applied in Dye-sensitized solar cell (DSSC) as hole block layer between dye coated mesoscopic TiO2 and FTO.17

Kogo et al.18 first reported c-Nb2O5

could be used as an effective blocking layer for mesoscopic perovskite solar cell and an efficiency of 8.8% has been achieved. Ling et al.

19

found room-temperature processed amorphous Nb2O5 (a-Nb2O5)

film could replace c-Nb2O5 as ETL for planar PSC and obtained a high efficiency of over 17% with MAPbI3 as absorber layer. Feng et al.20 prepare amorphous Nb2O5 ETL by E-beam evaporation and an efficiency of 18.53% has been obtained by using MA2/6FA4/6Pb(Br1/6I5/6)3 as absorber layer. However, compared with c-Nb2O5, the lower Femi level and carrier mobility of a-Nb2O5 leads to a lower open circuit voltage which limits the further improvement of a-Nb2O5 based perovskite solar cell. A major challenge in the application of metal oxides (such as TiO2) in perovskite solar cell21, 22 was the presence of defects related to nonstoichiometric oxygen/meta ratio. Oxygen vacancy may easily form in an 2


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oxygen-loss environment especially with a room deposition process. Thus, it’s important to research the effect of oxygen vacancy in properties of room temperature deposited Nb2O5 and its relation with efficiency of PSCs based on Nb2O5 ETL . PCBM is a common organic electron transportation material for organic solar cell and inverted planar perovskite solar cell

23, 24

. The lowest unoccupied molecular orbital (LUMO) of PCBM is close

to the conduction band minimum (CBM) of perovskite which allows electron transfer between perovskite and PCBM more efficiently. 25 Recently, Kegelmann et al.26 used a PCBM modified TiO2 as ETL to fabrication planar PSC with an efficiency of 18.4%. Zhang et al.27 inserted PCBM between pervskite and ZnO interface and achieved a high efficiency of 19.07%. However, for conventional n-i-p structure PSC, precursor solution cannot be well spread on the PCBM layer due to bad hydrophilic and PCBM can be dissolved by precursor solution during spin-coating.

28

Ionic liquid (IL), in virtue of its

excellent physical properties including high electrical conductivity and carrier mobility, has been widely used in supercapacitor

29, 30

and organic solar cell.

31

Yang et al.32 used 1-benzyl-3-meth-yli

midazolium chloride as an independent ETL material for flexible PSCs and achieved an efficiency of 16.09%. Huang et al.33 used methyltrioctylammonium trifluoromethanesulfonate to passivate the interface of perovskite and PCBM achieving an enhanced device efficiency of 17.51%.Wu et al.34 used a cheap and air-stable solid solid-state ionic liquid ([EMIM]PF6) to modify TiO2 and a high efficiency of 19.59% has been achieved. As reported, EMIM]PF6 could increase the Femi level and wettability of TiO2 which improve the quality of perovskite film. In this work, we first study the effect of oxygen vacancy on photovoltaic properties of PSC with room temperature deposited Nb2O5 film as ETL. The photovoltaic properties of PSC could be improved by reducing oxygen vacancies in Nb2O5 film. We further used PCBM, IL and PCBM/IL as buffer layer to modify Nb2O5 film. The effects of inserted buffer layers on structure, morphological properties of perovskite films were investigated and discussed in detail. Planar perovskite solar cell with PCBM/Nb2O5, IL/Nb2O5 and IL/PCBM/Nb2O5 film as ETL were also fabricated. The PSC with IL/PCBM/Nb2O5 ETL obtained a relatively high champion efficiency of 18.8%, as compared to PSC with PCBM/Nb2O5 ETL (16.6%) and PSC with Nb2O5 ETL (16.2%).

2. Results and discussion

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Figure 1. (a) Optical transmittance of bare FTO and Nb2O5 films deposited under Ar or Ar+O2 environment;(b) J–V curves of PSCs with Nb2O5 ETL deposited under Ar or Ar+O2 environment;(c) XPS elemental spectral (Nb 3d) for Nb2O5 films deposited under Ar or Ar+O2 environment;(d) XPS elemental spectral (O 1s) for Nb2O5 films deposited under Ar or Ar+O2 environment

Table 1 The champion and average J–V characteristics of PSCs with Nb2O5 ETL deposited under Ar or Ar+O2 environment Sample

-2

Jsc(mAcm )

Voc(V)

FF

PCE(%)

Nb2O5 in Ar

20.3

0.91

0.75

13.9

Average

20.2±0.6

0.89±0.04

0.7±0.03

12.7±0.9

Nb2O5 in Ar+O2

21.5

0.99

0.75

16.2

Average

21.8±0.2

0.96±0.03

0.71±0.03

15±0.9

Figure 1a reveals the optical transmittance of Nb2O5 film deposited in pure Ar or Ar+O2 environment and bare FTO substrate (dash line). As seen in Figure 1a, device based on pure Ar sputtered Nb2O5 shows a low transmittance (below 70%). In contrast, a high optical transmittance 4


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(above 80%) is observed for Nb2O5 sputtered in Ar+O2 environment. The low transmittance of ETL layer could impede light into perovskite absorb layer which may decrease the light-current in solar cell device35. In order to distinguish the valence and composition, XPS was conducted for Nb2O5 films sputtered in Ar or Ar+O2. Figure 1c shows the XPS spectroscopy of the Nb 3d spectrum. It can be seen that both film show two peaks around binding energies of 207.4 and 210.2 eV, which can be indexed as Nb5+ oxidation states for Nb2O5.36 Figure 1d shows the XPS spectroscopy of O 1s spectrum. Compare with Ar sputtered Nb2O5 film, Ar+O2 sputtered Nb2O5 film represents a stronger oxygen peak at binding energy of 530.0 eV which can be indexed as O2- oxidation states. Figure 1b and Table 1 shows J–V curves and photovoltaic parameter of champion devices based on Ar or Ar+O2 sputtered Nb2O5 and statistic parameter based on 10 devices. The Ar/O2 ratio in sputtering has been optimized and fixed at 20:1, as shown in Figure S1. As seen in Figure 1b, PSC with Ar sputtered Nb2O5 ETL shows a lower short-circuit current density (Jsc) in comparison with PSC with Ar+O2 sputtered Nb2O5 ETL, which could be explained by decreased optic transmittance as revealed in Figure 1a.Except for Jsc ,PSC with Ar+O2 sputtered Nb2O5 ETL also shows an obvious improvement in average open-circuit voltage (Voc) from 0.89±0.04V to 0.96±0.03V. The existence of oxygen vacancies in Nb2O5 bulk and surface could lead to increased trap defects which may affect the transportation ability of electrons resulting decreased open-circuit voltage of PSC. Ling et al.19 also reported an improved Voc for Nb2O5 based perovskite solar cell by annealed in 500℃ for 40min to decrease of oxygen vacancies in Nb2O5. Since

the thickness of the ETL could significantly affect the device performance, the thickness related average PCEs have been summarized in Figure S2 and the optimum thickness for the Nb2O5 ETL is about 100 nm. An average PCE of 15±0.9% was achieved for PSC with Ar+O2 sputtered Nb2O5 ETL which shows significant improvement in comparison with device with Ar sputtered Nb2O5 ETL (12.7±0.9%). Although a champion PCE of 16.2% has been obtained, it is still much lower than that of TiO2 based PSC (over 20%). Thus, buffer layers (PCBM,IL and PCBM/IL) were added in sequence at interface of perovskite and Nb2O5 ETL in order to improve the efficiency of PSC based on Nb2O5 ETL.

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Figure 2.(a) Optical transmittance of Nb2O5, PCBM/Nb2O5 and IL/PCBM/Nb2O5 films;(b) X-ray

diffraction patterns of perovskite films deposited on Nb2O5/FTO, PCBM/Nb2O5/FTO and IL/PCBM/Nb2O 5/FTO substrates

Since the light-current could be affected by the optical transmittance of ETL layer in solar cell device, the optical transmittance of PCBM and IL modified Nb2O5 films were measured to decide whether the buffer layers would hinder the light absorption of perovskite layer. The optic transmittance spectrums for Nb2O5, PCBM/Nb2O5, IL/PCBM/Nb2O5 film are presented in Figure 2a. As seen in Figure 2a, compared with pure Nb2O5 film, the transmittance of PCBM and IL/PCBM modified Nb2O5 films suffer a small drop. Nevertheless, relatively high average optical transmittance (above 75%) were observed for modified Nb2O5 films which may guarantee the larger part of the solar energy transport into perovskite layer. Figure 2b reveals the XRD patterns of perovskite film deposited on PCBM or IL/PCBM modified Nb2O5 film. Two obvious peaks around 14°, and 28.44° can be observed which belong to (110), (220) reflections of CH3NH3PbIxCl3-x respectively. Since the intensity of CH3NH3PbIxCl3-x peak is too strong which may cover the information of Nb2O5 film, XRD patterns of as deposited Nb2O5 film on glass substrate has been measured and shown in Figure S3. No obvious diffraction peaks can be found which indicates its amorphous structure. Figure S4 shows AFM pictures of Nb2O5, PCBM/Nb2O5, IL/PCBM/Nb2O5 films. As shown in Figure S4a, pure Nb2O5 film shows a relatively high root-mean-square (RMS) roughness of 6.22 nm. When PCBM or IL/PCBM was modified on Nb2O5 film, roughness decreases to 2.76 nm or 2.83 nm.

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Figure 3. Contacting angle images for (a) PCBM/Nb2O5 film;(b) IL/PCBM/Nb2O5 film; Scanning

microscopic images for (c) Perovskite/PCBM/Nb2O5 film; (d) Perovskite/IL/PCBM/Nb2O5 film The surface wettability of ETL layer is a very important factor for precursor solution spreads on substrate which could affect perovskite film growth during spin-coating process. Figure 3a shows water contacting angle of PCBM modified Nb2O5 film. The large contacting angle (87.2°) indicates the poor wettability of PCBM modified Nb2O5 film which may deteriorate the quality of perovskite film deposited on it. Moreover, PCBM could dissolve in perovskite precursor solution (DMF) during spin-coating process due to its good solubility in DMF. Figure 3c depicts the surface morphology of perovskite film deposited on PCBM modified Nb2O5 film. Large pin holes are observed which indicates bad coverage of perovskite film on substrate. By contrast, IL/PCBM modified Nb2O5 film shows a moderate water contacting angle (63.4°) as seen in Figure 3b,which may facilitate the spread of precursor solution. And IL layer could decrease the dissolution of PCBM in DMF. Figure 3d depicts the surface morphology of perovskite film deposited on IL/PCBM modified Nb2O5 film. The high coverage surface with less cracks could decrease the direct contact of ETL and hole transportation layer (HTL) which may improve the separation ability of photon-generated carriers.

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Figure 4. (a) Energy level diagram for band alignment of planar PSC with IL, PCBM and Nb2O5 as ETL; (b) Cross-sectional SEM image for PSC with IL/PCB M/Nb2O5 film Figure 4a gives the energy level diagram of PSC device with IL, PCBM and Nb2O5 as electron

transportation layer. Figure S5 reveals the UPS measurement results of sputtered Nb2O5 film on FTO substrate. The work function value (-4.7eV) and valence band maximum (VBM) value (-8.2eV) of sputtered Nb2O5 film were measured and calculated by UPS measurement results. Moreover, the conduction band minimum (CBM) value (-4.35eV) was calculated using the optical band gap value (3.85eV) of Nb2O5 from the optical absorption spectra. The CBM value for perovskite, PCBM and FTO is -3.9eV, -3.95 eV and -4.6 eV according to references 26, 38. As observed in the energy level diagram, PCBM owns relatively higher CBM value by comparison with Nb2O5, which may decrease the band offset between perovskite and ETL and accelerate the electron transfer from perovskite film to the ETL. As for IL, the cation [EMIM]+ have a high affinity to the perovskite bottom surface due to its organic−inorganic hybrid characterization, while the anion PF6− tend to be attached by the oxygen vacancies in ETL34. The incorporation of IL layer could induce a spontaneous dipolar polarization between perovskite and ETL with an increase electric field to separate carriers. Planar perovskite solar

cells were fabricated using IL,PCBM and IL/PCBM modified Nb2O5 films as ETL and the cross-sectional SEM image of representative device is given in Figure 4b.

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Figure 5. (a) J–V curves for PSCs with IL/Nb2O5, PCBM/Nb2O5 and IL/PCBM/Nb2O5 films; (b)

Statistic efficiency for PSCs with IL/Nb2O5, PCBM/Nb2O5 and IL/PCBM/Nb2O5 ETL Table 2 The champion and average J–V characteristics for PSCs with IL/Nb2O5, PCBM/Nb2O5 and IL/PCBM/Nb2O5 films Sample

Jsc(mAcm-2)

Voc(V)

FF

PCE(%)

IL/Nb2O5

22.2

1.05

0.70

16.4

Average

22.4±0.4

1.03±0.02

0.67±0.02

15.8±0.5

PCBM/Nb2O5

21

1.09

0.73

16.6

Average

20.1±1.2

1.04±0.02

0.74±0.03

15.8±0.4

IL/PCBM/Nb2O5

21.9

1.09

0.79

18.8

Average

22.1±0.6

1.08±0.01

0.76±0.02

18.1±0.3

J–V curves for PSCs with IL,PCBM and IL/PCBM modified Nb2O5 films as ETL are shown in Figure 5a and related parameters are presented in Table 2. Although PSC with IL modified Nb2O5 ETL shows the highest Jsc, the lowest Voc and fill factor (FF) limit its conversion efficiency with a low average value of 15.8±0.5%.Compare with PSC with IL modified Nb2O5 ETL, PSC with PCBM modified Nb2O5 ETL exhibits enhanced Voc and FF but a significant reduction of Jsc is also observed which is not benefit for further improvement of conversion efficiency. Unlike the PSC with single buffer layer modified Nb2O5 film, PSC with double buffer layer (IL/PCBM) modified Nb2O5 film obtains the highest Voc and FF accompanying with a modest Jsc and a champion PCE of 18.8% has been obtained. Statistic PCEs from 10 solar cell devices with IL, PCBM and IL/PCBM modified Nb2O5 ETL were measured and shown in Figure 5b and Table 2 to confirm the reproducibility. Solar cell device 9



with IL/PCBM modified Nb2O5 shows an average PCE of 18.1±0.3% which is considerably higher than that of devices with PCBM (15.8±0.4%) or IL (15.8±0.5%) modified Nb2O5 ETL. Besides, PCE of PSC with buffer layer PCBM or IL/PCBM as independent ETL has been measured and shown in Figure S6 and a significant increase could be found for PSC with IL/PCBM ETL compared with PSC with PCBM ETL.

Figure 6. (a) J-V curves for PSCs with Nb2O5 and IL/PCBM modified Nb2O5 films; (b) EQE of

corresponding PSC devices; (c) Steady state photoluminescence spectral of perovskite films on Nb2O5 and IL/PCBM modified Nb2O5 films; (d) TRPL spectral of perovskite films based on Nb2O5 and IL/PCBM modified Nb2O5 films Figure 6a shows J–V curves of PSCs with Nb2O5 and IL/PCBM modified Nb2O5 films. A huge improvement of Voc for PSC with IL/PCBM modified Nb2O5 film is observed, which leads to a corresponding promotion of conversion efficiency. The possible reason behind this may be the minimized band-offset between CBM of perovskite and ETL as analyzed in Figure 6a,which could 10


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reduce the recombination of photon-generated carriers resulting increased Voc. Same improvement was observed by Zhou et al. 39 when using fullerene modified TiO2 as ETL for PSCs. It is interesting that

the introduction of IL/PCBM buffer layer not only improved Voc and FF of PSC device, but also Jsc. As mentioned above, the IL/PCBM buffer layer could decrease the optical transmittance of Nb2O5 film, therefor a reduced Jsc in PSC with IL/PCBM modified Nb2O5 films was expected. Since the Jsc of PSC device is concerned with its external quantum efficiency (EQE), IPCE for these devices were surveyed and shown in Figure 6b.From the EQE of low wavelength (less than 550nm) in Figure 6b, one can find a mild decline for PSC with IL/PCBM modified Nb2O5 film as ETL suggesting lower photon current arises in this area, which is compatible with reduced optical transmittance as uncovered in Figure 6a. Instead, in long wavelength area (550-800nm), PSC with IL/PCBM modified Nb2O5 film shows a more stable and high EQE spectral indicating a more efficient photon-to-electron conversion ability with less recombination. The calculated Jsc from EQE spectral for PSC with Nb2O5 film or IL/PCBM modified Nb2O5 film is 21.30 mAcm-2 and 21.86 mAcm-2, which is in agreement with data from Table 1and Table 2.In order to diminish the inaccuracy in J-V measurement, J-V curves for PSCs with Nb2O5, PCBM/Nb2O5 and IL/PCBM/Nb2O5 ETL were given from forward and reverse scanning direction and shown in Figure S7. As seen in Figure S7, PSC with pristine Nb2O5 ETL exhibits a large hysteresis phenomenon with lower FF and Voc from forward direction than reverse direction. The most common cause for hysteresis in planar PSC with metal oxide ETL can be attributed to defects at metal oxide surface which lower the carrier collection rate. As seen in Figure S7, interface modification by using PCBM and IL/PCBM could significantly reduce hysteresis which suggesting the carrier traps between the perovskite and ETL have been reduced. The charge transfer ability between perovskite and ETL was studied by steady-state PL of perovskite film on FTO/Nb2O5, and FTO/ Nb2O5/PCBM/IL shown in Figure 6c. A reduced fluorescence intensity is observed by adding the buffer layer IL/PCBM between Nb2O5 film and perovskite, indicating a more efficient electron extraction for double buffer layer modified Nb2O5 film than pristine Nb2O5 film. Figure 6c shows TRPL measurement results for perovskite films deposited on different ETL to judge the carrier transportation ability. A bi-exponential rate law was used to fitting TRPL decay curves for detailed analysis of recombination dynamics, as following:

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() = −

+ − +

Typically, represents the interface recombination information and reflects the information for radiative recombination process.40

Average lifetime has been calculated to analyses the whole

recombination process, as following:

= ∑ /∑ As revealed in Figure 6d, FTO/Nb2O5/perovskite film has a relative long value of average lifetime (31 ns) while a significant decreased value (6.88 ns) is observed for FTO/Nb2O5/PCBM/IL/perovskite film. The decreased lifetime suggests that electron transportation rate of perovskite to Nb2O5/PCBM/IL is faster than that of perovskite to pristine Nb2O5, which may help the process of light-induced carriers collection in devices. 3. Conclusion In conclusion, Nb2O5 films were prepared by a room temperature sputtering method and used as ETL for planer perovskite solar cell. Oxygen-assist sputtering process could decrease oxygen vacancy in Nb2O5 and improve the efficiency of PSC. The modification with the PCBM, IL and PCBM/IL interlayer were used to optimize the interface between Nb2O5 ETL and perovskite. The lower band-offset and faster electron transfer were achieved via interface engineering. A champion efficiency of 18.8% with less hysteresis was achieved resulting from the insertion of PCBM/IL double buffer layer. The results indicate the room temperature deposited Nb2O5 films with appropriate interface modification can be suitable ETL for high efficient planar PSC devices. 4. Acknowledgments This work was supported by the Major State Basic Research Development Program of China (Grant No. 2013CB922300),National Natural Science Foundation of China (Grant No. 61376129,No. 61604055 and No. 61704057),Knowledge Innovation Program of the Chinese Academy of Sciences (Y2K4401DG0) and Shanghai Pujiang Program (16PJ14-02600).

Supporting Information Experimental Section; Statistics PCEs by using different Ar/O2 ratio sputtered Nb2O5 ETL; 12


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Statistic PCEs of PSCs based on Nb2O5 films with different thickness; X-ray diffraction patterns of sputtered Nb2O5 film on glass substrate; AFM Images for Nb2O5, PCBM/Nb2O5 and IL/PCBM/Nb2O5 film ;UPS data; J-V curve of PSCs with PCBM and IL/PCBM film; hysteresis behavior are available in supporting information.

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Interface Modification for Planar Perovskite Solar Cell Using Room

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