Efficiency Enhancement of Perovskite Solar Cells through Fast ...
Efficiency Enhancement of Perovskite Solar Cells through Fast Electron Extraction: the Role of Graphene Quantum Dots Zonglong Zhu,
†‡
Jiani Ma,
‡
⊥
‡
‡
Zilong Wang, Cheng Mu, Zetan Fan,§ Lili Du, ‡
Bai, Louzhen Fan,*§ He Yan, David Lee Phillips, †
Nano Science and Technology Program,
‡
⊥
⊥
Yang
†‡
and Shihe Yang*
Department of Chemistry, The Hong
Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ⊥
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong
Kong S.A.R., P. R. China §
Department of Chemistry, Beijing Normal University, Beijing, 100875, China
S1
Supporting Information Experiment Details 1.
Materials synthesis and device fabrication
Organometal halide perovskite (CH3NH3PbI3) solution synthesis. CH3NH3I was synthesized according to a reported procedure.ref For a typical synthesis, 0.3 mol (38 mL) methylamine CH3NH2) solution (33wt% in absolute ethanol) was reacted with equimolar (40 mL) hydroiodic acid (HI) (57 wt% in water) with stirring at ice bath for about 2 h to synthesis methylammoniium iodide (CH3NH3I). Crystallization of CH3NH3I was achieved using a rotary evaporator at 60 °C for 2~3 h. The obtained CH3NH3I power with equimolar lead(II) iodide (PbI2) were dissolved in γ -butyrolactone with stirring at 60 °C for over-night to produce a 2.5 mM CH3NH3PbI3 precursor solution. Perovskite TiO2 Mesoporous Structured Solar Cell Fabrication. First, fluorine doped tin oxdie (F:SnO2) coated glass (FTO) was patterned by laser etching. The patterned FTO were cleaned by ultrasonication and rinsed in the deionized water and mixed solution ethanol and acetone (v:v ~ 1:1). An 30~40 nm thick TiO2 compact layer was then deposited on the substrates by aerosol spray pyrolysis at 50 mM titanium diisopropoxide bis(acetylacetonate) solution (75% in 2-propanol,) diluted in ethanol. The nanocrystalline TiO2 paste was deposited by screen-printed method and calcined at 500 °C for 1 h. The films then immersed in 40 mM of TiCl4 aqueous solution at 60 °C for 1 h and heat-treated at 500 °C for 30 min. The CH3NH3PbI3 solution synthesis above was then coated onto the TiO2 meso-structure by spin-coating at
S2
3,000 r.p.m. for 60 s, and dried on a hot plate at 100 °C for 20 min. The hole transport material (HTM) was deposited on the CH3NH3PbI3/TiO2 meso-structure film by spin coating at 3,000 r.p.m for 30s. The HTM solution was prepared by dissolving spiro-OMeTAD (Merck) in chlorobenzene (110 mg/1 ml), 26 ul tert-butylpyridin (TBP) solution and 35 ul Li-bis(trifluoromethanesulfonyl) imide (Li-TFSI)/acetonitrile (170 mg/1 ml). For the metal electrode, 60 nm thickness of gold was deposited on the top of the HTM by a thermal evaporation through a shadow mask to define the active area of the devices (~12.6 mm2) and to form a top anode. When the mask was applied, the defined active area was changed to 6 mm2 (1.5mm*4mm). We also comparatively tested 15 cells using the mask and without using the mask, and the resulting data have now been collected in Table S2 below. The cell was packaged by scribbling UV-glue on the top and covered a glass slides, then the cell was exposed to UV light radiation for 10 min to make the glue solidify. The device testing was carried out the glove box after packaging.. 2.
Materials characterization
The morphologies of the device sample were characterized by a field emission scanning electron microscope (FE-SEM; JEOL 6700F) operated at 5 kV. The structure analysis of graphene quantum dots were carried out using a transmission electron microscope (TEM; JEOL 2010F) operated at 200 kV. X-ray diffraction (XRD) patterns of the samples were obtained using a diffractometer (Philips, PW 1830) with Cu Ka radiation at scan rate of 4°/min under operation condition of 30 kV and 40 mA.
S3
Elemental compositions of GQDs on TiO2 mesoporous film were investigated by energy dispersive X-ray spectroscope (EDS; Oxford INCA Energy) attached to the SEM and the distribution of the GQDs on TiO2 was analyzed by the mapping technique. Conduction band and valence band. The energy levels and optical band gaps of the system relative to vacuum of each component in Figure 1B were determined by methods reported previously1,2. Cyclic voltammetry (CV) was used to characterized energy level of GQDs as described in the following references3,4. The CV was carried out by a standard three-electrode system, with a glassy carbon disk working electrode, a platinum wire for the counter electrode, Ag/AgCl for the reference electrode, 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6, Aldrich) in acetonitrile as supporting electrolyte. The GQD was dropped on the glassy carbon electrode. 3.
Solar cell characterization Fabricated photovoltaic cells were characterized by current-voltage (J-V)
characteristics and incident photon-to-current conversion efficiency (IPCE). Photocurrent and voltage were measured by a solar simulator (Oriel, 450 W Xe lamp, AM 1.5 global filter) equipped with an electrochemical workstation (Zanher, Zennium). The light source was calibrated to 1 sun (100 mW/cm2) using an optical power meter (Newport, model 1916-C) equipped with a Newport818P thermopile detector. The IPCE measurements were carried out with a Zahner Zennium CIMPS-PCS
system
established
with
S4
the
tunable
light
source
(TLS).
Absorbed-photon-to-current conversion efficiency (APCE) according to Eq. (1) to more accurately reflect the carrier extraction efficiency. % %/1 10
4.
(1)
Optical spectroscopy
The sample prepared process is the same with the cell fabrication except using a sapphire substrate instead of FTO. Optical absorption measurements were carried out on a Perkin- Elmer spectrophotometer (model Lambda 20). The steady state photoluminescence
measurements
were
carried
out
on
Micro-Raman/Photoluminescence System (Renishaw) with a laser source operated at 514.5 nm and 20 mW. 5.
Transient absorption measurements
The sample preparation process is the same with the optical test above. Femtosecond transient absorption (fs-TA) measurements were performed using a femotosecond regenerative amplified Ti: sapphire laser system (Spectra Physics, Spitfire-Pro) and an automated data acquisition system (ultrafast systems, Helios). The TA traces were collected with a fixed wavelength pump laser operating at 400 nm (3.1 eV) generated from the second harmonic of the fundamental of a Ti-Sapphire laser (800 nm, 120 fs pulses) with a regenerative amplifier, and a well synchronized, variable time delayed probe beam (white light continuum).5 The amplifier was seeded with the oscillator output. The probe pulse was generated using about 5% of the amplified 800 nm output to generate a white-light continuum (350-800 nm) in a CaF2 crystal. The probe beam was split into two before passing through the sample. One beam travels through the
S5
sample; the other was sent to the reference spectrometer monitoring the fluctuations in the probe beam intensity. Fiber optics was coupled to a multichannel spectrometer. The film samples to be tested were loaded on the quartz substrates.
S6
Fig. S1. TEM (A), high-resolution TEM (B), and photograph (C) images of the homogeneous aqueous solution (0.5 mg/mL) of the GQDs sample.
Fig. S2. (A) UV-vis absorption spectrum of the GQD aqueous solution. (B) Photoluminescence spectrum of the GQD aqueous solution at the 400 nm excitation wavelength.
Fig. S3. EDS elemental mapping images and compositional analysis of a TiO2 mesoporous film covered with an ultrathin GQD layer. Top left: SEM image. S7
Fig. S4. X-ray diffraction (XRD) patterns of GQDs/TiO2 and CH3NH3PbI3/GQDs/TiO2 samples. Note: The XRD pattern of CH3NH3PbI3 sensitized film shows diffraction peaks at 14.05o, 28.45o, 31.87o, 40.45o and 43.13o, which can be assigned to the (110), (220), (310), (224) and (314) planes of the tetragonal perovskite structure.
Fig. S5. Three dimensional scheme (A) and photograph (B) of a typical patterned batch of PV cells.
S8
Fig. S6. Device performance as a function of concentration of the GQDs aqueous solutions.
Fig. S7. The integrated photocurrent densities of the IPCE spectra. CH3NH3PbI3/GQDs/TiO2 (red line) and CH3NH3PbI3/TiO2 (black line) cells were measured under AM 1.5 G illumination at 100 mW/cm2 (solid line).
S9
Fig. S8. Transient absorption spectra of pure TiO2 mesoporous film (A) and GQD/TiO2 film (B) with excitation at 400 nm. Normalized photoabsorption (PA) kinetic traces at 650 nm for pure TiO2 mesoporous film (C) and GQD/TiO2 film (D).
S10
Table S1. Average device performance parameters extracted from the measured current-voltage characteristics. A total number of 60 devices were fabricated for each batch of testing. Errors represent the standard deviations in each batch of data.
Concentration GQD (mg/mL)
Jsc (mA/cm2)
Voc (V)
0.00
15.20
0.78
0.909
0.032
0.589
0.029
8.58
0.54
0.1
15.83
0.79
0.912
0.043
0.614
0.031
8.72
0.56
0.5
16.81
0.83
0.917
0.038
0.618
0.031
9.76
0.58
1
16.51
0.81
0.909
0.034
0.601
0.030
9.02
0.57
FF
η (%)
Table S2. Mean cell performance parameters obtained from the current-voltage curves. Note that original represents the data without using aperture, whereas mask means with aperture. Jsc (mA/cm2)
Voc (V)
FF
η (%)
Original
16.81
0.83
0.917
0.038
0.618
0.031
9.76
0.58
Mask
16.91
1.91
0.914
0.064
0.620
0.070
9.62
1.27
Table S3. Summary of the parameters from fits to the transient absorption data in Figure 4C and 4D. The fitting functions of double exponential equation.
!
"#
$
!
% " $. &
Feature
Sample
A1
τ1 (ps)
A2
τ2 (ps)
Photobleach
CH3NH3PbI3/GQDs/TiO2
0.625
90.4
0.462
2268.7
(760 nm)
CH3NH3PbI3/TiO2
0.423
260.4
0.482
2230.5
Photoabsorption
CH3NH3PbI3/GQDs/TiO2
0.382
106.8
0.313
2073.4
(530 nm)
CH3NH3PbI3/TiO2
0.509
307.1
0.362
2329.3
S11
Reference (1) Zhang, M.; Bai, L.; Shang, W.; Xie, W.; Ma, H.; Fu, Y.; Fang, D.; Sun, H.; Fan, L.; Han, M.; Liu, C.; Yang, S. J. Mater. Chem.2012, 22, 7461. (2) Abrusci, A.; Stranks, S. D.; Docampo, P.; Yip, H. L.; Jen, A. K. Y.; Snaith, H. J. Nano Letter. 2013, 13, 3124. (3) Li, Y.; Hu, Y.; Zhao, Y.; Shi, G.; Deng, L.; Hou, Y.; Qu, L. Adv. Mater. 2011, 23, 776. (4) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater. 2011, 23, 2367. (5) Ma, J.; Su, T.; Li, M-D.; Du, W.; Huang, J.; Guan, X.; Phillips, D. L. J. Am. Chem. Soc. 2012, 134, 14858.
S12
†‡
Jiani Ma,
‡
⊥
‡
‡
Zilong Wang, Cheng Mu, Zetan Fan,§ Lili Du, ‡
Bai, Louzhen Fan,*§ He Yan, David Lee Phillips, †
Nano Science and Technology Program,
‡
⊥
⊥
Yang
†‡
and Shihe Yang*
Department of Chemistry, The Hong
Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ⊥
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong
Kong S.A.R., P. R. China §
Department of Chemistry, Beijing Normal University, Beijing, 100875, China
S1
Supporting Information Experiment Details 1.
Materials synthesis and device fabrication
Organometal halide perovskite (CH3NH3PbI3) solution synthesis. CH3NH3I was synthesized according to a reported procedure.ref For a typical synthesis, 0.3 mol (38 mL) methylamine CH3NH2) solution (33wt% in absolute ethanol) was reacted with equimolar (40 mL) hydroiodic acid (HI) (57 wt% in water) with stirring at ice bath for about 2 h to synthesis methylammoniium iodide (CH3NH3I). Crystallization of CH3NH3I was achieved using a rotary evaporator at 60 °C for 2~3 h. The obtained CH3NH3I power with equimolar lead(II) iodide (PbI2) were dissolved in γ -butyrolactone with stirring at 60 °C for over-night to produce a 2.5 mM CH3NH3PbI3 precursor solution. Perovskite TiO2 Mesoporous Structured Solar Cell Fabrication. First, fluorine doped tin oxdie (F:SnO2) coated glass (FTO) was patterned by laser etching. The patterned FTO were cleaned by ultrasonication and rinsed in the deionized water and mixed solution ethanol and acetone (v:v ~ 1:1). An 30~40 nm thick TiO2 compact layer was then deposited on the substrates by aerosol spray pyrolysis at 50 mM titanium diisopropoxide bis(acetylacetonate) solution (75% in 2-propanol,) diluted in ethanol. The nanocrystalline TiO2 paste was deposited by screen-printed method and calcined at 500 °C for 1 h. The films then immersed in 40 mM of TiCl4 aqueous solution at 60 °C for 1 h and heat-treated at 500 °C for 30 min. The CH3NH3PbI3 solution synthesis above was then coated onto the TiO2 meso-structure by spin-coating at
S2
3,000 r.p.m. for 60 s, and dried on a hot plate at 100 °C for 20 min. The hole transport material (HTM) was deposited on the CH3NH3PbI3/TiO2 meso-structure film by spin coating at 3,000 r.p.m for 30s. The HTM solution was prepared by dissolving spiro-OMeTAD (Merck) in chlorobenzene (110 mg/1 ml), 26 ul tert-butylpyridin (TBP) solution and 35 ul Li-bis(trifluoromethanesulfonyl) imide (Li-TFSI)/acetonitrile (170 mg/1 ml). For the metal electrode, 60 nm thickness of gold was deposited on the top of the HTM by a thermal evaporation through a shadow mask to define the active area of the devices (~12.6 mm2) and to form a top anode. When the mask was applied, the defined active area was changed to 6 mm2 (1.5mm*4mm). We also comparatively tested 15 cells using the mask and without using the mask, and the resulting data have now been collected in Table S2 below. The cell was packaged by scribbling UV-glue on the top and covered a glass slides, then the cell was exposed to UV light radiation for 10 min to make the glue solidify. The device testing was carried out the glove box after packaging.. 2.
Materials characterization
The morphologies of the device sample were characterized by a field emission scanning electron microscope (FE-SEM; JEOL 6700F) operated at 5 kV. The structure analysis of graphene quantum dots were carried out using a transmission electron microscope (TEM; JEOL 2010F) operated at 200 kV. X-ray diffraction (XRD) patterns of the samples were obtained using a diffractometer (Philips, PW 1830) with Cu Ka radiation at scan rate of 4°/min under operation condition of 30 kV and 40 mA.
S3
Elemental compositions of GQDs on TiO2 mesoporous film were investigated by energy dispersive X-ray spectroscope (EDS; Oxford INCA Energy) attached to the SEM and the distribution of the GQDs on TiO2 was analyzed by the mapping technique. Conduction band and valence band. The energy levels and optical band gaps of the system relative to vacuum of each component in Figure 1B were determined by methods reported previously1,2. Cyclic voltammetry (CV) was used to characterized energy level of GQDs as described in the following references3,4. The CV was carried out by a standard three-electrode system, with a glassy carbon disk working electrode, a platinum wire for the counter electrode, Ag/AgCl for the reference electrode, 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6, Aldrich) in acetonitrile as supporting electrolyte. The GQD was dropped on the glassy carbon electrode. 3.
Solar cell characterization Fabricated photovoltaic cells were characterized by current-voltage (J-V)
characteristics and incident photon-to-current conversion efficiency (IPCE). Photocurrent and voltage were measured by a solar simulator (Oriel, 450 W Xe lamp, AM 1.5 global filter) equipped with an electrochemical workstation (Zanher, Zennium). The light source was calibrated to 1 sun (100 mW/cm2) using an optical power meter (Newport, model 1916-C) equipped with a Newport818P thermopile detector. The IPCE measurements were carried out with a Zahner Zennium CIMPS-PCS
system
established
with
S4
the
tunable
light
source
(TLS).
Absorbed-photon-to-current conversion efficiency (APCE) according to Eq. (1) to more accurately reflect the carrier extraction efficiency. % %/1 10
4.
(1)
Optical spectroscopy
The sample prepared process is the same with the cell fabrication except using a sapphire substrate instead of FTO. Optical absorption measurements were carried out on a Perkin- Elmer spectrophotometer (model Lambda 20). The steady state photoluminescence
measurements
were
carried
out
on
Micro-Raman/Photoluminescence System (Renishaw) with a laser source operated at 514.5 nm and 20 mW. 5.
Transient absorption measurements
The sample preparation process is the same with the optical test above. Femtosecond transient absorption (fs-TA) measurements were performed using a femotosecond regenerative amplified Ti: sapphire laser system (Spectra Physics, Spitfire-Pro) and an automated data acquisition system (ultrafast systems, Helios). The TA traces were collected with a fixed wavelength pump laser operating at 400 nm (3.1 eV) generated from the second harmonic of the fundamental of a Ti-Sapphire laser (800 nm, 120 fs pulses) with a regenerative amplifier, and a well synchronized, variable time delayed probe beam (white light continuum).5 The amplifier was seeded with the oscillator output. The probe pulse was generated using about 5% of the amplified 800 nm output to generate a white-light continuum (350-800 nm) in a CaF2 crystal. The probe beam was split into two before passing through the sample. One beam travels through the
S5
sample; the other was sent to the reference spectrometer monitoring the fluctuations in the probe beam intensity. Fiber optics was coupled to a multichannel spectrometer. The film samples to be tested were loaded on the quartz substrates.
S6
Fig. S1. TEM (A), high-resolution TEM (B), and photograph (C) images of the homogeneous aqueous solution (0.5 mg/mL) of the GQDs sample.
Fig. S2. (A) UV-vis absorption spectrum of the GQD aqueous solution. (B) Photoluminescence spectrum of the GQD aqueous solution at the 400 nm excitation wavelength.
Fig. S3. EDS elemental mapping images and compositional analysis of a TiO2 mesoporous film covered with an ultrathin GQD layer. Top left: SEM image. S7
Fig. S4. X-ray diffraction (XRD) patterns of GQDs/TiO2 and CH3NH3PbI3/GQDs/TiO2 samples. Note: The XRD pattern of CH3NH3PbI3 sensitized film shows diffraction peaks at 14.05o, 28.45o, 31.87o, 40.45o and 43.13o, which can be assigned to the (110), (220), (310), (224) and (314) planes of the tetragonal perovskite structure.
Fig. S5. Three dimensional scheme (A) and photograph (B) of a typical patterned batch of PV cells.
S8
Fig. S6. Device performance as a function of concentration of the GQDs aqueous solutions.
Fig. S7. The integrated photocurrent densities of the IPCE spectra. CH3NH3PbI3/GQDs/TiO2 (red line) and CH3NH3PbI3/TiO2 (black line) cells were measured under AM 1.5 G illumination at 100 mW/cm2 (solid line).
S9
Fig. S8. Transient absorption spectra of pure TiO2 mesoporous film (A) and GQD/TiO2 film (B) with excitation at 400 nm. Normalized photoabsorption (PA) kinetic traces at 650 nm for pure TiO2 mesoporous film (C) and GQD/TiO2 film (D).
S10
Table S1. Average device performance parameters extracted from the measured current-voltage characteristics. A total number of 60 devices were fabricated for each batch of testing. Errors represent the standard deviations in each batch of data.
Concentration GQD (mg/mL)
Jsc (mA/cm2)
Voc (V)
0.00
15.20
0.78
0.909
0.032
0.589
0.029
8.58
0.54
0.1
15.83
0.79
0.912
0.043
0.614
0.031
8.72
0.56
0.5
16.81
0.83
0.917
0.038
0.618
0.031
9.76
0.58
1
16.51
0.81
0.909
0.034
0.601
0.030
9.02
0.57
FF
η (%)
Table S2. Mean cell performance parameters obtained from the current-voltage curves. Note that original represents the data without using aperture, whereas mask means with aperture. Jsc (mA/cm2)
Voc (V)
FF
η (%)
Original
16.81
0.83
0.917
0.038
0.618
0.031
9.76
0.58
Mask
16.91
1.91
0.914
0.064
0.620
0.070
9.62
1.27
Table S3. Summary of the parameters from fits to the transient absorption data in Figure 4C and 4D. The fitting functions of double exponential equation.
!
"#
$
!
% " $. &
Feature
Sample
A1
τ1 (ps)
A2
τ2 (ps)
Photobleach
CH3NH3PbI3/GQDs/TiO2
0.625
90.4
0.462
2268.7
(760 nm)
CH3NH3PbI3/TiO2
0.423
260.4
0.482
2230.5
Photoabsorption
CH3NH3PbI3/GQDs/TiO2
0.382
106.8
0.313
2073.4
(530 nm)
CH3NH3PbI3/TiO2
0.509
307.1
0.362
2329.3
S11
Reference (1) Zhang, M.; Bai, L.; Shang, W.; Xie, W.; Ma, H.; Fu, Y.; Fang, D.; Sun, H.; Fan, L.; Han, M.; Liu, C.; Yang, S. J. Mater. Chem.2012, 22, 7461. (2) Abrusci, A.; Stranks, S. D.; Docampo, P.; Yip, H. L.; Jen, A. K. Y.; Snaith, H. J. Nano Letter. 2013, 13, 3124. (3) Li, Y.; Hu, Y.; Zhao, Y.; Shi, G.; Deng, L.; Hou, Y.; Qu, L. Adv. Mater. 2011, 23, 776. (4) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater. 2011, 23, 2367. (5) Ma, J.; Su, T.; Li, M-D.; Du, W.; Huang, J.; Guan, X.; Phillips, D. L. J. Am. Chem. Soc. 2012, 134, 14858.
S12
