Different from band edge alignment, the Fermi level mismatch induced by band bending can manipulate charge collection at the ITO/(CH_3NH_3)_(1−x)Cu_xPbI_3 heterojunctions. In this work, we employed a feasible spin-coating process to prepare copper defect compensation in CH_3NH_3PbI_3. The related work function was shown to shift with the copper doping density by Kelvin probe force microscopy (KPFM). Next, we applied transient surface photovoltage (TSPV) spectroscopy and first-order series reactions simulations to confirm that interface charge recombination at the ITO/perovskite junction can be eliminated through Cu+ doping. Nanoelectric photoconductive AFM analysis showed enhanced charge transfer and a higher photovoltage at the ITO/Cu-perovskite junction. Owing to efficient Fermi level alignment, the ITO/(CH_3NH_3)_(1−x)Cu_xPbI_3/PCBM/Ag devices displayed high power conversion efficiencies of 15.14 ± 0.67% at ambient conditions for inverted perovskite solar cells without any hole transport layer

Jia, Zuxiao

Lei, Yan

Liu, Jiang

Liu, Rui

Lu, Kai

Qi, Ruijuan

Xiang, Yong

Yang, Xiaogang

Zheng, Zhi

Caltech Authors - Main

1
Supporting Information
Fermi Level Alignment by Copper Doping for Efficient ITO/Perovskite Junction 
Solar Cells
Kai Lu1,2#, Yan Lei1#, Ruijuan Qi3#, Jiang Liu4, Xiaogang Yang1*, Zuxiao Jia1, 
Rui Liu5, Yong Xiang2, Zhi Zheng1*
1Key Laboratory for Micro-Nano Energy Storage and Conversion Materials of Henan Province, 
College of Advanced Materials and Energy, Institute of Surface Micro and Nano Materials, Xuchang 
University, Henan 461000, China.
2School of Energy Science and Engineering, University of Electronic Science and Technology of China, 
Chengdu 611731, China.
3Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic 
Engineering, East China Normal University, Shanghai 200241, China
4Chengdu Green Energy and Green Manufacturing R&D Center, Chengdu Development Center of 
Science and Technology, China Academy of Engineering Physics, Chengdu, 610207, China.
5Joint Center for Artificial Photosynthesis, California Institute of Technology, Division of Chemistry 
and Chemical Engineering, Pasadena, CA 91125, United States
# These authors contributed equally.
*Corresponding Author email: xiaogang.yang@gmail.com (X.Y.) and zzheng@xcu.edu.cn (Z. Z.)
Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.
This journal is © The Royal Society of Chemistry 2017
2
Figure S1
Figure S1. CuI (~103 nm) after the Cu film reacting with I2 atmosphere on ITO substrate. The pattern is 
in good agreement with the cubic copper iodide in JCPDS card (No. 75-0831).
Figure S2
Figure S2. Cross-section FESEM images: a) ITO/CuI film; b) ITO/Cu20-perovskite film; c) assembled 
inverted ITO/(CH3NH3)1-xCuxPbI3/PCBM/Ag device; d-f) EDS elemental maps of Cu, Pb and I 
obtained at cross-section of ITO/Cu-perovskite. Figure S2a is the SEM image of the ITO/CuI cross-
section converted from 20 nm elemental Cu, where the thickness of CuI layer about 103 nm. The 
resolution of Figure S2b is lower for the larger working distance during EDS mapping. The EDS 
mapping of Cu (d) showed the Cu vertically distributed from bottom to the top surface, which had 
lower uniformity due to the Cu concentration, sample thickness and instrument resolution.
3
Figure S3
Figure S3. SEM images of the top surface of the Cu-perovskite films with CuI underlayer: (a) Cu10-
perovskite; (b) Cu30-perovskite; (c) Cu40-perovskite. Higher resolution image and cross-section image 
are inserted on the top-right and middle-right, respectively.   From the SEM images of the Cu doped 
perovskite films, we could find all the surfaces are conformal on the substrate.  This high quality of 
the films can reduce the short circuit of the device in nanoscale.  Moreover, the performance variation 
under different Cu doping level was less likely due to the film quality.  
Figure S4
Figure S4. XRD pattern of perovskite on Si (200) wafer for lattice measurements: (a) full pattern, (b) 
Si(400) peaks. Cu10, Cu20, Cu30 and Cu40 corresponding to the various doping level of Cu in the 
perovskite films. XRD of Perovskite on Si wafer: Si(400) was set to 69.29, corresponding to the card 
(JCPDS. No. 72.2108). (c)  the parameter a and volume shrinkage based on the Si/perovskite.
4
Table S1
Table S1. The Lattice parameters of Cu doped and undoped perovskite on Si(400), calculated by using 
a Cell software (Copyright © 1986 by K. Dwight) and CuK1 (=1.5406 Å).
Perovskites a (Å) c (Å) RMS errors V (Å3)  V (%)
Cu free 8.8760 12.4899 1.10E-04 984.00 0.000
Cu10 8.8745 12.4859 1.20E-04 983.35 -0.066
Cu20 8.8725 12.4864 1.30E-04 982.95 -0.107
Cu30 8.8706 12.4892 1.40E-04 982.74 -0.128
Cu40 8.8682 12.4891 1.50E-04 982.20 -0.182
XRD showed the slight lattice shrinkage, due to the Cu+ occupying the large size of CH3NH3+, which is 
quite different from the other reports as the interstitial defects or partial substitution of Cu at Pb sites.1 
Although this lattice parameters are obtained on the perovskite deposited on Si substrate, we assumed 
the crystal quality and orientation were the same as those on ITO substrate.  This lattice shrinkage 
suggested that the CuI acted as dopant rather than composites or blend in the perovskite matrix.
5
Figure S5
Figure S5. XPS spectra of the Cu-perovskite films: a) XPS survey spectra of Cu20-perovskite (upper 
line) and ITO interface after the removal of Cu20-perovskite by DMF; b) N1s; c) C1s and d) Pb4f.  
Based on the peak fitting as in Figure S4 and Cu2p in Figure 2f, the elements ratios were calculated 
according to the peak area and their sensitive factors.  In Figure S5a, the N1s showed a shoulder peak 
close to 401 eV, which is attributed to the formation of CH3NH2 from CH3NH3.2, 3 The I/Pb =2.5 is due 
to the deficiency of I elements, which is coincided with the literature reports.4-6
Figure S6. 
Figure S6. Elemental composition (vs. Pb) of the undoped and Cu-doped perovskite films through XPS.
Figure S7. 
6
Figure S7. Raman spectra of the pure and Cu doped perovskite films.  No peaks due to PbI2 or CuI 
could be found.  It was noting that the Raman spectra collection should be carried out at extremely 
low laser intensity.  Otherwise, the perovskite films would be sintered by the laser irradiation, or 
convert to PbI2 impurities. The spectra also suggested no PbI2 impurities were detected from the fresh 
prepared perovskite.
7
Figure S8. 
Figure S8. (a) Height image and (b) KPFM image of the Cu-free perovskite film on glass substrate; (c) 
height image and (d) KPFM image of the ITO substrate; (e) height image and (f) KPFM image of the 
Cu20-perovskite film on glass substrate.  Inserted images were the Potential-Y section on the 
corresponded films.  The potentials were measured versus Pt/Ir AFM tip.  Blue cycles in the V~Y 
figures indicated the scanning starting point from top to bottom.
8
Figure S9. 
Figure S9. (a) Height image and (b) KPFM image of the Cu10-perovskite film on glass substrate; (c) 
height image and (d) KPFM image of the Cu30-perovskite film on glass substrate; (e) height image and 
(f) KPFM image of the Cu40-perovskite film on glass substrate. Inserted were the Potential-Y section 
on the corresponded films. The potentials were measured versus Pt/Ir AFM tip. Blue cycles in the V~Y 
figures indicated the scanning starting point from top to bottom.
Table S2. The CPDs of Cu-free perovskite and Cu doped perovskite from KPFM via the AFM tip.
Perovskite Cu free Cu10 Cu20 Cu30 Cu40 ITO
CPD (V)a 0.14 -0.07 -0.21 -0.30 -0.39 -0.22
Fermi Level (eV) -5.44 -5.23 -5.09 -5.0 -4.91 -5.08
a The CPD values were calculated by using the equation of .  Due tip sample sample tipCPD E E    
to the air sensitive surface, the value was taken from the starting point of potential scanning (<3 min 
out from glove box).
9
Figure S10. 
Figure S10. Experimental and simulated TSPV curves of the ITO/perovskite thin films with various Cu 
content: (a) Cu10; (b) Cu30 and (c) Cu40. Black dashed circle in (a) showing the slight negative signal 
due to the misalignment of the band structures of Cu10-perovskite and ITO substrate.  It was found 
the simulated curves agreed well with the experiment curves in the spectra.  
We speculated that the first order kinetic processes in the TSPV curves.  Firstly, the surface 
charges density Q is proportional to the transient surface potential (V) detected at the surface, obeying 
the parallel plate capacitor property ( , where C is the capacitance for the TSPV setup).7  Q C V 
Secondly, the charge separation is mainly due to the collection of photon-excited charges separated 
from the bulk of the perovskite film, where the separation rate is proportional to the excited charges in 
bulk ( ).  Thirdly, after the TSPV reached to a minimum (negative peak), the charge sep exck Q
separation rate is comparable and the charge recombination rate is proportional to the charge absorbed 
at the solid/air surface, where it is much slower than the other bulk recombination modes.  According 
to this charge separation and recombination assumption, we can write a consecutive reaction (Eq. 1) for 
the charge evolution (or TSPV) in the semiconductor:
               (1)sep reck kexc sep recQ Q Q 
10
where Qexc, Qsep and Qrec corresponded to the excited charges in the bulk semiconductor, the separated 
charges detected by TSPV, and the surface recombined charge, respectively.  ksep and krec are the rate 
constants for the charge separation and recombination, respectively.  Obviously, only the separated 
charge Qsep contributes to the TSPV signal, while the Qexc and Qrec don’t.  For the boundary condition 
(t=0, Qsep,0=0, and krec≠ksep), the separated charges can be expressed as Eq. (2):8
             (2) sep recsep exc,0sep
rec sep
k t k tk QQ e e
k k
  

Therefore, the transient surface photovoltage V can be written as Eq. (3):
           (3)
   
sep recsep exc,0
rec sep
k t k tk QV e e
k k C
   

Table S3. Kinetics parameters of the Cu-free and Cu doped ITO/Perovskite junctions through TSPV.
Perovskite Cu free Cu10 Cu20 Cu30 Cu40
Tmax (s) 7.16×10-5 3.15×10-5 2.32×10-5 1.93×10-5 1.17×10-5
ksep (s-1) 5.42×104 1.24×105 1.80×105 2.15×105 4.11×105
krec (s-1) 1.22×103 2.52×103 2.95×103 3.60×103 3.52×103
Qexc,0 (a.u.)a 0.45 1.82 2.12 3.68 3.60
a: the relative value of the Qexc,0 were shown here.
11
Figure S11
Figure S11. Photoluminescence of the undoped and Cu20 doped perovskite films excited at 660 nm: a) 
PL emission peaks of glass/Cu0-perovskite (black solid line), glass/Cu20-perovskite (red solid line), 
ITO/Cu0-perovskite (black dashed line) and ITO/Cu20-perovskite (red dashed line); b) zoomed in 
image of PL curves of the ITO/Cu0-perovskite (black dashed line) and ITO/Cu20-perovskite (red 
dashed line); c) normalized time-resolved PL transient decay of the ITO/Cu0-perovskite (black square) 
and ITO/Cu20-perovskite (red circle), the emission measured at 775 nm. The simulation was based on 
double exponential components decay.     
The PL intensity of the perovskite on ITO was much lower than that on glass due to the easier charge 
transferred to ITO.  When Cu introduced into the perovskite films, the PL intensity was further 
suppressed due to the better charge separation and collection at the ITO electrode.  Transient PL 
showed double exponential decay on the samples: for shorter lifetime (1) the ITO/Cu20-perovskite 
showed slight shorter than the ITO/perovskite sample due to the charge separation; while the longer 
lifetime (2) may ascribed to surface trap states.
Figure S12
12
Figure S12. Electrochemical impedance spectra of the undoped and Cu20 doped perovskite films on 
ITO with Au as counter electrode: a) Niquist plot of the ITO/Cu0-perovskite and ITO/Cu20-perovskite 
samples measured under dark condition at 0 V (short circuit); b) Niquist plot of the ITO/Cu0-
perovskite and ITO/Cu20-perovskite samples measured under AM 1.5 illumination (1 sun) condition at 
0 V (short circuit).
The semicircle on ITO/Cu20-perovskite sample was smaller than ITO/Cu0-perovskite sample under 
dark condition; while there are one arc at higher frequency region and a half-developed semi-cycle at 
lower frequency region under light condition. The signal measured here corresponded to the charge 
transfer behavior at the interface and overlapping partially with the charge transport in the cells.9, 10 
Figure S13. 
13
Figure S13. Dark conductive AFM images: a) ITO/perovskite film; b) ITO/Cu-perovskite film.  The 
dark current of the conductive AFM showed a slight lower dark current (~0.7 pA) on the ITO/Cu-
perovskite film than ~1 pA on ITO/perovskite film.  
Table S4. Parameters of inverted pristine and Cu doped perovskite solar cell devices. a
Perovskite 
solar cells
Voc (V) Jsc (mA/cm2) FF (%) PCE (%)
Average 
PCE (%)
Champion 
PCE (%)
Cu free 0.875 16.59 44.54 6.46 6.05±0.64 6.76
Cu10 0.956 21.65 56.81 11.76 11.42±0.45 11.78
Cu20 1.038 22.73 64.54 15.23 15.14±0.67 15.84
Cu30 1.034 22.67 62.02 14.54 14.38±0.79 15.22
Cu40 0.848 20.31 63.58 10.95 10.83±0.87 12.03
 a the parameters were based on the forward scan.
Figure S14. 
14
Figure S14. UV-Vis absorption of perovskites.  The Cu doping lead to a slight increase of the 
absorption and a small positive shift of the band absorption edge.
Figure S15
Figure S15. Demonstrating ITO/Cu-perovskite/PCBM/Ag devices in series to power an electric fan: a) 
stable devices during power out under illumination; b) five devices connected in series.  It was shown 
the devices can power stably the fans under simulated solar light easily under ambient condition, even 
the devices were not sealed.
Figure S16
15
Figure S16. The hysteresis behavior of the perovskite solar cells: a) intrinsic perovskite solar cell; b) 
Cu20 doped perovskite solar cell.  Under ambient condition, the devices showed a larger hysteresis on 
pristine perovskite on ITO substrate when the potential sweeping forward (from short circuit to open 
circuit) or reverse.  This might be due to the not perfect charge collection as those HTL layers are 
absent.  
Figure S17
Figure S17.  IR thermal images of the hotplate for perovskite film preparation by using a IKA C-
MAG HS 7 Digital hotplate and a FLIR T335 camera: a) bare hotplate; b) hotplate with a ITO glass; c) 
bare hotplate after the ITO glass was removed; d) hotplate with less uniform distribution during the 
heating; e) digital camera image of additional metal plate on hotplate (horizontal probe is thermal 
couple); f) contaminated surface of metal plate on hotplate; g) clean surface of metal plate on hotplate; 
h) clean surface of metal on hotplate with an ITO glass; i) clean surface of metal on hotplate after the 
ITO glass was removed.  The color of the metal plate is lower than the ceramic surface hotplate is due 
to the less IR emission difference on metal.
16
References:
1. Y. Shirahata and T. Oku, AIP Conference Proceedings, 2017, 1807, 020008.
2. J. J. Chen and N. Winograd, Surface Science, 1995, 326, 285-300.
3. A. Calloni, A. Abate, G. Bussetti, G. Berti, R. Yivlialin, F. Ciccacci and L. Duò, J. Phys. 
Chem. C, 2015, 119, 21329-21335.
4. J. H. Yun, I. Lee, T.-S. Kim, M. J. Ko, J. Y. Kim and H. J. Son, Journal of Materials 
Chemistry A, 2015, 3, 22176-22182.
5. C. Li, S. Tscheuschner, F. Paulus, P. E. Hopkinson, J. Kießling, A. Köhler, Y. Vaynzof and S. 
Huettner, Advanced Materials, 2016, 28, 2446-2454.
6. G. Sadoughi, D. E. Starr, E. Handick, S. D. Stranks, M. Gorgoi, R. G. Wilks, M. Bär and H. J. 
Snaith, ACS Applied Materials & Interfaces, 2015, 7, 13440-13444.
7. L. Kronik and Y. Shapira, Surf. Sci. Rep., 1999, 37, 1-206.
8. P. Atkins and J. d. Paula, Atkins’ Physical Chemistry, W. H. Freeman and Company, 8th edn., 
2005.
9. W. Yan, Y. Li, Y. Li, S. Ye, Z. Liu, S. Wang, Z. Bian and C. Huang, Nano Energy, 2015, 16, 
428-437.
10. H. Wei, J. Xiao, Y. Yang, S. Lv, J. Shi, X. Xu, J. Dong, Y. Luo, D. Li and Q. Meng, Carbon, 
2015, 93, 861-868.


English

Fermi level alignment by copper doping for efficient ITO/perovskite junction solar cells

1Supporting Information
Fermi Level Alignment by Copper Doping for Efficient ITO/Perovskite Junction 
Solar Cells
Kai Lu1,2#, Yan Lei1#, Ruijuan Qi3#, Jiang Liu4, Xiaogang Yang1*, Zuxiao Jia1, 
Rui Liu5, Yong Xiang2, Zhi Zheng1*
1Key Laboratory for Micro-Nano Energy Storage and Conversion Materials of Henan Province, 
College of Advanced Materials and Energy, Institute of Surface Micro and Nano Materials, Xuchang 
University, Henan 461000, China.
2School of Energy Science and Engineering, University of Electronic Science and Technology of China, 
Chengdu 611731, China.
3Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic 
Engineering, East China Normal University, Shanghai 200241, China
4Chengdu Green Energy and Green Manufacturing R&D Center, Chengdu Development Center of 
Science and Technology, China Academy of Engineering Physics, Chengdu, 610207, China.
5Joint Center for Artificial Photosynthesis, California Institute of Technology, Division of Chemistry 
and Chemical Engineering, Pasadena, CA 91125, United States
# These authors contributed equally.
*Corresponding Author email: xiaogang.yang@gmail.com (X.Y.) and zzheng@xcu.edu.cn (Z. Z.)
Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.
This journal is © The Royal Society of Chemistry 2017
2Figure S1
Figure S1. CuI (~103 nm) after the Cu film reacting with I2 atmosphere on ITO substrate. The pattern is 
in good agreement with the cubic copper iodide in JCPDS card (No. 75-0831).
Figure S2
Figure S2. Cross-section FESEM images: a) ITO/CuI film; b) ITO/Cu20-perovskite film; c) assembled 
inverted ITO/(CH3NH3)1-xCuxPbI3/PCBM/Ag device; d-f) EDS elemental maps of Cu, Pb and I 
obtained at cross-section of ITO/Cu-perovskite. Figure S2a is the SEM image of the ITO/CuI cross-
section converted from 20 nm elemental Cu, where the thickness of CuI layer about 103 nm. The 
resolution of Figure S2b is lower for the larger working distance during EDS mapping. The EDS 
mapping of Cu (d) showed the Cu vertically distributed from bottom to the top surface, which had 
lower uniformity due to the Cu concentration, sample thickness and instrument resolution.
3Figure S3
Figure S3. SEM images of the top surface of the Cu-perovskite films with CuI underlayer: (a) Cu10-
perovskite; (b) Cu30-perovskite; (c) Cu40-perovskite. Higher resolution image and cross-section image 
are inserted on the top-right and middle-right, respectively.   From the SEM images of the Cu doped 
perovskite films, we could find all the surfaces are conformal on the substrate.  This high quality of 
the films can reduce the short circuit of the device in nanoscale.  Moreover, the performance variation 
under different Cu doping level was less likely due to the film quality.  
Figure S4
Figure S4. XRD pattern of perovskite on Si (200) wafer for lattice measurements: (a) full pattern, (b) 
Si(400) peaks. Cu10, Cu20, Cu30 and Cu40 corresponding to the various doping level of Cu in the 
perovskite films. XRD of Perovskite on Si wafer: Si(400) was set to 69.29, corresponding to the card 
(JCPDS. No. 72.2108). (c)  the parameter a and volume shrinkage based on the Si/perovskite.
4Table S1
Table S1. The Lattice parameters of Cu doped and undoped perovskite on Si(400), calculated by using 
a Cell software (Copyright © 1986 by K. Dwight) and CuK1 (=1.5406 Å).
Perovskites a (Å) c (Å) RMS errors V (Å3)  V (%)
Cu free 8.8760 12.4899 1.10E-04 984.00 0.000
Cu10 8.8745 12.4859 1.20E-04 983.35 -0.066
Cu20 8.8725 12.4864 1.30E-04 982.95 -0.107
Cu30 8.8706 12.4892 1.40E-04 982.74 -0.128
Cu40 8.8682 12.4891 1.50E-04 982.20 -0.182
XRD showed the slight lattice shrinkage, due to the Cu+ occupying the large size of CH3NH3+, which is 
quite different from the other reports as the interstitial defects or partial substitution of Cu at Pb sites.1 
Although this lattice parameters are obtained on the perovskite deposited on Si substrate, we assumed 
the crystal quality and orientation were the same as those on ITO substrate.  This lattice shrinkage 
suggested that the CuI acted as dopant rather than composites or blend in the perovskite matrix.
5Figure S5
Figure S5. XPS spectra of the Cu-perovskite films: a) XPS survey spectra of Cu20-perovskite (upper 
line) and ITO interface after the removal of Cu20-perovskite by DMF; b) N1s; c) C1s and d) Pb4f.  
Based on the peak fitting as in Figure S4 and Cu2p in Figure 2f, the elements ratios were calculated 
according to the peak area and their sensitive factors.  In Figure S5a, the N1s showed a shoulder peak 
close to 401 eV, which is attributed to the formation of CH3NH2 from CH3NH3.2, 3 The I/Pb =2.5 is due 
to the deficiency of I elements, which is coincided with the literature reports.4-6
Figure S6. 
Figure S6. Elemental composition (vs. Pb) of the undoped and Cu-doped perovskite films through XPS.
Figure S7. 
6Figure S7. Raman spectra of the pure and Cu doped perovskite films.  No peaks due to PbI2 or CuI 
could be found.  It was noting that the Raman spectra collection should be carried out at extremely 
low laser intensity.  Otherwise, the perovskite films would be sintered by the laser irradiation, or 
convert to PbI2 impurities. The spectra also suggested no PbI2 impurities were detected from the fresh 
prepared perovskite.
7Figure S8. 
Figure S8. (a) Height image and (b) KPFM image of the Cu-free perovskite film on glass substrate; (c) 
height image and (d) KPFM image of the ITO substrate; (e) height image and (f) KPFM image of the 
Cu20-perovskite film on glass substrate.  Inserted images were the Potential-Y section on the 
corresponded films.  The potentials were measured versus Pt/Ir AFM tip.  Blue cycles in the V~Y 
figures indicated the scanning starting point from top to bottom.
8Figure S9. 
Figure S9. (a) Height image and (b) KPFM image of the Cu10-perovskite film on glass substrate; (c) 
height image and (d) KPFM image of the Cu30-perovskite film on glass substrate; (e) height image and 
(f) KPFM image of the Cu40-perovskite film on glass substrate. Inserted were the Potential-Y section 
on the corresponded films. The potentials were measured versus Pt/Ir AFM tip. Blue cycles in the V~Y 
figures indicated the scanning starting point from top to bottom.
Table S2. The CPDs of Cu-free perovskite and Cu doped perovskite from KPFM via the AFM tip.
Perovskite Cu free Cu10 Cu20 Cu30 Cu40 ITO
CPD (V)a 0.14 -0.07 -0.21 -0.30 -0.39 -0.22
Fermi Level (eV) -5.44 -5.23 -5.09 -5.0 -4.91 -5.08
a The CPD values were calculated by using the equation of .  Due tip sample sample tipCPD E E    
to the air sensitive surface, the value was taken from the starting point of potential scanning (<3 min 
out from glove box).
9Figure S10. 
Figure S10. Experimental and simulated TSPV curves of the ITO/perovskite thin films with various Cu 
content: (a) Cu10; (b) Cu30 and (c) Cu40. Black dashed circle in (a) showing the slight negative signal 
due to the misalignment of the band structures of Cu10-perovskite and ITO substrate.  It was found 
the simulated curves agreed well with the experiment curves in the spectra.  
We speculated that the first order kinetic processes in the TSPV curves.  Firstly, the surface 
charges density Q is proportional to the transient surface potential (V) detected at the surface, obeying 
the parallel plate capacitor property ( , where C is the capacitance for the TSPV setup).7  Q C V 
Secondly, the charge separation is mainly due to the collection of photon-excited charges separated 
from the bulk of the perovskite film, where the separation rate is proportional to the excited charges in 
bulk ( ).  Thirdly, after the TSPV reached to a minimum (negative peak), the charge sep exck Q
separation rate is comparable and the charge recombination rate is proportional to the charge absorbed 
at the solid/air surface, where it is much slower than the other bulk recombination modes.  According 
to this charge separation and recombination assumption, we can write a consecutive reaction (Eq. 1) for 
the charge evolution (or TSPV) in the semiconductor:
               (1)sep reck kexc sep recQ Q Q 
10
where Qexc, Qsep and Qrec corresponded to the excited charges in the bulk semiconductor, the separated 
charges detected by TSPV, and the surface recombined charge, respectively.  ksep and krec are the rate 
constants for the charge separation and recombination, respectively.  Obviously, only the separated 
charge Qsep contributes to the TSPV signal, while the Qexc and Qrec don’t.  For the boundary condition 
(t=0, Qsep,0=0, and krec≠ksep), the separated charges can be expressed as Eq. (2):8
             (2) sep recsep exc,0sep
rec sep
k t k tk QQ e e
k k
  
Therefore, the transient surface photovoltage V can be written as Eq. (3):
           (3)   sep recsep exc,0rec sep k t k t
k Q
V e e
k k C
   
Table S3. Kinetics parameters of the Cu-free and Cu doped ITO/Perovskite junctions through TSPV.
Perovskite Cu free Cu10 Cu20 Cu30 Cu40
Tmax (s) 7.16×10-5 3.15×10-5 2.32×10-5 1.93×10-5 1.17×10-5
ksep (s-1) 5.42×104 1.24×105 1.80×105 2.15×105 4.11×105
krec (s-1) 1.22×103 2.52×103 2.95×103 3.60×103 3.52×103
Qexc,0 (a.u.)a 0.45 1.82 2.12 3.68 3.60
a: the relative value of the Qexc,0 were shown here.
11
Figure S11
Figure S11. Photoluminescence of the undoped and Cu20 doped perovskite films excited at 660 nm: a) 
PL emission peaks of glass/Cu0-perovskite (black solid line), glass/Cu20-perovskite (red solid line), 
ITO/Cu0-perovskite (black dashed line) and ITO/Cu20-perovskite (red dashed line); b) zoomed in 
image of PL curves of the ITO/Cu0-perovskite (black dashed line) and ITO/Cu20-perovskite (red 
dashed line); c) normalized time-resolved PL transient decay of the ITO/Cu0-perovskite (black square) 
and ITO/Cu20-perovskite (red circle), the emission measured at 775 nm. The simulation was based on 
double exponential components decay.     
The PL intensity of the perovskite on ITO was much lower than that on glass due to the easier charge 
transferred to ITO.  When Cu introduced into the perovskite films, the PL intensity was further 
suppressed due to the better charge separation and collection at the ITO electrode.  Transient PL 
showed double exponential decay on the samples: for shorter lifetime (1) the ITO/Cu20-perovskite 
showed slight shorter than the ITO/perovskite sample due to the charge separation; while the longer 
lifetime (2) may ascribed to surface trap states.
Figure S12
12
Figure S12. Electrochemical impedance spectra of the undoped and Cu20 doped perovskite films on 
ITO with Au as counter electrode: a) Niquist plot of the ITO/Cu0-perovskite and ITO/Cu20-perovskite 
samples measured under dark condition at 0 V (short circuit); b) Niquist plot of the ITO/Cu0-
perovskite and ITO/Cu20-perovskite samples measured under AM 1.5 illumination (1 sun) condition at 
0 V (short circuit).
The semicircle on ITO/Cu20-perovskite sample was smaller than ITO/Cu0-perovskite sample under 
dark condition; while there are one arc at higher frequency region and a half-developed semi-cycle at 
lower frequency region under light condition. The signal measured here corresponded to the charge 
transfer behavior at the interface and overlapping partially with the charge transport in the cells.9, 10 
Figure S13. 
13
Figure S13. Dark conductive AFM images: a) ITO/perovskite film; b) ITO/Cu-perovskite film.  The 
dark current of the conductive AFM showed a slight lower dark current (~0.7 pA) on the ITO/Cu-
perovskite film than ~1 pA on ITO/perovskite film.  
Table S4. Parameters of inverted pristine and Cu doped perovskite solar cell devices. a
Perovskite 
solar cells
Voc (V) Jsc (mA/cm2) FF (%) PCE (%)
Average 
PCE (%)
Champion 
PCE (%)
Cu free 0.875 16.59 44.54 6.46 6.05±0.64 6.76
Cu10 0.956 21.65 56.81 11.76 11.42±0.45 11.78
Cu20 1.038 22.73 64.54 15.23 15.14±0.67 15.84
Cu30 1.034 22.67 62.02 14.54 14.38±0.79 15.22
Cu40 0.848 20.31 63.58 10.95 10.83±0.87 12.03
 a the parameters were based on the forward scan.
Figure S14. 
14
Figure S14. UV-Vis absorption of perovskites.  The Cu doping lead to a slight increase of the 
absorption and a small positive shift of the band absorption edge.
Figure S15
Figure S15. Demonstrating ITO/Cu-perovskite/PCBM/Ag devices in series to power an electric fan: a) 
stable devices during power out under illumination; b) five devices connected in series.  It was shown 
the devices can power stably the fans under simulated solar light easily under ambient condition, even 
the devices were not sealed.
Figure S16
15
Figure S16. The hysteresis behavior of the perovskite solar cells: a) intrinsic perovskite solar cell; b) 
Cu20 doped perovskite solar cell.  Under ambient condition, the devices showed a larger hysteresis on 
pristine perovskite on ITO substrate when the potential sweeping forward (from short circuit to open 
circuit) or reverse.  This might be due to the not perfect charge collection as those HTL layers are 
absent.  
Figure S17
Figure S17.  IR thermal images of the hotplate for perovskite film preparation by using a IKA C-
MAG HS 7 Digital hotplate and a FLIR T335 camera: a) bare hotplate; b) hotplate with a ITO glass; c) 
bare hotplate after the ITO glass was removed; d) hotplate with less uniform distribution during the 
heating; e) digital camera image of additional metal plate on hotplate (horizontal probe is thermal 
couple); f) contaminated surface of metal plate on hotplate; g) clean surface of metal plate on hotplate; 
h) clean surface of metal on hotplate with an ITO glass; i) clean surface of metal on hotplate after the 
ITO glass was removed.  The color of the metal plate is lower than the ceramic surface hotplate is due 
to the less IR emission difference on metal.
16
References:
1. Y. Shirahata and T. Oku, AIP Conference Proceedings, 2017, 1807, 020008.
2. J. J. Chen and N. Winograd, Surface Science, 1995, 326, 285-300.
3. A. Calloni, A. Abate, G. Bussetti, G. Berti, R. Yivlialin, F. Ciccacci and L. Duò, J. Phys. 
Chem. C, 2015, 119, 21329-21335.
4. J. H. Yun, I. Lee, T.-S. Kim, M. J. Ko, J. Y. Kim and H. J. Son, Journal of Materials 
Chemistry A, 2015, 3, 22176-22182.
5. C. Li, S. Tscheuschner, F. Paulus, P. E. Hopkinson, J. Kießling, A. Köhler, Y. Vaynzof and S. 
Huettner, Advanced Materials, 2016, 28, 2446-2454.
6. G. Sadoughi, D. E. Starr, E. Handick, S. D. Stranks, M. Gorgoi, R. G. Wilks, M. Bär and H. J. 
Snaith, ACS Applied Materials & Interfaces, 2015, 7, 13440-13444.
7. L. Kronik and Y. Shapira, Surf. Sci. Rep., 1999, 37, 1-206.
8. P. Atkins and J. d. Paula, Atkins’ Physical Chemistry, W. H. Freeman and Company, 8th edn., 
2005.
9. W. Yan, Y. Li, Y. Li, S. Ye, Z. Liu, S. Wang, Z. Bian and C. Huang, Nano Energy, 2015, 16, 
428-437.
10. H. Wei, J. Xiao, Y. Yang, S. Lv, J. Shi, X. Xu, J. Dong, Y. Luo, D. Li and Q. Meng, Carbon, 
2015, 93, 861-868.


https://authors.library.caltech.edu/83064/2/c7ta07828e1_si.pdf

Fermi level alignment by copper doping for efficient ITO/perovskite junction solar cells

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main