Supporting informationSynergistic Effect of Electron Transport Layer and Colloidal Quantum Dot Solid Enable

PbSe Quantum Dot Solar Cell Achieving over 10 % Efficiency

Long Hu,* Xun Geng, Simrjit Singh, Junjie Shi, Yicong Hu, Shaoyuan. Li, Xinwei Guan,

Tengyue He, Robert Patterson, Xiaoning Li, Zhenxiang Cheng, Shujuan Huang* and Tom

Wu

Dr. L. Hu, X. Geng, S. Singh, J. Shi, X. Guan, T. He and Prof. Tom Wu

School of Materials Science and Engineering, University of New South Wales (UNSW),

Sydney, NSW, 2052, Australia

Email: long.hu@unsw.edu.au

Y. Hu, R. Patterson

Australian Centre for Advanced Photovoltaics, University of New South Wales, Sydney,

Australia.

Dr. S. Li

Australian Centre for Advanced Photovoltaics, University of New South Wales, Sydney,

Australia.

Faculty of Metallurgical and Energy Engineering/State Key Laboratory of Complex

Nonferrous Metal Resources Clean Utilization Kunming University of Science and

Technology Kunming China

A/Prof. S. Huang

Australian Centre for Advanced Photovoltaics, University of New South Wales, Sydney,

Australia.

School of Engineering, Macquarie University, Sydney, NSW 2109, Australia

Email: shujuan.huang@mq.edu.au

Xiaoning Li, Prof. Z. Cheng

Australian Institute for Innovative Materials and Institute for Superconducting and Electronic

Materials, University of Wollongong, Wollongong, NSW, 2522, Australia

Experimental Section:

PbSe CQDs were synthesized and purified according to previous reports. [1, 2] PbS CQDs used for HTL were synthesized and purified according to previous works.[3]

Solution Phase Ligand Exchange: 10 ml PbSe CQDs in octane (10 mg/ml) and 10 ml PbI2 in DMF (0.2 M) was mixed under ambient condition. Upon shaking, the PbSe CQDs were transferred from nonpolar octane phase into polar DMF phase, then octane was used to wash this layered phase three times to remove the exchanged organic ligands. Subsequently, the PbSe CQDs capped by PbI2 were precipitated by centrifugation and dried by pumping. For film deposition, the PbI2 capped PbSe CQDs were dissolved into mixed solvent of DMF and butylamine (5:5 v/v) with concentration of 450 mg/ml.

Device fabrication: SnO2 layer serving as ETL was deposited according to previously reported literature.[4] SnO2 precursor was prepared by dissolving SnCl2·2H2O into isopropanol under stirring for 2 hours. For SnO2 layer, the filtered SnO2 precursor solution was spin-cast on O2 plasma-cleaned ITO glasses at 4000 rpm/min for 30 s, then this spin-cast film was baked on hotplate at 180 oC in air for 30 min. This process was repeated 3 times to obtain SnO2 layer of roughly 70 nm thickness. For SnO2/PCBM layer, 2 mg/ml PCBM chloroform solution was spin-cast on SnO2 layer in N2 filled glovebox at 3000 rpm/min for 30 s. The PbI2-capped PbSe CQDs in mixed solvent was spin-cast on SnO2 or SnO2/PCBM layer in N2

filled glovebox at 2000 rpm for 30 s, followed by annealing at 90 oC for 2 min. For the PbS-EDT as HTL layer, PbS CQDs octane solution was pin-cast on PbSe-PbI2 layer in fume cupboard, treated with EDT acetonitrile solution (0.02% v/v) and rinsed with pure acetonitrile. This process was repeated 2 times. Finally, 80 nm thickness gold electrode was prepared by thermal evaporation. We fabricated PbSe-LBL film using identical process reported by recent literature.[1]

Characterizations:

The PL spectra measurements were performed at room temperature by a homemade laser PL spectroscopy system (CrystaLaser, Model BLC-050-405). The laser pulse width was 130 fs, and the repetition rate was 76 MHz. The excitation wavelength for both PL and TRPL measurements is 800 nm. Scanning electron microscope (SEM) measurements were carried out by using an FEI Nova Nano SEM 450. Transmission electron micrcoscope (TEM) is performed on a JEOL JEM-2010. Atomic force microscopy measurements were performed using SPM9700. X-ray photoelectron spectroscopy (XPS) measurement was carried out with a Genesis system (EDAX Inc.). X-ray diffraction (XRD) patterns were measured using the Empyriean thin film Xpert Materials Research diffractometer system with a triple axis goniometer. The absorption spectra of the solution-based samples were measured by Perkin Elmer Lambda1050 UV–vis–NIR spectrophotometer in ambient conditions. Fourier transform infrared spectroscopy (FT-IR) was performed on a Thermo Fisher FTIR6700. Ultraviolet photoelectron spectroscopy (UPS) measurement (Specs UVLS) conducted in

ultrahigh vacuum photoemission spectroscopy system with a He I excitation, was used to study the energy levels of samples, and 21.2 eV was referenced to the Fermi edge of argon etched gold. TPV and TPC measurements were performed by using identical method in previous work.[1, 4] C-V measurements were carried out using Agilent 4200A at a frequency of 10 kHz and an AC signal of 50 mV, scanning from -1 to +1 V, with a step of 50 mV. Contact angle measurements were conducted by a Data physics OCA-20 system at room temperature in ambient atmosphere.

The current density–voltage characteristics of devices were measured using Keithley 2400 (I-V) digital source meter under a simulated AM 1.5G solar irradiation at 100 mW/cm2

(Newport, AAA solar simulator, 94023A-U).

Figure S1. (a) absorption curve of pristine PbSe QDs in octane and (b) HR-TEM image of PbSe QDs capped by PbI2 serving as absorber layer

Figure S2 (a) Before ligand exchange, PbSe CQDs in hexane float in the upper layer. (b) After ligand exchange upon stirring, PbSe CQDs transfer into under layer. (c) The as-obtained PbSe QDs after centrifugation and supernatant discarding. (d) PbSe QDs re-dispersed in mixed solvent of DMF and butylamine.

Figure S3. XRD patterns of pristine PbSe QDs and PbSe-PTLE film

Figure S4. XPS signal of a detailed O 1s peak fitting in (a) PbSe-OA film, (b) PbSe-LBL film and (c) PbSe-PTLE.

Figure S5. Magnified UPS spectra showing secondary edge (ESE) for determining the work function (WF) and Fermi level (EF) using Equation 1. (b) The magnified UPS spectra showing the onset energy (Eonset) to determine valence band maximum energy (EVBM) using Equation 2. The conductive band energy (ECBM) can be then calculated by Equation 3. Above calculation methods refered to Reference 1 and 2.

WF=EF=21.2-ESE (1)

EVB=EF+Eonset (2)

ECBM=EVBM-Ebandgap (3)

Figure S6. The transmittance spectra of SnO2 and SnO2/PCBM films.

Figure S7. The cross-sectional SEM image of solar cell with (a) device configuration of ITO/SnO2/PbSe-PTLE/PbS-EDT/Au and (b) device configuration of

ITO/SnO2/PbSe-LBL/PbS-EDT/Au.

Figure S8. J-V curve of solar cell with device configuration of ITO/SnO2/PbS-EDT/Au

Figure S9. (a) PbS CQD absorption curve in hexane

Figure S10. Long-term air stability of devices with architecture of ITO/SnO2/PbSe-PTLE/PbS-EDT/Au and ITO/SnO2/PCBM/PbSe-PTLE/PbS-EDT/Au.

[1] L. Hu, Z. Zhang, R. J. Patterson, S. B. Shivarudraiah, Z. Zhou, M. Ng, S. Huang, J. E. Halpert, Solar RRL 2018, 2, 1800234.[2] Z. Zhang, Z. Chen, L. Yuan, W. Chen, J. Yang, B. Wang, X. Wen, J. Zhang, L. Hu, J. A. Stride, Advanced Materials 2017, 29, 1703214.[3] L. Hu, Z. Zhang, R. J. Patterson, Y. Hu, W. Chen, C. Chen, D. Li, C. Hu, C. Ge, Z. Chen, Nano energy 2018, 46, 212; L. Hu, R. J. Patterson, Y. Hu, W. Chen, Z. Zhang, L. Yuan, Z. Chen, G. J. Conibeer, G. Wang, S. Huang, Advanced Functional Materials 2017, 27, 1703687; L. Hu, D. B. Li, L. Gao, H. Tan, C. Chen, K. Li, M. Li, J. B. Han, H. Song, H. Liu, Advanced Functional Materials 2016, 26, 1899.[4] J. Khan, X. Yang, K. Qiao, H. Deng, J. Zhang, Z. Liu, W. Ahmad, J. Zhang, D. Li, H. Liu, Journal of Materials Chemistry A 2017, 5, 17240.