Efficient planar structured perovskite solar cells with enhanced open

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Efficient planar structured perovskite solar cells with enhanced open-circuit voltage and suppressed charge recombination based on slow grown perovskite layer from lead acetate precursor Cong Li, Qiang Guo, Zhibin Wang, Yiming Bai, Lin Liu, Fuzhi Wang, Erjun Zhou, Tasawar Hayat, Ahmed Alsaedi, and Zhan'ao Tan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15229 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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ACS Applied Materials & Interfaces

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Efficient planar structured perovskite solar cells with enhanced open-

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circuit voltage and suppressed charge recombination based on slow

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grown perovskite layer from lead acetate precursor

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Cong Lia, Qiang Guoa, Zhibin Wanga, Yiming Baia, Lin Liua, Fuzhi Wanga, Erjun Zhoub,

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Tasawar Hayatc,d, Ahmed Alsaedid and Zhan’ao Tan *,a

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a

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Sources, School of Renewable Energy, North China Electric Power University, Beijing

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102206, China.

State Key Laboratory of Alternate Electrical Power System with Renewable Energy

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b

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Nanoscience and Technology, Beijing 100190, China.

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c

Department of Mathematics, Quiad-I-Azam University, Islamabad 44000, Pakistan

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d

NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589,

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Saudi Arabia.

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*Corresponding author:

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E-mail: [email protected] (Z. A. Tan)

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for

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1 ACS Paragon Plus Environment


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ABSTRACT: For planar structured organic-inorganic hybrid perovskite solar cells

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(PerSCs) with PEDOT:PSS hole transportation layer, the open-circuit voltage (Voc) of the

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device is limited to be about 1.0 V, resulting inferior performance in comparison with

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TiO2 based planar counterparts. Therefore, increasing Voc of the PEDOT:PSS based

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planar device is an important way to enhance the efficiency of the PerSCs. Herein, we

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demonstrate a novel approach for perovskite film formation, and the film is formed by

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slow-growing from lead acetate precursor via one-step spin-coating process without

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thermal annealing process. Since the perovskite layer grows slowly and naturally, high

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quality perovskite film can be achieved with larger crystalline particles, less defects and

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smoother surface morphology. Ultraviolet absorption (UV), X-ray diffraction (XRD),

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scanning electron microscopy (SEM), steady state fluorescence spectroscopy (PL) and

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time-resolved fluorescence spectroscopy (TRPL) are used to clarify the crystallinity,

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morphology and internal defects of perovskite thin films. The PCE of p-i-n PerSCs based

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on slow-grown film (16.33%) shows greatly enhanced performance compared with

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control device based on traditional thermally annealed perovskite film (14.33%).

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Furthermore, the Voc of slow-growing device reaches 1.12 V, which is 0.1 V higher than

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that of the thermal annealing device. These findings indicate that slow growing of the


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perovskite layer from lead acetate precursor is a promising approach to achieve high

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quality perovsikte film for high performance PerSCs.

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KEYWORDS: perovskite solar cells, lead acetate precursor, high Voc, non-radiative

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recombination, slow-growing

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1. INTRODUCTION

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Thin film of organic-inorganic hybrid perovskite has been extensively studied as the

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main optical absorption materials for perovskite solar cells (PerSCs).1-3 The power

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conversion efficiency (PCE) of PerSCs has been remarkably boosted from 3.8% to a

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certified 22.1%.4-6 The perovskite materials can be achieved by various techniques,

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including one-step solution deposition,7-9 sequential solution deposition,10 low-

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temperature vapor-assisted solution process,11-13 and vacuum-evaporation deposition.14-16

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The outstanding photovolatic performance of PerSCs should attribute to the unique

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properties of perovskite materials such as long and balanced carrier diffusion length,17

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high defect tolerance along with low charge recombination, high light absorption

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coefficient over the visible spectra range with a sharp absorption onset.10, 18 These unique

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merits have allowed them to be extremely promising for the next-generation

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photovoltaics.



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Currently, two structures of n-i-p and p-i-n are commonly used for constructing

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perovskite solar cells. Compared with the high temperature sintering (TiO2, ZnO and

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Al2O3) in n-i-p structure, the hole transport layer (PEDOT:PSS or PTAA) in p-i-n

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structure is generally prepared by low temperature solution processing, which is time and

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energy saving. However, the work function of PEDOT:PSS is lower than that of Spiro-

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OMeTAD, which is closer to the valence band (VB) of perovskite (CH3NH3PbI3),

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inducing a lower Voc of the PEDOT:PSS based devices.19-21 The Voc of the Spiro-

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OMeTAD based device is about 1.1 V, while the Voc of the PEDOT:PSS based device is

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only 1.0 V, which limits the upgrade of PCE. Therefore, increasing the Voc of the

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PEDOT:PSS based device is an important way to improve the overall efficiency of

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PerSCs. There have been some studies on improving the Voc in n-i-p structures. For

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example, Michael Saliba reported a Voc of 1.20 V by way of organic cation stoichiometry

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modification and a Voc of 1.24 V by Rb incorporation, which further suppresses phase

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defects in the perovskite material.22 In addition, using SnO2 as the electron transport layer,

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the Voc of n-i-p planar device also exceed 1.20 V.23, 24 How to obtain higher Voc and PCE

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in PEDOT:PSS based structure is of great significance for exploiting the inherent

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advantages of p-i-n structure.


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In the perovskite solar cell with p-i-n structure, the Voc of the device with lead acetate

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as lead source is generally higher than that from lead iodide.7, 9, 25 In reaction of lead

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acetate (Pb(Ac)2) with ammonium salt (MAI), byproduct of methylamine acetate (MAAc)

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with sublimation point of only 60 will be produced.25 Both film smoothness and surface

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coverage of the perovskite film could be improved by using Pb(Ac)2 as lead source

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because of the facile removal of MAAc.7,

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perovskite films with lead acetate by one-step spin-coating method.9 Ultrasmooth

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perovskite films were obtained by low temperature and a shorter thermal annealing time,

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a power conversion efficiency (PCE) of 15% was achieved in the n-i-p planar

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heterostructure perovskite solar cells.9 Hypophosphorous acid as additive in lead acetate

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precursor solution can improve the performance of the device, and finally a PCE of about

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16% was obtained.26 In addition, a small amount of fullerene derivatives27, MABr20, 28

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and lead chloride (PbCl2)29 can also be incorporated into lead acetate precursor solution

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to improve the device performance by optimizing the quality of perovskite thin film.

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There are reports on the fabrication of

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In this study, we demonstrate a novel approach for perovskite film formation, and the

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film is formed by slow-growing from lead acetate precursor via one-step spin-coating

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process without thermal annealing process. Since the perovskite layer grows slowly and

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naturally, high quality perovskite film can be achieved with larger crystalline particles, 5 ACS Paragon Plus Environment


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less defects and smoother surface morphology. Thus, the non-radiative recombination

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due to the defect state is reduced, and the unnecessary loss of the built-in potential is

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avoided. The Voc of the slow-growing device reaches 1.12 V, which has 0.1 V higher than

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that of the thermal annealing device. The perovskite solar cells prepared by the slow-

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growing process has the highest PCE of 16.33%, and almost no hysteresis. Compared

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with the thermal annealing device (14.33%), the device performance of slow-growing has

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been significantly improved.

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2. EXPERIMENTAL SECTION

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2.1 Material: Patterned fluorine doped tin oxide (FTO) glass with sheet resistance

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of 14 Ωsq−1 was purchased from Wuhan Geao Instruments Science & Technology Co.,

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Ltd (China). Methylamine solution (40 wt% in methanol), hydriodic acid (HI, 57 wt% in

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water), anhydrous N,N-Dimethylformamide (DMF), Pb(Ac)2 and 75% (titanium

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(diisopropoxide) bis(2,4-pentanedionate)) TIPD isopropanol solution were purchased

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from Alfa Aesar. [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was purchased

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from Nano-C Inc. Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)

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aqueous solution (Clevious P VP AI 4083) was purchased from H. C. Stark. These

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commercially available materials were used directly without further purification.

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Methylammonium iodide (CH3NH3I) was synthesized according previous report.12 6 ACS Paragon Plus Environment

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Typically, 27.86 mL of methylamine (CH3NH2) and 30 mL of hydroiodic acid (HI) were

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mixed in a 100 mL round bottomed flask at 0

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mixture was rotation-evaporated at 50

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resulting powder was collected after dried in a vacuum oven at 60

and then stirred for 2 h. The reaction

for 1 h, yielding white precipitate. Finally, the overnight.

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2.2 Device fabrication: FTO coated glass substrates were ultrasonic cleaned twice

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with detergent, deionized water, acetone and isopropanol in sequence for 15 min. The

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pre-cleaned FTO substrates were then cleaned by UV-ozone (Shanghai Guoda UV

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Equipment Co., Ltd) for 15 min to remove the residual organic contaminant. The hole

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collection layer of PEDOT:PSS was spin-coated from its aqueous solution filtered with a

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0.45 µm filter on the cleaned FTO substrate at 2000 rpm for 35 s, and then thermal

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annealed at 150

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nm. The modified substrates were transferred into a nitrogen-filled glove-box for

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following procedures. The perovskite thin films were obtained through one-step spin-

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coating method, and share same precursor solution. Mixture of 162.5 mg Pb(Ac)2, 238

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mg MAI, 12 mg MABr in 500 µL DMF was spined coated on FTO/PEDOT:PSS

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substrate at 6000rpm for 40s. For TA devices, the precursor films were annealed at 85

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for 10 min. For SG devices, the as prepared films are removed from the glove-box and

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placed for 70-90 min at ambient temperature and humidity of 20-40%. After that, SG

for 15 min in air. The thickness of the PEDOT:PSS layer is around 35



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perovskite film can be obtained. Then the electron collection layer of PCBM was

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obtained by spin-coating its chlorobenzene solution (20 mg/mL) onto the perovskite layer

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at 1000 rpm for 30 s in a nitrogen-filled glove-box. The TIPD electron collection layer

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was prepared by spin-coating (2000 rpm) a 3.75 wt% TIPD isopropanol solution on the

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PCBM layer and then baking it at 150

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thermally deposited under a base pressure of 5×10−5 Pa. The active area of the device was

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fixed at 4 mm2.

for 10 min. Finally, a 100 nm Al electrode was

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2.3 Device characterization: The current density-voltage (J-V) measurements of the

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devices were conducted on a computer-controlled Keithley 2400 Source Measure Unit

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(SMU). Device characterization was done in a glove-box under simulated AM1.5G

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irradiation (100 mW/cm2) using a xenon-lamp-based solar simulator (AAA grade, SAN-

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EI ELECTRIC Co., Ltd). The external quantum efficiency (EQE) was measured using a

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Systems model QE-DLI lock-in amplifier coupled with a QE-M110 monochromator and

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75W xenon lamp (Enli Technology Co., Ltd.). The light intensity at each wavelength was

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calibrated with a standard single-crystal Si photovoltaic cell. The EQE measurements

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were performed under ambient conditions at room temperature.

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2.4 Instrumentation. The absorption spectra were measured by UV-2450 UV-VIS

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Spectrophotometer. The surface and the cross-section morphology of the samples were 8 ACS Paragon Plus Environment

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observed by Scanning electron microscopy (SEM) of FEI Quanta 200F at an accelerating

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voltage of 10 kV. X-ray diffraction (XRD) spectra of the films were acquired by using a

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D8 Advance (Bruker) diffractometer with an X-ray tube for Cu Kα radiation (λ = 0.1542

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nm) to analyze the structures of films. A Dektak XT (Bruker) surface profilometer was

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used to measure the thickness of the films involved in the devices. For time-resolved

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photoluminescence (TRPL) measurement, the spectrograph was equipped with a fast

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single-photon avalanche photodiode (PDM, Picoquant), and the temporal resolution for

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the detection is 50 ps. The surface morphology and phase images of the perovskite

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derived from SG process for different slow-growing time were analyzed using an Agilent

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5500 atomic force microscope (AFM) operated in the tapping mode under ambient

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atmosphere at room temperature. An ESCA Lab220i-XL electron spectrometer from VG

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Scientific using 300 W Al Kα radiation operated at a base pressure of 3×10−9 mbar was

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used to obtain XPS data. The binding energies were referenced to adventitious carbon

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(C1s line at 284.9 eV).

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3. RESULTS AND DISCUSSION

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The fabrication procedures of p-i-n structured perovskite solar cells with lead acetate

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by thermal annealing (TA) and slow-growing (SG) are illustrated in Fig. 1a. Firstly, the



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perovskite precursor film was prepared by spin-coating (4000 rpm) the DMF mixture

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solution of Pb(Ac)2 and MAI (1:3 molar, 44wt%) on FTO/PEDOT:PSS substrate.

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Secondly, the precursor perovskite film were subsequently heated at 85

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(TA process), or remain stationary out of glove box for some time (SG process), and then

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the perovskite films can be obtained. The third step is to spin the electron transport layer

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PC60BM and the cathode buffer layer TIPD12,

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perovskite solar cell was obtained after evaporating the Al electrode in a vacuum

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chamber. Among them, the TA device is the reference device using traditional fabrication

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process. Fig. 1b and c shows the absorption curves and photos of the perovskite films

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derived from SG process on quartz glass for different time. As can be seen from Fig. 1b,

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the absorption curve (black and red) of the obtained film is mainly governed by the

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absorption characteristic peaks of PbI2 (about 450 nm) in the first 20 minutes,31 then

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Pb(Ac)2 gradually reacts with MAI to yield PbI2 and MAAc. The pictures of the samples

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in the first 10 minutes are obvious yellowish as shown in Fig. 1c. The color of slow-

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growing sample for 20 minutes is much darker than that for 10 minutes, because the

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sublimation point of MAAc is very low, and it is easy to leave the film. During 30-60

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minutes, the PbI2 peaks gradually disappear, and the absorption intensity of the film is

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enhanced. However, there is still no obvious MAPbI3 perovskite characteristic peak

30

for 10 minutes

successively, and then the p-i-n


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(about 760 nm).32 Therefore, it is possible that the film during 30-60 minutes could be a

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MAI-PbI2-DMF intermediate film. The photos in Fig. 1c also show that the color of the

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film is getting darker and darker between 30-60 minutes. To further confirm the

4

composition of the intermediate states, we characterize the intermediate states by XRD,

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as shown in Fig. S1. The main characteristic peaks are consistent with the results in the

6

literature.33,

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minutes, the absorption intensity of thin films (Fig. 1b) is significantly enhanced, and the

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MAPbI3 perovskite characteristic peaks appear. It means that the perovskite has been

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formed when slow-growing for 70 minutes, and the potential transition processes are

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34

It is proved that the intermediate state is MAI-PbI2-DMF. After 70-80

shown in Fig. 1d.

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Figure 1. (a) The fabrication procedures of p-i-n perovskite solar cells with lead acetate

2

by thermal annealing (TA) and slow-growing (SG); (b) The absorption curves of films

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fabricated by SG process on quartz glass for different time; (c) Photos of films prepared

4

by slow-growing process for different time; (d) Schematic diagram of SG process.

5

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Tapping model atomic force microscopy (AFM) is used to monitor the morphology

7

and phase change of perovskite film during slow-growing. Fig. S2 shows AFM top view

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(a-l) and AFM phase (ap-lp) images of perovskite intermediate film with different slow-

9

growing time. The morphology of the film slow-growing for ten minutes shows many

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vein-like shapes, since the intermediate film is relatively soft, and the film has not been

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crystallized. The phase changes significantly in the 15 to 20 minutes as shown in Fig. S2.

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Combining the results of photographs and absorption spectra at this period (Fig. 1b, c), it

13

shows that this process is the most acute reaction stage from the intermediate to the

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perovskite. After 20 minutes, the phase and the corresponding photograph and absorption

15

spectra gradually change to the perovskite. To further identify whether there is residual

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MAAc presence in the final perovskite film obtained by slow growth. X-ray

17

photoelectron spectroscopy (XPS) measurement was used to detect the characteristic


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group of OC=O in MAAc. As shown in Fig. S3a, the C 1s peak of the perovskite film is

2

located at 284.9 eV, which should be assigned to the carbon in -C-H group, and the very

3

weak O1s peak at 532.3 eV should be attributed to the absorbed oxygen from ambient

4

atmosphere as shown in Fig. S3b. 35 While the characteristic binding energy for C 1s and

5

O 1s in OC=O group is located at 288.9 and 532.2 eV,36 respectively. The core level XPS

6

spectra of O1s, and C 1s indicate that there is no residual MAAc left in slow-growing

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perovskite film. Thus, the SG process can be simply summarized into MAI-MAAc-PbI2-

8

DMF, MAI-PbI2-DMF and MAPbI3 these three processes, as shown in Fig. 1d.

9

Furthermore, for comparison, both perovskite films derived from PbI2 source and

10

Pb(Ac)2 source were fabricated followed by slow-growing for different time, as shown in

11

Fig. S4. It can be seen clearly that the PbI2 sourced film is gradually changing gray, and

12

finally the perovskite films display poor surface morphology. However, the perovskite

13

films derived from Pb(Ac)2 source demonstrate smooth mirror-like surface and deep dark

14

color (70 min), just like thermally annealed perovskite derived from PbI2 source. During

15

the experiment, we found that the reaction time is related to the humidity and temperature

16

of the environment. Under certain temperature and humidity, by changing the spin

17

coating conditions of precursor solution, the volatilization of solvent DMF can be

18

controlled, so as to prepare high quality perovskite film. Photos of slow-growing 13 ACS Paragon Plus Environment


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perovskite films with different spin coating conditions at 27

2

given in Fig. S5. As shown in Fig. S5, the speed, acceleration and time during the spin

3

coating process have a tremendous impact on the quality of the slow-growing perovskite

4

film.

and 35% humidity are

5

Fig. 2 shows the SEM images of cross section and top view of the perovskite films

6

derived from thermal annealing and slow-growing. As can be seen from Fig. 2, the

7

perovskite thin film derived from SG process is more compact and has larger particles

8

than TA perovskite film. In Fig. 2a, the perovskite film fabricated by TA process has

9

higher density of grain boundaries among the particles, which should be the non-radiative

10

recombination centers of electron and hole,37 and could lead to the loss of the built-in

11

potential.38 The SG perovskite layer shows larger particles and less boundaries, which

12

could reduce the energy loss of the photo-generated charges. With the SEM images in

13

Fig. 2 and the slow-growing process in Fig. 1d, the crystallization of SG perovskite

14

should be slow and time-consuming process. With slow-growing process, the perovskite

15

particles could grow larger and denser, thus the generation probability of defects could be

16

effectively reduced. However, in the process of thermal annealing, a large number of

17

defects are possibly produced because of the fast crystallization process, which is

18

unfavorable to the crystallization of perovskite. 14 ACS Paragon Plus Environment

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Figure 2. SEM images of (a, b) cross section and (c, d) top view of the perovskite films

3

derived from thermal annealing (TA) and slow-growing (SG).

4

5

To investigate the crystalline quality of perovskite thin films, X-ray diffraction (XRD)

6

spectroscopy is introduced as shown in Fig. 3. For comparison, the XRD patterns of the

7

CH3NH3I powder (Fig. 3a), Pb(Ac)2 powder (Fig. 3b), PbI2 powder (Fig. 3c) as well as

8

FTO/PEDOT:PSS substrate (Fig. 3d) are also given. Fig. 3e displays the XRD patterns of

9

the CH3NH3PbI3 perovskite film grown on the FTO/PEDOT:PSS substrate derived from

10

TA process. A series of new diffraction peaks appear in comparison with PbI2 and

11

CH3NH3I at 14.1º (110), 19.8º (112), 25.2º (202), 28.4º (220), 32.5º (312), 41.3º (224)

12

and 43.8º (330), assigned to the tetragonal phase of the CH3NH3PbI3 perovskite, and the

13

diffraction peaks are in good agreement with previous reports.7,


12

There is no PbI2


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characteristic peaks appear in perovskite films fabricated by thermal annealing or slow

2

growth, indicating that there is no PbI2 yield during the formation of perovskite film from

3

Pb(Ac)2 precursor. Only (110), (220) and (330) three characteristic peaks can be observed

4

in SG perovskite thin films. No characteristic peaks of the substrate could be found,

5

which means that the SG perovskite thin film is very dense and no substrate exposed. In

6

addition, compared the intensity of two main characteristic peaks of (110) and (220), the

7

ratios of the two peaks are 1.10 and 0.866, respectively. This shows that the orientation of

8

SG perovskite crystal has changed. In other words, the SG perovskite thin film is more

9

compact than the annealed films, and it is no longer consistent with TA perovskite in the

10

crystalline direction.


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1 2

Figure 3. X-ray diffraction spectra of (a) CH3NH3I powder, (b) Pb(Ac)2 powder, (c) PbI2

3

powder, (d) FTO/PEDOT:PSS substrate, as well as perovskite thin film derived from (e)

4

thermal annealing and (f) slow-growing on FTO/PEDOT:PSS substrate.

5

6

The defects inside perovskite could indirectly reflect on the optical properties of thin

7

films. Therefore, the absorption and steady-state photoluminescence (PL) spectra of

8

perovskite films derived from TA and SG are investigated. As shown in Fig. 4a, the

9

absorption intensity of the SG perovskite thin film is equal to that of the TA perovskite



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

thin film, while the PL intensity of SG perovskite thin film is much stronger than that

2

from TA. The possible reason for the enhanced PL should ascribe to water molecule

3

infiltration to perovskite during slow-growing in ambient atmosphere, which is beneficial

4

to the crystallization of perovskite films. At present, several groups have employed first

5

principles calculation and molecular dynamics to understand the degradation pathways

6

and the atomic interaction between perovskite and water molecules.39-41 The water

7

molecules can spontaneously permeate into perovskite lattice and form hydrogen bonds

8

with MA cations and iodine ions, and photoluminescence could be enhanced after water

9

infiltration.40-43 According to the aforementioned SEM and XRD data, SG perovskite thin

10

film has larger particle size and fewer boundaries between particles, inducing longer

11

carrier diffusion length and decreased non-radiative recombination, thereby increasing

12

the radiation recombination and enhancing the fluorescence.44 To detect the

13

recombination of photogenerated carriers during the operation of PerSCs device, time-

14

resolved photoluminescence (TRPL) decays of the perovskite films are measured at the

15

peak emission of 770 nm. The tested results were fitted with a biexponential model.44 The

16

specific numerical fast decay component τ1 and the long decay component τ2 is inserted

17

into Fig. 4b, which respectively represent the non-radiative recombination defect states

18

and carrier recombination radiation.45,

46

The average PL lifetime for slow-growing 18

ACS Paragon Plus Environment

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1

perovskite is 95.24 ns and that for thermal annealing perovskite is 52.96 ns. The radiation

2

recombination of free carriers is enhanced in SG perovskite film, which corresponds to

3

the results of PL enhancement. Meanwhile, the enhanced radiative recombination of

4

carriers is beneficial to the increase of average PL lifetime τavg. The proportion of non-

5

radiative recombination in SG perovskite thin film (46%) is less than TA perovskite thin

6

film (73%). This result indicates that SG perovskite thin film has fewer defects and a

7

longer carrier diffusion length. This is in good agreement with the findings in SEM and

8

XRD since the uniform and dense film with high crystallinity, large particle size and less

9

boundaries provides a guarantee for long distance travel before carrier recombination.

10 19 ACS Paragon Plus Environment


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1

Figure 4. (a) Absorbance and steady-state photoluminescence (PL) spectra of the

2

perovskite thin film derived from TA and SG on quartz glasses; (b) The time-resolved

3

photoluminescence of the corresponding perovskite films spin-coated on quartz glasses,

4

inset shows the two-component exponential fitted carrier lifetime.

5

6

To further verify the photovoltaic performance of SG PerSCs, p-i-n type devices

7

with structure of FTO/PEDOT:PSS/Perovskite/PCBM/TIPD/Al are fabricated. For

8

comparison, both perovskite layers are fabricated by thermal annealing (TA) and by

9

slow-growing (SG). Current density-voltage (J-V) curves of champion perovskite solar

10

cells measured starting with reverse scan and continuing with forward scan are shown in

11

Fig. 5a. The device parameters of open-circuit voltage (Voc), short-circuit current density

12

(Jsc), filling factor (FF), and PCE are listed in Table 1. PerSCs derived from TA gives a

13

PCE of 14.48% at forward scan, and a PCE of 14.72% at reverse scan. While the device

14

fabricated by SG present negligible hysteresis. The PCE obtained at the forward is

15

16.33%, with a Voc of 1.12 V, a Jsc of 19.79 mA/cm2, and an FF of 73%, and the PCE

16

obtained at the reverse scan keeps at 16.25%, with almost unchanged Voc, Jsc, and FF.

17

The thermal annealing (TA) process based perovskite solar cells (PerSC)s still show a


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1

minor hysteresis compared to those based on SG process. This should ascribe to the

2

different defect density, since SG process could induce high quality perovskite film with

3

less defects and longer carrier lifetime as shown in Fig. 4.

4

The stabilized current/power output of the PerSCs based on slow-grown and thermal

5

annealed perovskite film is monitored at the maximum power point with working voltage

6

of 0.90 and 0.86 V, respectively. As shown in Fig. 5c, the SG device demonstrates stable

7

output PCE of 16.02% and Jsc of 19.25 mA/cm2, much higher than that (14.30% and

8

19.16 mA/cm2) of TA based devices. The high reproducibility of the PerSCs fabricated

9

by slow-growing and thermal annealing were also investigated by statistical assessment

10

of the five device parameters (Voc, Jsc, FF, PCE, and Jsc integrated from EQE) distribution

11

within 30 individual devices as shown in Fig. S8. Obviously, the five parameters of the

12

devices based on perovskite film obtained by slow growth are much higher than that of

13

the control device based on perovskite film obtained by thermal annealing. Comparing

14

the performance parameters of the devices derived from TA and SG. The values of Jsc are

15

close and have no obvious difference, that is, the charge transport properties of the SG

16

perovskite are comparable to that of the conventional perovskite. However, in terms of

17

Voc and FF, SG devices exhibit distinct advantages. In particular, the open circuit voltage

18

has an increase of 0.1 V, which proves that the built-in potential in the PerSCs is 21 ACS Paragon Plus Environment


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1

reinforced. The depletion of the built-in potential is mainly due to the recombination

2

center formed by trap states.5, 47 As the most important photoactive layer in perovskite

3

solar cells, the crystallinity of perovskite determines the number of trap states. Therefore,

4

the Voc with 0.1 V increase shows that the potential loss of PerSCs is suppressed, that is,

5

the crystallization of perovskite layer is perfect and has few trap states. EQE spectra of

6

devices are shown in Fig. 5b. The integrating Jsc for TA and SG devices are 19.08 and

7

19.20 mA/cm2, respectively, which is close to the Jsc derived from J-V curves.

8

With slow-growing process, high quality larger area perovskite film with dimension

9

of 1.5 cm by 1.5 cm can also be achieved as shown in Fig. S4 and Fig. S5. The J-V

10

curves of the large area (1cm×1cm) device based on SG process perovskite film are

11

shown in Fig. S6, and the parameters of device is given in Table S1. Control Spiro-

12

OMeTAD based PerSCs (FTO/TiO2/perovskite/Spiro-OmeTAD/Au) by SG and TA

13

processes were also fabricated. The J-V curves of the devices are shown in Fig. S7, and

14

the parameters of devices are listed in Table S2. Obviously, the devices based on SG

15

perovskite film still demonstrate higher Voc in comparison with the TA based devices.

16

This finding is in good agreement with the observation in PEDOT:PSS based devices as

17

shown in Fig. 5a.


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1 2

Figure 5. (a) Current density-voltage (J-V) curves of champion perovskite solar cells

3

measured starting with reverse scan and continuing with forward scan; (b) IPCE spectra

4

of the corresponding devices; (c) Stabilized Jsc and PCE at maximum power point of the

5

PerSCs.



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Page 24 of 31

1

Table 1. Photovoltaic performance of the PerSCs based on the perovskite thin films

2

derived from TA and SG under AM1.5G, 100mW/cm2 illumination. Voc (V)

Jsc (mA/cm2)

FF（%）

PCE（%）

Forward

1.02

19.94

71

14.48

Reverse

1.02

20.07

71

14.72

Forward

1.12

19.79

73

16.33

Reverse

1.12

19.77

73

16.25

Condition

TA

SG

3

4

CONCLUSION

5

In summary, we successfully demonstrate a novel approach for achieving perovskite

6

film by slow-growing from lead acetate precursor via one-step spin-coating process

7

without thermal annealing process. More uniform and denser perovskite thin films with

8

larger crystalline particles, fewer defects were obtained by this method as confirmed by

9

TRPL and SEM tests. The precursor-perovskite transition processes were evidenced by

10

film absorption. The PerSCs based on slow grown film exhibit no hysteresis and achieve

11

the champion PCE of 16.33%, which is significantly improved compared with the

12

thermal annealed PerSCs. Furthermore, the Voc of the device based on slow grown film is

13

0.1 V higher than the control device based on thermal annealed film, mainly due to the 24 ACS Paragon Plus Environment

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1

greatly supressed defects generation in the process of naturally slow-growing of the

2

perovsikte film, which decreases the non-radiative recombination of the defect states and

3

minimizes the built-in potential loss. Our study offers a simple but effective approach

4

towards high efficiency and simply prepared PerSCs.

5

AUTHOR INFORMATION

6

*Corresponding author:

7

E-mail: [email protected] (Z. A. Tan)

8 9

Notes

10

The authors declare no competing financial interest.

11

ACKNOWLEDGMENTS

12

This work was supported by the NSFC (51573042), the National Natural Science

13

Foundation of Beijing (2162045), and Fundamental Research Funds for the Central

14

Universities, China (JB2015RCJ02, 2016YQ06, 2016MS50, 2016XS47, 2016XS49,

15

2017XS084).

16

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Table of contents (Toc)

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Efficient planar structured perovskite solar cells with enhanced open

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