Article Cite This: J. Phys. Chem. C 2017, 121, 24903-24908
pubs.acs.org/JPCC
Unraveling the Impact of Rubidium Incorporation on the TransportRecombination Mechanisms in Highly Efficient Perovskite Solar Cells by Small-Perturbation Techniques Abdulrahman Albadri,*,† Pankaj Yadav,‡ Mohammad Alotaibi,§ Neha Arora,‡ Ahmed Alyamani,† Hamad Albrithen,†,∥ M. Ibrahim Dar,‡ Shaik M. Zakeeruddin,‡ and Michael Graẗ zel*,‡ †
National Center for Nanotechnology, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Federale de Lausanne, Station 6, CH-1015 Lausanne, Switzerland § National Center for Petrochemical, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia ∥ Physics Department, Science College, King Saud University, P.O. Box 6086, Riyadh 11442, Saudi Arabia ‡
S Supporting Information *
ABSTRACT: We applied intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) techniques to explore the effect of rubidium (Rb) incorporation into lead halide perovskite films on the photovoltaic parameters of perovskite solar cells (PSC). IMPS responses revealed the transport mechanisms at the TiO2/ perovskite interface and inside the perovskite absorber films. For recombination time constants, IMVS showed that the two perovskite solar cells differ in terms of trap densities that are responsible for recombination loss. Impedance spectroscopy carried out under illumination at open circuit for a range of intensities showed that the cell capacitance was dominated by the geometric capacitance of the perovskite layer. Our systematic studies revealed that Rb containing PSCs exhibit enhanced charge transport, slower charge recombination, faster photocurrent transient response, and lower capacitance than the Rb-free samples.
■
INTRODUCTION
an ionic radius of 1.67 Å can help the suppression of nonperovskite formation of the mixed MAFA composition.9 It also reduces residual PbI2 in the perovskite film. The stability improvement with adding inorganic elements (Cs) to the unstable organic elements (MA and FA) motivated the exploration of other inorganic elements such as Rubidium (Rb). Rb in RbPbI3 with an ionic radius of 1.52 Å corresponds to a Goldschmidt factor of 0.8 which is just below the tolerance value.10 However, Rb was reported to improve the performance and photostability of CsFAMA based PSCs without compromising on the formation of the black perovskite phase.12 The incorporation of Rb cations in the CsFAMA films substantially improved the photovoltage. However, questions surround the role of these Rb cations toward realizing such a remarkable advancement. We show that Rb incorporation into triple cation PSCs enables faster electron transport within the mesoporous TiO2 electrodes and provides lower recombination processes at the TiO2/perovskite interface. As a result, Rb incorporated PSCs
Since the first report on perovskite solar cells (PSCs) with an efficiency of 3.8%,1 many research groups have been active in developing higher efficiency with a current world record of 22.1%.2 The impressive achievements together with their potential of replacing silicon solar cells have put PSCs at the spotlight over the last several years. PSCs fabricated using solution-based easy and cost-effective methods have advantages in comparison to other types of solar cells that require either high temperature processes or vacuum systems.3,4 The main remarkable properties of perovskite materials are a high absorption coefficient and a large built-in voltage that lead to high short circuit current densities and high open circuit voltages, respectively.5 Although the latest progress and developments in the PSCs make this technology very promising and commercially viable, the susceptibility of perovskite toward moisture renders PSCs with their current architecture impractical. Lately, incorporation of inorganic cations into the perovskite framework has been found to enhance the moisture resistivity and photostability of PSCs. Adding an inorganic A cation to the organic methylammonium (MA) and formamidinium (FA) cations can improve the perovskite film crystallinity and stability.6−8 For example, cesium (Cs) with © 2017 American Chemical Society
Received: May 19, 2017 Revised: July 19, 2017 Published: August 21, 2017 24903
DOI: 10.1021/acs.jpcc.7b04766 J. Phys. Chem. C 2017, 121, 24903−24908
Article
The Journal of Physical Chemistry C
bis(trifluoromethylsulfonyl)imide lithium salt, tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)- cobalt(III) tris(bis(trifluoromethylsulfonyl) imide) (FK 209, from Dyenamo) and 4-tert-butylpyridine in a molar ratio of 0.5, 0.05 and 3.3, respectively. The device fabrication was completed by thermally evaporating 80 nm of gold layer as a back contact. For light J−V measurements, Oriel VeraSol-2 class AAA LED solar simulator was employed.14 The active area was fixed to 0.16 cm2 using a mask and the measurements were recorded in reverse bias (from Voc) at a scan speed of 50 mV/s. The impedance measurements were performed using a Bio-Logic SP-300 potentiostat.15 A dc potential bias was applied and overlaid by a sinusoidal ac potential perturbation over a frequency range of 1 MHz to 1 Hz. The applied dc potential bias was changed by 100 mV steps from 1100 to 0 mV. IMVS and IMPS measurements were done by using a frequency response analyzer (FRA) combined with Bio-Logic SP-300 potentiostat. The modulation current was set to be 10% of the DC background illumination intensity. The modulated cool white-LED array (12 V, 10 W) light source was driven by a galvanostatic mode of Biologic SP-300 which can provide both DC and AC components of illumination.
achieve substantially higher cell performance compared to the reference triple cation PSCs by improving the power conversion efficiency (PCE) from 18.64 to 19.53%. This enhancement was mainly due to better surface passivation that boosted the open circuit voltage (Voc) from 1.11 to 1.18 V. In this paper, we investigate the solar cell properties using electrochemical impedance spectroscopy (EIS) along with intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) techniques to understand the impact of Rb incorporation on the photovoltaic parameters of quadruple cation PSCs (Rb0.05(Cs0.05(MA0.17FA0.83)0.95)0.95Pb(I0.83Br0.17)3) in comparison to triple cation PSCs (Cs 5 (MA 0.17 FA 0.83 ) 0.95 Pb(I0.83Br0.17)3), specifically, the open circuit voltage of PSCs.
■
EXPERIMENTAL SECTION Fluorine-doped tin oxide (FTO, TCO glass, NSG 10, Nippon sheet glass, Japan)-coated glass substrates were first laser etched to isolate the two electrodes. FTO glass substrates were cleaned by ultrasonication in Hellmanex solution (2%, deionized water) and rinsed thoroughly with deionized water and then ultrasonicated in ethanol and acetone, and finally treated in oxygen plasma for 15 min. A 30 nm TiO2 compact layer was subsequently deposited on cleaned FTO coated glass substrates via spray pyrolysis at 450 °C using a commercial titanium diisopropoxide bis (acetylacetonate) solution (75% in 2propanol, Sigma-Aldrich) diluted in anhydrous ethanol (1:9, volume ratio) as precursor and oxygen as a carrier gas. A mesoporous TiO2 scaffold layer was then deposited using diluted TiO2 nanoparticle paste of ∼30 nm (Dyesol 30NRD: ethanol, 1:6 wt. ratio) by spin coating at 4000 rpm. Subsequently, the photoanodes were sequentially sintered for 5 min at 125 °C, 5 min at 325 °C, 5 min at 375 °C, 15 min at 450 °C, and 15 min at 500 °C. Lithium doping of mesoporous TiO2 was accomplished by spin coating a freshly prepared solution of Li-TFSI in acetonitrile (10 mg/mL) at a spin speed of 3000 rpm for 20 s, after a loading time of 5 s.12 This was followed by second sintering step at 450 °C for 30 min. After cooling down to 150 °C, the substrates were transferred into a dry air glovebox. Deposition of Perovskite Layer. The perovskite films composed of CsMAFA/Pb/[I x Br 1−x ] 3 perovskite and RbCsMAFA/Pb/[IxBr1−x]3 were synthesized using the antisolvent technique.13 The precursor solution containing FAI (1M), PbI2 (1.1 M), MABr (0.2 M) and PbBr2 (0.2 M) in anhydrous dimethylformamide/dimethyl sulfoxide (4:1 (v:v)) was prepared. Thereafter, CsI (Acros 99.9%), (5%, 1.5 M DMSO) was added to the precursor solution for triple cation perovskite precursor solution. For Rb incorporated perovskite, both CsI and RbI (Sigma-Aldrich, 99.9%), (5%, 1.5 M DMSO) were added to obtain quadruple cation perovskite precursor solution. The perovskite films were deposited by spin coating the precursor solution onto the mesoporous TiO2 films in a two-step program at 1000 and 6000 rpm for 10 and 30 s, respectively. During the second step, 100 μL of chlorobenzene was dropped on the spinning substrate 10 s prior to the end of the program. The substrates were then annealed at 100 °C for 1 h in a dry air glovebox with relative humidity
