Multifunctional dopamine-assisted preparation of efficient and stable perovskite solar cells

doi:10.1016/j.jechem.2020.05.061

Journal of Energy Chemistry

Volume 54, March 2021, Pages 291-300

https://doi.org/10.1016/j.jechem.2020.05.061 Get rights and content

Abstract

Perovskite solar cells (PSCs) show great potential for next-generation photovoltaics, due to their excellent optical and electrical properties. However, defects existing inside the perovskite film impair both the performance and stability of the device. Uncoordinated Pb²⁺, uncoordinated I⁻, and metallic Pb (Pb⁰) are the main defects occur during perovskite film preparation and device operation, due to the volatilization of organic cationic components. Passivating these defects is a desirable task, because they are non-radiative recombination centers that cause open-circuit voltage (V_OC) loss and degradation of the perovskite layer. Herein, the multifunctional bioactive compound dopamine (DA) is introduced for the first time to control the perovskite film formation and passivate the uncoordinated Pb²⁺ defects via Lewis acid-base interactions. The Pb⁰ and I⁻ defects are effectively suppressed by the DA treatment. At the same time, the DA treatment results in a stronger crystal orientation along the (110) plane and upshifts the valence band of perovskite closer to the highest occupied molecular orbital (HOMO) of the hole transport layer (2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamine)-9,9′-spirobifluorene, spiro-OMeTAD), which is beneficial for charge separation and transport processes. Consequently, the stability of MAPbI₃ (MA = CH₃NH₃) PSCs prepared with the DA additive (especially the thermal stability) is effectively improved due to the better crystallinity and lower number of defect trap states of the perovskite film. The optimized MAPbI₃ PSCs maintain approximately 90% of their original power conversion efficiency (PCE) upon annealing at 85 °C for 120 h. The best performance triple-cation perovskite (Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃) (FA = formamidinium) solar cell with ITO/SnO₂/Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃:DA/spiro-OMeTAD/MoO₃/Ag (ITO = indium tin oxide) structure shows a PCE of 21.03% with negligible hysteresis, which is dramatically enhanced compared to that of the control device (18.31%). Therefore, this work presents a simple and effective way to improve the efficiency and stability of PSCs by DA treatment.

Graphical abstract

Dopamine was introduced for the first time into the perovskite precursor solution to control the perovskite film formation and suppress the formation of ionic defects. The modified devices show improved performance (PCE = 21.03%) and stability.

Introduction

Organic-inorganic hybrid halide perovskite materials have attracted much attention due to their excellent photoelectric properties, including long carrier lifetime, high charge carrier mobility, tunable band gap, and high light absorption coefficients, which can be exploited in a variety of fields [1], [2], [3], [4]. Perovskite solar cells (PSCs) based on organic–inorganic hybrid perovskite materials have undergone extraordinarily rapid developments in the last decade, with their certified power conversion efficiency (PCE) increasing from 3.8% to 25.2% [5], [6], [7], [8]. Although the certified PCE is constantly increasing, the poor long-term stability against moisture and high temperatures remains a huge challenge for the commercialization of PSCs. Previous studies revealed that the degradation of PSCs is mainly caused by corrosive moisture, heat, and UV illumination [9], [10], [11]. Strategies including additive engineering [12], interfacial modification engineering [13], [14], [15], and encapsulation can prolong the device lifetime by isolating these external environmental factors [16], [17].

Non-radiative recombination processes including defect-assisted recombination, Auger recombination, interface-induced recombination and band-tail recombination are detrimental to the performance of PSCs. Among these processes, defect-assisted recombination is inevitable for polycrystalline perovskite materials prepared by solution processing, due to the large amounts of defects and grain boundaries. This process has negative impact on the electronic and optoelectronic properties of the perovskite layer, which limits the performance enhancements of the PSCs [18]. In addition, these defects not only serve as charge carrier recombination centers causing device performance loss, but also accelerate the decomposition process in the perovskite layer [19], [20]. Uncoordinated Pb²⁺, uncoordinated I⁻, and metallic Pb (Pb⁰) species are considered the main deep-level trap defects existing in MAPbI₃ (MA = CH₃NH₃) films [21], [22]. The uncoordinated Pb²⁺ ions are easily oxidized to Pb⁰ under preparation, thermal annealing, and operation conditions. Previous studies have shown Pb⁰ to be the primary defect causing deterioration of device performance as well as unsatisfactory long-term durability [23], [24]. At the same time, uncoordinated I⁻ ions generally form during the thermal annealing process, due to volatilization of organic cationic components. Their migration toward the cathode to form AgI was observed to impair both the performance and long-term durability of the device [25]. Thus, passivating the defects at surfaces and grain boundaries is a desirable task to further enhance both the performance and stability of PSCs. Electron donor Lewis bases such as thiophene and pyridine were introduced to passivate uncoordinated Pb²⁺ ions through coordinate bonding, resulting in decreased nonradiative carrier recombination and extended long-term stability [26], [27].

In addition, the matching between energy levels influences the charge transport process. Mismatching energy levels lead to charge accumulation at the interfaces and exacerbate charge carrier recombination. 2,2′,7,7′-tetrakis(N,N′-di-p methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) is commonly used as hole transport layer (HTL) in n-i-p planar devices. However, the energy mismatch between the valence band maximum (VBM) of the perovskite layer and the highest occupied molecular orbital (HOMO) of spiro-OMeTAD hinders the transfer of charge carriers and causes recombination at the perovskite/HTL interface. Thus, it is necessary to narrow the energy gap between perovskite layer and HTL.

Dopamine (DA), a bioactive small molecule with catechol structure and an active amino group, was previously used to modify Poly(3,4-ethylenedioxythiophene)- polystyrenesulfonate (PEDOT:PSS) films in order to improve the hole extraction capability and passivate surface traps, leading to improved performance and stability [28]. Furthermore, DA was shown to be an effective interfacial layer between perovskite and electron transport layer (ETL, TiO₂ and SnO₂), which is beneficial for improving charge extraction and reducing charge recombination, thus enhancing the photovoltaic performance and stability of the modified devices [29], [30]. Although these interfacial engineering approaches facilitate the development of PSCs, they require an additional spin-coating process, which is unfriendly to the commercial development of PSCs.

In this work, we developed a simple strategy for adding DA into the perovskite precursor solution, based on the following advantages: 1) the C = O bonds of DA effectively passivate the uncoordinated Pb²⁺ defects through Lewis acid-base interactions; 2) the Pb⁰ defects can be effectively suppressed due to the strong reducibility of DA; 3) moderate DA amounts can narrow the mismatching band gap between perovskite and spiro-OMeTAD layers, thus facilitating charge transport; 4) the amino group of DA can passivate the I⁻ defects. Consequently, the DA additive enhanced the device efficiency (up to 21.03%) and thermal stability, which is superior to those of the pristine devices.

Section snippets

Materials

Etching indium tin oxide (ITO) glass sheets (sheet resistance, R_S ≈ 10 Ω/sq) were purchased from CSG Holding Co., Ltd. Methyl ammonium iodide (MAI, 99.5%) was purchased from Borun New Material Technology. Lead iodide (PbI₂, 99.99%), 4-tert-butylpyridine (96%), and spiro-OMeTAD (99.8%) were purchased from Xi'an Polymer Light Technology Corp. Chlorobenzene (99.9%), dimethylformamide (DMF, 99%), and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI, 99.95%) were purchased from Sigma-Aldrich.

Results and discussion

The chemical structure of DA is shown in Fig. 1(a); the molecule has two hydroxyl groups and one ethyl amino group on the benzene ring. It is generally accepted that dopamine can be easily oxidized via the Michael addition reaction [31], as shown in Fig. S1. In order to explore the chemical bond between DA and MAPbI₃, the Fourier transform infrared (FTIR) spectra of DA, MAPbI₃, and MAPbI₃ with DA additive (MAPbI₃:DA) are shown in Fig. 1(b). Except for the stretching vibration related to N–H

Conclusions

In summary, DA was introduced for the first time into the perovskite precursor solution to control the crystallization process of the perovskite film and passivate uncoordinated Pb²⁺ defects via Lewis acid-base interactions. Pb⁰ defects were effectively suppressed due to the strong interaction between C = O bonds and uncoordinated Pb²⁺. Uncoordinated I⁻ defects were passivated by the amino group of DA. At the same time, the DA treatment improved the crystallinity and facilitated crystal growth

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No. 61974045), the Natural Science Foundation of Guangdong Province (Nos. 2019A1515012092 and 2017A030313), the Key Laboratory of Functional Molecular Solids, Ministry of Education (No. FMS201905), the Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (No. Y909kp1001), and a project funded by the Science and Technology Bureau of the Dongguan Government (No.

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Multifunctional dopamine-assisted preparation of efficient and stable perovskite solar cells

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusions

Declaration of Competing Interest

Acknowledgments

J. Energy Chem.

J. Energy Chem.

Electrochim. Acta

iScience

Nano Energy

Joule

Sol. Energy

Org. Electron.

J. Energy Chem.

Electrochim. Acta

Nature

Adv. Energy Mater.

Nature

Science

J. Am. Chem. Soc.

Adv. Mater.

Adv. Energy Mater.

Adv. Energy Mater.

, Adv. Energy Mater.

Nanoscale

J. Mater. Chem. A

Adv. Mater.

Nat. Rev. Mater.

Adv. Mater.

Adv. Energy Mater.

Science

RSC Adv.

Science

Science