The best-performing cell, which contains the FTO/Bl-TiO2/mp-TiO2+CH3NH3PbBr3 (~2-nm QDs)/PTAA/Au configuration shows the following results:

open-circuit voltage (VOC)=1.110 V,

current density (JSC)=14.07 mA cm−2,

fill factor=0.73

and an 11.40% PCE.

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NPG Asia Materials (2015) 7, e208; doi:10.1038/am.2015.86
Published online 14 August 2015

Highly stable and efficient solid-state solar cells based on methylammonium lead bromide (CH₃NH₃PbBr₃) perovskite quantum dots

Sawanta S Mali¹, Chang Su Shim¹ and Chang Kook Hong¹

¹Polymer Energy Materials Laboratory, School of Applied Chemical Engineering, Chonnam National University, Gwangju, South Korea

Correspondence: Professor Dr CK Hong, Polymer Energy Materials Laboratory, School of Applied Chemical Engineering, Chonnam National University, Gwangju 500757, South Korea. E-mail: hongck@chonnam.ac.kr

Introduction

Methylammonium lead halide (MAPbX) (X=I, Br or Cl)-based perovskite solar cells (PSCs) open new approaches for the fabrication of efficient and stable solid-state dye-sensitized solar cells. The pioneering work on alkali-metal lead and tin halides was performed by Wells.¹ The chemical formula of the compound that he used was CsPbX₃ (X=Cl, Br or I). However, 94 years later, D. Weber et al. successfully replaced cesium (Cs) with methylammonium cations (CH₃NH₃⁺) and studied various compositions of the first three-dimensional organic–inorganic hybrid perovskites by tuning their crystal structures and phases.^{2, 3} Specifically, MAPbBr₃perovskite shows the cubic phase I (Pm3m), tetragonal phase II (I4/mcm) and orthorhombic phase IV (Pnma, Z=4) systems. This is a p-type semiconducting material with a direct band gap of 1.93-2.3 eV that corresponds to an absorption onset of less than or equal to 550 nm, which makes this material an excellent light harvester.^4,5 After successful synthesis of CH₃NH₃PbI₃ perovskite quantum dots (QDs), Im et al.demonstrated a 6.5% power conversion efficiency (PCE) via the ex situ method.⁶Degradation of perovskite in a liquid electrolyte was solved by Gratzel et al. in 2013 using solid-state PSCs with a 13% PCE.⁷

The perovskite degradation problem has been nullified by implementing solid-state hole-transporting material (HTMs), for example, 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-MeOTAD).⁷ Thus, for the last 2 years, methylammonium lead halide perovskite semiconductors have enabled of low cost solution-processed photovoltaic technology. On average, the PCE is typically boosted to 16.6% (certified), with the highest reported efficiency of ~19.3% (uncertified) for a CH₃NH₃PbI_3−xCl_x perovskite with a planar geometry and without an antireflective coating.⁸

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Perovskite materials with different compositions have been successfully synthesized using in situ, ex situ, solvent engineering, compositional chemical management and vapor deposition techniques. Recently, Snaith et al. synthesized a mixed halide perovskite CH₃NH₃PbI_3−xCl_x using the dual-source vapor deposition technique to fabricatea planar heterojunction PSC.⁹ An open-circuit voltage (V_OC) of 1.07 V (the highest reported so far), a high current density (J_SC) of 21.5 mA cm⁻², and a 15.4%PCE were achieved. These results were achieved using the pinhole-free perovskite layer deposition technique, which enables thermal evaporation with a uniform HTM layer and a high diffusion length (L_D; 1069 nm).^{9, 10} In addition, the V_OC of the perovskite material can be controlled with electron-transporting layers (ETLs) such as TiO₂, Al₂O₃ and Yttrium-doped TiO₂ (Y-TiO₂).^{8, 11} Y. H. Hu discussed the state of the art of novel meso-superstructured solar cells that are based on an insulating ETL (Al₂O₃) and a lead-free perovskite (MASnI₃) for solar cell applications.¹²Recently, we have synthesized CH₃NH₃PbI₃ perovskite nanoparticles from γ-butyrolactone solvent and successfully developed low temperature-processed one-dimensional TiO₂ nanorod arrays with a 13.45% PCE.^{13, 14} Low temperature flexible PSCs have also been demonstrated by Yang et al. using phenyl-C₆₁-butyric acid methyl ester (PCMB) as the HTM material, resulting in a 9.2% PCE.¹⁵ However, a uniform deposition and thickness optimization of the phenyl-C₆₁-butyric acid methyl ester is difficult to achieve.¹⁶

Recently, Seok et al. demonstrated a 16.2% PCE using solvent engineering and toluene drip casting treatment for perovskite materials.¹⁷ This treatment helps to form an intermediate MAI–PbI₂–dimethyl sulfoxide (DMSO) phase, which retards the rapid reaction between PbI₂ and MAI during the evaporation step and results in a highly pure crystalline CH₃NH₃PbI₃ perovskite layer. However, these devices suffer from a low fill factor (FF) and hysteresis issues. The optimization of each layer is a major challenge for MAPbI₃-based PSCs, but the hysteresis issue can be solved by varying the thickness of the mesoporous TiO₂ (mp-TiO₂) layer. Seok et al. studied different compositions of methylammonium lead iodide (MAPbI₃), formamidinium lead iodide (FAPbI₃) and methylammonium lead bromide (MAPbBr₃) as effective light harvesters for PSCs. From this study, they concluded that the incorporation of MAPbBr₃ into FAPbI₃ stabilized the perovskite phase of FAPbI₃ and improved the PCE of the solar cell. The authors¹⁸ demonstrated a >18% PCE for mixed (FAPbI₃)_1−x (MAPbBr₃)_x, where x=0, 0.05 or 0.15. Therefore, a study of the bare MAPbBr₃ perovskite will open new opportunities in the architecture design of bilayer PSCs. Moreover, a high FF and high stability are the most important key features of MAPbBr₃-based PSCs. It is well known that MAPbX₃ nanoparticles/QDs with sizes <10 nm enable the preparation of new device architectures that could further enhance solar cell performance and elucidate the perovskite operating mechanisms.¹⁹ Therefore, MAPbBr₃ QDs-based PSCs open new approaches towards the development of stable and efficient solar cells. The optical properties of MAPbBr₃ and MAPbI₃ were investigated by Tanaka et al.²⁰. It was determined that for MAPbBr₃, the exciton binding energy and the exciton Bohr radius were 76 meV and 20 Å, respectively; for MAPbI₃, the exciton binding energy and exciton Bohr radius were 50 meV and 22 Å, respectively.²¹ The absorption coefficients of MAPbBr₃ and MAPbI₃ are 10⁵ cm⁻¹ and >4.3 × 10⁵ cm⁻¹ (at 300 nm), respectively.

Cai et al. demonstrated a 3.04% PCE using poly[N-9-hepta-decanyl-2,7-carbazole-alt-3,6-bis(thiophen-5-yl)-2,5-dioctyl-2,5-dihydropyrrolo[3,4–]pyrrole-1,4-dione](PCBTDPP) as the HTM.²² However, the V_OC can be tuned by the band gap and the HTM of the materials; the authors demonstrated a V_OC of 1.15 and 1.3 V for PCBTDPP and N,N′-dialkylperylenediimide as the HTMs, respectively. Recently, E. Edri et al. demonstrated a 0.72% PCE with a V_OC of 1.06 V using phenyl-C₆₁-butyric acid methyl ester as the HTMs²³ for MAPbBr₃ PSCs. Moreover, Seok et al.demonstrated band tuning by incorporating MAPbBr₃ (that is, Br doping) into MAPbI₃ to achieve a PCE of 12.3% and a V_OC of 1.13 V using polytriarylamine (PTAA) as the hole conductor.²⁴ The incorporation of MAPbBr₃ improved the V_OCand the current density from 0.87 to 1.13 V and from 5 to 18 mA cm⁻², respectively.

Im et al. fabricated a planar MAPbBr₃ hybrid PSC using P3HT, PTAA and poly-indenofluoren-8-triarylamine (PIF8-TAA) as the HTMs. The MAPbBr₃ perovskites were synthesized in a dimethylformamide solvent, and the influence of H₂O content was studied. The authors showed a 7.3%, 9.3% and 10.4% PCE with a V_OC of 1.09, 1.35 and 1.51 V for the P3HT, PTAA and PIF8-TAA HTMs, respectively.²⁰ To the best of our knowledge, there are no reports describing the synthesis of the MAPbBr₃QDs with <3 nm sizes in either the dimethylformamide or DMSO solvents that are used for the solar cell application.

In the current study, we report the preparation of highly stable and high conversion efficiency mp-TiO₂-based solid-state PSCs using MAPbBr₃ QDs. We synthesized highly crystalline MAPbBr₃ QDs with different sizes by the ex situ process in a DMSO solution followed by crystallization on mp-TiO₂. Two different types of HTMs, spiro-MeOTAD and PTAA, were used to study the performance of the solar cell devices. Because of the formation of very small (~2–3 nm) MAPbBr₃ perovskite QDs, the current density was drastically increased to 14.07 mA cm⁻². This resulted in an 11.14% conversion efficiency at 1 sun illumination. The resolution of the hysteresis issue is also discussed in detail.

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