Surface Defect Passivation and Energy Level Alignment Engineering with a Fluorine-Substituted Hole Transport Material for Efficient Perovskite Solar Cells

Abstract

The surface and boundary defects present in the perovskite film are reported to be nonradiative recombination and degradation centers, restricting further improvement of the power conversion efficiency (PCE) and long-term stability of perovskite solar cells. To address this problem, herein, we introduce a fluorine-substituted small molecular material 2FBTA-1 as a bifunctional buffer layer to efficiently passivate the surface defects of perovskite and tune the energy level alignment between the perovskite/2,2′,7,7′-tetrakis(N,N-di-(pmethoxyphenyl)amino)-9,9′-spirobifluorene (Spiro-OMeTAD) interface. X-ray photoelectron spectroscopy shows that with the insertion of 2FBTA-1 between perovskite and Spiro-OMeTAD, the metallic Pb 0 defects and uncoordinated Pb 2+ defects are well restricted. Consequently, the average PCE is distinctly improved from 18.4 ± 0.51 to 20.3 ± 0.40%. Moreover, the long-term stability of unencapsulated devices with 2FBTA-1 treatment under ambient conditions (relative humidity 40−60%) is effectively enhanced, retaining 87% of the initial efficiency after storage for 500 h.

Indolocarbazole-core linked triphenylamine as an interfacial passivation layer for perovskite solar cells

“…The metallic Pb is formed due to the reduction of Pb 2+ by V I . 19 This suggests that there are a lot of surface defects existing in the perovskite film, which will decrease the efficiency and ruin the stability of PSCs. However, the XPS peaks for metallic Pb disappear in the XPS spectra of the TM5 - and TM6 -coated perovskite films.…”

“…Such an energy level gradient is bene-cial to enhance hole extraction. 19 The Gaussian 09 package with B3LYP/6-31G+(d,p) was used to perform density functional theory (DFT) calculations, from which the surface electrostatic potential (ESP) diagrams for TM5 (Fig. 2a) and TM6 (Fig.…”

“…To passivate the surface defects of perovskite films, we herein design and synthesize two novel P-type organic molecules 19–22 through the combination of indolocarbazole and triphenylamine: 4,4′-(5,11-dioctyl-5,11-dihydroindolo[3,2- b ]carbazole-6,12-diyl)bis( N , N -bis(4-methoxyphenyl)aniline) ( TM5 ) and 4,4′-(5,11-dioctyl-5,11-dihydroindolo[3,2- b ]carbazole-6,12-diyl)bis( N , N -bis(4-(methylthio)phenyl)aniline) ( TM6 ) to coat onto the perovskite surface. The structures of TM5 and TM6 are shown in Scheme 1.…”

“…To passivate the surface defects of perovskite lms, we herein design and synthesize two novel P-type organic molecules [19][20][21][22] through the combination of indolocarbazole and triphenylamine:…”

Methods of Stability Control of Perovskite Solar Cells for High Efficiency

“…These materials are easy and inexpensive to process as compared to pure metals. The most common method for adjusting energy levels is the introduction of a dipolar layer between the perovskite and the charge transfer layers, for example, by functionalizing the surface [31]. This concept is based on changing the WF of transparent conducting oxides and the position of the Fermi level of the oxides with respect to the charge transfer levels of the semiconductor.…”