Fused-ring electron acceptor as an efficient interfacial material for planar and flexible perovskite solar cells

Highlights

• A fused-ring electron acceptor ITIC is employed as the interfacial material for rigid and flexible perovskite solar cells.

• ITIC not only suppresses the surface defects of TiO₂ film but also promotes the growth of perovskite crystals.

• ITIC improves the electron extraction and reduces the charge recombination which contributes to the performance of PSCs.

• The highest power conversion efficiency of 19.61% is achieved for the rigid PSCs after the introduction of ITIC.

• Flexible PSC with ITIC obtains a power conversion efficiency of 14.87%, and the device stability is greatly improved.

Abstract

Perovskite solar cell (PSC) has attracted great attention due to its high power conversion efficiency (PCE), low cost and solution processability. The well-designed interface and the modification of electron transport layer (ETL) are critical to the PCE and long-term stability of PSCs. In this article, a fused-ring electron acceptor is employed as the interfacial material between TiO₂ and the perovskite in rigid and flexible PSCs. The modification improves the surface of TiO₂, which decreases the defects of ETL surface. Moreover, the modified surface has lower hydrophilicity, and thus is beneficial to the growth of perovskite with large grain size and high quality. As a result, the interfacial charge transfer is promoted and the interfacial charge recombination can be suppressed. The highest PCE of 19.61% is achieved for the rigid PSCs after the introduction of ITIC, and the hysteresis effect is significantly reduced. Flexible PSC with ITIC obtains a PCE of 14.87%, and the device stability is greatly improved. This study provides an efficient candidate as the interfacial modifier for PSCs, which is compatible with low-temperature solution process and has a great practical potential for the commercialization of PSCs.

Introduction

Perovskite solar cells (PSCs) have become a research hotspot in the field of photovoltaics due to their fast-growing power conversion efficiency (PCE) [1], low cost, and solution processability [2,3]. Recently, the certificated PCE of PSCs reaches over 25% [[4], [5], [6]], which is exceeding copper indium gallium selenium and cadmium telluride solar cells and approaching the record of silicon-based solar cells. Inorganic-organic perovskite possesses advantages of adjustable band gap and high carrier mobility of both electrons and holes, so it is suitable for fabricating the photovoltaics devices with planar architecture. In a typical planar PSCs, perovskite layer is sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL) to construct an n-i-p stacked structure. Traditional electron transport materials include TiO₂, SnO₂, ZnO and fullerene and its derivatives [7]. The transport layers are found to be critical to the PCE and long-term stability of PSCs [8,9]. For example, as the most widely used ETL, anatase-type TiO₂ has been extensively employed for highly efficient PSCs due to its excellent optical transmittance, semiconductor characteristics, and chemical stability [[10], [11], [12]]. However, its rough surface with a large number of surface defects leads to serious charge recombination and large leakage current, resulting in efficiency loss and nonnegligible hysteresis effect [13,14]. Besides, TiO₂ would catalyze the decomposition of perovskite and decrease the stability of PSCs under ultraviolet irradiation. Therefore, many efforts have been devoted to improving the properties and modifying the surface of TiO₂ to obtain PSCs with high efficiency and stability. The approaches of optimization include element doping, surface treatment, nanocomposite assembly and interface engineering [[15], [16], [17], [18]]. Interface engineering focuses on the construction of an interlayer between TiO₂ and perovskite, such as fullerene and its derivatives, to promote electron extraction and passivate the trap states [[19], [20]]. Recently, several non-fullerene fused-ring electron acceptors (FREAs) have also been introduced into n-i-p perovskite solar cells as the interfacial modifier [[21], [22], [23]], which are less expensive and have better affinity to the substrate than fullerenes. The FREAs are used as the electron acceptors in organic solar cells and their structures are based on an electron donating fused ring with strong electron withdrawing groups, which contributes to adjustable optical band gap and energy level structure, good electronic transfer capability, excellent thermal and photochemical stability [21,22]. The application of FREAs in PSCS not only increases the PCE but also improves the long-term stability of devices, which is regarded as one of the effective approaches to further improve the PSCs performance.

In this paper, a typical FREAs, ITIC, is deposited on top of TiO₂ film as the interfacial modifier for planar PSCs with forward structure. ITIC has a well-match LUMO energy level (−3.93 eV) to the perovskite (Fig. 1) and good electron transport ability [23]. The influence of ITIC on the morphology of perovskite crystals and the electrical properties of the device are studied comprehensively. The devices with the structure of ITO/TiO₂/ITIC/(FAPbI₃)_X(MAPbCl₃)_1−x/spiro-OMeTAD/Ag are successfully fabricated. The PSCs on the rigid substrate obtain a highest PCE of 19.61% which is much higher than 16.99% of the reference device without ITIC, and the hysteresis effect is significantly reduced. The modified ETL is low-temperature solution processable, which is compatible with the plastic substrate. The ITIC-based flexible devices is thus fabricated and a champion PCE of 14.87% is achieved. The flexible device remains 80% of its original PCEs after 500h stored in ambient humidity of 30–40%, which shows significantly superior stability to the unmodified device.