Dopant‐Free Two‐Dimensional Hole Transport Small Molecules Enable Efficient Perovskite Solar Cells

Abstract

due to their advantages of high performance, low cost, and easy fabrication of large-scale flexible devices. [5][6][7][8] The primary obstacle hindering their commercial applications is the stability issue. [9,10] So far, the 2,2″,7,7″-tetrakis-(N,N-dip-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) has become the dominated hole transport material (HTM) in n-i-p PSCs. The deliquescent dopants, such as 4-tertbutylpyridine (TBP) and lithium bis(trifluoromethyl sulfonyl) imide (LiTFSI), are required to achieve high conductivity and hole mobility of Spiro-OMeTAD film, which not only make the device fabrication complicated but also inevitably accelerate device deterioration. [11] Hence, developing dopant-free HTMs to replace doped Spiro-OMeTAD is a promising but challenging choice to accelerate the commercialization of this technology. Small molecule HTM with a well-defined structure is ascendent in device reproducibility as opposed to those polymeric counterparts. In 2014, Han et al. reported the first small molecule HTM and achieved an efficiency of 11.03%. [12] However, so far, the PCEs of PSCs using dopant-free small molecule HTMs are still lower than that using doped Spiro-OMeTAD HTM or dopant-free polymer HTMs. [13][14][15][16][17][18][19][20] Conjugated small molecules with donor-acceptor (D-A) structures are promising candidates for dopant-free HTMs due to their high charge carrier mobility and fine-tunable energy levels. However, the small molecule HTMs with a Developing dopant-free hole transport materials (HTMs) to replace Spiro-OMeTAD is a challenging but urgent issue for commercialization of stateof-the-art n-i-p structured perovskite solar cells (PSCs). Here, this work proposes an effective two-dimensional conjugate engineering strategy to tune molecular stacking orientation and improve the hole mobility of dopant-free small molecule HTMs. For the first time, triphenylamine (TPA) groups are incorporated as side chains of benzo [1,2-b:4,5-b′]dithiophene (BDT) unit to extend the longitudinal conjugate, achieving two donor-acceptor-acceptor type 2D small molecules, namely XF2 and XF3, which show a dominant face-on orientation and better hole transport mobility than the linear small molecule XF1. The incorporation of alkoxy Lewis base groups makes XF3 a more effective defect passivator for perovskite surfaces. As a result, the PSCs using pristine XF3 HTM show a dramatically improved efficiency of 20.59% along with improved long-term stability compared to that of XF1 HTM (power conversion efficiency (PCE) = 18.84%). A champion efficiency of 21.44% is achieved through device engineering for dopant-free XF3-based PSCs. The results show that the building block with longitudinal conjugate extension in small molecules plays an essential role in the face-on orientation morphology and elucidates a key design rule for the dopant-free small molecule HTMs for high-performance PSCs.

Target Therapy for Buried Interface Enables Stable Perovskite Solar Cells with 25.05% Efficiency

“…These features indicate a strong interaction between perovskite and FOA due to the chemical reaction between Pb 2+ and oxalate (Figure S16, Supporting Information). [32,49] The electron-only devices with the structure of ITO/SnO 2 /perovskite/PC 61 BM/Ag were fabricated to evaluate the trap density of perovskite deposited on different substrates. The current-voltage curves of electron-only devices in dark revealed that the corresponding N t s were determined to be 1.89 × 10 15 and 4.02 × 10 14 cm −3 for the SnO 2 and SnO 2 -FOAbased devices, respectively (Figure 4g).…”

Selenophene-Based 2D Ruddlesden-Popper Perovskite Solar Cells with an Efficiency Exceeding 19%

“…47,48 Interface passivation is a simple and effective method for improved charge carrier transport, reduced nonradiative recombination, and enhanced PCE. 49,50 To achieve more efficient and stable 2D RP PSCs, we propose a predeposition transport layer (PDTL) strategy aimed at passivating the surface defects and enhancing the ETL properties through the establishment of a reinforced PCBM coverage over the perovskite layer. The flowchart outlining the PDTL processing is shown in Figure 2a.…”

Configuration Engineering of Unsymmetrical Structural Hole Transport Material for Efficient and Stable Regular/Inverted Perovskite Solar Cells

“…6 Compared to r-PSCs, the i-PSCs possess advantages of negligible photocurrent hysteresis, low-temperature preparation technology, and splendid compatibility in fabricating flexible and tandem cells. 7 Both r-PSCs and i-PSCs are fabricated as a sandwiched combination of electron transport materials, perovskite absorber, hole transport materials (HTMs), and electrodes, in which the HTM layers are indispensable in facilitating hole extraction/transport and restraining charge recombination occurring at the perovskite/charge transport interfaces. 8−10 Thus, a successful hole transport material should have the characteristics of high mobility, good film-forming property, energy level matching, high stability, etc.…”

“…The film morphology of HTMs plays a vital role in device efficiency and stability as it significantly affects the perovskite crystallization because the perovskite absorber is coated on the surface of the HTM layer. 7,9,11,13 A host of reported HTMs are applicable only for either r-PSCs or i-PSCs (e.g., spiro-OMeTAD is superior in r-PSCs, while performs poorly in the i-PSCs), which quite increases the research cost of devices and is not conducive to mass production. Therefore, seeking efficient HTMs performing well in both r-PSCs and i-PSCs is urgently needed, and the key for achieving this goal is in well modulating the two key elements (i.e., film morphology and hole mobility).…”

“…The r-PSCs own good structural compatibility and energy level matching with perovskite layers and charge transport layers, so champion PCEs are always attained by the r-PSCs . Compared to r-PSCs, the i-PSCs possess advantages of negligible photocurrent hysteresis, low-temperature preparation technology, and splendid compatibility in fabricating flexible and tandem cells …”