Reducing Energy Disorder of Hole Transport Layer by Charge Transfer Complex for High Performance p–i–n Perovskite Solar Cells

Abstract

Charge-transport layers (CTLs), which significantly influence charge extraction and recombination, play an important role in improving photovoltaic performance of perovskite solar cells (pero-SCs). [1-3] Solution-processed organic semiconductors with versatile molecular structures, finetuned energy levels, and low-temperature processing have been intensively explored as both hole-transport materials (HTMs) and electron-transport materials (ETMs) for the pero-SCs. [4-7] However, their electronic states in thin films are dramatically affected by molecular stacking, and their weak intermolecular interactions usually have a high degree of energy disorder resulting in low carrier mobility and severe interface recombination. [8-11] Therefore, the performance of p-in pero-SCs, which is highly dependent on OS-CTLs, still lags that of n-i-p pero-SCs. [12,13] Very few OS-CTLs of pero-SCs addressing energy disorder of the CTLs were reported in the literature. Thus far, only the electron transport layer (ETL) has demonstrably reduced energy disorder by enhancing molecular ordering. Huang and co-workers [14] found that the energetic disorder in ETLs such as in [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) induced band tail and additional electronic states, leading to reduced quasi-Fermi level splitting and low efficiency. They proposed that solvent-annealed PCBM ETL with high ordering could mitigate its energy disorder and increase the power conversion efficiency (PCE) of p-in planar pero-SCs. Similarly, Miyano and co-workers [15] synthesized a highly-crystalline fullerene derivative (C 60 MC 12) to replace amorphous PCBM for enhancing the crystallization of ETL. The highly crystalline ETL could efficiently reduce energy disorder. In comparison, HTMs usually have more flexible chains required for increasing their solubility. These long and flexible chains make it difficult to improve molecular ordering through a simple external treatment, resulting in larger energy disorder and more serious surface recombination. [16] However, few attention has been paid to tuning the energy disorder of the HTL, which is likely more critical to the performance of p-in pero-SCs. Solution-processed organic semiconductor charge-transport layers (OS-CTLs) with high mobility, low trap density, and energy level alignment have dominated the important progress in p-in planar perovskite solar cells (pero-SCs). Unfortunately, their inevitable long chains result in weak molecular stacking, which is likely to generate high energy disorder and deteriorate the charge-transport ability of OS-CTLs. Here, a charge-transfer complex (CTC) strategy to reduce the energy disorder in the OS-CTLs by doping an organic semiconductor, 4,4′-(4,8-bis(5-(trimethylsilyl)thiophen-2-yl) benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(N,N-bis(4-methoxyphenyl) aniline) (BDT-Si), in a commercial hole-transport layer (HTL), poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine (PTAA), is proposed. The formation of the CTC makes the PTAA conjugated backbone electron-deficient, resulti...

A Regularity‐Based Fullerene Interfacial Layer for Efficient and Stable Perovskite Solar Cells via Blade‐Coating

“…Organic-inorganic hybrid perovskite solar cells (PVSCs) have drawn extensive research attention as the next-generation photovoltaic devices, [1][2][3][4][5][6][7][8][9] owing to their impressively photoelectric characteristics, such as ideal absorption coefficient, [10] high carrier mobility, [11,12] long carrier lifetime, and unique defect reduce the energy disorder of PTAA hole transport layer (HTL) due to the enhanced intermolecular stacking and ordering, thus delivering a PCE of 21.87% and robust operating stability. [41] Meng and co-workers employed naphthalene diimide derivative (NDI-BN) to tailor the PC 61 BM/Ag interface, and found that the densely molecular packed structure of NDI-BN by π-π stacking force had the ability to effectually hinder ion migration. [42] Boschloo and co-workers investigated the fabrication of MAPbI 3−x Cl x and MAPbI 3 via solvent engineering on TiO 2 /glass substrates, which enable observe a long-term conversion from disordered arrangement to more crystalline for both perovskite from X-ray diffraction (XRD) patterns over 5 weeks.…”

Coexistent Integer Charge Transfer and Charge Transfer Complex in F4-TCNQ-Doped PTAA for Efficient Flexible Organic Light-Emitting Diodes

“…Consistent with the changing of F4-TCNQ – peak intensity in absorption spectra, the EPR signal also shows a tendency to decrease when the dopant’s concentration exceeded 15%. These features may be attributed to transitions from bonding to antibonding molecular hybridized orbitals . Notably, there are new sub-bandgap absorption peaks at about 2.4 eV in doped films, which is consistent with the hybridized state, and the variation trend of its intensity is positively correlated to the change of the doping concentration.…”

“…Figure c shows that CTC already exists at 1% low doping concentration (one dopant per 96 host molecules) in PTAA:F4-TCNQ, but the nitrile-stretching mode intensity of ICT reaches saturation at 15% doping concentration. This trend is different with the P3HT:F4-TCNQ system, in which ICT preferentially saturates in low doping concentration. , The formation of CTC makes the p−π conjugated PTAA backbone electron-deficient, resulting in a quinoidal and stiffer character, which is likely to planarize the PTAA backbone and create a conformation appropriate for ICT process in the nanoscale.…”

A dopant-free hole transport material boosting the performance of inverted methylamine-free perovskite solar cells

“…The increment of the WF is conducive to enhancing the V OC . 15 In this respect, PFDTS is more suitable than PTAA as a HTL for inverted PSCs. Fig.…”

“…The HTL can adjust the energy level difference between the electrode and the active layer to reduce energy loss. 14–16 (3) Regulating perovskite crystallization. The HTL can interact with the perovskite, which may regulate the crystallization of the perovskite, thereby affecting the morphology of perovskite.…”