penghuang

The role of DMAI in fabricating high quality CsPbI₃ inorganic perovskite thin films is demonstrated to be a volatile crystal growth additive rather than dopant. With optimal DMAI additive and PTACl passivation, a PTACl‐CsPbI₃ based champion photovoltaic device exhibits a record efficiency of 19.03 %.

Abstract

The controllable growth of CsPbI₃ perovskite thin films with desired crystal phase and morphology is crucial for the development of high efficiency inorganic perovskite solar cells (PSCs). The role of dimethylammonium iodide (DMAI) used in CsPbI₃ perovskite fabrication was carefully investigated. We demonstrated that the DMAI is an effective volatile additive to manipulate the crystallization process of CsPbI₃ inorganic perovskite films with different crystal phases and morphologies. The thermogravimetric analysis results indicated that the sublimation of DMAI is sensitive to moisture, and a proper atmosphere is helpful for the DMAI removal. The time‐of‐flight secondary ion mass spectrometry and nuclear magnetic resonance results confirmed that the DMAI additive would not alloy into the crystal lattice of CsPbI₃ perovskite. Moreover, the DMAI residues in CsPbI₃ perovskite can deteriorate the photovoltaic performance and stability. Finally, the PSCs based on phenyltrimethylammonium chloride passivated CsPbI₃ inorganic perovskite achieved a record champion efficiency up to 19.03 %.

A 2D metal–organic framework In‐Aipa‐derived N‐rich porous carbon material with rich pyridinic‐N and graphitic‐N is first introduced into the hole transport layers of perovskite solar cells as an auxiliary additive, contributing to the significantly improved power conversion efficiency from 16.47% to 18.51%, as well as the enhanced long‐term stability of over 85% efficiency retention under exposure to air for 720 h.

As the standard bidopants of hole transport layers (HTLs) in perovskite solar cells (PSCs), bis(trifluoromethane)sulfonimide lithium salt (Li‐TFSI) and 4‐tert‐butylpyridine not only induce adverse influence on the quality of thin films, but also seriously impair the long‐term stability of devices. Herein, a metal–organic framework‐derived 2D graphitic N‐rich porous carbon (NPC) is first introduced into the HTLs as an effective auxiliary additive. The introduction of NPC significantly reduces the aggregation of lithium salts and the formation of HTL defects, optimizing film quality for rapid hole extraction and migration. Furthermore, inherent porosity and hydrophobicity of NPCs are extremely beneficial to restrict the permeation of Li⁺ ions and anode metals, and prevent the moisture from eroding the HTLs and perovskite layers, enhancing the stability of PSCs. As expected, the PSCs with NPC realize a satisfactory fill factor of 0.76 and power conversion efficiency (PCE) of 18.51%, apparently higher than that of pristine devices (0.70% and 16.47%). In addition, over 85% of the initial PCE for optimized PSCs is maintained after 720 h of exposure to air. Obviously, an innovative strategy for highly efficient and long‐term stable PSC devices is provided.

A dopant‐free polymeric hole transport material (HTM) is synthesized to fabricate perovskite solar cells. The carbonyl groups can passivate defects of under‐coordinated Pb atoms that exist in the surface of perovskite films. A PBT1‐C based device shows a power conversion efficiency of 19.06% with a fill factor of 81.22%, which is the highest value among the dopant‐free polymeric HTMs.

Abstract

Although perovskite solar cells (PVSCs) have achieved rapid progress in the past few years, most of the high‐performance device results are based on the doped small molecule hole‐transporting material (HTM), spiro‐OMeTAD, which affects their long‐term stability. In addition, some defects from under‐coordinated Pb atoms on the surface of perovskite films can also result in nonradiative recombination to affect device performance. To alleviate these problems, a dopant‐free HTM based on a donor‐acceptor polymer, PBT1‐C, synthesized from the copolymerization between the benzodithiophene and 1,3‐bis(4‐(2‐ethylhexyl)thiophen‐2‐yl)‐5,7‐bis(2‐alkyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione units is introduced. PBT1‐C not only possesses excellent hole mobility, but is also able to passivate the surface traps of the perovskite films. The derived PVSC shows a high power conversion efficiency of 19.06% with a very high fill factor of 81.22%, which is the highest reported for dopant‐free polymeric HTMs. The results from photoluminescence and trap density of states measurements validate that PBT1‐C can effectively passivate both surface and grain boundary traps of the perovskite.

Structural, electronic band structure, and electrical properties of a series of charge‐transfer cocrystals based on F₆TNAP and six planar donors are presented. Density functional theory calculations afford large conduction bandwidths and low effective masses for all six cocrystals. A few cocrystals exhibit charge‐carrier mobilities in excess of 1 cm² V⁻¹ s⁻¹, as estimated from space‐charge limited current measurements.

Abstract

The crystal structures of the charge‐transfer (CT) cocrystals formed by the π‐electron acceptor 1,3,4,5,7,8‐hexafluoro‐11,11,12,12‐tetracyanonaphtho‐2,6‐quinodimethane (F₆TNAP) with the planar π‐electron‐donor molecules triphenylene (TP), benzo[b]benzo[4,5]thieno[2,3‐d]thiophene (BTBT), benzo[1,2‐b:4,5‐b′]dithiophene (BDT), pyrene (PY), anthracene (ANT), and carbazole (CBZ) have been determined using single‐crystal X‐ray diffraction (SCXRD), along with those of two polymorphs of F₆TNAP. All six cocrystals exhibit 1:1 donor/acceptor stoichiometry and adopt mixed‐stacking motifs. Cocrystals based on BTBT and CBZ π‐electron donor molecules exhibit brickwork packing, while the other four CT cocrystals show herringbone‐type crystal packing. Infrared spectroscopy, molecular geometries determined by SCXRD, and electronic structure calculations indicate that the extent of ground‐state CT in each cocrystal is small. Density functional theory calculations predict large conduction bandwidths and, consequently, low effective masses for electrons for all six CT cocrystals, while the TP‐, BDT‐, and PY‐based cocrystals are also predicted to have large valence bandwidths and low effective masses for holes. Charge‐carrier mobility values are obtained from space‐charge limited current (SCLC) measurements and field‐effect transistor measurements, with values exceeding 1 cm² V⁻¹ s¹ being estimated from SCLC measurements for BTBT:F₆TNAP and CBZ:F₆TNAP cocrystals.

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Jigeon Kim, Bonkee Koo, Wook Hyun Kim, Jongmin Choi, Changsoon Choi, Sung Jun Lim, Jong-Soo Lee, Dae-Hwan Kim, Min Jae Ko, Younghoon Kim

Graphical abstract

We demonstrate that sodium acetate (NaOAc) directly generates short-chain OAc ions to exchange the long-chain oleate ligands of CsPbI₃ perovskite quantum dots (CsPbI₃-PQDs). NaOAc-based ligand exchange enables preservation of CsPbI₃-PQD size, minimization of surface trap states, and enhancement of electronic coupling in the resultant CsPbI₃-PQD solids. Consequently, NaOAc-treated CsPbI₃-PQD solar cells show improved device performance with 12.4% power conversion efficiency.

Recent progress in bandgap engineering strategies including the two main, widely used impurity and pressure as well as intermediate band, external electric field, and steric methods are reviewed comprehensively. Their underlying mechanism, achievements, and challenges are outlined. Additionally, future research directions are provided to realize direct and gap size continually tunable perovskites for further enhancing solar cell performance.

Metal halide perovskites are attractive for highly efficient solar cells. As most perovskites suffer large or indirect bandgap compared with the ideal bandgap range for single‐junction solar cells, bandgap engineering has received tremendous attention in terms of tailoring perovskite band structure, which plays a key role in light harvesting and conversion. In this Review, various reported bandgap engineering strategies are summarized. The recently widely used two main strategies including impurity and pressure as well as their underlying mechanisms are reviewed comprehensively. In addition, intermediate band and external electric field for bandgap engineering are also investigated. Moreover, future research directions are outlined to guide the further investigation.

Upscaling loss for perovskite devices is higher than for any other photovoltaic technology. Herein, electroluminescence, dark lock‐in thermography, microphotoluminescence spectroscopy, and electron spectroscopy are used to investigate upscaling losses, focusing on layer inhomogeneities for modules with an aperture area up to 100 cm². Analysis helps in identification of processing pitfalls and strategies for overcoming or minimizing their effects.

Hybrid metal‐halide perovskite‐based thin‐film photovoltaics (PVs) have the potential to become the next generation of commercialized PV technology with certified power conversion efficiencies reaching 24% on devices having 0.1 cm² area. Recent efforts in upscaling this technology result in an efficiency of 12.6% for 354 cm² modules. However, upscaling loss for perovskite‐based PVs is higher than for any other PV technology. In this study, upscaling losses of devices with aperture area 0.1, 4, and 100 cm² are investigated, with a focus on layer inhomogeneities. Electroluminescence, dark lock‐in thermography, microphotoluminescence spectroscopy, and electron spectroscopy are used to analyze and group layer inhomogeneities with a minimal size of 10 μm and to compare loss mechanisms for radial and linear deposition techniques. Analysis results help to identify current processing pitfalls, where understanding and control of perovskite crystal formation plays the crucial role.

Photocurrent–voltage hysteresis in perovskite solar cells (PSCs) induced by ion migration combined with nonradiative recombination near the interface depends on perovskite composition and device structure. Among the methods used in the attempt to reduce the hysteresis, potassium‐ion doping is found to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition.

Abstract

Current‐density–voltage (J–V) hysteresis in perovskite solar cells (PSCs) is a critical issue because it is related to power conversion efficiency and stability. Although parameters affecting the hysteresis have been already reported and reviewed, little investigation is reported on scan‐direction‐dependent J–V curves depending on perovskite composition. This review investigates J–V hysteric behaviors depending on perovskite composition in normal mesoscopic and planar structure. In addition, methodologies toward hysteresis‐free PSCs are proposed. There is a specific trend in hysteresis in terms of J–V curve shape depending on composition. Ion migration combined with nonradiative recombination near interfaces plays a critical role in generating hysteresis. Interfacial engineering is found to be an effective method to reduce the hysteresis; however, bulk defect engineering is the most promising method to remove the hysteresis. Among the studied methods, KI doping is proved to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition. It is proposed from the current studies that engineering of perovskite film near the electron transporting layer (ETL) and the hole transporting layer (HTL) is of vital importance for achieving hysteresis‐free PSCs and extremely high efficiency.