Efficient Inverted Perovskite Solar Cells Research Articles

2D-Self-Assembled Organic Materials in Undoped Hole Transport Bilayers for Efficient Inverted Perovskite Solar Cells.

Interfaces between photoactive perovskite layer and selective contacts play a key role in the performance of perovskite solar cells (PSCs). The properties of the interface can be modified by the introduction of molecular interlayers between the halide perovskite and the transporting layers. Herein, two novel structurally related molecules, 1,3,5-tris(α-carbolin-6-yl)benzene (TACB) and the hexamethylated derivative of truxenotris(7-azaindole) (TTAI), are reported. Both molecules have the ability to self-assemble through reciprocal hydrogen bond interactions, but they have different degrees of conformational freedom. The benefits of combining these tripodal 2D-self-assembled small molecular materials with well-known hole transporting layers (HTLs), such as PEDOT:PSS and PTAA, in PSCs with inverted configuration are described. The use of these molecules, particularly the more rigid TTAI, enhanced the charge extraction efficiency and reduced the charge recombination. Consequently, an improved photovoltaic performance was achieved in comparison to the devices fabricated with the standard HTLs.

A novel perylene diimide-based ionene polymer and its mixed cathode interlayer strategy for efficient and stable inverted perovskite solar cells

Inverted (p–i–n) perovskite solar cells (PerSCs) have attracted much attention owing to their low temperature processability, less hysteresis effect and easy integration as a subunit for the tandem device. The unsatisfactory interface contacts and energy level barrier between adjacent interlayers on the cathode side are one of the key challenges for the development of p–i–n PerSCs. Herein, perylene diimide-based (PDI) ionene polymer was synthesized and developed as a cathode interlayer (CIL) to enhance interface contact, reduce the energy level barrier and prevent the migration of I- ions. The compact PNPDI CIL with high conductivity and appropriate lowest unoccupied molecular orbital (LUMO) level, resulted in a high efficiency device (20.03%), which is higher than the control device with bathophenanthroline (Bphen) (19.48%). Bphen-based CIL shows better adjusting ability of the work function of cathode metal but exhibits poor film-forming property. So, the synergistic effect of 1 + 1 > 2 can be obtained by combining Bphen and PNPDI into one CIL. As expected, the device performance was further improved by using the mixed CIL of Bphen and PNPDI, and 21.46% power conversion efficiency (PCE) was achieved. What’s more, the compact and hydrophobic mixed CIL dramatically enhanced the resistance to I- ions and moisture, which led to much enhanced device stability.

Surface Regulation with Polymerized Small Molecular Acceptor Towards Efficient Inverted Perovskite Solar Cells

AbstractOptimizing the interface between the perovskite and transport layers is an efficient approach to promote the photovoltaic performance of inverted perovskite solar cells (IPSCs). Given decades of advances in bulk materials optimization, the performance of IPSCs has been pushed to its limits by interface engineering with a power conversion efficiency (PCE) over 25% and excellent stability. Herein, an n‐type polymeric semiconducting material, PY‐IT, that has shown remarkable performance in organic photovoltaics, is introduced as an interface regulator between perovskite and ETL. Encouragingly, this polymerized small molecular acceptor (PSMA) exhibits significant effectiveness in both passivation defects and electron transfer facilitation properties with the merits of strong planarity and rotatable linkers, which significantly optimizes perovskite grain growth orientation and added charge transport channels. As a result, the PSMA‐treated IPSC devices obtain an optimal efficiency of 23.57% with a fill factor of 84%, among the highest efficiency among PSMA‐based IPSCs. Meanwhile, the photo‐stability of PSMA devices is eye‐catching, maintaining ≈80% of its initial PCE after 1000 h of simulated 1‐sun illumination under maximal power point tracking. This work combines the achievements of polymer science and IPSC device engineering to provide a new insight into interface regulation of efficient and stable devices.

Modulating buried interface with multi-fluorine containing organic molecule toward efficient NiOx-based inverted perovskite solar cell

Inverted perovskite solar cells (p-i-n PSCs) with nickel oxide (NiOx) as hole transport layer (HTL) have attracted tremendous attention due to the stable performance and ease of preparation. However, the NiOx/perovskite interface in the inverted perovskite solar cells (p-i-n PSCs) usually suffers from energy level mismatch, low conductivity, lattice mismatch, existence of surface defects etc., limiting the improvement of power conversion efficiency (PCE) and durability. Fluorine-containing materials have desired interface modification effect and are popular in the bulk and upper interface of perovskite, but rarely appear at NiOx/perovskite interface to address above issues. Herein, an interfacial modification strategy with a multi-fluorine organic molecule 6FPPY, is proposed to manage the buried interface of NiOx-based p-i-n PSC. Theoretical calculation and experimental results show that 6FPPY bridges the NiOx/perovskite interface through moieties with F atoms, producing a NiOx film with higher hole transport efficiency, releasing the residual strain of perovskite film, passivating the NiOx/perovskite interface defects, and suppressing the detrimental reaction between NiOx and perovskite. Consequently, p-i-n PSCs with 6FPPY-modified NiOx achieve a champion PCE of 24.0%, which is superior to the reference device. After 6FPPY modification, PSC can retain above 90% of the initial PCE after 200 h illumination at maximum power point in N2 atmosphere and exhibit better stability than the reference device during 1080 h storage under 60% humidity and 30–35 ℃.

Lowering the dielectric mismatch for efficient inverted perovskite solar cells through incorporating cyano-functionalized fullerene additive

Fullerene derivatives especially [1,1]-phenyl-C61-butyric-acid-methyl-ester (PC61BM) have been commonly used as electron transport layers and additives of inverted (p-i-n) perovskite solar cells (PVKSCs); however the dielectric constant (the εr) of PC61BM (3.8) is much smaller than those of organolead halide hybrid perovskites (6.5 for CH3NH3PbI3 [MAPbI3]), resulting in unfavorable exciton dissociation and impeded interfacial charge transport and consequently undesirable nonradiative recombination. Herein, a novel cyanofunctionalized fullerene in which two cyano groups are incorporated via a rhodanine moiety (abbreviated C60-Rhd-CN) was synthesized and introduced into the MAPbI3 active layer to construct inverted PVKSCs via an anti-solvent method. The involvement of two electron-withdrawing cyano groups within C60-Rhd-CN obviously increases the εr value (5.1) relative to PC61BM (3.8), hence lowering the dielectric mismatch between MAPbI3 and fullerene. Besides, the cyano groups enable strong coordination interactions with the Pb2+ ions of MAP-bI3, rendering efficient defect passivation of perovskite. As a result, C60-Rhd-CN incorporation facilitates exciton dissociation and interfacial charge transport and suppresses the nonradiative recombination, affording dramatic power conversion efficiency (PCE) enhancement of inverted PVKSC devices from 18.43% to 20.81%. Furthermore, C60-Rhd-CN incorporation helps to improve the ambient and thermal stabilities of PVKSC devices. To elucidate the critical role of cyano groups in increasing the the εr value and efficiency enhancement, another analogous novel rhodanine-functionalized fullerene with the two cyano groups substituted by a sulfur atom (abbreviated C60-Rhd-S) was also synthesized, which delivered a lower PCE of 19.45% than C60-Rhd-CN-incorporated devices due mainly to its lower the εr value (4.0).

Deciphering the Roles of MA-Based Volatile Additives for α-FAPbI3 to Enable Efficient Inverted Perovskite Solar Cells

Functional additives that can interact with the perovskite precursors to form the intermediate phase have been proven essential in obtaining uniform and stable α-FAPbI3 films. Among them, Cl-based volatile additives are the most prevalent in the literature. However, their exact role is still unclear, especially in inverted perovskite solar cells (PSCs). In this work, we have systematically studied the functions of Cl-based volatile additives and MA-based additives in formamidinium lead iodide (FAPbI3)-based inverted PSCs. Using in situ photoluminescence, we provide clear evidence to unravel the different roles of volatile additives (NH4Cl, FACl, and MACl) and MA-based additives (MACl, MABr, and MAI) in the nucleation, crystallization, and phase transition of FAPbI3. Three different kinds of crystallization routes are proposed based on the above additives. The non-MA volatile additives (NH4Cl and FACl) were found to promote crystallization and lower the phase-transition temperatures. The MA-based additives could quickly induce MA-rich nuclei to form pure α-phase FAPbI3 and dramatically reduce phase-transition temperatures. Furthermore, volatile MACl provides a unique effect on promoting the growth of secondary crystallization during annealing. The optimized solar cells with MACl can achieve an efficiency of 23.1%, which is the highest in inverted FAPbI3-based PSCs.

Rational design of Lewis base molecules for stable and efficient inverted perovskite solar cells.

Lewis base molecules that bind undercoordinated lead atoms at interfaces and grain boundaries (GBs) are known to enhance the durability of metal halide perovskite solar cells (PSCs). Using density functional theory calculations, we found that phosphine-containing molecules have the strongest binding energy among members of a library of Lewis base molecules studied herein. Experimentally, we found that the best inverted PSC treated with 1,3-bis(diphenylphosphino)propane (DPPP), a diphosphine Lewis base that passivates, binds, and bridges interfaces and GBs, retained a power conversion efficiency (PCE) slightly higher than its initial PCE of ~23% after continuous operation under simulated AM1.5 illumination at the maximum power point and at ~40°C for >3500 hours. DPPP-treated devices showed a similar increase in PCE after being kept under open-circuit conditions at 85°C for >1500 hours.

Modulating the strength of acceptor in D-A-D type hole transport materials for efficient inverted perovskite solar cells

We report herein a design strategy to improve the hole mobility of D-A-D-based hole transport material (HTM) by modifying the accepting core of the reference HTMs based on triphenylamine (TPA) and bithieno thiophene (BTTI) units. Eight HTMs are designed by replacing the BTTI unit in BTTI-TPA HTM with several commonly available diones and imides as acceptors. The structural, electronic, optical, and charge transport properties are studied using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The designed molecules are extensively examined for their structural, electronic, and absorption properties to extract the features that improve their hole mobility using the MPW1PW91/6–31 g(d,p) method. We identified four carefully designed BTTD-TPA, DBTT-TPA, NDTTI-TPA, and NDTI-TPA molecules as the most promising HTMs. This work exemplifies the role of the methyl group on electron acceptors on hole reorganization energies. Results obtained from this study provide valuable guidelines for designing efficient HTMs.

Impact of diethylene glycol chains on indolo [3,2-b]indole based small molecule as dopant-free hole transporting materials for efficient and stable inverted perovskite solar cells

Impact of diethylene glycol chains on indolo [3,2-b]indole based small molecule as dopant-free hole transporting materials for efficient and stable inverted perovskite solar cells

Photovoltaics literature survey (No. 181)

Photovoltaics literature survey (No. 181)

Tuning Surface Oxidation States of Nickel Oxide for Efficient Inverted Perovskite Solar Cells

Nickel oxide (NiOx), as an inorganic p-type semiconductor, has been widely adopted as hole-transporting layers in perovskite solar cells. Despite its superior material stability, the poor charge extraction and multi-vacancies greatly restrict the photovoltages and efficiencies. Here, we propose a facile method to tune the surface oxidation states of NiOx films by a lithium salt treatment for photovoltage enhancement. X-ray photoelectron spectroscopy measurement indicates that the lithium treatment only reduces the Ni3+ sites at the top region rather than in the bulk of NiOx films. This graded distribution allows more efficient charge extraction at the NiOx/perovskite interface as revealed by photoluminescence studies. Through a combination of capacitance–voltage and drive-level capacitance profiling measurements, we confirm enhanced built-in potentials and decreased interface defect densities in lithium-modified devices. Further modifying the interface with a self-assembly monolayer, the energy offsets at the interface can be largely reduced. Based on these enhanced properties, the modified devices achieve a high-power conversion efficiency of 22.4% (0.07 cm2) with a 120 mV enhancement in photovoltage in comparison with the untreated devices.

Self-Assembled Molecules for Hole-Selective Electrodes in Highly Stable and Efficient Inverted Perovskite Solar Cells with Ultralow Energy Loss.

Good selective contacts are necessary for solar cells that are efficient and have long-term stability. Since 1998, with the advent of solid-state dye sensitized solar cells (DSSC), Spiro-OMeTAD has become the reference hole-transporting material. Yet, for efficient solar cells Spiro-OMeTAD must be partially oxidized with chemical dopants, which compromises the long-term stability of the solar cell. Alternatively, semiconductor polymers such as PTAA have been also studied, matching or improving the solar cell characteristics. However, PTAA-based devices lack long-term stability. Moreover, both Spiro-OMeTAD and PTAA are expensive materials to synthesize. Hence, approaches toward increasing the solar cell stability without compromising the device efficiency and decreasing the manufacturing cost are very desirable. In this work we have modified Spiro-OMeTAD, by an easy-to-use methodology, by introducing a carboxylic acid anchoring group (Spiro-Acid), thereby allowing the formation of self-assembled monolayers (SAMs) of the hole-transporting material in dopant-free p-i-n hybrid perovskite solar cells (iPSCs). The resulting device showed a champion efficiency of 18.15% with ultralow energy loss, which is the highest efficiency among Spiro-OMeTAD-based iPSCs, and a remarkable fill factor of over 82%, as well as excellent long-term illumination stability. Charge transfer and charge carrier dynamics are studied by using advanced transient techniques to understand the interfacial kinetics. Our results demonstrate that the Spiro-OMeTAD-based SAMs have a great potential in producing low-cost iPSC devices, due to lower material usage, good long-term stability, and high performance.

4-tert-butylpyridine induced Ni3+/Ni2+ ratio modulation in NiOx hole transport layer towards efficient and stable inverted perovskite solar cells

NiOx plays a vital role in fabricating efficient and stable inverted perovskite solar cells (PSCs). However, the relatively low conductivity and inefficient hole extraction of NiOx hole transport layer (HTL) due to the low Ni3+/Ni2+ ratio limit its use in high-efficiency PSCs. Here, we report a facile strategy to remarkably enhance the electrical conductivity and hole extraction capability of NiOx HTL by simply introducing the small molecule 4-tert-butylpyridine (tBP) as additive in the NiOx precursor solution. It is revealed that the tBP efficient improves the charge selectivity of the NiOx interface, inducing the preferable p-type carrier concentration on the surface. Consequently, more oxygen is involved in the process of Ni2+ to Ni3+ conversion, and the resulting tBP: NiOx HTL exhibits significantly enhanced hole extraction and transport property. In addition, a thin layer of LiF is applied to passivate the surface of NiOx, which enables improved hole migration ability by the dipole effect of LiF and reduced trap state density of the surface. The inverted PSCs (0.16 cm2) fabricated with tBP: NiOx HTL delivers an enhanced efficiency of 20.2% relative to the control devices (17.5%). tBP: NiOx devices with a large area of 1 cm2 are also demonstrated, which achieve a decent efficiency of 16% with excellent long-term stability.

Cation exchange strategy to construct nanopatterned Zn:NiOx electrode with highly conductive interface for efficient inverted perovskite solar cells

Inorganic hole-transporting materials (HTMs) with excellent hole extraction and transfer properties are highly desirable for inverted perovskite solar cells (PSCs). However, the widely employed oxide HTMs (e.g., NiOx nanocrystal) often suffer from the low p-type conductivity and severe trap-assisted recombination at HTMs/perovskite interface, which significantly limits the final performance of PSCs. Herein, the facile template-assisted cation exchange strategy is developed to fabricate nanopatterned Zn:NiOx as efficient HTMs for PSCs, for the first time. Promisingly, by virtue of the advantageous 1D nanoscale architecture and synergistic substitutional Zn doping, high conductivity, high hole mobility and low interfacial charge trap states density have been achieved by such nanopatterned Zn:NiOx HTMs. These features endow the highly conductive interface for PSCs to accelerate hole extraction and minimize recombination loss. As a consequence, PSCs with nanopatterned Zn:NiOx deliver a champion efficiency of ≈20 % with open-circuit voltage of as high as 1.14 V. This work demonstrates and paves a new insight and avenue for the rational design of efficient oxide HTMs.

Recent progress of inverted organic-inorganic halide perovskite solar cells

In recent years, inverted perovskite solar cells (IPSCs) have attracted significant attention due to their low-temperature and cost-effective fabrication processes, hysteresis-free properties, excellent stability, and wide application. The efficiency gap between IPSCs and regular structures has shrunk to less than 1%. Over the past few years, IPSC research has mainly focused on optimizing power conversion efficiency to accelerate the development of IPSCs. This review provides an overview of recent improvements in the efficiency of IPSCs, including interface engineering and novel film production techniques to overcome critical obstacles. Tandem and integrated applications of IPSCs are also summarized. Furthermore, prospects for further development of IPSCs are discussed, including the development of new materials, methods, and device structures for novel IPSCs to meet the requirements of commercialization.

Crystallization manipulation and holistic defect passivation toward stable and efficient inverted perovskite solar cells

Synergistic modification of new intermediate phase assisted crystallization and upper interface passivation for enhanced crystallization, reduced defect density, and released stress in perovskite films, enabling 24.61% efficient inverted devices.

Low-temperature solution-processed LaNiO3 hole-transport layer for UV-stable inverted perovskite solar cells.

We report a solution-processing method to prepare an inorganic LaNiO3 (LNO) hole-transport layer (HTL) under low temperature (<150 °C) for the first time. The inverted PSCs prepared with LNO exhibit high UV-stability and promising efficiency (17.15%). Our preliminary results show great potential for LNO HTL in the fabrication of efficient and photostable inverted perovskite solar cells (PSCs).

Finite perovskite hierarchical structures via ligand confinement leading to efficient inverted perovskite solar cells

Finite 2D/3D hierarchical perovskites are effectively constructed via interfacial ligand confinement to obtain efficient and stable inverted perovskite solar cells and modules.

Defect passivation via a multifunctional organic additive toward efficient and stable inverted perovskite solar cells.

A multifunctional group molecule, namely MATC, was first introduced into a Cs/FA-based perovskite used as an additive. An impressive PCE of 21.51% was achieved for the inverted PSCs with reduced defect states and improved perovskite film quality. Moreover, MATC passivation considerably enhanced the stability of the PSC devices.

Efficient Inverted Perovskite Solar Cells via Improved Sequential Deposition.

Inverted-structure metal halide perovskite solar cells (PSCs) have attractive advantages like low-temperature processability and outstanding device stability. The two-step sequential deposition method shows the benefits of easy fabrication and decent performance repeatability. Nevertheless, it is still challenging to achieve high-performance inverted PSCs with similar or equal power conversion efficiencies (PCEs) compared to the regular-structure counterparts via this deposition method. Here, an improved two-step sequential deposition technique is demonstrated via treating the bottom organic hole-selective layer with the binary modulation system composed of a polyelectrolyte and an ammonium salt. Such improved sequential deposition method leads to the spontaneous refinement of up and buried interfaces for the perovskite films, contributing to high film quality with significantly reduced defect density and better charge transportation. As a result, the optimized PSCs show a large enhancement in the open-circuit voltage by 100mV and a dramatic lift in the PCE from 18.1% to 23.4%, delivering the current state-of-the-art performances for inverted PSCs. Moreover, good operational and thermal stability is achieved upon the improved inverted PSCs. This innovative strategy helps gain a deeper insight into the perovskite crystal growth and defect modulation in the inverted PSCs based on the two-step sequential deposition method.