JIALINGBO

The rate of charge transfer (CT) state dissociation in cyanine/fullerene solar cells strongly depends on the electric field but is temperature independent. CT states dissociate via a temperature-independent electron tunneling mechanism through a thin, high-energy potential barrier at the donor–acceptor interface. The results support a new mechanism for charge generation in organic solar cells via carrier tunneling from CT states.

Abstract

Charge transfer (CT) states play a key role in the functioning of organic solar cells; however, understanding the mechanism by which CT states dissociate efficiently into free charges remain a conceptual challenge. Here, the electric field dependent dynamics of charge generation in planar cyanine/fullerene photovoltaic cells is probed over a wide temperature range using time-resolved Stark effect experiments, transient absorption, and photocurrent measurements. Results indicate that dissociation of thermalized CT states is the rate-limiting step for all temperatures. The dissociation rate strongly depends on the field, but is temperature independent. The results also suggest that the yield of generated charges is temperature independent. Model electrostatic calculations illustrate that specific orientations of the cyanine crystal relative to C₆₀ create a repulsive potential for an electron near the interface that is largely due to the quadrupole moment of the unit cell. In combination with the electron-hole coulomb attraction and the electric field-induced barrier lowering, a high-energy potential barrier forms with a narrow width of a few nanometers. It is proposed that charge separation occurs via a field-dependent electron tunneling mechanism through that barrier, which is temperature independent. The results support a thus far overlooked pathway for CT state dissociation via carrier tunneling.

A ligand molecule containing carbonyls (carboxyl and amide) and a long hydrophobic alkyl chain is incorporated into a perovskite precursor to achieving improved crystallinity, reduced trap state density, and inhibited ion migration. This strategy enables an impressive power conversion efficiency exceeding 23% with inhibited hysteresis.

Abstract

The nonradiative recombination losses resulting from the trap states at the surface and grain boundaries directly hinder the further enhancement of power conversion efficiency (PCE) and stability of perovskite solar cells. Consequently, it is highly desirable to suppress nonradiative recombination through modulating perovskite crystallization and passivating the defects of perovskite films. Here, a simple and effective multifunctional additive engineering strategy is reported where 11 Maleimidoundecanoic acid (11MA) units with carbonyls (carboxyl and amide) and long hydrophobic alkyl chain are incorporated into a perovskite precursor solution. It is revealed that improved crystallinity, reduced trap state density, and inhibited ion migration are achieved, which is ascribed to the strong coordination interaction between the carbonyl groups at both sides of 11MA molecules and Pb²⁺. As a result, improved efficiency and stability are achieved simultaneously after introducing 11MA additive. The device with 11MA additive delivers a champion PCE of 23.34% with negligible hysteresis, which is significantly higher than the 18.24% of the control device. The modified device maintains around 91% of its initial PCE after aging under ambient conditions for 3000 h. This work provides a guide for developing multifunctional additive molecules for the purpose of simultaneous improvement of efficiency and stability.

A dual-functional polymer is introduced into a [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) layer to simultaneously modify the PCBM electron transport layer (ETL) and passivate the surface defects in the perovskite. The hybrid ETL exhibits excellent electronic properties and the ability to suppress the ion migration in perovskite solar cells, leading to a high power conversion efficiency of 21.13% and long-term stability.

Herein, the use of the polymer poly([N, N ′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)) (PNDI-2T) as both a dopant in the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) electron transport layer (ETL) of inverted perovskite solar cells (PSCs) and a surface passivant on the perovskite layer is reported. The PNDI-2T doping is found to be crucial for improving the homogeneity of the ETL film with complete surface coverage, enhancing the electron mobility of the ETL, and promoting the energy level match between the perovskite and ETL, thereby significantly facilitating electron transport in the PSC devices. Furthermore, as a passivant, the PNDI-2T in the ETL tends to pin on the top surface of the perovskite layer via PbS coordination. The surface passivation is also beneficial for the suppression of ion migration and diffusion of metal species from the electrodes. In consequence, PSCs with the PCBM:PNDI-2T ETL reached an efficiency of 21.13% and retain 90% of their original performance after 860 h light soaking. This work will inspire new efforts to take advantage of multifunctional ETLs as a simple and effective method to enhance the performance and long-term stability of PSCs.

Publication date: August 2021

Source: Nano Energy, Volume 86

Author(s): LiangLe Wang, Md. Shahiduzzaman, Ersan Y. Muslih, Masahiro Nakano, Makoto Karakawa, Kohshin Takahashi, Koji Tomita, Jean Michel Nunzi, Tetsuya Taima

Publication date: August 2021

Source: Nano Energy, Volume 86

Author(s): Ningning Ma, Jizhong Jiang, Yan Zhao, Lijuan He, Yao Ma, Hailu Wang, Lili Zhang, Chongxin Shan, Liang Shen, Weida Hu

Publication date: August 2021

Source: Nano Energy, Volume 86

Author(s): Muhammad Fahim, Irum Firdous, Weihai Zhang, Walid A. Daoud

Iodine-terminated self-assembled monolayer (I-SAM) was used in perovskite solar cells (PSCs) to achieve a 50% increase of adhesion toughness at the interface between the electron transport layer (ETL) and the halide perovskite thin film to enhance mechanical reliability. Treatment with I-SAM also increased the power conversion efficiency from 20.2% to 21.4%, reduced hysteresis, and improved operational stability with a projected T80 (time to 80% initial efficiency retained) increasing from ~700 hours to 4000 hours under 1-sun illumination and with continuous maximum power point tracking. Operational stability–tested PSC without SAMs revealed extensive irreversible morphological degradation at the ETL/perovskite interface, including voids formation and delamination, whereas PSCs with I-SAM exhibited minimal damage accumulation. This difference was attributed to a combination of a decrease in hydroxyl groups at the interface and the higher interfacial toughness.

The interface passivation and performance enhancements of perovskite solar cell (PSCs) devices are realized by introducing few-layer nonmetallic element sulfur-doped graphite carbon nitride nanosheets in the SnO₂-based electron transport layer (ETL) at the first time, which is attributed to the enhanced electron mobility and conductivity of CNS-modified ETL, reduced interfacial trap state density, and improved crystallinity of perovskite film.

High-quality electron transport layer (ETL) is beneficial to improve the charge extraction and transport, which determines the performance of perovskite solar cells (PSCs). However, the unbalanced charge extraction and interface problems commonly occur in the tin oxide (SnO₂) ETL. Herein, the sulfur-doped graphite carbon nitride (CNS) nanosheets are prepared and utilized for modifying the SnO₂ ETL to fabricate high-performance PSCs. The CNS-modified SnO₂ ETL exhibits enhanced electron mobility and conductivity, and matched energy level with perovskite, which promotes the extraction and transport of charge carriers at the interface, and balances charge extraction with the hole transport layer. In addition, interfacial carrier recombination is significantly reduced through effective interface passivation of sulfur atoms in CNS with the undercoordinated lead ions in perovskite films. Meanwhile, the introduction of an interfacial control material CNS also contributes to improve the crystalline quality of perovskite films with increasing grain size and light absorption intensity. As a consequence, an outstanding improvement in power conversion efficiency (PCE) from 18.98% to 20.33% is achieved after introducing CNS into the SnO₂ ETL, as well as an enhancement in stability against humidity, retaining near 90% of the initial PCE after aging in the ambient atmosphere for 30 days.

Mixing chlorobenzene (CB) and H₂O in the perovskite precursor is an effective method to improve the direct contact between poly-TPD and the perovskite active layer. Inverted (p–i–n) perovskite solar cells based on the modified perovskite display efficient hole-interface charge transfer and suppression of the bulk and interfacial nonradiative recombination, thereby achieving an excellent power conversion efficiency of 22.1%.

Inverted perovskite solar cells (IPSCs) suffer from perishing interface contact due to the non-wetting hole-transport layer (HTL). Herein, the several classes of solvent to the perovskite precursor (the process is defined as solvent-additive engineering) for achieving an improvement in the interface contact between nonwetting HTL and active perovskite layer, suitably achieving improved hole-interface charge transfer, are mixed. Also, a high-quality perovskite layer with high crystallinity, large grain distribution, and flat surface morphology is obtained based on solvent-additive engineering, which affords a lower bulk and interface trap density. IPSCs with the modified perovskite layer show suppression of nonradiative recombination on the surface and in the bulk of the perovskite, thereby achieving an outstanding power conversion efficiency of 20.6%. In addition, IPSCs using a mixed-cation perovskite (FA_0.83Cs_0.07MA_0.13PbI_2.64Br_0.39) are also fabricated and a highest efficiency of 22.1%, visualizing the broad applicability of this method, is achieved. This simple, low-cost, and efficient solvent-additive strategy can solve interface contact problems and improve perovskite quality, thus potentially giving rise to other applications.

The formation of a conformal oxide overlayer without damage to the perovskite layer is challenging. This study presents a highly dispersive ligand-off NiO (NiO_X) colloidal solution for n-i-p halide perovskite solar cells (HPSCs) using the designed solvent. The NiO_X overlayer shows outstanding charge extraction properties, and the NiO-HPSCs exhibit an efficiency of over 19.1% with excellent thermal stability.

Abstract

In n-i-p halide perovskite solar cells (HPSCs), the development of p-type oxides is one of the most noteworthy approaches as hole transport materials (HTMs) for long-term stability and mass production. However, the deposition of oxide HTMs through a solution process over the perovskite layer without damage to the perovskite layer remains a major challenge. Here, the colloidal dispersion of ligand-off NiO nanoparticles (NPs) to form the HTM overlayer on perovskite using appropriate solvents that do not damage the underlying perovskite layer is reported. Monodispersed NiO NPs are synthesized using oleylamine (OLA) ligands via the solvothermal method, and the OLA ligands are then removed to form ligand-off NiO NPs. Based on the Hansen solubility theory, appropriate mixed solvents are found for both the dispersion of NiO NPs without ligands and coating without perovskite damage. The colloidal dispersion form a compact and uniform NiO NPs layer of 30 nm thickness on the perovskite layer, allowing n-SnO₂/Halide/p-NiO HPSCs to be successfully fabricated. The HPSC shows a record power conversion efficiency under one sun illumination for an n-i-p oxide/halide/oxide structure and excellent thermal stability maintaining 98% of the initial efficiency for 580 h under 85 °C and 10% relative humidity condition.

Herein, the passivation and the selectivity of a range of common electron and hole transport layers to wide-bandgap perovskite solar cells are systematically quantified by comparing their internal voltage iV _oc, external voltage V _oc, and surface photovoltage SPV. The origins of voltage losses in these devices are explained and what limits the performance of the carrier transport layers is identified.

Excellent contact passivation and selectivity are prerequisites to realize the full potential of high-material-quality perovskite solar cells, first to maximize the internal voltage (or quasi-Fermi-level separation) iV within the absorber, then to translate this high internal voltage into a high external voltage V. Experimental quantification of contact passivation and selectivity is, thus, key to improving device performance. Here, open-circuit measurements of iV _oc and V _oc, combined with surface photovoltage measurements, are used to systematically quantify the passivation—using iV _oc as a metric—and the selectivity—defined as S _oc = V _oc/iV _oc—of a range of common carrier transport layers to wide-bandgap (1.67 eV) perovskite absorbers. The resulting solar cells suffer from large voltage deficits, particularly when NiO _x is used as the hole transport layer, even though it provides better passivation than its polymer-based counterparts (PTAA and PTAA/PFN). This indicates a poor selectivity of NiO _x (S _oc < 0.81 for NiO _x -based devices), whereas devices using polymer-based hole transport layers exhibit high selectivity (S _oc = 0.94–0.95). In agreement with recent reports, this low selectivity is attributed to the formation of an interlayer of non-perovskite material with high resistance to holes at the perovskite/NiO _x interface. These measurements also imply that the selectivity of the C60-based electron transport layers is relatively good.

A templated growth approach, in which the crystallization of a 3D perovskite is guided by a dynamically dominant 2D structure, was established for the fabrication of a low-dimensional perovskite thin film with an out-of-plane orientation and a large grain size. The template growth is enabled by the reduced crystallization barrier of the 2D PEA₂SnI_4−x SCN _x intermediate.

Abstract

The manipulation of the dimensionality and nanostructures based on the precise control of the crystal growth kinetics boosts the flourishing development of perovskite optoelectronic materials and devices. Herein, a low-dimensional inorganic tin halide perovskite, CsSnBrI_2−x (SCN) _x , with a mixed 2D and 3D structure is fabricated. A kinetic study indicates that Sn(SCN)₂ and phenylethylamine hydroiodate can form a 2D perovskite structure that acts as a template for the growth of the 3D perovskite CsSnBrI_2−x (SCN) _x . The film shows an out-of-plane orientation and a large grain size, giving rise to reduced defect density, superior thermostability, and oxidation resistance. A solar cell based on this low-dimensional film reaches a power conversion efficiency of 5.01 %, which is the highest value for CsSnBr _x I_3−x perovskite solar cells. Furthermore, the device shows enhanced stability in ambient air.

The dominant deep defect states in freshly prepared CsPbI_{3− x} Br _x films are mainly antisite defect pairs (Pb_I and I_Pb) and interstitial defects (Pb_i). All these defects are reduced because of self-regulation process after resting the films overnight in the dark. Based on this strategy, the reduced-defect high quality CsPbI_{3− x} Br _x films can be obtained and thus higher photovoltaic performance.

Abstract

Deep defects often act as Shockley–Read–Hall recombination centers in semiconductor materials, degrading the photoelectric performance and long-term stability of assembled photovoltaic devices. In this report, deep level transient spectroscopy is probed to determine defect concentrations and defect energy levels in all-inorganic CsPbI_{3− x} Br _x perovskite solar cells. Combining that data with the density functional theory calculation, the dominant deep defect states are assigned to antisite defect pairs (Pb_I and I_Pb) and interstitial defects (Pb_i) in freshly prepared CsPbI_{3− x} Br _x films. Astonishingly, all these defects are reduced by approximately one or two orders of magnitude after resting the films overnight, in excellent agreement with the defect-reduced trends from the fluorescence spectra, transient photovoltage, and space-charge-limited current measurements. The reduced defect concentrations are proposed to be connected with their self-regulation during the storage. To assess the thermodynamics possibilities, two reaction procedures are designed to calculate their formation enthalpies and negative Gibbs energy change revealed their spontaneous processes. Then, strain relief is the direct driving force for ion migration, thus defect-regulation by tracing the X-ray diffraction patterns. Furthermore, the power conversion efficiency is improved and the J–V hysteresis is suppressed due to reduced ion migration via relaxed strain.