Chen Weijie

Publication date: January 2023

Source: Journal of Energy Chemistry, Volume 76

Author(s): Baibai Liu, Ru Li, Qixin Zhuang, Xuemeng Yu, Shaokuan Gong, Dongmei He, Qian Zhou, Hua Yang, Xihan Chen, Shirong Lu, Zong-Xiang Xu, Zhigang Zang, Jiangzhao Chen

Revealing the multifunctional effects of NH₄ ⁺ through enhancing photo-carrier dynamics compared to Cl, creates a self-assembled surface trap state that causes non-radiative recombination at the interface. The NH₄ ⁺ pre-treatment on indium tin-oxide is an effective way to modify bifacial interfaces improving crystallinity and charge carrier transport, which leads to an increased power conversion efficiency and device stability in perovskite solar cells.

Abstract

Passivation is a popular method to increase power conversion efficiency (PCE), reduce hysteresis related to surface traps and defects, and adjust mismatched energy levels. In this paper, an approach is reported using ammonium chloride (AC) to enhance passivation effects by controlling chlorine (Cl) and ammonium ions (NH₄ ⁺) on the front and back side of tin oxides (SnO₂). AC pre-treatment is applied to indium tin-oxide (ITO) prior to SnO₂ deposition to advance the passivation approaches and compare the completely separated NH₄ ⁺ and Cl passivation effects, and sole NH₄ ⁺ is successfully isolated on the SnO₂ surface, the counterpart of AC-post-treatment, generating ammonia (NH₃) and Cl. It is demonstrated that multifunctional healing effects of NH₄ ⁺ are ascribed from AC-pre-treatment being the basis of SnO₂ crystallization and adjusting bifacial interface energy levels at ITO/SnO₂ and SnO₂/perovskite to enhance photo-carrier transport. As calculated by density functional theory, how the change of the passivation agent from Cl to NH₄ ⁺ more effectively suppresses non-radiative recombination ascribed to hydrated SnO₂ surface defects is explained. Consequently, enhancement of photo-carrier transport significantly improves a superior open-circuit voltage of 1.180 V and suppresses the hysteresis, which leads to the PCE of 22.25% in an AC-pre-treated device 3.000% higher than AC-post-treated devices.

A strategy of utilizing built-in field is proposed to break constraints on the application of the bilayer electron transport layer (ETL) in high-efficiency perovskite solar cells (PSCs), through which a bilayer ETL of C-TiO₂/SnO₂ with Li-doping is successfully developed and applied in high-efficiency PSCs demonstrating comprehensive advantages in improving V _OC, FF, and J _SC, and a PCE of 24.3% is achieved.

Abstract

Combining two kinds of electron transport layer (ETL) which have complementary advantages into a bilayer structure to form a bilayer ETL is an effective way to transcend inherent limitations of single-layer ETL, which is very helpful in the development of perovskite solar cells (PSCs). In this work, a strategy is proposed to break constraints on the application of the staggered bilayer ETL in high-efficiency PSC, namely utilizing a built-in field to overcome the dilemma in E _CBM making it possible to improve V _OC and FF simultaneously by tuning the Fermi level of ETLs properly. According to the strategy, a bilayer ETL structure comprised of C-TiO₂ and SnO₂ layer and corresponding Li-doping process are developed, and the characterization results confirm the effectiveness of the strategy, making the potentials of the C-TiO₂(Li)/SnO₂ bilayer ETL fully released for its application in high-efficiency PSCs: a V _OC of 1.201 V for an ordinary triple-cation-perovskite-based PSC and a photoelectric conversion efficiency of 24.3% for a low-bandgap-perovskite-based PSC with high haze FTO superstrate are successfully achieved, indicating that the C-TiO₂(Li)/SnO₂ bilayer ETL is a successful application paradigm of the proposed strategy and very promising in the application of high-efficiency PSCs.

The Spiro-BD-2OEG with composition-conditioning agent functionality is designed to improve the composition stability in the doped-Spiro-OMeTAD hole transport layer (HTL). By employing this strategy, the HTL shows a pinhole-free and smooth morphology with an enhanced Spiro-OMeTAD ordering. Finally, the resultant perovskite solar cells show an excellent power conversion efficiency of 24.19% and improved thermal, moisture, and operational stabilities.

Abstract

The doped Spiro-OMeTAD hole transport layer (HTL) formed using the lithium bis(trifluoromethane) sulfonimide salt and 4-tert-butylpyridine with phenethylammonium iodide surface treatment on a perovskite film has continuously dominated the record power conversion efficiencies (PCEs) of perovskite solar cells (pero-SCs). However, unstable HTL compositions and iodide salts can cause severe device degradation. In this study, an HTL composition-conditioning agent (CCA), Spiro-BD-2OEG, is designed, which contains a Spiro-OMeTAD-like backbone, functional pyridine units, and oligo (ethylene glycol) chains. This finely designed CCA presents good miscibility with Spiro-OMeTAD and its dopants and acts as a conditioning agent through weak bond interactions. As a result, the CCA-regulated HTL shows a pinhole-free and smooth morphology with enhanced Spiro-OMeTAD ordering and improves dopant stability. In addition, the gradient-distributed CCA in the HTL can narrow the energy level offset with the valence band of the perovskite. The resultant pero-SCs exhibit an excellent PCE of 24.19% without any interface treatment and weak size dependence. A remarkable PCE of 22.63% is obtained even for a 1.004-cm² device. Importantly, the strategy shows good universality and significantly promotes the long-term stability of the pero-SCs based on the classical doped Spiro-OMeTAD.

Publication date: 1 December 2022

Source: Nano Energy, Volume 103, Part B

Author(s): Sajjad Ahmad, Ruixue Lu, Yang Liu, Xuan Liu, Qing Yang, Xin Guo, Can Li

Two key interfaces on either side of the metal-halide perovskite thin film in flexible perovskite solar cells (f-PSCs) are reinforced simultaneously. This new class of dual-interface-reinforced f-PSCs has an unprecedented combination of high efficiency (21.03%), improved operational stability (1000 h T ₉₀), and enhanced mechanical reliability (10 000 cycles n ₈₈). The scientific underpinnings of these synergistic enhancements are elucidated.

Abstract

Two key interfaces in flexible perovskite solar cells (f-PSCs) are mechanically reinforced simultaneously: one between the electron-transport layer (ETL) and the 3D metal-halide perovskite (MHP) thin film using self-assembled monolayer (SAM), and the other between the 3D-MHP thin film and the hole-transport layer (HTL) using an in situ grown low-dimensional (LD) MHP capping layer. The interfacial mechanical properties are measured and modeled. This rational interface engineering results in the enhancement of not only the mechanical properties of both interfaces but also their optoelectronic properties holistically. As a result, the new class of dual-interface-reinforced f-PSCs has an unprecedented combination of the following three important performance parameters: high power-conversion efficiency (PCE) of 21.03% (with reduced hysteresis), improved operational stability of 1000 h T ₉₀ (duration at 90% initial PCE retained), and enhanced mechanical reliability of 10 000 cycles n ₈₈ (number of bending cycles at 88% initial PCE retained). The scientific underpinnings of these synergistic enhancements are elucidated.

The efficiency potential of inorganic n–i–p and p–i–n CsPbI₂Br solar cells is quantified using intensity-dependent photoluminescence measurements. A detailed loss analysis reveals the contribution of each charge transport layer to the voltage and fill factor losses. This study provides insights into the loss mechanisms and presents strategies for improvement.

Inorganic perovskite solar cells show excellent thermal stability, but the reported power conversion efficiencies are still lower than for organic–inorganic perovskites. This is mainly caused by lower open-circuit voltages (V _OCs). Herein, the reasons for the low V _OC in inorganic CsPbI₂Br perovskite solar cells are investigated. Intensity-dependent photoluminescence measurements for different layer stacks reveal that n–i–p and p–i–n CsPbI₂Br solar cells exhibit a strong mismatch between quasi-Fermi level splitting (QFLS) and V _OC. Specifically, the CsPbI₂Br p–i–n perovskite solar cell has a QFLS–e ·V _OC mismatch of 179 meV, compared with 11 meV for a reference cell with an organic–inorganic perovskite of similar bandgap. On the other hand, this study shows that the CsPbI₂Br films with a bandgap of 1.9 eV have a very low defect density, resulting in an efficiency potential of 20.3% with a MeO–2PACz hole-transporting layer and 20.8% on compact TiO₂. Using ultraviolet photoelectron spectroscopy measurements, energy level misalignment is identified as a possible reason for the QFLS–e ·V _OC mismatch and strategies for overcoming this V _OC limitation are discussed. This work highlights the need to control the interfacial energetics in inorganic perovskite solar cells, but also gives promise for high efficiencies once this issue is resolved.

A multifunctional ligand, namely 2-hexyl-thiophene (2HT), is employed as an efficient additive. The addition of 2HT results in improved crystal growth, longer carrier lifetime, lower defect density, and thus improved film quality and stability. The device with 2HT provides an enhanced power conversion efficiency (PCE) of 20.61% and more environmental stability than that of the pristine device (PCE = 18.65%).

Fabricating high-quality perovskite films with large grain sizes and low-defect densities is critical for developing efficient and stable perovskite solar cells (PeSCs). Herein, a simple and effective multifunctional additive engineering strategy is reported for producing high-quality perovskite films. By including 2-hexyl-thiophene (2HT) with a thiophene electron-pair donor and a long hydrophobic alkyl chain as an additive in the perovskite precursor solution, a perovskite film with high crystallinity, decreased trap density, and hindered ion migration is successfully obtained. These features are attributed to the strong coordinative interaction between the sulfur atom in 2HT and Pb²⁺ in the perovskite film. The long alkyl chain of the 2HT additive assisted the production of a superior perovskite film with enlarged grain size, smooth surface topography, and hydrophobicity. Consequently, improved efficiency and prolonged operational stability are realized simultaneously for an inverted MAPbI₃PeSC with the 2HT additive. The device with 2HT delivers a higher power conversion efficiency of 20.61% compared with that of the control device (18.65%) and exhibits negligible hysteresis. Moreover, the stability of the device with the 2HT additive is superior to that of the control device under various testing conditions.

Two novel nonconjugated small-molecule zwitterions, 4-(1-methyl piperidin-1-ium-1-yl)butane-1-sulfonate (MPBS) and 3-(1-methylpiperidin-1-ium-1-yl)-propane-1-sulfonate (MPPS), are synthesized as the morphology control additives for the cathode interlayers of nonfullerene organic solar cells. Both these zwitterions are able to form intense interaction with amino terminal-substituted perylene diimide (PDIN). It improves the morphology of the cathode interlayer and helps the formation of the electron transfer network, which significantly improve the photovoltaic performance.

The cathode interlayer (CIL) has significant impact on the performance of organic solar cells (OSCs), including the abilities to align energy levels, form ohmic contacts between the active layer and the electrode, and promote the electron extraction and transportation. However, the developments of CILs are still far behind the rapid explosion of active layer materials, especially for these nonfullerene acceptors (NFAs). This research provides a brand-new CIL optimization approach by applying the nonconjugated small-molecule zwitterion (NSMZ) as the morphology control additives (MCAs). Two novel NSMZs, 4-(1-methyl piperidin-1-ium-1-yl)butane-1-sulfonate (MPBS) and 3-(1-methylpiperidin-1-ium-1-yl)-propane-1-sulfonate, are developed as MCAs for the improvements of short-circuit current, fill factor, and power conversion efficiency (PCE) in various solar cell systems of the classic active layers and CILs. The interaction mechanism is systematically investigated. With the reduced energy barrier and suppressed electron recombination, a champion PCE of 18.65% on binary NFA–OSCs is achieved by incorporating the zwitterion of MPBS into the cathode interlayer as the additive, which is among the highest efficiency of the reported binary OSCs. The application of the zwitterion as MCAs for the cathode interlayer opens a novel avenue for highly efficient OSCs.

Herein, two wide-bandgap A₂-A₁-D-A₁-A₂-type nonfullerene acceptors with different central cores, namely BTA501 and BTA502, are developed to improve the photovoltaic performance of high-voltage organic solar cells. Consequently, the device of J52-F: BTA501 achieves an open circuit voltage (V _OC) of 1.037 V with a PCE of 11.82%, which are among the highest values for high-voltage devices with V _OC above 1.0 V.

In recent years, the organic solar cells (OSCs) research hotspot is the modification of end groups and alkyl side chains of A-DA’D-A-type nonfullerene acceptors (NFAs). However, the development of novel NFAs by changing the different bridged atom substitution of the central core is lagging behind. Herein, two wide-bandgap A₂-A₁-D-A₁-A₂-type NFAs with different central cores, namely BTA501 and BTA502, are developed to improve the photovoltaic performance of high-voltage OSCs. BTA501 adopted an indacenodithiophene (IDT) core, whereas BTA502 applied a silaindacenodithiophene (SiIDT) core. Expectedly, the SiIDT-based BTA502 exhibits a higher lowest unoccupied molecular orbital level and wider bandgap than BTA501, which thus enhances the open-circuit voltage (V _OC) but slightly decreases the short-circuit current density (J _SC) of OSCs. Moreover, the stronger self-aggregation characteristics and weaker π–π stacking of BTA502 severely affect the exciton dissociation and charge transport. When blended with two classic p-type polymers J52-F and PTB7-Th, both combinations based on BTA502 exhibit inferior device performance compared with BTA501. Excitingly, the device of J52-F: BTA501 achieves a V _OC of 1.037 V with a power conversion efficiency of 11.82% and a J _SC of 15.89 mA cm⁻², which are among the highest values for high-voltage OSCs with V _OC above 1.0 V.

Incorporation of 3,5-diflubenzylamine (DFBA) as an A-site insertion group allows to realize a thus far unreported DFBA perovskite with fascinating well lattice-matching heterojunction structure, which is experimentally observed to suppress the defect states and thereby significantly improve the stability of Sn-based perovskites.

Despite tin perovskite showing excellent optoelectronic properties such as ideal bandgap and high carrier mobility, the intrinsic instability of tin perovskite due to the reaction with water and oxygen that significantly shadows its application. Herein, a molecule substituted with two fluorine atoms for the construction of low-dimensional tin perovskite is explored. Tin perovskite film based on this molecule shows much enhanced hydrophobicity, bringing much enhanced stability in atmosphere. The unencapsulated device can be kept over 200 h in atmospheric environment. In comparison to the instantaneous degradation of device performance for traditional 3D solar cells, no obvious degradation of device efficiency is observed under continuous operation in atmosphere without encapsulation over 10 h. Furthermore, the low-dimensional perovskite film exhibits a unique two-layer structure that can effectively passivate defects of the tin perovskite film. Accordingly, the power conversion efficiency (PCE) of Sn-perovskite solar cells realizes a remarkable enhancement by a factor of 45% and yields a PCE of 8.38%. This work indicates that molecular design of low-dimensional structure is a promising approach to improve the stability of tin perovskite.

Electrodeposition is used for the first time for the deposition of mixed perovskite. Thanks to a maturation step, the performances can be significantly improved, to reach record efficiencies of 10%. An improvement of the stability under air at 40 °C is also observed. Electrodeposition seems to be an alternative method to develop large-area perovskite layers for application in solar devices.

Electrodeposition is investigated in this work as an alternative method to develop a large-area perovskite active layer for solar device application. In addition to the single perovskite MAPbI₃, the deposition of mixed perovskites MAPbI _x Cl_1−x and MA_1−y FA _y PbI_3−x Br _x is studied. This study is unique since these mixed perovskites have never been developed by electrodeposition before. It is noticed that depending on the ratios of FA, Br, and Cl infiltrated into the perovskite lattice, different microstructures and optical and chemical properties are obtained, having an impact on the photovoltaic performance of the perovskite layer. In fact, increasing the percentages of FA, Br, and Cl in the mixed perovskite improves the photovoltaic performance. Thanks to a maturation step carried out under vacuum at 40 °C for 500 h, the performances can be significantly improved, at best by 60%, to reach record efficiencies of 10%. An improvement of the stability under air at 40 °C is also observed. All the results obtained are compared with recent results in the literature, which makes it possible to appreciate the originality and innovation of the study proposed in this article. In summary, this work illustrates the importance of electrodeposition and its flexibility to develop different types of perovskites.

A champion Dion–Jacobson 2D perovskite photovoltaic device with a high open circuit voltage (V _oc) of 1.21 V and an efficiency over 17.6% is achieved through a thermally induced crystallization strategy. The highest V _oc at 120 °C is mainly attributed to the smallest energy loss ΔE ₃, which is originated from suppression of nonradiative recombination and reduction of trap density.

Quasi-2D perovskite solar cells (PSCs), with impressive stability and tunable optoelectronic properties, have become a promising alternative to 3D PSCs. However, the lack of understanding about rationally promoting the film quality of 2D perovskites has significantly undermined their power conversion efficiency (PCE). Herein, a thermally induced crystallization strategy to process the Dion–Jacobson (DJ) 2D perovskite films based on (BDA)(MA)₄Pb₅I₁₆ (n = 5) afforded by thermal treatments is reported, realizing high-quality perovskite films with increased crystallinity and dense structure. The photovoltaic performance of PSCs improved by thermal treatments is found to be mainly attributed to the significantly suppressed nonradiative recombination, more efficient charge generation, and restricted interfacial charge accumulation. As a result, the optimal device processed with thermal treatments produces a high open-circuit voltage of 1.21 V and a PCE of over 17.6% with suppressed energy loss down to 0.42 eV. Moreover, the treated devices without encapsulation show a satisfactory stability with <20% PCE degradation after 1000 h under maximum power point tracking. The demonstrated strategy in this work offers a promising route for the performance enhancement of 2D DJ PSCs toward realistic energy conversion applications.

This work proposes a large area MXene:Nafion–Si heterojunction solar cell with high performance and low cost. Herein, the organic Nafion solution is selected to solubilize the Ti₂CT _x MXene nanosheets, which not only enhance its dispersity but also increase the work function to 5.17 eV. Finally, the high power conversion efficiency of 14.21% is achieved with a large area of 7.29 cm².

2D transition metal carbon/nitrides (MXenes) have lately attracted increased attention in photovoltaic (PV) because of their good optical and electrical properties. MXenes–silicon solar cells as a novel PV device are considered to be a low-cost strategy for boosting Si-based device development. Nevertheless, power conversion efficiency (PCE) and large area preparation are limited in their future applications. Here, the solubilization of Ti₂CT _x (T = O, F, OH) MXene multilayer nanosheets is reported and thus the ability of scaled coating of Ti₂CT _x MXene thin films using an organic Nafion polymer solution is enhanced. It is demonstrated that an effect of termination by Nafion to Ti₂CT _x MXene groups (MXene:Nafion) dramatically enhanced the work function of the Ti₂CT _x MXene from 3.96 to 5.17 eV. Due to the excellent passivation ability of the organic Nafion species, the MXene:Nafion increases the effective carrier lifetime of silicon wafers to 2.52 ms compared with unpassivated control (0.03 ms). These results finally contribute to the high PCE of 14.21% of MXene:Nafion–Si solar cell with a large area of 7.29 cm². This work proposes a large area MXene:Nafion–Si heterojunction solar cell with high performance and low cost, which points to the great promise of such a device for future application.

Publication date: 1 December 2022

Source: Nano Energy, Volume 103, Part A

Author(s): Dan Zhou, Jianru Wang, Zhentian Xu, Haitao Xu, Jianwei Quan, Jiawei Deng, Yubing Li, Yongfen Tong, Bin Hu, Lie Chen