Chen Weijie

Publication date: August 2020

Source: Nano Energy, Volume 74

Author(s): Hyeon Seok Lee, Min Kyu Kim, Seong Ryul Pae, Daehan Kim, HyeJi Seo, Passarut Boonmongkolras, Issam Gereige, Steve Park, Byungha Shin

Publication date: August 2020

Source: Nano Energy, Volume 74

Author(s): Qing Guo, Ji Lin, Haiqin Liu, Xingliang Dong, Xia Guo, Long Ye, Zaifei Ma, Zheng Tang, Harald Ade, Maojie Zhang, Yongfang Li

Publication date: August 2020

Source: Nano Energy, Volume 74

Author(s): Kohei Nishimura, Muhammad Akmal Kamarudin, Daisuke Hirotani, Kengo Hamada, Qing Shen, Satoshi Iikubo, Takashi Minemoto, Kenji Yoshino, Shuzi Hayase

Publication date: August 2020

Source: Nano Energy, Volume 74

Author(s): Lisha Xie, Zhiyuan Cao, Jianwei Wang, Aili Wang, Shurong Wang, Yuying Cui, Yong Xiang, Xiaobin Niu, Feng Hao, Liming Ding

Microwave irradiation can accelerate the annealing period of perovskite films from the minute to second scale. The microwave annealed perovskite possesses >1 µm crystal grain size, eliminates miscellaneous phases, and facilitates >21% power‐conversion efficiency.

Abstract

Rapid processing technologies of perovskite solar cells (PSCs) offer an exciting approach to raise the rate of production. Herein, a rapid microwave‐annealing process (MAP) is reported to replace the traditional hotplate annealing process (HAP) and the processing period of perovskite is reduced to less than 1 min. Benefiting from the penetrability and simultaneity of microwave irradiation, the MAP method can effectively eliminate miscellaneous phases and thus achieve >1 µm large‐size crystal grains in perovskite films. These MAP treated perovskite films exhibit pure crystalline phase, long charge‐carrier lifetime, and low defect density, which can substantially improve the PSC efficiency without requiring an additional enhancer/passivation layer. The inverted planar PSCs present enhanced power conversion efficiency from 18.33% (HAP) to 21.59% (MAP) and good stability of >1000 h lifetime without encapsulation under ambient conditions. In addition, MAP can be applied to a large‐size (10 cm × 10 cm) perovskite film fabrication as well as a broader tolerance in environmental temperature and precursor concentration, making it a reliable method for repeatably practical fabrication of perovskite photovoltaics.

Deuterium oxide as a solvent additive enhances the power conversion efficiency of triple‐cation perovskite solar cells. It passivates the defects, thus enhancing the charge carrier lifetimes and diffusion lengths. Partial formamidinium deuteration also helps to stabilize the PbI₆ structure. This facile approach based on selective isotope exchange could possibly be extended to other perovskite devices to improve their optoelectronic properties.

Abstract

Heavy water or deuterium oxide (D₂O) comprises deuterium, a hydrogen isotope twice the mass of hydrogen. Contrary to the disadvantages of deuterated perovskites, such as shorter recombination lifetimes and lower/invariant efficiencies, the serendipitous effect of D₂O as a beneficial solvent additive for enhancing the power conversion efficiency (PCE) of triple‐A cation (cesium (Cs)/methylammonium (MA)/formaminidium (FA)) perovskite solar cells from ≈19.2% (reference) to 20.8% (using 1 vol% D₂O) with higher stability is reported. Ultrafast optical spectroscopy confirms passivation of trap states, increased carrier recombination lifetimes, and enhanced charge carrier diffusion lengths in the deuterated samples. Fourier transform infrared spectroscopy and solid‐state NMR spectroscopy validate the N–H₂ group as the preferential isotope exchange site. Furthermore, the NMR results reveal the induced alteration of the FA to MA ratio due to deuteration causes a widespread alteration to several dynamic processes that influence the photophysical properties. First‐principles density functional theory calculations reveal a decrease in PbI₆ phonon frequencies in the deuterated perovskite lattice. This stabilizes the PbI₆ structures and weakens the electron–LO phonon (Fröhlich) coupling, yielding higher electron mobility. Importantly, these findings demonstrate that selective isotope exchange potentially opens new opportunities for tuning perovskite optoelectronic properties.

Nature, Published online: 29 April 2020; doi:10.1038/s41586-020-2284-y

A 250 meV energetic barrier is found to lead to detrimental interfacial charge carrier recombination at the interfaces of the two photoactive layers in a perovskite/organic bulk heterojunction solar cell. Based on the energetic requirement identified, new device design strategies are suggested to eliminate losses in integrated perovskite solar cells and hence to achieve higher efficiencies than those observed in perovskite‐only devices.

Abstract

Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (J _sc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in J _sc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices.

The amorphous–crystalline heterophase SnO₂ electron transport bilayer (Bi‐SnO₂) exhibits improved surface morphology, fewer oxygen defects, and better energy band alignment with the perovskite, which enables more efficient electron extraction. The use of Bi‐SnO₂ boosts the efficiency of small‐area (0.09 cm²) and large‐area (3.55 cm²) perovskite solar cells up to 20.39% and 14.93%, respectively.

Abstract

Improving the ohmic contact and interfacial morphology between an electron transport layer (ETL) and perovskite film is the key to boost the efficiency of planar perovskite solar cells (PSCs). In the current work, an amorphous–crystalline heterophase tin oxide bilayer (Bi‐SnO₂) ETL is prepared via a low‐temperature solution process. Compared with the amorphous SnO₂ sol–gel film (SG‐SnO₂) or the crystalline SnO₂ nanoparticle (NP‐SnO₂) counterparts, the heterophase Bi‐SnO₂ ETL exhibits improved surface morphology, considerably fewer oxygen defects, and better energy band alignment with the perovskite without sacrificing the optical transmittance. The best PSC device (active area ≈ 0.09 cm²) based on a Bi‐SnO₂ ETL is hysteresis‐less and achieves an outstanding power conversion efficiency of ≈20.39%, which is one of the highest efficiencies reported for SnO₂‐triple cation perovskite system based on green antisolvent. More fascinatingly, large‐area PSCs (active areas of ≈3.55 cm²) based on the Bi‐SnO₂ ETL also achieves an extraordinarily high efficiency of ≈14.93% with negligible hysteresis. The improved device performance of the Bi‐SnO₂‐based PSC arises predominantly from the improved ohmic contact and suppressed bimolecular recombination at the ETL/perovskite interface. The tailored morphology and energy band structure of the Bi‐SnO₂ has enabled the scalable fabrication of highly efficient, hysteresis‐less PSCs.

Three asymmetric small‐molecule acceptors are developed by changing the fluorine atoms on the terminal group of Y6 to chlorine atoms, namely SY1, SY2, and SY3, with Y6, and Y6‐4Cl are utilized as the reference. Organic solar cells based on the PM6:SY1 blend demonstrate a champion power conversion efficiency of 16.83%. This work can provide a deeper and more comprehensive understanding of applying the asymmetric molecule design method.

Abstract

Small‐molecule acceptors (SMAs)‐based organic solar cells (OSCs) have exhibited great potential for achieving high power conversion efficiencies (PCEs). Meanwhile, developing asymmetric SMAs to improve photovoltaic performance by modulating energy level distribution and morphology has drawn lots of attention. In this work, based on the high‐performance SMA (Y6), three asymmetric SMAs are developed by substituting the fluorine atoms on the terminal group with chlorine atoms, namely SY1 (two F atoms and one Cl atom), SY2 (two F atoms and two Cl atoms), and SY3 (three Cl atoms). Y6 (four F atoms) and Y6‐4Cl (four Cl atoms) are synthesized as control molecules. As a result, SY1 exhibits the shallowest lowest unoccupied molecular orbital energy level and the best molecular packing among these five acceptors. Consequently, OSCs based on PM6:SY1 yield a champion PCE of 16.83% with an open‐circuit voltage (V _OC) of 0.871 V, and a fill factor (FF) of 0.760, which is the best result among the five devices. The highest FF for the PM6:SY1‐based device is mainly ascribed to the most balanced charge transport and optimal morphology. This contribution provides deeper understanding of applying asymmetric molecule design method to further promote PCEs of OSCs.

Publication date: 17 June 2020

Source: Joule, Volume 4, Issue 6

Author(s): Shengfan Wu, Jie Zhang, Zhen Li, Danjun Liu, Minchao Qin, Sin Hang Cheung, Xinhui Lu, Dangyuan Lei, Shu Kong So, Zonglong Zhu, Alex.K.-Y. Jen

Recent progress in inorganic lead‐based and lead‐free CsBX₃ perovskite solar cells using various strategies is reviewed and their prospects and challenges in the future are discussed in detail.

Abstract

All‐inorganic perovskite semiconductors have recently drawn increasing attention owing to their outstanding thermal stability. Although all‐inorganic perovskite solar cells (PSCs) have achieved significant progress in recent years, they still fall behind their prototype organic–inorganic counterparts owing to severe energy losses. Therefore, there is considerable interest in further improving the performance of all‐inorganic PSCs by synergic optimization of perovskite films and device interfaces. This review article provides an overview of recent progress in inorganic PSCs in terms of lead‐based and lead‐free composition. The physical properties of all‐inorganic perovskite semiconductors as well as the hole/electron transporting materials are discussed to unveil the important role of composition engineering and interface modification. Finally, a discussion of the prospects and challenges for all‐inorganic PSCs in the near future is presented.

The interplay between lead vacancy and water rationalizes the positive and negative effects of water on the charge carrier lifetimes in the organic–inorganic perovskite MAPbI₃. The obtained results provide a theoretical understanding of how the complex charge dynamics in perovskites are affected by defects and water.

Abstract

The perovskite CH₃NH₃PbI₃ excited‐state lifetimes exhibit conflicting experimental results under humid environments. Using ab initio nonadiabatic (NA) molecular dynamics, we demonstrate that the interplay between lead vacancy and water can rationalize the puzzle. The lead vacancy reduces NA coupling by localizing holes, slowing electron–hole recombination. By creating a deep electron trap state, the coexistence of a neutral lead vacancy and water molecules enhances NA coupling, accelerating charge recombination by a factor of over 3. By eliminating the mid‐gap state by accepting two photoexcited electrons, the negatively charged lead vacancy interacting with water molecules increases the carrier lifetime over 2 times longer than in the pristine system. The simulations rationalize the positive and negative effects of water on the solar cell performance exposure to humidity.

Publication date: 20 May 2020

Source: Joule, Volume 4, Issue 5

Author(s): Yan Jiang, Shih-Chi Yang, Quentin Jeangros, Stefano Pisoni, Thierry Moser, Stephan Buecheler, Ayodhya N. Tiwari, Fan Fu

Kinetic Monte Carlo simulations are used to describe the current–voltage characteristics of an organic bulk heterojunction solar cell. Excellent agreement between model and experiment is obtained by calibrating the injection barriers, the blend morphology, and the charge transfer recombination rate with data from independent measurement techniques.

Kinetic Monte Carlo (KMC) simulations are a powerful tool to study the dynamics of charge carriers in organic photovoltaics. However, the key characteristic of any photovoltaic device, its current–voltage (J–V ) curve under solar illumination, has proven challenging to simulate using KMC. The main challenges arise from the presence of injecting contacts and the importance of charge recombination when the internal electric field is low, i.e., close to open‐circuit conditions. Herein, an experimentally calibrated KMC model is presented that can fully predict the J–V curve of a disordered organic solar cell. It is shown that it is crucial to make experimentally justified assumptions on the injection barriers, the blend morphology, and the kinetics of the charge transfer state involved in geminate and nongeminate recombination. All of these properties are independently calibrated using charge extraction, electron microscopy, and transient absorption measurements, respectively. Clear evidence is provided that the conclusions drawn from microscopic and transient KMC modeling are indeed relevant for real operating organic solar cell devices.