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Publication date: July 2020

Source: Nano Energy, Volume 73

Author(s): Jian Kang, Shan Chen, Xiaole Zhao, Huajie Yin, Weiping Zhang, Mohammad Al-Mamun, Porun Liu, Yun Wang, Huijun Zhao

A CsBr buffer layer is inserted between NiO _x hole transport layer and perovskite layer to relieve the lattice mismatch induced interface stress and induce more ordered crystal growth. The results show that the addition of the CsBr buffer layer optimizes the interface between the perovskite and NiO _x , reduces interface defects and traps, and enhances the hole extraction/transfer.

Abstract

Recent research shows that the interface state in perovskite solar cells is the main factor which affects the stability and performance of the device, and interface engineering including strain engineering is an effective method to solve this issue. In this work, a CsBr buffer layer is inserted between NiO _x hole transport layer and perovskite layer to relieve the lattice mismatch induced interface stress and induce more ordered crystal growth. The experimental and theoretical results show that the addition of the CsBr buffer layer optimizes the interface between the perovskite absorber layer and the NiO _x hole transport layer, reduces interface defects and traps, and enhances the hole extraction/transfer. The experimental results show that the power conversion efficiency of optimal device reaches up to 19.7% which is significantly higher than the efficiency of the device without the CsBr buffer layer. Meanwhile, the device stability is also improved. This work provides a deep understanding of the NiO _x /perovskite interface and provides a new strategy for interface optimization.

A nonhalogen organic salt sodium p‐toluenesulfonate (STS) is applied during the surface modification of perovskite films for the first time, yielding an obvious enhancement of power conversion efficiency from 18.70% to 20.05% for perovskite solar cells, which originates from the surface passivation of the perovskite film with reduced trap state densities and suppressed interfacial charge recombination.

Ionic defects at the surfaces of organolead halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells (PSCs). Herein, sodium p‐toluenesulfonate (STS) is applied during the surface modification of perovskite layer for the first time, leading to the efficient surface passivation of the perovskite film and consequently significant enhancements in both efficiency and stability of mixed‐cation PSC devices. Upon incorporating STS atop the perovskite layer, the power conversion efficiency of the Cs_0.05MA_0.12FA_0.83PbI_2.55Br_0.45 (abbreviated as CsMAFA) mesoporous‐structure mixed‐cation PSC devices improves from 18.70% to 20.05% with reduced hysteresis. The sulfonate (–SO₃ ⁻) anion of STS coordinates with the Pb²⁺ of CsMAFA perovskite, and the Na⁺ cation of STS electrostatically interacts with the anions (I⁻/Br⁻) of CsMAFA perovskite, resulting in the surface passivation of the CsMAFA perovskite film with reduced electron and hole trap state densities. In addition, STS modification induces an upshift of the valence band of perovskite, facilitating hole extraction from the perovskite layer to the hole transport layer with suppressed interfacial charge recombination. Moreover, such a trap state passivation of perovskite film leads to improvement of the ambient stability of PSC devices.

In article https://doi.org/10.1002/aenm.2019033261903326, Prashant Sonar and co‐workers review the state‐of‐the‐art for dopant‐free organic hole transporting materials (HTMs) in perovskite solar cells. Depicted, are different device architectures using the small molecular HTM, ACE‐QA‐ACE, which has shown enhanced stability over doped HTMs.

Hysteresis in perovskite solar cells is suppressed with the insertion of a phenyl‐C61‐butyric acid methyl ester (PCBM) layer. In situ PL imaging is employed to observe the ionic migration in the perovskite layer, perovskite/PCBM bilayer and PPCBM bilayer. The mobilizable PCBM molecules are able to diffuse into the perovskite and therein passivate iodine ions/vacancies, thus reducing the hysteresis.

Abstract

The power conversion efficiency of inorganic–organic hybrid lead halide perovskite solar cells (PSCs) is approaching that of those made from single crystalline silicon; however, they still experience problems such as hysteresis and photo/electrical‐field‐induced degradation. Evidences consistently show that ionic migration is critical for these detrimental behaviors, but direct in‐situ studies are still lacking to elucidate the respective kinetics. Three different PSCs incorporating phenyl‐C61‐butyric acid methyl ester (PCBM) and a polymerized form (PPCBM) is fabricated to clarify the function of fullerenes towards ionic migration in perovskites: 1) single perovskite layer, 2) perovskite/PCBM bilayer, 3) perovskite/PPCBM bilayer, where the fullerene molecules are covalently linked to a polymer backbone impeding fullerene inter‐diffusion. By employing wide‐field photoluminescence imaging microscopy, the migration of iodine ions/vacancies under an external electrical field is studied. The polymerized PPCBM layer barely suppresses ionic migration, whereas PCBM readily does. Temperature‐dependent chronoamperometric measurements demonstrate the reduction of activation energy with the aid of PCBM and X‐ray photoemission spectroscopy (XPS) measurements show that PCBM molecules are viable to diffuse into the perovskite layer and passivate iodine related defects. This passivation significantly reduces iodine ions/vacancies, leading to a reduction of built‐in field modulation and interfacial barriers.

Publication date: July 2020

Source: Nano Energy, Volume 73

Author(s): Kevin J. Rietwyk, Boer Tan, Adam Surmiak, Jianfeng Lu, David P. McMeekin, Sonia R. Raga, Noel Duffy, Udo Bach

SnO₂ and perovskite have been bridged with multifunctional graphdiyne. Such delicate interface modification boosted the performance of solar cells in energy band alignment, electron mobility improvement, controllable perovskite growth inducement, and interface defect passivation.

Abstract

The matching of charge transport layer and photoactive layer is critical in solar energy conversion devices, especially for planar perovskite solar cells based on the SnO₂ electron‐transfer layer (ETL) owing to its unmatched photogenerated electron and hole extraction rates. Graphdiyne (GDY) with multi‐roles has been incorporated to maximize the matching between SnO₂ and perovskite regarding electron extraction rate optimization and interface engineering towards both perovskite crystallization process and subsequent photovoltaic service duration. The GDY doped SnO₂ layer has fourfold improved electron mobility due to freshly formed C−O σ bond and more facilitated band alignment. The enhanced hydrophobicity inhibits heterogeneous perovskite nucleation, contributing to a high‐quality film with diminished grain boundaries and lower defect density. Also, the interfacial passivation of Pb−I anti‐site defects has been demonstrated via GDY introduction.

Nonfullerene Organic Solar Cells

In article number 1900552, Chao‐Chao Qin, Xiao‐Tao Hao, and co‐workers investigate the influence of the synergetic effects of fluorination and chlorination on multiple temporal‐scale photocarrier dynamics based on a 2 × 2 matrix of organic solar cells. Both fluorination and chlorination have a positive effect on exciton diffusion, exciton dissociation, charge carrier transport, and collection, contributing to the enhancement of photovoltaic performance.

Precursor Preparation

In article number 1900463, Anita W. Y. Ho‐Baillie and co‐workers report the use of two precursor preparation methods for the deposition of phenethylammonium‐containing organic‐inorganic hybrid perovskite fi lms for photovoltaic a pplications. The fi lm properties, photovoltaic device performance, and stability differ depending on the precursor preparation methods. These new insights are important for optimising precursor preparations for lower dimensional perovskite fi lms to achieve the best device performance and stability.

The partial substitution of methylammonium (MA) and formamidinium (FA) cations by cesium (Cs) or rubidium (Rb) cations in multiple‐cation lead mixed halide perovskites, (FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃), reduces the trap‐assisted (k₁) and radiative (k₂) charge carrier recombination rate. Furthermore, Urbach energies are reduced, indicating improved perovskite film microstructure. Consequently, photovoltaic devices with Cs/Rb‐incorporated perovskites exhibit improved power conversion efficiency.

Incorporating cesium (Cs) or rubidium (Rb) cations into multiple‐cation lead mixed halide perovskites (FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃) increases their photovoltaic performance. Herein, the fundamental photophysics of perovskites are investigated by steady‐state and transient optical spectroscopy and the reasons for the performance increase are revealed. Cs/Rb‐cation incorporation slightly increases the bandgap, whereas exciton binding energies remain in the range of a few meV. Urbach energies are reduced, suggesting improved perovskite microstructure upon Cs/Rb incorporation. Carrier density‐induced broadening of the photo‐bleaching following the Burstein–Moss model is observed, and the effective carrier masses are determined to be a few tenths of the electron rest mass. From fits of the high‐energy tail of the perovskite's photo bleach to Boltzmann's distribution, subpicosecond hot‐carrier cooling is revealed, implying strong carrier–phonon coupling. Importantly, the charge carrier recombination dynamics indicate that Cs/Rb‐incorporation reduces both the first‐order (trap‐assisted) and the second‐order (radiative) recombination, which appears to be the main reason for the observed performance increase upon Cs/Rb‐cation incorporation. Overall, this work presents a detailed study of the photophysics of multiple‐cation mixed halide lead perovskites and develops a concise picture of the impact of cesium/rubidium incorporation on the photophysics and device performance.

By varying thermal annealing conditions, a thermally induced perovskite crystal control process of the wide‐bandgap perovskite films provides an opportunity to exploit both lead‐iodide passivation and perovskite orientation strategies with a fixed E _g of 1.73 eV. Based on this concept, the device efficiency is improved from 15.76% to 18.60% and the operational photostability is also enhanced without any encapsulation in ambient conditions.

Wide‐bandgap perovskite solar cells (WBG PSCs) have gained attention as promising tandem partners for silicon solar cells due to their complementary absorption, superb open‐circuit voltage, and an easy solution process. Recently, both their performance and stability have been improved by compositional engineering or defect passivation strategies, due to the modulation of perovskite crystal size and reduction of crystal defects. Herein, a report on the thermally induced phase control (TIPC) strategy is provided, which enables efficient and photostable WBG PSCs without compositional engineering by exploring a thermal annealing process window (100–175 °C and 3–60 min) of the WBG perovskite films. Within this window, a key annealing regime is found that produces preferred crystal orientations of lead iodide and the WBG perovskite, suppressing phase segregation and reducing charge recombination in the perovskites. The WBG PSCs (composition of FA_0.75MA_0.15Cs_0.1PbI₂Br and E _g of 1.73 eV) optimized by TIPC exhibit an excellent power conversion efficiency (PCE) of 18.60% and improved operational stability, maintaining >90% of the maximum PCE (during maximum power point tracking) without encapsulation after 12 h of operation (air mass 1.5 global irradiation in ambient air conditions) or after 500 h of operation (white light‐emitting diode irradiation (100 mW cm⁻²) in N₂ conditions).

A novel material called phenylhydrazinium iodide (PHAI) is effective for defects minimization, surface passivation, and efficient charge transportation in hybrid perovskite solar cells. It plays multiple roles in controlled crystallization, stabilizing under‐coordinated ions, and as a self‐supported moisture barrier in perovskite films.

Abstract

In recent years, hybrid perovskite solar cells (HPSCs) have received considerable research attention due to their impressive photovoltaic performance and low‐temperature solution processing capability. However, there remain challenges related to defect passivation and enhancing the charge carrier dynamics of the perovskites, to further increase the power conversion efficiency of HPSCs. In this work, the use of a novel material, phenylhydrazinium iodide (PHAI), as an additive in MAPbI₃ perovskite for defect minimization and enhancement of the charge carrier dynamics of inverted HPSCs is reported. Incorporation of the PHAI in perovskite precursor solution facilitates controlled crystallization, higher carrier lifetime, as well as less recombination. In addition, PHAI additive treated HPSCs exhibit lower density of filled trap states (10¹⁰ cm⁻²) in perovskite grain boundaries, higher charge carrier mobility (≈11 × 10⁻⁴ cm² V⁻¹ s), and enhanced power conversion efficiency (≈18%) that corresponds to a ≈20% improvement in comparison to the pristine devices.

Purely vibrationally excited lead–iodide perovskites are prepared using off‐resonance, infrared optical excitation far below the bandgap. The transient optical response manifested as bandgap oscillations below and above the static bandgap is attributed to the A _g optical phonon mode at 25 cm⁻¹. This mode, arising from antiphase octahedral rotations, is observed in both 3D perovskite CH₃NH₃PbI₃ and layered 2D perovskite [CH₃(CH₂)₃NH₃]₂PbI₄.

Abstract

Hybrid organic–inorganic perovskites such as methylammonium lead iodide have emerged as promising semiconductors for energy‐relevant applications. The interactions between charge carriers and lattice vibrations, giving rise to polarons, have been invoked to explain some of their extraordinary optoelectronic properties. Here, time‐resolved optical spectroscopy is performed, with off‐resonant pumping and electronic probing, to examine several representative lead iodide perovskites. The temporal oscillations of electronic bandgaps induced by coherent lattice vibrations are reported, which is attributed to antiphase octahedral rotations that dominate in the examined 3D and 2D hybrid perovskites. The off‐resonant pumping scheme permits a simplified observation of changes in the bandgap owing to the A _g vibrational mode, which is qualitatively different from vibrational modes of other symmetries and without increased complexity of photogenerated electronic charges. The work demonstrates a strong correlation between the lead–iodide octahedral framework and electronic transitions, and provides further insights into the manipulation of coherent optical phonons and related properties in hybrid perovskites on ultrafast timescales.

The inverse temperature‐dependencies of the photovoltaic parameters in MAPbI₃ perovskite solar cells lead to obtaining a peak efficiency of 21.4% at 220 K. These T ‐varied behaviors are related to combined properties of improved interfacial charge extraction, reduced charge trap density, and suppressed nonradiative recombination at lower temperatures.

Abstract

Understanding the factors that limit the performance of perovskite solar cells (PSCs) can be enriched by detailed temperature (T )‐dependent studies. Based on p‐i‐n type PSCs with prototype methylammonium lead triiodide (MAPbI₃) perovskite absorbers, T ‐dependent photovoltaic properties are explored and negative T ‐coefficients for the three device parameters (V _OC, J _SC, and FF) are observed within a wide low T ‐range, leading to a maximum power conversion efficiency (PCE) of 21.4% with an impressive fill factor (FF) approaching 82% at 220 K. These T ‐behaviors are explained by the enhanced interfacial charge transfer, reduced charge trapping with suppressed nonradiative recombination and narrowed optical bandgap at lower T . By comparing the T ‐dependent device behaviors based on MAPbI₃ devices containing a PASP passivation layer, enhanced PCE at room temperature is observed but different tendencies showing attenuating T ‐dependencies of J _SC and FF, which eventually leads to nearly T ‐invariable PCEs. These results indicate that charge extraction with the utilized all‐organic charge transporting layers is not a limiting factor for low‐T device operation, meanwhile the trap passivation layer of choice can play a role in the T ‐dependent photovoltaic properties and thus needs to be considered for PSCs operating in a temperature‐variable environment.

Publication date: 15 April 2020

Source: Joule, Volume 4, Issue 4

Author(s): Wenzhu Liu, Liping Zhang, Xinbo Yang, Jianhua Shi, Lingling Yan, Lujia Xu, Zhuopeng Wu, Renfang Chen, Jun Peng, Jingxuan Kang, Kai Wang, Fanying Meng, Stefaan De Wolf, Zhengxin Liu

A new passivator, 4‐tert‐butylbenzylammonium iodide (tBBAI), is introduced, which accelerates charge extraction while retarding nonradiative recombination, boosting the power conversion efficiency of perovskite solar cells (PSCs) from 20% to 23.5% and reducing the hysteresis to barely detectable levels. tBBAI‐passivated PSCs also show excellent stability, retaining over 95% of their initial PCE after 500 h full‐sun illumination under maximum‐power‐point tracking.

Abstract

Passivation of interfacial defects serves as an effective means to realize highly efficient and stable perovskite solar cells (PSCs). However, most molecular modulators currently used to mitigate such defects form poorly conductive aggregates at the perovskite interface with the charge collection layer, impeding the extraction of photogenerated charge carriers. Here, a judiciously engineered passivator, 4‐tert‐butyl‐benzylammonium iodide (tBBAI), is introduced, whose bulky tert‐butyl groups prevent the unwanted aggregation by steric repulsion. It is found that simple surface treatment with tBBAI significantly accelerates the charge extraction from the perovskite into the spiro‐OMeTAD hole‐transporter, while retarding the nonradiative charge carrier recombination. This boosts the power conversion efficiency (PCE) of the PSC from ≈20% to 23.5% reducing the hysteresis to barely detectable levels. Importantly, the tBBAI treatment raises the fill factor from 0.75 to the very high value of 0.82, which concurs with a decrease in the ideality factor from 1.72 to 1.34, confirming the suppression of radiation‐less carrier recombination. The tert‐butyl group also provides a hydrophobic umbrella protecting the perovskite film from attack by ambient moisture. As a result, the PSCs show excellent operational stability retaining over 95% of their initial PCE after 500 h full‐sun illumination under maximum‐power‐point tracking under continuous simulated solar irradiation.

In article number https://doi.org/10.1002/adfm.2019087621908762, Haiguang Zhao, Zhiming M. Wang, Federico Rosei, and Gurpreet Singh Selopal report on colloidal core/shell quantum dots (QDs) that exhibit promising optical and electrical properties. The recent developments in the engineering of the structure of core/shell QDs to tune exciton dynamics so as to improve the performance of QDs sensitized solar cells are presented.