JIALINGBO

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.

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.

A strategy is proposed to precisely control CsPbIBr₂ crystallization behaviors by incorporating sulfamic acid sodium salt (SAS), thus resulting in a high‐quality film. More importantly, SAS in perovskite possibly introduces an additional internal electric field effect that favors the electron transport and injection. Encouragingly, a higher efficiency of 10.57% is achieved with this strategy.

Abstract

Excellent power conversion efficiency (PCE) and stability are the primary forces that propel the all‐inorganic cesium‐based halide perovskite solar cells (PSCs) toward commercialization. However, the intrinsic high density of trap state and internal nonradiative recombination of CsPbIBr₂ perovskite film are the barriers that limit its development. In the present study, a facile additive strategy is introduced to fabricate highly efficient CsPbIBr₂ PSCs by incorporating sulfamic acid sodium salt (SAS) into the perovskite layer. The additive can control the crystallization behaviors and optimize morphology, as well as effectively passivate defects in the bulk perovskite film, thereby resulting in a high‐quality perovskite. In addition, SAS in perovskite has possibly introduced an additional internal electric field effect that favors electron transport and injection due to inhomogeneous ion distribution. A champion PCE of 10.57% (steady‐output efficiency is 9.99%) is achieved under 1 Sun illumination, which surpasses that of the contrast sample by 16.84%. The modified perovskite film also exhibits improved moisture stability. The unencapsulated device maintains over 80% initial PCE after aging for 198 h in air. The results provide a suitable additive for inorganic perovskite and introduce a new conjecture to explain the function of additives in PSCs more rationally.

Back once again with the IL behavior : To solve the basic problems of low interfacial electron extraction and significant interfacial charge recombination in electron transport layer‐free perovskite solar cells, an ionic liquid is self‐assembled onto the surface of conductive substrates. The power conversion efficiency is thereby improved remarkably from 9.01 % to 17.31 % upon self‐assembly of the ionic liquid.

Abstract

Electron transport layer (ETL)‐free perovskite solar cells (PSCs) are attractive because they have fewer layers and hence are lower in cost, but their inferior photovoltaic performance, as compared to ETL‐containing PSCs, greatly restricts their practical application. This study concerns the design and synthesis of a hydroxyethyl‐functionalized imidazolium iodide ionic liquid, the determination of its single crystal structure, and its self‐assembly on a conductive substrate for ETL‐free PSCs. The self‐assembly of the ionic liquid on the conductive substrate is found to lower the work function of the conductive substrate and enhance interfacial electron extraction while retarding interfacial charge recombination. As a consequence, the power conversion efficiency is improved remarkably from 9.01 % to 17.31 % upon self‐assembly of the ionic liquid on the conductive substrate. This finding provides a new way to assemble highly efficient ETL‐free PSCs.

Stable ion doping ! The effect of Ag doping on the stability of organometal halide perovskites was investigated. Ag doping improves the chemical stability of perovskites against moisture and heat. The improved stability was attributed to morphological and thermodynamic changes.

Abstract

Organometal halide perovskite (OHP) solar cells have been intensively studied because of their promising optoelectronic features, which has resulted in high power conversion efficiencies >23 %. Although OHP solar cells exhibit high power conversion efficiencies, their relatively poor stability is a significant obstacle to their practical use. We report that the chemical stability of OHP solar cells with respect to both moisture and heat can be improved by adding a small amount of Ag to the precursor. Ag doping increases the size of the OHP grains and reduces the size of the amorphous intergranular regions at the grain boundaries, and thereby hinders the infiltration of moisture into the OHP films and their thermal degradation. Quantum mechanical simulation reveals that Ag doping increases the energies of both the hydration reaction and heat‐induced vacancy formation in OHP crystals. This procedure also improves the power conversion efficiencies of the resulting solar cells.

SnO₂ has recently emerged as an attractive n‐type layer for perovskite solar cells, with advantages of high optical transparency, high electron mobility, UV‐stabilized properties as well as low‐temperature processing. Here, a detailed study of structure and morphology of a critical aspect of these devices is reported—the electron transport layer (ETL)—demonstrating improved energy level alignment, reduced hysteresis, and interfacial recombination, which translates to enhanced device performance and stability.

Abstract

Nanostructured tin (IV) oxide (SnO₂) is emerging as an ideal inorganic electron transport layer in n–i–p perovskite devices, due to superior electronic and low‐temperature processing properties. However, significant differences in current–voltage performance and hysteresis phenomena arise as a result of the chosen fabrication technique. This indicates enormous scope to optimize the electron transport layer (ETL), however, to date the understanding of the origin of these phenomena is lacking. Reported here is a first comparison of two common SnO₂ ETLs with contrasting performance and hysteresis phenomena, with an experimental strategy to combine the beneficial properties in a bilayer ETL architecture. In doing so, this is demonstrated to eliminate room‐temperature hysteresis while simultaneously attaining impressive power conversion efficiency (PCE) greater than 20%. This approach highlights a new way to design custom ETLs using functional thin‐film coatings of nanomaterials with optimized characteristics for stable, efficient, perovskite solar cells.

In article number https://doi.org/10.1002/adfm.2019059511905951, Jong H. Kim, Sang Hyuk Im, O‐Pil Kwon, and co‐workers report a series of electron transporting homochiral and heterochiral stereoisomers of naphthalene diimide crystalline materials and show simultaneous achievement of low‐temperature solution processability, high device performance, and long‐term temporal device stability.

Herein, polyethylene glycol (PEG) is used as an additive for the morphology control of lead‐reduced perovskite films. The power conversion efficiency of lead‐reduced perovskite solar cells with PEG additive improves from 13.7% to 16.1% without J –V hysteresis due to pinhole elimination of the perovskite film.

The organic–inorganic halide perovskite solar cells (PSCs) are rapidly developed in just a few years due to its high power conversion efficiency. However, it still faces some critical issues, one of which is the presence of toxic lead (Pb²⁺). Recent researches show that barium (Ba²⁺) can partially replace the Pb²⁺ in perovskite structure and achieve a promising device performance because of its adequate ionic radius. However, the optimal replacement amount of Ba²⁺ in perovskite is still limited. Herein, the methylammonium (MA)/formamidinium (FA) mixed‐cation perovskite is used as the active layer in PSCs and Pb²⁺ is partially substituted with Ba²⁺. Compared with the pure MA system, the best device efficiency can be achieved using higher Ba²⁺ replacement ratio. In addition, while introducing the appropriate polymer additive, the replacement ratio can be further increased without compromise of device efficiency. Using polyethylene glycol as polymer additive, 10.0 mol% Ba‐doped MA/FA mixed‐cation PSC with an efficiency of 16.1% can be realized. It is believed that this report provides an effective strategy to fabricate high‐performance lead‐reduced PSCs.

The introduction of F5PEA⁺ to partially replace PEA⁺ as the 2D perovskite passivation agent, with a strong noncovalent interaction between the two bulky cations and enhanced charge transport, is reported to improve the performance (from 19.58% to 21.10%) and stability of the corresponding wide‐bandgap perovskite solar cells.

The replacement of a small amount of organic cations with bulkier organic spacer cations in the perovskite precursor solution to form a 2D perovskite passivation agent (2D‐PPA) in 3D perovskite thin films has recently become a promising strategy for developing perovskite solar cells (PSCs) with long‐term stability and high efficiency. However, the long, bulky organic cations often form a barrier, hindering charge transport. Herein, for the first time, 2D‐PPA engineering based on wide‐bandgap (≈1.68 eV) perovskites are reported. Pentafluorophenethylammonium (F5PEA⁺) is introduced to partially replace phenylethylammonium (PEA⁺) as the 2D‐PPA, forming a strong noncovalent interaction between the two bulky cations. The charge transport across and within the planes of pure 2D perovskites, based on mixed ammoniums, increases by a factor of five and three compared with that of mono‐cation 2D perovskites, respectively. The perovskite films based on mixed‐ammonium (F5PEA⁺‐PEA⁺) 2D‐PPA exhibit similar surface morphology and crystal structure, but longer carrier lifetime, lower exciton binding energy, less trap density and higher conductivity, in comparison with those using mono‐cation (PEA⁺) 2D‐PPA. The performance of PSCs based on mixed‐cation 2D‐PPA is enhanced from 19.58% to 21.10% along with improved stability, which is the highest performance for reported wide‐bandgap PSCs.

Nano‐TiO₂ is unprecedentedly used to load the commercial dye N719 forming N719@TiO₂ nanoparticles which promotes charge extraction and suppress shallow defects in the fully printable carbon‐based perovskite solar cells due to surface carboxyl groups comprising the ligands of the N719 dye. Accordingly, the optimal power conversion efficiency increases from 12.00% (control) to 13.95% (N719@TiO₂).

Shallow defects are one of the energy states that trap photoexcited electrons leading to charge recombination and limit the increase in the photocurrent of perovskite solar cells (PSCs). Due to the large perovskite thickness and uncontrollable crystallization processes, suppressing shallow defects, especially methylamine (MA) vacancies, has become a key challenge for fully printable PSCs. Herein, nano‐TiO₂ is unprecedentedly used to load the commercial dye N719, forming N719@TiO₂ nanoparticles, which crucially improves the passivation effect of MA vacancies on the surface of perovskite and charge extraction, by the unbounded carboxyl group of N719 as a shell on the surface of TiO₂. Meanwhile, the core TiO₂ serves as a centre to bind the dyes, assisting the perovskite crystallization and enhancing the passivation effect. It is found that the charge extraction increases to 1.8007 × 10⁻⁹ C for the devices based on N719@TiO₂ from 1.5507 × 10⁻⁹ C for the control group. Simultaneously, the short‐circuit current density (J _sc) is significantly enhanced to 23.58 mA cm⁻² in the device containing N719@TiO₂ over that of the control device (21.95 mA cm⁻²). This opens up a novel pathway to reduce shallow defects in PSCs via organic passivator with carboxyl anchoring group loaded on n‐type semiconductors (nano‐TiO₂).

We report the profiling of spatial and energetic distributions of trap states in metal halide perovskite single-crystalline and polycrystalline solar cells. The trap densities in single crystals varied by five orders of magnitude, with a lowest value of 2 x 10¹¹ per cubic centimeter and most of the deep traps located at crystal surfaces. The charge trap densities of all depths of the interfaces of the polycrystalline films were one to two orders of magnitude greater than that of the film interior, and the trap density at the film interior was still two to three orders of magnitude greater than that in high-quality single crystals. Suprisingly, after surface passivation, most deep traps were detected near the interface of perovskites and hole transport layers, where a large density of nanocrystals were embedded, limiting the efficiency of solar cells.

A 2D/3D perovskite heterostructure passivation is employed for double‐cation wide‐bandgap PSCs with an engineered bandgap (1.65 eV ≤ E _g ≤ 1.85 eV), which results in improved stabilized PCEs and open‐circuit voltages for opaque and semitransparent perovskite solar cells. Four‐terminal perovskite/c‐Si and perovskite/CIGS tandem solar cells with stabilized PCEs of up to 25.7% and 25.0%, respectively, are demonstrated.

Abstract

Wide‐bandgap perovskite solar cells (PSCs) with optimal bandgap (E _g) and high power conversion efficiency (PCE) are key to high‐performance perovskite‐based tandem photovoltaics. A 2D/3D perovskite heterostructure passivation is employed for double‐cation wide‐bandgap PSCs with engineered bandgap (1.65 eV ≤ E _g ≤ 1.85 eV), which results in improved stabilized PCEs and a strong enhancement in open‐circuit voltages of around 45 mV compared to reference devices for all investigated bandgaps. Making use of this strategy, semitransparent PSCs with engineered bandgap are developed, which show stabilized PCEs of up to 25.7% and 25.0% in four‐terminal perovskite/c‐Si and perovskite/CIGS tandem solar cells, respectively. Moreover, comparable tandem PCEs are observed for a broad range of perovskite bandgaps. For the first time, the robustness of the four‐terminal tandem configuration with respect to variations in the perovskite bandgap for two state‐of‐the‐art bottom solar cells is experimentally validated.

A 2D/3D layered heterostructure with 2D perovskites self‐assembled atop 3D MAPbI₃ via a one‐step printing process is reported. The 2D perovskite capping layer significantly suppresses nonradiative recombination of the devices, leading to a remarkably high open‐circuit voltage of 1.2 V. Moreover, notable enhancement in light, thermal, and moisture stability is obtained as a result of the protective barrier of 2D perovskites.

Abstract

As perovskite solar cells (PSCs) are highly efficient, demonstration of high‐performance printed devices becomes important. 2D/3D heterostructures have recently emerged as an attractive way to relieving the film inhomogeneity and instability in perovskite devices. In this work, a 2D/3D ensemble with 2D perovskites self‐assembled atop 3D methylammonium lead triiodide (MAPbI₃) via a one‐step printing process is shown. A clean and flat interface is observed in the 2D/3D bilayer heterostructure for the first time. The 2D perovskite capping layer significantly suppresses nonradiative charge recombination, resulting in a marked increase in open‐circuit voltage (V _OC) of the devices by up to 100 mV. An ultrahigh V _OC of 1.20 V is achieved for MAPbI₃ PSCs, corresponding to 91% of the Shockley–Queisser limit. Moreover, notable enhancement in light, thermal, and moisture stability is obtained as a result of the protective barrier of the 2D perovskites. These results suggest a viable approach for scalable fabrication of highly efficient perovskite solar cells with enhanced environmental stability.

Self‐crystallized multifunctional 2D perovskite (M2P) is formed on top of a 3D perovskite light absorber. The M2P layer performs as a hole‐transfer facilitator and a surface‐trap passivator in perovskite solar cells (PSCs). PSCs using the developed 3D/2D perovskites achieve a power conversion efficiency of 20.79% with highly improved long‐term stability compared to devices without M2P.

Abstract

Recently, perovskite solar cells (PSC) with high power‐conversion efficiency (PCE) and long‐term stability have been achieved by employing 2D perovskite layers on 3D perovskite light absorbers. However, in‐depth studies on the material and the interface between the two perovskite layers are still required to understand the role of the 2D perovskite in PSCs. Self‐crystallization of 2D perovskite is successfully induced by deposition of benzyl ammonium iodide (BnAI) on top of a 3D perovskite light absorber. The self‐crystallized 2D perovskite can perform a multifunctional role in facilitating hole transfer, owing to its random crystalline orientation and passivating traps in the 3D perovskite. The use of the multifunctional 2D perovskite (M2P) leads to improvement in PCE and long‐term stability of PSCs both with and without organic hole transporting material (HTM), 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) compared to the devices without the M2P.