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Hybrid tin halides with formamidinium cations become a promising candidate for eco‐friendly solar cells, which achieves the best photovoltaic performance among various lead‐free perovskite solar cells (PSCs). Unfortunately, the low quality of perovskite layers and the oxidation of Sn2+ ions degrade the performance of devices, leading to an obviously lower power conversion efficiency (PCE) compare to hybrid lead halides. Additive engineering is a facile strategy for improving the quality of perovskite layers. In this work, the 2‐aminopyrazine (APZ) additive is investigated for depositing FA0.8PEA0.2SnI2.8Br0.2 layers, and corresponding PSCs are examined. It is found that the melting point of APZ additives drops below the annealing temperature of perovskite layers by the intermolecular interaction with formamidinium iodide (FAI). The melt phase at the grain boundaries encourages the diffusion of ions in the annealing process, leading to an improved quality of perovskite layers. And the formation of APZ‐SnI2 complex also inhibits the oxidation of Sn2+ ions. Consequently, the defect density and the recombination are significantly reduced in PSCs, resulting in an increase of PCEs from 11.4% to 13.0% and a simultaneous improvement of device stability. This work demonstrates a strategy for improving tin‐based PSCs based on melt assisted annealing and coordination interaction. [ABSTRACT FROM AUTHOR]

In inverted perovskite solar cells (PSCs), metal oxides become kind of promising hole transport layers for their facile synthesis and low cost. For conventional hole transport materials, the valence band match between metal oxides and perovskite layers is usually necessary for the hole extraction process. Ga2O3 is an emerging semiconductor material with ultrawide bandgap, but a significant energy level mismatch exists at Ga2O3/perovskite interfaces. In this work, Cu‐doped Ga2O3 (Ga2O3:Cu) nanocrystals are synthesized by the hydrothermal method and used as the hole transport material of inverted PSCs for the first time. It is found that Cu dopants can substantially improve the performance of Ga2O3 layers, and the efficiency of PSCs is increased from 7.6% to 19.5%. This improvement can be attributed to the additional hole transport channels from impurity levels of Cu dopants, which exactly match with the valence band of perovskite layers. As a consequence, Ga2O3:Cu layers can effectively extract holes and inhibit the recombination in perovskite layers. This work also provides an alternative route for the design of hole transport materials. [ABSTRACT FROM AUTHOR]

Mesoporous n–i–p perovskite solar cells (PeSCs) demonstrate attractive potential for obtaining high power conversion efficiencies (PCEs), by employing inorganic electron transport layers (ETLs). However, these ETLs composed of dual layers (a condense layer and a mesoporous layer) suffer the composite process and high sintering temperature. Here, we demonstrate a simple and efficient process to improve the device performance of PeSCs by using a textured SnO2 film. Self-aged sol-gel SnO2 solution after spin coating results in a textured structure without sacrificing the surface coverage. Excellent light trapping ability is achieved by optimizing the aged time of sol-gel SnO2 solution, which mimics the evolution of the conventional mesoporous layer. Such a SnO2 textured structure provides a large contact area for rapid charge extraction, and alleviates interfacial recombination loss. Therefore, this PeSC yields an optimal PCE of 19%, which is prominent in state-of-the-art SnO2-based devices. These results indicate that one-step solution processed SnO2 with a textured structure offers a simple and efficient way to improve the device performance of PeSCs without a complex process. [ABSTRACT FROM AUTHOR]

All‐inorganic halide perovskite (AIHP) is becoming one of the most promising generation materials of perovskite photovoltaics for commercialization due to its thermodynamic stability and soared efficiency. Depending on material properties, grain boundary (GB) has detrimental or beneficial effect on device photovoltaic performance. However, less attention is paid to GB behavior in AIHPs. Herein, it is concluded that the microscopic GBs are the major sites for photocarrier generation and transport, as well as the ionic pathway dominating the current hysteresis behavior in AIHP solar cells. Kelvin probe force microscopy (KPFM) measurements reveal a lower surface potential at GBs as compared to grain interiors (GIs), suggesting a significant upward band bending around GBs. Conductive atomic force microscopy (c‐AFM) measurements show a higher current flow in the vicinity of GBs, indicating enhanced carrier separation and collection taking place at GBs. Furthermore, the existence of ion motion is evidenced both by the single‐point c‐AFM measurement and voltage‐controlled KPFM measurement, which accounts for the common current hysteresis behavior in AIHP solar cells. These investigations provide an advanced understanding of the role of GBs in AIHP and further inspiration on how to optimize device performance up to the levels of organic–inorganic hybrid perovskite. [ABSTRACT FROM AUTHOR]

Organic–inorganic hybrid halide perovskites are highly promising candidates for efficient photovoltaics, however, the long-term stability in ambient air still limits their further practical applications. Herein, the enhancement of both performance and stability of perovskite solar cells is reported by employing two-dimension (2D)/three-dimension (3D) planar perovskite films. The 2D/3D planar perovskites combine the advantages of the high-performance from the 3D perovskite and the stability from 2D-perovskite. It is found that the benefits of the 2D perovskite are attributed to ultrathin coverage on the 3D perovskite surface, which not only passivates the grain boundaries of the underlayer 3D perovskite, but also aligns the energy level between the perovskite and hole transport layers from a microscopic point of view. This work demonstrates that modulating 2D/3D planar films by hybridizing different species of perovskite is a feasible way to simultaneously enhance both efficiency and stability of perovskite solar cells. Image 1 • The enhancement of both performance and stability of perovskite solar cells is reported. • 2D perovskite is formed by the reaction between PEAI and PbI 2 from 3D perovskite. • 2D/3D perovskite passivates grain boundaries and optimized energy band alignment. • Ultrathin 2D perovskite/3D perovskite layer exerts the function like the 2D:3D mixed layer. [ABSTRACT FROM AUTHOR]

Solution process is the most widely used method to prepare perovskite films for efficient solar cells due to its facile fabrication and low-cost. However, the aged perovskite precursor solutions usually suffer from degradation, which impedes the reproducibility of the solution. In this paper, we demonstrate the direct formed tri-iodide ions stabilizing colloidal precursor solution for efficient halide perovskite solar cells. The added I 2 molecules dissolved in perovskite precursor sustain the dynamic balance between I− ions and I 3 − ions by the reversible reaction: I− + I 2 ⇌ I 3 −, which effectively stabilizes the whole I− ion concentration in perovskite solution and prevents the colloidal particles from aggregation. The resulting perovskite films with better crystallization have less bulk defects that act as carrier recombination centers. As a result, the devices using the aged solution with the additional tri-iodide ions show the less efficiency drop from ∼19% to ∼16% with the increased aging time up to 15 days, compared with the devices using the as-prepared precursor solution degrading from ∼18% to ∼6%. Our results represent an important advance in the stabilization of perovskite colloidal precursor and promote the reproducibility of perovskite solar cells by solution process. • I 3 − ions are used to stabilize the perovskite precursor solution. • The added I 3 − ions prevent the colloidal particles from aggregation. • The perovskite solution with I 3 − ions result in high-quality films for high efficiency. • The devices using the aged solution with I 3 − ions show the less efficiency drop. [ABSTRACT FROM AUTHOR]

Abstract A uniform and compact hole blocking layer is necessary for high-performance perovskite solar cells, since it not only serves as an electron collector but also suppresses the carrier recombination. So far, high efficient perovskite solar cells have been obtained by using a blocking layer TiO 2 that requires an additional TiCl 4 treatment. In this paper, a comprehensive understanding of TiCl 4 treatment on the TiO 2 layer was investigated in planar perovskite solar cells. Scanning electron microscope, atomic force microscope, and Kelvin probe force microscopy were applied to investigate the morphology of the TiCl 4 treated TiO 2 , and steady-state photo luminescence spectroscopy and electrical impedance spectroscopy were conducted to study charge carrier dynamics. The TiCl 4 treated TiO 2 layer can result in high efficiency over 20% in planar perovskite solar cells. Our results demonstrate that the TiCl 4 treatment on the compact TiO 2 prior to the perovskite deposition was necessary for achieving high-performance solar cells. Graphical abstract Image 1 Highlights • TiCl 4 treatment on the TiO 2 layer was investigated comprehensively. • TiCl 4 treatment fills the cracks and pinholes of the TiO 2 layer. • TiCl 4 treatment depresses the surface work function of the TiO 2 layer. • Perovskite solar cells with the TiCl 4 treated TiO 2 result in high efficiency over 20%. [ABSTRACT FROM AUTHOR]

All inorganic CsPbX 3 perovskites have gained great attentions owing to their excellent carrier transport property and thermal stability. However, the presence of X vacancy in the CsPbX 3 lattice results in a high trap density within CsPbX 3 films, and subsequently affects the photovoltaic performance of solar cells. Here, we use phenylethylammonium iodide (PEAI) and phenylethylammonium bromide (PEABr) to manipulate the film quality of inorganic perovskites, which significantly improve the moisture-resistance of the CsPbI 2 Br film and photovoltaic performance. It is found that there is no so-called two-dimensional PEA 2 PbI 4 and PEA 2 PbBr 4 materials formed after a high temperature annealing, but iodine or bromide vacancies can be filled by external anions. The improved film quality with a low trap density renders the enhancement of the perovskite phase stability and the device efficiency of the CsPbI 2 Br solar cells. This work shows that filling vacancies by external anions can promote perovskite photovoltaic performance and stability. Image 1 • PEAI and PEABr manipulate the film quality of inorganic CsPbI 2 Br perovskite. • Iodine or bromide vacancies within CsPbI 2 Br can be filled by external anions. • The fused grain boundaries render the larger grains with a low trap density. • Highly efficient and stable CsPbI 2 Br photovoltaics with moisture-resistance are obtained. [ABSTRACT FROM AUTHOR]

In perovskite solar cells (PSCs), hybrid perovskite:fullerene phases are proposed to suppress macroscopic current hysteresis behavior by alleviating ion migration. However, the understanding of how fullerenes exactly alleviate the current hysteresis and what is the influence of fullerenes in such hybrid phases are still unclear from a microscopic viewpoint. Herein, the intentional incorporation of fullerene into perovskite is used to examine how fullerene exactly reduces the macroscopic current hysteresis. The location and distribution of fullerenes in the hybrid phase are confirmedly visualized using conductive atomic force microscopy and Kelvin probe force microscopy measurements. Fullerenes located at grain boundaries function as a source of beneficial effect on choking the channels of ion migration and also as the electron traps that compromise the photocarrier extraction. Macroscopic current hysteresis originating from the influxes of all nanoscopic grain boundary current signals is avoided in PSCs based on the hybrid perovskite:fullerene phases. These results not only provide a strong correlation between nanoscopic and macroscopic current hysteresis behaviors but also clearly clarify how fullerenes play a role in reducing the current hysteresis in hybrid phases and thus prototype devices. [ABSTRACT FROM AUTHOR]

• a-CsPbI 3 light harvester layers are prepared at low temperature without hydrogen iodide additive. • Lead acetate is used as the lead source of precursor solutions. • Ethylenediaminium iodide can cooperates with the lead acetate precursor. • The grain size of CsPbI 3 layers is increased and their trap density is reduced. • The power conversion efficiency and photo-stability of devices are improved. CsPbI 3 triggers strong interest in the photovoltaic community for their promising photoelectric property and nonvolatility. However, CsPbI 3 layers with a-phase crystal structure are difficult for preparation, and these layers easily degrade to non-photoactive δ-phase at room temperature. Tuning the composition of precursor solutions is an important method for improving the properties of CsPbI 3 layers. Herein, we prepare inorganic a-CsPbI 3 perovskite solar cells using lead acetate as the lead source, which is different from conventional precursors with lead iodide as the lead source. The lead acetate precursor needs neither incorporating hydrogen iodide nor preparing PbI 2 complex, and it is shown to enlarge the grain size, improve the crystallinity and reduce the trap density of CsPbI 3 layers. By cooperating the lead acetate precursor with EDAI 2 additive, the power conversion efficiency (PCE) of devices increases from 9.2% to 12.9% and the photo-stability of these devices is improved simultaneously. [ABSTRACT FROM AUTHOR]

Hybrid halide perovskite based on CH3NH3PbI3 and related materials has emerged as the most exciting development in the next generation photovoltaic technologies. There is still requirement for an effective method to establish a relationship between the charge transfer behaviors and photovoltaic properties. This study presents Kelvin probe force microscopy and conductive atomic force microscopy measurements of versatile perovskite films that participate in the formation of different heterojunctions, exploring local current, contact potential difference (CPD), and charge activities at the nanoscale. By comparing the values of CPD and current of these perovskite films in dark and under illumination, the charge transfer behaviors are locally illustrated, suggesting that the perovskite roles in these heterojunctions are strictly dependent on the contact layers. Furthermore, the average difference (ΔV) of the CPD values obtained in dark and under illumination for each heterojunction can be set to analyze the efficacy of the perovskite/contact layer interfaces. The ΔV polarity is related to the type of charge carrier (hole or electron), while the ΔV magnitude is related to the number of charge carrier. These results emphasize the importance of understanding of these heterojunction systems that could guide the design and optimization of the photovoltaic configuration. Kelvin probe force microscopy and conductive atomic force microscopy are applied to explore local contact potential difference (CPD), current, and charge activities of the perovskite films with different contact layers. The average difference of the CPD values of the perovskite films obtained in dark and under illumination signifies the efficacy of the perovskite/contact layer heterojunctions. [ABSTRACT FROM AUTHOR]

For efficient perovskite solar cells (PSCs), hole transport layers (HTLs) are necessary components that can suppress recombination by collecting holes and blocking electrons. Inorganic hole transport materials (HTMs) are attractive because of their ease of preparation, low production cost and high stability. Although metal oxides and halides have been developed as the HTMs, metal hydroxides have not yet been reported as the HTMs of PSCs. In this work, copper doped lanthanum hydroxide (La x Cu y (OH) 3) nanorods were firstly synthesized by the hydrothermal method as a new kind of HTM for PSCs. Although the valence band edge of La(OH) 3 is slightly mismatch with that of perovskite layers, Cu2+ dopants improve the energy level alignment at HTL/perovskite interfaces. As an acceptor dopant for La(OH) 3 , Cu2+ ions also increase the hole transport ability of La x Cu y (OH) 3 HTLs. Additionally, La x Cu y (OH) 3 HTLs tend to encourage the crystallization of perovskite layers. As a consequence, Cu2+ dopants lead to an increase of PCE from 11.3 % to 20.4 % for La x Cu y (OH) 3 based PSCs. This promotion can be related to the reduced recombination process and the elevated charge extraction process. This work firstly applied metal hydroxide as the HTMs of PSCs, and demonstrates the regulation of these HTMs by the doping method. [Display omitted] • La(OH) 3 nanorods with copper dopants (La x Cu y (OH) 3) were synthesized. • La x Cu y (OH) 3 films were deposited by spin-coating at a low temperature of 100 °C. • Energy level and hole transport ability were improved by incorporating Cu2+ ions. • The Cu2+ dopants can also promote the crystallization of perovskite layers. • Cu2+ dopants lead to an improvement of PCEs from 11.3 % to 20.4 % for PSCs. [ABSTRACT FROM AUTHOR]

In situ transformation and dissolution–crystallization mechanisms play a competing role in determining the characteristics of perovskite films that greatly affect the device performance of perovskite solar cells in the sequential two-step process. Herein, we develop a facile solution engineering to balance the transformation from PbI 2 to CH 3 NH 3 PbI 3 and dissolution–crystallization of CH 3 NH 3 PbI 3 crystal growth, producing pure phase CH 3 NH 3 PbI 3 crystals for high-efficient planar-structure solar cells. Low concentration of CH 3 NH 3 I in a mixed solvent of isopropanol/cyclohexane with low polarity is applied to suppress dissolution–crystallization (Ostwald ripening growth) of perovskite, while increases the transformation time from PbI 2 to CH 3 NH 3 PbI 3 . Combination of porous PbI 2 and temperature-assistance effectively promote the transformation from PbI 2 to CH 3 NH 3 PbI 3 and reduce the time of Ostwald ripening growth of perovskite. This solution engineering reconciles the complete PbI 2 transformation and dissolution–crystallization of CH 3 NH 3 PbI 3 , resulting in a pure phase perovskite without any residual PbI 2 in a short time. This strategy exemplified here can serve in the design and development of more sophisticated perovskites based on planar-structure applications without mesoporous TiO 2 scaffold. [ABSTRACT FROM AUTHOR]

Electron transport layers can collect photo-generated electrons and suppress interfacial recombination, which are crucial for efficient perovskite solar cells. Tin oxide is a suitable electron transport material for its wide bandgap, high electron mobility, low photocatalytic activity and appropriate energy band offsets. For the deposition of tin oxide layers, SnCl 2 precursor is usually used for the conventional solution spin-coating method. In this work, we developed an in-situ synthesized precursor, and ultrathin tin oxide layers were obtained by the combustion process at 150 °C. Furthermore, the devices with these tin oxide layers can achieve the PCE of 19.6%, while conventional tin oxide layers with SnCl 2 precursor lead to the PCEs of 16.9% and 14.9% with and without plasma treatment. This improvement can be related to the enhanced photoelectron collection and superior interfacial contact. This work provides a facile method for fabricating SnO 2 electron transport layers of perovskite solar cells at low temperature. [Display omitted] • A novel SnO 2 precursor solution was developed by the in-situ synthesis from tin powder. • SnO 2 ETLs with high purity were prepared by the combustion process at low temperature. • Ultrathin SnO 2 ETLs were obtained with enhanced charge collection and interfacial contact. • PSCs with newly developed SnO 2 ETLs were superior to devices by conventional SnCl 2 precursor. [ABSTRACT FROM AUTHOR]