Multifunctional Polymer-Regulated SnO2 Nanocrystals Enhance Interface Contact for Efficient and Stable Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) have rapidly developed and achieved power conversion efficiencies of over 20% with diverse technical routes. Particularly, planar-structured PSCs can be fabricated with low-temperature (≤150 °C) solution-based processes, which is energy efficient and compatible with flexible substrates. Here, the efficiency and stability of planar PSCs are enhanced by improving the interface contact between the SnO₂ electron-transport layer (ETL) and the perovskite layer. A biological polymer (heparin potassium, HP) is introduced to regulate the arrangement of SnO₂ nanocrystals, and induce vertically aligned crystal growth of perovskites on top. Correspondingly, SnO₂–HP-based devices can demonstrate an average efficiency of 23.03% on rigid substrates with enhanced open-circuit voltage (V_OC) of 1.162 V and high reproducibility. Attributed to the strengthened interface binding, the devices obtain high operational stability, retaining 97% of their initial performance (power conversion efficiency, PCE > 22%) after 1000 h operation at their maximum power point under 1 sun illumination. Besides, the HP-modified SnO₂ ETL exhibits promising potential for application in flexible and large-area devices.     

Suppressed Voltage Deficit and Degradation of Perovskite Solar Cells by Regulating the Mineralization of Lead Iodide

Both the uncoordinated Pb²⁺ and excess PbI₂ in perovskite film will create defects and perturb carrier collection, thus leading to the open-circuit voltage (V_OC) loss and inducing rapid performance degradation of perovskite solar cells (PSCs). Herein, an additive of 3-aminothiophene-2-carboxamide (3-AzTca) that contains amide and amino and features a large molecular size is introduced to improve the quality of perovskite film. The interplay of size effect and adequate bonding strength between 3-AzTca and uncoordinated Pb²⁺ regulates the mineralization of PbI₂ and generates low-dimensional PbI₂ phase, thereby boosting the crystallization of perovskite. The decreased defect states result in suppressed nonradiative recombination and reduced V_OC loss. The power conversion efficiency (PCE) of modified PSC is improved to 22.79% with a high V_OC of 1.22 V. Moreover, the decomposition of PbI₂ and perovskite films is also retarded, yielding enhanced device stability. This study provides an effective method to minimize the concentration of uncoordinated Pb²⁺ and improve the PCE and stability of PSCs.     

Novel Bilayer SnO2 Electron Transport Layers with Atomic Layer Deposition for High-Performance α-FAPbI3 Perovskite Solar Cells

Perovskite solar cells (PSCs) based on the SnO₂ electron transport layer (ETL) have achieved remarkable photovoltaic efficiency. However, the commercial SnO₂ ETLs show various shortcomings. The SnO₂ precursor is prone to agglomeration, resulting in poor morphology with numerous interface defects. Additionally, the open circuit voltage (V_oc) would be constrained by the energy level mismatch between the SnO₂ and the perovskite. And, few studies designed SnO₂-based ETLs to promote crystal growth of PbI₂, a crucial prerequisite for obtaining high-quality perovskite films via the two-step method. Herein, we proposed a novel bilayer SnO₂ structure that combined the atomic layer deposition (ALD) and sol-gel solution to well address the aforementioned issues. Due to the unique conformal effect of ALD-SnO₂, it can effectively modulate the roughness of FTO substrate, enhance the quality of ETL, and induce the growth of PbI₂ crystal phase to develop the crystallinity of perovskite layer. Furthermore, a created built-in field of the bilayer SnO₂ can help to overcome the electron accumulation at the ETL/perovskite interface, leading to a higher V_oc and fill factor. Consequently, the efficiency of PSCs with ionic liquid solvent increases from 22.09% to 23.86%, maintaining 85% initial efficiency in a 20% humidity N2 environment for 1300 h.     

Cesium Lead Inorganic Solar Cell with Efficiency beyond 18% via Reduced Charge Recombination

Cesium‐based inorganic perovskite solar cells (PSCs) are promising due to their potential for improving device stability. However, the power conversion efficiency of the inorganic PSCs is still low compared with the hybrid PSCs due to the large open‐circuit voltage (V_OC) loss possibly caused by charge recombination. The use of an insulated shunt‐blocking layer lithium fluoride on electron transport layer SnO₂ for better energy level alignment with the conduction band minimum of the CsPbI_3‐xBr_x and also for interface defect passivation is reported. In addition, by incorporating lead chloride in CsPbI_3‐xBr_x precursor, the perovskite film crystallinity is significantly enhanced and the charge recombination in perovksite is suppressed. As a result, optimized CsPbI_3‐xBr_x PSCs with a band gap of 1.77 eV exhibit excellent performance with the best V_OC as high as 1.25 V and an efficiency of 18.64%. Meanwhile, a high photostability with a less than 6% efficiency drop is achieved for CsPbI_3‐xBr_x PSCs under continuous 1 sun equivalent illumination over 1000 h.     

Mitigating Ion Migration with an Ultrathin Self-Assembled Ionic Insulating Layer Affords Efficient and Stable Wide-Bandgap Inverted Perovskite Solar Cells

Wide-bandgap perovskite solar cells (PSCs) are attracting increasing attention because they play an irreplaceable role in tandem solar cells. Nevertheless, wide-bandgap PSCs suffer large open-circuit voltage (V_OC) loss and instability due to photoinduced halide segregation, significantly limiting their application. Herein, a bile salt (sodium glycochenodeoxycholate, GCDC, a natural product), is used to construct an ultrathin self-assembled ionic insulating layer firmly coating the perovskite film, which suppresses halide phase separation, reduces V_OC loss, and improves device stability. As a result, 1.68 eV wide-bandgap devices with an inverted structure deliver a V_OC of 1.20 V with an efficiency of 20.38%. The unencapsulated GCDC-treated devices are considerably more stable than the control devices, retaining 92% of their initial efficiency after 1392 h storage under ambient conditions and retaining 93% after heating at 65 °C for 1128 h in an N₂ atmosphere. This strategy of mitigating ion migration via anchoring a nonconductive layer provides a simple approach to achieving efficient and stable wide-bandgap PSCs.     

Tailoring C60 for Efficient Inorganic CsPbI2Br Perovskite Solar Cells and Modules

Although inorganic perovskite solar cells (PSCs) are promising in thermal stability, their large open-circuit voltage (V_OC) deficit and difficulty in large-area preparation still limit their development toward commercialization. The present work tailors C₆₀ via a codoping strategy to construct an efficient electron-transporting layer (ETL), leading to a significant improvement in V_OC of the inverted inorganic CsPbI₂Br PSC. Specifically, tris(pentafluorophenyl)borane (TPFPB) is introduced as a dopant to lower the lowest unoccupied molecular orbital (LUMO) level of the C₆₀ layer by forming a Lewis acidic adduct. The enlarged free energy difference provides a favorable enhancement in electron injection and thereby reduces charge recombination. Subsequently, a nonhygroscopic lithium salt (LiClO₄) is added to increase electron mobility and conductivity of the film, leading to a reduction in the device hysteresis and facilitating the fabrication of a large-area device. Finally, the as-optimized inorganic CsPbI₂Br PSCs gain a champion power conversion efficiency (PCE) of 15.19%, with a stabilized power output (SPO) of 14.21% (0.09 cm²). More importantly, this work also demonstrates a record PCE of 14.44% for large-area inorganic CsPbI₂Br PSCs (1.0 cm²) and reports the first inorganic perovskite solar module with the excellent efficiency exceeding 12% (10.92 cm²) by a self-developed quasi-curved heating method.     

Nanoporous p-type NiOx electrode for p-i-n inverted perovskite solar cell toward air stability

Designing air-stable perovskite solar cells (PSCs) is a recent trend in low-cost photovoltaic technology. Metal oxide-based electron transporting layers (ETLs) and hole transporting layers (HTLs) have attracted tremendous attention in PSCs, because of their excellent air stability, high electron mobility, and optical transparency. Herein, we report a co-precipitation method for the synthesis of p-type nanoporous nickel oxide (np-NiOx) thin films as the HTL for inverted (p-i-n) PSCs. The best-performing p-i-n PSC having np-NiOx HTL, (FAPbI₃)_0.85(MAPbBr₃)_0.15 (herein FAPbI₃ stands for formamidinium lead iodide and MAPbBr₃ stands for methylammonium lead bromide) perovskite and phenyl-C₆₁-butyric acid methyl ester (PCBM)/ZnO ETL exhibited a 19.10% (±1%) power conversion efficiency (PCE) with a current density (J_SC) of 22.76?mA?cm^?2, open circuit voltage (V_OC) of 1.076?V and fill factor (FF) of 0.78 under 1?sun (100?mW?cm^?2). Interestingly, the developed p-i-n PSCs based on p-type NiOx and n-type ZnO could retain >80% efficiency after 160?days, which is much higher than conventional PEDOT:PSS HTL-based PSCs. Our findings provide air-stable perovskite solar cells with high efficiency.     

Red-Carbon-Quantum-Dot-Doped SnO2 Composite with Enhanced Electron Mobility for Efficient and Stable Perovskite Solar Cells

An efficient electron transport layer (ETL) plays a key role in promoting carrier separation and electron extraction in planar perovskite solar cells (PSCs). An effective composite ETL is fabricated using carboxylic-acid- and hydroxyl-rich red-carbon quantum dots (RCQs) to dope low-temperature solution-processed SnO₂, which dramatically increases its electron mobility by ≈20 times from 9.32 × 10⁻⁴ to 1.73 × 10⁻² cm² V⁻¹ s⁻¹. The mobility achieved is one of the highest reported electron mobilities for modified SnO₂. Fabricated planar PSCs based on this novel SnO₂ ETL demonstrate an outstanding improvement in efficiency from 19.15% for PSCs without RCQs up to 22.77% and have enhanced long-term stability against humidity, preserving over 95% of the initial efficiency after 1000 h under 40–60% humidity at 25 °C. These significant achievements are solely attributed to the excellent electron mobility of the novel ETL, which is also proven to help the passivation of traps/defects at the ETL/perovskite interface and to promote the formation of highly crystallized perovskite, with an enhanced phase purity and uniformity over a large area. These results demonstrate that inexpensive RCQs are simple but excellent additives for producing efficient ETLs in stable high-performance PSCs as well as other perovskite-based optoelectronics.     

Simultaneous Bottom‐Up Interfacial and Bulk Defect Passivation in Highly Efficient Planar Perovskite Solar Cells using Nonconjugated Small‐Molecule Electrolytes

Recent perovskite solar cell (PSC) advances have pursued strategies for reducing interfacial energetic mismatches to mitigate energy losses, as well as to minimize interfacial and bulk defects and ion vacancies to maximize charge transfer. Here nonconjugated multi‐zwitterionic small‐molecule electrolytes (NSEs) are introduced, which act not only as charge‐extracting layers for barrier‐free charge collection at planar triple cation PSC cathodes but also passivate charged defects at the perovskite bulk/interface via a spontaneous bottom‐up passivation effect. Implementing these synergistic properties affords NSE‐based planar PSCs that deliver a remarkable power conversion efficiency of 21.18% with a maximum V_OC = 1.19 V, in combination with suppressed hysteresis and enhanced environmental, thermal, and light‐soaking stability. Thus, this work demonstrates that the bottom‐up, simultaneous interfacial and bulk trap passivation using NSE modifiers is a promising strategy to overcome outstanding issues impeding further PSC advances.     

Interface Engineering for All‐Inorganic CsPbI2Br Perovskite Solar Cells with Efficiency over 14%

In this work, a SnO₂/ZnO bilayered electron transporting layer (ETL) aimed to achieve low energy loss and large open‐circuit voltage (V_oc) for high‐efficiency all‐inorganic CsPbI₂Br perovskite solar cells (PVSCs) is introduced. The high‐quality CsPbI₂Br film with regular crystal grains and full coverage can be realized on the SnO₂/ZnO surface. The higher‐lying conduction band minimum of ZnO facilitates desirable cascade energy level alignment between the perovskite and SnO₂/ZnO bilayered ETL with superior electron extraction capability, resulting in a suppressed interfacial trap‐assisted recombination with lower charge recombination rate and greater charge extraction efficiency. The as‐optimized all‐inorganic PVSC delivers a high V_oc of 1.23 V and power conversion efficiency (PCE) of 14.6%, which is one of the best efficiencies reported for the Cs‐based all‐inorganic PVSCs to date. More importantly, decent thermal stability with only 20% PCE loss is demonstrated for the SnO₂/ZnO‐based CsPbI₂Br PVSCs after being heated at 85 °C for 300 h. These findings provide important interface design insights that will be crucial to further improve the efficiency of all‐inorganic PVSCs in the future.     

Precise Control of Crystallization and Phase-Transition with Green Anti-Solvent in Wide-Bandgap Perovskite Solar Cells with Open-Circuit Voltage Exceeding 1.25 V

Wide-bandgap perovskite solar cells (PSCs) have attracted a lot of attention due to their application in tandem solar cells. However, the open-circuit voltage (V_OC) of wide-bandgap PSCs is dramatically limited by high defect density existing at the interface and bulk of the perovskite film. Here, an anti-solvent optimized adduct to control perovskite crystallization strategy that reduces nonradiative recombination and minimizes V_OC deficit is proposed. Specifically, an organic solvent with similar dipole moment, isopropanol (IPA) is added into ethyl acetate (EA) anti-solvent, which is beneficial to form PbI₂ adducts with better crystalline orientation and direct formation of α-phase perovskite. As a result, EA-IPA (7-1) based 1.67 eV PSCs deliver a power conversion efficiency of 20.06% and a V_OC of 1.255 V, which is one of the remarkable values for wide-bandgap around 1.67 eV. The findings provide an effective strategy for controlling crystallization to reduce defect density in PSCs.     

Bifacial Contact Junction Engineering for High‐Performance Perovskite Solar Cells with Efficiency Exceeding 21%

Ordered 1D metal oxide structure is desirable in thin film solar cells owing to its excellent charge collection capability. However, the electron transfer in 1D electron transporting layer (ETL)‐based devices is still limited to a submicrometer‐long pathway that is vertical to the substrate. Here, an innovative closely packed rutile TiO₂ nanowire (CRTNW) network parallel to the facet of fluorine‐doped tin oxide (FTO) substrate is reported, which can serve as a 1D nanoscale electron transport pathway for efficient perovskite solar cells (PSCs). The PSC constructed using newly prepared CRTNW ETL achieves an impressive power conversion efficiency of 21.10%, which can be attributed to the facilitated electron extraction induced by the favorable junctions formed at FTO/ETL and ETL/perovskite interfaces and also the suppressed charge recombination originating from improved perovskite morphology with large grains, flat surface, and good surface coverage. The bifacial contact junctions engineering also enables large‐area device fabrication. The PSC with 1 cm² aperture yields an efficiency of 19.50% under one sun illumination. This work highlights the significance of controlling the orientation and packing density of the ordered 1D oxide nanostructured thin films for highly efficient optoelectronic devices in a large‐scale manner.     

Reducing Hysteresis and Enhancing Performance of Perovskite Solar Cells Using Low‐Temperature Processed Y‐Doped SnO2 Nanosheets as Electron Selective Layers

Despite the rapid increase of efficiency, perovskite solar cells (PSCs) still face some challenges, one of which is the current–voltage hysteresis. Herein, it is reported that yttrium‐doped tin dioxide (Y‐SnO₂) electron selective layer (ESL) synthesized by an in situ hydrothermal growth process at 95 °C can significantly reduce the hysteresis and improve the performance of PSCs. Comparison studies reveal two main effects of Y doping of SnO₂ ESLs: (1) it promotes the formation of well‐aligned and more homogeneous distribution of SnO₂ nanosheet arrays (NSAs), which allows better perovskite infiltration, better contacts of perovskite with SnO₂ nanosheets, and improves electron transfer from perovskite to ESL; (2) it enlarges the band gap and upshifts the band energy levels, resulting in better energy level alignment with perovskite and reduced charge recombination at NSA/perovskite interfaces. As a result, PSCs using Y‐SnO₂ NSA ESLs exhibit much less hysteresis and better performance compared with the cells using pristine SnO₂ NSA ESLs. The champion cell using Y‐SnO₂ NSA ESL achieves a photovoltaic conversion efficiency of 17.29% (16.97%) when measured under reverse (forward) voltage scanning and a steady‐state efficiency of 16.25%. The results suggest that low‐temperature hydrothermal‐synthesized Y‐SnO₂ NSA is a promising ESL for fabricating efficient and hysteresis‐less PSC.     

Matching Charge Extraction Contact for Wide‐Bandgap Perovskite Solar Cells

Efficient wide‐bandgap (WBG) perovskite solar cells are needed to boost the efficiency of silicon solar cells to beyond Schottky–Queisser limit, but they suffer from a larger open circuit voltage (V_OC) deficit than narrower bandgap ones. Here, it is shown that one major limitation of V_OC in WBG perovskite solar cells comes from the nonmatched energy levels of charge transport layers. Indene‐C60 bisadduct (ICBA) with higher‐lying lowest‐unoccupied‐molecular‐orbital is needed for WBG perovskite solar cells, while its energy‐disorder needs to be minimized before a larger V_OC can be observed. A simple method is applied to reduce the energy disorder by isolating isomer ICBA‐tran3 from the as‐synthesized ICBA‐mixture. WBG perovskite solar cells with ICBA‐tran3 show enhanced V_OC by 60 mV, reduced V_OC deficit of 0.5 V, and then a record stabilized power conversion efficiency of 18.5%. This work points out the importance of matching the charge transport layers in perovskite solar cells when the perovskites have a different composition and energy levels.     

Reduced Interface‐Mediated Recombination for High Open‐Circuit Voltages in CH3NH3PbI3 Solar Cells

Perovskite solar cells with all‐organic transport layers exhibit efficiencies rivaling their counterparts that employ inorganic transport layers, while avoiding high‐temperature processing. Herein, it is investigated how the choice of the fullerene derivative employed in the electron‐transporting layer of inverted perovskite cells affects the open‐circuit voltage (V_OC). It is shown that nonradiative recombination mediated by the electron‐transporting layer is the limiting factor for the V_OC in the cells. By inserting an ultrathin layer of an insulating polymer between the active CH₃NH₃PbI₃ perovskite and the fullerene, an external radiative efficiency of up to 0.3%, a V_OC as high as 1.16 V, and a power conversion efficiency of 19.4% are realized. The results show that the reduction of nonradiative recombination due to charge‐blocking at the perovskite/organic interface is more important than proper level alignment in the search for ideal selective contacts toward high V_OC and efficiency.     

SnO2: A Wonderful Electron Transport Layer for Perovskite Solar Cells

The highest power conversion efficiency of perovskite solar cells is beyond 22%. Charge transport layers are found to be critical for device performance and stability. A traditional electron transport layer (ETL), such as TiO₂, is not very efficient for charge extraction at the interface, especially in planar structure. In addition, the devices using TiO₂ suffer from serious degradation under ultraviolet illumination. SnO₂ owns a better band alignment with the perovskite absorption layer and high electron mobility, which is helpful for electron extraction. In this Review, recent progresses in efficient and stable perovskite solar cells using SnO₂ as ETL are summarized.     

20.67%-Efficiency Inorganic CsPbI3 Solar Cells Enabled by Zwitterion Ion Interface Treatment

All-inorganic CsPbI₃ perovskite solar cells (PSCs) have been extensively studied due to their high thermal stability and unprecedented rise in power conversion efficiency (PCE). Recently, the champion PCE of CsPbI₃ PSCs has reached up to 21%; however, it is still much lower than that of organic–inorganic hybrid PSCs. Interface modification to passivate surface defects and minimize charge recombination and trapping is important to further improve the efficiency of CsPbI₃ PSCs. Herein, a new zwitterion ion is deposited at the interface between electron transporting layer (ETL) and perovskite layer to passivate the defects therein. The zwitterion ions can not only passivate oxygen vacancy (V_O) and iodine vacancy (V_I) defects, but also improve the band alignment at the ETL-perovskite interface. After the interface treatment, the PCE of CsPbI₃ device reaches up to 20.67%, which is among the highest values of CsPbI₃ PSCs so far. Due to the defect passivation and hydrophobicity improvement, the PCE of optimized device remains 94% of its original value after 800 h storing under ambient condition. These results provide an efficient way to improve the quality of ETL-perovskite interface by zwitterion ions for achieving high performance inorganic CsPbI₃ PSCs.     

<Emphasis Type="Italic">In situ</Emphasis> fabrication of 2D SnS<Subscript>2</Subscript> nanosheets as a new electron transport layer for perovskite solar cells

To promote commercialization of perovskite solar cells (PSCs), low-temperature processed electron transport layer (ETL) with high carrier mobility still needs to be further developed. Here, we reported two-dimensional (2D) tin disulfide (SnS₂) nanosheets as ETL in PSCs for the first time. The morphologies of the 2D SnS₂ material can be easy controlled by the in situ synthesized method on the conductive fluorine-doped tin oxide (FTO) substrate. We achieved a champion power conversion efficiency (PCE) of 13.63%, with the short-circuit current density (J_SC) of 23.70 mA/cm², open-circuit voltage (V_OC) of 0.95 V, and fill factor (FF) of 0.61. The high J_SC of PSCs results from effective electron collection of the 2D SnS₂ nanosheets from perovskite layer and fast electron transport to the FTO. The low V_OC and FF are the results of the lower conduction band of 2D SnS₂ (4.23 eV) than that of TiO₂ (4.0 eV). These results demonstrate that 2D material is a promising candidate for ETL in PSCs.

Open image in new window

     

Superfast Room‐Temperature Activation of SnO2 Thin Films via Atmospheric Plasma Oxidation and their Application in Planar Perovskite Photovoltaics

The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has now exceeded 20%; thus, research focus has shifted to establishing the foundations for commercialization. One of the pivotal themes is to curtail the overall fabrication time, to reduce unit cost, and mass‐produce PSCs. Additionally, energy dissipation during the thermal annealing (TA) stage must be minimized by realizing a genuine low‐temperature (LT) process. Here, tin oxide (SnO₂) thin films (TFs) are formulated at extremely high speed, within 5 min, under an almost room‐temperature environment (<50 °C), using atmospheric Ar/O₂ plasma energy (P‐SnO₂) and are applied as an electron transport layer of a “n–i–p”‐type planar PSC. Compared with a thermally annealed SnO₂ TF (T‐SnO₂), the P‐SnO₂ TF yields a more even surface but also outstanding electrical conductivity with higher electron mobility and a lower number of charge trap sites, consequently achieving a superior PCE of 19.56% in P‐SnO₂‐based PSCs. These findings motivate the use of a plasma strategy to fabricate various metal oxide TFs using the sol–gel route.     

Significant efficiency enhancement of carbon-based CsPbI2Br perovskite solar cells enabled by optimizing tin oxide electron transport layer

Carbon-based perovskite solar cells (C-PSCs) have been popular for achieving low-cost and stable photovoltaics. To overcome an obstacle of high-temperature annealing process for producing titanium dioxide (TiO₂), CsPbI₂Br C-PSCs based on a device structure of FTO/tin oxide (SnO₂)/CsPbI₂Br/carbon electrode can be fabricated at the low-temperature annealing process of 280 °C for 180 s, where SnO₂ is used as the electron transporting layer (ETL). Experimental results showed that the suitable concentration of SnO₂ ETL could yield smooth surface CsPbI₂Br films with free-pinhole and larger grain-sized crystallization. In combination with prolonging annealing time, a champion power conversion efficiency of 9.68% with a larger open-circuit voltage (V_oc) of 1.14 V was obtained for CsPbI₂Br C-PSC based on SnO₂ ETL. Here, a simple low-temperature fabricating process of SnO₂ ETL can be adapted to flexible substrates for C-PSCs and furtherly reduce the manufacturing cost.