shuhui

Nature Energy, Published online: 19 August 2019; doi:10.1038/s41560-019-0446-7

A general approach for lab‐to‐manufacturing translation is developed to achieve high‐performance flexible organic solar modules without obvious efficiency loss. The shear impulse during the coating/printing process is applied to control the morphology evolution of the bulk heterojunction layer for both fullerene and nonfullerene acceptor systems. A quantitative transformation factor of shear impulse between slot‐die printing and spin‐coating is detected.

Abstract

The blossoming of organic solar cells (OSCs) has triggered enormous commercial applications, due to their high‐efficiency, light weight, and flexibility. However, the lab‐to‐manufacturing translation of the praisable performance from lab‐scale devices to industrial‐scale modules is still the Achilles' heel of OSCs. In fact, it is urgent to explore the mechanism of morphological evolution in the bulk heterojunction (BHJ) with different coating/printing methods. Here, a general approach to upscale flexible organic photovoltaics to module scale without obvious efficiency loss is demonstrated. The shear impulse during the coating/printing process is first applied to control the morphology evolution of the BHJ layer for both fullerene and nonfullerene acceptor systems. A quantitative transformation factor of shear impulse between slot‐die printing and spin‐coating is detected. Compelling results of morphological evolution, molecular stacking, and coarse‐grained molecular simulation verify the validity of the impulse translation. Accordingly, the efficiency of flexible devices via slot‐die printing achieves 9.10% for PTB7‐Th:PC₇₁BM and 9.77% for PBDB‐T:ITIC based on 1.04 cm² . Furthermore, 15 cm² flexible modules with effective efficiency up to 7.58% (PTB7‐Th:PC₇₁BM) and 8.90% (PBDB‐T:ITIC) are demonstrated with satisfying mechanical flexibility and operating stability. More importantly, this work outlines the shear impulse translation for organic printing electronics.

Publication date: 16 October 2019

Source: Joule, Volume 3, Issue 10

Author(s): Yuanyuan Fan, Junjie Fang, Xiaoming Chang, Ming-Chun Tang, Dounya Barrit, Zhuo Xu, Zhiwu Jiang, Jialun Wen, Huan Zhao, Tianqi Niu, Detlef-M. Smilgies, Shengye Jin, Zhike Liu, Er Qiang Li, Aram Amassian, Shengzhong (Frank) Liu, Kui Zhao

Context & Scale

Summary

Graphical Abstract

Diboron‐treated SnO₂ exhibits some Sn³⁺ species, which serve as electron donors with more n‐type nature, resulting in the higher Fermi level on the surface of SnO₂, promoting electron extraction and reducing carrier recombination in the electron transport layer (ETL)/perovskite interface. A power‐conversion efficiency of 22.04% is obtained in an n‐i‐p structure perovskite solar cell.

Energy‐level modulation between perovskite and carrier transport layers to obtain a promoted carrier extraction and reduced charge recombination is an effective way to achieve high‐efficiency perovskite solar cells. Here, diboron is used as an effective interfacial modifier between SnO₂ and perovskite. By taking advantage of the higher Fermi level on the surface of SnO₂ after diboron treatment, a power‐conversion efficiency of 22.04% in a solar cell device based on two‐step solution‐processed planar n‐i‐p structure is obtained. With the help of thorough characterizations, it is argued that the diboron‐treated SnO₂ exhibits some Sn³⁺ species, which serve as electron donors with a more n‐type nature, promoting electron extraction and reducing carrier recombination in the electron transport layer (ETL)/perovskite interface. Further analysis speculates that the formation of surface diboron–oxygen Lewis pair induces a reducing state of diboron complexes, resulting in the spontaneous electron redistribution and the formation of Sn³⁺−O^–• species. This provides an effective chemical approach to tune the energy alignment between the oxide ETL and absorber.

A systematic study of structurally similar fullerene derivatives shows that even minor modifications in their structure have a strong impact on their performance as electron transport layer (ETL) materials for perovskite solar cells. The best ETL significantly improves ambient stability of the devices for >800 h presumably due to an optimal size/shape of the solubilizing addend enabling compact molecular packing.

It is known that the operation lifetime of perovskite solar cells can be extended by orders of magnitude if properly selected hole‐transport and electron transport layers provide good isolation for the perovskite absorber preventing evaporation of volatile species (e.g., photoinduced) from the active layer and blocking the diffusion of aggressive moisture and oxygen from the surrounding environment. Herein, a systematic study of a family of structurally similar fullerene derivatives as electron transport layer (ETL) materials for p‐i‐n perovskite solar cells is presented. It is shown that even minor modifications of the molecular structure of the fullerene derivatives have a strong impact on their electrical performance and, particularly, ambient stability of the devices. Indeed, an optimally functionalized fullerene derivative applied as an ETL enables stable operation of perovskite solar cells when exposed to air for >800 h, which is manifested in retention of 90% of the original photovoltaic performance. In contrast, the reference devices with phenyl‐C₆₁‐butyric acid methyl ester as the ETL degraded almost completely within less than 100 h of air exposure. Most probably, the side chains of the best‐performing fullerene ETL materials are filling the gaps between the carbon spheres, thus preventing the diffusion of oxygen and moisture inside the device.

The theme of this review is the progress of microcavity (MC) in organic solar cells (OSCs) in recent years. The principle of MC is described in detail. In addition, the application of MC in other photo‐electronic conversion devices is also briefly introduced. Finally, the summary and prospect of microcavity organic solar cells (MCOSCs) are given.

In recent decades, organic solar cells (OSCs) have drawn increasing interest due to their unique properties such as low cost, solution‐processing, flexibility, semitransparency, and nontoxicity. Due to some shortcomings of limited optical absorption in organic semiconductors as well as low carrier mobility and short exciton diffusion length, light‐trapping technologies such as surface plasmon resonance, photonic crystals, and microcavities (MCs) have been widely developed to improve device performance. Among these methods, the MC effect is liable to form and has unneglectable influences on the device efficiency. However, few reports systematically summarize the development of MC‐based OSCs. Herein, the principle of the MC effect is introduced first, and subsequently, the application and the development of MCs in single and multi‐junction OSCs are described in detail. Furthermore, in addition to the traditional MCs‐enhanced light absorption, other applications based on the MC structure in OSCs and other photo‐electronic conversion devices are also represented. Finally, the problems that need to be solved and the development directions of MC‐based OSCs in the future are outlined. It is believed that this review can provide new thinking for achieving high‐performance OSCs with optical means.

Various strategies related to light management and photocarrier collection are developed to enhance perovskite solar cell performance. The exploration of novel plasmonic nanostructures with predesigned size and shape is needed in the field. Herein, a bioinspired nanostructure of Au nanorod–nanoparticle dimers with structural darkness is used to enhance the light harvesting and performance of perovskite solar cells.

Hybrid perovskites have recently attracted enormous attention for photovoltaic applications, and various strategies related to light management and photocarrier collection are developed to enhance their performance. As an effective route toward near‐field light enhancement, metal nanostructures with subwavelength dimensions can couple incident photons with conduction electrons, giving rise to localized surface plasmon resonances. However, efficiency enhancements through plasmonic routes are limited to the short wavelength range corresponding to metal extinction wavelength. Thus, the exploration of novel plasmonic nanostructures with predesigned sizes and shapes is needed to advance this field. Herein, for the first time, a bioinspired nanostructure of Au nanorod–nanoparticle dimers with structural darkness is exploited to enhance the light harvesting and performance of perovskite solar cells. Differing from conventional metallic nanoparticles, biometric nanoparticles introduce geometric singularity to the system, providing a broadband response for energy harvesting. By embedding the core–shell gold dimers in the perovskite solar cells, a notable enhancement of broadband light absorption is observed, and sequentially, the efficiency of perovskite solar cells increases by 16%.

An ingenious surface chlorination treatment method is used to passivate the interface defects of perovskite/zinc oxide (ZnO), which effectively reduces the interface charge recombination loss and improves the poor interface chemical characteristics. Thus, the fabricated zinc oxide–chlorine (ZnO–Cl)‐based device achieves an enhanced efficiency and suppressed hysteresis, as well as strengthened stability in perovskite solar cells.

Defect states on the zinc oxide (ZnO) surface cause severe interfacial charge recombination and perovskite decomposition during device operation, which inevitably leads to efficiency loss and poor device stability, making the usage of ZnO in perovskite solar cells (PSCs) problematic. Herein, a simple and effective method of inorganic chlorination treatment is used to passivate the surface defects of the ZnO electron transport layer. It is shown that chlorine (Cl) effectively fills the oxygen vacancy defects of ZnO, suppressing charge recombination and facilitating charge transport at the perovskite/ZnO interface. Therefore, the resulting CH₃NH₃PbI₃‐based device achieves an enhanced power conversion efficiency with suppressed hysteresis. Meanwhile, the chlorination of the ZnO surface protects the perovskite layer from decomposition, thus improving device stability. Herein, an ingenious method is developed to further improve the device performance of ZnO‐based PSCs and useful guidance is provided for the development of other perovskite optoelectronics, especially those with ZnO as the charge transport layer.

Pb(SCN)₂ functions at the grain boundaries and pinholes to in situ polish the perovskite film surface. A 425 nm‐thick CsPbI₂Br film with high crystalline, smooth, and uniform surface morphology is obtained, with an efficiency of 10.44% for a low cost and stable carbon‐based perovskite solar cell processed under low‐temperature (150 °C).

Improvement in stability and an economical processing technique are the main aspects of the commercialization of perovskite solar cells (PSCs). In this study, a 425 nm‐thick CsPbI₂Br film with a high crystalline, smooth, and uniform surface morphology is obtained by Pb(SCN)₂ passivating the grain boundaries under low temperature (150 °C). The results of a series of electrochemical analyses, including space‐charge‐limited‐current (SCLC), open‐circuit voltage decay (OCVD), electrical impedance spectroscopy (EIS), intensity‐modulated photocurrent spectroscopy (IMPS), and intensity‐modulated photovoltage spectroscopy (IMVS), demonstrate that the trap density of the CsPbI₂Br film is greatly reduced with Pb(SCN)₂, which effectively inhibits the interface recombination and promotes charge transport in CsPbI₂Br PSC. Efficiencies of 12.22% and 10.44% are achieved for low‐temperature‐processed CsPbI₂Br planar‐architecture PSCs with ITO/SnO₂/CsPbI₂Br/ poly(3‐hexylthiophene) (P3HT)/Ag and ITO/SnO₂/CsPbI₂Br/carbon, respectively. This low‐cost, high‐efficiency carbon‐based inorganic PSC shows potential industrial application, especially for tandem solar cells.

A liquid crystal (LC) molecule (4′‐heptyl‐4‐biphenylcarbonitrile) is first used as a “binding agent” to connect grain boundaries of perovskite. The crystal orientation of perovskite grains is controlled and the electron transport process is accelerated after treating with LC; these are reflected by the significant improvement in power conversion efficiency and high fill factor. Remarkably, the LC greatly contributes to the humid‐stability of perovskite solar cells.

Hybrid perovskites have rapidly emerged as highly promising optoelectronic materials for perovskite solar cells (PSCs), whereas solution‐processed perovskite films usually contain a large amount of grain‐boundary network, which is unbeneficial for efficient film function, including charge transport and environmental stability. Herein, a liquid crystal (LC) molecule is first used as a “binding agent” to connect grains and fill grain boundaries of perovskite. The LC molecule (4′‐heptyl‐4‐biphenylcarbonitrile) interacts with PbI₂ to control the crystal orientation for fine and oriented perovskite grains, which accelerates electron transport and enhances environmental stability. Consequently, compared with the pristine devices, the power conversion efficiency of the LC‐based device increases from 17.14% to 20.19% with a high fill factor (over 80%). Remarkably, the LC‐based PSCs retain 92% of their initial efficiency at 25 °C, and a relative humidity of 70% after 500 h, whereas the control samples are almost degraded completely under the same conditions.

Herein, the open‐circuit voltage losses and bias‐dependent photo‐ and electroluminescence of high‐performance 2D/3D perovskite solar cells, which exhibit outstanding optoelectronic properties, are investigated. These are state‐of‐the‐art photovoltaic devices. Results suggest that by reducing nonradiative recombination processes in the absorber, the power conversion efficiency of the studied photovoltaic devices can be improved.

Herein, the optoelectronic properties of interface‐engineered perovskite 2D|3D‐heterojunction structure solar cells are reported. The reciprocity theorem is applied to determine the maximum open‐circuit voltage (V _oc) the device can deliver under solar illumination. A V _oc of 1.295 V is found, analyzing the measured external quantum efficiency and assuming only radiative recombination. For comparison, the experimental open‐circuit voltage found for the studied 2D|3D heterojunctions is 1.15 V. The contribution of nonradiative recombination is explored by measuring the electroluminescence quantum yield. A quantum yield of 0.4% is found at current densities equivalent to 1 sun illumination. This translates into a V _oc loss of ≈140 mV, which is in very good agreement with the experimental findings. In addition, the fundamental correlation between luminescence intensity and the chemical potential predicted by the generalized Planck law is confirmed for the photoluminescence measured at different light intensities when the device is operated under open‐circuit conditions and for the electroluminescence when operated under a forward bias. The investigations in this study suggest that further efficiency improvements can be achieved by reducing the nonradiative recombination in the studied solar cell. At the same time, a high‐performance near IR light emitting diode can be realized.

A versatile alkaline earth metals doping strategy is utilized to engineer the electronic structure of NiO _x contacts for inverted planar perovskite solar cells, which demonstrates a power conversion efficiency of 19.49% with a high open‐circuit voltage of 1.14 V. Enhanced charge extraction and conductivity are responsible for the high‐performance devices.

Organometallic halide perovskite solar cells (PSCs) are rapidly evolving as the promising photovoltaic technologies with high record efficiency over 24%. The inorganic p‐type semiconductor NiO _x is extensively used as important hole transport layers for the realization of stable and hysteresis‐free solar cells due to their good electronic properties, facile fabrication, and excellent chemical endurance. However, the critical issues of NiO _x films including poor intrinsic conductivity and mismatched band alignment limit further improvement of the device performance. Herein, it is demonstrated that a versatile alkaline earth metal (Mg, Ca, Sr, and Ba) doping strategy can effectively engineer the electronic properties of NiO _x contacts in inverted planar PSCs. Alkaline earth metal doping can deepen valence band maximum and enhance the hole conductivity of NiO _x films, which better aligns the energy band in solar cells. The champion device based on Sr‐doped NiO _x films attains a power conversion efficiency of 19.49% with a high open‐circuit voltage (V _OC) of 1.14 V for NiO _x ‐based CH₃NH₃PbI₃ devices. The resulted device shows negligible hysteresis and high stability as well. This finding provides a systematic doping strategy to further improve the performance of inverted planar PSCs.

A polymer solar cell (PSC) with a large active area of 216 cm² and high power conversion efficiency of 7.7% is presented, involving a nonfullerene acceptor and the solution‐processable ZrOx interfacial layer made by blade coating. This represents the highest reported efficiency for PSCs with an active area more than 10 cm². More encouragingly, the large‐area PSC shows good long‐term thermal stability as well.

A polymer solar cell involving a nonfullerene acceptor is made by blade coating. In the ternary bulk‐heterojunction layer, the donor is poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b’]dithiophene))‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐ 5,7‐bis(2‐ethylhexyl)benzo[1,2‐c:4,5‐c’]dithiophene‐4,8‐dione)] (PBDB‐T) and the acceptor is a mixture of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2’,3’‐d’]‐s‐indaceno[1,2‐b:5,6‐b’]dithiophene) (ITIC) and [6,6]‐phenyl C₇₁‐butyric acid methyl ester (PC₇₁BM). The device structure is an indium tin oxide (ITO)‐coated glass substrate/PEDOT:PSS/ternary active layer/interfacial layer/Al. For a small active area of 0.04 cm², the best power conversion efficiency is 9.8% with the LiF interfacial layer. For a large active area of 216 cm², the best efficiency is 7.7% with the ZrOx interfacial layer. After annealing at 85 °C for 30 days, the large‐area device keeps 75% of the initial efficiency. The efficiency of 4.9% is achieved for a large‐area semi‐transparent device.

Propane‐1,3‐diammonium cations are first adopted to construct cesium–formamidinium (Cs–FA) perovskite solar cells (PSCs) with an efficiency of 18.1% and much enhanced device stability, and the opposing effects induced by the diammonium cation are resolved.

Incorporating diammonium cations, which electrostatically connect the adjacent inorganic slabs ([PbI₆]⁴⁻), into 3D perovskite is recently proposed to develop high‐performance perovskite solar cells (PSCs). However, due to limited studies, the effects of these organic cations on the perovskite structural and optoelectronic properties are yet to be understood. Herein, a diammonium cation, propane‐1,3‐diammonium (PDA), is first proposed to modulate the cesium–formamidinium (Cs–FA)‐mixed cation perovskite. By increasing the PDA content, the efficiency of the Cs_0.15FA_{0.85 − x} PDA _x PbI₃ PSC first increases and then drastically decreases. The highest power conversion efficiency (PCE) of 18.10% obtained by Cs_0.15FA_0.83PDA_0.02PbI₃ is superior to that of the Cs_0.15FA_0.85PbI₃ (16.82%). Through systematic investigations, it is revealed that the PDA content–dependent efficiency is attributed to a competition between the enhanced defect passivation and emerged excitonic effect with an increased PDA content. Moreover, the encapsulated Cs_0.15FA_0.83PDA_0.02PbI₃ device exhibits almost 1.5 times increased stability than the Cs_0.15FA_0.85PbI₃ counterpart, with 83% of its initial efficiency retained after 500 h exposure, under continuous light soaking at 60 °C in ambient air. This study provides a practical strategy to enhance the device stability without sacrificing the efficiency and deepens our understanding on effects of diammonium cation incorporated in 3D perovskite.

2D/3D perovskite heterostructures or composites have recently been recognized as efficient strategy to improve the stability of perovskite solar cells. In this work, we demonstrated a novel solution process to develop 2D/3D perovskites with modulated diffusion passivation by introducing phenylethylammonium iodide (PEAI) and N,N‐dimethylformamide (DMF) additive, which could effectively enhance device performance and long‐term stability. Compared with conventional device, the device with PEAI and DMF solvent additive treatment exhibited enhanced charge transport, improved charge extraction and suppressed non‐radiative carrier recombination. The solar cells with an optimal 2D/3D perovskite passivation treatment exhibited an extremely high fill factor of 83.6% and an average power conversion efficiency of 21.4% (21.3% by using integrated photocurrent from IPCE spectra) based on NiO_x hole transport layer. Furthermore, the unencapsulated device exhibited excellent stability under continuously simulated sunlight illumination and outstanding air stability after 1000 h storage under ambient air condition.

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Bi₂Te₃ nanoplates with a tunable energy structure are introduced in inorganic perovskite solar cells (PSCs), accelerating hole transport by the matched band alignment. Confirmed by systematic measurements, charge recombination is largely suppressed due to lower trap density and higher carrier mobility. The optimal PSC with Bi₂Te₃ exhibits highly decreased V _OC loss and enhanced long‐term stability over 50 days.

To solve the thermal instability issue of organic–inorganic hybrid perovskites, all‐inorganic perovskite solar cells (PSCs) have been featured in the spotlight. However, their power conversion efficiencies (PCEs) are far from satisfactory due to the substantially radiative and nonradiative recombination of charge carriers in the common‐structured devices. Herein, bismuth telluride (Bi₂Te₃) nanoplates are designed as an interlayer between cesium lead halide (CsPbBrI₂) and 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,90‐spirobifluorene (Spiro‐OMeTAD) to reduce the notorious trap states and charge recombination. Confirmed by systematic electrochemical and photoelectrical techniques, the Bi₂Te₃ interlayer optimizes hole extraction and transport efficiency because of the matched band level structure and drastically reduces trap defect densities. Prolonged effective lifetime and shorter diffusion time induced by the Bi₂Te₃ interlayer reveal less electron–hole recombination and more efficient carrier transport, which lead to a larger photocurrent and less open circuit voltage loss of PSCs. The all‐inorganic PSCs with the optimal Bi₂Te₃ interlayer exhibit a highly enhanced PCE of 11.96%. Moreover, Bi₂Te₃ also acts as a blocking layer for the migration of iodide ions, silver, and moisture, resulting in a considerable device stability of more than 70% of initial PCE after 50 days without extra encapsulation. This low‐cost and facile method for efficient and stable all‐inorganic PSCs offers great promise as a next‐generation renewable energy source.

Carbon‐electrode based perovskite solar cells (CPSCs) are well known for their low cost and sound stability. However, the highest power conversion efficiency of these devices is only about 70% of that demonstrated by metal electrode‐based PSCs, leaving a gap of about 30%. Bulk engineering and interface engineering is helpful in narrowing the gap. Herein, these two strategies are summarized for CPSCs.

Carbon electrodes have been adopted widely in perovskite solar cells (PSCs). Due to its suitable work function (though not high enough), the carbon electrode itself could extract photogenerated holes and has helped to achieve a power conversion efficiency of ≈16% in the absence of hole‐transporting material. Meanwhile, due to the inert chemical nature and the micrometer‐sized film thickness (≈10 μm), carbon electrodes can prolong the stability of PSCs. These merits are appealing for the commercialization of PSCs. However, the efficiency of carbon‐electrode PSCs is relatively low. A gap of ≈30% remains when comparing with PSCs using evaporated metal films as the electrode. Herein, the progresses in the efficiency of the four kinds of carbon‐electrode based PSCs (mesoscopic, embedment, planar, and quasi‐planar) are reviewed and compared to metal‐electrode based PSCs. Then, the role of bulk engineering and interface engineering in the progress of efficiency is discussed. Finally, outlooks are described in accordance with the discussions.

Here, studies on regulation of the interfacial charge balance in SnO₂‐based planar perovskite solar cells are reported. SnO₂ with optimum thickness exhibits enhanced charge balance. Moreover, trap‐assisted carrier recombination is significantly suppressed by using diethylenetriaminepentaacetic acid as a passivator. As a result, the champion device demonstrates a promising efficiency of 21.28% with negligible hysteresis and much improved environmental stability.

Control of dynamics at the electron transport layer–perovskite interface, such as charge transfer and recombination, is essential in achieving high‐efficiency planar perovskite solar cells (PSCs). Herein, it was observed that the trade‐off between unfavorable electron transport of a thick SnO₂ film and serious electron recombination at thin SnO₂ film/perovskite interfaces is essential for the performance of SnO₂‐based planar PSCs. The optimized efficiency of devices beyond 20% is obtained by using a two‐step deposition of SnO₂. Moreover, trap‐assisted carrier recombination is significantly suppressed by using the diethylenetriaminepentaacetic acid passivator via the formation of coordination with undercoordinated Sn and Pb²⁺ ions. As a result, the champion device demonstrates a promising efficiency of 21.28% with negligible hysteresis and much improved environmental stability, i.e., retaining 98% of the initial efficiency under ambient atmosphere over 1000 h.

The large grains and high crystallinity of Pb(Ac)₂‐doped α‐CsPbI₂Br active layers with CsBr passivation is realized by a two‐step annealing process. The corresponding planar all‐inorganic CsPbI₂Br perovskite solar cells exhibit a long‐term ultrahigh power conversion efficiency of 16.37%, with a substantially improved V _OC of 1.271 V.

All‐inorganic CsPbI₂Br perovskite has attracted increasing attention, owing to its outstanding thermal stability and suitable bandgap for optoelectronic devices. However, the substandard power conversion efficiency (PCE) and large energy loss (E _loss) of CsPbI₂Br perovskite solar cells (PSCs) caused by the low quality and high trap density of perovskite films still limit the application of devices. Herein, the post‐treatment of evaporating cesium bromide (CsBr) is utilized on top of the perovskite surface to passivate the CsPbI₂Br–hole‐transporting layer interface and reduce E _loss. The results of microzone photoluminescence indicate that the evaporated CsBr gathered at the grain boundaries of CsPbI₂Br layers and Br‐enriched perovskites (CsPbI _x Br_3−x , x < 2) are formed, which can provide protection for CsPbI₂Br. Therefore, the gaps between crystal grains are filled up, and the recombination loss of the all‐inorganic CsPbI₂Br PSCs is reduced accordingly. The champion device exhibits high open‐circuit voltage and a PCE of 1.271 V and 16.37%, respectively. This is the highest reported PCE among all‐inorganic CsPbI₂Br PSCs reported so far. In addition, the stability of CsPbI₂Br PSCs is effectively improved by CsBr passivation, and the device without encapsulation can retain 86% of its initial PCE after 1368 h of storage, which is beneficial for practical applications.

A facile method is reported for preparing α‐CsPbI₃ perovskite films at room temperature by introducing ascorbic acid (AA) in the CsPbI₃ precursor solution. The champion device not only showed a high efficiency of 11.44% but also had excellent stability, retaining more than 76% of its initial efficiency after aging in ambient conditions for 250 h without encapsulation.

The all‐inorganic α‐CsPbI₃ perovskite with superb thermal stability and suitable band gap for light harvesting has been considered as a promising candidate for efficient perovskite solar cells (PSCs). However, the photoactive black α‐CsPbI₃ is thermodynamically unstable and transforms spontaneously into nonphotoactive yellow δ‐phase at room temperature. Herein, a facile method is reported to prepare α‐CsPbI₃ perovskite films with high stability at room temperature by mixing a small amount of ascorbic acid (AA) in the CsPbI₃ precursor solutions. It is revealed that the interaction of AA with the CsPbI₃ precursors could effectively inhibit the rapid crystallization of CsPbI₃ and reduce the size of the coordination colloidal, and thus decrease the grain size of CsPbI₃ for preparing long‐term stable α‐CsPbI₃ films. The PSCs based on the AA‐stabilized CsPbI₃ films exhibit reproducible photovoltaic performance with a champion efficiency of up to 11.44% and stable output of 11.30%, along with excellent stability, retaining more than 76% of its initial efficiency after aging in ambient conditions for 250 h without encapsulation. Most importantly, such low‐cost, solution‐processable inorganic PSCs with high performance also show promising potential for large‐scale preparation.

Though great success has been achieved in perovskite solar cells (PSCs), it still suffers from several drawbacks in terms of stability and higher efficiency. Doping as effective method to modify the optical and electronic properties of the materials has been hotly studied in lead halide perovskites (LHPs). In this mini review, we discuss the Pb‐site doping in organic‐inorganic hybrid perovskites (OIH‐LHPs) and inorganic CsPbX₃ based materials. We conclude the doping has three functions toward PSCs: participating in crystalline process, modifying the energy states in LHPs and acting important role on the stability of PSCs. Issues about further improvements are raised and perspectives for further investigation are presented at last.

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A simple coalloying strategy is applied to partly substitute HC(NH₂)₂/CH₃NH₃ (FA/MA) and I⁻ in FA_0.85MA_0.15PbI₃ perovskite by Cs⁺ and Ac⁻ respectively, which is an effective way to improve the tolerance factor, crystallinity, electronic properties, and band structure of FA_0.85MA_0.15PbI₃ materials. Consequently, the coalloyed perovskite solar cells yield a champion power conversion efficiency of 21.95% with negligible hysteresis and high stability.

A simple coalloying strategy is applied to improve the efficiency and stability of FA_0.85MA_0.15PbI₃ perovskite solar cells (PSCs) by using cesium acetate (CsAc) as an additive. It is found that the simultaneous incorporation of cation (Cs⁺) and anion (Ac⁻) into the FA_0.85MA_0.15PbI₃ film is an effective approach to realize lattice contraction, grain size enlargement, photoelectric properties improvement, band structure modulation, and therefore the optimization of the efficiency and stability of PSCs. At optimal CsAc alloying, the FA_0.85MA_0.15PbI₃ PSCs achieve a maximum power conversion efficiency (PCE) of 21.95% and an average of over 21%. In addition, the alloyed PSCs retain 97% of their initial PCE values after aging for 55 days in air without encapsulation.

An interconnected SnO₂ thin film (composed of presynthesized SnO₂ nanocrystals interconnected by amorphous phase SnO _x ) is proposed as an electron transport layer for efficient flexible perovskite solar cells. The interconnected SnO₂ thin film enables fast electron extraction from the perovskite layer and retards nonradiative charge carrier recombination. Corresponding flexible solar cells demonstrate a power conversion efficiency as high as 16.29%.

This study reports on interconnected SnO₂ electron transport layers (composed of presynthesized SnO₂ nanocrystals interconnected by amorphous phase SnO _x ) processed at low temperature (120 °C) for highly efficient flexible perovskite solar cells. Herein, the amorphous phase SnO _x serves as an effective binder to connect the SnO₂ nanocrystals to obtain ultra‐smooth electron transport layers. Further characterization of the charge carrier kinetics at the perovskite/electron transport layer interface confirms that the interconnected SnO₂ nanocrystals layer facilitates electron extraction and retards nonradiative charge carrier recombination. Consequently, a power conversion efficiency of 16.29% is achieved for flexible perovskite solar cells using the interconnected SnO₂ electron transport layer on indium tin oxide/polyethylene terephthalate substrates.

Herein, a simple cation exchange growth (CEG) method is demonstrated that replaces the organic MA⁺ cation with Cs⁺ to produce a high‐quality black γ‐phase CsPbI₃ perovskite device, enhancing both power conversion efficiency and stability. As a result, the device fabricated using the optimized CEG method yields efficiency up to 14.1%.

Abstract

Inorganic lead halide perovskites have attracted attention due to their tolerance to higher processing temperature and higher bandgap suitable for tandem solar cell application. Not only do they improve cell stability and efficiency, they also reveal many interesting and un‐anticipated material qualities. This work reports a simple cation exchange growth (CEG) method for fabricating inorganic high‐quality cesium lead iodide (CsPbI₃) by adding methylammonium iodide (MAI) additive in the precursor. X‐ray diffraction results reveal a multi‐stage film formation process whereby i) MAPbI₃ perovskite first formed that acts as a perovskite template for ii) subsequent ion exchange whereby the MA⁺ ions in the MAPbI₃ are replaced by Cs⁺ (as temperature ramps up) and iii) form g‐phase perovskite CsPbI₃. Optical microscopy, photoluminescence, and electrical characterizations reveal that the CEG process produces high‐quality film with better absorption, uniform and dense film with better interface, lower defects, and better stability. Using the CEG approach, the power conversion efficiency of the best CsPbI₃ solar cell is significantly increased up to 14.1% for the device fabricated using 1.0 m MAI additive. The outcome is beneficial for further improvement of inorganic perovskite solar cells and their application in perovskite‐silicon tandem devices.

Photocurrent–voltage hysteresis in perovskite solar cells (PSCs) induced by ion migration combined with nonradiative recombination near the interface depends on perovskite composition and device structure. Among the methods used in the attempt to reduce the hysteresis, potassium‐ion doping is found to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition.

Abstract

Current‐density–voltage (J–V) hysteresis in perovskite solar cells (PSCs) is a critical issue because it is related to power conversion efficiency and stability. Although parameters affecting the hysteresis have been already reported and reviewed, little investigation is reported on scan‐direction‐dependent J–V curves depending on perovskite composition. This review investigates J–V hysteric behaviors depending on perovskite composition in normal mesoscopic and planar structure. In addition, methodologies toward hysteresis‐free PSCs are proposed. There is a specific trend in hysteresis in terms of J–V curve shape depending on composition. Ion migration combined with nonradiative recombination near interfaces plays a critical role in generating hysteresis. Interfacial engineering is found to be an effective method to reduce the hysteresis; however, bulk defect engineering is the most promising method to remove the hysteresis. Among the studied methods, KI doping is proved to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition. It is proposed from the current studies that engineering of perovskite film near the electron transporting layer (ETL) and the hole transporting layer (HTL) is of vital importance for achieving hysteresis‐free PSCs and extremely high efficiency.

Vacuum‐deposited methylammonium lead iodide can adopt a perovskite structure with a stable cubic lattice at room temperature. Reducing the metallic salt evaporation rate leads to a tetragonal phase structure. This room‐temperature cubic perovskite circumvents the tetragonal to cubic phase transition resulting at ≈55 °C, and leads to photovoltaic devices with efficiencies above 19%.

Abstract

Methylammonium lead triiodide (MAPI) has emerged as a high‐performance photovoltaic material. Common understanding is that at room temperature, it adopts a tetragonal phase and it only converts to the perfect cubic phase around 50–60 °C. Most MAPI films are prepared using a solution‐based coating process, yet they can also be obtained by vapor‐phase deposition methods. Vapor‐phase‐processed MAPI films have significantly different characteristics than their solvent‐processed analogous, such as relatively small crystal‐grain sizes and short excited‐state lifetimes. However, solar cells based on vapor‐phase‐processed MAPI films exhibit high power‐conversion efficiencies. Surprisingly, after detailed characterization it is found that the vapor‐phase‐processed MAPI films adopt a cubic crystal structure at room temperature that is stable for weeks, even in ambient atmosphere. Furthermore, it is demonstrated that by tuning the deposition rates of both precursors during codeposition it is possible to vary the perovskite phase from cubic to tetragonal at room temperature. These findings challenge the common belief that MAPI is only stable in the tetragonal phase at room temperature.

An efficient and neutral colored semitransparent organic photovoltaic cell (ST‐OPV) is realized by utilizing a near‐infrared (NIR) absorbing ternary cell combined with a thin, semitransparent, high‐conductivity Cu–Ag alloy electrode. A combination of optical outcoupling and antireflection coatings leads to enhanced visible transmission, while reflecting the NIR back into the cell where it is absorbed.

Abstract

Building‐integrated photovoltaics employing transparent photovoltaic cells on window panes provide an opportunity to convert solar energy to electricity rather than generating waste heat. Semitransparent organic photovoltaic cells (ST‐OPVs) that utilize a nonfullerene acceptor‐based near‐infrared (NIR) absorbing ternary cell combined with a thin, semitransparent, high conductivity Cu‐Ag alloy electrode are demonstrated. A combination of optical outcoupling and antireflection coatings leads to enhanced visible transmission, while reflecting the NIR back into the cell where it is absorbed. This combination of coatings results in doubling of the light utilization efficiency (LUE), which is equal to the product of the power conversion efficiency (PCE) and the average photopic transparency, compared with a conventional semitransparent cell lacking these coatings. A maximum LUE = 3.56 ± 0.11% is achieved for an ST‐OPV with a PCE = 8.0 ± 0.2% at 1 sun, reference AM1.5G spectrum. Moreover, neutral colored ST‐OPVs are also demonstrated, with LUE = 2.56 ± 0.2%, along with Commission Internationale d'Eclairage chromaticity coordinates of CIE = (0.337, 0.349) and a color rendering index of CRI = 87.