Chen Weijie

Interfacial phenomena play a critical role in organic electronic devices. Scanning transmission electron microscopy nanoanalysis and secondary ion mass spectrometry measurements on high‐voltage organic tandem solar cells comprising a fluorinated absorber material and an aluminum‐containing recombination layer reveal the accumulation of fluorine in the recombination layer.

Abstract

The choice of the optimum combination of materials for the absorber layers, electrodes, as well as interfacial layers is highly important to enhance further advances in the field of organic photovoltaics. Usually, these materials are assumed to be stable under the applied processing steps for the fabrication of the solar cells. Herein, organic tandem solar cells are examined with fluorine‐containing absorber layers consisting of the fluorinated polymer donor PTB7‐Th and the indacenodithiophene‐type small molecule acceptor O‐IDTBR and MoO₃/Al/PFN‐Br as recombination layer. Although both subcells comprise the same low bandgap absorber materials, the tandem solar cells reveal high open‐circuit voltage values approaching 2 V. However, using a combination of scanning transmission electron microscopy nanoanalysis techniques and secondary ion mass spectrometry with depth profiling, an unexpected phenomenon is disclosed. It is found that significant amounts of fluorine are accumulated in the recombination layer region which originates very likely from alumina–aryl fluoride interactions responsible for a partial defluorination of the conjugated polymer in the absorber layer.

Thionation is a straightforward strategy to dramatically boost the performance of dopant‐free polymeric hole‐transporting materials (HTMs) for perovskite solar cells. Upon HTM thionation, a nearly 40% enhancement in the power conversion efficiency of the corresponding devices is observed. Such an increase is attributed to the enhancement of both the hole transport within the HTM and the interfacial hole transfer dynamics.

Abstract

To date, the most efficient perovskite solar cells (PSCs) require hole‐transporting materials (HTMs) that are doped with hygroscopic additives to improve their performance. Unfortunately, such dopants negatively impact the overall PSCs stability and add cost and complexity to the device fabrication. Hence, there is a need to investigate new strategies to boost the typically modest performance of dopant‐free HTMs for efficient and stable PSCs. Thionation is a simple and single‐step approach to enhance the carrier‐transport capability of organic semiconductors, yet still completely unexplored in the context of HTMs for PSCs. In this work, a novel polymeric semiconductor, P1, based on a diketopyrrolopyrrole (DPP) moiety, is proposed as a dopant‐free HTM. Its modest performance in PSCs (power conversion efficiency (PCE) = 7.1%) is significantly enhanced upon thionation of the DPP moiety. The resulting dithioketopyrrolopyrrole‐based HTM, P2, leads to PSCs with nearly 40% performance improvement (PCE = 9.7%) compared to devices based on the nonthionated HTM (P1). Furthermore, thionation also remarkably boosts the shelf‐storage and thermal stability with respect to traditional 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene‐based PSCs. This work provides useful insights to further design effective dopant‐free HTMs employing the straightforward one‐step thionation strategy for efficient and stable PSCs.

Small‐molecule electrolyte (C6‐E‐OTs) hybridized ZnO layer is provided as the electron transporting layer. The device based on the blend of PTB7 and PC₇₁BM as the active layer shows an enhanced power conversion efficiency (PCE) from 7.6% based on ZnO to 8.8% using the C6‐E‐OTs hybridized ZnO layer. Hybridized ZnO layer process can overcome the limitation faced by the thickness tolerance of interlayer.

Abstract

Small‐molecule electrolyte (C6‐E‐OTs) hybridized ZnO layer is provided as the electron transporting layer. The device based on the blend of PTB7 and PC₇₁BM as the active layer shows an enhanced power conversion efficiency (PCE) from 7.6% based on ZnO to 8.8% using the C6‐E‐OTs hybridized ZnO layer. The device can be further improved by simultaneously using a C6‐E‐OTs hybridized ZnO layer and a 5 nm thick C6‐E‐OTs as the interlayer. The synergy effect of hybridization and interlayer enhanced the PCE of the device to 8.9%, which is a 17.1% increase in comparison with the device based on ZnO. The presence of C6‐E‐OTs hybridized ZnO and a 5 nm of C6‐E‐OTs as the interlayer in the device with PTB7‐Th as the donor significantly improves the PCE from 8.0% based on ZnO to 9.4%, resulting in a 17.5% enhancement. Main contribution for enhancing the PCE of the device is the improved J _sc, which results from the reduction of energy offset at the cathode interface. Thus, hybridized ZnO layer process can overcome the limitation faced by the thickness tolerance of interlayer.

Interfacial phenomena play a critical role in organic electronic devices. Scanning transmission electron microscopy nanoanalysis and secondary ion mass spectrometry measurements on high‐voltage organic tandem solar cells comprising a fluorinated absorber material and an aluminum‐containing recombination layer reveal the accumulation of fluorine in the recombination layer.

Abstract

The choice of the optimum combination of materials for the absorber layers, electrodes, as well as interfacial layers is highly important to enhance further advances in the field of organic photovoltaics. Usually, these materials are assumed to be stable under the applied processing steps for the fabrication of the solar cells. Herein, organic tandem solar cells are examined with fluorine‐containing absorber layers consisting of the fluorinated polymer donor PTB7‐Th and the indacenodithiophene‐type small molecule acceptor O‐IDTBR and MoO₃/Al/PFN‐Br as recombination layer. Although both subcells comprise the same low bandgap absorber materials, the tandem solar cells reveal high open‐circuit voltage values approaching 2 V. However, using a combination of scanning transmission electron microscopy nanoanalysis techniques and secondary ion mass spectrometry with depth profiling, an unexpected phenomenon is disclosed. It is found that significant amounts of fluorine are accumulated in the recombination layer region which originates very likely from alumina–aryl fluoride interactions responsible for a partial defluorination of the conjugated polymer in the absorber layer.

2D materials have shown great potential for use as photovoltaic materials owing to their outstanding properties. The application of a wide variety of emerging 2D materials for efficient, scalable, and stable perovskite solar cells is reviewed. Interface engineering, energy level alignment, film morphology control, instability issues, hysteresis phenomena, and other key factors are discussed.

Abstract

Perovskite solar cells (PSCs) are now at the forefront of the state‐of‐the‐art photovoltaic technologies due to their high efficiency and low fabrication costs. To further realize the potential of this fascinating class of solar cells, nanostructured functional materials have been playing important roles. 2D layered materials have attracted a great deal of interest due to their fascinating properties and unique structure. Recently, the exploration of a wide range of novel 2D materials for use in PSCs has seen considerable progress, but still a lot remains to be done in this field. In this progress report, the advancements that have recently been made in the application of these emerging 2D materials, beyond graphene, for PSCs are presented. Both the advantages and challenges of these 2D materials for PSCs are highlighted. Finally, important directions for the future advancements toward efficient, low‐cost, and stable PSCs are outlined.

The interface and bulk defects of perovskite solar cells are distinguished and quantified, and are for the first time traced in situ using an expanded admittance model. A fullerene derivative [6, 6]‐phenyl‐C61‐butyric acid (PCBA) is introduced into the TiO₂/perovskite interface to release the interface stress.

Abstract

The stability issue that is obstructing commercialization of the perovskite solar cell is widely recognized, and tremendous effort has been dedicated to solving this issue. However, beyond the apparent thermal and moisture stability, more intrinsic semiconductor mechanisms regarding defect behavior have yet to be explored and understood. Herein, defects are quantified; especially interface defects, within the cell to reveal their impact on device performance and especially stability. Both the bulk and interface defects are distinguished and traced in situ using an expanded admittance model when the cell degrades in its efficiency under illumination or voltage. The electric field‐induced interface, rather than bulk defects, is found to have a direct correlation to stability. Releasing the interface strain using a fullerene derivative is an effective way to suppress interface defect formation and improve stability. Overall, this work provides a quantitative approach to probing the semiconductor mechanism behind the stability issue, and the inherent correlation discovered here among the electric field, interface strain, interface defects, and cell stability has important implications for ongoing device stability engineering.

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.

Rapid alternative deposition provides new approaches for Cs‐based perovskite fabrication. The thousand‐layer CsPbI₂Br thin films obtained through this method exhibit a smooth surface and high crystallinity, and the solar cell devices deliver excellent performance under both 1‐sun solar and fluorescent light illumination.

Abstract

The novel growth of cesium lead halide perovskite thin films, which are prepared through thousand‐layer rapid alternative deposition, is performed by developing an active perovskite film consisting of a layer‐by‐layer structure. This method is considerably more difficult to be implemented from the solution process. The obtained thin film morphology and characteristics are distinguished from that of the traditional a few layers and two‐material codeposition. These alternative deposited perovskites are integrated with vacuum‐deposited carrier‐transporting layers and electrodes, and all vacuum‐sublimed perovskite solar cells exhibit an outstanding power conversion efficiency of 13.0%. The use of these devices for environmental light energy harvesting provides a power conversion efficiency of 33.9% under fluorescent light illumination of 1000 lux.

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.

Calculations of Shockley–Queisser limits for perovskite solar cells under artificial light sources reveal the existence of an unusual zone, in which the bandgaps (E _g) of commonly used perovskite materials are too small to harvest photonic energy efficiently. Accordingly, increasing the value of E _g of the perovskite solar cell, by incorporating Br⁻ ions, improves the power conversion efficiencies under indoor lighting conditions.

Abstract

Indoor photovoltaics (IPVs) are attracting renewed interest because they can provide sustainable energy through the recycling of photon energy from household lighting facilities. Herein, the Shockley–Queisser model is used to calculate the upper limits of the power conversion efficiencies (PCEs) of perovskite solar cells (PeSCs) for two types of artificial light sources: fluorescent tubes (FTs) and white light–emitting diodes (WLEDs). An unusual zone is found in which the dependence of the PCEs on the bandgap (E _g) under illumination from the indoor lighting sources follows trends different from that under solar irradiation. In other words, IPVs exhibiting high performance under solar irradiation may not perform well under indoor lighting conditions. Furthermore, the ideal bandgap energy for harvesting photonic power from these indoor lighting sources is ≈1.9 eV—a value higher than that of common perovskite materials (e.g., for CH₃NH₃PbI₃). Accordingly, Br⁻ ions are added into the perovskite films to increase their values of E _g. A resulting PeSC featuring a wider bandgap exhibits PCEs of 25.94% and 25.12% under illumination from an FT and a WLED, respectively. Additionally, large‐area (4 cm²) devices are prepared for which the PCE reaches ≈18% under indoor lighting conditions.

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.

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.

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.

Alkali metals are reported to influence the device performance of Cu(In,Ga)Se₂ (CIGSe) solar cells substantially. The present work shows that although Na and K are known to segregate to grain boundaries (GBs) in polycrystalline CIGSe thin films, no indications can be found that these alkali metals exhibit any passivating effect on GBs.

Thin‐film solar cells based on Cu(In,Ga)Se₂ (CIGSe) absorber layers reach conversion efficiencies of above 20%. One key to this success is the incorporation of alkali metals, such as Na and K, into the surface and the volume of the CIGSe thin film. The present work discusses the impact of Na and K on the grain‐boundary (GB) properties in CIGSe thin films, i.e., on the barriers for charge carriers, Φ_b, and on the recombination velocities at the GBs, s _GB. First, the physics connected with these two quantities as well as their impact on the device performance are revised, and then the values for the barrier heights and recombination velocities are provided from the literature. The s _GB values are measured by means of a cathodoluminescence analysis of Na‐/K‐free CIGSe layers as well as on CIGSe layers on Mo/sapphire substrates, which are submitted to only NaF or only KF postdeposition treatments. Overall, passivating effects on GBs by neither Na nor K can be confirmed. The GB recombination velocities seem to remain on the same order of magnitude, in average about 10³–10⁴ cm s⁻¹, irrespective of whether CIGSe thin films are Na‐/K‐free or Na‐/K‐containing.

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.

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.

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 comprehensive theoretical analysis of two‐terminal and four‐terminal perovskite/copper indium gallium selenide (CIGS) tandem solar cells is investigated from optical and electrical aspects. According to different optical absorptions, the current matching points of different halide components are obtained. Under the condition of current matching, an optimal performance up to 31.13% can be obtained by using two‐terminal CH₃NH₃PbI₂Br/CIGS tandem structure.

Perovskite/copper indium gallium selenide (CIGS) tandem solar cells represent an attractive configuration to obtain ultrahigh efficiency. A detailed theoretical analysis is crucial for further improving the performance of tandem solar cells. Herein, four‐terminal and two‐terminal perovskite/CIGS tandem solar cells are intensively researched. For four‐terminal perovskite/CIGS tandem solar cell, the optimal thicknesses of CH₃NH₃PbI₃ and CIGS are 0.5 and 3 μm, respectively, according to the simulation result. Reducing the thickness of TiO₂ and Spiro‐OMeTAD can minimize the short‐wavelength parasitic absorption and long‐wavelength parasitic absorption, respectively. Meanwhile, using antireflection coating, such as 100 nm MgF₂, is beneficial to increase the photon absorption. For two‐terminal perovskite/CIGS tandem solar cells, the thicknesses of perovskite and CIGS are tuned to meet the current matching. To further improve the efficiency of two‐terminal tandem cells, FTO thickness is reduced to minimize reflection, and the optimal doping concentration of CIGS (1 × 10¹⁸ cm⁻³) is used. In addition, results show that the quality of perovskite films should be improved by enlarging the grain size to decrease the trap states at grain boundary. Finally, the optimal efficiency of two‐terminal CH₃NH₃PbI₂Br/CIGS tandem solar cells reaches 31.13%.

Perovskite solar cells are very promising for their high efficiency and solution‐process feasibility. Herein, some fabrication methods for gaining a high‐quality perovskite layer with long‐term stability are reviewed. These approaches significantly enhance the stability of perovskites, which makes it applicable for commercialization. However, these methods have some issues and it still leaves much room for further optimization.

Organic–inorganic hybrid perovskites (OIHPs) are one of the hottest fields on account of their immense potential for photovoltaics. As one of the most promising OIHPs, formamidinium (FA)‐based perovskites have been developed very fast in the past few years. The power conversion efficiency (PCE) has reached certified 24.2%, which is comparable with that of monocrystalline silicon solar cells. However, the easy formation of nonperovskite δ‐phase formamidinium lead triiodide (FAPbI₃) at a low temperature needs to be solved when fabricating a high‐quality light absorber layer. Several strategies have been used to avoid the formation of δ‐phase FAPbI₃ and improve phase stability in recent years such as tolerance factor adjustment, dimensional engineering, addictive processing, interfacial modification, defects passivation, and in situ growth. These approaches can enhance the phase stability to some extent; however, their contribution to long‐term stability and especially their real mechanism is still unknown. Herein, the relationships among the tolerance factors, the structure of FAPbI₃, and the phase transition phenomenon are summarized. In addition, various methodologies and potential mechanisms for stabilizing α‐phase FAPbI₃ at room temperature (RT) are discussed. In conclusion, a series of challenges in the popular processings of perovskite solar cells and their corresponding solutions that help achieve commercialization faster are summarized.

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

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.

The photochemistry of halide‐related defects affects the optoelectronic properties of lead–halide perovskite semiconductors and their reactivity to external stimuli such as light and environmental molecules.

Abstract

The presence of various types of chemical interactions in metal‐halide perovskite semiconductors gives them a characteristic “soft” fluctuating structure, prone to a wide set of defects. Understanding of the nature of defects and their photochemistry is summarized, which leverages the cooperative action of density functional theory investigations and accurate experimental design. This knowledge is used to describe how defect activity determines the macroscopic properties of the material and related devices. Finally, a discussion of the open questions provides a path towards achieving an educated prediction of device operation, necessary to engineer reliable devices.

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.

Nature Photonics, Published online: 19 August 2019; doi:10.1038/s41566-019-0505-4

Nature Energy, Published online: 19 August 2019; doi:10.1038/s41560-019-0448-5

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