shuhui

Large energy loss has been a major obstacle for further efficiency improvement of inorganic perovskite solar cells. This review provides a basic understanding of energy loss and may inspire new designs or more impactful methods for further minimizing the energy loss of inorganic perovskite solar cells.

Abstract

Though the cesium‐based inorganic perovskite solar cells (IPSCs) have developed rapidly in recent two years, the power conversion efficiency (PCE) is still far away from the Shockley–Queisser limit due to the large open‐circuit voltage (V _oc) deficit, which results from the large energy loss (E _loss). Large E _loss has been a major obstacle for further efficiency improvement of IPSCs. In this review, the authors, for the first time, focus on investigating the E _loss of IPSCs and start from discussing the essence and origin of the E _loss. Then, the reported efficient methods for reducing the band tails and energy disorder are systematically summarized and reviewed, including crystallization optimization, defect passivation, and interface engineering. Finally, the authors offer an overall perspective on managing E _loss in IPSCs and point out the possible ways to reduce the E _loss and promote the efficiency. This review provides a basic understanding of E _loss and may inspire new designs or more impactful methods for further minimizing the E _loss of IPSCs.

The role of DMAI in fabricating high quality CsPbI₃ inorganic perovskite thin films is demonstrated to be a volatile crystal growth additive rather than dopant. With optimal DMAI additive and PTACl passivation, a PTACl‐CsPbI₃ based champion photovoltaic device exhibits a record efficiency of 19.03 %.

Abstract

The controllable growth of CsPbI₃ perovskite thin films with desired crystal phase and morphology is crucial for the development of high efficiency inorganic perovskite solar cells (PSCs). The role of dimethylammonium iodide (DMAI) used in CsPbI₃ perovskite fabrication was carefully investigated. We demonstrated that the DMAI is an effective volatile additive to manipulate the crystallization process of CsPbI₃ inorganic perovskite films with different crystal phases and morphologies. The thermogravimetric analysis results indicated that the sublimation of DMAI is sensitive to moisture, and a proper atmosphere is helpful for the DMAI removal. The time‐of‐flight secondary ion mass spectrometry and nuclear magnetic resonance results confirmed that the DMAI additive would not alloy into the crystal lattice of CsPbI₃ perovskite. Moreover, the DMAI residues in CsPbI₃ perovskite can deteriorate the photovoltaic performance and stability. Finally, the PSCs based on phenyltrimethylammonium chloride passivated CsPbI₃ inorganic perovskite achieved a record champion efficiency up to 19.03 %.

Application of the proposed slow post‐annealing for layered 2D perovskite solar cells based on BA₂MA₃Pb₄I₁₃ photo‐absorber leads to a favorable alignment on the multi‐perovskite phases and resultant champion power conversion efficiency to 17.26%, showing simultaneously enhanced open‐circuit voltage and short‐circuit current.

Abstract

Layered Ruddlesden–Popper (RP) phase (2D) halide perovskites have attracted tremendous attention due to the wide tunability on their optoelectronic properties and excellent robustness in photovoltaic devices. However, charge extraction/transport and ultimate power conversion efficiency (PCE) in 2D perovskite solar cells (PSCs) are still limited by the non‐eliminable quantum well effect. Here, a slow post‐annealing (SPA) process is proposed for BA₂MA₃Pb₄I₁₃ (n = 4) 2D PSCs by which a champion PCE of 17.26% is achieved with simultaneously enhanced open‐circuit voltage, short‐circuit current, and fill factor. Investigation with optical spectroscopy coupled with structural analyses indicates that enhanced crystal orientation and favorable alignment on the multiple perovskite phases (from the 2D phase near bottom to quasi‐3D phase near top regions) is obtained with SPA treatment, which promotes carrier transport/extraction and suppresses Shockley–Read–Hall charge recombination in the solar cell. As far as it is known, the reported PCE is so far the highest efficiency in RP phase 2D PSCs based on butylamine (BA) spacers (n = 4). The SPA‐processed devices exhibit a satisfactory stability with <4.5% degradation after 2000 h under N₂ environment without encapsulation. The demonstrated process strategy offers a promising route to push forward the performance in 2D PSCs toward realistic photovoltaic applications.

Publication date: 20 November 2019

Source: Joule, Volume 3, Issue 11

Author(s): Enbing Bi, Wentao Tang, Han Chen, Yanbo Wang, Julien Barbaud, Tianhao Wu, Weiyu Kong, Peng Tu, Hong Zhu, Xiaoqin Zeng, Jinjin He, Shin-ichi Kan, Xudong Yang, Michael Grätzel, Liyuan Han

Context & Scale

Summary

Graphical Abstract

Recent progress of inorganic perovskite materials and photovoltaic solar cells is summarized, including materials design, methods for preparing high‐quality perovskite films, phase instabilities, nanocrystals, quantum dots, lead‐free perovskites, device process, and upscaling. In addition, the energy loss mechanisms within the device are discussed and relevant methods are proposed accordingly.

Abstract

All‐inorganic perovskites are considered to be one of the most appealing research hotspots in the field of perovskite photovoltaics in the past 3 years due to their superior thermal stability compared to their organic–inorganic hybrid counterparts. The power‐conversion efficiency has reached 17.06% and the number of important publications is ever increasing. Here, the progress of inorganic perovskites is systematically highlighted, covering materials design, preparation of high‐quality perovskite films, and avoidance of phase instabilities. Inorganic perovskites, nanocrystals, quantum dots, and lead‐free compounds are discussed and the corresponding device performances are reviewed, which have been realized on both rigid and flexible substrates. Methods for stabilization of the cubic phase of low‐bandgap inorganic perovskites are emphasized, which is a prerequisite for highly efficient and stable solar cells. In addition, energy loss mechanisms both in the bulk of the perovskite and at the interfaces of perovskite and charge selective layers are unraveled. Reported approaches to reduce these charge‐carrier recombination losses are summarized and complemented by methods proposed from our side. Finally, the potential of inorganic perovskites as stable absorbers is assessed, which opens up new perspectives toward the commercialization of inorganic perovskite solar cells.

Low‐cost and efficient organic small molecules are desired as hole transporting materials for high‐performance perovskite solar cells. Two new molecules containing a benzodithiophene core and triphenylamine side chains are synthesized from cheap starting materials by a simple and low‐cost method. Lead‐free, tin‐based perovskite solar cells employing these new benzodithiophene‐based hole transporting materials achieve good efficiencies.

Abstract

Developing efficient interfacial hole transporting materials (HTMs) is crucial for achieving high‐performance Pb‐free Sn‐based halide perovskite solar cells (PSCs). Here, a new series of benzodithiophene (BDT)‐based organic small molecules containing tetra‐ and di‐triphenyl amine donors prepared via a straightforward and scalable synthetic route is reported. The thermal, optical, and electrochemical properties of two BDT‐based molecules are shown to be structurally and energetically suitable to serve as HTMs for Sn‐based PSCs. It is reported here that ethylenediammonium/formamidinium tin iodide solar cells using BDT‐based HTMs deliver a champion power conversion efficiency up to 7.59%, outperforming analogous reference solar cells using traditional and expensive HTMs. Thus, these BDT‐based molecules are promising candidates as HTMs for the fabrication of high‐performance Sn‐based PSCs.

Laminated perovskite layers with different crystal sizes and optical and electrical characteristics are achieved by using aniline as the solvent in the perovskite precursor solution. Inverted planar perovskite solar cells with the laminated films as active layers achieve an average power conversion efficiency of 20.65%, originating from the high V _OC 1.112 V and fill factor of 80.8%.

Abstract

Laminated multilayers of perovskite films with different optical and electronic characteristics will easily realize high‐performance optoelectronic devices because it is widely demonstrated that differential distribution of film properties in the vertical direction of devices plays particularly important roles in device performance. However, the existing laminated perovskite films are hardly prepared by a solution process because there is no solvent with sufficient selectivity of solubility for different perovskite materials. Here, it is demonstrated that aniline (AN) has a largely different solubility toward the perovskite MAPbI₃ and the MAPbI₃ blend with an additive of hydrochloride diethylammonium chloride. By using AN as the solvent in the perovskite precursor solution, two laminated perovskite layers with different crystal size and optical and electrical characteristics are achieved. Inverted perovskite solar cells with the laminated films as active layers achieve an averaged power conversion efficiency of 20.65% originating from the high V _OC 1.112 V and fill factor of 80.8%. The devices maintain 98% efficiency after 400 h under 65% RH. This work provides a very simple and feasible method for production of laminated perovskite films to achieve high‐performance perovskite solar cells.

Nature Materials, Published online: 09 September 2019; doi:10.1038/s41563-019-0478-1

All inorganic: The crystal structure, phase transition, band gap and optical properties of perovskite‐phase RbPbBr₃ were analyzed theoretically and experimentally. This new perovskite microsphere can serve as a gain medium and microcavity to achieve broadband (475–540 nm) single‐mode lasing with a high Q of about 2100.

Abstract

Rubidium lead halides (RbPbX₃), an important class of all‐inorganic metal halide perovskites, are attracting increasing attention for photovoltaic applications. However, limited by its lower Goldschmidt tolerance factor t≈0.78, all‐inorganic RbPbBr₃ has not been reported. Now, the crystal structure, X‐ray diffraction (XRD) pattern, and band structure of perovskite‐phase RbPbBr₃ has now been investigated. Perovskite‐phase RbPbBr₃ is unstable at room temperature and transforms to photoluminescence (PL)‐inactive non‐perovskite. The structural evolution and mechanism of the perovskite–non‐perovskite phase transition were clarified in RbPbBr₃. Experimentally, perovskite‐phase RbPbBr₃ was realized through a dual‐source chemical vapor deposition and annealing process. These perovskite‐phase microspheres showed strong PL emission at about 464 nm. This new perovskite can serve as a gain medium and microcavity to achieve broadband (475–540 nm) single‐mode lasing with a high Q of about 2100.

Recent progress of inorganic perovskite materials and photovoltaic solar cells is summarized, including materials design, methods for preparing high‐quality perovskite films, phase instabilities, nanocrystals, quantum dots, lead‐free perovskites, device process, and upscaling. In addition, the energy loss mechanisms within the device are discussed and relevant methods are proposed accordingly.

Abstract

All‐inorganic perovskites are considered to be one of the most appealing research hotspots in the field of perovskite photovoltaics in the past 3 years due to their superior thermal stability compared to their organic–inorganic hybrid counterparts. The power‐conversion efficiency has reached 17.06% and the number of important publications is ever increasing. Here, the progress of inorganic perovskites is systematically highlighted, covering materials design, preparation of high‐quality perovskite films, and avoidance of phase instabilities. Inorganic perovskites, nanocrystals, quantum dots, and lead‐free compounds are discussed and the corresponding device performances are reviewed, which have been realized on both rigid and flexible substrates. Methods for stabilization of the cubic phase of low‐bandgap inorganic perovskites are emphasized, which is a prerequisite for highly efficient and stable solar cells. In addition, energy loss mechanisms both in the bulk of the perovskite and at the interfaces of perovskite and charge selective layers are unraveled. Reported approaches to reduce these charge‐carrier recombination losses are summarized and complemented by methods proposed from our side. Finally, the potential of inorganic perovskites as stable absorbers is assessed, which opens up new perspectives toward the commercialization of inorganic perovskite solar cells.

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.

A power conversion efficiency of 13.36% in ternary organic photovoltaics is obtained by carefully picking materials with good compatibility and complementary absorption spectra, as well as well‐matched energy levels with efficient energy transfer.

Abstract

Organic photovoltaics (OPVs) are fabricated with PM6 as donor and T6Me, IT‐2F, or their mixture as acceptor. A 13.36% power conversion efficiency (PCE) is achieved from the optimized ternary OPVs with 50 wt% IT‐2F in acceptors, which is attributed to the enhanced photon harvesting of ternary active layers and improved exciton utilization efficiency through energy transfer from IT‐2F to T6Me. The efficient energy transfer from IT‐2F to T6Me can be confirmed from the photoluminescence spectra of neat and blend films, which may provide additional channels to enhance exciton utilization efficiency for achieving short‐circuit current density (J _SC) improvement of ternary OPVs. It should be highlighted that the fill factor (FF) of ternary OPVs can be monotonously increased along with the incorporation of IT‐2F, indicating the gradually optimized phase separation degree of ternary active layers. The third component IT‐2F plays a key role in optimizing phase separation as a morphology regulator. Over 8% PCE improvement is achieved in the optimized ternary OPVs compared with the over 12% PCEs of the corresponding binary OPVs, respectively. This work indicates that the performance of ternary OPVs can be well optimized by carefully picking materials with good compatibility and complementary absorption spectra, as well as the appropriate energy levels.

A series of wide bandgap donor polymers are designed and synthesized by incorporating a monothiophene functionalized with both a fluorine atom and an ester group. Fabricated from nonhalogenated solvent, power conversion efficiencies of 11.39% and 12.11% are achieved for binary and ternary nonfullerene solar cells, respectively.

Abstract

Significant progress has been made in nonfullerene small molecule acceptors (NF‐SMAs) that leads to a consistent increase of power conversion efficiency (PCE) of nonfullerene organic solar cells (NF‐OSCs). To achieve better compatibility with high‐performance NF‐SMAs, the direction of molecular design for donor polymers is toward wide bandgap (WBG), tailored properties, and preferentially ecofriendly processability for device fabrication. Here, a weak acceptor unit, methyl 2,5‐dibromo‐4‐fluorothiophene‐3‐carboxylate (FE‐T), is synthesized and copolymerized with benzo[1,2‐b:4,5‐b′]dithiophene (BDT) to afford a series of nonhalogenated solvent processable WBG polymers P1‐P3 with a distinct side chain on FE‐T. The incorporation of FE‐T leads to polymers with a deep highest occupied molecular orbital (HOMO) level of −5.60−5.70 eV, a complementary absorption to NF‐SMAs, and a planar molecular conformation. When combined with the narrow bandgap acceptor ITIC‐Th, the solar cell based on P1 with the shortest methyl chain on FE‐T achieves a PCE of 11.39% with a large V _oc of 1.01 V and a J _sc of 17.89 mA cm⁻². Moreover, a PCE of 12.11% is attained for ternary cells based on WBG P1, narrow bandgap PTB7‐Th, and acceptor IEICO‐4F. These results demonstrate that the new FE‐T is a highly promising acceptor unit to construct WBG polymers for efficient NF‐OSCs.

A high power conversion efficiency of 11.76%, the best efficiency for all‐polymer solar cells, is achieved by printing fabrication based on PTzBI‐Si:N2200 processing with 2‐methyltetrahydrofuran. A Multi‐length‐scaled morphology is found in the bulk heterojunctions, which ensures fast transfer of carriers and facilitates exciton separation, and boosts carrier mobility and current density, thus improving the device performance.

Abstract

All‐polymer solar cells (all‐PSCs) exhibit excellent stability and readily tunable ink viscosity, and are therefore especially suitable for printing preparation of large‐scale devices. At present, the efficiency of state‐of‐the‐art all‐PSCs fabricated by the spin‐coating method has exceeded 11%, laying the foundation for the preparation and practical utilization of printed devices. A high power conversion efficiency (PCE) of 11.76% is achieved based on PTzBI‐Si:N2200 all‐PSCs processing with 2‐methyltetrahydrofuran (MTHF, an environmentally friendly solvent) and preparation of active layers by slot die printing, which is the top efficient for all‐PSCs. Conversely, the PCE of devices processed by high‐boiling point chlorobenzene is less than 2%. Through the study of film formation kinetics, volatile solvents can freeze the morphology in a short time, and a more rigid conformation with strong intermolecular interaction combined with the solubility limit of PTzBI‐Si and N2200 in MTHF results in the formation of a fibril network in the bulk heterojunction. The multilength scaled morphology ensures fast transfer of carriers and facilitates exciton separation, which boosts carrier mobility and current density, thus improving the device performance. These results are of great significance for large‐scale printing fabrication of high‐efficiency all‐PSCs in the future.

The spintronic properties of different hybrid organic–inorganic perovskites (HOIPs) are studied in spin valve devices, including spin diffusion length and spin lifetime, as well as the impact of the chemical components on these properties. This study aims at demonstrating promising spintronic applications of HOIPs, and providing a clear path for engineering spintronic devices based on HOIPs.

Abstract

The hybrid organic–inorganic perovskites (HOIPs) form a new class of semiconductors which show promising optoelectronic device applications. Remarkably, the optoelectronic properties of HOIP are tunable by changing the chemical components of their building blocks. Recently, the HOIP spintronic properties and their applications in spintronic devices have attracted substantial interest. Here the impact of the chemical component diversity in HOIPs on their spintronic properties is studied. Spin valve devices based on HOIPs with different organic cations and halogen atoms are fabricated. The spin diffusion length is obtained in the various HOIPs by measuring the giant magnetoresistance (GMR) response in spin valve devices with different perovskite interlayer thicknesses. In addition spin lifetime is also measured from the Hanle response. It is found that the spintronic properties of HOIPs are mainly determined by the halogen atoms, rather than the organic cations. The study provides a clear avenue for engineering spintronic devices based on HOIPs.

Grain boundaries (GBs) play an important role in most polycrystalline solar cells. In this essay, three important questions are explored: Do GBs affect: 1) recombination and thus open‐circuit voltage? Not dramatically, if at all; 2) current–voltage hysteresis? Most studies show that hysteresis is dominated by defects at GBs; and 3) long‐term durability? Yes, GBs definitely help increase the rate of perovskite degradation.

Abstract

Grain boundaries (GBs) play an important role in most polycrystalline solar cells. In perovskite solar cells, the research community is just starting to understand their effects on performance and long‐term durability. In this essay, three important questions are explored: Do GBs affect: 1) recombination and thus open‐circuit voltage? Not dramatically, if at all; 2) current–voltage hysteresis? Most studies show that hysteresis is dominated by defects at GBs; and 3) long‐term durability? Yes, GBs definitely help increase the rate of perovskite degradation. In this essay, the latest reports are summarized and the authors' perspective on this very important subject is given.

A new method is developed to synthesize SnO _x ‐Cl colloids and to realize an in situ and spontaneous ion‐exchange reaction during the perovskite film crystallization process. It is found that such ion exchange can perfectly passivate the interface defects and reduce energy loss at the interface.

Abstract

Interface engineering is of great concern in photovoltaic devices. For the solution‐processed perovskite solar cells, the modification of the bottom surface of the perovskite layer is a challenge due to solvent incompatibility. Herein, a Cl‐containing tin‐based electron transport layer; SnO _x ‐Cl, is designed to realize an in situ, spontaneous ion‐exchange reaction at the interface of SnO _x ‐Cl/MAPbI₃. The interfacial ion rearrangement not only effectively passivates the physical contact defects, but, at the same time, the diffusion of Cl ions in the perovskite film also causes longitudinal grain growth and further reduces the grain boundary density. As a result, an efficiency of 20.32% is achieved with an extremely high open‐circuit voltage of 1.19 V. This versatile design of the underlying carrier transport layer provides a new way to improve the performance of perovskite solar cells and other optoelectronic devices.