awn

ASnX₃ perovskite solar cells (Sn‐PSC) have the potential to deliver the most efficient solar cell technology using Sn, a less hazardous element for health and environment than Pb. This review gives a comprehensive introduction to the field, in a suitable fashion for nonexpert readers, gradually narrowing and detailing the subject with the most recent and significant advances.

Abstract

ASnX₃ perovskite solar cells (Sn‐PSC) have the potential to deliver the most efficient solar cell technology with safe materials. In this review, a comprehensive introduction of the field is given, that is suitable for nonexperts, gradually leading the reader to a narrower and detailed analysis of the most recent and significant advances. A brief description is given of the leading alternatives for lead‐free PSC and the reasons for ASnX₃ compounds' status as one of the most promising candidates are presented. The last part of the review focuses on the stabilization of ASnX₃, which is the most compelling challenge to achieve the highest efficiency PSCs. The most promising approaches toward stable and efficient ASnX₃ PSCs are identified and discussed.

A betaine‐based zwitterionic polymer poly sulfobetaine methacrylate (PSBMA) is employed as interfacial material in p‐i‐n perovskite solar cells. Through improving the interfacial affinity and regulating the energy level at the anode and cathode, respectively, the power conversion efficiency as well as storage stability of the devices greatly improve. In addition, PSBMA also shows advantages in large active area devices.

To improve the performance of perovskite solar cells (Pero‐SCs), a betaine‐based zwitterionic polymer poly(sulfobetaine methacrylate) (denoted by PSBMA) is employed as interlayers at both the anode and cathode in p‐i‐n Pero‐SCs. 1) At the anode side, PSBMA acts as a glue to stitch the two interfacially unfavorable materials: perovskite and poly(bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine), by which the quality of perovskite films as well as the corresponding device performance greatly improve. 2) At the cathode side, PSBMA smoothes the energy levels between PC₆₁BM and Al, and thus facilitates the electron injection efficiency. The power conversion efficiency (PCE) is promoted from 17.31% to 19.16% after PSBMA is introduced as both anode and cathode sides of the p‐i‐n Pero‐SCs. More importantly, PSBMA also shows great potential for large active area (1 cm × 1 cm) Pero‐SCs, and a PCE as high as 15.7% is achieved.

A spirobixanthene‐based dendrimer, DH1, is designed and synthesized. DH1 with the hyperbranched structure shows a large molecular size of up to 1.9 nm. The amorphous DH1 is the first dendrimer‐type HTM applied for MAPbI₃ perovskite solar cells, obtaining a power conversion efficiency of 17.13%. This work demonstrates that a quasiglobular dendrimer with a large molecular size is a promising design approach for excellent HTMs.

A dendrimer based on a spirobixanthene core, termed DH1, is designed and synthesized as a hole‐transporting material (HTM) for perovskite solar cells (PSCs). DH1 showing a hyperbranched structure with methoxydiphenylamine carbazole dendrons stretching outward along the para‐phenylene spacer acquires a large molecular size of up to 1.9 nm, which favors good thermal stability and amorphous property. The thus obtained DH1‐based pinhole‐free film as a hole‐transport layer results in a power conversion efficiency of 17.13% and reduced hysteresis behavior of MAPbI₃‐based planar PSCs. This work provides the first example of the use of dendrimer‐type HTM for PSC application, demonstrating a promising approach to design HTMs in a quasiglobular dendrimer with a large molecular size.

The power conversion efficiency of N2200‐based all‐polymer solar cells (all‐PSCs) can be drastically enhanced from ≈1% to ≈11% by simply changing the solvent from chlorobenzene and 2‐methyltetrahydrofuran (Me‐THF). In‐depth investigations reveal that the preaggregation of donor (PTzBI) and acceptor (N2200) polymers in 2‐Me‐THF is the key to enable such high performance for N2200‐based all‐PSC device.

Herein, all‐polymer solar cells (all‐PSCs) are studied based on PTzBI:N2200 system processed from two different solvents, chlorobenzene (CB) and 2‐methyltetrahydrofuran (Me‐THF). It is found that the preaggregation of the donor and acceptor polymers in Me‐THF is the key factor that enables a drastic enhancement in cell efficiency from ≈1% (processed by CB) to ≈11% (processed by Me‐THF). When using CB as the solvent, both donor and acceptor polymers are well dissolved and mostly disaggregated. In contrast, the donor and acceptor polymers both exhibit strong aggregation in Me‐THF. As a result, the donor and acceptor blend films processed from Me‐THF exhibit pure domains with appropriate molecular packing structure, which leads to high charge mobilities (10⁻³–10⁻⁴ cm² V⁻¹ s⁻¹) and fill factors (FFs; 75%), whereas the blend films processed by CB suffer from highly miscible and impure domains, hence decreasing the charge mobilities by 1–2 orders of magnitude compared with those of the corresponding pure films. The current work reveals that the polymer preaggregation is the key reason enabling optimal morphology and high performance in N2200‐based all‐PSCs, and this strategy may be potentially applied in other systems to optimize the morphology and performance of all‐PSCs.

This review presents the present status and the future perspectives of Sn‐based perovskite solar cells. The strategies to find the breakthrough of highly efficient and robust Sn‐perovskite solar cells are discussed by focusing on current fabrication processes and defect physics scenario including compositional and dimensional engineering.

Sn‐based halide perovskites have attracted much interest due to their highly valuable electrical and optical properties. The promising optical and electrical properties of Sn‐based perovskites have enticed a lot of research to focus on developing the strategies and explore the in‐depth material characteristics. Sn‐halide perovskites exhibit apparent merits and demerits. The ideal electrical and optical properties are even better than that of Pb‐analogs, namely close‐to‐optimal bandgap, strong optical absorption, and good carrier mobilities. However, the present achievement of Sn‐halide perovskite solar cells is not satisfactory, which is commonly attributed to relatively low defect tolerance, fast crystallization, and oxidative instability. The efficiency of Sn‐based perovskites is far ahead, with a 9% power conversion efficiency (PCE), than the other (Ge, Bi, Sb, Cu, etc.) Pb‐free options but simultaneously lagging far behind Pb‐based analogs that have a 25.2% PCE. This review is aimed at presenting milestone works and revealing the pros and cons of Sn‐halide perovskites. In addition, the defect physics of Sn‐based perovskites is described. The improvement of open‐circuit voltage is a critical issue for Sn‐halide perovskites to compete with Pb‐based perovskites. The understanding of defect physics plays an instrumental role in designing strategies for efficient and robust Sn‐halide perovskite solar cells.

Machine learning (ML) is used to predict the material bandgap and perovskite solar cell device performances. The findings from the ML model matches well with the trend in the solar cell theory derived from the “Shockley and Queisser limit.” Other findings, which are beneficial for the fabrication of high‐performing perovskite solar cells are also discussed.

Abstract

Perovskite solar cells (PSCs) have recently received considerable attention due to the high energy conversion efficiency achieved within a few years of their inception. However, a machine learning (ML) approach to guide the development of high‐performing PSCs is still lacking. In this paper ML is used to optimize material composition, develop design strategies, and predict the performance of PSCs. The ML models are developed using 333 data points selected from about 2000 peer reviewed publications. These models guide the design of new perovskite materials and the development of high‐performing solar cells. Based on ML guidance, new perovskite compositions are experimentally synthesized to test the practicability of the model. The ML model also shows its ability to predict underlying physical phenomena as well as the performance of PSCs. The PSC model matches well with the theoretical prediction by the Shockley and Queisser limit, which is almost impossible for a human to find from an ensemble of data points. Moreover, strategies for developing high‐performing PSCs with different bandgaps are also derived from the model. These findings show that ML is very promising not only for predicting the performance, but also for providing a deeper understanding of the physical phenomena associated with the PSCs.

The polystyrene is introduced into perovskite solar cells as the buffer layer between the SnO₂ and perovskite, which can release the stress during the perovskite annealing. A large lattice, fewer defect, and low ion‐immigration‐energy perovskite can be obtained by releasing stress. Finally, 21.89% efficiency is obtained and the cell can maintain almost 97% of the initial efficiency after 5 days.

Abstract

The mixed halide perovskites have become famous for their outstanding photoelectric conversion efficiency among new‐generation solar cells. Unfortunately, for perovskites, little effort is focused on stress engineering, which should be emphasized for highly efficient solar cells like GaAs. Herein, polystyrene (PS) is introduced into the perovskite solar cells as the buffer layer between the SnO₂ and perovskite, which can release the residual stress in the perovskite during annealing because of its low glass transition temperature. The stress‐free perovskite has less recombination, larger lattices, and a lower ion migration tendency, which significantly improves the cell's efficiency and device stability. Furthermore, the so‐called inner‐encapsulated perovskite solar cells are fabricated with another PS capping layer on the top of perovskite. As high as a 21.89% photoelectric conversion efficiency (PCE) with a steady‐state PCE of 21.5% is achieved, suggesting that the stress‐free cell can retain almost 97% of its initial efficiency after 5 days of “day cycle” stability testing.

Incorporating dicyanobenzothiadiazole into polymer yields an n‐type semiconductor DCNBT‐IDT, which exhibits a narrow bandgap of 1.43 eV and a high absorption coefficient of 6.15 × 10⁴ cm⁻¹. The DCNBT‐IDT‐based all‐polymer solar cells achieve a remarkable power conversion efficiency of 8.32% with a small energy loss of 0.53 eV and a photoresponse of up to 870 nm.

Abstract

Currently, n‐type acceptors in high‐performance all‐polymer solar cells (all‐PSCs) are dominated by imide‐functionalized polymers, which typically show medium bandgap. Herein, a novel narrow‐bandgap polymer, poly(5,6‐dicyano‐2,1,3‐benzothiadiazole‐alt‐indacenodithiophene) (DCNBT‐IDT), based on dicyanobenzothiadiazole without an imide group is reported. The strong electron‐withdrawing cyano functionality enables DCNBT‐IDT with n‐type character and, more importantly, alleviates the steric hindrance associated with typical imide groups. Compared to the benchmark poly(naphthalene diimide‐alt‐bithiophene) (N2200), DCNBT‐IDT shows a narrower bandgap (1.43 eV) with a much higher absorption coefficient (6.15 × 10⁴ cm⁻¹). Such properties are elusive for polymer acceptors to date, eradicating the drawbacks inherited in N2200 and other high‐performance polymer acceptors. When blended with a wide‐bandgap polymer donor, the DCNBT‐IDT‐based all‐PSCs achieve a remarkable power conversion efficiency of 8.32% with a small energy loss of 0.53 eV and a photoresponse of up to 870 nm. Such efficiency greatly outperforms those of N2200 (6.13%) and the naphthalene diimide (NDI)‐based analog NDI‐IDT (2.19%). This work breaks the long‐standing bottlenecks limiting materials innovation of n‐type polymers, which paves a new avenue for developing polymer acceptors with improved optoelectronic properties and heralds a brighter future of all‐PSCs.

The use of liquid exfoliated 2D WS₂ and MoS₂ as hole‐transporting layers (HTLs) in ultrahigh efficiency organic solar cells is reported. WS₂ yields cells with higher power conversion efficiency (PCE), fill‐factor, and short‐circuit current than MoS₂ and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate). When WS₂ is introduced as HTL in PBDB‐T‐2F:Y6:PC₇₁BM organic solar cells, a maximum PCE value of 17% is achieved.

Abstract

The application of liquid‐exfoliated 2D transition metal disulfides (TMDs) as the hole transport layers (HTLs) in nonfullerene‐based organic solar cells is reported. It is shown that solution processing of few‐layer WS₂ or MoS₂ suspensions directly onto transparent indium tin oxide (ITO) electrodes changes their work function without the need for any further treatment. HTLs comprising WS₂ are found to exhibit higher uniformity on ITO than those of MoS₂ and consistently yield solar cells with superior power conversion efficiency (PCE), improved fill factor (FF), enhanced short‐circuit current (J _SC), and lower series resistance than devices based on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) and MoS₂. Cells based on the ternary bulk‐heterojunction PBDB‐T‐2F:Y6:PC₇₁BM with WS₂ as the HTL exhibit the highest PCE of 17%, with an FF of 78%, open‐circuit voltage of 0.84 V, and a J _SC of 26 mA cm⁻². Analysis of the cells' optical and carrier recombination characteristics indicates that the enhanced performance is most likely attributed to a combination of favorable photonic structure and reduced bimolecular recombination losses in WS₂‐based cells. The achieved PCE is the highest reported to date for organic solar cells comprised of 2D charge transport interlayers and highlights the potential of TMDs as inexpensive HTLs for high‐efficiency organic photovoltaics.

In article number https://doi.org/10.1002/adma.2019032391903239, Junsheng Yu, Wei Huang, Ziyi Ge, Tobin J. Marks, Antonio Facchetti, and co‐workers present a spontaneous passivation method to greatly improve the performance of perovskite solar cells (PSCs) by using a zwitterionic small‐molecule electrolyte. This bottom‐up passivation is a novel and promising strategy to overcome outstanding issues impeding PSC advances in the future.

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 %.

A fully automated spray‐coated technology for the commercial large‐scale solution‐based processing of colloidal CsPbI₃ quantum dot films is achieved. The solar cells based on such films show a high power conversion efficiency of 11.2%.

Abstract

A fully automated spray‐coated technology with ultrathin‐film purification is exploited for the commercial large‐scale solution‐based processing of colloidal inorganic perovskite CsPbI₃ quantum dot (QD) films toward solar cells. This process is in the air outside the glove box. To further improve the performance of QD solar cells, the short‐chain ligand of phenyltrimethylammonium bromide (PTABr) with a benzene group is introduced to partially substitute for the original long‐chain ligands of the colloidal QD surface (namely PTABr‐CsPbI₃). This process not only enhances the carrier charge mobility within the QD film due to shortening length between adjacent QDs, but also passivates the halide vacancy defects of QD by Br⁻ from PTABr. The colloidal QD solar cells show a power conversion efficiency (PCE) of 11.2% with an open voltage of 1.11 V, a short current density of 14.4 mA cm⁻², and a fill factor of 0.70. Due to the hydrophobic surface chemistry of the PTABr–CsPbI₃ film, the solar cell can maintain 80% of the initial PCE in ambient conditions for one month without any encapsulation. Such a low‐cost and efficient spray‐coating technology also offers an avenue to the film fabrication of colloidal nanocrystals for electronic devices.

In article number https://doi.org/10.1002/aenm.2019018051901805, Doojin Vak, Seok‐In Na and co‐workers report a highly efficient single‐junction ternary polymer solar cell (PSC) based on PTB7‐Th, PC₇₁BM, and COi8DFIC using slot‐die coating. This approach is readily translated into large‐area module and roll‐to‐roll processed PSCs, which produce the highest power conversion efficiency among the printing‐based PSCs.

In article number https://doi.org/10.1002/aenm.2019017191901719, Jooho Moon and co‐workers demonstrate a cold antisolvent bathing method for fabricating highly efficient large‐area perovskite solar cells. This temperature tuned antisolvent bathing adjusts the nucleation and growth of the perovskite materials, leading to large grain‐sized perovskite films with preferred crystal orientation and fewer crystal defects. Reduced recombination and facilitated extraction of photogenerated charge carriers are observed, exhibiting champion efficiency of 18.50%.

A dopant‐free polymeric hole transport material (HTM) is synthesized to fabricate perovskite solar cells. The carbonyl groups can passivate defects of under‐coordinated Pb atoms that exist in the surface of perovskite films. A PBT1‐C based device shows a power conversion efficiency of 19.06% with a fill factor of 81.22%, which is the highest value among the dopant‐free polymeric HTMs.

Abstract

Although perovskite solar cells (PVSCs) have achieved rapid progress in the past few years, most of the high‐performance device results are based on the doped small molecule hole‐transporting material (HTM), spiro‐OMeTAD, which affects their long‐term stability. In addition, some defects from under‐coordinated Pb atoms on the surface of perovskite films can also result in nonradiative recombination to affect device performance. To alleviate these problems, a dopant‐free HTM based on a donor‐acceptor polymer, PBT1‐C, synthesized from the copolymerization between the benzodithiophene and 1,3‐bis(4‐(2‐ethylhexyl)thiophen‐2‐yl)‐5,7‐bis(2‐alkyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione units is introduced. PBT1‐C not only possesses excellent hole mobility, but is also able to passivate the surface traps of the perovskite films. The derived PVSC shows a high power conversion efficiency of 19.06% with a very high fill factor of 81.22%, which is the highest reported for dopant‐free polymeric HTMs. The results from photoluminescence and trap density of states measurements validate that PBT1‐C can effectively passivate both surface and grain boundary traps of the perovskite.

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.

Cheaper and greener ILs: A one‐pot synthetic process using water as the only solvent is designed to obtain ionic liquids (ILs) in a yield of approximately 95 mol % and purity greater than 99.3 wt % in a processing time of only 1 h. Moreover, no heating is required to carry out the reaction. The one‐pot ILs have similar properties to ILs prepared by conventional procedures and show high performance without any further purification through sorbents.

Abstract

An innovative one‐pot synthetic process that uses water as the only processing solvent was used to obtain ionic liquids (ILs) in a yield of approximately 95 mol % and purity greater than 99.3 wt % (<2 ppm each of lithium, bromide and moisture) in a processing time of 1 h. Since no heating is needed for carrying out the reaction and no purification through sorbents is required, energy, time and chemicals can be saved to minimize waste production. The physicochemical and electrochemical validation, including tests in batteries, reported herein shows that the above‐mentioned ILs have properties analogous to those of ILs prepared by standard reported procedures and show high performance without any further purification step through sorbents. These characteristics, in combination with low cost, easy execution and scale‐up, sustainability and versatility, make the one‐pot process even more appealing, especially for industrial‐scale applications.

Nonfullerene acceptors‐based terpolymer, SMD2, is designed and synthesized to continuously fabricate high‐performance organic solar cell (OSC) modules, and multifunctional hole transport layers are developed, and applied to flexible modules via an all‐solution process. the flexible OSC modules fabricated in an industrial production line have a PCE of 5.25% (P _max = 419.6 mW) on an area of 80 cm².

Abstract

To ensure laboratory‐to‐industry transfer of next‐generation energy harvesting organic solar cells (OSCs), it is necessary to develop flexible OSC modules that can be produced on a continuous roll‐to‐roll basis and to apply an all‐solution process. In this study, nonfullerene acceptors (NFAs)‐based donor polymer, SMD2, is newly designed and synthesized to continuously fabricate high‐performance flexible OSC modules. Also, multifunctional hole transport layers (HTLs), WO₃/HTL solar bilayer HTLs, are developed and applied via an all‐solution process called “ProcessOne” into inverted structure. SMD2, the donor terpolymer, has a deep highest occupied molecular orbital (HOMO) level and can achieve a power conversion efficiency (PCE) of 11.3% with NFAs without any pre‐/post‐treatment because of its optimal balance between crystallinity and miscibility. Furthermore, the integration of multifunctional HTLs enables the recovery of the drop in open circuit voltage (V _OC) caused by a mismatch in energy levels between the deep HOMO level of the NFAs‐based bulk‐heterojunction layer and the solution‐processed HTLs. Also, the photostability under ultraviolet‐exposure necessary for “ProcessOne” is greatly improved because of the integration of multifunctional HTLs. Consequently, because of the synergistic effects of these approaches, the flexible OSC modules fabricated in an industrial production line have a PCE of 5.25% (P _max = 419.6 mW) on an active area of 80 cm².