Chen Weijie

Integrated perovskite/bulk‐heterojunction (BHJ) organic solar cells have shown great potential to further improve their performance by combining the advantages of perovskite solar cells and NIR BHJ organic solar cells. Combining with the maintained high V _OC, higher efficiencies are expected by fully optimizing the perovskite layers and NIR BHJ layers through device engineering and materials innovations.

Abstract

The recently emerged integrated perovskite/bulk‐heterojunction (BHJ) organic solar cells (IPOSCs) without any recombination layers have generated wide attention. This type of device structure can take the advantages of tandem cells using both perovskite solar and near‐infrared (NIR) BHJ organic solar materials for wide‐range sunlight absorption and the simple fabrication of single junction cells, as the low bandgap BHJ layer can provide additional light harvesting in the NIR region and the high open‐circuit voltage can be maintained at the same time. This progress report highlights the recent developments in such IPOSCs and the possible challenges ahead. In addition, the recent development of perovskite solar cells and NIR organic solar cells is also covered to fully underline the importance and potential of IPOSCs.

Organic solar cells achieve over 15% efficiency through the use of a copolymer donor, and simultaneously enhanced open‐circuit voltage and short‐circuit current density are obtained. High‐performance solar cells are adaptable for environment‐friendly solvents using a blade‐coating method, while showing better photostability than the corresponding ternary solar cells.

Abstract

Ternary blending and copolymerization strategies have proven advantageous in boosting the photovoltaic performance of organic solar cells. Here, 15% efficiency solar cells using copolymerization donors are demonstrated, where the electron‐withdrawing unit, ester‐substituted thiophene, is incorporated into a PBDB‐TF polymer to downshift the molecular energy and broaden the absorption. Copolymer‐based solar cells suitable for large‐area devices can be fabricated by a blade‐coating method from a nonhalogen and nonaromatic solvent mixture. Although ternary solar cells can achieve comparable efficiencies, they are not suitable for environment‐friendly processing conditions and show relatively low photostability compared to copolymer‐based devices. These results not only demonstrate high‐efficiency organic photovoltaic cells via copolymerization strategies but also provide important insights into their applications in practical production.

An easy and scalable methylamine (MA) gas healing method was realized for inorganic cesium based perovskite (CsPbX₃) layer by incorporating a certain amount of MAX (X=I or Br) initiator into the raw film. It was found that the excess MAX could accelerate the absorption rate of the MA gas into the CsPbX₃ film and quickly turn it into a liquid intermediate phase. Through the healing process, a high uniform and high crystallinity CsPbX₃ film with enhanced device performance was achieved. Moreover, the chemical interaction between a series of halides and MA gas molecules were studied and the results could offer guidance in further optimization of the healing strategy.

A simple single‐source evaporation method is employed to a fabricate high‐quality Sb₂(Se_1‐xS_x)₃film. Based on this single‐source evaporation method, combined with a H₂O₂ treatment on CdS surface, the CdS/Sb₂(Se_1‐xS_x)₃ solar cells achieve a device efficiency of 6.30%, which is the highest reported efficiency of Sb₂(Se_1‐xS_x)₃ thin‐film solar cells without hole‐transporting layer.

The properties of Sb₂(Se_1‐xS_x)₃ alloy films can be continuously tuned by changing its composition, enabling potentially better device efficiency compared to the Sb₂S₃ or Sb₂Se₃ counterparts. Fabrication of Sb₂(Se_1−xS_x)₃ films with good uniformity and orientation are prerequisites to realize their full potential. Here, pure‐phase and high‐uniformity Sb₂(Se_1‐xS_x)₃ absorbers are successfully produced via a vapor transport deposition method employing a single evaporation source. Increasing the travelling distance of vapor particles can improve the morphology and crystallinity of Sb₂(Se_1‐xS_x)₃ films, and as a result, compact Sb₂(Se_1‐xS_x)₃ films with strong preferential [221] orientation and uniform composition throughout the whole film are obtained. Further, the CdS surface is treated with H₂O₂ solution and a device with a champion efficiency of 6.30% is obtained, a new efficiency record for Sb₂(Se_1‐xS_x)₃ based thin‐film solar cells without a hole‐transporting layer.

Publication date: April 2019

Source: Nano Energy, Volume 58

Author(s): Jia Li, Kang Liu, Tianpeng Ding, Peihua Yang, Jiangjiang Duan, Jun Zhou

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Publication date: April 2019

Source: Nano Energy, Volume 58

Author(s): Fei Wu, Hao Lin, Zhenhai Yang, Mingdun Liao, Zilei Wang, Zhengping Li, Pingqi Gao, Jichun Ye, Wenzhong Shen

Abstract

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Publication date: April 2019

Source: Nano Energy, Volume 58

Author(s): Xiaodong Li, Wenxiao Zhang, Wenjun Zhang, Hai-Qiao Wang, Junfeng Fang

Abstract

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Spontaneous grain polymerization strategy is proposed to fabricate efficient and stable perovskite solar cells through the incorporation of ethyl 2-cyanoacrylate (E2CA) additive. E2CA chemically anchors to grain boundaries and then spontaneously polymerizes to hydrophobic polymer when exposed in air, thus passivating defects and blocking moisture penetration. The resulted devices exhibit high efficiency of 21.03% with good air and thermal stability.

Publication date: May 2019

Source: Nano Energy, Volume 59

Author(s): Heng Guo, Haiyuan Chen, Haiyan Zhang, Xu Huang, Jian Yang, Bojun Wang, Yulan Li, Liping Wang, Xiaobin Niu, Zhiming Wang

Abstract

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High‐quality perovskite films are fabricated via in situ substrate‐heating‐assisted deposition in air. The in situ substrate temperature significantly influences the morphology, composition, and band gap of resulted perovskite films. The perovskite solar cells fabricated in air show the efficiency up to 18.38%. The fabrication process and device structure offer the advantages of well matching with low‐cost, large‐scale printing techniques.

It is a great challenge to process highly efficient perovskite solar cells (PSCs) under ambient conditions, which limits their potential commercialization. Herein, high‐quality triple cation mixtures Cs_0.21FA_0.56MA_0.23(I_0.98Br_0.02)₃ perovskite films are fabricated through two‐step solution processes via in situ substrate‐heating‐assisted deposition under ambient conditions with a relative humidity of about 40%. The in situ substrate temperature during the deposition significantly influences the grain size and compactness of lead iodide films, and therefore greatly affects the morphology, composition, and band gap of resulted perovskite films. Based on the optimization of substrate temperature and the thickness of perovskite layer, PSCs with a planar heterojunction configuration of ITO/SnO₂/perovskite/Spiro‐OMeTAD/Ag are fabricated, which deliver a power conversion efficiency up to 18.38%. These results suggest that high‐performance PSCs can be fabricated under ambient conditions instead of an inert environment, providing a fundamental step for accelerating PSC commercialization.

Antimony chalcogenide is a promising absorber material for photovoltaic cell. The authors achieve a power conversion efficiency (PCE) of 4.57% for spray coated Sb₂(S1_‐xS_ex)₃ film solar cells with a post selenylation and prepare a module with a large‐scale active area of 21 cm² with a PCE of 3.19%，suggesting this technique is promising for industrial application.

Antimony chalcogenide is a promising absorber material for photovoltaic (PV) cells because of its tunable band gap, strong optical absorption, earth‐abundant, and nontoxic constituents. Here, a novel hydrazine‐free approach for fabricating antimony chalcogenide films prepared via spray pyrolysis deposition with further selenylation, as well as high performance solar cells are reported. The fabricated antimony chalcogenide solar cells exhibits remarkably high open‐circuit voltage (V _OC) comparing with that of Sb₂Se₃ and Sb₂S₃ devices. The distribution of Se during the selenylation process via refering to the Fick's second law is discussed. The results show that the selenized absorber has tremendously improved absorbance for visible light and a cascade‐type energy level from surface to bulk, resulting in an ideal energy level matching for p‐n junction. By introducing an efficient buffer layer, the detrimental short‐circuit is avoided by reducing the cracks. Finally, 4.57% of the power conversion efficiency (PCE) of the solar cell is achieved. Furthermore, a module with a large‐scale active area of 21 cm² is successfully fabricated, which shows a PCE of 3.19%, and a V _OC of 3.42 V. The present work suggests that the antimony chalcogenide solar cells are promising for industrial application.

Tetrafluoroborate (BF₄ ⁻) anion can be successfully incorporated into a mixed‐ion perovskite crystal frame, leading to lattice relaxation and a longer photoluminescence lifetime, higher recombination resistance, and 1–2 orders magnitude lower trap density in prepared perovskite solar cells. These advantages result in an improved power conversion efficiency of 20.16% from 17.55% due to enhanced open‐circuit voltage (V _OC) and fill factor.

Abstract

Composition engineering is a particularly simple and effective approach especially using mixed cations and halide anions to optimize the morphology, crystallinity, and light absorption of perovskite films. However, there are very few reports on the use of anion substitutions to develop uniform and highly crystalline perovskite films with large grain size and reduced defects. Here, the first report of employing tetrafluoroborate (BF₄ ⁻) anion substitutions to improve the properties of (FA = formamidinium, MA = methylammonium (FAPbI₃)_0.83(MAPbBr₃)_0.17) perovskite films is demonstrated. The BF₄ ⁻ can be successfully incorporated into a mixed‐ion perovskite crystal frame, leading to lattice relaxation and a longer photoluminescence lifetime, higher recombination resistance, and 1–2 orders magnitude lower trap density in prepared perovskite films and derived solar cells. These advantages benefit the performance of perovskite solar cells (PVSCs), resulting in an improved power conversion efficiency (PCE) of 20.16% from 17.55% due to enhanced open‐circuit voltage (V _OC) and fill factor. This is the highest PCE for BF₄ ⁻ anion substituted lead halide PVSCs reported to date. This work provides insight for further exploration of anion substitutions in perovskites to enhance the performance of PVSCs and other optoelectronic devices.

A specific bidentate molecule, 2‐mercaptopyridine, is demonstrated to substantially enhance anchoring strength at surface of metal halide perovskites, which improves the passivation efficacy and stability synchronously relative to monodentate counterparts. The highly stable bidentate anchoring based passivation on CH₃NH₃PbI₃ not only advances power conversion efficiency from 18.35% to 20.28%, but also leads to a champion lifetime in humid air.

Abstract

Chemical passivation is an effective approach to suppress the grain surface dominated charge recombination in perovskite solar cells (PSCs). However, the passivation effect is usually labile on perovskite crystal surface since most passivating agents are weakly anchored. Here, the use of a bidentate molecule, 2‐mercaptopyridine (2‐MP), to increase anchoring strength for improving the passivation efficacy and stability synchronously is demonstrated. Compared to monodentate counterparts of pyridine and p‐toluenethiol, 2‐MP passivation on CH₃NH₃PbI₃ film results in twofold improvement of photoluminescence lifetime and remarkably enhanced tolerance to chlorobenzene washing and vacuum heating, which improve the power conversion efficiency of n–i–p planar structured PSCs from 18.35% to 20.28%, with open‐circuit voltage approaching 1.18 V. Moreover, the CH₃NH₃PbI₃ films passivated with 2‐MP exhibit unprecedented humid‐stability that they can be exposed to saturated humidity for at least 5 h, mainly due to the passivation induced surface deactivation, which renders the unencapsulated devices retaining 93% of the initial efficiency after 60 days aging in air with relative humidity of 60–70%.

A negligible‐Pb‐waste and upscalable perovskite film processing method combining raster ultrasonic spray coating and chemical vapor deposition is developed. Perovskite solar module shows a power conversion efficiency of 14.7% on 12 cm² active area, much lower substrate size dependence than the spin‐coating method, and outstanding operational stability near the maximum power point under 1 sun illumination (T ₈₀ lifetime of 388 h).

Abstract

An upscalable perovskite film deposition method combining raster ultrasonic spray coating and chemical vapor deposition is reported. This method overcomes the coating size limitation of the existing stationary spray, single‐pass spray, and spin‐coating methods. In contrast with the spin‐coating method (>90% Pb waste), negligible Pb waste during PbI₂ deposition makes this method more environmentally friendly. Outstanding film uniformity across the entire area of 5 cm × 5 cm is confirmed by both large‐area compatible characterization methods (electroluminescence and scattered light imaging) and local characterization methods (atomic force microscopy, scanning electron microscopy, photoluminescence mapping, UV–vis, and X‐ray diffraction measurements on multiple sample locations), resulting in low solar cell performance decrease upon increasing device area. With the FAPb(I_0.85Br_0.15)₃ (FA = formamidinium) perovskite layer deposited by this method, champion solar modules show a power conversion efficiency of 14.7% on an active area of 12.0 cm² and an outstanding shelf stability (only 3.6% relative power conversion efficiency decay after 3600 h aging). Under continuous operation (1 sun light illumination, maximum power point condition, dry N₂ atmosphere with <5% relative humidity, no encapsulation), the devices show high light‐soaking stability corresponding to an average T ₈₀ lifetime of 535 h on the small‐area solar cells and 388 h on the solar module.

The optical absorption in monolithic perovskite/silicon tandem solar cells with flat Si front‐side is improved. The successful tailoring and incorporation of a nanocrystalline silicon oxide composite interlayer with tuneable refractive index is demonstrated on device by experiments and optical simulations. Improved short‐circuit current density (38.7 mA cm⁻²) combined with excellent contact properties lead to a cell with a certified stabilized conversion efficiency of 25.2%.

Abstract

Perovskite/silicon tandem solar cells are attractive for their potential for boosting cell efficiency beyond the crystalline silicon (Si) single‐junction limit. However, the relatively large optical refractive index of Si, in comparison to that of transparent conducting oxides and perovskite absorber layers, results in significant reflection losses at the internal junction between the cells in monolithic (two‐terminal) devices. Therefore, light management is crucial to improve photocurrent absorption in the Si bottom cell. Here it is shown that the infrared reflection losses in tandem cells processed on a flat silicon substrate can be significantly reduced by using an optical interlayer consisting of nanocrystalline silicon oxide. It is demonstrated that 110 nm thick interlayers with a refractive index of 2.6 (at 800 nm) result in 1.4 mA cm⁻² current gain in the silicon bottom cell. Under AM1.5G irradiation, the champion 1 cm² perovskite/silicon monolithic tandem cell exhibits a top cell + bottom cell total current density of 38.7 mA cm⁻² and a certified stabilized power conversion efficiency of 25.2%.

Ring fusion and backbone fluorination yield a novel ladder‐type building block f‐FBTI2, a desirable “stronger acceptor” for enabling n‐type electron‐acceptor polymers. The resulting polymer semiconductor f‐FBTI2‐T shows an excellent power conversion efficiency of 8.1% with a very small energy loss of 0.53 eV in all‐polymer solar cell devices.

Abstract

A novel imide‐functionalized arene, di(fluorothienyl)thienothiophene diimide (f‐FBTI2), featuring a fused backbone functionalized with electron‐withdrawing F atoms, is designed, and the synthetic challenges associated with highly electron‐deficient fluorinated imide are overcome. The incorporation of f‐FBTI2 into polymer affords a high‐performance n‐type semiconductor f‐FBTI2‐T, which shows a reduced bandgap and lower‐lying lowest unoccupied molecular orbital (LUMO) energy level than the polymer analog without F or with F‐functionalization on the donor moiety. These optoelectronic properties reflect the distinctive advantages of fluorination of electron‐deficient acceptors, yielding “stronger acceptors,” which are desirable for n‐type polymers. When used as a polymer acceptor in all‐polymer solar cells, an excellent power conversion efficiency of 8.1% is achieved without any solvent additive or thermal treatment, which is the highest value reported for all‐polymer solar cells except well‐studied naphthalene diimide and perylene diimide‐based n‐type polymers. In addition, the solar cells show an energy loss of 0.53 eV, the smallest value reported to date for all‐polymer solar cells with efficiency > 8%. These results demonstrate that fluorination of imide‐functionalized arenes offers an effective approach for developing new electron‐deficient building blocks with improved optoelectronic properties, and the emergence of f‐FBTI2 will change the scenario in terms of developing n‐type polymers for high‐performance all‐polymer solar cells.

One source of instability in perovskite solar cells (PSCs) is interfacial defects, particularly those that exist between the perovskite and the hole transport layer (HTL). We demonstrate that thermally evaporated dopant-free tetracene (120 nm) on top of the perovskite layer, capped with a lithium-doped Spiro-OMeTAD layer (200 nm) and top gold electrode, offers an excellent hole-extracting stack with minimal interfacial defect levels. For a perovskite layer interfaced between these graded HTLs and a mesoporous TiO₂ electron-extracting layer, its photoluminescence yield reaches 15% compared to 5% for the perovskite layer interfaced between TiO₂ and Spiro-OMeTAD alone. For PSCs with graded HTL structure, we demonstrate efficiency of up to 21.6% and an extended power output of over 550 hours of continuous illumination at AM1.5G, retaining more than 90% of the initial performance and thus validating our approach. Our findings represent a breakthrough in the construction of stable PSCs with minimized nonradiative losses.

Author Correction: Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites

Author Correction: Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites, Published online: 14 February 2019; doi:10.1038/s41563-019-0316-5