Chen Weijie

Publication date: 16 January 2019

Source: Joule, Volume 3, Issue 1

Author(s): David Kiermasch, Lidón Gil-Escrig, Henk J. Bolink, Kristofer Tvingstedt

Context & Scale

Summary

Graphical Abstract

Design and application of volatilizable solid additives in non-fullerene organic solar cells

Design and application of volatilizable solid additives in non-fullerene organic solar cells, Published online: 07 November 2018; doi:10.1038/s41467-018-07017-z

The latest progress in exciton–photon coupling of perovskite materials is reviewed. Polaritons in planar and nanowire Fabry–Pérot microcavities are discussed predominantly in terms of materials and photophysics. Large Rabi‐splitting energy (≈656 meV) is achieved in CsPbBr₃. These large values enable polariton condensation and polariton lasers to be realized at high temperature or in low‐Q cavities.

Abstract

The semiconductor exciton–polariton, arising from the strong coupling between excitons and confined cavity photon modes, is not only of fundamental importance in macroscopic quantum effects but also has wide application prospects in ultralow‐threshold polariton lasers, slowing‐light devices, and quantum light sources. Very recently, metallic halide perovskites have been considered as a great candidate for exciton–polariton devices owing to their low‐cost fabrication, large exciton oscillator strength, and binding energy. Herein, the latest progress in exciton–polaritons and polariton lasers of perovskites are reviewed. Polaritons in planar and nanowires Fabry–Pérot microcavities are discussed with particular reference to material and photophysics. Finally, a perspective on the remaining challenges in perovskite polaritons research is given.

The chloride coated‐PbSe quantum dots are passivated using CsPbX3 (X = Br and I) perovskite quantum dots via halide ion exchange in solution. Solar cells using PbSe quantum dots passivated by CsPbBr_0.5I_2.5 mixed halide perovskites achieve a 9.2% power conversion, the highest reported efficiency for a PbSe QD device to date.

PbSe quantum dots (QDs) have stronger electronic coupling resulting from a large Bohr exciton radius, suggesting PbSe QDs may be able to achieve superior charge separation and transport in optoelectronic devices compared with PbS QDs. However, PbS QDs solar cell have achieved a certified 12.01% power conversion efficiency (PCE), whereas PbSe QD photovoltaics lag behind at 8.2% PCE. One reason for this difference is that there has been significantly less work done on surface passivation of PbSe QDs. Here, the surface passivation of chlorinated PbSe QDs is optimized via a halide ion exchange treatment using mixed halide CsPb(Br/I)₃ perovskite nanocrystals. Champion devices made from treated QDs achieved a PCE of 9.2%, V _oc of 0.56 V, J _sc of 25.7 mA cm⁻², and fill factor of 64%. Average PCEs for optimized cells are 8.9%. Detailed physical characterizations including capacitance‐voltage (C‐V), V _oc, and J _sc as a function of light intensity, transient photovoltage, and photocurrent measurements are all carried out to investigate the mechanism of the improvement in the PCE and to understand the role of the mixed halide perovskites in providing superior surface passivation for PbSe solar cells. At this time, 9.2% is the highest PCE yet reported for PbSe QDs solar cells.

A novel thermionic emission–based interconnecting layer (ICL) for all‐perovskite tandem solar cells is demonstrated with solution‐processed light absorbers, wide absorption, and high efficiency. The novel ICL structure employs a new hybrid system of fluoride silane–incorporated polyethylenimine ethoxylated for solvent resistance and defect passivation.

Abstract

All‐perovskite tandem cells have been considered a potential candidate for bringing the power conversion efficiency (PCE) beyond the Shockley–Queisser limit of single‐junction device while retaining the advantages of earth‐abundant materials and solution processability. However, a challenging issue with regard to realizing such solution‐processed devices is the fulfillment of complex and coupled requirements of the interconnecting layer (ICL), including solvent resistance to protect underlying perovskite film, high electrical properties for carrier transport and recombination, and high optical transmission. In this work, a new thermionic emission–based ICL with enhanced solvent resistance features is demonstrated. Fundamentally, the thermionic emission plays a critical role in the electron transport process in the ICL, which is confirmed through both experimental and theoretical studies. Besides achieving high optical transmission and electrical properties, the new ICL chemically protects the underlying perovskite film by introducing a fluoride silane–incorporated polyethylenimine ethoxylated hybrid system that also passivates the surface defects to reduce electrical loss. The monolithic all‐perovskite tandem cells demonstrate highest PCE of 17.9% (from current density–voltage scan) and the highest steady‐state efficiency is 16.1% for a typical device. Consequently, this work contributes to not only understanding the fundamental mechanism of ICLs but also promotes robust and low‐cost photovoltaics.

Near‐infrared nonfullerene acceptors (NIR NFAs, T1–T4) with fluorinated regioisomeric A–Aπ–D–Aπ–A backbones are developed for high‐performance polymer solar cells (PSCs), in which proximal NFAs with varied F‐atoms (T1–T3) largely outperform distal NFA (T4). Particularly, single‐junction PSCs with a PTB7‐Th:T2 blend can achieve 10.87% power conversion efficiency (PCE), and tandem PSCs through integrating with an ITIC:PBDB‐T blend reach a PCE of 14.64%.

Abstract

Solar photon‐to‐electron conversion with polymer solar cells (PSCs) has experienced rapid development in the recent few years. Even so, the exploration of molecules and devices in efficiently converting near‐infrared (NIR) photons into electrons remains critical, yet challenging. Herein presented is a family of near‐infrared nonfullerene acceptors (NIR NFAs, T1–T4) with fluorinated regioisomeric A–Aπ–D–Aπ–A backbones for constructing efficient single‐junction and tandem PSCs with photon response up to 1000 nm. It is found that the tuning of the regioisomeric bridge (Aπ) and fluoro (F)‐substituents on a molecular skeleton strongly influences the backbone conformation and conjugation, leading to the optimized optoelectronic and stable stacking of resultant NFAs, which eventually impacts the performance of derived PSCs. In PSCs, the proximal NFAs with varied F‐atoms (T1–T3) mostly outperform than that of distal NFA (T4). Notably, single‐junction PSC with PTB7‐Th:T2 blend can reach 10.87% power conversion efficiency (PCE), after implementing a solvent additive to improve blend morphology. Moreover, efficient tandem PSCs are fabricated through integrating such NIR cells with mediate bandgap nonfullerene‐based subcells, to achieve a PCE of 14.64%. The results reveal the structural design of organic semiconductor and device with improved photovoltaic performance.

A novel cryogenic process, described by Charles Surya and co‐workers in article number 1804402 has universal applicability to grow organometal halide perovskites. A cryogenic temperature inhibits premature coalescence of nuclei into large crystallites, decoupling nucleation and crystallization phases. The enhanced control over the growth process yields excellent film and high‐performance solar cells.

APSCs offer all: All‐polymer solar cells have attracted great attention, owing to rational design, improved morphology, strong absorption, enhanced stability etc. This Minireview highlights the opportunities of APSCs, selected polymer families suitable for these devices with optimization to enhance the performance further, and discusses the challenges facing APSC development for commercial applications.

Abstract

For over two decades bulk‐heterojunction polymer solar cell (BHJ‐PSC) research was dominated by donor:acceptor BHJ blends based on polymer donors and fullerene molecular acceptors. This situation has changed recently, with non‐fullerene PSCs developing very rapidly. The power conversion efficiencies of non‐fullerene PSCs have now reached over 15 %, which is far above the most efficient fullerene‐based PSCs. Among the various non‐fullerene PSCs, all‐polymer solar cells (APSCs) based on polymer donor‐polymer acceptor BHJs have attracted growing attention, due to the following attractions: 1) large and tunable light absorption of the polymer donor/polymer acceptor pair; 2) robustness of the BHJ film morphology; 3) compatibility with large scale/large area manufacturing; 4) long‐term stability of the cell to external environmental and mechanical stresses. This Minireview highlights the opportunities offered by APSCs, selected polymer families suitable for these devices with optimization to enhance the performance further, and discusses the challenges facing APSC development for commercial applications.

Increasing markets and decreasing package weight for high-specific-power photovoltaics

Increasing markets and decreasing package weight for high-specific-power photovoltaics, Published online: 05 November 2018; doi:10.1038/s41560-018-0258-1

Previously unreported nanoscale defects are observed in solution‐processed methylammonium lead tri‐iodide (MAPI) perovskite solar cells. A novel methodology is introduced that eliminates these defects, modifies MAPI crystallinity and enhances power conversion efficiency >30%. In‐situ optoelectronic characterization correlates performance enhancements to improvements in charge collection efficiency, reduced electron–hole recombination, and an overall decrease of trap‐state density.

Abstract

Here, previously unobserved nanoscale defects residing within individual grains of solution‐processed methylammonium lead tri‐iodide (CH₃NH₃PbI₃, MAPI) thin films are identified. Using scanning transmission electron microscopy (STEM), the defects inherently associated with the established solution‐processing methodology are identified, and a facile processing modification to eliminate these defects is introduced. Specifically, defect elimination is achieved by coannealing the as‐deposited MAPI layer with the electron transport layer (phenyl‐C₆₁‐butyric acid methyl, PCBM) resulting in devices that significantly outperform devices prepared using the established methodology—with power conversion efficiencies increasing from 13.6% to 17.4%. The use of transmission electron microscopy allows the correlation of performance enhancements to improved intragrain crystallinity and shows that highly coherent crystallographic orientation results within individual grains when processing is modified. Detailed optoelectronic characterization reveals that the improved intragrain crystallinity drives an improvement of charge collection and a reduction of PEDOT:PSS/perovskite interfacial recombination. The study suggests that the microstructural defects in MAPI, owing to a lack of structural coherence throughout the thickness of thin film, are a significant cause of interfacial recombination.

Publication date: January 2019

Source: Nano Energy, Volume 55

Author(s): Chih-Yu Chang, Bo-Chou Tsai, Yu-Cheng Hsiao, Min-Zhen Lin, Hsin-Fei Meng

Abstract

Graphical abstract

Efficient and stable perovskite tandem solar cells are demonstrated by incorporating novel conductive cross-linkable interfacial layer. The resulting devices afford a power conversion efficiency (PCE) up to 18.69%, which represents the highest PCE among all reported monolithic all-perovskite tandem cells. Additionally, the encapsulated devices maintain ≈ 91% of their initial PCE after 9300 h of air exposure.

Publication date: February 2019

Source: Nano Energy, Volume 56

Author(s): Rui Lin, Hang Lei, Dan Ruan, Kui Jiang, Xiang Yu, Zilong Wang, Wenjie Mai, He Yan

Abstract

Graphical abstract

Publication date: January 2019

Source: Nano Energy, Volume 55

Author(s): Sajid Sajid, Ahmed Mourtada Elseman, Dong Wei, Jun Ji, Shangyi Dou, Hao Huang, Peng Cui, Meicheng Li

Abstract

Graphical abstract

By introducing an intermediate MoO_3−x layer, this work demonstrates unambiguously a 1) reduced decomposition reaction between CZCTS and Mo; 2) accelerated potassium diffusion, facilitating grain growth; and 3) modified back contact band alignment, resulting in better minority carrier collection length and reduced recombination rate. Finally, with further optimized conditions, the power conversion efficiency can reach 8.98% (the active area efficiency is 9.26%).

Kesterite Cu₂ZnSnS₄ (CZTS) has been investigated intensively as a promising absorber material for thin film solar cells. However, the reported best power conversion efficiency (PCE) is still low due to the intrinsic limitations of CZTS defects and the unfavorable front and back contact interfaces. In this study, an intermediate MoO_3−x layer is introduced as a primary back contact prior to the Cd‐doped Cu₂ZnSnS₄ (CZCTS) absorber layer growth. It has been demonstrated that the insertion of the MoO_3−x layer can suppress the decomposition reaction between CZCTS and Mo, reducing secondary phases and voids and improving the rear interface quality. The MoO_3−x layer can also accelerate potassium diffusion into the CZCTS layer and interfaces, which facilitates the grain growth, passivates interfaces and hence yields better crystallinity. Moreover, the band alignment at the back contact is modified by the MoO_3−x layer, resulting in better minority carrier collection and improvement of open‐circuit voltage (V _OC). With the optimal MoO_3−x layer, the PCE of CZCTS solar cells has increased from ≈5.43 to ≈7%, largely attributed to the ≈60 mV V _OC increment. Through the modification of the CZCTS precursor and optimizing the anti‐reflection coating layer, the best PCE achieved is 8.98%, with an active area efficiency of 9.26%.

A graded bilayered inorganic hole‐transporting layer (including compact NiO _x and mesoporous CuGaO₂) is developed for inverted perovskite solar cells. The resulting devices demonstrate both high efficiency, with the champion one giving a stabilized efficiency of ≈20% and superior thermal stability with >80% of the initial efficiency being retained subject to 1000 hours' thermal aging at 85 °C.

Abstract

The unstable feature of the widely employed organic hole‐transporting materials (HTMs) (e.g., spiro‐MeOTAD) significantly limits the practical application of perovskite solar cells (PSCs). Therefore, it is desirable to design new structured PSCs with stable HTMs presenting excellent carrier extraction and transfer properties. This work demonstrates a new inverted PSC configuration. The new PSC has a graded band alignment and bilayered inorganic HTMs (i.e., compact NiO_x and mesoporous CuGaO₂). In comparison with planar‐structured PSCs, the mesoporous CuGaO₂ can effectively extract holes from perovskite due to the increased contact area of the perovskite/HTM. The graded energy alignment constructed in the ultrathin compact NiO_x, mesoporous CuGaO₂, and perovskite can facilitate carrier transfer and depress charge recombination. As a result, the champion device based on the newly designed mesoscopic PSCs yields a stabilized efficiency of ≈20%, which is considered one of the best results for inverted PSCs with inorganic HTMs. Additionally, the unencapsulated PSC device retains more than 80% of its original efficiency when subjected to thermal aging at 85 °C for 1000 h in a nitrogen atmosphere, thus demonstrating superior thermal stability of the device. This study may pave a new avenue to rational design of highly efficient and stable PSCs.

A novel SnO₂‐in‐polymer matrix is demonstrated to be an excellent electron‐selective layer in perovskite solar cells. The polymer is uniformly dispersed in SnO₂ colloidal ink and promotes the nanoparticle disaggregation in the ink. Planar‐structure perovskite solar cells based on this SnO₂‐in‐polymer matrix show a high efficiency of 20.8% with negligible hysteresis and superior reproducibility.

Abstract

Understanding interfacial loss and the ways to improving interfacial property is critical to fabricate highly efficient and reproducible perovskite solar cells (PSCs). In SnO₂‐based PSCs, nonradiative recombination sites at the SnO₂–perovskite interface lead to a large potential loss and performance variation in the resulting photovoltaic devices. Here, a novel SnO₂‐in‐polymer matrix (i.e., polyethylene glycol) is devised as the electron transporting layer to improve the film quality of the SnO₂ electron transporting layer. The SnO₂‐in‐polymer matrix is fabricated through spin‐coating a polymer‐incorporated SnO₂ colloidal ink. The polymer is uniformly dispersed in SnO₂ colloidal ink and promotes the nanoparticle disaggregation in the ink. Owing to polymer incorporation, the compactness and wetting property of SnO₂ layer is significantly ameliorated. Finally, photovoltaic devices based on Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45 perovskite sandwiched between SnO₂ and Spiro‐OMeTAD layer are fabricated. Compared with the averaging power conversion efficiency of 16.2% with 1.2% deviation for control devices, the optimized devices exhibit an improved averaging efficiency of 19.5% with 0.25% deviation. The conception of polymer incorporation in the electron transporting layer paves a way to further increase the performance of planar perovskite solar cells.

High‐efficiency (21.06%) and durable 2D–3D vertical aligned perovskite solar cells (PSCs) with phase pure 2D perovskite are demonstrated. The phase pure 2D perovskite minimizes photo‐generated charge‐carrier localization in the low‐dimensional perovskite; the dominant vertical alignment does not affect charge‐carrier extraction. The traditional constraint of trade‐off between efficiency and stability in PSC is overcome.

Abstract

Three‐dimensional (3D) metal‐halide perovskite solar cells (PSCs) have demonstrated exceptional high efficiency. However, instability of the 3D perovskite is the main challenge for industrialization. Incorporation of some long organic cations into perovskite crystal to terminate the lattice, and function as moisture and oxygen passivation layer and ion migration blocking layer, is proven to be an effective method to enhance the perovskite stability. Unfortunately, this method typically sacrifices charge‐carrier extraction efficiency of the perovskites. Even in 2D–3D vertically aligned heterostructures, a spread of bandgaps in the 2D due to varying degrees of quantum confinement also results in charge‐carrier localization and carrier mobility reduction. A trade‐off between the power conversion efficiency and stability is made. Here, by introducing 2D C₆H₁₈N₂O₂PbI₄ (EDBEPbI₄) microcrystals into the precursor solution, the grain boundaries of the deposited 3D perovskite film are vertically passivated with phase pure 2D perovskite. The phases pure (inorganic layer number n = 1) 2D perovskite can minimize photogenerated charge‐carrier localization in the low‐dimensional perovskite. The dominant vertical alignment does not affect charge‐carrier extraction. Therefore, high‐efficiency (21.06%) and ultrastable (retain 90% of the initial efficiency after 3000 h in air) planar PSCs are demonstrated with these 2D–3D mixtures.