shuhui

A thermodynamically favored crystal preferable orientation growth along the (001) crystal plane is explored in formamidinium/methylammonium mixed perovskites, and the origin is found to be the reduction of surface energy. Combined with the (001) plane lying parallel to the substrate, it affects the charge transportation and collection in the resultant perovskite solar cells, resulting in a power conversion efficiency of 21.2%.

Abstract

Crystal orientation has a great impact on the properties of perovskite films and the resultant device performance. Up to now, the exquisite control of crystal orientation (the preferred crystallographic planes and the crystal stacking mode with respect to the particular planes) in mixed‐cation perovskites has received limited success, and the underlying mechanism that governs device performance is still not clear. Here, a thermodynamically favored crystal orientation in formamidinium/methylammonium (FA/MA) mixed‐cation perovskites is finely tuned by composition engineering. Density functional theory calculations reveal that the FA/MA ratio affects the surface energy of the mixed perovskites, leading to the variation of preferential orientation consequently. The preferable growth along the (001) crystal plane, when lying parallel to the substrates, affects their charge transportation and collection properties. Under the optimized condition, the mixed‐cation perovskite (FA_{1– x} MA _x PbI_2.87Br_0.13 (Cl)) solar cells deliver a champion power conversion efficiency over 21%, with a certified efficiency of 20.50 ± 0.50%. The present work not only provides a vital step in understanding the intrinsic properties of mixed‐cation perovskites but also lays the foundation for further investigation and application in perovskite optoelectronics.

A group of new 2D Ruddlesden–Popper perovskites (RPPs) containing cyclohexane methylamine is presented. The films exhibit reverse‐graded quantum well distribution, which facilitates internal charge transfer and photon collection. This structure enables the vulnerable large n RPPs to be protected by outer more resistant small n RPPs. Consequently, a 15.05% efficient solar cell with significantly improved stability in the presence of humidity is achieved.

Abstract

2D Ruddlesden–Popper perovskites (RPPs) have emerged as a promising solar cell material. A group of novel RPPs with cyclohexane methylamine (CMA) as a spacer cation is presented. Unlike previously reported RPPs, the deposited films of (CMA)₂(MA) _{n −1}Pb _n I_{3 n +1} (MA is CH₃NH₃ ⁺, n = 1, 2, 3, …) exhibit multiple phases with reverse‐graded quantum well (QW) distribution; small n (n = 2) RPPs are located at the surface and large n (n ≥ 10) RPPs at the bottom. This has three advantages: (a) The outer, more moisture resistant, small n RPPs create a stable barrier that protects the vulnerable large n RPP lattice from being attacked by water molecules. (b) It forms a type‐II band alignment between different phases, which favors self‐driven charge transport. (c) The natural structure of graded QWs expands the range of photon collection. Attributed to these properties, the best efficiency of 15.05%, with high open‐circuit voltage (V _oc) of 1.10 V for a first‐generation solar cell containing (CMA)₂(MA)₈Pb₉I₂₈, is achieved. A notable enhancement in short wavelength is observed in the Incident photon‐to‐current conversion efficiency spectra. This device shows significantly improved long‐term stability, retaining ≈95% of the initial efficiency after 4600 h exposure in ambient conditions with 40–70% relative humidity.

Cu₂ZnSnS₄ (CZTS) absorbers with different bandgaps are produced by variation of thermal treatments. These CZTS absorbers are combined with Zn_1−x Sn _x O _y (ZTO) or CdS buffer layers. A high open circuit voltage of 809 mV is obtained for an ordered CZTS absorber with CdS, while a 9.7% device is obtained utilizing a Cd free ZTO buffer layer.

Abstract

Cu₂ZnSnS₄(CZTS) thin‐film solar cell absorbers with different bandgaps can be produced by parameter variation during thermal treatments. Here, the effects of varied annealing time in a sulfur atmosphere and an ordering treatment of the absorber are compared. Chemical changes in the surface due to ordering are examined, and a downshift of the valence band edge is observed. With the goal to obtain different band alignments, these CZTS absorbers are combined with Zn_1−x Sn _x O _y (ZTO) or CdS buffer layers to produce complete devices. A high open circuit voltage of 809 mV is obtained for an ordered CZTS absorber with CdS buffer layer, while a 9.7% device is obtained utilizing a Cd free ZTO buffer layer. The best performing devices are produced with a very rapid 1 min sulfurization, resulting in very small grains.

The 4‐fluorophenethylammonium iodide based quasi‐2D perovskite (n = 5) solar cell shows a power conversion efficiency of 14.5% with excellent stability in air, with a humidity of 40–50%, maintaining 90% of the original efficiency after 40 d.

Abstract

The recent development of quasi‐2D perovskite solar cells have drawn significant attention due to the improved stability of these materials and devices against moisture compared to their 3D counterparts. However, the optoelectronic properties of 2D perovskites need to be optimized in order to achieve high efficiency. In this work, the effect of spacer cations, i.e., phenethylammonium (PEA), 4‐fluorophenethylammonium (F‐PEA), and 4‐methoxyphenethylammonium (MeO‐PEA) on the optoelectronic properties and device performance of quasi‐2D perovskites is systematically studied. It is observed that both larger and more hydrophobic cations can improve perovskite stability against moisture, while larger size can adversely influence the device performance. Interestingly, with F‐PEA or MeO‐PEA, distribution of n value can be shifted toward high 3D content in quasi‐2D perovskite layers, which enables lower bandgaps and possibly lower exciton binding energy. Due to the best charge transport and lowest bandgap, the F‐PEAI‐based quasi‐2D perovskite (n = 5) solar cell shows a highest power conversion efficiency (PCE) of 14.5% with excellent stability in air with a humidity of 40–50%, keeping 90% of the original PCE after 40 d. It is believed that the approach may open a way for the design of new organic spacer cations for stable low‐dimensional hybrid perovskites with high performance.

Remarkable advancement has been made in the efficiency of organic solar cells (OSCs) in recent times, mostly due to novel fused ring electron acceptors (FREAs). Here, structural evolution of FREAs to enhance efficiency is comprehensively discussed. Moreover, recent progress in polymer design, semi‐transparent OSCs, ternary, and tandem OSCs is provided. The challenges and future development of FREAs are briefly addressed.

Abstract

The quest for sustainable energy sources has led to accelerated growth in research of organic solar cells (OSCs). A solution‐processed bulk‐heterojunction (BHJ) OSC generally contains a donor and expensive fullerene acceptors (FAs). The last 20 years have been devoted by the OSC community to developing donor materials, specifically low bandgap polymers, to complement FAs in BHJs. The current improvement from ≈2.5% in 2013 to 17.3% in 2018 in OSC performance is primarily credited to novel nonfullerene acceptors (NFA), especially fused ring electron acceptors (FREAs). FREAs offer unique advantages over FAs, like broad absorption of solar radiation, and they can be extensively chemically manipulated to tune optoelectronic and morphological properties. Herein, the current status in FREA‐based OSCs is summarized, such as design strategies for both wide and narrow bandgap FREAs for BHJ, all‐small‐molecule OSCs, semi‐transparent OSC, ternary, and tandem solar cells. The photovoltaics parameters for FREAs are summarized and discussed. The focus is on the various FREA structures and their role in optical and morphological tuning. Besides, the advantages and drawbacks of both FAs and NFAs are discussed. Finally, an outlook in the field of FREA‐OSCs for future material design and challenges ahead is provided.

Inorganic perovskite quantum dots can be used as efficient luminescent down converting layers for ultraviolet blocking and conversion in traditional perovskite solar cells. In this work, a new cell configuration by integrating CsPbBr₃ inside of device structure is demonstrated. The modified devices could exhibit weak hysteresis, improved photoelectric performance, and multiple colors of fluorescence.

Abstract

Photovoltaic devices employing lead halide perovskites as the photoactive layer have attracted enormous attention due to their commercialization potential. Yet, there exists challenges on the way to the practical use of perovskite solar cells (PSCs), such as light stability and current–voltage (J–V ) hysteresis. Inorganic perovskite nanocrystals (IPNCs) are promising candidates for high‐performance photovoltaic devices due to their simple synthesis methods, tunable bandgap, and efficient photon downshifting effect for ultraviolet (UV) light blocking and conversion. In this work, CsPbBr₃ IPNCs modification could give rise to the vapor phase and solution‐processed PSCs with a power conversion efficiency (PCE) of 16.4% and 20.8%, respectively, increased by 11.6% and 5.6% compared to the control devices for more efficient UV utilization and carrier recombination suppression. As far as is known, 11.6% is the most effective enhanced factor for PSCs based on photon downshifting effect inside of devices. The CsPbBr₃ layer could also significantly retard light‐induced degradation, leading to the lifetime over 100 h under UV illumination for PSCs. Additionally, the modified PSCs exhibit weak hysteresis and multiple colors of fluorescence. These results shed light on the future design of combining a photon downshifting layer and carrier interfacial modification layer in the applications of perovskite optoelectronic devices.

Publication date: 15 May 2019

Source: Joule, Volume 3, Issue 5

Author(s): Masoud Ghasemi, Huawei Hu, Zhengxing Peng, Jeromy James Rech, Indunil Angunawela, Joshua H. Carpenter, Samuel J. Stuard, Andrew Wadsworth, Iain McCulloch, Wei You, Harald Ade

Context & Scale

Summary

Graphical Abstract

Inorganic perovskite quantum dots can be used as efficient luminescent down converting layers for ultraviolet blocking and conversion in traditional perovskite solar cells. In this work, a new cell configuration by integrating CsPbBr₃ inside of device structure is demonstrated. The modified devices could exhibit weak hysteresis, improved photoelectric performance, and multiple colors of fluorescence.

Abstract

Photovoltaic devices employing lead halide perovskites as the photoactive layer have attracted enormous attention due to their commercialization potential. Yet, there exists challenges on the way to the practical use of perovskite solar cells (PSCs), such as light stability and current–voltage (J–V ) hysteresis. Inorganic perovskite nanocrystals (IPNCs) are promising candidates for high‐performance photovoltaic devices due to their simple synthesis methods, tunable bandgap, and efficient photon downshifting effect for ultraviolet (UV) light blocking and conversion. In this work, CsPbBr₃ IPNCs modification could give rise to the vapor phase and solution‐processed PSCs with a power conversion efficiency (PCE) of 16.4% and 20.8%, respectively, increased by 11.6% and 5.6% compared to the control devices for more efficient UV utilization and carrier recombination suppression. As far as is known, 11.6% is the most effective enhanced factor for PSCs based on photon downshifting effect inside of devices. The CsPbBr₃ layer could also significantly retard light‐induced degradation, leading to the lifetime over 100 h under UV illumination for PSCs. Additionally, the modified PSCs exhibit weak hysteresis and multiple colors of fluorescence. These results shed light on the future design of combining a photon downshifting layer and carrier interfacial modification layer in the applications of perovskite optoelectronic devices.

Triimide‐functionalized n‐type polymers are synthesized and used as the acceptor materials in all‐polymer solar cells (all‐PSCs), and a remarkable power conversion efficiency of 8.98% is achieved, which is among the highest values in all‐PSCs. The results demonstrate the positive effects of increasing imide number in polymer acceptors on improving all‐polymer solar cell performance.

Two triimide‐functionalized n‐type acceptor polymers are designed and synthesized, which show narrower bandgap, lower‐lying frontier molecular orbital energy levels, and improved film morphology than the diimide‐functionalized analogue polymers. When blended with a p‐type donor polymer semiconductor PTB7‐Th, an outstanding power conversion efficiency of 8.98% with a remarkable open‐circuit voltage of 1.03 V is attained. This efficiency is among the highest values in all‐polymer solar cells (all‐PSCs) reported till today, surpassing that (6.85%) of the diimide‐functionalized analogue polymers by a big margin and even higher than that (8.69%) of the fullerene‐based solar cells. The results demonstrate that the triimide‐functionalized f‐BTI3 is an excellent building block for developing n‐type polymer semiconductors, and the polymer f‐BTI3‐T is among the best‐performing n‐type polymers for applications in all‐PSCs. The structure–property correlations of these imide‐functionalized polymer semiconductors offer important guides for developing high‐performance n‐type polymer semiconductors.

Leadless hybrid perovskites are obtained by the epitaxial synthesis of CsPbX₃ (X = Cl, Br, I) perovskite quantum dots through surface chemical conversion of Cs₂GeF₆ double perovskites with PbX₂ (X = Cl, Br, I). The obtained CsPbBr₃/Cs₂GeF₆ products show high color purity and enhanced stability, indicating their potential application in lighting devices.

Abstract

Lead halide perovskites (LHPs) have received increased attention owing to their intriguing optoelectronic and photonic properties. However, the toxicity of lead and the lack of long‐term stability are potential obstacles for the application of LHPs. Herein, the epitaxial synthesis of CsPbX₃ (X = Cl, Br, I) perovskite quantum dots (QDs) by surface chemical conversion of Cs₂GeF₆ double perovskites with PbX₂ (X = Cl, Br, I) is reported. The experimental results show that the surface of the Cs₂GeF₆ double perovskites is partially converted into CsPbX₃ perovskite QDs and forms a CsPbX₃/Cs₂GeF₆ hybrid structure. The theoretical calculations reveal that the CsPbBr₃ conversion proceeds at the Cs₂GeF₆ edge through sequential growth of multiple PbBr₆ ⁴⁻ layers. Through the conversion strategy, luminescent and color‐tunable CsPbX₃ QDs can be obtained, and these products present high stability against decomposition due to anchoring effects. Moreover, by partially converting red emissive Cs₂GeF₆:Mn⁴⁺ to green emissive CsPbBr₃, the CsPbBr₃/Cs₂GeF₆:Mn⁴⁺ hybrid can be employed as a low‐lead hybrid perovskite phosphor on blue LED chips to produce white light. The leadless CsPbX₃/Cs₂GeF₆ hybrid structure with stable photoluminescence opens new paths for the rational design of efficient emission phosphors and may stimulate the design of other functional CsPbX₃/Cs‐containing hybrid structures.

A series of room‐temperature and ecofriendly solvent processable polymers are designed and synthesized by backbone modification of the high temperature processable PNTz4T polymer. The resulting random terpolymer (PNTz4T‐5MTC)‐based solar cells show 9.66% power conversion efficiency (PCE) on a small area (0.12 cm²) and an outstanding 6.61% PCE on large‐area module (54.45 cm²) at room temperature processing conditions in air.

Abstract

The room temperature (RT) processability of the photoactive layers in polymer solar cells (PSCs) from halogen‐free solvent along with their highly reproducible power conversion efficiencies (PCEs) and intrinsic thickness tolerance are extremely desirable for the large‐area roll‐to‐roll (R2R) production. However, most of the photoactive materials in PSCs require elevated processing temperatures due to their strong aggregation, which are unfavorable for the industrial R2R manufacturing of PSCs. These limiting factors for the commercialization of PSCs are alleviated by synthesizing random terpolymers with components of (2‐decyltetradecyl)thiophen‐2‐yl)naphtho[1,2‐c:5,6‐c′]bis[1,2,5]thiadiazole and bithiophene substituted with methyl thiophene‐3‐carboxylate (MTC). In contrast to the temperature‐dependent PNTz4T polymer, the resulting random terpolymers (PNTz4T‐MTC) show better solubility, slightly reduced crystallinity and aggregation, and weaker intermolecular interaction, thus enabling PNTz4T‐MTC to be processed at RT from a halogen‐free solvent. Particularly, the PNTz4T‐5MTC‐based photoactive layer exhibits an excellent PCE of 9.66%, which is among the highest reported PCEs for RT and ecofriendly halogen‐free solvent processed fullerene‐based PSCs, and a thickness tolerance with a PCE exceeding 8% from 100 to 520 nm. Finally, large‐area modules fabricated with the PNTz4T and PNTz4T‐5MTC polymer have shown 4.29% and 6.61% PCE respectively, with an area as high as 54.45 cm² in air.

Cu₂ZnSnS₄ (CZTS) absorbers with different bandgaps are produced by variation of thermal treatments. These CZTS absorbers are combined with Zn_1−x Sn _x O _y (ZTO) or CdS buffer layers. A high open circuit voltage of 809 mV is obtained for an ordered CZTS absorber with CdS, while a 9.7% device is obtained utilizing a Cd free ZTO buffer layer.

Abstract

Cu₂ZnSnS₄(CZTS) thin‐film solar cell absorbers with different bandgaps can be produced by parameter variation during thermal treatments. Here, the effects of varied annealing time in a sulfur atmosphere and an ordering treatment of the absorber are compared. Chemical changes in the surface due to ordering are examined, and a downshift of the valence band edge is observed. With the goal to obtain different band alignments, these CZTS absorbers are combined with Zn_1−x Sn _x O _y (ZTO) or CdS buffer layers to produce complete devices. A high open circuit voltage of 809 mV is obtained for an ordered CZTS absorber with CdS buffer layer, while a 9.7% device is obtained utilizing a Cd free ZTO buffer layer. The best performing devices are produced with a very rapid 1 min sulfurization, resulting in very small grains.

An effective approach to low temperature, solution‐processed ZnO electron‐transport layers (ETLs) for perovskite solar cells by combustion synthesis is developed. Due to the intrinsic passivation effects, high crystallinity, matched energy levels, ideal surface topography, and good chemical compatibility with the perovskite layer, combustion‐processed ZnO electron transport layers enable power conversion efficiencies approaching 17–20% for three representative perovskite systems without ETL doping or surface functionalization.

Abstract

Perovskite solar cells (PSCs) have advanced rapidly with power conversion efficiencies (PCEs) now exceeding 22%. Due to the long diffusion lengths of charge carriers in the photoactive layer, a PSC device architecture comprising an electron‐ transporting layer (ETL) is essential to optimize charge flow and collection for maximum performance. Here, a novel approach is reported to low temperature, solution‐processed ZnO ETLs for PSCs using combustion synthesis. Due to the intrinsic passivation effects, high crystallinity, matched energy levels, ideal surface topography, and good chemical compatibility with the perovskite layer, this combustion‐derived ZnO enables PCEs approaching 17–20% for three types of perovskite materials systems with no need for ETL doping or surface functionalization.

Publication date: 15 May 2019

Source: Joule, Volume 3, Issue 5

Author(s): Xinbo Yang, Wenzhu Liu, Michele De Bastiani, Thomas Allen, Jingxuan Kang, Hang Xu, Erkan Aydin, Lujia Xu, Qunyu Bi, Hoang Dang, Esra AlHabshi, Konstantinos Kotsovos, Ahmed AlSaggaf, Issam Gereige, Yimao Wan, Jun Peng, Christian Samundsett, Andres Cuevas, Stefaan De Wolf

Context & Scale

Summary

Graphical Abstract

Through a laminar air‐knife‐assisted room‐temperature meniscus coating, assisted by in situ UV–vis and microscopes, fundamental mechanisms of perovskite nucleation and crystal growth are investigated. Upon in‐depth film formation understanding, hysteresis‐free perovskite solar cell with a power conversion efficiency of 20.26% for 0.06 cm² and 18.76% for 1 cm² devices are successfully demonstrated by manufacturing a friendly room‐temperature meniscus coating process.

Abstract

Perovskite solar cells (PSCs) are ideally fabricated entirely via a scalable solution process at low temperatures to realize the promise of simple manufacturing, low‐cost processing, compatibility with flexible substrates, and perovskite‐based tandem solar cells. However, high‐quality photoactive perovskite thin films under those processing conditions is a challenge. Here, a laminar air‐knife‐assisted room‐temperature meniscus coating approach that enables one to control the drying kinetics during the solidification process and achieve high‐quality perovskite films and solar cells is devised. Moreover, this approach offers a solid model platform for in situ UV–vis and microscopic investigation of the perovskite film drying kinetics, which provide rich insights correlating the degree of supersaturation, the nucleation, and growth rate during the kinetic drying process, and ultimately, the film morphology and performance of the solar cell devices. Manufacturing friendly, antisolvent‐free room‐temperature coating of hysteresis‐free PSCs with a power conversion efficiency of 20.26% for 0.06 cm² and 18.76% for 1 cm² devices is demonstrated.

An innovative interfacial modifier, namely, 3‐phenyl‐2‐propen‐1‐amine (t‐PPEA) is developed for perovskite solar cells to overcome the dilemma of the trade‐off between transport and stability of the device, with a unique “quasi‐coplanar” rigid geometrical configuration and distinct electron delocalization characteristic.

Abstract

Interfacial ligand passivation engineering has recently been recognized as a promising avenue, contributing simultaneously to the optoelectronic characteristics and moisture/operation tolerance of perovskite solar cells. To further achieve a win‐win situation of both performance and stability, an innovative conjugated aniline modifier (3‐phenyl‐2‐propen‐1‐amine; PPEA) is explored to moderately tailor organolead halide perovskites films. Here, the conjugated PPEA presents both “quasi‐coplanar” rigid geometrical configuration and distinct electron delocalization characteristics. After a moderate treatment, a stronger dipole capping layer can be formed at the perovskite/transporting interface to achieve favorable banding alignment, thus enlarging the built‐in potential and promoting charge extraction. Meanwhile, a conjugated cation coordinated to the surface of the perovskite grains/units can form preferably ordered overlapping, not only passivating the surface defects but also providing a fast path for charge exchange. Benefiting from this, a ≈21% efficiency of the PPEA‐modified solar cell can be obtained, accompanied by long‐term stability (maintaining 90.2% of initial power conversion efficiency after 1000 h testing, 25 °C, and 40 ± 10 humidity). This innovative conjugated molecule “bridge” can also perform on a larger scale, with a performance of 18.43% at an area of 1.96 cm².

A low‐cost, high‐yield synthesis of a chorine‐substituted thiophene donor polymer material P(Cl) is presented. The material is characterized, and its performance within a polymer solar cell (PSC) with a nonfullerene acceptor is evaluated. Excellent power‐conversion efficiency combined with a low synthetic complexity is demonstrated for P(Cl), and hence, it is a promising candidate for the development in commercial PSC applications.

The industrialization of polymer solar cells (PSCs) requires high‐performance devices with high efficiencies and stabilities. Although high‐performance PSCs are achieved via outstanding research into their component materials and device structures, several challenges still need to be overcome, including the synthetic complexity (SC) of producing the active material. In this study, donor polymers based on two heterocyclic rings and simple donor–acceptor structures are designed to obtain a low‐cost material for PSCs. An inexpensive and high‐performance donor polymer P(Cl) is realized by the introduction of a chlorine‐atom substitution. P(Cl), which has lower SC than commercial donor polymers, has many advantages, such as high overall yield, low number of synthetic steps, and inexpensive raw materials. Moreover, fabricated P(Cl)‐based PSCs exhibit a high power‐conversion efficiency (PCE) of 12.14%. Through the shelf protocol of the international summit on organic photovoltaics stability in dark testing‐1 (ISOS‐D‐1) measurements, superior long‐term stability is demonstrated for P(Cl)‐based devices both without and with encapsulation; their PCEs are maintained at 91% and 100% of the initial values for up to 2002 and 2858 h, respectively, under ambient conditions. Therefore, P(Cl) is a promising donor polymer for commercial PSC applications.

Publication date: 17 April 2019

Source: Joule, Volume 3, Issue 4

Author(s): Feng Gao