Chen Weijie

In an inverted planar perovskite solar cell employing a hole transporting NiO thin film, photovoltaic performance is found to depend significantly on annealing atmosphere when preparing the NiO film. The best power conversion efficiency can be achieved from the NiO film annealed at the oxygen partial pressure of 30% in mixture of O₂ and N₂.

The effect of annealing atmosphere and importance of oxygen partial pressure upon annealing NiO film for achieving high efficiency inverted perovskite solar cells (PSCs) is reported. The solution‐process NiO films are deposited on an FTO (fluorine‐doped tin oxide) substrate and annealed at 400 °C under different atmospheres of air, O₂, N₂, and Ar. The devices using air‐ and O₂‐annealed NiO films show better photovoltaic performance than the N₂‐ and Ar‐annealed ones, mostly due to large difference in photocurrent density (J _sc) of ≈20 mA cm⁻² for air and O₂ vs ≈15 mA cm⁻² for N₂ and Ar. Oxygen‐excess condition leads to more p‐type characteristics along with better electrical and interfacial properties, leading to higher photovoltaic performance. When comparing air and O₂ condition, the air‐annealed NiO film shows slightly better power conversion efficiency (PCE) (15.68% for air vs. 14.93% for O₂), being indicative of importance of oxygen partial pressure. By carefully modifying oxygen content, the best photovoltaic performance is achieved from the NiO film annealed at the O₂/(O₂+N₂) ratio of 30%, delivering a PCE of 16.32%.

Publication date: April 2019

Source: Nano Energy, Volume 58

Author(s): Saripally Sudhaker Reddy, Veera Murugan Arivunithi, Vijaya Gopalan Sree, Haeun Kwon, Juyun Park, Yong-Cheol Kang, Huihui Zhu, Yong-Young Noh, Sung-Ho Jin

Abstract

Graphical abstract

Here a method to grow wafer‐size thin halide perovskite multiple crystals on aqueous solution surface is reported. The efficiency of lateral‐structure solar cells based on the single‐crystalline perovskite wafer reaches 5.9%.

Abstract

Solar‐grade single or multiple crystalline wafers are needed in large quantities in the solar cell industry, and are generally formed by a top‐down process from crystal ingots, which causes a significant waste of materials and energy during slicing, polishing, and other processing. Here, a bottom‐up technique that allows the growth of wafer‐size hybrid perovskite multiple crystals directly from aqueous solution is reported. Single‐crystalline hybrid perovskite wafers with centimeter size are grown at the top surface of a perovskite precursor solution. As well as saving raw materials, this method provides unprecedented advantages such as easily tunable thickness and rapid growth of the crystals. These crystalline wafers show high crystallinity, broader light absorption, and a long carrier recombination lifetime, comparable with those of bulk single crystals. Lateral‐structure perovskite solar cells made of these crystals demonstrate a record power conversion efficiency of 5.9%.

Additive‐free all‐polymer solar cells (all‐PSCs) devices based on TTFQx‐T1:N2200 are fabricated and the active layer (TTFQx‐T1:N2200) processed with THF and CHCl₃. The optimized device processed with THF exhibited better photovoltaic properties than that processed with CHCl₃. This work afforded a feasible strategy for constructing high‐performance all‐PSCs via a simple eco‐friendly processing method.

In this work, a medium bandgap quinoxaline (Qx) based polymer, named TTFQx‐T1, and a narrow bandgap n‐type polymer, named N2200, are employed to fabricate all‐polymer solar cells (all‐PSCs), which exhibited good light absorption for high short circuit current density (J _sc), well‐matched molecular energy level for good charge separation and high open circuit voltage (V _oc). Chlorinated solvents are harmful to both the environment and human beings; therefore, it is important to develop environmentally friendly solvents. Considering this, the green solvent tetrahydrofuran (THF) was employed to process all‐PSCs. The blend films based on TTFQx‐T1:N2200 processed with THF and thermal annealing (TA) exhibited better phase separation and preferential face‐on orientation toward the substrate, which benefited the exciton dissociation and charge carrier mobilities for higher FF and J _sc. The optimized device based on TTFQx‐T1:N2200 delivered an efficient power conversion efficiency of 8.63%, which is the highest value for all‐PSCs from Qx based polymers.

A one‐pot solution method to synthesize lead‐free 2D perovskite (BA)₂SnI_{4− x} Cl _x crystals with various Cl incorporation concentrations is reported. The Cl element is successfully incorporated into the crystal lattice. The Cl incorporation greatly alters the morphology, optical properties, phase transition temperature, and charge transport behavior of the as‐synthesized crystals.

Abstract

The incorporation of chloride (Cl) into methylammonium lead iodide (MAPbI₃) perovskites has attracted much attention because of the significantly improved performance of the MAPbI₃‐based optoelectronic devices with a negligible small amount of Cl incorporation. It is expected that the Cl incorporation in 2D perovskites with layered nature would be much more efficient and thus can greatly alter the morphology, optical properties, phase transition, and charge transport; however, studies on those aspects in 2D perovskites remain elusive up to date. Here, a one‐pot solution method to synthesize the Cl‐doped lead‐free 2D perovskite (BA)₂SnI₄ with various Cl incorporation concentrations is reported and how the Cl incorporation affects the morphology change, photoluminescence, phase transition, and charge transport is investigated. The Cl element is successfully incorporated into the crystal lattice in the solution‐processed perovskite materials, confirmed by X‐ray photoelectron spectroscopy and energy dispersive X‐ray spectroscopy measurements. The temperature‐dependent photoluminescence studies indicate that the emission properties and phase transition behavior in (BA)₂SnI_{4− x} Cl _x can be tuned by varying the Cl incorporation concentration. Electrical measurement suggests that the charge transport behavior can also be greatly altered by the Cl doping concentration and the electrical conductivity can be significantly improved under a higher Cl incorporation concentration.

Eu‐porphyrin complex is introduced into perovskite film to perfectly fabricate 2D perovskite inlaying the grain boundary of 3D polycrystalline. Such a modified device significantly increases humid, heating and UV light stability.

Abstract

The formation of defects at surfaces and grain boundaries (GBs) during the fabrication of solution‐processed perovskite film are thought to be responsible for its instability. Herein, Eu‐porphyrin complex (Eu‐pyP) is directly doped into methylammonium lead triiodide (MAPbI₃) precursor, perfectly fabricating 2D (Eu‐pyP)_0.5MA _{n ‐1}Pb _n I_{3 n +1} platelets inlaying the GBs of 3D polycrystalline interstices in this protocol. The device based on Eu‐pyP doped perovskite film possesses a champion efficiency of 18.2%. More importantly, the doped perovskite solar cells device shows beyond 85% retention of its pristine efficiency value, whereas the pure MAPbI₃ device has a rapid drop in efficiency down to 10% within 100 h under 45% humidity at 85 °C in AM 1.5 G. The above acquired perovskite films reveal an unpredictable thermodynamic self‐healing ability. Consequently, the findings provide an avenue for defect passivation to synchronously improve resistibility to moisture, heat, and solar light including UV.

Thermochromic lead‐free double perovskites that have potential applications in smart windows and temperature sensors are demonstrated. The anharmonic fluctuations and associated strong electron–phonon coupling, combined with the spin–orbit coupling effect, are responsible for the thermochromism. The findings on the structure modulation‐induced bandgap narrowing of Cs₂AgBiBr₆ provide new insights for the development of optoelectronic devices based on double perovskites.

Abstract

Lead‐free halide double perovskites with diverse electronic structures and optical responses, as well as superior material stability show great promise for a range of optoelectronic applications. However, their large bandgaps limit their applications in the visible light range such as solar cells. In this work, an efficient temperature‐derived bandgap modulation, that is, an exotic fully reversible thermochromism in both single crystals and thin films of Cs₂AgBiBr₆ double perovskites is demonstrated. Along with the thermochromism, temperature‐dependent changes in the bond lengths of AgBr (R _AgBr) and BiBr (R _BiBr) are observed. The first‐principle molecular dynamics simulations reveal substantial anharmonic fluctuations of the R _AgBr and R _BiBr at high temperatures. The synergy of anharmonic fluctuations and associated electron–phonon coupling, and the peculiar spin–orbit coupling effect, is responsible for the thermochromism. In addition, the intrinsic bandgap of Cs₂AgBiBr₆ shows negligible changes after repeated heating/cooling cycles under ambient conditions, indicating excellent thermal and environmental stability. This work demonstrates a stable thermochromic lead‐free double perovskite that has great potential in the applications of smart windows and temperature sensors. Moreover, the findings on the structure modulation‐induced bandgap narrowing of Cs₂AgBiBr₆ provide new insights for the further development of optoelectronic devices based on the lead‐free halide double perovskites.

2D Dion–Jacobson perovskites have better carrier charge transport because of the closer interlayer distance. Solar cells based on Dion–Jacobson perovskites having mixed organic cations and using solvent‐engineering methods and hydriodic acid additive achieve higher efficiencies with high fill factors. Most importantly, the Dion–Jacobson perovskite solar cells exhibit better environmental stability compared with butylammonium‐based perovskites and 3D analogs.

Abstract

Hybrid halide 2D perovskites deserve special attention because they exhibit superior environmental stability compared with their 3D analogs. The closer interlayer distance discovered in 2D Dion–Jacobson (DJ) type of halide perovskites relative to 2D Ruddlesden–Popper (RP) perovskites implies better carrier charge transport and superior performance in solar cells. Here, the structure and properties of 2D DJ perovskites employing 3‐(aminomethyl)piperidinium (3AMP²⁺) as the spacing cation and a mixture of methylammonium (MA⁺) and formamidinium (FA⁺) cations in the perovskite cages are presented. Using single‐crystal X‐ray crystallography, it is found that the mixed‐cation (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃ perovskite has a narrower bandgap, less distorted inorganic framework, and larger PbIPb angles than the single‐cation (3AMP)(MA)₃Pb₄I₁₃. Furthermore, the (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃ films made by a solvent‐engineering method with a small amount of hydriodic acid have a much better film morphology and crystalline quality and more preferred perpendicular orientation. As a result, the (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃‐based solar cells exhibit a champion power conversion efficiency of 12.04% with a high fill factor of 81.04% and a 50% average efficiency improvement compared to the pristine (3AMP)(MA)₃Pb₄I₁₃ cells. Most importantly, the 2D DJ 3AMP‐based perovskite films and devices show better air and light stability than the 2D RP butylammonium‐based perovskites and their 3D analogs.

Rapid advancement of perovskite solar cells confronts the challenges of reliable measurement, which is important for data analysis and results reproduction. Major measurement methods and the key factors affecting evaluation are summarized. A measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.

Abstract

Perovskite solar cells (PSCs) have undergone an incredibly fast development and attracted intense attention worldwide owing to their high efficiency and low‐cost fabrication. However, it is challenging to make a reliable measurement of PSCs, which creates great difficulty for researchers to compare and reproduce published results. Herein, the major measurement methods and key factors affecting evaluation of PSCs are summarized, such as hysteresis in current–voltage measurement, calibration of solar simulators for less mismatch in spectra and light intensity, and the area for the calculation of current density and power conversion efficiency. PSCs are also compared with n–i–p or p–i–n structures that exhibit different feedback under the same measurement methods. Finally, a measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.

A synergistic effect is proposed by employing a dielectric mirror and a ternary strategy to precisely tune the color perception as well as semitransparent organic solar cell (ST‐OSC) performance. It results in the highest efficiency reported for neutral‐color ST‐OSCs to date.

Abstract

Neutral‐colored semitransparent organic solar cells (ST‐OSCs) have attracted considerable attention owing to their unique application in no‐visual‐obstacle building‐integrated photovoltaics. Toward this promising potential application, a synergistic effect is first proposed by employing a dielectric mirror and ternary photoactive layer with near‐infrared absorption to tune the color perception as well as ST‐OSC performance precisely. As a result, a neutral‐color ST‐OSC with high average transmittance of over 21% is successfully constructed, and a remarkable color‐rendering index approaching 100 and high power conversion efficiency (PCE) of 9.37% are simultaneously achieved. To the best of our knowledge, this is the highest PCE reported for neutral‐color ST‐OSCs to date. Importantly, this synergistic effect is demonstrated to be a universal strategy that is not only suitable for various photoactive layer systems, but can also be implanted in flexible substrate. The resulting neutral‐color flexible ST‐OSCs also show a promising PCE of 8.76%.

Publication date: April 2019

Source: Nano Energy, Volume 58

Author(s): Kang Wang, Zhiwen Jin, Lei Liang, Hui Bian, Haoran Wang, Jiangshan Feng, Qian Wang, Shengzhong (Frank) Liu

Abstract

Graphical abstract

Here, γ-CsPbI₃ is developed using chlorine doping. Characterizations show that the chlorine is effective on the morphological, crystallite orientation and composition change of the γ-CsPbI₃ film. Moreover, the optimized film shows higher conductivities, lower trap density and longer carrier mobility. As a result, the optimized PSC gives PCE of 16.07% with much improved stability.

Publication date: 20 February 2019

Source: Joule, Volume 3, Issue 2

Author(s): David P. McMeekin, Suhas Mahesh, Nakita K. Noel, Matthew T. Klug, JongChul Lim, Jonathan H. Warby, James M. Ball, Laura M. Herz, Michael B. Johnston, Henry J. Snaith

Context & Scale

Summary

Graphical Abstract

Three polymers L24, L68, and L810 are developed as donor materials for organic solar cells. As the alkyl side chain of the fluorobenzotriazole (FTAZ) unit increases, the L810‐based device exhibits lower energy loss, better molecular face‐on orientation, and a higher absorption coefficient. Consequently, the power conversion efficiency is improved to 12.1%, which is one of the highest values for FTAZ‐based devices.

Abstract

The fluorobenzotriazole (FTAZ)‐based copolymer donors are promising candidates for nonfullerene polymer solar cells (PSCs), but suffer from relatively low photovoltaic performance due to their unsuitable energy levels and unfavorable morphology. Herein, three polymer donors, L24, L68, and L810, based on a chlorinated‐thienyl benzodithiophene (BDT‐2Cl) unit and FTAZ with different branched alkyl side chain, are synthesized. Incorporation of a chlorine (Cl) atom into the BDT unit is found to distinctly optimize the molecular planarity, energy levels, and improve the polymerization activity. Impressively, subtle side chain length of FTAZ realizes a dramatic improvement in all the device parameters, as revealed by the short‐current density (J _sc) improved from 7.41 to 20.76 mA cm⁻², fill‐factor from 36.3 to 73.5%, and even the open‐circuit voltage (V _oc) from 0.495 to 0.790 V. The best power conversion efficiency (PCE) of 12.1% is obtained from the L810‐based device, which is one of the highest values reported for FTAZ‐based PSCs so far. Notably, the corresponding external quantum efficiency curve keeps a very prominent value up to 80% from 500 to 800 nm. The notable performance is discovered from the reduced energy loss, improved molecular face‐on orientation, the down‐shifted energy levels, and optimized absorption coefficient regulated by side‐chain engineering.

An electron‐selective TiO _x based heterocontact is developed and trialed as a dopant‐free partial rear contact in high efficiency silicon solar cells. This cell not only reaches an efficiency of above 23% but also maintains its performance after a short anneal at 400 °C—setting new benchmarks of performance and thermal stability for this cell architecture.

Abstract

Over the past five years, there has been a significant increase in both the intensity of research and the performance of crystalline silicon devices which utilize metal compounds to form carrier‐selective heterocontacts. Such heterocontacts are less fundamentally limited and have the potential for lower costs compared to the current industry dominating heavily doped, directly metalized contacts. A low temperature (≤230 °C), TiO _x /LiF _x /Al electron heterocontact is presented here, which achieves mΩcm² scale contact resistivities ρ_c on lowly doped n‐type substrates. As an extreme demonstration of the potential of this heterocontact, it is trialed in a newly developed, high efficiency n‐type solar cell architecture as a partial rear contact (PRC). Despite only contacting ≈1% of the rear surface area, an efficiency of greater than 23% is achieved, setting a new benchmark for n‐type solar cells featuring undoped PRCs and confirming the unusually low ρ_c of the TiO _x /LiF _x /Al contact. Finally, in contrast to previous versions of the n‐type undoped PRC cell, the performance of this cell is maintained after annealing at 350–400 °C, suggesting its compatibility with conventional surface passivation activation and sintering steps.

Publication date: 17 April 2019

Source: Joule, Volume 3, Issue 4

Author(s): Jun Yuan, Yunqiang Zhang, Liuyang Zhou, Guichuan Zhang, Hin-Lap Yip, Tsz-Ki Lau, Xinhui Lu, Can Zhu, Hongjian Peng, Paul A. Johnson, Mario Leclerc, Yong Cao, Jacek Ulanski, Yongfang Li, Yingping Zou

Context & Scale

Summary

Graphical Abstract

Three A₂‐A₁‐D‐A₁‐A₂ type small molecules are used as electron acceptors for fullerene‐free organic solar cells, and the cyano‐containing A₂ segments have a large effect on the open‐circuit voltage (V _OC) and power conversion efficiency (PCE). By utilizing the “same‐acceptor‐strategy” and chosing J71 as donor polymer, the dicyano containing molecule of BTA3 affords the highest PCE of 8.60% with a V _OC of 1.20 V.

To achieve efficient organic solar cells (OSCs), the design of promising non‐fullerene small molecular acceptors (SMAs) is crucially important and the relationship between the chemical structure and optoelectronic properties needs to be further investigated. Herein, an A₂‐A₁‐D‐A₁‐A₂ molecular skeleton is adopted to study the effect of end‐capped A₂ groups containing different numbers of cyano units, where D and A₁ are fixed as indacenodithiophene (IDT) and benzotriazole (BTA) units, respectively. Utilizing the “same‐acceptor‐strategy,” three BTA‐based SMAs, named as BTA701, BTA3, and BTA703, are paired with a BTA‐based p‐type polymer J71. The open‐circuit voltage (V _OC) gradually decreases with the enhancement of electron‐accepting ability of terminal A₂ units, from 1.32 V (BTA701) to 1.20 V (BTA3) and to 0.85 V (BTA703). The device J71:BTA3 eventually shows the best power conversion efficiency (PCE) of 8.60% with a V _OC up to 1.2 V because of the complementary light absorption, high and balanced hole and electron mobility, suitable phase separation, and crystallinity. This study indicates that appropriate cyano‐containing units in BTA‐based SMAs can effectively modulate the absorption, energy levels, charge mobility and surface free energy, which can provide valuable insights to the further design of SMAs. In addition, these results prove that the “same‐acceptor‐strategy” is simple and effective to realize a V _OC as high as 1.2V.

New‐type 2D/3D perovskites are designed by first introducing two hydrophobic ammonium salt cations with halogen functional groups into 3D perovskite. The 2D/3D perovskite devices exhibit optimal power conversion efficiency as high as 20.08% under 1 sun irradiation and superior stability when exposed to humidity, temperature, and continuous UV irradiation.

Abstract

2D perovskites have attracted extensive attention due to their excellent stability compared with 3D perovskites. However, the intrinsic hydrophilicity of introduced alkylammonium salts effects the humidity stability of 2D/3D perovskites. Devices based on longer chain alkylammonium salts show improvement in hydrophobicity but lower efficiency due to the poorer charge transport among various layers. To solve this issue, two hydrophobic short‐chain alkylammonium salts with halogen functional groups (2‐chloroethylamine, CEA⁺ and 2‐bromoethylamine, BEA⁺) are introduced into (Cs_0.1FA_0.9)Pb(I_0.9Br_0.1)₃ 3D perovskites to form 2D/3D perovskite structure, which achieve high‐quality perovskite films with better crystallization and morphology. The optimal 2D/3D perovskite solar cells (PSCs) with 5% CEA⁺ display a power conversion efficiency (PCE) as high as 20.08% under 1 sun irradiation. Because of the notable hydrophobicity of alkylammonium cations with halogen functional groups and the formed 2D/3D perovskite structure, the optimal PSCs exhibit superior moisture resistance and retain 92% initial PCE after aging at 50 ± 5% relative humidity for 2400 h. This work opens up a new direction for the design of new‐type 2D/3D PSCs with improved performance by employing proper alkylammonium salts with different functional groups.

Protonation of polyethylenimine ethoxylated (PEIE) can effectively passivate the chemical reaction between the PEIE and a nonfullerene (NF) active layer. As a result, the PEIE can work very efficiently as a low‐work‐function interface for NF solar cells. These flexible solar cells exhibit power conversion efficiency up to 12.5% with a room‐temperature‐processed PEIE interface.

Abstract

Nonfullerene (NF) organic solar cells (OSCs) have been attracting significant attention in the past several years. It is still challenging to achieve high‐performance flexible NF OSCs. NF acceptors are chemically reactive and tend to react with the low‐temperature‐processed low‐work‐function (low‐WF) interfacial layers, such as polyethylenimine ethoxylated (PEIE), which leads to the “S” shape in the current‐density characteristics of the cells. In this work, the chemical interaction between the NF active layer and the polymer interfacial layer of PEIE is deactivated by increasing its protonation. The PEIE processed from aqueous solution shows more protonated N⁺ than that processed from isopropyl alcohol solution, observed from X‐ray photoelectron spectroscopy. NF solar cells (active layer: PCE‐10:IEICO‐4F) with the protonated PEIE interfacial layer show an efficiency of 13.2%, which is higher than the reference cells with a ZnO interlayer (12.6%). More importantly, the protonated PEIE interfacial layer processed from aqueous solution does not require a further thermal annealing treatment (only processing at room temperature). The room‐temperature processing and effective WF reduction enable the demonstration of high‐performance (12.5%) flexible NF OSCs.