Chen Weijie

The thermomechanical properties of donor–acceptor polymers with systematically tuned side‐chain lengths are investigated. A predictive linear mass‐per‐flexible bond model is introduced to capture the side‐chain length effect on their glass transition temperature. This provides guidance toward the design of future application‐driven conjugated polymers with desired thermomechanical performances.

Abstract

Semiconducting donor–acceptor (D–A) polymers have attracted considerable attention toward the application of organic electronic and optoelectronic devices. However, a rational design rule for making semiconducting polymers with desired thermal and mechanical properties is currently lacking, which greatly limits the development of new polymers for advanced applications. Here, polydiketopyrrolopyrrole (PDPP)‐based D–A polymers with varied alkyl side‐chain lengths and backbone moieties are systematically designed, followed by investigating their thermal and thin film mechanical responses. The experimental results show a reduction in both elastic modulus and glass transition temperature (T _g) with increasing side‐chain length, which is further verified through coarse‐grained molecular dynamics simulations. Informed from experimental results, a mass‐per‐flexible bond model is developed to capture such observation through a linear correlation between T _g and polymer chain flexibility. Using this model, a wide range of backbone T _g over 80 °C and elastic modulus over 400 MPa can be predicted for PDPP‐based polymers. This study highlights the important role of side‐chain structure in influencing the thermomechanical performance of conjugated polymers, and provides an effective strategy to design and predict T _g and elastic modulus of future new D–A polymers.

The energy level alignment of the lead halide perovskite thin film is studied, while subjected to a cycle of atmospheres relevant to industrial production. The atmospheric‐induced doping and dedoping is found to affect the photovoltaic properties significantly. The impact of the environment implies careful control of the atmosphere is necessary for achieving the best device performance.

Abstract

Solar cells based on metal halide perovskites have emerged as a promising low‐cost photovoltaic technology. In contrast to inert atmospheres where most of the lab‐scale devices are made to date, large‐area low‐cost production of perovskite solar cells often involves processing of perovskites in various atmospheres including ambient air, nitrogen, and/or vacuum. Herein, the impact of atmosphere on the energy levels of methylammonium lead halide perovskite films is systematically investigated. The atmosphere is varied to simulate the typical fabrication process. Through a comprehensive analysis combining the Fermi level evolution, surface photovoltage, photoluminescence properties, photovoltaic performance, and device simulation, an overall landscape of the energy diagram of the perovskite layer is able to be determined. The findings have direct implications for real‐world devices under typical atmospheres, and provide insights into the fabrication‐process design and optimization. Furthermore, a universal Fermi level shift under vacuum for lead halide‐based perovskites revealed in this study, urges a refreshed view on the energetics studies conducted without considering the atmospheric effect.

Interface energetics in 2D Ruddlesden–Popper perovskite solar cells are systematically investigated. The potential gradient across ligands that significantly decreases surface work function, promotes separation of the photogenerated charge carriers with electron transferring from perovskite crystal to ligand at the interface, suppressing the charge recombination and thus enhancing the open‐circuit voltage.

Abstract

2D Ruddlesden–Popper perovskites (RPPs) are emerging as potential challengers to their 3D counterpart due to superior stability and competitive efficiency. However, the fundamental questions on energetics of the 2D RPPs are not well understood. Here, the energetics at (PEA)₂(MA) _{n −1}Pb _n I_{3 n +1}/[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) interfaces with varying n values of 1, 3, 5, 40, and ∞ are systematically investigated. It is found that n–n junctions form at the 2D RPP interfaces (n = 3, 5, and 40), instead of p–n junctions in the pure 2D and 3D scenarios (n = 1 and ∞). The potential gradient across phenethylammonium iodide ligands that significantly decreases surface work function, promotes separation of the photogenerated charge carriers with electron transferring from perovskite crystal to ligand at the interface, reducing charge recombination, which contributes to the smallest energy loss and the highest open‐circuit voltage (V _oc) in the perovskite solar cells (PSCs) based on the 2D RPP (n = 5)/PCBM. The mechanism is further verified by inserting a thin 2D RPP capping layer between pure 3D perovskite and PCBM in PSCs, causing the V _oc to evidently increase by 94 mV. Capacitance–voltage measurements with Mott–Schottky analysis demonstrate that such V _oc improvement is attributed to the enhanced potential at the interface.

Publication date: September 2020

Source: Nano Energy, Volume 75

Author(s): M. Laska, Z. Krzemińska, K. Kluczyk-Korch, D. Schaadt, E. Popko, W.A. Jacak, J.E. Jacak

Implementation of nanoparticles (NPs) with plasmonic effects is an effective strategy for photon and charge dynamic management in perovskite solar cells (PSCs). The outstanding effects of plasmonic nanostructures such as Ag NPs decorated on TiO₂ nanowires in electron transport materials as well as localized surface plasmon resonance of Au NPs in hole transport materials enhance the photovoltaic response of PSCs.

Abstract

Perovskite solar cells (PSCs) have emerged recently as promising candidates for next generation photovoltaics and have reached power conversion efficiencies of 25.2%. Among the various methods to advance solar cell technologies, the implementation of nanoparticles with plasmonic effects is an alternative way for photon and charge carrier management. Surface plasmons at the interfaces or surfaces of sophisticated metal nanostructures are able to interact with electromagnetic radiation. The properties of surface plasmons can be tuned specifically by controlling the shape, size, and dielectric environment of the metal nanostructures. Thus, incorporating metallic nanostructures in solar cells is reported as a possible strategy to explore the enhancement of energy conversion efficiency mainly in semi‐transparent solar cells. One particularly interesting option is PSCs with plasmonic structures enable thinner photovoltaic absorber layers without compromising their thickness while maintaining a high light harvest. In this Review, the effects of plasmonic nanostructures in electron transport material, perovskite absorbers, the hole transport material, as well as enhancement of effective refractive index of the medium and the resulting solar cell performance are presented. Aside from providing general considerations and a review of plasmonic nanostructures, the current efforts to introduce these plasmonic structures into semi‐transparent solar cells are outlined.

The bromination of non‐fullerene acceptors provides a promising alternative approach for the creation of high‐performance organic solar cells. BTIC‐2Br‐m ‐based devices exhibit an outstanding power conversion efficiency of 16.11% with an elevated open circuit voltage of 0.88 V, representing one of the highest efficiencies in brominated non‐fullerene acceptors.

Abstract

The concept of bromination for organic solar cells has received little attention. However, the electron withdrawing ability and noncovalent interactions of bromine are similar to those of fluorine and chlorine atoms. A tetra‐brominated non‐fullerene acceptor, designated as BTIC‐4Br, has been recently developed by introducing bromine atoms onto the end‐capping group of 2‐(3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene) malononitrile and displayed a high power conversion efficiency (PCE) of 12%. To further improve its photovoltaic performance, the acceptor is optimized either by introducing a longer alkyl chain to the core or by modulating the numbers of bromine substituents. After changing each end‐group to a single bromine, the BTIC‐2Br‐m ‐based devices exhibit an outstanding PCE of 16.11% with an elevated open‐circuit voltage of V _oc = 0.88 V, one of the highest PCEs reported among brominated non‐fullerene acceptors. This significant improvement can be attributed to the higher light harvesting efficiency, optimized morphology, and higher exciton quenching efficiencies of the di‐brominated acceptor. These results demonstrate that the substitution of bromine onto the terminal group of non‐fullerene acceptors results in high‐efficiency organic semiconductors, and promotes the use of the halogen‐substituted strategy for polymer solar cell applications.

Publication date: 11 June 2020

Source: Chem, Volume 6, Issue 6

Author(s): Chunqing Ma, Nam-Gyu Park

Transient photovoltage (TPV) experiments are performed on perovskite solar cells with different defect concentrations. The voltage decay time does not represent recombination dynamics but an RC‐limited charging and discharging process. Devices with inhomogeneous generation and recombination profiles can show a pronounced double‐exponential behavior. Drift‐diffusion device simulations reproduce these results and show the relevance of transport limitations in TPV.

Abstract

In all kinds of solar cells, transient photovoltage (TPV) decay measurements have been used to determine charge carrier lifetimes and to quantify recombination processes and orders. However, in particular, for thin‐film devices with a high capacitance, the time constants observed in common TPV measurements do not describe recombination dynamics but RC (R: resistance, C: capacitance) times for charging the electrodes. This issue has been revisited for organic and perovskite solar cells in the recent literature. Here, these discussions are extended by analyzing a perovskite model system (Bi defects in Cs_0.1FA_0.9Pb(Br_0.1I_0.9)₃ in which defect recombination can be tuned. It is found that TPV, intensity‐modulated photovoltage spectroscopy, and impedance spectroscopy yield the same time constants that do not describe recombination dynamics but are limited by the differential resistance of the diode and the geometric capacitance in common light intensity ranges (<1 sun). By employing numerical device simulations, it is found that low charge carrier mobility can furthermore limit the TPV time constants. In samples with spatially nonuniform recombination dynamics, two time constants are measured, which depend on the charge carrier generation profile that can be tuned by the wavelength of the incident light. In that case, numerical simulation provides insights into recombination and charge transport processes in the device.

The doping of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with dopamine is reported. The doping of dopamine endows PEDOT:PSS with enhanced work function and conductivity. This work provides an efficient strategy to enhance the performances of organic solar cells.

Abstract

Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been is applied as hole transport material in organic electronic devices for more than 20 years. However, the redundant sulfonic acid group of PEDOT:PSS has often been overlooked. Herein, PEDOT:PSS‐DA is prepared via a facile doping of PEDOT:PSS with dopamine hydrochloride (DA·HCl) which reacts with the redundant sulfonic acid of PSS. The PEDOT:PSS‐DA film exhibits enhanced work function and conductivity compared to those of PEDOT:PSS. PEDOT:PSS‐DA‐based devices show a power conversion efficiency of 16.55% which is the highest in organic solar cells (OSCs) with (poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)‐4‐fluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithio‐phene))‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)‐benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione))] (PM6):(2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐ethylhexyl)‐3,9‐diundecyl‐12,13‐dihydro‐[1,2,5]thiadiazolo[3,4‐e]thieno[2′′,3′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2‐g]thieno[2′,3′:4,5]thieno[3,2‐b]indole‐2,10‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile) (Y6) as the active layer. Furthermore, PEDOT:PSS‐DA also exhibits enhanced performance in three other donor/acceptor systems, exhibiting high compatibility in OSCs. This work demonstrates that doping PEDOT:PSS with various amino derivatives is a potentially efficient strategy to enhance the performance of PEDOT:PSS in organic electronic devices.

The hygroscopic characteristics of dopants in 2,2′,7,7′‐tetrakis(N ,N ‐di‐p ‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐MeOTAD) hole‐transporting layers (HTLs) result in the degradation of both HTL morphology and device performance. A detailed study on the effects of initial morphology is presented. Accumulated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is the key factor causing poor stability. Performing thermal annealing on HTL can improve the air stability greatly.

Abstract

Doped 2,2′,7,7′‐tetrakis(N ,N ‐di‐p ‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐MeOTAD), which acts as a hole‐transporting layer (HTL), endows perovskite solar cells (PSCs) with excellent performance. However, the intrinsically hygroscopic nature of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dopants also aggravates the moisture instability of PSCs. In this work, the origins of the moisture instability of spiro‐MeOTAD HTLs are explored and strategies to enhance moisture resistance are proposed. After 780 h of aging in air, 52% of the initial power conversion efficiency (PCE) can be sustained by prolonging the mixing time of the precursor solution of spiro‐MeOTAD to reduce accumulated LiTFSI. In contrast, only 7% of the initial PCE remains if the precursor solution is mixed briefly. By thermally annealing an HTL to evaporate residual tBP in spiro‐MeOTAD, pinholes are completely eliminated and 65% of the initial PCE remains after the same aging time. In this study, the significance of the initial morphology of spiro‐MeOTAD HTLs on device stability is analyzed and strategies based on physical morphology for controlling PSC moisture instability induced by HTL dopants are developed.

Improved defect passivation of perovskite quantum dots (PQDs) is reported using glycine as a dual‐passivation ligand, which can simultaneously fill the A‐site (cesium) and iodine vacancies on the PQD surface. The enhanced photovoltaic performance is obtained in the glycine‐based PQD solar cells (PQDSCs) compared with that of the traditional Pb(NO₃)₂‐based PQDSCs, resulting from increased charge carrier extraction.

Abstract

Inorganic CsPbI₃ perovskite quantum dot (PQD) receives increasing attention for the application in the new generation solar cells, but the defects on the surface of PQDs significantly affect the photovoltaic performance and stability of solar cells. Herein, the amino acids are used as dual‐passivation ligands to passivate the surface defects of CsPbI₃ PQDs using a facile single‐step ligand exchange strategy. The PQD surface properties are investigated in depth by combining experimental studies and theoretical calculation approaches. The PQD solid films with amino acids as dual‐passivation ligands on the PQD surface are thoroughly characterized using extensive techniques, which reveal that the glycine ligand can significantly improve defect passivation of PQDs and therefore diminish charge carrier recombination in the PQD solid. The power conversion efficiency (PCE) of the glycine‐based PQD solar cell (PQDSC) is improved by 16.9% compared with that of the traditional PQDSC fabricated with Pb(NO₃)₂ treating the PQD surface, owning to improved charge carrier extraction. Theoretical calculations are carried out to comprehensively understand the thermodynamic feasibility and favorable charge density distribution on the PQD surface with a dual‐passivation ligand.

In this work, we demonstrated a simple random ternary copolymerization strategy to synthesize a series of polymer acceptors PTPBT‐ET x by polymerizing a small molecule acceptor unit modified from Y6 with thiophene connecting unit and controlled amount of 3‐ethylesterthiophene (ET) unit. Compared PTPBT of only Y6‐like units and thiophene units, PTPBT‐ET x (where x represents the molar ratio of ET unit) with incorporating ET unit in the ternary copolymers show up‐shifted LUMO energy level, increased electron mobility and improved blend morphology in the blend film with polymer donor PBDB‐T. And the all‐PSC based on PBDB‐T:PTPBT‐ET 0.3 achieved high power conversion efficiency (PCE) over 12.5%. In addition, the PTPBT‐ET 0.3 based all‐PSC device also exhibits long‐term photostability over 300 hours. These results demonstrate that the random ternary copolymerization of a small molecule acceptor unit with a third functional unit is a simple but effective strategy to develop efficient polymer acceptors for all‐PSCs.

Publication date: September 2020

Source: Nano Energy, Volume 75

Author(s): Yongjoon Cho, Tanya Kumari, Seonghun Jeong, Sang Myeon Lee, Mingyu Jeong, Byongkyu Lee, Jiyeon Oh, Youdi Zhang, Bin Huang, Lie Chen, Changduk Yang

Publication date: September 2020

Source: Nano Energy, Volume 75

Author(s): Jie Xu, Hua Dong, Jun Xi, Yingguo Yang, Yue Yu, Lin Ma, Jinbo Chen, Bo Jiao, Xun Hou, Jingrui Li, Zhaoxin Wu

Publication date: September 2020

Source: Nano Energy, Volume 75

Author(s): Junjie Ma, Yuhao Li, Jing Li, Minchao Qin, Xiao Wu, Ziyu Lv, Yao-Jane Hsu, Xinhui Lu, Yichu Wu, Guojia Fang

Publication date: September 2020

Source: Nano Energy, Volume 75

Author(s): Lingyi Meng, Zhuangzhuang Wei, Tao Zuo, Peng Gao

An ultrathin PbI₂ shell is constructed on the surface of triple cation based lead halide perovskite via the method of ZnI₂ aided in situ transformation. This PbI₂ layer can prolong excited‐state lifetime of perovskite and attenuate the heat‐accelerated interface charge recombination, leading to better power conversion efficiency and improved thermostability of perovskite solar cells.

Abstract

Judicious tailoring of a robust interlayer is central to maintain the durable operation of optoelectronic devices. In this paper, an ultrathin, compact, and uniform PbI₂ shell on the surface of perovskite via the method of ZnI₂ aided in situ transformation is produced. The resultant PbI₂ interlayer can prolong the excited‐state lifetime of perovskite and attenuate the recombination kinetics of separated charges, leading to an improvement of power conversion efficiency up to 22.5% for perovskite solar cells (PSCs) at the AM 1.5G conditions. Moreover, the PSC with PbI₂ interlayer exhibits an enhanced thermostability, retaining 87% of initial efficiency after aging at 60 °C for 1000 h.

A phase segregation effect in triple‐cation mixed‐halide perovskite under continuous light illumination is reported. The halide ion migration leads to the formation of smaller bandgap iodide‐rich and larger bandgap bromide‐rich domains in the perovskite film. The relationship between photoinduced phase segregation and an unusual increase in carrier lifetime under illumination (>1 µs) is highlighted. These photoinduced changes are thermally activated and fully reversible under darkness.

Abstract

Mixed‐halide hybrid perovskite semiconductors have attracted tremendous attention as a promising candidate for efficient photovoltaic and light‐emitting devices. However, these perovskite materials may undergo phase segregation under light illumination, thus affecting their optoelectronic properties. Here, photoexcitation induced phase segregation in triple‐cation mixed‐halide perovskite films that yields to red‐shift in the photoluminescence response is reported. It is demonstrated that photoexcitation induced halide migration leads to the formation of smaller bandgap iodide‐rich and larger bandgap bromide‐rich domains in the perovskite film, where the phase segregation rate is found to follow the excitation power‐density as a power law. Results confirm that charge carrier lifetime increases due to the trapping of photoexcited carriers in the segregated smaller bandgap iodide‐rich domains. Interestingly, these photoinduced changes are fully reversible and thermally activated when the excitation power is turned off. A significant difference in activation energies for halide ion migration is observed during phase segregation and recovery process. Additionally, the emission linewidth broadening is investigated as a function of temperature which is governed by the exciton–optical phonon coupling. The mechanism of photoinduced phase segregation is interpreted based on exciton–phonon coupling strength in both mixed and demixed (segregated) states of perovskite films.

The aza[5]helicene‐based hole‐transporter is superior to its congener with the planar N‐annulated perylene π‐linker. This study has highlighted that the use of a helical π‐linker for donor−π linker−donor typed organic semiconductors can retain stronger intermolecular π⋅⋅⋅π interactions and attenuated interface charge recombination, leading to better power conversion efficiency of perovskite solar cells.

Abstract

The superior role of helical π‐linkers is demonstrated for the design of donor−π linker−donor typed molecular semiconductors in perovskite solar cells (PSCs). Flat N‐annulated perylene (NP) and contorted aza[5]helicene (A5H) are side‐functionalized with methoxyphenyl and end‐capped with dimethoxydiphenylamine electron‐donor to afford two small‐molecule hole‐transporters J3 and J4. For methoxyphenyl functionalized π‐linkers, intermolecular π⋅⋅⋅π interactions in planar NP exist more extensively than those in helical A5H. However, for the dimethoxydiphenylamine derived hole‐transporters with high highest occupied molecular orbital energy levels, a part of the π⋅⋅⋅π interaction remains for J4 with A5H, while this desirable effect for charge transport is completely deprived for J3 with NP. Thus, the theoretically predicted hole mobility of J4 single‐crystal is even over two times higher than that of J3 one. Because of the larger size of the molecular aggregate, the hole mobility of the spin‐coated J4 thin film is also over three times as high as that of the J3 analog. Due to the reduced transport resistance and enhanced recombination resistance, PSCs with J4 exhibit a power conversion efficiency of 21.0% at standard air mass 1.5 global conditions, which is higher than that of 19.4% with J3 and that of 20.3% with spiro‐OMeTAD control.

The undercoordinated ionic defects at heterojunction interfaces remain challenges that limit the performances and stability of perovskite photoelectric devices. A self‐phase separated doping strategy is developed to link multilayer heterojunction interfaces including both the energy level and trap states, paving a novel route for nonequilibrium distributed dopants to solve the key challenge of interface defects.

Abstract

In perovskite solar cells (PSCs), the interfaces of the halide perovskite/electron transport layer (ETL) and ETL/metal oxide electrode (MOE) always attract and trap free carriers via the surface electrostatic force, altering quasi‐Fermi level (E _Fq) splitting of contact interfaces, and significantly limit the charge extraction efficiency and intrinsic stability of devices. Herein, a graded “bridge” is first reported to link the MOE and perovskite interfaces by self vertical phase separation doping (PSD), diminishing the side effect of notorious ionic defects via both reinforced interface E _bi and the vacancies filling. Experimental and theoretical results prove that the inhomogeneous distribution of CsF in the bulk or surface of PC₆₁BM would not only form metal–oxygen (M–O) dipole on MOE, reinforcing the interface E _bi, but also create a graded energy bridge to alleviate the disadvantage of band offset raised by the enhanced interface E _bi, which significantly avoid the carrier accumulation and recombination at defective interfaces. Employing PSD, the power conversion efficiency of the devices approaches 21% with a high open‐circuit voltage (1.148 V) and delivers a high stability of 89% after aging 60 days in atmosphere without encapsulation, which is the highest efficiency of organic electron transport layers for n–i–p PSCs.