awn

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.

A low‐temperature seed‐assisted growth (SAG) method for high‐quality CsPbIBr₂ perovskite films through reducing the formation energy by introducing methylammonium halides is demonstrated. The device fabricated using optimized SAG‐based film yields a power conversion efficiency of 10.47% with a remarkable open circuit voltage (V _oc) of 1.21 V.

Abstract

All‐inorganic CsPbIBr₂ perovskite has recently received growing attention due to its balanced band gap and excellent environmental stability. However, the requirement of high‐temperature processing limits its application in flexible devices. Herein, a low‐temperature seed‐assisted growth (SAG) method for high‐quality CsPbIBr₂ perovskite films through reducing the crystallization temperature by introducing methylammonium halides (MAX, X = I, Br, Cl) is demonstrated. The mechanism is attributed to MA cation based perovskite seeds, which act as nuclei lowering the formation energy of CsPbIBr₂ during the annealing treatment. It is found that methylammonium bromide treated perovskite (Pvsk‐Br) film fabricated at low temperature (150 °C) shows micrometer‐sized grains and superior charge dynamic properties, delivering a device with an efficiency of 10.47%. Furthermore, an efficiency of 11.1% is achieved for a device based on high‐temperature (250 °C) processed Pvsk‐Br film via the SAG method, which presents the highest reported efficiency for inorganic CsPbIBr₂ solar cells thus far.

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.

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

Radio frequency magnetron sputtered SnO₂ is used as an electron transport layer (ETL) for PbS quantum dot solar cells with an efficiency of 8.4%. Further to modify the SnO₂ surface, a thin sol–gel ZnO layer is spin‐coated on top of SnO₂ forming a SnO₂–ZnO double ETL. The best device with double ETL achieves an efficiency over 10%.

Herein, for the first time, it is successfully demonstrated that radio frequency (RF) magnetron sputtered SnO₂ can be a qualified alternative electron transport layer (ETL) for a high‐efficiency PbS quantum dot (QD) solar cell. The highest performing device using such a SnO₂ ETL obtains an efficiency of 8.4%, which is comparable to the sol–gel ZnO‐based one (8.8%). The excellent performance mainly results from the improved current density, which is attributed to the superior properties of the SnO₂ ETL, such as high electron mobility and excellent optical transmittance. However, it is also found that the sputtered SnO₂‐based devices show smaller voltage and fill factor due to the unsatisfied surface morphology and energy level alignment. By combining a thin (around 10 nm) sol–gel ZnO film on top of a sputtered SnO₂ film to form the double ETL, the best efficiency of 10.1% is obtained, which is the highest efficiency using SnO₂ ETL in a PbS QD solar cell. The work not only provides a new avenue to improve the efficiency of PbS QD solar cells but also offers the possibility to use an industry compatible sputtering technique for PbS QD solar cells.

A novel non‐conjugated polymer based on the polyethylene backbone, PVCz‐OMeTPA, with suitable energy levels, good hole mobility, as well as excellent film‐forming ability is developed as an efficient dopant‐free hole‐transporting material (HTMs) for inverted quasi‐2D perovskite solar cells (PSCs). Quasi‐2D PSCs using the dopant‐free PVCz‐OMeTPA as HTM exhibit an excellent power conversion efficiency of 17.22% and long‐term environmental stability.

Quasi‐2D perovskites with excellent stability have been recognized as an alternative to 3D counterparts for perovskite solar cells (PSCs). Although the power conversion efficiency (PCE) of quasi‐2D PSCs has increased over 18% by the compositional controlling and solvent engineering of perovskites, fewer studies have been conducted to exploit charge transport layers and investigate their interface relationships with quasi‐2D perovskites. To achieve high efficiency and good long‐term stability for quasi‐2D PSCs, hole‐transporting materials (HTMs) with matched energy levels and good chemical compatibility with quasi‐2D perovskites are explored and investigated. Herein, a novel non‐conjugated polymer based on polyethylene backbone, poly[3,6‐(4,4′‐dimethoxytriphenylamino)‐9‐vinyl‐9H‐carbazole] (PVCz‐OMeTPA), is easily synthesized and investigated as a promising dopant‐free HTM for quasi‐2D PSCs. Due to its more suitable energy levels, good hole mobility, as well as excellent film‐forming ability to assist the formation of high‐quality quasi‐2D perovskite films, the optimized p–i–n structured quasi‐2D PSCs based on PVCz‐OMeTPA exhibit the best PCE of 17.22%. The unencapsulated quasi‐2D PSCs based on PVCz‐OMeTPA maintain 82% of the initial efficiency after 1400 h under a relative humidity of ≈40% and sustain over 81% of the original efficiency after aging for 600 h upon 70 °C of continuous annealing.

A simple yet highly compatible interconnecting layer for organic tandem solar cell is presented. All double‐junction tandem devices with different active layers show high reproducibility and efficiencies in several laboratories. Among these tandem devices, an excellent PCE of 16.1% is achieved. In addition, most of the tandem devices achieve more than 40% enhancement from the single‐junction organic photovoltaic device.

Abstract

Tandem structure provides a practical way to realize high efficiency organic photovoltaic cells, it can be used to extend the wavelength coverage for light harvesting. The interconnecting layer (ICL) between subcells plays a critical role in the reproducibility and performance of tandem solar cells, yet the processability of the ICL has been a challenge. In this work the fabrication of highly reproducible and efficient tandem solar cells by employing a commercially available material, PEDOT:PSS HTL Solar (HSolar), as the hole transporting material used for the ICL is reported. Comparing with the conventional PEDOT:PSS Al 4083 (c‐PEDOT), HSolar offers a better wettability on the underlying nonfullerene photoactive layers, resulting in better charge extraction properties of the ICL. When FTAZ:IT‐M and PTB7‐Th:IEICO‐4F are used as the subcells, a power conversion efficiency (PCE) of 14.7% is achieved in the tandem solar cell. To validate the processability of these tandem solar cells, three other research groups have successfully fabricated tandem devices using the same recipe and the highest PCE obtained is 16.1%. With further development of donor polymers and device optimization, the device simulation results show that a PCE > 22% can be realized in tandem cells in the near future.

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.

Tapered cross‐section photoelectron spectroscopy is introduced as a new method to analyze complete electronic devices in thermodynamic equilibrium and in operation. The power of the method is demonstrated by analyzing a state‐of‐the‐art mixed ion perovskite solar cell in the dark and under illumination.

Abstract

The purpose of this article is twofold. On the one hand the method of spacial resolved photoemission spectroscopy on small angle tapered cross‐sections (TCS) of complete devices is introduced to analyze simultaneously the chemical and electronic structure. On the other hand, a specific working principle of the analyzed cell type is revealed. Solar cells of 18% efficiency are prepared from a single precursor (FAPbI₃)_0.85(MAPbBr₃)_0.15 with excess of 15% PbI₂. It is shown that TCS‐phototoelectron spectroscopy allows to determine the chemical composition as well as the potential distribution across the full device in the dark and in operation. The energy converting contact is the hole extraction back contact. Interestingly the photopotential in the analyzed cell type is predominantly created within the hole extraction layer and not in the n‐doped perovskite absorber. With the addition of measured core level to valence band maximum positions of the respective layers, TCS line scans lead to the band diagram for the full device. In addition, depth variations of the chemical composition are found: the bromide concentration increases while the iodide concentration is reduced near and within the mesoporous TiO₂ layer.

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.

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 of nonequilibrium distributed dopant 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.

Organic bulk heterojunction and perovskite solar cells can be applied to windows, exterior walls, and roofs of houses to produce electrical energy, and the generated energy can easily charge various electronic devices (i.e., mobile phones, laptops, headsets, cameras, etc.). Since the use of internet of things (IoT) devices is increasing recently, the strategy of utilizing building energy will become more important as time goes by. Therefore, as a strategy to produce more energy, Changhee Lee, Hyeok Kim and co‐workers intend in article number https://doi.org/10.1002/admi.2019020031902003 to use Al doped TiO₂ as the electron extraction layer of the solar cell to improve the power conversion efficiency.

A general decomposition pathway from tetragonal CH₃NH₃PbI₃ and cubic CH₃NH₃PbBr₃ to lead halides is revealed, through the formation of an intermediate superstructure CH₃NH₃PbX_2.5 with ordered vacancies. A carbon coating is demonstrated to be effective in stabilizing the perovskite framework, and thus slowing down the decomposition.

Abstract

Organic–inorganic hybrid perovskites (OIHPs) have generated considerable excitement due to their promising photovoltaic performance. However, the commercialization of perovskite solar cells (PSCs) is still plagued by the structural degradation of the OIHPs. Here, the decomposition mechanism of OIHPs under electron beam irradiation is investigated via transmission electron microscopy, and a general decomposition pathway for both tetragonal CH₃NH₃PbI₃ and cubic CH₃NH₃PbBr₃ is uncovered through an intermediate superstructure state of CH₃NH₃PbX_2.5, X = I, Br, with ordered vacancies into final lead halides. Such decomposition can be suppressed via carbon coating by stabilization of the perovskite structure framework. These findings reveal the general degradation pathway of OIHPs and suggest an effective strategy to suppress it, and the atomistic insight learnt may be useful for improving the stability of PSCs.

Publication date: 17 June 2020

Source: Joule, Volume 4, Issue 6

Author(s): Jianyu Yuan, Abhijit Hazarika, Qian Zhao, Xufeng Ling, Taylor Moot, Wanli Ma, Joseph M. Luther

Disorderly conduct : A crystal‐engineering strategy has been introduced to narrow the band gap of benchmark double perovskite Cs₂AgBiBr₆. The band gap of Cs₂AgBiBr₆ crystals can be reduced from 1.98 eV to 1.72 eV, reaching the smallest reported band gap for Cs₂AgBiBr₆ under ambient conditions. DFT calculations indicate that Ag–Bi disorder in the crystal structure could lead to band‐gap narrowing.

Abstract

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs₂AgBiBr₆, shows attractive optical and electronic features, making it promising for high‐efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal‐engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs₂AgBiBr₆ under ambient conditions. The band‐gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first‐principles calculations indicate that enhanced Ag–Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band‐gap narrowing effect. This work provides new insights for achieving lead‐free double perovskites with suitable band gaps for optoelectronic applications.

Incorporation of a small portion of a novel polymer donor named PBT(E)BTz with a deeper highest occupied molecular orbital level than that of the host materials is proven promising to construct highly efficient ternary polymer solar cells (PSCs). In addition to the role of a “solid additive” for ternary PSCs, PBT(E)BTz shows great potential to be a thermal and light stabilizer in ternary PSCs.

Abstract

Ternary strategies have attracted extensive attention due to their potential in improving power conversion efficiencies (PCEs) of single‐junction polymer solar cells (PSCs). In this work, a novel wide bandgap polymer donor (E _g ^opt ≈ 2.0 eV) named PBT(E)BTz with a deep highest occupied molecular orbital (HOMO) level (≈−5.73 eV) is designed and synthesized. PBT(E)BTz is first incorporated as the third component into the classic PBDB‐T‐SF:IT‐4F binary PSC system to fabricate efficient ternary PSCs. A higher PCE of 13.19% is achieved in the ternary PSCs with a 5% addition of PBT(E)BTz over binary PSCs (12.14%). Similarly, addition of PBT(E)BTz improves the PCE for PBDB‐T:IT‐M binary PSCs from 10.50% to 11.06%. The study shows that the improved PCE in ternary PSCs is mainly attributed to the suppressed charge carrier recombination and more balanced charge transport. The generality of PBT(E)BTz as a third component is further evidenced in another efficient binary PSC system—PBDB‐TF:BTP‐4Cl: an optimized PCE of 16.26% is realized in the ternary devices. This work shows that PBT(E)BTz possessing a deep HOMO level as an additional component is an effective ternary PSC construction strategy toward enhancing device performance. Furthermore, the ternary device with 5% PBT(E)BTz displays better thermal and light stability over binary devices.