Chen Weijie

Ion migration in tin-halide perovskites is investigated with a full range of bromide ratios under light illumination or electric field. The migration in these systems is negligible compared to lead-halide perovskites owing to a higher ion migration activation energy (E _a). Using mixed lead-tin perovskites with different E _a, the threshold E _a for observation of ion migration in perovskite devices is determined to be around 0.65 eV.

Abstract

Ion migration is a notorious phenomenon observed in ionic perovskite materials. It causes several severe issues in perovskite optoelectronic devices such as instability, current hysteresis, and phase segregation. Here, we report that, in contrast to lead halide perovskites (LHPs), no ion migration or phase segregation was observed in tin halide perovskites (THPs) under illumination or an electric field. The origin is attributed to a much stronger Sn-halide bond and higher ion migration activation energy (E _a) in THPs, which remain nearly constant under illumination. We further figured out the threshold E _a for the absence of ion migration to be around 0.65 eV using the CsSn_yPb_1-y(I_0.6Br_0.4)₃ system whose E _a varies with Sn ratios. Our work shows that ion migration does not necessarily exist in all perovskites and suggests metallic doping to be a promising way of stopping ion migration and improving the intrinsic stability of perovskites.

Highly efficient and stable flexible inverted perovskite solar cells are developed through modifying the interface between perovskite and hole transport layer via pentylammonium acetate molecule, which achieve a record power conversion efficiency of 23.68% (0.08 cm², certified: 23.35%) and excellent mechanical stability.

Abstract

Among the emerging photovoltaic technologies, rigid perovskite solar cells (PSCs) have made tremendous development owing to their exceptional power conversion efficiency (PCE) of up to 25.7%. However, the record PCE of flexible PSCs (≈22.4%) still lags far behind their rigid counterparts and their mechanical stabilities are also not satisfactory. Herein, through modifying the interface between perovskite and hole transport layer via pentylammonium acetate (PenAAc) molecule a highly efficient and stable flexible inverted PSC is reported. Through synthetic manipulation of anion and cation, it is shown that the PenA⁺ and Ac⁻ have strong chemical binding with both acceptor and donor defects of surface-terminating ends on perovskite films. The PenAAc-modified flexible PSCs achieve a record PCE of 23.68% (0.08 cm², certified: 23.35%) with a high open-circuit voltage (V _OC) of 1.17 V. Large-area devices (1.0 cm²) also realized an exceptional PCE of 21.52%. Moreover, the fabricated devices show excellent stability under mechanical bending, with PCE remaining above 91% of the original PCE even after 5000 bends.

Polymer solar cells are shown with a hole transport layer based on the same polymer as the one used in the bulk heterojunction. By doping, its work function is strongly affected to maximize the open-circuit voltage. This homojunction approach is promising and applies to recent trending semiconducting polymers with deep-lying energetics.

Hole transporting layers (HTL) in polymer solar cells remain a subject of importance to enable enhanced efficiency and stability compared to the benchmark PEDOT:PSS. The design of an interlayer based on the same polymer as the one used in the bulk heterojunction (BHJ) is reported here. In this HTL, the polymer is doped, thus forming a so-called homojunction. The conductivity of PTQ10 doped with magic blue is optimized for varying dopant concentrations. The resulting solar cells show competing power conversion efficiency as the widely used PEDOT:PSS and improved stability. This strategy opens the route toward the development of deep-lying work function HTL and is promising for future BHJ materials with deep-lying highest occupied molecular orbital polymers.

The incorporation of CdI₂ results in simultaneous lattice engineering and defect control of inorganic perovskite, which is found to reduce nonradiative recombination and prolonged charge carrier lifetime. As a result, the power conversion efficiency of inorganic perovskite solar cells is significantly improved from 19.5% to 20.8%, together with improved device ambient stability.

Abstract

Doping of all-inorganic lead halide perovskites to enhance their photovoltaic performance and stability has been reported to be effective. Up to now most studies have focused on the doping of elements in to the perovskite lattice. However, most of them cannot be doped into the perovskite lattice and the roles of these dopants are still controversial. Herein,the authors introduce CdI₂ as an additive into CsPbI_3−x Br _x and use it as active layer to fabricate high-performance inorganic perovskite solar cells (PSCs). Cd with a smaller radius than Pb can partially substitute Pb in the perovskite lattice by up to 2 mol%. Meanwhile, the remaining Cd stays on the surface and grain boundaries (GB) of the perovskite film in the form of Cs₂CdI_4−x Br_−x , which is found to reduce non-radiative recombination. These effects result in prolonged charge carrier lifetime, suppressed defect formation, decreased GBs, and an upward shift of energybands in the Cd-containing film. A champion efficiency of 20.8% is achieved for Cd-incorporated PSCs, together with improved device ambient stability. This work highlights the importance of simultaneous lattice engineering, defectcontrol and atomic-level characterization in achieving high-performance inorganic PSCs with well-defined structure-property relationships.

P1 modifies the perovskite surface, and the Förster energy transfer occurs at the P1/FAPbI₃ interface, further triggering Dexter energy transfer in FAPbI₃. Thus exciton “recycling” is realized. At the same time, the compressive stress introduced by P1 releases the residual stress on the surface of perovskite, realizing the power conversion efficiency of 23.51% (rigid) and 21.4% (flexible).

Abstract

Formamidinium lead triiodide (FAPbI₃) has been demonstrated as the most efficient perovskite system to date, due to its excellent thermal stability and an ideal bandgap approaching the Shockley-Queisser limit. Whereas, there are intrinsic quantum confinement effects in FAPbI₃, which lead to unwanted non-radiative recombination. Additionally, the black α-phase of FAPbI₃ is unstable under room temperature due to the significant residual tensile stress in the film. To simultaneously address the above issues, a thermally-activated delayed fluorescence polymer P1 is designed in the study to modify the FAPbI₃ film. Owing to the spectral overlap between the photoluminescence of P1 and absorption of the above-bandgap quantum wells of FAPbI₃, the Förster energy transfer occurs at the P1/FAPbI₃ interface, which further triggers the Dexter energy transfer within FAPbI₃. The exciton “recycling” can thus be realized, which reduces the non-radiative recombination losses in perovskite solar cells (PSCs). Moreover, P1 is found to introduce compressive stress into FAPbI₃, which relieves the tensile stress in perovskite. Consequently, the PSCs with P1 treatment achieve an outstanding power conversion efficiency (PCE) of 23.51%. Moreover, with the alleviation of stress in the perovskite film, flexible PSCs (f-PSCs) also deliver a high PCE of 21.40%.

Publication date: 15 December 2022

Source: Nano Energy, Volume 104, Part B

Author(s): Xianyong Zhou, Luozheng Zhang, Hang Hu, Zhengyan Jiang, Deng Wang, Jiabang Chen, Yaru Li, Jiawen Wu, Yong Zhang, Meiqing Zhang, Chang Liu, Yuanjun Peng, Xingzhu Wang, Baomin Xu

A solvent-induced anti-aggregation (SIAA) strategy is proposed to cope with the severe aggregation action of the small-molecule electron-transporting layer via the mixing of ethanol and trifluoroethanol solvents at an optimal volume ratio. A champion power conversion efficiency of 19.0% is yielded based on the PM6:L8-BO system after the SIAA treatment.

Abstract

The severe aggregation property of the small molecule electron-transporting layer (ETL) not only deteriorates the photovoltaic performance and operational reliability but also constrains its compatibility with large-scale coating techniques. Herein, by applying N,N′-Bis{3-[3-(Dimethylamino)propylamino]propyl}perylene-3,4,9,10-tetracarboxylic diimide (PDINN) (a well-known ETL) as a demo, a solvent-induced anti-aggregation (SIAA) strategy is proposed to cope with these hurdles via the mixing of ethanol and trifluoroethanol solvents at an optimal volume ratio. In situ photoluminescence and dynamic light scattering synergistically reveals the suppressed aggregation behavior of the SIAA-treated PDINN dispersion during the film-forming process. Owing to this amendment, the film quality and electron-transport capability of the PDINN layer are remarkably enhanced. In consequence, based on the PM6:L8-BO system, a champion power conversion efficiency (PCE) of 19.0% together with an impressive fill factor of 80.6% is harvested. A 1 cm² device with an excellent PCE of 16.6% is also fabricated using the doctor-blading SIAA-treated PDINN ink. More strikingly, this SIAA treatment impels better reliability under long-term shelf-lifetime and thermal stress periods. This work provides a promising and tractable approach to address the inherent self-aggregation issue of electron-transporting materials, which is beneficial for the development of efficient and stable organic optoelectronic devices.

Publication date: 21 December 2022

Source: Joule, Volume 6, Issue 12

Author(s): Zhengjie Zhu, Kaitian Mao, Kai Zhang, Wei Peng, Jieqi Zhang, Hongguang Meng, Shuang Cheng, Tieqiang Li, Hongzhen Lin, Qi Chen, Xiaojun Wu, Jixian Xu

A polyfluoroalkyl-containing guest acceptor (EH-C₈F₁₇) enables self-stratification in the active layer of bulk-heterojunction organic solar cells. The favorable vertical phase separation and molecular stacking increases the mobility, increases carrier lifetimes, and reduces trap-assisted recombination, leading to significantly improved device performance.

Abstract

The elaborate control of the vertical phase distribution within an active layer is critical to ensuring the high performance of organic solar cells (OSCs), but is challenging. Herein, a self-stratification active layer is realised by adding a novel polyfluoroalkyl-containing non-fullerene small-molecule acceptor (NFSMA), EH-C₈F₁₇, as the guest into PM6:BTP-eC9 blend. A favourable vertical morphology was obtained with an upper acceptor-enriched thin layer and a lower undisturbed bulk heterojunction layer. Consequently, a power conversion efficiency of 18.03 % was achieved, higher than the efficiency of 17.40 % for the device without EH-C₈F₁₇. Additionally, benefiting from the improved charge transport and collection realised by this self-stratification strategy, the OSC with a thickness of 350 nm had an impressive PCE of 16.89 %. The results of the study indicate that polyfluoroalkyl-containing NFSMA-assisted self-stratification within the active layer is effective for realising an ideal morphology for high-performance OSCs.

A straddling type-I energy level alignment is found for 2D/3D interfaces of phenylethylammonium lead quaternary iodide and methylammonium lead triiodide, enabling efficient energy transfer from the 2D to the 3D component.

Abstract

The success of using 2D Ruddlesden-Popper metal halide perovskites (MHPs) in optoelectronic devices has ignited great interest as means for energy level tuning at the interface with 3D MHPs. Inter alia, the application of 2D phenylethylammonium lead quaternary iodide (PEA₂PbI₄)/3D MHPs interfaces has improved various optoelectronic devices, where a staggered type-II energy level alignment is often assumed. However, a type-II heterojunction seems to contradict the enhanced photoluminescence observed for 2D PEA₂PbI₄/3D MHP interfaces, which raises fundamental questions about the electronic properties of such junctions. In this study, using direct and inverse photoelectron spectroscopy, it is revealed that a straddling type-I energy level alignment is present at 2D PEA₂PbI₄/3D methylammonium lead triiodide (MAPbI₃) interfaces, thus explaining that the photoluminescence enhancement of the 3D perovskite is induced by energy transfer from the 2D perovskite. These results provide a reliable fundamental understanding of the electronic properties at the investigated 2D/3D MHP interfaces and suggest careful (re)consideration of the electronic properties of other 2D/3D MHP heterostructures.

Two organic molecules, 1,2-bis(4-(3,6-bis(bis(4-methoxyphenyl)amino)-9H-carbazol-9-yl)phenyl)ethane-1,2-dione (DB) and 3,6-bis(3,6-bis(bis(4-methoxyphenyl)amino)-9H-carbazol-9-yl)phenanthrene-9,10-dione (PQ), are developed as hole transport materials for perovskite solar cells. The two molecules exhibit similar structures and the same defect passivation groups, but with different configurations. DB with a flexible central structure can adjust its configuration to better interact with perovskite. Consequently, the DB-based device exhibits a higher efficiency of 22.21% and long-term stability.

Abstract

The development of hole-transporting materials (HTMs) that can passivate defects in perovskite is of great significance in improving the efficiency and long-term stability of perovskite solar cells. To date, the investigation on HTMs mainly focus on exploring new structures, while molecular configuration is seldomly concerned. In this work, two small molecules are developed as HTMs with benzil and phenanthrene quinone as the core structure, respectively. With similar structure and the same defect passivation groups, whereas, the two molecules exhibit different configurations, thus distinct properties. Compared to 3,6-bis(3,6-bis(bis(4-methoxyphenyl)amino)-9H-carbazol-9-yl)phenanthrene-9,10-dione (PQ) with a rigid core structure, the benzil group in 1,2-bis(4-(3,6-bis(bis(4-methoxyphenyl)amino)-9H-carbazol-9-yl)phenyl)ethane-1,2-dione (DB) is flexible and can adjust molecular configuration to efficiently interact with the underlying perovskite material, which is confirmed from both experimental results and theoretical simulations. The DB-based device exhibits a high power conversion efficiency of 22.21% with excellent long-term stability, superior to the PQ-based device (20.22%). This study demonstrates that molecular configuration engineering will directly affect the properties of hole transport materials, as well as their interactions with perovskite, which should also be taken into consideration when devising HTMs.

A combined homo hydrocarbon solvent sequential deposition process is demonstrated for all-polymer solar cells (all-PSCs) fabrication. A champion power conversion efficiency (PCE) of 17.7% with a remarkably high fill factor of 77.7% is achieved for rigid all-PSCs based on PM6:PY-V-γ, while the corresponding flexible device also enables an impressive PCE of 14.5% with excellent storage, thermal, photo, and mechanical stabilities.

Abstract

All-polymer solar cells (all-PSCs) have achieved impressive progress in photovoltaic performance and stabilities recently. However, their power conversion efficiencies (PCEs) still trail that of small-molecular acceptor-based organic solar cells (>19%) mainly because of the inferior fill factor (FF). Herein, a combined homo hydrocarbon solvent and sequential deposition (SD) strategy is presented to boost the FF of rigid all-PSCs to 77.7% and achieve a superior PCE of 17.7% with excellent stability, which is among the highest efficiencies reported for all-PSCs thus far. Meanwhile, a remarkable PCE of 14.5% is realized for flexible all-PSCs with outstanding mechanical stability. The blend film morphologies measurements suggest that the SD method enables the formation of an ideal pseudo-bilayer film with bicontinuous interdigitated structure and ordered polymer packing. The numerical simulation result indicates that the FF enhancement mainly results from the efficient exciton diffusion dynamics, increased carrier mobilities, and more balanced electron/hole mobility ratio induced by the developed SD method. This is also confirmed by the FF loss analysis, which manifests that the reduced series resistance and increased shunt resistance are the main reasons for the reduction of FF loss. This work provides a promising strategy to fabricate highly efficient and stable all-PSCs to promote their future development and practical manufacturing.

A semiconductor spacer, namely bithiophenedimethylamine (BThDMA), is successfully developed for 2D Dion–Jacobson (DJ) perovskite solar cells (PSCs) and high photovoltaic performance is demonstrated. An important finding is that there are strong orbital interactions between the conjugated organic spacer and the adjacent inorganic layers, whereas no such interactions exist in DJ perovskites using an insulating aliphatic spacer with similar length.

Abstract

2D Dion–Jacobson (DJ) perovskites have become emerging photovoltaic materials owing to their intrinsic structure stability. However, as insulating aliphatic cations are widely used as spacers, the interactions between the spacers and inorganic layers in DJ perovskites have rarely been studied. Here, an organic semiconductor spacer with two covalently connected thiophene rings, namely bithiophene dimethylammonium (BThDMA), is successfully developed for 2D DJ perovskite solar cells (PSCs). An important finding is that there are strong orbital interactions between the conjugated organic spacer and adjacent inorganic layers, whereas no such interactions exist in DJ perovskite using an aliphatic octane-1,8-diaminium (ODA) spacer with similar length. The BThDMA spacer with multiple conjugated aromatic rings can also induce crystal growth with large grain size and preferred vertical orientation, resulting in reduced trap density and improved charge-carrier mobility. As a result, the optimized device based on (BThDMA)MA _{n −1}Pb _n I_{3 n +1} (nominal n = 5) shows an excellent PCE of 18.1% with negligible hysteresis, which is a record efficiency for 2D DJ PSCs using a spacer with two or more covalently linked aromatic rings. These findings provide a novel and important insight on achieving efficient and stable 2D DJ perovskite solar cells by developing organic semiconductor spacers.

Publication date: 21 December 2022

Source: Joule, Volume 6, Issue 12

Author(s): Yanna Sun, Li Nian, Yuanyuan Kan, Yi Ren, Zhihao Chen, Lei Zhu, Ming Zhang, Hang Yin, Huajun Xu, Jianfeng Li, Xiaotao Hao, Feng Liu, Ke Gao, Yuliang Li

The nanoimprint lithography combined with donor/acceptor sequential deposition dual-functionalized regulation strategy can optimize the vertical composition gradient of the active layer. High-quality PM6 nanograting is fabricated, which is beneficial to precisely controlling vertical bicontinuous donor/acceptor network and building directional charge transport channels, thus endowing the imprinted pseudo-planar heterojunction organic solar cells with an improved power conversion efficiency of 17.36%.

Abstract

Although suitable vertical phase separation morphology in organic solar cells (OSCs) can be obtained by the donor/acceptor sequential deposition (SD) method, the lack of precisely adjusting vertical composition gradient and molecular crystallinity is a key limitation. Here, nanoimprint lithography (NIL) combined with SD dual-functionalized regulation strategy is first used to fabricate high-performance pseudo-planar heterojunction (PPHJ) OSCs, which is conducive to constructing vertical bi-continuous donor/acceptor network to provide sufficient charge separation interface area and orderly charge transport channels. PM6 donor with regular periodic nanograting structure and improved crystallinity is formed via NIL, effectively avoiding the erosion problem ascribed from the subsequent depositing of the Y6 acceptor. Furthermore, the finite-different time-domain (FDTD) measurement is employed to confirm the vertical composition gradient of the donor/acceptor, revealing a strong regular light absorption and reduced voltage loss. As a result, the best-imprinted device enables a power conversion efficiency as high as 17.36%, which is higher than the control SD-based device (15.46%). It is the first time to obtain high-quality PM6 nanograting by NIL, which can provide an avenue to form favorable phase separation morphology and adjust the vertical composition gradient for the high-performance PPHJ OSCs.