Chen Weijie

A perovskite solar cell with both high efficiency and high thermal stability is examined. The optimized device achieved by engineering perovskite composition exhibits 92% power conversion efficiency retention in a stress test conducted at 85 °C/85% RH while exceeding 20% power conversion efficiency (certified efficiency of 20.8% at 1 cm²). These results reveal a great potential for future practical use.

Abstract

Perovskite solar cells have received great attention because of their rapid progress in efficiency, with a present certified highest efficiency of 23.3%. Achieving both high efficiency and high thermal stability is one of the biggest challenges currently limiting perovskite solar cells because devices displaying stability at high temperature frequently suffer from a marked decrease of efficiency. In this report, the relationship between perovskite composition and device thermal stability is examined. It is revealed that Rb can suppress the growth of PbI₂ even under PbI₂‐rich conditions and decreasing the Br ratio in the perovskite absorber layer can prevent the generation of unwanted RbBr‐based aggregations. The optimized device achieved by engineering perovskite composition exhibits 92% power conversion efficiency retention in a stress test conducted at 85 °C/85% relative humidity (RH) according to an international standard (IEC 61215) while exceeding 20% power conversion efficiency (certified efficiency of 20.8% at 1 cm²). These results reveal the great potential for the practical use of perovskite solar cells in the near future.

Amino‐functionalized TiO₂ nanoparticles are synthesized in situ by a facile onestep, low‐temperature, nonhydrolytic approach, and are applied as the electrontransport layer of regular‐structure planar heterojunction perovskite solar cells, offering a dramatic performance increase due to the passivation of the surface trap states of the perovskite film.

Abstract

Titanium oxide (TiO₂) has been commonly used as an electron transport layer (ETL) of regular‐structure perovskite solar cells (PSCs), and so far the reported PSC devices with power conversion efficiencies (PCEs) over 21% are mostly based on mesoporous structures containing an indispensable mesoporous TiO₂ layer. However, a high temperature annealing (over 450 °C) treatment is mandatory, which is incompatible with low‐cost fabrication and flexible devices. Herein, a facile one‐step, low‐temperature, nonhydrolytic approach to in situ synthesizing amino‐functionalized TiO₂ nanoparticles (abbreviated as NH₂‐TiO₂ NPs) is developed by chemical bonding of amino (‐NH₂) groups, via TiN bonds, onto the surface of TiO₂ NPs. NH₂‐TiO₂ NPs are then incorporated as an efficient ETL in n‐i‐p planar heterojunction (PHJ) PSCs, affording PCE over 21%. Cs_0.05FA_0.83MA_0.12PbI_2.55Br_0.45 (abbreviated as CsFAMA) PHJ PSC devices based on NH₂‐TiO₂ ETL exhibit the best PCE of 21.33%, which is significantly higher than that of the devices based on the pristine TiO₂ ETL (19.82%) and is close to the record PCE for devices with similar structures and fabrication procedures. Besides, due to the passivation of the surface trap states of perovskite film, the hysteresis of current–voltage response is significantly suppressed, and the ambient stability of devices is improved upon amino functionalization.

Publication date: March 2019

Source: Nano Energy, Volume 57

Author(s): Bo Yang, Ming Wang, Xiaofei Hu, Tingwei Zhou, Zhigang Zang

Abstract

Graphical abstract

In article no. 1800232, Yaqing Feng, Gui Yu, Jin‐Song Hu, and co‐workers report a hydrophobic conjugated polymer with a high hole mobility as a new multifunctional interlayer to protect sensitive perovskite from moisture and additives and enhance hole collection and transport, leading to air‐stable efficient perovskite solar cells.

A sequential slot‐die (SSD) coating technique can lead the polymer to form pre‐aggregates, resulting in enhanced crystallinity and face‐on orientation which is superior for charge transport. As a result, large area (1 cm²) flexible devices with a power conversion efficiency of 7.11% are obtained. Therefore this approach offers a new strategy for the fabrication of large area flexible organic solar cells.

For the preparation of flexible organic solar cells (OSCs), the Roll‐to‐Roll slot‐die coating technique is preferable. Herein, a sequential slot‐die (SSD) coating strategy to fabricate flexible OSCs using non halogenated solvent under ambient atmosphere, is developed. The coating order of the active layer materials shows great influence on the performance of OSCs. It is found that, compared with the one‐step coating, the power conversion efficiency (PCE) of devices with an area of 0.75 cm², and fabricated by SSD coating process with the polymer as the first layer, is enhanced from 4.86 to 5.51% for the binary system, whereas from 6.09 to 7.32% for the ternary system, showing an increase of 13 and 20% in PCE, respectively. For the devices with a standard small area of 4 mm² and large area of 1 cm², PCE as high as 9.36 and 7.11% are obtained, respectively, which are among the top value for flexible devices fabricated by slot‐die coating. It turns out that the SSD coating process with the polymer as the first layer assists pre‐aggregation of the polymer to form better crystal domains with face‐on orientation. Therefore, a sequential deposition strategy could provide a new means for manufacturing the high efficiency flexible OSCs.

Phase transition effects on thermal stability of planar perovskite solar cells are illuminated. Large carrier trap densities are observed in the methylammonium lead triiodide‐based solar cells aged under high operating temperatures. These carrier traps are detrimental to long‐term stability. Perovskite alloys with mixed both cations and anions could effectively avoid the formation of carrier traps and result in better device stability.

Abstract

A power conversion efficiency of over 20% has been achieved in CH₃NH₃PbI₃‐based perovskite solar cells (PSC), however, low thermal stability associated with the presence of a phase transition between tetragonal and cubic structures near room temperature is a major issue that must be overcome for future practical applications. Here, the influence of the phase transition on the thermal stability of PSCs is investigated in detail by comparing four kinds of perovskite films with different compositions of halogen atoms and organic components. Thermally stimulated current measurements reveal that a large number of carrier traps are generated in solar cells with the perovskite CH₃NH₃PbI₃ as a light absorber after operation at 85 °C, which is higher than the phase‐transition temperature. Electrochemical impedance spectroscopy measurements further exclude effects of a possible morphology change on the formation of carrier traps. These carrier traps are detrimental to the thermal stability. The thermogravimetric analysis does not show a decomposition for any of the materials in the temperature range relevant for operation. The perovskite alloys do not have this phase transition, resulting in effectively suppressed formation of carrier traps. PSCs with improved thermal stability under the standard thermal cycling test are demonstrated.

In article number 1801079, Chuanjiang Qin, Chihaya Adachi, and co‐workers adopt thermally stimulated current and electrochemical impedance spectroscopy measurements to reveal that carrier traps are generated in solar cells with CH₃NH₃PbI₃ as a light absorber after operation at 85°C, which is higher than its phase transition temperature. Perovskite alloys‐based solar cells with improved thermal stability under a standard thermal cycling test are demonstrated.

Single material organic solar cells (SMOSCs) based on ambivalent donor acceptor materials are the ultimate stage of simplification of organic solar cells and a possible solution to the instability of multicomponent cells. Impressive progress in both polymeric and molecular SMOSCs has been recently reported and the related perspectives and fundamental issues are critically discussed here.

Abstract

Single material organic solar cells (SMOSCs) are based on ambivalent materials containing electron donor (D) and acceptor (A) units capable to ensure the basic functions of light absorption, exciton dissociation, and charge transport. Compared to bicomponent bulk heterojunctions, SMOSCs present several major advantages such as considerable simplification of cell fabrication and a strong stabilization of the morphology of the D/A interface, and thus of the cell lifetime. In addition to these technical issues, SMOSCs pose fundamental questions regarding the possible formation, and dissociation of excitons on the same molecular D–A architecture. SMOSCs are developed with various approaches, namely “double‐cable” polymers, block copolymers, oligomers, and molecules that differ by the donor platform: polymer or molecule, the nature of A, the D–A connection, and the intra‐ and intermolecular interactions of D and A. Although for several years the maximum efficiency of SMOSCs has remained limited to 1.0–1.5%, impressive progress has been recently accomplished leading to SMOSCs with 4.0–5.0% efficiency. Here, recent advances in the synthesis of D–A materials for SMOSCs are presented in the broader context of the chemistry of organic photovoltaic materials in order to discuss possible directions for future research.

Quaternary organic solar cells based on PBDB‐T:PTB7‐Th:ITIC:FOIC are reported to deliver a parallel‐alloy morphology mode in non‐fullerene‐based devices. In the quaternary system, the parallel‐like donors and alloy‐like acceptors together facilitate transfer kinetics, optimize blend morphology, and drive the device efficiency toward over 12.5%, which indicates the great potential of quaternary organic solar cells.

Abstract

Two compatible donors (PBDB‐T and PTB7‐Th) and two miscible acceptors (ITIC and FOIC) are employed to deliver a parallel‐alloy morphology model in non‐fullerene‐based quaternary organic solar cells. PBDB‐T and PTB7‐Th form a parallel link with a slight adjustment of molecular packing into enhanced face‐on crystallites while ITIC disperses into discontinuous FOIC microcrystal regions to form continuous and ordered alloy‐like acceptor phases. Characterization of blend morphology highlights the parallel‐alloy model—enabled by the introduction of PBDB‐T and ITIC, which contributes to improved molecular packing and reduced domain size resulting in efficient charge generation and consistent transport channels. This successful parallel‐alloy quaternary blend morphology demonstrates an enhanced optical absorption, optimized domain size, and nanostructures toward simultaneous improvement in charge transfer and transport. Therefore, a power conversion efficiency of 12.52% is realized for a quaternary device which is 6% higher than the ternary device (PBDB‐T:PTB7‐Th:FOIC) and 12% higher than the binary device (PTB7‐Th:FOIC). Domination of quaternary devices over ternary and binary blends, which is another feasible way to realize highly efficient devices through further investigation of quaternary OSCs, is presented.

Solution-processed intermediate-band solar cells with lead sulfide quantum dots and lead halide perovskites

Solution-processed intermediate-band solar cells with lead sulfide quantum dots and lead halide perovskites, Published online: 10 January 2019; doi:10.1038/s41467-018-07655-3