Chen Weijie

A method for the determination of an optical diode factor for bare absorbers is derived and verified experimentally. By combining this with quasi Fermi level splitting measurements, it is possible to predict the VOC and FF of a solar cell before device finishing. Comparing the optical diode factor to the diode factor of devices allows one to unveil limitations by interfaces.

In the search for new materials for solar cells, a fast feedback is needed. Radiative efficiency measurements based on photoluminescence (PL) are the tool of choice to screen the voltage a material is capable of. Additionally the dependence of the radiative efficiency on excitation density contains information on the diode ideality factor, which determines in turn the fill factor of the solar cell. Both parameters are immediate ingredients of the efficiency of a solar cell and can be determined from PL measurements, which allow fast feedback. The method to determine the optical diode ideality factor from PL measurements and compare to electrical measurements in finished solar cells are discussed.

The theoretical power conversion efficiency of all‐inorganic CsPbI₂Br perovskite solar cells is predicted to be 22.1%, and only by taking both material chemistry and device physics into consideration can researchers achieve this goal.

Cesium‐based all‐inorganic perovskite solar cells (PSCs), especially for CsPbI₂Br component‐based devices, have attracted increasing attention due to its advantage of superior thermal and phase stability. Since the pioneering study reported in 2016, more than 30 papers have been published, reporting the rapid boost in the power conversion efficiency (PCE) of PSCs to 14.81%. The CsPbI₂Br PSC is one of the most remarkable research hotspots in the field of perovskite photovoltaics. In this progress report, the recent advances in CsPbI₂Br PSCs are systematically reviewed, which in turn introduces the basic property and stability of active layers, and the performance improvements in these devices. The challenges as well as the possible solutions toward better‐performing CsPbI₂Br PSCs are also discussed. The theoretical calculation results point out that there is much room for further device performance enhancement, particularly in open‐circuit voltages. This progress report focuses on CsPbI₂Br material properties and summarizes recent strategies to improve the corresponding device's PCE, in order to open new perspectives toward commercial utility of PSCs.

All-inorganic cesium lead iodide perovskite solar cells with stabilized efficiency beyond 15%

All-inorganic cesium lead iodide perovskite solar cells with stabilized efficiency beyond 15%, Published online: 31 October 2018; doi:10.1038/s41467-018-06915-6

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.

The sp²‐nitrogen positions in the n‐type (D–A₁–D–A₂) conjugated polymers have a significant impact on the photovoltaic properties of p–i–n perovskite solar cells when they are used as an electron transporting layer. pBTTz with the HOMO and LUMO levels well‐matched with the valence and conduction bands of the perovskite layer, respectively, shows excellent power conversion efficiency and high stability.

Abstract

It is highly desirable to employ n‐type polymers as electron transporting layers (ETLs) in inverted perovskite solar cells (PSCs) due to their good electron mobility, high hydrophobicity, and simplicity of film forming. In this research, the capability of three n‐type donor–acceptor₁–donor–acceptor₂ (D–A₁–D–A₂) conjugated polymers (pBTT, pBTTz, and pSNT) is first explored as ETLs because these polymers possess electron mobilities as high as 0.92, 0.46, and 4.87 cm² (Vs)⁻¹ in n‐channel organic transistors, respectively. The main structural difference among pBTT, pBTTz, and pSNT is the position of sp²‐nitrogen atoms (sp²‐N) in the polymer main chains. Therefore, the effect of different substitution positions on the PSC performances is comprehensively studied. The as‐fabricated p–i–n PSCs with pBTT, pBTTz, and pSNT as ETLs show the maximum photoconversion efficiencies of 12.8%, 14.4%, and 12.0%, respectively. To be highlighted, pBTTz‐based device can maintain 80% of its stability after ten days due to its good hydrophobicity, which is further confirmed by a contact angle technique. More importantly, the pBTTz‐based device shows a neglected hysteresis. This study reveals that the n‐type polymers can be promising candidates as ETLs to approach solution‐processed highly‐efficient inverted PSCs.

A nearly formamidinium (FA) lead–tin (Pb–Sn) mixed perovskite FAPb_0.75Sn_0.25I₃ is exploited to fabricate a low‐bandgap perovskite solar cell. By combination with a NiO _x hole transport layer, a power conversion efficiency of 17.25% is obtained. This low‐bandgap perovskite solar cell maintains about 91% of its original efficiency at 80 °C for 20 h, which demonstrates good thermal stability.

Abstract

Low bandgap lead–tin (Pb–Sn) mixed perovskite solar cells have achieved high power conversion efficiency in excess of 17%. However, methylammonium (MA) cation is usually contained, and the thermal stability of MA is always a great concern. In this work, according to composition engineering, a nearly formamidinium (FA) based low‐bandgap Pb–Sn mixed perovskite FAPb_0.75Sn_0.25I₃ is being tried to explore as the absorber layer. Combined with interface engineering by replacing poly(3,4‐ethylenedioxythiophene)‐polystyrenesulfonic acid (PEDOT:PSS), layer with NiO _x as hole transport layer, a power conversion efficiency of 17.25% is obtained. This low‐bandgap perovskite solar cell maintains about 91% of its original efficiency at 80 °C for 20 h, and 92% of its initial performance after 46 days storage at the room temperature. The good thermal stability of nearly FA based low‐bandgap perovskite could be good for delivering efficient and stable perovskite‐perovskite tandem solar cells.

The sulfonic zwitterion combines the functions of morphology tailoring and defect passivation together into one kind of functional molecule, and this “all‐in‐one” system provides a facile but effective pathway for the fabrication of high‐performance perovskite solar cells.

Abstract

Uniform and high‐electronic‐quality perovskite thin films are essential for high‐performance perovskite devices. Here, it is shown that the 3‐(decyldimethylammonio)‐propane‐sulfonate inner salt (DPSI), which is a sulfonic zwitterion, plays dual roles in tuning the crystallization behavior and passivating the defects of perovskites. The synergistic effect of crystallization control and defect passivation remarkably suppresses pinhole formation, reduces the charge trap density, and lengthens the carrier recombination lifetime, and thereafter boosts the small‐area (0.08 cm²) planar perovskite device efficiency to 21.1% and enables a high efficiency of 18.3% for blade‐coating large‐area (1 cm²) devices. The device also shows good light stability, which remains at 88% of the initial efficiency under continuous unfiltered AM 1.5G light illumination for 480 h. These findings provide an avenue for simultaneous crystallization control and defect passivation to further improve the performance of perovskite devices.

The impact of Rashba effects in halide perovskites is still under debate. Using femtosecond transient absorption and photoluminescence, it is shown that luminescence from hot carriers is weaker than that of cold carriers, as expected from strongly radiative transitions in direct gap semiconductors. Several possible resolutions to this, including lattice dynamics that overcome Rashba splittings at room temperature are considered.

Abstract

The generation and recombination of charge carriers in semiconductors through photons controls photovoltaic and light‐emitting diode operation. Understanding of these processes in hybrid perovskites has advanced, but remains incomplete. Using femtosecond transient absorption and photoluminescence, it is observed that the luminescence signal shows a rise over 2 ps, while initially hot photogenerated carriers cool to the band edge. This indicates that the luminescence from hot carriers is weaker than that of cold carriers, as expected from strongly radiative transitions in direct gap semiconductors. It is concluded that the electrons and holes show a strong overlap in momentum space, despite recent proposals that Rashba splitting leads to a band offset suppressing such an overlap. A number of possible resolutions to this, including lattice dynamics that remove the Rashba splitting at room temperature, and localization of luminescence events to length scales below 10 nm are considered.

Publication date: 16 January 2019

Source: Joule, Volume 3, Issue 1

Author(s): Wanchun Xiang, Zaiwei Wang, Dominik J. Kubicki, Wolfgang Tress, Jingshan Luo, Daniel Prochowicz, Seckin Akin, Lyndon Emsley, Jiangtao Zhou, Giovanni Dietler, Michael Grätzel, Anders Hagfeldt

Context & Scale

Summary

Graphical Abstract