Bichen Li

Boosting the performance of planar heterojunction perovskite solar cell by controlling the precursor purity of perovskite materials

Perovskite solar cells (PSCs) have received great attention due to their high power conversion efficiency and low fabrication cost. The perovskite layer is usually prepared from a solution of precursors. We found that the PbI2 purity has a significant effect on the crystallinity, charge carrier dynamics, and photovoltaic properties of the perovskite films. Planar heterojunction PSCs using highly pure PbI2 showed a high power conversion efficiency (PCE) of 16.4%, which was higher than that of control PSCs with low purity PbI2 by 30–40%. Steady-state photoluminescence (PL), time-resolved PL (TR-PL) and femtosecond transient absorption measurements (FS-TA) revealed that impurities can lower the electron lifetime and increase the non-radiative recombination. This study implies that the PCEs of the perovskite solar cell devices could be further boosted by controlling the precursor purity.

High performance planar perovskite solar cells with a perovskite of mixed organic cations and mixed halides, MA1−xFAxPbI3−yCly

Hybrid organic–inorganic perovskite solar cells (PSCs) have attracted great interest owing to their low fabrication costs and high power conversion efficiency. Most studies have focused on the devices with methylammonium lead trihalide perovskites. Here, we explore a new perovskite with mixed organic cations and mixed halides, MA1−xFAxPbI3−yCly. MA1−xFAxPbI3−yCly films can be fabricated by annealing at a temperature of 80–110 °C. Planar heterojunction PSCs using this perovskite as the active material can exhibit a high power conversion efficiency (PCE) of up to 18.14% with short-circuit photocurrent density (Jsc) of 21.55 ± 0.55 mA cm−2, open-circuit voltage (Voc) of 1.100 ± 0.010 V, and fill factor (FF) of 0.75 ± 0.02. The PCE is much higher than those of the control devices with other commonly employed perovskites including MAPbI3, MAPbI3−yCly, MAPbI3−yBry, and MA1−xFAxPbI3. The superior performance is mainly attributed to the enhancement of Jsc, which is a result of long charge diffusion lengths due to the presence of mixed organic cations and mixed halides. In addition, there is no obvious hysteresis in the J–V curves along the forward and reverse scan directions. The formation of undesirable δ-phase perovskite that has a band gap of 2.8 eV is not observed in the MA1−xFAxPbI3−yCly films. These findings pave the way for the design of new hybrid perovskites with stronger light absorption over a wide range, lower charge recombination, and improved charge transport properties through compositional engineering.

Self-assembled atomically thin hybrid conjugated polymer perovskites with two-dimensional structure

We show that self-assembly of protonated polymer chains and metal halide network gives rise to formation of two-dimensional (2D) hybrid perovskites, which incorporate protonated polyaniline as the conjugated organic cation component and PbI6 octahedra as the inorganic component. Single- and few-unit-cell-thick layers of these perovskites are obtained by mechanical exfoliation. Owing to the semiconducting behaviour of conjugated compounds, the atomically thin hybrid conjugated polymer perovskites reported here are expected to be considerably different from the quantum well systems based on unconjugated organic cation incorporating hybrid perovskites, in which the insulating organic layers act as potential barriers and the semiconducting inorganic layers act as potential wells. Besides, these materials are highly flexible in terms of chemical composition unlike conventional inorganic 2D materials, such as graphene, transition metal chalcogenides, transition metal oxides, boron nitride and black phosphorus. Different functional polymers/macromolecules could be self-assembled with a metal halide network in the same manner for the design of novel 2D hybrid perovskites for target applications. We also demonstrate that the conjugated polymer perovskites have tunable optical band gaps and they are highly stable against humidity.

Room temperature ferroelectricity of hybrid organic–inorganic perovskites with mixed iodine and bromine

Ferroelectricity has been reported in organic–inorganic perovskites, and it can enhance the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) because ferroelectricity can facilitate charge carrier separation and charge transport through the perovskite layer. However, the existence of ferroelectricity in perovskites has been in hot debate, particularly at room temperature. Here, we report the ferroelectric polarization switching of MAPb(I1−xBrx)3 which showed a high dependence on its composition. The ferroelectric behavior of MAPbI3-50% PbBr2 is confirmed with domain switching by Piezoelectric Force Microscopy (PFM) imaging and bias-off “butterfly-like” amplitude loops and piezoresponse hysteresis loops at room temperature. The possible factors, such as film electrical properties attributed to the enhanced room-temperature ferroelectricity in MAPb(I1−xBrx)3 films are clarified by using conductive atomic force microscopy (C-AFM). In addition, the charge separation and charge transport in the perovskite MAPb(I1−xBrx)3 films are further investigated by Kelvin probe force microscopy (KPFM). Finally, the possible influences of polarization orientations, trapping effects and ion migrations within the MAPb(I1−xBrx)3 films on the J–V characteristics of PSC devices are discussed in detail. It discovers that the polarization switching under the positive tip biased condition in the PSCs with 50% PbBr2, which could hinder the photovoltaic performance. These findings help better understand the electronic structure of hybrid organic–inorganic perovskites and provide guidance for the improvement of the PSC performance and other electronic applications of perovskites.

The Effect of Methylammonium Iodide on the Supersaturation and Interfacial Energy of the Crystallization of Methylammonium Lead Triiodide Single Crystals

It is very important to study the crystallization of hybrid organic-inorganic perovskites because their thin films are usually prepared from solution. The investigation on the growth of perovskite films is however limited by their polycrystallinity. In this work, methylammonium lead triiodide single crystals grown from solutions with different methylammonium iodide (MAI):lead iodide (PbI2 ) ratios were investigated. We observed a V-shaped dependence of the crystallization onset temperature on the MAI:PbI2 ratio. This is attributed to the MAI effects on the supersaturation of precursors and the interfacial energy of the crystal growth. At low MAI:PbI2 ratio (<1.7), more MAI leads to the supersaturation of the precursors at lower temperature. At high MAI:PbI2 ratio, the crystal growing plans change from (100)-plane dominated to (001)-plane dominated. The latter have higher interfacial energy than the former, leading to a higher crystallization onset temperature.

PEDOT:PSS monolayers to enhance the hole extraction and stability of perovskite solar cells

A hole transport layer (HTL) plays a key role in efficient hole extraction and transfer in inverted planar perovskite solar cells. A 10–20 nm thick poly (3,4-ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS) layer is the most popular HTL in such a device structure. But is it essential to construct such a thick PEDOT:PSS layer? To address this question, herein self-assembled PEDOT:PSS monolayers are obtained on the indium tin oxide (ITO) surface through a facile water rinsing process. Perovskite solar cells with water rinsed PEDOT:PSS as a HTL yield improved power conversion efficiency (PCE) from 13.4% to 18.0%, compared with the control cells with as-cast PEDOT:PSS. The main contribution is from the open-circuit voltage (Voc) and fill factor (FF). Characterization indicates that the majority of PEDOT:PSS is washed away, but an ultra-thin layer of PEDOT:PSS can attach strongly onto ITO via In–O–S chemical bonds between the PSS chain and ITO. Subsequently, PEDOT and PSS form a bilayered structure due to Coulomb interaction. Such an arrangement induces an oriented electric field from positively charged PEDOT to negatively charged PSS, which can accelerate the process of hole extraction. Moreover, the oriented arrangement of PEDOT:PSS monolayers provides higher work function and stronger hydrophobicity, leading to the enhancement in Voc and stability in the ambient environment. This work suggests that there is still room for the efficiency improvement of perovskite solar cells by optimizing the traditional functional layers.