Fengxian Xie

Thermally Stable MAPbI3 Perovskite Solar Cells with Efficiency of 19.19% and Area over 1 cm2 achieved by Additive Engineering

Solution-processed perovskite (PSC) solar cells have achieved extremely high power conversion efficiencies (PCEs) over 20%, but practical application of this photovoltaic technology requires further advancements on both long-term stability and large-area device demonstration. Here, an additive-engineering strategy is developed to realize a facile and convenient fabrication method of large-area uniform perovskite films composed of large crystal size and low density of defects. The high crystalline quality of the perovskite is found to simultaneously enhance the PCE and the durability of PSCs. By using the simple and widely used methylammonium lead iodide (MAPbI3 ), a certified PCE of 19.19% is achieved for devices with an aperture area of 1.025 cm2 , and the high-performing devices can sustain over 80% of the initial PCE after 500 h of thermal aging at 85 °C, which are among the best results of MAPbI3 -based PSCs so far.

Vertical recrystallization for highly efficient and stable formamidinium-based inverted-structure perovskite solar cells

Formamidinium (FA)-based perovskite materials show an extended absorption spectrum to 840 nm, which enables high power conversion efficiencies of over 20% compared with normal-structure perovskite solar cells (PSCs). However, it is rarely possible to obtain high performance in inverted-structure PSCs owing to the unbalanced electron–hole transport properties. To achieve desirable electronic qualities in a FA perovskite film, it is necessary to substantially improve the crystallinity of the perovskite films. A new perovskite growth method is presented here using methylammonium chloride (MACl) to assist vertical recrystallization in a formamidinium perovskite film. The obtained film consists of highly crystallized vertically-orientated grains, which minimize the vertical grain boundary and trap site in the films, and later contribute to a power conversion efficiency above 20% in inverted-structure PSCs. Most importantly, the highly crystalline, phase-pure morphology and low MA content in formamidinium perovskite films can contribute to the light-soaking stability and thermal stability up to 500 h in solar-cell devices.

Soft-cover deposition of scaling-up uniform perovskite thin films for high cost-performance solar cells

Low-cost and high energy conversion efficiency are the crucial factors for large scale application of solar cells. In recent years, a promising high cost-performance photovoltaic technology, organometal halide perovskite solar cells (PSCs), has attracted great attention. However, most of the reported high efficiencies were obtained on a small working area of about 0.1 cm2 with the material utilization ratio of only 1% during film deposition, which actually hinders the advancement in future application of PSCs. Here we present the soft-cover deposition (SCD) method where surface wettability, solution viscosity and thermal crystallization are the processing key factors for the deposition of uniform perovskite films with high material utilization ratios. Scaling-up, pinhole-free, large crystal grains and rough-border-free perovskite films were obtained over a large area of 51 cm2, which were processed continuously in ambient air with a significant enhancement in the material utilization ratio up to ∼80%. Highly reproducible power conversion efficiencies up to 17.6% were achieved in unit cells with a working area of 1 cm2, leading to a high overall cost-performance. We believe that the present SCD technology will benefit the low-cost fabrication of highly efficient perovskite solar cells and open up a route for the deposition of other solution processed thin-films.

Enhanced Stability of Perovskite Solar Cells through Corrosion‐Free Pyridine Derivatives in Hole‐Transporting Materials

The molecular structure of pyridine derivatives is critical to perovskite solar cell performance, especially stability. Most of the pyridine additives easily form complexes with perovskite. A new pyridine additive with a long alkyl chain substituted at its o-position does not corrode perovskite. The stability of devices containing this additive is the highest among the investigated cells.

Annealing-free perovskite films by instant crystallization for efficient solar cells

Organic–inorganic perovskite solar cells (PSCs) have attracted considerable attention around the world because they can be fabricated easily and inexpensively by solution-based processes. The key to the fabrication of high-performance PSCs is the crystallinity and morphology of the perovskite film, and thermal annealing is usually required to achieve a film with the necessary properties. Herein, we introduce a technique for instant crystallization of perovskite films without the need for thermal annealing. Specifically, a solution of methylammonium iodide and lead iodide was spin-coated onto a substrate, and ethyl acetate was dripped onto the film during spinning to induce instant crystallization of a CH3NH3PbI3 perovskite film. The resulting crystalline film exhibited large crystal grains and a high carrier lifetime. PSCs fabricated with annealing-free films prepared by means of this technique exhibited performance comparable to that of PSCs fabricated with annealed films and showed much higher efficiency than did reference cells fabricated with annealing-free films. This new instant-crystallization technique offers a way to shorten the device fabrication time, which will reduce the cost of manufacturing efficient PSCs.

A comparative study of o,p-dimethoxyphenyl-based hole transport materials by altering π-linker units for highly efficient and stable perovskite solar cells

Two easily synthesized o,p-dimethoxyphenyl-based hole transport materials (HTMs) containing biphenyl (HL-1) and carbazole (HL-2) in the π-system, respectively, have been designed and studied for perovskite solar cells (PSCs). A higher efficiency of 18.34% for the HL-2 based device was obtained compared to that of HL-1 showing a lower efficiency of 16.14%. A small hysteresis was also observed in the HL-2 based device while the HL-1 based device displayed a significant hysteresis. As a carbazole unit has a stronger electron-donating ability than biphenyl, HL-2 shows a higher hole mobility. The steady-state photoluminescence characteristics confirm that HL-2 can efficiently extract charge carrier at the perovskite/HTM interface rather than HL-1. Meanwhile, a compact HL-2 film without pin-holes effectively suppressed the non-radiative recombination at the interface, resulting in the improvement of the fill factor and open voltage. Most importantly, the steric hindrance due to the long hexyl chain of HL-2 could restrain the halogen migration from the perovskite to the Ag electrode. Thus, the HL-2 based device without encapsulation showed an advanced thermal stability at 85 °C after storing for 100 h compared to the HL-1. These results indicate that the o,p-dimethoxyphenyl unit is a promising alternative to develop small molecular HTMs for highly efficient and stable PSCs.

Low‐Temperature Soft‐Cover Deposition of Uniform Large‐Scale Perovskite Films for High‐Performance Solar Cells

Large-scale high-quality perovskite thin films are crucial to produce high-performance perovskite solar cells. However, for perovskite films fabricated by solvent-rich processes, film uniformity can be prevented by convection during thermal evaporation of the solvent. Here, a scalable low-temperature soft-cover deposition (LT-SCD) method is presented, where the thermal convection-induced defects in perovskite films are eliminated through a strategy of surface tension relaxation. Compact, homogeneous, and convection-induced-defects-free perovskite films are obtained on an area of 12 cm2 , which enables a power conversion efficiency (PCE) of 15.5% on a solar cell with an area of 5 cm2 . This is the highest efficiency at this large cell area. A PCE of 15.3% is also obtained on a flexible perovskite solar cell deposited on the polyethylene terephthalate substrate owing to the advantage of presented low-temperature processing. Hence, the present LT-SCD technology provides a new non-spin-coating route to the deposition of large-area uniform perovskite films for both rigid and flexible perovskite devices.

Accurate and fast evaluation of perovskite solar cells with least hysteresis

It is of great importance to evaluate the performance of perovskite solar cells (PSCs) accurately, especially to avoid the errors induced by hysteresis during current density–voltage measurement. Here we found that PSCs with n–i–p and p–i–n structures exhibit very different hysteresis behavior. A longer delay time leads to small hysteresis in n–i–p PSCs, whereas it shows little effect in p–i–n ones. In contrast, a smaller voltage step is preferred for reduced hysteresis in p–i–n PSCs, which is further supported by a transient photocurrent study. Finally, we proposed fast and accurate evaluation methods for PSCs based on these two structures.

Effect of thermal-convection-induced defects on the performance of perovskite solar cells

Thermal-convection-induced defects can cause huge loss in the power conversion efficiency of solution-processed perovskite solar cells. We investigated two types of convection in perovskite solution during the formation of perovskite films. By balancing the convection via special configurations of surface tension and boiling point in mixed γ-butyrolactone (GBL) and dimethylsulfoxide (DMSO), we removed microscopic defects such as rings, bumps, and crevices. The deposited perovskite films were smooth and dense, which enabled a high power conversion efficiency of 17.7% in a 1 cm2 cell area. We believe that the present strategy for controlling the convection can be helpful in improving the perovskite film quality for solvent-rich scalable solution processes of solar cells such as doctor blading, soft-cover deposition, printing, and slot-die coating.