Beauty of halide perovskite polymorphs

The ambient phase stability of the inorganic CsPbI3 cell was achieved through a molecular additive. The fabricated β−γ−CsPbI3-based PHS achieves a power conversion efficiency (PCE) of 21.59% , which are among the highest-reported device characteristics to date as far as this strategy is concerned.
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Beauty of halide perovskite polymorphs
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Introduction

Chonnam National University (CNU) South Korea research team Dr. Sawanta S. Mali, Prof. Chang Kook Hong and their team have developed new technology to fabricate an inorganic halide perovskite solar cells, while using simple hot-air and thermal evaporation approach to improve the stability of the perovskite itself. We built an all-inorganic perovskite solar cells with some bis(pentafluorophenyl)zinc (Zn(C6F5)2) molecular additives and guanidinium iodide (GAI) organic additive respectively for β−CsPbI3 and γ−CsPbI3 solar absorber, which apparently increases their phase stability.

Why Phase-heterojunction solar cells?

Implementing the conventional single p–n junction silicon solar cell concept and its implementation in PSCs will be the best priority to improve the charge transport ability. Previously it was demonstrated that the formation of a p–n homojunction by optimizing the different cation to anion ratios followed by thermal evaporation of a p-type top layer or capsaicin additive, which produced 21.38% and 21.88% PCE, respectively. This controlled transformation from its original intrinsic type to the p- or n-type helps to better match energy levels with the respective hole and electron transport layer (HTL and ETL) interfaces. Therefore, this identical type of conductivity plays a critical role in improving charge transport at the electron-collecting interface and enhancing device performance. However, self-doping and its transformation into either type to p or n-type conductivity is a more complicated process with a shorter annealing window and cation-to-anion ratio. Although Sb3+  and Ag+ doping, respectively, have demonstrated n-type and p-type doping abilities, a clear bilayer p–n junction is still controversial or not yet successfully revealed. 

Apart from the abovementioned promising properties, different polymorphs are the unique characteristics of halide perovskite. Generally, CsPbI3 exists in the cubic (α), tetragonal (β) and orthorhombic (γ) and non-perovskite (δ) phases, which completely depend on its fabrication
protocols.

Key point 

“The regular periodic arrangement of atoms in a crystal decides its quality. However, if we modify or twist their crystal orientations without damaging their congenital properties. Then we can tune its optoelectronic and crystal stability properties.”

Searching for an alternative approach in halide perovskites, all-inorganic cesium lead iodide (CsPbI3), as a promising alternative to conventional metal halide hybrid PSCs, has recently attracted the attention of researchers because of its skyrocketing efficiency (3.2 to 20.37%) within the last five years. In addition, their high thermal stability, free-from-halide segregation and ability to deposit by anti-solvent-free techniques in ambient conditions make
them potential candidates for practical applications. Taking the benefits of promising optoelectronic and cost-effective properties of halide perovskites, we developed a phase-heterojunction all-inorganic perovskite solar cells by utilizing beta (β) and gamma (γ) phases together with the help of the hot-air method and thermal evaporation method respectively. In addition, we also used molecular doping and some effective charge-transporting metal oxides to stabilize the perovskite phase in the ambient conditions,”

Key Findings

Our main approach was to interface two different polymorphs of CsPbI3 (such as β−CsPbI3 and γ−CsPbI3) perovskites to achieve PHS device configuration using a suitable deposition technique. However, all solution-processed perovskite deposition techniques are not suitable
for underneath the perovskite absorber due to the requirement of polar solvents for both polymorphs of CsPbI3. Therefore, to achieve PHS architecture, we used a joint deposition method that is a combination of a single-step DHA method for the deposition of the front β−CsPbI3 absorber layer followed by thermal evaporation of the γ−CsPbI3 perovskite layer.

Figure 1. The fabrication process of phase-heterojunction all-inorganic perovskite solar cells (PHS) using hot-air and thermal evaporation method. 

We fabricated the cell with a perovskite known as CsPbI3, which has a remarkable energy bandgap of 1.69 eV. However, it is also known for stability issues. Its phase stability in ambient air still needs improvement due to the role moisture plays in accelerating the perovskite-to-non-perovskite phase transformation,” they explained, noting that the passivating alternative layer (γ-CsPbI3) on the underneath perovskite crystal extend its lifetime. They used hot-air and thermal evaporation methods to form novel phase-heterojunction device architecture. This perovskite solar cell has fluorine-doped tin oxide coated (FTO) substrates, an electron transport layer (ETL) based on mesoporous titanium oxide (TiO2), the perovskite layer, a hole transporting layer (HTL) made of the polymer regioregular poly(3-hexylthiophene) (P3HT), and a top electrode based on gold (Au), (Figure 1). “This cell configuration achieved a power conversion efficiency of 21.59 %, which is the world's best record to date.” Importantly, these devices scale up to 25 cm2 and ~170 cm2 in module scale using the developed protocol. The device was also able to retain more than 90% of its initial efficiency after 200 hours of aging cycles one sun illumination. 

Figure 2. Large-area fabrication and stability analysis. a, Best-performing device J–V characteristics of a PHS based on β−CsPbI3-Zn(C6F5)2/γ−CsPbI3-GAI-based PHS with a 1 cm × 1 cm active area under simulated AM 1.5 G solar illumination of 100 mW cm−2 in forward and reverse scans. b, Shelf stability of the encapsulated champion device under ambient conditions with a 1 cm × 1 cm active area. Data are presented as mean values ± SD. For shelf-stability measurements, devices were stored in the dark at ambient conditions with relative humidity 30−35%.

Suitable for alternative compositions?

Generally speaking, the suitability of the developed method for alternative perovskite
compositions are equally important. For this, we further explore the feasibility of our PHS approach in alternative polymorphs of organic-inorganic hybrid perovskites phase heterojunction (H-PHS). However, this type of two different compositions is not pure phase heterojunction but heterojunction or bilayered or 3D/PP-2D bilayer perovskite device has been reported previously. In this section, we implemented all-inorganic CsPbI2Br and organic-inorganic hybrid MAPbI3, Cs0.05FA0.95PbI3 perovskite composition in order to check the PHS concept for alternative composition. For this, used γ-CsPbI2Br front absorber deposited by the dynamic hot-air (DHA) method in ambient conditions, followed by thermal evaporation of α-MAPbI3 or α-Cs0.05FA0.95PbI3 rear absorber.

Representative JV curves show our γ-CsPbI2Br/α-MAPbI3-based device exhibited a VOC of 1.175 V, a JSC of 16.75 mA cm-2, and a FF of 78.1 %, yielding 15.36 % PCE. In addition, the champion device based on γ-CsPbI2Br/α-Cs0.05FA0.95PbI3 configuration showed an excellent FF of 80%, with a JSC of 19.20 mA cm-2, and a VOC of 1.147 V, yielding efficiency of 17.62 %,  (Figure 2a). 

Upscaling 

To check this method’s suitability towards the cell-to-module scale, we deposited the front absorber by the DHA method16, followed by thermal evaporation of the rear absorber onto a laser-scribed (5 cm × 5 cm) fluorine-doped tin oxide (FTO) substrate. Our champion device has an 18.08 cm2 active area (geometrical fill factor 94.2%). The J–V curves recorded from the PSM based on a β−CsPbI3/γ−CsPbI3 PHS exhibited a PCE of 18.43%, which is close to our 1 × 1 cm2 area device. The shelf-life stability of encapsulated PMS maintains 80% of their original PCEs after ten days of aging under ambient conditions. The efficiency statistics of 20 PMSs show the average aperture efficiency of the PHS-based PMSs crossing 18.5%. Suitability towards the ultra-large (13 cm × 13 cm) module using this triple-source thermal evaporation method has also been demonstrated.

Figure  3 Photovoltaic performance of the β−CsPbI3-Zn(C6F5)2/γ−CsPbI3-GAIbased PSM. a, Schematic structure of the all-inorganic perovskite mini module. b, Photographs of all-inorganic perovskite mini-module

Conclusions

Our demonstration of the formation of β–CsPbI3/γ–CsPbI3 PHS method has great potential in perovskite photovoltaic technologies. To this end, we successfully demonstrated this PHS concept by fabricating polymorphic phases of CsPbI3 by implementing a DHA and triple-source
thermal evaporation methods, which show superior performance over conventional single-junction perovskite solar cells. Crucially for phase stabilization, we introduce molecular and organic additives-based approaches, while maintaining their photovoltaic performance. We 
developed anti-solvent-free and interconnecting layer-free PHS, which would facilitate improved charge extraction and its further development in multijunction solar cells. This PHS concept provides a simplified approach towards efficient PSCs with high stability. In future
work, different polymorphic phases of either organic–inorganic and/or all-inorganic halide perovskites could also be the best alternatives for highly efficient PHS devices.

Next Steps

“We are planning to make a 15 cm x 15 cm (225 cm2) module using by combining hot-air, blade coating and air-knife assisted slot-die coating and trying to implement in building photovoltaics,”

Photo caption. From left, Prof. Chang Kook Hong, Dr. Sawanta S. Mali and Dr. Jyoti V. Patil showing the all-inorganic perovskite solar cell module fabricated at the Polymer Energy Materials Laboratory, Chonnam National University, South Korea, which approaches 20% efficiency. (Photo credit: Mr. Chang-Seok Ryu)

For more details, please check out our paper "Phase-heterojunction all-inorganic
perovskite solar cells surpassing 21.5% efficiency" 

Link: https://doi.org/10.1038/s41560-023-01310-y 

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