Elsevier

Nano Energy

Volume 90, Part A, December 2021, 106597
Nano Energy

Ambient processed and stable all-inorganic lead halide perovskite solar cells with efficiencies nearing 20% using a spray coated Zn1−xCsxO electron transport layer

https://doi.org/10.1016/j.nanoen.2021.106597Get rights and content

Highlights

  • Ambient conditions processed all-inorganic perovskite solar cells.

  • Functional revision/doping of chemically unstable ZnO ETL, which can now be easily spray coated.

  • Demonstration of an exotic, highly stable γ−CsPb1−xCdxI2.5Br0.5 inorganic absorbing layer.

  • PCE nearing ~20% PCE with >300 h thermal stability at 85 ℃.

Abstract

Achieving ambient-processed and stable all-inorganic halide perovskite solar cells with high conversion efficiencies is a well-established goal within the halide perovskite research community. In striving for this, electron transporting layers based on common TiO2 and SnO2 nanoparticles have been widely deployed, however, can stifle device performance due to their requirement for high temperature processing and non-uniform layer deposition. Using a low-temperature processed Cs-doped ZnO nanocrystalline electron transport layer and a narrow-bandgap (~1.70 eV) all-inorganic absorber (Cd-doped CsPbI2.5Br0.5), we report ambient-processed solar cells which exhibit high conversion efficiencies (>19.75%) and stable performance under ambient condition (>300 h). A smooth interface is established by combining spray-deposition of the electron transport layer and injection of hot-air during the perovskite deposition, which crystallizes smooth, compact perovskite thin films directly from solution. The high performance is attributed to preserving a solar-friendly and phase-stable perovskite layer, along with improved charge carrier management.

Introduction

Clean and inexhaustible solar energy is regarded as a promising substitute for fossil fuels and lead halide perovskite solar cells (PSCs) – compounds based on a APbX3 general formula, where A represents (in)organic cations and X = Cl, Br and I – are an emerging low-cost technology with the potential to disrupt the mature silicon photovoltaic market. In just a few years, the power conversion efficiency (PCE) of PSCs now exceed 25% and 29% for single junction and perovskite/silicon monolithic tandem devices, respectively [1], [2], [3], [4]. While the rate of improvement for organic PSCs (A-site represents organic cations such as formamidinium, FA, or methylammonium, MA) is slowing, the promise for all-inorganic cesium lead halide perovskite devices has steadily increased, due to their ability to combine efficient solar harvesting (PCE ~ 20%) with long-term thermal stability [5], [6], i.e. the use of organic cations MA and FA renders PSCs sensitive to ambient light, moisture and heat [7], [8], [9]. The inability to both fabricate and operate organic lead halide PSCs under ambient conditions is currently seen as the major obstacle toward technological success.

All-inorganic CsPbI3 perovskite exhibits a relatively narrow, solar-friendly bandgap (Eg≈1.7 eV), however forms a thermodynamically unstable perovskite structure at room temperature. Relatively direct stabilizing routes involve the partial replacement of the I-sites with Br. However, the added stability comes at the cost unfavorably widening of the bandgap, e.g. at a composition of CsPbI2Br the bandgap is ~1.9 eV, making it suboptimal for single junction PSC configurations. Therefore, beyond the use of additives in the precursor solution [10], [11], surface passivating ligands [12], reduced crystal dimensions [13], the application of hot air during deposition (so-called hot-air method) and B-site metal ion doping have become appealing strategies toward securing a stable, all-inorganic CsPbI3-based perovskite thin films [14], [15], [16].

To ensure CsPbI3-based PSCs performances continue to rise closer to their thermodynamic limits, device engineering and choosing an appropriate electron transport layers (ETL) is crucial. Though widely adopted, the conventional TiO2 is not the best choice due to its relatively low conductivity and problematic photocatalytic activity. Thus, the discovery of novel ETLs is currently key to on-going progress; a number of ETL compositions have been utilized within all-inorganic PSCs with varying degrees of success (PCE), namely, SnO2 (>18.06%) [17], ZnO (>16.84%) [18], In2S3 (5.59%) [19], Nb2O5 (11.74%) [20] and ZnO@SnO2 core-shell (14.35%) [21]. The use of ETLs based on ZnO rarely attract attention though it has been applied within PSCs. This is because ZnO can deprotonate MA+ and FA+ cations and induce the degradation of hybrid organic-inorganic perovskites. However, due to its excellent electron mobility, ZnO is considered a promising candidate and worth deeper exploration. Although a few reports have demonstrated successful implementation in perovskite PV based on ZnO ETLs for normal planar type “n-i-p” type devices, it unfortunately also suffered from serious surface perovskite degradation during normal thermal stress [15], [22], [23]. This perovskite degradation for ZnO ETL based PSCs arises due to its inherent nature to act as a base (i.e. proton acceptor). Therefore, pristine ZnO cannot be used as the ETL in PSCs. However, this degradation issue can be ressolved by passivating methylammonium chloride (MACl) [24], implementation of bilayer ETLs [25] or passivation or doping of stable metal oxides [15], [26]. Cs doping is one of the best alternatives because of its high conductivity and suitability for deposition at low temperature. Therefore, mixing Cs with ZnO to form a non-basic alloy opens a new approach towards stable ETLs for PSCs. Further, in order to disregard the perovskite/ETL interface recombination, it is necessary to extract the excited electrons from the perovskite absorber layer quickly(See Table 1).

Here we report ambient processed all-inorganic PSCs based on a narrow bandgap γ−CsPb1−xCdxI2.5Br0.5 (Eg=1.71 eV) perovskite and a Zn1−xCsxO ETL, facilitating ambient-stable PCE nearing 20%. Tuning the Cs inclusion within the ZnO ETL allows for the band alignment to be optimized toward enhanced charge extraction and mitigates the degrading influence of its pristine surface. We further demonstrate that a smooth and compact Zn1−xCsxO ETL is realized using a simple and scalable spray coating approach. The use of Cd2+ doping in organic halide perovskite (MA/FAPbI3) has showed improved grain growth [27], suppression of atomic vacancies [28] and modulation of optoelectronic properties [29], and we have adopted this strategy to bolster the phase stability of hot-air deposited CsPbI3-based this film, retaining a narrow bandgap and its all-inorganic nature. Our champion PSCs exhibited > 19.75% PCE stable performance in ambient condition with > 300 h air stability based on Zn0.9Cs0.1O ETLs. The stabilized efficiency of the unencapsulated hero device independently certified at the KITECH is 18.63 %. Additionally, we realize PCE of 18.46% in large-area 1 × 1 cm2 device configurations, retaining > 95% of the initial efficiency over 300 h under continuous heating at 80 °C.

Section snippets

Results and discussion

We begin by detailing the physical profile of our Zn1−xCsxO colloidal nanocrystals synthesized by a modified version of our previously reported chemical route [15]. The use of Zn1−XCsxO as an ETL recently yielded promising results when implemented within PbS colloidal quantum dot (CQD) solar cells [30]. Instead of conventional spin coating, we developed a spray coating deposition technique in order to get high quality Zn1−xCsxO thin films at scale. The colloidal nanocrystals solution was

Conclusions

In summary, we showcased a low temperature processed Zn0.9Cs0.1O material as a promising ETL using spray coating technique, for all-inorganic γ − CsPb0.99Cd0.01I2.5Br0.5-based planar n–i–p PSCs. Our fabrication enables highly efficient and air-stable planar all-inorganic PSCs operating under ambient conditions. A champion device comprised of a γ − CsPb0.99Cd0.01I2.5Br0.5 (Eg=1.705 eV) perovskite absorber with Zn0.9Cs0.1O ETL exhibited 19.75% PCE. The impressive device performance originates

Materials and methods

Full experimental procedures provided in the Supplemental Information.

Supplemental Information

Supplemental Information includes materials and methods, characterizations, Twenty-six Figs. S1-S26, and eight Table S1-Table S8.

CRediT authorship contribution statement

Dr. Sawanta S. Mali: Conceptualization, Methodology, Writing - original draft preparation, Writing - review & editing, Data curation, investigation. Dr. Jyoti V. Patil: Data curation, Visualization investigations and Validation. Dr. Julian A. Steele: Structural analysis. Prof. Chang-Kook Hong: Supervision, Fund acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2018R1A6A1A03024334). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018R1C1B6008218). This work was also supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of

Dr. Sawanta S. Mali received his Ph. D. degree from Shivaji University Kolhapur, India in 2013 under the supervision of Pro Vice-Chancellor Prof. Pramod S. Patil. Currently he is working as a Associate Research Professor at Chonnam National University, South Korea. He is recipient of prestigious “Best Ph. D. thesis Award” by International Solvothermal and Hydrothermal Association (ISHA)−2014. Moreover, he is recipient of Outstanding Young Researcher Award by KIECS-2015. He has been awarded as

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    Dr. Sawanta S. Mali received his Ph. D. degree from Shivaji University Kolhapur, India in 2013 under the supervision of Pro Vice-Chancellor Prof. Pramod S. Patil. Currently he is working as a Associate Research Professor at Chonnam National University, South Korea. He is recipient of prestigious “Best Ph. D. thesis Award” by International Solvothermal and Hydrothermal Association (ISHA)−2014. Moreover, he is recipient of Outstanding Young Researcher Award by KIECS-2015. He has been awarded as an outstanding overseas young researcher by Korean Research Foundation (Brain-Pool) from 2016 to 2021. Since 2013, he is working on optimizing different recipes for halide perovskite solar cells. His current research interest is mainly focuses the synthesis of 2D/3D all-inorganic perovskite solar cells towards phase stabilization and its commercialization using air-processing techniques.

    Dr. Jyoti V. Patil, is currently working as a Research Professor at Polymer Energy Materials Laboratory, Chonnam National University, Gwangju. She received her Ph. D. degree in sensitized solar cells based on electrospun TiO2 nanofibers from Shivaji University Kolhapur in 2018 under the guidance of Pro Vice-Chancellor Prof. Pramod S. Patil. Currently, her research interest is mainly focuses on air-processed all-inorganic perovskite solar cells through rare earth metal ion doping and its stability analysis.

    Dr. Julian A. Steele received his Ph.D. in physics from The Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong, before joining the group of Prof. Roeffaers (KU Leuven) as a postdoctoral researcher with financial support from the Belgium government (FWO), where his work focuses on nanoscale optical materials. From 2019−2020, he undertook a research stay in the laboratory of Prof. Peidong Yang, at UC Berkeley, to work on phase transition phenomena within metal halide perovskites. A central theme of his research is to discover and develop deep structure-property relationships for emerging optoelectronic materials and devices.

    Prof. Dr. Chang Kook Hong completed his Ph. D. from The University of Akron (USA) in Polymer Engineering in 2001. Afterwards he joined University of Delaware and The University of Southern Mississippi as a Postdoctoral Fellow during 2001–2004. He worked for Samsung Electronics until 2007. Currently he is a full Professor at School of Chemical Engineering. He is also working as a Vice-Dean of Faculty of Science at Chonnam National University. His main research focuses on energy devices, such as perovskite solar cells and secondary batteries, using polymeric materials and polymer nanocomposites for various applications.

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