Elsevier

Organic Electronics

Volume 98, November 2021, 106298
Organic Electronics

High performance inverted perovskite solar cells using PEDOT:PSS/KCl hybrid hole transporting layer

https://doi.org/10.1016/j.orgel.2021.106298Get rights and content

Highlights

  • KCl is used to modify the PEDOT:PSS film.

  • Island and dendrite KCl structures are observed in the PEDOT:PSS/KCl films.

  • Charge carrier recombination is suppressed using the PEDOT:PSS/KCl films.

  • Champion power conversion efficiency of 17.09 % has been demonstrated.

Abstract

PEDOT:PSS is one of the most widely used hole transporting layer for inverted perovskite solar cells. Yet the performances of the corresponding perovskite solar cells are not satisfactory. Here, we demonstrate that KCl modified PEDOT:PSS film can promote the crystallization of perovskite film and enlarge the perovskite crystals. At the same time, KCl can diffuse into the perovskite film and effectively passivate the defects. As a result, inverted perovskite solar cells fabricated on 10 mg mL−1 PEDOT:PSS/KCl films exhibit an average power conversion efficiency of 16.24 %, which is enhanced by 17.77 % compared with the reference perovskite solar cells. Open circuit voltage of 1.009 V and power conversion efficiency of 17.09 % have also been demonstrated using the optimized 10 mg mL−1 PEDOT:PSS/KCl films.

Introduction

The application of organic-inorganic hybrid perovskite as promising light absorbing materials in solar cells has been intensively explored in the last decade owing to the inherently excellent opto-electronic properties of the perovskite materials. As a result, power conversion efficiencies (PCEs) of perovskite solar cells have been boosted from 3.8 % in 2009 to 25.5 % in 2020 [1], approaching that of the commercial single crystal silicon solar cells and demonstrating great prospect of commercialization. Inverted p-i-n structure perovskite solar cells have recently drawn plenty of attention due to their simple structure, easy and low temperature fabrication process, negligible hysteresis, and compatible with flexible perovskite solar cells [[2], [3], [4]]. Thus, a lot of effects have been made to promote the performances of the inverted perovskite solar cells.

PEDOT:PSS is the most widely studied hole transporting layer in the inverted perovskite solar cells due to its low temperature solution processability, high optical transparency, and moderate conductivity. However, the PCEs of inverted perovskite solar cells employing PEDOT:PSS hole transporting layer are not satisfactory compared with that of inverted perovskite solar cells employing PTAA or NiO hole transporting layer. The open circuit voltage deficiency (Voc ≤ 1.0 V) is the main factor limiting the PCEs of perovskite solar cells employing PEDOT:PSS hole transporting layers, which is caused by surface recombination at the perovskite/PEDOT:PSS interfaces [2,[4], [5], [6]]. Therefore, various methods have been proposed to modify the properties of the PEDOT:PSS layers in order to optimize the performances of the perovskite solar cells. Alkali metal salts are highly soluble in water, thus alkali metal salts modified PEDOT:PSS films can be easily prepared by spin-coating the PEDOT:PSS aqueous solution with addition of alkali metal salts. At the same time, the opto-electronic properties of alkali metal salts modified PEDOT:PSS films can be effectively tuned to enhance the hole extraction efficiency and suppress charge recombination, leading to enhanced PCEs compared with the pristine PEDOT:PSS films [3,5,7,8]. NaCl doping has been demonstrated to enhance the conductivity and hole extraction capability of the PEDOT:PSS film, which boosts the average PCE from 15.1 % for pristine PEDOT:PSS film to 17.1 % for PEDOT:PSS film with 5 mg mL−1 NaCl [7]. CsI doped PEDOT:PSS film also exhibits superior hole transportation and extraction properties, at the same time, energy level of PEDOT:PSS is modified to better match that of the perovskite light absorbing layer, thus charge recombination at the perovskite/PEDOT:PSS interface is suppressed. As a result, perovskite solar cells employing CsI doped PEDOT:PSS film have achieved a high Voc of 1.084 V and excellent PCE of 20.22 % [5]. KCl has been demonstrated to successfully modify the commonly used SnO2 electron transporting layer and NiO hole transporting layers, which can improve the quality of the perovskite films, reduce the trap density, and suppress the interfacial recombination at the same time [[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. Therefore, electron or hole transporting layers modified with KCl can effectively boost the PCEs of the perovskite solar cells. Considering the trap passivating capability of KCl and its chemical similarity with CsI and NaCl, KCl might be a potential candidate to effectively modify the PEDOT:PSS film in order to pursue superior PCEs, yet literatures focusing on application of KCl modified PEDOT:PSS film in inverted perovskite solar cells are scarce.

In this study, the influence of KCl addition on the morphologies of PEDOT:PSS films and the performances of the resulting perovskite solar cells are studies carefully. Optical images confirm that island or dendrite shaped KCl structures are formed when KCl is added into PEDOT:PSS films, which might promote the heterogeneous crystallization of the perovskite and results in larger perovskite crystals. At the same time, K+ and Cl ions can diffuse into the perovskite film when annealed at 100 °C, which can effectively passivate the perovskite/PEDOT:PSS interface, leading to less trap density and suppressed charge carrier recombination. As a result, perovskite solar cells fabricated on PEDOT:PSS/KCl films exhibit enhanced performances. Average PCE is boosted from 13.79 % for the reference solar cells to 16.24 % for perovskite solar cells fabricated on 10 mg mL−1 PEDOT:PSS/KCl films.

Section snippets

Fabrication of the perovskite solar cells

Inverted perovskite solar cells are fabricated according to our previous report with minor modification [20]. In brief, FTO substrates are thoroughly ultrasonic cleaned using deionized water and ethanol before use. PEDOT:PSS/KCl aqueous solutions are prepared by diluting the Clevios PVP AI 4083 with deionized water (v:v = 1:1) and then proper amounts of KCl are added into the diluted solutions. The final KCl concentrations are 0 mg mL−1, 5 mg mL−1, 10 mg mL−1 and 15 mg mL−1 respectively. The

Results and discussions

Transmittance spectra of the PEDOT:PSS/KCl films deposited on FTO substrates are displayed in supporting information Fig. S1. All the PEDOT:PSS/KCl films exhibit nearly the same transmittance in the wavelength range from 300 nm to 900 nm and are slightly higher than that of bare FTO substrate. UV–vis absorbance spectra of the perovskite films deposited on the PEDOT:PSS/KCl films are presented in Fig. 1a. All the perovskite films exhibit nearly the same absorbance spectra in the wavelength range

Conclusion

In summary, we have demonstrated that PEDOT:PSS/KCl film can be a better hole transporting material compared with the reference PEDOT:PSS film. Optical microscope, AFM and SEM measurements confirm that KCl can form island or dendrite structures on the PEDOT:PSS films leading to slightly enlarged perovskite crystals. At the same time, KCl can diffuse into the perovskite films during the subsequently thermal treatment and effectively passivate the defects in the perovskite films, which has been

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.

Acknowledgements

This work receives financial support from Doctoral Scientific Research Foundation of Xinyang Normal University, Nanhu Scholars Program for Young Scholars of XYNU, Key Project of Henan Province Colleges and Universities Plan (No. 19A430022), and Natural Science Foundation of Henan Province (No. 202300410334).

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