Boost the performance of inverted perovskite solar cells with PEDOT:PSS/Graphene quantum dots composite hole transporting layer
Graphical abstract
Introduction
Perovskite solar cells have become the hotspot in the field of thin film solar cells due to its high power conversion efficiencies and low fabrication cost. Power conversion efficiencies have been promoted from 3.8% in 2009 to 25.2% in 2019 during a period of about 10 years [1], this amazing growth rate in power conversion efficiencies is unprecedented compared with the other type of solar cells. Perovskite solar cells can be roughly divided into conventional n-i-p structure and the inverted p-i-n structure. The conventional structure has demonstrated superior power conversion efficiencies [[2], [3], [4], [5]], however, the commonly used TiO2 electron transporting layer needs high temperature annealing and not suitable for flexible solar cells. On the contrary, the inverted structure solar cells exhibit power conversion efficiencies slightly lower than the conventional structure [6,7], but they can be fabricated in a low temperature process, thus are very promising in wearable devices and flexible solar cells.
In the inverted perovskite solar cells, PEDOT:PSS is the most commonly used hole transporting layer due to its high transparency, moderate conductivity, and feasible low temperature solution process [[8], [9], [10]]. Yet, the highest power conversion efficiencies of perovskite solar cells fabricated on PEDOT:PSS are reported to be around 19% [7,11], which are lower than most high performance perovskite solar cells. The work function of pristine PEDOT:PSS is much lower than the valence band edge of perovskite light absorbing layer, this mismatch of band alignment results in energy losses during the hole extraction process and a low Voc less than 1.0 V is usually observed [8,12]. At the same time, conductivity of PEDOT:PSS is lower than that of the widely used [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) electron transporting layer. The unbalanced charge transportation in the hole transporting layer and the electron transporting layer can create charge carrier accumulation, leading to large leakage current and low fill factor [13,14].
To solve the above mentioned problems, various methods have been proposed, such as solvent treatment, employment of an additive, and formation of PEDOT:PSS/nanomaterial composite hole transporting layer [8,10,[15], [16], [17], [18]]. All these methods can enhance the performances of inverted perovskite solar cells fabricated on PEDOT:PSS films due to one or more of the following factors: (1) modifying the morphology of the resulting PEDOT:PSS film, and gradually leading to perovskite films with higher crystallinity and less traps or defects, (2) adjusting the work function of PEDOT:PSS film to better match that of the perovskite film, and leading to an increase of the open circuit voltage, (3) increasing the conductivity of the PEDOT:PSS film, and facilitating hole extraction and transportation.
Composite hole transporting layers containing PEDOT:PSS and an extra nanomaterial are widely studied as potential candidate to replace the pristine PEDOT:PSS film in order to pursue superior performances. Metal nanoparticles, CrO3, CuSCN, V2O5, WO3 et al. have been employed to form composite hole transporting films together with PEDOT:PSS [[19], [20], [21], [22], [23]]. Besides the above mentioned materials, carbon materials, such as carbon black [24], carbon nanotubes [16], carbon dot [17], graphene oxide [18] et al. have also been successfully explored to form composite hole transporting film with PEDOT:PSS. Graphene quantum dots (GQDs) are single, double and few layered graphene sheets with dimensions less than 100 nm [[25], [26], [27], [28]]. Usually, GQDs are functionalized with hydroxyl and carboxylic acid groups, and a schematic structure of single layered GQDs is illustrated in supporting information Fig. S1. Due to its high conductivity, excellent solubility, and stable photoluminescence, GQDs have gained widespread interests. GQDs have been studied to serve as hole transporting layer in organic solar cells [29], yielding power conversion efficiencies comparable to that of PEDOT:PSS. Later, PEDOT:PSS films incorporating GQDs are demonstrated to be superior hole transporting layer both in organic solar cells and Si/PEDOT:PSS hybrid solar cells [30,31]. GQDs are believed to strongly interact with the PEDOT and PSS chain, leading to a phase separation between PEDOT and PSS, thus conductivity of the PEDOT:PSS/GQDs composite film is enhanced [32]. The resulting better hole transporting capability of the PEDOT:PSS/GQDs composite film is the main reason for the enhancement of the performances of the above mentioned solar cells.
In this report, PEDOT:PSS/GQDs composite films are prepared by spin coating a mixture of commercial PEDOT:PSS solution and GQDs solution, the performances of the inverted perovskite solar cells with different GQDs concentrations are studied. The optimized PEDOT:PSS/GQDs composite film can boost the average power conversion efficiency to 15.24%, which is enhanced by 19% compared with the PEDOT:PSS film. A champion power conversion efficiency of 16.16% is also obtained with the optimized PEDOT:PSS/GQDs composite film.
Section snippets
Materials
GQDs aqueous solution (1 mg mL−1) was purchased from Tanfeng Graphene Technology (Suzhou) Co. Ltd. The average size of the GQDs is about 15 nm, the thicknesses of the GQDs are between 0.5 nm and 2.0 nm according to the product specification. Clevios PVP AI 4083 PEDOT:PSS was bought from Heraeus (Germany). PbI2, CH3NH3I (MAI) and PC61BM were purchased from Xi'an Polymer Light Technology Corp. To prepare the PEDOT:PSS/GQDs composite films, 1 mL pristine PEDOT:PSS solution was mixed with x mL GQDs
Results and discussions
GQDs are incorporated into the PEDOT:PSS films by mixing different volume of GQDs aqueous solution with 1 mL commercial PEDOT:PSS solution. To ensure that the final film thicknesses of the PEDOT:PSS/GQDs films are nearly the same, deionized water is added and the final volumes of the mixed PEDOT:PSS/GQDs solutions are the same. The PEDOT:PSS/GQDs films are prepared using a simple spin coating process. Perovskite solar cells with a p-i-n structure are fabricated on these PEDOT:PSS/GQDs films and
Conclusion
In conclusion, PEDOT:PSS/GQDs composite films are employed as hole transporting layer for inverted structure perovskite solar cells. Residual PbI2 is detected in the perovskite films fabricated on PEDOT:PSS/GQDs films, which might be helpful to passivate the defects of the perovskite films. Current-voltage curves of the PEDOT:PSS/GQDs films confirm that the conductivity of the films are increased when GQDs are incorporated into the PEDOT:PSS films. This enhanced conductivity of the
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, and Key Project of Henan Province Colleges and Universities Plan (No. 19A430022).
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