High performance carbon-based planar perovskite solar cells by hot-pressing approach
Graphical abstract
Carbon electrode-based planar perovskite solar cells prepared by hot-pressing method at 80 °C demonstrated impressive device performance and excellent reproducibility due to the enhanced conductivity and interfacial contact.
Introduction
Organic-inorganic hybrid lead halide perovskite solar cells (PSCs) are emerging as a promising photovoltaic technology for cost-effective solar electricity, owning to their skyrocketing progress of power conversion efficiency (PCE) achieved to more than 25% within just 10 years [1]. Meanwhile, in order to solve the critical stability issue and to further reduce the cost of PSCs, carbon materials have been utilized as back contact to replace the expensive noble metal contact such as gold owing to the advantages of low material cost, good stability in ambient environment, inertness to ions dissociated from the perovskite and hydrophobicity [2,3]. Generally, PSCs using carbon electrode (C–PSCs) adopt two configurations, mesoporous structure and planar structure. Most of the C–PSCs reported in literature adopted a mesoporous structure using mesoporous TiO2 as an electron transport layer (ETL) [[4], [5], [6]]. The fabrication of TiO2 mesoporous layer requires high temperature annealing process, which may limit the application of C–PSCs on flexible conductive polymer based substrates. In contrast, planar structure can be made at low temperature using simpler fabrication procedure, which is more attractive for practical application [7]. Nevertheless, the performance of planar C–PSCs is normally lower than its mesoporous counterpart, which is even much lower compared to the state-of-the-art PSCs that use organic semiconductor based hole transport materials such as spiro-OMeTAD and gold as current collector.
Generally speaking, three methods have been reported to fabricate carbon electrodes for C–PSCs including doctor-blading/screen-printing [4,7] and pressing transfer [8]. Doctor-blading/screen-printing is the mainly used technique in fabrication of carbon electrode for mesoporous C–PSCs or planar C–PSCs and over 15% of PCE was reported with champion devices [7,9]. In this method, a carbon paste containing carbon black, graphite, organic binder or insolating metal oxide is doctor-bladed/printed directly onto the TiO2/ZrO2 mesoporous layer or perovskite light absorber, followed by annealing at relatively high temperature to form carbon electrode [9]. This technique makes it feasible for large scale production of C–PSCs due to its simplicity, low cost and good reproducibility.
However, organic solvents contained in the carbon slurry for both doctor-balding and screen-printing methods can potentially damage the underneath material (perovskite or organic HTL), which impedes their application in C–PSCs with organic HTL, such as conventional spiro-OMeTAD and P3HT [10]. Alternative approach like pressing a free-standing carbon film onto perovskite or HTL can resolve these problems in C–PSCs. In this approach, a free-standing carbon film is firstly made which is followed by being pressed directly onto a perovskite layer or HTL under a certain pressure to make C–PSCs [8]. Nevertheless, the quality of interfacial contact between the carbon electrode and underneath layer is often poor, leading to lower device performance and severe hysteresis phenomenon observed in the current density-voltage (J-V) plot.
Herein, we report a hot-pressing approach to fabricate planar C–PSCs by pressing free-standing carbon films directly onto CuSCN based HTL under heating. The schematic is as depicted in Fig. 1. After carefully optimizing the applied temperature, we found the conductivity of carbon electrode and the adhesion of carbon film to the HTL were remarkably improved. As a result, the CuSCN based C–PSCs made by hot-pressing technique at 80 °C yielded a champion efficiency of 15.3% and an average PCE of 14.6% with eliminated hysteresis behaviour and excellent device stability. Furthermore, it was also demonstrated that this universal hot-pressing process is valid to spiro-OMeTAD based organic HTL, which is widely used in metal electrode based PSCs but rarely applied in C–PSCs. The carbon electrode made by the hot-pressing method demonstrates a great potential in the application of perovskite solar cells.
Section snippets
Materials preparation
All materials were purchased from Sigma-Aldrich and used as received without further purification unless otherwise stated. Materials such as ammonium bromide (MABr) and formamidinium iodide (FAI) were provided by Greatcell Solar for the preparation of triple cation perovskite film. Lead iodide (PbI2, 99.9%) was ordered from Youxuan Tech, China. 2, 2′, 7, 7′-Tetrakis-(N, N-di-4-methoxyphenylamino)-9, 9'spirobifluorene (Spiro-OMeTAD) was supplied by Borun New Material, China. The carbon paste was
Results and discussion
The surface morphology and thickness of the carbon films after going through different pressing process were compared firstly. Fig. 2a-c shows the surface morphology by SEM of as-prepared carbon film, the carbon film after pressing under 30 MPa at room temperature (22 °C, normal pressing) and the carbon film made by pressing under 30 MPa at 80 °C by (hot-pressing), respectively. As seen in Fig. 2a, the carbon film without any pressing shows a mesoporous structure with plenty of micron-sized
Conclusions
We demonstrated a new method to prepare carbon electrode for planar perovskite solar cells, which involves hot-pressing a free-standing carbon film on hole transport material of PSCs. After optimizing the applied temperature, we found both the electrical and mechanical properties of the carbon electrode were significantly enhanced by using the hot-pressing process at 80 °C, leading to remarkable enhancement of the device performance. An outstanding PCE of 15.3% with high reproducibility and
CRediT authorship contribution statement
Yang Yang: Writing - original draft, Investigation. Minh Tam Hoang: Investigation. Disheng Yao: Investigation. Vincent Tiing Tiong: Investigation. Xiaoxiang Wang: InvestigationNgoc Duy Pham: Investigation, Writing - review & editing. Wentao Sun: Supervision. Hongxia Wang: Supervision, Writing - review & editing.
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
The authors thank the financial support to this work by Australian Research Council Discovery Project (DP190102252). Y.Y acknowledges Queensland University of Technology (QUT) postgraduate scholarship. The data of SEM, UPS reported in this paper were obtained at the Central Analytical Research facility (CARF), QUT. Access to CARF was supported by the generous funding from Science and Engineering faculty, QUT.
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