Free-standing flexible carbon electrode for highly efficient hole-conductor-free perovskite solar cells
Introduction
Methylammonium lead halide perovskites as light absorbers for thin film solar cells have attracted great attention in the past few years due to their superb optical and electric properties [1], [2]. From the liquid-type to the solid-state, the perovskite solar cells (PSCs) have experienced a rapid development [3], [4], [5]. Most recently, a series of high power conversion efficiencies (PCEs) about 15–19% have been obtained by optimizing fabrication process and device structure, and the certified record PCE of 20.1% was also reported [6], [7], [8], [9], [10], [11], [12], [13], [14]. Simple fabrication process and high conversion efficiency make PSCs competitive in future commercialization. In views of decreasing materials cost and simplifying the cell structure, hole transporting material (HTM)-free PSCs are also developed [15], [16], [17], [18], [19], [20], [21]. With Au electrode, the PCE of the cell has already exceeded 10% by improving the perovskite film deposition and back contact, showing the promising prospect of HTM-free PSCs [18], [19], [20], [21]. However, the noble mental Au counter electrode (CE) is still widely used in these high performance photovoltaic devices, which is derived from thermal evaporation under high vacuum condition, limiting its future application. Therefore, it is necessary to develop low-cost materials to replace the costly metallic CEs (Au or Ag).
Conductive carbon materials, especially graphite and carbon black (CB), are abundantly available and low-cost, which have been applied in dye-sensitized solar cells [22], [23], [24]. Recently, several research groups reported PSCs with carbon materials [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. Han et al. post-deposited perovskite materials into the mesoporous TiO2/ZrO2/carbon monolithic structure of the HTM-free PSC, and achieved a high PCE of 12.8%, however, the carbon layer needs to be sintered at 400 °C, which limited their fabrication on a plastic substrate [26]. Yang et al. attempted different carbon materials (such as candle soot, single-layered or multilayered graphene) as hole extractors for perovskite solar cells, and obtained 11% PCE. Their efforts have been paid on building an excellent contact between perovskite and graphene, however the cell efficiency was still unsatisfactory [30], [31]. Wong and Mhaisalkar et al fabricated semitransparent CH3NH3PbI3/spiro-OMeTAD-CNTs solar cells with 9.90% PCE by directly transferring the freestanding CNT film on the perovskite surface with several drops of toluene to increase adhesion, however, higher sheet resistance of CNT films leading to unsatisfied fill factor [32]. Ma and Sun et al. directly deposited commercial conductive carbon pastes onto the perovskite layers of HTM-free devices with 8–9% PCEs, respectively [33], [34]. But the existence of the solvents in carbon pastes may destroy the perovskite layer and limit the application in the devices with organic/polymer HTMs.
Herein, a free-standing thermoplastic carbon film with good flexibility and conductivity has been fabricated by a simple and low-temperature (<100 °C) process. The carbon films are directly hot-pressed onto perovskite film for HTM-free PSCs. It is revealed that the carbon films components and hot-press pressure have significant influence on the back contact and the performance of HTM-free PSCs. By optimization toward the carbon film components and hot-press pressure, up to 13.53% of PCE has been achieved with an average PCE of 12.03%, which is one of the highest efficiencies for HTM-free PSCs. This kind of flexible carbon electrodes with the advantages of low-cost, simple and low-temperature (<100 °C) preparation process and high efficiency, show great potential application in perovskite solar cells.
Section snippets
Materials
PbI2 (99%) and N,N-dimethylformide (DMF, 99.7%) were purchased from Sigma–Aldrich and Alfar Aesar, respectively. Graphite (D50 < 400 nm, 99.95%) was purchased from Aladdin. Polyvinyl acetate (PVAc) was purchased from Sinopharm Chemical Reagent Co. Ltd. All the chemicals were directly used without further purification. CH3NH3I was synthesized as literature described [5]. Fluorine-doped tin oxide conducting glass (FTO, Pilkington, thickness 2.2 mm, sheet resistance 14 Ω/square) was used as substrates
Results and discussion
The fabrication procedure of our perovskite solar cells with carbon CE is shown in Fig. 1. Four types of carbon films with different weight ratios of graphite flakes and CB particles are obtained. The carbon film with pure graphite flakes is defined as A, and the carbon films with graphite and CB weight ratio of 5:1, 3:1 and 2:1 are defined as B, C and D, respectively. The morphologies of as-prepared carbon films are shown in Fig 2(a)–(f). Comparison of the SEM images of carbon film A before
Conclusions
In summary, a free-standing thermoplastic carbon film with good flexibility and conductivity has been prepared by a simple and low-temperature process. The carbon films are directly hot-pressed onto perovskite film to be used as the CE for HTM-free PSCs. It is revealed that, the carbon film components and hot-press pressure have significant influence on the back contact and the cell performance. By careful optimization toward the carbon components and hot-press pressures, the back contact of
Acknowledgements
This work was supported by National Key Basic Research Program (Nos. 2012CB932903 and 2012CB932904), Natural Science Foundation of China (Nos. 91433205, 51421002, 51072221, 51372270, 51372272, 21173260, 11474333 and 91233202), and the Knowledge Innovation Program of the Chinese Academy of Sciences.
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