All-inorganic, hole-transporting-layer-free, carbon-based CsPbIBr2 planar perovskite solar cells by a two-step temperature-control annealing process

https://doi.org/10.1016/j.mssp.2019.104870Get rights and content

Highlights

  • The effect of different first-step temperature during a two-step annealing process on PCEs of PSCs was studiedfor the first time.

  • When the first-step annealing temperature was 150 °C and the second-step annealing temperature was 280 °C, the highest efficiency was 8.31%.

  • Compared with solar cells annealed by a one-step annealing process, there is an increase of 66.9 % in PCE.

  • The study on why the PSCs by a two-step annealing process has a higher PCE was done for the first time.

Abstract

In this paper, all-inorganic CsPbIBr2 thin films were annealed by a two-step temperature-control process. All-inorganic, hole-transporting-layer-free, carbon-based planar perovskite solar cells (PSCs) with these CsPbIBr2 thin films (FTO/c-TiO2/CsPbIBr2/C) were fabricated. The effect of different first-step annealing temperatures during the two-step temperature-control process (50 °C/280 °C, 80 °C/280 °C, 100 °C/280 °C, 150 °C/280 °C, 180 °C/280 °C) on the photovoltaic conversion efficiency (PCE) of PSCs was investigated for the first time. When the first-step annealing temperature was 150 °C and the second-step annealing temperature was 280 °C, the highest efficiency of 8.31% was obtained. Without encapsulation, the solar cell could retain 97% of the initial PCE, when it was stored at 80 °C and zero humidity for 8 days. For comparison, solar cells with CsPbIBr2 films annealed by a one-step temperature-control process were fabricated. The highest PCE was 4.98%. From this, we could see that there is an increase of 66.9% in PCE, through using a two-step temperature-control annealing process. And also, in order to investigate why the PSCs by a two-step temperature-control annealing process has a higher PCE, all kinds of measurements were done for the first time. According to the results of the measurements, the perovskite film annealed by a two-step temperature-control process has a bigger crystal size, fewer grain boundaries, stronger PL and UV–vis absorption intensities, longer lifetime of minority carriers, less energy loss for hole transporting.

Introduction

Energy harvesting and environmental pollution are still two of the most challenged problems for humankind [1]. In order to resolve these problems, mankind focuses on new energy field, such as Li-ion battery [2,3], sodium-air batteries [[4], [5], [6]], Photocatalytic degradation [[7], [8], [9]], and so on. Owing to be abundant, renewable and sustainable, solar energy has been paid much attention [10]. To utilize these advantages, solar cells are fabricated to directly convert the solar energy to the electricity [11]. To date, there are three types of solar cells. First type is the commercial bulk silicon solar cell [12]. Second type is the partially commercial thin film solar cell,for examples, amorphous silicon solar cells [13], copper zinc tin sulfur solar cells [[14], [15], [16]], CdTe solar cells [17]. Third type is the new solar cell, such as dye-sensitized solar cells [18], perovskite solar cells [19,20], and so on. Organic-inorganic hybrid perovskite solar cells (PSCs) have been attracted much attention, because their photovoltaic conversion efficiency (PCE) can compete with that of the commercial bulk silicon solar cells [19]. Until now, the highest PCE of the perovskite solar cells is 24.2% [21]. However, the PSCs with the high efficiency usually contain the organic cations, for examples, formamidinium (FA+) and methyl ammonium (MA+) [[21], [22], [23]], which would influence the long-term stability. The long-term instability of organic-inorganic PSCs is an obstacle for their commercialization [19].

In order to resolve this problem, all-inorganic perovskite materials have been explored for solar cells. Park et al. fabricated Cs3Bi2I9 PSCs by a one-step spin-coating method and obtained a PCE of over 1% [24]. Correa-Baena et al. fabricated Rb3Sb2I9 PSCs with a PCE of 0.76% [25]. Chen et al. fabricated Cs2TiBr6 PSCs with a PCE of 3.3% [26]. The most promising and investigated all-inorganic perovskite is CsPbX3 (X = I, Br), for examples, CsPbI3 [27], CsPbBr3 [28,29], CsPbI2Br [30], and CsPbIBr2 [19,31,32]. Among CsPbX3, CsPbBr3 and CsPbIBr2 are more stable than CsPbI3 and CsPbI2Br at ambient atmosphere [19,[27], [28], [29], [30], [31], [32]]. However, CsPbBr3 possesses a large bandgap of 2.3 eV, resulting in limiting the utilization of solar light, so that the PCE of CsPbBr3 PSCs is limited [28,29]. CsPbIBr2, with a lower bandgap of 2.05 eV than that of CsPbBr3, is regarded as a relatively ideal all-inorganic perovskite material for PCSs [19,31,32].

At present, PSCs with high efficiencies usually require expensive hole-transporting materials (HTMs), such as spiro-OMeTAD [33], which results in the degradation of the PCE for PSCs against the thermal stress [21]. The perovskites could be used as the HTMs, due to their long hole-transporting lengths [34]. Considering cost saving, the HTMs could be omitted in PSCs. Carbon is regarded as the most promising electrode material to substitute for costly metal electrode materials [19,32,[35], [36], [37], [38], [39], [40]]. Therefore, many researchers have paid much attention on the HTMs-free carbon-based PSCs [19,32,[36], [37], [38], [39]]. Till now, the best efficiency of the HTMs-free carbon based PSCs is over 15% [40], indicating that the HTMs free carbon based PSC is promising.

In this paper, we fabricated the HTMs-free carbon-based PSCs--- glass/FTO/c-TiO2/CsPbIBr2/C by an intermolecular exchange method [19]. Liu et al. presented a two-step temperature-control method to anneal CsPbI2Br and CsPbIBr2 thin films. [30,31] In these two papers, there are no discussions about the effect of different annealing temperatures at the first step on the PCEs of PSCs. In this work, the effect of different first-step annealing temperatures during the two-step temperature-control process (50 °C/280 °C, 80 °C/280 °C, 100 °C/280 °C, 150 °C/280 °C, 180 °C/280 °C) on the photovoltaic conversion efficiency (PCE) of PSCs was investigated for the first time. The highest efficiency of PSCs, 8.31%, was obtained at 150 °C for the first step and 280 °C for the second step. The open-circuit voltage (Voc) is 1.05 V. The short-circuit current density (Jsc) is 17.08 mA/cm2. The fill factor (FF) is 0.48. Without encapsulation, the solar cell could retain 97% of the initial PCE when it was stored at 80 °C in zero humidity for 8 days. For comparison, solar cells with CsPbIBr2 films annealed by a one-step temperature-control process were fabricated. The highest PCE was 4.98%. The open-circuit voltage (Voc) is 1.03 V. The short-circuit current density (Jsc) is 9.96 mA/cm2. The fill factor (FF) is 0.48. From this we could see that there is an increase of 66.9% in PCE, through using a two-step temperature-control process. And also, in order to investigate why the PSCs by a two-step temperature-control process has a higher PCE, all kinds of measurements were done.

Section snippets

Materials

Cesium iodide (CsI, anhydrous, 99.999%), lead(Ⅱ) bromide (PbBr2, anhydrous, 99.999%), Dimethyl sulfoxide (DMSO, anhydrous, ≥ 99.9%), Titanium diisopropoxide bis (acetylacetonate) (75 wt % in isopropanol) were bought from Sigma-Aldrich. Isopropanol (IPA, anhydrous, 99.5%) was bought from Macklin. Methanol (anhydrous, 99.9%), ethanol (anhydrous, GR) was bought from Aladdin. Titanium chloride (TiCl4, 99.0%–99.6%) was purchased from Labgogo. Carbon electrode paste was bought from Shanghai MaterWin

Results and discussion

Fig. 1a shows the schematic diagram of glass/FTO/c-TiO2/CsPbIBr2/C solar cells. Corss-sectional SEM of all-inorganic perovskite solar cell is shown in Figrue S1. Fig. 1b presents typical J-V curves of the best cells fabricated at 50 °C/280 °C, 80 °C/280 °C, 100 °C/280 °C, 150 °C/280 °C and 180 °C/280 °C, respectively. Table 1 shows the open-circuit voltage (Voc), the short-circuit current density (Jsc), fill factor (FF) and PCE for the champion cells fabricated at 50 °C/280 °C, 80 °C/280 °C,

Conclusions

In this paper, the all-inorganic CsPbIBr2 thin films were prepared by an intermolecular exchange method and then were annealed by a two-step temperature-control process. All-inorganic, HTL-free, carbon-based planar solar cells with these CsPbIBr2 thin films were fabricated. We present the effect of different annealing temperatures at the first step on the PCE of PSCs for the first time. When the first-step temperature was 150 °C and the second-step temperature is 280 °C, the highest PCE of

Statement to specify the contribution of each co-author

  • (1)

    Cheng Wang: My master. His contribution includes putting forward the experimental scheme, preparation of materials, fabrication of devices, discussion, analysis, writing the paper.

  • (2)

    Junsen Zhang: My master. His contribution includes preparation of materials, fabrication of devices, discussion.

  • (3)

    Jipeng Duan: My postgraduate. His contribution includes preparation of materials, fabrication of devices, discussion.

  • (4)

    Li Gong: corresponding author.

  • (5)

    Jie Wu: co-researcher. His contribution includes I–V

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 National Natural Science Foundation of China, China (Contract no.51302021), the Postgraduate Research Innovation Project of Changsha University of Technology, China (Contract no. CX2019SS41) and Research-based Learning and Innovative Experiment Program for College Students in Hunan Province in 2018, China.

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