Elsevier

Organic Electronics

Volume 87, December 2020, 105958
Organic Electronics

Origin of temperature-dependent performance of hole-transport-layer-free perovskite solar cells doped with CuSCN

https://doi.org/10.1016/j.orgel.2020.105958Get rights and content

Highlights

  • The origin of low annealing temperature in CuSCN:MAPI PSCs was investigated.

  • The highest PCE was achieved with 60 °C annealing.

  • Electron conduction was significantly enhanced with 60 °C annealing.

  • CuSCN dopants aggregated in the CuSCN:MAPI layer annealed at 100 °C.

  • The large electron transport barrier was formed with 100 °C annealing.

Abstract

Recently, a hole-transport-layer (HTL)-free structure was proposed to decrease the cost of organic lead halide perovskite solar cell (PSC) fabrication. In HTL-free PSCs, instead of using an HTL insertion, the HTL material can be added directly into a perovskite precursor solution to improve hole transport. For example, copper thiocyanate (CuSCN) is used for p-type doping of methylammonium lead triiodide (MAPI) via spin coating from a mixed solution. However, the optimum annealing temperature for CuSCN-doped MAPI (CuSCN:MAPI) PSCs is lower than the 100 °C that is typical for undoped MAPI PSCs. In this study, the origin of such lower annealing temperatures of CuSCN:MAPI PSCs is investigated. The highest power conversion efficiency (PCE) and enhanced electron transport in CuSCN:MAPI are obtained with annealing at 60 °C. Using transmission electron microscopy-energy-dispersive X-ray spectroscopy, it is revealed that annealing at 60 °C results in the uniform distribution of CuSCN, while the annealing at 100 °C induces the aggregation of CuSCN with a diameter of ~20 nm. A large energy barrier formed by the shallow-lying conduction band minimum of these CuSCN clusters hinders electron transport. The lower PCE of CuSCN:MAPI PSCs with annealing at 100 °C is attributed to this deterioration in the electron transport.

Introduction

Organic lead halide perovskite solar cells (PSCs) have developed rapidly in the recent decade. The highest power conversion efficiency (PCE) of PSCs is now reported as higher than 25% [1]. The low fabrication cost via the solution process is also an important advantage of PSCs compared to traditional solar cells (e.g., crystalline Si) [[2], [3], [4], [5]]. However, to replace thermal and nuclear power generation viably, the cost should be reduced further. One of the approaches to decrease the cost of solar cell fabrication is simplifying the manufacturing process [6]. In the PSC structure, a transparent conducting oxide, metal electrode, and perovskite light-absorbing layer are the essential components that cannot be omitted. However, a use of a charge transport layer is not mandatory if efficient charge transport can be obtained from perovskite to electrodes.

In this regard, PSCs without a hole transport layer (HTL) have been studied [7,8]. In these HTL-free PSCs, HTL material can be incorporated into the perovskite precursor solution as a dopant that enhances the hole transport ability [[9], [10], [11], [12]]. These dopants are distributed within the perovskite layer by spin coating. However, fine control of the dopant distribution in the perovskite layer is difficult owing to the solution process. Although these dopants improve charge transport ability, they deteriorate the charge transport if they are improperly dispersed. As the dopant implantation technique has developed much in the case of Si-based semiconductor technology, methods to regulate the dopants in perovskite should also be developed to improve the PCE of the PSCs. For this, the dopant distribution, which is affected by film fabrication conditions, should be understood well.

Copper thiocyanate (CuSCN) is an efficient HTL material that yields a high PCE in PSCs [[13], [14], [15], [16], [17], [18]]. Interestingly, the CuSCN-doped PSC shows a PCE similar to the PSC with a CuSCN HTL [19,20]. Meanwhile, a high-quality methylammonium lead triiodide (CH3NH3PbI3, MAPI) film can be fabricated by annealing the intermediate phase of MAPI-dimethyl sulfoxide (DMSO) [21,22]. The most frequently used temperature to anneal the deposited film is 100 °C [[21], [22], [23], [24], [25]]. However, for PSCs with CuSCN-doped MAPI (CuSCN:MAPI), much lower annealing temperatures of 50–70 °C are optimal [19,20]. This implies that the annealing temperature plays a crucial role in the doping process. However, the origin of such low annealing temperatures is not clearly understood.

In this paper, we identify the reason why the CuSCN:MAPI PSC shows optimum annealing temperature at 60 °C, which is considerably lower than the conventional annealing at 100 °C for the undoped MAPI PSC. Using transmission electron microscopy-energy-dispersive X-ray spectroscopy (TEM-EDX), we found that the CuSCN dispersion in a CuSCN:MAPI layer varies considerably with the annealing temperature. The charge transport mechanism in CuSCN:MAPI is presented based on an energy-level diagram derived from ultraviolet photoelectron spectroscopy (UPS) results.

Section snippets

Sample fabrications

An indium tin oxide (ITO) glass substrate was cleaned by ultrasonication in deionized water, detergent, acetone, and ethanol. After that, ITO was dried by blowing N2, followed by UV-ozone treatment for 15 min at 100 °C. A quantity of 159 mg of methylammonium iodide (Dyesol), 461 mg of PbI2 (Alfa Aesar, purity 99.9985%), and 71 μL of DMSO (purity 99.5%, Sigma-Aldrich) were dissolved in 0.9 mL of N,N-dimethylformamide (purity 99.8%, Sigma-Aldrich) to prepare a MAPI solution. To dope MAPI, 4.8 mg

Results and discussion

To determine the optimum annealing temperature of CuSCN:MAPI PSCs, we fabricated PSCs having the structure Ag/BCP/C60/CuSCN:MAPI/ITO (from top to bottom, the inset in Fig. 1). Before the main experiments, we verified that the PCEs of the CuSCN:MAPI PSCs are higher than those of the MAPI PSCs (Fig. S2 and Table S2 in Supplementary Material). Also, the PCE of the CuSCN:MAPI PSC is slightly higher than that of the MAPI PSC with the CuSCN HTL (Fig. S3 in Supplementary Material). Fig. 1 shows the JV

Conclusion

In summary, we studied the origin of the changes in the performance of CuSCN:MAPI PSCs depending on the annealing temperature. The highest PCE of CuSCN:MAPI PSCs was achieved with 60 °C annealing, which is a temperature lower than the typical 100 °C for undoped MAPI PSCs. In the JV curves of the HOD and EOD, a significant improvement in electron conduction was obtained with 60 °C annealing, while hole conduction was not greatly changed. The TEM-EDX images showed that CuSCN dopants are

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 study was supported by the National Research Foundation of Korea [NRF-2020R1A2C2014644, 2017R1A5A1014862 (SRC program: vdWMRC center), 2018R1D1A1B07051050, and 2018R1A6A1A03025582]; Samsung Display Company; and Industry-Academy joint research program between Samsung Electronics and Yonsei University.

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