Cerium-doped indium oxide transparent electrode for semi-transparent perovskite and perovskite/silicon tandem solar cells

doi:10.1016/j.solener.2019.12.040

Solar Energy

Volume 196, 15 January 2020, Pages 409-418

https://doi.org/10.1016/j.solener.2019.12.040 Get rights and content

Highlights

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ICO thin films have been prepared by magnetron sputtering at room temperature.
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High mobility and low absorption is obtained at room temperature.
•
Complex refractive index advantages in the effective wavelength range.
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Electrical and optical improvement were well reflected in the cells performance.
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An 8.96% and 8.06% relative efficiency increment was achieved in ST-PSCs and perovskite/SHJ solar cells respectively.

Abstract

Perovskite and perovskite/silicon tandem solar cells hold great promise for commercialization for their high efficiency and low cost. One of the major challenges for semi-transparent perovskite and tandem devices is the availability of suitable transparent electrodes. Here, we report an alternative high-performance cerium-doped indium oxide (ICO) transparent electrode with high mobility, low carrier concentration and high transmittance by Radio frequency (RF) magnetron sputtering at room temperature (RT) for thus applications. A high mobility of 51.6 cm²/Vs, a low resistivity of 5.74 × 10⁻⁴ Ω cm as well as a high average transmittance of 83.5% ranging from 400 nm to 1800 nm were obtain at RT. The spectral response of a planar semi-transparent perovskite solar cell (ST-PSC) proportion accelerated in 450–700 nm range and resulted in an absolute 1.1 mA/cm² (from 17.38 to 18.48 mA/cm²) and 1.47 mA/cm² (from 13.63 to 15.10 mA/cm²) short-circuit current improvement by replacing commercial ITO electrodes with ICO at different illuminate sides. Finally, the preliminary perovskite/silicon-heterojunction (SHJ) two-terminal tandem solar cell achieves a relative 8.06% improvement in power conversion efficiency (PCE) (from 18.85% to 20.37%) in the use of ICO transparent electrode, illustrating a promising alternative transparent electrode to further improve high-efficiency semi-transparent perovskite and its tandem solar cells.

Introduction

Extensive research interest has focused on perovskite solar cells (PSCs) and its tandem devices due to its high efficiency, wide bandgap, and low fabrication cost (Green, 2016). Semi-transparent perovskite solar cells (ST-PSCs) are currently the focus of research studies due to their high power conversion efficiency (PCE) combined with crystalline silicon (c-Si) (Dewi et al., 2019, Hossain et al., 2019, Kim et al., 2019, Duong et al., 2017) or chalcogenide solar cells (Han et al., 2018, Bailie et al., 2015). Within the last two years, there have been several encouraging demonstrations of perovskite/silicon tandem devices with a PCE superior to that of their (high-performance) c-Si bottom-cells. Furthermore, the latest record PCE of a perovskite/silicon tandem has reached to a certified 28% (Oxford, 2019), thereby surpassing the best-in-class reported for single-junction c-Si (26.7%) (NREL, 2018). Parasitic absorption in transparent electrodes is one of the main challenges to enableing PCEs for perovskite-based tandem solar cells beyond 30% (Jacobs et al., 2019). Additionally, in devices with soft materials such as perovskite, the high temperatures during deposition and post-annealing may accelerate methylammonium iodide evolution and result in irreversible damage of the perovskite active layer and the organic carrier extraction layers (Loper et al., 2015). Thus, one of the major challenges for ST-PSCs is the availability of suitable transparent electrode material in a low temperature procedure. Importantly, the transparent electrode is not only the first necessary development for a semi-transparent perovskite device, but also the missing building block for the integration in a monolithic device (Loper et al., 2015).

Ideally, transparent electrodes feature simultaneously a high lateral conductivity and broadband transparency to minimize resistive losses and to increase the performance throughout the solar spectrum on device level, respectively. Significant efforts have been made (Loper et al., 2015, You et al., 2015), and among them sputtered transparent conductive oxides (TCOs) can be regarded as the ideal candidates because of their high transmittance in the visible and near-infrared (NIR) region, low sheet resistance, good chemical stability and process reproducibility (Ellmer, 2012). To fulfill the seemingly contradictive transparency and conductivity requirement, strategies to increase the carrier mobility are required. It is critical that the transparent electrodes should combine a high conductivity with a minimal free-carrier absorption. Losses associated with the parasitic absorption caused by free-carrier can be reduced by decreasing the carrier density of the transparent electrode (Koida et al., 2007). On the other hand, increase in the carrier mobility is required to maintain a high conductivity. Motivated by these criteria, some TCOs based on doped In₂O₃ have already been explored for tandem solar cells, such as ITO, Zr-doped In₂O₃ (IZRO) and ITO/H-doped In₂O₃ (In₂O₃:H) bilayer electrodes. In this, ITO, whereas being well established, is of limited appeal because of its low mobility and high parasitic absorption especially in the NIR range (Morales-Masis et al., 2017), or it needs a post-annealing process (at 100 °C for 10–15 min) to enable its mobility >40 cm²/Vs (Ramos et al., 2018, Bush et al., 2017). IZRO has been explored as another promising TCO with a high mobility and NIR transparency, resulting in improved short circuit current density, compared to ITO in silicon heterojunction solar cells (Morales-Masis et al., 2018) and perovskite/silicon tandem device (Aydin et al., 2019). Unfortunately, this IZRO still has limitations because of its low mobility as-deposited and need a post-annealing process (at 200 °C for 25 min) (Aydin et al., 2019). Conversely, crystalline In₂O₃:H has a very high mobility and optical transparency (Koida et al., 2007). However, this TCO may degrade the device performance due to water vapor effusion (Koida et al., 2009).

Recently, Ce-doped In₂O₃ (ICO) has been explored as a promising electrode with micro-strain in the vicinity to the dopant sites and density of oxygen vacancies could be reduced by CeO₂ doping. With these benefits, excellent optoelectronic properties such as high transparency, wide bandgap (>3.5 eV), and high conductivity, combined with high NIR transparency were obtained to improve crystallinity at 150 °C by DC arc-discharge ion-plating and post-annealing at 200 °C (Kobayashi et al., 2014, Eiji et al., 2016). Therefore, a procedure at room temperature (RT) makes it competitive for the application in perovskite and its tandem solar cells.

In this study, we thoroughly studied the structural and optoelectronic properties of radio frequency (RF) sputtered ICO films, especially at room deposition temperature. Improvements of light transmittance in 300–1800 nm range and carrier mobility are detected in these films deposited at RT compared to a conventional ITO transparent electrode. Each promotion is tested in external quantum efficiency (EQE) and J-V detections. An initial PCE was then measured of more than 14% and 20% respectively to confirm its applicability for high-efficiency ST-PSCs and perovskite/SHJ two-terminal tandem solar cells. Thus, this high-quality ICO transparent electrode with improved full spectrum performance and carrier mobility at RT make it a promising electrode for perovskite single junction and perovskite-based tandem solar cells.

Section snippets

Experimental methods

ICO and ITO films about 100 nm (±5 nm) thick were deposited on Eagle XG glass substrates by RF magnetron sputtering in a KJLC Lab-18 sputtering system. The source materials for TCO deposition in this study were a ceramic In₂O₃:CeO₂ target (3 wt%) and ceramic In₂O₃:SnO₂ target (10 wt%), respectively. We maintained the sputtering chamber at a base pressure less than 1 × 10⁻⁵ Pa before each deposition. The distance between the substrate and target was maintained at 190 mm. We applied both low

Results and discussion

We obtained XRD patterns of ICO and ITO films prepared at different substrate temperatures. As shown in Fig. 1(a), no peaks were observed for ICO films that were deposited below 100 °C. As the substrate temperature increased, the amorphous phase changed to polycrystalline structure, with the most prominent peak of (2 2 2) and weak planes of (2 1 1) and (3 3 2). The prominent (2 2 2) peak maximized intensity at 150 °C, whereas the (2 1 1) peak was inclined to a higher temperature. Additionally,

Conclusion

In summary, using magnetron sputtering at RT, we achieved a high-performance ICO transparent electrode to effectively enhance the electricity and full-spectrum performance of the transparent electrode for high-efficiency ST-PSCs and perovskite/SHJ tandem solar cells. A high mobility of 51.6 cm²/Vs as well as a reduced resistivity of 5.74 × 10⁻⁴ Ω cm were obtain at RT. The optical transmittance in the entire sensitive wavelength region was improved with an average transmittance boost of 5%

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.

Acknowledgment

This work was supported by the National Key Research Program of China (2018YFB1500104).

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However, there are still relatively few studies on ICO thin films. An et al. obtained ICO films with mobility of 51.6 cm2 V−1s−1 by using RF magnetron sputtering at room temperature [11]. Substrate heating or post-annealing treatment can enhance the mobility of films.
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Cerium-doped indium oxide transparent electrode for semi-transparent perovskite and perovskite/silicon tandem solar cells

Highlights

Abstract

Introduction

Section snippets

Experimental methods

Results and discussion

Conclusion

Declaration of Competing Interest

Acknowledgment

Thin Solid Films

Nat. Energy

Sol. Energy Mater. Sol. Cells

Surf. Coat. Tech.

Thin Solid Films

Sci. Bull.

Sens. Actuator B-Chem.

Nano Energy

Zr-doped indium oxide (IZRO) transparent electrodes for perovskite-based tandem solar cells

Adv. Funct. Mater.

Semi-transparent perovskite solar cells for tandems with silicon and CIGS

Energ. Environ. Sci.

Ce 3d XPS investigation of cerium oxides and mixed cerium oxide (CexTiyOz)

Surf. Interface Anal.

The influence of deposition parameters on room temperature ozone sensing properties of InOx films

Sens. Actuator B-Chem.

23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability

Nat. Energy

Highly efficient semitransparent perovskite solar cells for four terminal perovskite-silicon tandems

ACS Appl. Mater. Interfaces

Highly transparent and conductive indium tin oxide thin films for solar cells grown by reactive thermal evaporation at low temperature

Appl. Phys. A

Rubidium Multication perovskite with optimized bandgap for perovskite-silicon tandem with over 26% efficiency

Adv. Energy Mater.

Cerium oxide and hydrogen co-doped indium oxide films for high-efficiency silicon heterojunction solar cells

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The influence of deposition parameters on room temperature ozone sensing properties of InO_x films