Oxygen passivation of silicon nanocrystals: Influences on trap states, electron mobility, and hybrid solar cell performance

doi:10.1016/j.nanoen.2014.09.031

Nano Energy

Volume 10, November 2014, Pages 322-328

https://doi.org/10.1016/j.nanoen.2014.09.031 Get rights and content

Highlights

Dangling bonds were observed on Si nanocrystals surface after surface modification.
Controlled oxidation has been proposed to passivate Si nanocrystals.
Electron mobility has been enhanced dramatically after dangling bond passivation.
Si nanocrystal-based hybrid solar cell with high efficiency has been achieved.

Abstract

Surface quality of nanostructures has a significant influence on related device performance. In this study, a large number of dangling bonds are detected on the silicon nanocrystal (Si NC) surface after surface modification. These dangling bonds work as carrier traps and degrade particle electrical properties. Therefore, controlled particle oxidization is proposed as a substitution method for surface passivation, which can reduce carrier traps easily and effectively. Electron mobility is improved dramatically after a 12-hour controlled oxidation, and as a result, Si NC/PTB7 hybrid solar cells (HSC) achieves an efficiency of 3.6%; this is more than twice the efficiency of devices fabricated with fresh Si NCs (without passivation), and the highest efficiency for Si NC-based HSCs reported to date. However, excessive oxidation should be avoided because it introduces an oxygen rich layer on the particle surface, which blocks carrier transport and deteriorates electrical properties inversely.

Graphical abstract

Introduction

As the attractive candidates of acceptor materials, inorganic semiconductor nanocrystals (NCs) are being increasingly introduced into organic-based bulk heterojunction solar cells [1], [2]. Compared to the organic counterparts, normally fullerene derivatives, NCs have promising potential in more efficient light-harvesting [3], [4]; whilst keeping the superiorities which organic materials possess such as lightweight, flexible and solution-processable. In addition, NCs have several unique advantages. First, the shape can be tailored through synthesis methods; this has been proven useful for light trapping, exciton dissociation, as well as carrier transportation [5], [6]. Also, when the size is comparable or smaller than its Bohr radius, the quantum effect makes NC work totally different with its bulk material. Indirect bandgap increases with decreasing size; this fact changes light absorption properties and provides the possibility of choosing the spectral window of the complementary absorption profile [7], [8], [9]. Meanwhile, continuous energy bands start to separate into discrete states [10], [11]. This limits the energy loss through thermal relaxation, and facilitates multiple exciton generation (MEG) process [12], [13], an effective strategy expected to promote solar cell power conversion efficiency (PCE) [14], [15].

Recently, remarkable accomplishments have been achieved in NC/polymer hybrid solar cells (HSCs). Among them, CdS and PbS-based HSCs with PCE of 4.1% and 7% have been achieved, respectively [16], [17], [18]. However, toxicity prohibits their vast applications. Even so, from a scientific view, it is still a worthwhile endeavor to gain a deeper understanding of HSCs. Employing wide bandgap metal-oxide NCs such as TiO₂ and ZnO has achieved a PCE as high as 2% [19], [20], [21]. Nevertheless, further promotion would be a big challenge due to the wide bandgap, which assists a little in light-harvesting.

In addition to those mentioned above, silicon nanocrystal (Si NC) has been extensively studied and shown feasibility as a proper acceptor material in HSCs [22], [23], [24], [25]. Silicon is environmentally benign and has strong absorption especially in the UV region [26], [27]. Furthermore, instead of the complicated solution process commonly employed for NC synthesis (e.g., ZnO, PbS, and CdS.), mass production can be realized conveniently by means of a nonthermal plasma [28], [29], [30], [31], [32]. Devices with PCE at about 2% have already been achieved [33], [34]. It is well known that surface quality of nanostructures has a significant influence on related device performance. Several pioneer works on Si NC-based device have shown that the performance can be further improved through proper Si NCs surface treatment [35], [36], [37], [38]. In this work, we demonstrate that controlled particle oxidization can be employed as an easy and effective way for surface passivation. It plays a critical role in reducing particle dangling bonds, enhancing electron mobility, and lastly PCE of Si NC-based HSCs. Nevertheless, excessive oxidation should be avoided as electron mobility deteriorates reversely. Device PCE as high as 3.6% has been achieved after an optimized oxidization, which is the best value for Si NC-based HSCs until now, and indicates that it is competitive with other types of HSCs.

Section snippets

Si NC synthesis

Si NCs were synthesized from silicon tetrachloride (SiCl₄) by using a nonthermal plasma as previously reported elsewhere [25], [30]. A quartz tube with inner diameter of 45 mm was employed as the reactor. Very high frequency (VHF) at 70 MHz was supplied on two copper electrodes surrounding the quartz tube. Particle properties such as crystallinity and size can be well controlled through changing synthesis parameters. Here, Si NCs were synthesized with 8 standard cubic centimeters per minute

Si NCs surface passivation

In this work, fresh Si NCs with hydrogen termination were employed directly. The FTIR spectrum of fresh Si NCs is shown in Figure 1a. Peaks around 668 cm⁻¹ and 2360 cm⁻¹ are assigned to CO₂ physisorbed during measurement, which does not influence particle properties and will be excluded from the following discussions [42]. The weak peak around 825 cm⁻¹ is related to Si-OH absorption [43]. In addition, strong Si-H_x peaks around 630 cm⁻¹,910 cm⁻¹ and 2100 cm⁻¹ can be observed, which are attributed to

Conclusions

In summary, it is found that surface dangling bonds exist on silicon nanocrystal surface after surface modification. These dangling bonds need to be passivated as they work as trap states and influence particle electrical properties considerably. However, surface passivation methods traditionally adopted for bulk silicon are not suitable for nanocrystals. Accordingly, controlled oxidation has been proposed here to passivate silicon nanocrystals with oxygen. Electron mobility has been improved

Acknowledgments

This work was financially supported by the Funding Program for Next Generation World-Leading Researchers (GR040). The authors gratefully acknowledge Prof. Ryo Ishikawa, Prof. Shirai Hajime (Saitama University, Japan), and Prof. Yasuyoshi Kurokawa (Tokyo Institute of Technology, Japan) for sharing equipment. We thank Prof. Xiaodong Pi (Zhejiang University, China) and Dr. Ryan Gresback (Tokyo Institute of Technology, Japan) for excellent discussion and comments. TEM analysis was supported by the

Yi Ding received his Ph.D. degree from Saitama University, Japan in 2010. He has worked for 2 years at National Institute for Materials Science, Japan. Since 2012, he works as a project researcher in Department of Mechanical Science and Engineering, Tokyo Institute of Technology, Japan. He mainly works on low dimensional semiconductor material fabrication and its applications in light emission diodes, transistors, solar cells, etcetera. Current research focuses on polymer-inorganic hybrid solar

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Shu Zhou got his B.S. degree in Department of Material Science and Engineering from Chongqing University in 2010 and M.S. in State Key Lab of Silicon Materials from Zhejiang University. He is currently a Ph.D. candidate under the direction of Prof. Tomohiro Nozaki in Graduate School of Science and Engineering, Tokyo Institute of Technology, Japan. His research interest is focused on doped Silicon nanocrystals and their applications in nanodevices, such as solar cells, transistors, and so forth.

Tomohiro Nozaki received B.E. (1993) and M.E. (1995) degrees in Energy Engineering from Toyohashi University of Technology, Japan. He is currently a Professor of Mechanical Sciences and Engineering at Tokyo Institute of Technology. His research focuses are plasma chemistry and application to greenhouse gas conversion, plasma catalysis, nanomaterials synthesis, and silicon quantum dots and application to photovoltaics. He is currently the vice chair of Plasma Electronics Div., Japan Society of Applied Physics. He has been an editorial board member of Plasma Chemistry and Plasma Processes (Springer) since 2010.

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Rapid communicationOxygen passivation of silicon nanocrystals: Influences on trap states, electron mobility, and hybrid solar cell performance

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Si NC synthesis

Si NCs surface passivation

Conclusions

Acknowledgments

Sol. Energ. Mater. Sol. Cell.

Sol. Energ. Mater. Sol. Cell.

Nano Energ.

Thin Solid Films.

Science

J. Phys. Chem. C.

Nano Lett.

Adv. Mater.

Nano Lett.

Science

Chem. Mater.

Nat. Nanotechnol.

J. Am. Chem. Soc.

Chem. Rev.

Nano Lett.

Nano Lett.

Science

Science

Nano Lett.

Adv. Mater.

Nat. Nanotechnol.

Energ. Environ. Sci.

J. Am. Chem. Soc.

Nat. Mater. 8

Green

Appl. Phys. Lett.

Nano Lett.

Jpn. J. Appl. Phys.

Rapid communication
Oxygen passivation of silicon nanocrystals: Influences on trap states, electron mobility, and hybrid solar cell performance