Elsevier

Solar Energy

Volume 202, 15 May 2020, Pages 80-85
Solar Energy

Improving light harvesting and charge extraction of polymer solar cells upon buffer layer doping

https://doi.org/10.1016/j.solener.2020.03.105Get rights and content

Highlights

Abstract

It is an important strategy to improve the performance of polymer solar cells (PSCs) by incorporating metal nanoparticles into functional layers. Herein, gold (Au)@titanium dioxide (TiO2) plasmonic core-shell nanoparticles (PCSNPs) are synthesized and doped into zinc oxide (ZnO) as hybrid electron transport layer in PSCs. Improved light trapping and electrical conductivity are achieved through localized surface plasma resonance of Au@TiO2 PCSNPs. As a result, the short-circuit current density is apparently enhanced, while remaining the unchanged open-circuit voltage. The maximum PCE of 8.801% is obtained when 1.5 wt% Au@TiO2 PCSNPs are introduced into the ZnO layer. This study provides a new inspiration for the development of high efficiency PSCs.

Introduction

Environmental problems caused by limited fossil fuels have motivated a rapid increase of research on solar energy. Polymer solar cells (PSCs) have received extensive attention due to their low cost, lightweight, good flexibility and so on (Nielsen et al., 2010, Edwards et al., 2012, Krebs et al., 2010, Brabec et al., 2010, Chen and Goodman, 2008, Li et al., 2015). However, the use of PSCs is still limited due to their low power energy conversion efficiency (PCE) from the perspective of commercial development. The PCE of PSCs is affected by many factors, such as internal quantum efficiency, deficient light absorption, the thickness of the active layer film and electron transport (Jo et al., 2009, Chen et al., 2013a, Schilinsky et al., 2002, Su et al., 2012, Liu et al., 2014). Among them, low absorption light is one of the important factors affecting the performance of PSCs. So a series of light capture technologies come into our sight and develop rapidly (Wu et al., 2011, Liu et al., 2016, Li et al., 2019, Chen et al., 2013b, Guo et al., 2014).

Introducing nanoparticles (NPs) into the transport layer or active layer to take advantage of the local plasma effect both can improve light absorption. These metal NPs can generate amplified local fields, while they also act as recombination centers for excitons, therefore they are usually located in the functional layer near the active layer to avoid direct contact with excitons. Recently, Au NPs are more widely used in solar cells due to their resonance peaks near the absorption center of polymer semiconductor and higher work function than silver (Jain et al., 2006, Park et al., 2014). However, pristine Au NPs are often unstable under reaction condition and easy to migrate or aggregate into large particles, resulting in the loss of special properties of original Au NPs (Corma and Garcia, 2008). The Au NPs is suitable to make a core–shell metal@semiconductor composite because of its adjustable longitudinal localization of the Surface Plasmon Resonance (SPR) band (Chen et al., 2013b). Therefore, unique anisotropy Au NPs can be assembled into many types of structures. As previous reports indicate (Kumara et al., 2001b, Tanaka, 2005, Kumara et al., 2001a, Chappel et al., 2002, Gu and Fan, 2017), it is expected to create an insulating barrier between NPs, preventing effective electron transport through the membrane. The metal oxide shell covering metal NPs is used to prevent recombination and back reaction, shielding the core metal NPs from the external environment (Qi et al., 2011, Cao and Banin, 1999). Some main properties of metal oxides are summarized in Table 1 (Palomares et al., 2003, Zhang et al., 2010). It is clear that the conduction band edge for other metal oxides is significantly negative on the zinc oxide (ZnO) conduction band edge. Consequently, these metal oxides should have the influence on preventing electron injection and charge recombination occurrence. Nevertheless, there are some significant differences in the zero charge point (PZC) of these metal oxides. Moreover, we found the PZC of these metal oxides has a strong correlation with the ability of overcoats fabricated to improve the performance of organic solar cells (Palomares et al., 2003). Titanium dioxide (TiO2) is an interesting composite material because of its synergistic role on promoting oxidation (Schwartz et al., 2004, Chen and Goodman, 2008) and photochemical reactions (Kamat, 2008). TiO2, rather than insulation materials, is used as the enclosure in this paper, which is also because the charge carrier can be easily transferred to the surrounding contact area with TiO2.

In this work, Au@TiO2 core–shell nanoparticles (PCSNPs) are synthetized and incorporated into ZnO electron transport layer (ETL) of PSCs. This method can greatly increase light absorption of the active layer and promote the electron transport in PSCs. As a result, we got an improved PCE of 8.801% with an increased short-circuit current density (Jsc) of 17.569 mA/cm2.

Section snippets

Synthesis of Au NPs

The Au NPs were made by a seed-growth method reported by Nikoobakht and ElSayed (Nikoobakht and El-Sayed, 2003). First, Au seed solution was prepared by rapidly adding 0.6 mL of 0.1 M aqueous sodium borohydride (NaBH4) into an aqueous solution containing 10 mL of 0.1 M cetyltrimethylammonium bromide (CTAB) and 0.1 mL of 25 mM auric chloride acid (HAuCl4). Then, the solution was stirred for 2 min. Au NPs were synthesized by mixing 0.8 mL 25 mM HAuCl4 with 20 mL aqueous solution containing 0.1 M

Results and discussion

The device configuration of PSCs and the core–shell Au@TiO2 structure are shown in Fig. 1a and b. The morphology and structure of Au@TiO2 PCSNPs studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are displayed in Fig. 2. It can be seen that the shapes of all products are roughly spherical and the sizes are relatively uniform. The core–shell structure can clearly be seen. It was found that the size of the synthesized Au@TiO2 PCSNPs were basically between 28

Conclusion

In summary, we investigated the effect of Au@TiO2 PCSNPs doping in the ZnO ETL on the performance of inverted PSCs from the aspects of optical absorption and electron transport. We conclude the improvement mainly be ascribed to the plasmon resonance of Au@TiO2 PCSNPs insertion. It results in the increased absorption and facilitates charge separation and transport. At the optimal doping concentration of 1.5 wt% Au@TiO2 PCSNPs, the maximum PCE of 8.801% was achieved. We believe this method has

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

The authors are grateful to the National Key Research and Development Program of China (No. 2019YFA0705900) funded by MOST, the National Natural Science Foundation of China (61875072), the Special Project of the Province-University Co-constructing Program of Jilin Province (SXGJXX2017-3), the National Postdoctoral Program for Innovative Talents (BX20190135), Scientific Research Planning Project of Education Department of Jilin Province (JJKH20200980KJ), Industrial Technology Research and

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