Elsevier

Electrochimica Acta

Volume 278, 10 July 2018, Pages 204-209
Electrochimica Acta

Efficiency enhancement of bifacial dye-sensitized solar cells through bi-tandem carbon quantum dots tailored transparent counter electrodes

https://doi.org/10.1016/j.electacta.2018.05.057Get rights and content

Highlights

  • Bi-tandem carbon quantum dots tailored transparent counter electrode is fabricated.

  • The catalytic ability of counter electrode has been enhanced under light irradiation.

  • The device yields a rear efficiency of 6.55% under solar irradiation.

Abstract

Bifacial light-harvesting technology is regarded as one effective strategy to enhance the power output of photovoltaic devices, in which the transparent electrode is crucial to determine the power conversion efficiency when illuminated from rear side. To date, the efficiency enhancement of the state-of-the-art bifacial mesoscopic solar cells is mainly focused on the rational design of high catalytic counter electrode without sacrificing the transparency. The cell performance maximum through increasing electron density at counter electrode surface to enhance its catalytic ability is still a challenging problem. In the current work, bi-tandem carbon quantum dots are made to tailor transparent CoSe counter electrode for bifacial dye-sensitized solar cells, achieving a maximized front power conversion efficiency of 8.54% and a rear efficiency of 6.55% in comparison with 7.87% and 5.03% for carbon quantum dots-free device, respectively. The mechanism behind this increment is stemmed from the photo-excited electrons of bi-tandem carbon quantum dots and therefore realizing the electron accumulation on the counter electrode surface.

Introduction

Creation of high-efficiency solar cells is a persistent objective to solve energy crisis and environmental pollution problems [1,2]. Among several potential solar cells, bifacial dye-sensitized solar cells (DSSCs) that can harvest solar energy from both sides have attracted tremendous scientific interests owing to the increased light-harvesting ability and power output [3,4]. When applied in photovoltaic station, the front sides of back-row solar panels reflect partial sunlight to back sides of front-row panels. The utilization of reflected light can significantly increase the overall performances of solar cells. However, the traditional DSSCs with precious platinum counter electrode (Pt CE) can only convert solar energy into electricity from front side because of the low optical transparency of precious Pt electrode. Although semi-transparent or transparent CEs have been used for bifacial DSSCs, the low electron concentration at electrode surface limits further enhancement of rear power conversion efficiency. The function of a CE is to collect electrons from external circuit and to catalyze the reduction reaction of I3 + 2e → 3I, therefore the electron density has significant impact to this charge conversion process. Till now, most of the bifacial DSSCs yield relatively lower rear irradiation efficiencies by only improving optical transmittance of CEs with sacrificing the catalytic activity [5,6]. Carbon quantum dots (CQDs) are new graphene materials with lateral size lower than 20 nm, broad absorption ranges [7], tunable bandgaps and high molar extinction coefficient [8]. These excellent performances make CQDs ideal light harvesters or interfacial modification materials for photoanodes of photovoltaics [9,10]. Previous work has demonstrated that the tandem structure from inorganic quantum dots is promising in extracting charges and increasing power conversion efficiency by building band alignment from multilayered quantum dots [[11], [12], [13], [14], [15]]. Inspired by this concept, here we make bi-tandem CQDs tailored transparent CoSe CEs (noted as CQDs12-CoSe) for bifacial DSSC applications. Upon illumination from rear side, the tandem CQDs absorb photons and excite electrons, enriching electron density at CE surface and therefore enhancing catalytic performances of a CE. The microstructures, photoelectric behaviors of CQDs and electrochemical performances of CQDs12-CoSe CEs are carefully characterized to understand the promotion to photovoltaic properties.

Section snippets

Preparation of m-TiO2/N719 photoanodes

TiO2 colloid was synthesized according to the detailed procedures reported previously [16]. Colloidal TiO2 films were fabricated by coating the TiO2 colloid onto freshly cleaned FTO glass substrate (12 Ω square−1) with a size of 2.5 × 2.5 cm2 by a doctor-blade method. Subsequently, the FTO glass supported colloidal TiO2 films were calcined in a muffle furnace at 450 °C for 30 min in air, obtaining mesoporous TiO2 film (m-TiO2). The resultant m-TiO2 electrodes with film thickness of around 10 μm

Results and discussion

Fig. 1a shows that both CQDs1 and CQDs2 aqueous solutions are yellow under daylight. When irradiated by a 365 nm UV lamp, the reagent solutions are both brighter green luminescence (Fig. 1b), demonstrating the obvious photoluminescence (PL) effect. The transmission electron microscopy (TEM) images of CQDs1 and CQDs2 in Fig. 1c reveal that the CQDs with an average diameters of 2–3 nm are homogeneously dispersed in solution. As shown in Fig. 1d, two characteristic UV–vis absorption peaks centered

Conclusions

In summary, the combination of tandem CQDs structure with transparent CoSe CE is an effective strategy for markedly enhancing catalytic activity of a CE and therefore enhancing power conversion efficiency of a bifacial DSSC. Arising from the photo-excited electrons of CQD tandem, the rear efficiency of bifacial DSSC with CQDs12-CoSe CE is enhanced to 6.55% in comparison with 6.03% for CoSe based device. The concept reported here may provide new pathways for bifacial DSSC platforms.

Acknowledgements

The authors gratefully acknowledge the financial supports from National Natural Science Foundation of China (61774139, 21503202 and 61604143) and Director Foundation from Qingdao National Laboratory for Marine Science and Technology (QNLM201702).

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