Article pubs.acs.org/JPCC
Enhancing the Efficiency of Multicrystalline Silicon Solar Cells by the Inkjet Printing of Silicon-Quantum-Dot Ink Xiaodong Pi,* Li Zhang, and Deren Yang* State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China ABSTRACT: Among all types of solar cells, multicrystalline silicon (Si) solar cells are the most widely produced. The enhancement of the efficiency of multicrystalline Si solar cells may help broaden the deployment of solar cells worldwide. Here we show that the efficiency of state-of-the-art commercially produced multicrystalline Si solar cells can be enhanced by a simple inkjet printing of Si-quantum-dot (Si-QD) ink at the solar cell surface. It is found that the efficiency enhancement results from both the down-shifting of Si QDs and the antireflection of porous Si-QD films at the solar cell surface. The current results demonstrate that Si-based nanotechnology can facilitate the continuous development of traditional Si solar cells.
solvents,9,10,13 enabling the inkjet printing of Si-QD ink. Given the fact that the short-wavelength response of multicrystalline Si solar cells is usually poorer than that of monocrystalline Si solar cells, it is expected that the beneficial effect of downshifting of Si QDs should be more pronounced for multicrystalline Si solar cells than monocrystalline Si solar cells.14 We will show that the down-shifting of Si QDs indeed clearly enhances the efficiency of state-of-the-art commercially produced multicrystalline Si solar cells together with the antireflection of Si-QD films.
1. INTRODUCTION The excellent balance between cost and efficiency of multicrystalline silicon (Si) solar cells leads to the fact that multicrystalline Si solar cells are currently the most widely produced solar cells in the world.1 The further increase of efficiency without incurring significant cost increase for multicrystalline Si solar cells should greatly boost the development of photovoltaics. After many years of development, however, it is now rather challenging to improve the balance between cost and efficiency for multicrystalline Si solar cells. To meet this challenge, researchers may turn to nanotechnology. As one type of the most important nanomaterials, quantum dots (QDs) hold great promise for the application of nanotechnology in photovoltaics.2−6 van Sark et al. have theoretically predicted that the efficiency of multicrystalline Si solar cells may be significantly enhanced by taking advantage of the down-shifting effect of CdSe QDs.6 On one hand, no experimental work has been carried out to test van Sark et al.’s prediction. On the other hand, CdSe QDs may not be the material of choice given the toxicity of Cd and the low abundance of Se.7,8 In contrast, Si QDs are now well positioned to enhance the performance of multicrystalline Si solar cells thanks to the recent development of Si-QD ink.9−11 We previously spincoated Si-QD ink at the surface of monocrystalline Si solar cells.12 The antireflection of the resulting porous Si-QD films led to the efficiency enhancement of monocrystalline Si solar cells. It was concluded that the down-shifting efficiency of Si QDs was not high enough to contribute to the enhancement of solar cell efficiency. In this work, the formulation of Si-QD ink has been improved by using hydrosilylated Si QDs. The hydrosilylation of Si QDs causes the down-shifting efficiency to be large under short-wavelength light excitation. In addition, hydrosilylated Si QDs can be excellently dispersed in organic © 2012 American Chemical Society
2. EXPERIMENTAL SECTION Freestanding Si QDs were synthesized by using SiH4-based plasma.12,15,16 The hydrosilylation of Si QDs was then performed in a mixture of 1-octadocene and mestilylene in the atmosphere of argon at 165 °C. Hydrosilylated Si QDs were dispersed in mestilylene to form Si-QD ink with a concentration of ∼1 mg/mL. The photoluminescence (PL) from Si QDs was measured in an integrating sphere, which was linked to a CCD spectrometer (Maya 2000PRO, Ocean Optics).9,17 A 370 nm light emitting diode (LED) was mounted inside the integrating sphere to excite Si QDs. Please note that Si QDs may very well absorb light with wavelengths around 370 nm,12,13 which is representative for short-wavelength sunlight. The light at 370 nm was first collected when only the solvent of mestilylene was inside the integrating sphere. As Si-QD ink was inside the integrating sphere the light at 370 nm was collected again. By comparing the above two results the absorption of Si QDs at 370 nm was determined. The PL quantum yield (QY) Received: July 17, 2012 Revised: September 11, 2012 Published: September 17, 2012 21240
dx.doi.org/10.1021/jp307078g | J. Phys. Chem. C 2012, 116, 21240−21243
The Journal of Physical Chemistry C
Article
of Si QDs was obtained by calculating the ratio of integrated PL intensity for Si QDs to the absorption of Si QDs. An inkjet printer (Dimatix DMP 2831) was employed to print the Si-QD ink at the surface of a batch of as-produced multicrystalline Si solar cells (size 156 × 156 mm2; efficiency ∼17.2%), which were fabricated by Hareonsolar (600401.SH) with texturized surface. The printing through one of the channels in the printhead gave rise to an about 80 μm wide line. Drop spaces in the range from 20 to 80 μm were selected to obtain Si-QD films with varying thickness and porosity after the evaporation of the solvent of Si-QD ink. As schematically shown in Figure 1, the drop space (D) was tuned by adjusting
Figure 2. Result for the subtraction of the luminescence spectrum obtained with only the solvent of mestilylene inside the integrating sphere from that obtained with Si-QD ink inside the integrating sphere. The negative peak corresponds to the absorption of Si QDs at 370 nm. The positive peak at ∼773 nm results from the light emission from Si QDs. The PL QY of Si QDs is ∼53% under the excitation of 370 nm light.
corresponds to the absorption of Si QDs at 370 nm.9 The positive peak at ∼773 nm results from the light emission from Si QDs. We calculate the PL QY for Si QDs to be ∼53%. In the context of the utilization of sunlight,14 the PL measurement indicates that Si QDs can efficiently down-shift shortwavelength (
