Optical simulations of P3HT/Si nanowire array hybrid solar cells
© Wang et al.; licensee Springer. 2014
Received: 23 January 2014
Accepted: 24 March 2014
Published: 14 May 2014
An optical simulation of poly(3-hexylthiophene) (P3HT)/Si nanowire array (NWA) hybrid solar cells was investigated to evaluate the optical design requirements of the system by using finite-difference time-domain (FDTD) method. Steady improvement of light absorption was obtained with increased P3HT coating shell thickness from 0 to 80 nm on Si NWA. Further increasing the thickness caused dramatic decrease of the light absorption. Combined with the analysis of ultimate photocurrents, an optimum geometric structure with a coating P3HT thickness of 80 nm was proposed. At this structure, the hybrid solar cells show the most efficient light absorption. The optimization of the geometric structure and further understanding of the optical characteristics may contribute to the development for the practical experiment of the promising hybrid solar cells.
KeywordsP3HT Si nanowire array Hybrid solar cells Finite-difference time-domain (FDTD) method
Solar cells based on polymer materials provide a promising route toward cost-effective, large-area, and flexible organic photovoltaic (OPV) solar cells [1–3]. Among all the photoactive polymer materials, poly(3-hexylthiophene) (P3HT) is one of the most widely used photoactive materials in fabricating organic solar cells. Considerable efforts have been focused on enhancing the energy conversion efficiency, where the best devices consist of a mixture of polythiophene derivatives (electron donor) and C60 fullerenes (electron acceptor), called a bulk heterojunction (BHJ) type, where the maximum power conversion efficiency (PCE) has reached approximately 7.5% to date . Another promising way for facilitating carrier collection is to fabricate nanostructure-based hybrid solar cells that use ordered semiconductor nanowire array (NWA) surrounded by photoactive organics. Benefitted from the ease of fabrication and cost-effectiveness, Si NWA is utilized to form P3HT/Si NWA hybrid solar cells. Over standard hybrid solar cells, it is expected that the Si NWA-based solar cells have the following advantages: On the electrical side, due to high carrier mobility and small dimensions, the Si NWA offers straight pathways for the carriers to escape the device as quickly as possible . On the optical side, the light absorption is extend to infrared below the bandgap of silicon, thereby more photons in the solar radiation can be harvested. Meanwhile, due to their sub-wavelength dimensions, the strong light trapping effects arising from light scattering, light guiding, and inherent antireflection properties make NWA constructed hybrid solar cells absorb more photons with less material consumption as compared with conventional planar structure [6–10].
Because of these advantages, researches focusing on hybrid solar cells of P3HT/Si NWA have been done by many groups [11, 12]. In the past few years, the reported devices' performances have been improved, but the published PCE of P3HT/Si NWA solar cells are still low. From the published reports of other inorganic semiconductor solar cells based on NWA, the property, especially optical absorptivity, of the photovoltaic device depends critically on the geometry of the sub-wavelength NWA structure [13–15]. The absence of properly optimized structure may be the main reason for the low PCE of the proposed hybrid solar cells. Thus, before practical fabrication of P3HT/Si NWA hybrid solar cells, the geometry of P3HT/Si NWA must be optimized. In view of this, in this paper, we do an optical simulation about P3HT/Si NWA hybrid solar cells to explore the optical characteristics of the system, so as to give an optical guidance for the practical fabrication of P3HT/Si NWA hybrid solar cells.
In this paper, an optical simulation about P3HT/Si NWA hybrid solar cells was investigated to explore the optical characteristics of the system. First, the influence of the thickness of P3HT on the optical absorption of solar cells has been thoroughly analyzed by using finite-difference time-domain (FDTD) method . Second, to further understand the optical absorption of the system, the optical generation rates in the x-z cross section of hybrid P3HT/Si NWA under optimized coated and uncoated Si NWA were obtained. Finally, to find an optimized geometry, the ultimate photocurrents were calculated to maximize the light absorption capability of the hybrid solar cells in the solar spectrum.
Results and discussion
From the above discussion, it is clear that the light absorption of the hybrid structure is quite sensitive to structural parameters. By proper choice of organic coating thickness, we find that the absorption of NWA is significantly enhanced. To further determine the optimized geometric configuration, the ultimate photocurrents were calculated for various thicknesses. We denoted the ultimate photocurrent by assuming perfect carrier extraction : Jph = (e / hc) ∫ λA(λ)I(λ)dλ, where e is the elementary charge, h is Plank's constant, c is the light speed, I(λ) is the AM1.5G spectrum, and A(λ) is the absorption of the solar cells.
From the discussion above, in the fabrication of P3HT/Si NWA hybrid solar cells, a conformal organic coating with thickness of 80 nm is proposed to be spun onto the surface of Si NWA to obtain the maximum optical absorption. But, this thickness is much larger than the exciton diffusion length (approximately 10 nm) in P3HT . Recently, Paulus et al. have presented their experimental and theoretical results on nano-heterojunction organic solar cells, in which the maximum photocurrent occurs at 60 to 65 nm of a P3HT photoactive layer due to bulk exciton sink in P3HT [21, 22]. Considering the P3HT/Si NWA hybrid structure has the same exciton dissociation mechanism as that proposed by Paulus et al., the thickness of the conformal P3HT thickness can be increased above the exciton diffusion length in the design of P3HT/Si NWA hybrid cells. Meanwhile, from Figure 4, good light absorption could still be maintained for a hybrid structure with a P3HT coating thickness slightly less than 80 nm. So, for practical fabrication of P3HT/Si NWA hybrid solar cells, the conformal coating with thickness of dozens of nanometers is propitious for the balance of the photon absorption, charge separation, and charge transport in the proposed P3HT/Si NWA hybrid solar cells.
In conclusion, an optical simulation was investigated to evaluate the optical design requirements for improving the efficiency of P3HT/Si NWA solar cells. It is found that as a photoactive material, the introduction of organic coating on Si NWA can further increase the absorptance of P3HT/Si NWA hybrid structure, leading to a better light absorption for wavelengths both below and above the absorption cutoff wavelength of P3HT. At optimized size, the proposed hybrid solar cells exhibit promising photo absorption efficiency. Moreover, we give a direct theoretical proof about the superior performance of the core-shell condition with conformal coating of P3HT as compared with full-infiltrated condition. These findings will play a significant role in realizing the most effective hybrid solar cells formed by organic and semiconductor NWAs in practical experiment. Combined with easy and superior fabrication of such hybrid solar cells, a breakthrough in cell efficiency of the proposed device may be achieved. Obviously, the combination of low-cost Si NWA and solution-processed photoactive organic coating makes this P3HT/Si NWA hybrid solar cell worthy of further investigation.
WW got his bachelors degree in Electronic Science and Technology in 2011 at Hunan University, China. Now, he is taking his master's degree at Solid State Physics Department at Hefei Institute of Physical Science, Chinese Academy of Sciences. He is working on fabrication and characterization of semiconductor nanostructure-based applications. XL received his Ph.D. degree in Solid State Physics at Hefei Institute of Physical Science, Chinese Academy of Sciences, in Hefei in 2007. He joined the Institute of Solid State Physics (ISSP) in Hefei, China in 2007. His main research field is dedicated to the physical characterization of semiconductor nanostructures and their application in hybrid solar cells. He is an author and a coauthor of more than 30 scientific publications in journals and conference proceedings related to micro and nano systems. LW got his Ph.D. degree in Condensed Matter Physics in Solid State Physics in 2013 at Hefei Institute of Physical Science, Chinese Academy of Sciences. At present, he has a post-doctoral position at the Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences. He is involved in semiconductor device design and characterization of nanowires and nanoparticles of both polymeric and inorganic materials for photovoltaic applications. YZ obtained his bachelors degree in Applied Physics from China University of Petroleum in 2011. Now, he studies Solid State Physics at Hefei Institute of Physical Science, Chinese Academy of Sciences for his master's degree. What he majors in are synthesis and characterization of III-V compound semiconductor nanowires and photovoltaic applications. HD received her bachelors degree in Applied Physics in 2012 at Changchun University of Science and Technology, China. At present, she is working on fabrication and characterization of semiconductor nanostructure-based applications at Solid State Physics at Hefei Institute of Physical Science, Chinese Academy of Sciences for a master's degree. BZ obtained his master's degree in The Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, in 2013. At present, he studies at the Solid State Physics Department at Hefei Institute of Physical Science, Chinese Academy of Sciences for a Ph.D. degree. He majors in the synthesis and characterization of semiconductor materials and semiconductor devices. TS received his Ph.D. degree at the Department of Physics of the University of Science and Technology of China in 2007. And now, he is a research associate at the Institute of Solid State Physics, Chinese Academy of Sciences. He has a background in X-ray absorption spectrum, polymer solar cells, and thin films coatings. XZ obtained his bachelors degree in Materials Science and Engineering in 2009 at Nanjing University, China. Now, he stays at Solid State Physics Department at Hefei Institute of Physical Science, China Academy of Sciences for a Ph.D. degree. He is working on fabrication and characterization of polymer semiconductor nanostructure. NL received his bachelors degree in Applied Physics in 2011 at Anhui University, China. At present, he is working on fabrication and characterization of polymer semiconductor at Solid State Physics Department at Hefei Institute of Physical Science, Chinese Academy of Sciences for his master's degree. YW obtained his Ph.D. degree from Columbia University in 1993. Now, he is a professor in Solid State Physics at Hefei Institute of Physical Science, Chinese Academy of Sciences. His research interests are wide-gap semiconductor materials, novel semiconductor devices, and semiconductor quantum structures.
power conversion efficiency
perfect match layer.
This work was supported by the Natural Science Foundation of China under Contract Nos. 11104271 and 1117904 and the Natural Science Foundation of Anhui Province under Contract No. 1308085MA10.
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