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Cite this: Nanoscale, 2014, 6, 6092
Inorganic/organic hybrid solar cells: optimal carrier transport in vertically aligned silicon nanowire arrays† Keisuke Sato,*ab Mrinal Duttaa and Naoki Fukata*a Inorganic/organic hybrid radial heterojunction solar cells that combine vertically-aligned n-type silicon nanowires (SiNWs) with poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) have great potential for replacing commercial Si solar cells. The chief advantage of such solar cells is that they exhibit higher absorbance for a given thickness than commercial Si solar cells, due to incident lighttrapping within the NW arrays, thus enabling lower-cost solar cell production. We report herein on the effects of NW length, annealing and surface electrode on the device performance of SiNW/PEDOT:PSS hybrid radial heterojunction solar cells. The power conversion efficiency (PCE) of the obtained SiNW/ PEDOT:PSS hybrid solar cells can be optimized by tuning the thickness of the surface electrode, and the
Received 10th February 2014 Accepted 25th March 2014
etching conditions during NW formation and post-annealing. The PCE of 9.3% is obtained by forming efficient transport pathways for photogenerated charge carriers to electrodes. Our approach is a
DOI: 10.1039/c4nr00733f
significant contribution to design of high-performance and low-cost inorganic/organic hybrid
www.rsc.org/nanoscale
heterojunction solar cells.
Introduction Most recently, solar cells using one-dimensional architecture, such as silicon (Si) wires1,2 and Si nanowires (SiNWs),3–7 have raised hope for the realization of highly efficient solar cell modules. The main advantage of SiNWs is that they show greater light absorption (minimal reectivity) due to incident light-trapping within the NW arrays. In short, nanostructured Si solar cells lead to higher absorbance per unit thickness than achieved by commercial Si solar cells,8–10 signicantly reducing the quantity of Si materials needed for cell fabrication.11,12 Other important advantages are that they permit the enlargement of radial p–n junction areas to improve carrier separation12 and to provide efficient transport pathways for photogenerated charge carriers to electrodes.13,14 In the design of such SiNW solar cells, the formation of vertically aligned SiNWs is of crucial importance. The vertically aligned SiNWs can assist with light-trapping and suppress photon reections on the surface, thus simultaneously improving both light absorption and carrier generation.15,16 The formation of metal-free SiNWs is also necessary for the production of highly efficient solar cells. We
a
World Premier International Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. E-mail: [email protected]; [email protected]
b
Department of Electrical and Electronic Engineering, Tokyo Denki University, 5 SenjuAsahi-cho, Adachi-ku, Tokyo 120-8551, Japan † Electronic supplementary 10.1039/c4nr00733f
information
6092 | Nanoscale, 2014, 6, 6092–6101
(ESI)
available.
See
DOI:
fabricated SiNWs by catalytic chemical vapour deposition (CVD) with vapour–liquid–solid (VLS) growth.17 Using this particular growth technique, the electrically active level derived from a metal (e.g., gold) used as catalyst has been found to be transferred onto the NW sidewall surfaces and into the NW volume due to metal diffusion from the NW tip during the growth process.18 When metal-contaminated SiNWs were used as the active layer in solar cells, this metal contamination proved detrimental to cell performance, causing a dramatic drop in power conversion efficiency (PCE). This is due to the trapping of most of the photogenerated charge carriers in the metal-related electronic levels, leading to carrier transport losses. This problem can be solved by employing a top-down approach, such as a solution-based etching process.19–21 Metal-assisted chemical etching,19,21 which is a simple and low-cost technique, has several advantages. For instance, it can enable not only easy and complete removal of the metal by post-treatment immersion in a solution, but also control of the morphology, including the diameter, length, and orientation of SiNWs relative to the substrate, allowing the fabrication of metal-free and vertically aligned SiNWs. A matter of key importance in the development of solar cells is how the p–n junction is formed. In SiNW solar cells, the p–n junction is normally formed by a thermal diffusion process using an n-type dopant, employing a furnace at 845–850 C for p-type NWs22,23 or by deposition of a p-type polycrystalline Si sheath using low-pressure CVD at 450 C and subsequent rapid thermal annealing at 1000 C for n-type NWs.4 As a step toward the practical use of SiNW solar cells, there has been signicant interest in forming p–n junctions using low-
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 01 April 2014. Downloaded by University of Chicago on 28/10/2014 23:16:44.
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temperature processes, since commercial Si solar cells, as typied by heterojunction with intrinsic thin-layer (HIT) solar cells, have been developed at process temperatures below 200 C.24 Hybrid solar cells with radial heterojunctions that combine an organic with an inorganic material not only require lowtemperature fabrication due to the low thermostability of the organic material and solution-based processes (e.g., coating and printing techniques) but also can realize lower manufacturing costs by utilizing inexpensive and abundant organic materials, in contrast to expensive p–n junctions consisting of only Si.16,25 In addition, this cell might be useful for applications where low weight, mechanical exibility and disposability are important.26,27 Making inorganic/organic hybrid solar cells commercially viable would help cut the market cost of PV and improve the adoption rate of solar cells. Herein, we describe our creation of inorganic/organic hybrid radial heterojunction solar cells that combine an organic material with metal-free and vertically aligned n-type SiNWs. We selected poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), also known as Clevios PH1000, manufactured by Heraeus (Leverkusen, Germany), as the organic material (i.e., p-type-like conjugated conducting polymer), since it is not only transparent and highly conductive (
Published on 01 April 2014. Downloaded by University of Chicago on 28/10/2014 23:16:44.
PAPER
View Journal | View Issue
Cite this: Nanoscale, 2014, 6, 6092
Inorganic/organic hybrid solar cells: optimal carrier transport in vertically aligned silicon nanowire arrays† Keisuke Sato,*ab Mrinal Duttaa and Naoki Fukata*a Inorganic/organic hybrid radial heterojunction solar cells that combine vertically-aligned n-type silicon nanowires (SiNWs) with poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) have great potential for replacing commercial Si solar cells. The chief advantage of such solar cells is that they exhibit higher absorbance for a given thickness than commercial Si solar cells, due to incident lighttrapping within the NW arrays, thus enabling lower-cost solar cell production. We report herein on the effects of NW length, annealing and surface electrode on the device performance of SiNW/PEDOT:PSS hybrid radial heterojunction solar cells. The power conversion efficiency (PCE) of the obtained SiNW/ PEDOT:PSS hybrid solar cells can be optimized by tuning the thickness of the surface electrode, and the
Received 10th February 2014 Accepted 25th March 2014
etching conditions during NW formation and post-annealing. The PCE of 9.3% is obtained by forming efficient transport pathways for photogenerated charge carriers to electrodes. Our approach is a
DOI: 10.1039/c4nr00733f
significant contribution to design of high-performance and low-cost inorganic/organic hybrid
www.rsc.org/nanoscale
heterojunction solar cells.
Introduction Most recently, solar cells using one-dimensional architecture, such as silicon (Si) wires1,2 and Si nanowires (SiNWs),3–7 have raised hope for the realization of highly efficient solar cell modules. The main advantage of SiNWs is that they show greater light absorption (minimal reectivity) due to incident light-trapping within the NW arrays. In short, nanostructured Si solar cells lead to higher absorbance per unit thickness than achieved by commercial Si solar cells,8–10 signicantly reducing the quantity of Si materials needed for cell fabrication.11,12 Other important advantages are that they permit the enlargement of radial p–n junction areas to improve carrier separation12 and to provide efficient transport pathways for photogenerated charge carriers to electrodes.13,14 In the design of such SiNW solar cells, the formation of vertically aligned SiNWs is of crucial importance. The vertically aligned SiNWs can assist with light-trapping and suppress photon reections on the surface, thus simultaneously improving both light absorption and carrier generation.15,16 The formation of metal-free SiNWs is also necessary for the production of highly efficient solar cells. We
a
World Premier International Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. E-mail: [email protected]; [email protected]
b
Department of Electrical and Electronic Engineering, Tokyo Denki University, 5 SenjuAsahi-cho, Adachi-ku, Tokyo 120-8551, Japan † Electronic supplementary 10.1039/c4nr00733f
information
6092 | Nanoscale, 2014, 6, 6092–6101
(ESI)
available.
See
DOI:
fabricated SiNWs by catalytic chemical vapour deposition (CVD) with vapour–liquid–solid (VLS) growth.17 Using this particular growth technique, the electrically active level derived from a metal (e.g., gold) used as catalyst has been found to be transferred onto the NW sidewall surfaces and into the NW volume due to metal diffusion from the NW tip during the growth process.18 When metal-contaminated SiNWs were used as the active layer in solar cells, this metal contamination proved detrimental to cell performance, causing a dramatic drop in power conversion efficiency (PCE). This is due to the trapping of most of the photogenerated charge carriers in the metal-related electronic levels, leading to carrier transport losses. This problem can be solved by employing a top-down approach, such as a solution-based etching process.19–21 Metal-assisted chemical etching,19,21 which is a simple and low-cost technique, has several advantages. For instance, it can enable not only easy and complete removal of the metal by post-treatment immersion in a solution, but also control of the morphology, including the diameter, length, and orientation of SiNWs relative to the substrate, allowing the fabrication of metal-free and vertically aligned SiNWs. A matter of key importance in the development of solar cells is how the p–n junction is formed. In SiNW solar cells, the p–n junction is normally formed by a thermal diffusion process using an n-type dopant, employing a furnace at 845–850 C for p-type NWs22,23 or by deposition of a p-type polycrystalline Si sheath using low-pressure CVD at 450 C and subsequent rapid thermal annealing at 1000 C for n-type NWs.4 As a step toward the practical use of SiNW solar cells, there has been signicant interest in forming p–n junctions using low-
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 01 April 2014. Downloaded by University of Chicago on 28/10/2014 23:16:44.
Paper
temperature processes, since commercial Si solar cells, as typied by heterojunction with intrinsic thin-layer (HIT) solar cells, have been developed at process temperatures below 200 C.24 Hybrid solar cells with radial heterojunctions that combine an organic with an inorganic material not only require lowtemperature fabrication due to the low thermostability of the organic material and solution-based processes (e.g., coating and printing techniques) but also can realize lower manufacturing costs by utilizing inexpensive and abundant organic materials, in contrast to expensive p–n junctions consisting of only Si.16,25 In addition, this cell might be useful for applications where low weight, mechanical exibility and disposability are important.26,27 Making inorganic/organic hybrid solar cells commercially viable would help cut the market cost of PV and improve the adoption rate of solar cells. Herein, we describe our creation of inorganic/organic hybrid radial heterojunction solar cells that combine an organic material with metal-free and vertically aligned n-type SiNWs. We selected poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), also known as Clevios PH1000, manufactured by Heraeus (Leverkusen, Germany), as the organic material (i.e., p-type-like conjugated conducting polymer), since it is not only transparent and highly conductive (
