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CdS quantum dots composited solar cell with efficient light harvesting and surface passivation
Accepted Manuscript Polycrystalline Si nanocorals/CdS quantum dots composited solar cell with efficient light harvesting and surface passivation Wuliang Feng, Yawen Wang, Jie Liu, Xibin Yu PII: DOI: Reference:
S0009-2614(14)00410-2 http://dx.doi.org/10.1016/j.cplett.2014.05.035 CPLETT 32180
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
13 January 2014 13 May 2014
Please cite this article as: W. Feng, Y. Wang, J. Liu, X. Yu, Polycrystalline Si nanocorals/CdS quantum dots composited solar cell with efficient light harvesting and surface passivation, Chemical Physics Letters (2014), doi: http://dx.doi.org/10.1016/j.cplett.2014.05.035
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polycrystalline Si nanocorals/CdS quantum dots composited solar cell with efficient light harvesting and surface passivation Wuliang Feng, Yawen Wang, Jie Liu, Xibin Yu* The Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Department of Chemistry, Shanghai Normal University, Shanghai 200234, People's Republic of China *Address correspondence to: [email protected] Telephone: +86-21-64322511 Keywords: nanocorals, quantum dots, Poly-Si, CdS Abstract We propose a composited Polycrystalline Si nanocorals (NCLs) solar cell with p-n active layer that integrated with CdS quantum dots (QDs). The composited solar cell achieved 22.6% improvement in power conversion efficiency (PCE), compared to the bare Si substrate. As studied by effective minority-carrier
lifetime,
photoluminescence and reflectance, the improvement of PCE contents two parts. Firstly, the prominent light trapping of Si NCLs with proper depth would dominant over the surface recombination velocity. Secondly, as the CdS QDs on the topside, the remarkable antireflection, passivation and down-conversion properties would contribute to a further enhanced PCE.
1
1. Introduction Optical, recombination, series resistance, and thermal or quantum losses are the four main losses that constrained the maximum theoretical efficiency of Si based solar cell efficiency to 31% [1, 2]. Bare Si substrate is a poor absorber that reflects approximately 35% of incident photons. To minimize optical losses, on one hand, different kinds of surface texturing processes have been carried out to provide remarkable diffuse reflection [3-5]. On the other, antireflection (AR) coating with refractive index intermediate between air and Si has been applied to the air-Si interface. The combination of surface texturing and SiNx AR coating can reduce the reflectance of incident photons to about 10% on Poly-Si. Both of the methods have also been achieved in commercial scale fabrication. However, ultraviolet (UV) energy is still highly reflected and the process to deposit SiNx AR coating in large area is highly cost [6-8]. In comparison, sub-wavelength Si nanostructure with excellent light trapping on top of the Si solar cell can also achieve the same reflectance character, even without any AR coating. Despite of the benefits from light absorbance, the high surface recombination velocity caused by surface defects restricts the enhancement of PCE [6]. Among the four main losses, thermal or quantum losses make up about half of the solar energy, in part due to the loss of photon energy that exceeds the Si bandgap, especially in UV region. Down-conversion materials can improve the utility of the whole solar spectrum, due to the extension of operating spectral range toward ultraviolet and emit lower energy photons. In recent decades, rare earth-doped luminescent materials with down-conversion property have been extensively studied [9, 10], but the integration of these materials on Si based solar cell is far from practical up till now. Apart from that, inorganic semiconductor quantum dots (QDs) also have down-conversion property to improve the utility of the full solar energy [11, 12]. Large bandgap QDs can help to utilize UV light effectively by the absorption of a high-energy photon and then convert it into two low-energy photons [13, 14]. Additional benefit of the QDs comes from the ability to slow down electronic relaxation [15, 16]. With a quantum confinement effect, hot carriers can be extracted 2
before they cool to the band edges. The impact ionization in QDs opens up a way to utilize electron-hole pairs with a single photon, known as the multiple exciton generation (MEG) phenomenon [17]. If all of the energy of the hot carriers were captured, solar-to-electric power conversion efficiencies could be theoretically increased to as high as 66% [18]. Here, Si coral like one-dimensional (1-D) nanostructure was synthesised. We investigated how this Si nanocorals (NCLs) affected the photovoltaic performance of Poly-Si with p–n homojunction, and its integration with CdS QDs afterwards. Firstly, optimum etching time of Si NCLs was explored by investigating the interaction of light absorption and effective minority-carrier lifetime. Subsequently, with the integration of ~3nm CdS QDs, the composited solar cell achieved 22.6% PCE enhancement compared to the bare Poly-Si substrate without Si NCLs and CdS QDs. 2. Experimental The Poly-Si wafers employed in this work were purchased from Changzhou Yijing Optoelectronics Technology Co., Ltd, followed with a standard commercial process including RCA surface clear, surface-texturization and phosphorus diffusion. The thickness of wafers was 200 µm with bulk p-n junction, the resistivity of Si wafer is in the range of 1-10 Ω.cm. Si NCLs shown in Figure 1a was prepared using metal-assisted electroless etching (MAE) by two steps [19-21]. Firstly, the Si substrate was etched in an aqueous solution of silver nitrate (0.005 M) and hydrofluoric acid (0.5 M) for 30s to deposit a layer of Ag nanoparticle masks. Then, the Si substrate with Ag nanoparticle masks was immersed into an aqueous solution of hydrogenperoxide (0.7 M), hydrofluoric acid (0.4 M). Two Poly-Si substrates were attached to each other using waterproof tape to protect the p-type surface from being etched. The two n-type surfaces of the wafers were etched simultaneously. The depth of Si NCLs was adjusted by changing etching time for 0, 10s, 20s, 30s and 40s. The CdS QDs were prepared by typical wet solution phase chemical syntheses with some modifications [22]. Afterwards, CdS QDs were spin coated (2000r/min) on the Si NCLs and then thermal treated under 200 °C with the protection of N2 for 6 min. 3
Si NCLs were characterized using FESEM (Hitachi, Japan, operated at 15 kV). CdS QDs were characterized by TEM (JEOL JEM-200CX microscope operating at 160 kV) The absorption and reflection spectra of the specimens were measured using CARY 500 Scan UV/VIS/NIR spectrophotometer, with an integrating sphere (Labsphere) in a wavelength range of 250-1200 nm. The effective recombination minority-carrier lifetime of the specimens were characterized by Semilab WT-2000PVN. Photoluminescence spectroscopy was carried out with a VARIAN Cary-Eclipse 500 fluorescence spectrophotometer equipped with a 60 W Xenon lamp as the excitation source, the sample was excited by a light beam with 650 nm. The absorption spectra of CdS QDs was characterized by Beckman Coulter DU 730 UV/VIS/NIR spectrophotometer. EQE measurement was carried out with CrownTech CCTH-150W. The total cell area measured was 9 cm2. Chemical vapor deposition (CVD) was employed to evaporate Al as backside electrode. Frontside Ag electrode was printed and dried at 120 °C, and went through rapid thermal annealing at 750 °C for 2s. The J-V characteristics of the solar cells were investigated under the illumination of AM1.5G (100 mW/cm2) and provided with Zennium electrochemical workstation (model: Xpot). The light intensity was calibrated with a silicon standard cell. 3. Results and discussion The absorption spectra of Si NCLs are shown in Figure 1b. It is found that the absorbance from 250 nm to 1100 nm enhanced rapidly with the increase of Si NCLs etching time, especially in the region of 250-650 nm, indicating that deeper Si NCLs have stronger diffuse reflection. Regardless of the light trapping benefits from the deeper Si NCLs, the higher surface recombination velocity is harmful to cell conversion efficiency. Si NCLs arrays with high aspect ratios have large surface recombination velocity that can lead to poor charge carrier collection efficiency. As the data reads in Figure 1c, the recombination lifetime decreases rapidly with the increases of etching time and this will probably leads to inferior Jsc. Thus, the recombination lifetime of minority-carrier is inversely proportional to the surface 4
recombination velocity, and Jsc is square root proportional to the minority-carrier recombination lifetime [23]. As the negative correlation displays, the light absorption benefits will be counteracted by the reduction of minority-carrier recombination lifetime to a certain extent and Jsc will peak at a specific etching time. As the consequence of the interaction between minority-carrier lifetime and light absorption, the highest Jsc is obtained by 20 seconds of electroless etching. It worth noting that a longer etching time on n-type Si surface may be harmful for the cell performance since the thickness of the bulk n-type Si is no more than 1µm. The J-V characteristics of Poly-Si without NCLs and with NCLs (20s etching) are shown in Figure 1d. The open-circuit voltage (Voc) exhibits ignorable change. Because the Si NCLs exceed Si Bohr radius in principle do not change the Fermi energy distribution in the p-n junction. Similarly, an ignorable change of FF (from 68.2% to 67.5%) indicates that Si NCLs on the top of the device do not affect the series resistance. The Jsc increased from 22.92mA/cm2 to 25.61mA/cm2 as proper Si NCLs applied, indicating that the benefit of light trapping greatly dominant over the surface recombination velocity. The PEC increased from 8.58% to 9.37%, mainly influenced by Jsc. Basing on the considerable Jsc enhancement that benefited from the remarkable light trapping. The PCE of Si NCLs solar cell still remains great potential to be further improved, which lies in the reduction of surface recombination velocity. So far, proper n-type semiconductor QDs have been extensively studied. Among that, CdS QDs, with high extinction coefficient, efficient charge separation, high stability under irradiation, and certain down-conversion properties also have the potential to improve the cell performance of Si solar cell with p-n active layer. The schematic illustration of the fabrication process of Si NCLs/CdS QDs composited solar cell is shown in Figure 2. TEM image in Figure 3a shows that the CdS QDs were uniform and mono-disperse with an average diameter of 3 nm. CdS QDs were spin coated on the Si NCLs to examine experimental characterization on the composited cell. As shown in XRD pattern, the peak at 2θ values of 26.54°, 36.58°, 43.76°, 57.76° can be indexed to the (002), (102), (110), (201) planes of CdS QDs, indicating that the pure 5
phase of CdS QDs were obtained without impurity peaks. Figure 3b shows the surface morphology of Si NCLs capped with CdS QDs. Large amount of CdS QDs were padded in the gap of Si NCLs, remained a porous surface. J-V characteristic of the composited solar cell is shown in Figure 1d. It is worth noting that the Voc displayed a slightly enhancement after the integration. It implies that the enhanced light utility increased the generation of electron-hole pairs, leading to the slight change of the Fermi energy distribution in the Si p-n junction [24]. Another observation of the J-V characteristic obtained from the slope of the curves indicates that the FF of the composited solar cell decreased to a certain extent (from 67.5% to 62.7%) as Si NCLs were capped with CdS QDs. It implies that CdS on the top side of the device affected the conductance of electrons between n-type Si and Ag electrode, thus produced extra series resistance. Despite of the drawback, Jsc increased dramatically from 25.61 to 29.59 mA/cm2. As the consequence, the composited cell achieved 10.52% in PCE, which is a 12.3% improvement compared to the non CdS capped Si NCLs counterpart. In total, that is a 22.6% PEC improvement compared to the bare Poly-Si substrate. In order to understand the underlying mechanism for the further enhanced PCE, we firstly investigated the down-conversion character of CdS QDs layer. The absorption and photoluminescence (PL) spectra are shown in Figure 4a. The absorption peak of the CdS QDs is observed at 431nm. The fluorescence spectrum shows a peak at 458nm with a narrow curve and 616nm with a broad curve. The Stokes shift is 27nm and 185nm respectively. The sample was excited by a light beam with 416nm and 423nm. The spectral response of the external quantum efficiency (EQE) was measured to understand the utility of photons with different wavelengths. As shown in Figure 4b, The composited cell shows an enhanced EQE spectral response in the whole range from 300nm to 1100 nm. According to Figure 4c, the EQE enhancement curve peaks at 440nm, which is quite accordance with the absorption peak of CdS QDs. It implies that down-conversion took place in the device [25]. To characterize the contribution of down-conversion effect, we calculated the effective current due to the CdS QDs down-conversion layer on the top of Si NCLs 6
solar cell. The carries can be estimated directly from Jsc of a solar cell [26, 27]. Jsc without and with CdS QDs can be calculated as follows: 1100 nm e λ × EQE (λ ) × I AM 1.5G (λ )d λ ×∫ 300 nm hc 431nm e J SC ,QDs = × (∫ [1 − A(λ )] × λ × EQE (λ ) × I AM 1.5G (λ )d λ 300 nm hc
J SC =
∫
431nm
∫
1100 nm
300 nm
431nm
(1)
A(λ ) × λ × EQE (λ ) × QY × I AM 1.5G (λ )d λ +
λ × EQE (λ ) × I AM 1.5G (λ )d λ )
(2)
Where c, h, e, λ represent for the speed of light, Plank’s constant, electric charge and the wavelength respectively. A(λ) is the absorption of ultraviolet light of the quantum dots, the average A(λ) is about 11%. QY is the quantum yield of the dots, the average QY is about 40%. The above equations tell that the portion of down-conversion photon make up to 8% of the Jsc enhancement. As the absorption depth is given by the inverse of the absorption coefficient, high-energy photons have relatively larger absorption coefficient and tend to have shallower absorption depth [6]. UV lights tend to produce electron-hole pairs near the surface of the Si NCLs solar cell where mass of surface defects concentrates. Photo generated carriers disappears easily through recombination. Nevertheless, with a down-conversion layer, the emitted lower energy photons were absorbed in the deeper region, leading to more photons absorbed in the depletion region. Photo generated electron-hole pairs will be separated immediately with the help of a build-in-electric field, thus contribute to the Jsc enhancement [28]. Reflectance spectra of Poly-Si substrate, Si NCLs solar cell, and the composited solar cell are shown in Figure 5a to characterize the antireflection property of CdS QDs layer. CdS QDs significantly reduced the reflectance spectral ranging from 250nm to 1100 nm, rather than just targeted on the CdS QDs absorption edge. With refractive index (n=2.5) between air and Poly-Si, the application of colloidal CdS QDs ink in large area fabrication is a potential way to develop the low-cost AR layer. The porous composited solar cell surface is probably another reason for the reduced reflectance. Previous researches have found that the nanohole [29] or nanoporous [30] 7
structures exhibited better light trapping properties than the 1-D nanowires with the same depth. Regardless of the smaller specific surface area of hole or porous nano structures, they possess stronger diffuse reflection, due to the unique geometrical optics structures. Obviously, the remarkable antireflection property of CdS QDs layer is a more important factor for the PCE enhancement. The effective recombination lifetime of Si NCLs capped with CdS QDs was also studied to understand the further enhanced cell efficiency. As shown in Figure 5b, after integrated with CdS QDs, the effective recombination minority-carrier lifetime of Si NCLs was improved remarkably, implying that the CdS QDs coating acted as a passivation layer. The enhanced recombination lifetime is probably due to the reduced surface area of porous structure, this process will reduce the recombination center. Besides, in the previous reports, CdS QDs have the ability to slow down electronic relaxation [31]. With such characteristic, CdS QDs can in principle able to extract hot carriers to electron or hole conductors before they cool to the band edges, it also probably contributes to the recombination lifetime enhancement. According to the two curves, it is interesting to find that the recombination lifetime enhancement increased more in longer etching time. The enhancements of Si NCLs capped with CdS QDs are 1.8%, 6.2%, 16.3%, 33.5% and 54.1% for Si NCLs with etching time of 0, 10, 20, 30, and 40 s, respectively. This variation tendency is probably due to the higher recombination velocity caused by the longer etching time. 4.conclusion A composited solar cell of Poly-Si NCLs & CdS QDs achieved 22.6% PCE enhancement compared to the bare Poly-Si substrate. The two-step improvement including: 1. Jsc enhancement of Si NCLs, as the result of remarkable light trapping dominant over the surface recombination velocity. 2. The benefit of antireflection, passivation and down-conversion properties in CdS QDs.
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Acknowledgment
This work is supported by the Shanghai Science & Technology Committee (12521102501, 11ZR1426500), the first-class discipline construction planning in Shanghai University, PCSIRT (RT1269 ), the Program of Shanghai Normal University (DZL124) and the Key Laboratory of Resource Chemistry of Ministry of Education of China.
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Figures
Figure 1. (a) FESEM image of Si NCLs. (b) Light absorption of Si NCLs, depending on electro-less etching time. (c) Minority-carrier lifetime and Jsc of Si NCLs solar cells, depending on electro-less etching time. (d) J-V characteristics of Poly-Si substrate, Si NCLs solar cell, and the composited solar cell (CdS QDs concentration is 12mg/mL, 5 layers).
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Figure 2. Schematic illustration to shown the fabrication process of Si NCLs/CdS QDs composited solar cell, corresponding with cross sectional FESEM images. The process contents: (a) Bare polycrystalline Si wafer with p-n homojunction. (b) Top-Down fabrication of Si NCLs by means of metal-assisted electroless etching, on n-type layer. (c) Deposition of CdS QDs using spin-coating. Large amount of CdS QDs are padded in the gap of Si NCLs.
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Figure 3. (a) TEM image and X-ray diffraction pattern of CdS QDs. (b) FESEM image of Si NCLs capped with CdS QDs. (CdS QDs concentration is 12mg/mL, 5 layers).
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Figure 4. (a) Absorption and photoluminescence spectra of CdS QDs. (b) EQE spectra of Si NCLs solar cell and Si NCLs/CdS QDs composited solar cell. (c) EQE enhancement of Si NCLs/CdS QDs composited solar cell in comparison with Si NCLs solar cell.
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Figure 5. (a) Reflectance spectra of Poly-Si substrate, Si NCLs solar cell, and the composited solar cell. (b) Effect of the etching time on recombination lifetime of Si NCLs and the integration with CdS QDs.
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The Poly-Si nanocorals/CdS quantum dots composited solar cell achieved 22.6% increase in power conversion efficiency compared to the bare Poly-Si substrate counterpart.
16
1. We proposed a Si NCLs/QDs composited structure to improve the efficiency of Poly-Si solar cell. 2. Si NCLs solar cell achieved a remarkably absorption and efficiency improvement. 3. After capped with CdS QDs, the composited solar cell achieved 22.6% efficiency enhancement. 4. Antireflection, passivation and down-conversion of CdS are the key points for the improvement.
17
S0009-2614(14)00410-2 http://dx.doi.org/10.1016/j.cplett.2014.05.035 CPLETT 32180
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
13 January 2014 13 May 2014
Please cite this article as: W. Feng, Y. Wang, J. Liu, X. Yu, Polycrystalline Si nanocorals/CdS quantum dots composited solar cell with efficient light harvesting and surface passivation, Chemical Physics Letters (2014), doi: http://dx.doi.org/10.1016/j.cplett.2014.05.035
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polycrystalline Si nanocorals/CdS quantum dots composited solar cell with efficient light harvesting and surface passivation Wuliang Feng, Yawen Wang, Jie Liu, Xibin Yu* The Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Department of Chemistry, Shanghai Normal University, Shanghai 200234, People's Republic of China *Address correspondence to: [email protected] Telephone: +86-21-64322511 Keywords: nanocorals, quantum dots, Poly-Si, CdS Abstract We propose a composited Polycrystalline Si nanocorals (NCLs) solar cell with p-n active layer that integrated with CdS quantum dots (QDs). The composited solar cell achieved 22.6% improvement in power conversion efficiency (PCE), compared to the bare Si substrate. As studied by effective minority-carrier
lifetime,
photoluminescence and reflectance, the improvement of PCE contents two parts. Firstly, the prominent light trapping of Si NCLs with proper depth would dominant over the surface recombination velocity. Secondly, as the CdS QDs on the topside, the remarkable antireflection, passivation and down-conversion properties would contribute to a further enhanced PCE.
1
1. Introduction Optical, recombination, series resistance, and thermal or quantum losses are the four main losses that constrained the maximum theoretical efficiency of Si based solar cell efficiency to 31% [1, 2]. Bare Si substrate is a poor absorber that reflects approximately 35% of incident photons. To minimize optical losses, on one hand, different kinds of surface texturing processes have been carried out to provide remarkable diffuse reflection [3-5]. On the other, antireflection (AR) coating with refractive index intermediate between air and Si has been applied to the air-Si interface. The combination of surface texturing and SiNx AR coating can reduce the reflectance of incident photons to about 10% on Poly-Si. Both of the methods have also been achieved in commercial scale fabrication. However, ultraviolet (UV) energy is still highly reflected and the process to deposit SiNx AR coating in large area is highly cost [6-8]. In comparison, sub-wavelength Si nanostructure with excellent light trapping on top of the Si solar cell can also achieve the same reflectance character, even without any AR coating. Despite of the benefits from light absorbance, the high surface recombination velocity caused by surface defects restricts the enhancement of PCE [6]. Among the four main losses, thermal or quantum losses make up about half of the solar energy, in part due to the loss of photon energy that exceeds the Si bandgap, especially in UV region. Down-conversion materials can improve the utility of the whole solar spectrum, due to the extension of operating spectral range toward ultraviolet and emit lower energy photons. In recent decades, rare earth-doped luminescent materials with down-conversion property have been extensively studied [9, 10], but the integration of these materials on Si based solar cell is far from practical up till now. Apart from that, inorganic semiconductor quantum dots (QDs) also have down-conversion property to improve the utility of the full solar energy [11, 12]. Large bandgap QDs can help to utilize UV light effectively by the absorption of a high-energy photon and then convert it into two low-energy photons [13, 14]. Additional benefit of the QDs comes from the ability to slow down electronic relaxation [15, 16]. With a quantum confinement effect, hot carriers can be extracted 2
before they cool to the band edges. The impact ionization in QDs opens up a way to utilize electron-hole pairs with a single photon, known as the multiple exciton generation (MEG) phenomenon [17]. If all of the energy of the hot carriers were captured, solar-to-electric power conversion efficiencies could be theoretically increased to as high as 66% [18]. Here, Si coral like one-dimensional (1-D) nanostructure was synthesised. We investigated how this Si nanocorals (NCLs) affected the photovoltaic performance of Poly-Si with p–n homojunction, and its integration with CdS QDs afterwards. Firstly, optimum etching time of Si NCLs was explored by investigating the interaction of light absorption and effective minority-carrier lifetime. Subsequently, with the integration of ~3nm CdS QDs, the composited solar cell achieved 22.6% PCE enhancement compared to the bare Poly-Si substrate without Si NCLs and CdS QDs. 2. Experimental The Poly-Si wafers employed in this work were purchased from Changzhou Yijing Optoelectronics Technology Co., Ltd, followed with a standard commercial process including RCA surface clear, surface-texturization and phosphorus diffusion. The thickness of wafers was 200 µm with bulk p-n junction, the resistivity of Si wafer is in the range of 1-10 Ω.cm. Si NCLs shown in Figure 1a was prepared using metal-assisted electroless etching (MAE) by two steps [19-21]. Firstly, the Si substrate was etched in an aqueous solution of silver nitrate (0.005 M) and hydrofluoric acid (0.5 M) for 30s to deposit a layer of Ag nanoparticle masks. Then, the Si substrate with Ag nanoparticle masks was immersed into an aqueous solution of hydrogenperoxide (0.7 M), hydrofluoric acid (0.4 M). Two Poly-Si substrates were attached to each other using waterproof tape to protect the p-type surface from being etched. The two n-type surfaces of the wafers were etched simultaneously. The depth of Si NCLs was adjusted by changing etching time for 0, 10s, 20s, 30s and 40s. The CdS QDs were prepared by typical wet solution phase chemical syntheses with some modifications [22]. Afterwards, CdS QDs were spin coated (2000r/min) on the Si NCLs and then thermal treated under 200 °C with the protection of N2 for 6 min. 3
Si NCLs were characterized using FESEM (Hitachi, Japan, operated at 15 kV). CdS QDs were characterized by TEM (JEOL JEM-200CX microscope operating at 160 kV) The absorption and reflection spectra of the specimens were measured using CARY 500 Scan UV/VIS/NIR spectrophotometer, with an integrating sphere (Labsphere) in a wavelength range of 250-1200 nm. The effective recombination minority-carrier lifetime of the specimens were characterized by Semilab WT-2000PVN. Photoluminescence spectroscopy was carried out with a VARIAN Cary-Eclipse 500 fluorescence spectrophotometer equipped with a 60 W Xenon lamp as the excitation source, the sample was excited by a light beam with 650 nm. The absorption spectra of CdS QDs was characterized by Beckman Coulter DU 730 UV/VIS/NIR spectrophotometer. EQE measurement was carried out with CrownTech CCTH-150W. The total cell area measured was 9 cm2. Chemical vapor deposition (CVD) was employed to evaporate Al as backside electrode. Frontside Ag electrode was printed and dried at 120 °C, and went through rapid thermal annealing at 750 °C for 2s. The J-V characteristics of the solar cells were investigated under the illumination of AM1.5G (100 mW/cm2) and provided with Zennium electrochemical workstation (model: Xpot). The light intensity was calibrated with a silicon standard cell. 3. Results and discussion The absorption spectra of Si NCLs are shown in Figure 1b. It is found that the absorbance from 250 nm to 1100 nm enhanced rapidly with the increase of Si NCLs etching time, especially in the region of 250-650 nm, indicating that deeper Si NCLs have stronger diffuse reflection. Regardless of the light trapping benefits from the deeper Si NCLs, the higher surface recombination velocity is harmful to cell conversion efficiency. Si NCLs arrays with high aspect ratios have large surface recombination velocity that can lead to poor charge carrier collection efficiency. As the data reads in Figure 1c, the recombination lifetime decreases rapidly with the increases of etching time and this will probably leads to inferior Jsc. Thus, the recombination lifetime of minority-carrier is inversely proportional to the surface 4
recombination velocity, and Jsc is square root proportional to the minority-carrier recombination lifetime [23]. As the negative correlation displays, the light absorption benefits will be counteracted by the reduction of minority-carrier recombination lifetime to a certain extent and Jsc will peak at a specific etching time. As the consequence of the interaction between minority-carrier lifetime and light absorption, the highest Jsc is obtained by 20 seconds of electroless etching. It worth noting that a longer etching time on n-type Si surface may be harmful for the cell performance since the thickness of the bulk n-type Si is no more than 1µm. The J-V characteristics of Poly-Si without NCLs and with NCLs (20s etching) are shown in Figure 1d. The open-circuit voltage (Voc) exhibits ignorable change. Because the Si NCLs exceed Si Bohr radius in principle do not change the Fermi energy distribution in the p-n junction. Similarly, an ignorable change of FF (from 68.2% to 67.5%) indicates that Si NCLs on the top of the device do not affect the series resistance. The Jsc increased from 22.92mA/cm2 to 25.61mA/cm2 as proper Si NCLs applied, indicating that the benefit of light trapping greatly dominant over the surface recombination velocity. The PEC increased from 8.58% to 9.37%, mainly influenced by Jsc. Basing on the considerable Jsc enhancement that benefited from the remarkable light trapping. The PCE of Si NCLs solar cell still remains great potential to be further improved, which lies in the reduction of surface recombination velocity. So far, proper n-type semiconductor QDs have been extensively studied. Among that, CdS QDs, with high extinction coefficient, efficient charge separation, high stability under irradiation, and certain down-conversion properties also have the potential to improve the cell performance of Si solar cell with p-n active layer. The schematic illustration of the fabrication process of Si NCLs/CdS QDs composited solar cell is shown in Figure 2. TEM image in Figure 3a shows that the CdS QDs were uniform and mono-disperse with an average diameter of 3 nm. CdS QDs were spin coated on the Si NCLs to examine experimental characterization on the composited cell. As shown in XRD pattern, the peak at 2θ values of 26.54°, 36.58°, 43.76°, 57.76° can be indexed to the (002), (102), (110), (201) planes of CdS QDs, indicating that the pure 5
phase of CdS QDs were obtained without impurity peaks. Figure 3b shows the surface morphology of Si NCLs capped with CdS QDs. Large amount of CdS QDs were padded in the gap of Si NCLs, remained a porous surface. J-V characteristic of the composited solar cell is shown in Figure 1d. It is worth noting that the Voc displayed a slightly enhancement after the integration. It implies that the enhanced light utility increased the generation of electron-hole pairs, leading to the slight change of the Fermi energy distribution in the Si p-n junction [24]. Another observation of the J-V characteristic obtained from the slope of the curves indicates that the FF of the composited solar cell decreased to a certain extent (from 67.5% to 62.7%) as Si NCLs were capped with CdS QDs. It implies that CdS on the top side of the device affected the conductance of electrons between n-type Si and Ag electrode, thus produced extra series resistance. Despite of the drawback, Jsc increased dramatically from 25.61 to 29.59 mA/cm2. As the consequence, the composited cell achieved 10.52% in PCE, which is a 12.3% improvement compared to the non CdS capped Si NCLs counterpart. In total, that is a 22.6% PEC improvement compared to the bare Poly-Si substrate. In order to understand the underlying mechanism for the further enhanced PCE, we firstly investigated the down-conversion character of CdS QDs layer. The absorption and photoluminescence (PL) spectra are shown in Figure 4a. The absorption peak of the CdS QDs is observed at 431nm. The fluorescence spectrum shows a peak at 458nm with a narrow curve and 616nm with a broad curve. The Stokes shift is 27nm and 185nm respectively. The sample was excited by a light beam with 416nm and 423nm. The spectral response of the external quantum efficiency (EQE) was measured to understand the utility of photons with different wavelengths. As shown in Figure 4b, The composited cell shows an enhanced EQE spectral response in the whole range from 300nm to 1100 nm. According to Figure 4c, the EQE enhancement curve peaks at 440nm, which is quite accordance with the absorption peak of CdS QDs. It implies that down-conversion took place in the device [25]. To characterize the contribution of down-conversion effect, we calculated the effective current due to the CdS QDs down-conversion layer on the top of Si NCLs 6
solar cell. The carries can be estimated directly from Jsc of a solar cell [26, 27]. Jsc without and with CdS QDs can be calculated as follows: 1100 nm e λ × EQE (λ ) × I AM 1.5G (λ )d λ ×∫ 300 nm hc 431nm e J SC ,QDs = × (∫ [1 − A(λ )] × λ × EQE (λ ) × I AM 1.5G (λ )d λ 300 nm hc
J SC =
∫
431nm
∫
1100 nm
300 nm
431nm
(1)
A(λ ) × λ × EQE (λ ) × QY × I AM 1.5G (λ )d λ +
λ × EQE (λ ) × I AM 1.5G (λ )d λ )
(2)
Where c, h, e, λ represent for the speed of light, Plank’s constant, electric charge and the wavelength respectively. A(λ) is the absorption of ultraviolet light of the quantum dots, the average A(λ) is about 11%. QY is the quantum yield of the dots, the average QY is about 40%. The above equations tell that the portion of down-conversion photon make up to 8% of the Jsc enhancement. As the absorption depth is given by the inverse of the absorption coefficient, high-energy photons have relatively larger absorption coefficient and tend to have shallower absorption depth [6]. UV lights tend to produce electron-hole pairs near the surface of the Si NCLs solar cell where mass of surface defects concentrates. Photo generated carriers disappears easily through recombination. Nevertheless, with a down-conversion layer, the emitted lower energy photons were absorbed in the deeper region, leading to more photons absorbed in the depletion region. Photo generated electron-hole pairs will be separated immediately with the help of a build-in-electric field, thus contribute to the Jsc enhancement [28]. Reflectance spectra of Poly-Si substrate, Si NCLs solar cell, and the composited solar cell are shown in Figure 5a to characterize the antireflection property of CdS QDs layer. CdS QDs significantly reduced the reflectance spectral ranging from 250nm to 1100 nm, rather than just targeted on the CdS QDs absorption edge. With refractive index (n=2.5) between air and Poly-Si, the application of colloidal CdS QDs ink in large area fabrication is a potential way to develop the low-cost AR layer. The porous composited solar cell surface is probably another reason for the reduced reflectance. Previous researches have found that the nanohole [29] or nanoporous [30] 7
structures exhibited better light trapping properties than the 1-D nanowires with the same depth. Regardless of the smaller specific surface area of hole or porous nano structures, they possess stronger diffuse reflection, due to the unique geometrical optics structures. Obviously, the remarkable antireflection property of CdS QDs layer is a more important factor for the PCE enhancement. The effective recombination lifetime of Si NCLs capped with CdS QDs was also studied to understand the further enhanced cell efficiency. As shown in Figure 5b, after integrated with CdS QDs, the effective recombination minority-carrier lifetime of Si NCLs was improved remarkably, implying that the CdS QDs coating acted as a passivation layer. The enhanced recombination lifetime is probably due to the reduced surface area of porous structure, this process will reduce the recombination center. Besides, in the previous reports, CdS QDs have the ability to slow down electronic relaxation [31]. With such characteristic, CdS QDs can in principle able to extract hot carriers to electron or hole conductors before they cool to the band edges, it also probably contributes to the recombination lifetime enhancement. According to the two curves, it is interesting to find that the recombination lifetime enhancement increased more in longer etching time. The enhancements of Si NCLs capped with CdS QDs are 1.8%, 6.2%, 16.3%, 33.5% and 54.1% for Si NCLs with etching time of 0, 10, 20, 30, and 40 s, respectively. This variation tendency is probably due to the higher recombination velocity caused by the longer etching time. 4.conclusion A composited solar cell of Poly-Si NCLs & CdS QDs achieved 22.6% PCE enhancement compared to the bare Poly-Si substrate. The two-step improvement including: 1. Jsc enhancement of Si NCLs, as the result of remarkable light trapping dominant over the surface recombination velocity. 2. The benefit of antireflection, passivation and down-conversion properties in CdS QDs.
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Acknowledgment
This work is supported by the Shanghai Science & Technology Committee (12521102501, 11ZR1426500), the first-class discipline construction planning in Shanghai University, PCSIRT (RT1269 ), the Program of Shanghai Normal University (DZL124) and the Key Laboratory of Resource Chemistry of Ministry of Education of China.
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Figures
Figure 1. (a) FESEM image of Si NCLs. (b) Light absorption of Si NCLs, depending on electro-less etching time. (c) Minority-carrier lifetime and Jsc of Si NCLs solar cells, depending on electro-less etching time. (d) J-V characteristics of Poly-Si substrate, Si NCLs solar cell, and the composited solar cell (CdS QDs concentration is 12mg/mL, 5 layers).
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Figure 2. Schematic illustration to shown the fabrication process of Si NCLs/CdS QDs composited solar cell, corresponding with cross sectional FESEM images. The process contents: (a) Bare polycrystalline Si wafer with p-n homojunction. (b) Top-Down fabrication of Si NCLs by means of metal-assisted electroless etching, on n-type layer. (c) Deposition of CdS QDs using spin-coating. Large amount of CdS QDs are padded in the gap of Si NCLs.
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Figure 3. (a) TEM image and X-ray diffraction pattern of CdS QDs. (b) FESEM image of Si NCLs capped with CdS QDs. (CdS QDs concentration is 12mg/mL, 5 layers).
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Figure 4. (a) Absorption and photoluminescence spectra of CdS QDs. (b) EQE spectra of Si NCLs solar cell and Si NCLs/CdS QDs composited solar cell. (c) EQE enhancement of Si NCLs/CdS QDs composited solar cell in comparison with Si NCLs solar cell.
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Figure 5. (a) Reflectance spectra of Poly-Si substrate, Si NCLs solar cell, and the composited solar cell. (b) Effect of the etching time on recombination lifetime of Si NCLs and the integration with CdS QDs.
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The Poly-Si nanocorals/CdS quantum dots composited solar cell achieved 22.6% increase in power conversion efficiency compared to the bare Poly-Si substrate counterpart.
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1. We proposed a Si NCLs/QDs composited structure to improve the efficiency of Poly-Si solar cell. 2. Si NCLs solar cell achieved a remarkably absorption and efficiency improvement. 3. After capped with CdS QDs, the composited solar cell achieved 22.6% efficiency enhancement. 4. Antireflection, passivation and down-conversion of CdS are the key points for the improvement.
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