Study on modification of single-walled carbon nanotubes on the surface of monocrystalline silicon solar cells Tiancheng Gong, Yong Zhu,* Wenbin Xie, Ning Wang, Jie Zhang, and Wenjie Ren The Key Laboratory of Optoelectronic Technology & System, Education Ministry of China, Chongqing University, Chongqing 400044, China *Corresponding author: [email protected] Received 6 June 2014; revised 20 August 2014; accepted 25 August 2014; posted 26 August 2014 (Doc. ID 213642); published 26 September 2014

Modification of single-walled carbon nanotubes (SWNTs) on the surface of monocrystalline silicon solar cells was investigated. The modification was realized by dropping a well-distributed mixture of SWNTs and ethanol with different dosages on the surface of monocrystalline silicon solar cells in the same effective area. The experimental results showed that the increasing rates of conversion efficiency, short-circuit current, and fill factor were 4.37%, 2.18%, and 2.11%, respectively; the open circuit voltage and series resistance decreased by 0.11% and 9.37% compared with the bare solar cell without an antireflection (AR) layer, when the modification reached the best state by dropping a 0.5 mL mixture solution with a concentration of 0.08 g∕L. With the energy-band diagrams of the heterojunction and p-n junction, the principles of the modification of SWNTs on monocrystalline silicon solar cells and the reasons for the change of electrical parameters were analyzed theoretically. Through experiments and theoretical analyses, the modification of SWNTs on solar cells is a potential and effective way to improve the performance of solar cells. © 2014 Optical Society of America OCIS codes: (040.5350) Photovoltaic; (160.4236) Nanomaterials; (040.6040) Silicon; (310.6805) Theory and design. http://dx.doi.org/10.1364/AO.53.006457

1. Introduction

Photovoltaic cell is one of the most efficient tools for harvesting solar energy. The energy conversion process typically involves: photon absorption, carrier separation, transportation, and collection, which would directly determine the conversion efficiency of solar cells [1]. In photovoltaic industry development, the primary goal is to improve the conversion efficiency. Effective approaches include obtaining more light absorption, more effective carrier separation and collection processes [2]. In recent years, many effective methods have been developed to reduce light reflection, such as creating silicon nanowire arrays 1559-128X/14/286457-07$15.00/0 © 2014 Optical Society of America

and fabricating the novel surface nanostructured silicon (black silicon) [3–5]. Three-dimensional nanowire arrays with their core–shell structures could provide direct transportation paths, thus enabling more effective separation and carrier collection [6–8]. The stability of solar cells is also an important factor, especially for cells with nanomaterials, nanostructures, or polymers, due to their sensitivity to changes in temperature, the moisture, and the light illumination [9]. Although nanomaterial/ nanostructure-based solar cells are evolving rapidly, able to increase light absorption, and separate or collect carriers to a large extent, it is difficult to fabricate solar cells with high efficiency, high stability, and low cost. Complex processes and special conditions are often required in producing and testing solar cells, which adds to the difficulty in production 1 October 2014 / Vol. 53, No. 28 / APPLIED OPTICS

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and raises the cost. Besides, ideal structure is not easily fabricated, and internal structure of solar cells may even be undermined which makes conversion efficiency lower and lower. Carbon nanotube (CNT) is a new photoelectric conversion material. It has attracted more and more attention due to its unique structure and properties [10–12]. Integrating CNTs with photovoltaic devices is a potential method for fabricating solar cells. Jia et al. reported several new types of nanotube– silicon heterojunction solar cells with the efficiency of 7.4% (CNT films with n-type silicon) [13], 10.9% (CNT films with oxide-silicon) [14,15], 13.8% (CNT films treated by nitric acid with n-type silicon) [16]. Zhang et al. assembled single-walled carbon nanotubes (SWNTs) onto the surface of p-n junction silicon solar cells and obtained an enhancement of 3.92% in conversion efficiency with the density of SWNTs increased from 50 to 400 tubes μm−2 [17]. Jung et al. reported a SWNT–silicon p-n junction solar cell and demonstrated an 11.2% efficient device [18]. CNT-based solar cells are a hotspot in recent years’ research and the conversion efficiency of solar cells of different junctions formed with different types of CNTs is increasing. However, these methods [13–18] have not been put into mass production due to complicated fabrication processes (preparation of nanomaterials, doping process, electrode fabrication, etc.) and high processing cost. They only stayed at the stage of preparing and testing in laboratories. Nanomaterial decorated on traditional siliconbased solar cells does not destroy the inner structure of solar cells. However, the above-mentioned research status shows that studies on this are relatively few. We explored a method of modifying SWNTs on monocrystalline silicon solar cells which could improve the conversion efficiency and cell performance easily with low fabrication cost. An effective enhancement of conversion efficiency by modification of SWNTs has been achieved after delicate experiments, in stark contrast to solar cells without modification of SWNTs. Besides, we compared the results of modification of different densities of SWNT film with monocrystalline silicon solar cells and established appropriate qualitative theoretical models (energy-band diagrams) in order to clarify the principles. 2. Experimental A.

Sample Preparations

Solar cells with/without a silicon nitride (Si3 N4 ) antireflection (AR) layer based on traditional monocrystalline silicon p-n junction solar cells were prepared. As illustrated in Fig. 1(a), several pieces of solar cells with an effective area of 19 cm2 were cut from the same piece of standard solar cell (125 mm × 125 mm) and experienced different experimental treatments, which were labeled as “Sample 1,” “Sample 2,” and “Sample 3.” “Sample 1” was the 6458

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Fig. 1. (a) Steps of sample fabrication. (b) Structures of silicon p-n solar cell and refitted PDMS–SWNTs–Si p-n solar cell.

original silicon solar cell with a Si3 N4 AR layer and did not go through any process, while “Sample 2” and “Sample 3” were immersed into a 5% hydrofluoric acid (HF) solution at 20°C for 30 min to remove the AR layer, and then thoroughly washed with deionized water and dried at 60°C under nitrogen. Furthermore, a well-distributed mixed solution of ethanol and SWNT powder (hereinafter referred to as “mixed solution”) of 0.08 g∕L [19] was prepared, and then dropped on “Sample 3” for 0.25, 0.5, 0.75, 1, and 1.25 mL, respectively. It is necessary to add some ethyl cellulose to the mixed solution to ensure formation of SWNT film. B. Sample Measurements

After evaporation of ethanol, the SEM characterization, ultraviolet–visible–near infrared (UV–Vis–NIR) test, open-circuit voltage (V oc ), short-circuit current density (J sc ), and J–V curve of each sample under the standard illumination condition (AM 1.5, 100 mW∕cm2 ) was measured. An AR layer of polydimethylsiloxane (PDMS) was encapsulated in order to reduce the light reflection and improve the efficiency of solar cells, and PDMS as an AR layer for solar cells has been reported in several literatures [20,21]. The advantage of using PDMS as an AR layer is that it costs less and does not require a complex coating process as compared with Si3 N4 . It can increase J sc efficiently and further improve the conversion efficiency (η) of solar cells. The PDMS layer was spin coated on solar cells with a speed of 1050 r∕ min , and then these samples were placed in a thermostatic drying chamber at a temperature of 35°C for 24 h, until the PDMS layer was completely cured. Finally, the results of reflectance and transmittance with the wavelength range from 300 to 1500 nm and J–V curve under AM 1.5 sunlight for each sample were measured. The structures of the original and refitted solar cells are illustrated in Fig. 1(b).

3. Results and Discussion A.

Structural Characterizations

Figure 2 shows the surface morphology of monocrystalline silicon solar cells with/without SWNTs modification of the mixed solution for 0.25, 0.75, and 1.25 mL, respectively. The samples were examined using a scanning electron microscopy (SEM) with an electron beam accelerated to 5.0 kV. The SEM images show that SWNT bundles distribute on the surface of silicon evenly and SWNTs are in good contact with silicon. The surface density of SWNTs is increasing with the increase of the mixed solution. B.

Transmittance and Absorptance

UV–Vis–NIR test of “Sample 1” to “Sample 3” were measured by a spectrophotometer (Hitachi HighTech U-4100) and the samples are produced without the process of printing electrodes, which means they have no top and bottom electrodes. Here, we only illustrate “Sample 3” for modification of the 0.5 mL mixed solution (see details in Section 3.C). Figure 3 shows the transmittance and absorptance with the wavelength range from 300 to 1500 nm for each sample. One can see that these samples of solar cells have low transmittance (lower than 0.4%). Besides, the absorptance of “Sample 3” was better than “Sample 2” in the wavelength range of 1000– 1500 nm, so SWNTs could enhance the light absorption of bare solar cells. The performance of “Sample 3 (with PDMS)” was better than “Sample 3 (without PDMS)” in the wavelength range of 300–1500 nm, so PDMS can further enhance the light absorption of the solar cells modified by SWNTs. The performance of “Sample 3 (with PDMS)” was better than “Sample 1” in the wavelength range of

Fig. 2. Configuration and SEM images of monocrystalline silicon solar cells and their surfaces with the modification of SWNTs mixed solution of 0.25, 0.75, and 1.25 mL, respectively. (a) Original solar cell. (b) SWNTs modified with 0.25 mL. (c) SWNTs modified with 0.75 mL. (d) SWNTs modified with 1.25 mL.

Fig. 3. Optical transmittance and absorptance measurements of “Sample 1,” “Sample 2,” and “Sample 3” (with/without PDMS).

300–1500 nm and the best absorptance was achieved. As we know, SWNTs perform well in light absorption in the wavelengths of UV–Vis–NIR [22,23], and PDMS is a transparent, viscous, flexible, and hydrophobic polymer-based material [24–26]. The transmittance of PDMS is high because it is transparent, thus solar cells could capture more photons with the PDMS layer. Due to the characteristic of viscosity and flexibility [25], no cracks emerge on the PDMS layer under illumination or high temperature. The PDMS layer can also enhance abrasion resistance and the stability of AR coating. And the hydrophobic of PDMS can enhance the antireflective stability by introducing hydrophobic methyl groups into coatings [25,26]. Eventually, the modification of SWNTs and coated with PDMS can enhance the light absorption of silicon solar cells. C.

Photovoltaic Properties

The photovoltaic properties of “Sample 1” to “Sample 3” were tested under standard illumination conditions (AM 1.5, 100 mW∕cm2 ) by a solar simulator (NBeT Solar-500 Xenon light source) and a high precision digital multimeter (Agilent 34410A). The data of the J–V curves of each sample provide the V oc , J sc , fill factor (FF), series resistance (Rs ), and η. Figure 4(a) shows the test system of solar cells and Fig. 4(b) illustrates the J–V curves of solar cells. Here, “Sample 3-x” represents the modification of the mixed solution for x times (x = the amount of mixed solution/0.25). In order to make the data more comparable, “Sample 2” is the same as “Sample 3-0” (with no SWNTs adding process). The repetitive experiments above were done and the typical J–V curves of “Sample 2” and “Sample 3” for 0.25– 1.25 mL mixed solution of SWNTs adding process are shown in Fig. 4(b). The best enhancement of performance was obtained under “Sample 3-2” with the mixed solution of 0.5 mL. The relationships among each sample and V oc ∕J sc ∕η∕Rs with error bars are shown in Fig. 5. The error bars indicate the average and standard deviation of the measurements at different times. The maximum error of V oc, J sc , η, and Rs is 1 October 2014 / Vol. 53, No. 28 / APPLIED OPTICS

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Table 1.

Fig. 4. (a) Test system of solar cells. (b) Typical J–V curves of “Sample 2” and “Sample 3” for 1–5 times the SWNTs adding process.

0.3 V, 0.04 mA∕cm2 , 0.02%, and 0.03 Ω · cm2 , respectively. Table 1 shows the average data of photovoltaic properties for each sample. We can make the following comparisons of these samples and obtain conclusions accordingly: I. The comparison between “Sample 3-2” (modified with SWNTs but no PDMS layer) and “Sample 2” (bare solar cell) shows that the V oc decreased slowly

No. of Sample

V oc (mV)

J sc (mA∕cm2 )

FF (%)

Rs (Ω · cm2 )

η (%)

1 2 3-1 3-2 3-3 3-4 3-5 3-2 (with PDMS)

551.5 550.3 550.3 549.7 549.2 549.1 548.9 551.9

33.80 27.94 28.50 28.55 28.56 28.39 28.07 34.26

53.8 52.1 52.1 53.2 52.1 52.1 51.0 53.3

1.87 2.98 2.92 2.69 2.91 2.92 4.10 2.44

10.02 8.01 8.17 8.36 8.17 8.12 7.86 10.07

from 550.3 to 548.9 mV, and J sc increased from 27.94 to 28.56 mA∕cm2 and then decreased to 28.07 mA∕cm2 , with increased times of SWNT dropping. “Sample 3-2” with the modification of 0.08 g∕L mixed solution for 0.5 mL had a better result and Rs decreased to 2.69 Ω · cm2 and η reached 8.36%. At this point, the increasing rates of η, J sc , and FF were 4.37%, 2.18%, and 2.11%, respectively, V oc and Rs decreased by 0.11% and 9.37%, respectively, which performed better than “Sample 2” without SWNTs. So this comparison proves that the modification of SWNTs can increase the short-circuit current and conversion efficiency of bare solar cells. II. Comparing “Sample 3-2” (modified with a SWNTs and PDMS layer), there is a small increase of V oc from 549.7 to 551.9 mV (↑0.4%), an increase of J sc from 28.55 to 34.26 mA∕cm2 (↑20%), and η increased from 8.36% to 10.07% (↑20.45%). So this comparison proves that coating with PDMS can further enhance the short-circuit current and conversion efficiency due to its AR effect. III. Comparing “Sample 3-2 (with PDMS)” with “Sample 1” (original sample cut from the same piece of solar cell), there is an increase of V oc from 551.5 to 551.9 mV (↑0.07%), an increase of J sc from 33.8 to 34.26 mA∕cm2 (↑1.36%), and η increased from 10.02% to 10.07% (↑0.5%). From the error bar of η, the maximum error is 0.02% and it is lower than 0.05%; the measurement accuracy of the J–V testing instrument is 0.003% and the stability of light intensity is less than 0.4%, so the efficiency of solar cells modified with SWNTs and coated with PDMS is better than traditional solar cells with AR coating. Finally, benefiting from the effect of SWNTs, we obtained a stable solar cell with a relative efficiency increased by 4.37% (improved from 8.01% to 8.36%, compared to the bare solar cells without a Si3 N4 AR layer). Benefiting from the SWNTs and PDMS layer, we obtained a relative efficiency increased by 0.5% (improved from 10.02% to 10.07%, compared to the original solar cells with a Si3 N4 AR layer). D.

Fig. 5. Photovoltaic properties of each sample. (a) V oc , (b) J sc , (c) η, and (d) Rs . 6460

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Photovoltaic Properties of Solar Cells under AM 1.5G, 100 mW∕cm2 Illumination

Theoretical Analysis

1. About the Effect of PDMS The experimental results of “Sample 3-2” (modified with a SWNTs and PDMS layer) and “Sample 3-2” (modified with SWNTs but no PDMS layer) in Fig. 3

and Table 1 show that the solar cell coated with a PDMS layer has lower reflectance and PDMS can further enhance J sc ↑20%, V oc ↑0.4%, and η↑20.45% due to its AR effect. Actually, the PDMS film provides a refractive index gradient to reduce Fresnel reflection loss as an AR layer [21,27]. We can explain the result with the effective medium theory (EMT) [28]. When a PDMS layer is coated on the solar cell, the light path length can be increased due to the scattering of the PDMS film. The refractive index decreased gradually from PDMS to air and the reflectance from air to solar cell can be efficiently reduced, because the PDMS layer prevents reflected wavelengths by using different interfaces to cancel each wavelength partially and alleviates the drastic change in the refractive index between the air and the surface of solar cell. Eventually, the effective refractive index (neff ) of the PDMS layer can be obtained as follows: 1

neff  n2PDMS f  n2air 1 − f 2 ;

(1)

where f is the filling factor of the PDMS layer, nPDMS and nair are refractive indices of PDMS and air, respectively. From the equation we can find that neff is proportional to f , and the value of f varies with the depth of the PDMS layer (from 0 to 1). So PDMS can enhance the effective refractive index and reduce the reflectance of solar cells, thus the values of J sc, V oc , and η have increased by different degrees. 2. About J sc The experimental results of “Sample 3-2” (modified with SWNTs but no PDMS layer) and “Sample 2” (bare solar cell) in Figs. 3–5 and Table 1 show that SWNTs can increase optical absorption, improve J sc ↑2.18%, Rs ↓9.73%, and η↑4.37%. But if the area density of the SWNT film is high, it will bring a negative impact on the solar cell, and it is mainly reflected in the decrease of J sc ↓1.72%, η↓5.98%, and the increase of Rs ↑52.42%. The SWNTs used in the experiment are n-type semiconductor in macroscopic view, tested by Hall effect measurements (bulk carrier concentration: −7.847 × 1019 ∕cm3 ). So the SWNT film is in touch with n-Si and forms n–n homotype heterojunction. A physical model

has been established to interpret the experimental phenomena above. The internal structure diagram of the SWNTs–Si solar cell is shown in Fig. 6(a). It is a fact that either heterojunction or p-n junction can generate photocurrent density J ph under light illumination, and J ph can be approximated by J ph  J sc  qGLn  W  Lp ;

(2)

where q refers to elementary charge, G refers to the generation rate of electron–hole pairs near the p-n junction under illumination, Ln is the diffusion length of the electron, Lp is the diffusion length of the hole, and W refers to the width of space charge region [29]. With the effect of built-in electric field, electrons moved to the area of n-Si and holes moved to the area of the SWNT film near the interface of the heterojunction to form the photocurrent density J ph1 from n-Si to SWNTs. Similarly, electrons moved to the area of n-Si and holes moved to the area of p-Si near the interface of the p-n junction to form the photocurrent density J ph2 from n-Si to p-Si. For the increase of J sc from 27.94 to 28.56 mA∕cm2 with the comparison of “Sample 3-2” (modified with SWNTs but no PDMS layer) and “Sample 2” (bare solar cell), three main reasons are listed: (a) increase optical absorption and generate more electrons and holes, (b) provide more photocarrier transport channels, and (c) modify the recombination of carriers. The fact that SWNTs can increase optical absorption is clearly illustrated in Fig. 3 and according to the relevant literature, SWNTs have two effects on solar cells: one is to be used as a part of heterojunction with n-Si and separate electron–hole pairs by generating a built-in electric field, and the other can be used as transparent electrodes to provide more photocarrier transport channels [14,30]. So points (a) and (b) can be established by the comparing “Sample 3-2” (modified with SWNTs but no PDMS layer) and “Sample 2” (bare solar cell). For the reason (c), the recombination of carriers can be divided into the following types: radiative recombination, Auger recombination, trap recombination, surface recombination, and grain boundary recombination. The first two types of recombination are mainly caused by the energy-band structure of semiconductor, and the others are mainly caused by the defect of solar cells

Fig. 6. (a) Internal structure diagram of the SWNTs–Si solar cell. The heterojunction is formed by the combination of SWNTs and n-Si. (b) Influence of the recombination rate on the solar cell simulated by PC1D. (c) Energy-band diagram of the SWNTs–silicon p-n solar cell. 1 October 2014 / Vol. 53, No. 28 / APPLIED OPTICS

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and releases energy in the form of heat [1,31]. SWNTs are considered to be a good material for improvement of the recombination of solar cells, because they provide large surface area support and also stabilize charge separation by trapping electrons transferred from semiconductor, thereby hindering recombination of carriers. Besides, the strong interaction between SWNTs and semiconductor results in a close contact to form a barrier junction which offers an effective route of reducing electron–hole recombination by improving the injection of electrons into the nanotube [32]. Finally, the consequence of (a), (b), and (c) leads to the increase of the value of G in Eq. (2) and the enhancement of J sc under the optimization value of the modification. For the decrease of J sc from 28.56 to 28.07 mA∕cm2 with the modification of SWNTs from 0.75 to 1.25 mL, more and more photons will be absorbed by SWNTs and mainly dissipated in the form of heat [33] with the increase of the area density of the SWNT film which will lead to decreased light illumination on the p-n junction and an increased recombination rate of carriers. In particular, we used the PC1D software (version 5.9) to simulate the impact of the recombination rate on solar cells. We loaded the typical file of a silicon solar cell (Pvcel.prm) in PC1D and the result is shown in Fig. 6(b). With the increase of the recombination rate (from 103 to 107 m∕s), a decrease of open-circuit voltage and short-circuit current occurred. Besides, it is obvious that J ph1 generated by heterojunction is not transmitted to an external circuit because the junction has no bottom electrode. Consequently, J ph2 will decrease with the increase of the area density of the SWNT film. Thus, if the area density of the SWNT film exceeds the optimization point of the modification, the hererojunction will reverse to weaken the effect of the p-n junction, which results in the decrease of J ph ∕J sc. 3. About V oc The experimental results of “Sample 2” and “Sample 3” in Figs. 4 and 5 and Table 1 show that V oc decreases from 550.3 to 548.9 mV with increasing area density of the SWNT film. The variation of V oc has been further analyzed. Figure 6(c) shows an energy-band diagram of the continuous n–n homotype heterojunction and p-n junction. The Fermi energy level will be split under light illumination. The built-in electric field of the heterojunction ED1 weakens the built-in electric field of the p-n junction ED2 . The photovoltage V ph of the SWNTs–Si p-n solar cell can be approximated by V ph  V oc

1  EFn-Si − EFp-Si  − ΔV; q

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4. Conclusion

To summarize, the modification of SWNTs on the surface of silicon solar cells has been studied. The experimental results as well as theoretical analysis suggested that the heterojunction formed with SWNTs and silicon can enhance transmission of carriers and improve the conversion efficiency of solar cells. Our experiment showed that the modification of 0.08 g∕L mixed solution for 0.5 mL would bring us the relative best conversion efficiency enhancement of 4.37% (increased from 8.01% to 8.36% without the PDMS layer) and 0.5% (increased from 10.02% to 10.07% with the PDMS layer). If the area density of the SWNT film exceeds the optimization point of the modification, the hererojunction will reverse to weaken the effect of the p-n junction, more photons will be absorbed by SWNT film and less will be received by the p-n junction, so the conversion efficiency decreases slowly. Due to the general silicon solar cells that we have used in the experiment, the fabrication accuracy in the laboratory cannot meet the industrial standard and the efficiency of our SWNT–PDMS silicon solar cell does not reach the best value of 18%. The result shows that SWNTmodified silicon solar cells can promote the absorption of light, the transmission of carriers, and Rs when the density of SWNTs is within a certain range. Our future work will focus on further optimization of the process of solar cells, including: (a) the density of the modification of CNTs, (b) improving the adding process to increase the uniformity, and (c) optimizing CNT-based materials (CNTs with single-walled and multi-walled, different functional groups, different length and chirality, etc.). It implicates that CNT materials have potential applications in photovoltaic cells. The work is funded by the National Natural Science Foundation of China (No. 61376121) and the Fundamental Research Funds for the Central Universities (106112013CDJZR 125502, 20003, 20008). References

(3)

where q refers to elementary charge, ΔV refers to the influence of the built-in electric field by heterojunction, and it is proportional to the area density of the SWNT film. Notice that ΔV is not equal to V ph 6462

generated by heterojunction, due to the particular structure of the SWNTs–Si p-n solar cell (the heterojunction has no bottom electrode). On the other hand, despite the fact that SWNTs can modify the recombination of carriers and lead to the increase of V oc, more photons will be absorbed by the SWNT film and less will be received by the p-n junction with the increase of area density of the SWNT film. Therefore, V ph or V oc will decrease slowly with the increase of the area density of the SWNT film.

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1 October 2014 / Vol. 53, No. 28 / APPLIED OPTICS

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Study on modification of single-walled carbon nanotubes on the surface of monocrystalline silicon solar cells.

Modification of single-walled carbon nanotubes (SWNTs) on the surface of monocrystalline silicon solar cells was investigated. The modification was re...
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