Improved performance of a-Si:H solar cell by using up-conversion phosphors

Journal of Alloys and Compounds 658 (2016) 848e853

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Improved performance of a-Si:H solar cell by using up-conversion phosphors Boyang Qu a, Yuechao Jiao a, *, Shangwen He b, Yongsheng Zhu a, Ping Liu a, Jun Sun a, Jingxiao Lu c, Xiaodan Zhang d a

School of Electronic and Information Engineering, ZhongYuan University of Technology, Zhengzhou 450007, China School of Mechanics & Engineering Science, Zhengzhou University, Zhengzhou 450052, China Key Laboratory of Materials Physics of Ministry of Education, Zhengzhou University, Zhengzhou 450052, China d Institute of Photo-electronic Thin Film Devices and Technology of Nankai University, Tianjin 300071, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 July 2015 Received in revised form 3 November 2015 Accepted 3 November 2015 Available online 6 November 2015

Up-conversion is a promising technique for harvesting sub-band-gap photons in photovoltaic devices. The implementation and characterization of rare-earth doped up-converters on a-Si:H solar cells are investigated. Three up-conversion phosphors, b-NaYF4:Yb3þ 20%/Er3þ 1%, b-NaYF4:Yb3þ 25%/Ho3þ 1% and b-NaYF4:Yb3þ 60%/Tm3þ 0.5%, are developed and employed as low energy convert materials for a-Si:H solar cells. Two different ways of adhering the up-converter to the a-Si:H solar cell have been implemented: by dissolving the powder in the polydimethylsiloxane (PDMS) and by compressing it into solid slice with a tablet pressing machine. The performance of a-Si:H solar cells integrating the up-converters is investigated under both 980 nm infrared laser diode and simulated solar light source (AM1.5G) illumination. The Current-Voltage measurements reveal that the b-NaYF4:Yb3þ 20%/Er3þ 1% up-conversion phosphor performs both up-conversion and scattering functions in improving the overall performance of a-Si:H solar cell. © 2015 Elsevier B.V. All rights reserved.

Keywords: Up-conversion A-Si:H solar cell b-NaYF4:Yb3þ/Ln3þ (Ln3þ ¼ Er3þ, Ho3þ,Tm3þ)

1. Introduction Transmission of the sub-band-gap light is one of the major loss mechanisms in conventional solar cells. Up-conversion (UC) is a promising approach for reducing transmission losses by converting the sub-band infrared photons to higher energy photons which can subsequently be directed to the solar cell with the help of an optical reflector located behind the solar cell and absorbed in the active layer [1,2]. The introduction of the up-converter containing UC phosphors allows the spectral response of the solar cell to be broadened, which has been proven to be an efficient way to harvest the near infrared light for solar cells [3,4]. Among the various UC phosphors, Yb3þ/Er3þ co-doped hexagonal (b) NaYF4 is regarded as an excellent green emission UC phosphor due to its near-perfect resonance between the specific crystal-field levels of Yb3þ and Er3þ ions [5e7]. Recently, NaYF4:Yb/Er (20/2 mol%) UC nanophosphors were employed as up-converters combined with organic solar cell and a short circuit current density enhancement

* Corresponding author. E-mail address: [email protected] (Y. Jiao). http://dx.doi.org/10.1016/j.jallcom.2015.11.024 0925-8388/© 2015 Elsevier B.V. All rights reserved.

of approximately 1.6% for the UC device was demonstrated under one sun irradiation [3]. UC phosphor NaYF4:Yb3þ, Er3þ doped TiO2 nanocrystalline films were used as electrode for dye-sensitized solar cells to increase the photoelectric conversion efficiency from 6.71% to 7.65% under AM1.5G illumination [4]. In addition, some multifunctional composites UC materials were synthesized for application in solar cells [8e10]. The coreeshell structural NaYF4:Yb3þ/Er3þ@SiO2 UC materials and Yb3þ/Er3þ/ Tm3þ co-doped b-NaYF4 UC nanoprisms were introduced into different solar cells respectively, the performances of solar cells were all obviously enhanced. Up to now, the implementation and characterization of the up-converters containing b-NaYF4:Yb3þ/ Er3þ UC phosphor on the rear of a-Si:H solar cells have been reported by several researchers [11e13]. However, no significant improvement has been reported due to the lower luminescent efficiency of existing b-NaYF4:Yb3þ/Er3þ UC phosphors. Therefore, more efforts need to be spent on designing and developing high luminescent efficiency b-NaYF4:Yb3þ/Er3þ UC phosphor. In this paper, three UC phosphors, such as b-NaYF4:Yb3þ 20%/ 3þ Er 1%, b-NaYF4:Yb3þ 25%/Ho3þ 1% and b-NaYF4:Yb3þ 60%/Tm3þ 0.5%, are synthesized via a facile hydrothermal method on the basis

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of our previous work [14,15]. The as-prepared UC phosphors are powders. In order to accomplish the integration of UC phosphors into the a-Si:H solar cell conveniently, the polydimethylsiloxane (PDMS) that acts as an intermediate carrier is adopted and the upconverter is fabricated using solidified mixture of PDMS and UC phosphor. On the other hand, the UC solid slice, as another upconverter form, is prepared to integrate into the a-Si:H solar cells. Our objective is to verify which up-converter can make the greatest contribution to the performance improvement of the solar cells. The influences of different up-converters on the performance of aSi:H solar cells under both 980 nm laser and AM1.5G illumination are investigated and compared. The un-doped b-NaYF4 material and b-NaYF4:Yb3þ/Er3þ UC phosphor are applied to the a-Si:H solar cells to further elaborate these influences. 2. Material and methods The b-NaYF4:Yb3þ 20%/Er3þ 1%, b-NaYF4:Yb3þ 25%/Ho3þ 1% and b-NaYF4:Yb3þ 60%/Tm3þ 0.5% UC phosphors were synthesized according to the procedure described in [14], which consist of Y3þ 79%, Yb3þ 20%, Er3þ 1% (molar ratio Y:Yb:Er ¼ 79:20:1), Y3þ 74%, Yb3þ 25%, Ho3þ 1% (molar ratio Y:Yb:Ho ¼ 74:25:1), and Y3þ 39.5%, Yb3þ 60%, Tm3þ 0.5% (molar ratio Y:Yb:Tm ¼ 39.5:60:0.5), respectively. For powder materials, it was necessary to process them into flaky up-converters by shaping process. The polydimethylsiloxane (PDMS, Sylgard184, it is usually under liquid state and will become a kind of elastomer after heated) is adopted as an intermediate carrier [11e13]. The as-prepared UC phosphor, such as bNaYF4:Yb3þ 20%/Er3þ 1%, was dissolved in PDMS with magnetic stirring for about 1 h to assure a homogeneous distribution. Then the mixture was introduced into a culture dish and dispersed uniformly. After that, the culture dish was solidified in a vacuumdryingoven at 90  C for 30 min. The as-prepared UC colloid slice was cut into an appropriate size for further using. In the second method, the pre-weighed UC phosphor was compressed into solid slice which was directly applied to a-Si:H solar cells as an upconverter. The process flow diagram of the integration of an upconverter into a a-Si:H solar cell is shown in Fig. 1. It was worthy to note that a group of Al gird lines are deposited on ZnO as the back electrode to collect and elicit the photocurrent. The shape of a-Si:H solar cells (1.2 cm  1.2 cm) used in experiments is shown in Fig. 2 (Provided by Nankai University, China). Fig. 3(a) shows the schematic structure of a p-i-n type a-Si:H solar cell with an up-converter under 980 nm infrared laser illumination with 300 mW power. An electrometer/high resistance meter (KEITHLEY-6517A, USA) was used to measure the photocurrent. It should be stressed that the laser can only irradiate along the direction of n-i-p through the a-Si:H solar cell and reach to the upconverter, because of the limitation of the experimental platform. While Fig. 3(b) shows the typical schematic structure of a p-i-n type

Fig. 1. Process flow of the integration of up-converters into a-Si:H solar cells.

Fig. 2. The shape of a-Si:H solar cell (1.2 cm  1.2 cm).

a-Si:H solar cell with an up-converter under AM1.5G illumination [11,12]. To investigate the influences of UC phosphors on performance of a-Si:H solar cells, spectral response and currentevoltage measurements are performed. 3. Results and discussion 3.1. Emission spectra The mechanisms for the frequency UC of b-NaYF4:Yb3þ/ (Ln3þ ¼ Er3þ, Ho3þ, Tm3þ) have been well established [16e18]. Fig. 4 shows the UC emission spectra of three UC phosphors samples excited by a 980 nm infrared laser with a pumping power of about 40 mW. Fig. 4(a) indicates that there are three emission bands existing in the b-NaYF4:Yb3þ 20%/Er3þ 1% UC phosphors sample. Green emission bands of approximately 518 nm and 536 nm originate from the 2H11/2 / 4I15/2 and 4S3/2 / 4I15/2 transitions of Er3þ ions, respectively. While a red emission band of approximately 650 nm originates from the 4F9/2 / 4I15/2 transitions of Er3þ ions [19e21]. Similarly, Green emission band of approximately 540 nm originates from the (5F4, 5S2) / 4I8 transitions of Ho3þ ions. Red (~644 nm) and near infrared (NIR) (~747 nm) emission bands originate from the 5F5 / 5I8 and 5I4 / 5I8 transitions of Ho3þ ions, respectively (Fig. 4(b)) [22,23]. As shown in Fig. 4(c), the b-NaYF4:Yb3þ 60%/Tm3þ 0.5% UC phosphor sample mainly has two emission bands: Blue (~473 nm) and NIR (~800 nm) emission bands which originate from the 1G4 / 3H6 and 3H4 / 3H6 transitions of Tm3þ ions respectively [24]. 3.2. Laser illumination The thicknesses of the as-prepared UC solid slices and UC colloid slices were fundamentally the same. When they were placed between the reflector and the solar cell (shown in Fig. 3(a)), the solar cell could rise to almost the same height. Only in this way could we keep the distance between the laser and the solar cell constant. The main purpose is to assure that the laser illuminates on the same position of solar cell and the power exciting the UC phosphors is almost the same. Before the laser was switched on, the current reading was Ia that indicated the dark current without lighting. When the power was adjusted to 300 mW, the photocurrent generated by the a-Si:H solar cell was Ib. When the up-converter was placed at the front side of a-Si:H solar cell, the current reading was Ic. So the photocurrent generated by the a-Si:H solar cell due to the introduction of up-converter was DI ¼ Ic e Ib.

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Fig. 3. Schematic structure of p-i-n type a-Si:H solar cells with up-converters under two light sources illumination (a) 980 nm infrared laser (300 mW), (b) AM1.5G, 25  C.

The experimental results listed in Table 1 indicate that the bNaYF4:Yb3þ 20%/Er3þ 1% UC solid slice makes the greatest contribution to enhancing the photocurrent of solar cell and the increasement in photocurrent in the NIR range is 142.8 mA. Similarly, for the b-NaYF4:Yb3þ 20%/Er3þ 1% UC colloid slice, a photocurrent of 72.9 mA is gained, which is better than the other two UC

colloid slices. Among these UC phosphors, the b-NaYF4:Yb3þ 20%/ Er3þ 1% UC phosphor manifests outstanding luminescent properties attributing to the higher energy transfer efficiency between Yb3þ and Er3þ ions. During the experiments, it was observed with the naked eyes that the relative UC photoluminescence intensity of b-NaYF4:Yb3þ 20%/Er3þ 1% UC solid slice excited by a 980 nm laser was stronger than the other two. This experimental phenomenon demonstrated that the UC phosphors with higher luminescent efficiency were in favor of improving the performance of solar cell. In addition, the effect of UC solid slice on enhancing the photocurrent of solar cell was better than that of UC colloid slice for the same UC phosphor. The main reason was that the organosilicone in UC colloid slice could absorb partially visible light emitted by UC phosphor and lead to the lower UC photoluminescence intensity. 3.3. AM1.5G illumination

Fig. 4. UC emission spectra of samples (a) b-NaYF4:Yb3þ 20%/Er3þ 1%, (b) bNaYF4:Yb3þ 25%/Ho3þ 1%, (c) b-NaYF4:Yb3þ 60%/Tm3þ 0.5% (980 nm, 40 mW).

Under AM1.5G illumination, the performance parameters of solar cell, such as open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and photoelectric conversion efficiency (h), were obtained by measuring the JeV curves of solar cells. The JeV curves of a-Si:H solar cell with and without up-converters are shown in Fig. 5eFig. 7, respectively. The performance-related parameters of solar cell are listed in Table 2. To illustrate the effect of up-converter on the performance of solar cell, the b-NaYF4:Yb3þ 20%/Er3þ 1% UC phosphor was taken as an example (Fig. 5and Table 2). As a result of the introduction of up-converters, the Jsc were increased by 0.887 mA/cm2 (UC colloid slice) and 2.211 mA/ cm2 (UC solid slice), respectively. Meanwhile, the h were improved by 0.183% (UC colloid slice) and 0.352% (UC solid slice), respectively. The effect of UC solid slice on enhancing the performance of solar cell was still better than that of UC colloid slice. Besides, the Voc increased slightly. In theory, the introduction of up-converter could not change the basic structure of p-i-n a-Si:H solar cell and the Voc should remain unchanged. It is well known that the Voc of a-Si:H solar cell is determined by the width of the band-gap, the quality of the intrinsic layer, and the performance of the doped layers. Nevertheless, we observed a slight variation of Voc and the reasons were further studied. By contrast, it was also found that the Jsc of a-Si:H solar cell were improved by DJsc (1) ¼ 27.655e25.444 ¼ 2.211 mA/cm2 (NaYF4:Yb3þ 20%/Er3þ 1% solid slice), DJsc (2) ¼ 27.216e25.109 ¼ 2.109 mA/cm2 (NaYF4:Yb3þ 25%/Ho3þ 1% solid slice) and DJsc

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Table 1 Improvements in the photocurrent of a-Si:H solar cell integrated with UC phosphors under 980 nm laser illumination. UC phosphor

No.

1. NaYF4:Yb3þ 20%/Er3þ 1%

1.1 1.2 2.1 2.2 3.1 3.2

2. NaYF4:Yb3þ 25%/Ho3þ 1% 3. NaYF4:Yb3þ 60%/Tm3þ 0.5%

solid slice colloid slice solid slice colloid slice solid slice colloid slice

Ia (mA)

Ib (mA)

Ic (mA)

D I (mA)

8.4 8.3 8.3 8.4 8.5 8.4

29.5 29.4 29.1 29.3 30.3 30.1

172.3 102.3 95.3 52.3 63.2 43.3

D I1 ¼ Ic e Ib ¼ 142.8 D I2 ¼ Ic e Ib ¼ 72.9 D I3 ¼ Ic e Ib ¼ 66.2 D I4 ¼ Ic e Ib ¼ 23.3 D I5 ¼ Ic e Ib ¼ 32.9 D I6 ¼ Ic e Ib ¼ 13.2

NaYF4:Yb3þ 20%/Er3þ 1% UC solid slice makes the greatest contribution to enhancing the photocurrent of solar cell. However, it didn't exhibit any preponderance under AM1.5G illumination. In term of above-mentioned experimental results, it is speculated that there are two possible reasons for the performance improvements of solar cell under AM1.5G illumination. On the one hand, the upconverter absorbs infrared light and emits visible light which is absorbed by solar cell again. On the other hand, the up-converter possibly scatters part of the visible light which penetrates the solar cell. When the scattered visible light is absorbed by the solar cell again, it can also contribute to the performance improvements. As a consequence, it is difficult to judge which portion dominates in the performance improvements according to the above-mentioned experimental results. 3.4. In-depth analysis

Fig. 5. JeV curves of reference a-Si:H solar cell (a) without UC, (b) UC colloid slice, (c) UC solid slice. (NaYF4:Yb3þ 20%/Er3þ 1%).

2 3þ 60%/Tm3þ 0.5% (3) ¼ 27.374e25.089 ¼ 2.285 mA/cm (NaYF4:Yb solid slice), respectively. Due to the light-induced degradation of aSi:H solar cells, a slight variation of its performance will happen in the experiments. Besides, the stability of simulator light source and compactness of electrodes contact between solar cells and electrometer/high resistance meter also can cause the experimental errors. Excluding these unfavorable experimental conditions, we considered that the improvements in photocurrent for three UC phosphors were almost the same. The experimental results described in Part 3.2 have clearly demonstrated that the b-

Fig. 6. JeV curves of reference a-Si:H solar cell (a) without UC, (b) UC colloid slice, (c) UC solid slice. (NaYF4:Yb3þ 25%/Ho3þ 1%).

In order to further elucidate our viewpoints, the transmission spectrum of a-Si:H solar cell used in the experiment was measured. As shown in Fig. 8, a part of the visible light with wavelengths in the range of 500e800 nm penetrates the a-Si:H solar cell. When scattered by the interface of the up-converter, it will be absorbed again and the performance parameters of solar cell also change. To illustrate which portion dominates the performance improvements, the b-NaYF4:Yb3þ 20%/Er3þ 1% and un-doped b-NaYF4 solid slice were applied to the a-Si:H solar cell. The JeV curve of reference solar cell is shown as the black line (Fig. 9). While the JeV curves of a-Si:H solar cell with b-NaYF4:Yb3þ 20%/Er3þ 1% and with un-doped b-NaYF4 solid slices are shown as the red and blue lines, respectively. Table 3 lists the performance-related parameters of the solar cell. As can be seen, the Jsc and h of the solar cell with bNaYF4 are higher than those of the reference solar cell and lower

Fig. 7. JeV curves of reference a-Si:H solar cell (a) without UC, (b) UC colloid slice, (c) UC solid slice. (NaYF4:Yb3þ 60%/Tm3þ 0.5%).

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Table 2 The detailed performance parameters of a-Si:H solar cell under AM1.5G illumination. UC phosphor

No.

Voc (V)

Jsc (mA/cm2)

FF (%)

h (%)

1. NaYF4:Yb3þ 20%/Er3þ 1%

1.(a) reference solar cell 1.(b) reference solar cell þ UC colloid slice 1.(c) reference solar cell þ UC solid slice 2.(a) reference solar cell 2.(b) reference solar cell þ UC colloid slice 2.(c) reference solar cell þ UC solid slice 3.(a) reference solar cell 3.(b) reference solar cell þ UC colloid slice 3.(c) reference solar cell þ UC solid slice

0.765 0.772 0.773 0.760 0.768 0.769 0.757 0.765 0.767

25.444 26.331 27.655 25.109 26.314 27.216 25.089 26.122 27.374

36.657 36.457 32.914 33.556 33.244 33.058 33.687 33.343 32.858

4.677 4.860 5.029 4.574 4.796 4.933 4.569 4.758 4.927

2. NaYF4:Yb3þ 25%/Ho3þ 1%

3. NaYF4:Yb3þ 60%/Tm3þ 0.5%

Fig. 8. Transmission spectrum of a-Si:H solar cell used in experiments.

Fig. 9. JeV curves of reference solar cell (a) without UC, (b) with NaYF4 solid slice, (c) with NaYF4:Yb3þ 20%/Er3þ 1% solid slice.

than those of solar cell with b-NaYF4:Yb3þ 20%/Er3þ 1% UC phosphor. The experimental results demonstrates that b-NaYF4 and bNaYF4:Yb3þ 20%/Er3þ 1% both can improve the performance of aSi:H solar cell. It has been reported that NaYF4 NPs can act as effective scattering centers in the range of 400 nm to the visible region, indicating the extension of light path-length within the electrodes and therefore the increase of light harvesting [25,26]. Therefore, the un-doped b-NaYF4 particles act as light scattering centers and also contribute to enhancing the performance of the aSi:H solar cell in our experiments. As a result, the current density is increased from 16.78 to 18.29 mA/cm2 (DJsc ¼ 1.51 mA/cm2) and the overall conversion efficiency is improved by 0.45%. When Yb3þ and Er3þ ions were introduced, a slight enhancement in Jsc and h was observed. The Jsc is increased from 18.29 to 18.52 mA/cm2 (DJsc ¼ 0.23 mA/cm2), while the h is improved by 0.055%. The improved absorption in the NIR region can be attributed to the UC effect of Yb3þ and Er3þ ions. Hence, we can conclude that the scattering effect dominates the performance improvements in aSi:H solar cell. The experimental results support to use UC phosphors to solar cells. As can be seen from the Fig. 10, the b-NaYF4:Yb3þ 20%/Er3þ 1% UC phosphor has a strong absorption band in 873e1056 nm

Fig. 10. Diffuse reflection spectrum of (a) un-doped b-NaYF4, (b) b-NaYF4:Yb3þ 20%/ Er3þ 1%.

Table 3 The detailed performance parameters of a-Si:H solar cell under AM1.5G illumination. No.

Voc (V)

Jsc (mA/cm2)

FF (%)

h (%)

a. reference solar cell b. reference solar cell þ NaYF4 slice c. reference solar cell þ with NaYF4:Yb3þ/Er3þ slice

0.863 0.867 0.864

16.78 18.29 18.52

38.41 37.28 37.28

5.561 5.911 5.966

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compared to the un-doped b-NaYF4, which originates from the 2F7/ 2 3þ ions) and 4I15/2 / 4I13/2 (Er3þ ions) energy level 2 / F5/2 (Yb transitions. A weaker absorption band of approximately 1523 nm originates from the 4I15/2 / 4F13/2 energy level transitions of Er3þ ions [27,28]. The NIR photons absorbed by Yb3þ and Er3þ ions can be transformed into visible photons and absorbed by solar cell, which will enhance the Jsc and h. However, the energy of these visible photons is low, because the absorption bands of Yb3þ and Er3þ ions are narrow and only a small portion of NIR in the solar spectrum can be converted to visible region. On the other hand, a typical two-photon process is involved in the energy-transfer UC (ETU) between Yb3þ and Er3þ ions and the overall luminescent efficiency of as-prepared b-NaYF4:Yb3þ 20%/Er3þ 1% UC phosphor is still low, which limits the performance improvements in the photovoltaic device. Hence, making good use of the infrared spectrum region remains a challenging journey.

4. Conclusions In summary, we have investigated three types of UC phosphors in a-Si:H solar cells for enhanced NIR light harvesting. The performance of the solar cells employing UC phosphors were found to be enhanced as compared to that of the solar cells without UC phosphors under both 980 nm laser and AM1.5G illumination. The properties of solar cell incorporating b-NaYF4:Yb3þ 20%/Er3þ 1% are much better, which is attributed to the higher energy transfer efficiency between Yb3þ and Er3þ ions. In addition, the scattering and UC effects of b-NaYF4:Yb3þ 20%/Er3þ 1% contribute to the enhanced absorption in the visible and NIR region. The experimental results provide evidence for the usefulness of applying UC phosphors to photovoltaic device.

Acknowledgments This work was supported by the National Basic Research Program of China (Grant No. 2011CBA00706), the National Natural Science Foundation of China (Grant No. 61305080, 60807001).

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