Band engineering of a Si quantum dot solar cell by modification of B-doping profile

Solar Energy Materials & Solar Cells 159 (2017) 80–85

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Band engineering of a Si quantum dot solar cell by modification of B-doping profile Gyea Young Kwak a,b, Seong Hyun Lee a, Jong Shik Jang a, Songwoung Hong a,b, Ansoon Kim a,b, Kyung Joong Kim a,b,n a b

Division of Industrial Metrology, Korea Research Institute of Science and Standards, Daejeon, South Korea Department of Nano Science, University of Science and Technology, Daejeon, South Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 2 March 2016 Received in revised form 29 August 2016 Accepted 1 September 2016

A Si quantum dot (QD) heterojunction solar cell with a p-type Si QD layer on an n-type crystalline Si wafer was developed, and the effect of the boron (B)-doping profile in the Si QD layer was investigated. The doping concentration of B in the p-type Si QD layer was optimized at 5.71
1021 atoms/cm3, and high power conversion efficiency (PCE) of 14.15% was achieved at the optimized B concentration. A Si QD solar cell having a Si QD layer with a double-step B-doping profile was suggested to enhance the vertical charge carrier transport in the Si QD layer. As a result of the B-doping profile change from a single-step to double-step, the PCE of the Si QD solar cell was increased from 14.41% to 14.98%. In the Si QD solar cell with a double-step B-doping profile, quantum efficiency analysis showed that the improvement in short circuit current within short wavelength region under 650 nm can be related with the additional built-in E-field caused by the band structure modification. & 2016 Published by Elsevier B.V.

Keywords: Band engineering Si Quantum dot Solar cell Electric Field Doping profile

1. Introduction Various kinds of new technologies and concepts have been proposed to overcome the efficiency limit of a crystalline Si solar cell with a single p-n junction. The power conversion efficiency (PCE) can be improved by effective absorption and utilization of solar light [1–4]. Multiple junction solar cells, intermediate band solar cells and hot carrier solar cells have been suggested as new types of solar cells to overcome the efficiency limit [5–10]. Silicon nanostructures have been studied as new materials to achieve high efficiency solar cells because the band-gap energies of the silicon nanostructures are different from that of bulk crystalline Si due to quantum confinement effect [11–16]. Among them, silicon quantum dot (Si QD) has been proposed as a promising material for the next-generation solar cell because of simple fabrication and easy modification of band-gap energy [17– 20]. The band-gap energy of Si QDs can be well controlled by varying the size of the Si QDs and changing the matrix material such as SiO2, Si3N4 and SiC [13,15,21,22]. An intrinsic drawback of a Si QD solar cell is the high series resistivity of the Si QD absorber layer due to the insulating property of the matrix material [23]. Doping of the Si QD layer can be n Corresponding author at: Division of Industrial Metrology, Korea Research Institute of Science and Standards, Daejeon, South Korea. E-mail address: [email protected] (K.J. Kim).

http://dx.doi.org/10.1016/j.solmat.2016.09.001 0927-0248/& 2016 Published by Elsevier B.V.

an additional source to improve the carrier transport in the Si QD layer by varying the band structure. So far, a-Si:H(n)/a-Si:H(i)/c-Si (p) solar cells having emitters with a gradient doping profile have been studied to improve the PCE of amorphous Si thin film solar cells [24]. However, there have been no enough studies for the effect of the gradient doping profile on the Si QD absorber layer to enhance carrier transport after electron-hole separation. In this study, the doping concentration of boron (B) in a Si QD solar cell with a single-step B doping profile (SS Si QDSC) was optimized at 5.71
1021 atoms/cm3. Furthermore, a Si QD solar cell with a double-step B doping profile (DS Si QDSC) was investigated to modify the energy band in the Si QD layer. As a result of the energy band engineering, the short circuit current (JSC) and open circuit voltage (VOC) were simultaneously improved, and the PCE was increased from 14.41% to 14.98%. An additional built-in electric field formed by energy band bending in the Si QD layer is believed to enhance the tunneling probability which is related with vertical charge carrier transport.

2. Experimental methods B-doped SiOx (SiOx:B) layers were grown on 6″ Si (100) substrates (n-type, 1  3 Ω cm) by ion beam sputtering deposition of a Si wafer using an Ar þ ion beam of 800 eV. The stoichiometry of the SiOx layers was controlled by varying the oxygen partial pressure and analyzed by in-situ x-ray photoelectron spectroscopy

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Fig. 1. SIMS concentration depth profile of a B-doped Si multilayer film grown at various locations of the B chip from the center (Cþ 0) to 5 cm away from the center (Cþ 5) of the sputtering area.

(XPS) [25]. The stoichiometry of the SiOx layers was fixed to SiO1.6 for which the density of the Si QDs is maximized as reported in the previous study [26]. The silicon substrates were rotated during film deposition to ensure the homogeneity in the thickness and the stoichiometry of the films. B doping of the SiOx:B layers was achieved by co-sputtering of Si and B using a combination target in which a boron chip is mounted on a Si wafer. B concentration was analyzed by secondary ion mass spectrometry (SIMS) depth profiling using a Cameca IMS-7F (Cameca, France). The doping concentration of the B-doped Si QD layer was controlled by varying the location of the B chip of the combination target from the center of the sputtering area. As shown in Fig. 1, the B concentration was maximized to 1.35
1022 atoms/cm3 when the B chip was located at the center (C þ 0) of the sputtering area and decreased to 3.06
1020 atoms/cm3 when the B chip was moved 5 cm away from the center (C þ5). The concentration of B was measured from the relative sensitivity factor of B determined from a SIMS depth profile of a B-implanted Si reference material (NIST SRM 2137). The B-doped SiOx films were annealed at 1100 °C for 60 min in a nitrogen atmosphere to form Si QDs. The

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distribution of Si-QDs was investigated by high resolution-transmission electron microscopy (HR-TEM) using a Cs corrected TEM, JEM-2100F (JEOL, Japan). The surface SiO2 layer formed during high temperature annealing was removed by dipping in a buffered oxide etchant (BOE, 50:1). The total thickness of the Si QD layers was controlled to be about 130 nm. Al electrodes of about 700 nm were grown at the front and rear sides by thermal evaporation. After that, the cells were annealed at 425 °C for 40 min in an ambient atmosphere to improve the electrical contact property. The Si QD films grown on 6 in. Si (100) wafers were mechanically diced to small chips of 10 mm
10 mm size and the area of exposed active area was 0.19 cm2. Photovoltaic properties of the solar cells were measured by a solar simulator (McScience K201) under AM 1.5 G illumination with a Xe lamp at room temperature. The PCE (η) of the solar cells was determined from the relation of η ¼JSC  VOC  FF/Pin, where JSC, VOC, FF and Pin are the short-circuit current density (mA/cm2), open-circuit voltage (mV), fill factor (%) and incident power density (100 mW/cm2) at room temperature, respectively. The external quantum efficiency (EQE), reflectance and internal quantum efficiency (IQE) were measured with an Oriel IQE-200 instrument with a scanning spectral range from 350 nm to 1100 nm.

3. Results and discussion In this study, two types of Si quantum dot (QD) heterojunction solar cells with a p-type Si QD layer were fabricated on n-type crystalline Si substrates to investigate the effect of the doping profile on the built-in electric field of the solar cells. Fig. 2 shows the two cases of Si QD solar cells. The first one is a Si QD solar cell with a single-step doping profile of B (SS Si QDSC) where the B doping profile is homogeneous along depth direction. However, the second one is a Si QD solar cell with a double-step B doping profile (DS Si QDSC). It has a non-homogeneous p þ/p configuration where a heavily doped surface Si QD layer with a 20 nm thickness is added onto the optimally doped Si QD absorber layer to improve the PCE. The formation of Si-QDs in the two solar cells was investigated by HR-TEM. As shown in the HR-TEM images, there is no significant difference in the size and density of Si QDs between the SS Si QDSC and the DS Si QDSC. As a result, the formation of Si QDs in SiO2 matrix may not be affected by the concentration of boron. Fig. 3 shows the SIMS depth profiles of the Si

Fig. 2. Schematic diagrams and cross-section HR-TEM images of (a) the SS Si QDSC and (b) the DS Si QDSC. (Si QD-1 and Si QD-2 represent the highly B-doped layer and the optimally B-doped layer in the DS Si QDSC, respectively).

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10

DS Si QD layer (as-dep)

6

SIMS Intensity (cps)

10

5

10

SS Si QD layer (as-dep) 4

10

3

10

2

10

1

10

0

100

200

300

400

500

600

700

800

Sputter Time (sec) Fig. 3. SIMS concentration depth profiles of B in the as-deposited SS Si QD layer and DS Si QD layer.

QD layers in the two cells. In the DS Si QDSC, the B concentration in the heavily doped layer is higher than that of the optimally doped layer. A spacer layer was inserted between the two Si QD layers in the DS Si QDSC to reduce boron diffusion from the heavily doped layer to the optimally doped Si QD layer during high temperature annealing. A sandwich structure of SiO1.2:B(2 nm)/SiO2(2 nm)/SiO1.2: B(2 nm) was selected as the spacer layer from our previous results [26,27]. Here, the role of the central SiO2 (2 nm) layer is to reduce the boron diffusion and that of SiO1.2 (2 nm) layer is to maintain the average stoichiometry to x ¼1.6. The density of the Si QDs is maximized in the SiO2/SiO1.2 multilayer [26]. Doping and activation of the dopants are essential for the performance of electronic devices. For effective activation of a B doped Si QD layer, the doped B atoms should be substituted to the four-fold coordinated crystalline Si (Siact) sites inside the Si QDs instead of the three-fold coordinated Si with dangling bond (SiDB) sites at the surface of the Si QDs [19]. Although most of the Si sites in the single crystalline bulk Si are Siact sites, the Si QD layer is composed of SiDB sites and Siact sites. In the Si QD layer, the ratio of the SiDB sites to the Siact sites increases as the size of the Si QDs decreases due to the increase in the surface-to-volume ratio. Substitution of B in a SiDB site is energetically preferable to that in a Siact site. This means that B atoms prefer substitution to a surface SiDB site instead of a Siact site inside the Si QDs. As a result, a much higher doping concentration of B is required for the effective activation in electronic devices using Si QDs than in conventional devices using bulk Si substrate. The B concentrations of 1.1
1020 atoms/cm3 and 3
1020 atoms/cm3 were reported as the minimum concentration for the activation of the Si nanostructures [19,28]. In this study, the doping concentration of the Si QD absorber layer was optimized in the high concentration range from 4.90
1020 atoms/cm3 to 1.35
1022 atoms/cm3 to improve the PCE of a Si QD solar cell with a single-step B doping profile (SS Si QDSC). Fig. 4 shows the average photovoltaic performance parameters of the SS Si QDSCs as a function of doping concentration of the Si QD layer. As the doping concentration increases, the opencircuit voltage (VOC) monotonically increases owing to the increment of the built-in potential, and a maximum VOC value was obtained at 5.71
1021 atoms/cm3.

Fig. 4. Average device values with the standard deviation for the VOC, JSC, FF, and PCE of the Si QD solar cells as a function of doping concentration of B, where the red squares are the average values of the three devices.

The relation between VOC and the doping concentration can be estimated from the following equations [29].

ρ=NA = NV e (EV − EF ) / kT

(1)

EV − EF = kT ln(NA/NV )

(2)

where ρ is the hole density in the valence band, NA is the density of acceptors, NV is the effective density of states in the valence band, k is the Boltzmann constant, T is kelvin, EV is the valence band

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Table 1 Average device parameters of the SS Si QDSCs as a function of B-doping concentration. The standard deviation was calculated by taking the data of three devices in each B-doping concentration. Samples

Doping concentration (atoms/cm3)

VOC (mV)

JSC (mA/cm2)

FF (%)

PCE (%)

1 2 3 4 5 6 7

4.90
1020 6.30
1020 1.12
1021 2.03
1021 3.47
1021 5.71
1021 1.35
1022

503.53 713.70 519.74 72.08 527.40 7 3.62 536.59 73.78 540.487 2.45 544.92 7 2.95 537.44 71.45

25.42 7 1.44 30.187 0.72 35.06 7 0.16 34.127 0.39 34.737 0.44 35.187 0.56 30.65 7 0.57

25.99 7 4.40 37.94 7 0.96 42.517 0.68 55.377 1.42 72.137 2.23 74.69 71.53 74.80 71.52

3.30 7 0.29 5.95 7 0.01 7.86 70.12 10.147 0.29 13.54 7 0.26 14.317 0.12 12. 327 0.46

40 SD-1 DD-1

35 30 25 2

Jsc (mA/cm )

energy level and EF is the Fermi level. The Fermi level splitting (EV-EF) will be increased by the increase of B concentration (NA) in Eq. (2). It will result in the increase of builtin potential (VOC) as shown in Fig. 4. The fill factor (FF) also shows a similar trend with the VOC. As a result, an optimal PCE of 14.31% (VOC ¼544.92 mV, JSC ¼35.18 mA/ cm2, FF¼74.69%) was achieved at a boron concentration of 5.71
1021 atoms/cm3 as shown in Table 1. At the doping concentration above 5.71
1021 atoms/cm3, although the fill factor (FF) is not changed, the VOC and JSC are decreased, which leads to a considerable decrease in the PCE. This decrease in the PCE is estimated to be highly related with the significant decrease in the JSC due to Auger recombination at the surface region of the SS Si QDSC. Accordingly, the minority carrier lifetime will be decreased with an increasing doping concentration by Auger recombination [30,31]. To improve the electrical properties, a Si QD solar cell with a double-step doping profile in the Si QD absorber layer (DS Si QDSC) was fabricated. The DS Si QDSC consisted of a heavily doped Si QD surface layer (1.35
1022 atoms/cm3) with a 20 nm thickness, a spacer layer of 6 nm and an optimally doped Si QD bottom layer (5.71
1021 atoms/cm3) of 104 nm as shown in Fig. 2(b). The average electrical parameters of the SS Si QDSC and the DS Si QDSC are shown in Table 2. The values of VOC, JSC and FF for the SS Si QDSC (SD) are 544.20 mV, 34.86 mA/cm2, and 75.24% and those of DS Si QDSC (DD) are 551.67 mV, 35.04 mA/cm2, and 76.58%, respectively. The PCE is increased from 14.28% to 14.80% by the increases in JSC and VOC in DD. Fig. 5 shows the J-V curves of the best SS Si QDSC (SD-1) and the best DS Si QDSC (DD-1). The PCE was increased from 14.41% (SD-1) to 14.98% (DD-1). This indicates that an additional built-in electric field (E-field) formed by the gradient doping profile (double-step B doping) contributes to the enhancement of VOC. In addition, the carrier transport in the Si-QD layer can be also improved by the additional E-field, which will result in the increase of JSC. Fig. 6(a) shows the external quantum efficiency (EQE) results of the SS Si QDSC (SD-1) and the DS Si QDSC (DD-1). In the short wavelength region (300  650 nm), the EQE values of DD-1 are higher than those of SD-1. The improvement of the EQE value in the short wavelength range can be related with in the energy band diagrams of the two different solar cells. The Si QD layers used in this study can be supposed to be a superlattice structure of a Si QD layer (2 nm) and a SiO2 layer (2 nm). The band gap of SiO2 is about 9 eV and that of the Si QDs in SiO2 matrix is known to be about 3.17 eV [23,32]. The experimental band gap of the Si QDs formed from a SiO1.6 layer is about 1.5 eV [26]. From these information, the band diagram can be

20 15 10

2

SD-1 34.53 mA/cm , 545.78 mV 2 DD-1 35.54 mA/cm , 552.87 mV

5 0

0

100

200

300 400 Voltage(mV)

500

600

Fig. 5. One sun AM 1.5 G illuminated J-V curves for the best cells of the SS Si QDSC (SD-1) and the DS Si QDSC (DD-1).

simply designed for the SS Si QDSC as shown in Fig. 7(a). In the case of DS Si QDSC, an additional built-in E-field will be induced from the Fermi level shift between the different doping regions as shown in Fig. 7(b). As a result, the vertical transport of the photogenerated carriers in the Si QD layer and the n-Si substrate will be improved by the additional built-in E-field and result in the increase of current density. As shown in Fig. 6(b), the internal quantum efficiency (IQE) value for the SS Si QDSC (SD-1) is lower than that for the DS Si QDSC (DD-1), which is similar with the EQE results. On the other hand, the reflectance results of the two solar cells are very similar as shown in Fig. 6(b). It is noteworthy that the increased current density in the short wavelength range may not come from the optical absorption enhancement but from the improved charge carrier transport in the Si QD layer because the IQE value increases in the short wavelength (300–650 nm) range while the change in reflectance is very small in the same wavelength region. In this study, the improvement in the power conversion efficiency (PCE) of Si QD solar cells was achieved by applying the double-step doping profile of B in the Si QD layer. For the additional improvement in the PCE of Si QD solar cells, band-gap energy tuning and doping profile modification of the Si QD layer can be applied together. It is well known that the band-gap energy of a Si QD layer can be controlled by varying the size of the Si QDs and

Table 2 Average device parameters of the SS Si QDSC (SD) and the DS Si QDSC (DD). The standard deviation was calculated from five devices. Sample

VOC (mV)

JSC (mA/cm2)

FF (%)

PCE (%)

Rsh (Ω)

Rs (Ω)

SD DD

544.20 7 2.83 551.677 2.46

34.86 7 0.64 35.04 7 0.39

75.277 1.36 76.58 7 0.59

14.28 7 0.10 14.80 7 0.12

12,554 7 6014 27,1347 10723

9.59 7 0.44 8.917 0.30

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1.0

changing the matrix material such as SiO2, Si3N4 and SiC [13,15,22]. The increase in the band-gap energy of the top area of the Si QD layer will result in the increase in PCE by effective absorption of solar light. The PCE of Si QD solar cells will be further improved by improved carrier transport due to the increased builtin E-field by stepwise doping profile of the Si QD layer.

(a)

EQE (a.u.)

0.8

0.6 4. Conclusion

0.4 EQE:SD-1 EQE:DD-1

0.2

0.0 300

400

500

600

700

800

900

1000 1100

Wavelength (nm)

1.0

(b)

IQE & R (a.u.)

0.8

0.6

0.4

IQE:SD-1 IQE:DD-1 R:SD-1 R:DD-1

0.2

A heterojunction Si QD solar cell with a single-step doping profile of B in the Si QD layer (SS Si QDSC) was fabricated, and the doping concentration of B in the Si QD layer was optimized. The maximum PCE of the SS Si QDSC was achieved at the optimized B concentration of 5.71
1021 atoms/cm3. A Si QD solar cell with a double-step doping profile of B (DS Si QDSC) was developed for the improvement of electrical property. As a result of the doping profile modification, the photovoltaic efficiency was increased from 14.41% (SS Si QDSC) to 14.98% (DS Si QDSC). This can be related with the band bending at the p-type Si QD layer due to the highly doped surface layer, which promotes the separation of the carriers and results in the increase in the JSC of 1 mA/cm2. The PCE of the Si QD solar cells developed in this study is still lower than that of the commercialized single crystalline Si solar cells because our devices were fabricated with simple structures without surface texturing, anti-reflection coating, surface passivation and back surface field. If these additional processes are applied, the PCE of the Si QD solar cell will exceed those of the commercialized single crystalline Si solar cells.

Acknowledgments

0.0 300

400

500

600

700

800

900

1000 1100

Wavelength (nm) Fig. 6. (a) External quantum efficiency (EQE), (b) internal quantum efficiency (IQE) and reflectance (R) of the SS Si QDSC (SD-1) and the DS Si QDSC (DD-1).

We would like to acknowledge the financial support from the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) / ISTK (Korea Research Council for Industrial Science and Technology) of Republic of Korea (Grant B551179-1201-00).

References

Fig. 7. Schematic energy band diagrams of the SS Si QDSC (SD-1) and the DS Si QDSC (DD-1).

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