Supporting Information Dopant-Free All-Back-Contact Si Nanohole ...
Supporting Information Dopant-Free All-Back-Contact Si Nanohole Solar Cells using MoOx and LiF Films
Han-Don Um, Namwoo Kim, Kangmin Lee, Inchan Hwang, Ji Hoon Seo, and Kwanyong Seo*
Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, Korea
* Corresponding authors. E-mail: [email protected]
S1
EXPERIMENTALS
Fabrication of planar solar cells with front-back contact To characterize the MoOx/n-Si junction, the planar junction Si solar cells were fabricated from Czochralski (CZ) n-type Si wafers (resistivity of 13 cm, 350-m thick). The heavily-doped region (n+) was formed by phosphorus diffusion via the spin-on-dopant (P509, Filmtronics) method at the front of the solar cell for an ohmic contact between Si and front electrode. The diffusion doping was carried out in a tube furnace under a mixed ambient of 20 % O2 and 80 % N2 at 900 C. The sheet resistance of the n+-Si region was 130150 /sq. After removing phosphorus glass and SiOx layer, a thin SiNx layer (60-nm-thick) was deposited by PE-CVD (PEH-600, SORONA) onto the heavily-doped (n+) Si. For the top contacts with the grid design, SiNx-deposited Si substrates were covered with photoresist (AZ4330E, AZ electronic materials, thickness of ~ 10 m) before the metal deposition using lithography process, followed by the thermal deposition of 300-nm-thick Al films. The MoOx/Si junction was formed onto the rear side of the device. The MoOx/Al contact was deposited by thermal evaporation using a solid MoO3 source, followed by the deposition of the 100-nm-thick Al film. In case of LiF/Al contact, the p-n junction was formed by diffusing the boron dopant (B155, Filmtronics) into the front of the n-Si substrate under the N2 ambient at 880 C, as the LiF/Al contact was for the collection of the majority carriers (electrons). The sheet resistance of the p+-Si region was 90110 /sq. After doping process, the SiNx film and top contact were produced as mentioned above. Subsequently, the LiF/Al contact was formed by thermally depositing the thin LiF film and the 100-nm-thick Al film onto the rear surface of the solar cells. S2
Fabrication of Si nanohole ABC solar cells. The Si nanohole arrays were fabricated by the metal-assisted chemical etching (MACE) onto the front of the Si substrate. The rear surface of the Si substrate was covered by the polymer film to prevent the formation of Si nanoholes. The Si substrate was dipped into the diluted HF solution to remove the native oxide. The Ag nanoparticles were first deposited onto a hydrogenterminated Si substrate by the electroless deposition in a mixed solution of 4.8 M hydrofluoric acid (HF) and 0.005 M silver nitrate (AgNO3) for 60 sec at the room temperature. After the deposition of the Ag nanoparticle, the substrate was immersed in an aqueous solution of 4.8 M HF and 0.6 M hydrogen peroxide (H2O2) for 30 sec at the room temperature. The Si nanoholes were then dipped in the diluted nitric acid (HNO3) for 10 min so as to remove the Ag nanoparticles. In order to minimize the surface recombination, the Si nanoholes were covered by the 10-nm-thick Al2O3 layer deposited using atomic layer deposition (Atomic premium, CN1) at 250 C. At the backside of the substrate, the interdigitated patterns for the hole and electron contacts were fabricated by photolithography. The center-to-center distance (pitch) between the hole and electron contacts was controlled from 225 to 1025 m while maintaining the fixed space of 50 m between contacts. For the pitch of 225 m, the widths of hole and electron contacts were 210 and 140 m, respectively, so as to form the hole contact with the areal ratio of approximately 60%. When the pitches were varied from 225 to 1025 m, we maintained the same area of the hole contact for the all devices. After the patterning, the MoOx (10-nmthick)/Al (500-nm-thick) and LiF (0.5-nm-thick)/Al (500-nm-thick) films were thermally deposited onto the hole and electron contacts, respectively.
S3
Fabrication of Si nanohole ABC solar cell by thermal doping processes Si nanoholes were formed onto the front of the Si substrate as mentioned above. For the selective diffusion of boron and phosphorus dopants, the diffusion barriers were required. After forming the Si nanoholes, 300-nm-thick SiO2 films were deposited onto both front and back of Si substrate via PE-CVD (PEH-600, SORONA). In order to form the patterned diffusion barrier for the hole contact, the SiO2 film at the backside of the substrate were patterned by the photolithography and SiO2 etch-back process. The hole contact (p+-Si) was formed by boron diffusion via the spin-on-dopant (SOD) method. The boron dopant source (B155, Filmtronics, Inc) was spin-coated onto the backside of the Si substrate. The doping process of boron dopant was conducted in a tube furnace under the N2 ambient at 880 C. The SiO2 diffusion barrier and boron silicate glass, remained after the SOD diffusion, were removed by using a diluted HF solution. For the electron contact (n+-Si), the phosphorus diffusion was conducted by the same process sequence of the boron diffusion including the SiO2 deposition, SiO2 etch-back process, phosphorus diffusion and HF treatment. The phosphorus diffusion (P509, Filmtronics, Inc) was carried out in the furnace under the mixed ambient of 20% O2 and 80% N2 at 900 C. After forming p+ and n+ regions, the Si substrate was covered by thin Al2O3 layer as the passivation layer. The metal electrodes for n+ and p+-Si were formed by the photolithography and the thermal deposition of 500-nm-thick Al film.
Characterization of Si nanohole ABC solar cell The surface morphologies of the MoOx and LiF films were characterized by atomic force microscopy (AFM, Veeco microscope). The saturation current densities were extracted by the lifetime measurement (WCT-120, Sinton). The photovoltaic properties of our solar cells were S4
investigated using a solar simulator (Class AAA, Oriel Sol3A, Newport) under AM 1.5G illumination. Incident flux was measured using a calibrated power meter, and double-checked using a NREL-calibrated solar cell (PV Measurements, Inc.). EQE was measured using a Xe light source and a monochromator in the wavelengths range of 400–1100 nm. Optical reflection measurements were performed over wavelengths of 400–1100 nm using a UV-Vis/NIR spectrophotometer (Cary 5000, Agilent) equipped with a 110 mm integrating sphere to account for total light (diffuse and specular) reflected from the samples.
S5
SUPPORTING FIGURES
(a) [Emitter formation] SiO2 deposition
[BSF formation]
[Metallization]
SiO2 deposition
Deposition of passivation layer
n-Si n+-Si
p+-Si Passivation layer
SiO2
Photolithography & SiO2 etch-back
Photolithography & SiO2 etch-back
Photolithography & etch-back
n+ -Si
p+-Si
photoresist
Boron diffusion
n+-Si
boron silicate glass (BSG)
Oxide removal
n+-Si
(b)
Phosphorus diffusion
n+-Si
n+-Si
p+ -Si
Phosphosilicate glass (PSG)
p+-Si
Al
Oxide removal
n+-Si
Deposition of passivation layer
Metal deposition & Lift-off
p+-Si
Photolithography & etch-back
MoOx/Al deposition & Lift-off
n-Si
MoOx
Passivation layer
Photoresist
LiF/Al deposition & Lift-off
MoOx
Photolithography & etch-back
LiF
Figure. S1. Process charts of (a) conventional and (b) dopant-free all-back-contact solar cells. S6
(a)
Vacuum level n-Si
MoOx
EC EF EV
MoOx
(b)
n-Si
Vacuum level
inversion ( B > Eg/2)
MoOx
electrons
B
n-Si
- - - - ++
holes
EC EF
Eg EV
MoOx
n-Si
Figure. S2. Energy band diagrams of a MoOx film adjacent to n-Si under thermal nonequilibrium condition (a) and MoOx/Si contact in thermal equilibrium (b).
S7
Figure. S3. Transmittance spectra of MoOx films with different thicknesses: 2.5 (black solid line with filled squares), 5 (red solid line with filled circles), 10 (blue solid line with filled triangles), and 20 nm (green solid line with filled inverse triangles).
S8
Figure. S4. AFM height-mode images of (a) 2.5-nm-thick and (b) 20-nm-thick MoOx films. AFM phase-mode images of (c) 2.5-nm-thick and (d) 20-nm-thick MoOx films.
S9
100
EQE (%)
80 60 40 20 0 400
w/o LiF LiF 0.5 nm 500
600
700
800
900
1000 1100
Wavelength (nm)
Figure. S5. EQE spectra of the front-back contact solar cells without (black solid line with filled squares) and with 0.5-nm-thick LiF film (green solid line with filled triangles).
S10
(a)
Vacuum level Al (4.08 eV)
B electrons
Al
(b)
n-Si
EC EF
EV
Vacuum level LiF/Al (3.3 eV)
tunneling B electrons
EC EF
EV
Al
LiF
n-Si
Figure. S6. Energy band diagrams of (a) Al-Si and (b) Al-LiF-Si contacts in thermal equilibrium.
S11
Figure. S7. AFM height-mode images of (a) 0.5-nm-thick and (b) 2-nm-thick LiF films. AFM phase-mode images of (c) 0.5-nm-thick and (d) 2-nm-thick LiF films.
S12
20
Si
1
1/eff 1/Auger (ms )
LiF
18
LiF
J0 qWn
slope
2
i
16 MoOx
14
MoOx/Si LiF/Si
Si MoOx 15
1x10
15
2x10
15
3x10
15
4x10
15
5x10
-3
15
6x10
excess carrier density n (cm ) Figure. S8. Inverse effective carrier lifetime reduced by inverse Auger carrier lifetime versus excess carrier density for MoOx/Si (black solid line with filled squares) and LiF/Si contacts (red solid line filled circles). Saturation current density (J0) can be extracted from the slope J0/qWni2, where q, W, ni2 represent the electron charge, the wafer thickness, the intrinsic concentration, respectively.
S13
2
Current density (mA/cm )
40
30
20
10
0 0.0
0.2
0.4
0.6
Voltage (V) Figure. S9. Current densityvoltage curve of the thermally doped all-back-contact solar cell.
S14
Han-Don Um, Namwoo Kim, Kangmin Lee, Inchan Hwang, Ji Hoon Seo, and Kwanyong Seo*
Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, Korea
* Corresponding authors. E-mail: [email protected]
S1
EXPERIMENTALS
Fabrication of planar solar cells with front-back contact To characterize the MoOx/n-Si junction, the planar junction Si solar cells were fabricated from Czochralski (CZ) n-type Si wafers (resistivity of 13 cm, 350-m thick). The heavily-doped region (n+) was formed by phosphorus diffusion via the spin-on-dopant (P509, Filmtronics) method at the front of the solar cell for an ohmic contact between Si and front electrode. The diffusion doping was carried out in a tube furnace under a mixed ambient of 20 % O2 and 80 % N2 at 900 C. The sheet resistance of the n+-Si region was 130150 /sq. After removing phosphorus glass and SiOx layer, a thin SiNx layer (60-nm-thick) was deposited by PE-CVD (PEH-600, SORONA) onto the heavily-doped (n+) Si. For the top contacts with the grid design, SiNx-deposited Si substrates were covered with photoresist (AZ4330E, AZ electronic materials, thickness of ~ 10 m) before the metal deposition using lithography process, followed by the thermal deposition of 300-nm-thick Al films. The MoOx/Si junction was formed onto the rear side of the device. The MoOx/Al contact was deposited by thermal evaporation using a solid MoO3 source, followed by the deposition of the 100-nm-thick Al film. In case of LiF/Al contact, the p-n junction was formed by diffusing the boron dopant (B155, Filmtronics) into the front of the n-Si substrate under the N2 ambient at 880 C, as the LiF/Al contact was for the collection of the majority carriers (electrons). The sheet resistance of the p+-Si region was 90110 /sq. After doping process, the SiNx film and top contact were produced as mentioned above. Subsequently, the LiF/Al contact was formed by thermally depositing the thin LiF film and the 100-nm-thick Al film onto the rear surface of the solar cells. S2
Fabrication of Si nanohole ABC solar cells. The Si nanohole arrays were fabricated by the metal-assisted chemical etching (MACE) onto the front of the Si substrate. The rear surface of the Si substrate was covered by the polymer film to prevent the formation of Si nanoholes. The Si substrate was dipped into the diluted HF solution to remove the native oxide. The Ag nanoparticles were first deposited onto a hydrogenterminated Si substrate by the electroless deposition in a mixed solution of 4.8 M hydrofluoric acid (HF) and 0.005 M silver nitrate (AgNO3) for 60 sec at the room temperature. After the deposition of the Ag nanoparticle, the substrate was immersed in an aqueous solution of 4.8 M HF and 0.6 M hydrogen peroxide (H2O2) for 30 sec at the room temperature. The Si nanoholes were then dipped in the diluted nitric acid (HNO3) for 10 min so as to remove the Ag nanoparticles. In order to minimize the surface recombination, the Si nanoholes were covered by the 10-nm-thick Al2O3 layer deposited using atomic layer deposition (Atomic premium, CN1) at 250 C. At the backside of the substrate, the interdigitated patterns for the hole and electron contacts were fabricated by photolithography. The center-to-center distance (pitch) between the hole and electron contacts was controlled from 225 to 1025 m while maintaining the fixed space of 50 m between contacts. For the pitch of 225 m, the widths of hole and electron contacts were 210 and 140 m, respectively, so as to form the hole contact with the areal ratio of approximately 60%. When the pitches were varied from 225 to 1025 m, we maintained the same area of the hole contact for the all devices. After the patterning, the MoOx (10-nmthick)/Al (500-nm-thick) and LiF (0.5-nm-thick)/Al (500-nm-thick) films were thermally deposited onto the hole and electron contacts, respectively.
S3
Fabrication of Si nanohole ABC solar cell by thermal doping processes Si nanoholes were formed onto the front of the Si substrate as mentioned above. For the selective diffusion of boron and phosphorus dopants, the diffusion barriers were required. After forming the Si nanoholes, 300-nm-thick SiO2 films were deposited onto both front and back of Si substrate via PE-CVD (PEH-600, SORONA). In order to form the patterned diffusion barrier for the hole contact, the SiO2 film at the backside of the substrate were patterned by the photolithography and SiO2 etch-back process. The hole contact (p+-Si) was formed by boron diffusion via the spin-on-dopant (SOD) method. The boron dopant source (B155, Filmtronics, Inc) was spin-coated onto the backside of the Si substrate. The doping process of boron dopant was conducted in a tube furnace under the N2 ambient at 880 C. The SiO2 diffusion barrier and boron silicate glass, remained after the SOD diffusion, were removed by using a diluted HF solution. For the electron contact (n+-Si), the phosphorus diffusion was conducted by the same process sequence of the boron diffusion including the SiO2 deposition, SiO2 etch-back process, phosphorus diffusion and HF treatment. The phosphorus diffusion (P509, Filmtronics, Inc) was carried out in the furnace under the mixed ambient of 20% O2 and 80% N2 at 900 C. After forming p+ and n+ regions, the Si substrate was covered by thin Al2O3 layer as the passivation layer. The metal electrodes for n+ and p+-Si were formed by the photolithography and the thermal deposition of 500-nm-thick Al film.
Characterization of Si nanohole ABC solar cell The surface morphologies of the MoOx and LiF films were characterized by atomic force microscopy (AFM, Veeco microscope). The saturation current densities were extracted by the lifetime measurement (WCT-120, Sinton). The photovoltaic properties of our solar cells were S4
investigated using a solar simulator (Class AAA, Oriel Sol3A, Newport) under AM 1.5G illumination. Incident flux was measured using a calibrated power meter, and double-checked using a NREL-calibrated solar cell (PV Measurements, Inc.). EQE was measured using a Xe light source and a monochromator in the wavelengths range of 400–1100 nm. Optical reflection measurements were performed over wavelengths of 400–1100 nm using a UV-Vis/NIR spectrophotometer (Cary 5000, Agilent) equipped with a 110 mm integrating sphere to account for total light (diffuse and specular) reflected from the samples.
S5
SUPPORTING FIGURES
(a) [Emitter formation] SiO2 deposition
[BSF formation]
[Metallization]
SiO2 deposition
Deposition of passivation layer
n-Si n+-Si
p+-Si Passivation layer
SiO2
Photolithography & SiO2 etch-back
Photolithography & SiO2 etch-back
Photolithography & etch-back
n+ -Si
p+-Si
photoresist
Boron diffusion
n+-Si
boron silicate glass (BSG)
Oxide removal
n+-Si
(b)
Phosphorus diffusion
n+-Si
n+-Si
p+ -Si
Phosphosilicate glass (PSG)
p+-Si
Al
Oxide removal
n+-Si
Deposition of passivation layer
Metal deposition & Lift-off
p+-Si
Photolithography & etch-back
MoOx/Al deposition & Lift-off
n-Si
MoOx
Passivation layer
Photoresist
LiF/Al deposition & Lift-off
MoOx
Photolithography & etch-back
LiF
Figure. S1. Process charts of (a) conventional and (b) dopant-free all-back-contact solar cells. S6
(a)
Vacuum level n-Si
MoOx
EC EF EV
MoOx
(b)
n-Si
Vacuum level
inversion ( B > Eg/2)
MoOx
electrons
B
n-Si
- - - - ++
holes
EC EF
Eg EV
MoOx
n-Si
Figure. S2. Energy band diagrams of a MoOx film adjacent to n-Si under thermal nonequilibrium condition (a) and MoOx/Si contact in thermal equilibrium (b).
S7
Figure. S3. Transmittance spectra of MoOx films with different thicknesses: 2.5 (black solid line with filled squares), 5 (red solid line with filled circles), 10 (blue solid line with filled triangles), and 20 nm (green solid line with filled inverse triangles).
S8
Figure. S4. AFM height-mode images of (a) 2.5-nm-thick and (b) 20-nm-thick MoOx films. AFM phase-mode images of (c) 2.5-nm-thick and (d) 20-nm-thick MoOx films.
S9
100
EQE (%)
80 60 40 20 0 400
w/o LiF LiF 0.5 nm 500
600
700
800
900
1000 1100
Wavelength (nm)
Figure. S5. EQE spectra of the front-back contact solar cells without (black solid line with filled squares) and with 0.5-nm-thick LiF film (green solid line with filled triangles).
S10
(a)
Vacuum level Al (4.08 eV)
B electrons
Al
(b)
n-Si
EC EF
EV
Vacuum level LiF/Al (3.3 eV)
tunneling B electrons
EC EF
EV
Al
LiF
n-Si
Figure. S6. Energy band diagrams of (a) Al-Si and (b) Al-LiF-Si contacts in thermal equilibrium.
S11
Figure. S7. AFM height-mode images of (a) 0.5-nm-thick and (b) 2-nm-thick LiF films. AFM phase-mode images of (c) 0.5-nm-thick and (d) 2-nm-thick LiF films.
S12
20
Si
1
1/eff 1/Auger (ms )
LiF
18
LiF
J0 qWn
slope
2
i
16 MoOx
14
MoOx/Si LiF/Si
Si MoOx 15
1x10
15
2x10
15
3x10
15
4x10
15
5x10
-3
15
6x10
excess carrier density n (cm ) Figure. S8. Inverse effective carrier lifetime reduced by inverse Auger carrier lifetime versus excess carrier density for MoOx/Si (black solid line with filled squares) and LiF/Si contacts (red solid line filled circles). Saturation current density (J0) can be extracted from the slope J0/qWni2, where q, W, ni2 represent the electron charge, the wafer thickness, the intrinsic concentration, respectively.
S13
2
Current density (mA/cm )
40
30
20
10
0 0.0
0.2
0.4
0.6
Voltage (V) Figure. S9. Current densityvoltage curve of the thermally doped all-back-contact solar cell.
S14
