An Investigation on a Crystalline-Silicon Solar Cell with Black Silicon Layer at the Rear
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Crystalline-Si (c-Si) solar cell with black Si (b-Si) layer at the rear was studied in order to develop c-Si solar cell with sub-band gap photovoltaic response. The b-Si was made by chemical etching. The c-Si solar cell with b-Si at the rear was found to perform far better than that of similar structure but with no b-Si at the rear, with the efficiency being increased relatively by 27.7%. This finding was interesting as b-Si had a large specific surface area, which could cause high surface recombination and degradation of solar cell performance. A graded band gap was found to form at the rear of the c-Si solar cell with b-Si layer at the rear. This graded band gap tended to expel free electrons away from the rear, thus reducing the probability of electron-hole recombination at b-Si and improving the performance of c-Si solar cell.
Keywordsc-Si solar cell Black silicon Graded band gap Surface recombination
Highly surface-etched Si that has been loaded or doped with metal or non-metal ions could exhibit strong and broadband absorptivity [1, 2, 3, 4, 5, 6]. This type of Si, or black Si (b-Si), has attracted much attention for its potential application in broadband response photovoltaics [7, 8, 9]. To date, investigations of b-Si solar cell have focused on such a configuration that the b-Si layer is at the front of the solar cell [10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. In this case, electron-hole pairs induced by the sub-band gap near infrared (NIR) absorption at the b-Si layer are far away from the PN junction zone and cannot be decomposed by the built-in field to become charge carriers, making the sub-band gap NIR photovoltaic response impossible. It is then conceived that if the b-Si layer is placed at the rear, the NIR absorption-induced electron-hole pairs could be decomposed by the Si/oxide interfacial field at the rear  or by a built-in field there if an interdigitated back contact (IBC) configuration is adopted , making the photovoltaic (PV) response of such a crystalline (c)-Si solar cell extend to the sub-band gap NIR range. Unfortunately, the large specific surface area of b-Si would usually cause high surface recombination, which would severely degrade the solar cell performance [10, 15, 22]. Hence, before we start to study the sub-band gap NIR response of c-Si solar cell, it is necessary to know how large the surface recombination of b-Si could be and how to minimize or avoid its influence . In this work, we studied the PV response of c-Si solar cell with b-Si at the rear and explored the physics underlying our observations.
P-type Si<100> wafer (CZ, double-side polish, 10 × 10 × 0.2 mm3 in size, 1–10 Ω cm) was used as the substrate. The Si wafer was ultrasonically cleaned and then dipped in dilute HF(1%), followed by etching in a NaOH/alcohol/H2O (0.5 g/200 ml/200 ml) solution at 90 °C for 15 min to slightly texture the surface for antireflection and then rinsing in de-ionized water. To prepare b-Si at the rear, a Ag layer with apparent thickness of 3 nm was evaporated onto one surface of Si substrate as catalyst by resistance heating in a home-made vacuum chamber with base pressure less than 5 × 10−4 Pa. After immersing the Si wafer in a HF(40%):H2O2(30%):H2O = 1:5:10 solution for 120 s at room temperature, a b-Si layer was formed at that Si surface or at the rear of the solar cell. A phosphorous paste was then deposited onto the other Si surface or the front of the solar cell, followed by annealing at 900 °C for 20 min in nitrogen to form a PN junction. A 20-nm-thick SiO2 layer was evaporated onto the front of the solar cell for surface passivation. For the rear surface passivation, a 10-nm-thick Al2O3 layer was deposited using the technique of atomic layer deposition (ALD) (Beneq TFS 200). An 80-nm-thick ITO layer was deposited onto the front surface as the front electrode. A 2-μm-thick Al layer was evaporated by resistance heating as the rear electrode. A thermal annealing in nitrogen at 425 °C for 5 min was conducted to finalize the preparation of c-Si solar cell. It should be pointed out that in this work, we focused on the effect of b-Si at the rear on the PV response; therefore, the front surface was only slightly textured and not highly etched to form b-Si.
The reflectance spectra were measured using a UV-vis-NIR spectrophotometer (Shimadzu, UV-3101PC). The surface morphology was measured with a scanning electron microscope (SEM) (Philips, XL 30). The PV parameters of the solar cell were obtained with a solar simulator (Oriel/Newport, model 94023A) under 1-Sun AM1.5G condition. The external quantum efficiency (EQE) of the solar cell was acquired on a QE system of Oriel/Newport. Transmission electron microscopy (TEM) measurements were carried out on a JEOL EM-3000 system. Surface-emitting photoluminescence (PL) spectra were recorded by a spectrophotometer (Ocean Optics USB2000), with a 325-nm He-Cd laser (Melles Griot, model series 74) as the excitation source. The surface potentials of p-type Si and b-Si were measured by a Kelvin probe system (KP Technology SKP5050), the so-called contact potential difference, or CPD identification.
Results and Discussion
Photovoltaic parameters for the solar cells of “wafer,” “wafer + Al2O3,” “b-Si,” and “b-Si + Al2O3”
V oc (mV)
J sc (mA/cm2)
Wafer + Al2O3
b-Si + Al2O3
We studied the c-Si solar cell with a b-Si layer at the rear. The c-Si solar cell of such a configuration showed a far better performance than a c-Si solar cell of similar structure but with no b-Si at the rear. This result was attributed to the formation of a graded band gap at the rear, which can largely reduce the probability of surface recombination at the rear, thus improving the performance of the c-Si solar cell. The finding of this work can be applied to developing a c-Si solar cell with broadband PV response, including the sub-band gap NIR response, in the future.
The authors would thank Jun-Yi Gong for CPD measurement.
This work was supported by the National Natural Science Foundation of China (51472051, 61275178) and the CIOMP-Fudan University joint foundation (FC2017-001).
ZQZ prepared all the samples and measured the I-V and EQE data. FH and WJZ measured the SEM data and TEM data. HYC helped to grow the Al2O3 passivation layer. LM and CZ measured the reflectance data and PL data. ML designed the experiments and wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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