A conversion efficiency of 20.23% of heterojunction with intrinsic thin layer (HIT) solar cell on 156 mm × 156 mm metallurgical Si wafer has been obtained. Applying AFORS-HET software simulation, HIT solar cell with metallurgical Si was investigated with regard to impurity concentration, compensation level, and their impacts on cell performance. It is known that a small amount of impurity in metallurgical Si materials is not harmful to solar cell properties.
It is always the ultimate goal for PV industry to achieve both high conversion efficiency and low cost. Heterojunction with intrinsic thin layer (HIT) solar cells realizes high open-circuit voltage and hence high conversion efficiency by applying hydrogen-rich a-Si passivation to N-type Si wafer. Panasonic (formerly Sanyo) realized the highest HIT conversion efficiency of 25.6% [
HIT solar cells using both Siemens and metallurgical N-type Si wafers (156 × 156 mm2) were fabricated using ULVAC plasma enhanced chemical vapor deposition (PECVD) for 5~10 nm a-Si films and Sumitomo reactive plasma deposition (RPD) for transparent conductive oxide (TCO) preparation. Finally, screen printing was applied to form silver metal contact to extract the light-generated current. The HIT solar cells were fabricated with four-bus-bar design and bifacial structure (Figure
(a) Front side fabricated HIT solar cell. (b) Schematic diagram of HIT solar cell structure.
Si materials produced by metallurgical method have the following properties: (1) impurity compensation phenomenon, for example, coexistence of boron and phosphorus and (2) higher concentration level of metal impurities, for example, iron (Fe) and copper (Cu). It is well known that Fe and Cu concentrations in Si by Siemens method are negligible. The metallurgical Si wafers used in this work had resistivity of 2.0 Ω·cm, minority carrier lifetime of 30
Input parameters for AFORS-HET simulation.
Parameters | a-Si:H(p) | a-Si:H(i) | c-Si(n) | a-Si:H(n+) |
---|---|---|---|---|
Thickness (nm) | 5 | 5 | 1.7 × 105 | 5 |
Dielectric constant | 11.9 | 11.9 | 11.9 | 11.9 |
Electron affinity (eV) | 3.8 | 3.8 | 4.05 | 3.8 |
Band gap (eV) | 1.70 | 1.74 | 1.12 | 1.70 |
Effective conduction band density |
1.0 × 1020 | 1.0 × 1020 | 2.8 × 1019 | 1.0 × 1020 |
Effective valence band density |
1.0 × 1020 | 1.0 × 1020 | 1.04 × 1019 | 1.0 × 1020 |
Electron mobility (cm2 |
10 | 20 | 1040 | 10 |
Hole mobility (cm2 |
1 | 2 | 412 | 1 |
Acceptor concentration (cm−3) | 1.0 × 1019 | 0 | 5 × 1015 | 0 |
Donor concentration (cm−3) | 0 | 0 | 7.03 × 1015 | 1.0 × 1019 |
Thermal velocity of electrons (cm/s) | 1.0 × 107 | 1.0 × 107 | 1.0 × 107 | 1.0 × 107 |
Thermal velocity of hole (cm/s) | 1.0 × 107 | 1.0 × 107 | 1.0 × 107 | 1.0 × 107 |
Layer density |
2.328 | 2.328 | 2.328 | 2.328 |
Band tail density of states ( |
2.0 × 1021 | 2.0 × 1021 | 2.0 × 1021 | |
Characteristic energy (eV) for donors, acceptors | 0.045, 0.037 | 0.045, 0.02 | 0.06, 0.037 | |
Capture cross-section for donor states, e, h (cm2) | 1.0 × 10−15, 1.0 × 10−17 | 1.0 × 10−15, 1.0 × 10−17 | 1.0 × 10−15, 1.0 × 10−17 | |
Capture cross-section for acceptor states, e, h (cm2) | 1.0 × 10−17, 1.0 × 10−15 | 1.0 × 10−17, 1.0 × 10−15 | 1.0 × 10−17, 1.0 × 10−15 | |
defect density of states at Gaussian peak energy (eV) | 1.0 × 1017~6.0 × 1019 | 1.0 × 1016~1.0 × 1019 | 8.0 × 1017~1.0 × 1020 | |
Standard deviation (eV) | 1.10, 1.35 | 0.725, 1.025 | 0.40, 0.65 | |
Capture cross-section for donor states, e, h (cm2) | 1.0 × 10−14, 1.0 × 10−15 | 1.0 × 10−14, 1.0 × 10−15 | 1.0 × 10−14, 1.0 × 10−15 | |
Capture cross-section for acceptor states, e, h (cm2) | 1.0 × 10−15, 1.0 × 10−14 | 1.0 × 10−15, 1.0 × 10−14 | 1.0 × 10−15, 1.0 × 10−14 |
Defect state distributions of different types of a-Si:H in the simulations.
Si purification by metallurgical method can be realized through gas blowing, slagging boron removal, electron beam refining phosphorus removal, acid leaching, and unidirectional solidification metals removal. The repeated processes were applied to remove the metal impurities in Si and reach SOG requirements. However, the metal impurities in Si with Siemens method are higher. The metal impurities (Fe, Cu, and Ni) lead to a decrease of minority carrier lifetime, reducing the light-generated current. In AFORS-HET, defects can be deliberately added to c-Si for simulating the impact of metal impurities on solar cell properties. In this work, only Fe and Cu are considered to be the two main metal impurities. Due to the directional solidification of Fe residue, electron beam processing with copper electrodes may be introduced. The energy levels of Fe and Cu in Si are 0.4 eV and 0.24 eV, respectively. The electron capture cross-sections for Fe and Cu are 4.5 × 10−14 cm−2 and 1 × 10−14 cm−2, respectively [
(a) Impact of Fe impurity on cell properties. (b) Impact of Fe + Cu on cell properties.
Fe
Fe + Cu
In metallurgical N-type Si wafer, there are certain amounts of boron doping besides phosphorus, which is a well-known compensation phenomenon, and the compensation is proportional to the concentration of boron. In AFORS-HET simulation, donor concentration
Influence of compensation level on solar cell properties.
In this work, the metallurgical Si wafers were used to fabricate 156 mm × 156 mm HIT solar cells in a pilot line at Shanghai Institute of Micro-System and Information Technology, Chinese Academy of Sciences. The same fabrication processes were used to prepare the same batch of Siemens Si HIT cell for contrast. The
Comparison of
The measured parameters for metallurgical Si were introduced into AFORS-HET software for simulation of HIT solar cell. Table
Comparison of HIT solar cell properties with metallurgical and Siemens Si samples.
Cell parameters | Siemens Si | Metallurgical Si | Simulated data | Optimized data |
---|---|---|---|---|
|
0.7294 | 0.6949 | 0.6841 | 0.7061 |
|
38.918 | 37.564 | 37.656 | 39.641 |
FF (%) | 78.37 | 77.48 | 75.08 | 81.08 |
|
22.25 | 20.23 | 19.58 | 21.17 |
TC (%/°C) | −0.225 | −0.3043 | −0.2000 | −0.2000 |
It can be concluded that although the material quality of metallurgical Si is relatively worse than that of Siemens method, HIT solar cells with competitive performance can still be achieved. The best performance of metallurgical HIT solar cell is
The authors declare no conflicts of interest regarding the publication of this paper.
The authors appreciate the financial sponsorship from Fujian Industrial Guidance Project (no. 2017H0038). Also thanks are given to Ningxia Power Group Co., Ltd., China, for providing N-type monocrystalline SOG silicon rods purified with metallurgical method.