Rear Side Structures Research Articles

An Investigation of Internal Quantum Efficiency of Bifacial Interdigitated Back Contact (IBC) Crystalline Silicon Solar Cell

In this article, we investigated the internal quantum efficiency (IQE) properties of n-type bifacial interdigitated back contact crystalline silicon solar cells using an IQE mapping system. In the cell structure, high and low IQE values were observed above the emitter and back surface field (BSF) regions. The IQE values above the BSF busbar were drastically reduced due to electrical shading loss. Line scan profiles at different wavelengths showed the detailed distribution of IQE values. The IQE values varied greatly depending not only on the difference between emitter and BSF regions but also on the rear side structure such as the electrode width and the distance between the emitter and BSF regions. On the other hand, the IQE spectra at over 950 nm improved by increasing the light absorbance ratio from the rear side. After module formation, the IQE spectra at short wavelengths were significantly reduced. The IQE properties were obtained from the front and rear sides. The difference in the short-circuit current between front side illumination and rear side illumination was mainly due to optical shading loss and carrier recombination loss at the BSF region. For a high cell efficiency, it is necessary to improve the passivation properties of the BSF region and optimize the electrode design.

Comparative Analyses Between Two Techniques To Understand Metal-Induced Recombination Losses In Industrial N-Type Bifacial PERT Solar Cells

Due to the high cell efficiency, low Light-Induced Degradation (LID) and high bifaciality, n-type bifacial passivated emitter and rear totally diffused (PERT) solar cells are being widely researched and currently being implemented in mass production. With the improvement of dopant profiles and passivation quality, J0,emitter and J0,bsf under passivation area could reduce to lower values. However, the Voc of cell is still not high enough which may be caused by the high recombination loss under metal area. In this paper, two different methods which are suitable for low cell efficiency diagnoses are investigated and compared to evaluate the J0,metal on both boron and phosphorus doping side of solar cells. For the first method, high resistivity n-type mono wafers are prepared with double front side or rear side structure of solar cell. Four different types of metallization ratio areas are designed on the one wafer of single side. By testing the J0,total of these four area, we can fit a line of J0,total with different contact fraction, so the J0,metal can be calculated by the expression of J0,total, J0,passivation and contact fraction. For the second method, Suns-Voc and Suns-PL are used for calculation. Four types of cell structures with metal, without metal, only with front metal and only with rear metal are fabricated to measure the probed Voc and PL derived Voc either use (i) different carrier injection levels or (ii) different light illuminations. A line can be fitted under different Suns, and by contrasting the line of cell structure and other structures, J0,metal of front metal and rear metal can be calculated by the bias of these lines. By comparing this two methods, we get the boron side Ag/Al paste J0,metal and the phosphorus side Ag paste J0,metal are around 2801 fA/cm2 and 964.9 fA/cm2, respectively. By comparing this two methods, the value of second method is lower than the first method and more appropriate for IV simulation using Quokka.

Evaluation of carrier collection probability in bifacial interdigitated-back-contact crystalline silicon solar cells by the internal quantum efficiency mapping method

Bifacial interdigitated-back-contact (IBC) silicon solar cells with a high bifaciality of 0.91 were fabricated. Screen printing and firing technology were used to reduce the production cost. For the first time, the relationship between the rear side structure and carrier collection probability was evaluated using internal quantum efficiency (IQE) mapping. The measurement results showed that the screen-printed electrode and back surface field (BSF) area led to low IQE. The low carrier collection probability by BSF area can be explained by electrical shading effects. Thus, it is clear that the IQE mapping system is useful to evaluate the IBC cell.

Development and Characterization of AlOx/SiNx :B Layer Systems for Surface Passivation and Local Laser Doping

This work aims to improve the rear-side properties of p -type monocrystalline silicon solar cells by using the passivated emitter and rear locally diffused (PERL) solar cell concept. To realize the rear side structure, the so-called PassDop approach was used combining both surface passivation and local doping. The concept utilizes a multifunctional, doped AlO x /SiN x :B layer stack; the localized structuring is achieved by local contact opening and doping by a laser process. Using AlO x /SiN x :B PassDop layers, an outstanding effective surface recombination velocity S eff of less than 4 cm/s was achieved after firing at the passivated area. The boron concentration in the PassDop layers did not show any significant influence on S eff . Laser doping resulted in highly doped regions in the silicon with a sheet resistance of below 20 Ω/sq and surface doping concentrations close to 1 × 1020 cm–3. Accordingly, calculations showed that the saturation current density at the laser doped areas can be as low as 900 fA/cm² for line-shaped contact structures.

Wave optical simulation of the light trapping properties of black silicon surface textures.

Due to their low reflectivity and effective light trapping properties black silicon nanostructured surfaces are promising front side structures for thin crystalline silicon solar cells. For further optimization of the light trapping effect, particularly in combination with rear side structures, it is necessary to simulate the optical properties of black silicon. Especially, the angular distribution of light in the silicon bulk after passage through the front side structure is relevant. In this paper, a rigorous coupled wave analysis of black silicon is presented, where the black silicon needle shaped structure is approximated by a randomized cone structure. The simulated absorptance agrees well with measurement data. Furthermore, the simulated angular light distribution within the silicon bulk shows that about 70% of the light can be subjected to internal reflection, highlighting the good light trapping properties.

Crystalline Silicon Solar Cells with Enhanced Light Trapping via Rear Side Diffraction Grating

Effective light trapping concepts are increasingly relevant for thinner crystalline silicon solar cells. Diffractive rear side structures can be designed to enhance optical light path lengths in the weakly absorbed infrared wavelength range. However, because of the rather difficult passivation of these nano-scale structures and possible plasma induced damages, the optical gain has so far not been shown on final device level for wafer-based crystalline silicon solar cells in combination with a high open circuit voltage.In this work, we fabricated and characterized silicon solar cells with an optically active layer at the rear side which is electrically decoupled from the silicon bulk by a thin, planar passivation layer. A binary diffraction grating was realized on the rear side via Nanoimprint Lithography (NIL) and plasma etching. For the rear side metallization a laser based foil process and photolithographically opened point contacts with evaporated aluminum were used as two alternative process routes. The best cell shows an EQE increase of up to 29 % absolute at 1100 nm compared to planar reference cells. This corresponds to a total Jsc gain of 1.2 mA/cm2 and a best cell efficiency of 19.4 % (Voc: 683.1 mV, Jsc: 35.4 mA/cm2, FF: 81.0 %) with planar front side and SiO2 antireflection coating.Optical simulations, which fit well to the experimental data, show that cell efficiencies of 20.8 % and 21.3 % can be reached with a planar front by using an optimized single layer (SiNxOy) or double layer (MgF, SiNx) ARC, respectively. By calculating Jsc for different cell thicknesses it is shown that a 100 μm thick solar cell incorporating the fabricated diffractive grating is expected to exhibit a higher Jsc than a planar cell with 250 μm thickness.

Picosecond Laser Microstructuring for Black Silicon Solar Cells

Sulfur-doping and broad band absorptive black silicon materials were fabricated by picosecond laser irradiation. Two kinds of microstructures, laser induced periodic surface structure (LIPSS) and conical spikes were obtained by changing parameters of laser scanning. Black silicon solar cells with back surface field were explored. Influences of different rear side structures to devices were presented and conversion efficiency of 9% is available.

20·5% efficient silicon solar cell with a low temperature rear side process using laser-fired contacts

The paper presents a rear side structure for crystalline silicon solar cells, which is processed at a maximum temperature of 220°C. Using two different material compositions for electrical and optical needs, the layer system has excellent passivation properties, enhances light trapping and allows for a good ohmic contact. With this structure we achieve an independently confirmed conversion efficiency η=20·5% on a 250 μm thick silicon solar cell. Due to the fact that the maximum process temperature is 220°C, this layer system enables new solar cell concepts. Copyright © 2006 John Wiley & Sons, Ltd.