In this chapter the theoretical description of up-conversion processes was summarized. Us-ing rate equations, the contribution of the different possible processes, such as stimulated and spontaneous transitions, non-radiative decay and the energy transfer mechanisms, was described. From these considerations, the influence of the concentration of the active ion on the up-conversion efficiency, the relation between up-conversion process and the shape of the excitation spectrum and the dependence of the intensity of emission as a result of up-conversion on the input power could be determined.
Solar Cell Processing
The application of an up-converter to the rear of a solar cell requires a cell structure that transmits light with wavelengths within the excitation range of the up-converter. In addition the cell has to be sensitive to the emission from the up-converter on the rear of the cell. Both requirements are fulfilled by bifacial solar cells.
Bifacial solar cells are primarily of interest for space applications due to the possibility of collection of the earth’s albedo light. Beside this, the advantages are lower absorbance of infrared light, thus lower operating temperatures and a high power to weight ratio [129].
Also in static concentrators [130] or when installed in a highly reflecting surrounding [131], bifacial cells are advantageous.
Early bifacial concepts had an emitter on both sides (n+pn+-structures) [129]. High ef-ficiencies with only one emitter and back surface field (BSF) were reached on n-type FZ silicon [132] with 18.1% and 19.1% and on Cz [133] with 18.0% and 18.3% under illumi-nation of the p+n side and n+n side respectively (both on an area of 4 cm2). On p-type silicon with a back contact concept, efficiencies of 20.6% and 19.3% on an area of 4 cm2 under illumination of the unmetallized and metallized side respectively were reached by Glunz et. al [134]. With n+pn+-structures, where the rear side metallization does not contact the n+-region (floating emitter) efficiencies of 21.3% and 19.8% on an area of 1 cm2 under illumination of the front and rear respectively were reached [135, 136].
The straightforward approach to replace the full metallization of the rear of a conventional cell (n+p) by a grid was proposed the first time in 1977 by Chambouleyron and Chevalier [137]. Using PECVD SiNx as passivation on both sides efficiencies of 20.1% and 17.2%
were reached with this concept on an area of 4 cm2 in 1987 [138].
Within this thesis a n+pp+ design was developed, allowing for application of an up-converter to the rear. As discussed in Section 1.3.3, the optical properties of the solar cell can be adapted to the application of an up-converter by choosing an adequate thickness of the antireflection coating. This is intrinsically tied with losses in conventional cell per-formance. A conventional high efficiency cell design is not appropriate for the application of an up-converter, since the front surface texture reduces the transmission of light with a wavelength within the excitation range of the up-converter to unacceptably low levels.
Therefore a compromise between a conventional design and a design fully adapted to the up-converter must be found, to maintain a high conventional performance while meeting the requirements of the up-converter. As a consequence, the cells processed for the
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Figure 3.1: Schematic drawing of the cell process. This process was applied to 0.5 Ωcm p-type float zone silicon.
plication of an up-converter within this thesis have no texturization, but a conventional antireflection coating. The lack of a front surface texture leads to loss in the short circuit current, but ensures a sufficient transmission in the excitation range of the up-converter so that it can be studied.
In this chapter the processing details of the bifacial solar cells are given. The results of the characterization including I-V-, spectral response, transmission, reflection and lifetime measurements are reviewed.
3.1 Cell Concept and Processing Details
The bifacial cell concept used for this work is shown in Figure 3.1. The starting material was 0.5 Ωcm p-type float zone silicon wafers with a thickness of 250 µm and a diameter of 100 mm, so that eight 2×2 cm2 cells resulted from each wafer.
After RCA cleaning1 a back-to-back 60 Ω/sq boron diffusion is carried out using boron
1The RCA cleaning procedure was proposed by Kern [139]. The cleaning consists of two steps. First organic contamination and particles are removed using NH3(25%)-H2O2(30%)-H2O-solution in a ratio of 1:1:5 at about 80◦C (all concentrations are given in percent by volume). The thin oxide that grows in this step is then removed in diluted HF (1%). In the second cleaning step, metal contamination is removed in HCl(37%)-H2O2(30%)-H2O (1:1:5), also followed by a HF-dip. An RCA clean was performed before each high temperature step (all diffusions and oxidations).
tribromide (BBr3). In the diffusion process an in-situ oxidation step is included to trans-form the boron rich layer into boron glass2, which is afterwards removed in HF (10%). A thermal oxide of about 90 nm thickness (thickness determined by ellipsometry measure-ments) is grown. The boron diffused side is shielded by the oxide, while on the undiffused side the oxide is removed lithographically to enable emitter diffusion. The emitter is cre-ated in a gas phase POCl3 diffusion leading to a sheet resistance of about 85 Ω/sq. Both the phosphorus glass and the masking oxide are etched away in diluted HF. A second thermal oxide is grown as passivation and antireflection coating with a thickness of about 110 nm.
On both sides the metallization is applied by evaporating a titanium-palladium-silver stack with thicknesses of 50 nm each after first defining the finger grid photolithographically on both sides3. A sufficient thickness of the metallization is reached using a combination of autocatalytic silver-plating and galvanically strengthening. In the plating process the thickness of the metallization is enhanced in a degree necessary to ensure the electrical contact of the rear side of the solar cell in the galvanical process. The plating is per-formed using the commercial plating solution “electroless silvermix ESM100” purchased from Polymer Kompositer. A galvanic deposition of silver performed afterwards ensures homogeneity and sufficient metallization. The fingers produced in this way have heights of 5 - 8 µm, widths of about 30 - 40 µm and a finger spacing of 800 µm, which results in a coverage of 5 - 6.5% including the busbar.
Sintering the cells for 30 minutes at 380◦C under Ar/H2 atmosphere, which is also called a forming gas anneal (FGA)4, results in a good surface passivation. The passivating properties are further enhanced by a post-metallization anneal (or alneal), which includes evaporating an aluminium layer of 1 µm thickness on both sides and sintering at 380◦C under Ar/H2 atmosphere. After the thermal treatment the aluminium is removed using phosphoric acid.
Since the emitter and BSF cover the whole wafer area and thus connect the single cells processed in parallel on the wafer, the pn junction is separated by dicing the wafers into 2×2 cm2 cells.