Table 7.1: Overview of six different POCl3emitters. The annealing time of diffusions in the second row is 90 min.
1 2 3
Diff 1 Diff 2 Indus-700
Tunload=700◦C Tunload=500◦C Tunload=700◦C
4 5 6
Diff 1 + anneal Diff 2 + anneal Indus-700 + anneal Tanneal=700◦C Tanneal=500◦C Tanneal=700◦C
also be induced by inhomogeneous doping profiles. Since the present analysis focuses on the difference between modified POCl3emitters on neighbored mc-Si samples with comparable grain structure, these changes in the absolute value ofLe f f,Qshould not be relevant here. In the following onlyLe f f,Q, which is simply referred to asLe f f, is investigated. All presented solar cells have the same rear side passivation which is the aluminum back surface field. Hence, differences between modified POCl3 emitters are expected not to be influenced by varying surface recombination at the rear.
7.2 Experimental Details
In Fig.7.1the temperature profiles of three different POCl3diffusions are shown.Diff 1is the reference emitter of this work with a modified unloading temperature of 700◦C. The Diff 2 process describes the same POCl3 diffusion but with the lower unloading temperature of 500◦C. The third diffusion is considerably shorter and has a smaller oxygen flow during the drive-in step referred to asIndus-700. It is the same diffusion process as theindustryprocess of Chapter3but with the unloading temperature of 700◦C. For the sake of clarity, all process names are depicted in italic letters2. In addition, three more diffusions are tested. These diffusions are basically the same diffusions as the already described ones but with a subsequent 90 min annealing in a nitrogenN2 ambient at the respective unloading temperature.
These processes are referred to asdiffusion name + anneal. An overview of all diffusions is given in Tab.7.1. All of them yield a sheet resistance of approximately 80Ω/2.
The contribution of the emitter to the total saturation current densityJ01is investigated according to equation7.2. Hence, the emitter saturation current densityJ0Eis measured for each of the six emitters on 200Ωcm FZ samples both-sided passivated with a fired PECVD SiNx:H layer. Note that the samples are symmetrical with the emitter being present on both sample sides underneath each SiNx:H layer. Fig.7.2 shows theJ0Evalues determined by the slope method of Kane and Swanson at an excess carrier density of 3×1015cm−3using the photoconductance decay (PCD) lifetime measurement in transient mode [196].
All values are plotted versus POCl3 diffusion. Neither the emitter quality of Diff 2nor the one of the Indus-700diffusion can be improved by annealing at the respective unloading temperatures. OnlyDiff 1 samples exhibit a slight decrease inJ0Edue to annealing, which means a lowered recombination within the emitter region. The emitter saturation current density J0E is lowered by 21 fA/cm2 down to the minimum value of 120 fA/cm2. This is a small value compared to the decrease of the total saturation current densityJ01 which decreases by 403 fA/cm2 due to annealing down to 1400 fA/cm2 (seeJ01 of cells from bottom ingot height in Fig.7.6a). Thus, the contribution of the emitter to the total saturation
2This representation has been selected only for this chapter since six different diffusion processes are compared in contrast to the lifetime based analysis wherein typically two different diffusions are compared.
POCl3
825°C -N2
POCl3 825°C
-N2
Temperature (a.u.)
Diff 2 with T
unload= 500°C
500°C 700°C 700°C
Diff 1 & Indus-700
Indus-700
Time (a.u.)
Diff 1 Indus-700 Drive-in duration (min) 60 5
Oxygen fl ow high low
Diff 1
Diff 2
Figure 7.1: Temperature profiles of three different POCl3diffusions: Diff 1,Diff 2andIndus-700. TheDiff 2process has a modified unloading temperature of 500◦C. The crucial differences betweenDiff 1andIndus-700are given in the table (see also Chapter3).
Figure 7.2: Emitter saturation current densityJ0Eversus POCl3diffusion.
current densityJ01is rather small in this case.
The ECV profiles of the Diff 1as well as the Indus-700 diffusions with and without annealing at the end are already depicted and discussed in Sec. 2.3.3. Nonetheless, they are repeatedly depicted in Fig. 7.3since they are necessary for the interpretation of the solar cell results. It is important to note that theIndus-700 emitter is more shallow than theDiff 1emitter. In addition, a smaller active phosphorus concentration [P+] in the kink region by 21% is observed for the latter emitter. This is particularly visible in the enlarged kink region of the ECV profiles shown in Fig. 7.3b. The strongest reduction of the P+kink
7.2 Experimental Details 117
(a) (b) Enlarged black rectangle sketched in (a).
Figure 7.3: ECV profiles of POCl3diffusions with and without a 90 min annealing atTunload=700◦C.
concentration yielded by the 90 min annealing atTunload=700◦C is revealed by theIndus-700 + anneal emitter. Herein, the P+kink concentration is reduced by 14.7% due to the annealing step. In agreement with the description in Sec.2.3.3, it is suggested that this reduction is explained by the deactivation of 14.7% of the P+kink concentration of theIndus-700emitter. The additional annealing allows the system to reach its equilibrium concentration, i.e., the lowered solubility limit atTunload=700◦C compared with the higher POCl3diffusion temperature.
Twelve 5×5 cm2 vertically neighbored sister samples originating from a mc-Si ingot are selected containing a comparable grain structure. Two sets of six solar cells on these sister samples are produced with the six different POCl3emitters acting on the same grain structure of the different sister samples.
Each given cell parameter in this analysis is, therefore, averaged over the values obtained from two cells.
The cells are lab-type solar cells which means their final size is 2×2 cm2. The cell process flow is given in Fig.7.4aand is described in more detail in Sec. 2.2.2. Note that samples are not textured within this experiment to exclude parasitic surface effects possibly induced by inhomogeneous texturization and to put a particular focus on the performance of the different POCl3emitters. Four solar cells are simulta-neously produced on the same 5×5 cm2 mc-Si wafer, namely a, b, c and d as shown in Fig.7.4b. As the penultimate processing step the four cells are cut out of the wafer. The cutting sketch in Fig.7.4b is shown on a MPCD (Microwave PhotoConductance Decay) lifetime map of a 15.6×15.6 cm2 wafer measured by an industrial partner. The material under investigation is the industrially produced mate-rial VI listed as the last mc-Si matemate-rial in Tab.2.1of Chapter2. Three regions a, b and d are located in the red zone of lower material quality closer to the crucible walls than region c.
In the following, cell results from two ingot heights (bottom and top) of an industrially produced mc-Si ingot will be presented. The lab-type cells are produced on wafers originating from the edge region of the ingot which is strongly affected by in-diffusing impurities from the crucible walls as shown by the red zone in Fig.7.4b. Note that the gettering behavior of such regions is of special interest for the Indus-700since these regions frequently limit cell performance. This is also demonstrated for material I by the industrial solar cell efficiencies3shown versus ingot height in Fig.3.10of Sec. 3.2.3.
3The respective solar cells are manufactured using a standard industrial cell process by the industrial project partner (Sun-ways AG) in the framework of SolarWinS (see the acknowledgement).
(a) Process flow
Figure 7.4: (a) Lab-type solar cell process flow. As the next-to-last processing step four 2×2 cm2cells are cut out of the 5×5 cm2sample: a, b, c and d. (b) A MPCD lifetime map of a 15.6×15.6 cm2mc-Si wafer out of a industrially crystallized mc-Si block is shown (as-cut). The cutting sketch of a 5×5 cm2sample is added with the four cell regions mainly located in the red zone of the mc-Si wafer.