Fabrication of polycrystalline silicon (poly-Si) thin-film solar cells (see Figure 1) involves high-temperature processing such as crystallization, dopant diffusion and activation, and defect annealing.1, 2 These processes require temperatures in excess of 900°C. Poly-Si material is thermally stable and its electronic quality benefits from high temperature treatments. However, glass, on which cells are fabricated, cannot withstand high temperatures for sufficiently long periods, thus limiting cell performance. The recent development of line-focus diode-lasers makes it possible to overcome this restriction. Diode-lasers can deliver power sufficient to heat silicon films up to melting point. A line-focus beam, which can be from a few millimeters to tens of centimeters long, is ideal for a large-area, thin-film device treatment, while laser scanning speeds allow control of the treatment time (or exposure) in the milliseconds range (1–100ms).3 In this range, the heat transfer is limited to a few tens of microns, and thus mostly confined to a thin film itself.4, 5 As a result, we can apply higher temperatures to silicon film while keeping the glass at a far lower temperature.
Line-focus diode-lasers can be used for liquid-phase crystallization of ultrathin (50–300nm) silicon films on glass to fabricate high crystal-quality seed layers for solar-cell applications.6 We developed three other applications of diode lasers using poly-Si cell technology. The first is defect annealing in pre-cystallized poly-Si films. The second is dopant activation and dopant diffusion in pre-crystallized poly-Si films to improve film electronic quality and create a solar-cell structure. The third is laser-induced liquid-phase crystallization of a few micron-thick silicon films to be used as a cell absorber (see Figure 2). Typical temperature profiles in silicon films during the laser treatment are shown in Figure 3 for different laser exposures and doses ('dose' here refers to power × exposure). The relative shape of the temperature profile does not depend on laser powers and/or scanning speeds, but it stays exactly the same for different laser conditions. This suggests that the profile is entirely determined by the laser beam shape. If a particular process requires a specific temperature profile, it can be achieved only by adjusting the beam shape accordingly.
Figure 1. Schematic structure of polycrystalline silicon (poly-Si) thin-film solar cells (arbitrary scale). p: p-type silicon (doped with boron), n: n-type (doped with phosphorous). SiN: silicon mononitride.
We achieved diode-laser-induced defect annealing/dopant activation in solid-phase crystallized (SPC) poly-Si thin-film solar cells by scanning a laser beam over the cell structure. Under the condition of 30–60J/cm2 laser dose and 3–6ms exposure, the resulting solar cell open-circuit voltages (Voc) are marginally higher than the voltages after standard rapid thermal annealing (RTA): 480–492mV versus 462–472mV. At the same time, the maximum glass temperature for the laser process is about 650°C, compared with 930°C for RTA.7
Figure 2. Snapshot of diode-laser-induced silicon film crystallization. The process is undertaken on several silicon films for use as a cell absorber.
We used dopant (boron or phosphorous) diffusion in poly-Si thin films to create a very thin (30–100nm) but heavily doped surface layer, which can serve either as a back-surface field (BSF) in SPC solar cells, or as an emitter in laser-crystallized (LC) cells, respectively. The process is very similar to that for diode- laser defect annealing, but we applied a spin-on-dopant source to the poly-Si film surface prior to the laser treatment. Boron- doped BSF diffusion requires 40–50J/cm2 of laser dose and 2–3ms exposure. The Voc of SPC poly-Si solar cells with laser-doped BSF is 450–470mV, similar to the Voc of the cells with as-deposited BSF. Phosphorous-doped emitter diffusion in LC cells is achieved at 96J/cm2 of laser dose and 2–4ms exposure. The Voc of 480–520mV for LC cells with laser-diffused emitter is comparable to the Voc of the cells with the thermally diffused emitter. In both BSF and emitter diffusion processes, the maximum temperature experienced by glass is reduced from about 900°C (thermal diffusion) to 650°C.
Figure 3. Temperature profiles in 2μm-thick poly-Si film on glass for different scanning speeds and doses: 0.075m/s (2.3ms, 21J/cm2); 0.100m/s (1.8ms, 17J/cm2); 0.133m/s (1.3ms, 12J/cm2).
Figure 4. (a) Backscatter electron microscopy image of linear grains in diode-laser crystallized silicon film; (b) Transmission electron microscope (TEM) image of defect-free grain in laser crystallized silicon film; (c) TEM image of grains in solid-phase crystallized silicon film.
Our most successful application of the diode laser was for liquid-phase crystallized poly-Si thin-film cells. SPC cells have a high defect density that limits cell Voc to about 500mV. We obtained a better silicon material quality by line-focus diode laser-induced melting and subsequent solidification. We achieved a continuous lateral crystal growth, whereby the growth front was seeded by the preceding crystallized region, forming high quality long parallel grains (see Figure 4a).8 Other researchers have developed a similar crystallization process using an electron beam as a heat source.9 A cross-sectional transmission electron microscopy (TEM) image of LC silicon film (see Figure 4b) shows grains with relatively low defect density, <1×106cm−2, as compared with 1×1010cm−2 in SPC poly-Si (see Figure 4c). Lower defect density means higher electronic quality and better final solar cell performance. After diffusing a phosphorous-doped emitter, we achieved LC cell Voc of up to 557mV and energy conversion efficiency of 8.7%.
The current cell performance is limited by a simplified research device design, while the electronic quality of LC silicon films is consistent with efficiencies above 13%.10 To achieve such high efficiency levels, we intend to introduce more advanced design features, for example, higher contact doping and better light-trapping, which are typical for commercial SPC poly-Si solar cells.
The research work in this article was funded by the Australian Research Council and Australian Solar Institute. The following researchers have also contributed to the presented results: J. Huang, K. H. Kim, M. Green of University of NSW, and O. Kunz, U. Schubert, R. Egan of Suntech R&D Australia.
Sergey Varlamov, Bonne Eggleston, Jonathon Dore
University of New South Wales
Daniel Ong, Rhett Evans
Suntech R&D Australia
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