Defect Study of Polycrystalline–silicon Seed Layers made by Aluminum Induced Crystallization

Defect etching revealed a very large density (∼109cm−2) of intragrain defects in polycrystalline silicon (pc-Si) layers obtained through aluminum-induced crystallization of amorphous Si and epitaxy. Electron-beam-induced current measurements showed a strong recombination activity at these defects. Cathodoluminescence measurements showed the presence of two deep-level radiative transitions (0.85 and 0.93eV) with a relative intensity varying from grain to grain. These results indicate that the unexpected quasi-independence on the grain size of the open-circuit voltage of these pc-Si solar cells is due to the presence of numerous electrically active intragrain defects.

The formation of device-grade polycrystalline silicon (poly-Si) films on standard glass at low temperature (i.e., below the softening point of the glass) using simple and fast processes is one of the major challenges for low-cost thin-film solar cells. In this paper, we report what is believed to be the first successful realisation of thick (/spl sim/5 /spl mu/m), large-grained (/spl sim/5 /spl mu/m), continuous poly-Si films on glass at T<650/spl deg/C. This technological breakthrough has been achieved by using ion-assisted deposition at 630/spl deg/C for epitaxially thickening a thin (/spl sim/0.4 /spl mu/m) seeding layer made on glass by aluminium-induced crystallisation.

A polycrystalline silicon (pc-Si) film is produced by low-temperature LPCVD process on a poly silicon seed layer. The properties of such film are characterized and compared with the films on a glass substrate and a silicon wafer. The poly crystalline silicon seed layers are fabricated by aluminum-induced crystallization (AIC) process with a glass/Al/Al 2 O 3 /a-Si structure, in which the Al layer was deposited on the glass substrate by a DC magnetron sputter, and a-Si film by PECVD method, respectively. After the AIC process with 400°C for 150min, the poly silicon film has the average grain size of about 10μm. The poly silicon seed layer is thickened (i.e. the poly silicon film is deposited on the seed layer) by LPCVD process with 600°C for 60min. As results, it is observed that the quality of the poly silicon film on the seed layer is better than that on the glass and worse than that on the silicon wafer. But the deposited poly silicon absorber layer could not reflect the property of the poly silicon seed layer, due to the relatively low temperature process.

In this work, the kinetics of aluminum-induced crystallization (AIC) of non-stoichiometric silicon oxide a-SiO0.25 was investigated for annealing temperatures of 370, 385 and 400 °C, as a result of which thin films of polycrystalline silicon were obtained. It is shown that for low annealing temperatures, the surface morphology of the crystalline material is represented by dendric structures corresponding to the growth model with diffusion-limited aggregation. In addition, with an increase in the annealing temperature, the nucleation density increases from 3 to 53 mm–2. From the Arrhenius plot, the activation energy of the AIC process of a-SiO0.25 was obtained for the first time, which was 3.7±0.4 eV.

Large-grained polycrystalline silicon on glass for thin-film solar cells

In this work, efficiency enhancement of wafer based polycrystalline silicon (pc-Si) solar cells has been investigated by using plasmonic silver nanoparticles (Ag NPs) on the top surface. Ag NPs has been fabricated by rapid thermal processing (RTP) of non-continuous silver thin film at low temperature regime (200–300 °C) for various RTP durations (5–30 min). The results obtained reveals that an optimum RTP durations of 20 min leads to drastic reflectance reduction of pc-Si wafers with minimum average particle size at all temperatures. At 20 min RTP duration, the total reflectance of the pc-Si wafers integrated with Ag NPs reduced from 35.1% of the raw pc-Si wafer to 12.97, 17.9 and 19.31% respectively at 200, 250 and 300 °C in the wavelength region 300–800 nm. Around 3% increase in conversion efficiency of pc-Si solar cells is observed due to incorporation of plasmonic Ag nanoparticles.

First AM conference joins MPIF's PM2014 and hardmetals events

Recently polycrystalline silicon (pc-Si) thin film transistors (TFT's) have emerged as the devices of choice for many applications. The TFT's made of a thin un-doped polycrystalline silicon film deposited on a glass substrate by the Low Pressure Chemical Vapor Deposition technique LPCVD have limits in the technological process to the temperature <; 600°C. The benefit of pc-Si is to make devices with large grain size. Unfortunately, according to the conditions during deposition, the pc-Si layers can consist of a random superposition of grains of different sizes, where grains boundaries parallels and perpendiculars appear. In this paper, the transfer characteristics IDS-VGS are simulated by solving a set of two-dimensional (2D) drift-diffusion equations together with the usual density of states (DOS: exponential band tails and Gaussian distribution of dangling bonds) localized at the grains boundaries. The impact of thickness of the active layer on the distribution of the electrostatic potential, the effect of density of intergranular traps states and grain size on the TFT's transfer characteristics IDS-VGS have been also investigated.

A new way to selectively remove Si islands from polycrystalline silicon seed layers made by aluminum-induced crystallization

Polycrystalline silicon thin films on glass substrate

For the reduction of optical loss and contact material cost in silicon heterojunction (SHJ) solar cells, copper plating has been considered as a suitable metallization technique. However, a plated copper contact on an indium-tin-oxide (ITO) generally has low a reliability of adhesion. For this reason, proper seed layer materials are required for adhesive copper plating. As a requirement for a suitable seed layer material, the contact resistance between the seed and the ITO is also important, as well as the adhesion, because high series resistance results in a low fill factor. In this research, we deposited a seed layer by co-evaporating copper with other metals (Cu-X). The contact resistivity (ρ c ) of the Cu-X deposited on ITO was evaluated by using the transfer length method (TLM). In order to confirm the influence of the work function of the Cu-X, we measured the contact potential difference (CPD) by using photoemission in an air system (PAS). Moreover, phase and we analyzed the crystallite size by using X-ray diffraction (XRD) measurement.

The fabrication of large-grain 1.25 μm thick polycrystalline silicon (poly-Si) films via two-stage aluminum-induced crystallization (AIC) for application in thin-film solar cells is reported. The induced 250 nm thick poly-Si film in the first stage is used as the seed layer for the crystallization of a 1 μm thick amorphous silicon (a-Si) film in the second stage. The annealing temperatures in the two stages are both 500°C. The effect of annealing time (15, 30, 60, and 120 minutes) in the second stage on the crystallization of a-Si film is investigated using X-ray diffraction (XRD), scanning electron microscopy, and Raman spectroscopy. XRD and Raman results confirm that the induced poly-Si films are induced by the proposed process.

Plasma hydrogenation is an efficient method to passivate intergrain and intragrain defects of polycrystalline silicon (pc-Si) solar cells. The hydrogenation experiments were carried out in hydrogen plasma generated in an electron cyclotron resonance system controlling different operating parameters such as microwave power (P MW), process time (t H) and hydrogenation temperature (T H) for a fixed hydrogen flux of 30 sccm. The hydrogenation of n+pp+ pc-Si solar cells resulted in an improvement in the open-circuit voltage. The improvement was correlated with the dopant deactivation due to the formation of boron–hydrogen bonding. This was demonstrated from the changes in the doping level after hydrogenation of n+p diode structures made using single crystalline silicon as a reference material. It was found that deactivation of boron was more pronounced at high microwave plasma power, in good agreement with the high open-circuit voltage values obtained on pc-Si mesa cells. On the other hand, the effect of longer hydrogenation time and higher temperature resulted in a decrease of boron deactivation, while an increase in V oc with a tendency of saturation at high T H was observed. Reasons for such behavior were thoroughly explained.

Application of phosphorus diffusion gettering process on upgraded metallurgical grade Si wafers and solar cells

Chemical-vapor deposited large-area multicrystalline epitaxial silicon solar cells on upgraded metallurgical silicon/graphite substrates