Kyung Kim

Carrier-Induced Degradation in Multicrystalline Silicon: Dependence on the Silicon Nitride Passivation Layer and Hydrogen Released During Firing

Carrier-induced degradation (CID) of multicrystalline silicon (mc-Si) solar cells has been receiving significant attention; however, despite this increasing interest, the defect (or defects) responsible for this degradation has not been determined yet. Previous studies have shown that the surface passivation layer and the firing temperature have a significant impact on the rate and extent of this degradation. In this paper, we further study this impact through an investigation of the CID behavior of the mc-Si wafers passivated with six different silicon nitride layers, each fired at four different peak temperatures. At low firing temperatures, no significant difference in the CID was identified between the samples with different passivation layers; however, a large range of degradation extents was observed at higher firing temperatures. Using Fourier transform infrared spectroscopy, a correlation was found between the degradation extent and the amount of hydrogen released from the dielectric during firing. We verified that no degradation of the surface passivation quality occurred, indicating that the degradation is primarily associated with a bulk defect.

Effect of deposition temperature on electron-beam evaporated polycrystalline silicon thin-film and crystallized by diode laser

The effects of the deposition temperature on the microstructure, crystallographic orientation, and electrical properties of a 10-μm thick evaporated Si thin-film deposited on glass and crystallized using a diode laser, are investigated. The crystallization of the Si thin-film is initiated at a deposition temperature between 450 and 550 °C, and the predominant (110) orientation in the normal direction is found. Pole figure maps confirm that all films have a fiber texture and that it becomes stronger with increasing deposition temperature. Diode laser crystallization is performed, resulting in the formation of lateral grains along the laser scan direction. The laser power required to form lateral grains is higher in case of films deposited below 450 °C for all scan speeds. Pole figure maps show 75% occupancies of the (110) orientation in the normal direction when the laser crystallized film is deposited above 550 °C. A higher density of grain boundaries is obtained when the laser crystallized film is deposited below 450 °C, which limits the solar cell performance by n = 2 recombination, and a performance degradation is expected due to severe shunting.

Low-Absorbing and Thermally Stable Industrial Silicon Nitride Films With Very Low Surface Recombination

Amorphous silicon nitride has become the state-of-the-art antireflection coating for silicon solar cells. Optimization of silicon nitride films requires consideration of both the films optical and electrical properties. It is commonly assumed that silicon-rich silicon nitride films (films with high refractive index) provide better surface passivation, compared to that obtained by films with lower indices. However, silicon-rich films are usually very absorptive in the short (and even medium) wavelength range. Development of low absorption silicon nitride films, that provide good surface passivation, is therefore highly valuable. In this study we compare nine different industrial silicon nitride films, all with similarly low refractive index of 2.09 ± 0.01 measured at 633 nm. We demonstrate that these films exhibit very different electrical, chemical, and optical properties despite their similar refractive index values and correlate these differences with the specific deposition conditions. As a result of this investigation, we have developed industrial thermally stable low-absorbing silicon nitride films that provide excellent surface passivation, with surface saturation current density of 7 fA/cm2 on both n- and p-type wafers. We demonstrate that the developed low absorption films provide surface passivation with equal quality to that obtained by industrial silicon-rich silicon nitride films.

Should the refractive index at 633 nm be used to characterize silicon nitride films

The refractive index at 633 nm is often used to characterize silicon nitride films. Besides providing information about the reflection at this particular wavelength, it is frequently used to indicate additional information regarding the films absorption and even regarding its surface passivation quality. In this study, we compare nine different silicon nitride films, all with a similar refractive index at 633 nm (2.09±0.01). We demonstrate that these films exhibit very different electrical, chemical and optical properties despite their similar refractive index values. As a result of this investigation, we have developed industrial low-absorption silicon nitride films that provide excellent surface passivation, with saturation current density of 7 fA/cm2 on both n- and p-type wafers. This surface passivation quality is equal to that obtained by industrial silicon-rich silicon nitride films. All the films developed in this study were fabricated using industrial equipment and are thermally stable.

Diode laser processed crystalline silicon thin-film solar cells

Line-focus diode laser is applied to advance crystalline silicon thin-film solar cell technology. Three new processes have been developed: 1) defect annealing/dopant activation; 2) dopant diffusion; 3) liquid phase crystallisation of thin films. The former two processes are applied to either create a solar cell device from pre-crystallised films or improve its performance while reducing the maximum temperature experienced by substrate. The later process is applied to amorphous silicon films to obtain high crystal and electronic quality material for thin-film solar cells with higher efficiency potential. Defect annealing/dopant activation and dopant diffusion in a few micron thick poly-Si films are achieved by scanning with line-focus 808 nm diode laser beam at 15-24 kW/cm2 laser power and 2~6 ms exposure. Temperature profile in the film during the treatment is independent from laser power and exposure but determined by beam shape. Solar cell open-circuit voltages of about 500 mV after such laser treatments is similar or even higher than voltages after standard rapid-thermal treatments while the highest temperature experienced by glass is 300C lower. Amorphous silicon films can be melted and subsequently liquid-phase crystallised by a single scan of line laser beam at about 20 kW/cm2 power and 10-15 ms exposure. Solar cells made of laser-crystallised material achieve 557 mV opencircuit voltage and 8.4% efficiency. Electronic quality of such cells is consistent with efficiencies exceeding 13% and it is currently limited by research-level simplified cell metallisation.

Outstanding As-deposited surface passivation by industrial PECVD aluminum oxide

Aluminum oxide has been highlighted as a promising surface passivation layer for p-type silicon surface. To-date, most of the studies have focused on aluminum oxide layers deposited with atomic layer deposition systems which have lower throughput than industrial plasma-based systems. In this study, the effects of deposition conditions on the electrical and optical properties of aluminum oxide deposited by an industrial plasma enhanced chemical vapor deposition system are presented. Low saturation current density of 1.9 fA/cm2 was achieved by as deposited layer on p-type Czochralski wafer. The most significant deposition process factor for high quality surface passivation was found to be the gas flow rate ratio between nitrous oxide and tri-methyl-aluminum.

High performance plasma hydrogenation for large-grained polycrystalline silicon thin film solar cells

Polycrystalline silicon thin-film solar cells with large grains up to 1 mm wide and 10 mm long created by diode laser crystallization require an enhanced high performance plasma hydrogenation process for better cell performance. Higher process temperature, longer process time and higher plasma power (800°C, 16 minutes and 3.9 kW, respectively) than those used for smaller grain polycrystalline silicon films were applied and their effects on the cell performance were evaluated. It is found that open circuit voltage increases from 417 mV to 451 mV for 0.1-suns and from 518 mV to 529 mV for 1-sun due to lower the diode saturation currents from 3.1e-5 A/cm2 to 1.2e-5 A/cm2 after such a hydrogenation process. This result shows that enhanced hydrogenation has a potential for application to various polycrystalline silicon films with wide range of grain size and defect density.

Selective high concentration doping of boron near absorber contacts of a laser crystallized silicon thin-film solar cell on glass

Liquid phase crystalline silicon solar cell on glass with lightly doped absorber layer has degradations in Voc and FF after contact bake. Experimental results provide evidence of elimination of Voc degradation with selective high concentration doping near absorber contacts. Comparison of cell performance between baseline processed and selectively doped samples are provided. After 43 days, the enhanced cell performance continued, with negligible deviations. In addition, a low series resistance of 1.5 Ω was obtained.

Effects of absorber doping and post-deposition treatments on the performance of n-type poly-Si thin film solar cells on glass

High temperature, 740 °C or 800 °C, hydrogenation treatment was applied to n-type polycrystalline silicon thin film solar cells on glass with absorber doping from 5×10¹⁶ cm^-3 to 5×10¹⁷ cm^-3, after rapid thermal annealing at 930 °C or 970 °C. Effects of hydrogenation and RTA temperature on the performance of n-type poly-Si thin film solar cells were studied. It is found that higher RTA temperature leads to higher open circuit voltage in all absorber doping range at low hydrogenation temperature. Higher hydrogenation temperature improves the open circuit voltage in the mediate absorber doping range significantly and this effect is independent of RTA temperature.

Material characteristics of crystalline Si thin-film solar cells on glass fabricated by diode laser crystallization

Diode laser crystallization was performed silicon thin film on glass. Large linear grains along the laser scanning direction were formed when the laser scanning speed of 150-1000 mm/min was used. First order □3 twin boundaries were found to be dominating grain boundaries. Pole figure measurement showed very uniform (100) texture can be formed when SiOx layer capping layer was used. Promising bulk resistivity of the as-crystallized films was resulted. Emitter was formed using spin on diffusion and subsequent RTP Suns Voc results after emitter formation exhibited n=1 recombination. Hydrogen plasma passivation effectively passivated grain boundaries.