Jonathon Dore

Progress in Laser-Crystallized Thin-Film Polycrystalline Silicon Solar Cells: Intermediate Layers, Light Trapping, and Metallization

Diode laser crystallization of thin silicon films on the glass has been used to form polycrystalline silicon layers for solar cells. Properties of an intermediate layer stack of sputtered SiOx/SiNx/SiOx between the glass and the silicon have been improved by reactively sputtering the SiNx layer, which result in enhanced optical and electrical performance. Light trapping is further enhanced by texturing the rear surface of the silicon prior to metallization. An initial efficiency of 11.7% with VOC of 585 mV has been achieved using this technique, which are the highest values reported for poly-Si solar cells on glass substrates. Cells suffer a short term, recoverable degradation of VOC, and fill factor. The magnitude of the degradation is reduced via the repeated thermal treatment. A selective p+ metallization scheme has been developed which eliminates the degradation altogether.

Intermediate Layer Development for Laser-Crystallized Thin-Film Silicon Solar Cells on Glass

The intermediate layer (IL) between the glass and silicon plays an important role in laser-crystallized thin-film silicon solar cells. SiO_X, SiN_X, and SiC_X deposited by RF sputtering or plasma-enhanced chemical vapor deposition, either as single layers or in stacks, have been tested as ILs with regard to silicon wettability and silicon crystal quality and the effect of hydrogen passivation. SiC_X is the best wetting layer, allowing a larger laser crystallization process window than SiO_X or SiN_X. SiN_X layers are limited by pinholing, which increases in severity with laser fluence. SiO_X ILs result in lower silicon grain-boundary density compared with SiC_x-based layers and to SiN_X-based layers. Hydrogen passivation of laser-crystallized silicon on single layer SiO_X has no impact on V_OC, while an improvement of around 60 mV is found for samples on SiO_x/SiN_x/SiO_x stacks. Diffusion of dopants from the IL are found to create a uniformly doped absorber with no evidence of a front-surface field.

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.

Data mining photovoltaic cell manufacturing data

So called “data mining” techniques comprise a broad family of statistical investigative and analysis techniques from informal exploratory and graphical methods through to sophisticated multivariate analysis. Data mining of photovoltaic (PV) cell manufacturing data can be used with an understanding of cell performance to isolate the variance in production associated with wafer material quality or other time based changes. This can lead to new insights for SPC and for understanding process variance.

Laser crystallised silicon-on-glass thin films and solar cells: Significance of the interface

Liquid-phase crystallised Si on-glass has emerged as a new PV material potentially capable of solar cell efficiencies comparable to that of mc-Si wafer cells. An interface between Si and glass is found to play a crucial role in realizing the efficiency potential. Transparent dielectrics, such as SiOx, SiCx, SiNx, are used as interface buffers to serve a few functions: Si wetting and adhesion; high transmission and antireflection; impurity diffusion control; interface passivation. Effects of different dielectrics and their combinations on Si film and solar cell properties are presented. The triple layer buffer of SiOx/SiNx/SiOx resulted in the best achieved efficiency of 11.7%.