Guangtao Yang

Design and application of ion-implanted polySi passivating contacts for interdigitated back contact c-Si solar cells

Ion-implanted passivating contacts based on poly-crystalline silicon (polySi) are enabled by tunneling oxide, optimized, and used to fabricate interdigitated back contact (IBC) solar cells. Both n-type (phosphorous doped) and p-type (boron doped) passivating contacts are fabricated by ion-implantation of intrinsic polySi layers deposited via low-pressure chemical vapor deposition and subsequently annealed. The impact of doping profile on the passivation quality of the polySi doped contacts is studied for both polarities. It was found that an excellent surface passivation could be obtained by confining as much as possible the implanted-and-activated dopants within the polySi layers. The doping profile in the polySi was controlled by modifying the polySi thickness, the energy and dose of ion-implantation, and the temperature and time of annealing. An implied open-circuit voltage of 721 mV for n-type and 692 mV for p-type passivating contacts was achieved. Besides the high passivating quality, the developed ...

Combined Optical and Electrical Design of Plasmonic Back Reflector for High-Efficiency Thin-Film Silicon Solar Cells

A back reflector (BR) that can efficiently scatter weakly absorbed light is essential to obtain high-efficiency thin-film silicon solar cells. We present the design routes of plasmonic BR based on self-assembled silver nanoparticles (Ag NPs) for high-efficiency thin-film silicon solar cells. Both optical and electrical effects on solar cells are considered. The shape of Ag NPs, the thickness of ZnO:Al spacer layers, materials on top of Ag NPs, and nanoparticle size are crucial for the performance of plasmonic BR. Increased annealing temperature lead to the formation of more appropriate shapes (more spherical and regular shapes) for a good light scattering and, thus, increase the photocurrent. The ZnO:Al layer between the Ag NPs and the Ag planar film has an optical effect on solar cells, while the ZnO:Al layer between the Ag NPs and the doped a-Si:H has both optical and electrical influence on the device. Larger NPs have less parasitic absorption and can preferentially scatter light into larger angles, thus increasing the spectral response in the solar cell. However, for larger Ag NPs, the fill factor deteriorates due to the rougher surface in the plasmonic BR, indicating a compromise between light trapping and electrical performance. Following the design routes, we obtained 8.4% high-efficiency plasmonic a-Si:H solar cell.

Periodic and Random Substrate Textures for Liquid-Phase Crystallized Silicon Thin-Film Solar Cells

A major limitation in current liquid-phase crystallized (LPC) silicon thin-film record solar cells is optical losses caused by their planar glass-silicon interface. In this study, silicon is grown on nanoimprinted periodically, as well as randomly textured glass substrates, and successfully implemented into state-of-the-art LPC silicon thin-film solar cells. Compared with an optimized planar reference device, both textures enhance absorption of light. Interlayer and process optimization allowed achieving a material quality comparable with the planar reference device. On the random texture, an open-circuit voltage above 630 mV was obtained, as well as an external quantum efficiency exceeding the planar reference device by +3 mA/cm2.

Determination of defect density of state distribution of amorphous silicon solar cells by temperature derivative capacitance-frequency measurement

In this contribution, we demonstrate the application temperature dependent capacitance-frequency measurements (C-f) to n-i-p hydrogenated amorphous silicon (a-Si:H) solar cells that are forward-biased. By using a forward bias, the C-f measurement can detect the density of defect states in a particular energy range of the interface region. For this contribution, we have carried out this measurement method on n-i-p a-Si:H solar cells of which the intrinsic layer has been exposed to a H2-plasma before p-type layer deposition. After this treatment, the open-circuit voltage and fill factor increased significantly, as well as the blue response of the solar cells as is concluded from external quantum efficiency. For single junction, n-i-p a-Si:H solar cells initial efficiency increased from 6.34% to 8.41%. This performance enhancement is believed to be mainly due to a reduction of the defect density in the i-p interface region after the H2-plasma treatment. These results are confirmed by the C-f measurements. Af...

Poly-crystalline silicon-oxide films as carrier-selective passivating contacts for c-Si solar cells

The poly-Si carrier-selective passivating contacts (CSPCs) parasitically absorb a substantial amount of light, especially in the form of free carrier absorption. To minimize these losses, we developed CSPCs based on oxygen-alloyed poly-Si (poly-SiOx) and deployed them in c-Si solar cells. Transmission electron microscopy analysis indicates the presence of nanometer-scale silicon crystals within such poly-SiOx layers. By varying the O content during material deposition, we can manipulate the crystallinity of the poly-SiOx material and its absorption coefficient. Also, depending on the O content, the bandgap of the poly-SiOx material can be widened, making it transparent for longer wavelength light. Thus, we optimized the O alloying, doping, annealing, and hydrogenation conditions. As a result, an extremely high passivation quality for both n-type poly-SiOx (J0¼3.0 fA/cm2 and iVoc¼740 mV) and p-type poly-SiOx (J0¼17.0 fA/cm2 and iVoc¼700 mV) is obtained. A fill factor of 83.5% is measured in front/back-contacted solar cells with both polarities made up of poly-SiOx. This indicates that the carrier transport through the junction between poly-SiOx and c-Si is sufficiently efficient. To demonstrate the merit of poly-SiOx layers’ high transparency at long wavelengths, they are deployed at the back side of interdigitated backcontacted (IBC) solar cells. A preliminary cell efficiency of 19.7% is obtained with much room for further improvement. Compared to an IBC solar cell with poly-Si CSPCs, a higher internal quantum efficiency at long wavelengths is observed for the IBC solar cell with poly-SiOx CSPCs, thus demonstrating the potential of poly-SiOx in enabling higher JSC.

Facing the challenge of liquid phase crystallizing silicon on textured glass substrates

A major limitation in current liquid phase crystallized (LPC) silicon thin-film record solar cells are optical losses caused by their planar glass-silicon interface. In this study, silicon is grown on nanoimprinted periodically as well as on randomly textured glass substrates and successfully implemented into state-of-the-art LPC silicon thin-film solar cell stacks. By systematically varying every layer the whole sample stack is optimized regarding its anti-reflection ability. Compared to an optimized planar reference device, a reduction of reflection losses by -3.5% (absolute) on the random and by -9.4% (absolute) on the periodic texture has been achieved in the wavelength range of interest.

Moly-poly solar cell: Industrial application of metal-oxide passivating contacts with a starting efficiency of 18.1%

We present large-area “moly-poly” cells, with a front side MoOx/a-Si:H(i) passivating contact and a rear-side poly-Si/SiOx stack, and we have demonstrated that MoOx based c-Si solar cell technology can be scaled to industrial wafer size. Excellent surface passivation was achieved using MoOx and poly-Si, leading to implied Voc values above 700 mV, and a final cell Voc of 687 mV. However, some care needs to be taken to avoid parasitic optical losses in the infra-red (IR) spectral range due to free-carrier absorption (FCA). These losses were investigated by comparing poly-Si layers of different thicknesses, deposited by low-pressure or plasma-enhanced chemical vapor deposition (LPCVD or PECVD), at the rear side of moly-poly cells. We found that ultra-thin PECVD layers are most suitable for solar cell applications due to a very good trade-off between surface passivation and reduced FCA. Based on this result, a 18.1% efficient 9.2×9.2 cm2 molypoly cell was made, which is the highest reported efficiency so far for moly-poly cells. Finally, we present a preliminary study of the parasitic IR losses in the MoOx layer itself, when deposited on either a-Si:H or SiOx passivation layers

Textured substrate for high-efficiency n-i-p μc-Si:H solar cells

In this paper, we investigate the light scattering properties of our randomly textured back reflector (BR) for n-i-p thin-film microcrystalline silicon μc-Si:H) solar cells. The texture-etched ZnO:Al (AZO) covered with Ag is used as BR. The substrate temperature during the sputtering of AZO and the etching time are crucial for the morphology of the BR. An optimized substrate temperature can improve the morphology of etched AZO and optical performance of the BR, thereby increasing the photocurrent of solar cells. After Ag evaporation we obtain small features superimposed on the large features of the etched AZO. As a result the angle distribution becomes much wider. Large features of BR can preferentially scatter light into larger angles. The optimized BR has a wide surface angle distribution with an average angle of 27 ° and exhibiting a peak at 32 °. The optimized BR has a very broad reflective angular intensity distribution in air (AIDair) with peak at around 45 ° for incident light from 400 nm to 800 nm, and has a very high haze in reflection. With this optimized BR we obtained 7.68% efficiency for rf-PECVD deposited n-i-p μc-Si:H thin film solar cells. The Jsc is 25.2 mA/cm2 with a very high quantum efficiency in the long wavelength region. And the VHF-PECVD deposited n-i-p μc-Si:H thin film solar cells prove that the optimized BR does not deteriorates the solar cells electrical performance and the obtained the best efficiency is 8.9%.