Baojie Yan

Relationship of deep defects to oxygen and hydrogen content in nanocrystalline silicon photovoltaic materials

We report measurements of the structural and compositional properties of a range of hydrogenated nanocrystalline films. We employed Raman spectroscopy for crystallinity and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) for impurity characterizations. The crystalline volume fractions and impurity levels are correlated with the deep state densities determined by drive level capacitance profiling. Those defects were found to have a thermal emission energy of 0.65±.05u2002eV. We found that the overall crystallinity correlated reasonably well with the density of such defect states and also found a strong correlation between the defect density and the levels of oxygen impurities. Possible origins of these defects are discussed.

Temperature-Dependent Open-Circuit Voltage Measurements and Light-Soaking in Hydrogenated Amorphous Silcon Solar Cells

We present temperature-dependent measurements of the open-circuit voltage V OC (T) in hydrogenated amorphous silicon nip solar cells prepared at United Solar. At room-temperature and above, VOC measured using near-solar illumination intensity differs by as much as 0.04 V for the as-deposited and light-soaked states; the values of V OC for the two states converge below 250 K. Models for V OC based entirely on recombination through deep levels (dangling bonds) do not account for the convergence effect. The convergence is present in a model that assumes the recombination traffic in the as-deposited state involves only bandtails, but which splits the recombination traffic fairly evenly between bandtails and defects for the light-soaked state at room-temperature. Recombination mechanisms are important in understanding light-soaking, and the present results are inconsistent with at least one well-known model for defect generation.

Light trapping effect from randomized textures of Ag/ZnO back reflector on hydrogenated amorphous and nanocrystalline silicon based solar cells

We report our progress in the optimization of Ag/ZnO back reflectors (BR) for a-Si:H and nc-Si:H solar cells. Theoretically, a BR with a smooth metal surface and a textured dielectric surface would be more desirable. A smooth metal/dielectric interface reduces the plasmonic resonance loss and parasitic losses due to light trapped in sharp angles; a textured dielectric/semiconductor interface provides scattering for light trapping. In order to obtain sufficient light scattering at the ZnO/silicon interface, a highly textured ZnO layer is normally used. However, a highly textured ZnO surface causes deterioration of nc-Si:H material quality. In addition, to make a highly textured ZnO surface, a thick ZnO layer is needed, which could introduce additional absorption in the bulk ZnO layer and reduce the photocurrent density. Therefore, Ag/ZnO BR structures for nc-Si:H solar cells needs to be optimized experimentally. In this study, we found that an optimized Ag/ZnO BR for nc-Si:H solar cells is constructed with textured Ag and thin ZnO layers. Although a textured Ag layer might cause certain losses resulting from plasmonic absorption, the enhanced light scattering by a moderately textured Ag layer makes it possible to use a thin ZnO layer, where the absorption in the ZnO layer is low. With such a BR, we achieved a short-circuit current density of over 29 mA/cm2 from a nc-Si:H single-junction solar cell. Using the high performance nc-Si:H cell in an a-Si:H/nc- Si:H/nc-Si:H triple-junction structure, we achieved an initial active-area efficiency of 14.5% with a total current density exceeding 30 mA/cm2.

Extraction of carrier transport parameters from hydrogenated amorphous and nanocrystalline silicon solar cells

Transport properties are very important for solar cells. The efficiency of solar cells is determined by the competition of carrier collection and recombination. The most important parameter is the carrier mobility-lifetime product. However, methods commonly used for measuring transport parameters require specially designed samples. The results are often not easily correlated to solar cell performance. In this paper, we present our studies of extraction of material properties from conventional current-voltage characteristics and quantum efficiency curves. First, we carried out analyses of shunt resistance as a function of the light intensity. For solar cells with no clear parasitic shunt resistance, the shunt resistance is inversely proportional to the short-circuit current, and its proportionality coefficient is related to the effective carrier mobility-lifetime product. For an a-Si:H solar cell made under an optimized condition with high hydrogen dilution, the effective mobility-lifetime product was estimated to be 1.2x10-8 cm2/V. For a-SiGe:H solar cells, the effective mobility-lifetime product depends on Ge content. For optimized a-SiGe:H bottom cells used in high efficiency a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structures, their values are ~5.0x10-9 cm2/V. For high efficiency nc-Si:H solar cells, the effective mobility-lifetime product is ~5.0x10-7 cm2/V. Second, we measured the quantum efficiency as a function of electrical bias and developed an analytical model to deduce the effective mobility-lifetime product. The results obtained from the second method are consistent with the values from the first method. We will present detailed analyses and interpretations of the transport parameters and their correlation to solar cell performance.

Correlation between Powder in the Plasma and Stability of High Rate Deposited a-Si:H

Hydrogenated amorphous silicon (a-Si:H) solar cells incorporating high deposition rate (8-10A/s) intrinsic layers were deposited using modified very high frequency (MVHF) plasma. We have monitored the light scattered from powder generated in the plasma using an Ar-laser and a silicon photodiode. This simple, non-invasive technique allows us to make measurements on the same reactor used to make the solar cells. First, we have varied the total flow rate and observed a maximum in the scattered light intensity from powder in the plasma during the deposition of the intrinsic layer, and correlated this with the degradation, as well as the stabilized performance of the solar cells. Then, we have studied the effects of varying the deposition temperature and/or the addition of germane to the gas mixture on the scattered light intensity due to powder in the plasma.

Correlation of Material Properties and Open-Circuit Voltage of Amorphous Silicon Based Solar Cells

Correlation of hydrogenated amorphous silicon (a-Si:H) alloy material properties and solar cell characteristics have been studied experimentally and by computer simulation. Simulation results show that all three solar cell parameters, short-circuit current density (J sc ), open-circuit voltage (V oc ), and fill factor (FF), decrease with increased defect density. For a given intrinsic layer thickness, a larger band gap (E g ) results in a higher V oc but a lower J sc . However, FF does not depend on band gap. This allows us to distinguish the effect of change in band gap from that in defect density on the variation in V oc . For solar cells with good interface characteristics, a linear relation FF = βV oc + γ is obtained by light soaking experiments and simulation with different defect densities. The slope β is in the range from 2 to 3 V -1 depending on cell properties and light soaking condition, and the intersect γ depends mainly on the band gap. Comparing cells made with high H 2 dilution to no H 2 dilution, we find that a 58 mV enhancement in V oc with H 2 dilution is due to both widening of band gap and reduced defect density. Simulation results also show that a narrower valence band tail leads to a higher V oc . We did not include this effect in the analysis due to lack of available data for correlation between H 2 dilution and band tail narrowing.