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High-efficiency amorphous silicon (a-Si:H) solar cells were deposited with different thicknesses of the p-type amorphous silicon carbide layer on substrates of varying roughness. We observed a light-induced open-circuit voltage (Voc) increase upon light soaking for thin p-layers, but a decrease for thick p-layers. Further, the Voc increase is enhanced with increasing substrate roughness. After correction of the p-layer thickness for the increased surface area of rough substrates, we can exclude varying the effective p-layer thickness as the cause of the substrate roughness dependence. Instead, we explain the observations by an increase of the dangling-bond density in both the p-layer--causing a Voc increase--and in the intrinsic absorber layer, causing a Voc decrease. We present a mechanism for the light-induced increase and decrease, justified by the investigation of light-induced changes of the p-layer and supported by Advanced Semiconductor Analysis simulation. We conclude that a shift of the electron quasi-Fermi level towards the conduction band is the reason for the observed Voc enhancements, and poor amorphous silicon quality on rough substrates enhances this effect. [ABSTRACT FROM AUTHOR]

Several amorphous silicon (a-Si:H) deposition conditions have been reported to produce films that degrade least under light soaking when incorporated into a-Si:H solar cells. However, a systematic comparison of these a-Si:H materials has never been presented. In the present study, different plasma-enhanced chemical vapor deposition conditions, yielding standard low-pressure VHF a-Si:H, protocrystalline, polymorphous, and high-pressure RF a-Si:H materials, are compared with respect to their optical properties and their behavior when incorporated into single-junction solar cells. A wide deposition parameter space has been explored in the same deposition system varying hydrogen dilution, deposition pressure, temperature, frequency, and power. From the physics of layer growth, to layer properties, to solar cell performance and light-induced degradation, a consistent picture of a-Si:H materials that are currently used for a-Si:H solar cells emerges. The applications of these materials in single-junction, tandem, and triple-junction solar cells are discussed, as well as their deposition compatibility with rough substrates, taking into account aspects of voltage, current, and charge collection. In sum, this contributes to answering the question, 'Which material is best for which type of solar cell?' [ABSTRACT FROM AUTHOR]

We study absorption enhancement by light scattering at periodically textured interfaces in thin film silicon solar cells. We show that the periodicity establishes resonant coupling to propagating waveguide modes. Ideally, such modes propagate in the high index silicon film where they are eventually absorbed, but waveguide modes exist also in the transparent front contact layer if the product of its refractive index and thickness exceeds half the wavelength. Taking into account that the absorption coefficient of realistic transparent conducing films exceeds the one of silicon close to its band gap, certain waveguide modes will enhance parasitic absorption in the transparent front contact. From an analysis based on the statistic distribution of energy among the available waveguide and radiation modes, we conclude that conventional thin film silicon solar cells with thick and nonideal contacts may fail to reach the previously noted bulk limit of 4nSi2; instead, a more conservative limit of 4(nSi2-nTCO2) applies. [ABSTRACT FROM AUTHOR]

Heterojunction with intrinsic thin layer or “HIT” solar cells are considered favorable for large-scale manufacturing of solar modules, as they combine the high efficiency of crystalline silicon (c-Si) solar cells, with the low cost of amorphous silicon technology. In this article, based on experimental data published by Sanyo, we simulate the performance of a series of HIT cells on N-type crystalline silicon substrates with hydrogenated amorphous silicon (a-Si:H) emitter layers, to gain insight into carrier transport and the general functioning of these devices. Both single and double HIT structures are modeled, beginning with the initial Sanyo cells having low open circuit voltages but high fill factors, right up to double HIT cells exhibiting record values for both parameters. The one-dimensional numerical modeling program “Amorphous Semiconductor Device Modeling Program” has been used for this purpose. We show that the simulations can correctly reproduce the electrical characteristics and temperature dependence for a set of devices with varying I-layer thickness. Under standard AM1.5 illumination, we show that the transport is dominated by the diffusion mechanism, similar to conventional P/N homojunction solar cells, and tunneling is not required to describe the performance of state-of-the art devices. Also modeling has been used to study the sensitivity of N-c-Si HIT solar cell performance to various parameters. We find that the solar cell output is particularly sensitive to the defect states on the surface of the c-Si wafer facing the emitter, to the indium tin oxide/P-a-Si:H front contact barrier height and to the band gap and activation energy of the P-a-Si:H emitter, while the I-a-Si:H layer is necessary to achieve both high Voc and fill factor, as it passivates the defects on the surface of the c-Si wafer. Finally, we describe in detail for most parameters how they affect current transport and cell properties. [ABSTRACT FROM AUTHOR]

The deposition of a stack of amorphous (a-Si:H) and microcrystalline (μc-Si:H) tandem thin film silicon solar cells (micromorph) requires at least twice the time used for a single junction a-Si:H cell. However, micromorph devices have a higher potential efficiency, thanks to the broader absorption spectrum of μc-Si:H material. High efficiencies can only be achieved by mitigating the nanocracks in the μc-Si:H cell and the light-induced degradation of the a-Si:H cell. As a result, μc-Si:H cell has to grow on a smooth substrate with large periodicity (>1 μm) and the a-Si:H cell on sharp pyramids with smaller feature size (∼350 nm) to strongly scatter the light in the weak absorption spectra of a-Si:H material. The asymmetric intermediate reflector introduced in this work uncouples the growth and light scattering issues of the tandem micromorph solar cells. The stabilized efficiency of the tandem n-i-p/n-i-p micromorph is increased by a relative 15% compared to a cell without AIR and 32% in relative compared to an a-Si:H single junction solar cells. The overall process (T<200 °C) is kept compatible with low cost plastic substrates. The best stabilized efficiency of a cell deposited on polyethylene-naphthalate plastic substrate is 9.8% after 1000 h of light soaking at Voc, 1 sun, and 50 °C. [ABSTRACT FROM AUTHOR]

We present a study of the optical mode structure in metal-dielectric multilayer structures that represent amorphous silicon thin film solar cells with metallic back contact. Knowledge of the modal structure represents a first step toward describing absorption enhancement by the interface texture in solar cells. We present a method for determining experimentally the dispersion relations of multilayer films by coupling polarized light in a spectral reflection measurement to eigenmodes, using a one-dimensional sinusoidal grating. Because the used grating represents only a minor perturbation that establishes the coupling, the experimental data is well explained by the modal structure of a geometry with flat interfaces. On the basis of the measured mode structure, we propose an explanation for the beneficial effect of a low index buffer layer between the silicon absorber and the metallic back reflector. [ABSTRACT FROM AUTHOR]

Hydrogenated microcrystalline silicon growth by very high frequency plasma-enhanced chemical vapor deposition is investigated in an industrial-type parallel plate R&D KAI™ reactor to study the influence of pressure and silane depletion on material quality. Single junction solar cells with intrinsic layers prepared at high pressures and in high silane depletion conditions exhibit remarkable improvements, reaching 8.2% efficiency. Further analyses show that better cell performances are linked to a significant reduction of the bulk defect density in intrinsic layers. These results can be partly attributed to lower ion bombardment energies due to higher pressures and silane depletion conditions, improving the microcrystalline material quality. Layer amorphization with increasing power density is observed at low pressure and in low silane depletion conditions. A simple model for the average ion energy shows that ion energy estimates are consistent with the amorphization process observed experimentally. Finally, the material quality of a novel regime for high rate deposition is reviewed on the basis of these findings. [ABSTRACT FROM AUTHOR]

The influence of the substrate surface morphology on the performance of microcrystalline silicon solar cells in the substrate or n-i-p (nip) configuration is studied in this paper. The experiments are carried out on glass substrates coated with naturally textured films of ZnO deposited by low pressure chemical vapor deposition which serves as backcontact and as template for the light trapping texture. The film surface morphology can be modified with a plasma treatment which smoothens the V-shaped valleys to a more U-shaped form. We investigate, first, the influence of different substrates morphologies on the performance of microcrystalline (μc-Si:H) thin film silicon solar cells deposited by very high frequency plasma enhanced chemical vapor deposition. The V-shaped morphologies are found to have strong light trapping capabilities but to be detrimental for the μc-Si:H material growth and lead to degraded open circuit voltage (Voc) and fill factor (FF) of the solar cells. Hence, in Sec. III B, we introduce a buffer layer with a higher amorphous fraction between the n doped and intrinsic layer. Our study reveals that the buffer layer limits the formation of voids and porous areas in the μc-Si:H material on substrates with strong light trapping capabilities. Indeed, this layer mitigates Voc and FF losses which enhances the performance of the μc-Si:H solar cell. Finally, by applying our findings, we report an efficiency of 9% for a nip μc-Si:H thin film silicon cell with a thickness of only 1.2 μm. [ABSTRACT FROM AUTHOR]

We study the influence of different textures and dielectric environments on the excitation of surface plasmon resonances on silver because textured metallic films often serve as back contacts of silicon thin film solar cells. For coupling between light and the surface plasmon excitation we use a periodic sinusoidal structure that enables us to sample the dispersion relation at well defined conditions with a simple spectral reflection measurement. We use three layer samples of amorphous silicon/ZnO/silver to mimic the behavior of the back contact in a thin film silicon solar cell; the measurements suggest that losses due to plasmon excitation can very well extend in the spectral region where optimum reflectance is desired. An appropriate thickness of ZnO is able to reduce absorption losses. Our findings on periodic structures are also found useful to explain the behavior of surface plasmon excitation on randomly textured ZnO/Ag reflector layers. [ABSTRACT FROM AUTHOR]

We investigate amorphous silicon (a-Si:H) thin film solar cells in the n-i-p or substrate configuration that allows the use of nontransparent and flexible substrates such as metal or plastic foils such as polyethylene-naphtalate (PEN). A substrate texture is used to scatter the light at each interface, which increases the light trapping in the active layer. In the first part, we investigate the relationship between the substrate morphology and the short circuit current, which can be increased by 20% compared to the case of flat substrate. In the second part, we investigate cell designs that avoid open-circuit voltage (Voc) and fill factor (FF) losses that are often observed on textured substrates. We introduce an amorphous silicon carbide n-layer (n-SiC), a buffer layer at the n/i interface, and show that the new cell design yields high Voc and FF on both flat and textured substrates. Furthermore, we investigate the relation between voids or nanocrack formations in the intrinsic layer and the textured substrate. It reveals that the initial growth of the amorphous layer is affected by the doped layer which itself is influenced by the textured substrate. Finally, the beneficial effect of our optical and electrical findings is used to fabricate a-Si:H solar cell on PEN substrate with an initial efficiency of 8.8% for an i-layer thickness of 270 nm. [ABSTRACT FROM AUTHOR]

Single-junction microcrystalline silicon (μc-Si:H) solar cells of selected i-layer crystalline volume fractions were light soaked (AM1.5, 1000 h at 50 °C) and subsequently annealed at increasing temperatures. The variations of subbandgap absorption during light soaking and during thermal annealing were monitored by Fourier transform photocurrent spectroscopy. The kinetics were shown to follow stretched exponential functions over long times such as 1000 h. The effective time constants appearing in the stretched exponential function decrease with decreasing crystalline volume fraction as well with increasing annealing temperature. Their Arrhenius-like dependence on temperature is characterized by a unique value of the activation energy. Furthermore, we demonstrate that the configuration of the solar cells (p-i-n or n-i-p) does not influence the degradation kinetics, as long as the average crystallinity of the intrinsic layer is of comparable value. [ABSTRACT FROM AUTHOR]

In a recent work, we showed that atomic force microscopy (AFM) is a powerful technique to image cross sections of polycrystalline thin films. In this work, we apply a modification of AFM, namely, electrostatic force microscopy (EFM), to investigate the electronic properties of cleaved II-VI and multijunction thin-film solar cells. We cleave the devices in such a way that they are still working with their nominal photovoltaic efficiencies and can be polarized for the measurements. This allows us to differentiate between surface effects (work function and surface band bending) and bulk device properties. In the case of polycrystalline CdTe/CdS/SnO[sub 2]/glass solar cells, we find a drop of the EFM signal in the area of the CdTe/CdS interface (±50 nm). This drop varies in amplitude and sign according to the applied external bias and is compatible with an n-CdS/p-CdTe heterojunction model, thereby invalidating the possibility of a deeply buried n-p CdTe homojunction. In the case of a triple-junction GaInP/GaAs/Ge device, we observe a variation of the EFM signal linked to both the material work-function differences and to the voltage bias applied to the cell. We attempt a qualitative explanation of the results and discuss the implications and difficulties of the EFM technique for the study of such thin-film devices. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The article presents a new approach, which is shown to be based on the Rayleigh-Sommerfeld integral and can be used for analyzing the light scattering taking from the micro- and the nanotextured surfaces. The methology is shown to be highly effective and accurate.