F.-J. Haug

Plasmonic absorption in textured silver back reflectors of thin film solar cells

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.

Optimization of amorphous silicon thin film solar cells for flexible photovoltaics

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.

Flexible micromorph tandem a-Si/μc-Si solar cells

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 ...

Influence of the ZnO buffer on the guided mode structure in Si/ZnO/Ag multilayers

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.

Light management in thin film silicon solar cells

Thin film silicon is an attractive and versatile material for photovoltaics whose manufacturing reached a high level of maturity. Owing to its moderate efficiency compared to crystalline technologies, it should target either power plants with low installation cost or applications with added value like building-integration. Since the technology relies on very thin films of a weakly absorbing material, light-management is, and always has been, a key aspect of the technology. In this review, we briefly describe the class of materials that is summed up under the name “thin film silicon”, point out requirements on device design, and discuss functionalities that enhance the absorption in the silicon films, addressing both their theoretical understanding as well as experimental realization.

Compositional study of defects in microcrystalline silicon solar cells using spectral decomposition in the scanning transmission electron microscope

The chemical compositions of defective regions in microcrystalline thin film Si solar cells are studied using energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope. Nanometer-resolved chemical analysis reveals the presence of ZnO in micrometer-long defective regions. Due to the recent application of unmixing algorithm to EELS, the chemical compositions of the defective regions are determined objectively, without introducing artefacts from the fitting procedures. It is shown that the defective regions in the Si layer are filled by ZnO, which diffuses along voids that propagate from the bottom up to the top ZnO contacts.

ZnO Transparent conductive oxide for thin film silicon solar cells

There is general agreement that the future production of electric energy has to be renewable and sustainable in the long term. Photovoltaic (PV) is booming with more than 7GW produced in 2008 and will therefore play an important role in the future electricity supply mix. Currently, crystalline silicon (c-Si) dominates the market with a share of about 90%. Reducing the cost per watt peak and energy pay back time of PV was the major concern of the last decade and remains the main challenge today. For that, thin film silicon solar cells has a strong potential because it allies the strength of c-Si (i.e. durability, abundancy, non toxicity) together with reduced material usage, lower temperature processes and monolithic interconnection. One of the technological key points is the transparent conductive oxide (TCO) used for front contact, barrier layer or intermediate reflector. In this paper, we report on the versatility of ZnO grown by low pressure chemical vapor deposition (ZnO LP-CVD) and its application in thin film silicon solar cells. In particular, we focus on the transparency, the morphology of the textured surface and its effects on the light in-coupling for micromorph tandem cells in both the substrate (n-i-p) and superstrate (p-i-n) configurations. The stabilized efficiencies achieved in Neuchâtel are 11.2% and 9.8% for p-i-n (without ARC) and n-i-p (plastic substrate), respectively.

Diffraction and absorption enhancement from textured back reflectors of thin film solar cells

We study light scattering and absorption in thin film solar cells, using a model system of a sinusoidally textured silver reflector and dielectric layers of ZnO and amorphous silicon. Experimental results are compared to a theoretical model based on a Rayleigh expansion. Taking into account the explicit interface profile, the expansion converges fast and can be truncated typically after three or four orders. At the same time, the use of realistic permittivity data correctly reproduces the intensity of diffracted orders as well as the coupling to guided modes and surface plasmon polariton resonances at the silver surface. The coupling phenomena behind the light trapping process can therefore be assessed in a simple, yet accurate manner.

Temperature dependence of hydrogenated amorphous silicon solar cell performances

Thin-film hydrogenated amorphous silicon solar (a-Si:H) cells are known to have better temperature coefficients than crystalline silicon cells. To investigate whether a-Si:H cells that are optimized for standard conditions (STC) also have the highest energy yield, we measured the temperature and irradiance dependence of the maximum power output (Pmpp), the fill factor (FF), the short-circuit current density (Jsc), and the open-circuit voltage (Voc) for four series of cells fabricated with different deposition conditions. The parameters varied during plasma-enhanced chemical vapor deposition (PE-CVD) were the power and frequency of the PE-CVD generator, the hydrogen-to-silane dilution during deposition of the intrinsic absorber layer (i-layer), and the thicknesses of the a-Si:H i-layer and p-type hydrogenated amorphous silicon carbide layer. The results show that the temperature coefficient of the Voc generally varies linearly with the Voc value. The Jsc increases linearly with temperature mainly due to temperature-inducedbandgap reduction and reduced recombination. The FFtemperature dependence is not linear and reaches a maximum at temperatures between 15 °C and 80 °C. Numerical simulations show that this behavior is due to a more positive space-charge induced by the photogenerated holes in the p-layer and to a recombination decrease with temperature. Due to the FF(T) behavior, the Pmpp (T) curves also have a maximum, but at a lower temperature. Moreover, for most series, the cells with the highest power output at STC also have the best energy yield. However, the Pmpp (T) curves of two cells with different i-layer thicknesses cross each other in the operating cell temperature range, indicating that the cell with the highest power output could, for instance, have a lower energy yield than the other cell. A simple energy-yield simulation for the light-soaked and annealed states shows that for Neuchâtel (Switzerland) the best cell at STC also has the best energy yield. However, for a different climate or cell configuration, this may not be true.

Optimization of the Asymmetric Intermediate Reflector Morphology for High Stabilized Efficiency Thin n-i-p Micromorph Solar Cells

This paper focuses on our latest progress in n-i-p thin-micromorph solar-cell fabrication using textured back reflectors and asymmetric intermediate reflectors, both deposited by low-pressure chemical vapor deposition of zinc oxide. We then present microcrystalline bottom cells with high crystallinity, which yield excellent long wavelength response for relatively thin absorber thickness. In a 1.5-μm-thick μc-Si:H single-junction n-i-p solar cell, we thus obtain a short-circuit current density of 25.9 mA·cm-2, resulting in an initial cell efficiency of 9.1%. Subsequently, the roughness of the intermediate reflector layer is adapted for the growth of high-performance amorphous silicon (a-Si:H) top cells. Combining bottom cells with high current, an optimal intermediate reflector morphology and a 0.22-μm-thick a-Si:H top cell, we reach high initial open-circuit voltages of 1.45 V, and we obtain a stabilized cell with an efficiency of 11.1%, which is our best stable efficiency for n-i-p solar cells.