Modulated surface-textured substrates for thin-film silicon solar cells exhibiting high haze in a broad range of wavelengths were fabricated. Glass substrates coated with different thicknesses of a sacrificial layer were wet-etched allowing the manipulation of the surface morphology with surface roughness ranging from 200 nm up to 1000 nm. Subsequently, zinc-oxide layers were sputtered and then wet-etched constituting the final modulated textures. The morphological analysis of the substrates demonstrated the surface modulation, and the optical analysis revealed broad angle intensity distributions and high hazes. A small anti-reflective effect with respect to untreated glass was found for etched glass samples. The performance of solar cells on high-haze substrates was evaluated. The solar cells outperformed the reference cell fabricated on a randomly-textured zinc-oxide-coated flat glass. The trend in the efficiency resembled the increased surface roughness and the anti-reflective effect was confirmed also in solar cell devices.@en

In thin-film silicon photovoltaic technology light management is important for achieving high efficiency of solar cells. Advanced light trapping techniques have to be employed to boost the solar cell efficiencies further. In this contribution we investigate the following advanced light trapping techniques implemented amorphous silicon solar cell: (i) periodic-like surface textures to achieve an angle-selective scattering, (ii) modulated surface textures for enhanced scattering in broad wavelength range, and (iii) one-dimensional photonic crystals used as dielectric back reflectors.@en

Hydrogenated microcrystalline silicon (¿c-Si:H) has become attractive for use in thin-film silicon solar cells. The external quantum efficiency (EQE) of ¿c-Si:H solar cells extends up to 1100 nm, which is exploited in tandem solar cells. Properties of p¿i interface are critical for performance as it affects carrier collection, which is visible in the blue response. Here, we report how ¿c-Si:H p- and i-layer material properties influence the p¿i interface of ¿c-Si:H solar cells. The effect of RF PECVD parameters of these layers on the p¿i interface was investigated. We find that the blue response of the solar cell is sensitive to the crystallinity of both the p- and i-layers. We demonstrate that transient depletion during i-layer deposition affects the blue response of ¿c-Si:H solar cell. We obtained a narrow process window for optimal solar-cell performance. At the optimal deposition pressure of 9 mbar and using transient depletion, an EQE at 400 nm of 0.6 was obtained, achieving 16% higher short-circuit current density. Reducing the diborane flow during p-layer deposition yielded 13% relative increase in efficiency.@en

We present modulated surface textures on glass substrates for improved light trapping in thin-film silicon solar cells. The surface morphology, the roughness spectral distribution, and the scattering properties of the modulated surface-textured substrates were characterized. We deposited solar cells on the modulated surface-textured substrates and observed enhancement in the performance compared to a solar cell deposited on textured ZnO:Al (reference) substrates.@en

Optical analysis of hydrogenated amorphous silicon (a-Si:H) solar cells with a periodic texture applied to the interfaces was carried out by two-dimensional optical simulator. The optical simulator solves the electromagnetic wave equations by means of finite element method using triangular elements for the discretization of space. The periodic texture with rectangular-like shape acts as a diffraction grating which scatters light into selective angles and thus gives a potential for significant prolongation of optical paths in thin absorber layers of the cells. Optimization of the geometrical parameters (period, height and duty-cycle) of the periodic texture was carried out in order to obtain the highest photocurrent from a-Si:H solar cells. The a-Si:H solar cell with the optimal periodic texture parameters (period of 300¿nm, height of 300¿nm and duty cycle of 50%) and the absorber layer thickness of 300¿nm generates up to 35% more photocurrent in comparison to the cell with flat interfaces. The optical analysis demonstrates that the optimal periodic texture in the a-Si:H solar cell results in the best trade-off between the antireflection effect at front interfaces, light scattering efficiency and the absorption losses at realistic metal back contact.@en

Diffusive dielectric materials such as white paint have been demonstrated as effective back reflectors in the photovoltaic technology. In this work, a one-dimensional (1D) optical modeling approach for simulation of white paint films is developed and implemented in a 1D optical simulator for thin-film solar cells. The parameters of white paint, such as the paint film thickness, the pigment volume concentration (PVC), and the pigment/binder refractive index ratio (RIR), are examined and optimized to achieve the required optical properties for back reflector application. The simulation trends indicate that white paint back reflectors with sufficient film thickness and higher PVC and RIR values exhibit improved reflectivity characteristics which results in an increased long-wavelength quantum efficiency of thin-film silicon solar cells. The simulation results based on the 1D model agree very well with the experimental data obtained from reflectance measurements of various white paint compositions and quantum efficiency measurements of amorphous silicon solar cells with white paint back reflectors.@en

Photon management is one of the key issues for improving the performance of thin-film silicon solar cells. An important part of the photon management is light trapping that helps to confine photons inside the thin absorber layers. At present light trapping is accomplished by the employment of the refractive-index matching layers at the front side and the high-reflective layers at the back contact of the solar cells and scattering of light at randomly surface-textured interfaces. In this article key issues and potential of light management in thin-film silicon solar cells are addressed. Novel approaches for light trapping are presented such as i) surface textures based on periodic diffraction gratings and modulated surface morphologies for enhanced scattering and anti-reflection, ii) metal nano-particles introducing plasmonic scattering, and iii) one-dimensional photonic-crystal-like structures for back reflectors.@en

In this work, we investigate the concept of transparent conductive oxide (TCO)/white paint (WP) back reflectors as an alternative to the conventional textured metal-based reflectors in thin-film silicon solar cells. The optical characterisation results show that WP back reflectors exhibit good reflectivity and excellent light scattering properties which can lead to enhanced light confinement within the thin absorber layers of the cell. To confirm this, ZnO:Al/WP reflectors with WP films of different pigment volume concentration and thickness are tested in thin-film amorphous silicon solar cells. The external quantum efficiency measurements indicate that by employing ZnO:Al/WP reflectors, good solar cell performance can be achieved, which is comparable to the performance of the cell with the conventional TCO/Ag reflector. Finally, optical simulations are employed to study the potential for further improvements in cell performance as a result of the optimisation of TCO/WP reflectors.@en

Substrates with a modulated surface texture were prepared by combining different interface morphologies. The spatial frequency surface representation method is used to evaluate the surface modulation. When combining morphologies with appropriate geometrical features, substrates exhibit an increased scattering level in a broad wavelength region. We demonstrate that the improved scattering properties result from a superposition of different light scattering mechanisms caused by the different geometrical features integrated in a modulated surface texture.@en

A concept of a modulated one-dimensional photonic-crystal (PC) structure is introduced as a back reflector for thin-film solar cells. The structure comprises two PC parts, each consisting of layers of different thicknesses. Using layers of amorphous silicon and amorphous silicon nitride a reflectance close to 100% is achieved over a broad wavelength region (700¿1300 nm). Based on this concept, a back reflector was designed for thin-film microcrystalline silicon solar cells, using n-doped amorphous silicon and ZnO:Al. Simulations show that the short-circuit current of the cell with a modulated PC back reflector closely resembles that of a cell with an ideal reflector. ©2009 American Institute of Physics@en

One-dimensional photonic crystals having desired broad region of high reflectance (R) were fabricated by alternating the deposition of amorphous silicon and amorphous silicon nitride layers. The effect of the deposition temperature and angle of incidence on the optical properties of photonic crystals deposited on glass substrate was determined and an excellent matching was found with the simulated results. The broad region of high R of photonic crystals deposited on flat and textured ZnO:Al substrates decreases when compared to the R of photonic crystals de-posited on glass. The performance of amorphous silicon solar cells with 1-D photonic crystals integrated as the back reflector was evaluated. The external quantum efficiency measurement demonstrated that the solar cells with the photonic crystals back reflector had an enhanced re-sponse in the long wavelength region (above 550 nm) compared to the cells with the Ag reflector.@en

Today amorphous and microcrystalline silicon based solar cells use surface-textured substrates for enhancing the light absorption and buffer and graded layers in order to improve the overall performance of the cells. Tandem and triple-junction configurations are utilized to assure better use of the solar spectrum and, thus, achieve higher conversion efficiencies of the devices. Resulting structures of the solar cells are complex and computer modeling has become an essential tool for a detailed understanding and further optimization of their optical and electrical behavior. The performance limits of tandem and triple-junction silicon based solar cells are studied by simulations using the optical simulator SunShine developed at Ljubljana University and the opto-electrical simulator ASA developed at Delft University of Technology. First, both simulators were calibrated with realistic optical and electrical parameters. Then, they were used to study the required scattering properties, absorption in non-active layers, antireflective coatings, the crucial role of the wavelength selective intermediate reflector, and a careful current matching in order to indicate the way for achieving a high photocurrent, more than 15 mA/cm(2) for a tandem a-Si:H//mu c-Si:H and I I mA/cm(2) for a triple-junction a-Si:H/a-SiGe:H/mu c-Si:H solar cells. By optimizing electrical properties of the layers and interfaces, for example using a p-doped a-SiC layer with a larger band gap (E-G > 2 eV) and introducing buffer layers at p/i interfaces, the extraction of the charge carriers, the open-circuit voltage and the fill factor of the solar cells are improved. The potential for achieving the conversion efficiency over 15% for the a-Si:H/mu c-Si:H and 17 % for the triple-junction a-Si:H/a-SiGe:H/mu c-Si:H solar cells is demonstrated.@en