W. Reetz

Development of highly efficient thin film silicon solar cells on texture-etched zinc oxide-coated glass substrates

Abstract ZnO films prepared by magnetron sputtering on glass substrates and textured by post-deposition chemical etching are applied as substrates for p–i–n solar cells. Using both rf and dc sputtering, similar surface textures can be achieved upon etching. Excellent light trapping is demonstrated by high quantum efficiencies at long wavelengths for microcrystalline silicon solar cells. Applying an optimized microcrystalline/amorphous p-layer design, stacked solar cells with amorphous silicon top cells yield similarly high stabilized efficiencies on ZnO as on state-of-the-art SnO 2 (9.2% for a-Si/a-Si). The efficiencies are significantly higher than on SnO 2 -coated float glass as used for module production.

A constructive combination of antireflection and intermediate-reflector layers for a-Si/μC-Si thin film solar cells

This device design approach combines sputter-deposited TiO2 antireflection layer (ARL) and plasma-enhanced chemical vapor deposition-grown SiOx intermediate-reflector layer (IRL) in superstrate a-Si∕μc-Si thin film solar cell. The loss of current from either the component cells with individual application of ARL and IRL has been recovered with their combined application. With both ARL and IRL in a-Si∕μc-Si cell, (a) the top cell current and (b) the sum of top and bottom cell current increases. An initial efficiency of 11.8% [Voc=1.42V, FF=0.74, Jsc (top)=11.5mAcm−2, Jsc (bottom)=11.2mAcm−2] is achieved from such an a-Si∕μc-Si cell with a total Si layer thickness less than 2μm.

Photoresponse enhancement in the near infrared wavelength range of ultrathin amorphous silicon photosensitive devices by integration of silver nanoparticles

We have investigated the contribution of localized surface plasmon polaritons (LSPPs) in silver nanoparticles with radii smaller than 20 nm to the photocurrent of ultrathin photosensitive devices based on amorphous silicon. An increased light absorption and an enhanced photocurrent are found for wavelengths between 600 nm and 1150 nm in presence of nanoparticles. As amorphous silicon absorbs light efficiently only at wavelengths up to 750 nm, the increased photocurrent in the near infrared range is explained in terms of LSPP-induced photoemission of electrons within and in close vicinity of the nanoparticles.

Color-sensitive photodetector based on porous silicon superlattices

Color-sensitivity of Si photodiodes was achieved by integrating porous silicon (PS) Bragg reflectors and Fabry–Perot filters. The PS was formed in the p+-type part of the p+n junction which required illumination of the samples during anodization. The optimal illumination power density turned out to be a compromise: high power densities are necessary to enable high anodization current densities, but this results in a degraded filter performance. The PS layers had no significant influence on the electrical characteristics of the photodiodes, but as expected they strongly modified the spectral response. The results are in good agreement with the reflectance spectra of the filters.

Plasmon-induced photoexcitation of “hot” electrons and “hot” holes in amorphous silicon photosensitive devices containing silver nanoparticles

We report on a plasmon-induced photocurrent in photosensitive devices based on hydrogenated amorphous silicon (a-Si:H) containing silver nanoparticles (NPs). The photocurrent is measured in a spectral region corresponding to optical transitions below the band gap of a-Si:H. Photoexcitation of “hot” electrons in the NPs or in defect states present in the vicinity of the NPs, resulting from plasmon decay in the NPs, is often cited as being responsible for this effect. In this study, we demonstrate that plasmon induced photogeneration of “hot” holes is also able to contribute to a photocurrent. A bifacial symmetrical transparent device was prepared in order to compare the internal quantum efficiency of both processes, the first based on the photogeneration of “hot” electrons and the second based on the photogeneration of “hot” holes.

Texture etched ZnO:Al films as front contact and back reflector in amorphous silicon p-i-n and n-i-p solar cells

This paper treats the use of texture etched ZnO:Al films in amorphous silicon solar cells. Chemically textured ZnO:Al films were implemented as a front TCO in p-i-n (superstrate) and n-i-p (substrate) solar cells, and in combination with Ag as a textured back reflector in n-i-p (substrate) solar cells. These cells exhibit excellent optical and light-trapping properties demonstrated by high short-circuit current densities. Adapted microcrystalline p-layers solve the ZnO/p-contact problem and thereby provide high fill factors and open-circuit voltages. The initial efficiencies so far obtained are close to 10% for p-i-n and 8% for n-i-p solar cells.

Quantum efficiency of a-Si:H p-i-n solar cells in the 250 to 375 K range-insights from defect pool modelling

A variation of the temperature T of a p-i-n solar cell shifts the Fermi level and thereby changes the occupation of the defect densities within the device according to the defect model used. Thus the T dependence of the quantum efficiency (QE) promises to give a clearer insight into the defect distribution and recombination within the device. With decreasing T both the red and the blue response of an a-Si:H p-i-n solar cell decrease, the loss in the red being due to the i layer gap shift. The drop on the short wavelength side with decreasing T is attributed to a change of charge state of the D/sup +/ in the nonequilibrium case within the first 100 nm of the i-layer. This negative space charge diminishes the field spike near the p/i interface and reduces the hole concentration there by recombination.

Thin-Film Barriers for Durable Thin-Film PV Modules

This contribution describes thin-film barrier coatings to enhance the stability of silicon thin-film solar modules. Thin-film barriers are applied to protect the modules from degradation in humid environment. Demonstrator modules were exposed to indoor and outdoor tests to evaluate failure modes by electrochemical corrosion during operation. Test results reveal some major failure modes, but we demonstrate long lifetimes under operational conditions for certain modules without conventional encapsulation. These modules without conventional encapsulation showed less than 1% relative degradation during 1000 hours of damp-heat tests with light exposure at open circuit voltage. These promising results will guide the way to new encapsulation concepts for longer lifetimes and/or new low-cost concepts. This approach might be applied to any thin-film technology and will be an important aspect for consumer electronics with less strict requirements for very long-term operation.

Growth of μc-Si active layer in a-Si/μc-Si solar cell: An optimization of growth process through modification of active layer near p/i interface

The growth of μc-Si active layer has been investigated in a-Si/μc-Si superstrate p-i-n solar cells. The growth of μc-Si i-layer seems to have considerable dependence on the immediate substrate it is growing on. In superstrate configuration, the p-layer of μc-Si cell in an a-Si/μc-Si cell should attain the required properties for doping, window, recombination layer, and for seed layer of μc-Si intrinsic layer growth in the bottom cell. An easy approach to fabricate μc-Si solar cells would be to decouple the requirements for a p-layer. In this article, a modification of the growth condition of μc-Si i-layer near the p/i interface has been demonstrated to fabricate a-Si/μc-Si solar cells starting with a non-optimized p-layer in terms of seeding requirements. The solar cell performance has been improved with this modification by decoupling the role of p-layer as seed layer for μc-Si growth. The structural properties of the i-layer have been investigated with Raman spectroscopy. An initial conversion efficiency of 11.3% has been achieved in this work.

Stability of Thin-Film Silicon Solar Cells

The long term stability of non-encapsulated amorphous (a-Si:H) and microcrystalline (muc-Si:H) silicon single and tandem cell structures was tested by means of light soaking (AM 1.5, T=50 degC), damp heat testing (T=85 degC, humidity=85%) and high temperature treatment (T=150 degC) up to 2000 h to simulate a variety of harsh environmental conditions. In order to study the influence of the TCO front contact and backside contact on the long term stability, cells deposited on different substrates and prepared with different backside configurations were examined. As prepared (non-encapsulated) a-Si:H and muc-Si:H diodes show very similar effects after light soaking, damp heat testing and temperature treatment. Both solar cell types show no significant variation of the solar cell parameters even after 2000 h of damp heat testing. After light soaking a-Si:H diodes exhibit the well known distinct degradation of the fill factor while the bulk properties of the investigated muc-Si:H diodes remain nearly unchanged