Photovoltaics

Photovoltaic solar energy represents one of the main components of future sustainable energy scenarios. With high average growth rates of at least 40% p.a. over the last 5 years, and decreasing cost, photovoltaics is expected to become a big business line this century.

Photovoltaics can be classified into so-called 1st, 2nd and 3rd generation. 1st generation PV is based on silicon wafers and represents the vast majority of the worldwide solar cell production. Concepts consuming less material are based on thin-film technology and are considered as second generation photovoltaics. Finally, third generation photovoltaics aims at overcoming the so-called Shockley Queisser limit. At IHT we pursue research on 1st and 3rd generation PV.

1st Generation PV

The research in the field of wafer-based solar cells concentrates on the development of innovative process technologies like texturization via nanoimprint lithography or the formation of selective emitters by screen printing, reactive ion etching (RIE) or laser doping. The development of these processes is accompanied by extensive device and process simulations. In addition, advanced optical and electrical measurements are used to characterize the solar cells and new techniques are being developed. For instance, carrier density imaging based on the lock in thermography was used for the first time at our institute to investigate screen-printed selective emitters. In close collaboration with solar cell manufacturers processes developed at IHT are evaluated with respect to their introduction into an industrial production environment. Furthermore, the analysis and disabling of the influence of defects (by e.g. gettering of impurities) on the solar cell plays a vital role within our current research topics. The whole photovoltaic activities at IHT are complemented by simulations of photovoltaic modules to increase the efficiency and optimize cell sorting and module design.

3rd Generation PV

The huge gap between the theoretical conversion efficiency of sunlight to electricity of 85% and the theoretical conversion efficiency for a single

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Fig.1. Transmission electron micrograph of a Si/SiO2 multilayer stack for 3rd generation lsoar cells.

junction silicon solar cell of 33% (the famous Shockley Queisser limit) stimulated the design of new 3rd generation solar cell concepts that shall allow to exceed the Shockley Queisser limit. At IHT, all-silicon third generation approaches are investigated. Here, the potential of nanotechnology and especially bandgap engineering via quantum confinement is explored in order to improve Si based cells. To this end, Si/SiO2 multi-layer stacks are deposited as shown in the high-resolution TEM image. If the Si layers are sufficiently thin, vertical quantization yields a substantially increased bandgap suitable for absorption of the short-wavelength portion of the solar spectrum without relaxation losses. Contacting the multi-layer stack laterally with n- and p-type contacts as illustrated in Fig.2 enables an efficient extraction of charges because of the separation of quantization and current transport directions.

Fig.2. Illustration of a laterally contacted multilayer solar cell.

A major challenge here is to create ultrathin Si layers with a high degree of crystallinity. Current research aims at improving the crystalline quality of the multi-layer stacks that would not only strongly increase the efficiency of 3rd generation solar cells but is also interesting for photonics applications.