Optics of Nanostructured Thin-Film Silicon Solar Cells

A solar cell is an optoelectronic device that converts light energy into electrical energy.This is based on the photovoltaic effect that was first investigated by Becquerel in 1837. Inthe Bell Laboratories the first solar cell was made in 1953 by Chapin, Fuller and Pearson. Thefirst commercial interest in solar cells was the power supply of satellites. In the meantimemany solar cell concepts have been developed [1-2]. At the moment the world market forsolar cells is dominated by crystalline solar cells [3]. Today several thin-film cell conceptsbased on amorphous silicon (a-Si:H), cadmium telluride (CdTe) and copper / indium / gallium/ (di)selenide (CIGS: Cu(In,Ga)(S,Se)) are going into commercial production as the thin-filmsolar cell technology is already providing similar or even lower costs per watt peak than thestandard crystalline silicon solar cell [4-7]. The steep “learning curve” for thin-filmtechnologies is expected to bring the production costs down even below 1 €/W(peak). Thiswould lead to the grid parity, the point at which photovoltaic electricity is equal to or cheaperthan grid power. Photovoltaic production has been doubling every two years, increasing by anaverage of ~ 50 percent per year since 2002, making it the world’s fastest-growing energytechnology. Until today cumulative global installations have reached 15200 megawatts. Foran unlimited growth of productions capacities of thin-film solar cells the absorber materialamorphous silicon plays a special role in contrast to CdTe or CIGS cells as it is not volumelimited like telluride or indium or toxic like cadmium [6]. A short energy payback time ofabout 1 year of the energy needed for the solar cell production has been reached for thin-filmsilicon modules [8]. Advantages under non ideal conditions like higher outdoor temperaturesor a high amount of non direct sun light lead to a plus of produced electricity per installedwatt (peak) for thin-film silicon in comparison to crystalline silicon modules [4]. The energyconversion efficiency of thin-film silicon solar cell modules is today mainly below 10 % andthus about half of the efficiency of crystalline silicon modules. The main reason is the use ofvery thin (and thus cheaper) silicon absorber layers with a total thickness of ~ 2 μm incomparison to an absorber thickness of ~ 200 μm for crystalline silicon cells. This leads to alower absorption of photons and less current is generated in the thin-film silicon solar cell. Asthe features of a thin-film solar cell are in the micrometer and sub-micrometer range newpreparation methods and analysis techniques, so called nanotechnologies, are necessary. Therealization of nanooptics with standard concepts is inhibited by the Abbe-limit. Innanostructures, like noble metal nano particles or nanogratings, high intensity electromagneticfields have been found very close to the structures. First explanations with strong [...