Tristan Köhler

Enhancement of photocurrent in an ultra-thin perovskite solar cell by Ag nanoparticles deposited at low temperature

Ultra-thin perovskite absorber layers have attracted increasing interest since they are suitable for application in semi-transparent perovskite and tandem solar cells. In this study, size and density controlled plasmonic silver nanoparticles are successfully incorporated into ultra-thin perovskite solar cells through a low temperature spray chemical vapor deposition method. Incorporation of Ag nanoparticles leads to a significant enhancement of 22.2% for the average short-circuit current density. This resulted in a relative improvement of 22.5% for the average power conversion efficiency. Characterization by surface photovoltage and photoluminescence provides evidence that the implemented silver nanoparticles can enhance the charge separation and the trapping of electrons into the TiO2 layer at the CH3NH3PbI3/TiO2 interface. The application of these silver nanoparticles therefore has promise to enhance the ultra-thin perovskite solar cells.

Non-vacuum chalcopyrite solar cell absorbers: The Spray-ILGAR approach

In this report, solar cells based on CuInS2 absorber films, which were prepared by the Spray-ILGAR method, are presented for the first time. All solar cells had a glass/Mo/CuInS2/CdS/i-ZnO/n-ZnO/Ni-Al structure. Solar cells were produced from Spray-ILGAR CuInS2 absorber films using various deposition parameters. To date, a maximum efficiency of 4.1 % has been achieved. The CuInS2 absorber films consist of two layers of different crystalline quality. The origin as well as the influence of this structure on the photovoltaic performance is discussed. Therefore, scanning electron microscopy and Raman spectroscopy were applied, in order to characterize the Spray-ILGAR CuInS2 absorber films.

Concentrating light in Cu(In,Ga)Se2 solar cells

Light concentration has proven beneficial for solar cells, most notably for highly efficient but expensive absorber materials using high concentrations and large scale optics. Here we investigate light concentration for cost efficient thinfilm solar cells which show nano- or microtextured absorbers. Our absorber material of choice is Cu(In,Ga)Se2 (CIGSe) which has a proven stabilized record efficiency of 22.6% and which - despite being a polycrystalline thin-film material - is very tolerant to environmental influences. Taking a nanoscale approach, we concentrate light in the CIGSe absorber layer by integrating photonic nanostructures made from dielectric materials. The dielectric nanostructures give rise to resonant modes and field localization in their vicinity. Thus when inserted inside or adjacent to the absorber layer, absorption and efficiency enhancement are observed. In contrast to this internal absorption enhancement, external enhancement is exploited in the microscale approach: mm-sized lenses can be used to concentrate light onto CIGSe solar cells with lateral dimensions reduced down to the micrometer range. These micro solar cells come with the benefit of improved heat dissipation compared to the large scale concentrators and promise compact high efficiency devices. Both approaches of light concentration allow for reduction in material consumption by restricting the absorber dimension either vertically (ultra-thin absorbers for dielectric nanostructures) or horizontally (micro absorbers for concentrating lenses) and have significant potential for efficiency enhancement.

Preparation of an amorphous optically transparent and hydrophobic Al 2 O 3 top-protective layer for first-surface CSP reflectors

In Concentrated Solar Power (CSP) application, top coatings are of tremendous importance as a way to protect the reflective layers form degradation and conserve the efficiency and durability of the mirrors. The choice of Al<inf>2</inf>O<inf>3</inf> as a top-protective coating for CSP reflectors is based on its high stability, hardness and transparency. In this study, high quality and stable CSP first-surface silvered thick glass mirrors were prepared with a transparent and hydrophobic amorphous Al<inf>2</inf>O<inf>3</inf> top-protective layer with different thicknesses (1µm − 4µm) in order to investigate the effect of the film-thickness on the optical properties and surface morphology of the samples. The spectrophotometric measurements were conducted using a Perkin Elmer Lambda 950 UV/VIS/NIR Spectrophotometer and showed no significant change in the optical properties of the amorphous Al<inf>2</inf>O<inf>3</inf> layers with different thicknesses. The surface morphology was characterized using scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results show that increasing the thickness of the Al<inf>2</inf>O<inf>3</inf> layer up to 3 µm increased the surface hydrophobicity of the mirrors whereas a decrease in the water contact angle was noticed with 4 µm thickness. The measured water contact angles (WCA) were 94°, 98°, 102° and 95° for 1 µm, 2 µm, 3 µm and 4 µm, respectively. A decrease in the water contact angle (WCA=33°) was noticed by achieving a phase transformation from amorphous to crystalline (γ-Al2O3) using annealing at 800°C for 2 hours, exhibiting a hydrophilic behavior.