Process Development of Silicon Heterojunction Interdigitated Back-Contacted (SHJ-IBC) Solar Cells Bonded to Glass

In imec’s i2-module concept, silicon heterojunction interdigitated back-contacted (SHJ-IBC) solar cells are fabricated on monocrystalline foils bonded to glass. The proposed technology allows for cell processing on thin wafers mechanically supported by the glass, increasing the yield of processing such thin wafers. A process sequence for SHJ-IBC cell fabrication that can be applied to bonded thin foils is described. We investigated and optimized individual process steps on thick wafers. Then the developed steps were integrated into a process flow to fabricate solar cells on wafers with different thicknesses and bonding agents. On wafers with a thickness of 190 μm, functional cells with efficiencies of 22.6% and 21.7% were made on freestanding and silicone bonded wafers, respectively. On thin wafers of 57 μm, our best SHJ-IBC cell on an EVA bonded wafer exhibits excellent Voc of 740 mV and efficiency of 20.0%, which demonstrates the high potential of the i2-module concept

Polymer based organic solar cells using ink-jet printed active layers

Ink-jet printing is used to deposit polymer:fullerene blends suitable as active layer for organic solar cells. We show that merging of separately deposited ink droplets into a continuous, pinhole-free organic thin film results from a balance between ink viscosity and surface wetting, whereas for certain of the studied solutions clear coffee drop effect occurs for single droplets; this can be minimized for larger printed areas, yielding smooth layers with minimal surface roughness. Resulting organic films are used as active layer for solar cells with power conversion efficiency of 1.4% under simulated AM1.5 solar illumination. (C) 2008 American Institute of Physics.status: publishe

The Effect of Dust Deposition on Photovoltaic Modules

The effect of dust settlement on the efficiency of photovoltaic modules was measured. To gain a deeper insight into the problem the physical properties of the collected dust were examined using a scanning electron microscope (SEM). A potential solution for the phenomenon, namely the usage of special coatings on the cover glass, was investigated. The results show a constant power loss between 3% and 4% for the optimal tilt angle and regular rainfall. Rain seems to have little cleaning effect on smaller dust particles , but on bigger particles (like pollen) the effect was clearly visible.status: publishe

Nanoimprinted semiconducting polymer films with 50 nm features and their application to organic heterojunction solar cells

Nanoimprint lithography is used to directly pattern the conjugated polymer semiconductor poly(3-hexylthiophene) (P3HT). We obtain trenches with aspect ratios up to 2 and feature sizes as small as 50 nm in this polymer. The application to organic solar cells is shown by creating an interpenetrated donor-acceptor interface, based on P3HT and N,N'-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C-13), deposited from the vapor phase to reduce shadow effects. A planarizing layer of spin-coated zinc oxide (ZnO) nanoparticles is used to reduce the roughness of the layer stack. The response of the photovoltaic devices follows the increased interface area, up to a 2.5-fold enhancement.status: publishe

The angular response of ultrathin film organic solar cells

We study the effect of oblique incident light on the optical interference effects and photocurrent of organic planar heterojunction solar cells. We find that the thin layers used in organic solar cells induce a complex light distribution within the device, further altered by incident angle. The responsivity can increase as a function of angle to values up to 15% larger than at normal incidence, and optical simulations are shown to match these trends. Furthermore, we show that the outdoor performance at different tilting angles is higher than expected, which improves the attractiveness of organic solar cells for building-integrated applications. (c) 2008 American Institute of Physics.status: publishe

Analytical model for the open-circuit voltage and its associated resistance in organic planar heterojunction solar cells

We derive an analytical formula for the open-circuit voltage (V-oc) of organic planar heterojunction solar cells under standard operating conditions. We find that the type of free carrier recombination at the interface between the donor and acceptor materials controls the slope of V-oc vs incident light intensity. By using the same derivation, an equation for the resistance around V-oc is obtained. From this, we investigate two parameters in more detail and compare them to experiments. The first is the work function of the cathode metal. We show that, within our model, V-oc does not depend on this work function, while the cell resistance around V-oc is strongly dependent on it. Second, we find that the asymptotic resistance around V-oc is a third-order power function of the thickness of the organic layers (acceptor or donor). The model provides insights to achieve low-resistivity high open-circuit voltage organic solar cells.status: publishe

Preparation of microflake ink for low cost printing of CIS-Se absorber layers

Mechanical alloying and bead milling techniques were used for the low cost preparation of precursor inks for printing CIS-Se absorber layers. The plate shaped microflake particle precursor is composed of nanostructured elemental Copper-Indium-Sulfur and Selenium. The high aspect ratio microflakes have a typical thickness around 200 nm and a diameter of 1-5 µm. The plate shape of the microflakes facilitates the deposition of a high quality film during the coating process. Moreover, the nanostructured nature of the microflakes improves the sintering behavior during selenization. The milling technique is relatively low cost, easily up-scalable, fast and very flexible in targeted composition.status: publishe