Investigation of the Potential Distribution within Organic Solar Cells by Scanning Kelvin Probe Microscopy

In this work the potential distribution within organic solar cells was investigated. Using a focused ion beam micrometer sized holes were milled into the cells such that the cross sections became accessible by scanning Kelvin probe microscopy (SKPM). SKPM measurements were performed on Poly(3-hexylthiophen) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) solar cells under illumination and under different bias voltages. In a bilayer solar cell the applied bias voltage drops at the interface between P3HT and anode and within the organic layer. In a bulk heterojunction solar cell the potential drops at the interface between P3HT and the anode and at the interface between the PCBM and cathode. In solar cells which were inverted due to altered contact materials there is no potential drop at the contacts, but the potential uniformly drops within the organic material. It can be concluded that an inverted device structure is more favorable for this morphology of the bulk heterojunction. The open circuit voltage exhibited a similar distribution within the device as an external applied bias voltage. Furthermore, SKPM measurements were performed on solar cells with S-shaped current-voltage characteristics. It was mapped that the S-shape behavior results from a transport barrier at the cathode interface

Mesoscale trumps nanoscale: metallic mesoscale contact morphology for improved light trapping, optical absorption and grid conductance in silicon solar cells

We report on a computational study exploring the design of mesoscale metallic front contacts for solar cells. We investigated silver contact structures with circle, triangle and square cross-sections for various length scales and surface coverages. We found that for ‘nanoscale’ contacts with widths between 10 nm and 1000 nm, resonant coupling actually impairs light absorption in the semiconductor. Conversely, for ‘mesoscale’ contact widths > 1000 nm, the light interaction is determined by the geometric shadowing. We find that mesoscale silver contacts with triangular cross-section outperform other nanostructure morphologies in reducing shadow losses and yield contact transparency of >99% percent with sheet resistance <0.2 Ω/sq. Surprisingly, very densely spaced mesoscale silver triangular cross-section contacts can enhance the absorption of thin silicon/silver structures by up to 15% at a front contact coverage of 83%, due to light trapping by the front contact. Such structures can also maintain up to 100% absorption within the silicon, at a front contact coverage of 50%, relative to the same structure without metal

Total Internal Reflection for Effectively Transparent Solar Cell Contacts

A new strategy for eliminating photocurrent losses due to the metal contacts on the front of a solar cell was proposed, simulated, and tested. By placing triangular cross-section lines of low refractive index on top of the contacts, total-internal reflection at the interface of the low-index triangles and the surrounding material can direct light away from the metal and into the photoactive absorber. Simulations indicated that losses can be eliminated for any incident angle, and that yearly energy production improvements commensurate with the metallized area are possible. Proof of principle experiments were carried out to eliminate the reflective losses of a commercial solar cell's busbar contact. Spatially resolved laser beam induced current measurements demonstrated that reflection losses due to the busbar were reduced by voids with triangular cross-section

Direct observation of the potential distribution within organic light emitting diodes under operation

We show the first direct measurement of the potential distribution within organic light emitting diodes (OLEDs) under operation and hereby confirm existing hypotheses about charge transport and accumulation in the layer stack. Using a focused ion beam to mill holes in the diodes we gain access to the cross section of the devices and explore the spatially resolved potential distribution in situ by scanning Kelvin probe microscopy under different bias conditions. In bilayer OLEDs consisting of tris(hydroxyquinolinato) aluminum (Alq_3)/N, N ′-bis(naphthalene-1-yl)-N,N ′-bis(phenyl) benzidine (NPB) the potential exclusively drops across the Alq_3 layer for applied bias between onset voltage and a given transition voltage. These findings are consistent with previously performed capacitance–voltage measurements. The behavior can be attributed to charge accumulation at the interface between the different organic materials. Furthermore, we show the potential distribution of devices with different cathode structures and degraded devices to identify the cathode interface as main culprit for decreased performance