Functional substrates for flexible organic photovoltaic cells

The use of organic semiconducting materials in solar cells offers great potential for new applications thanks to its low production cost and mechanical flexibility. One particularly promising concept is the so-called bulk heterojunction solar cell. The most widely-used transparent electrode material for organic solar cells is indium tin oxide (ITO). Because of growing consumption of ITO in flat-panel technologies, the price of indium has jumped more than tenfold in the past three years and is sure to be a major factor in the cost of organic solar cells as demand continues to increase.

Efficiency remains limited by the small charge-carrier mobilities and insufficient overlap between the solar spectrum and the absorption of the organic photoactive components. Efficiencies up to 4.8% are reported for this type of organic solar cell. Motivated by the demand for efficient and cost-effective organic solar cells, we developed cell architectures based on holographic microstructured substrates.

In principle, the geometry of the cell architecture can be described by a folded planar solar cell (see Figures 1 and 2). This set-up offers two advantages compared to the standard organic solar cell. First, the ITO electrode is replaced by a highly-conductive polymer layer with a supporting metal grid located in the valley of the structure. Second, the microprism structure contributes to an increased light absorption due to a two-fold reflection of the incident light.

The dimensions of the microstructure strongly affect the efficiency of the solar cell. Ohmic losses have to be minimized by a sufficiently-small grid distance, whereas the ratio between the lattice distance of the grid and the width of the conducting lines should be as large as possible to minimize the shadowing effect.

Based on the results of electrical calculations, we chose a structure period of 20μm in combination with a 2μm-wide metal grid for the microprism cells. The microprism structures with rounded tips were generated by interference lithography and subsequent micro-replication. The metal grid was made by a combination of evaporation processes under inclined incident angles using the microstructure as a self-aligning shadow mask. Gold grid lines 50nm thick and 2-3μm-wide were obtained. As a result, the effective sheet resistance of the grid was ≈10 Ω/□, which is comparable to the sheet resistance of commercially available ITO.

We were able to realize solar cells based on microprism substrates with efficiencies comparable to planar ITO-based devices. To prove the light trapping effect that was predicted from simulations, the homogeneity and the thickness of the photoactive layer has to be optimized.

Buried nanoelectrodes are vertically-orientated electrodes embedded in the photo-active layer of the solar cell. The substitution of both planar electrodes with buried nanoelectrodes results in an interdigital electrode set-up (see Figure 3).

Since the dimensions of these structures are in the range of the wavelength of light, near-field optics plays an important role, and we investigated these with optical simulation calculations. The substrate for the buried nanoelectrodes is made by the replication of holographically-created structures into an acrylic ultraviolet-curable photopolymer. The dimensions of the microstructure are a lamellae period of 720nm, a depth of approximately 400nm, and a cavity width of 400nm. Two electrodes can be realised by oblique evaporation of different metals (see Figure 4 left). One pre-requisite for good contact is matching the electrode workfunction to the respective quasi Fermi potentials of the semiconductor. We chose gold and titanium as hole and electron contacts respectively.