Karen Wilken

Light Management in Flexible Thin-Film Solar Cells—The Role of Nanoimprinted Textures and Tilted Surfaces

We present the application of ultraviolet (UV) nanoimprint lithography for the replication of advanced light management schemes in flexible thin-film solar cells. The approach is maintained entirely at low temperatures, which are required for the development of flexible solar cells on low-cost transparent polymer films. Light-scattering properties are significantly improved by this technique, and thin-film silicon solar cells prepared on these substrates show a substantial improvement in performance due to the nanoimprinted texture. We further investigate the effect of various incident angles of the light on the short-circuit current density (Jsc) of the solar cell and evaluate the corresponding performance of a flexible solar cell in a bent state. Our results show that in the case of imprinted texture, the Jsc and efficiency is reduced within 5% in a bent case of a semicircle when a reduction of the effective illumination area with angle is not taken into account. Overall, the solar cell on imprint-textured polyethylene terephthalate (PET) film shows an increased Jsc for the entire range of incident angles and bent states compared with the nonimprinted PET substrate.

Thin-film silicon solar cells fabricated at low temperature: A versatile technology for application on transparent flexible plastic substrates and in integrated photoelectrochemical water splitting modules

Amorphous silicon (a-Si:H) solar cells in p-i-n configuration were developed at a low deposition temperature of 140 °C, suitable for application on transparent flexible plastic substrates. Deteriorated electronic properties of the p-layer with decreasing temperature were identified as the main reason for reduced solar cell performance. Optimization of the p-layer properties resulted in an efficiency of 8.2 % for a solar cell fabricated entirely at 140 °C. As a parallel application scenario, a-Si:H/a-Si:H tandem solar cells are designed for application in integrated photoelectrochemical water splitting modules. Here we benefit from the increased open circuit voltages with values around 1.9 V which provides ample margin for possible overpotential losses in water splitting modules.

Mechanical Properties of ZTO, ITO, and a-Si:H Multilayer Films for Flexible Thin Film Solar Cells

The behavior of bi- and trilayer coating systems for flexible a-Si:H based solar cells consisting of a barrier, an electrode, and an absorption layer is studied under mechanical load. First, the film morphology, stress, Young’s modulus, and crack onset strain (COS) were analyzed for single film coatings of various thickness on polyethylene terephthalate (PET) substrates. In order to demonstrate the role of the microstructure of a single film on the mechanical behavior of the whole multilayer coating, two sets of InSnOx (indium tin oxide, ITO) conductive coatings were prepared. Whereas a characteristic grain–subgrain structure was observed in ITO-1 films, grain growth was suppressed in ITO-2 films. ITO-1 bilayer coatings showed two-step failure under tensile load with cracks propagating along the ITO-1/a-Si:H-interface, whereas channeling cracks in comparable bi- and trilayers based on amorphous ITO-2 run through all constituent layers. A two-step failure is preferable from an application point of view, as it may lead to only a degradation of the performance instead of the ultimate failure of the device. Hence, the results demonstrate the importance of a fine-tuning of film microstructure not only for excellent electrical properties, but also for a high mechanical performance of flexible devices (e.g., a-Si:H based solar cells) during fabrication in a roll-to-roll process or under service.

Improved flexible thin-film solar cells with nanoimprinted light management textures

A nanoimprint based approach to achieve efficient light management for solar cells on low temperature transparent polymer (PET) films is presented. The imprint-textured PET substrates show excellent light scattering properties and lead to significantly improved incoupling and trapping of the light in the solar cell, resulting in a current density of 12.9 mA/cm2, similar to that on a glass substrate with texture-etched ZnO:Al and only 0.2 mA/cm2 lower than that on Asahi(U)-type TCO glass. An efficiency of 6.9 % was achieved for a flexible thin-film silicon solar cell on low cost transparent PET substrate.