Fs-laser micro-machining for µ-TLM resistivity test structures in TCO top contact multilayers for perovskite heterojunction tandem solar cellsWednesday (24.06.2020) 10:40 - 11:00 Room 3
Laser ablation processes are used for layer-selective micro-structuring of various thin film systems on rigid or flexible substrates ranging from display technology, LED devices to organic and inorganic PV thin films. PV concepts like perovskite silicon tandem solar cells consisting of thin film multilayer stacks are able to use the sunlight spectrum much more efficiently. Therefore, an efficient electro-optical matching of perovskite and silicon sub-cells is needed. The use of several transparent conductive oxide (TCO) layers play a central role for increasing the electro-optical performance. Measuring sheet resistance of TCO sublayers as well as material properties is a challenge to established industrial measuring.
In this work we adapted a new approach of µ-TLM test structures for resistivity measurements based on fs-laser micro machining. This method is applied to highly resistive interfacial and conductive bulk multilayer systems in TCO top contact for perovskite hetero-junction tandem samples. Besides electrical sheet and contact resistivity values of the individual layers, the heat affected zones (HAZ) at the laser structures are characterized by high resolution transmission electron microscopy (TEM). The µ-TLM test structures consist of isolating trenches and homogenous ablation areas in µm dimensions prepared with reasonable effort using fs-laser processes. The usage of ultrashort pulses with 200 fs pulse duration and with a laser wavelength in the IR range (λ = 1.03 µm) allows selective removal of the top ITO layer of the TCO multilayer stack by a minimized thermal influence (HAZ) on the layers beneath and in the adjacent region of the laser trenches. The small effective optical penetration and ablation depth were achieved by an ultrafast non-thermal ablation mechanism via free carrier absorption at the interface of each multilayer systems (e.g. ITO/SnOx). For our experiments, we use multilayer stacks of perovskite-HJT top cell contact layers. The laser patterning of TLM structures were used to compare sheet and contact resistance of the samples. Therefore, a layer-selective and sub-µm homogenous area removal of Ag and ITO without damaging underlying layer is required. By using optimized fs-laser pulses intensity regarding different ablation thresholds and spot-to-spot overlap optimization by beam shaping methods, reproducible test structures and electrical resistivity data can be achieved and were investigated by profilometry.