Arnaud Walter

Efficient Monolithic Perovskite/Silicon Tandem Solar Cell with Cell Area >1 cm2

Monolithic perovskite/crystalline silicon tandem solar cells hold great promise for further performance improvement of well-established silicon photovoltaics; however, monolithic tandem integration is challenging, evidenced by the modest performances and small-area devices reported so far. Here we present first a low-temperature process for semitransparent perovskite solar cells, yielding efficiencies of up to 14.5%. Then, we implement this process to fabricate monolithic perovskite/silicon heterojunction tandem solar cells yielding efficiencies of up to 21.2 and 19.2% for cell areas of 0.17 and 1.22 cm(2), respectively. Both efficiencies are well above those of the involved subcells. These single-junction perovskite and tandem solar cells are hysteresis-free and demonstrate steady performance under maximum power point tracking for several minutes. Finally, we present the effects of varying the intermediate recombination layer and hole transport layer thicknesses on tandem cell photocurrent generation, experimentally and by transfer matrix simulations.

Laser-Scribing Patterning for the Production of Organometallic Halide Perovskite Solar Modules

Efficiencies of solar cells based on organometallic halide perovskite absorber material have dramatically increased over the past few years. Most of efficiencies reported so far have, however, been obtained on solar cells with very small lab-scale area of less than 0.3 cm2. Only a handful of studies addressed the performances of minimodules based on perovskite, and all of them showed relatively large dead areas between the solar cell segments. In this study, we used laser-scribing techniques to pattern the module segment, reduce the dead area, and optimize the aperture area efficiency. The fraction of the dead area in the module is less than 16%, which proves that the laser-scribing technology can be adopted for monolithic serial interconnected perovskite modules and paves the way to improving module efficiency.

Zinc tin oxide as high-temperature stable recombination layer for mesoscopic perovskite/silicon monolithic tandem solar cells

Perovskite/crystalline silicon tandem solar cells have the potential to reach efficiencies beyond those of silicon single-junction record devices. However, the high-temperature process of 500 °C needed for state-of-the-art mesoscopic perovskite cells has, so far, been limiting their implementation in monolithic tandem devices. Here, we demonstrate the applicability of zinc tin oxide as a recombination layer and show its electrical and optical stability at temperatures up to 500 °C. To prove the concept, we fabricate monolithic tandem cells with mesoscopic top cell with up to 16% efficiency. We then investigate the effect of zinc tin oxide layer thickness variation, showing a strong influence on the optical interference pattern within the tandem device. Finally, we discuss the perspective of mesoscopic perovskite cells for high-efficiency monolithic tandem solar cells.

Progression towards high efficiency perovskite solar cells via optimisation of the front electrode and blocking layer

The effects of a fluorine doped tin oxide (FTO) electrode, titanium dioxide (TiO2−x) blocking layer (BL) and perovskite (methyl ammonium lead triiodide) preparation on the overall properties of the photovoltaic cells have been studied. The FTO electrode was deposited by atmospheric pressure chemical vapour deposition (APCVD) and the hole blocking layer by spin coating, atomic layer deposition (ALD) or sputtering. We have shown the importance of obtaining uniform thin films of FTO, with low sheet resistance to aid the formation of pin hole free uniform TiO2−x blocking layers and hence well adhered, perovskite layers. The optimal BL thickness was 20 nm, while thicker films gave decreased shunt resistance and thinner a greater number of pin holes through the layers. We also showed that the conformal nature of ALD and magnetron sputtering, along with their increased uniformity control over spin coating again improved cell efficiency. The main improvement comes for the smaller Roc, attributed to an improved electrical transport through particularly the sputtered TiO2−x blocking layer. After identifying the optimised parameters, all the properties were combined to fabricate large solar cells (1 cm2) yielding power conversion efficiencies beyond 16%.

1 cm2 CH3NH3PbI3 mesoporous solar cells with 17.8% steady-state efficiency by tailoring front FTO electrodes

In this article, we investigate the effects of atmospheric-pressure chemical vapour deposited fluorine doped tin oxide (FTO) thin films as front electrodes for the fabrication of mesoporous perovskite solar cells with an active area of 1 cm2 and compare them with the use of a commonly used commercial transparent conducting oxide. The effects of sheet resistance (Rs) and surface roughness are both closely linked to the film thickness. In order to separate out these effects the characteristics of the deposited FTOs were carefully controlled by changing the fluorine doping levels and the number of passes under the coating head to give films of specific thicknesses or Rs. Under AM 1.5 Sun illumination and maximum power point tracking, the optimised FTOs yielded a steady-state power conversion efficiency of 17.8%, higher than that of the reference cell fabricated from the commercial FTO. We attribute the improved cell efficiency to increased fill factor and a lower series resistance resulting from the lower Rs and increased thickness of these FTO substrates. This low-cost and viable methodology is the first such type of study looking independently at the significance of FTO roughness and resistance for highly efficient mesoporous perovskite solar cells.

Laser induced forward transfer of graphene

Transfer of graphene and other two-dimensional materials is still a technical challenge. The 2D-materials are typically patterned after transfer, which leads to a major loss of material. Here, we present laser induced forward transfer of chemical vapor deposition grown graphene layers with well-defined shapes and geometries. The transfer is based on photo-decomposition of a triazene-based transfer layer that produces N2 gas, which propels a graphene layer from the donor to the acceptor substrate. The functionality of the graphene-metal junction was verified by realizing functional bottom contact bottom gate field-effect transistors.

High-efficiency perovskite/silicon heterojunction tandem solar cells

Silicon solar cells reached record efficiencies close to their practical efficiency limit. A promising approach to further increase performance at low cost lies in combining silicon and perovskite solar cells to form a tandem device. Here, we present high-efficiency perovskite/silicon heterojunction tandem cells. From 4-terminal measurements, a total steady-state efficiency of up to 24.4% was obtained using maximum power point tracking. For monolithic tandems, which have the potential for higher performance but are more challenging to fabricate, efficiencies of 19.2% and 21.2% were reached on cell sizes of 1.22 and 0.17 cm2, respectively. These efficiencies are steady during >300 s of maximum power point tracking.