Combining a perovskite top-layer with a silicon heterojunction solar cell as bottom cell can provide silicon photovoltaics with an important efficiency boost, which can help overcome their practical efficiency limit of about 27 %. This is possible because the top cell helps to reduce thermalization losses: It absorbs UV and visible light and is transparent to near-infrared (NIR) radiation which is absorbed in the bottom cell. Thanks to the low-cost production potential of perovskite cells, these high-efficiency tandem cells are expected to further reduce the cost of solar energy.
Compatible production process achieved
While perovskite cells seem to be the perfect partner for silicon heterojunction cells for tandem applications, their fabrication, so far, has been challenging: The most common production process for perovskite cells involves processing steps with temperatures exceeding 200°C, which would deteriorate the performance of the bottom cell. CHEOPS partner EPFL (École Polytechnique Fédérale de Lausanne) thus developed a low-temperature recipe for planar perovskite cells with excellent uniformity, where the fabrication takes place strictly below 200°C.
In this recipe, the silicon bottom cell is first covered with a recombination junction, an electron transport material and a lead iodide (PbI₂) layer by physical vapour deposition. Subsequently, an organohalide (e.g., CH₃NH₃I) solution is spin-coated onto the PbI₂ layer in an inert atmosphere and thermally annealed to form the perovskite material by interdiffusion. Ultimately, the samples are covered with a hole transport material and receive metal contacts.
Details of this recipe have been published (and are freely available for download) in Werner et al., ACS Energy Lett. 1, 474 (2016). This publication compares the performance and characteristics of two tandem cell configurations made with the same recipe: Monolithic tandem cells, where the perovskite cell is processed directly on top of the bottom cell, and mechanically stacked tandem cells, where the perovskite cell is fabricated separately and placed on top of the silicon cell afterwards.
A further improved recipe for single-junction planar perovskite cells, the reduction of primary reflection at the air/glass interface with a microtextured antireflection foil, and the optimization of the electrical contacts yielded a steady-state efficiency of 16.3% on a cell processed at temperatures below 200°C and with an active area larger than 1 cm². These results are currently prepared for a publication.
Further optimizations needed
While this production process is an important step towards high-efficiency tandem cells, further development will be necessary. CHEOPS will optimize the perovskite material’s bandgap to make it an ideal complement to the silicon heterojunction bottom cell, adapt production processes to be compatible with textured substrates and implement optimized charge transport layers. With these improvements, the CHEOPS consortium ultimately aims to achieve monolithic tandem cells with an active area of 2x2 cm2 and efficiencies above 29 %.