Nanocrystalline silicon oxide interlayer in monolithic perovskite/silicon heterojunction tandem solar cells with total current density >39 mA/cm² Bernd Stannowski 1 , Luana Mazzarella 1 , Yen-Hung Lin 2 , Simon Kirner 3 , Anna B. Morales-Vilches 1 , Lars Korte 1 , Steve Albrecht 1 , Ed Crossland 3 , Chris Case 3 , Henry Snaith 2 , Rutger Schlatmann 1 1 Helmholtz-Zentrum Berlin, Schwarzschildstr. 3, 12489 Berlin, Germany. 2 Clarendon Laboratory, Oxford University, Parks Road, Oxford OX1 3PU, UK. 3 Oxford PV, Begbroke Science Park, Woodstock Rd., Oxford OX51PF, UK. Abstract — Silicon heterojunction solar cells are implemented as bottom cells in monolithic perovskite/silicon tandem solar cells. Commonly they are processed with a smooth front side to facilitate wet processing of the lead-halide perovskite cell on top. The inherent drawback of this design, namely, enhanced reflection of the cell, can be significantly reduced by replacing the amorphous or nanocrystalline silicon front side n layer of the silicon cell by a nanocrystalline silicon oxide n layer. It is deposited with the same commonly used plasma-enhanced chemical vapor deposition and can be tuned to feature opto-electrical properties for enhanced light coupling into the Si bottom cell, namely, low parasitic absorption and an intermediate refractive index of ~2.6. We demonstrate that a 80 – 100 nm thick layer results in 0.9 mA/cm² current gain in the bottom cell yielding tandem cells with a top cell + bottom cell total current above 39 mA/cm². These first nc-SiOx:H-coupled tandem cells reach an efficiency >23.5 %. Index Terms — photovoltaic cells, silicon, heterojunctions, hybrid junctions. I. INTRODUCTION Silicon heterojunction (SHJ) solar cells are attractive for use as bottom cells in monolithic tandem cells with lead-halide perovskite top cells. This is because they have a high efficiency with an open circuit voltage above 730 mV combined with a high spectral response in the NIR regime. Moreover, from a processing point of view the device structure ideally matches the requirements for processing the perovskite top cell, namely, the top layer usually is a tin-doped indium oxide (ITO), which is a good substrate and contact material for the perovskite top cell. Due to the commonly used spin coating process for the perovskite top cell, it has to be deposited on a flat (polished) silicon wafer in order to obtain homogeneous cells without shunts. An early tandem cell reached an efficiency of 19.9 %, strongly limited by the bottom cell current [1]. Later a certified stable tandem efficiency of 23.6 % was presented [2] featuring better current matching. This cell, however, was still limited by the short circuit current of 18.1 mA/cm² and a total current (top + bottom) below 38 mA/cm². The limited total current was due to high reflection losses, partially as result of the flat interfaces. To overcome the drawback of the flat front side, namely, light reflection out of the tandem cell at the interface between the perovskite (n @ 800 nm = 2.4) and the silicon (n @ 800nm = 3.7) sub cell, optical light coupling into the bottom cell by using an intermediate-refractive index material with low parasitic light absorption for wavelengths >700 nm is required. One option is plasma-enhanced chemical vapor deposited (PECVD) (doped) nanocrystalline silicon oxide (nc-SiOx:H), which has been developed over the past years to replace the doped amorphous silicon contact layers in silicon heterojunction solar cells [3]. Due to its mixed-phase morphology with doped silicon (nano)crystals embedded in an amorphous silicon (sub)oxide matrix [4,5] its optical properties can be tuned over a wide range by varying the oxygen content, without deteriorating the electrical (contact) properties in the cell. We previously showed the superior properties in both-side textured rear p-type emitter SHJ solar cells featuring <10 nm thin n-type nc-SiOx:H front contacts, namely, an enhanced short circuit current with excellent electrical properties ( = 22.6%, Jsc = 38.3 mA/cm², Voc = 731 mV, FF = 80.6 %) [6]. Front-side flat cells of the same type and an optically adapted 17-nm thick nc- SiOx:H but no ARC on the front TCO reached an efficiency of  = 20.4 %, with jsc = 35.7 mA/cm², Voc = 719 mV, FF = 79.6 %. By adding a PECVD based SiO2 AR layer on top of the TCO the cell current could be increased by 1.3 mA/cm² leading to 21.2 % cell efficiency. In a recent paper we proposed the use of nc-SiOx:H as an optical interlayer in perovskite / silicon tandem cells [7]. Optical simulations predicted ideal light coupling with nc-SiOx:H having a refractive index (at 800 nm) of 2.6 and a thickness of 90 nm, which would facilitate a current matched tandem cell with Jsc = 20.1 mA/cm², which is a high value for a SHJ cell with flat front interface and only the rear side being textured. Tandem cells with an efficiency up to 30 % are achievable and even higher values appear feasible with improved perovskite bandgap [7]. To validate this predicted high current, we realized tandem cells varying the interlayer refractive index and thickness as given in Table 1. We present the cell results. II. EXPERIMENTAL DETAILS The tandem device structure realized in the present study is depicted in Figure 1. Rear-emitter silicon heterojunction cells brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by HZB Repository