Processing and characterisation of tandem solar cells from crystalline silicon materials

We present the first crystalline Si-based tandem solar cell device consisting of a monolithically interconnected c-Si wafer bottom and a Si nanocrystal (NC) top solar cell. The major part of this work has been realised within a European consortium in the frame of the FP7 project NASCEnT. Within this project we have been able to (i) develop new nanomaterials with photovoltaic compatible technologies, to (ii) improve the theoretical understanding of transport, recombination and dopant incorporation in these quantum confined absorber materials and to (iii) design devices which enable us to prove quantum confinement. In this publication we show that although solid phase crystallisation at 1100 °C for around 30 min is necessary to generate Si NCs of proper quality the wafer bottom cell as well as the c-SiC(n+) tunnel recombination junction layer can withstand such harsh conditions. Besides simulations to determine the influence of the high thermal budget on dopant profiles in wafer and c-SiC lifetime measurements on parallel processed wafers still show minority carrier lifetimes of 600 ms. The bifacial character of the tandem device enables us to clearly distinguish between bottom and top cell contributions. With a NC layer thickness of initially 3 nm Voc values as high as 978 mV could be achieved at 1 sun illumination from the front-side. Under rear-side illumination the short circuit current Jsc is with 1.0 mA/cm2 4 times higher that under front side illumination due to the better absorption and carrier lifetime of the bottom cell but still limited by the series resistance of the intrinsic NC absorber material. However, at 510 mV the Voc is much lower, indicating the difference in Voc is due to photogenerated voltage in the NC top cell which can only be created by illumination with low-wavelength light, i.e. front illumination. This very first tandem solar cell device including quantum dot absorber layers produced with photovoltaic compatible processes is the next step to successfully overcome the Shockley-Queisser limit for wafer based Si solar cells.