Perovskite silicon tandem solar cells have the potential to overcome the efficiency limit of 29.4% [1] of single junction silicon solar cells by reduction of thermalisation losses. In particular, we aim for monolithic tandem devices as it requires a lower number of transversely conductive layers and allows for more facile module integration compared to 4-terminal devices.

We use an n-type heterojunction silicon bottom solar cell with a pyramidal rear side texture. On both sides an intrinsic amorphous silicon layer is deposited by plasma enhanced chemical vapour deposition (PECVD) followed by a p doped and n doped amorphous silicon layer on the front and rear side, respectively. This silicon solar cell exhibits an implied open-circuit voltage (V_OC) of over 700 mV. A perovskite top solar cell with regular n-i-p architecture is connected to the bottom solar cell via an indium doped tin oxide (ITO) recombination layer. To prevent degradation of the bottom solar cell, a low-temperature process is deployed for the top cell. As an electron contact we use evaporated compact TiO₂ and a UV-treated mesoporous TiO₂ scaffold [2]. Subsequently, a passivation layer of PCBM and PMMA is spin coated [3] followed by deposition of the stable mixed cation mixed halide perovskite FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃ with an optical bandgap of 1.7 eV, which is in the optimal range for monolithic silicon-based tandem devices [4]. The front contact consists of Spiro-OMeTAD and directly sputtered ITO (sheet resistance 44 Ohm/sq). As we use a soft ITO deposition process no additional buffer layer is needed. Finally, a MgF₂ antireflection coating was evaporated. The thicknesses of ITO and MgF₂ have been optimized in order to achieve highest transmission. The perovskite top solar cells reach V_OC values of ~1150 mV.

Our tandem devices achieve very high V_OC values of >1.8 V and power conversion efficiencies (PCE) over 20% with negligible hysteresis on a defined cell area of 0.25 cm². The champion device exhibits over 21% PCE measured under 1 sun illumination at fixed maximum power point voltage for 30 min in ambient air.

The remarkably high V_OC values exceed the V_OC of recent record efficiency devices [5]. The slightly lower PCE is due to the Spiro-OMeTAD hole transport layer: parasitic absorption and the inappropriate refractive index causing reflection losses limit the short-circuit current of our devices. To overcome this limitation, we investigate alternative hole transport materials.