SnOx electron selective layers for perovskite/silicon tandem solar cells using atomic layer deposition

In this work the application of ALD deposited SnOx films as electron transport layers in perovskite solar cells is analysed. Processes to fabricate homogeneous, transparent and conductive tin oxide films were developed on an Oxford Instruments FlexAL tool using a TDMASn precursor and H2O as oxidiser. Two process regimes were investigated; an ALD regime, where the precursor gases are fully separated by long purging steps and a pulsed-CVD regime, where short purge times allow for continuous reactions. Both process regimes were analysed at deposition temperatures from 100 – 250°C and showed a decrease in growth rate with an increase in refractive index for higher temperatures. In terms, of optical properties highly transparent films in the visible range (> 80%) were obtained for all analysed processes. The samples with the lowest absorption were SnOx films deposited at low temperatures in the pulsed-CVD regime. Films with low absorption also exhibited improved conductivity in the range of 200 – 500 Ωcm, which decreased further when the samples were heated. All investigated films were amorphous with a tin rich atomic composition of SnOx. The processes were performed to be compatible with n-i-p and inverted perovskite single junction solar cells as well as tandem devices on textured silicon bottom cells, due to conformal coating at low deposition temperatures and no need for thermal annealing steps. For the application on cell level, perovskite single junction solar cells in a n-i-p architecture were fabricated with a ~15 nm SnOx film as electron transport layer. To improve electron extraction properties different organic interlayers and mesoporous TiO2 were investigated below the perovskite absorber. It was seen that the use of PCBM on top of SnOx improved the solar cell performance of devices with a co-evaporated MAPbI3 absorber. Solar cells with efficiencies close to 6% were fabricated which exhibited a moderately high Voc of ~990 mV but low Jsc of < 10 mA/cm2. For devices with wet-chemically deposited perovskite absorber materials, the fullerene solutions did not form a closed film due to wettability issues on SnOx and the risk of washing away by the spin-coated perovskite solution. SEM-images confirmed that no closed interlayers were formed in the wet-chemical devices which could be the cause of poor reproducibility for devices with a planar structure and SnOx as electron contact. The best performing device was achieved with SnOx and mesoporous TiO2 deposited by spin-coating and a MAPbI3 absorber. It showed a mean PCE from forward and reverse scans of 12.8% with a Voc > 990 mV and a Jsc close to 20 mA/cm2. Compared to the TiO2 reference cells the devices using SnOx showed lower efficiencies but improved reproducibility and reduced hysteresis in the mesoporous structure. The produced cells serve as an initial proof of concept for the use of SnOx by ALD in the analysed solar cell structure. To analyse the potential for commercialisation of perovskite based photovoltaic technologies a techno-economic analysis was performed. Taking into account up-scaled manufacturing processes for perovskite modules, manufacturing costs of 21.0 $/m2 were calculated. This cost is below the calculated allowed extra costs for the top cell of a tandem device with 30 % efficiency, estimated at 30 – 80 $/m2. Projections of the LCOE showed that perovskite single junction cells with a PCE of 15% and a lifetime of 25 years could achieve an LCOE of 5.2 c/kWh. For two-terminal tandem devices with a similar lifetime and an efficiency of 27% an LCOE of 6.6 c/kWh could potentially be achieved, making both technologies competitive with conventional energy technologies in Germany. An overview of literature on life cycle assessments showed that despite the use of lead based absorber materials, perovskite technologies have a minor environmental impact and are considered more sustainable than other photovoltaic technologies.