Less is more: Thinner electron transporting layer increases performance of perovskite modules

CHEOPS assessed a new production process for bigger (5×5 cm2) perovskite solar cells. The process significantly improves the cells’ performance and spatial uniformity – both is key to the scale-up of the technology.

To increase the photovoltaic performance of perovskite solar cells, CHEOPS researchers conducted several experiments to assess the effect of a thinner electron transporting layer (compact titanium dioxide blocking layer, BL-TiO2) on photovoltaic parameters of perovskite solar cells.

50% thickness = 10% performance increase

Results show that, in this case, less is more:
Reducing the thickness of BL-TiO2 from 40-50 nm to 20-30 nm increased the open circuit voltage by 10.36 % on average. The fill factor was increased by an average of 3.79 % (absolute increase). The picture below shows a cross-sectional scanning electron microscope (SEM) image of the optimized device stack.

These figures are important indicators for solar cell performance: The open circuit voltage is the maximum voltage available from a solar cell and it occurs at zero current. The fill factor then compares the actual maximum power of a solar cell to the theoretical power that would result with an open circuit voltage and a short circuit current. Simply put: The higher the fill factor, the higher is the quality of the solar cell.

Cross-sectional scanning electron microscope (SEM) images of the optimized device stack at different magnifications. The layer highlighted by the blue lines is the electron transporting material (compact titanium dioxide) that was reduced to 20–30 nm, which resulted in a significant increase photovoltaic performance. The upper layer (pink) is the hole transport material (Spiro-MeOTAD), the yellow layer is perovskite. The green layer is mesoporous titanium dioxide on which the perovskite crystals can grow.

Blade-coating results in uniform cells

Perovskite depositions are very sensitive to the realization technique, the curing and the annealing steps, which means that what works for the production of lab-scale cells may not work for larger cells. This is why, in addition to photovoltaic performance, spatial uniformity is key for the scale-up of perovskite technology and characterization of the perovskite homogeneity is mandatory. CHEOPS researchers did so by using light beam induced current (LBIC) measurements to assess the homogeneity of 5×5 cm2 modules, produced with three different coating techniques: Two-step blade-coating, anti-solvent quenching and spin-coating. As the LBIC map below indicates, modules fabricated by using the two-step procedure and the automated blade coater (for both perovskite and hole transport layer) are the most uniform. Blade coating also allows production in air without controlling the environment and facilitates scale-up to larger substrates.

The red light regions in the LBIC maps indicate regions with lower photocurrent, deep red regions indicate regions with higher current. A homogenous, deep red is the most desirable result. The module with blade-coated perovskite and Spiro-OMeTADon (right) comes closest to such a result.

Next Steps

CHEOPS will continue to optimize photovoltaic performance by improving the perovskite formulation passing from methylammonium-based perovskite to mixed-cation perovskites with optimized morphologies and thickness. This formulation will be applied to a larger substrate (10×10 cm2) which comes closer to the size required for industrial production.