The photovoltaic (PV) industry has evolved tremendously since the first practical silicon solar cell was created in the 1950s. In 2018, more solar PV capacity was installed globally than any other power generation technology, even more than all fossil fuels and nuclear together, and more than all other renewables combined. Despite significant advances, solar energy’s share of the global power output for 2018 was still only 2 %. To penetrate the market, significant increases in energy conversion efficiency are required to enable proportional reductions in costs downstream. Perovskites are among the most promising materials gaining attention. With the support of the Marie Skłodowska-Curie programme, the POSITS project applied high-tech characterisation and processing methods to advance technology for exploiting them.
The important potential of perovskites
While silicon PV technology has 65 years under its belt, perovskites have been in the limelight for only about a decade. Nevertheless, according to project fellow Dr Terry Chien-Jen Yang, “Perovskite solar cells have made unprecedented progress in terms of power conversion efficiency from 3.8 % in 2009 to a certified 25.2 % in 2019.” Efficiency of these cells can be further increased by combining materials with different bandgaps to enable a better use of the solar spectrum. For instance, tandem solar cells combining silicon with a bandgap of 1.12 eV and perovskites in the bandgap range of 1.65-1.7 eV have the potential to achieve efficiencies greater than 30 %, values beyond the reach of silicon alone.
Nurturing a nascent technology and overcoming challenges
The most effective method to tune the perovskite bandgap is via halide substitution. However, this can lead to instabilities. As Yang explains, “One of the key problems is photoinduced phase segregation, a phenomenon causing the separation of the perovskite material into iodine-rich and bromine-rich domains. This lowers the voltage and efficiency of the high-bandgap perovskite cells.” POSITS set out to characterise this segregation and determine ways to ameliorate it. Researchers reviewed a number of perovskite materials as candidates to improve stability and reduce phase segregation. Using optical and structural methods, the team characterised complex refractive indices of high-bandgap perovskites. Extensive research also investigated phase segregation kinetics, identifying rate-controlling mechanisms as well the impact of the process on luminescence yield. Perhaps most importantly, POSITS aided in the development of a two-step hybrid deposition technique to yield functional cells on textured rather than with flat silicon bottom cells. It demonstrated the importance of texture in creating higher short-circuit currents compared to perovskite-on-flat-silicon tandem designs. The most recent results of POSITS have been submitted for publication. As Yang summarises, “The use of commercially relevant textured rather than flat silicon bottom cells could be the future of perovskite-on-silicon tandem solar cell technology. We are still a step away from our goal of 30 % efficiency, but we have strongly enhanced our knowledge of materials and devices required for achieving it.” While performance and long-term stability improvements are still required, perovskites are on their way to playing an important role in the future of solar cells and potentially other optoelectronic devices that we use today.
POSITS, solar, perovskite, solar cell, silicon, efficiency, tandem, photovoltaic (PV), bandgap