To achieve the net-zero emission, we must consider sustainable forms to convert and store energy. Solar cells involving perovskite materials, either alone or as perovskite/Si-tandem devices, emerging as potential alternatives considering high efficiency (~ 29-30%) and cost-effectiveness could become leading renewable-energy technologies. Despite enormous performance progress, their structural instability limited commercialization. Structural fluctuations in the broadly studied ‘soft’ semiconductors like perovskites were shown to play an important role in driving their exceptional optoelectronic properties, while this also introduces structural instability. Intense research interests are now geared towards improving structural dynamics of perovskites as well as electronic properties/dynamics separately, while owing to the soft lattice character, electronic and structural dynamics are highly entangled and time dependent. This entanglement leads to the formation of mixed ‘hybrid’ electronic-vibrational states (i.e. polarons). Characterization of those states calls for a development of an experimental toolkit which would rely on simultaneous probing of electronic response under vibrational stimulation with ultrafast time-resolution.
Keeping this in mind, we develop action spectroscopy in which the interaction of light with the system under study is evaluated based on measuring the ‘outcome’ of the interaction i.e. either photocurrent or photoluminescence in this case. Interaction of the light with the system was implemented through double-resonant excitation (infrared pre-excitation followed by a visible laser) based vibrationally promoted electronic resonance (VIPER) spectroscopy. Development of this new research methodology is anticipated to become a versatile experimental tool in future to study the carrier-phonon coupling in soft semiconductor class.
The main goal of the project is to capture and control this electronic-vibration coupling of hybrid perovskites such that we can build a design principle towards structure-function relationship in perovskites. Finding from this project reveals vibrational mode-selective coupling of the organic cations with the inorganic lattice for a short time-period and this coupling disappears when the organic cation reorients and breaks the H-bonding interaction with the halides. In a larger picture, this coupling is responsible for the intrinsic non-radiative loss channels in the hybrid perovskites. Therefore, our findings demonstrate for the first time that apart from extrinsic loss-factors (like grain boundaries, various trap-centers), controlling the intrinsic vibronic coupling strength through manipulation of the organic cation structure one could be able to increase the efficiency of perovskite solar cells.
