Energy Level Engineering, Co-doping and Extended Conjugation in Heptazine Frameworks: Towards Efficient Solar Energy Conversion using Carbon Nitride-Based Heterojunctions

Carbon nitride (C_xN_y) is a layered crystalline polymeric organic semiconductor consisting of nanosheets of <i>sp</i>²-hybridized triazine heterocycles. C_xN_y aggregates can either be large (3D crystals or sub-micron sized nanosheets) or small (truncated nanosheets or small spherical nanoparticles resembling quantum dots). The conjugated nanosheets allow for relatively fast charge transport, a high active surface area and the customizability of surface reaction sites. Consequently, carbon nitrides have shown remarkable performance as electrocatalysts, photocatalysts, optoelectronic sensors and carrier transport layers for a range of applications including, but not limited to - water splitting, CO₂ reduction, photovoltaics, photodetectors, water treatment and fuel cells. Carbon nitride combines the aforementioned advantages with scalable synthesis compatible with the use of agricultural wastes as precursors and an elemental constitution made of earth abundant elements. Insofar as the electronic applications of C_xN_yare concerned, further enhancements in the performance of based optoelectronic devices require the following conditions to be met: (i) Narrowing of the electronic bandgap (ii) Increase in carrier mobility (iii) Availability of high quality interfaces for exciton dissociation and charge transfer and (iv) Achievement of high photoluminescence and/or electroluminescence quantum yields. These technical challenges are interconnected and not entirely independent of each other. Herein we present a sequence of materials synthesis, device fabrication and compositional doping strategies to address these challenges and achieve the desired enhancements. We show that incorporation of graphene-like <i>sp</i>² carbon regions within C_xN_ynanosheets through the formation of carbon-rich carbon nitride extends π-conjugation and improves carrier transport. A secondary positive effect of such carbon-rich regions in C_xN_y is the reduction in intra-sheet and inter-sheet hydrogen bonding, which is widely believed to be a source of non-radiative recombination in C_xN_y. Doping and co-doping C_xN_y by highly polarizable main group elements such as S, P, Se, etc enriches the electron density in the nanosheets and increases the dielectric constant of carbon nitride, which in turn increases carrier mobility and reduces the exciton binding energy. We found that main group element-doped carbon-rich carbon nitrides exhibit narrow bandgaps as low as 1.5 eV and generate photocurrent densities as large as 5 mA/cm² for photoelectrochemical water splitting under AM1.5G one sun illumination. Finally, we tacked the issue of heterojunction formation through operando matrix-assisted solid-state condensation polymerization and electrophoretic anodization. The device testing data has been backed up by extensive characterization using XANES, SSNMR, HRXPS and HRTEM.