Phase-separated disordered bulk-heterojunction nanostructures for photoelectrochemical hydrogen generation

Donor:Acceptor bulk-heterojunction (BHJ) electrodes are expected to play significant roles in electrode design aimed to increase the efficiency of photoactive materials with low exciton lifetime and diffusion length. While planar BHJ electrodes are well-studied in literature, poor control over donor and acceptor domain sizes during electrode preparation renders it difficult to achieve consistent device performance. Herein, as an alternate to planar BHJ electrodes, nanoparticle BHJ is proposed where the domain size is externally controlled during the preparation of the nanoparticles, which are subsequently used to make the photoelectrode. This approach preserves the domain size and hence is expected to offer better control over planar BHJ electrode design. Based on the donor and acceptor phase distribution, a nanoparticle-based electrode can be categorized as (i) core-shell, (ii) blended, (iii) randomly-distributed, and (iv) Janus-type structures. Owing to the lack of continuous charge transport pathways in other categories, both within and across the nanoparticle interface, it is hypothesized that randomly-distributed nanoparticle-based BHJ electrodes perform better than other configurations. Hence, to validate, photoelectrochemical behaviour of randomly-distributed BHJ nanoparticulate electrode is examined using organic semiconductors P3HT and PCBM serving as electron donor and acceptor, respectively for hydrogen generation applications. Individual P3HT and PCBM nanoparticles were prepared by the miniemulsion technique, which are subsequently used to fabricate distinct donor-acceptor, phase-separated, core-shell structure free bulk-heterojunction photoelectrodes. The optoelectronic, morphological and photoelectrochemical H2 generation examinations revealed that the prepared nanoparticulate electrode performs better than the other nanoparticle-based BHJ electrodes under similar conditions. Following this, BHJ electrodes comprising CuO and TiO2 nanoparticles were fabricated to determine if similar results can be extrapolated to inorganic semiconductor photoelectrodes. However, on the contrary, the inorganic nanoparticle-based BHJ electrodes displayed poor photoelectrochemical characteristics. The decreased performance is determined to be due to: (i) high electron-hole recombination at the donor and acceptor interface, (ii) increased electrode resistance upon TiO2 addition, and (iii) photocorrosion.