초록/요약

Organic-inorganic hybrid bulk heterojunction (BHJ) solar cells, which consist of conjugated polymers as electron donors and colloidal quantum dots (QDs, so called nanocrystals, NCs) as electron acceptors, have advantages in solution-based simple and cost-effective fabrication process and feasibility for use in flexible and large-scale renewable energy platform. This type of solar cell possesses the combined advantages of both material classes; for example, polymers present film-forming property and high absorption coefficient, while QDs present unique properties of size-tunable bandgap modulation, multiple exciton generation, strong and broad absorption profile, high dielectric constant, and high carrier mobility. However, the reported power conversion efficiencies (PCEs) of currently existing hybrid BHJ solar cells lag behind those of all-organic and all-inorganic solar cells, which reflects the necessities for improving their performance through extensive researches in donor-acceptor (D-A) materials, hybrid materials interfaces, film morphologies, processing conditions, device structures, and so on. In this dissertation, the development of low-bandgap QD-based hybrid solar cells and several scientific approaches to increase their efficiencies are demonstrated. It begins with a general introduction to solar cells, operating principles, state of the art, and strategies for improving photocurrent generation of hybrid solar cells. The results of systematic investigations depending on several parameters of semiconductor QD materials (e.g., PbS, PbSxSe1-x, HgTe, CuInS2, FeS2, and CdSe), polymer-QD HJ types, device architectures, polymer:QD blending ratios, QD surface-capping ligands, shapes and sizes of QDs, and nanopatterning are demonstrated. Notably, promising PCE values of greater than 3% are achieved in hybrid solar cells, consisting of nanocomposites of PbS QDs and poly[2,6-(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-alt-4,7(2,1,3-benzothiadiazole) (PSBTBT), by systematically optimizing band alignment at QD-polymer hybrid interfaces as a result of the QD size manipulation. PCEs have reached to ~3.4% by incorporating both QDs and nanorods (NRs) of PbSxSe1-x in the low-bandgap polymer matrix, benefiting from the straight pathways for photogenerated carriers within NRs that are conductively interconnected by spherical QDs. A further improvement in the PCEs approaching 4.56% is obtained as optimizing D-A concentration ratios and employing a new device structure, which contains D/D:A/A′ photoactive layers for balanced p-n junctions in the device. The PCEs achieved here are to date the highest value in hybrid polymer:PbS BHJ solar cells, and are comparable to general efficiencies in all-organic BHJ devices, suggesting a great potential of QDs to replace organic acceptors. Next, universal optical approaches are demonstrated as a means to improve the efficiency of solar cells. Hybrid BHJ solar cells fabricated on flexible polymer-based microlens array (MLA) substrates show increased optical path in the photoactive layer, resulting in the overall PCE increase. Our micro-controlling techniques of the solar light, which incorporate the MLA or dome-shaped microstructures on the light incident surface of the solar cells, exhibit great potential of the optical devices to further improve PCEs of hybrid solar cells. Finally, summary and future outlooks in hybrid heterojunction solar cells are given. In appendix part, synthesis of QDs, fabrication of devices, and characterization of photovoltaic performances are described.