Charge Collection Control via Interface Modification for High Performance Organic/Inorganic Solar Cells

DoctorInterface engineering determines the built-in potential, charge carrier collection, and thereby overall photovoltaic performance of the organic and perovskite thin film solar cells. General idea for the interface modification is introducing an interlayer between a photoactive layer and an electrode. Transparent interlayer with adequate energy level and high carrier mobility would maximize the performance of the solar cells. However, understanding the nature of interface is complex, and interfacial factors are not solely elucidated. Herein, this thesis is dedicated to understanding the nature of the interface comprehensively and optimizing the charge-transferring interfaces by systematically modifying the energy levels, conductivities, elastic modulus, and molecular interactions of the interlayers for the organic/inorganic thin film solar cells. In Chapter 2, organic/metal contact is modified through the C70-fullerene-based self-assembled-monolayer (SAM) which introduces the covalent bonding and C70-fullerene which is interactive with fullerene-based acceptor at the organic/metal contact. This interface reduces energy barrier for electron transfer, and enhances electron extraction efficiency. Furthermore, this C70-fullerene-covered interface prevents direct contact between the donor polymer in the photoactive layer and the cathode, blocking the recombination between holes from the donor polymer and electrons from the cathode. Trap states at the cathodic interface are effectively passivated with C70-fullerene SAM, reducing the trap-assisted recombination. As a result of efficient electron extraction and reduced charge carrier recombination, power conversion efficiency of the organic solar cells with C70-fullerene SAM records 10.82%, especially with the short-circuit current density of 20.56 mA cm-2. In Chapter 3, ZnO/CH3NH3PbI3 contact is modified through the hybridization of ZnO with a polar insulating polymer, poly(ethylene glycol) (PEG). PEG successfully passivates the oxygen defects on ZnO and prevents direct contact between CH3NH3PbI3 and defects on ZnO. Uniform CH3NH3PbI3 film is formed on soft ZnO:PEG layer after dispersing the residual stress from the volume expansion during CH3NH3PbI3 conversion. PEG also increases the work of adhesion of CH3NH3PbI3 film on ZnO:PEG layer and holds CH3NH3PbI3 film with hydrogen bonding. Furthermore, PEG tailors the interfacial electronic structure of ZnO, reducing the electron affinity of ZnO. As a result, selective electron-collection cathode is formed with reduced electron affinity and deep-lying valence band of ZnO, which significantly enhances carrier lifetime (473 µs) and photovoltaic performance (15.5%). The mechanically and electrically durable ZnO:PEG/CH3NH3PbI3 interface maintains the sustainable performance of the solar cells over 1 year. In Chapter 4, thiophene-based π-bridges are introduced in push-pull type conjugated polymer, alternately copolymerized electron-rich and electron-deficient monomers, to elongate π-conjugation along the hole-transporting polymer backbone. Elongated π-conjugation, thus delocalized electrons, enhances intermolecular π-π interaction and increases crystallinity of the polymer with dominant face-on orientation which is advantageous for the vertical charge transport inside the film. Elongated π-conjugation and increased crystallinity directly improve the hole mobility. Moreover, the highest occupied molecular orbital level increases to be similar to the valence band of the perovskite. As a result, efficient hole extraction at peroskite/hole transport layer (HTL) occurs with reduced charge carrier recombination. In Chapter 5, two major determinants of the efficient charge extraction, charge transport and interfacial charge transfer, are decoupled at the HTL, and only charge transport is examined. A simple physical tuning of a representative polymeric HTL, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), provides a wide range of charge conductivities from 10-4 to 103 S cm-1 without significant modulations in their energy levels, thereby enabling the decoupling of charge transport and transfer properties at HTLs. The transient photovoltaic response measurement reveals that the facilitation of hole transport through the highly conductive HTL promotes the elongation of charge carrier lifetimes within the PeSCs up to 3 times, leading to enhanced photocurrent extraction and finally 25% higher power conversion efficiency