Interface Engineering Of Polymer Solar Cells

Recently, the power conversion efficiency （PCE） of polymer solar cells over10%has been achieved with the new materials and novel device structures, which shows thepotential as the commercial techniques for solar energy conversion. Althoughtremendous efforts have been applied to develop novel photo-sensitive donor materialsfor high-efficiency devices, a majority of energy has been compromised due to theenergy barrier at the interface between the donor （or acceptor） and electrode. Thus, it isessential to create an ohmic contact between donor and anode and between acceptor andcathode for efficient hole and electron collection, which certainly improve the overallperformance of the solar devices.In this thesis, vacuum thermal deposition and spin coating technology are deployedto fabricate polymer solar cells based on poly（3-hexylthiophene）:（6,6）-phenyl-C60-butyric acid methyl ester （P3HT:PCBM）. There are mainly threemethods proposed:1) Anode modification with chloroform;2) cathode modificationwith Alq₃;3) cathode interface engineering with n-type doped Bphen. We investigatethe current density-voltage characteristics, external quantum efficiency （EQE） of thedevices, and explore the working mechanisms. The corresponding works reported inthis thesis are shown as followed:1. Enhanced performance of polymer solar cells based on apoly（3-hexylthiophene）:（6,6）-phenyl C61butyric acid methyl ester bulk heterojunction isreported by modifying the indium tin oxide （ITO） anode with chloroform solution.Instead of the traditional UV-ozone treatment, the chloroform modification on ITOanode can result in the PCE enhancement resulting from an increase in the photocurrentwith negligible change in the open-circuit voltage. An increase of20%in powerconversion efficiency is realized over a UV-ozone treated device. The performanceenhancement is attributed to the increase in the work function of the ITO substratethrough the chloroform treatment, and thus improved charge collection efficiency. 2. Highly efficient inverted heterojunction polymer solar cells are developedusing a tris（8-hydroxyquinolinato） aluminum interlayer between the active layer and theITO cathode, in which the active layer is composed of a blended P3HT:PCBM bulkheterojunction. With post-processing light irradiation for40minutes, the PCE of thedevice could be improved irreversibly from2.56%to3.33%under AM1.5illumination（100mW/cm2）. The significant performance enhancement is attributed to the removal ofsurface potential of vacuum-deposited Alq₃layer on ITO substrate duo to lightirradiation-induced molecular dipolar reorientation, which lowers the charge transportbarrier and thus improves the charge collection efficiency.3. An efficient inverted polymer solar cell is enabled by incorporating an n-typedoped wide-gap organic electron transporting layer （ETL） between the ITO cathode andthe photoactive layer for electron extraction. The ETL is formed by a thermal-depositedcesium carbonate-doped4,7-diphenyl-1,10-phenanthroline （Cs₂CO₃:BPhen） layer. Thecell response parameters critically depended on the doping concentration and filmthickness of the Cs₂CO₃:BPhen ETL. Inverted polymer solar cell with an optimizedCs₂CO₃:BPhen ETL exhibits a PCE of4.12%as compared to1.34%for the device witha pristine BPhen ETL. The enhanced performance in the inverted device is associatedwith the favorable energy level alignment between Cs₂CO₃:BPhen and theelectron-acceptor material, as well as increased conductivity in the doped organic ETLfor electron extraction. The method reported here provides a facile approach to optimizethe performance of inverted polymer solar cells in terms of easy control of filmmorphology, chemical composition, conductivity at low processing temperature, as wellas compatibility with fabrication on flexible substrates.