Electrode Modification Of Efficient Polymer Solar Cells And Performance Study Of Photovoltaic Detectors

Polymer solar cells(PSCs) are new kind of photovoltaic device that can directly convert solar photons into electricity. Due to their perceived advantages such as lightweight, low-cost and unlimited materials resource, currently PSCs are being investigated extensively in worldwide. Usually, photovoltaic devices can be classified as photovoltaic cell and photovoltaic detector. The former type function as energy source and can convert solar energy into electricity, while the latter can response to photons and converts them to electrical signals.To date, the maximum power conversion efficiency(PCE) of PSCs has exceeded 10%. However, the efficiency still needs to be further improved to meet the demand ofcommercialization.Therefore, present studies of PSCsaremainly focused on boostingtheir efficiencies. In the past decade, interface engineering on the interfaces between photoactive layer and electrodes has been demonstrated as one of the most effective approach to increase the PCE of the devices.In this thesis, the Ph.D candidate successfully demonstrated highly efficient inverted PSCs with efficiency of 6.56% from low bandgap donor materialby using conjugated polyelectrolytes(CPEs) as the cathode interlayer. By combining the resultsof comparative studies, we found that both the devices from commonly used Zn O interlayer and CPEs cathode layer have small series resistance, although theirdevice performance differ from each other. As shown by capacitance-voltage(C-V) measurements, the incorporation of PCEN and PFN lead to increased built-in potential across of the cell, which is beneficial for exciton separation and charge carrier transport. Moreover, the results of transient photovoltage measurementunveilthe existence of two decay routes through which charge carriers undergo recombination loss. It is worthy to mention, the interfacial trap states can be effectively suppressed upon the incorporation of CPEs interlayer, as suggested by the results of dependence of the slope of Voc on illumination intensity.In the second chapter of the thesis, the candidate described the use of solution-processible n-type PBI J-aggregates thin film as cathode buffer layer for highly efficient inverted-type PSCs. Nano-sized, rod-like PBI-1 aggregates with J-type excitonic coupling were deposited on ITO substrate via self-assembly by spin-coatingfrom anhydrous THF, whichcan effectively resist the erosion from the common organic solvent. As a result of enhancements in short current density, fill factor and open circuit voltage, the PSCs with PBI-1 buffer layer show enhanced power conversion efficiency of > 9%.The results reported here clearly indicate that n-type solution-processible PBIs areone of the most promising interfacial materials for highly efficient PSCs.In chapter 3, we demonstrated that solution- processed vanadium oxide(VOx) can be used as efficient hole-extracting layer(HEL) for high performance polymer solar cells. When solution- processed vanadium oxide(VOx) was used in combination of PEDOT:PSS, the device performance of the resulted devices showed an a remarkable improvement. The devices based on the solution-processed HEL exhibited a high power conversion efficiency of 3.96%, while the value can be enhanced to 4.06% and 4.16% in the double layer configuration.All the VOX-based devices showed a high fill factor(FF) over 70%, which was ascribed to efficient hole extracting efficiency associated with the solution-processed VOX hole-extracting layer. The origins of the improvement were also studied by transmission spectra, atomic force microscope(AFM), and capacitance-voltage characteristics.Despite the tremendous progresses in photovoltaic cell applications, the exploration of diketopyrrolopyrrole(DPP) based low bandgap polymers as photosensors for IR detection has been seldom studied. In chapter 4, we fabricated a series of high performance near-infrared photodetectors in which DPP-containing polymers(PDPP3T, PDPPBT and PDPPT) were used as the key components. All of the DPP derivatives based devices exhibit high responsivities, detectivities and broad LDR. Specifically,PDPP3 T based detectors showed a responsivity of 310 m A/W at 855 nm, a LDR value of 130 d B, whichare close to the highest valuereported to date in the literature. Moreover, we demonstrated highly efficient flexible photodetectors by replacing the rigid glass with PEN transparent substrate. Photoresponse tests show that our DPP-containing photodetectors possess high response speed, with a rise time than 1 millisecond. What’s more, our flexible detectorsexhibited outstanding mechanical flexibility and can find applications in various areas(e.g., portable devices, aerospace science, nightvision,and many other military and commercial fields).