A Study On Applications Of Poly Bulk-heterojunction Photovol

Polymer-fullerene bulk-heterojunction (BHJ) photovoltaic devices have attracted great attention in recent years due to their low-cost, simple manufacting process to realize large area devices and technical compatibility with flexible devices. However, Organic semiconductor devices always exhibit the poor transient response characteristic and the low efficiency of photo/electric conversion because of their low charge-carrier mobility, short exciton diffusing length and short charge carrier lifttime. The poor performance in charge transportation restricts the widely application of organic semiconductors. Now, a simple, yet successful technique is the solution-processed bulk-heterojunction that leads the organic materials to a potential alternate to the inorganic counterpart in photovoltaic devices.In Chapter 2, A high-performance polymer ultraviolet (UV) detector based on a simple sandwich structure consisting of the wide-band-gap conjugated polymer of poly(3,6-(N-2-ethylhexyl)carbazole) (PCz) as the electron donor (D) blending with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the electron acceptor (A) was fabricated. The detector with the structure of ITO/PEDOT:PSS/PCz:PCBM(2:1 by w/w)/Al is sensitive to UV light, and the zero-bias photoresponsivity in UV range reaches as high as about 50 mA/W. The corresponding quantum efficiency is 18% (el/ph) under illumination with 346 nm UV light. The ratio of the photocurrent to dark current (Dynamic range) is 1.3×105 at zero bias under illumination of 355 nm UV light with power of 1 mW/cm2. The transient behavior and stability of the detector were also characterized and discussed.A charge carrier mobility of polymer films with the time-of-flight (TOF) technique by using a fullerene layer was measured and the TOF photocurrent waveform can be remarkably improved. The 80-nm-thick fullerene layer is functioned as a charge-separation layer (CSL) which was placed between ITO electrode and the polymer layer of MEH-PPV (poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene]). In the CSL, the photo-generated holes and electrons can be efficiently separated, resulting in an enhanced current signal and great improvement of TOF waveform. The sample structure with fullerene layer exhibits a great advantage to measure hole mobilities of polymers with low energy band gap.Hole mobilities of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4- phenylenevinylene]( MEH-PPV )films fabricated by spin-casting and drop-casting have been measured by time-of-flight (TOF) technique. The non-dispersive hole transport current waveform is obtained. The results exhibit that the hole mobility of MEH-PPV prepared by drop-casting is higher than that from the film prepared by spin-casting. The solar cells based on MEH-PPV and fullerene derivative blend films prepared by spin-casting and drop-casting, respectively, were fabricated. The power-conversion efficiency (PCE) of the drop-casting device has a great improvement over 35%, compared with that of spin-casting one. The improvement attributes to the stronger absorption and more balance of electrons and holes transport in MEH-PPV polymer films.In Chapter 5, we have modeled experimental short-circuit photocurrent action spectra of thin film bulk-heterojunction photovoltaic cells. Modeled was based on the assumption that the photocurrent generation process is the result of the creation and diffusion of photogenerated exctions, which are dissociated by charge transfer action at the donor/acceptor interface. To calculate the distribution of optical electrical field inside cells, we have adopted the matrix modeling technique, which allows the accuracy simulation of all of the interference effects in the multilayered thin films. For P3HT: PCBM bulk-heterojunction photovoltaic cells, we have shown that the experimental power-conversion efficiency (PCE) is modulated by varying the thickness of active layer, which obeys calculation results of optical electric field distribution. The calculated internal optical electric field distribution can be used for studying the influence of the geometrical structure on the performance and optimizing the PCE, in the thin film photovoltaic cells. Additionally, we found that the internal optical electric field distribution would be reformed by introducing the optical spacer into bulk-heterojunction photovoltaic cells. Therefore, by introducing the suitable optical spacer would optimize the photocurrent action spectra to realize the higher PCE of photovoltaic cells.