| As the global energy demand continues to increase every year, the limiting supply of today's main energy sources (i.e. oil, coal, natural gas) and their detrimental long-term effects on the natural balance on our planet, force people develop some renewable energy sources. Harvesting energy directly from the sunlight using photovoltaic (PV) technology is being widely recognized as an essential component of future global energy production. Up to now, the photovoltaic cells based on inorganic materials (mainly silicon) have been proved to convert sunlight to electricity efficiently. But the high cost for manufacture limits them to be widely used. Organic and polymer solar cells, based on organic and polymer materials as active layer, possess the advantage of light weight, plenty of choices for active layer, flexibility, low cost and easily being large-scale etc., which attracts great attention in recent years.Organic and polymer heterojunction solar cells compose of cathode, anode and active layer (donor and acceptor). Light is converted to electricity by the solar cells through five processes sequentially, (i) photons are absorbed by the active layer and exitons form; (ii) excitons diffuse in the active layer; (iii) charge transfer when excitons reach donor/acceptor interface and electron-hole (e-h) geminate pair forms; (iv) e-h geminate pairs dissociate with field assisted and free charge carriers are produced; (v) finally, the free carriers are transported through their respective phases to the electrodes in order to be extracted. The photovoltaic performance of devices is strongly dependent on the light harvesting, energy levels of materials, the morphology of active layers, electrodes, and so on. This thesis is mainly discussing how the above factors such as molecular structure, morphology of active layer and device structure affect photovoltaic performance of solar cells. We used a class of donor-acceptor (D-A) molecules and a class of X-shaped oligothiophene as donors and PCBM as acceptor and investigated the relation between the molecular structure and their photovoltaic performance; we investigated the photocurrent generation of active layers with different morphology which were controlled by thermal annealing and processed by different solvents; We used high-conductivity PEDOT:PSS as anode to replace typical indium tin oxide (ITO) for preparing polymer solar cells on flexible substrates. Based on the flexible and polymer-anode devices, we constructed V- and W- shaped solar cells which can trap light more efficiently than the planar devices; we fabricated inverted semitransparent vacuum-free solar cells using ITO as cathode and PEDOT as anodes. More details are now listed below,1. In the Introduction part, the basic concepts of organic and polymer heterojunction solar cells are described, including the device structure, work principle, important steps during the development history, characterization and related instruments, influencing factors on the performance, and reviewed the recent work focusing on materials of active layers, device physics and processing, novel device structure, and so on.2. A class (three molecules) of donor-acceptor molecules and a class (four molecules) of X-shaped oligothiophene were used as donors separately, PCBM as acceptor for preparing solar cells. In the donor-acceptor molecules, the donor unit is carbazole unit or phenothiazine and the acceptor unit is malononitrile derivative or 3-(1,3-dithiolan-2-yl)pentane-2,4-dione. The results of the devices based on D-A molecules shows that the open-circuit voltage (VOC) and short-circuit current (JSC) can be adjusted by changing the electron-donating ability of donor unit and electron-withdrawing ability of acceptor unit. The absorption from the internal charge transition (ICT) locates in the long-wavelength region and can contribute the photocurrent efficiently. The results of the devices based on X-shaped oligothiophenes shows that as the numbers of branched thiophene unit increase, the conjugated interaction becomes stronger resulting in absorption towards longer wavelength, film-forming ability become better, which improve both the VOC and ISC and enhance the power conversion efficiency (PCE) from 0.008% to 0.8% under 100mW/cm2 white light illumination.3. The morphology of blend films consisting of MEH-PPV as donor and perylenediimide derivative as acceptor was investigated by atomic force microscocy (AFM). The blend films were thermally annealed and processed from chloroform and chlorobenzene solution. AFM measurement shows in the blend films large crystal-like perylenediimide derivative aggregates appear during the thermal annealing and when processed from chlorobenzene solution. The aggregates exhibit as more efficient sensitizer for photocurrent generation under low electric field because the e-h pairs are more loosely bound in the aggregates.4. We fabricated polymer heterojunction solar cells on flexible substrate using bilayer PEDOT as anode with the power conversion efficiency (PCE) reaching about 2.2% in small area under AM 1.5 100mW/cm2 illumination. Based on the flexible polymer-anode solar cells, we constructed relatively large-area V- and W- shaped solar cells. The PCE can be enhanced by about 60% with the folded opening angle of 30°comparing to the unfolding one.5. We demonstrated vacuum-free polymer solar cells where PEO-coated ITO as cathode and bilayer PEDOT:PSS as anode. The work function of ITO was measured to be 4.4eV and decreased to be 3.9eV when PEO coated on it. The device ITO/PEO/active layer/bilayer PEDOT was fabricated by sequentially spin-coating from fluids without vacuum processing. Its PCE reaches 0.54% and can be enhanced to 0.7%, by using a white paper as reflector on the backside of devices. The property of the photocurrent is sensitive to the light reflector backside makes the cells potentially used for image detection.
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