Recently, a number of research efforts have focused on the fabrication of organic photovoltaic (OPV) devices by solution processes, which can meet the requirements for low-cost fabrication. As one of the solution processes, the spin coating has been predominantly utilized for the fabrication of bulk hetero-junction OPV devices. However, the limitations of spin coating technique upon forming patterns freely and processing with large-area substrates make it difficult for the commercialization. Printing is, of course, the most straightforward technology for creating patterned films. As a representative printing technique, in this study, screen printing technique is employed to achieve the low-cost fabrication process for solar cells. In this study, the screen printing technique was utilized to produce bulk heterojunction OPV devices based on P3HT and PCBM. The conditions for organic solar cells are optimized, including the conditions of screen printer, solvents used for screen printing and so on. After optimization of these conditions, the screen printed organic solar cell exhibited the highest PCE of 4.23% for single cell with the work area of 0.09 cm2 under AM1.5 simulated light at 1,000 W/m2 in ambient condition with chloroform as solvent. The PCE obtained in this study is close to the highest value up to date and is comparable to that produced by the conventional spin coating. The relationship between surface morphology of the active layer and solar cells performance is discussed in this paper. Moreover, with the comparable performance to conventional spin coating, screen printing was used to fabricate the series in situ connected solar cells. Preliminary data on the fabrication of in situ series connected polymer solar cells are presented. All the processes but the deposition of aluminum cathode layer employed screen printing technique through a patterned screen mask. Chloroform was used as solvent for the screen-printing process of active layer. Nine individual solar cells with an active area of 1 cm2 were fabricated by one screen print step. Every three cells were connected in series in situ as a group. For one group, the series connection of three individual cells gave a solar cell module measuring 12 mm×50 mm (6 cm2). The active area was 50% of the total area. The remaining 50% of the area was used for interconnections between cells to make sure efficient current extraction and low connection resistance. With the described architecture, the module gave an open circuit voltage (VOC) of 1.77V, a short circuit current (ISC) of 7.8 mA, a fill factor (FF) of 52% and a power conversion efficiency of 2.38% under AM1.5 illumination at 1000 W/m2. Advantages of screen printing technique for commercial application are revealed by the fabrication of in situ series connected solar cells. With the screen printed series connection structure, large area polymer solar cell modules with 48 single cells are fabricated. And the proto type solar cell modules are used as power for stopwatch and LED lights. As well as conventional structure polymer solar cells, invert structure polymer solar cells were also fabricated with a MoO3 hole selective layer, a sol-gel derived ZnO electron selective layer, and the structure of ITO/ZnO/P3HT:PCBM/MoO3/Al. A maximum efficiency of 3.3 % was achieved with optimized buffer layer thickness.