This dissertation reports a research done on the stability and photovoltaic performancernstudies of bulk heterojunction polymer solar cells. Different techniques were used, in orderrnto study the stability and performance of polymer solar cells fabricated using differentrnpolymers and fullerenes.rnTwo fullerene derivatives with different electron affinity were used to fabricate polymerrnsolar cell based on P3HT as donor. Results obtained from UV-visible, FTIR and J-Vrnmeasurement showed that the P3HT:PCBM film and solar cells fabricated fromrnP3HT:PCBM as active layer have much better air stability than those made fromrnP3HT:ICBA as active layer. Due to lower electron affinity of ICBA, the blend ofrnP3HT:ICBA photobleached at a faster rate compared to PCBM.rnEffect of sol-gel synthesized TiOx layer on the stability and performance of P3HT:ICBArnbased solar cells were investigated by spectroscopic and electrical measurement methods.rnResults from J-V measurements indicates that P3HT:ICBA/TiOx solar cell has betterrnperformance and stability compared to the reference device. The higher value of shortcircuitrncurrent density and open-circuit voltage for P3HT:ICBA/TiOx solar cell indicate thernrole of TiOx layer as the optical spacer and hole blocking layer, respectively. The resultsrnfrom UV-visible absorbance, J-V and C-V measurement on light irradiatedrnP3HT:ICBA/TiOx solar cell demonstrates that TiOx layer can be used to protectrnP3HT:ICBA active layer from oxygen and water attack under ambient air and to improvernthe working lifetime of P3HT:ICBA based device.rnInvestigation on the effect of two fullerene derivatives with similar electron affinity on thernphotovoltaic performance and stability of P3HT based solar cells is presented. From UVvisiblernmeasurements it is found out that P3HT mixed with both PC61BM and PC71BM has good ambient air photochemical stability. However, device based on P3HT:PC61BM hasrnbetter stability compared to P3HT:PC71BM based device.rnThe impact of two different fullerenes with similar electron affinity on the photovoltaicrnperformance and stability of PCDTBT based solar cells were studied. Results from UVvisiblernabsorbance indicate that unlike that of pure film of PCDTBT, blend of PCDTBTrnwith PC61BM, PC71BM and 1:1 mixture of PC61BM and PC71BM has relatively constantrnabsorbance under light exposure. The results show the stabilizing power of acceptorrnfullerenes when mixed with PCDTBT. The results from J-V measurement indicates that thernsolar cells made from PCDTBT:PC61BM and PCDTBT:PC61+71BM have better stabilityrnunder light exposure in ambient air compared to the device based on PCDTBT:PC71BM.rnWhen only PC71BM is used as acceptor the device has higher rate of decay of PCE.rnThe effect of DIM on the PCE and stability of PSCs made of PCDTBT:PC71BM isrninvestigated. PCE of the device based on PCDTBT:PC71BM processed with DIM is higherrnthan the reference device. In terms of device stability, the PSCs processed with DIMrnshowed poor stability at longer light exposure time. For the device without DIM especiallyrnas the light exposure time was increased, the device stability was better. The result from ISrnmeasurement shows that for pristine PCDTBT:PC71BM devices with DIM, the active layerrnresistance is lower compared to device without DIM. However, after irradiating the devicernfor 5 hr the resistance of the device processed with DIM is higher and it is consistent withrndecreased PCE of aged device.rnThe investigation presented on the application of NIL for performance improvement ofrnorganic solar cells shows that, NIL is really promising technique for improving efficiencyrnof polymer solar cells. Nanopatterning of the PEDOT:PSS electrode can be used as a wayrnto improve the cell efficiency. In our case relative increase of the 60%, from 0.8% to 1.3%rnof efficiency is obtained. The increased interfacial area and higher hole collectionrncontributed for the improved photovoltaic performance of nanopatterned device.rnKeywords: polymer solar cells, degradation, photochemical stability, lifetime, chargerncarrier mobility, P3HT, PCDTBT, ICBA, PC61BM, PC71BM, TiOx, DIM, nanopatterning