Microscopic characterization of charge carriers in solar cells is useful for high-performance cell fabrication because the formation and accumulation of charges in cells greatly affect the device performance. Electron spin resonance (ESR) is suitable for such characterization because it can directly observe charge carriers with spins in these cells. In this work, the ESR method is applied to organic thin-fi lm solar cells to investigate charge formation in such devices. Heterojunction cells of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/pentacene/C 60 /bathocuproine (BCP)/Al are investigated. Clear ESR signals are observed by inserting a typical PEDOT:PSS hole buffer layer. From analysis of the dependence of the ESR characteristics on the external magnetic fi eld direction, the bias voltage, and the duration of solar-simulated irradiation, the charges (mobile holes) in pentacene layers are successfully identifi ed and it can be deduced that these holes are formed at the PEDOT:PSS/pentacene interface during device fabrication. This ESR analysis provides useful knowledge for understanding device operation and improving device performance at the microscopic level. 592 www.MaterialsViews.com www.advenergymat.de

We have investigated the dependence of device characteristics of bulk-heterojunction organic thin-film solar cells on the concentration of glycerol and sorbitol addition in poly(3,4-ethylenedioxy thiophene):poly(4-styrene sulfonate) (PEDOT:PSS) solutions for fabricating buffer layers. The device structure is ITO/buffer/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C 61-butyric acid methylester (PCBM)/ Al . Glycerol addition is effective for increasing power conversion efficiency (PCE) from 1.25 to 1.41% because of the increase in short-circuit current density (J sc ) without decreasing open-circuit voltage (V oc ). On the other hand, sorbitol addition decreases PCE from 1.25 to 1.04%, owing to the decrease in V oc . This difference in V oc behavior is ascribed to different work function of PEDOT:PSS with glycerol and sorbitol treatment.

No abstract

The degradation mechanism of pentacene/C 60 heterojunction solar cells under light irradiation was investigated by evaluating the current density-voltage (J-V) and the incident photon to current conversion efficiency (IPCE) characteristics. The solar cells with a structure of indium tin oxide (ITO)/hole transport layer (HTL)/pentacene/C 60 /BCP/Al were fabricated using the HTL of poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) or glycerol-added PEDOT:PSS (G-PEDOT:PSS). A p-type semiconductor pentacene, an ntype semiconductor C 60 , and an exciton blocking layer bathocuproine (BCP) were used for the fabrication. From the analysis of the IPCE characteristics for the cells without the HTLs, the device degradation is found to be caused by the absorption in pentacene at the wavelength range between 320 and 370 nm. Moreover, the insertion of the HTLs into the cells is found to be effective for preventing the performance degradation under light irradiation.