No abstract

Morphological control over the bulk heterojunction (BHJ) microstructure of a high-effi ciency small molecule photovoltaic system is demonstrated using both thermal treatment and solvent additive processing. Single crystal X-ray diffraction is utilized to understand molecular interactions in the solid state and the BHJ morphology is examined using bright fi eld, high-resolution, and cross-section transmission electron microscopy techniques. Controlling the domain size, while maintaining good molecular order within the semiconducting donor material, is found to be crucial in achieving high performance and over 90% internal quantum effi ciency exhibited under the optimized conditions.

Charge transport and nongeminate recombination are investigated in two solution-processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)-based donor molecules, mono-DPP and bis-DPP, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PCBM). While the bis-DPP system exhibits a high fi ll factor (62%) the mono-DPP system suffers from pronounced voltage dependent losses, which limit both the fi ll factor (46%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current-voltage characteristics indicate that the mono-DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most signifi cant factor leading to the difference in fi ll factor is the comparatively poor hole transport properties in the mono-DPP system (2 × 10 − 5 cm 2 V − 1 s − 1 versus 34 × 10 − 5 cm 2 V − 1 s − 1 ). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination.

Recent measurements in poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films show that capacitance scales with film volume. We discuss the ramifications of this finding and propose a simple model that describes capacitance in terms of sites in which ions injected from the electrolyte replace holes that are extracted from the film by a metal contact. We propose that volumetric capacitance is inversely proportional to the average distance between these sites.

In this report, we demonstrate that parasitic leakage currents dominate the current voltage characteristics of organic solar cells measured under illumination intensities less than one sun when the device shunt resistance is too low (<106 Ω cm2). The implications of such effects on common interpretations of the light intensity dependence of the solar cell open circuit voltage, fill factor, short circuit current, and power conversion efficiency are discussed in detail.