Effect of Temperature and Illumination on the Electrical Characteristics of Polymer–Fullerene Bulk‐Heterojunction Solar Cells

The current–voltage characteristics of ITO/PEDOT:PSS/OC₁C₁₀‐PPV:PCBM/Al solar cells were measured in the temperature range 125–320 K under variable illumination, between 0.03 and 100 mW cm^–2 (white light), with the aim of determining the efficiency‐limiting mechanism(s) in these devices, and the temperature and/or illumination range(s) in which these devices demonstrate optimal performance. (ITO: indium tin oxide; PEDOT:PSS: poly(styrene sulfonate)‐doped poly(ethylene dioxythiophene); OC₁C₁₀‐PPV: poly2‐methoxy‐5‐(3,7‐dimethyl octyloxy)‐1,4‐phenylene vinylene]; PCBM: phenyl‐C₆₁ butyric acid methyl ester.) The short‐circuit current density and the fill factor grow monotonically with temperature until 320 K. This is indicative of a thermally activated transport of photogenerated charge carriers, influenced by recombination with shallow traps. A gradual increase of the open‐circuit voltage to 0.91 V was observed upon cooling the devices down to 125 K. This fits the picture in which the open‐circuit voltage is not limited by the work‐function difference of electrode materials used. The overall effect of temperature on solar‐cell parameters results in a positive temperature coefficient of the power conversion efficiency, which is 1.9 % at T = 320 K and 100 mW cm^–2 (2.5 % at 0.7 mW cm^–2). The almost‐linear variation of the short‐circuit current density with light intensity confirms that the internal recombination losses are predominantly of monomolecular type under short‐circuit conditions. We present evidence that the efficiency of this type of solar cell is limited by a light‐dependent shunt resistance. Furthermore, the electronic transport properties of the absorber materials, e.g., low effective charge‐carrier mobility with a strong temperature dependence, limit the photogenerated current due to a high series resistance, therefore the active layer thickness must be kept low, which results in low absorption for this particular composite absorber.