Temperature dependent characteristics of poly(3 hexylthiophene)-fullerene based heterojunction organic solar cells

Electrical and optical properties of poly(3-hexylthiophene-2.5diyl) (P3HT) used as the main component in a polymer/fullerene solar cell were studied. From the study of space-charge limited current behavior of indium-tin-oxide (ITO)/P3HT/Au hole-only devices, the hole mobility and density were estimated to range from ${\text{1.4×10}}^{\text{−6}}\hspace{0.17em}{\mathrm{cm}}^2/\mathrm{V s}$ and ${\text{5.3×10}}^{14}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}$ at 150 K to ${\text{8.5×10}}^{\text{−5}}\hspace{0.17em}{\mathrm{cm}}^2/\mathrm{V s}$ and ${\text{1.1×10}}^{15}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}$ at 250 K, respectively. The highest occupied to lowest occupied molecular orbital energetic difference was estimated from absorption spectrometry to be about 2.14 eV. Strong quenching of photoluminescence when the polymer was mixed with $\text{[6,6]-}{\mathrm{phenyl-C}}_{61}$ butyric acid methyl ester (PCBM), provided evidence of photoinduced charge transfer from P3HT to PCBM. Characterization of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells was done by analyzing the dependence of current density–voltage characteristics on temperature and illumination intensity. The main solar cell characteristics recorded at 300 K under 100 mW/cm² white-light intensity were: Open-circuit voltage 0.48 V, current density 1.28 mA/cm², with an efficiency of 0.2%, and fill factor of 30.6%. Open-circuit voltage decreased almost linearly with increasing temperature, while short circuit current density increased with temperature, saturating at around 320 K, and decreased thereafter. Power conversion efficiency and fill factor were maximum around 3 mW/cm² due to the poor bulk transport properties of the active layer.