Temperature and irradiance effect on the photovoltaic parameters of a fullerene/conjugated-polymer solar cell

Bulk donor-acceptor heterojunctions between conjugated polymers and fullerene derivatives have been utilized successfully for photovoltaic devices showing monochromatic efficiencies above 1%. The present paper reports the temperature and irradiance dependencies of full-spectrum photovoltaic parameters for such devices. The measurements were performed under real sun conditions and under a solar simulator. The sun provided a light source stable in intensity to within ±1% and closely approximating a true AM1.5 spectrum, whereas the simulator enabled the light intensity to be varied in the range 80-600 W m^-2. The most interesting feature that was observed for these devices is that above a cell temperature of 20 °C the positive temperature coefficient observed for the short-circuit current exceeds in magnitude the negative temperature coefficient that was found for the open-circuit voltage. This means that, unlike the situation for conventional PV devices, these cells actually exhibit an increase in efficiency with increasing temperature (reaching a value of 0.63% at 40 °C). We suggest that the observed behavior originates from the temperature dependence of the conductivity of the conjugated polymers-fullerene composite. This hypothesis is confirmed by the irradiance-resolved measurements performed at different cell temperatures. We observe a linear increase in the short-circuit current with light intensity over the whole ranges of irradiances and temperatures but maximum temperature influence is observed at highest light intensity.