Temperature Behavior of Bulk Heterojunction Solar Cells

7 Temperature Behavior of Bulk Heterojunction Solar Cells 

In spite of the existence of such a standard, all kinds of efficiencies have been reported for organic solar cells, based on measurements performed under a wide variety of test conditions [148]. This section reports a detailed study of 

Additional outdoor measurements of Voc made while continuously varying the cell temperature, without recording the entire I/V curve, confirm this behavior (Fig. 5.49). For all samples, the observed linear decrease has a temperature coefficient in the range dVoc/dT ss 1.40-1.65 mV/K. This is comparable with corresponding values observed for familiar inorganic solar 

In order to try and understand the physical mechanisms which may be responsible for the observed temperature dependence of Voc in the high and low temperature ranges, it is instructive to start with an analysis of the Voc behavior of conventional inorganic semiconductor solar cells with a p-n junction [150]  

The observed experimental result that Voc decreases linearly for bulk heterojunction solar cells allows us to conclude that, at least in the high temperature range (T 200 K), these solar cells may be described by a diode model with Ip exp(E/kT). Here E is a parameter analogous to Eg for conventional semiconductors. For conjugated polymer/fullerene bulk heterojunction solar cells, E should correspond to the energy difference between the HOMO level of the donor and the LUMO level of the acceptor components of the active layer [as also suggested by the extrapolated value of V oc(0 K)]. 

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