To measure the spatial temperature gradients, a 3 ? 3 transistor array was fabricated on a 2 ? 2 cm2 ITO/glass substrate (Fig. 5a). Fig. 5b presents the distribution of values of IDS of the individual OTFTs in the array. The insignificant variations in color indicated that all of the devices maintained almost uniform values of IDS, with highest and lowest values of –5.79 and –5.19 µA, respectively. The slight variations in electrical performance of the OTFTs within the array were presumably caused by differences in the crystallinity of the organic semiconducting layer 40. Supplementary Fig. S4 presents the output characteristics of the nine devices. The spatial temperature mapping was determined by heating one corner of the glass substrate to 45 °C; the electrical performance of each transistor was recorded and plotted in two dimensions, with the intensity of the color representing the measured performance. Figs. 5c–e display the values of IDS, the mobility, and the gradient in the values of Vth while heating the upper corner of the substrate. The device closest to the heating source exhibited the highest values of IDS and µ and the largest shift in the value of Vth; the devices next to it were also slightly affected.