Parallel polymer tandem solar cells containing comb-shaped common electrodes

The poor optical transmittance of common electrodes based on thin metal films in three-terminal parallel tandem solar cells poses a major hindrance to attaining high power conversion efficiency (PCE). High optical transmittance and electrical conductivity of the common electrodes are crucial to achieving high PCE. We report a parallel polymer tandem solar cell that contains a comb-shaped electrode (CSE) based on gold (Au), which provides both high optical transmittance and low electrical resistance. We studied the performance of the tandem cell devices as a function of number of teeth N of the CSEs with tooth dimensions of 50 μm (width)×30 nm (thickness)×3 mm (length). A maximum PCE of 3.72%, which amounts to 85% of the PCE of an ideal tandem cell, was obtained with N=5 in the tandem structure ITO/ZnO/P3HT:PC71BM/PEDOT:PSS+Triton X-100/Au CSE/PEDOT:PSS/PTB7:PC71BM/LiF/Al, where ITO is indium tin oxide, P3HT is regio-regular poly(3-hexylthiophene), PC71BM is [6,6]-phenyl-C71-butyric acid methyl ester, PEDOT:PSS is poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid), Triton X-100 is 4-(1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol, and PTB7 is thieno[3,4-b]-thiophene/benzodithiophene. The combination of high optical transmittance and low electrical resistance of the Au CSE and the conductivity-enhanced hole transport layer of PEDOT:PSS+Triton X-100 resulted in a 20% higher PCE than that obtained using conventional common electrodes based on 12 nm thick Au films. We analyzed the resistance of the combined anode, which consists of the Au CSE and the PEDOT:PSS+Triton X-100 film, theoretically. The theoretical results enabled us to predict the optimum number of teeth required for the Au CSEs to give the maximum PCE in three-terminal parallel tandem cells, and showed excellent agreement with the experimental results.