Effects of buffer layer properties and annealing process on bulk heterojunction morphology and organic solar cell performance

The performance of polymer–fullerene bulk heterojunction (BHJ) solar cells is strongly dependent onthe vertical distribution of the donor and acceptor regions within the BHJ layer. In this work, weinvestigate in detail the effect of the hole transport layer (HTL) physical properties and the thermalannealing on the BHJ morphology and the solar cell performance. For this purpose, we have preparedsolar cells with four distinct formulations of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)(PEDOT:PSS) buffer layers. The samples were subjected to thermal annealing, applied either before(pre-annealing) or after (post-annealing) the cathode metal deposition. The effect of the HTL and theannealing process on the BHJ ingredient distribution – namely, poly(3-hexylthiophene) (P3HT) and[6,6]-phenyl C61 butyric acid methyl ester (PCBM) – has been studied by spectroscopic ellipsometryand atomic force microscopy. The results revealed P3HT segregation at the top region of the films,which had a detrimental effect on all pre-annealed devices, whereas PCBM was found to accumulate atthe bottom interface. This demixing process depends on the PEDOT:PSS surface energy; the morehydrophilic the surface the more profound is the vertical phase separation within the BHJ. At the sametime those samples suffer from high recombination losses as evident from the analysis of the J–Vmeasurements obtained in the dark. Our results underline the significant effect of the HTL–active andactive–ETL (electron transport layer) interfacial composition that should be taken into account duringthe optimization of all polymer–fullerene solar cells.