into the active layer, [6,7] the background doping, typically p-type, is believed to arise from polymer synthesis and is difficult to control. [3,9] In a recent report, we demonstrate that OPV active layers deposited on top of a MoO x hole transport layer (HTL) are p-doped, with the background hole concentration depending on the work function of MoO x and the active layer material. [8] Here we show that the electron transport layer (ETL) could also induce doping in the OPV active layer. In particular, poly(3hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) films deposited on top of branched polyethylenimine (PEI) ETL convert from typical lightly p-doped to strongly n-doped, with devices displaying substantially lower short-circuit current density (J sc ) and a strikingly different external quantum efficiency (EQE) spectral shape when compared with devices made on ZnO ETL. Using drift-diffusion and transfer matrix method (TMM) simulations to elucidate the experimental results, we establish that a combination of carrier type and concentration in the active layer fully explain the observed J sc and EQE spectral differences. Finally, we show that the origin of n-doping is due to the high solubility of PEI in the active layer processing solvent. The understanding is confirmed by device behaviors for PEI and another polymeric ETL, polyethylenimine ethoxylated (PEIE), and the dependence on active layer thickness and the energy of negative integer charge-transfer state. This research provides a comprehensive understanding of the effects of doping type and concentration on OPV device behaviors. Specifically, it provides an explanation for the wide variation of EQE spectra reported in the literature for OPV devices made with the same active layers but different interfacial contact layers. [10,11]
Results and DiscussionsPEI and ZnO have been reported to be good ETL candidates in OPVs. [12,13] ZnO is an effective metal oxide ETL in OPV because of its low work function that enhances built-in field and deep valence band maximum that blocks holes to the cathode. [13,14] On the other hand, PEI is a polymeric ETL that functions as an interfacial modifier to reduce the cathode work function. [12] Figure 1a and Table 1 show the device current density versus Doping in an organic photovoltaic (OPV) device can largely impact its performance. In this work, it is discovered that n-type electrical doping in the poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester active layer can be induced by a certain electron transport layer (ETL), particularly branched polyethylenimine (PEI). Consequently, OPV devices with different ETLs exhibit dramatically different current density-voltage behaviors and external quantum efficiency (EQE) spectra. Using drift-diffusion modeling, Hall effect, and capacitance-voltage measurements, it is shown that the difference in EQE spectra originates from the different background carrier type and concentration in the OPV active layer, dictated by the specific properties of ETL. Fa...