P. W. M. Blom

The charge injection across a polymeric heterojunction of a poly-p-phenylene vinylene derivative (injecting layer) and poly(9,9-dioctylfluorene) (accepting layer) is investigated. The electric field in the accepting layer is obtained after correcting the applied voltage for the voltage drop across the injecting layer due to the buildup of space charge. At high electric fields, the current across the polymeric heterojunction exhibits only a weak dependence on the field due to the absence of image force effects, in agreement with model predictions. The strong dependence at low fields can be explained by taking the increase of the Fermi level into account, which effectively modifies the barrier for charge carriers waiting for a jump across the heterojunction.

The dependence of the performance of OC₁C₁₀‐PPV:PCBM (poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene):methanofullerene [6,6]‐phenyl C₆₁‐butyric acid methyl ester)‐based bulk heterojunction solar cells on their composition has been investigated. With regard to charge transport, we demonstrate that the electron mobility gradually increases on increasing the PCBM weight ratio, up to 80 wt.‐%, and subsequently saturates to its bulk value. Surprisingly, the hole mobility in the PPV phase shows an identical behavior and saturates beyond 67 wt.‐% PCBM, a value which is more than two orders of magnitude higher than that of the pure polymer. The experimental electron and hole mobilities were used to study the photocurrent generation of OC₁C₁₀‐PPV:PCBM bulk‐heterojunction (BHJ) solar cells. From numerical calculations, it is shown that for PCBM concentrations exceeding 80 wt.‐% reduced light absorption is responsible for the loss of device performance. From 80 to 67 wt.‐%, the decrease in power conversion efficiency is mainly due to a decreased separation efficiency of bound electron–hole (e–h) pairs. Below 67 wt.‐%, the performance loss is governed by a combination of a reduced generation rate of e–h pairs and a strong decrease in hole transport.

A fundamental limitation of the photocurrent of solar cells based on a blend of poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene) (MDMO‐PPV) and [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) is caused by the mobility of the slowest charge‐carrier species, the holes in the MDMO‐PPV. In order to allow the experimentally observed photocurrents electrostatically, a hole mobility of at least 10^–8 m² V^–1 s^–1 is required, which exceeds the observed hole mobility in pristine MDMO‐PPV by more than two orders of magnitude. However, from space‐charge‐limited conduction, admittance spectroscopy, and transient electroluminescence measurements, we found a hole mobility of 2 × 10^–8 m² V^–1 s^–1 for the MDMO‐PPV phase in the blend at room temperature. Consequently, the charge‐carrier transport in a MDMO‐PPV:PCBM‐based solar cell is much more balanced than previously assumed, which is a necessary requirement for the reported high fill factors of above 50 %.

It has recently been reported that, after electrical conditioning, an ohmic hole contact is formed in poly(9,9‐dioctylfluorene) (PFO)‐based polymer light‐emitting diodes (PLED), despite the large hole‐injection barrier obtained with a poly(styrene sulfonic acid)‐doped poly(3,4‐ethylenedioxythiophene) (PEDOT:PSS) anode. We demonstrate that the initial current at low voltages in a PEDOT:PSS/PFO‐based PLED is electron dominated. The voltage at which the hole injection is enhanced strongly depends on the electron‐transport properties of the device, which can be modified by the replacement of reactive end groups by monomers in the synthesis. Our measurements reveal that the switching voltage of the PLED is governed by the electron concentration at the PEDOT:PSS/PFO contact. The switching effect in PFO is only observed for a PEDOT:PSS hole contact and not for other anodes such as indium tin oxide or Ag.

The current–voltage characteristics of methanofullerene [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM)‐based devices are investigated as a function of temperature. The occurrence of space–charge limited current enables a direct determination of the electron mobility. At room temperature, an electron mobility of μ_e = 2 × 10^–7 m² V^–1 s^–1 has been obtained. This electron mobility is more than three orders of magnitude larger than the hole mobility of donor‐type conjugated polymer poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene) (OC₁C₁₀‐PPV). As a result, the dark current in PCBM/OC₁C₁₀‐PPV based devices is completely dominated by electrons. The observed field and temperature‐dependence of the electron mobility of PCBM can be described with a Gaussian disorder model. This provides information about the energetic disorder and average transport‐site separation in PCBM.

The performance of polymer light-emitting diodes (PLEDs) consisting of various poly-p(phenylene vinylene) (PPV)-based derivatives has been investigated. It is demonstrated that an increased charge carrier mobility gives rise to an enhancement of the quenching of the electroluminescence at the metallic cathode. As a result the power efficiency of PPV-based LEDs under operating conditions exhibits a distinct optimum at a mobility of about 10−11 m2/V s.