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.

Hole transport in poly(phenylene vinylene)/methanofullerene bulk-heterojunction solar cells Melzer, C.; Koop, E.J.; Mihailetchi, V.D.; Blom, P.W.M.

No abstract

Wearable pressure sensors have demonstrated great potential in detecting pulse pressure waves on the skin for the noninvasive and continuous diagnosis of cardiac conditions. However, difficulties lie in positioning conventional single-point sensors on an invisible arterial line, thereby preventing the detection of adequate signal amplitude for accurate pulse wave analysis. Herein, we introduce the spatiotemporal measurements of arterial pulse waves using wearable active-matrix pressure sensors to obtain optimal pulse waveforms. We fabricate thin-film transistor (TFT) arrays with high yield and uniformity using inkjet printing where array sizes can be customizable and integrate them with highly sensitive piezoresistive sheets. We maximize the pressure sensitivity (16.8 kPa −1 ) and achieve low power consumption (10 1 nW) simultaneously by strategically modulating the TFT operation voltage. The sensor array creates a spatiotemporal pulse wave map on the wrist. The map presents the positional dependence of pulse amplitudes, which allows the positioning of the arterial line to accurately extract the augmentation index, a parameter for assessing arterial stiffness. The device overcomes the positional inaccuracy of conventional single-point sensors, and therefore, it can be used for medical applications such as arterial catheter injection or the diagnosis of cardiovascular disease in daily life.

We present a closed description of the charge carrier injection process from a conductor into an insulator. Common injection models are based on single electron descriptions, being problematic especially once the amount of charge-carriers injected is large. Accordingly, we developed a model, which incorporates space charge effects in the description of the injection process. The challenge of this task is the problem of self-consistency. The amount of charge-carriers injected per unit time strongly depends on the energy barrier emerging at the contact, while at the same time the electrostatic potential generated by the injected charge-carriers modifies the height of this injection barrier itself. In our model, self-consistency is obtained by assuming continuity of the electric displacement and the electrochemical potential all over the conductor/insulator system.The conductor and the insulator are properly taken into account by means of their respective density of state distributions. The electric field distributions are obtained in a closed analytical form and the resulting current-voltage characteristics show that the theory embraces injectionlimited as well as bulk-limited charge-carrier transport. Analytical approximations of these limits are given, revealing physical mechanisms responsible for the particular current-voltage behavior.In addition, the model exhibits the crossover between the two limiting cases and determines the validity of respective approximations. The consequences resulting from our exactly solvable model are discussed on the basis of a simplified indium tin oxide/organic semiconductor system.