Jairo C. Nolasco

Polymer/cathode interface barrier limiting the open circuit voltage in polymer:fullerene organic bulk heterojunction solar cells: A quantitative analysis

By exploring the origin of the saturation current (J0), we show that the polymer/cathode interface determines the open circuit voltage (Voc) in polymer:fullerene bulk heterojunction solar cells. We fabricate and characterize P3HT:PCBM solar cells and compare their electrical characteristics using as reference P3HT/cathode Schottky diodes. We demonstrate with thermionic emission theory that J0 has the same origin in both devices. Thus, a general barrier-dependent expression for Voc, valid for non-ohmic cathode contacts is proposed and discussed.

Effect of Density of States on Mobility in Small-Molecule n-Type Organic Thin-Film Transistors Based on a Perylene Diimide

Bottom-contact organic thin-film transistors with active layers made of n-type small-molecule perylene diimide Polyera ActivinkTM N1400 were fabricated and were compared with those of 6,13-bis (triisopropylsilylethinyl)pentacene and poly(triarylamine) (PTAA). For the three devices, the mobility values and the distribution of the density of states (DOS) were estimated from current-voltage measurements. The mobility value of the N1400 samples increases up to four orders of magnitude within the measured voltage range, reaching the PTAA mobility values at high gate voltages. Finally, we analyze the effect of the distribution of the DOS on the mobility by studying the free-charge sheet concentration.

Organic thin-film transistor bias-dependent capacitance compact model in accumulation regime

The authors present an analytical and continuous model for the total charges at the gate, drain and source electrodes for organic thin-film transistors (OTFTs), from which analytical expressions of the total capacitances are obtained. Under the quasi-static approximation, the model parameters are extracted using the previously developed unified model and parameter extraction method (UMEM). The capacitance model is valid above threshold voltage. It guarantees continuity of the expressions for the capacitance at the transition between linear and saturation regimes, as well as takes into account the overlap capacitance. Comparisons between modelled and experimental C GG values are shown.

Performance of P3HT/c-Si hybrid solar cell

The perfornmance of Au / P3HT (poly [3-hexylthiophene]) / n-type crystalline silicon (n-c-Si) / Al heterojuntion hybrid solar cells are presented. A P3HT layer with 80 nm thick was spin-coated on silicon substrate followed by thermal annealing and metal deposition. The capacitance-voltage measurements showed that the junction is abrupt. The conduction mechanism over temperature range from 300K to 360K is multi-step tunneling for medium voltage values from 0.2 V to 0.5V. Afterwards, the current-voltage characteristic is affected by a relatively high series resistance. The solar cell performance under 60mW/cm2 illumination was: VOC = 0.40 V, Isc = 10.0 mA/cm2, FF = 0.37 with the conversion efficiency of 2.46 %.

Conduction mechanisms of P3HT: PCBM solar cell

In order to get a deeper understanding of the conduction mechanisms limiting the electrical characteristics of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells, dark current-voltage measurements at different temperatures were analyzed using a compact electrical equivalent circuit previously used in p/n junctions. Between 0.2 V and 0.6 V, the current-voltage characteristic is modeled by an exponential term which can be described by Multi-Tunneling Capture Emission process. For larger voltage, the model takes into account Space-Charge Limited process and series resistance. In addition, the model is useful to calculate the built in potential of the solar cell using only dark current-voltage-temperature measurements.

Study of frequency dispersion effects to consider in a capacitance model for OTFTs

This work presents a study of the frequency dispersion effects of the capacitance-voltage characteristics of organic PMMA on P3HT Thin Film Transistors (TFT) using simulated curves. For validation, simulated CV characteristics at 1 MHz are compared with experimental curves, measured at the same frequency.

Compact Capacitance Model for OTFTs at Low and Medium Frequencies

We present a compact capacitance model for organic thin-film transistors (OTFTs), which is valid not only in the accumulation regime but also in the partial and total depletion regimes. The parameters applied in the model are analytically extracted from the current-voltage characteristics of the devices, using a previously developed unified model and the parameter extraction method. The overlap capacitance effect is taken into account, and the frequency dependence is considered empirically by means of the insulator permittivity. Comparisons between modeled and experimental gate-to-channel capacitances of staggered upper contact p-channel OTFTs based on P3HT-PMMA and PCDTBT-PMMA show the validity of the model at low and medium frequencies.

Performance and degradation of organic solar cells with different P3HT:PCBM[70] blend composition

In this work, we investigate the performance of Bulk Heterojunction (BHJ) Organic Solar Cells fabricated with P3HT films blended with varying weight fractions of PCBM[70]. To determine the optimal composition, P3HT:PCBM[70] blends at three different ratios (1:0.84, 1:1 and 1:1.21, wt%) were fabricated. The electrical parameters were extracted from the current — voltage characteristics (I-V) under dark and illuminated conditions. The degradation mechanisms involved in the devices were studied.

Analytical energy-barrier-dependent Voc model for amorphous silicon solar cells

We show that the open circuit voltage (Voc) in hydrogenated amorphous silicon (a-Si:H) solar cells can be described by an analytical energy-barrier-dependent equation, considering thermionic emission as the physical mechanism determining the recombination current. For this purpose, the current-voltage characteristics of two device structures, i.e., a-Si:H(n)/a-Si:H(i)/a-Si:H(p)/AZO p-i-n solar cells with different p-doping concentrations and a-Si:H(n)/a-Si:H(i)/AZO Schottky structures with different intrinsic layer thicknesses, were analyzed in dark and under illumination, respectively. The calculated barrier in the p-i-n devices is consistent with the difference between the work function of the p-layer and the conduction band edge of the i-layer at the interface in thermal equilibrium.

Study of the interface area effect on the density of states in PTAA-Cytop® OTFTs

In this paper we show that the mobility reduces for OTFTs with similar channel length when the channel width is increased. The effect is shown in staggered bottom contact organic thin film transitors (OTFTs) made of the P-Type semiconductor Poly(Triarylamine) PTAA and Cytop® as insulator. It can also be seen from experiment, that this mobility reduction is associated to an increase in the density of localized traps present in the active layer material. An interpretation of this effect is presented.