Effects In Organic Solar Cells Research Articles

Photomask Effect in Organic Solar Cells With ZnO Cathode Buffer Layer

In this letter, we report on the performance of conventional bulk heterojunction organic solar cell (OSC) incorporating a solution-processed zinc oxide (ZnO) spin coated on the photoactive layer regioregular poly(3-hexylthiophene):indene-C60 bis adduct. We show a significant improvement in short-circuit current density (Jsc) upon an introduction of ZnO, and this is further evidenced by the reduction of Jsc leading to a lower PCE in the device without the presence of ZnO. Furthermore, we also examined the “photomask” on the performance of ZnO-based device yielding a decrease in both Jsc and open-circuit voltage Voc (of about 13.42% and 0.73%, respectively) but an increase in filling factor. Results demonstrate that ZnO plays an important role in the improvement of OSCs' performance.

Enhanced Performance of SubPC/C60 Solar Cells by Annealing and Modifying Surface Morphology

The performance effect of organic solar cells with subphthalocyanine (SubPC)/fullerene (C60) bilayer was investigated with thermal treatment while changing the vacuum deposition rate of SubPC. The thermal annealing at 100 degrees C increases the optical absorption intensity of SubPC film at the spectral range of 550-630 nm. The X-ray diffraction (XRD) patterns indicates that the thermally annealed film formed the much-ordered morphology, as compared to the non-annealed film. Consequently, thermally treated solar cell exhibited almost 10% higher power conversion efficiency (PCE) compared to the non-annealed device. The fill factor (FF) and PCE of the devices were increased as the deposition rate of SubPC was increased up to 5 A/s and then saturated at higher deposition rates (> 5 A/s). The surface roughness of SubPC films, measured with an atomic force microscope, increased from 1.1 to 5 nm as the deposition rate increased from 1 to 7 A/s. These results imply that rough surface increases the interfacial area between SubPC and C60 and thereby improves the separation of photogenerated electron and hole pairs at the SubPC/C60 interface.

Organic solar cells: A new look at traditional models

Traditional inorganic solar cell models, originating with the work of Shockley, are widely used in understanding bulk heterojunction (BHJ) organic solar cell response (organic solar cells are also referred to as organic photovoltaics, or OPVs). While these models can be useful, there are several key points of departure from traditional solar cell behavior. In this Perspective, we discuss three important areas: (1) geminate pair and bimolecular recombination, (2) effects of interfacial layers inserted between the electrodes and active layer, and (3) resistance effects. Since organic solar cell materials typically have large Coulombic exciton binding energies (e.g., ∼0.3–0.5 eV), limited dissociation of photogenerated charge carriers can be a significant limitation in these cells that is not observed in traditional silicon solar cells. Additionally, the active layer morphology of BHJ organic solar cells allows free charge carriers to recombine before extraction from the cell, creating another photocurrent loss mechanism. Interfacial layers serve a unique role in BHJ organic solar cells; in addition to conventional functions such as photon transmission and charge injection, interfacial layers often act as “blocking” layers, ensuring that charge carriers are collected at their respective electrodes (i.e., electrons at the cathode and holes at the anode). Additionally, resistance effects in organic solar cells differ from traditional models in both field and cell area dependencies. Organic semiconductor mobilities and charge densities exhibit significant sensitivity to field strength, with mobility varying by ∼10x over typical cell voltage test ranges (1 V). This creates the need for alternative models to describe cell internal resistance. Finally, resistance losses are also sensitive to cell area, due to the limited conductivities of the transparent electrode materials used. Therefore, accommodation of the above deviations from traditional models is imperative for the design and synthesis of new generation high efficiency organic solar cell materials.

Device annealing effect in organic solar cells with blends of regioregular poly(3-hexylthiophene) and soluble fullerene

Here we report enhanced efficiency bulk heterojunction organic solar cells using blend films of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) that are subjected to a thermal annealing process. Blend films (P3HT:PCBM=1:1 by weight) were prepared using chlorobenzene and 1,2-dichlorobenzene in order to investigate the role of the solvent. Irrespective of the chosen solvent, the optimal device annealing temperature was found to be 140 °C. The highest power conversion efficiency, 3% under air mass 1.5 simulated solar illumination (100mW∕cm2), was achieved by device annealing at 140 °C for 15 min using blend films prepared from chlorobenzene (2.3% for 1,2-dichlorobenzene).