Optical management in organic photovoltaic devices

Due to their potentials in light‐weight,flexible,and semitransparent devices,organic photovoltaics are of great significance in the field of renewable energy.However,the narrow intrinsic absorption spectrum of organic materials hinders the full utilization of solar energy.To fabricate a highly efficient opaque solar cell,it is greatly necessary to modify the optical properties of the device to improve light absorption.In addition,the growing interest in building‐integrated photovoltaics drives the development of semitransparent devices.The preparation of semitransparent solar cells with excellent performance imposes high requirements on the high efficiency and appropriate visible light transmittance of effective optical management.In this review,the recent research progress of optical management in organic photovoltaics is reviewed,including the design of light‐absorbing materials,the modification of different layers,adding a lighttrapping structure,and changing the light absorption capabilities of specific materials,so as to provide strategies of how to improve the performance of organic photovoltaic devices and present the prospect of the area.

Enhanced performance in organic photovoltaic devices with a KMnO_4 solution treated indium tin oxide anode modification

The properties of poly（3-hexylthiophene）：（6,6）-phenyl C61 butyric acid methyl ester （P3HT：PCBM） organic pho- tovoltaic devices （OPVs） with an indium tin oxide （ITO） anode treated by a KMnO4 solution are investigated. The optimized KMnO4 solution has a concentration of 50 rag/L, and ITO is treated for 15 min. The modification of ITO anode results in an enhancement of the power conversion efficiency （PCE） of the device, which is responsible for the increase of the photocurrent. The performance enhancement is attributed to the work function modification of the ITO substrate through the strong oxygenation of KMnO4, and then the charge collection efficiency is improved.

Flexible organic photovoltaic devices based on oligothiophene derivatives

For the purpose of developing flexible organic photovoltaic devices, we have fabricated two flexible devices using 5-formyl- 2,2′：5′,2″：5″,2′″-quaterthiophene （4T-CHO）, 5-formyl-2,2′：5′, 2″：5″,2′″：5′″,2″″-quinquethiophene （5T-CHO） and 3,4,9,10-perylenetertracarboxylic dianhydride （PTCDA）. The PET-ITO/4T-CHO/PTCDA/A1 device has an open circuit voltage （Voc） of 1.56 V, photoelectric conversion efficiency of 0.77%. The PET-ITO/5T-CHO/PTCDA/A1 device has a Voc of 1.70 V, photoelectric conversion efficiency of 0.84%. The two flexible devices have high Voc （1.56 and 1.70 V）. It is possible that intermolecular hydrogen bonding between -CHO group of nT-CHO and carboxylic dianhydride of PTCDA contributes to enhancing the efficiency by promoting interfacial electron transfer and eliminating the subconducting band trap sites.

A thermal stable cathode buffer based on an inexpensive tetranuclear zinc(Ⅱ) complex for organic photovoltaic devices

An inexpensive material, i.e., tetranuclear zinc（Ⅱ） complex, （Zn40（A/D）6） [AID = 7-azaindolate], was utilized as a cathode buffer in organic photovoltaic （OPV） devices, leading to the improvement of device performance. Compared to OPV devices based on a conventional cathode buffer of TPBi （1,3,5-tris（2-N-phenylbenzimidazolyl）benzene）, although the freshly prepared devices showed similar performance, when heated to a series of high temperatures under air, the short circuit current and the open circuit voltage of the Zn40（AID）6 devices dropped more slowly, indicating the superiority of using Zn40（AID）6 as a cathode buffer over TPBi in OPV devices.

Organic photovoltaic device enhances the neural differentiation of rat bone marrow-derived mesenchymal stem cells

Electrical stimulation is known to be involved in stem cell differentiation,particularly neural differentiation.Various electrical stimulation systems and devices have been developed for neural tissue engineering.The organic photovoltaic materials PM6 and Y6 have showed high-power conversion efficiency.In this study,we used PM6 and Y6 to develop an organic photovoltaic device(OPD)to supply electrical stimulation.The photoelectric stimulation by the OPD showed no impact on cell viability.We tested the neural differentiation potential of rat bone marrow-derived mesenchymal stem cells(rBMSCs)under light induced electrical stimulation.The changes in cell morphology suggested that photoelectric stimulation significantly increased the neurite length and the number of extremities of differentiated neural cells.In addition,genes of neuron markers and neurotrophic factors were upregulated when rBMSCs were under photoelectric stimulation.Furthermore,the calcium influx of differentiated cells responding to acetylcholine and the phosphorylation of extracellular-signal-regulated kinase(ERK)1 and 2,protein kinase B(AKT)and mammalian target of rapamycin(mTOR)were significantly elevated after photoelectric stimulation.These findings demonstrated that PM6:Y6 based OPD could provide photoelectric stimulation to enhance rBMSCs neural differentiation,which might be an alternative approach to electrically manipulate stem cells differentiation into neural cells in vitro.

UV-ozone-treated MoO_3 as the hole-collecting buffer layer for high-efficiency solution-processed SQ:PC_(71) BM photovoltaic devices

The enhanced performance of a squaraine compound, with 2,4-bis[4-（N,N-diisobutylamino）-2,6-dihydroxyphenyl] squaraine as the donor and [6,6]-phenyl-C71-butyric acid methyl ester （PC71BM） as the acceptor, in solution-processed or- ganic photovoltaic devices is obtained by using UV-ozone-treated MoO3 as the hole-collecting buffer layer. The optimized thickness of the MoO3 layer is 8 nm, at which the device shows the best power conversion efficiency （PCE） among all devices, resulting from a balance of optical absorption and charge transport. After being treated by UV-ozone for 10 min, the transmittance of the MoO3 film is almost unchanged. Atomic force microscopy results show that the treated surface morphology is improved. A high PCE of 3.99% under AM 1.5 G illumination （100 mW/cm2） is obtained.

Synthesis and photovoltaic properties of new europium complex Eu(DBM)3(CPyBM)

A new europium（Ⅲ） complex, tris（dibertzoylmethanate）{ 1-[9-hexyl-9-carbazole]-2-（2-pyridyl）-bertzimidazole}europium（Ⅲ） [Eu（DBM）a（CPyBM）] was synthesized and used as an electron-acceptor and electron-transport layer in organic photovoltaic （PV） device. Power conversion efficiency achieved from the device was 1.04% under illumination with 365 nm UV light at 1.6 mW/cm^2. Compared with the previous reported devices based on Eu（Ⅲ） complexes, the PV performances were improved. The working mechanism of the organic PV device was discussed.

Synthesis and Photovoltaic Properties of Novel Porphyrin Derivatives

The porphyrin derivatives, 5,10,15,20-tetra（4-（N-pentane-carboxamide） phenyl） porphyrin（4 NC5-TPP）, 5,10,15,20-tetra（4-（N-dodecane-carboxamide） phenyl） porphyrin（4 NC12-TPP） and their zinc-complexes（4 NC5-TPPZn and 4 NC12-TPPZn）, have been synthesized. Their thermal properties and morphologies were investigated via thermal gravity analysis（TGA）, differential scanning calorimetry（DSC） and polarized optical microscopy（POM）. It was found that the 4 NC5-TPP was amorphous and the 4 NC5-TPPZn was crystalline at room temperature, while the 4 NC12-TPP formed the columnar liquid crystal and the 4 NC12-TPPZn showed the spherulite texture. The electron state density distributions and the optimum configuration of the porphyrin derivatives were calculated by chemical simulation. The electrochemical oxidation and reduction abilities of the porphyrin derivatives were studied by cyclic voltammetry（CV）. It was indicated that the porphyrin derivatives had the potential to develop organic photovoltaic（OPV） devices. Using the porphyrin derivatives as donor materials and the 3,4,9,10-perylenetetracarboxylic dianhydride（PTCDA） as the acceptor material, the OPV devices were fabricated. The device structure is ITO/porphyrin derivatives：PTCDA/Al. The relationship between the morphology and performance of OPV was studied. It was found that the crystalline morphology of the film was beneficial to improve the efficiency of the devices.

Organic thin-film solar cells:Devices and materials

In recent years, the performance of organic thinfilm solar cells has gained rapid progress, of which the power conversion efficiencies （r/p） of 3%-5% are commonly achieved, which were difficult to obtain years ago and are improving steadily now. The r/p of 7.4% was achieved in the year 2010, and r/p of 9.2% was disclosed and confirmed at website of Mitsubishi Chemical in April, 2011. The promising future is that the r/p of 10% is achievable according to simulation results. Apparently, these are attributed to material innovations, new device structures, and also the better understanding of device physics. This article summarizes recent progress in organic thinfilm solar cells related to materials, device structures and working principles. In the device functioning part, after each brief summary of the working principle, the methods for improvements, such as absorption increment, organic/electrode interface engineering, morphological issues, are addressed and summarized accordingly. In addition, for the purpose of increasing exciton diffusion efficiency, the benefit from triplet exciton, which has been proposed in recent years, is highlighted. In the active material parts, the chemical nature of materials and its impact on device performance are discussed. Particularly, emphasis is given toward the insight for better understanding device physics as well as improvements in device performance either by development of new materials or by new device architecture.