Inverted Organic Photovoltaics Research Articles

Light Trapping in Inverted Organic Photovoltaics With Nanoimprinted ZnO Photonic Crystals

Zinc oxide photonic crystal (ZnO PC) formed via facile nanoimprinting was employed on the ZnO electron selective layer of inverted organic photovoltaics (OPV). Optimized inverted OPV fabricated with these highly ordered periodic structures provided effective light trapping, which resulted in increased incident light absorption in the active layer. Consequently, OPVs with the ZnO PC layers show a 23% current density improvement compared with OPVs with planar ZnO layer. Finite-difference time-domain simulation studies show that the electric field intensity is significantly higher in the active layer for devices with ZnO PC structures in comparison with reference devices with planar ZnO electron selective layer. Nanoimprinted ZnO PC is, thus, a viable method for light absorption and efficiency enhancement in OPVs.

Development of Inverted Organic Photovoltaics with Anion doped ZnO as an Electron Transporting Layer

In this study, 3-dimensional ripple structured anion (chlorine) doped ZnO thin film are developed, and used as electron transporting layer (ETL) in inverted organic photovoltaics (I-OPVs). Optical and electrical characteristics of ZnO:Cl ETL are investigated depending on the chlorine doping ratio and optimized for high efficient I-OPV. It is found that optimized chlorine doping on ZnO ETL enhances the ability of charge transport by modifying the band edge position and carrier mobility without decreasing the optical transmittance in the visible region, results in improvement of power conversion efficiency of I-OPV. The highest performance of 8.79 % is achieved for I-OPV with ZnO:Cl-x (x=0.5wt%), enhanced ~10% compared to that of ZnO:Cl-x (x=0wt%).

Enhancement of the power conversion efficiency due to the plasmonic resonant effect of Au nanoparticles in ZnO nanoripples

Au-ZnO nanoripples (NRs) were synthesized by using a sol-gel method for utilization as an electron transport layer (ETL) in inverted organic photovoltaic (OPV) cells. Absorption spectra showed that the plasmonic broadband light absorption of the ZnO NRs was increased due to the embedded Au nanoparticles (NPs). In particular, as compared to regular inverted OPV cells with a ZnO NR ETL, the incident photon-to-current efficiency of the inverted OPV cells with a Au-ZnO NR ETL was significantly enhanced due to the localized surface plasmon resonance (LSPR) effect of the Au NRs. The enhancement of the short-circuit current density (10.05 mA/cm2) of the inverted OPV cells with a Au-ZnO NR ETL was achieved by the insertion of the Au NPs into the ZnO NRs. The power conversion efficiency (PCE) of the OPV cells with Au-ZnO NRs was 3.25%. The PCE of the inverted OPV cells fabricated with a Au-ZnO NR ETL was significantly improved by 20.37% in comparison with that of inverted OPV cells fabricated with a ZnO NR ETL. This improvement can mainly be attributed to an increase in light absorption in the active layer due to the generation of the LSPR effect resulting from the existence of the Au NPs embedded in the ZnO NRs.

High-Performance Inverted Organic Photovoltaics Without Hole-Selective Contact.

A detailed investigation of the functionality of inverted organic photovoltaics (OPVs) using bare Ag contacts as the top electrode is presented. The inverted OPVs without a hole-transporting layer (HTL) exhibit a significant gain in hole-carrier selectivity and power-conversion efficiency (PCE) after exposure in ambient conditions. Inverted OPVs comprised of ITO–ZnO–poly(3-hexylthiophene-2,5-diyl)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM)–Ag demonstrate over 3.5% power conversion efficiency only if the devices are exposed in air for over 4 days. As concluded through a series of measurements, the oxygen presence is essential to obtaining fully operational solar cell devices without HTL. Moreover, accelerated stability tests under damp heat conditions (RH = 85% and T = 65 °C) performed to nonencapsulated OPVs demonstrate that HTL-free inverted OPVs exhibit comparable stability to the reference inverted OPVs. Importantly, it is shown that bare Ag top electrodes can be efficiently used in inverted OPVs using various high-performance polymer–fullerene bulk heterojunction material systems demonstrating 6.5% power-conversion efficiencies.

Optimization of organic photovoltaic device performance via exciton generation profile adjustment

We analyzed the conversion performance of conventional organic photovoltaic (OPV) and inverted organic photovoltaic (IOPV) devices with an active layer of polymer, PTB7: PC70BM. We computed the current density–voltage (JV) curves, short-circuit current density (Jsc), open-circuit voltage (Voc), maximum current density (Jmax), maximum power density (Pmax), and fill-factor (FF) under various scenarios. We employed the one-dimensional optical transfer matrix theory to calculate the light intensity that was then used as the input at the active layer for optical carrier generation. Then we obtained electrical performance parameters from the JV curves plotted by solving Poisson and charge transport equations. The effects of adjusting the exciton generation profile by tuning the active layer width and optical spacer thickness under 100 mW·cm−2 air mass 1.5 global (AM 1.5G) illumination are also analyzed. In addition, the effect on the conversion performance by using different electron and hole mobility relations in the polymers composing the active layer is computed. To identify the optimal performance, we proposed an exciton generation profile that maintains a constant amplitude when shifted through the active layer. Subsequently, by adjusting the active layer width, optical spacer thickness, and electron and hole mobility, we found that the OPV structure achieved performance characteristics previously reported only for IOPV structures.

ITO surface modification for inverted organic photovoltaics

The work function (WF) of indium-tin-oxide (ITO) substrates plays an important role on the inverted organic photovoltaic device performance. And electrode engineering has been a useful method to facilitate carrier extraction or charge collection to enhance organic photovoltaic (OPV) performance. By using self-assembly technique, we have deposited poly(dimethyl diallylammonium chloride) (PDDA) layers onto ITO coated glass substrates. The results indicate that the surface WF of ITO is reduced by about 0.3 eV after PDDA modification, which is attributed to the modulation in electron affinity. In addition, the surface roughness of ITO substrate became smaller after PDDA modification. These modified ITO substrates can be applied to fabricate inverted OPVs, in which ITO works as the cathode to collect electrons. As a result, the photovoltaic performance of inverted OPV is substantially improved, mainly reflecting on the increase of short circuit current density.

Efficiency enhancement of inverted organic photovoltaic cells due to an embedded Ce-doped ZnO electron transport layer synthesized by using a sol–gel process

Ce-doped ZnO layers were synthesized by using a sol–gel method for applications as electron transport layers (ETLs) in inverted organic photovoltaic (OPV) cells. X-ray photoelectron spectroscopy spectra, energy-dispersive X-ray analysis spectra, and atomic force microscopy and scanning electron microscopy images showed that the formed samples were Ce-doped ZnO layers with smooth surfaces. The inverted OPV cell based on a poly (3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester bulk heterojunction containing a Ce-doped ZnO ETL was fabricated for enhanced efficiency. Current density–voltage results showed that the power conversion efficiency of the fabricated inverted OPV cell with a Ce-doped ZnO ETL was 0.87 times larger than that with a ZnO ETL due to the enhanced absorption of the Ce-doped ZnO ETL at a near-ultraviolet/blue light region between 300 and 500 nm. Device structure and current density–voltage (J–V) characteristic curves for inverted organic photovoltaic (OPV) cells with a ZnO or a Ce-doped ZnO electron transport layer (ETL) under AM 1.5 stimulated illumination at an intensity of 100 mW/cm2. The enhancement of the power conversion efficiency (PCE) values of the inverted OPV cells with a Ce-doped ZnO ETL is attributed to increases in the short-circuit current density, open-circuit voltage, and fill factor. The results indicate that the PCEs of the ZnO-based OPV cells can be improved by doping Ce into the ZnO layer.

Charge transport studies in donor-acceptor block copolymer PDPP-TNT and PC71BM based inverted organic photovoltaic devices processed in room conditions

Diketopyrrolopyrole-naphthalene polymer (PDPP-TNT), a donor-acceptor co-polymer, has shown versatile behavior demonstrating high performances in organic field-effect transistors (OFETs) and organic photovoltaic (OPV) devices. In this paper we report investigation of charge carrier dynamics in PDPP-TNT, and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) bulk-heterojunction based inverted OPV devices using current density-voltage (J-V) characteristics, space charge limited current (SCLC) measurements, capacitance-voltage (C-V) characteristics, and impedance spectroscopy (IS). OPV devices in inverted architecture, ITO/ZnO/PDPP-TNT:PC71BM/MoO3/Ag, are processed and characterized at room conditions. The power conversion efficiency (PCE) of these devices are measured ∼3.8%, with reasonably good fill-factor 54.6%. The analysis of impedance spectra exhibits electron’s mobility ∼2 × 10−3 cm2V−1s−1, and lifetime in the range of 0.03-0.23 ms. SCLC measurements give hole mobility of 1.12 × 10−5 cm2V−1s−1, and electron mobility of 8.7 × 10−4 cm2V−1s−1.

General method to synthesize ultrasmall metal oxide nanoparticle suspensions for hole contact layers in organic photovoltaic devices

Abstract

Modification on ITO to Fabricate Low Work Function Electrode in Inverted Organic Photovoltaics

The PCE has a great relationship with the work function of conductors in organic and printed electronics devices. And organic photovoltaics require an electrode with a work function (WF) that is low enough to either facilitate the transport of electrons in and out of various optoelectronic devices or collect electrons from the lowest unoccupied molecular orbital (LUMO) of a given organic semiconductor. In inverted organic photovoltaics, the ITO is normally used as cathode to collect electrons .By using PDDA deposition, the surface work function of ITO can be decreased by 0.3 eV, which is able to improve the electrons transport and the PCE in OPV, as it has been proved that the surface electronic potential of ITO is very sensitive to the presence of self-assembled molecular layers.

High efficiency PTB7-based inverted organic photovoltaics on nano-ridged and planar zinc oxide electron transport layers

Consistently high PCEs of ~8% were achieved for the PTB7-based inverted organic photovoltaics on a variety of nano-ridged and planar ZnO electron transport layers (ETLs) as long as the optimal active layer thickness was carefully controlled for each underlying ETL.

Evaporation-free inverted organic photovoltaics using a mixture of silver nanoparticle ink formulations for solution-processed top electrodes

We report an investigation of inkjet-printed silver (Ag) nanoparticle inks combined with a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) formulation for solution-processed top electrodes in inverted organic photovoltaics (OPVs) employing the poly(3-hexylthiopehene):phenyl-C61-butyric acid methyl ester material system. We propose a suitable mixture of Ag nanoparticle inks to control the printability and electrical conductivity of the solution-processed top electrode. Based on the proposed solution-processed hole-selective contact, a power conversion efficiency in the range of 3% is reported for evaporation-free inverted OPVs.

A hybrid copper:tungsten suboxide window electrode for organic photovoltaics.

A new type of window electrode for organic photovoltaics (OPVs) based on an ultra-thin bilayer of copper and amorphous tungsten suboxide, which derives its remarkable optical and electrical properties from spontaneous diffusion of copper into the oxide layer. As the window electrode in efficient inverted OPVs, this unpatterned electrode is shown to perform as well as indium-tin oxide glass.

Photovoltaic analysis of the effects of PEDOT:PSS-additives hole selective contacts on the efficiency and lifetime performance of inverted organic solar cells

Solution processed inverted organic photovoltaics (OPVs) usually use (Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) PEDOT:PSS derivatives as hole selective contact. In this study the effect of different PEDOT:PSS formulations, Al4083, PH and PH500 in inverted structured OPVs is investigated. Through detailed device physics analysis PEDOT:PSS PH is proposed as most suitable hole selective contact for inverted OPVs device function. Furthermore, PEDOT:PSS PH hole selective contact is treated with 3 different wetting agents, Zonyl FS-300 fluorosurfactant (Zonyl), 2,5,8,11-tetramethyl-6-dodecyn-5,8-diol ethoxylate (Dynol) and Zonyl:Dynol mixture and the corresponding non-encapsulated inverted OPVs investigated under accelerated humidity lifetime conditions. The inverted OPVs incorporating PEDOT:PSS:Zonyl hole selective contact shown limitations on humidity lifetime performance due to the poorest adhesion properties of Zonyl-treated PEDOT:PSS PH compared with Dynol and Zonyl/Dynol mixture treaded PEDOT:PSS PH.

Self-assembled, aligned ZnO nanorod buffer layers for high-current-density, inverted organic photovoltaics.

Two different soft-chemical, self-assembly-based solution approaches are employed to grow zinc oxide (ZnO) nanorods with controlled texture. The methods used involve seeding and growth on a substrate. Nanorods with various aspect ratios (1-5) and diameters (15-65 nm) are grown. Obtaining highly oriented rods is determined by the way the substrate is mounted within the chemical bath. Furthermore, a preheat and centrifugation step is essential for the optimization of the growth solution. In the best samples, we obtain ZnO nanorods that are almost entirely oriented in the (002) direction; this is desirable since electron mobility of ZnO is highest along this crystallographic axis. When used as the buffer layer of inverted organic photovoltaics (I-OPVs), these one-dimensional (1D) nanostructures offer: (a) direct paths for charge transport and (b) high interfacial area for electron collection. The morphological, structural, and optical properties of ZnO nanorods are studied using scanning electron microscopy, X-ray diffraction, and ultraviolet-visible light (UV-vis) absorption spectroscopy. Furthermore, the surface chemical features of ZnO films are studied using X-ray photoelectron spectroscopy and contact angle measurements. Using as-grown ZnO, inverted OPVs are fabricated and characterized. For improving device performance, the ZnO nanorods are subjected to UV-ozone irradiation. UV-ozone treated ZnO nanorods show: (i) improvement in optical transmission, (ii) increased wetting of active organic components, and (iii) increased concentration of Zn-O surface bonds. These observations correlate well with improved device performance. The devices fabricated using these optimized buffer layers have an efficiency of ∼3.2% and a fill factor of 0.50; this is comparable to the best I-OPVs reported that use a P3HT-PCBM active layer.

Solution-processed nanostructured zinc oxide cathode interfacial layers for efficient inverted organic photovoltaics

Inverted organic photovoltaic (OPV) cells based on poly(3-hexylthiophene) (P3HT) as an electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an electron acceptor, were fabricated and characterized. To improve the photovoltaic performance, interface control using either dense or nanostructured ZnO films as cathode buffer layers for effective electron transport was demonstrated, while an under-stoichiometric transition metal oxide, such as MoOx, was employed as the anode buffer layer for efficient hole extraction. Incorporation of a nanostructured ZnO interlayer enhanced electron–hole dissociation by enabling a larger interfacial contact with the active layer, that results in increased short-circuit current density (Jsc) and eventually contributing to higher power conversion efficiency (PCE).

High performance organic photovoltaics with zinc oxide and graphene oxide buffer layers

We report air stable inverted organic photovoltaics (OPVs) incorporating graphene oxide (GO) and solution processed zinc oxide (ZnO) as hole transport and electron transport layers, respectively. Both the hole transport layer and the electron transport layer (HTL and ETL) are of advantage in high transparency and environmental stability. The use of GO and ZnO in poly(2,7-carbazole) derivative (PCDTBT):fullerene derivative (PC₇₀BM)-based inverted OPVs leads to an improved device stability and enhanced high open circuit voltage (V(oc)) of 0.81 V, a short-circuit current density (J(sc)) of 14.10 mA cm(-2), and a fill factor (FF) of 54.44 along with a power conversion efficiency of 6.20%.

Photovoltaic properties of organic solar cell with octafluorophthalocyanine as electron acceptors

Organic photovoltaic (OPV) cells were fabricated using copper-2,3,9,10,16,17,23,24-octafluorophthalocyanine (F8CuPc) as an electron acceptor. Normal and inverted OPV cells using F8CuPc were showed rectification in dark and photovoltaic characteristics under illumination. The inverted-type cell had good durability. The power conversion efficiency (η) of the inverted-type cell was 0.11%, approximately 6.5 times higher than that of the normal-type cell. The cell using F8CuPc was more stable than that using fullerene in air. These results indicate the possibility of using F8CuPc as an electron acceptor for air-stable OPV cells.

Tuning indium tin oxide work function with solution-processed alkali carbonate interfacial layers for high-efficiency inverted organic photovoltaic cells

Selective electron collection by an interfacial layer modified indium tin oxide cathode is critically important for achieving high-efficiency inverted structure organic photovoltaic (OPV) cells. Here, we demonstrate that solution-processed alkali carbonates, such as Li2CO3, Na2CO3, K2CO3, Rb2CO3, Cs2CO3, are good interfacial layer materials. Both carbonate concentration and annealing conditions can affect cathode work function and surface roughness. By proper optimization, different alkali carbonates can be almost equally effective as the cathode interfacial layer. Furthermore, good device performance can be achieved at a low annealing temperature (<50 ° C), which allows for potential applications in solution-processed inverted OPV cells on plastic substrates. This work indicates that alkali carbonates, not just cesium carbonate, are valid choices as the cathode interlayer in inverted OPV devices.

Organic photovoltaic with PEDOT:PSS and V2O5 mixture as hole transport layer

In this study, we report on organic photovoltaics based on inverted structure with the photoactive layer sandwiched between lithium zinc oxide electron transport layer and PEDOT:PSS doped with V2O5 hole transport layer. The efficiency of the PEDOT:PSS device increases from 4.04 to 4.20% as we doped with V2O5. This inverted organic photovoltaic can retain efficiency with only 1.9% degradation after 3 weeks storage in ambient conditions. The degradation rate for PEDOT:PSS device is significantly higher (11.6%) compared to PEDOT:PSS doped with V2O5. The good stability in our device can be attributed to the air stable PEDOT:PSS doped with V2O5 HTL as the interfacial layers. It is found that the degradation rate decreases with increasing V2O5 concentration in PEDOT:PSS.