An easily prepared Ag8GeS6 nanocrystal and its role on the performance enhancement of polymer solar cells

Novel Ag₈GeS₆ nanocrystal materials (AGS NCs) have recently earned affectionate attention due to its bulk band-gap of 1.4 eV, which makes it ideal as a broad-spectrum absorber material for both semiconductor photocatalyst and photovoltaic devices. In this paper, we investigated the role of AGS NCs as molecular dopant on solution-processed polymer solar cells (PSCs). Argyrodite AGS NCs was prepared via a colloidal synthesis process using simple inorganic compounds as precursors. Incorporating AGS NCs into PSCs leads to not only improved light absorption of active layer but also increased phase separation of donor and acceptor. Moreover, the doping effect of AGS NCs was also confirmed by nanoscale morphology and photocurrent generation mechanism analysis, revealing that AGS NCs could serve as both exciton dissociation centers and charge transfer medium. This study shows that employment of AGS NCs is a facile way to improve the electrical and optical properties of organic photovoltaic devices.     

Understanding the role of organic polar solvent induced nanoscale morphology and electrical evolutions of P3HT:PCBM composite film

We investigated the effect of organic polar solvent on the properties of [6,6]-phenyl-C₇₁-butyric acid methyl ester (PCBM) films and poly(3-hexylthiophene) (P3HT):PCBM blend films employed as active layer in organic photovoltaic. The nanoscale morphology and the electrical characteristics of the P3HT:PCBM film can be controlled through organic polar solvent exposure, which exhibited with a short-circuit current density of 8.64 mA/cm², an open circuit voltage of 0.63 V, and a power conversion efficiency of 3.29% under AM 1.5 illumination with a light intensity of 100 mW/cm². By exposing the active layer films to organic polar solvent a favorable phase separation in the P3HT:PCBM films is obtained. The improved power conversion efficiency can be to the high conductivity and high surface area of the P3HT:PCBM layer treated with organic polar solvent.     

Organic photovoltaic devices with enhanced efficiency processed from non-halogenated binary solvent blends

The development of processing routes to fabricate organic photovoltaic devices (OPVs) using non-halogenated solvents is a necessary step towards their eventual commercialisation. To address this issue, we have used Hansen solubility parameter analysis to identify a non-halogenated solvent blend based on a mixture of carbon disulphide and acetone. This solvent blend was then used to deposit a donor–acceptor polymer–fullerene thin-film that was then used as the active layer of bulk-heterojunction OPV. For the benchmark polymer:fullerene system PCDTBT:PC₇₀BM, a power conversion efficiency of 6.75% was achieved; a 20% relative improvement over reference cells processed using the chlorinated-solvent chlorobenzene. Improvements in device efficiency are attributed to an increase in electron and hole conductivity resulting from enhanced fullerene crystallisation; a property that leads to enhanced device efficiency through improved charge extraction.     

Highly efficient thieno[3,4-c]pyrrole-4,6-dione-based solar cells processed from non-chlorinated solvent

To obtain high performance bulk heterojunction organic solar cells, the selection of solvents to prepare the donor/acceptor blend is as important as the choice of the donor/acceptor materials themselves. State-of-the-art lab-scale polymer solar cells have evolved around chlorinated solvents such as chloroform, chlorobenzene and o-dichlorobenzene. However, for large scale applications, benign processing solvents may become inevitable. In this work, we used a mixture of Xylenes (a chlorine-free solvent), methyl naphthalene (MeN) and 1,8-diiodoctane (DIO) to modulate the nano-scale morphology of poly(4,4-bis(2-ethylhexyl)-dithieno[3,2-b:2′,3′-d]silole-alt-1,3-(5-octylthieno[3,4-c]pyrrole-4,6-dione) (PDTSTPD)/PCBM blend, one of the most efficient active layer in polymeric solar cell. Power conversion efficiencies up to 5.5% (with PC₆₁BM) and 6.2% (with PC₇₁BM) were obtained for photovoltaic devices with an active area of 1.0 cm².     

Synthesis and photophysical properties of semiconductor molecules D1-A-D2-A-D1-type structure based on derivatives of quinoxaline and dithienosilole for organics solar cells

A novel small molecule with D1-A-D2-A-D1 structure denoted as DTS(QxHT₂)₂ based on quinoxaline acceptor and dithienosilone donor units was synthesized and its optical and electrochemical properties were investigated. The thin film of DTS(QxHT₂)₂ showed a broad absorption profile covering the solar spectrum from 350 nm to 780 nm with an optical bandgap of 1.63 eV. The energy levels estimated from the cyclic voltammetry indicate that this small molecule is suitable as donor along with PC₇₁BM as acceptor for the fabrication solution processed bulk heterojunction solar cells for efficient exciton dissociation and high open circuit voltage. The organic solar cells based on optimized DTS(QxHT2)2:PC₇₁BM active layers processed with chloroform and DIO/CF showed overall power conversion efficiency of 3.16% and 6.30%, respectively. The higher power conversion efficiency of the solar cell based on the DIO/CF processed active layer is attributed to enhanced short circuit photocurrent and fill factor may be related to better phase separation between donor and acceptor in the active layer and more balanced charge transport, induced by the solvent additive. The power conversion efficiency of the organic solar cell was further improved up to 7.81% based on active layer processed with solvent additive, using CuSCN as hole transport layer instead of PEDOT:PSS and mainly attributed to increased fill factor and open circuit voltage due the formation of better Ohmic contact between the active layer and the CuSCN layer.     

Topographical and morphological aspects of spray coated organic photovoltaics

Herein we discuss the topographical and nanomorphological aspects of spray deposited organic photovoltaics. We show that the solvent properties have a massive impact on the topography, but less on the nanomorphology formation of composites based on the electron donor poly(3-hexylthiophene) (P3HT) and the electron acceptor [6,6]-phenyl C61 butyric acid methyl ester (PCBM). An adapted solvent mixture consisting of ortho-dichlorobenzene (oDCB) and 1,3,5-trimethylbenzene (mesitylene) allows us to demonstrate spray coated organic photovoltaic devices with 3.1% power conversion efficiency (PCE). Moreover, we show that spray coating is a feasible technology to deposit all solution processable layers of organic solar cells, including the hole transporting layer poly(3,4-ethylene dioxythiophene) doped with polystyrene sulphonic acid (PEDOT:PSS) as well and demonstrate fully spray coated devices with 2.7% PCE.     

Influence of functional derivatives of an amino-coumarin/MWCNT composite organic hetero-junction on the photovoltaic characteristics

A layer of a coumarin derivative\multiwall carbon nanotube (MWCNT) composite was sandwiched between an indium tin oxide (ITO) layer and a top metal electrode. This layer acted as the active hetero-junction (middle hetero-junction) in a triple hetero-junction organic solar cell. The composition of the organic material in the middle hetero-junction component, second from the ITO layer, was varied, keeping the weight percentage of the MWCNTs constant, to investigate the photovoltaic properties of the cell. The choice of amino-coumarin and its derivatives were dictated by the need for a high efficiency organic up-converter. Amino-coumarin is a two photon absorbing organic donor whereas CNTs are acceptors and ballistic charge transport media. For those reasons, amino-coumarin-CNT composite materials were selected as a photo active layer to will absorb the sub-band photons effectively. Additionally, the radial self-assembling of the CNTs inside the organic matrix (during synthesis) enhances the photovoltaic characteristics of the material. The photo induced charge transport mechanism between the MWCNTs and organics was analyzed using photo luminescence (PL) measurements. By the use of 15 wt% of CNTs with the organics, more than 90% of the PL signal was quenched, indicating an ultrafast transport mechanism between the donor and acceptor materials.     

Molecular engineering to improve carrier lifetimes for organic photovoltaic devices with thick active layers

The morphology of the bulk heterojunction absorber layer in an organic photovoltaic (OPV) device has a profound effect on the electrical properties and efficiency of the device. Previous work has consistently demonstrated that the solubilizing side-chains of the donor material affect these properties and device performance in a non-trivial way. Here, using Time-Resolved Microwave Conductivity (TRMC), we show by direct measurements of carrier lifetimes that the choice of side chains can also make a substantial difference in photocarrier dynamics. We have previously demonstrated a correlation between peak photoconductance measured by TRMC and device efficiencies; here, we demonstrate that TRMC photocarrier dynamics have an important bearing on device performance in a case study of devices made from donor materials with linear vs. branched side-chains and with variable active layer thicknesses. We use Grazing-Incidence Wide Angle X-ray Scattering to elucidate the cause of the different carrier lifetimes as a function of different aggregation behavior in the polymers. Ultimately, the results help establish TRMC as a technique for screening OPV donor materials whose devices maintain performance in thick active layers (>250 nm) designed to improve light harvesting, film reproducibility, and ease of processing.     

Interpenetrating heterojunction photovoltaic cells based on C60 nano-crystallized thin films

An interpenetrating heterojunction (IHJ) structure facilitates efficient charge separation and transport in the active layer of organic photovoltaic cells (OPVs). Additionally, the recombination of generated carriers in IHJs is reduced as these networks exhibit high carrier transport with minimal recombination sites. We have developed a simple method to fabricate nanocrystallized fullerene (C₆₀) films, which are produced by subjecting evaporated C₆₀ films to either solvent spin-coating or solvent vapor annealing (SVA). The size of the rod-shaped nanocrystals in the films were controlled by changing the solvent and annealing time.An 80-nm-diameter size nanocrystallized C₆₀ film that was fabricated using SVA with ethanol was incorporated as an acceptor material in an inverted IHJ OPV cell. Tetraphenyldibenzoperiflanthene (DBP) was evaporated onto the nanocrystallized C₆₀ film as the donor material. The power conversion efficiency of an IHJ OPV cell (ITO/TiOx/nanocrystallized C₆₀ film/DBP/MoO₃/Au) increased from 1.79% to 2.12%, when compared with the conventional PHJ OPV cell.     

On the Role of Bathocuproine in Organic Photovoltaic Cells

The effect of bathocuproine (BCP) on the optical and electrical properties of organic planar heterojunction photovoltaic cells is quantified by current–voltage characterization under 1 sun AM 1.5D simulated solar illumination and spectral response at short‐circuit conditions. By inserting a 10 nm BCP layer in an indium tin oxide (ITO)/subphthalocyanine (SubPc)/buckminsterfullerene (C₆₀)/BCP/Al thin‐film structure, an increase in power‐conversion efficiency from 0.05 to 3.0% is observed, mostly reflected in the enhanced open‐circuit voltage up to 920 mV. Furthermore, the incorporation of a 10‐nm BCP layer in an ITO/C₆₀/BCP/Al structure leads to an increase in built‐in potential from 250 to 850 mV, as demonstrated by electroabsorption. It is argued that BCP passivates C₆₀ such that a 10‐nm layer provides a sufficient buffer layer that prohibits Al contacting the C₆₀ where it would otherwise create donor states.     

Efficient Organic Photovoltaic Cells Based on Nanocrystalline Mixtures of Boron Subphthalocyanine Chloride and C60

The electrical and structural behavior of uniformly mixed films of boron subphthalocyanine chloride (SubPc) and C₆₀ and their performance in organic photovoltaic cells is explored. Device performance shows a strong dependence on active‐layer donor–acceptor composition, and peak efficiency is realized at 80 wt.% C₆₀. The origin of this C₆₀‐rich optimum composition is elucidated in terms of morphological changes in the active layer upon diluting SubPc with C₆₀. While neat SubPc is found to be amorphous, mixed films containing 80 wt.% C₆₀ show clear nanocrystalline domains of SubPc. Supporting electrical characterization indicates that this change in morphology coincides with an increase in the hole mobility of the SubPc:C₆₀ mixture, with peak mobility observed at a composition of 80 wt.% C₆₀. Organic photovoltaic cells constructed using this optimum SubPc:C₆₀ ratio realize a power conversion efficiency of (3.7 ± 0.1)% under 100 mW cm^?2 simulated AM1.5G solar illumination.     

Soluble carbon nanotubes/phthalocyanines transparent electrode and interconnection layers for flexible inverted polymer tandem solar cells

In this study, we focus on transparent electrodes for organic solar cells prepared from aqueous solutions consisting of single wall carbon nanotubes (SWCNTs) and a copper-phthalocyanine derivative (TSCuPc). We first investigated their electrical conductivity and optical properties. The high solubility level of the TSCuPc/SWCNTs enables the production of stable inks with high conductivity which allows obtaining flexible photovoltaic devices based TSCuPc/SWCNTs films with good performances. A power conversion efficiency of 3.2% was achieved in a device with a blend of poly (3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as the active layer with a TSCuPc/SWCNTs sprayed electrode on a polyethylene terephthalate (PET) substrate. TSCuPc and TSCuPc/SWCNTs were also employed as the electron transport layer (ETL) and the interconnecting layer (ICL) in an inverted tandem organic solar cell based on front P3HT–ICBA and back [70]PCBM–PCDTBT active layers, achieving an efficiency of 7.40%.     

Highly Efficient Hybrid Solar Cells Based on an Octithiophene–GaAs Heterojunction

We report a new type of hybrid heterojunction solar cell based on rod‐like octithiophene (8T) as the organic p‐type semiconductor and GaAs(111) as the inorganic n‐type semiconductor. By using a semitransparent gold layer as the front contact deposited onto the 8T films, solar‐energy conversion efficiencies of up to 4.2 % could be obtained. The reduction in the contact resistance at the Au/8T interface induced by iodine doping is found to be a very crucial factor for the high efficiency. Furthermore, we demonstrate that hybrid solar cells can be successfully used to investigate the photovoltaic properties of organic semiconductors in detail. By means of external quantum efficiency (EQE) measurements, the influence of film morphology on the photocurrent collection length in 8T films is studied. The results show that, in hybrid solar cells using highly ordered microcrystalline 8T films, an active contribution of the organic‐layer semiconductor to the total photocurrent exists. A very large photocurrent collection length of up to 100 nm has been estimated from EQE measurements, indicating that exciton diffusion is very efficient in microcrystalline 8T. On the other hand, the use of nanocrystalline 8T leads to high photocurrent losses in the organic part of the hybrid solar cell. The strong influence of the film morphology on the photocurrent collection in 8T is attributed to a reduction in the exciton diffusion length due to a high trap density in nanocrystalline 8T films. Thus, our results reveal the importance of high crystalline order for obtaining efficient photocurrent collection in 8T films.     

Nano-engineering of hybrid organic heterojunctions with carbon nanotubes to improve photovoltaic performance

Organic–inorganic hybrid photovoltaics are beginning to show significant promise as a low cost highly efficient route towards renewable energy generation. Of the hybrid architectures available, carbon nanotube incorporated organic photovoltaics is considered to be among the most promising. Herein, the optical and electronic effects of localizing multiwalled carbon nanotubes in the donor polymer is investigated in comparison to its incorporation into the bulk heterojunction architecture (triple heterojunction scheme) through photoluminescence quenching and dark diode characteristics analysis. A significant improvement in photoluminescence quenching is observed when the nanotubes are localized in the donor polymer where the active layer is formed through a sequential deposition route in comparison to the triple heterojunction scheme. However, the former architecture also leads to a higher recombination of carriers due to the introduction of trap states as observed through space charge limited conduction analysis. In comparison, the triple heterojunction scheme shows a lower dark current and hence a significantly improved photovoltaic device performance (3.8% in comparison to 2.6% for the sequentially deposited architecture). This indicates that the formation of the triple heterojunction is the more ideal scheme for improving device performances in organic–inorganic hybrid architectures.     

The working mechanism of organic photovoltaic cell by using copper phthalocyanine as exciton blocking layer

We demonstrate the working mechanism of organic photovoltaic (OPV) cell with copper phthalocyanine (CuPc) as exciton blocking layer (EBL). The new EBL material CuPc, commonly has been used as electron donor in the organic solar cells due to its electron-donating and hole-transporting properties. But here we proves that the α-polymorph CuPc layer can transfer electrons to Al cathode through the half-filled b_1g level, this mechanism is different from that of general EBL material with larger band gap and electron-transporting property, which is based on damage states induced by the heat of evaporating Al.     

Enhanced efficiency of organic solar cells by mixed orthogonal solvents

In this work, the effects of mixed solvents on donor–acceptor vertical phase separation and light absorption was investigated. By using mixed orthogonal solutions of 1,2 o-dichlorobenzene (o-DCB) and dichloromethane (DCM), a PCBM([6,6]-phenyl-C₆₁-butyric acid methyl ester)-rich top layer was induced in typical poly(3-hexylthiophene-2,5-diyl)(P3HT):PCBM bulk heterojunction structure. By carefully adjusting the o-DCB:DCM volume ratio, the contact between active layer and the Al cathode was significantly improved due to the precipitation of PCBM on the top surface, which resulting in an electron transport preferable interface between the active layer and cathode. Meanwhile, light absorption was also effectively improved due to the increased crystallinity of polymers under mixed solvents. Overall, the short circuit current was greatly increased, and the efficiency was improved from 3.07% in the control sample to 3.97% by adding 30% DCM. The detailed mechanism of the formation of PCBM-rich layer and enhanced light absorption with o-DCB:DCM solution was expatiated. Our findings suggest a facile spin coating method to fabricate efficient BHJ solar cells, which can pave the way for the large scale application of organic photovoltaic devices (OPVs) in the future.     

A review of recent plasmonic nanoparticles incorporated P3HT: PCBM organic thin film solar cells

An optimum thickness of organic active layer of 100 nm or possibly less results in poor optical absorption in organic photovoltaic cells (OPV). The optical absorption can be improved by using a thick organic active layer, but the charge carrier collection efficiency will decrease due to low charge carrier mobility for most of the polymeric organic semiconductor. This phenomenon imposes a trade-off between optical absorption and charge carriers transport inside OPV. Recently, metallic nanostructures such as gold (Au) and silver (Ag) with various sizes and morphologies have been identified as an alternative route to boost the performance of OPV at this specific limited thickness (ie. ≤100 nm). Multiple plasmonic effects such as optical and electrical effects are induced upon introducing metallic nanoparticle(s), NP(s) into OPV. This review highlights recent progress in plasmonic-enhanced poly(3-hexylthiophene-2,5-diyl): phenyl-C61-butyric acid methyl ester (P3HT: PCBM)-based OPV with NP(s) located either inside organic active layer or carrier transport layer (CTL) or at various interfaces within the OPV cell architecture. With understanding of the physical plasmonic effects for Au and Ag in OPV, such plasmonic NP(s) act as a new class of strategy for performance optimization.     

Organic solar cells using plasmonics of Ag nanoprisms

We demonstrate plasmonic effects in bulk heterojunction organic solar cells (OSCs) consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) by incorporating silver (Ag) triangular shaped nanoparticles (nanoprisms; NPSs) into a poly(3,4-ethylenedioxythiophene) buffer layer. The optical absorption and geometric characteristics of the Ag NPSs were investigated in terms of their tunable in-plane dipole local surface plasmon resonance (LSPR) bands. The photovoltaic characteristics showed that the power conversion efficiency (PCE) of the plasmonic OSCs was enhanced by an increase of short circuit current (J_sc) compared to that of the reference cells without any variation in electrical properties. The enhanced J_sc is directly related to the enhancement of optical absorption efficiency by the LSPR of the Ag NPSs. We measured the photovoltaic characteristics of the plasmonic OSCs with various distances between the Ag NPSs and the P3HT:PCBM active layer, in which the PCEs of the plasmonic OSCs decreased with increasing distance. This suggests that the increase of photocurrent and optical absorption was due to near field enhancement (i.e., intensified incident light on the active layer) by the LSPR of the Ag NPSs.     

Heterojunction Ambipolar Organic Transistors Fabricated by a Two‐Step Vacuum‐Deposition Process

Ambipolar organic field‐effect transistors (OFETs) are produced, based on organic heterojunctions fabricated by a two‐step vacuum‐deposition process. Copper phthalocyanine (CuPc) deposited at a high temperature (250 °C) acts as the first (p‐type component) layer, and hexadecafluorophthalocyaninatocopper (F₁₆CuPc) deposited at room temperature (25 °C) acts as the second (n‐type component) layer. A heterojunction with an interpenetrating network is obtained as the active layer for the OFETs. These heterojunction devices display significant ambipolar charge transport with symmetric electron and hole mobilities of the order of 10^–4 cm² V^–1 s^–1 in air. Conductive channels are at the interface between the F₁₆CuPc and CuPc domains in the interpenetrating networks. Electrons are transported in the F₁₆CuPc regions, and holes in the CuPc regions. The molecular arrangement in the heterojunction is well ordered, resulting in a balance of the two carrier densities responsible for the ambipolar electrical characteristics. The thin‐film morphology of the organic heterojunction with its interpenetrating network structure can be controlled well by the vacuum‐deposition process. The structure of interpenetrating networks is similar to that of the bulk heterojunction used in organic photovoltaic cells, therefore, it may be helpful in understanding the process of charge collection in organic photovoltaic cells.     

Electric-field controlled light-emissive characteristics in nanoscale for polymer/fullerene organic solar cells

Bulk-hetero-junction (BHJ) organic photovoltaic cells (OPVCs) consisting of a poly(3-hexylthiophene) (P3HT) as a donor and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) as an acceptor were fabricated and their light-emissive characteristics as a function of applied bias were investigated. The nanoscale luminescence spectra at different positions on the P3HT/PCBM based photovoltaic cells were measured using a laser confocal microscope (LCM) with a high spatial resolution. For the P3HT/PCBM OPVCs with a relatively thin active layer, the light-emissive characteristics were changed considerably with varying applied bias. We observed that the luminescence intensity increased with increasing reverse bias under light illumination, this result was confirmed by the LCM photoluminescence mapping images. This result originates from the increase of free charges due to the de-trapping effect of trapped charge transfer excitons near the interface, through the external electric-field and incident light.