Tuning optical properties of poly(3-hexylthiophene) nanoparticles through hydrothermal processing

Abstract

Poly(3-hexylthiophene) (P3HT) nanoparticles (NPs) were prepared by a reprecipitation method. Hydrothermal processing applied external pressure to the pristine P3HT NPs at temperatures ranging from 60 to 150 °C. Optical absorption and photoluminescence (PL) spectra for the hydrothermally treated P3HT NPs varied markedly with the processing temperature. With increasing treatment temperature, the absorption peak broadened and the peak position shifted from 510 to 623 nm; moreover, the intensity ratio of the 0–1 to 0–0 emission varied. These changes were caused by interactions between the P3HT main chains and alkyl side groups and conformational modifications induced by the high pressure during the hydrothermal process. The evolution of the optical absorption spectra of the P3HT NPs during the hydrothermal processing was strongly correlated with the variation of PL excitation spectra and with the PL emission spectra of a single NP.     

Tuning optical properties of poly(3-hexylthiophene) nanoparticles through hydrothermal processing

Poly(3-hexylthiophene) (P3HT) nanoparticles (NPs) were prepared by a reprecipitation method. Hydrothermal processing applied external pressure to the pristine P3HT NPs at temperatures ranging from 60 to 150 °C. Optical absorption and photoluminescence (PL) spectra for the hydrothermally treated P3HT NPs varied markedly with the processing temperature. With increasing treatment temperature, the absorption peak broadened and the peak position shifted from 510 to 623 nm; moreover, the intensity ratio of the 0–1 to 0–0 emission varied. These changes were caused by interactions between the P3HT main chains and alkyl side groups and conformational modifications induced by the high pressure during the hydrothermal process. The evolution of the optical absorption spectra of the P3HT NPs during the hydrothermal processing was strongly correlated with the variation of PL excitation spectra and with the PL emission spectra of a single NP.     

Effect of cadmium sulphide quantum dot processing and post thermal annealing on P3HT/PCBM photovoltaic device

The present study demonstrates the effect on photovoltaic performance of poly(3-hexylthiophene) (P3HT) on doping of cadmium sulphide (CdS) quantum dots (QDs). The P3HT/CdS nanocomposite shows a 10 nm blue shift in the UV-vis absorption relative to the pristine P3HT. The blue shift in the absorption of the P3HT/CdS nanocomposite can be assigned to the quantum confinement effect from the CdS nanoparticles. Significant PL quenching was observed for the nanocomposite films, attributed to additional decaying paths of the excited electrons through the CdS. Solar cell performance of pure P3HT and dispersed with CdS QDs have been studied in the device configuration viz indium tin oxide (ITO)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/Al and ITO/PEDOT:PSS/ P3HT:CdS:PCBM/Al, respectively. Incorporation of CdS QDs in the P3HT matrix results in the enhancement in the device efficiency (?) of the solar cell from 0.45 to 0.87%. Postproduction thermal annealing at 150 °C for 30 min improves device performance due to enhancement in the device parameters like FF, V_OC and improvement in contact between active layer and Al.     

P3HT:PCBM, best seller in polymer photovoltaic research

In the field of polymer‐based photovoltaic cells, poly(3‐hexylthiophene) (P3HT) and 1‐(3‐methoxycarbonyl)propyl‐1‐phenyl[6,6]C₆₁ (PCBM) are, to date, the most‐studied active materials around the world for the bulk‐heterojunction structure. Various power‐conversion efficiencies are reported up to approximately 5%. This Research News article is focused on a survey of the tremendous literature published between 2002 and 2010 that exhibits solar cells based on blends of P3HT and PCBM.     

Comparative study: the effect of annealing conditions on the properties of P3HT:PCBM blends

This paper presents a detailed study on the role of various annealing treatments on organic poly(3-hexylthiophene) and [6]-phenyl-C₆₁-butyric acid methyl ester blends under different experimental conditions. A combination of analytical tools is used to study the alteration of the phase separation, structure and photovoltaic properties of the P3HT:PCBM blend during the annealing process. Results showed that the thermal annealing yields PCBM “needle-like” crystals and that prolonged heat treatment leads to extensive phase separation, as demonstrated by the growth in the size and quantity of PCBM crystals. The substrate annealing method demonstrated an optimal morphology by eradicating and suppressing the formation of fullerene clusters across the film, resulting in longer P3HT fibrils with smaller diameter. Improved optical constants, PL quenching and a decrease in the P3HT optical bad-gap were demonstrated for the substrate annealed films due to the limited diffusion of the PCBM molecules. An effective strategy for determining an optimized morphology through substrate annealing treatment is therefore revealed for improved device efficiency.     

The synergistic effect of nanocrystal integration and process optimization on solar cell efficiency

This paper investigates the roles of semiconducting single-walled carbon nanotubes (SWNTs) and metallic SWNTs in the SWNT/poly(3-hexylthiophene) (P3HT)-based photovoltaic conversion system. SWNTs containing different fractions of semiconducting nanotubes were conjugated with P3HT by virtue of π-π interaction. The energy transfer and carrier transport mechanisms in the photovoltaic composites were experimentally investigated by optical absorption spectroscopy, photoluminescence spectroscopy and carrier mobility measurements. At low loading of SWNTs, a high percentage of semiconducting nanotubes result in diminished non-radiative decay of exciton and lower carrier mobility, causing higher open circuit voltage and lower photocurrent. At an optimized morphology, SWNT/P3HT/phenyl-C61-butyric acid methyl ester (PCBM) hybrid-based solar cells demonstrated much higher photocurrent than a reference solar cell (P3HT:PCBM) due to the improved carrier mobility. Further thermal annealing of the devices significantly increased the open circuit voltage to 610?mV, resulting in an 80% increase of power conversion efficiency in comparison to the reference solar cell. These results are expected to lay a foundation for the integration of various nanocrystals into solar cells for efficient photovoltaic conversion.     

Control of Shell Morphology in p–n Heterostructured Water‐Processable Semiconductor Colloids: Toward Extremely Efficient Charge Separation

This article describes p–n heterostructured water‐borne semiconductor naonoparticles (NPs) with unique surface structures via control of shell morphology. The shell particles, comprising PC60–[6,6]‐phenyl‐C61‐butyric acid methyl ester (PC₆₁BM) composite, having n‐type semiconductor characteristics, notably influence the charge carrier behavior in the core–shell NPs. A one‐ or two‐phase methodology based on a PC60 surfactant‐water phase and PC₆₁BM n‐type semiconductor‐organic phase provides highly specific control over the shell structure of the NPs, which promote their superior charge separation ability when combined with poly‐3‐hexyl‐thiophene (P3HT). Moreover, the resulting water‐borne NP exhibits shell morphology‐dependent carrier quenching and stability, which is characterized via luminescence studies paired with structural analysis. Corresponding to the results, outstanding performances of photovoltaic cells with over 5% efficiency are achieved. The results suggest that the surrounding shell environments, such as the shell structure, and its electronic charge density, are crucial in determining the overall activity of the core–shell p–n heterostructured NPs. Thus, this work provides a new protocol in the current fields of water‐based organic semiconductor colloids.     

Anomalous thickness-dependence of photocurrent explained for state-of-the-art planar nano-heterojunction organic solar cells

Due to their simple geometry and design, planar heterojunction (PHJ) solar cells have advantages both as potential photovoltaics with more efficient charge extraction than their bulk heterojunction (BHJ) counterparts, and as idealized interfaces to study basic device operation. The main reason for creating BHJs was the limited exciton diffusion length in the active materials of the PHJ: if an exciton is generated at a distance greater than its diffusion length from the hetero-interface of the PHJ, it would be very unlikely to be able to contribute to the photocurrent. Based on this argument one expects a maximum in the photocurrent of PHJs for a thickness of the active layer equal to the exciton diffusion length (~10 nm). However, in two recently developed PHJs that have appeared in the literature, a maximum photocurrent is observed for 60-65 nm of poly(3-hexylthiophene) (P3HT). In this work, we explore this anomaly by combining both an optical T-matrix and a kinetic Monte Carlo simulation that tracks the exciton behavior in the PHJs. The two systems considered are a P3HT/single walled carbon nanotube (SWNT) device, and a P3HT/phenyl-C61-butyric acid methyl ester (PCBM) device. The model demonstrates how a bulk exciton sink can explain the shifted maximum in the P3HT/SWNT case, whereas in the P3HT/PCBM case the maximum is mainly determined by PCBM molecules interdiffusing in the P3HT upon annealing. Based upon the results of this model it will be possible to more intelligently design nanostructured photovoltaics and optimize them toward higher efficiencies.     

The role of solvent and morphology on miscibility of methanofullerene and poly(3-hexylthiophene)

A bicontinuous, percolating bulk heterojunction morphology is integral to organic polymer solar cells. Understanding the factors affecting the miscibility of photovoltaic polymers with a fullerene electron acceptor molecule is a key to controlling the morphology. Starting from discreet pure phases - a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) bilayer film - the evolution of the P3HT-PCBM interface was studied with particular attention to the role of residual solvent in P3HT on PCBM interdiffusion. This investigation shows that in the bilayer geometry PCBM can rapidly diffuse into amorphous P3HT, but phase separation is maintained if the P3HT layer is cast from a very volatile solvent or if it is annealed prior to casting the PCBM overlayer to complete the bilayer geometry.     

Formation of phenyl-C<Subscript>61</Subscript>-butyric acid methyl ester nanoscale aggregates after supercritical carbon dioxide annealing

In this study, we successfully developed a novel method to create [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) nanoscale aggregates using supercritical carbon dioxide (scCO₂) annealing and fabricated bulk heterojunction (BHJ) solar cells with the nanoscale PCBM to improve device performance. PCBM forms nanoscale aggregates with a size of approximately 70 nm after scCO₂ annealing at 11 MPa and 50 °C for 60 min. However, PCBM remains amorphous after thermal annealing (TA) at 150 °C for 5 min. The morphology, structure, and crystallinity of poly(3-hexylthiophene) (P3HT) in the scCO₂-treated P3HT film are nearly the same as those in the TA-treated P3HT film. In the P3HT/PCBM blend, the formation of PCBM nanoscale aggregates by scCO₂ treatment decreases the disturbance for P3HT crystallization and improves diffusion and regular packing of P3HT molecular chains. This increases the crystallinity of P3HT so that it becomes higher than that in the TA-treated blend film. The nanoscale aggregates of PCBM and the higher crystallinity of P3HT give the scCO₂-treated P3HT/PCBM BHJ solar cells a maximum power conversion efficiency (PCE) of 2.74%, which is much higher than that of the as-cast device (PCE is 1.70%) and a little higher than the highest PCE (2.64%) of thermally annealed devices. These results indicate that scCO₂ is an effective, mild, and environmental method to modulate the nanoscale aggregates of PCBM and to improve the PCE of BHJ solar cells. However, the size of the PCBM aggregates is a little larger than the most suitable size of the exciton diffusion length, leading to limited improvement of the PCE.     

Photovoltaic contribution of photo-generated excitons in acceptor material of organic solar cells

Contribution of exciton generation in acceptor material to the photovoltaic performance of three bulk-heterojunction organic solar cells (BHJ OSCs), PTB7:PC₇₁BM, P3HT:ICBA and P3HT:PC₆₁BM are studied. Singlet and triplet rates of absorption and dissociation and diffusion lengths are calculated and compared with those when excitons are generated in the donor of these BHJ OSCs. It is found that the rates of absorption and dissociation and diffusion lengths are comparable whether excitons are generated in donor or acceptor of these BHJ OSCs. Therefore, it is proposed that the contribution of exciton generation in acceptor may not be negligible.     

Preparation of poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester Langmuir-Blodgett bulk hetero junctions on indium tin oxide

Mono-layers of aggregated Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) molecules were obtained by using solutions of P3HT, PCBM and P3HT-PCBM mixture without stabilizers such as stearates in chloroform at an air-water interface. 1 to 10 cycle-lifted LB films of P3HT and PCBM were successfully transferred to cleaned bare indium-tin-oxide coated glass substrate by vertical lifting method excluding the first 1 to 2 cycle layer. The dependence of P3HT and PCBM film thickness on the transfer cycles has been explained by the molecular sizes, where four edge-on P3HT molecular and six PCBM molecular stacking which result in thickness was taken into account. Work functions of deposited LB-layers were consistent with those of the ordinary casted films. P3HT and PCBM LB-layers showed optical activity in both infra-red (IR) and visible absorption regions of the spectrum. P-polarized IR absorption owing to C=C and C=O stretching vibrations observed in LB-layered films clearly indicate the enhancement of the orientation of these bonds perpendicular to the substrate surface in contrast to the spin-coated one. Visible optical absorption intensity was increased well in proportion with the lift cycle-numbers of both P3HT and PCBM LB films. The photovoltaic characteristics have been observed in the devices fabricated with P3HT (5 cycles-layer)/PCBM (5 cycles-layer) LB hetero structure as an active layer of the solar cells. The surface pressure of LB compression for the mixture of P3HT and PCBM, that is, bulk hetero mixtures, has also been well built up to 30 mN/m.     

Effect of thermal annealing on the efficiency of heterojunction photovoltaic cells fabricated using poly(3-hexylthiophene) and methanofullerene, [6,6]-phenyl C61-butyric acid methyl ester

The effects of thermal annealing on the efficiency of heterojunction photovoltaic (PV) cells that were fabricated using poly(3-hexylthiophene) (P3HT) and methanofullerene, [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were investigated. The absorption spectra showed that the absorption intensity of the P3HT:PCBM layer that was annealed for 5 min had the highest value among the several samples with different annealing temperatures. The atomic force microscopy image showed that the P3HT:PCBM layer that was annealed for 5 min had the best surface morphology. The X-ray photoelectron spectroscopy demonstrated that the P3HT:PCBM layer that was annealed at 140 degrees C for 10 min enhanced the PCBM aggregation on the surface Al layer that was covered by the P3HT:PCBM layer. The efficiencies of the PV cells that were annealed at 3, 5, and 10 min were approximately 2.7, 4.2, and 3.5%, respectively. Based on the experiment results, the variations in the efficiency of the PV cells due their thermal treatment were described.     

Efficient and stable, structurally inverted poly(3-hexylthiopen): [6,6]-phenyl-C61-butyric acid methyl ester heterojunction solar cells with fibrous like poly(3-hexylthiopen)

We investigated an inverted organic photovoltaic device structure in which a densely packed ~ 100 nm thin TiO₂ layer on fluorine doped conducting glass serves as anode and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)/Au layer on top of the active layer serves as cathode. The active layer is comprised of a blend of poly(3-hexylthiopene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The rectification behavior of such a device is improved significantly and injection losses are minimized compared to devices without any compact TiO₂ layer. Moreover, nanostructured P3HT active layer was achieved in-situ by spin coating concentrated pure P3HT and P3HT:PCBM blend and solar cell performances on thickness of the active layer were also investigated. For the inverted solar cells constructed with different concentrations of P3HT and PCBM keeping the P3HT:PCBM ratio 1:0.8 (wt.%), the highest short circuit current and efficiency was observed when the P3HT and PCBM concentration was equal to 1.5 (wt.%) and 1.2 (wt.%) respectively. This leads to highly stable and reproducible power conversion efficiency above 3.7% at 100 mW/cm² light intensity under AM 1.5 conditions.     

Enhancement of the power conversion efficiency of polymer solar cells by functionalized single-walled carbon nanotubes decorated with CdSe/ZnS core–shell colloidal quantum dots

In this paper, we demonstrated an enhanced performance of polymer solar cells by incorporating functionalized single-walled carbon nanotubes (SWCNTs) decorated with CdSe/ZnS core–shell colloidal quantum dots (CQDs) into copolymers of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as active layer. Short-circuit current density and power conversion efficiency of the ITO/PEDOT:PSS/P3HT:PCBM:(CdSe/ZnS-SWCNTs)/Al solar cells can be enhanced by more than 31 and 23 %, respectively, as compared with the control device ITO/PEDOT:PSS/P3HT:PCBM/Al. This enhancement is due to the high electron-transporting ability of SWCNTs and the increased absorption of CdSe/ZnS CQD in visible region. It shows an applicable way to improve the efficiency of polymer solar cells by incorporating suitable quantity of CQDs-decorated SWCNTs with suitable kinds of CQDs and suitable acid treatment to the SWCNTs.     

Surface modification of ZnO nanorods with CdS quantum dots for application in inverted organic solar cells: effect of deposition duration

Incorporating cadmium sulfide quantum dots (CdS QDs) onto ZnO nanorod (ZNRs) has been investigated to be an efficient approach to enhance the photovoltaic performance of the inverted organic solar cell (IOSC) devices based on ZNRs/poly (3-hexylthiophene) (P3HT). To synthesize CdS/ZNRs, different durations of deposition per cycle from 1 to 9 min were used to deposit CdS via SILAR technique onto ZNRs surface grown via hydrothermal method at low temperature on FTO substrate. In typical procedures, P3HT as donor polymer were spun-coating onto CdS/ZNRs to fabricate IOSC devices, followed by Ag deposition as anode by magnetron sputtering technique. Incorporation of CdS QDs has modified the morphological, structural, and optical properties of ZNRs. Incorporation of CdS QDs onto ZNRs also led to higher open circuit voltage (V_oc) and short circuit current density (J_sc) of optimum ZNRs/CdS QDs devices due to the increased interfacial area between ZNRs and P3HT for more efficient exciton dissociation, reduced interfacial charge carrier recombination as a result of lower number of oxygen defects which act as electron traps in ZnO and prolonged carrier recombination lifetime. Therefore, the ZNRs/CdS QDs/P3HT device exhibited threefold higher PCE (0.55%) at 5 min in comparison to pristine ZNR constructed device (0.16%). Overall, our study highlights the potential of ZNRs/CdS QDs to be excellent electron acceptors for high efficiency hybrid optoelectronic devices.     

P3HT聚集体结构对有机太阳电池效率的影响

为探究有机太阳电池活性层聚集体结构与器件效率的关系,本文以丙酮和正己烷为不良溶剂,诱导聚(3-己基噻吩)(P3HT)分子结晶,通过改变两种溶剂的滴加时长,制备了一系列尺寸均匀、分散性好以及分子排列有序的P3HT纳米颗粒。利用原子力显微镜(AFM),紫外可见光谱仪(UVVis),X射线衍射仪(XRD)等手段表征了纳米颗粒的尺寸、形貌和结晶有序性。将P3HT纳米颗粒和[6,6]-苯基-C61-丁酸甲酯(PCBM)混合成膜制备了本体异质结有机太阳电池,发现基于尺寸相近但沿π-π堆砌方向结晶更有序的P3HT纳米颗粒的器件显示更高的光电转换效率。     

Localized Surface Plasmon Resonance Enhanced Light Absorption in AuCu/CsPbCl3 Core/Shell Nanocrystals

Localized surface plasmon resonance (LSPR) is shown to be effective in trapping light for enhanced light absorption and hence performance in photonic and optoelectronic devices. Implementation of LSPR in all-inorganic perovskite nanocrystals (PNCs) is particularly important considering their unique advantages in optoelectronics. Motivated by this, the first success in colloidal synthesis of AuCu/CsPbCl₃ core/shell PNCs and observation of enhanced light absorption by the perovskite CsPbCl₃ shell of thickness in the range of 2–4 nm, enabled by the LSPR AuCu core of an average diameter of 7.1 nm, is reported. This enhanced light absorption leads to a remarkably enhanced photoresponse in PNCs/graphene nanohybrid photodetectors using the AuCu/CsPbCl₃ core/shell PNCs, by more than 30 times as compared to the counterparts with CsPbCl₃ PNCs only (8–12 nm in dimension). This result illustrates the feasibility in implementation of LSPR light trapping directly in core/shell PNCs for high-performance optoelectronics.     

Nanoarchitectonics of a Au nanoprism array on WO3 film for synergistic optoelectronic response

Abstract

A layered photoelectrode consisting of a conductive indium tin oxide substrate, a WO₃ nanocrystalline film and an array of Au nanoprisms was fabricated via a multistep process. Scanning electron microscopy and atomic force microscopy showed that the Au nanoprisms had a uniform size and shape and formed periodic hexagonal patterns on the WO₃ film. The optical absorption of the photoelectrode combined the intrinsic absorption of WO₃ and plasmonic absorption of Au. Using this photoelectrode, we investigated the effect of the Au nanoprism array on the optoelectronic conversion performance of the WO₃ film. Photoelectrochemical measurement indicated that the array substantially enhanced the photocurrent in the WO₃ film. Electrochemical impedance measurements revealed that the Schottky junctions formed between Au and WO₃ can facilitate the separation of photogenerated carriers as well as the interfacial carrier transfer. In this study, we demonstrate that covering a semiconductor with plasmonic noble metal nanoparticles can improve its optoelectronic conversion efficiency.     

Improvement of organic solar cell efficiency by solution-processed doping of pentacene in PEDOT:PSS layer

In the current research, organic solar cells (OSCs) with various concentrations of pentacene in Poly(ethylenedioxythiopene):Poly(styrenesulfonate) (PEDOT:PSS) interface layer were investigated for better hole extraction. The ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al-fabricated solar cell fabricated via brush coating provides superior photovoltaic, electrical and optical characteristics when compared with the ITO/PEDOT:PSS/P3HT:PCBM/Al solar cell. The ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al solar cells deliver a V_OC ~350?mV and 2.57% efficiency. It is observed that the optimized concentration of pentacene doping in PEDOT:PSS layer, along with an active layer of P3HT and PC₆₀BM, doubles the efficiency of the device, when compared with pristine PEDOT:PSS layer. The degradation studies of the fabricated bulk heterojunction OSCs reveal that the degrading abilities of ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al solar cells are 60% more better than those of ITO/PEDOT:PSS/P3HT:PCBM/Al devices. Thus, this work will ultimately contribute toward fully solution processed painted device, which will provide low-cost manufacturing and improved stability of pentacene-based organic photovoltaics.