Effect of film and device annealing in polymer:polymer solar cells with a LiF nanolayer

We report the effect of thermal annealing on the performance of polymer:polymer solar cells with a lithium fluoride (LiF) nanolayer inserted between active layer and electron-collecting electrode. The active layer was prepared using blend films of regioregular poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). Film annealing and device annealing were separately performed at 150 °C for 30 min to understand the influence of the existence of LiF nanolayer during thermal annealing. Results showed that both film and device annealing did considerably improve the power conversion efficiency (PCE) of P3HT:F8BT solar cells though the PCE was higher in the case of device annealing. The surface analysis suggested that the improved device performance by thermal annealing is attributed partly to the formation of planar p-n junction structure in the P3HT:F8BT blend film during thermal annealing.     

Thermal annealing effect on the nanomechanical properties and structure of P3HT:PCBM thin films

Nanostructured polymer-fullerene thin films are among the most prominent materials for application in high efficient polymer solar cells. Specifically, poly(3-hexylthiophene) (P3HT) and fullerene derivatives (PCBM) blends are used as the donor/acceptor materials forming a bulk heterojunction. Although P3HT:PCBM properties have been extensively studied, less light has been set on its nanomechanical properties, which affect the device service life. In this work Atomic Force Acoustic Microscopy (AFAM), Atomic Force Spectroscopy and Nanoindentation were used to study the effect of the fullerene presence and the annealing on the P3HT:PCBM nanomechanical behavior. The P3HT:PCBM thin films were prepared by spin coating on glass substrates and then annealed at 100 °C and 145 °C for 30 min. Large phase separation was identified by optical and Atomic Force Microscopy (AFM) for the annealed samples. Needle-like PCBM crystals were formed and an increase of the polymer crystallinity degree with the increase of the annealing temperature was confirmed by X-ray diffraction. AFAM characterization revealed the presence of aggregates close to stiff PCBM crystals, possibly consisting of amorphous P3HT material. AFM force-distance curves showed a continuous change in stiffness in the vicinity of the PCBM crystals, due to the PCBM depletion near its crystals, and the AFM indentation provided qualitative results about the changes in P3HT nanomechanical response after annealing.     

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.     

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.     

Preparation and characterization of regioregular poly(3-octylthiophene-2,5-diyl)/copper indium disenillide/titania heterojunction polymer solar cells

By optimizing the P3OT/CISe ratio, TiO₂ content in the P3OT/CISe active layer, annealing temperature and time, this study investigated hybrid Al/Ca/P3OT:CISe:TiO₂/PEDOT:PSS/ITO thin film solar cells with improved efficiency. Due to an increase in charge-carrier transport and a decrease of electron-hole recombination, it is possible to increase the efficiency of hybrid solar cells by adding TiO₂ nanoparticles to the P3OT:CISe active film. Also, performance enhancement of the solar cells can occur with an increase of CISe content in P3OT as well as the addition of a PEDOT:PSS layer to the cell structure. The optimum TiO₂ content in P3OT:CISe layer is 15 wt.%. The optimum annealing temperature and time are 125 °C and 30 min, respectively. The formation of large CISe and TiO₂ aggregates that reduce charge mobility may cause the decrease of efficiency. The rough surface may effectively reduce the charge-transport distance and provide nanoscale phase separation that further enhances internal light scattering and light absorption. The best results for the open circuit voltages (V_oc), short-circuit current density (J_sc), fill factor (FF), and efficiency (η_e) of Al/Ca/POCT15/PEDOT:PSS/ITO hybrid solar cells obtained at optimized conditions were V_oc = 0.49, J_sc = 3.20, FF = 42.96, and η_e = 0.674, respectively.     

Preparation and characterization of P3HT:CuInSe2:TiO2 thin film for hybrid solar cell applications

Improved efficiency of hybrid Al/Ca/P3HT:CISe:TiO₂/PEDOT:PSS/ITO thin film solar cells was obtained by optimizing P3HT/CISe ratio. This study also investigated the effects of TiO₂ content in the P3HT:CISe active layer, and altering annealing temperature conditions. The optimum TiO₂ content and annealing temperature for solar cell efficiency is 25 wt.% and 150 °C, respectively. The optimal results for the open circuit voltages (V_OC), short-circuit current density (J_SC), fill factor (FF), and efficiency (η) of the prepared hybrid thin film solar cell were V_OC = 0.335 V, J_SC = 8.07 mA/cm², FF = 52.75, and η = 1.425.     

Evolution of vertical phase separation in P3HT:PCBM thin films induced by thermal annealing

The achievement of the desirable morphology at the nanometer scale of bulk heterojunctions consisting of a conjugated polymer with fullerene derivatives is a prerequisite in order to optimize the power conversion efficiency of organic solar cells. The various experimental conditions such as the choice of solvent, drying rates and annealing have been found to significantly affect the blend morphology and the final performance of the photovoltaic device. In this work, we focus on the effects of post deposition thermal annealing at 140 °C on the blend morphology, the optical and structural properties of bulk heterojunctions that consist of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM). The post thermal annealing modifies the distribution of the P3HT and the PCBM inside the blend films, as it has been found by Spectroscopic Ellipsometry studies in the visible to far-ultraviolet spectral range. Phase separation was identified by AFM and GIXRD as a result of a slow drying process which took place after the spin coating process. The increase of the annealing time resulted to a significant increase of the P3HT crystallinity at the top regions of the blend films.     

Thermal effects and plasma damage upon encapsulation of polymer solar cells

A barrier structure consisting of silicon oxide and silicon nitride films was deposited via plasma-enhanced chemical vapor deposition (PECVD) for the encapsulation of polymer solar cells (PSCs). The total concentration of the solution and the ratio of P3HT and PCBM on the performance of polymer solar cells were studied by UV-Vis absorption spectroscopy, atomic force microscopy and photocurrent measurement. Base on these measurements, there is a compromise between light absorption and phase separation with increasing blend concentration. The PSCs were annealed at 80, 100, 120 and 140 °C for 10-60 min to investigate the thermal effects and to estimate the best deposition temperature of the barrier layers. Nevertheless, the devices with the encapsulation of barrier layers had relatively low power conversion efficiencies (PCE) of 0.98% comparing to the devices heated in the PECVD system (1.57%) at the same condition of 80 °C for 45 min due to the plasma damage during the film deposition process. After inserting a 5-nm TiO_x layer between Al/barrier structure and active layer against the plasma damage, the annealed devices presented an average PCE of 2.26% and demonstrated over 50% of their initial value after constant exposure to ambient atmosphere and sunlight for 1500 h.     

Organic solar cells with p-type amorphous chromium oxide thin film as hole-transporting layer

Efficient organic solar cells based on the blends of poly (3-hexylthiophene) (P3HT):fullerene derivative [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) composites have been fabricated by using the sputtered amorphous chromium oxide (ACO) film on fluorine-doped tin oxide (FTO) coated glass substrates as a hole-transporting layer (HTL). Through ACO layer sputtering temperature, film thickness and oxygen flow ratio optimization, the highest power conversion efficiency of 3.28% of FTO/ACO/P3HT:PCBM/Al solar cells on glass has been achieved under AM1.5G 100 mW/cm² illumination. It is found that the device with 10 nm thick ACO sputtered at 473 K and oxygen flow ratio f(O₂) (O₂/O₂ + Ar) = 40% shows the best photovoltaic properties. The photovoltaic properties in these devices are discussed in terms of the band diagrams and series resistance of the devices, and characteristics of ACO HTL. It is concluded that ACO is a suitable alternative to poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) as a HTL.     

Direct writing of semiconducting polythiophene and fullerene derivatives composite from bulk heterojunction solar cell by inkjet printing

The direct writing approach of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) composite from bulk-heterojunction (BHJ) solar cell was efficiently addressed by inkjet printing technology using conventional chlorobenzene ink solution. The structure of inkjet-printed P3TH:PCBM BHJ film was fabricated by the repetitive direct writing of new line overlapped partially on former line. The best structure of P3HT:PCBM film for BHJ solar cell was observed from inkjet printing condition of around 50% of droplet overlaps with 2 wt.% BHJ ink at 25 °C of substrate temperature. The maximum power conversion efficiency reached 2.83% with an open circuit voltage of 0.62 V, a short circuit current density of 8.60 mA/cm², and a fill factor of 0.53 under air mass 1.5 G irradiation (100 mW/cm²).     

Solution-processable zinc oxide for the polymer solar cell based on P3HT:PCBM

The device performance of polymer solar cells with zinc oxide (ZnO) nanoparticles inserted as an electron injection layer between the poly(3-hexylthiopene) (P3HT):phenyl-C60-butyric acid methyl ester (PCBM) active layer and the Al electrode was studied. The polymer solar cell consists of molybdenum-oxide (MoO3) as a hole injection layer, P3HT:PCBM bulk heterojunction as an active layer, and ZnO NPs as an electron injection layer. The ZnO layer was formed from a precursor solution on the top part of the P3HT:PCBM film (1:0.8 weight ratio) via sol-gel spin-coating, and was annealed at a low temperature (150 degrees C). The crystallinity, the atomic ratio of Zn and O, the absorption spectra, and the surface morphology of the ZnO thin films were studied. The device with a ZnO layer showed 9-11% higher J(SC) and 8-9% higher PCE compared to the devices without a ZnO layer. These improved device properties are attributed to the efficient electron extraction and the decreased reflectivity owing to the use of a ZnO layer.     

Growth and characterization of germanium epitaxial film on silicon (001) using reduced pressure chemical vapor deposition

High quality germanium (Ge) epitaxial film is grown directly on silicon (001) substrate using a “three-step growth” approach in a reduced pressure chemical vapor deposition system. The growth steps consist of sequential low temperature (LT) at 400 °C, intermediate temperature ramp (LT-HT) of ~ 6.5 °C/min and high temperature (HT) at 600 °C. This is followed by post-growth anneal in hydrogen at temperature ranging from 680 to 825 °C. Analytical characterizations have shown that the Ge epitaxial film of thickness ~ 1 μm experiences thermally induced tensile strain of 0.20% with a threading dislocation density of < 10⁷ cm^− 2 under optical microscope and root mean square roughness of ~ 0.9 nm. Further analysis has shown that the annealing time at high temperature has an impact on the surface morphology of the Ge epitaxial film. Further reduction in the RMS roughness can be achieved either through chemical mechanical polishing or to insert an annealing step between the LT-HT ramp and HT steps.     

Effect of layer thickness on structural quality of Ge epilayers grown directly on Si(001)

A study of Ge epilayer growth directly on a Si(001) substrate is presented, following the two temperature Ge layer method. In an attempt to minimize the overall thickness while maintaining a good quality Ge epilayer, we have investigated the effect of varying the thickness of both the low and high temperature Ge layers, grown at 400 °C and 670 °C, respectively, by reduced pressure chemical vapor deposition. We find that the surface of the low temperature (LT) seed layer has a threading dislocation density (TDD) to the order of 10¹¹ cm^− 2. On increasing the LT layer thickness from 30 nm to 150 nm this TDD decreases by a factor of 2, while its roughness doubles and degree of relaxation increases from 82% to 96%. Growth of the high temperature (HT) layer reduces the TDD level to around 10⁸ cm^− 2, which is also shown to decrease with increasing layer thickness. Both the surface roughness and degree of relaxation reach stable values for which increasing the thickness beyond about 700 nm has no effect. Finally, annealing the HT layer is shown to reduce the TDD, without affecting the degree of relaxation. However, unless a thick structure is used the surface roughness increases significantly on annealing.     

Effect of annealing on the characteristics of Au/Cr bilayer films grown on glass

Au layers with thickness of about 110 nm were sputter-deposited on unheated glass substrates coated with a Cr layer about 20 nm thick. The chamber was evacuated to a pressure of 2 Pa and then sputtering was carried out at Ar pressure of 4 Pa. The Au/Cr bilayer films were annealed in a vacuum of 5×10⁻⁴ Pa at 170°C, 180°C, 200°C and 250°C for from 5 to 120 min, respectively. Atomic force microscopy was used to observe the structural characteristic of the bilayer films. Auger electron spectroscopy was used to analyze the composition inside the Au layers. The sheet resistance of the films was measured using the four-point probe technique. The grain size of the bilayer film gradually increases with an increase in annealing temperature while its average surface roughness ranging from 4.5 to 6.8 nm does not show any systematic change with annealing temperature and time. No impurities such as carbon, nitrogen and oxygen are detected inside all of the Au layers. When the annealing temperature reaches 200°C and the annealing time exceeds 30 min, chromium atoms markedly diffuse into the Au layer. Furthermore, for the bilayer films annealed at 250°C, chromium atoms have markedly diffused into the Au layer even for annealing time of 5 min. Regardless of the increase in grain size of the Au layer, the diffusion of chromium atoms into the Au layer causes an increase in the resistivity of the bilayer film.     

Study on the antimony tin oxide as a hole injection layer for polymer light emitting diodes

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) is commonly used as a hole transfer layer in polymer light emitting diodes (PLEDs). However, Indium tin oxide transparent electrodes are corroded by poly(styrenesulfonate) and the erupted indium diffuses into the active layer, which in turn decreases the brightness, efficiency and lifetime of the device. In this study, therefore, antimony tin oxide (ATO) was introduced as a hole injection layer (HIL) in PLEDs. The work function and pH of ATO were − 5.1 eV and ~ 7.5, respectively. When annealed at 200 °C, high conductivity (~ 0.18 S/cm) was observed, which represents good HIL characteristics. Here, the maximum luminance (26,114 cd/m²) and maximum efficiency (1.55 cd/A) of the PLEDs were increased by 33% and 20% respectively. Their stability improved as well.     

Effect of annealing on the characteristics of Au layers grown on the high-temperature deposited Ni50Fe50 layers

80 nm-thick Ni₅₀Fe₅₀ layers were sputter-deposited on glass substrates at 400 °C and then Au layers were sputter-deposited on the Ni₅₀Fe₅₀ layers. The Au/Ni₅₀Fe₅₀ bilayer films were annealed in a vacuum of 5×10⁻⁴ Pa from 250 to 450 °C for 30 min or 90 min. The characteristics of the Au layers were studied by Auger electron spectroscopy, field emission scanning electron microscopy, X-ray diffraction and a four-point probe technique. When the annealing temperature reaches 450 °C, Fe and Ni atoms diffuse markedly into the Au layer and the Fe content is more than the Ni content. When the annealing temperature is lower than 450 °C, the grain size of the Au layers does not change markedly with annealing temperature. However, as the annealing temperature reaches 450 °C, the annealing promotes the grain growth of the Au layer. As the annealing temperature exceeds 300 °C, the resistivity of the bilayer films increases with increasing annealing temperature. The diffusion of Fe and Ni atoms into the Au layer results in an increase in the resistivity of the annealed bilayer film. Large numbers of Fe and Ni atoms diffusing into the Au layer of the annealed Au/Ni₅₀Fe₅₀ bilayer film lead to a significant decrease in the lattice constant of the Au layer.     

MOCVD indium sulphide for application as a buffer layer in CIGS solar cells

Indium sulphide has a high potential as a buffer layer material in Cu(In,Ga)Se₂ (CIGS) solar devices. In this work a metal organic vapour deposition (MOCVD) process was investigated for the indium sulphide deposition. Trimethyl-indium and t-butyl-thiol were applied as precursor sources. The films produced with different process conditions were characterised by scanning electron microscopy. We also present the first results of CIGS laboratory cells with an MOCVD indium sulphide buffer layer. The best device showed an efficiency of 12.3% (CdS reference = 13.0%) at a deposition temperature of 300 °C and a deposition time of 20 min, combined with a 5 min post-annealing treatment at 200 °C in air.     

Fluorine doped indium oxide films for silicon solar cells

The effect of conditions of preparation of the In₂O₃:F(IFO)/(pp⁺)Si solar cell (SC) by pyrosol method was systematically studied with the goal to maximize its photovoltage. Heterojunction IFO/(pp⁺)Si SC was obtained with the efficiency of 16.6% and photovoltage of 617 mV as well as the IFO/(n⁺pp⁺)Si SC with the efficiency of 19.2% using the following obtained optimal conditions: film-forming solution: 0.2 M InCl₃ + 0.05 M NH₄F + 0.1 M H₂O in methanol; carrier gas — Ar + 5% O₂; deposition temperature — 480 °C; duration of deposition — 2 min; two-minute annealing in argon with sprayed methanol at a temperature of 380 °C.     

Characterization and visible light germicidal efficacy of nitrogen doped TiO2 film crystallized by microwave irradiations

The visible light bactericidal ability of nitrogen doped TiO₂ (TiON) film on Si (100) has been enhanced greatly (by 22%) through a single-mode microwave irradiation annealing (f = 2.45 GHz, 20 s) in comparison with the conventional heat treatment at the same temperature (500 °C, 300 s). Analyses of X-ray photoelectron spectroscopy and transmission electron microscopy showed that the microwave annealed TiON film had a higher nitrogen concentration and much better crystallinity, both of which contribute to its prior bactericidal ability under visible light. The optimized annealing parameters of microwave irradiation are 500 °C within 20 s. Increase of annealing temperature and irradiation time resulted in the decrease of nitrogen concentration within the film. The crystallized TiON film has an anatase but not a rutile structure at the annealing temperature up to 800 °C.     

Annealing optimization of silicon nitride film for solar cell application

Hydrogenated films of silicon nitride (SiNx:H) is commonly used as an antireflection coating as well as passivation layer in crystalline silicon solar cell. SiNx:H films deposited at different conditions in Plasma Enhanced Chemical Vapor Deposition (PECVD) reactor were investigated by varying annealing condition in infrared (IR) heated belt furnace to find the optimized condition for the application in silicon solar cells. By varying the gases ratio (R = NH₃/SiH₄ + NH₃) during deposition, the SiNx:H films of refractive indices 1.85-2.45 were obtained. Despite the poor deposition rate, the silicon wafer with SiNx:H film deposited at 450 °C showed the best effective minority carrier lifetime. The film deposited with the gases ratio of 0.57 shows the best peak of carrier lifetime at the annealing temperature of 800 °C. The single crystalline silicon solar cells fabricated in conventional industrial production line applying the optimized film deposition and annealing conditions on large area substrates (125 mm × 125 mm) were found to have the conversion efficiencies as high as 17.05 %. Low cost and high efficiency single crystalline silicon solar cells fabrication sequence employed in this study has also been reported in this paper.