Optical and electrical properties of hybrid photovoltaic devices from poly (3-phenyl hydrazone thiophene) (PPHT) and TiO2 blend films

Bulk heterojunction photovoltaic devices based on blends of a conjugated polymer poly (3-phenyl hydrazone thiophene) (PPHT) as electron donor and titanium dioxide (TiO₂) particles as an electron acceptor (n-type wide band gap semiconductor) have been studied. The blend films were spin coated from a common solvent mixture. The absorption peak and shape of the absorption spectra of PPHT:TiO₂ (40 vol%) indicate that it is a superposition of the absorption of PPHT and TiO₂. From the cyclic voltammetry measurements, we have estimated the values of levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for PPHT. A strong fluorescence quenching indicates that the exciton dissociation and charge separation occurs successfully at PPHT:TiO₂ interfaces formed in the bulk. This also enhances the possibility that the separated charges will reach the electrodes before recombining. Experimentally observed current–voltage characteristics of this device has been explained employing the metal–insulator–metal (MIM) model, where the intimate mixture of electron accepting and hole accepting materials is treated as a homogeneous intrinsic semiconductor. In this device, holes are solely transported along the pure donor phase and electrons transported along the paths of acceptor phase. Effect of TiO₂ concentrations in the blend and thermal annealing of device has been described is detail. For low concentration of TiO₂ below 20% the device performance is poor than that for pure PPHT, while for higher concentration of TiO₂ significant improvement was obtained. The thermal annealing of the device also improves the photovoltaic response of the device, which may be due to the reduction in the recombination process.     

Omnidirectional light‐harvesting enhancement of dye‐sensitized solar cells with ZnO nanorods

In this work, the chemical solution method was used to prepare one‐dimensional (1‐D) ZnO nanorod (NR) photoelectrodes, which were subsequently used in dye‐sensitized solar cells (DSSCs). The effects of ZnO NRs on the omnidirectional light‐harvesting performance of DSSCs were investigated by growing ZnO NRs with varying lengths as photoelectrodes. On the basis of field‐emission scanning electron microscopy and ultraviolet (UV)–vis‐near infrared (NIR) spectroscopy measurements on the ZnO NR photoelectrodes of varying lengths, it was observed that the dye adsorption and light‐scattering properties of NRs are affected by their length. In addition, DSSCs were prepared using ZnO NRs of varying lengths. These DSSCs were examined via electrochemical impedance spectroscopy, monochromatic incident photon‐to‐electron conversion efficiency measurements, and solar simulations to measure their photovoltaic efficiencies, carrier lifetimes, and device characteristics in omnidirectional antireflection measurements. The highest photovoltaic efficiency between these DSSCs was 0.33%. Omnidirectional antireflection measurements were performed on DSSCs with different ZnO NR lengths, and it was observed that the smallest change in efficiency between angles of incidence of 0° and 60° was 23%. Therefore, the light‐scattering properties of ZnO NR photoelectrodes improve the omnidirectional antireflection light capture characteristics of DSSCs.     

Effect of annealing on the optical, electrical, and photovoltaic properties of bulk hetero-junction device based on PPAT:TY blend

Polythiophenes are the promising candidates for polymer photovoltaic devices due to their stability and absorption in near-red region of visible spectrum. In this paper, the optical, electrical, and photovoltaic properties of device fabricated with the blend of poly(phenyl azo methane thiophene) (PPAT) (donor) and thiazole yellow (TY) (acceptor) (70:30 by weight ratio), sandwiched between ITO and Al electrodes have been reported. The observed strong quenching in the fluorescence of PPAT after blending it with TY indicates the rapid and effective separation of photo-induced electron and hole and subsequent transfer of electron from PPAT to TY and holes from TY to PPAT. The absorption spectrum of the blend shows a super-imposition of both components indicating that there is no charge transfer at the ground state. The thermal annealing shows significant improvement in both optical and photoelectrical properties of the blend, as can be seen from the recorded current–voltage characteristics (J–V) of the device in dark and under illumination. This change is also evidenced by the modification in absorption spectrum on annealing resulting in red shifts, due to the donor component. The appearance of red shift may by attributed to the molecular diffusion of TY out to the PPAT matrix on annealing, resulting in a rise to the TY clusters. The growth of TY clusters lead to the formation of percolation paths and therefore improves photocurrent. By annealing the device under applied external voltage, both the electrical and photovoltaic properties of the device have been improved. The enhancement in photocurrent, quantum collection efficiency (EQE), and fill factor of annealed device may be due to an increase in carrier mobility after annealing the device.     

Polymeric solar cells based on P3HT:PCBM: Role of the casting solvent

Polymeric photovoltaic (PV) solar cells have been fabricated using six solvents: chloroform (CHCl₃), toluene (T), chlorobenzene (CB), orthodichlorobenzene (ODCB), 1,2,3,4-tetrahydronaphthalene (THN) and 1,2,4-trichlorobenzene (TCB). The active layers were composed of poly(3-hexyl)thiophene (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Special care has been taken to keep all experimental parameters constant (thickness of the active layers, donor/acceptor weight ratio, area of active surface and electrodes) in order to avoid artefacts and truly study the effect of solvents. Studies using atomic force microscopy (AFM) and optical absorption (UV-vis) showed the relationship between the photovoltaic performance and the evaporation rate of solvents. The use of solvents with high boiling point results in a higher degree of organization in the structure of P3HT. A direct comparison with devices processed with thermal treatment has also been performed. As often reported thermal annealing increases photo-conversion efficiency of devices created from common solvents, due to better separation of phase between the two materials of the blend. In the case of solvents with high boiling point such as THN and TCB, neither phase separation nor modification of P3HT crystallization induced by thermal annealing has been observed. However thermal treatment appears to enhance performance, ensuing the evaporation of remaining solvent in the active layers. An overview of the effect of solvent on the electrical properties of films containing pure P3HT and P3HT:PCBM blend reported in the literature has been completed for the discussion.     

Dye-sensitized solar cell based on nanocrystalline ZnO thin film electrodes combined with a novel light absorbing dye Coomassie Brilliant Blue in acetonitrile solution

In this work, characterization of dye-sensitized solar cells (DSSC) using nanocrystalline ZnO thin film electrodes combined with a novel light absorbing dye Coomassie Brilliant Blue (CBB), in acetonitrile solution is reported. The absorption spectrum of this dye in acetonitrile solution indicates appreciable absorption in the range of 500–700 nm with a sharp peak at 597 nm indicating its possible use as a photosensitizer for ZnO. The current–voltage and efficiency characteristics of a DSSC based on this dye and ZnO acceptor are measured for two methods of depositing the ZnO. Better response is achieved for nanocrystalline ZnO thin films than for sprayed films in terms of cell output.     

P3HT/ZnO bulk-heterojunction solar cell sensitized by a perylene derivative

In this work, a soluble perylene-derivative dye, N, N′-didodecyl-3,4,9,10-perylene tetracarboxylic diimide (PDI), was used to improve the photovoltaic performance of poly(3-hexylthiophene) (P3HT)/ZnO bulk heterojunction cells through blending with the composite. Results show that by incorporation of PDI in the P3HT/ZnO composite, the light absorption and exciton separation can be significantly improved. The photocurrent under white-light irradiation can be increased from 6.35 to 9.55 mA/cm². Solar decay experiment shows that V_OC of the ITO/PEDOT:PSS/P3HT:ZnO:PDI/Al device decreases rapidly to almost zero in 1 h under persistent white-light illumination. After placing a 420 nm cutoff filter between the cell and the xenon lamp, the stability of the cell can be significantly improved. The device performance can maintain about 80% of the original value within 30 h and I_SC degraded to zero after 142 h. The addition of PDI into the P3HT/ZnO device up to 5 wt% does not show observable effect on the solar cell decay behavior.     

Charge generation and photovoltaic properties of hybrid solar cells based on ZnO and copper phthalocyanines (CuPc)

Photovoltaic properties and charge transfer process within the blend of p-type organic semi conducting copper phthalocyanine (CuPc) and inorganic n-type semiconductor zinc oxide (ZnO) nanoparticle have been determined. The photo-induced electron transfer mechanism in the composite thin film structure is predicted on the basis of its optical absorption behavior, photo-action spectra and photoluminescence (PL) characteristics, supported by current–voltage characteristics in dark and under illumination. An efficient electron transfer between donor (CuPc) and acceptor (ZnO) nanoparticle is explained by analyzing the photo-action spectra of the device and respective quenching of photoluminescence (PL) in composite thin film structure. The increase in photocurrent in composite blend based device is justified in terms of formation of bulk heterojunction between CuPc and ZnO nanoparticles. Also, the subsequent increase in photovoltaic response is ascribed to the formation of interconnecting network in between CuPc and ZnO nanoparticles. Our experimental results supports that photo-generation of charge carriers is induced by the production of excitons and their subsequent dissociation of into free carriers at ZnO–CuPc interface which is spread over the bulk region sandwiched between Al and ITO. Additionally, the optical and electrical properties of the composite blend based device have been discussed in details. The capacitance–voltage characteristics of the device support the formation of bulk heterojunction between CuPc and ZnO nanoparticles.     

Optimization of process parameters for high-efficiency polymer photovoltaic devices based on P3HT:PCBM system

Here, we report the fabrication of high-efficiency poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) blend photovoltaic device. Process parameters like solvent, solvent drying conditions, electron donor to acceptor ratio and cathodes structures are optimized in making the devices. For the first time, we used cosolvent systems to make active layer of P3HT:PCBM composite and G-PEDOT:PSS, made by mixing 6 wt% glycerol to PEDOT:PSS, is used as a buffer layer. Highest efficiency of 4.64% was obtained for the device made with 1:0.7 ratio of P3HT to PCBM, o-dichlorobenzene:chloroform cosolvent, newly developed slow process and G-PEDOT:PSS. Film morphology is evaluated by atomic force microscopy (AFM). Time-of-flight (TOF) and incident photon-to-current conversion efficiency (IPCE) measurements are also performed for the best device.     

Electrochemically grown ZnO nanorods for hybrid solar cell applications

A hybrid solar cell is designed and proposed as a feasible and reasonable alternative, according to acquired efficiency with the employment of zinc oxide (ZnO) nanorods and ZnO thin films at the same time. Both of these ZnO structures were grown electrochemically and poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester; (P3HT:PCBM) was used as an active polymer blend, which was found to be compatible to prepared indium-tin-oxide (ITO) substrate base. This ITO base was introduced with mentioned ZnO structure in such a way that, the most efficient configuration was optimized to be ITO/ZnO film/ZnO nanorod/P3HT: PCBM/Ag. Efficiency of this optimized device is found to be 2.44%. All ZnO works were carried out electrochemically, that is indeed for the first time and at relatively lower temperatures.     

Synthesis conditions, light intensity and temperature effect on the performance of ZnO nanorods-based dye sensitized solar cells

We present in this work a careful study of the different parameters affecting vertically-aligned ZnO-nanorods (NRs) based dye sensitized solar cells (DSCs). We analyze the effect of synthesis conditions, light intensity, UV light and working temperature, and correlated them to the final photovoltaic properties of the DSC. Although similar studies can be found in the literature for DSCs based on TiO₂, this work is, to our knowledge, the first detailed study carried out for DSC based on vertically-aligned ZnO nanorods. The ZnO NRs were grown between 1.6 and 5.2 μm long. Electrodes made with 1.6 ± 0.2 μm thickness were used to analyze parameters such as synthesis conditions, light intensity (800-1500 W m⁻²), UV light irradiation and temperature (25-75 °C). We have also carried out initial analysis of the solar cell lifetime under continuous light irradiation at 45 °C, and analyzed the ZnO electrode before and after testing. The best photovoltaic response was characterized by a power conversion efficiency of 1.02%, with J_sc of 3.72 mA cm⁻², V_oc of 0.603 V and 45% FF (at 72 °C), for a ZnO NR electrode of 5.2 μm thickness. Comparison of our power conversion efficiency values with published data is also presented, as well as a brief discussion on the possible reasons behind the low power conversion efficiency observed for these type of solar cells.     

Photovoltaic properties of poly(benzothiadiazole-thiophene-co-bithiophene) as donor in polymer solar cells

We report the photovoltaic properties of a D-A copolymer, poly(benzothiadiazole-thiophene-co-bithiophene) (PBTTbT), containing the donor (D) unit of oligothiophene with a hexyl side chain and the acceptor (A) unit of 2,1,3-benzothiadiazole (BT) with a methyl side chain. The geometry, electronic and absorption spectroscopic properties of bithiophene-benzothiadiazole-thiophene monomer (M1) of the polymer were investigated theoretically by the density functional theory (DFT) method for deep understanding the relationship of the structure and properties of the polymer. Polymer solar cells (PSCs) were fabricated with PBTTbT as an electron donor blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₀BM) as an electron acceptor. The power conversion efficiency (PCE) of PSC is 0.87% for an optimized PBTTbT:PC₇₀BM weight ratio of 1:3, under the illumination of AM 1.5, 100 mW/cm². With the additive of 1% 1,8-dioctanedithiol and thermal annealing at 130 °C for 15 min, the PCE of the device was improved to 1.98%. The efficiency improvement of the device was ascribed to a better morphology of the PBTTbT:PC₇₀BM active layer with the additive and thermal annealing.     

Effect of functional groups of acceptor material on photovoltaic response of bulk hetero-junction organic devices based on tin phthalocyanine (SnPc)

We have fabricated and characterized the bulk hetero-junction photovoltaic devices based on the donor and acceptor blends. For the present investigation, we have used SnPc as donor material, whereas a series of acceptor materials with identical backbone but different acceptor strength were used to make the blend. The role of functional groups, attached to backbone of acceptor material on the optical, electrical and photovoltaic properties of the devices was investigated. The origin of open-circuit voltage (V_oc) and short-circuit photocurrent (J_sc) has been discussed considering the ionization potential of donor and electron affinity of the acceptor material. The photocurrent of the device depends on choice of acceptor materials, i.e. number of acceptor group attached to the acceptor molecules, showing the importance of energy mismatch between lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels of donor and acceptor molecules. Here, we also investigated the effect of thermal annealing on the optical, electrical and photovoltaic properties of the bulk hetero-junction device employing tin phthalocyanine (SnPc) and Rose Bengal (RB) used as donor and acceptor, respectively. From the impedance spectroscopy, we conclude that the change in bulk resistance and dielectric constant of the active material due to the illumination has a direct relevance to the photocurrent generated by the device.     

Influence of processing additives to nano-morphology and efficiency of bulk-heterojunction solar cells: A comparative review

Research and development towards high efficiency plastic solar cells have been accelerating in recent years. Polymer-based bulk heterojunction solar cells are offering an attractive and inexpensive concept for large scale production by solution processing as well as advantageous flexible and aesthetic form factors. The thin film nano-morphology of bulk-heterojunction solar cells has been shown to dramatically influence the photovoltaic performance of the devices. This article reviews the different methods used to control the film nano-morphology of bulk-heterojunction solar cells focussing on the chemical additives during solution processing. All power conversion efficiency limiting mechanisms of bulk-heterojunction solar cells are discussed in detail. It is shown, how the formation of optimal percolation pathways between donor and acceptor influences the photovoltaic device performance. It is explained how the film nano-morphology relates to light absorption, free charge carrier generation as well as charge transport to the electrodes.     

Improvement in light harvesting and performance of P3HT:PCBM solar cell by using 9,10-diphenylanthracene

We have studied the effect of 9,10-diphenylanthracene (DPA) as a conjugated dye with different concentrations on light harvesting and performance of solar cell composed from poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁butyric acid methyl ester (PCBM) blend films. The dye concentration effect was investigated with optical absorption spectroscopy, photocurrent spectroscopy, and current density-voltage characteristic measurements on devices under AM1.5 white light illumination with intensity of 100 mW/cm². The incorporation of the conjugated DPA inside P3HT:PCBM blend improved the light harvesting, slightly, and conjugation length indicated from the optical absorption and external quantum efficiency spectra. By adding specific amounts of the DPA into P3HT:PCBM blend, the external quantum efficiency and solar cell performance parameters, i.e., short circuit current density, fill factor, and power conversion efficiency improved as a result of improvement in the light harvesting and charge carrier transfer taking place between P3HT and PCBM through the conjugated DPA molecules.     

Improving power conversion efficiency in polythiophene/fullerene-based bulk heterojunction solar cells

In the present work, we have studied photovoltaic devices fabricated from a blend of regioregular poly (3-hexlthiophene) (P3HT) and Buckminster fullerene. The solvent and composite ratio have been selected to obtain best morphology and minimum degradation. Buffer layers of poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS) at the anode and of LiF at the cathode were used to improve the device performance. It was further found that post-annealing of the devices for an optimum duration and temperature improves the solar cells, and the power conversion efficiency of the devices increases to 2.1% at AM1.5. Though the efficiency using [6,6]-phenyl C60 butyric acid methyl ester (PCBM) as the composite acceptor instead of C₆₀ can be higher, it was not used because of its very high cost as compared to C₆₀.     

A new organic dye bearing aldehyde electron-withdrawing group for dye-sensitized solar cell

A new metal-free dye (I) with a diketopyrrolopyrrole (DPP) core was synthesized, in which triphenylamine was used as electron donor, thiophene units as the π-conjugated bridge, aldehyde units as electron acceptor. The corresponding dye II containing carboxy group as the electron-withdrawing acceptor for the purpose of comparison was also synthesized. The absorption spectra, electrochemical and photovoltaic properties of I and II were extensively investigated. Electrochemical measurements data indicate that the tuning of HOMO and LUMO energy levels can be conveniently accomplished by alternating electron acceptor. The short-circuit photocurrent density and conversion efficiency of solar cell based on aldehyde-containing dye is more dominant than that bear a carboxy group as the electron withdrawing anchoring group. The new sensitizer I exhibited a photovoltaic performance: a short-circuit photocurrent density (J_sc) of 6.07 mA cm^?2, an open-circuit photovoltage (V_oc) of 568 mV, and a fill factor (FF) of 0.66, corresponding to an overall conversion efficiency of 2.27% under standard global AM 1.5 solar light condition. This work suggests that aldehyde units as new type of electron withdrawing anchoring group are promising candidates for improvement of the performance of DSSCs.     

A photoelectrochemical solar cell based on ZnO/dye/polypyrrole film electrode as photoanode

Nanostructured ZnO film electrodes were prepared. A preliminary PEC solar cell based on nanostructured Zno/dye/polypyrrole (PPy) film electrode was fabricated. A fill factor of 0.754 and a high overall light to electricity conversion efficiency of 1.3% for this PEC solar cell were obtained. The sensitization mechanisms of the nanostructured ZnO electrodes were also discussed.     

Synthesis and characterization of ZnO nanowires grown on different seed layers: The application for dye-sensitized solar cells

Zinc oxide (ZnO) nanowire electrodes which were grown on different seed layers and examination of their significant effects on the performance of dye sensitized solar cells were studied. Through chemical bath deposition process, the ZnO nanowires were grown on an indium tin oxide (ITO) coated glass using sputter-deposited aluminum doped zinc oxide (AZO) and ZnO seed layers. Afterward, main parameters such as solution concentration, growth temperature, and time were systematically investigated based on morphology of nanowires. The X-ray diffraction (XRD), field emission scanning microscopy (FESEM), and photoluminescence (PL) were applied to investigate the characteristics of the samples. The results showed ZnO nanowires, which were grown by AZO seed layer, had a high density array with hexagonal wurtzite structure distributed vertically and uniformly on ITO coated glass. The mentioned zinc-oxide nanowires grown under an optimum condition on different seed layer were used to fabricate dye solar cells afterward. The seed layer was effective on morphologic, optical, and structural features. The overall light-conversion efficiency of dye sensitized solar cell with ZnO nanowires grown on AZO seed layer was almost 2 times higher than that of those grown on ZnO seed layer. Electrochemical impedance spectroscopy analysis was measured under standard light to investigate the electron transport properties in the both ZnO-NW DSSCs. As the results showed, photoanode electron recombination rate with electrolyte was 6.02 Hz for dye solar cells of zinc oxide (ZnO-NWDSSC) produced by ZnO seed layer, which is 2.5 times faster than cells with AZO seed layer.     

Effects of intrinsic ZnO buffer layer based on P3HT/PCBM organic solar cells with Al-doped ZnO electrode

Organic solar cell devices were fabricated using poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), which play the role of an electron donor and acceptor, respectively. The transparent electrode of organic solar cells, indium tin oxide (ITO), was replaced by Al-doped ZnO (AZO). ZnO has been studied extensively in recent years on account of its high optical transmittance, electrical conduction and low material cost. This paper reports organic solar cells based on Al-doped ZnO as an alternative to ITO. Organic solar cells with intrinsic ZnO inserted between the P3HT/PCBM layer and AZO were also fabricated. The intrinsic ZnO layer prevented the shunt path in the device. The performance of the cells with a layer of intrinsic ZnO was superior to that without the intrinsic ZnO layer.     

Antireflection layer of ZnO nanorod embedded in PDMS film for enhancing omnidirectional photovoltaic performance of CIGS photovoltaic cell

In this study, a flexible polydimethylsiloxane (PDMS) ARL with embedded ZnO‐NRs was fabricated on the surfaces of Cu(In,Ga)Se₂ (CIGS) solar cells using the chemical solution method. Under the conditions of an AM1.5G solar spectrum, the application of the PDMS ARL of embedded ZnO‐NRs on the surface of a CIGS solar cell can effectively increase the conversion efficiency of the solar cell from 7.23% to 7.79%. In addition, broadband and omnidirectional light harvesting are important as the sun moves over time; therefore, omnidirectional anti‐reflection applications of various ARLs on CIGS photovoltaic cells were also studied.