Efficient bilayer heterojunction polymer solar cells with bumpy donor–acceptor interface formed by facile polymer blend

We demonstrated a facile method for the fabrication of bilayer polymer solar cells with a controlled heterojunction structure via simple polymer blends. The spontaneous phase separation of poly(3-hexylthiophene)/polyethylene glycol blends provides a bumpy electron-donor layer with characteristic circular depressions. The diameter and depth of the circular depressions can be controlled by varying the PEG content of the blend. The deposition of -phenyl-C61-butyric acid methyl ester as an electron-acceptor layer then creates an interpenetrating donor–acceptor interface for bilayer heterojunction polymer solar cells. The bumpy morphology of the interface results in a significant enhancement in the power conversion efficiency over that of the bilayer polymer solar cells with a typical planar interface, which is mainly due to an increase of photocurrent. An estimation of the field-dependent possibility of charge separation indicates that charge extraction is more efficient than charge recombination in the bilayer devices and the increase in the interfacial area of solar cells with a bumpy-interface leads to generate more electron-hole pairs at the interface.     

MoO3–Au composite interfacial layer for high efficiency and air-stable organic solar cells

Efficient and stable polymer bulk-heterojunction solar cells based on regioregular poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PC₆₁BM) blend active layer have been fabricated with a MoO₃–Au co-evaporation composite film as the anode interfacial layer (AIL). The optical and electrical properties of the composite MoO₃–Au film can be tuned by altering the concentration of Au. A composite film with 30% (weight ratio) Au was used as the AIL and showed a better performance than both pure MoO₃ and PEDOT:PSS as AIL. The surface morphology of the MoO₃–Au composite film was investigated by atomic force microscopy (AFM) and showed that the originally rough ITO substrate became smooth after depositing the composite film, with the root mean square roughness (RMS) decreased from 4.08 nm to 1.81 nm. The smooth surface reduced the bias-dependent carrier recombination, resulting in a large shunt resistance and thus improving the fill factor and efficiency of the devices. Additionally, the air stability of devices with different AILs (MoO₃–Au composite, MoO₃ and PEDOT:PSS) were studied and it was found that the MoO₃–Au composite layer remarkably improved the stability of the solar cells with shelf life-time enhanced by more than 3 and 40 times compared with pure MoO₃ layer and PEDOT:PSS layer, respectively. We argue that the stability improvement might be related with the defect states in MoO₃ component.     

Analysis of quantum efficiency and optical enhancement in amorphous Si p–i–n solar cells

The effect of i-layer thickness, tin oxide texture, and back reflector (BR) on optical enhancement has been systematically studied in a series of 20 a-Si p–i–n solar cells. The internal quantum efficiency has been analyzed by a simple model based on the work of Schade and Smith. The enhancement of optical absorption is characterized by m, a wavelength-dependent fitting parameter representing the increase in optical pathlength relative to the i-layer thickness d. Solar cells with an Al BR have negligible optical enhancement, with m < 1.5, consistent with large parasitic absorption at the Al/Si interface as reported by others. Solar cells on highly textured SnO₂ with ZnO/Al or ZnO/Ag BR have peak values of m ∼ 3–4, with ZnO/Ag having slightly larger values than ZnO/Al. It was found that m has a strong dependence on the product αd, and that maximum values of m increase with reflectivity of the BR. It is shown that a major source of parasitic absorption loss at long wavelengths is light trapping in the textured SnO₂ front contact. Copyright © 2002 John Wiley & Sons, Ltd.     

Different depositing amount of CuInS2 on TiO2 nanoarrays for polymer/CuInS2–TiO2 solar cells

This paper reports the deposition of CuInS₂ on TiO₂ nanoarrays, with different depositing amounts and demonstrates the application of TiO₂–CuInS₂ composites in polymer-based solar cells. The composites of TiO₂–CIS1 and TiO₂–CIS2 were prepared by the deposition of CuInS₂ on TiO₂ with one-step or two-step solvethermal reactions, respectively, and characterized by XRD, SEM, TEM, absorption spectrum and PL spectrum. Results showed that TiO₂–CIS1 displayed the higher light-harvesting ability and PL quenching efficiency compared to TiO₂–CIS2, although less CuInS₂ was deposited on TiO₂ surface. As a result, polymer/TiO₂–CIS1 solar cells displayed much higher J_sc correlated with the increased absorptivity and charge separation efficiency, and the higher V_oc was originated from the presence of strong interaction between TiO₂ and CuInS₂ in TiO₂–CIS1 resulting in the effective modification of TiO₂ surface by CuInS₂.     

Investigation of donor–acceptor copolymer films and their blends with fullerene in the active layers of bulk heterojunction solar cells by Raman microspectroscopy

Thin films made of three low-band gap donor–acceptor copolymers (CDTF, CDTDP and CDTDOP) composed of 4,6-bis(3′-dodecylthiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole-5′,5′-diyl as an electron-acceptor structural unit and various electron-donor structural units, such as 9,9-bis(2-ethylhexyl)fluorene-2,7-diyl, 2,5-didodecyl-1,4-phenylene and 2,5-didodecyloxy-1,4-phenylene, respectively, and thin films of their blends with various ratios of a soluble fullerene derivative [6,6]-phenyl C₆₁-butyric acid methyl ester ([60]PCBM) as an active layer for bulk heterojunction solar cells were studied by means of UV–vis absorption spectroscopy and Raman microspectroscopy. The molecules of CDTDP and CDTDOP possess the same main chains; they differ in the side-chain oxygen only, which changes the donor strength of the donor units. UV–vis and Raman studies allow us to show differences in the hindering of molecule planarization and aggregation in the blends. Absorption of the polymer films covered the whole visible spectral region and extended up to near infrared for CDTDOP. The absorption behavior of the CDTDP blend films qualitatively differed from the absorption behavior of the blend films of CDTF or CDTDOP. The Raman measurements were performed at two different laser excitation wavelengths (633 and 785 nm), which enabled the photoluminescence of both components in the Raman spectra to be distinguished. The Raman study was performed in different parts of the films, including the separated areas. It was proven that the separated areas in the blend films had higher contents of [60]PCBM than the rest of the films.     

Conversion efficiency improvement mechanisms of polymer solar cells by balance electron–hole mobility using blended P3HT:PCBM:pentacene active layer

To improve the power conversion efficiency of polymer solar cells, the blended P3HT:PCBM:pentacene active layer was used to balance hole–electron mobility and roughen surface. Using space-charged-limited current model to analyze the hole-only devices and the electron-only devices, the P3HT:PCBM:pentacene (weight ratio = 1:0.8:0.09) active layer exhibited balance hole–electron mobility. Compared with the power conversion efficiency of 3.46% of the conventional polymer solar cells using P3HT:PCBM (1:0.8) active layer, the power conversion efficiency of 4.42% was obtained. In other words, the power conversion efficiency was improved about 27.5%.     

Effect of TiO2 nanoparticle content in sol–gel derived TiO2 paste on the photovoltaic properties of TiO2 photoanode of dye-sensitized solar cells

In the present work, the effect of the amount of TiO₂ nanoparticles, added to the sol–gel derived paste, on the photovoltaic properties of fabricated dye-sensitized solar cells (DSSCs) was investigated. A titanium sol (Ti-sol) was synthesized using a Pechini type sol–gel method, and different pastes were prepared by adding various amounts of TiO₂ nanoparticles to the obtained Ti-sol. The pastes were used to fabricate the mesoporous TiO₂ semiconducting layers for DSSCs. It was observed that by increasing the mass ratio (MR) of TiO₂ nanoparticles to Ti-sol the thickness of TiO₂ layer increases. This led to the more adsorption of dye molecules per unit area of active TiO₂ layer, which were determined by UV–vis spectrophotometry. Also, micro-cracks were observed in TiO₂ layers obtained from pastes with low MR values. But their amount and size decreased with increasing MR, which was due to the decrease of paste surface tension (σ). As a result, short circuit current density (I_sc) showed continuous increase with increasing MR, which was due to the more dye adsorption. Open circuit voltage (V_oc) first increased and then decreased by enhancing MR, which was explained by considering the electron–hole recombination rate. Finally, the DSSC fabricated from the paste with MR=0.65 showed the maximum conversion efficiency (η).     

Effect of the chemical composition of Cu–In–Ga–Se layers on the photoconductivity and conversion efficiency of CdS/CIGSe solar cells

The effect of the [Ga]/[In+Ga] ratio of gallium and indium on the microwave photoconductivity of Cu–In–Ga–Se (CIGSe) films and on the efficiency of solar cells fabricated in accordance with the same technology is investigated. According to the observations of a field-emission scanning electron microscopy (FESEM), the grain size decreases with increasing Ga content. With increasing gallium content in the samples, the photogenerated-electron lifetime and the activation energy of the microwave photoconductivity also decrease. The changes in the activation energy of the through conduction in darkness are less than 20%. Analysis of the obtained data shows that the known effect of the gallium gradient on the efficiency should be associated with modification of the internal structure of grains instead of with their boundaries.     

Effect of UV light irradiation on photovoltaic characteristics of inverted polymer solar cells containing sol–gel zinc oxide electron collection layer

An inverted organic bulk-heterojunction solar cell containing a zinc oxide (ZnO) based electron collection layer with a structure of ITO/ZnO/[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM): regioregular poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxylenethiophene): poly(4-styrene sulfonic acid)/Au (ZnO cell) was fabricated. We examined the relationship between the heating temperature of the ZnO layer and the device performance under irradiation by simulated sunlight while cutting the UV light. The effects of the UV light contained in simulated sunlight were investigated by photocurrent–voltage (I–V) and alternating current impedance spectroscopy (IS) measurements. When the ZnO cells were irradiated with simulated sunlight, they exhibited a maximum power conversion efficiency (PCE) of over 3%, which hardly varied with the heating temperature of ZnO layers treated at 250 °C, 350 °C, and 450 °C. In contrast, when the ZnO cells were irradiated with simulated sunlight without UV content, their photovoltaic characteristics were very different. In the case of the cell with ZnO prepared by heating at 250 °C, PCE of 2.7% was maintained even under continuous irradiation with simulated sunlight without UV. However, for the cells with ZnO prepared by heating at 350 °C and 450 °C, the shapes of the I–V curves changed with the UV-cut light irradiation time, accompanying an increase in their series resistance. Overall, after UV-cut light irradiation for 1 h, the PCE of the cell with ZnO prepared by heating at 350 °C decreased to 1.80%, while that of the cell with ZnO prepared by heating at 450 °C fell to 1.35%. The photo IS investigations suggested that this performance change was responsible for the formation of charge-trapping sites at the ZnO/PCBM:P3HT interface which act as recombination centers for photo-produced charges in the PCBM:P3HT layer.     

Effect of sol–gel MgO spin-coating on the performance of TiO2-based dye-sensitized solar cells

This paper is concerned with the improvement of dye-sensitized solar cell (DSSC) efficiency upon MgO post-treatment of the TiO₂ electrode. A simple sol–gel technique, involving magnesium acetate as precursor, ethanol as solvent and nitric acid as stabilizer, is applied to prepare a solution of suspended MgO nanoparticles. A single drop of MgO sol at 0.1 M precursor concentration was spin-coated at 3000 rpm for 30 s onto the TiO₂ electrode and sintered at 500 K for 1 h. Dye-loading using N3-dye was applied for 6 h. An increase in the average efficiency of the DSSC from 2.5% to 3.9% (over 50% enhancement) was recorded. Measurements of the dark I–V characteristics, the open circuit voltage decays, the SEM images and the dye absorbance spectra, for both uncoated and MgO-coated electrodes were examined. The improvement of the DSSC efficiency was attributed to an upward shift of the TiO₂ flat band energy and a reduction of the rate of back-transport and recombination.     

Effects of ultraviolet–ozone treatment on organic-stabilized ZnO nanoparticle-based electron transporting layers in inverted polymer solar cells

Electron transporting layers (ETLs) in inverted polymer solar cells (I-PSCs) were fabricated by spin coating a colloidal dispersion of ZnO nanoparticles (NPs), and the effects of ultraviolet–ozone (UVO) treatment on the ZnO NP ETLs were investigated. The brief UVO treatment (<5 min) could considerably improve the performance of the resulting I-PSCs (∼30% increase in power conversion efficiency); whereas, excessive UVO treatment (>10 min) caused significant degradation. The characterization of the ZnO ETLs as a function of the UVO treatment duration revealed that brief treatment can remove the residual organic stabilizer molecules on the surface of the ZnO films by UV induced decomposition mechanism. However, excessive treatment can generate additional defects on/within the ZnO films, which can induce charge recombination. This effect was further confirmed by the thermal treatment of the ZnO ETLs at a high temperature (280 °C) at which the organic surfactants could be removed. Flexible I-PSCs were also fabricated using indium doped tin oxide coated plastic substrates and the usefulness of the room temperature UVO treatment was further confirmed in view of its potential applicability in flexible devices.     

Capacitance–voltage characteristics of P3HT:PCBM bulk heterojunction solar cells with ohmic contacts and the impact of single walled carbon nanotubes on them

Capacitance–voltage (C–V) characteristics of P3HT:PCBM devices of two different thicknesses are correlated with current density–voltage (J–V) characteristics. The rising portion of the C–V characteristics coincides with the exponential current density below the built-in voltage. The negative capacitance (NC) of these devices is a low frequency phenomenon and it occurs in trap-free space charge limited current (SCLC) regime. The onset frequencies of NC for devices with and without SWNTs also do not follow direct relation with effective mobility. The NC in thin devices has non-monotonic change with voltage for thin devices showing that interface state kinetics can be the reason for its occurrence. The NC of thick devices, on the other hand, increases monotonically with voltage showing that bulk properties dominate in these. Addition of SWNTs to these devices for efficiency improvement does not modify their built-in voltage. Also, the SWNTs do not affect the forward NC behaviour. However, the devices containing SWNTs show NC in reverse bias also which has different frequency dependence with voltage. The reverse bias NC is attributed to the large non-linear reverse current by charge injection into the additional energy levels introduced by SWNTs.     

Covalent bond–grafted soluble poly(o-methoxyaniline)-graphene oxide composite materials fabricated as counter electrodes of dye-sensitised solar cells

Covalent bond–grafted soluble poly(o-methoxyaniline)–functional graphene oxide (POMA-FGO) nanocomposite materials were prepared and doped with 1S-(+)-camphorsulfonic acid (CSA) as counter electrodes (CEs) for realising dye-sensitised solar cells (DSSCs). DSSCs with the POMA-FGO and POMA-FGO-CSA CEs exhibited substantially reduced interfacial impedance, which may be attributed to dual improvement from the well-dispersed FGO and the doping effect of CSA. Therefore, the efficiency levels of the DSSCs fabricated with the POMA-FGO CEs were higher than those of the DSSCs fabricated with the POMA CEs. When CSA was doped with the CEs, the efficiency of the DSSC was further enhanced. In particular, the efficiency of a DSSC based on the POMA-FGO (0.5%)-CSA (16%) CEs reached 8.81%, which was higher than that of a DSSC with a conventional platinum electrode. Therefore, POMA-FGO-CSA–based CEs can serve as a potential alternative to expensive platinum CEs.     

Role of the donor material and the donor–acceptor mixing ratio in increasing the efficiency of Schottky junction organic solar cells

Schottky junction organic solar cells (OSCs) employ a high work-function anode and an active layer comprised of fullerene and low concentrations of donor. In this study, the roles of the donor material and the donor–acceptor mixing ratio in Schottky junction OSCs are explored. The results show that the high short circuit current (J_sc) seen in Schottky junction OSCs at low donor concentrations arises primarily from photocurrent contributions from charge-transfer intermolecular states in C₆₀ aggregates. These aggregates absorb light at 400–600 nm and are thus well matched to the solar spectrum. The presence of the donor molecules is shown to be necessary for the dissociation of the C₆₀ aggregate excitons, which ultimately allows for enhanced photocurrents. The exciton dissociation process is governed primarily by the highest occupied molecular orbial (HOMO) energy level difference between the donor and C₆₀, and is only efficient when this difference is large enough for the energetically favorable transfer of holes from C₆₀ to the donor material. Increasing the donor concentration beyond a certain threshold hinders C₆₀ aggregate formation and thus removes its contribution to photocurrent completely. Furthermore, the V_oc is shown to be strongly influenced by the choice of donor material, indicating that it is not set by the Schottky junction barrier height as previously thought. In spite of this influence on V_oc, the choice of donor in the active layer does not appear to play a significant role in the extraction of holes from the Schottky junction organic solar cells. Optimized chlorine indium phthalocyanine (ClInPc) doped C₇₀ Schottky cells were fabricated to demonstrate a peak power conversion efficiency of 3.6%.     

Synthesis of ZnO and Fe2O3 nanoparticles by sol–gel method and their application in dye-sensitized solar cells

ZnO and Fe₂O₃ nanoparticles have been formed in a silica matrix, through the sol–gel method and were used as a photoanode to fabricate dye-sensitized solar cells (DSCs). The obtained oxides were characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscope and UV–visible absorption spectroscopy. The results indicate that ZnO and Fe₂O₃ prepared by this method may be used as photoanodes in photo-electro-chemical energy conversion systems. DSSCs have been built using eosin Y as photosensitizer and their photocurrent, open-circuit voltage, fill factor and efficiency have been measured under direct sunlight illumination (1000 Wcm^?2). A ZnO-film solar cell had the best performance with an open-circuit voltage of V_oc=0.7 V and short-circuit current density of I_sc=490 μA/cm². This was attributed to high optical gap energy and transparency of ZnO compared to Fe₂O₃. The effects of annealing temperature and concentration of Fe₂O₃ on conversion efficiency of the Fe₂O₃ based solar cell were also studied.     

Development of a new diindenopyrazine–benzotriazole copolymer for multifunctional application in organic field-effect transistors,polymer solar cells and light-emitting diodes

A new donor–acceptor (D?A) copolymer (PIPY–DTBTA) containing 6,12-dihydro-diindeno[1,2-b;1′,2′-e]pyrazine donor and benzotriazole acceptor was synthesized and characterized for multifunctional applications in organic field-effect transistors (OFETs), polymer solar cells (PSCs) and polymer light-emitting diodes (PLEDs). The polymer exhibits high molecular weights, excellent film-forming ability, a deep HOMO energy level, and good solution processability. Solution-processed thin film OFETs based on this polymer revealed good p-type characteristic with a high hole mobility up to 0.0521 cm² V^?1 s^?1. Bulk-heterojunction PSCs comprising this polymer and PC₆₁BM gave a power conversion efficiency (PCE) of 0.77%. The single-layer PLEDs based on PIPY–DTBTA emitted a yellow–red light with a maximum brightness of 385 cd m^?2 at the turn-on voltage of 6 V.     

Indoline as electron donor unit in “Push–Pull” organic small molecules for solution processed organic solar cells: Effect of the molecular π-bridge on device efficiency

In this work we have synthesized and characterized four indoline-based small organic molecules for their use as electron donor moiety in bulk-heterojunction solution processed organic solar cells combined with PC₇₀BM as electron acceptor. Our results show a wide range of light to energy efficiencies from 0.8% to 3.5% under standard measurement conditions. An initial analysis suggests that the main limitation is the device photocurrent due to the device film thickness. Yet, charge transfer dynamics were studied to correlate charge loss mechanisms to π-bridge structural variations and, moreover, mobility measurements were also carried out to fully explain these device limitations.     

Improved performance of surface functionalized TiO2/activated carbon for adsorption–photocatalytic reduction of Cr(VI) in aqueous solution

TiO₂ coated on activated carbon (TiO₂/AC) was synthesized by a facile sol-hydrothermal preparation. The as-prepared TiO₂/AC was modified by sequential treatment of nitric acid and thionyl chloride, followed by surface attaching of diethylenetriamine. The obtained materials were characterized by transmission electron microscopy, Fourier transform-infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, BET adsorption, and their adsorption–photocatalytic reduction activities were evaluated by the removal of aqueous Cr(VI). The results showed that TiO₂ particles were homogeneously deposited on the surface of activated carbon, and diethylenetriamine was grafted covalently onto the surface of TiO₂/AC through amide linkage. The diethylenetriamine-functionalized TiO₂/AC (TiO₂/AC-DETA) exhibits high adsorption capability for Cr(VI) through electrostatic, coordinative and hydrogen bonding interactions. The significant enhancement of photocatalytic activity of TiO₂/AC-DETA for reduction of Cr(VI) was observed, which was mainly attributed to the enrichment of Cr(VI) onto the surface of AC and successive fast interfacial transfer of Cr(VI) and photogenerated electrons resulted from the intimate contact between AC and TiO₂ through dense heterojunctions by forming of C–O–Ti linkages.     

Evaluation of the conversion efficiency of thin-film single-junction (a-Si:H) and tandem (μc-Si:H + a-Si:H) solar cells by analysis of the experimental dark and load current-voltage (I–V) characteristics

The aim of the study is to apply a method commonly used to determine the efficiency of multi-junction nanoheterostructure III?CV solar cells by analysis of the dark current-voltage (I?CV) characteristics to such an unconventional semiconducting material as amorphous silicon. a-Si:H and a-Si:H/??c-Si:H p-i-n structures without a light-scattering sublayer or an antireflection coating are studied. The results of measurements of the dark I?CV characteristics demonstrate that the voltage dependence of the current has several exponential portions. The conversion efficiency of solar cells (SCs) is calculated for each portion of the dark I?CV characteristic. This yields a dependence of the potential SC efficiency on the generation current density or on the photon flux. The observed agreement between the data derived from the experimental characteristics and results of calculations can be considered satisfactory and acceptable, thus the method suggested for measurement and analysis of dark I?CV characteristics and tested earlier on SCs based on crystalline III?CV compounds acquires a universal nature. The analysis of the characteristics of p-i-n amorphous silicon structures and the calculation of potential efficiencies, based on this analysis, extend the authors?? understanding of this class of devices and make it possible to improve the technology and photoconversion efficiency of SCs of this kind.