PCBM Solar Cells Research Articles

刮涂法制备聚合物薄膜太阳能电池

The poly(3-hexylthiophene):-phenyl C61-butyric acid methyl ester(P3HT∶PCBM) bulk-heterojunction polymer solar cells fabricated with doctor-blade coating were studied.It is found that the active layer thickness increases with increasing the coating speed or lowing substrate temperature,which is attributed to the changes of solution viscosity and surface tension.Moreover,the P3HT∶PCBM active layer produced by doctor-blade coating demonstrates fine donor-acceptor nanoscale phase separation favoring enhancing exciton dissociation and charge collection.A 4.2% P3HT∶PCBM solar cell was fabricated with doctor-blade coating technique.

Improving the electron mobility of TiO2 nanorods for enhanced efficiency of a polymer–nanoparticle solar cell

The poly(3-hexyl thiophene):TiO2 nanorod (P3HT:TiO2) solar cell has a better thermal stability than the P3HT:PCBM solar cell; however, the former has a lower power conversion efficiency (PCE) than the latter. We would like to enhance the PCE of P3HT:TiO2 solar cell by improving the electron mobility of anatase TiO2 nanorods. Two novel approaches: (1) ripening and (2) boron doping for TiO2 nanorods were explored. TiO2 nanorods were synthesized first by sol–gel process in the presence of an oleic acid surfactant at 98 °C for 10 h. The size of the TiO2 nanocrystal is about 35 nm in length and 5 nm in diameter. The insulating oleic acid on the TiO2 nanorods was replaced by pyridine (as-synthesized TiO2) for good compatibility and charge transport between P3HT and TiO2 in the application of hybrid P3HT:TiO2 nanorod solar cells. The crystallinity of the as-synthesized TiO2 nanorods was increased through ripening (120 °C, 24 h) by using an autoclave reactor while the size of the nanocrystals was not significantly changed. Boron doped TiO2 nanorods (B-doped TiO2) were synthesized using the same sol–gel process of as-synthesized TiO2 nanorods but by replacing 0.7 at.% Ti with B using boron n-butoxide instead of titanium tetraisopropoxide. The UV-Vis spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses indicate the B is present in TiO2 nanorods as substitutional defects which can be either Ti–O–B or O–Ti–B bonding, with a B 1 s binding energy of 192.1 eV. The ripening process is more effective at increasing the crystallinity of TiO2 nanorods than boron doping, as shown by XRD and Raman spectroscopy. The electron mobility of the TiO2 nanorods is improved from 6.21×10−5 to 2.33×10−4 (cm2 V−1 s) and 5.27×10−4 (cm2 V−1 s) for ripened TiO2 and B-doped TiO2, respectively, as compared with as-synthesized TiO2. The PCE of P3HT:TiO2 solar cells was increased by 1.31 times and 1.79 times under A. M. 1.5 illumination for ripened and B-doped TiO2, respectively, as compared with as-synthesized TiO2. The B-doped TiO2 has the highest mobility and PCE, mainly due to the presence of partially reduced Ti4+ by boron atom with delocalized electrons.

The effect of side-chain length on regioregular poly[3-(4-n-alkyl)phenylthiophene]/PCBM and ICBA polymer solar cells

The photovoltaic, charge transport, light absorption and morphology properties of a series of poly[3-(4-n-alkyl)phenylthiophene] (PAPT):acceptor (phenyl-C61-butyric acid methyl ester (PCBM) or indene-C60 bisadduct (ICBA)) blend films are reported as a function of alkyl side-chain length. Regioregular poly[3-(4-n-butyl)phenylthiophene] (PBPT), poly[3-(4-n-hexyl)phenylthiophene] (PHPT), poly[3-(4-n-octyl)phenylthiophene] (POPT), and poly[3-(4-n-decyl)phenylthiophene] (PDPT) were successfully synthesized by a modified Grignard metathesis (GRIM) polymerization method. The effects of the alkyl side-chain length on the optical, electrochemical and structural properties of the polymers were carefully investigated to establish a relationship between the molecular structure and device function. Bulk heterojunction solar cells made of PAPTs blended with either PCBM or ICBA showed a strong photovoltaic property dependence on the alkyl side-chain length. Among all PAPTs used in this study, the best performance was observed in PHPT-based devices. This is the first report of the use of PHPT in polymer solar cells. In addition, PAPT devices blended with ICBA generally showed higher open-circuit voltages (Voc) and power conversion efficiencies than PCBM-based devices. For example, a PHPT:ICBA photovoltaic device showed a Voc of 0.79 V and a power conversion efficiency of 3.7%, which is the highest performance reported thus far using PAPT polymers. While the optical properties of PAPT/electron acceptor films exhibit the strongest effect on the short-circuit current of the devices, a balance between the optical, electrical and morphological properties of blended films caused by the alkyl side-chain length determines the overall photovoltaic performance of these devices. Therefore, our work provides a fundamental understanding of the molecular structure–device function relationship, especially with respect to changes in the alkyl side-chain length in conjugated polymers.

Non-Langevin bimolecular recombination in a silole-based polymer:PCBM solar cell measured by time-resolved charge extraction and resistance-dependent time-of-flight techniques

The silole-based non-Langevin conjugated polymer KP115 has been used to demonstrate that circuit resistance is a crucial parameter in time-of-flight measurements of organic photovoltaic cells, providing a resistance-dependent bimolecular recombination coefficient. The origin of this behaviour is the biphasic decay dynamics present in KP115:PCBM devices (observed using a novel time-dependent charge extraction technique), which time-of-flight cannot accurately characterise.

Bis-EH-PFDTBT:PCBM solar cells: A compositional, thickness, and light-dependent study

Poly{[2,7-(9,9-bis-(2-ethylhexyl)-fluorene)]-alt-[5,5-(4,7-di-2′-thienyl-2,1,3-benzothiadiazole)]}:PCBM bulk heterojunction solar cells were fabricated and characterized under different incident light power intensities. Charge-trapping effects take place at low fullerene content in the photoactive blend; an efficient polymer fullerene intermixing with formation of continuous phases is reached at a donor:acceptor ratio of 1:4. For an optimized active layer thickness of 100 nm, a power-conversion efficiency of 2.57% was obtained. Photocurrent measurements under reverse-bias conditions show that a high percentage of the photogenerated excitons does not lead to the formation of free carriers, thus representing the major limiting factor for the device’s efficiency.

Solution‐Processed Metallic Nanowire Electrodes as Indium Tin Oxide Replacement for Thin‐Film Solar Cells

AbstractSolution processed silver nanowire (Ag NW) films are introduced as transparent electrodes for thin‐film solar cells. Ag NW electrodes were processed by doctor blade‐coating on glass substrates at moderate temperatures (less than 100 °C). The morphological, optical, and electrical characteristics of these electrodes were investigated as a function of processing parameters. For solar‐cell application, Ag NW electrodes with an average transparency of 90% between 450 and 800 nm and a sheet resistivity of ≈10 Ω per square were chosen. The performance of poly(3‐hexylthiophen‐2,5‐diyl):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) solar cells on Ag NW electrodes was found to match the performance of otherwise identical cells on indium tin oxide. Overall, P3HT:PCBM solar cells with an efficiency of 2.5% on transparent Ag NW electrodes have been realized.

Thermal Stability of Poly[2-methoxy-5-(2′-phenylethoxy)-1,4-phenylenevinylene] (MPE-PPV):Fullerene Bulk Heterojunction Solar Cells

To improve the thermal stability of polymer:fullerene bulk heterojunction solar cells, a new polymer, poly[2-methoxy-5-(2′-phenylethoxy)-1,4-phenylenevinylene] (MPE-PPV), has been designed and synthesized, which showed an increased glass transition temperature (Tg) of 111 °C. The thermal characteristics and phase behavior of MPE-PPV:[6,6]-phenyl C61-butyric acid methyl ester ([60]PCBM) blends were investigated by means of modulated temperature differential scanning calorimetry and rapid heating–cooling calorimetry. The thermal stability of MPE-PPV:[60]PCBM solar cells was compared with devices based on the reference MDMO-PPV material with a Tg of 45 °C. Monitoring of the photocurrent–voltage characteristics at elevated temperatures revealed that the use of high-Tg MPE-PPV resulted in a substantial improvement of the thermal stability of the solar cells. Furthermore, a systematic transmission electron microscope study of the active polymer:fullerene layer at elevated temperatures likewise demonstrated a more stable morphology for the MPE-PPV:[60]PCBM blend. Both observations indicate that the use of high-Tg MPE-PPV as donor material leads to a reduced free movement of the fullerene molecules within the active layer of the photovoltaic device. Finally, optimization of the PPV:fullerene solar cells revealed that for both types of devices the use of [6,6]-phenyl C71-butyric acid methyl ester ([70]PCBM) resulted in a substantial increase of current density and power conversion efficiency, up to 3.0% for MDMO-PPV:[70]PCBM and 2.3% for MPE-PPV:[70]PCBM.

Post-fabrication annealing effects on the performance of P3HT:PCBM solar cells with/without ZnO nanoparticles

In this study, P3HT:PCBM organic photovoltaic (OPV) devices, with or without ZnO nanoparticles buffer layer between the photoactive layer (P3HT:PCBM) and the cathode (Al top electrode), were fabricated. The devices were annealed at 145°C either before or after depositing the top electrode. The objective of this study was to investigate the effects of the ZnO buffer layer and pre-/post-fabrication annealing on the general performance of these devices. The short-circuit current density (JSC), open-circuit voltage (VOC) and the external quantum efficiency (EQE) of the OPV devices were improved by the insertion of the ZnO layer and post-fabrication annealing. The post-fabrication annealed devices, with or without the ZnO layer, exhibited higher values of JSC, VOC and EQE than those of similar devices annealed before depositing the Al metal. This can be attributed to, among other things, improved charge transport across the interface between the photoactive layer and the Al top electrode as a result of post-annealing induced modification of the interface morphology.

Dielectric response of doped organic semiconductor devices: P3HT:PCBM solar cells

Dielectric response of doped organic semiconductor devices: P3HT:PCBM solar cells

Effect of blend composition in BisEH-PFDTBT:PC 70BM solar cells

The photovoltaic properties of bulk-heterojunction solar cells with an active layer made of an alternating fluorene copolymer, poly{[2,7-(9,9-bis-(2-ethylhexyl)-fluorene)]-alt-[5,5-(4,7-di-2′-thienyl-2,1,3-benzothiadiazole)]}, and [6,6]-phenyl-C 71-butyric acid methylester are investigated by varying the blend composition and the incident light power intensity. An efficient donor–acceptor intermixing is reached with donor to acceptor weight ratios of at least 1:3, while at lower acceptor content charge trapping effects are evidenced by the light intensity dependent study. Reverse bias analysis demonstrates that a major limiting factor for the performance of the investigated solar cells is represented by the poor ability of the generation of free charge carriers, attributed to the low hole mobility in the polymer phase.

Area and Light Intensity Dependence of Buffer Layers on P3HT:PCBM Solar Cells

Polymer solar cells have been fabricated with buffer layers to enhance the charge extraction toward the electrodes. We investigated the active area and the light intensity dependences in poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells with different hole buffer layers, molybdenum oxide (MoO3) and poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The short-circuit current (JSC) for the MoO3-based device is nearly independent of the active area and the light intensity whereas the JSC and light intensity dependence for PEDOT:PSS-based device are strongly affected by the active areas below 20 mm. These dependences on the active area and the light intensity can be explained by the lateral conductivity characteristics of the hole buffer layers, and compared to PEDOT:PSS, MoO3 can provide efficient and stable hole extraction regardless of active area and light intensity.

Intra‐Molecular Donor–Acceptor Interaction Effects on Charge Dissociation, Charge Transport, and Charge Collection in Bulk‐Heterojunction Organic Solar Cells

AbstractThis article reports experimental studies on internal charge dissociation, transport, and collection by using magnetic field effects of photocurrent (MFEPC) and light‐assisted dielectric response (LADR) in highly‐efficient organic solar cells based on photovoltaic polymer PTB2 and PTB4 with intra‐molecular “donor–acceptor” interaction. The MFEPC at low‐field (< 150 mT) indicates that intra‐molecular “donor‐acceptor” interaction generates charge dissociation in un‐doped PTB2 and PTB4 films, which is similar to that in lightly doped P3HT (Poly(3‐hexylthiophene)) with 5 wt% PCBM (1‐(3‐methyloxycarbonyl)‐propyl‐1‐phenyl (6,6) C61). After PTB2 and PTB4 are mixed with PCBM to form bulk‐heterojunctions, the MFEPC at high‐field (> 150 mT) reveals that the charge‐transfer complexes formed at PTB2:PCBM and PTB4:PCBM interfaces have much lower binding energies due to stronger electron‐withdrawing abilities, as compared to the P3HT:PCBM device, towards the generation of photocurrent. Furthermore, the light‐assisted dielectric response: LADR indicates that the PTB2:PCBM and PTB4:PCBM solar cells exhibit larger capacitances relative to P3HT:PCBM device under photoexcitation. This reflects that the PTB2:PCBM and PTB4:PCBM bulk heterojunctions have more effective charge transport and collection than the P3HT:PCBM counterpart. As a result, our experimental results indicate that intra‐molecular “donor‐acceptor” interaction plays an important role to enhance charge dissociation, transport, and collection in bulk‐heterojunction organic solar cells.

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 TiO 2 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 TiO 2 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/cm 2 light intensity under AM 1.5 conditions.

On Voltage, Photovoltage, and Photocurrent in Bulk Heterojunction Organic Solar Cells

The interpretation of voltage, photovoltage, and photocurrent in polymer/fullerene bulk heterojunction (BHJ) solar cells is discussed in terms of fundamental device models and results of capacitance spectroscopy. First we establish the relationship between the applied voltage (which is the difference of Fermi levels) and the variation of electrostatic potential that governs the drift field. We then show the most common distribution of carriers and Fermi levels in the blend layer, supported on experimental results of impedance spectroscopy of P3HT:PCBM solar cells. We arrive at the conclusion that charge separation and charge transportation has very little to do with a built-in electric field between metal contacts, while kinetics plays a major role in photocurrent production. Finally, we discuss the key factors relevant to understand device properties and power conversion efficiencies of the BHJ solar cells: recombination, charge generation, and the current–potential curve, based on the suggested model that emphasizes mobile electrons and holes (that we term quasifree carriers) contributing to the respective Fermi levels.

Increasing the efficiency of polymer solar cells by silicon nanowires

Silicon nanowires have been introduced into P3HT:[60]PCBM solar cells, resulting inhybrid organic/inorganic solar cells. A cell efficiency of 4.2% has been achieved, which is arelative improvement of 10% compared to a reference cell produced without nanowires.This increase in cell performance is possibly due to an enhancement of the electrontransport properties imposed by the silicon nanowires.In this paper, we present a novel approach for introducing the nanowires by mixing theminto the polymer blend and subsequently coating the polymer/nanowire blend onto asubstrate. This new onset may represent a viable pathway to producing nanowire-enhancedpolymer solar cells in a reel to reel process.

Performance improvement of MEH-PPV:PCBM solar cells using bathocuproine and bathophenanthroline as the buffer layers

In this work, bathocuproine (BCP) and bathophenanthroline (Bphen), commonly used in small-molecule organic solar cells (OSCs), are adopted as the buffer layers to improve the performance of the polymer solar cells (PSCs) based on poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction. By inserting BCP or Bphen between the active layer and the top cathode, all the performance parameters are dramatically improved. The power conversion efficiency is increased by about 70% and 120% with 5-nm BCP and 12-nm Bphen layers, respectively, when compared with that of the devices without any buffer layer. The performance enhancement is attributed to BCP or Bphen (i) increasing the optical field, and hence the absorption in the active layer, (ii) effectively blocking the excitons generated in MEH-PPV from quenching at organic/aluminum (Al) interface due to the large band-gap of BCP or Bphen, which results in a significant reduction in series resistance (Rs), and (iii) preventing damage to the active layer during the metal deposition. Compared with the traditional device using LiF as the buffer layer, the BCP-based devices show a comparable efficiency, while the Bphen-based devices show a much larger efficiency. This is due to the higher electron mobility in Bphen than that in BCP, which facilitates the electron transport and extraction through the buffer layer to the cathode.

Study of the Cesium Carbonate (Cs2CO3) Inter Layer Fabricated by Solution Process on P3HT:PCBM Solar Cells

In this paper, we studied the effect of the electron injection layer, Cesium carbonate (Cs2CO3), thickness on the performance of organic solar cell (OSC) based on blends of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester fullerene derivative (PCBM). The polymer solar cell consists of molybdenum-oxide (MoO3) as a hole injection layer, P3HT and PCBM bulk hetero junction as an active layer, and Cesium carbonate (Cs2CO3) as an electron injection layer. We measured each device by current-voltage measurement and impedance spectroscopy which is widely used for equivalent circuit analysis of solid state structures. The device with the Cs2CO3 layer showed about 8–10% higher JSC and about 6–8% higher power conversion efficiency compared with the devices without the Cs2CO3 layer.

Improved efficiency of MEH-PPV:PCBM solar cells by the use of ZnS nano-particles

Zinc sulphide (ZnS) nano-particles (~10 nm) have been synthesized and incorporated in poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) for photovoltaic applications. Incorporation of ZnS nano-particles in MEH-PPV improved the photovoltaic performance, as they work as electron acceptors and provide better electron transport in the matrix. ZnS nano-particles have also been dispersed in MEH-PPV along with phenyl [6,6′] C61 butyric acid methyl ester (PCBM). Incorporation of optimum amount of ZnS nano-particles in MEH-PPV:PCBM system resulted into further increase in the power conversion efficiency compared to that in their absence. This improvement in the efficiency has been attributed to additional better pathways for electron transportation.

ITO-free flexible inverted organic solar cell modules with high fill factor prepared by slot die coating

We report on the fabrication of inverted ITO-free P3HT:PCBM solar cell modules on glass and PET foil as substrate where the organic functional layers are deposited with slot die coating, a reel to reel compatible coating technique. The active layers have been processed in ambient atmosphere, which will be of advantage in a future production and is especially remarkable as the metallic cathode is already deposited on the substrate at this stage of fabrication. The modules comprise two busses of 11 cell elements connected in series each. The series connection leads to an open circuit voltage of up to 6.88 V on glass substrate, which translates to 625 mV per cell element, a very competitive value for P3HT:PCBM based solar cells on glass. Although the designated area is as large as 41 cm 2 and the active area 26.4 cm 2, we obtain fill factors of up to 65% for these modules, which again is a typical value for small area laboratory cells. Remarkably the values for PET foil as substrate with an open circuit voltage of 6.5 V and a fill factor of 64% are very close to the results on glass and to our knowledge the highest fill factors for flexible organic solar cells, even if compared to small area devices. The short circuit current densities and therefore efficiencies are also comparable to small area devices, if only the photoactive area is accounted for. Therefore we have demonstrated that the scale up of organic solar cells can be achieved with a suited circuitry scheme.

Transient photocurrent measurements of PCDTBT:PC70BM and PCPDTBT:PC70BM Solar Cells: Evidence for charge trapping in efficient polymer/fullerene blends

We report measurements of the turn-on and turn-off photocurrent dynamics as a function of applied voltage for efficient polymer/fullerene bulk heterojunction solar cells composed of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT): [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT):PC70BM blends. In particular we present evidence for charge trapping that facilitates recombination in these systems. For the PCDTBT:PC70BM system, an initial transient photocurrent peak 5–10 μs after turn-on is observed for operating voltages between 0.5 V and open-circuit. Furthermore, a long photocurrent tail is observed in the decay dynamics of PCDTBT:PC70BM devices with charge still being extracted hundreds of microseconds after turn-off. These features in the PCDTBT:PC70BM device are attributed to trapping and detrapping of charge on the microsecond time scale, with charge trapping facilitating recombination either through trap-assisted recombination or space-charge effects. For the PCPDTBT:PC70BM system, evidence for charge trapping is also observed albeit on a faster time scale. No initial transient photocurrent peak is observed, however the faster PCPDTBT:PC70BM decay dynamics show only a weak voltage dependence consistent with rapid trapping and recombination of charge. For both systems the amount of extracted charge as a function of applied voltage follows a similar form to the measured current-voltage curves providing evidence that photocurrent is hampered by the extraction, and not just the separation, of charge in these systems. The origin of charge trapping and the nature of recombination is discussed, along with the influence of additives on charge transport in the PCPDTBT:PC70BM system.