PCBM Solar Cells Research Articles

Recent Modeling of MDMO-PPV: PCBM Solar Cells versus Mixture Ratio, Electric Field and Incidence Angle with Hopping Model

It is a matter of controversy why excitons can efficiently dissociate into free carriers at an intrinsic polymer/fullerene interface. While extensive characterization is performed in the course of many reported experimental studies, correlation of performance and physical parameters among studies done in different laboratories is low, pointing out the need to address some aspects of BHJSC active materials that have received relatively some attention. In this paper, we discuss the modeling of MDMO-PPV/PCBM(Poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene)/ soluble C60 derivative, methanofullerene, [6,6]-phenyl C61-butyric acid methyl ester organic devices. In our approach, we apply two recent methods so called transfer matrix method and hopping model to calculate the exciton dissociation probability, and photocurrent density versus mixture ratio, electric field and angle of incidence. The results show that EDP (exciton dissociation probability) in solar cells without PEDOT-PSS (Poly (ethylendioxythiophene)-Poly(styrene sulfonic acid)) HTL hole transporting layer is better than the cells with additional layer in enhancing the performance of MDMO-PPV/PCBM solar cells. When the weight ratio of MDMO-PPV is less than 3:5 and 2:5 respectively, the best exciton dissociation probability, and photocurrent density of solar cell is obtained.

Influence of bridging atom on the vertical phase separation of low band gap bulk heterojunction solar cells

Vertical phase separation of the polymer and fullerene molecules in bulk heterojunction organic solar cells influences the exciton dissociation, charge carrier transport and collection. This work compares the vertical phase separation of poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b:3,4-b′]dithiophene-2,6-diyl]] (C-PCPDTBT):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) and poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b:3,4-b′]dithiophene-siloe2,6-diyl]] (Si-PCPDTBT):PC71BM blend films, using X-ray photoemission spectroscopy depth profiles. The difference between the two polymers is the bridging atom, which is carbon for C-PCPDTBT and silicon for Si-PCPDTBT. Si-PCPDTBT exhibits enhanced polymer chain packing and crystallinity. We believe this enhanced chain packing provides a driving force during film drying which alters the vertical morphology. The different nature of vertical phase separation plays a role in determining the increased device performance observed for Si-PCPDTBT:PC71BM solar cells. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Enhanced performance of polymer:fullerene bulk heterojunction solar cells upon graphene addition

Graphene has potential for applications in solar cells. We show that the short circuit current density of P3HT (Poly(3-hexylthiophene-2,5-diyl):PCBM((6,6)-Phenyl C61 butyric acid methyl ester) solar cells is enhanced by 10% upon the addition of graphene, with a 15% increase in the photon to electric conversion efficiency. We discuss the performance enhancement by studying the crystallization of P3HT, as well as the electrical transport properties. We show that graphene improves the balance between electron and hole mobilities with respect to a standard P3HT:PCBM solar cell.

Hybrid organic solar cells using both ZnO and PCBM as electron acceptor materials

In this paper, we studied the effect of the addition of zinc oxide (ZnO) nanoparticles to the active layer of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) solar cells. Devices with varying ratios of ZnO and PCBM were fabricated while maintaining a fixed concentration of P3HT. This effect was investigated in different solvents, specificaly chlorobenzene (CB) and 1,2-dichlorobenzene (DCB). The addition of ZnO nanoparticles was found to significantly increase the power conversion efficiency (PCE) in the P3HT:PCBM solar cells. The mixing of ZnO in the active layer using CB as the solvent introduced a red shift in the absorption spectra and enhanced the absorption in the visible region. The incorporation of ZnO nanoparticles was also found to increase the surface roughness of the active layer. ZnO nanoparticles agglomerated as their concentration relative to PCBM increased and completely agglomerated in the absence of the fullerene derivative.

Fine Tuning of Polymer Properties by Incorporating Strongly Electron-Donating 3-Hexyloxythiophene Units into Random and Semi-random Copolymers

Two series of copolymers containing varying percentages of strongly electron-donating 3-hexyloxythiophene units (3HOT), namely the random poly(3-hexylthiophene-co-3-hexyloxythiophene)s (P3HT-co-3HOTs) and the semi-random poly(3-hexylthiophene–thiophene–3-hexyloxythiophene–diketopyrrolopyrrole)s (P3HTT-HOT-DPPs), were synthesized via Stille polymerization. The influence of 3HOT content on UV–vis absorption, HOMO energy levels, polymer crystallinity, and polymer:PC61BM solar cell performance, especially the open-circuit voltage (Voc), was investigated. Importantly, introduction of the strong donor 3HOT results in regularly decreased band gaps and broadened absorption compared to the corresponding parent polymers, regioregular poly(3-hexylthiophene) (P3HT) and semi-random poly(3-hexylthiophene–thiophene–diketopyrrolopyrrole) (P3HTT-DPP). The HOMO energies of both random P3HT-co-3HOT and semi-random P3HTT-HOT-DPP copolymers increase significantly with 3HOT incorporation, from −5.2 eV to around −4.95 eV with h...

Charge carrier generation and transport in different stoichiometry APFO3:PC61BM solar cells.

In this paper we studied carrier drift dynamics in APFO3:PC61BM solar cells of varied stoichiometry (2:1, 1:1, and 1:4 APFO3:PC61BM) over a wide time range, from subpicoseconds to microseconds with a combination of ultrafast optical electric field probing and conventional transient integrated photocurrent techniques. Carrier drift and extraction dynamics are strongly stoichiometry dependent: the speed of electron or hole drift increases with higher concentration of PC61BM or polymer, respectively. The electron extraction from a sample with 80% PC61BM takes place during hundreds of picoseconds, but slows down to sub-microseconds in a sample with 33% PC61BM. The hole extraction is less stoichiometry dependent: it varies form sub-nanoseconds to tens of nanoseconds when the PC61BM concentration changes from 33% to 80%. The electron extraction rate correlates with the conversion efficiency of solar cells, leading to the conclusion that fast electron motion is essential for efficient charge carrier separation preventing their geminate recombination.

Improved efficiency of P3HT:PCBM solar cells by incorporation of silver oxide interfacial layer

In recent years, a substantial amount of research has been focused on identifying suitable interfacial layers in organic light-emitting diodes and organic solar cells which has efficient charge transport properties. In this work, a very thin layer of AgOx is deposited on top of the ITO layer along with PEDOT:PSS and is observed that the solar cells having the AgOx interfacial layer showed a 28% increase in power conversion efficiency in comparison to that of the control cell. The enhancement in efficiency has been ascribed to improvements in fill factor as well as the increase in shunt resistance and decrease in the series resistance of the solar cells. An equivalent circuit model is also provided to understand the changes in the series and shunt resistances in the AgOx modified devices.

Improved performance of P3HT:PCBM solar cells by both anode modification and short-wavelength energy utilization using Tb(aca)3phen

The performance of P3HT:PCBM solar cells was improved by anode modification using spin-coated Tb(aca)3phen ultrathin films. The modification of the Tb(aca)3phen ultrathin film between the indium tin oxide (ITO) anode and the PE-DOT:PSS layer resulted in a maximum power conversion efficiency (PCE) of 2.99% compared to 2.66% for the reference device, which was due to the increase in the short-circuit current density (Jsc). The PCE improvement could be attributed to the short-wavelength energy utilization and the optimized morphology of the active layers. Tb(aca)3phen with its strong down-conversion luminescence properties is suitable for the P3HT:PCBM blend active layer, and the absorption region of the ternary blend films is extended into the near ultraviolet region. Furthermore, the crystallization and the surface morphology of P3HT:PCBM films were improved with the Tb(aca)3phen ultrathin film. The ultraviolent—visible absorption spectra, atomic force microscope (AFM), and X-ray diffraction (XRD) of the films were investigated. Both anode modification and short-wavelength energy utilization using Tb(aca)3phen in P3HT:PCBM solar cells led to about a 12% PCE increase.

Electron Collection as a Limit to Polymer:PCBM Solar Cell Efficiency: Effect of Blend Microstructure on Carrier Mobility and Device Performance in PTB7:PCBM

The poor photovoltaic performance of state‐of‐the‐art blends of poly[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl] (PTB7) and [6,6]‐phenyl‐C61‐butyric acid (PCBM) at large active layer thicknesses is studied using space‐charge‐limited current mobility and photovoltaic device measurements. The poor performance is found to result from relatively low electron mobility. This is attributed to the low tendency of PTB7 to aggregate, which reduces the ability of the fullerene to form a connected network. Increasing the PCBM content 60–80 wt% increases electron mobility and accordingly improves performance for thicker devices, resulting in a fill factor (FF) close to 0.6 at 300 nm. The result confirms that by improving only the connectivity of the fullerene phase, efficient electron and hole collection is possible for 300 nm‐thick PTB7:PCBM devices. Furthermore, it is shown that solvent additive 1,8‐diiodooctane (DIO), used in the highest efficiency PTB7:PCBM devices, does not improve the thickness dependence and, accordingly, does not lead to an increase in either hole or electron mobility or in the carrier lifetime. A key challenge for researchers is therefore to develop new methods to ensure connectivity in the fullerene phase in blends without relying on either a large excess of fullerene or strong aggregation of the polymer.

Cyclic potential growth mechanism for electropolymerized polythiophenes as anode buffer layers in P3HT–PCBM solar cells

A new method for electropolymerization of polythiophenes as anode buffer layer in combination with a PEDOT:PSS buffer in P3HT–PCBM solar cells is introduced. This approach uses a stack of PEDOT:PSS and electropolymerized polythiophene (ePT) as an anode buffer layer. A significant improvement in the short circuit current is seen in solar cells with additional thin buffer layers of ePT grown on top of PEDOT:PSS. The cells with ePT buffer layer grown with constant potential mode compared to solar cells without additional ePT buffer layer have an 18.7% increase in power conversion efficiency. However, the cells with ePT grown with cyclic potential mode result in an additional enhancement in short circuit current with a further increase of 11.7% in power conversion efficiency. This improvement is attributed to better quality of the buffer layer obtained by cyclic potential electropolymerization technique with respect to film quality and area coverage.

Origin of the fill factor loss in bulk-heterojunction organic solar cells

In this paper, the decrease of fill factor with increase of light illumination intensity was investigated through light assisted impedance studies based on ITO/poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate)/[poly(3-hexylthiophene):1-(3-methyloxycarbonyl)-propyl-1-phenyl(6,6)C61 (P3HT:PCBM)]/Ca/Al solar cells. It was found that the charge accumulation at the electrode interface causes a decrease in parallel resistance (Rp), consequently a reduction in fill factor. Moreover, it has also been found that the photocurrent loss near the open-circuit voltage through bimolecular recombination also leads to the reduction of fill factor with increasing light intensity in P3HT:PCBM solar cells.

Degradation of electrical properties of PTB1:PCBM solar cells under different environments

We investigate the degradation of bulk heterojunction solar cells based on the polymer poly((4,8-bis (octyloxy) benzo (1,2-b:4,5-b׳) dithiophene-2,6-diyl) (2-((dodecyloxy) carbonyl) thieno(3,4-b) thiophenediyl)) (PTB1) and the fullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) under different environments: dry nitrogen atmosphere (H2O<0.1ppm, O2<0.1ppm), air (60±5% relative humidity) and under encapsulation in accordance with established ISOS-D-1 protocols. The evolution of the electrical measurements under dark and illumination conditions is used to analyze the degradation process during 5300h and its relationship with the physical mechanisms. The degradation in the efficiency of the solar cells is mainly due to a reduction of short circuit current density (JSC) and fill factor (FF) while open circuit voltage (VOC) is the most stable parameter in all studied conditions. The TS80 lifetimes for power conversion efficiency (PCE) of samples exposed to nitrogen and air environments and under encapsulation were 990h, 4h and 48h, respectively. The analysis of the PCE decay permits to identify and evaluate the intensity of different degradation mechanisms (intrinsic polymer chemical reactions or by diffused environmental oxygen or water) in the different conditions.

Plasmonic ITO-free polymer solar cell.

The aluminum and sliver multilayered nano-grating structure is fabricated by laser interference lithography and the intervals between nanoslits is filled with modified PEDOT:PSS. The grating structured transparent electrode functions as the anti-reflection layer which not only decreases the reflected light but also increases the absorption of the active layer. The performances of P3HT:PC₆₁BM solar cells are studied experimentally and theoretically in detail. The field intensities of the transverse magnetic (TM) and transverse electrical (TE) waves distributed in the active layer are simulated by rigorous coupled wave analysis (RCWA). The power conversion efficiency of the plasmonic ITO-free polymer solar cell can reach 3.64% which is higher than ITO based polymer solar cell with efficiency of 3.45%.

Improvement in photovoltaic performance of anthracene-containing PPE–PPV polymer-based bulk heterojunction solar cells with silver nanoparticles

In this study, we investigated the effect of silver nanoparticles (Ag NPs) in the active layer of anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene):phenyl-C61-butyric acid methyl ester (AnE-PVstat:PCBM)-based bulk heterojunction solar cells. By incorporating Ag NPs of 6 nm in diameter, the power conversion efficiency of AnE-PVstat:PCBM solar cells was improved to 3.10 % from a value of 2.46 % for the solar cells fabricated without nanoparticles. Moreover, ISOS-L-1 stability test showed that the lifetime of the cells was also significantly improved by doping Ag NPs into the active layer.

Polymer/cathode interface barrier limiting the open circuit voltage in polymer:fullerene organic bulk heterojunction solar cells: A quantitative analysis

By exploring the origin of the saturation current (J0), we show that the polymer/cathode interface determines the open circuit voltage (Voc) in polymer:fullerene bulk heterojunction solar cells. We fabricate and characterize P3HT:PCBM solar cells and compare their electrical characteristics using as reference P3HT/cathode Schottky diodes. We demonstrate with thermionic emission theory that J0 has the same origin in both devices. Thus, a general barrier-dependent expression for Voc, valid for non-ohmic cathode contacts is proposed and discussed.

Determination of the intrinsic and the injection dependent charge carrier density in organic solar cells using the Suns-VOC method

A method is presented to calculate the intrinsic and injection dependent average charge carrier density from the transport resistance of an organic solar cell. The latter is determined using a combination of a Suns-VOC and a current-voltage measurement under illumination which allows to split the total series resistance of the solar cell into an (ohmic) contribution of the circuitry and an injection dependent part which is caused by the transport of the charge carriers through the photoactive layer. In the derivation of the formula for the average charge carrier density, spatially homogeneous generation rate and gradients of the quasi Fermi levels as well as balanced mobilities had to be assumed. However, numerical simulations revealed that even for strongly inhomogeneous generation and unbalanced mobilities the results are reasonably accurate proving the practical applicability of the presented method. Using an inverted ITO-free P3HT:PCBM solar cell, we determined a value for the intrinsic charge carrier density of ni = 2.88 × 1011 cm−3 at T = 300 K and for the average density of mobile charge carriers at “1 sun” under open circuit conditions we obtained nav = 3.2 × 1016 cm−3.

Describing the light intensity dependence of polymer:fullerene solar cells using an adapted Shockley diode model

Solar cells are generally optimised for operation under AM1.5 100 mW cm(-2) conditions. This is also typically done for polymer solar cells. However, one of the entry markets for this emerging technology is portable electronics. For this market, the spectral shape and intensity of typical illumination conditions deviate considerably from the standard test conditions (AM1.5, 100 mW cm(-2), at 25 °C). The performance of polymer solar cells is strongly dependent on the intensity and spectral shape of the light source. For this reason the cells should be optimised for the specific application. Here a theoretical model is presented that describes the light intensity dependence of P3HT:[C60]PCBM solar cells. It is based on the Shockley diode equation, combined with a metal-insulator-metal model. In this way the observed light intensity dependence of P3HT:[C60]PCBM solar cells can be described using a 1-diode model, allowing fast optimization of polymer solar cells and module design.

High efficiency P3HT:PCBM solar cells with an inserted PCBM layer

By inserting a PCBM (phenyl-C61-butyric acid methyl ester) layer between a P3HT (poly(3-hexylthiophene)):PCBM blend and a Ca/Al cathode, the performance of P3HT:PCBM bulk-heterojunction polymer solar cells is greatly improved. The maximum power conversion efficiency reached 4.24%, which is much higher than that of the traditional standard P3HT:PCBM based device (3.57%). By exploring various experimental techniques including absorption spectroscopy, X-ray photoelectron spectroscopy and impedance spectroscopy, we have found that the enhancement of the device performance can mainly be attributed to two reasons: (1) the inserted-PCBM layer can enhance the overall light absorption of the whole device and hence improve the photocurrent. (2) The inserted-PCBM layer can increase the amount of PCBM at the interface between the active layer and the cathode electrode, and hence suppress carrier recombination and facilitate electron extraction.

Effect of Organically-Modified Titania Nanoparticles on the Performance of Poly(3-hexythiophene): PCBM Bulk Heterojunction Solar Cells

ABSTRACTThe modified TiO2 nanoparticles were incorporated into the Bulk heterojunction system of P3HT:PCBM to improve the performance of P3HT:PCBM bulk heterojunction organic solar cells. The organically-modified TiO2 nanoparticle compounds were synthesized in aqueous media at room temperature. These TiO2 compounds in various solution concentrations were deposited on the top of the P3HT:PCBM active layer by spin coating. The performance of organic solar cells was carefully investigated in the respect of the scattering and the localized surface plasmon resonance (LSPR) that couple strongly to the incident light. In addition to the device, P3HT:PCBM solar cells with the use of the TiO2 nanoparticles, enhanced Fill Factor (FF) due mainly to improved shunt resistance (Rsh). The TiO2 plays a critical role in improving the interface between P3HT:PCBM active layer and Al electrode.

Benefits of very thin PCBM and LiF layers for solution-processed p–i–n perovskite solar cells

Optimal thickness of a PCBM layer and insertion of the LiF interlayer on a well-controlled flat surface of the perovskite film are essential for fabricating planar perovskite–PCBM solar cells.