PCBM Solar Cells Research Articles

Influence of β-linkages on the morphology and performance of DArP P3HT–PC61BM solar cells

Direct arylation polymerization (DArP) has emerged as a greener and more atom-efficient alternative to Stille polymerization. Despite the attractiveness of this method, DArP is known to produce β-linkages in polymers, which have β-protons available for activation. Here, we report the influence of the β-defect content in DArP poly(3-hexylthiophene) (P3HT) on the performance of bulk-heterojunction solar cells and the morphology of pristine polymers and their blends with PC61BM in thin films and compare with Stille P3HT containing 0% β-defects as a reference point. The optical and electrochemical properties as well as the hole mobilities of pristine polymers remain virtually the same when the amount of β-defects is limited to 0.75% or lower, as evidenced by UV–visible absorption spectra, cyclic voltammetry and space-charge-limited current (SCLC) mobility measurements. However, an increase of β-defect concentration to 1.41% significantly affects the oxidation onset, UV–visible absorption profile and hole mobility of P3HT. The key result of this study is that the photovoltaic performance of DArP P3HT with 0% β-defects is remarkably close to that of Stille P3HT, whereas the performance of DArP P3HT with 0–0.75% β-defects does not differ dramatically from that of Stille P3HT and could potentially be improved upon by individual optimization of the processing conditions.

A transition solvent strategy to print polymer:fullerene films using halogen-free solvents for solar cell applications

Inkjet printing is a mask-less non-contact deposition technique that is potentially suited for prototyping and manufacturing of thin-film polymer organic semiconductor devices from digital images. However new strategies are needed to achieve films with good macromorphology (i.e., high-fidelity footprint and uniform cross-section) and nanomorphology on unstructured substrates using a conventional ink-jet. Here we report a new transition solvent strategy to provide the desired film macromorphology and ultrafine nanomorphology in regioregular poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) model films, without using chlorinated solvents. This strategy employs a good volatile solvent in combination with a miscible poor solvent that is much less volatile, which is the reverse of the usual low−high boiling-point solvent method. The good solvent suppresses premature aggregation in the ink head. Its removal by evaporation on the substrate leaves the poor solvent that triggers early π-stacking ordering and/or gelation of the polymer matrix that immobilizes the printed fluid on the substrate, suppressing both contact-line depinning and evaporation-induced solvent flow effects. The resultant donor–acceptor nanomorphology is further improved by vacuum drying at an optimal rate that avoids bubble formation. We have systematically characterized P3HT:PCBM films deposited with different solvents and platen temperatures to identify key macro- and nano-morphology determining processes. High-performance printed P3HT:PCBM solar cells were realized. These findings are applicable also to other printing and coating techniques based on low-viscosity inks.

Effects of PCBM Loading and Thermal Annealing on Nanomorphology of Blend of Polymer/Fullerene Thin Films Solar Cells: Impact on Charge Carrier Mobility and Efficiency

Blend of P3HT/Fullerene thin films solar cell with two different percentage ratio of PCBM loading is investigated. Optical absorption spectroscopy is employed to elucidate the nature of PCBM cluster formation upon thermal annealing. Sandwich structures comprising of ITO/ Cs2CO3/ P3HT: PCBM/ LiF/ Al (electron only device), and ITO/ PEDOT:PSS/ P3HT:PCBM/ Au (hole only device) are fabricated using spin coating for the investigations concerning electron and hole mobilities. The impact of charge carrier mobilities on bimolecular recombination and ultimately the power conversion efficiency for two different PCBM loading is also investigated. A direct correlation between Langev in recombination rate and short circuit current density as a function of thermal annealing is realized. The maximum power conversion efficiency is measured at 150°C for P3HT: PCBM (1:1) solar cell.

Hole transit in P3HT:PCBM solar cells with embedded gold nanoparticles

The electrical properties of polymer solar cells (PSCs) with gold nanoparticles (AuNPs) embedded in the hole transport layers (HTLs) were investigated. The PSCs with AuNP-embedded HTLs exhibited 8%, 9.5%, and 22% enhancement in short-circuit current density, open-circuit voltage, and power conversion efficiency, respectively. The low contact resistance at the active layer/HTL interface resulted in the high collection efficiency and low electron–hole recombination. The downshift of the Fermi energy in AuNP-embedded HTLs lowered the energy barrier and thinned the depletion layer at the active layer/HTL interface, accounting for the enhanced collection efficiency.

A DMF-assisted solution process boosts the efficiency in P3HT:PCBM solar cells up to 5.31%

To achieve ideal phase separation in P3HT:PCBM (1:1, w/w) blend films, a selective second solvent, DMF, which cannot dissolve P3HT but slightly dissolves PCBM, was added into 1,2-dichlorobenzene. Addition of 10% DMF led to better charge transport, better morphology and average power conversion efficiency (PCE) enhancement from 3.75% to 4.29%. To form an acceptor rich layer near the cathode and achieve effective vertical phase separation, PCBM solution in DMF was spin-coated on the top surface of the P3HT:PCBM active layer. The PCBM rich layer enhanced hole blocking and electron transport, leading to an average PCE improvement from 4.29% to 4.83% (for the PCBM rich layer formed by a 2 mg ml−1 PCBM solution). Finally, to obtain more uniform and smooth films with better contact of Ca/PCBM and BHJ/PCBM interfaces, the films were thermally annealed at 120 ° C for 10 min after spin coating the PCBM rich layer, leading to an average PCE enhancement from 4.83% to 5.17% (the best PCE was 5.31%).

Fine tuning of the PCDTBT-OR:PC71BM blend nanoscale phase separation via selective solvent annealing toward high-performance polymer photovoltaics

Solution-processable polymer solar cells show great promise for providing a cost-effective route to create lightweight and flexible solar energy conversion devices. The photoactive layer comprising the conjugated polymer donor and fullerene derivative acceptor must be optimized to form bicontinuous nanoscale phase separation in order for efficient exciton dissociation and charge collection due to the short exciton diffusion length of organic semiconductors. The donor polymer poly[9-(heptadecan-9-yl)-9H-carbazole- 2,7-diyl-alt-(5,6-bis(hexyloxy)-4,7-di(thiophen-2- yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] (PCDTBT-OR) has a deeper highest occupied molecular orbital level compared to its counterpart PCDTBT, and shows promise in increasing the open-circuit voltage and power conversion efficiency (PCE) of polymer solar cells. The phase separation evolution of the PCDTBT-OR:PC71BM blend with various weight ratios under tetrahydrofuran (THF) vapor annealing and its influence on the photovoltaic performance is investigated in detail. It is found that THF vapor annealing can promote the acceptor PC71BM aggregation from the donor PCDTBT-OR matrix to form nanoscale donor/acceptor phase separation for efficient exciton dissociation and charge collection depending on the donor/acceptor weight ratio and the annealing time. The THF vapor-annealed PCDTBT-OR:PC71BM solar cells exhibit remarkable enhancement, with a PCE of 7.01% compared to 3.25% of the as-cast solar cells with the same active layer thickness. This work provides a general methodology to construct nano-interpenetrating networks for homogeneous polymer/fullerene blends and is potentially applicable to the roll-to-roll manufacturing of polymer solar cells.

Effect of Morphological Changes on Presence of Trap States in P3HT:PCBM Solar Cells Studied by Cross-Sectional Energy Filtered TEM and Thermally Stimulated Current Measurements

A combination of energy filtered transmission electron microscopy (EFTEM) and thermally stimulated current (TSC) was used in order to investigate the effect of thermal annealing on the performance of an organic solar cell based on P3HT and PCBM as a well-studied reference system. By probing specific elements, EFTEM allowed spectroscopic imaging with enhanced resolution compared to standard TEM techniques. Here, we applied EFTEM to cross-sections of pristine and thermally annealed organic solar cells to probe the sulfur concentration as a measure for the P3HT distribution within the photoactive layer. Thermal annealing for 10 min at 130 °C resulted in a reordering of P3HT and PCBM into better defined domains. The effect of the morphological changes on the presence of trap states was studied by TSC measurements. The TSC spectra recorded for the pristine and the thermally annealed solar cells showed three contributions, respectively, that could be assigned to the neat materials P3HT and PCBM as well as the blend. The pristine solar cell revealed a significantly lower density of trap states in the P3HT phase compared to the annealed solar cell. In combination with our EFTEM measurements, we were able to attribute this finding to the increased number of P3HT rich domains present in the annealed device. Annealing of P3HT:PCBM solar cells had a beneficial impact not only on the local molecular order, but in particular on providing percolation paths for both charge carrier types.

Investigation of optical and electrical properties of new aromatic polyazomethine with thiophene and cardo moieties toward application in organic solar cells

New polyazomethine (25Th-cardo) based on thiophene and cardo moieties was synthesized, characterized and tested as active layer in solar cells. The structure of polymer was characterized by means of FTIR and 1H NMR spectroscopy. The temperatures of 5% weight loss of the polyazomethine range from 435 to 455°C in nitrogen, depending on the heating rate applied. 25Th-cardo has electrochemical energy band gap of approximately ∼1.85eV, and HOMO energy level at −5.21eV. Optical absorption properties of pure 25Th-cardo and of its mixtures with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were investigated on different substrates: glass or glass/PEDOT:PSS. Electrical properties of the two kinds of devices: ITO/25Th-cardo:PCBM/Al and ITO/PEDOT:PSS/25Th-cardo:PCBM/Al were tested by impedance spectroscopy. The investigated polymer solar cells contained different amounts of PCBM. Bulk heterojunction solar cells with the architecture ITO/PEDOT:PSS/25Th-cardo:PCBM/Al gave a power conversion efficiency 3.5 times higher for the mixture of 25Th-cardo to PCBM 1:2 (w/w) than for 1:1 (w/w) under 100mW/cm2 AM1.5G sunlight illumination in air. The external quantum efficiency value of 25Th-cardo:PCBM solar cell was the highest (about 0.3%) in the near UV range from 3.1 to 3.7eV.

Probing the annealing induced molecular ordering in bulk heterojunction polymer solar cells using in-situ Raman spectroscopy

We have carried out temperature dependent in-situ Raman as well as optical microscopy of poly(3-hexylthiophene) and [6,6]-phenyl-C71 butyric acid methyl ester (P3HT:PCBM) films to investigate the structural and morphological evolution during heating and cooling cycles of thermal annealing. The full-width half-maximum of symmetric CåC stretching mode, CåC/C–C intensity ratio and Raman shift of CåC mode of P3HT measured during heating upto 130°C as well as subsequent cooling revealed that: (i) molecular disorder in P3HT chains increase during heating cycle and (ii) on cooling the molecular ordering enhances rapidly at ~100°C. However, heating to higher temperatures (>150°C) leads to the formation of PCBM aggregates. The mechanism of morphology evolution in P3HT:PCBM films envisaged through in-situ Raman and optical microscopy has been confirmed by other ex-situ techniques, such as, atomic force microscopy, scanning electron microscopy, UV–vis spectroscopy and X-ray elemental mapping. A correlation between the morphological evolution and various photovoltaic parameters of the fabricated P3HT:PCBM solar cells has been established.

Optimization of solution-processed oligothiophene:fullerene based organic solar cells by using solvent additives

The optimization of solution-processed organic bulk-heterojunction solar cells with the acceptor-substituted quinquethiophene DCV5T-Bu4 as donor in conjunction with PC61BM as acceptor is described. Power conversion efficiencies up to 3.0% and external quantum efficiencies up to 40% were obtained through the use of 1-chloronaphthalene as solvent additive in the fabrication of the photovoltaic devices. Furthermore, atomic force microscopy investigations of the photoactive layer gave insight into the distribution of donor and acceptor within the blend. The unique combination of solubility and thermal stability of DCV5T-Bu4 also allows for fabrication of organic solar cells by vacuum deposition. Thus, we were able to perform a rare comparison of the device characteristics of the solution-processed DCV5T-Bu4:PC61BM solar cell with its vacuum-processed DCV5T-Bu4:C60 counterpart. Interestingly in this case, the efficiencies of the small-molecule organic solar cells prepared by using solution techniques are approaching those fabricated by using vacuum technology. This result is significant as vacuum-processed devices typically display much better performances in photovoltaic cells.

Mapping Nanoscale Variations in Photochemical Damage of Polymer/Fullerene Solar Cells with Dissipation Imaging

We use frequency-modulated electrostatic force microscopy to track changes in cantilever quality factor (Q) as a function of photochemical damage in a model organic photovoltaic system 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 3'H-cyclopropa[8,25][5,6]fullerene-C71-D5h(6)-3'-butanoic acid, 3'-phenyl-, methyl ester (PC71BM). We correlate local Q factor imaging with macroscopic device performance and show that, for this system, changes in cantilever Q correlate well with changes in external quantum efficiency and can thus be used to monitor local photochemical damage over the entire functional lifetime of a PTB7:PC71BM solar cell. We explore how Q imaging is affected by the choice of cantilever resonance frequency. Finally, we use Q imaging to elucidate the differences in the evolution of nanoscale structure in the photochemical damage occurring in PTB7:PC71BM solar cells processed with and without the solvent additive 1,8-diiodooctane (DIO). We show that processing with DIO not only yields a preferable morphology for uniform performance across the surface of the device but also enhances the stability of PTB7:PC71BM solar cells-an effect that can be predicted based on the local Q images.

Influence of morphology of PCDTBT:PC71BM on the performance of solar cells

Because of the restriction of low energy difference between the highest occupied molecular orbital of P3HT and the lowest unoccupied molecular orbital of PCBM, the obtained power conversion efficiency of P3HT:PCBM solar cells is merely half the ideal value. In this paper, we have fabricated bulk heterojunction solar cells based on PCDTBT and PC71BM (structure: ITO/PEDOT:PSS/PCDTBT:PC71BM/LiF (0.8 nm)/Al (80 nm)). In order to optimize the performance of the cells, the weight ratio of PCDTBT to PC71BM, the thickness of the active layer and thermal annealing are investigated. When the weight ratio of PCDTBT to PC71BM is 1:2 and the thickness of the active layer is 73 nm, a short circuit current density of 10.36 mA/cm2, an open-circuit voltage of 0.91 V, a fill factor of 55.06 % and a power conversion efficiency of 5.19 % can be achieved. Moreover, we probe the influence of annealing temperature on the performance of organic solar cells, and find that the thermal treatment methodology (apart from the removal of trapped casting solvent) is of limited benefit.

Nongeminate and Geminate Recombination in PTB7:PCBM Solar Cells

A combination of transient photovoltage (TPV), voltage dependent charge extraction (CE), and time delayed collection field (TDCF) measurements is applied to 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):[6,6]‐phenyl‐C71‐butyric acid (PC71BM) bulk heterojunction solar cells to analyze the limitations of photovoltaic performance. Devices are processed from pure chlorobenzene (CB) solution and a subset is optimized with 1,8‐diiodooctane (DIO) as co‐solvent. The dramatic changes in device performance are discussed with respect to the dominating loss processes. While in the devices processed from CB solution severe geminate and nongeminate recombination is observed, the use of DIO facilitates efficient polaron pair dissociation and minimizes geminate recombination. Thus, from the determined charge carrier decay rate under open circuit conditions and the voltage dependent charge carrier densities n(V), the nongeminate loss current Jloss of the samples with DIO alone enables the reconstruction of the current/voltage (j/V) characteristics across the whole operational voltage range. Geminate and nongeminate losses are considered to describe the j/V response of cells prepared without additive, but lead to a clearly overestimated device performance. The deviation between measured and reconstructed j/V characteristics is attributed to trapped charges in isolated domains of pure fullerene phases.

The effects of MoO3 treatment on inverted PBDTTT-C:PC71BM solar cells

In recent years, organic solar cells have become the most potential technologies of the photovoltaic devices. In selection of the hole transport layer (HTL) in the inverted structure of organic solar cells, the use of molybdenum oxide (MoO3) has attracted more attention. However, the deposition of thermal evaporation in HTL fabrication requires highly vacuumed process, so it reduces the advantages in organic solar cells of large-area manufacture and low cost. Therefore, the solution process in MoO3 has been investigated. In this work, we dissolved MoO3 in isopropyl alcohol (IPA), spin-coated it on active layer Poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4–b]thiophene))-2,6-diyl]): [6,6]-phenyl-C71-butyric acid methyl ester acid methyl ester (PBDTTT-C:PC71BM) as the anode modification layer in the inverted structure, and then modified further by air plasma before Ag(silver) electrodes were deposited. As a result, controlling the conditions of treating time, pressure and power of plasma, the efficiency of device with MoO3 solution process is improved from 2.50% to 4.65% under the plasma conditions 1.5Torr/18.0 W/1 min.

Slot‐Die Coating of a High Performance Copolymer in a Readily Scalable Roll Process for Polymer Solar Cells

Copolymers based on dithieno[3,2‐b:2′,3′‐d]silole (DTS) and dithienylthiazolo[5,4‐d]thiazole (TTz) are synthesized and tested in an all‐solution roll process for polymer solar cells (PSCs). Fabrication of polymer:[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) solar cells is done on a previously reported compact coating/printing machine, which enables the preparation of PSCs that are directly scalable with full roll‐to‐roll processing. The positioning of the side‐chains on the thiophene units proves to be very significant in terms of solubility of the polymers and consequently has a major impact on the device yield and process control. The most successful processing is accomplished with the polymer, PDTSTTz‐4, that has the side‐chains situated in the 4‐position on the thiophene units. Inverted PSCs based on PDTSTTz‐4 demonstrate high fill factors, up to 59%, even with active layer thicknesses well above 200 nm. Power conversion efficiencies of up to 3.5% can be reached with the roll‐coated PDTSTTz‐4:PCBM solar cells that, together with good process control and high device yield, designate PDTSTTz‐4 as a convincing candidate for high‐throughput roll‐to‐roll production of PSCs.

Order of decay of mobile charge carriers in P3HT:PCBM solar cells

The charge carrier dynamics of organic solar cells are strongly influenced by trapping and allow to draw conclusions on the loss mechanisms limiting the photovoltaic performance. In this study, we derive the recombination order Δ of mobile charge carriers. For annealed P3HT:PCBM solar cells, it allows us to pinpoint the dominant recombination of mobile with trapped charge carriers in tail states. While the characteristic tail state energy of about 40 meV rises to about 100 meV for 30 h oxygen exposure under illumination, Δ decreases only weakly from 1.70 to 1.62. This corresponds to a slight shift towards trap-assisted recombination.

Photodegradation in Encapsulated Silole‐Based Polymer: PCBM Solar Cells Investigated using Transient Absorption Spectroscopy and Charge Extraction Measurements

AbstractLight induced degradation has been observed in the performance of organic solar cells in the absence of oxygen and a detailed analysis of the effect of this photodegradation on optical and electrical features has been accomplished. This photodegradation study has been performed on encapsulated photovoltaic blend devices comprised of the silole‐based donor–acceptor polymer KP115 blended with [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM). Photodegradation induces an almost 20% decrease in power conversion efficiency, primarily as a result of a reduction in short circuit current, JSC. The initial burn‐in phase of the photodegradation has been examined using a combination of transient absorption spectroscopy and charge extraction measurements, including photo‐CELIV (charge extraction by linearly increasing voltage) and time‐resolved charge extraction using a nanosecond switch. These measurements reveal a bimodal KP115 polaron population, comprised of both delocalised and localised/trapped charge carriers. The photodegradation results are consistent with an alteration of this bimodal KP115 polaron population, with the polarons becoming trapped in a broader, deeper density of localised states. Under laser illumination and at open circuit conditions, this enhanced trapping after light soaking inhibits charges from undergoing bimolecular recombination, leading to higher extracted charge densities at long times. At the lower charge densities operating at short circuit conditions and under continuous white light illumination, where bimolecular recombination is much less significant, the JSC decreases after light soaking due to a reduction in the efficiency of trapped charge carrier extraction.

Improved Photovoltaic Performance of MEH‐PPV/PCBM Solar Cells via Incorporation of Si Nanocrystals

AbstractWe have studied the effect of silicon nanocrystals (SiNCs) as a third component on performance of organic bulk heterojunction solar cells composed of poly[2‐methoxy,5‐(2′‐ethylhexyloxy)‐1,4‐phenylene vinylene] (MEH‐PPV):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) blend film. By adding suitable amounts of SiNCs into MEH‐PPV:PCBM blend, the device performance such as external quantum efficiency, short circuit current density (JSC), and power conversion efficiency (PCE) improved. Incorporation of 2.5% SiNCs in the blend led to 13.6% improvement of JSC, which in turn resulted in 18% improvement of PCE up to 2.28%. The improved performance was mainly due to the improvements both in the charge generation from the interface of MEH‐PPV/SiNCs and the charge collection at the cathode.

Thermal annealing effect on internal electrical polarization in organic solar cells

This article reports experimental studies on internal electrical polarization effects by using optical absorption and photoexcitation assisted capacitance–voltage (C–V) measurements based on morphological development in the standard P3HT:PCBM solar cell. We observe that morphological development can increase absorption intensity upon thermal annealing. The increase of absorption intensity essentially reflects an enhancement of absorption coefficient of local donor and acceptor structures. We attribute the enhancement of absorption coefficient to the well-known morphological developments: increased polymer crystallinity and molecular aggregations caused by thermal annealing. Furthermore, the enhancement in absorption coefficient indicates stronger electrical polarizations in the P3HT:PCBM film. The C–V studies find that increasing local electrical polarizations can lead to an enhancement on the generation of charge carriers at donor:acceptor interfaces and the transport of generated charge carriers to respective electrode interfaces in the P3HT:PCBM device. Our experimental findings suggest that local electrical polarizations play an important role in the development of photovoltaic processes in organic solar cells.

Small molecule dye rubrene doped organic bulk heterojunction solar cells

Poly(3-hexylthiphene) (P3HT):[6,6]-phenyl C60-butyric acid methyl ester (PCBM) bulk heterojunction solar cells doped with a small molecule dye of rubrene were fabricated. The effect of different doping concentration on the performance of solar cells was investigated. The results showed that by doping 3.5wt% rubrene, the short circuit current and the power conversion efficiency (PCE) of the cell were improved by 23.1% and 23.4%, respectively. The increased light absorption and effective charge separation of blend films contributed to the enhancement of PCE in doping devices. Moreover, by simulating the photocurrent of solar cells using Onsager–Braun theory, it was found that doping rubrene was responsible for the enhancement of charge transfer generation and dissociation, thereby improving the performance of P3HT:PCBM solar cells.