Performance characteristics of pentacene-based organic photovoltaic cells

We demonstrate the maximum power conversion efficiency of 3.89% from organic photovoltaic cells using pentacene as a hole transport layer with PIN structure of ITO/poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS)-glycerol/pentacene/pentacene:C₆₀/C₆₀/BCP (bathocuproine)/Al under standard AM 1.5 illumination (100 mW/cm²). To achieve high power conversion efficiency, the optimization of thickness of pentacene and glycerol-doped poly(3,4-ethylenedioxy-thiophene)–poly(styrene sulfonate) (G-PEDOT:PSS) as well as pentacene:C₆₀ (1:1) thin film as an active layer was accomplished. Our results show that the PIN structure with enlarged interface between pentacene and C₆₀ thin films increases the power conversion efficiency of the devices than the PN devices. The morphology of pentacene thin film with various thicknesses and glycerol-doped PEDOT:PSS layers crucially affected the performance characteristics of pentacene-based photovoltaic cells.     

Engineering vertical morphology with nanoparticulate organic photovoltaic devices

Sequential deposition of monolayers, composed of nanoparticles with varied donor-acceptor concentration ratios, has allowed the fabrication of organic photovoltaic (OPV) active layers with engineered vertical morphology. The performance of polymer-polymer poly(9,9-dioctylfluorene-co-bis-N,N-phenyl-1,4-phenylenediamine):poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (PFB:F8BT) and polymer-fullerene poly(3-hexylthiophene):phenyl C61 butyric acid methyl ester (P3HT:PCBM) nanoparticulate (NP), graded nanoparticulate (GNP) and bulk heterojunction (BHJ) OPV devices have been studied. For both material systems the highest device V_OC is observed for the graded structure. Furthermore, thermal treatments can be used to alleviate parasitic series resistance in the GNP devices, thus improving device J_SC and efficiency. Overall, this work shows that the nanoparticle approach provides a new experimental lever for morphology control in OPV devices.     

Enhancing photovoltaic response of organic solar cells using a crystalline molecular template

We demonstrate that a crystalline pentacene molecular templating layer considerably changes the morphology of the subsequently deposited lead phthalocyanine (PbPc) layer, resulting in an improved crystallinity at the early stages of growth of the PbPc film and a higher content of the triclinic phase. For bilayer PbPc (20 nm)/C₆₀ (40 nm) organic solar cells with or without the pentacene templating layer, the use of the pentacene templating layer leads to a 48% enhancement in the short-circuit current without noticeably affecting the solar cell open-circuit voltage or fill factor. A copper or zinc phthalocyanine molecular templating layer also leads to enhanced photovoltaic response from the PbPc/C₆₀ cells, though less significant than the pentacene template. The improved device performance originates from stronger absorption by the triclinic PbPc phase in the near infrared and the enhanced internal quantum efficiency over the entire spectrum where PbPc absorbs.     

Photocarrier behavior in organic heterojunction photovoltaic cells

We have investigated the bias dependence of photocurrent in several organic heterojunction cells to elucidate the behavior of photogenerated charge carriers. Both the planar and planar-mixed heterojunction devices are shown to always have negative photocurrent even at large forward biases; this phenomena has been attributed to an increased driving force for carrier diffusion away from the heterointerface as the applied bias increases. In contrast, the drift current generally dominates in mixed heterojunction devices due to distributed nature of charge generation throughout the active layer, leading to a photocurrent that is highly dependent on the internal electric field. This dependence gives rise to the reversal of the photocurrent direction at high biases when compared to that at the short-circuit condition. However, the voltage yielding zero photocurrent shows appreciable wavelength dependence, which is strongly correlated to the detailed charge carrier generation profile within the active layer.     

High performance organic solar cells enabled by an iodinated additive

Additive engineering is a simple and effective strategy to enhance the efficiency of organic solar cells (OSCs). However, traditional additives such as 1,8-diiodooctane (DIO) or 1-chloronaphthalene (CN), suffer from inferior stability, concentration sensitivity, and need additional thermal treatments, which are not desirable for industrial application. Here we introduce a simple, effective and versatile solid additive 1,3-diiodobenzene (1,3-DIB) into the OSCs. In comparison to the control devices, the 1,3-DIB treated OSCs exhibit significantly improved performance with a power conversion efficiency (PCE) of 16.90% for polymer OSCs and a PCE of 14.35% for binary all-small-molecule OSCs. Mechanism studies reveal that 1,3-DIB can improve charge transport and extraction, decrease charge recombination, enhance crystallinity and improve the phase separation. Furthermore, no thermal annealing is needed in PM6:Y6 based OSCs and the 1,3-DIB treated devices show excellent stability and reproducibility in both polymer and small molecule OSCs. Our results demonstrated that additive engineering is a powerful method to enhance the OSC performance.     

Performance enhancement in organic photovoltaic devices using plasma-polymerized fluorocarbon-modified Ag nanoparticles

In this work, Ag nanoparticles were modified by an ultra-thin plasma-polymerized fluorocarbon film (CF_X) to form a composite CF_X-modified Ag nanoparticles/indium tin oxide (ITO) anode for application in organic photovoltaic (OPV) devices. A CF_X-modified Ag nanoparticles/ITO anode exhibited a superior surface work function of 5.4 eV suited for application in OPV devices. The performance of zinc phthalocyanine:fullerene-based OPV devices showed a significant improvement when the structural identical cells are made with the CF_X-modified Ag nanoparticles/ITO. This work yielded a promising power conversion efficiency of 3.5 ± 0.1%, notably higher than that with a bare ITO anode (2.7 ± 0.1%).     

Effects of doping at the ppm level in Simple n+p-homojunction organic photovoltaic cells

The effects of doping at concentrations at the ppm level in organic photovoltaic cells were clarified using simple n⁺p-homojunctions. With doping from 0 to 10 ppm, the fill factor increased due to the appearance of majority carriers. From 10 to 100 ppm, the photocurrent density increased due to an increase in the built-in potential, i.e., the formation of an n⁺p-homojunction. The photocurrent was increased by a factor of 1.3 by directly doping the photoactive co-deposited layer with acceptor molecules at a concentration of 100 ppm.     

Solution-processable star-shaped photovoltaic organic molecules based on triphenylamine and benzothiadiazole with longer pi-bridge

Two solution-processable star-shaped D-π-A organic molecules with triphenylamine (TPA) as donor unit, benzothiadiazole (BT) as acceptor unit and 4-hexyl-thienylenevinylene as pi conjugated bridge, S(TPA-TBTT) and S(TPA-TBTT-TPA), have been designed and synthesized for the application as donor materials in bulk-heterojunction organic solar cells (OSCs). The two molecules possess broader absorption from 350 to 700 nm benefitted from the longer pi-bridge in the molecules but weaker absorbance and poorer solubility in comparison with their corresponding organic molecules with shorter vinylene pi-bridge. The OSC based on S(TPA-TBTT): PC₇₀BM (1:3, w/w) exhibited J_sc of 6.41 mA/cm², V_oc of 0.75 V, FF of 39.0% and power conversion efficiency of 1.90%, under the illumination of AM 1.5, 100 mW/cm².     

Doped organic single-crystal photovoltaic cells

Doped rubrene single-crystal photovoltaic cells were fabricated. Whole photocurrent generated in the pn-homojunction with the macroscopic area of 2 mm × 1 mm was confirmed to be collected through the p-doped homoepitaxial layer. A single-crystal substrate collected excitons to the pn-homojunction with the collection efficiency reaching 46%, owing to the long exciton diffusion length of 2.7 μm.     

Morphology optimization of organic solar cells enabled by interface engineering of zinc oxide layer with a conjugated organic material

A diphenylphosphine-oxide-based conjugated organic molecule, ((1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T), was doped into ZnO to improve the characteristics of the electron transport layer (ETL) in inverted organic solar cells (OSCs). A series of characterization techniques were carried out to demonstrate the function of PO-T2T in film aspect, including transmittance, atomic force microscopy (AFM), transmission electron microscopy (TEM), water contact angle and grazing incidence wide angle X-ray scattering (GIWAXS). Light dependent, space-charge-limited current, exciton dissociation possibility were aimed to explore the influence of PO-T2T for internal carrier behaviors based on PTB7-Th: PC₇₁BM system. It's found that the PO-T2T doped ETLs played a role in morphology optimization of ETL and undermined the trap-assistant recombination through filling the defects ZnO itself had, simultaneously. Besides, the electron mobility was also improved. With the optimized functionalities, the OSCs' efficiency based on fullerene system Poly[4,8- bis(5-(2-Ethylhexyl)thiophen-2-yl) benzo [1,2-b:4,5-b′] dithiophene-co-3-fluorothieno [3,4-b] thiophene-2-carboxylate] (PTB7-Th): [6,6]-Phenyl C71 butyric acid methyl ester (PC₇₁BM) was improved from 9.03% to 9.84%. Finally, when this strategy was applied into another hot-topic system, poly((2,6-(4,8-bis(5-(2-ethylhexyl-3- fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5- (1′,3′-di-2-thienyl)-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′] dithiophene-4,8-dione)) (PBDB-TF):2,2′-((2Z,2′Z)-((12,13-bis(2- ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e] thieno[2,″3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5] thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6), a high PCE of 16.34% was obtained. These results demonstrated that the PO-T2T had a positive role in OSC performance improvement.     

Partitioning of the organic layers for the fabrication of high efficiency organic photovoltaic devices

Lateral partitioning of hole extraction layer with insulating walls improved the power conversion efficiency of organic photovoltaic device. When the conductivity of the hole extraction layer is low, no improvement is obtained by partitioning. However, when the conductivity is high, a significant improvement was obtained in the partitioned cells, showing the estimated power conversion efficiency of 4.58% compared to the 3.54% of the single cell structure. This improvement, carefully corrected by masking at measurement, could be explained by the reduction of series resistance. Although accurate estimation of device area at partitioned device might be difficult, its effectiveness on the properties of large area organic photovoltaic device is clear, as shown in the result of 1 cm²-size cell with the same manner.     

Interpenetrating heterojunction photovoltaic cells based on C60 nano-crystallized thin films

An interpenetrating heterojunction (IHJ) structure facilitates efficient charge separation and transport in the active layer of organic photovoltaic cells (OPVs). Additionally, the recombination of generated carriers in IHJs is reduced as these networks exhibit high carrier transport with minimal recombination sites. We have developed a simple method to fabricate nanocrystallized fullerene (C₆₀) films, which are produced by subjecting evaporated C₆₀ films to either solvent spin-coating or solvent vapor annealing (SVA). The size of the rod-shaped nanocrystals in the films were controlled by changing the solvent and annealing time.An 80-nm-diameter size nanocrystallized C₆₀ film that was fabricated using SVA with ethanol was incorporated as an acceptor material in an inverted IHJ OPV cell. Tetraphenyldibenzoperiflanthene (DBP) was evaporated onto the nanocrystallized C₆₀ film as the donor material. The power conversion efficiency of an IHJ OPV cell (ITO/TiOx/nanocrystallized C₆₀ film/DBP/MoO₃/Au) increased from 1.79% to 2.12%, when compared with the conventional PHJ OPV cell.     

Improved photovoltaic characteristics of organic cells with heterointerface layer as a hole-extraction layer inserted between ITO anode and donor layer

We have fabricated an improved organic photovoltaic (OPV) cell in which organic heterointerface layer is inserted between indium-tin-oxide (ITO) anode and copper-phthalocyanine (CuPc) donor layer in the conventional OPV cell of ITO/CuPc/fullerene (C₆₀)/bathophenanthroline (Bphen)/Al to enhance the power conversion efficiency (PCE) and fill factor (FF). The inserted ITO-buffer layer consists of electron-transporting layer (ETL) and hole-transporting layer (HTL). We have changed the ETL and HTL materials variously and also changed their layer thickness variously. It is confirmed that ETL materials with higher LUMO level than the work function of ITO give low PCE and FF. All the double layer buffers give higher PCE than a single layer buffer of TAPC. The highest PCE of 1.67% and FF of 0.57% are obtained from an ITO buffer consisted of 3 nm thick ETL of hexadecafkluoro-copper-phthalocyanine (F₁₆CuPc) and 3 nm thick HTL of 1,1-bis-(4-methyl-phenyl)-aminophenylcyclohexane (TAPC). This PCE is 1.64 times higher than PCE of the cell without ITO buffer and 2.98 times higher than PCE of the cell with single layer ITO buffer of TAPC. PCE is found to increase with increasing energy difference (ΔE) between the HOMO level of HTL and LUMO level of F₁₆CuPc in a range of ΔE < 0.6 eV. From the ΔE dependence of PCE, it is suggested that electrons moved from ITO to the LUMO level of the electron-transporting F₁₆CuPc are recombined, at the F₁₆CuPc/HTL-interface, with holes transported from CuPc to the HOMO level of HTL in the double layer ITO buffer ETL, leading to efficient extraction of holes photo-generated in CuPc donor layer.     

Controlled donor-accepter ratio for application of organic photovoltaic cells by alternative intermittent electrospray co-deposition

Because co-deposition method has been utilized in a conventional thermal evaporation process to realize graded donor-acceptor architectures, we investigated an alternative intermittent electrospray co-deposition method for solution-processed organic photovoltaic cells. In this method, two solutions of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C₆₁ butyric acid methyl ester (PCBM) were alternatively deposited using high-voltage pulse. Thus, the P3HT:PCBM blend thin film could be deposited even in a vacuum-free experimental setup. The optimum pulse width was found to be greater than 6 s to avoid an unexpected charge to the adjacent glass capillary, which causes the instable electrospray. The P3HT molecular ordering estimated from Raman spectroscopy and grazing incidence X-ray diffraction patterns was comparable to that estimated from the spin-coated device. In addition, the P3HT:PCBM ratio of the deposited thin film could be controlled by changing the ratio of the pulse width for the P3HT and PCBM solutions and was evaluated from the ultraviolet–visible absorption spectrum. Finally, a two layered bulk heterojunction structure with P3HT:PCBM was successfully demonstrated, leading to a maximum photoconversion efficiency of 3.1%. This value was 1.4-fold higher than that of the uniformly mixed bulk heterojunction device because of the high carrier-collection efficiency.     

Efficiency limit of excitonic photovoltaic cells under phosphor-based white LED illumination

The limit of energy conversion of excitonic photovoltaic cells working under white light illumination generated by phosphor-based LED is analysed using the modified Giebink approach. Particularly, the impact of the optical energy gap and energy loss associated with the excitons dissociation at the heterojunction interface on power conversion efficiency of the device are discussed. From the results of our study it follows that the optimal optical energy gap value of organic materials equals 1.87–1.91 eV for the cool light and 1.80–1.82 eV for the warm light. The value of maximum power efficiency reaches 50%, if the energy loss related to excitons dissociation at the interface ED/EA is smaller than 0.3 eV and it decreases up to 40%, if the energy loss reaches the value of 0.5 eV. The obtained results reveal the direction of further improvement of efficiency of organic photovoltaic solar cells for indoor applications.     

Enhanced power conversion efficiency of organic photovoltaic devices due to the surface plasmonic resonance effect generated utilizing Au-WO3 nanocomposites

Au-WO₃ nanocomposites (NCs) were used as a hole transport layer (HTL) to enhance the power conversion efficiency (PCE) of organic photovoltaic (OPV) cells. The photon absorption of the active layer in the OPV cells was increased due to the plasmonic effect caused by the Au-WO₃ NCs, resulting in an enhanced short-circuit current density for the OPV cells with the Au-WO₃ NC HTL. The value of the root-mean-square roughness of the Au-WO₃ NC film was smaller than that of the WO₃ NP film, resulting in a more efficient transport of holes from the active layer. The PCE of the OPV cell with an Au-WO₃ NCs HTL with an Au NP concentration of 10 wt% was improved by 60.37% in comparison with that with WO₃ nanoparticles. The enhancement of the PCE was attributed to both an increase in the efficiency of the hole transport at an Au-WO₃ NCs HTL with an Au NP concentration of 10 wt%/active layer heterointerface and an enhanced photon absorption due to the localized surface plasmon resonance effect of the Au-WO₃ NCs.     

Degradation of self-assembled monolayers in organic photovoltaic devices

Self-assembled monolayers (SAMs) based on n-octylphosphonic acid (C8PA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA) were investigated for application as an anode buffer layer in C₆₀-based organic photovoltaic (OPV) devices. We found that the degradation of the OPV efficiency with respect to air exposure was significantly reduced with the perfluorinated PFOPA compared to the aliphatic C8PA. We attribute the OPV degradation to moisture diffusion from the top aluminum electrode and the lowering of the anode work function as a result of hydrolysis of the SAM buffer layer.     

Detecting 6 MV X-rays using an organic photovoltaic device

An organic photovoltaic (OPV) device has been used in conjunction with a flexible inorganic phosphor to produce a radiation tolerant, efficient and linear detector for 6 MV X-rays. The OPVs were based on a blend of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). We show that the devices have a sensitivity an order of magnitude higher than a commercial silicon detector used as a reference. Exposure to 360 Grays of radiation resulted in a small (2%) degradation in performance demonstrating that these detectors have the potential to be used as flexible, real-time, in vivo dosimeters for oncology treatments.     

Hydrophilic polyurethane acrylate and its physical property for efficient fabrication of organic photovoltaic cells via stamping transfer

Recently, stamping transfer process using by soft mold or film has been considered by promising technology to solve the drawbacks of spin coating such as deposition of large area and specific region, reducing the material loss, and multi-staking device structures. For the previous researches, polyurethane acrylate (PUA) stamp was essentially treated the 1H, 1H, 2H, 2H-Perfluorooctyltrichlorosilane (FOTS) for self-assembled monolayer (SAM) onto the Si wafer to modify surface energy. Because the FOTS is known as corrosive material, it is necessary to develop the intrinsic property of PUA with environment friendly. In this research, we investigates non-FOTS based PUA stamping transfer and the different surface energy properties that result in various physical phenomena when used for organic photovoltaics. To transfer the material, the energy release rate (G) between the PUA and the coated material should be smaller than the G between the coated material and the substrate. As a result, hydrophilic PUA was used to reduce the interaction between the PUA and the organic bulk heterojunction (BHJ) layer to transfer the BHJ layer from the PUA stamp to a PEDOT:PSS-coated ITO-substrate. 2-Hydroxyethyl methacrylate (HEMA) is included as the reactive diluent to reduce the PUA viscosity, and the contact angle was measured to compare the surface property between the reference PUA and the HEMA-PUA. The stamping-transferred BHJ device exhibits a 95% relative efficiency (2.9%) when compared to that obtained when using a spin-coating process, which is considered as a good alternative to fabricate optoelectronic devices. More importantly, we have found a decrease in the fill factor (64%–58%) and a comparable performance (3.0%–2.9%) derived from the increase in the charge recombination and resistance during the stamping transfer.     

Understanding the effect of triplet sensitizers in organic photovoltaic devices

The effect of PtOEP as a dopant on the performance of MEH-PPV/C₆₀ photovoltaic devices was studied. Bilayer heterojunction devices with various compositions and layer structures were used to determine the possible pathways by which the photogeneration efficiency is enhanced. A key finding is that photocurrent generation enhancement always occurs in the MEH-PPV absorption region, regardless of the PtOEP dopant concentration or the MEH-PPV layer thickness. This result suggests that the presence of PtOEP in the donor MEH-PPV layer is primarily responsible for increasing the triplet exciton diffusion length of MEH-PPV by acting as a triplet sensitizer, rather than as an additional absorber for direct photogeneration. Values obtained from simulation show that the enhancement of exciton diffusion length of MEH-PPV can be more than a factor of 2 with optimal PtOEP concentrations. Further support for the role of PtOEP as a triplet sensitizer in MEH-PPV was obtained in experiments incorporating a blocking layer between MEH-PPV and C₆₀, whereby the various exciton transfer processes can be differentiated.