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1.
    
A high electron mobility polymer, poly{[N,N’‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5’‐(2,2’‐bithiophene) (P(NDI2OD‐T2)) is investigated for use as an electron acceptor in all‐polymer blends. Despite the high bulk electron mobility, near‐infrared absorption band and compatible energy levels, bulk heterojunction devices fabricated with poly(3‐hexylthiophene) (P3HT) as the electron donor exhibit power conversion efficiencies of only 0.2%. In order to understand this disappointing photovoltaic performance, systematic investigations of the photophysics, device physics and morphology of this system are performed. Ultra‐fast transient absorption spectroscopy reveals a two‐stage decay process with an initial rapid loss of photoinduced polarons, followed by a second slower decay. This second slower decay is similar to what is observed for efficient P3HT:PCBM ([6,6]‐phenyl C61‐butyric acid methyl ester) blends, however the initial fast decay that is absent in P3HT:PCBM blends suggests rapid, geminate recombination of charge pairs shortly after charge transfer. X‐ray microscopy reveals coarse phase separation of P3HT:P(NDI2OD‐T2) blends with domains of size 0.2 to 1 micrometer. P3HT photoluminescence, however, is still found to be efficiently quenched indicating intermixing within these mesoscale domains. This hierarchy of phase separation is consistent with the transient absorption, whereby localized confinement of charges on isolated chains in the matrix of the other polymer hinders the separation of interfacial electron‐hole pairs. These results indicate that local, interfacial processes are the key factor determining the overall efficiency of this system and highlight the need for improved morphological control in order for the potential benefit of high‐mobility electron accepting polymers to be realized.  相似文献   

2.
    
The light intensity dependence of the main photoelectrical parameters of the nonfullerene small‐molecule bulk heterojunction (BHJ) solar cells p‐DTS(FBTTh2)2:perylene diimide (T1:PDI) shows that the nongeminate recombination losses play an important role in this system. A simple approach for the quantitative analysis of capacitance spectroscopy data of the organic BHJ solar cells, which allows to determine the density of free charge carriers as a function of applied bias under standard working conditions, is demonstrated. Using the proposed capacitance spectroscopic technique, the nongeminate recombination losses in the T1:PDI solar cells are quantitatively characterized in the scope of the bimolecular‐ and trap‐assisted recombination mechanisms. Their contributions are separately analyzed within a wide range of the applied bias.  相似文献   

3.
    
Bimolecular recombination in bulk heterojunction organic solar cells is the process by which nongeminate photogenerated free carriers encounter each other, and combine to form a charge transfer (CT) state which subsequently relaxes to the ground state. It is governed by the diffusion of the slower and faster carriers toward the electron donor–acceptor interface. In an increasing number of systems, the recombination rate constant is measured to be lower than that predicted by Langevin's model for relative Brownian motion and the capture of opposite charges. This study investigates the dynamics of charge generation, transport, and recombination in a nematic liquid crystalline donor:fullerene acceptor system that gives solar cells with initial power conversion efficiencies of >9.5%. Unusually, and advantageously from a manufacturing perspective, these efficiencies are maintained in junctions thicker than 300 nm. Despite finding imbalanced and moderate carrier mobilities in this blend, strongly suppressed bimolecular recombination is observed, which is ≈150 times less than predicted by Langevin theory, or indeed, more recent and advanced models that take into account the domain size and the spatial separation of electrons and holes. The suppressed bimolecular recombination arises from the fact that ground‐state decay of the CT state is significantly slower than dissociation.  相似文献   

4.
    
A three‐dimensional indium tin oxide (ITO) nanohelix (NH) array is presented as a multifunctional electrode for bulk heterojunction organic solar cells for simultaneously improving light absorption and charge transport from the active region to the anode. It is shown that the ITO NH array, which is easily fabricated using an oblique‐angle‐deposition technique, acts as an effective antireflection coating as well as a light‐scattering layer, resulting in much enhanced light harvesting. Furthermore, the larger interfacial area between the electrode and the active layer, together with the enhanced carrier mobility through highly conductive ITO NH facilitate transport and collection of charge carriers. The optical and electrical improvements enabled by the ITO NH electrode result in a 10% increase in short‐circuit current density and power‐conversion efficiency of the solar cells.  相似文献   

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6.
    
Charge transport in organic photovoltaic (OPV) devices is often characterized by space‐charge limited currents (SCLC). However, this technique only probes the transport of charges residing at quasi‐equilibrium energies in the disorder‐broadened density of states (DOS). In contrast, in an operating OPV device the photogenerated carriers are typically created at higher energies in the DOS, followed by slow thermalization. Here, by ultrafast time‐resolved experiments and simulations it is shown that in disordered polymer/fullerene and polymer/polymer OPVs, the mobility of photogenerated carriers significantly exceeds that of injected carriers probed by SCLC. Time‐resolved charge transport in a polymer/polymer OPV device is measured with exceptionally high (picosecond) time resolution. The essential physics that SCLC fails to capture is that of photo­generated carrier thermalization, which boosts carrier mobility. It is predicted that only for materials with a sufficiently low energetic disorder, thermalization effects on carrier transport can be neglected. For a typical device thickness of 100 nm, the limiting energetic disorder is σ ≈71 (56) meV for maximum‐power point (short‐circuit) conditions, depending on the error one is willing to accept. As in typical OPV materials the disorder is usually larger, the results question the validity of the SCLC method to describe operating OPVs.  相似文献   

7.
    
Solution‐processed organic BHJ solar cells based on 3,6‐bis[5‐(benzofuran‐2‐yl)thiophen‐2‐yl]‐2,5‐bis(2‐ethylhexyl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione (DPP(TBFu)2) or poly(3‐hexylthiophene) blended with [6,6]‐phenyl‐C60(70) ‐butyric acid methyl ester (PC60(70) BM) behave differently under various irradiation intensities. Small molecule‐based DPP(TBFu)2:PC60 BM solar cells show up to 5.2% power conversion efficiency and a high fill factor at low light intensity. At 100 mW cm?2 illumination, the efficiency and fill factor decrease, resulting in stronger power losses. Impedance spectroscopy at various light intensities reveals that high charge recombination is the cause of the low fill factor in DPP(TBFu)2:PC60 BM.  相似文献   

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10.
    
A novel technique based upon injection‐charge extraction by linearly increasing voltage (i‐CELIV) in a metal‐insulator‐semiconductor (MIS) diode structure is described for studying charge transport in organic semiconductors. The technique (MIS‐CELIV) allows selective measurement of both electron and hole mobilities of organic solar cells with active layers thicknesses representative of operational devices. The method is used to study the model high efficiency bulk heterojunction combination poly[N‐9′′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) and [6,6]‐phenyl C70‐butyric acid methyl ester (PC70BM) at various blend ratios. The absence of bipolar transport in PCDTBT‐and‐PC70BM‐only diodes is shown and strongly imbalanced carrier mobility is found in the most efficient “optimized” blend ratios. The mobility measurements are correlated with overall device performance and it is found that balanced and high charge carrier mobility are not necessarily required for high efficiencies in thin film organic solar cells.  相似文献   

11.
    
The photoinduced open‐circuit voltage (Voc) loss commonly observed in bulk heterojunction organic solar cells made from amorphous polymers is investigated. It is observed that the total charge carrier density and, importantly, the recombination dynamics are unchanged by photoinduced burn‐in. Charge extraction is used to monitor changes in the density of states (DOS) during degradation of the solar cells, and a broadening over time is observed. It is proposed that the Voc losses observed during burn‐in are caused by a redistribution of charge carriers in a broader DOS. The temperature and light intensity dependence of the Voc losses can be described with an analytical model that contains the amount of disorder broadening in a Gaussian DOS as the only fit parameter. Finally, the Voc loss in solar cells made from amorphous and crystalline polymers is compared and an increased stability observed in crystalline polymer solar cells is investigated. It is found that solar cells made from crystalline materials have a considerably higher charge carrier density than those with amorphous materials. The effects of a DOS broadening upon aging are suppressed in solar cells with crystalline materials due to their higher carrier density, making crystalline materials more stable against Voc losses during burn‐in.  相似文献   

12.
    
Poly(benzo[1,2‐b:4,5‐b′]dithiophene–alt–thieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) polymer donors with linear side‐chains yield bulk‐heterojunction (BHJ) solar cell power conversion efficiencies (PCEs) of about 4% with phenyl‐C71‐butyric acid methyl ester (PC71BM) as the acceptor, while a PBDTTPD polymer with a combination of branched and linear substituents yields a doubling of the PCE to 8%. Using transient optical spectroscopy it is shown that while the exciton dissociation and ultrafast charge generation steps are not strongly affected by the side chain modifications, the polymer with branched side chains exhibits a decreased rate of nongeminate recombination and a lower fraction of sub‐nanosecond geminate recombination. In turn the yield of long‐lived charge carriers increases, resulting in a 33% increase in short circuit current (J sc). In parallel, the two polymers show distinct grazing incidence X‐ray scattering spectra indicative of the presence of stacks with different orientation patterns in optimized thin‐film BHJ devices. Independent of the packing pattern the spectroscopic data also reveals the existence of polymer aggregates in the pristine polymer films as well as in both blends which trap excitons and hinder their dissociation.  相似文献   

13.
    
The high power conversion efficiencies (PCEs) of laboratory‐scale polymer‐based organic solar cells are yet to translate to large area modules because of a number of factors including the relatively large sheet resistance of available transparent conducting electrodes (TCEs), and the high defect densities associated with thin organic semiconductor junctions. The TCE problem limits device architectures to narrow connected strips (<1 cm) causing serious fabrication difficulties and extra costs. Thin junctions are required because of poor charge transport (imbalanced mobilities) in the constituent organic semiconductors. These issues are addressed using a combination of approaches to create thick junctions conformally coated on low sheet resistance metal grid TCEs. An essential feature of these thick junctions is balanced carrier mobilities, which affords high fill factors and efficient carrier extraction. Conformal coating is achieved by promoting enhanced intermolecular interactions in the coating solution using a high molecular weight polymeric semiconductor and appropriate solvent system. This combination of balanced mobilities, conformal coating and metallic grid TCEs is a simple and generic approach to the fabrication of defect‐free large area organic solar cells (OSCs). The approach is demonstrated with 25 cm2 monolithic devices possessing aperture‐corrected power conversion efficiencies of 5% and fill factors exceeding 0.5.  相似文献   

14.
    
Environmental friendly aqueous‐processed solar cells have become one of the most promising candidates for the next‐generation photovoltaic devices. Researchers have made lots of progress in designing active materials with novel structures, manipulating the defects in active materials, optimizing device architecture, etc. However, it has long been a challenge to control the width of the depletion region and enhance carrier extraction ability. Fabrication of a thick bulk heterojunction (BHJ) film is an effective strategy to address these issues but difficult to realize. Herein, the thicker BHJ film of ZnO:CdTe is successfully fabricated and incorporated into CdTe‐poly(p‐phenylenevinylene) hybrid solar cells. As expected, this BHJ film enhances light absorption, extends the width of the depletion region, prolongs carrier lifetime, and promotes carrier extraction ability. Moreover, the electron transport layer of sol–gel ZnO with excellent transmittance and electrical conductivity boosts electron generation, transport, and injection, which further improves the device performance. As a result, the highest short current density (Jsc) of 19.5 mA cm?2, power conversion efficiency of 6.51%, and the widest depletion region (177 nm) are obtained in aqueous‐processed hybrid solar cells.  相似文献   

15.
    
Charge‐transfer (CT) state electroluminescence is investigated in several polymer:fullerene bulk heterojunction solar cells. The ideality factor of the electroluminescence reveals that the CT emission in polymer:fullerene solar cells originates from free‐carrier bimolecular recombination at the donor‐acceptor interface, rather than a charge‐trap‐mediated process. The fingerprint of the presence of nonradiative trap‐assisted recombination, a voltage‐dependent CT electroluminescence quantum efficiency, is only observed for the P3HT:PCBM system, which is explained by a reduction of the competing bimolecular recombination rate. These results are in agreement with measurements of the illumination‐intensity dependence of the open‐circuit voltage.  相似文献   

16.
    
A family of porphyrins and benzoporphyrins bearing phenyl, thiophenyl, or bithiophenyl groups at their meso‐positions are synthesized and systematically investigated for their potential use in bulk heterojunction solar cells (BHJ‐SCs). Comparative studies of these compounds show that the introduction of the thiophenyl and bithiophenyl groups, and the extension of the porphyrin π‐conjugated system significantly affect both photophysical and electrochemical properties. Binary conventional and ternary converted BHJ‐SCs based on these compounds are fabricated and studied. Results show that remarkable enhancement of the device efficiency is achieved by using the thiophene‐containing benzoporphyrin derivatives as additives for a poly(3‐hexylthiophene) (P3HT):phenyl‐C61‐butyric acid methyl ester blend in the inverted BHJ‐SCs. The optimum BHJ‐SC exhibits a maximum energy conversion efficiency of 4.3%, corresponding to 19% enhancement of the conversion efficiency as compared with the benchmark BHJ‐SCs.  相似文献   

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18.
    
We report the fabrication of high performance organic solar cells by spray‐coating the photoactive layer in air. The photovoltaic blends consist of a blend of carbazole and benzothiadiazole based donor–acceptor copolymers and the fullerene derivative PC70BM. Here, we formulate a number of photovoltaic inks using a range of solvent systems that we show can all be deposited by spray casting. We use a range of techniques to characterize the structure of such films, and show that spray‐cast films have comparable surface roughness to spin‐cast films and that vertical stratification that occurs during film drying reduces the concentration of PCBM towards the underlying PEDOT:PSS interface. We also show that the active layer thickness and the drying kinetics can be tuned through control of the substrate temperature. High power conversion efficiencies of 4.3%, 4.5% and 4.6% were obtained for solar cells made from a blend of PC70BM with the carbazole‐based co‐polymers PCDTBT, P2 and P1. By applying a low temperature anneal after the deposition of the cathode, the efficiency of spray‐cast solar‐cells based on a P2:PC70BM blend is increased to 5.0%. Spray coating holds significant promise as a technique capable of fabricating large‐area, high performance organic solar cells in air.  相似文献   

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20.
    
Charge transport and recombination are studied for organic solar cells fabricated using blends of polymer poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole)‐5,5′‐diyl] (Si‐PCPDTBT) with [6,6]‐phenyl‐C61‐butyric acid methyl ester (mono‐PCBM) and the bis‐adduct analogue of mono‐PCBM (bis‐PCBM). The photocurrent of Si‐PCPDTBT:bis‐PCBM devices shows a strong square root dependence on the effective applied voltage. From the relationship between the photocurrent and the light intensity, we found that the square‐root dependence of the photocurrent is governed by the mobility‐lifetime (μτ) product of charge carriers while space‐charge field effects are insignificant. The fill factor (FF) and short circuit current density (Jsc) of bis‐PCBM solar cells show a considerable increase with temperature as compared to mono‐PCBM solar cells. SCLC analysis of single carrier devices proofs that the mobility of both electrons and holes is significantly lowered when replacing mono‐PCBM with bis‐PCBM. The increased recombination in Si‐PCPDTBT:bis‐PCBM solar cells is therefore attributed to the low carrier mobilities, as the transient photovoltage measurements show that the carrier lifetime of devices are not significantly altered by using bis‐PCBM instead of mono‐PCBM.  相似文献   

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