Bulk Heterojunction Organic Photovoltaic Devices Research Articles

Enhanced Photovoltaic Properties of Bulk Heterojunction Organic Photovoltaic Devices by an Addition of a Low Band Gap Conjugated Polymer.

In this study, we fabricated organic photovoltaics (OPVs) by introducing the polymer additive HTh6BT into the photoactive layer of a poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) system. The HTh6BT had a relatively low band gap energy of 1.65 eV and a molecular and crystalline structure similar to that of P3HT. In the photoactive layer, the HTh6BT and P3HT can both act as donors. In such parallel-type bulk heterojunctions, each donor can form excitons and generate charges while being separated from the donor/acceptor interface. Changes in the photovoltaic property of the OPV device by the addition of HTh6BT were evaluated, and the optical characteristics of the photoactive layer, as well as the surface morphology, polymer ordering, and crystallinity of the P3HT:PCBM film were analyzed. Compared to a device without HTh6BT, all short-circuit current densities, open-circuit voltages, and fill factors were enhanced, leading to the enhancement of the power conversion efficiency by 36%.

Dual substitution at 4,9-positions of carbazole in donor-π-acceptor copolymer enhances performance of bulk-heterojunction organic solar cells

Several carbazole-2,7-diyl units with alkoxy substituents at the 4-position in addition to branched (1-alkyl)alkyl substituents at the 9-position were developed and combined with difluorinated dithienylbenzothiadiazole by using a Suzuki coupling reaction to synthesize donor–π–acceptor-type copolymers. X-ray diffraction analyses of these copolymers revealed that the 4,9-disubstituted carbazole unit contributed to form a balanced interdigitated structure, which resulted in smaller π-stacking distances (4.0–3.5 Å) of the polymer backbone, a narrower bandgap (2.0–1.8 eV), and higher hole mobilities (4.2 × 10−4 to 2.9 × 10−3 cm2 V−1 s−1) than those of the carbazole copolymer without the 4-substituent (PCDT2FBT). A bulk heterojunction organic photovoltaic device with a configuration of ITO/PEDOT:PSS/polymer:PC70BM/LiF/Al exhibited a power conversion efficiency of 5.46%, a short-circuit current density of 12.4 mA cm−2, a fill factor of 0.58, and an open-circuit voltage of 0.76 V. The photovoltaic performance was much higher than that of PCDT2FBT. These results demonstrate that incorporation of the side chains into the 4,9-positions of carbazole is a novel approach to obtain self-organizing carbazole polymers, enabling improvement in light absorption, hole mobility, and photovoltaic performance.

Double acceptor donor–acceptor alternating conjugated polymers containing cyclopentadithiophene, benzothiadiazole and thienopyrroledione: toward subtractive color organic photovoltaics

Double acceptor copolymers comprising benzothiadiazole (BT), thieno[3,4-c]pyrrole-4,6-dione (TPD) and two cyclopentadithiophene (CPDT) units, and a series of copolymers comprising CPDT-A-CPDT (A: acceptor=BT or TPD) with different numbers of thiophene units have been synthesized by combinations of Suzuki and Stille coupling, direct arylation and oxidative polymerization. Ultraviolet–visible absorption spectra and cyclic voltammograms were measured to compare the optical and electrochemical properties of the polymers. The double acceptor copolymer exhibited hybridized features resulting from the presence of both BT, as well as TPD. On the other hand, an increase in the number of electron donor units resulted in an increase in the band gap for both the TPD and BT series. Organic photovoltaics (OPV) devices were fabricated using the polymers and a fullerene derivative. These donor–acceptor alternating copolymers showed higher OPV performance (power conversion efficiency=3–4%) than other polymers. The active layer comprising the double acceptor polymer and a fullerene derivative showed subtractive color resulting from wide-range visible light absorption, which is advantageous for developing transparent building-integrated OPVs. The OPV performance of BT-based copolymers synthesized via direct arylation was comparable or lower compared with those prepared by conventional cross-coupling polymerizations. Double acceptor donor–acceptor alternating conjugated copolymers comprising thienopyrroledione, benzothiadiazole, two cyclopentadithiophene groups and their analogs with thiophene or bithiophene units have been synthesized by combinations of direct arylation, Suzuki and Stille coupling and oxidative polymerization. These polymers were used to fabricate bulk heterojunction organic photovoltaic (OPV) devices with fullerene derivatives. The active layer comprising the double acceptor polymer with fullerene derivatives showed a subtractive color resulting from wide-range visible light absorption, which is advantageous for practical OPV applications.

Correction to Bis(trin-hexylsilyl oxide) Silicon Phthalocyanine: A Unique Additive in Ternary Bulk Heterojunction Organic Photovoltaic Devices.

Correction to Bis(trin-hexylsilyl oxide) Silicon Phthalocyanine: A Unique Additive in Ternary Bulk Heterojunction Organic Photovoltaic Devices.

Quantitative Analyses of Competing Photocurrent Generation Mechanisms in Fullerene-Based Organic Photovoltaics

The performance of fullerene-based organic photovoltaic devices (OPVs) with low donor concentrations is not limited by the trade-off between short-circuit current density (Jsc) and open-circuit voltage (Voc), unlike bulk heterojunction OPVs. While the high Voc in this novel type of OPVs has been studied, here we investigate the mechanisms that govern Jsc, which are not well understood. Three mechanisms, diffusion limited exciton relaxation, geminate recombination during exciton dissociation, and nongeminate recombination during charge transport, are studied analytically by combining various experimental techniques and transfer matrix simulation. We find that exciton dissociation at donor/acceptor interfaces is the dominant factor to produce high Jsc, and at low P3HT concentrations exciton relaxation limits photocurrent generation. With more P3HT inclusion, the creation of interfaces promotes exciton dissocation but also reduces fullerene crystallinity, weakening the driving force for charge separation, an...

Electronic and structural properties in thermally annealed PSiF-DBT:PC71BM blends for organic photovoltaics

We have performed X-ray Photoelectron Spectroscopy (XPS) and NK Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy on PSiF-DBT:PC71BM, which is a low bandgap polymer blended with a C70 fullerene derivative, aiming at understanding the change in electronic and structural properties caused by thermal annealing, since these are crucial parameters for the efficiency of bulk-heterojunction organic photovoltaic devices. We observe polymer segregation and a preferred lying-down orientation of the benzothiadazole unit for blends that underwent annealing. An optimization of the film orientational order is essential for a better charge and exciton mobility, thus, enhancing the efficiency of the device. In addition, theoretical calculations at Configuration Interaction with Singles and Doubles excitations (CISD) level, including the relaxation of the core orbitals due to the hole formation are used to interpret the photoabsorption spectra.

Probing the Energy Level Alignment and the Correlation with Open-Circuit Voltage in Solution-Processed Polymeric Bulk Heterojunction Photovoltaic Devices.

Energy level alignment at the organic donor and acceptor interface is a key to determine the photovoltaic performance in organic solar cells, but direct probing of such energy alignment is still challenging especially for solution-processed bulk heterojunction (BHJ) thin films. Here we report a systematic investigation on probing the energy level alignment with different approaches in five commonly used polymer:[6,6]-phenyl-C71-butyric acid methyl ester (PCBM) BHJ systems. We find that by tuning the weight ratio of polymer to PCBM the electronic features from both polymer and PCBM can be obtained by photoemission spectroscopy. Using this approach, we find that some of the BHJ blends simply follow vacuum level alignment, but others show strong energy level shifting as a result of Fermi level pinning. Independently, by measuring the temperature-dependent open-circuit voltage (VOC), we find that the effective energy gap (Eeff), the energy difference between the highest occupied molecular orbital of the polymer donor (EHOMO-D) and lowest unoccupied molecular orbital of the PCBM acceptor (ELUMO-A), obtained by photoemission spectroscopy in all polymer:PCBM blends has an excellent agreement with the extrapolated VOC at 0 K. Consequently, the photovoltage loss of various organic BHJ photovoltaic devices at room temperature is in a range of 0.3-0.6 V. It is believed that the demonstrated direct measurement approach of the energy level alignment in solution-processed organic BHJ will bring deeper insight into the origin of the VOC and the corresponding photovoltage loss mechanism in organic photovoltaic cells.

Rapid and Checkable Electrical Post-Treatment Method for Organic Photovoltaic Devices.

Post-treatment processes improve the performance of organic photovoltaic devices by changing the microscopic morphology and configuration of the vertical phase separation in the active layer. Thermal annealing and solvent vapor (or chemical) treatment processes have been extensively used to improve the performance of bulk-heterojunction (BHJ) organic photovoltaic (OPV) devices. In this work we introduce a new post-treatment process which we apply only electrical voltage to the BHJ-OPV devices. We used the commercially available P3HT [Poly(3-hexylthiophene)] and PC61BM (Phenyl-C61-Butyric acid Methyl ester) photovoltaic materials as donor and acceptor, respectively. We monitored the voltage and current applied to the device to check for when the post-treatment process had been completed. This electrical treatment process is simpler and faster than other post-treatment methods, and the performance of the electrically treated solar cell is comparable to that of a reference (thermally annealed) device. Our results indicate that the proposed treatment process can be used efficiently to fabricate high-performance BHJ-OPV devices.

Sequentially Different AB Diblock and ABA Triblock Copolymers as P3HT:PCBM Interfacial Compatibilizers for Bulk-Heterojunction Photovoltaics

The P3HT:PCBM (P3HT = poly(3-hexylthiophene, PCBM = phenyl-C61-butyric acid methyl ester) bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells using the AB diblock and ABA triblock copolymers (A = polystyrene derivative with donor-acceptor units (PTCNE) and B = P3HT) as compatibilizers were fabricated. Under the optimized blend ratio of the block copolymer, the power conversion efficiency (PCE) was enhanced. This PCE enhancement was clearly related to the increased short-circuit current (J(sc)) and fill factor (FF). The incident photon to current efficiency (IPCE) measurement suggested that the P3HT crystallinity was improved upon addition of the block copolymers. The increased P3HT crystallinity was consistent with the increased photovoltaic parameters, such as J(sc), FF, and consequently the PCE. The surface energies of these block copolymers suggested their thermodynamically stable location at the interface of P3HT:PCBM, showing the efficient compatibilizing performance, resulting in enlarging and fixing the interfacial area and suppressing the recombination of the generated carriers. Grazing incidence X-ray scattering (GIXS) results confirmed the superior compatibilizing performance of the ABA triblock copolymer when compared to the AB diblock copolymer by the fact that, after blending the ABA triblock copolymer in the P3HT:PCBM system, the enhanced crystallinity of matrix P3HT was observed in the excluded areas of the less-aggregated PCBM domains, changing the P3HT crystalline domain orientation from "edge-on" to "isotropic". This is, to the best of our knowledge, the first sequential effect (AB vs ABA) of the block copolymers on the compatibilizing performances based on BHJ OPV device systems.

Donor–acceptor polymers with a regioregularly incorporated thieno[3,4-b]thiophene segment as a π-bridge for organic photovoltaic devices

A series of donor-acceptor (D–A) polymers with regioregularly incorporated a thieno[3,4-b]thiophene as a π-bridge between electron donor and electron acceptor segments were successfully synthesized for the first time. The optical, thermal, and electrochemical properties of the D–A polymers with the thieno[3,4-b]thiophene as a π-bridge were characterized by UV/vis spectroscopy, thermogravimetric analysis, and cyclic voltammetry measurements. They exhibited low energy bandgap because of extended π-conjugation length and increased electron density through conjugated polymer backbones. Moreover, they showed suitable HOMO and LUMO energy levels for photovoltaic applications. The bulk heterojunction organic photovoltaic devices based on the D–A polymers with the regioregularly incorporated thieno[3,4-b]thiophene segment exhibited about 10% higher power conversion efficiency (PCE) than the D–A polymer with thienyl segment as a π-bridge due to enhanced light harvesting ability and excellent nanoscale network. This study implies that the D–A polymers with the regioregularly incorporated thieno[3,4-b]thiophene segment as a π-bridge can achieve high PCE through structural modification of the polymers.

Doped Interlayers for Improved Selectivity in Bulk Heterojunction Organic Photovoltaic Devices

Poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is less selective for holes in an inverted‐architecture organic photovoltaic (OPV) than it is in a conventional‐architecture OPV device due to differences between the interfacial‐PSS concentration at the top and bottom of the PEDOT:PSS layer. In this work, thin layers of polysulfonic acids are inserted between the poly(3‐hexylthiophene) (P3HT):indene‐C60 bisadduct (ICBA) bulk heterojunction (BHJ) active layer and PEDOT:PSS to create a higher concentration of acid at this interface and, therefore, mimic the distribution of materials present in a conventional device. Upon thermal annealing, this acid layer oxidizes P3HT, creating a thin p‐type interlayer of P3HT+/acid− on top of the BHJ. Using X‐ray absorption spectroscopy, Kelvin probe, and ellipsometry measurements, this P3HT+/acid− layer is shown to be insoluble in water, indicating it remains intact during the subsequent deposition of PEDOT:PSS. Current density–voltage measurements show this doped interlayer reduces injected dark current while increasing both open‐circuit voltage and fill factor through the creation of a more hole selective BHJ‐PEDOT:PSS interface.

Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells

The bulk heterojunction (BHJ) organic photovoltaic (OPV) architecture has dominated the literature due to its ability to be implemented in devices with relatively high efficiency values. However, a simpler device architecture based on a single organic semiconductor (SS-OPV) offers several advantages: it obviates the need to control the highly system-dependent nanoscale BHJ morphology, and therefore, would allow the use of broader range of organic semiconductors. Unfortunately, the photocurrent in standard SS-OPV devices is typically very low, which generally is attributed to inefficient charge separation of the photogenerated excitons. Here we show that the short-circuit current density from SS-OPV devices can be enhanced significantly (∼100-fold) through the use of inverted device configurations, relative to a standard OPV device architecture. This result suggests that charge generation may not be the performance bottleneck in OPV device operation. Instead, poor charge collection, caused by defect-induced electric field screening, is most likely the primary performance bottleneck in regular-geometry SS-OPV cells. We justify this hypothesis by: (i) detailed numerical simulations, (ii) electrical characterization experiments of functional SS-OPV devices using multiple polymers as active layer materials, and (iii) impedance spectroscopy measurements. Furthermore, we show that the collection-limited photocurrent theory consistently interprets typical characteristics of regular SS-OPV devices. These insights should encourage the design and OPV implementation of high-purity, high-mobility polymers, and other soft materials that have shown promise in organic field-effect transistor applications, but have not performed well in BHJ OPV devices, wherein they adopt less-than-ideal nanostructures when blended with electron-accepting materials.

Relating Nongeminate Recombination to Charge-Transfer States in Bulk Heterojunction Organic Photovoltaic Devices

The importance of nongeminate recombination of free photogenerated carriers through charge-transfer (CT) states compared with at the electrode interfaces in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices was investigated using impedance spectroscopy (IS) and low-energy external quantum efficiency (EQE) measurement. Thin (∼60 nm) poly(di(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-co-octylthieno[3,4-c] pyrrole-4,6-dione):[6,6]-phenyl-C70-butyric acid methyl ester (PBDT-TPD:PC71BM) OPVs processed with varying concentrations of solvent additive 1-chloronaphthalene (CN) provide a well-controlled system in which only the donor–acceptor interfacial area, hence CT concentration, is changed. We found that additive inclusion resulted in increased CT state concentration, measured by low-energy EQE spectroscopy, and higher nongeminate recombination, determined from IS. In contrast, the energetic disorder in the BHJ, as determined from the dependence of carrier density on open-circuit voltage, did not show a dependence on CN concentration, suggesting that it is not related to changes in donor–acceptor morphology. The correlated relationship between nongeminate recombination strength and CT state concentrations presents unambiguous evidence of CT states as the major channels in nongeminate recombination loss in these BHJ OPV devices.

Enhanced Stability of Aluminum Nanoparticle-Doped Organic Solar Cells.

Enhancement of the stability of bulk heterojunction (BHJ) organic photovoltaic (OPV) devices is reported by the addition of surfactant-free aluminum (Al) nanoparticles (NPs) into the photoactive layer. The universality of the effect is demonstrated for two different BHJ systems, namely, the well-studied poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as well as the high efficient poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC71BM). It is shown that the lifetime of the devices with Al NPs, operating under continuous one-sun illumination in ambient conditions, is more than three times longer compared to the reference devices. Using complementary analytical techniques for in situ studies, we have explored the underlying mechanisms behind the observed stability improvement in the case of the P3HT:PCBM system. In particular, laser-induced fluorescence (LIF), photoluminescence decay and Fourier transform infrared (FTIR) spectroscopy experiments were performed and complemented with device degradation electrical measurements. It is found that the embedded Al NPs act as performance stabilizers, giving rise to enhanced structural stability of the active blend. Furthermore, it is revealed that the observed improvement can also be ascribed to NP-mediated mitigation of the photo-oxidation effect. This study addresses a major issue in OPV devices, that is, photoinduced stability, indicating that the exploitation of Al NPs could be a successful approach toward fabricating OPVs exhibiting long-term operating lifetimes.

Thickness-dependent internal quantum efficiency of narrow band-gap polymer-based solar cells

Active layer thickness-dependent internal quantum efficiencies (IQE) of photocurrent within bulk heterojunction organic photovoltaic (OPV) devices were characterized. These active layers consisted of an electron-donating narrow band-gap polymer and an electron-accepting fullerene derivative. In order to calculate IQE spectra as a function of active layer thickness, incident photon-to-current conversion efficiency (IPCE) spectra and internal absorptance spectra of active layers with various thicknesses were estimated in these solar cell devices. The transfer matrix method (TMM) was used to calculate the internal absorptance spectra of active layers by using experimental optical constants of thin layers typical for these types of OPV devices including narrow band-gap polymer blend film. In addition, spatially resolved absorptance spectra were used to analyze obtained IPCE spectra as well as OPV device parameters (e.g., short circuit current density) at various active layer thicknesses. Finally, charge-carrier-collecting probability as a function of active layer thickness was suggested with which the relationship between initial exciton generation and final power conversion efficiency can be more quantitatively described.

Photoelectric Memory Effect in an Organic Bulk Heterojunction Device

We report on a memory effect observed in an inverted bulk heterojunction organic photovoltaic device, where the electron-collecting electrode is ZnO nanowires grown on an indium tin oxide (ITO) substrate. The device presented a unique response to light by switching from a rectifying behavior in the dark to a resistive response under illumination. After cessation of light, the device slowly stabilized back to its rectifying nature after ∼270 min, introducing a volatile photoelectric memory effect. The reversibility of the response is verified through multiple cycles of light exposure and placing the device in the dark. The device is also illuminated with different light intensities to study the photovoltaic response through I–V characterization. It is found that the time constant associated with the transition between the rectifying and resistive characteristics is independent from the light intensity. Further study revealed that there is a hysteresis loop in the I–V curve in the dark, but the loop vanished in the resistive mode under illumination. A mechanism based on oxygen absorption–desorption has been suggested to explain the observed effect. Such a memory effect can be used in various optoelectronic devices to save the optical information for an extended time.

The interplay between structure, processing, and properties in organic photovoltaic devices: how to translate recent laboratory-scale developments to modules

The design of π-conjugated molecules and polymers has driven the increase in efficiency of bulk heterojunction organic photovoltaic devices from <1% to over 12%. The pathways to generation of free charge carriers are still being uncovered. By focusing on blends of conjugated polymers with fullerenes, recent work has highlighted the impact of the design of donor–acceptor polymers on optoelectronic properties and phase-separated morphologies. This morphology of the active layer is largely controlled by processing conditions, such as use of processing additives. Developing a deep understanding of the impact of polymer chemistry and processing at the laboratory scale is key to translating the technology of organic photovoltaics from the research scale to large-area modules.

Absence of Structural Impact of Noble Nanoparticles on P3HT:PCBM Blends for Plasmon-Enhanced Bulk-Heterojunction Organic Solar Cells Probed by Synchrotron GI-XRD.

The incorporation of noble metal nanoparticles, displaying localized surface plasmon resonance, in the active area of donor-acceptor bulk-heterojunction organic photovoltaic devices is an industrially compatible light trapping strategy, able to guarantee better absorption of the incident photons and give an efficiency improvement between 12% and 38%. In the present work, we investigate the effect of Au and Ag nanoparticles blended with P3HT: PCBM on the P3HT crystallization dynamics by synchrotron grazing incidence X-ray diffraction. We conclude that the presence of (1) 80 nm Au, (2) mix of 5 nm, 50 nm, 80 nm Au, (3) 40 nm Ag, and (4) 10 nm, 40 nm, 60 nm Ag colloidal nanoparticles, at different concentrations below 0.3 wt% for Au and below 0.1% for Ag in P3HT: PCBM blends, does not affect the behaviour of the blends themselves.

Quinoxalineimide as a Novel Electron-accepting Building Block for Organic Optoelectronics

Abstract Organic semiconductors with a donor–acceptor–donor architecture based on the novel acceptor unit of quinoxalineimide (QI) were designed and synthesized. They absorbed wider wavelength light compared to PC61BM, and functioned as acceptor materials for solution-processed bulk-heterojunction organic photovoltaic devices. In addition, the relatively shallow LUMO levels, due to the electron-withdrawing pyrazine and imide groups of the QI unit, resulted in a high open-circuit voltage of the devices.

Plasmonic Bulk Heterojunction Solar Cells: The Role of Nanoparticle Ligand Coating

There has been a lot of interest regarding the influence of active-layer-incorporated plasmonic nanoparticles (NPs) in the performance of bulk heterojunction organic photovoltaic (OPV) devices, while both an increase and decrease in performance have been reported. In this paper, following a systematic approach, we demonstrate strong evidence of the critical importance of the NPs’ ligand shell on the device performance. In particular, it is argued that the plasmonic effect accountable for the performance enhancement takes place only in the case in which the NP’s core is in direct contact with the active layer polymer donor. This can be achieved with the utilization of either ligand-free NPs or NPs terminated with the same polymer donor as the active layer. Using this concept we achieved an enhanced efficiency of 7.16% in OPV devices incorporating the poly(3-hexylthiophene-2,5-diyl) (P3HT):indene-C60 bisadduct (ICBA) active layer. On the contrary, devices with ligand-terminated Au NPs show lower performance...