Photocurrent Extraction Efficiency near Unity in a Thick Polymer Bulk Heterojunction

The detailed characterization of a dialkoxyphenylene‐difluorobenzothiadiazole based conjugated polymer poly[(2,5‐bis(2‐hexyldecyloxy)phenylene)‐alt‐(5,6‐difluoro‐4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) is reported. PPDT2FBT closely tracks theoretical photocurrent production while maintaining a high fill factor in remarkably thick films. In order to understand the properties that enable PPDT2FBT to function with thick active layers, the effect of film thickness on the material properties and device parameters was carefully studied and compared to three benchmark polymers. Optical modeling, grazing incidence wide angle X‐ray scattering, cross‐sectional transmission electron microscopy, transient photoconductivity, and extensive device work were carried out and have clarified the key structural features and properties that allow such thick active layers to function efficiently. The unique behavior of thick PPDT2FBT films arises from high vertical carrier mobility, an isotropic morphology with strong, vertical π–π stacking, and a suitable energy band structure. These physical characteristics allow efficient photocurrent extraction, internal quantum efficiencies near 100% and power conversion efficiencies over 9% from exceptionally thick active layers in both conventional and inverted architectures. The ability of PPDT2FBT to function efficiently in thick cells allows devices to fully attenuate incident sunlight while providing a pathway to defect‐free film processing over large areas, constituting a major advancement toward commercially viable organic solar cells.     

Semiconducting Carbon Nanotubes for Improved Efficiency and Thermal Stability of Polymer–Fullerene Solar Cells

The effects of the incorporation of semiconducting single‐walled nanotubes (sc‐SWNTs) with high purity on the bulk heterojunction (BHJ) organic solar cell (OSC) based on regioregular poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (rr‐P3HT:PCBM) are reported for the first time. The sc‐SWNTs induce the organization of the polymer phase, which is evident from the increase in crystallite size, the red‐shifted absorption characteristics and the enhanced hole mobility. By incorporating sc‐SWNTs, OSC with a power conversion efficiency (PCE) as high as 4% can be achieved, which is ≈8% higher than our best control device. A novel application of sc‐SWNTs in improving the thermal stability of BHJ OSCs is also demonstrated. After heating at 150 °C for 9 h, it is observed that the thermal stability of rr‐P3HT:PCBM devices improves by more than fivefold with inclusion of sc‐SWNTs. The thermal stability enhancement is attributed to a more suppressed phase separation, as shown by the remarkable decrease in the formation of sizeable crystals, which in turn can be the outcome of a more controlled crystallization of the blend materials on the nanotubes.     

Charge Density Dependent Nongeminate Recombination in Organic Bulk Heterojunction Solar Cells

Apparent recombination orders exceeding the value of two expected for bimolecular recombination have been reported for organic solar cells in various publications. Two prominent explanations are bimolecular losses with a carrier concentration dependent prefactor due to a trapping limited mobility and protection of trapped charge carriers from recombination by a donor–acceptor phase separation until re‐emission from these deep states. In order to clarify which mechanism is dominant temperature‐ and illumination‐dependent charge extraction measurements are performed under open circuit and short circuit conditions at poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C₆₁ butyric acid methyl ester (P3HT:PC₆₁BM) and PTB7:PC₇₁BM (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]]) solar cells in combination with current–voltage characteristics. It is shown that the charge carrier density n dependence of the mobility μ and the recombination prefactor are different for P3HT:PC₆₁BM at temperatures below 300 K and PTB7:PC₇₁BM at room temperature. Therefore, in addition to μ(n), a detrapping limited recombination in systems with at least partial donor–acceptor phase separation is required to explain the high recombination orders.     

A Versatile Fluoro‐Containing Low‐Bandgap Polymer for Efficient Semitransparent and Tandem Polymer Solar Cells

The versatility of a fluoro‐containing low band‐gap polymer, poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b’]dithiophene)‐alt‐4,7‐(5‐fluoro‐2,1,3‐benzothia‐diazole)] (PCPDTFBT) in organic photovoltaics (OPVs) applications is demonstrated. High boiling point 1,3,5‐trichlorobenzene (TCB) is used as a solvent to manipulate PCPDTFBT:[6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) active layer morphology to obtain high‐performance single‐junction devices. It promotes the crystallization of PCPDTFBT polymer, thus improving the charge‐transport properties of the active layer. By combining the morphological manipulation with interfacial optimization and device engineering, the single‐junction device exhibits both good air stability and high power‐conversion efficiency (PCE, of 6.6%). This represents one of the highest PCE values for cyclopenta[2,1‐b;3,4‐b’]dithiophene (CPDT)‐based OPVs. This polymer is also utilized for constructing semitransparent solar cells and double‐junction tandem solar cells to demonstrate high PCEs of 5.0% and 8.2%, respectively.     

Comparison of Two D−A Type Polymers with Each Being Fluorinated on D and A Unit for High Performance Solar Cells

For the purpose of investigating the effect of fluorination position on D?A type conjugated polymer on photophysical and photovoltaic properties, two types of fluorinated polymere are synthesized, HF with fluorination on electron‐donating unit and FH with fluorination on electron‐accepting unit. Compared to non‐fluorinated polymer, fluorinated polymers exhibit deeper HOMO energy levels without change of bandgap and stronger vibronic shoulder in UV?visible absorption, indicating that fluorination enhances intermolecular interaction. HF with fluorinated D unit exhibits well‐developed fibril network, low bimolecular recombination and high hole mobility, which lead a high PCE of 7.10% in conventional single‐junction solar cells, which is higher than the PCE (6.41%) of FH with fluorinated A unit. Therefore, this result demonstrates that fluorination on electron‐donating unit in D?A polymers could be a promising strategy for achieving high performance polymer solar cells.     

Enhanced Performance in Polymer Solar Cells by Surface Energy Control

Enhanced performance of an inverted‐type polymer solar cell is reported by controlling the surface energy of a zinc oxide (ZnO) buffer layer, on which a photoactive layer composed of a polymer:fullerene‐derivative bulk heterojunction is formed. With the approach based on a mixed self‐assembled monolayer, the surface energy of the ZnO buffer layer can be controlled between 40 mN m^?1 and 70 mN m^?1 with negligible changes in its work function. For the given range of surface energy the power conversion efficiency increases from 3.27% to 3.70% through enhanced photocurrents. The optimized morphology obtained by surface energy control results in the enhanced photocurrent and transmission electron microscopy analysis verifies the correlation between the surface energy and the phase morphology of the bulk heterojunction. These results demonstrate that surface energy control is an effective method for further improving the performance of polymer solar cells, with potentially important implications for other organic devices containing an interface between a blended organic active layer and a buffer or an electrode layer.     

Recombination‐Limited Photocurrents in Low Bandgap Polymer/Fullerene Solar Cells

The charge transport and photogeneration in solar cells based on the low bandgap‐conjugated polymer, poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b; 3,4‐b′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] (PCPDTBT) and fullerenes is studied. The efficiency of the solar cells is limited by a relatively low fill factor, which contradicts the observed good and balanced charge transport in these blends. Intensity dependent measurements display a recombination limited photocurrent, characterized by a square root dependence on effective applied voltage, a linear dependence on light intensity and a constant saturation voltage. Numerical simulations show that the origin of the recombination limited photocurrent stems from the short lifetime of the bound electron‐hole pairs at the donor/acceptor interface.     

Quinacridone‐Based Electron Transport Layers for Enhanced Performance in Bulk‐Heterojunction Solar Cells

A water/alcohol‐soluble small molecule based on the commercially available pigment quinacridone is employed as an electron transport layer in organic photovoltaics. The quinacridone derivative is utilized in solution‐processed bulk‐heterojunction solar cells to improve primarily the fill factor of the devices, contributing to an upwards of 19% enhancement in the power conversion efficiency relative to the control devices with no electron transport layer. The facile synthesis of the quinacridone derivative coupled with the ease of device fabrication via solution processing provide a simple, yet effective means of improving the performance of existing organic photovoltaic cells.     

Alternating Copolymers of Cyclopenta[2,1‐b;3,4‐b′]dithiophene and Thieno[3,4‐c]pyrrole‐4,6‐dione for High‐Performance Polymer Solar Cells

A series of alternating copolymers of cyclopenta[2,1‐b;3,4‐b′]dithiophene (CPDT) and thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) have been prepared and characterized for polymer solar cell (PSC) applications. Different alkyl side chains, including butyl (Bu), hexyl (He), octyl (Oc), and 2‐ethylhexyl (EH), are introduced to the TPD unit in order to adjust the packing of the polymer chain in the solid state, while the hexyl side chain on the CPDT unit remains unchanged to simplify discussion. The polymers in this series have a simple main chain structure and can be synthesized easily, have a narrow band gap and a broad light absorption. The different alkyl chains on the TPD unit not only significantly influence the solubility and chain packing, but also fine tune the energy levels of the polymers. The polymers with Oc or EH group have lower HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels, resulting higher open circuit voltages (V_oc) of the PSC devices. Power conversion efficiencies (PCEs) up to 5.5% and 6.4% are obtained from the devices of the Oc substituted polymer (PCPDTTPD‐Oc) with PC₆₁BM and PC₇₁BM, respectively. This side chain effect on the PSC performance is related to the formation of a fine bulk heterojunction structure of polymer and PCBM domains, as observed with atomic force microscopy.     

Improved Performance of Molecular Bulk‐Heterojunction Photovoltaic Cells through Predictable Selection of Solvent Additives

Solvent additives provide an effective means to alter the morphology and thereby improve the performance of organic bulk‐heterojunction photovoltaics, although guidelines for selecting an appropriate solvent additive remain relatively unclear. Here, a family of solvent additives spanning a wide range of Hansen solubility parameters is applied to a molecular bulk‐heterojunction system consisting of an isoindigo and thiophene containing oligomer as the electron donor and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) as the electron acceptor. Hansen solubility parameters are calculated using the group contribution method and compared with the measured solubilities for use as a screening method in solvent additive selection. The additives are shown to alter the morphologies in a semipredictable manner, with the poorer solvents generally resulting in decreased domain sizes, increased hole mobilities, and improved photovoltaic performance. The additives with larger hydrogen bonding parameters, namely triethylene glycol (TEG) and N‐methyl‐2‐pyrrolidone (NMP), are demonstrated to increase the open circuit voltage by ~0.2 V. Combining a solvent additive observed to increase short circuit current, poly(dimethylsiloxane), with TEG results in an increase in power conversion efficiency from 1.4 to 3.3%.     

Synthesis and Characterization of New Thieno[3,4‐c]pyrrole‐4,6‐dione Derivatives for Photovoltaic Applications

A new class of low‐bandgap copolymers based on benzodithiophene (BDT) and thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) units is reported. Chemical modifications of the conjugated backbone promote both high molecular weights and processability while allowing for tuning of the electronic properties. Copolymers with substituted thiophene spacers (alkyl chains facing the BDT unit) or unsubstituted thiophene spacers tend to have low power conversion efficiencies (PCE less than 1%) due to a bad morphology of the active layer, whereas copolymers with substituted thiophene spacers (alkyl chains facing TPD unit) show enhanced morphology and PCEs up to of 3.9%. Finally, BDT‐TPD copolymers without any thiophene spacers still show the best performances with power conversion efficiencies up to 5.2%.     

Highly Efficient Inverted Organic Solar Cells Through Material and Interfacial Engineering of Indacenodithieno[3,2‐b]thiophene‐Based Polymers and Devices

A synergistic approach combining new material design and interfacial engineering of devices is adopted to produce high efficiency inverted solar cells. Two new polymers, based on an indacenodithieno[3,2‐b]thiophene‐difluorobenzothiadiazole (PIDTT‐DFBT) donor–acceptor (D–A) polymer, are produced by incorporating either an alkyl thiophene (PIDTT‐DFBT‐T) or alkyl thieno[3,2‐b]thiophene (PIDTT‐DFBT‐TT) π‐bridge as spacer. Although the PIDTT‐DFBT‐TT polymer exhibits decreased absorption at longer wavelengths and increased absorption at higher energy wavelengths, it shows higher power conversion efficiencies in devices. In contrast, the thiophene bridged PIDTT‐DFBT‐T shows a similar change in its absorption spectrum, but its low molecular weight leads to reduced hole mobilities and performance in photovoltaic cells. Inverted solar cells based on PIDTT‐DFBT‐TT are explored by modifying the electron‐transporting ZnO layer with a fullerene self‐assembled monolayer and the MoO₃ hole‐transporting layer with graphene oxide. This leads to power conversion efficiencies as high as 7.3% in inverted cells. PIDTT‐DFBT‐TT's characteristic strong short wavelength absorption and high efficiency suggests it is a good candidate as a wide band gap material for tandem solar cells.     

Open‐Circuit Voltage and Effective Gap of Organic Solar Cells

The open‐circuit voltage (V_OC) of an organic solar cell is limited by the donor‐acceptor material system. The effective gap E_g^eff between the electron affinity of the acceptor and the ionization potential of the donor is usually regarded as the upper limit for V_OC, which is only reached for T → 0 K. This relation is confirmed for a number of small‐molecule bulk heterojunction p‐i‐n type solar cells by varying the temperature and illumination intensity. With high precision, the low temperature limit of V_OC is identical to E_g^eff. Furthermore, the influence of the hole transport material in a p‐doped hole transport layer and the donor‐acceptor mixing ratio on this limit V₀ is found to be negligible. Varying the active material system, the quantitative relation between V₀ and E_g^eff is found to be identity. A comparison of V₀ in a series of nine different donor‐acceptor material combinations opens a pathway to quantitatively determine the ionization potential of a donor material or the electron affinity of an acceptor material.     

Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post‐Treatment for ITO‐Free Organic Solar Cells

Highly conductive poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand‐alone electrodes for organic solar cells have been optimized using a solvent post‐treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm^?1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO‐free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C₆₀ bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre‐heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post‐treatment can be a very promising electrode material for highly efficient flexible ITO‐free organic solar cells.     

Nongeminate Recombination and Charge Transport Limitations in Diketopyrrolopyrrole‐Based Solution‐Processed Small Molecule Solar Cells

Charge transport and nongeminate recombination are investigated in two solution‐processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)‐based donor molecules, mono‐DPP and bis‐DPP, blended with [6,6]‐phenyl‐C71‐butyric acid methyl ester (PCBM). While the bis‐DPP system exhibits a high fill factor (62%) the mono‐DPP system suffers from pronounced voltage dependent losses, which limit both the fill factor (46%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current‐voltage characteristics indicate that the mono‐DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most significant factor leading to the difference in fill factor is the comparatively poor hole transport properties in the mono‐DPP system (2 × 10^?5 cm² V^?1 s^?1 versus 34 × 10^?5 cm² V^?1 s^?1). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination.     

Enhanced Omnidirectional Photovoltaic Performance of Solar Cells Using Multiple‐Discrete‐Layer Tailored‐ and Low‐Refractive Index Anti‐Reflection Coatings

An optimized four‐layer tailored‐ and low‐refractive index anti‐reflection (AR) coating on an inverted metamorphic (IMM) triple‐junction solar cell device is demonstrated. Due to an excellent refractive index matching with the ambient air by using tailored‐ and low‐refractive index nanoporous SiO₂ layers and owing to a multiple‐discrete‐layer design of the AR coating optimized by a genetic algorithm, such a four‐layer AR coating shows excellent broadband and omnidirectional AR characteristics and significantly enhances the omnidirectional photovoltaic performance of IMM solar cell devices. Comparing the photovoltaic performance of an IMM solar cell device with the four‐layer AR coating and an IMM solar cell with the conventional SiO₂/TiO₂ double layer AR coating, the four‐layer AR coating achieves an angle‐of‐incidence (AOI) averaged short‐circuit current density, J_SC, enhancement of 34.4%, whereas the conventional double layer AR coating only achieves an AOI‐averaged J_SC enhancement of 25.3%. The measured reflectance reduction and omnidirectional photovoltaic performance enhancement of the four‐layer AR coating are to our knowledge, the largest ever reported in the literature of solar cell devices.     

Solvent‐Assisted Low‐Temperature Crystallization of SnO2 Electron‐Transfer Layer for High‐Efficiency Planar Perovskite Solar Cells

A high‐quality polycrystalline SnO₂ electron‐transfer layer is synthesized through an in situ, low‐temperature, and unique butanol–water solvent‐assisted process. By choosing a mixture of butanol and water as a solvent, the crystallinity is enhanced and the crystallization temperature is lowered to 130 °C, making the process fully compatible with flexible plastic substrates. The best solar cells fabricated using these layers achieve an efficiency of 20.52% (average 19.02%) which is among the best in the class of planar n–i–p‐type perovskite (MAPbI₃) solar cells. The strongly reduced crystallization temperature of the materials allows their use on a flexible substrate, with a resulting device efficiency of 18%.     

Formation Mechanism of Fullerene Cation in Bulk Heterojunction Polymer Solar Cells

The charge carrier dynamics in blend films of [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) and conjugated polymers with different ionization potentials are measured using transient absorption spectroscopy to study the formation mechanism of PCBM radical cation, which was previously discovered for blend films of poly[2‐methoxy‐5‐(3,7‐dimethyloctyloxy)‐1,4‐phenylenevinylene] (MDMO‐PPV) and PCBM. On a nanosecond time scale after photoexcitation, polymer hole polaron and PCBM radical anion are observed but no PCBM radical cation is found in the blends. Subsequently, the fraction of polymer hole polarons decreases and that of PCBM radical cations increases with time. Finally, the fraction of PCBM radical cations becomes constant on a microsecond time scale. The final fraction of PCBM radical cation is dependent on the ionization potential of polymers but independent of the excitation wavelength. These findings show that the formation of PCBM radical cation is due to hole injection from polymer to PCBM domains. Furthermore, the energetic conditions for such hole injection in polymer/PCBM blend films are discussed on the basis of Monte Carlo analysis for hole hopping in a disordered donor/acceptor heterojunction with varying energetic parameters.     

Nanoscale Morphology of Conjugated Polymer/Fullerene‐Based Bulk‐ Heterojunction Solar Cells

The relation between the nanoscale morphology and associated device properties in conjugated polymer/fullerene bulk‐heterojunction “plastic solar cells” is investigated. We perform complementary measurements on solid‐state blends of poly[2‐methoxy‐5‐(3,7‐dimethyloctyloxy)]‐1,4‐phenylenevinylene (MDMO‐PPV) and the soluble fullerene C₆₀ derivative 1‐(3‐methoxycarbonyl) propyl‐1‐phenyl [6,6]C₆₁ (PCBM), spin‐cast from either toluene or chlorobenzene solutions. The characterization of the nanomorphology is carried out via scanning electron microscopy (SEM) and atomic force microscopy (AFM), while solar‐cell devices were characterized by means of current–voltage (I–V) and spectral photocurrent measurements. In addition, the morphology is manipulated via annealing, to increase the extent of phase separation in the thin‐film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials under thermal treatment. Furthermore the photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase‐separated blends cast from toluene. For toluene‐cast films the scale of phase separation depends strongly on the ratio of MDMO‐PPV to PCBM, as well as on the total concentration of the casting solution. Finally we observe small beads of 20–30 nm diameter, attributed to MDMO‐PPV, in blend films cast from both toluene and chlorobenzene.     

Polymer/Nanocrystal Hybrid Solar Cells: Influence of Molecular Precursor Design on Film Nanomorphology,Charge Generation and Device Performance

In this work, molecular tuning of metal xanthate precursors is shown to have a marked effect on the heterojunction morphology of hybrid poly(3‐hexylthiophene‐2,5‐diyl) (P3HT)/CdS blends and, as a result, the photochemical processes and overall performance of in situ fabricated hybrid solar cells. A series of cadmium xanthate complexes is synthesized for use as in situ precursors to cadmium sulfide nanoparticles in hybrid P3HT/CdS solar cells. The formation of CdS domains is studied by simultaneous GIWAXS (grazing incidence wide‐angle X‐ray scattering) and GISAXS (grazing incidence small‐angle X‐ray scattering), revealing knowledge about crystal growth and the formation of different morphologies observed using TEM (transmission electron microscopy). These measurements show that there is a strong relationship between precursor structure and heterojunction nanomorphology. A combination of TAS (transient absorption spectroscopy) and photovoltaic device performance measurements is used to show the intricate balance required between charge photogeneration and percolated domains in order to effectively extract charges to maximize device power conversion efficiencies. This study presents a strong case for xanthate complexes as a useful route to designing optimal heterojunction morphologies for use in the emerging field of hybrid organic/inorganic solar cells, due to the fact that the nanomorphology can be tuned via careful design of these precursor materials.