Three‐Dimensional Bulk Heterojunction Morphology for Achieving High Internal Quantum Efficiency in Polymer Solar Cells

Here, an investigation of three‐dimensional (3D) morphologies for bulk heterojunction (BHJ) films based on regioregular poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) is reported. Based on the results, it is demonstrated that optimized post‐treatment, such as solvent annealing, forces the PCBM molecules to migrate or diffuse toward the top surface of the BHJ composite films, which induces a new vertical component distribution favorable for enhancing the internal quantum efficiency (η_IQE ) of the devices. To investigate the 3D BHJ morphology, novel time‐of‐flight secondary‐ion mass spectroscopy studies are employed along with conventional methods, such as UV‐vis absorption, X‐ray diffraction, and high‐resolution transmission electron microscopy studies. The η_IQE of the devices are also compared after solvent annealing for different times, which clearly shows the effect of the vertical component distribution on the performance of BHJ polymer solar cells. In addition, the fabrication of high‐performance P3HT:PCBM solar cells using the optimized solvent‐annealing method is reported, and these cells show a mean power‐conversion efficiency of 4.12% under AM 1.5G illumination conditions at an intensity of 100 mW cm^?2.     

Quantum well model of a conjugated polymer heterostructure solar cell

In current study we analyzed the performance of a conjugated polymer heterostructured solar cell using quantum well representation for 100 Å thin polymer layers. The electrical field across the polymer layer is extremely strong to dissociate all excitons generated by sunlight. The behavior of free electrons appearing as a result of exciton dissociation was analyzed using wavefunctions and set of available energy levels. We came to the conclusion that small but finite probability exists to collect free electrons by the anode of the solar cell. The analyzed device was comprised of layers of bulk heterojunctions, namely, 100 nm layers of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene]:phenyl C61-butyric acid methylester MDMO-PPV:PCBM (1:4), (poly-3-hexylthiophene): phenyl C61-butyric acid methylester P3HT:PCBM (1:1) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopen[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] PCPDTBT:PCBM (1:3). Set of these layers are designed to be connected using transparent cathodes of lithium fluoride/aluminium/gold LiF/Al/Au, which are compatible with PCBM LUMO.     

Dramatically enhanced performances and ideally controlled nano-morphology via co-solvent processing in low bandgap polymer solar cells

The device performance of photovoltaics with a polymer:fullerene bulk heterojunction (BHJ) structure, consisting of DT-PDPP2T-TT donor polymer and poly(3-hexylthiophene):[6,6]phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) acceptor compound, was investigated as a function of co-solvent composition. An enhancement of the photocurrent density and fill factor is observed in diodes made by spin-coating with chloroform mixed with ortho-dichlorobenzene, which allows a significantly higher device efficiency of 5.55% compared to diodes made from neat chloroform (efficiency = 3.61%). To clarify the role of the co-solvent, we investigated the nanoscale morphology with AFM, TEM and 2D-GIWAXS techniques and also the free-charge carrier mobility via space-charge limited current theory. We obtained the result that, under such supersaturated conditions, co-solvents induce increased polymer crystalline aggregation into a 3D phase structure and boost charge-carrier transport characteristics. This provides a rational basis for the development of ideally-controlled BHJ films that yield efficient DT-PDPP2T-TT:PCBM solar cells. Therefore, carefully selecting solvent mixtures provides an approach toward efficient low bandgap polymer solar cells.     

Compositional Dependence of the Performance of Poly(p‐phenylene vinylene):Methanofullerene Bulk‐Heterojunction Solar Cells

The dependence of the performance of OC₁C₁₀‐PPV:PCBM (poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene):methanofullerene [6,6]‐phenyl C₆₁‐butyric acid methyl ester)‐based bulk heterojunction solar cells on their composition has been investigated. With regard to charge transport, we demonstrate that the electron mobility gradually increases on increasing the PCBM weight ratio, up to 80 wt.‐%, and subsequently saturates to its bulk value. Surprisingly, the hole mobility in the PPV phase shows an identical behavior and saturates beyond 67 wt.‐% PCBM, a value which is more than two orders of magnitude higher than that of the pure polymer. The experimental electron and hole mobilities were used to study the photocurrent generation of OC₁C₁₀‐PPV:PCBM bulk‐heterojunction (BHJ) solar cells. From numerical calculations, it is shown that for PCBM concentrations exceeding 80 wt.‐% reduced light absorption is responsible for the loss of device performance. From 80 to 67 wt.‐%, the decrease in power conversion efficiency is mainly due to a decreased separation efficiency of bound electron–hole (e–h) pairs. Below 67 wt.‐%, the performance loss is governed by a combination of a reduced generation rate of e–h pairs and a strong decrease in hole transport.     

Bulk heterojunction bipolar field-effect transistors processed with alkane dithiol

Bipolar filed-effect transistors (BiFETs) fabricated from bulk heterojunction (BHJ) materials comprised of various ratios of the small bandgap 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 the soluble fullerene, [6,6]-phenyl-C71butyric acid methyl ester (PC₇₁BM) are reported. We focus on the effect of the addition of small concentrations of the processing additive, 1,8-octanedithiol (ODT), on the gate-induced transport properties. Processing with the ODT additive increased the mobilities of holes (on the PCPDTBT) and electrons (on the PC₇₁BM). If, however, the ODT was not completely removed from the BHJ films, the hole mobility actually decreased, implying that residual ODT functions as a hole trap.     

Polyacetylene-based polyelectrolyte as a universal interfacial layer for efficient inverted polymer solar cells

Here we report that poly(N-dodecyl-2-ethynylpyridiniumbromide) (PDEPB) interlayers between electron-collecting zinc oxide (ZnO) layers and bulk heterojunction (BHJ) layers act as a universal interfacial layer for improving the performances of inverted-type polymer:fullerene solar cells. Three different BHJ layers, poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), poly[(4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl-alt-(N-2-ethylhexylthieno[3,4-c]pyrrole-4,6-dione)-2,6-diyl]] (PBDTTPD):PC₆₁BM, and poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), were employed so as to prove the role of the PDEPB interlayers. Results showed that the power conversion efficiency (PCE) of polymer:fullerene solar cells with the three different BHJ layers increased in the presence of the PDEPB interlayers prepared from 0.5 mg/ml solutions. The improved PCE was attributed to the conformal coating of the PDEPB layers on the ZnO layers (by atomic force microscopy measurement), lowered work functions of ZnO induced by the PDEPB layers (by Kelvin probe measurement), and reduced interface resistance (by impedance spectroscopy measurement), as supported by the noticeable change in the atom environments of both the ZnO and PDEPB layers (by X-ray photoelectron spectroscopy measurement).     

Photoconductivity of a Low‐Bandgap Conjugated Polymer

The photoconductive properties of a novel 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, with an optical energy gap of E_g ～ 1.5 eV, have been studied. The results of photoluminescence and photoconductivity measurements indicate efficient electron transfer from PCPDTBT to PCBM ([6,6]‐phenyl‐C61 butyric acid methyl ester, a fullerene derivative), where PCPDTBT acts as the electron donor and PCBM as the electron acceptor. Electron‐transfer facilitates charge separation and results in prolonged carrier lifetime, as observed by fast (t > 100 ps) transient photoconductivity measurements. The photoresponsivities of PCPDTBT and PCPDTBT:PCBM are comparable to those of poly(3‐hexylthiophene), P3HT, and P3HT:PCBM, respectively. Moreover, the spectral sensitivity of PCPDTBT:PCBM extends significantly deeper into the infrared, to 900 nm, than that of P3HT. The potential of PCPDTBT as a material for high‐efficiency polymer solar cells is discussed.     

Electrostatic Self‐Assembly Conjugated Polyelectrolyte‐Surfactant Complex as an Interlayer for High Performance Polymer Solar Cells

A simple method is demonstrated to improve the film‐forming properties and air stability of a conjugated polyelectrolyte (CPE) without complicated synthesis of new chemical structures. An anionic surfactant, sodium dodecybenzenesulfonate (SDS), is mixed with cationic CPEs. The electrostatic attraction between these two oppositely‐charged materials provides the driving force to form a stable CPE‐surfactant complex. Compared with a pure CPE, this electrostatic complex is not only compatible with highly hydrophobic bulk‐heterojunction (BHJ) films, e.g. poly(3‐hexylthiophene):[6,6]‐phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM), but also works well with other low bandgap polymer‐based BHJ films. Using this complex as a cathode interface layer, a high power conversion efficiency of 4% can be obtained in P3HT:PCBM solar cells together with improved stability in air. Moreover, ～20% performance enhancement can also be achieved when the complex is used as an interlayer to replace calcium metal for low bandgap polymer‐based BHJ systems.     

Effect of Alkyl Side‐Chain Length on Photovoltaic Properties of Poly(3‐alkylthiophene)/PCBM Bulk Heterojunctions

The morphological, bipolar charge‐carrier transport, and photovoltaic characteristics of poly(3‐alkylthiophene) (P3AT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) blends are studied as a function of alkyl side‐chain length m, where m equals the number of alkyl carbon atoms. The P3ATs studied are poly(3‐butylthiophene) (P3BT, m = 4), poly(3‐pentylthiophene) (P3PT, m = 5), and poly(3‐hexylthiophene) (P3HT, m = 6). Solar cells with these blends deliver similar order of photo‐current yield (exceeding 10 mA cm^?2) irrespective of side‐chain length. Power conversion efficiencies of 3.2, 4.3, and 4.6% are within reach using solar cells with active layers of P3BT:PCBM (1:0.8), P3PT:PCBM (1:1), and P3HT:PCBM (1:1), respectively. A difference in fill factor values is found to be the main source of efficiency difference. Morphological studies reveal an increase in the degree of phase separation with increasing alkyl chain length. Moreover, while P3PT:PCBM and P3HT:PCBM films have similar hole mobility, measured by hole‐only diodes, the hole mobility in P3BT:PCBM lowers by nearly a factor of four. Bipolar measurements made by field‐effect transistor showed a decrease in the hole mobility and an increase in the electron mobility with increasing alkyl chain length. Balanced charge transport is only achieved in the P3HT:PCBM blend. This, together with better processing properties, explains the superior properties of P3HT as a solar cell material. P3PT is proved to be a potentially competitive material. The optoelectronic and charge transport properties observed in the different P3AT:PCBM bulk heterojunction (BHJ) blends provide useful information for understanding the physics of BHJ films and the working principles of the corresponding solar cells.     

Time‐Dependent Morphology Evolution by Annealing Processes on Polymer:Fullerene Blend Solar Cells

Changes in the nanoscale morphologies of the blend films of poly (3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), for high‐performance bulk‐heterojunction (BHJ) solar cells, are compared and investigated for two annealing treatments with different morphology evolution time scales, having special consideration for the diffusion and aggregation of PCBM molecules. An annealing condition with relatively fast diffusion and aggregation of the PCBM molecules during P3HT crystallization results in poor BHJ morphology because of prevention of the formation of the more elongated P3HT crystals. However, an annealing condition, accelerating PCBM diffusion after the formation of a well‐ordered morphology, results in a relatively stable morphology with less destruction of crystalline P3HT. Based on these results, an effective strategy for determining an optimized annealing treatment is suggested that considers the effect of relative kinetics on the crystallization of the components for a blend film with a new BHJ materials pair, upon which BHJ solar cells are based.     

Quantifying space charge accumulation in organic bulk heterojunctions by nonlinear optical microscopy

In this work, we apply electric field induced second harmonic generation microscopy to directly observe and quantify space charge accumulation in operating organic bulk heterojunction photovoltaic cells comprised of poly(4,4-dioctyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) (PSBTBT) mixed with phenyl-C61-butyric acid methyl ester (PCBM). We adjust the effective electron and hole mobility within these blends by altering the relative composition of PSBTBT and PCBM, and observe dramatic shifts in space charge accumulation. The PSBTBT rich device shows strong electron accumulation (2.8 × 10¹⁴ e⁻/cm³) and the PCBM rich device shows strong hole accumulation (5.5 × 10¹⁴ h⁺/cm³).     

Understanding Local and Macroscopic Electron Mobilities in the Fullerene Network of Conjugated Polymer‐based Solar Cells: Time‐Resolved Microwave Conductivity and Theory

The efficiency of bulk heterojunction (BHJ) organic photovoltaics is sensitive to the morphology of the fullerene network that transports electrons through the device. This sensitivity makes it difficult to distinguish the contrasting roles of local electron mobility (how easily electrons can transfer between neighboring fullerene molecules) and macroscopic electron mobility (how well‐connected is the fullerene network on device length scales) in solar cell performance. In this work, a combination of density functional theory (DFT) calculations, flash‐photolysis time‐resolved microwave conductivity (TRMC) experiments, and space‐charge‐limit current (SCLC) mobility estimates are used to examine the roles of local and macroscopic electron mobility in conjugated polymer/fullerene BHJ photovoltaics. The local mobility of different pentaaryl fullerene derivatives (so‐called ‘shuttlecock’ molecules) is similar, so that differences in solar cell efficiency and SCLC mobilities result directly from the different propensities of these molecules to self‐assemble on macroscopic length scales. These experiments and calculations also demonstrate that the local mobility of phenyl‐C₆₀ butyl methyl ester (PCBM) is an order of magnitude higher than that of other fullerene derivatives, explaining why PCBM has been the acceptor of choice for conjugated polymer BHJ devices even though it does not form an optimal macroscopic network. The DFT calculations indicate that PCBM's superior local mobility comes from the near‐spherical nature of its molecular orbitals, which allow strong electronic coupling between adjacent molecules. In combination, DFT and TRMC techniques provide a tool for screening new fullerene derivatives for good local mobility when designing new molecules that can improve on the macroscopic electron mobility offered by PCBM.     

Optical,electrical and mechanical properties of indium tin oxide on polyethylene terephthalate substrates: Application in bulk-heterojunction polymer solar cells

We investigated optical, electrical and mechanical properties of indium tin oxide (ITO) on flexible polyethylene terephthalate (PET) substrate, considering bulk-heterojunction (BHJ) polymer solar cells applications. Encapsulation of flexible solar cells with the architecture PET/ITO/PEDOT:PSS/P3HT:PCBM (or P3HT:PCBM:AZ-NDI-4)/Al was done by direct brush-painting with nail enamel. Active cell layer blends of [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) with regioregular or regiorandom poly(3-hexylthiophene-2,5-diyl) (P3HT) were applied. Additionally for this role the mixture of regioregular P3HT:PCBM with naphthalene diimide–imine with four thiophene rings AZ-NDI-4 was tested. Obtained photovoltaic (PV) and optical (UV–vis) results of the flexible polymer solar cells were compared with the same architecture of devices on the glass/ITO substrate.     

Effect of residual catalyst on solar cells made of a fluorene-thiophene-benzothiadiazole copolymer as electron-donor: A combined electrical and photophysical study

The effects of residual catalyst impurities (palladium) on the hole mobility of a fluorene-thiophene-benzothiadiazole copolymer (poly{[4′-(9,9-bis(2-ethylhexyl)fluoren-2-yl)-2′,1′,3′-benzothiadiazole-7,7′-diyl]-co-[2′-(9,9-bis(2-ethylhexyl)fluoren-2-yl)thien-7,5′-diyl]}) (PFB-co-FT), as well as on its photovoltaic and photophysical response when blended with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), are investigated. Two samples of the copolymer, only differing for the Pd content (9 and 3360 ppm), are considered and compared. The transport of positive carriers is characterized by a lower mobility and a higher dispersion in the Pd-rich PFB-co-FT sample. The photovoltaic parameters of PFB-co-FT:PCBM solar cells show a significant dependence on the residual catalyst impurities, attributed to a different concentration of trap states. Variations in charge mobility and trapping induced by impurities was confirmed also by ESR (Electron Spin Resonance) experiments: an increased concentration of trapped charges in the presence of a higher level of metal impurities was revealed by light induced ESR, while the variation of polaron mobility correlates with the lifetime variation of the photogenerated PCBM triplet state detected by time-resolved ESR. All experimental evidences point to a strong effect of Pd impurities on the transport properties of charge carriers.     

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.     

Photo-physics of PTB7, PCBM and ICBA based ternary solar cells

Indene-C₆₀bisadduct (ICBA) is one of the rare acceptors which can supersede commonly used phenyl-C71-butyric acid methyl ester (PCBM70) in enhancing the performance of bulk heterojunction (BHJ) solar cells owing to its shallower lowest unoccupied molecular orbital (LUMO) level. However, ICBA tends to decrease the photocurrent for most of the low band-gap polymers synthesized to date. Here we examine the interaction of ICBA with the one of the popular low band-gap polymers poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7), at femtosecond level, to explore key dynamics governing the operation of BHJ cells involving ICBA. The photo-physics of binary and ternary systems based on PTB7 coupled with PCBM70 and/or ICBA are studied by means of transient absorption spectroscopy (TAS) and electrochemical impedance spectroscopy (EIS) and supported by morphology analysis. Our study suggests that both inefficient charge-separation and poor charge transport of ICBA is responsible for relatively low photocurrent generation.     

Bipolar Charge Transport in PCPDTBT‐PCBM Bulk‐Heterojunctions for Photovoltaic Applications

An experimental study of the transport properties of a low‐bandgap conjugated polymer giving high photovoltaic quantum efficiencies in the near infrared spectral region (E_g‐opt ～ 1.35 eV) is presented. Using a organic thin film transistor geometry, we demonstrate a relatively high in‐plane hole mobility, up to 1.5 · × 10^?2 cm² V^?1 s^?1 and quantify the electron mobility at 3 × · 10^?5 cm² V^?1 s^?1 on a SiO₂ dielectric. In addition, singular contact behavior results in bipolar quasi‐Ohmic injection both from low and high workfunction metals like LiF/Al and Au. X‐ray investigations revealed a degree of interchain π‐stacking that is probably embedded in a disordered matrix. Disorder also manifests itself in a strong positive field dependence of the hole mobility from the electric field. In blends made with the electron acceptor methanofullerene [6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM), the transistor characteristics suggest a relatively unfavorable intermixing of the two components for the application to photovoltaic devices. We attribute this to a too fine dispersion of [C60]‐PCBM in the polymer matrix, that is also confirmed by the quenching of the photoluminescence signal measured in PCPDTBT [C60]‐PCBM films with various composition. We show that a higher degree of phase separation can be induced during the film formation by using 1,8‐octanedithiol (ODT), which leads to a more efficient electron percolation in the [C60]‐PCBM. In addition, the experimental results, in combination with those of solar cells seem to support the correlation between the blend morphology and charge recombination. We tentatively propose that the drift length, and similarly the electrical fill factor, can be limited by the recombination of holes with electrons trapped on isolated [C60]‐PCBM clusters. Ionized and isolated [C60]‐PCBM molecules can modify the local electric field in the solar cell by build‐up of a space‐charge. The results also suggest that further improvements of the fill factor may also be limited by a strong electrical‐field dependence of the hole transport.     

Comparative study of spectral and morphological properties of blends of P3HT with PCBM and ICBA

We report a comparative study on spectral and morphological properties of two blend systems for polymer solar cells: the donor material poly(3-hexylthiophene) (P3HT) in combination with the acceptor material of either [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) or indene-C₆₀ bisadduct (ICBA) that was reported to enhance efficiencies of polymer solar cells. Optical microscopy and grazing incidence X-ray scattering reveal the stronger tendency of PCBM to from larger and more ordered domains/grains than ICBA either in pure or blend films. Compared to PCBM, the presence of ICBA also substantially perturbs the organization and longer-range ordering of P3HT in increasing the ICBA ratio in blends. With larger and more ordered phase-separated domains, the P3HT/PCBM blend films exhibit significant optical scattering at higher PCBM ratios. Yet, such optical scattering is not significant for P3HT/ICBA blends (even with high ICBA ratios). Overall, results here suggest the reported higher efficiencies of P3HT/ICBA solar cells (vs. P3HT/PCBM cells) cannot be attributed to larger and/or more ordered phase-separated donor–acceptor domains and other characteristics play more important roles in this case.     

Luminescent cathode buffer layer for enhanced power conversion efficiency and stability of bulk-heterojunction solar cells

We report high photovoltaic efficiency of over 9% in solution-processed, small-molecule (SPSM) 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c]1,2,5]thiadiazole) p-DTS(FBTTh₂)₂:[6-6]-phenyl C70 butyric acid methyl ester (PC₇₀BM) blend based inverted BHJ solar cell by incorporating luminescent zinc oxide doped with sodium (ZnO:Na) quantum dots (QD) (l-ZnO) as a cathode buffer layer (CBL) in inverted bulk-heterojunction (BHJ) solar cells for the first time. The l-ZnO absorbs ultraviolet (UV) light and down-converts it to visible light. The l-ZnO layer's emission overlaps significantly with the absorption of p-DTS(FBTTh₂)₂, leading to an enhanced absorption by p-DTS(FBTTh₂)₂. This resulted in a significant enhancement of photo-current from 15.4 to 17.27 mA/cm² and efficiency from 8% to 9.2% for ZnO and l-ZnO based devices, respectively. This is among one of the highest efficiency values reported so far in the case of SPSM based single junction BHJ solar cells. The luminescent ZnO layer also protects the active layer from UV-induced degradation as solar cells show high stability under constant solar light illumination retaining more than 90% (∼28 h) of its initial efficiency, whereas BHJ solar cells without the luminescent ZnO layer degraded to ∼50% of its initial value under same conditions. Since ZnO is an essential part of inverted organic solar cells, the luminescent l-ZnO CBL has great potential in inverted organic solar cells.     

Modification of PCBM Crystallization via Incorporation of C60 in Polymer/Fullerene Solar Cells

The morphological effects of the incorporation of C₆₀ into blended thin‐films of poly(3‐hexylthiophene) and [6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM) are investigated. The results show that addition of C₆₀ readily alters the growth‐rate and morphology of PCBM crystallites under different environmental conditions. The effect of C₆₀ on the growth of large PCBM crystallites is thoroughly characterized using optical microscopy, electron microscopy and UV‐visible absorption spectroscopy. Results show that C₆₀ incorporation modifies fullerene aggregation and crystallization and greatly reduces the average crystallite size at C₆₀ loadings of ≈50 wt% in the fullerene phase. Organic field‐effect transistors (OFETs) are prepared to evaluate the electron mobility of PCBM/C₆₀ films and organic solar cells (OSCs) are fabricated from mixed‐fullerene active layers to evaluate their performance. It is demonstrated that the use of fullerene mixtures in organic electronic applications is a viable approach to produce more stable devices and to control the growth of micrometer‐sized fullerene crystals.