Bulk-heterojunction solar cells are reported with an enhanced power conversion efficiency (PCE) based on a newly designed semiconducting selenophene-thienopyrrolodione (TPD) copolymer blended with [6,6]-phenyl C₇₁ butyric acid methyl-ester. The solar cells are fabricated using simple solution processing (implying low-cost fabrication). The relatively deep highest occupied molecular orbital (HOMO) level leads to a correspondingly high open-circuit voltage of 0.88 V. The PCE approaches 5.8% when Clevious P VP AI4083 is used as the hole-transport interlayer, with an optimized active layer thickness of approximately 95 nm, and a donor-acceptor blend ratio of 1:1. A fill factor (FF) of 0.62 is achieved. The use of additives does not seem to be beneficial in this blended system, due to the achievement of proper phase separation in the as-cast films. Also, the BHJ devices with a 3% ratio of a 1-chloronaphthalene (CN) additive exhibit much more severe oxidative degradation from the decreased FF with a high series resistance than BHJ devices without additive. The selenophene-TPD based BHJ solar cell is a promising candidate for high-performance single cells with a low-cost additive-free fabrication and a long-term stable operation.

A new semiconducting polymer based on terthiophene-thienopyrrolodione alternating building blocks with a deep HOMO energy level (5.66 eV) is presented. The polymer is prepared by a direct heteroarylation polycondensation reaction, which is a low-cost and green alternative to the standard Stille coupling reaction and thus avoids any use of toxic stannyl derivatives. Integrating the polymer into bulk heterojunction solar cells with [6,6]-phenyl C71-butyric acid methyl ester ([70]PCBM) leads to a PCE of over 6% and a high open-circuit voltage of up to 0.94 V. To obtain these results a unique processing additive, 1-chloronaphthalene, is used, and a relatively low concentration of [70]PCBM is used in the blend solution.

This article presents a critical discussion of the various physical processes occurring in organic bulk heterojunction (BHJ) solar cells based on recent experimental results. The investigations span from photoexcitation to charge separation, recombination, and sweep-out to the electrodes. Exciton formation and relaxation in poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) and poly-3(hexylthiophene) (P3HT) are discussed based on a fluorescence up-conversion study. The commonly accepted paradigm describing the conversion of incident photons into charge carriers in the BHJ material is re-examined in light of these femtosecond time-resolved measurements. Transient photoconductivity, time-delayed collection field, and time-delayed dual pulse experiments carried out on BHJ solar cells demonstrate the competition between carrier sweep-out by the internal field and the loss of photogenerated carriers by recombination. Finally, an emerging hypothesis is discussed: that bimolecular recombination accounts for the majority of recombination from short circuit to open circuit in optimized solar cells, and that bimolecular recombination is bias- and charge-density-dependent. The study of recombination loss processes in organic solar cells leads to insights into what must be accomplished to achieve the “ideal” solar cell.

A simple, solution-processed route to the development of MoO_x thin-films using oxomolybdate precursors is presented. The chemical, structural, and electronic properties of these species are characterized in detail, within solution and thin-films, using electrospray ionization mass spectrometry, grazing angle Fourier transform infrared spectroscopy, thermogravimetric analysis, atomic force microscopy, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy. These analyses show that under suitable deposition conditions the resulting solution processed MoO_x thin-films possess the appropriate morphological and electronic properties to be suitable for use in organic electronics. This is exemplified through the fabrication of poly(3-hexylthiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PC₆₁BM) bulk heterojunction (BHJ) solar cells and comparisons to the traditionally used poly(3,4-ethyldioxythiophene)/poly(styrenesulfonate) anode modifying layer.

Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device, including reducing the open circuit voltage, the short circuit current and the fill factor. Steady state studies show a dramatic increase in the trap-assisted recombination rate when [6,6]-phenyl C₈₄ butyric acid methyl ester (PC₈₄BM) is introduced as a trap site in polymer bulk heterojunction solar cells made of a blend of the copolymer poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) and the fullerene derivative [6,6]-phenyl C₆₁ butyric acid methyl ester (PC₆₀BM). The trap density dependent recombination studied here can be described as a combination of bimolecular and Shockley–Read–Hall recombination; the latter is dramatically enhanced by the addition of the PC₈₄BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cell devices and gives insight into the mechanism of the trap-induced recombination loss.

Improved performance of p-type organic light-emitting transistors (OLETs) is demonstrated by introducing a conjugated polyelectrolyte (CPE) layer and symmetric high work function (WF) source and drain metal electrodes. The OLET comprises a tri-layer film consisting of a hole transporting layer, an emissive layer, and a CPE layer as an electron injection layer. The thickness of the CPE layer is critical for achieving good performance and provides an important structural handle for consideration in future optimization studies. We also demonstrate for the first time, good performance solution-processed blue-emitting OLETs. These results further demonstrate the simplification of device fabrication and improved performance afforded by integrating CPE interlayers into organic optoelectronic devices.

Increasing the molecular weight of the low-bandgap semiconducting copolymer, poly[(4,4-didoecyldithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl], Si-PDTBT, from 9 kDa to 38 kDa improves both photoresponsivity and charge transport properties dramatically. The photocurrent measured under steady state conditions is 20 times larger in the higher molecular weight polymer (HM_n Si-PDTBT). Different decays of polarization memory in transient photoinduced spectroscopy measurements are consistent with more mobile photoexcitations in HM_n Si-PDTBT relative to the lower molecular weight counterpart (LM_n Si-PDTBT). Analysis of the current-voltage characteristics of field effect transistors reveals an increase in the mobility by a factor of 700 for HM_n Si-PDTBT. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy and grazing incidence small angle X-ray scattering (GISAXS) measurements demonstrate that LM_n Si-PDTBT forms a disordered morphology throughout the depth of the film, whereas HM_n Si-PDTBT exhibits pronounced π-π stacking in an edge-on configuration near the substrate interface. Increased interchain overlap between polymers in the edge-on configuration in HM_n Si-PDTBT results in the higher carrier mobility. The improved optical response, transport mobility, and interfacial ordering highlight the subtle role that the degree of polymerization plays on the optoelectronic properties of conjugated polymer based organic semiconductors.

The development of high-efficiency plastic solar cells is rapidly accelerating as the need for economically viable alternative energy sources becomes evident. Polymer-based bulk-heterojunction (BHJ) solar cells are attractive in that they can be coated from solution onto flexible substrates by a variety of techniques and thus inexpensive large-volume manufacturing should be possible. Further, the inherent flexibility of the polymeric materials combined with thin photovoltaic active layers results in devices that can be adapted to a variety of unique aesthetics and form factors. Recent advances in key relationships between thin-film casting methods, bulk-heterojunction morphology, and device performance have occurred in tandem with the synthesis of novel polymer semiconductors that possess increased optical-absorption breadth and optoelectronic performance. This Research News article highlights a few techniques developed to optimize the BHJ nanomorphology and performance of solar cells fabricated by various solution-processing methods.

Enhanced performance of n-channel organic field-effect transistors (OFETs) is demonstrated by introducing a titanium sub-oxide (TiO_x) injection layer. The n-channel OFETs utilize [6,6]-phenyl-C₆₁ butyric acid methyl ester (PC₆₁BM) or [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM) as the semiconductor in the channel. With the TiO_x injection layer, the electron mobilities of PC₆₁BM and PC₇₁BM FET using Al as source/drain electrodes are comparable to those obtained from OFETs using Ca as the source/drain electrodes. Direct measurement of contact resistance (R_c) shows significantly decreased R_c values for FETs with the TiO_x layer. Ultraviolet photoelectron spectroscopy (UPS) studies demonstrate that the TiO_x layer reduces the electron injection barrier because of the relatively strong interfacial dipole of TiO_x. In addition to functioning as an electron injection layer that eliminates the contact resistance, the TiO_x layer acts as a passivation layer that prevents penetration of O₂ and H₂O; devices with the TiO_x injection layer exhibit a significant improvement in lifetime when exposed to air.