Improving the Photovoltage of Dithienopyrrole Dye‐Sensitized Solar Cells via Attaching the Bulky Bis(octyloxy)biphenyl Moiety to the Conjugated π‐Linker

The judicious design of 3D giant organic dye molecules to enable the formation of a porous photoactive layer on the surface of titania is one of the viable tactics to abate the adverse interfacial charge recombination in dye‐sensitized solar cells (DSCs) employing outer‐sphere redox couples. Here 2′,6′‐bis(octyloxy)‐biphenyl substituted dithieno[3,2‐b:2′,3′‐d]pyrrole segment is constructed and employed as the π‐linker of a high molar absorption coefficient organic push‐pull dye. With respect to its congener possessing the hexyl substituted dithieno[3,2‐b:2′,3′‐d]pyrrole linker, the new dye can self‐assemble on the surface of titania to afford a porous organic coating, which effectively slow down the kinetics of charge recombination of titania electrons with both outer‐sphere tris(1,10‐phenanthroline)cobalt(III) ions and photooxidized dye molecules, improving the cell photovoltage. In addition, the diminishments of charge recombination via modulating the microstructure of interfacial functional zone can also overcompensate the disadvantageous impact of reduced light‐harvesting and evoke an enhanced photocurrent output, bringing forth an efficiency improvement from 7.5% to 9.3% at the 100 mW cm^?2, simulated AM1.5 conditions.     

Unveiling the Role of Dopant Polarity in the Recombination and Performance of Organic Light‐Emitting Diodes

The recombination of charges is an important process in organic photonic devices, because the process influences the device characteristics such as the driving voltage, efficiency, and lifetime. Here, by using various homoleptic and heteroleptic Ir complexes as dopants, it is reported that the stationary dipole moment (μ₀) of the dopant rather than the trap depth (ΔE_t ) is a major factor determining the recombination mechanism in dye‐doped organic light‐emitting diodes (OLEDs). Dopants with large μ₀ (e.g., homoleptic Ir(III) dyes) induce large charge trapping on them, resulting in high driving voltage and trap‐assisted recombination‐dominated emission. On the other hand, dyes with small μ₀ (e.g., heteroleptic Ir(III) dyes) show Langevin recombination‐dominated emission characteristics with much less charge trapping on them no matter what ΔE_t is, leading to lower driving voltage and higher efficiencies. This finding will be useful in any organic photonic devices such as phosphorescent or thermally assisted delayed fluorescent dye sensitized fluorescent OLEDs where trapping and recombination mechanisms play key roles.     

Features of the exciplex mechanism of charge photogeneration in amorphous molecular semiconductors doped with the dye rhodamine 6G

Results of investigation of the processes of charge carrier photogeneration and relaxation within the exciplex absorption region in films of poly-N-epoxypropylcarbazole doped with the dye rhodamine 6G are given. When the films are illuminated with light from the dye absorption region, new centres of charge photogeneration appear that absorb within the exciplex absorption region. It is shown for the first time that these centres have a long lifetime at room temperature and their temporal concentration changes correlate with EPR photosignal changes. The concentration of centres is independent of the external electric field strength during their generation. These centres are identified as triplet exciplexes. © 1997 John Wiley & Sons, Ltd.     

Surface Design in Solid‐State Dye Sensitized Solar Cells: Effects of Zwitterionic Co‐adsorbents on Photovoltaic Performance

In solid‐state dye sensitized solar cells (SSDSCs) charge recombination at the dye‐hole transporting material interface plays a critical role in the cell efficiency. For the first time we report on the influence of dipolar co‐adsorbents on the photovoltaic performance of sensitized hetero‐junction solar cells. In the present study, we investigated the effect of two zwitterionic butyric acid derivatives differing only in the polar moiety attached to their common 4 carbon‐chain acid, i.e., 4‐guanidinobutyric acid (GBA) and 4‐aminobutyric acid (ABA). These two molecules were implemented as co‐adsorbents in conjunction with Z907Na dye on the SSDSC. It was found that a Z907Na/GBA dye/co‐adsorbent combination increases both the open circuit voltage (V_oc) and short‐circuit current density (J_sc) as compared to using Z907Na dye alone. The Z907Na/ABA dye/co‐adsorbent combination increases the J_sc. Impedance and transient photovoltage investigations elucidate the cause of these remarkable observations.     

An Effective Approach for High‐Efficiency Photoelectrochemical Solar Cells by Using Bifunctional DNA Molecules Modified Photoanode

This paper firstly reports the effect of deoxyribonucleic acid (DNA) molecules extracted from chickpea and wheat plants on the injection/recombination of photogenerated electrons and sensitizing ability of dye‐sensitized solar cells (DSSCs). These high‐yield DNA molecules are applied as both linker bridging unit as well as thin tunneling barrier (TTB) at titanium dioxide (TiO₂ )/dye interface, to build up high‐efficient DSSCs. With its favorable energy levels, effective linker bridging role, and double helix structure, bifunctional DNA modifier shows an efficient electron injection, suppressed charge recombination, longer electron lifetime, and higher light harvesting efficiency, which leads to higher photovoltaic performance. In particular, a photoconversion efficiency (PCE) of 9.23% is achieved by the binary chickpea and wheat DNA‐modified TiO₂ (CW@TiO₂) photoanode. Furthermore, time‐resolved fluorescence spectroscopy measurements confirm a better electron transfer kinetics for DNA‐modified TiO₂ photoanodes, implying a higher electron transfer rate (k_ET). This work highlights a great contribution for the photoanodes that are linked with DNA molecule, which act as both bridging unit and TTB to control the charge recombination and injection dynamics, and hence, boost the photovoltaic performance in the DSSCs.     

Photoinduced Electron Transfer in Dye‐Sensitized SnO2 Nanowire Field‐Effect Transistors

Electron transfer from excited dye molecules (chlorophyll or fluorescein) to a semiconductor is demonstrated by photoaction and photoluminescence spectra on field‐effect transistors consisting of dye‐sensitized individual SnO₂ nanowires. The photoaction spectrum shows a much better resolution for nanowires non‐covalently functionalized with dye molecules than for dyes deposited on SnO₂ nanoparticle‐films. Possible reasons for the deviation between the photoaction spectra and ordinary optical absorption spectra as well as for the current‐tail appearing along the falling edge are addressed. In dye‐sensitized nanowires, electron transfer from photo‐excited dyes to nanowires is analyzed by comparing gate‐voltage dependences in photoaction and photoluminescence spectra. The importance of this study is in the understanding of electron injection and recombination provided, as well as the performance optimization of nanowire‐based dye‐sensitized solar cells.     

Emission Color Tuning in Ambipolar Organic Single‐Crystal Field‐Effect Transistors by Dye‐Doping

The effect of dye‐doping in ambipolar light‐emitting organic field‐effect transistors (LE‐OFETs) is investigated from the standpoint of the carrier mobilities and the electroluminescence (EL) characteristics under ambipolar operation. Dye‐doping of organic crystals permits not only tuning of the emission color but also significantly increases the efficiency of ambipolar LE‐OFETs. A rather high external EL quantum efficiency (～0.64%) of one order of magnitude higher than that of a pure p‐distyrylbenzene (P3V2) single crystal is obtained by tetracene doping. The doping of tetracene molecules into a host P3V2 crystal has almost no effect on the electron mobility and the dominant carrier recombination process in the tetracene‐doped P3V2 crystal involves direct carrier recombination on the tetracene molecules.     

Non uniform recombination in thin silicon-on-sapphire films

Recombination parameters of SOS films are deduced from the study of the magnetoconcentration effect in double-injecting structures. The method of measurement is based on an original theory succintly developed; it takes into account general SRH bulk and surface recombination laws; moreover inhomogeneous distributions of recombination centers are considered.Experimental results (current-voltage characteristics of such “magnetodiodes”), when analysed according to the proposed method, lead to more realistic values of the global recombination parameters (τ_v, S₁, S₂) of the SOS film. It is proved, for example, that the previous simplified analysis overestimates the carrier recombination velocity on the Si-Al₂O₃ surface. On the contrary our method gives both a moderate value for this recombination velocity and a lower carrier lifetime near the Sapphire interface, which well agrees with the continuity of recombination rates at the surface and in the underlying bulk; the higher recombination region is found to be 100–1000 Å thick.     

Enhanced efficiency of phenothiazine derivative organic dye‐sensitized ionic liquid solar cells on aging

We have used electrochemical impedance to investigate the improvement in photovoltaic performance in aging of ionic liquid dye‐sensitized solar cells using a high‐absorption coefficient organic dye (2E)‐2‐cyano‐3‐(5‐(5‐((E)‐2‐(10‐(2‐ethylhexyl)‐10H‐phenothiazin‐7‐yl)vinyl)thiophen‐2‐yl)thiophen‐2‐yl)acrylic acid, which is in contrast to N719‐based devices. It was found that the enhancement is due to reduced recombination of the photoexcited electrons. The decreased recombination plausibly originates from molecular re‐orientation along with cation adsorption, with Fourier transform infrared spectra lending support to the former mechanism. After aging, the photovoltaic device using the organic dye outperforms the counterpart by the ruthenium complex dye and achieves an impressive efficiency of 5.6% under AM 1.5 100 mW/cm² illumination. Copyright © 2011 John Wiley & Sons, Ltd.     

A Near‐Infrared cis‐Configured Squaraine Co‐Sensitizer for High‐Efficiency Dye‐Sensitized Solar Cells

A cis‐configured squaraine dye (HSQ1), synthesized by incorporation of a strongly electron‐withdrawing dicyanovinyl group into the central squaric acid moiety, is employed in dye‐sensitized solar cells (DSCs). In solution, HSQ1 displays an intense absorption in the near‐infrared region with a maximum at 686 nm and when the dye is adsorbed on a TiO₂ surface, the absorption spectrum broadens in both the blue and the near‐infrared regions, which is favorable for efficient light harvesting over a broad wavelength range. A solar cell sensitized with HSQ1 shows a broader incident photon‐to‐current conversion efficiency (IPCE) spectrum (from 400 to 800 nm) and a higher IPCE in the long‐wavelength region (71% at 700 nm) than a cell sensitized with squaraine dye SQ1. Furthermore, a solar cell co‐sensitized with HSQ1 and N3 dye shows remarkably improved short‐circuit current density and open‐circuit voltage compared to those of a DSC based on N3 alone and fabricated under the same conditions. The energy‐conversion efficiency of the co‐sensitized DSC is 8.14%, which is the highest reported efficiency for a squaraine dye–based co‐sensitized DSC without using Al₂O₃ layer.     

Enhanced Light‐Harvesting Capability of a Panchromatic Ru(II) Sensitizer Based on π‐Extended Terpyridine with a 4‐Methylstylryl Group for Dye‐Sensitized Solar Cells

A novel Ru π‐expanded terpyridyl sensitizer, referred to as HIS‐2, is prepared based on the molecular design strategy of substitution with a moderately electron‐donating 4‐methylstyryl group onto the terpyridyl ligand. The HIS‐2 dye exhibits a slightly increased metal‐to‐ligand charge transfer (MLCT) absorption at around 600 nm and an intense π–π* absorption in the UV region compared with a black dye. Density functional theory calculations reveal that the lowest unoccupied molecular orbital (LUMO) is distributed over the terpyridine and 4‐methylstyryl moieties, which enhances the light‐harvesting capability and is appropriate for smooth electron injection from the dye to the TiO₂ conduction band. The incident photon‐to‐electricity conversion efficiency spectrum of HIS‐2 exhibits better photoresponse compared with black dye over the whole spectral region as a result of the extended π‐conjugation. A DSC device based on black dye gives a short‐circuit current (J_SC) of 21.28 mA cm^?2, open‐circuit voltage (V_OC) of 0.69 V, and fill factor (FF) of 0.72, in an overall conversion efficiency (η) of 10.5%. In contrast, an HIS‐2 based cell gives a higher J_SC value of 23.07 mA cm^?2 with V_OC of 0.68 V, and FF of 0.71, and owing to the higher J_SC value of HIS‐2, an improved η value of 11.1% is achieved.     

Control of Solid‐State Dye‐Sensitized Solar Cell Performance by Block‐Copolymer‐Directed TiO2 Synthesis

Hybrid dye‐sensitized solar cells are typically composed of mesoporous titania (TiO₂), light‐harvesting dyes, and organic molecular hole‐transporters. Correctly matching the electronic properties of the materials is critical to ensure efficient device operation. In this study, TiO₂ is synthesized in a well‐defined morphological confinement that arises from the self‐assembly of a diblock copolymer—poly(isoprene‐b‐ethylene oxide) (PI‐b‐PEO). The crystallization environment, tuned by the inorganic (TiO₂ mass) to organic (polymer) ratio, is shown to be a decisive factor in determining the distribution of sub‐bandgap electronic states and the associated electronic function in solid‐state dye‐sensitized solar cells. Interestingly, the tuning of the sub‐bandgap states does not appear to strongly influence the charge transport and recombination in the devices. However, increasing the depth and breadth of the density of sub‐bandgap states correlates well with an increase in photocurrent generation, suggesting that a high density of these sub‐bandgap states is critical for efficient photo‐induced electron transfer and charge separation.     

Significant Improvement of Dye‐Sensitized Solar Cell Performance by Small Structural Modification in π‐Conjugated Donor–Acceptor Dyes

Two donor‐π‐acceptor (D‐π‐A) dyes are synthesized for application in dye‐sensitized solar cells (DSSC). These D‐π‐A sensitizers use triphenylamine as donor, oligothiophene as both donor and π‐bridge, and benzothiadiazole (BTDA)/cyanoacrylic acid as acceptor that can be anchored to the TiO₂ surface. Tuning of the optical and electrochemical properties is observed by the insertion of a phenyl ring between the BTDA and cyanoacrylic acid acceptor units. Density functional theory (DFT) calculations of these sensitizers provide further insight into the molecular geometry and the impact of the additional phenyl group on the photophysical and photovoltaic performance. These dyes are investigated as sensitizers in liquid‐electrolyte‐based dye‐sensitized solar cells. The insertion of an additional phenyl ring shows significant influence on the solar cells' performance leading to an over 6.5 times higher efficiency (η = 8.21%) in DSSCs compared to the sensitizer without phenyl unit (η = 1.24%). Photophysical investigations reveal that the insertion of the phenyl ring blocks the back electron transfer of the charge separated state, thus slowing down recombination processes by over 5 times, while maintaining efficient electron injection from the excited dye into the TiO₂‐photoanode.     

Recombination in small-gap Pb1−xSnxTe

Instrinsic and extrinsic recombination mechanisms in small-gap Pb_1−xSn_xTe are investigated by means of stationary and non-stationary laser-excited photoeffects, carried out under conditions with weak as well as strong deviation from equilibrium. A straightforward analysis of radiative recombination within the Kane type two-band model is given and supported by absorption measurements. This analysis is combined with a treatment of Auger recombination based on the parameters determined from the photoeffects. The conclusion is drawn that at high carrier concentrations (equilibrium or non-equilibrium) strong competition between radiative and Auger recombination takes place. The consequences for the threshold current of double hetero-junction lasers are outlined.     

Effect of superstoichiometric components on the spectral and kinetic characteristics of the luminescence of ZnSe crystals with isovalent impurities

The spectral and kinetic parameters of the X-ray luminescence of ZnSe crystals doped with Zn, Se, and Te were investigated during the growth process at temperatures in the range 80–500 K, and also after annealing in Zn vapor. ZnSe crystals grown from a stoichiometric mixture, or mixture containing chalcogenide impurities, typically produce the minimum level of afterglow and a rapid rise of X-ray luminescence, as well as a shift of its peak from the infrared region toward shorter wavelengths after annealing in zinc. ZnSe crystals grown from material with excess of Zn have a relatively low X-ray luminescence yield and a substantial level of afterglow. It is assumed that the growth of Te-activated crystals is accompanied by the development of thermally stable complexes of the form V _ZnTe_Se that act as radiative recombination centers. The introduction of excess Zn into the initial mixture produces a reduction in the concentration of V _Zn and, hence, in the concentration of radiative recombination centers. It is shown that, for free-electron concentrations n<10¹⁸ cm⁻³, the afterglow time constant τ can be described as a function of n by a model of radiative recombination that involves a single impurity level, whereas for n>10¹⁸ cm⁻³, the time constant decreases with increasing n, which cannot be explained in terms of the simple model. It is suggested that radiative recombination centers of a new type are produced as a result of prolonged annealing in Zn vapor. Fiz. Tekh. Poluprovodn. 31, 1211–1215 (October 1997)     

Systematic Efficiency Improvement for Cu2ZnSn(S,Se)4 Solar Cells By Double Cation Incorporation with Cd and Ge

The performance of kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cell is known to be severely limited by the nonradiative recombination near the heterojunction interface and within the bulk of the CZTSSe absorber resulting from abundant recombination centers and limited carrier collection efficiency. Herein, nonradiative recombination is simultaneously reduced by incorporating small amounts of Ge and Cd into the CZTSSe absorber. Incorporation of Ge effectively increases the p-type doping, thus successfully improving the bulk conductance and reducing the recombination in the CZTSSe bulk via enhanced quasi-Fermi level splitting, while the incorporation of Cd greatly reduces defects near the junction region, enabling larger depletion region width and better carrier collection efficiency. The combined effects of Cd and Ge incorporation give rise to systematic improvement in open-circuit voltage (V_OC), short-circuit current density (J_SC), and fill factor (FF), enabling a high conversion efficiency of 11.6%. This study highlights the multiple cation incorporation strategy for systematically manipulating the opto-electronic properties of kesterite materials, which may also be applicable to other semiconductors.     

Enhanced‐Light‐Harvesting Amphiphilic Ruthenium Dye for Efficient Solid‐State Dye‐Sensitized Solar Cells

A ruthenium sensitizer (coded C101, NaRu (4,4′‐bis(5‐hexylthiophen‐2‐yl)‐2,2′‐bipyridine) (4‐carboxylic acid‐4′‐caboxylate‐2,2′‐bipyridine) (NCS)₂) containing a hexylthiophene‐conjugated bipyridyl group as an ancillary ligand is presented for use in solid‐state dye‐sensitized solar cells (SSDSCs). The high molar‐extinction coefficient of this dye is advantageous compared to the widely used Z907 dye, (NaRu (4‐carboxylic acid‐4′‐carboxylate) (4,4′‐dinonyl‐2,2′‐bipyridine) (NCS)₂). In combination with an organic hole‐transporting material (spiro‐MeOTAD, 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine) 9, 9′‐spirobifluorene), the C101 sensitizer exhibits an excellent power‐conversion efficiency of 4.5% under AM 1.5 solar (100 mW cm^?2) irradiation in a SSDSC. From electronic‐absorption, transient‐photovoltage‐decay, and impedance measurements it is inferred that extending the π‐conjugation of spectator ligands induces an enhanced light harvesting and retards the charge recombination, thus favoring the photovoltaic performance of a SSDSC.     

A systematical investigation of non-fullerene solar cells based on diketopyrrolopyrrole polymers as electron donor

Diketopyrrolopyrrole (DPP)-based conjugated polymers have been successfully applied in high performance field-effect transistors and fullerene-based solar cells, but show limited application in non-fullerene solar cells. In this work, we use four DPP polymers as electron donor and a perylene bisimide dye as electron acceptor to construct non-fullerene solar cells. The donors and acceptor have complementary absorption spectra in visible and near-infrared region, resulting in broad photo-response from 300 nm to 1000 nm. The solar cells were found to provide relatively low power conversion efficiencies of 1.6–2.6%, which was mainly due to low photocurrent and fill factor. Further investigation reveals that the low performance is originated from the high charge recombination in photo-active layers. Our systematical studies will help better understand the non-fullerene solar cells based on DPP polymers and inspire new researches toward efficient non-fullerene solar cells with broad photo-response.     

The effect of grain boundary shunt currents on mis solar cell performance

The degradation of solar cells by grain boundaries can take any of three forms: recombination of minority carriers, forward current due to recombination in the space charge region of the junction or Schottky barrier, or forward currents due to shunting. There is no doubt that minority carrier recombination occurs and degrades the short circuit current. There seems little doubt that grain boundaries also degrade the solar cell open circuit voltage, but whether the degradation is due to recombination in the space charge region or due to shunting is not clear. To date most attention has been paid to space charge recombination. There is data, however, that shunt currents can flow, especially in doped regions of the grain boundaries with the conductivity along the grain boundary estimated from that data to be 10⁻¹² to 10⁻⁵ mhos/square. We will present an analysis assuming such grain boundary conductivities and show that the predictions of such a shunt model are in agreement with experiment. Specifically the shunt current is predicted to increase exponentially as qV/2kT where V is the forward bias, the shunt current significantly lowers the open circuit voltage, and it has negligible effect on the short circuit current.