Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopy

Enhanced photocurrent by the co-sensitization of ZnO with dye and CuInSe nanocrystals

ZnO@Cu_0.28In_1.72Se_2.72 (5–10 nm) was synthesised for the first time using a template-free method and a vacuum one-pot-nanocasting process without long-chain ligands.

Understanding the role of the dye/oxide interface via SnO2-based MK-2 dye-sensitized solar cells

The interfacial blocking layer between dye and oxide was found to play crucial roles in retarding recombination, increasing diffusion, accelerating dye regeneration and narrowing the density of states.

A strategy to enhance the efficiency of dye-sensitized solar cells by the highly efficient TiO2/ZnS photoanode

In dye-sensitized solar cells (DSSCs), the TiO₂/ZnS photoanode film plays an important role in increasing the power conversion efficiency than bare TiO₂.

Enhanced performance of PTB7:PC₇₁BM solar cells via different morphologies of gold nanoparticles

The effects of gold nanoparticles (AuNPs) incorporated in the hole transporting layer (HTL) of poly[[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b'] dithiophene-2, 6-diyl] [3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophened iyl]] (PTB7): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) based solar cells are being systematically investigated in terms of the optical properties, electrical properties, and photovoltaic performance. The impacts of AuNPs on the optical response of the devices are modeled by finite-difference time-domain (FDTD) simulation. The size of the AuNPs used in this work is around 50-70 nm, so that 10-20 nm penetrated from the HTL into the active layer. We found that the power conversion efficiencies (PCEs) of the devices with AuNPs are significantly enhanced from 7.5%, for the control device, to 8.0%, 8.1%, and 8.2% for Au nanosphere-, nanorod-, and nanocube-incorporated devices, respectively. Among the photovoltaic parameters of the AuNP devices, the short circuit current density (JSC) exhibits the largest improvement, which can be attributed to the improved optical properties of the devices. On the basis of the calculation results, the scattering cross section for the samples in the presence of AuNPs can be enhanced by a factor of ∼10(10)-10(13) and Au nanocubes exhibit superior scattering cross section compared to the Au nanospheres and nanorods with the same linear dimension. From the experimental impedance spectroscopy results, we found that the addition of AuNPs had little effect on the electrical properties of the device. The device performance is also found to be sensitive to the concentration and morphology of the AuNPs.

Compact layer free perovskite solar cells with 13.5% efficiency

The recent breakthrough of organometal halide perovskites as the light harvesting layer in photovoltaic devices has led to power conversion efficiencies of over 16%. To date, most perovskite solar cells have adopted a structure in which the perovskite light absorber is placed between carrier-selective electron- and hole-transport layers (ETLs and HTLs). Here we report a new type of compact layer free bilayer perovskite solar cell and conclusively demonstrate that the ETL is not a prerequisite for obtaining excellent device efficiencies. We obtained power conversion efficiencies of up to 11.6% and 13.5% when using poly(3-hexylthiophene) and 2,2',7,7'-tetrakis(N,N-di(4-methoxyphenyl)amino)-9,9'-spirobifluorene, respectively, as the hole-transport material. This performance is very comparable to that obtained with the use of a ZnO ETL. Impedance spectroscopy suggests that while eliminating the ZnO leads to an increase in contact resistance, this is offset by a substantial decrease in surface recombination.

An approach for an advanced anode interfacial layer with electron-blocking ability to achieve high-efficiency organic photovoltaics

The interfacial properties of PEDOT:PSS, pristine r-GO, and r-GO with sulfonic acid (SR-GO) in organic photovoltaic are investigated to elucidate electron-blocking property of PEDOT:PSS anode interfacial layer (AIL), and to explore the possibility of r-GO as electron-blocking layers. The SR-GO results in an optimized power conversion efficiency of 7.54% for PTB7-th:PC71BM and 5.64% for P3HT:IC61BA systems. By combining analyses of capacitance-voltage and photovoltaic-parameters dependence on light intensity, it is found that recombination process at SR-GO/active film is minimized. In contrast, the devices using r-GO without sulfonic acid show trap-assisted recombination. The enhanced electron-blocking properties in PEDOT:PSS and SR-GO AILs can be attributed to surface dipoles at AIL/acceptor. Thus, for electron-blocking, the AIL/acceptor interface should be importantly considered in OPVs. Also, by simply introducing sulfonic acid unit on r-GO, excellent contact selectivity can be realized in OPVs.

Incorporation of Cl into sequentially deposited lead halide perovskite films for highly efficient mesoporous solar cells

Organic-inorganic lead halide perovskites have been widely used as absorbers on mesoporous TiO2 films as well as thin films in planar heterojunction solar cells, yielding very high photovoltaic conversion efficiencies. Both the addition of chloride and sequential deposition methods were successfully employed to enhance the photovoltaic performance. Here, both approaches are combined in a sequential method by spincoating PbCl2 + PbI2 on a mesoporous TiO2 film followed by the perovskite transformation. The role of Cl in determining the optical, electrical, structural and morphological properties is correlated with the photovoltaic performance. The highest photovoltaic efficiency of 14.15% with the V(oc), FF and J(sc) being 1.09 V, 0.65 and 19.91 mA cm(-2) respectively was achieved with 10 mol% of PbCl2 addition due to an increase of the film conductivity induced by a better perovskite morphology. This is linked to an improvement of the hysteresis and reproducibility of the solar cells.

Improved performance of colloidal CdSe quantum dot-sensitized solar cells by hybrid passivation

A hybrid passivation strategy is employed to modify the surface of colloidal CdSe quantum dots (QDs) for quantum dot-sensitized solar cells (QDSCs), by using mercaptopropionic acid (MPA) and iodide anions through a ligand exchange reaction in solution. This is found to be an effective way to improve the performance of QDSCs based on colloidal QDs. The results show that MPA can increase the coverage of the QDs on TiO2 electrodes and facilitate the hole extraction from the photoxidized QDs, and simultaneously, that the iodide anions can remedy the surface defects of the CdSe QDs and thus reduce the recombination loss in the device. This hybrid passivation treatment leads to a significant enhancement of the power conversion efficiency of the QDSCs by 41%. Furthermore, an optimal ratio of iodide ions to MPA was determined for favorable hybrid passivation; results show that excessive iodine anions are detrimental to the loading of the QDs. This study demonstrates that the improvement in QDSC performance can be realized by using a combination of different functional ligands to passivate the QDs, and that ligand exchange in solution can be an effective approach to introduce different ligands.

Iron pyrite thin film counter electrodes for dye-sensitized solar cells: high efficiency for iodine and cobalt redox electrolyte cells

Iron pyrite has been the material of interest in the solar community due to its optical properties and abundance. However, the progress is marred due to the lack of control on the surface and intrinsic chemistry of pyrite. In this report, we show iron pyrite as an efficient counter electrode (CE) material alternative to the conventional Pt and poly(3,4-ethylenedioxythiophene (PEDOT) CEs in dye-sensitized solar cells (DSSCs). Pyrite film CEs prepared by spray pyrolysis are utilized in I3(-)/I(-) and Co(III)/Co(II) electrolyte-mediated DSSCs. From cyclic voltammetry and impedance spectroscopy studies, the catalytic activity is found to be comparable with that of Pt and PEDOT in I3(-)/I(-) and Co(III)/Co(II) electrolyte, respectively. With the I3(-)/I(-) electrolyte, photoconversion efficiency is found to be 8.0% for the pyrite CE and 7.5% for Pt, whereas with Co(III)/Co(II) redox DSSCs, efficiency is found to be the same for both pyrite and PEDOT (6.3%). The excellent performance of the pyrite CE in both the systems makes it a distinctive choice among the various CE materials studied.

Facile water-based spray pyrolysis of earth-abundant Cu2FeSnS4 thin films as an efficient counter electrode in dye-sensitized solar cells

A novel approach to produce earth-abundant Cu2FeSnS4 (CFTS) thin film using spray pyrolysis of nontoxic aqueous precursors followed by sulfurization is reported. The CFTS phase formation was confirmed by both Raman spectroscopy and X-ray diffraction techniques. Hall measurements of these films reveal p-type conductivity with good charge carrier density and mobilities appropriate for solar harvesting devices. To the best of our knowledge, this is the first report on the electrical properties of solution-processed Cu2FeSnS4 thin films estimated using Hall measurements. Dye-sensitized solar cells (DSSC) fabricated with CFTS thin film as a photocathode in iodine/iodide electrolyte exhibit good power conversion efficiency, 8.03%, indicating that CFTS would be a promising cheaper alternative to replace Pt as a counter electrode in DSSCs.

Inkjet printing and instant chemical transformation of a CH3NH3PbI3/nanocarbon electrode and interface for planar perovskite solar cells

A planar perovskite solar cell that incorporates a nanocarbon hole-extraction layer is demonstrated for the first time by an inkjet printing technique with a precisely controlled pattern and interface. By designing the carbon plus CH3NH3I ink to transform PbI2 in situ to CH3NH3PbI3, an interpenetrating seamless interface between the CH3NH3PbI3 active layer and the carbon hole-extraction electrode was instantly constructed, with a markedly reduced charge recombination compared to that with the carbon ink alone. As a result, a considerably higher power conversion efficiency up to 11.60% was delivered by the corresponding solar cell. This method provides a major step towards the fabrication of low-cost, large-scale, metal-electrode-free but still highly efficient perovskite solar cells.

Electrochemical determination of the density of states of nanostructured NiO films

Mesoporous p-type NiO films were prepared by aerosol-assisted chemical vapor deposition (AACVD) and characterized by X-ray diffraction (XRD). The nanostructure of the films was investigated by field emission gun scanning electron microscopy (FEG-SEM). The density of states (DOS) in these nanostructured films has been determined by means of electrochemical impedance spectroscopy and cyclic voltammetry. The analysis reveals an exponential distribution of band gap states above the valence band that extends around 1.5 eV. In addition, monoenergetic states were also identified which overlap with the exponential distribution. This distribution of states has an enormous influence in the electronic processes of the devices in which NiO electrodes are employed (electrochromism, water splitting or energy storage). Especially, in p-type dye-sensitized solar cells (p-DSCs), it is thought that intra-band-gap states are responsible for the fast observed recombination processes, whose existence and distribution has not been clearly determined yet and are now confirmed and quantified by our analysis. This provides a better comprehension of the recombination events which represent one of the main losses in p-DSCs.

Incorporation of carbon nanotubes in a hierarchical porous photoanode of tandem quantum dot sensitized solar cells

The incorporation of multi-walled carbon nanotubes (MWCNT) in quantum dot (QD) sensitized solar cells (QDSC) based on CdSe QDs and quantum rods (QRs) is investigated. The composite hierarchical porous photoanode of titania/CNT is synthesized by sol-gel induced phase separation and QDs/QRs are prepared by the modified solvothermal method. The QDs and QRs form a tandem structure on the hierarchical porous photoanode after deposition by the electrophoretic method. Incorporation of MWCNT in the QDSC photoanode in optimum content (0.32 wt%) causes appreciable enhancement in cells efficiency (about 41% increase). This improvement in efficiency mainly emerges from the beneficial role of MWCNTs in charge injection and collection. The MWCNTs result in longer electron lifetime and higher electron diffusion length, which is confirmed by electrochemical impedance spectroscopy.

Effect of a long alkyl group on cyclopentadithiophene as a conjugated bridge for D-A-π-A organic sensitizers: IPCE, electron diffusion length, and charge recombination

The option of using conjugated π-linkers is critical for rational molecular design toward an energy-level strategy for organic sensitizers. To further optimize photovoltaic performance, methyl- and octyl-substituted 4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) are introduced into D-A-π-A featured sensitizers. Along with CPDT, instead of thiophene as conjugated bridge, WS-39 and WS-43 exhibit an extended spectral response due to the excellent conjugation and coplanarity of CPDT. Specifically, we focused on the critical effect of length of the alkyl group linked to the bridging carbon atoms of CPDT on the photovoltaic performances. Octyl-substituted WS-39 shows a broader IPCE onset with an enhanced photovoltage relative to the analogue WS-5. In contrast, WS-43, with methyl substituted on the CPDT moiety, presents a relatively low quantum conversion efficiency within the whole spectral response region, along with low photocurrent density. WS-43 displays a distinctly low IPCE platform, predominately arising from the short electron diffusion length with significant electron loss during the electron transport. The relative movement of the conduction band edge (E(CB)) and charge transfer resistance as well as lifetime of injected electrons are studied in detail. Under standard AM 1.5 conditions, WS-39-based solar cells show a promising photovoltaic efficiency of 9.07% (J(SC) = 16.61 mA cm(-2), V(OC) = 770 mV, FF = 0.71). The octyl chains attached on CPDT can provide dual protection and exhibit a high propensity to prevent binding of the iodide-triiodide redox couple, producing an efficient shielding effect to retard the charge recombination and resulting in improvement of V(OC). Our research paves the way to explore more efficient sensitizers through ingenious molecular engineering.

Effect of blocking layer to boost photoconversion efficiency in ZnO dye-sensitized solar cells

The effect of a ZnO compact blocking layer (BL) in dye-sensitized solar cells (DSSCs) based on ZnO photoanodes is investigated. BL is generated through spray deposition onto fluorine-doped tin oxide (FTO) conducting glass before the deposition of a ZnO active layer. The functional properties of dye-sensitized solar cells (DSSCs) are then investigated as a function of the thickness of the BL for two different kinds of ZnO active layer, i.e., hierarchically self-assembled nanoparticles and microcubes composed of closely packed ZnO sheets. Presence of BL leads to the improvement of photoconversion efficiency (PCE), by physically insulating the electrolyte and the FTO. This effect increases at increasing BL thickness up to around 800 nm, while thicker BL results in reduced cell performance. Remarkable increase in Jsc is recorded, which doubles as compared to cells without blocking layer, leading to PCE as high as 5.6% in the best cell under one sun irradiation (AM 1.5 G, 100 mW cm(-2)). Electrochemical impedance spectroscopy (EIS) elucidates the mechanism boosting the functional features of the cells with BL, which relies with enhanced chemical capacitance together with an almost unchanged recombination resistance, which are reflected in an increased electron lifetime. The results foresee a straightforward way to significantly improve the performance of ZnO-based DSSCs.

Evaluation of back contact in spray deposited SnS thin film solar cells by impedance analysis

The role of back metal (M) contact in sprayed SnS thin film solar cells with a configuration Glass/F:SnO2/In2S3/SnS/M (M = Graphite, Cu, Mo, and Ni) was analyzed and discussed in the present study. Impedance spectroscopy was employed by incorporating constant phase elements (CPE) in the equivalent circuit to investigate the degree of inhomogeneity associated with the heterojunction and M/SnS interfaces. A best fit to Nyquist plot revealed a CPE exponent close to unity for thermally evaporated Cu, making it an ideal back contact. The Bode phase plot also exhibited a higher degree of disorders associated with other M/SnS interfaces. The evaluation scheme is useful for other emerging solar cells developed from low cost processing schemes like spray deposition, spin coating, slurry casting, electrodeposition, etc.

Universal low-temperature MWCNT-COOH-based counter electrode and a new thiolate/disulfide electrolyte system for dye-sensitized solar cells

A new thiolate/disulfide organic-based electrolyte system composed of the tetrabutylammonium salt of 2-methyl-5-trifluoromethyl-2H-[1,2,4]triazole-3-thiol (S(-)) and its oxidized form 3,3'-dithiobis(2-methyl-5-trifluoromethyl-2H-[1,2,4]triazole) (DS) has been formulated and used in dye-sensitized solar cells (DSSCs). The electrocatalytic activity of different counter electrodes (CEs) has been evaluated by means of measuring J-V curves, cyclic voltammetry, Tafel plots, and electrochemical impedance spectroscopy. A stable and low-temperature CE based on acid-functionalized multiwalled carbon nanotubes (MWCNT-COOH) was investigated with our S(-)/DS, I(-)/I3(-), T(-)/T2, and Co(II/III)-based electrolyte systems. The proposed CE showed superb electrocatalytic activity toward the regeneration of the different electrolytes. In addition, good stability of solar cell devices based on the reported electrolyte and CE was shown.

A green route and rational design for ZnO-based high-efficiency photovoltaics

In this work, at room-temperature and without any organic surfactants we reported two green and facile approaches for rapid synthesis of ZnO nanorods (NRs) and nanosheet-based ZnO hierarchical structures (NSHSs). Based on their structural advantages, the quasi-solid ZnO-DSCs achieved a record photoelectric conversion efficiency (PCE) of 6.83%.

Recombination Study of Combined Halides (Cl, Br, I) Perovskite Solar Cells

We report on the preparation of a series of solution-processed perovskite solar cells based on methylammonium (MA) lead halide derivatives, MAPbX3, which show tunable optical properties depending on the nature and ratio of the halides employed (X = Cl, Br, and I). Devices have been prepared with different cell architecture, thin film, and mesoporous scaffold (TiO2 and Al2O3). We have analyzed different sample sets focusing on the characterization of the charge recombination by means of impedance spectroscopy (IS). On the one hand, our study discloses that the insertion of both Cl and Br in the perovskite lattice reduces the charge recombination rates in the light absorber film, thus determining the open circuit voltage (Voc) of the device. The samples prepared on a mesoporous Al2O3 electrode present lower charge recombination rates than those devices prepared on mesoporous TiO2. Furthermore, the addition of Br in the perovskite structure was demonstrated to improve slightly the lifetime of the devices; in fact, the efficiencies of all devices tested remained at least at the 80% of the initial value 1 month after their preparation. These results highlight the crucial role of the charge-recombination processes on the performance of the perovskite solar cells and pave the way for further progress on this field.

Combined strategy to realize efficient photoelectrodes for low temperature fabrication of dye solar cells

We implemented a low-temperature approach to fabricate efficient photoanodes for dye-sensitized solar cells, which combines three different nanoarchitectures, namely, a highly conductive and highly transparent AZO film, a thin TiO2-blocking layer, and a mesoporous TiO2 nanorod-based working electrode. All the components were processed at T≤200°C. Both the AZO and the TiO2 blocking layers were deposited by reactive sputtering, whereas the TiO2 nanorods were synthesized by surfactant-assisted wet-chemical routes and processed into photoelectrodes in which the native geometric features assured uniform mesoporous structure with effective nanocrystal interconnectivity suitable to maximize light harvesting and electron diffusion. Because of the optimized structure of the TiO2-blocking/AZO bilayer, and thanks to the good adhesion of the TiO2 nanorods over it, a significant enhancement of the charge recombination resistance was demonstrated, this laying on the basis of the outstanding power conversion efficiency achievable through the use of this photoanode's architecture: a value of 4.6% (N719) was achieved with a 4-μm-thick electrode processed at T=200°C. This value noticeably overcomes the current literature limit got on AZO-based cells (N719), which instead use Nb-doped and thicker blocking layers, and thicker nanostructured photoanodes, which have been even sintered at higher temperatures (450-500°C).

Strategy to improve photovoltaic performance of DSSC sensitized by zinc prophyrin using salicylic acid as a tridentate anchoring group

Three new zinc porphyrin dyes attached to ethynyl benzoic acid as an electron transmission and anchoring group have been designed, synthesized, and well-characterized. The performances of their sensitized solar cells have been investigated by optical, photovoltaic, and electrochemical methods. The photoelectric conversion efficiency of the solar cells sensitized by the dye with salicylic acid as an anchoring group demonstrated obvious enhancement when compared with that sensitized by the dye with carboxylic acid as an anchoring group. The density functional theory calculations and the electrochemical impedance spectroscopies revealed that tridentate binding modes could increase the efficiency of electron injection from dyes to the TiO2 nanoparticles by more electron pathways.

Simple way to engineer metal-semiconductor interface for enhanced performance of perovskite organic lead iodide solar cells

A thin wide band gap organic semiconductor N,N,N',N'-tetraphenyl-benzidine layer has been introduced by spin-coating to engineer the metal-semiconductor interface in the hole-conductor-free perovskite solar cells. The average cell power conversion efficiency (PCE) has been enhanced from 5.26% to 6.26% after the modification and a highest PCE of 6.71% has been achieved. By the aid of electrochemical impedance spectroscopy and dark current analysis, it is revealed that this modification can increase interfacial resistance of CH3NH3PbI3/Au interface and retard electron recombination process in the metal-semiconductor interface.

Modification of TiO₂ electrode with organic silane interposed layer for high-performance of dye-sensitized solar cells

Back electron transfer from the TiO2 electrode surface to the electrolyte is the main reason behind the low-open circuit potential (Voc) and the low-fill factor (FF) of the dye-sensitized solar cells (DSSCs). Modifications to the TiO2 electrode, fabricated using {010}-faceted TiO2 nanoparticles with six different kinds of silane, are reported to decrease the back electron transfer on the TiO2 surface. The effect of alkyl chain length of hydrocarbon silanes and fluorocarbon silanes on adsorption parameters of surface coverage and adsorption constant, interfacial resistance, and photovoltaic performances were investigated. Adsorption isotherms, impedance analysis, and photovoltaic measurements were used as the investigation techniques. The reduction of back electron transfer depended on the TiO2 surface coverage by silane, alkyl chain length, and the molecular structure of the silane. Even though Voc and FF were improved, significant reduction in short-circuit photocurrent density (Jsc) was observed after silanization because of desorption of dye during silanization. A new approach, sequential adsorption process of silane and dye, was introduced to enhance Voc and FF without lowering Jsc. Heptadecafluorodecyl trimethoxy-silane showed the highest coverage on the surface of the TiO2 and had the highest effect on the performance improvement of the DSSC, where Voc, FF, and efficiency (η) were improved by 22, 8.0, and 22%, respectively.

An expression for the bridge-mediated electron transfer rate in dye-sensitized solar cells

We have derived an expression for the rate of electron transfer between a semiconductor and a redox centre connected to the semiconductor via a molecular bridge. This model is particularly useful to study the charge recombination (CR) process in dye-sensitized solar cells, where the dye is often connected to the semiconductor by a conjugated bridge. This formalism, designed to be coupled with density functional theory electronic structure calculations, can be used to explore the effect of changing the bridge on the rate of interfacial electron transfer. As an example, we have evaluated the CR rate for a series of systems that differ in the bridge length.

Conditions for diffusion-limited and reaction-limited recombination in nanostructured solar cells

The performance of Dye-sensitized solar cells (DSC) and related devices made of nanostructured semiconductors relies on a good charge separation, which in turn is achieved by favoring charge transport against recombination. Although both processes occur at very different time scales, hence ensuring good charge separation, in certain cases the kinetics of transport and recombination can be connected, either in a direct or an indirect way. In this work, the connection between electron transport and recombination in nanostructured solar cells is studied both theoretically and by Monte Carlo simulation. Calculations using the Multiple-Trapping model and a realistic trap distribution for nanostructured TiO2 show that for attempt-to-jump frequencies higher than 10(11)-10(13) Hz, the system adopts a reaction limited (RL) regime, with a lifetime which is effectively independent from the speed of the electrons in the transport level. For frequencies lower than those, and depending on the concentration of recombination centers in the material, the system enters a diffusion-limited regime (DL), where the lifetime increases if the speed of free electrons decreases. In general, the conditions for RL or DL recombination depend critically on the time scale difference between recombination kinetics and free-electron transport. Hence, if the former is too rapid with respect to the latter, the system is in the DL regime and total thermalization of carriers is not possible. In the opposite situation, a RL regime arises. Numerical data available in the literature, and the behavior of the lifetime with respect to (1) density of recombination centers and (2) probability of recombination at a given center, suggest that a typical DSC in operation stays in the RL regime with complete thermalization, although a transition to the DL regime may occur for electrolytes or hole conductors where recombination is especially rapid or where there is a larger dispersion of energies of electron acceptors.

Influence of the donor size in D-π-A organic dyes for dye-sensitized solar cells

We report two new molecularly engineered push-pull dyes, i.e., YA421 and YA422, based on substituted quinoxaline as a π-conjugating linker and bulky-indoline moiety as donor and compared with reported IQ4 dye. Benefitting from increased steric hindrance with the introduction of bis(2,4-dihexyloxy)benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO2 surface is reduced, especially in redox electrolyte employing Co(II/III) complexes as redox shuttles. It was found that the open circuit photovoltages of IQ4, YA421, and YA422 devices with cobalt-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, respectively. Moreover, the cells employing graphene nanoplatelets on top of gold spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to platinum as a CE. Consequently, YA422 devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochemical impedance spectroscopy and transient absorption spectroscopy analysis were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor-π-acceptor dyes. Laser flash photolysis experiments indicate that even though the dye regeneration of YA422 is slower than that of the other two dyes, the slower back electron transfer of YA422 contributes to the higher device performance.

Influence of structural variations in push-pull zinc porphyrins on photovoltaic performance of dye-sensitized solar cells

We designed and synthesized two new zinc porphyrin dyes for dye-sensitized solar cells (DSCs). Subtle molecular structural variation in the dyes significantly influenced the performance of the DSC devices. By utilizing these dyes in combination with a cobalt-based redox electrolyte using a photoanode made of mesoporous TiO2 , we achieved a power conversion efficiency (PCE) of up to 12.0 % under AM 1.5 G (100 mW cm(-2)) simulated solar light. Moreover, we obtained a high PCE of 6.4 % for solid-state dye-sensitized solar cells by using 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene as a hole-transporting material.

Role of additional PCBM layer between ZnO and photoactive layers in inverted bulk-heterojunction solar cells

In order to induce greater light absorption, nano-patterning is often applied to the metal-oxide buffer layer in inverted bulk-heterojunction(BHJ) solar cells. However, current homogeneity was significantly disturbed at the interface, leading to an efficiency that was not fully optimized. In this work, an additional PC61BM layer was inserted between the ZnO ripple and the photoactive layer to enhance the electron extraction. The insertion of additional PC61BM layer provided substantial advantages in the operation of inverted BHJ solar cells; specifically, it enhanced current homogeneity and lowered accumulation and trapping of photogenerated charges at the ZnO interface. Inclusion of the additional PC61BM layer led to effective quenching of electron-hole recombination by a reduction in the number of accumulated charges at the surface of ZnO ripples. This resulted in a 16% increase in the efficiency of inverted BHJ solar cells to 7.7%, compared to solar cells without the additional PC61BM layer.