Temperature Dependence of the Electron Diffusion Coefficient in Electrolyte-Filled TiO2

The temperature and photoexcitation density dependences of the electron transport dynamics in electrolytefilled mesoporous TiO2 nanoparticle films were investigated by transient photocurrent measurements. The thermal activation energy of the diffusion coefficient of photogenerated electrons ranged from 0.19–0.27 eV, depending on the specific sample studied. The diffusion coefficient also depends strongly on the photoexcitation density; however, the activation energy has little, if any, dependence on the photoexcitation density. The light intensity dependence can be used to infer temperature-independent dispersion parameters in the range 0.3–0.5. These results are inconsistent with the widely used transport model that assumes multiple trapping of electrons in an exponential conduction-band tail. We can also exclude a model allowing for widening of a band tail with increased temperature. Our results suggest that structural, not energetic, disorder limits electron transport in mesoporous TiO2. The analogy between this material and others in which charge transport is limited by structural disorder is discussed

Temperature Dependence of the Electron Diffusion Coefficient in Electrolyte-Filled TiO2 Nanoparticle Films: Evidence Against Multiple Trapping in Exponential Conduction-Band Tails

The temperature and photoexcitation density dependences of the electron transport dynamics in electrolytefilled mesoporous TiO2 nanoparticle films were investigated by transient photocurrent measurements. The thermal activation energy of the diffusion coefficient of photogenerated electrons ranged from 0.19–0.27 eV, depending on the specific sample studied. The diffusion coefficient also depends strongly on the photoexcitation density; however, the activation energy has little, if any, dependence on the photoexcitation density. The light intensity dependence can be used to infer temperature-independent dispersion parameters in the range 0.3–0.5. These results are inconsistent with the widely used transport model that assumes multiple trapping of electrons in an exponential conduction-band tail. We can also exclude a model allowing for widening of a band tail with increased temperature. Our results suggest that structural, not energetic, disorder limits electron transport in mesoporous TiO2. The analogy between this material and others in which charge transport is limited by structural disorder is discussed

A faux hawk fullerene with PCBM-like properties

Reaction of C60, C6F5CF2I, and SnH(n-Bu)3 produced, among other unidentified fullerene derivatives, the two new compounds 1,9-C60(CF2C6F5)H (1) and 1,9-C60(cyclo-CF2(2-C6F4)) (2). The highest isolated yield of 1 was 35% based on C60. Depending on the reaction conditions, the relative amounts of 1 and 2 generated in situ were as high as 85% and 71%, respectively, based on HPLC peak integration and summing over all fullerene species present other than unreacted C60. Compound 1 is thermally stable in 1,2-dichlorobenzene (oDCB) at 160 °C but was rapidly converted to 2 upon addition of Sn2(n-Bu)6 at this temperature. In contrast, complete conversion of 1 to 2 occurred within minutes, or hours, at 25 °C in 90/10 (v/v) PhCN/C6D6 by addition of stoichiometric, or sub-stoichiometric, amounts of proton sponge (PS) or cobaltocene (CoCp2). DFT calculations indicate that when 1 is deprotonated, the anion C60(CF2C6F5)− can undergo facile intramolecular SNAr annulation to form 2 with concomitant loss of F−. To our knowledge this is the first observation of a fullerene-cage carbanion acting as an SNAr nucleophile towards an aromatic C–F bond. The gas-phase electron affinity (EA) of 2 was determined to be 2.805(10) eV by low-temperature PES, higher by 0.12(1) eV than the EA of C60 and higher by 0.18(1) eV than the EA of phenyl-C61-butyric acid methyl ester (PCBM). In contrast, the relative E1/2(0/−) values of 2 and C60, −0.01(1) and 0.00(1) V, respectively, are virtually the same (on this scale, and under the same conditions, the E1/2(0/−) of PCBM is −0.09 V). Time-resolved microwave conductivity charge-carrier yield × mobility values for organic photovoltaic active-layer-type blends of 2 and poly-3-hexylthiophene (P3HT) were comparable to those for equimolar blends of PCBM and P3HT. The structure of solvent-free crystals of 2 was determined by single-crystal X-ray diffraction. The number of nearest-neighbor fullerene–fullerene interactions with centroid⋯centroid (⊙⋯⊙) distances of ≤10.34 Å is significantly greater, and the average ⊙⋯⊙ distance is shorter, for 2 (10 nearest neighbors; ave. ⊙⋯⊙ distance = 10.09 Å) than for solvent-free crystals of PCBM (7 nearest neighbors; ave. ⊙⋯⊙ distance = 10.17 Å). Finally, the thermal stability of 2 was found to be far greater than that of PCBM

Device Performance of Emerging Photovoltaic Materials (Version 3)

Following the 2nd release of the “Emerging PV reports,” the best achievements in the performance of emerging photovoltaic devices in diverse emerging photovoltaic research subjects are summarized, as reported in peer-reviewed articles in academic journals since August 2021. Updated graphs, tables, and analyses are provided with several performance parameters, e.g., power conversion efficiency, open-circuit voltage, short-circuit current density, fill factor, light utilization efficiency, and stability test energy yield. These parameters are presented as a function of the photovoltaic bandgap energy and the average visible transmittance for each technology and application, and are put into perspective using, e.g., the detailed balance efficiency limit. The 3rd installment of the “Emerging PV reports” extends the scope toward triple junction solar cells

Device Performance of Emerging Photovoltaic Materials (Version 3)

Following the 2nd release of the “Emerging PV reports,” the best achievements in the performance of emerging photovoltaic devices in diverse emerging photovoltaic research subjects are summarized, as reported in peer-reviewed articles in academic journals since August 2021. Updated graphs, tables, and analyses are provided with several performance parameters, e.g., power conversion efficiency, open-circuit voltage, short-circuit current density, fill factor, light utilization efficiency, and stability test energy yield. These parameters are presented as a function of the photovoltaic bandgap energy and the average visible transmittance for each technology and application, and are put into perspective using, e.g., the detailed balance efficiency limit. The 3rd installment of the “Emerging PV reports” extends the scope toward triple junction solar cells

Device Performance of Emerging Photovoltaic Materials (Version 2)

Following the 1st release of the “Emerging photovoltaic (PV) reports”, the best achievements in the performance of emerging photovoltaic devices in diverse emerging photovoltaic research subjects are summarized, as reported in peer-reviewed articles in academic journals since August 2020. Updated graphs, tables, and analyses are provided with several performance parameters, e.g., power conversion efficiency, open-circuit voltage, short-circuit current density, fill factor, light utilization efficiency, and stability test energy yield. These parameters are presented as a function of the photovoltaic bandgap energy and the average visible transmittance for each technology and application and are put into perspective using, e.g., the detailed balance efficiency limit. The 2nd instalment of the “Emerging PV reports” extends the scope toward tandem solar cells and presents the current state-of-the-art in tandem solar cell performance for various material combinations.</p

Device Performance of Emerging Photovoltaic Materials (Version 1)

Emerging photovoltaics (PVs) focus on a variety of applications complementing large scale electricity generation. Organic, dye‐sensitized, and some perovskite solar cells are considered in building integration, greenhouses, wearable, and indoor applications, thereby motivating research on flexible, transparent, semitransparent, and multi‐junction PVs. Nevertheless, it can be very time consuming to find or develop an up‐to‐date overview of the state‐of‐the‐art performance for these systems and applications. Two important resources for recording research cells efficiencies are the National Renewable Energy Laboratory chart and the efficiency tables compiled biannually by Martin Green and colleagues. Both publications provide an effective coverage over the established technologies, bridging research and industry. An alternative approach is proposed here summarizing the best reports in the diverse research subjects for emerging PVs. Best performance parameters are provided as a function of the photovoltaic bandgap energy for each technology and application, and are put into perspective using, e.g., the Shockley–Queisser limit. In all cases, the reported data correspond to published and/or properly described certified results, with enough details provided for prospective data reproduction. Additionally, the stability test energy yield is included as an analysis parameter among state‐of‐the‐art emerging PVs

Formation and properties of metastable defects induced by pulsed laser irradiation in hydrogenated amorphous silicon

THE AIM OF THIS THESIS IS THE STUDY OF THE KINETICS OF CREATION OF METASTABLE DEFECTS IN HYDROGENATED AMORPHOUS SILICON (Α-SI:H) AS WELL AS THE EFFECT OF THESE DEFECTS ON THE TRANSPORT PROPERTIES. WE MEASURED THE KINETICS OF CREATIONOF METASTABLE DEFECTS WITH SHORT, INTENCE LASER PULSES IN VARIOUS SAMPLES AND COMPARED TO RESULTS OBTAINED WITH CONTINOUS ILLUMINATION. THE RECOMBINATION OF PHOTOCARRIES DURING THE LASER PULSE WAS STUDIED FOR THE FIRST TIME. WE ALSO STUDIED, IN RELATION TO THE DEFECT DENSITY, THE CREATION OF A SPACE CHARGE REGION NEAR THE CATHODE IN PHOTOCONDUCTING Α-SI:H. THE MAIN CONCLUSIONS ARE: 1) THE KINETICS OF DEFECT CREATION CHANGES WITH DOPING, 2)THE PHOTOCONDUCTIVITY IS NOT A SINGLE-VALUED FUNCTION OF DEFECT DENSITY, 3) RECOMBINATION OF PHOTOCARRIERS DURING THE HIGH INTENSITY PULSES IN INDEPENDENT OF DEFECTS AND OF DOPING.ΣΤΟΧΟΣ ΤΗΣ ΔΙΑΤΡΙΒΗΣ ΕΙΝΑΙ Η ΜΕΛΕΤΗ ΤΟΣΟ ΤΗΣ ΚΙΝΗΤΙΚΗΣ ΤΗΣ ΔΗΜΙΟΥΡΓΙΑΣ ΜΕΤΑΣΤΑΘΩΝ ΑΤΕΛΕΙΩΝ ΣΤΟ ΑΜΟΡΦΟ ΥΔΡΟΓΟΝΩΜΕΝΟ ΠΥΡΙΤΙΟ (Α-SI:H), ΟΣΟ ΚΑΙ ΤΗΣ ΕΠΙΔΡΑΣΗΣ ΑΥΤΩΝ ΣΤΗΝ ΕΠΑΝΑΣΥΝΔΕΣΗ ΤΩΝ ΦΩΤΟΦΟΡΕΩΝ. ΠΡΑΓΜΑΤΟΠΟΙΗΘΗΚΑΝ ΜΕΤΡΗΣΕΙΣ ΚΙΝΗΤΙΚΗΣ ΔΗΜΙΟΥΡΓΙΑΣ ΑΤΕΛΕΙΩΝ ΜΕ ΙΣΧΥΡΟ ΠΑΛΜΙΚΟ LASER ΚΑΙ ΕΓΙΝΕ ΣΥΓΚΡΙΣΗ ΜΕ ΜΕΤΡΗΣΕΙΣΚΑΤΩ ΑΠΟ ΣΥΝΕΧΗ ΦΩΤΙΣΜΟ. ΕΠΙΣΗΣ, ΠΡΑΓΜΑΤΟΠΟΙΗΘΗΚΑΝ ΜΕΤΡΗΣΕΙΣ ΦΩΤΟΑΓΩΓΙΜΟΤΗΤΑΣΣΤΗ ΔΙΑΡΚΕΙΑ ΤΟΥ ΙΣΧΥΡΟΥ ΠΑΛΜΟΥ LASER. ΜΕΛΕΤΗΘΗΚΕ ΚΑΙ ΕΡΜΗΝΕΥΤΗΚΕ Η ΕΜΦΑΝΙΣΗ ΠΕΡΙΟΧΗΣ ΦΟΡΤΙΟΥ ΧΩΡΟΥ ΣΕ ΔΕΙΓΜΑΤΑ Α-SI:H ΚΑΙ Η ΕΠΙΔΡΑΣΗ ΤΗΣ ΣΤΗ ΦΩΤΟΑΓΩΓΙΜΟΤΗΤΑ ΤΟΥ ΥΛΙΚΟΥ. ΚΥΡΙΑ ΣΥΜΠΕΡΑΣΜΑΤΑ ΕΙΝΑΙ : 1) Η ΚΙΝΗΤΙΚΗ ΔΗΜΙΟΥΡΓΙΑΣ ΑΤΕΛΕΙΩΝΜΕ LASER ΑΛΛΑΖΕΙ ΣΕ ΔΕΙΓΜΑΤΑ ΜΕ ΠΡΟΣΜΕΙΞΕΙΣ, 2) Η ΦΩΤΟΑΓΩΓΙΜΟΤΗΤΑ ΔΕΝ ΕΙΝΑΙΜΟΝΟΤΙΜΗ ΣΥΝΑΡΤΗΣΗ ΤΗΣ ΠΥΚΝΟΤΗΤΑΣ ΑΤΕΛΕΙΩΝ, 3) Η ΕΠΑΝΑΣΥΝΔΕΣΗ ΤΩΝ ΦΩΤΟΦΟΡΕΩΝ ΣΤΗ ΔΙΑΡΚΕΙΑ ΤΟΥ ΠΑΛΜΟΥ LASER ΕΙΝΑΙ ΑΝΕΞΑΡΤΗΤΗ ΤΗΣ ΠΥΚΝΟΤΗΤΑΣ ΑΤΕΛΕΙΩΝ ΚΑΙ ΠΡΟΣΜΙΞΕΩΝ

Photoinduced electron transfer in composites of conjugated polymers and dendrimers with branched colloidal nanoparticles

Charge generation and separation dynamics in donor:acceptor systems based on composites of branched CdSe nanoparticles with a phenyl-cored thiophene-containing dendrimer (4G1-3S), or a low-bandgap conjugated polymer (PCPDTBT) are reported upon exclusive excitation of the donor or the acceptor. Time-resolved microwave conductivity is used to study the dynamics of either transfer of holes from the nanoparticle to dendrimer, or conversely the transfer of electrons from the polymer to the nanoparticle. Higher photoconductance signals and longer decay-times are correlated with device efficiencies, where composites with higher nanoparticle concentration exhibit higher solar photovoltaic power conversion efficiencies and an increase in external quantum efficiencies. This work evaluates the contribution of both components to device performance, but specifically the role of photoexcited nanoparticles.15 page(s

Fullerenes and carbon nanotubes as acceptor materials in organic photovoltaics

The development of new acceptor materials has played an important role in advancing the efficiency of organic photovoltaics (OPV). In this review we survey two types of acceptors, fullerene derivatives and single-walled carbon nanotubes (SWCNTs). The former has been the most successful type of acceptor in OPV, while the latter is a new type of acceptor that has shown encouraging spectroscopic results but its implementation in efficient devices is work in progress. We present some of the essential properties of an OPV acceptor, and the main experimental methodology used to characterize the photoconductivity of an OPV active layer comprising a donor-acceptor blend. We then discuss different aspects of donor-acceptor blends, or bulk heterojunctions, with a polymer donor and a fullerene or SWCNT acceptor. We show the basic charge-generation and recombination properties of a prototypical polymer-fullerene system, and discuss the significance of the molecular structure of the fullerene in terms of the mixing of the two components and its effect on photocarrier dynamics. In the case of the fullerene acceptors we also compare to OPV device efficiency. We discuss the role of charge and energy transfer from the polymer in exciton quenching in polymer-SWCNT blends, and show that the former results in the generation of long-lived charge carriers, similar to observations in polymer-fullerene blends. We show that SWCNT enrichment, to yield samples with only a few percent of metallic SWCNTs results in an enhancement of the photoconductance and carrier lifetime in P3HT-SCWNT blends. We also discuss the role that SWCNT diameter plays on the driving force for carrier generation, and suggest that smaller SWCNTs may be critical for efficient OPV device performance. Finally, we discuss the implications of the results of the photoconductivity studies on OPV device performance, and outline future challenges and open questions pertaining to the optimization of acceptors in OPV.11 page(s