Thomas Dittrich

Charge Separation in Type II Tunneling Multilayered Structures of CdTe and CdSe Nanocrystals Directly Proven by Surface Photovoltage Spectroscopy

Charge separation and diffusion in type II multilayered structures of CdTe and CdSe nanocrystals with a polymer spacer are unambiguously proven by surface photovoltage spectroscopy. Holes accumulate in CdTe nanocrystal layers, and the electrons in CdSe nanocrystal layers. An increase of thickness of the polymer spacer strongly decreases the charge separation efficiency. Surface photovoltage transients demonstrate diffusion of the separated charges over several layers of the same kind of nanocrystals.

Determination of spatial charge separation of diffusing electrons by transient photovoltage measurements

It is shown that the surface photovoltage produced by the photoinjected carriers in a layer can be split up into two contributions: total amount of charge, and distance between the centers of charge of the positive and negative carriers. This fact allows us to extract information directly about spatial charge separation of photoinduced charge and its time evolution from surface photovoltage transients. Two cases of particular experimental relevance are analyzed in detail to show the generality of the method: Diffusion photovoltage and tunneling recombination in layers with thickness less than the screening length, and in layers thicker than the screening length, considering also the limit case of diffusion in a semi-infinite space.

It Takes Two to Tango—Double-Layer Selective Contacts in Perovskite Solar Cells for Improved Device Performance and Reduced Hysteresis

Solar cells made from inorganic-organic perovskites have gradually approached market requirements as their efficiency and stability have improved tremendously in recent years. Planar low-temperature processed perovskite solar cells are advantageous for possible large-scale production but are more prone to exhibiting photocurrent hysteresis, especially in the regular n-i-p structure. Here, a systematic characterization of different electron selective contacts with a variety of chemical and electrical properties in planar n-i-p devices processed below 180 °C is presented. The inorganic metal oxides TiO2 and SnO2, the organic fullerene derivatives C60, PCBM, and ICMA, as well as double-layers with a metal oxide/PCBM structure are used as electron transport materials (ETMs). Perovskite layers deposited atop the different ETMs with the herein applied fabrication method show a similar morphology according to scanning electron microscopy. Further, surface photovoltage spectroscopy measurements indicate comparable perovskite absorber qualities on all ETMs, except TiO2, which shows a more prominent influence of defect states. Transient photoluminescence studies together with current-voltage scans over a broad range of scan speeds reveal faster charge extraction, less pronounced hysteresis effects, and higher efficiencies for devices with fullerene compared to those with metal oxide ETMs. Beyond this, only double-layer ETM structures substantially diminish hysteresis effects for all performed scan speeds and strongly enhance the power conversion efficiency up to a champion stabilized value of 18.0%. The results indicate reduced recombination losses for a double-layer TiO2/PCBM contact design: First, a reduction of shunt paths through the fullerene to the ITO layer. Second, an improved hole blocking by the wide band gap metal oxide. Third, decreased transport losses due to an energetically more favorable contact, as implied by photoelectron spectroscopy measurements. The herein demonstrated improvements of multilayer selective contacts may serve as a general design guideline for perovskite solar cells.

Inorganic solid state solar cell with ultra-thin nanocomposite absorber based on nanoporous TiO2 and In2S3

The increase of the diffusion length of a wet chemically prepared absorber by introducing a nanocomposite morphology and the resulting potential for reaching high energy conversion efficiencies were demonstrated.

Defect study of Cu2ZnSn(SxSe1−x)4 thin film absorbers using photoluminescence and modulated surface photovoltage spectroscopy

Defect states in Cu2ZnSn(SxSe1� x)4 thin films with x ¼0.28, 0.36, and 1 were studied by combining photoluminescence (PL) and modulated surface photovoltage (SPV) spectroscopy. A single broad band emission in the PL spectra was observed and can be related to quasi-donor-acceptor pair transitions. The analysis of the temperature dependent quenching of the PL band (x ¼0.28, 0.36, and 1) and SPV (x ¼0.28) signals resulted in activation energies below 150meV for PL and about 90 and 300meV for SPV. Possible intrinsic point defects that might be associated with these observed activation energies are discussed. V C 2015 AIP Publishing LLC.

Visualizing the Nano Cocatalyst Aligned Electric Fields on Single Photocatalyst Particles

The cocatalysts or dual cocatalysts of photocatalysts are indispensable for high efficiency in artificial photosynthesis for solar fuel production. However, the reaction activity increased by cocatalysts cannot be directly ascribed to the accelerated catalytic kinetics, since photogenerated charges are involved in the elementary steps of photocatalytic reactions. To date, diverging views about cocatalysts show that their exact role for photocatalysis is not well understood yet. Herein, we image directly the local separation of photogenerated charge carriers across single crystals of the BiVO4 photocatalyst which loaded locally with nanoparticles of a MnOx single cocatalyst or with nanoparticles of a spatially separated MnOx and Pt dual cocatalyst. The deposition of the single cocatalyst resulted not only in a strong increase of the interfacial charge transfer but also, surprisingly, in a change of the direction of built-in electric fields beneath the uncovered surface of the photocatalyst. The additive electric fields caused a strong increase of local surface photovoltage signals (up to 80 times) and correlated with the increase of the photocatalytic performance. The local electric fields were further increased (up to 2.5 kV·cm-1) by a synergetic effect of the spatially separated dual cocatalysts. The results reveal that cocatalyst has a conclusive effect on charge separation in photocatalyst particle by aligning the vectors of built-in electric fields in the photocatalyst particle. This effect is beyond its catalytic function in thermal catalysis.

Optical and Electronic Properties of Pyrite Nanocrystal Thin Films: the Role of Ligands

Pyrite nanocrystals are currently considered as a promising material for large scale photovoltaic applications due to their non-toxicity and large abundance. While scalable synthetic routes for phase-pure and shape controlled colloidal pyrite nanocrystals have been reported, their use in solar cells has been hampered by the detrimental effects of their surface defects. Here, we report a systematic study of optical and electronic properties of pyrite nanocrystal thin films employing a series of different ligands varying both the anchor and bridging group. The effect of the ligands on the optical and electronic properties is investigated by UV-vis/NIR absorption spectroscopy, current voltage characteristic measurements and surface photovoltage spectroscopy. We find that the optical absorption is mainly determined by the anchor group. The absorption onset in the thin films shifts up to ∼100 meV to the red. This is attributed to changes in the dielectric environment induced by different anchors. The conductivity and photoconductivity, on the other hand, are determined by combined effects of anchor and bridging group, which modify the effective hopping barrier. Employing different ligands, the differential conductance varies over four orders of magnitude. The largest redshift and differential conductance are observed for ammonium sulfides and thiolated aromatic linkers. Pyridine and long chain amines, on the other hand, lead to smaller modifications. Our findings highlight the importance of surface functionalization and interparticle electronic coupling in the use of pyrite nanocrystals for photovoltaic devices.

Photoinduced charge separation in donor–acceptor spiro compounds at metal and metal oxide surfaces: application in dye-sensitized solar cell

Modulated charge separation has been investigated by surface photovoltage spectroscopy in the fixed capacitor arrangement for four different fluorene compounds adsorbed from highly diluted solutions at ultra-thin nanoporous TiO2 (np-TiO2), Au and ITO surfaces. Donor (–N(C6H5)2) and acceptor moieties (–CN, –COOH) were linked by fluorene or spirobisfluorene cores and the chain length has been changed by introducing thiophene. Modulated charge separation by electron injection and intramolecular transport has been separated and directed adsorption of spiro compounds at Au surfaces has been demonstrated. Striking differences between the interaction of linking –CN and –COOH groups and the different substrates were observed. The capability as TiO2 spectral sensitizer of spirobisfluorene donor–acceptor molecules with extended conjugation has been demonstrated. Solar cells exhibit 5.6% energy conversion efficiency, confirming that spiro-configured geometry is a promising route to the design of metal free dyes.

Optically induced switch of the surface work function in TiO2/porphyrin–C60 dyad system

The TiO2/P–C6060 (porphyrin–C60 fullerene dyad) system has been investigated by surface photovoltage spectroscopy under steady state (in the Kelvin-probe arrangement) and under chopped light illumination (in the capacitor arrangement). In contrast to TiO2 with adsorbed porphyrins (M-5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrin) and N3 (Ru(dcbpyH2)2(NCS)2) molecules, the TiO2/P–C6060 dyad system can be switched by light between negative and positive surface charging depending on the photon energy. A mechanism involving the participation of a photoinduced intramolecular charge transfer state in the P–C6060 dyad is proposed in order to explain the observed changes in the photovoltage.

Acceptor-Enhanced Local Order in Conjugated Polymer Films.

Disorder in conjugated polymers is a general drawback that limits their use in organic electronics. We show that an archetypical conjugated polymer, MEH-PPV, enhances its local structural and electronic order upon addition of an electronic acceptor, trinitrofluorenone (TNF). First, acceptor addition in MEH-PPV results in a highly structured XRD pattern characteristic for semicrystalline conjugated polymers. Second, the surface roughness of the MEH-PPV films increases upon small acceptor addition, implying formation of crystalline nanodomains. Third, the low-frequency Raman features of the polymer are narrowed upon TNF addition and indicate decreased inhomogeneous broadening. Finally, the photoinduced absorption and surface photovoltage spectroscopy data show that photoexcited and dark polymer intragap electronic states assigned to deep defects disappear in the blend. We relate the enhanced order to formation of a charge-transfer complex between MEH-PPV and TNF in the electronic ground state. These findings may be of high importance to control structural properties as they demonstrate an approach to increasing the order of a conjugated polymer by using an acceptor additive.