No abstract

Nanocrystal quantum dots (QD) show great promise toward improving solar cell efficiencies through the use of quantum confinement to tune absorbance across the solar spectrum and enable multi-exciton generation. Despite this remarkable potential for high photocurrent generation, the achievable open-circuit voltage (Voc) is fundamentally limited due to non-radiative recombination processes in QD solar cells. Here we report the highest open-circuit voltages to date for colloidal QD based solar cells under one sun illumination. This Voc of 692 ± 7 mV for 1.4 eV PbS QDs is a result of improved passivation of the defective QD surface, demonstrating as a function of the QD bandgap (Eg). Comparing experimental Voc variation with the theoretical upper-limit obtained from one diode modeling of the cells with different Eg, these results clearly demonstrate that there is a tremendous opportunity for improvement of Voc to values greater than 1 V by using smaller QDs in QD solar cells.

Metal halide perovskite solar cells have progressed rapidly over the past decade, providing an exceptional opportunity for space photovoltaic (PV) power applications. However, the solar cells to be used for space power have to demonstrate a stable operation under extreme conditions, particularly concerning harsh radiations. In contrast to previously reported superior stability of low PV performance perovskite solar cells against high-energy radiation, we investigate the effects of high-energy electron beam irradiation on the degradation of perovskite solar cells with a high-power conversion efficiency exceeding 20%. We find very high remaining factors of >87.7% in the open-circuit voltage (V OC ) and >93.5% in the fill factor (FF) and a significantly decreased short-circuit current density (J SC ) after the exposure to high-fluence electron irradiations of 10 15 e/cm 2 . The pronounced loss of J SC is due to the decreasing transmittance of the soda-lime glass substrate and the partial decomposition of the perovskite absorber layers. The irradiated cells retained superior remaining factors in both V OC and FF, demonstrating a superior tolerance of perovskite solar cells after the exposure to the electron irradiation. These results show that perovskite solar cells hold great potential for space PV power applications if stable perovskite compositions and space-suitable substrates are employed.

We report here an improved efficiency, up to 4.8% with a high fill factor of ϳ63% under AM 1.5G spectral illumination and 100 mW/ cm 2 intensity, for poly͑3-hexylthiophene͒ and ͓6,6͔-phenyl C 61 butyric acid methyl ester bulk heterojunction photovoltaic ͑PV͒ devices with a 1:0.8 weight ratio using surface modifications to the indium tin oxide ͑ITO͒ anodes through plasma oxidized silver. Here, an enhanced short-circuit current density was achieved without significant loss in the open-circuit voltage ͑Ͼ0.6 V͒ nor the fill factor ͑Ͼ63% ͒, leading to an efficiency jump from 4.4% in the control devices to 4.8% with the surface modified ITO anode. The enhanced short-circuit density is attributed to an interface energy step between the ITO and the polymer hole transporting layer. It has been theorized that the introduction of an interface energy step could alter the charge collection efficiency, resulting in an improved overall efficiency in PV devices. In our study, the current density-voltage characteristics under darkness clearly show an increased current density, especially under forward bias, for the anode treated cell, suggesting the presence of an interface energy step between the ITO and the hole transporting layer with surface modified ITO anodes.

Solution-synthesized inorganic cadmium telluride nanocrystals (∼4 nm; 1.45 eV band gap) are attractive elements for the fabrication of thin-film-based low-cost photovoltaic (PV) devices. Their encapsulating organic ligand shell enables them to be easily dissolved in organic solvents, and the resulting solutions can be spray-cast onto indium-tin oxide (ITO)-coated glass under ambient conditions to produce photoactive thin films of CdTe. Following annealing at 380 °C in the presence of CdCl2(s) and evaporation of metal electrode contacts (glass/ITO/CdTe/Ca/Al), Schottky-junction PV devices were tested under simulated 1 sun conditions. An improved PV performance was found to be directly tied to control over the film morphology obtained by the adjustment of spray parameters such as the solution concentration, delivery pressure, substrate distance, and surface temperature. Higher spray pressures produced thinner layers (<60 nm) with lower surface roughness (<200 nm), leading to devices with improved open-circuit voltages (Voc) due to decreased surface roughness and higher short-circuit current (Jsc) as a result of enhanced annealing conditions. After process optimization, spray-cast Schottky devices rivaled those prepared by conventional spin-coating, showing Jsc = 14.6 ± 2.7 mA cm(-2), Voc = 428 ± 11 mV, FF = 42.8 ± 1.4%, and Eff. = 2.7 ± 0.5% under 1 sun illumination. This optimized condition of CdTe spray deposition was then applied to heterojunction devices (ITO/CdTe/ZnO/Al) to reach 3.0% efficiency after light soaking under forward bias. The film thickness, surface morphology, and light absorption were examined with scanning electron microscopy, optical profilometry, and UV/vis spectroscopy.

Films of nanocrystalline PbSe were fabricated with a set of structurally varied short-chain dicarboxylic acids. Oxidation rates were studied via NIR spectroscopy to determine the effect of the structure of the diacid ligands on film stability under ambient conditions. Ligands favoring a non-bridging bonding mode were found to provide the best protection against oxidation, while among ligands expected to bridge between adjacent nanocrystals in the films, those with shorter chain lengths conferred better oxidative stability. Electronic coupling was observed as a red shift in the optical data of the ground excitonic peak of the PbSe films and found to be strongly influenced by the structure of the ligand. Transport measurements were made in air using thin-film transistors that were treated with a thin Al 2 O 3 coating via remote plasma ALD. Films prepared using fumaric, maleic, and oxalic acids yielded mobility numbers of 2.5 × 10 −5 , 3.7 × 10 −5 , and 1.6 × 10 −3 cm 2 / V•s, respectively. Results suggest that the internanocrystal distance is the major contributor to electron mobility through the nanocrystalline films, while the electronic coupling is heavily influenced by multiple factors related to the structure of the surface ligands in addition to the internanocrystal distance.

Room-temperature electroluminescence from electron-hole plasmas in the metal-oxide-silicon tunneling diodes Conjugated polymers, with molecular orbitals delocalized along the polymer chain, are useful organic semiconductors that provide the possibility of molecular electronics for low-power organic-based memory and logic. Quantum functional devices based upon carrier tunneling processes open vistas into very efficient and low-power consumption circuitry that would be ideal for these applications. We demonstrate here strong room temperature negative differential resistance ͑NDR͒ for poly͓2-methoxy-5-͑2Ј-ethyl-hexyloxy͒-1,4-phenylenevinylene͔ ͑MEH-PPV͒ polymer tunnel diodes ͑PTD͒ using a thin TiO 2 tunneling layer ͑ϳ2-8 nm͒ sandwiched between the MEH-PPV and the indium tin oxide anode. A key advantage is the pronounced NDR using a thick polymer layer with a large active area, circumnavigating the need for molecularly-sized junctions. Current-voltage measurements show large and reproducible NDR with a PVCR as high as 53 at room temperature. We also demonstrate basic logic circuit operation using a pair of these PTDs connected in series to form a monostable-bistable transition logic element ͑MOBILE͒ latch.

Soluble inorganic nanocrystals offer a potential route to the fabrication of all-inorganic devices using solution deposition techniques. Spray processing offers several advantages over the more common spin- and dip-coating procedures, including reduced material loss during fabrication, higher sample throughput, and deposition over a larger area. The primary difference observed, however, is an overall increase in the film roughness. In an attempt to quantify the impact of this morphology change on the devices, we compare the overall performance of spray-deposited versus spin-coated CdTe-based Schottky junction solar cells and model their dark current-voltage characteristics. Spray deposition of the active layer results in a power conversion efficiency of 2.3 ± 0.3% with a fill factor of 45.7 ± 3.4%, Voc of 0.39 ± 0.06 V, and Jsc of 13.3 ± 3.0 mA/cm(2) under one sun illumination.