Nanocrystal Solar Cells Research Articles

PbS nanocrystal solar cells with high efficiency and fill factor

We report on the fabrication of efficient PbS solar cells, showing power conversion efficiencies approaching 4% and fill factors of 60% under AM1.5 illumination. The effect of the size of two different nanocrystals (NCs) on the performance and key parameters of the devices are discussed together with peculiar features of device functioning. The results prove that the devices are not under space-charge limitation and the device performance is influenced by charge trapping which is dependent on the size of the NCs.

Effects of nanocrystal size and device aging on performance of hybrid poly(3-hexylthiophene):CdSe nanocrystal solar cells

We have studied hybrid solar cells based on the polymer poly(3-hexylthiophene) (P3HT) and colloidal CdSe nanocrystals. Using CdSe nanospheres with varying size, we have found that the power conversion efficiency ( η P ) of these devices increases monotonically with the CdSe nanocrystal size, from η P =(0.39±0.04)% under AM1.5G solar illumination for 4.0±0.2 nm size nanospheres to η P =(1.9±0.2)% for 6.8±0.5 nm size nanospheres. The efficiency increase with nanocrystal size is mostly due to a significant increase in the short-circuit current, whereas the open-circuit voltage and fill factor of the solar cells are less affected. The devices also exhibit abnormal initial aging behavior when exposed to air, as an increase in both the short-circuit current and open-circuit voltage during the first 30 min leads to a significant increase in η P .

Stability Assessment on a 3% Bilayer PbS/ZnO Quantum Dot Heterojunction Solar Cell

We provide the first NREL-certified efficiency measurement on an all-inorganic, solution-processed, nanocrystal solar cell. The 3% efficient device is composed of ZnO nanocrystals and 1.3 eV PbS quantum dots with gold as the top contact. This configuration yields a stable device, retaining 95% of the starting efficiency after a 1000-hour light soak in air without encapsulation.

Improvement in photovoltaic performance for hybrid P3HT/elongated CdS nanocrystals solar cells with F-doped SnO2 arrays

Hybrid CdS/P3HT photovoltaic devices using F-doped SnO2 (FTO)-coated ZnO nanorod arrays as electrodes were studied in this work. The crystalline FTO, made using low-cost spray pyrolysis deposition (SPD), displayed a nominally complete and uniform coating over the entire outer surface of the ZnO nanorods. The photovoltaic performance of the CdS/P3HT photovoltaic devices increased with the FTO-coated ZnO nanorod length and thickness of the FTO layer, due to improvement in collecting photogenerated electrons, which results in the increased short circuit current density (JSC) and fill factor (FF) of the devices. The incorporation of CdS into the P3HT greatly enhanced the JSC of the devices with a nanorod FTO electrode. The power conversion efficiency (PCE) of the device with a ZnO nanorod length of 320 nm was increased from 0.37% for P3HT without CdS to 1.8% with CdS of AR = 1 (spherical shape). With a further increase of the AR of the CdS nanocrystal to 4, the PCE was further increased up to 2.6%. However, longer CdS nanocrystals conversely caused deterioration in PCE as a result of the nanorod array morphology. These results indicate that, although increased nanorod length could improve the photocurrent and efficiency, other factors, such as P3HT infiltration, nanorod array morphology and CdS nanocrystal length are required for obtaining the optimal performance of these devices.

Low temperature synthesis of thiol‐functionalized CdTe nanoclusters with different tellurium contents

AbstractIn recent years thiol‐functionalized cadmium telluride nanoparticles act as a core layers for obtaining high photovoltaic efficiency in conjugated polymer/inorganic nanocrystal solar cells and also provide easy tuning of quantum size effect by varying thiol groups and processing temperature. In this work, thiol functionalized CdTe nanoclusters were prepared for different tellurium concentrations at low temperature and its impact has been discussed. Structural parameters estimated by X‐ray diffraction revealed hexagonal phases at lower tellurium concentration and cubic phase at higher tellurium concentrations. Surface morphology of the samples analyzed by transmission electron microscopy, exhibited cluster type of morphology. Optical band gap energy estimated by diffused electron microscopy showed an increase with Te content and band edge emission has been observed in the photoluminescence spectra of CdTe nanoclusters around 620 nm. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical and optical characteristics of silicon nanocrystal solar cells

We investigated the relationship between the absorption in silicon nanocrystals (Si NCs) and the photocurrent of Si NC solar cells. Here, the absorption of Si NCs in the blue and green light regions was enhanced by up to 14 times compared to bulk Si. In addition, the photocurrent in Si NC solar cells was found to originate from absorption in the Si NCs. The photocurrent of the Si NC solar cells, however, was smaller than the predicted value due to the slow carrier transport in the silicon nitride film containing the Si NCs.

Hybrid Photovoltaics Based on Semiconductor Nanocrystals and Amorphous Silicon

Semiconductor nanocrystals (NCs) are promising materials for applications in photovoltaic (PV) structures that could benefit from size-controlled tunability of absorption spectra, the ease of realization of various tandem architectures, and, perhaps, increased conversion efficiency in the ultraviolet region through carrier multiplication. The first practical step toward utilization of the unique properties of NCs in PV technologies could be through their integration into traditional silicon-based solar cells. Here, we demonstrate an example of such hybrid PV structures that combine colloidal NCs with amorphous silicon. In these structures, NCs and silicon are electronically coupled, and the regime of this coupling can be tuned by altering the alignment of NC energy states with regard to silicon band edges. For example, using wide-gap CdSe NCs we demonstrate a photoresponse which is exclusively due to the NCs. On the other hand, in devices comprising narrow-gap PbS NCs, both the NCs and silicon contribute to photocurrent, which results in PV response extending from the visible to the near-infrared region. The hybrid silicon/PbS NC solar cells show external quantum efficiencies of approximately 7% at infrared energies and 50% in the visible and a power conversion efficiency of up to 0.9%. This work demonstrates the feasibility of hybrid PV devices that combine advantages of mature silicon fabrication technologies with the unique electronic properties of semiconductor NCs.

Determining the Internal Quantum Efficiency of PbSe Nanocrystal Solar Cells with the Aid of an Optical Model

We determine the internal quantum efficiency (IQE) of the active layer of PbSe nanocrystal (NC) back-contact Schottky solar cells by combining external quantum efficiency (EQE) and total reflectance measurements with an optical model of the device stack. The model is parametrized with the complex index of refraction of each layer in the stack as calculated from ellipsometry data. Good agreement between the experimental and modeled reflectance spectra permits a quantitative estimate of the fraction of incident light absorbed by the NC films at each wavelength, thereby yielding well-constrained QE spectra for photons absorbed only by the NCs. Using a series of devices fabricated from 5.1+/-0.4 nm diameter PbSe NCs, we show that thin NC cells achieve an EQE and an active layer IQE as high as 60+/-5% and 80+/-7%, respectively, while the QE of devices with NC layers thicker than about 150 nm falls, particularly in the blue, because of progressively greater light absorption in the field-free region of the films and enhanced recombination overall. Our results demonstrate that interference effects must be taken into account in order to calculate accurate optical generation profiles and IQE spectra for these thin film solar cells. The mixed modeling/experimental approach described here is a rigorous and powerful way to determine if multiple exciton generation (MEG) photocurrent is collected by devices with EQE<100%. On the basis of the magnitudes and shapes of the IQE spectra, we conclude that the 1,2-ethanedithiol treated NC devices studied here do not produce appreciable MEG photocurrent.

Exciton escape in CdSe core-shell quantum dots: Implications for the development of nanocrystal solar cells

Exciton escape in CdSe core-shell quantum dots: Implications for the development of nanocrystal solar cells

Enhanced photovoltaic response of organic solar cell by singlet-to-triplet exciton conversion

Ir complex was doped to conjugated polymers, and the photoinduced absorption of triplet excitons in host materials was examined. A greatly enhanced intersystem crossing rate was observed, despite the decrease in triplet exciton lifetime. The authors find that the steady-state triplet exciton population in host polymer would increase by an order of magnitude. Conjugated polymer/colloidal CdSe nanocrystal hybrid solar cells were fabricated and the effect of Ir-complex doping on photovoltaic response was studied. It was found that due to the enhanced singlet-to-triplet conversion, greatly enhanced photovoltaic response of these hybrid organic solar cells was observed. The results suggest that triplet solar cells may be achieved by doping conventional photovoltaic materials with transition-metal complexes.

Air-Stable All-Inorganic Nanocrystal Solar Cells Processed from Solution

We introduce an ultrathin donor-acceptor solar cell composed entirely of inorganic nanocrystals spin-cast from solution. These devices are stable in air, and post-fabrication processing allows for power conversion efficiencies approaching 3% in initial tests. This demonstration elucidates a class of photovoltaic devices with potential for stable, low-cost power generation.