Bias-dependent Quantum Research Articles

Fast Field-Insensitive Charge Extraction Enables High Fill Factors in Polymer Solar Cells.

Fill factor (FF) is a determining parameter for the power conversion efficiency (PCE) of organic solar cells (OSC). So far, nonfullerene (NF) OSCs with state-of-the-art PCEs exhibit FFs <0.8, lower than the values of Si or perovskite solar cells. The FFs directly display the dependence of photocurrent on bias, meaning that the competition between charge extraction and recombination is modulated by internal electric potential (Vin). Here, we report a study to understand key parameters/properties affecting the device FF based on seven groups of NF-OSCs consisting of widely used PBDBT-2F or PTB7-Th donors and representative NF-acceptors with FFs ranging from 0.60 to 0.78 and PCEs from 10.27 to 16.09%. We used field-dependent transient photocurrent measurements to reveal that fast and field-insensitive charge extraction at low Vin is an essential prerequisite for obtaining high FFs (0.75-0.8), which is enabled by balanced charge transport in steady and reduced bimolecular charge recombination in high purity phases. With bias-dependent quantum efficiency analysis, we further show that the recombination loss at low Vin in the devices with low FFs tends to be more significant involving excitons generated in the donor phase of blends. Our results provide relevance for how to improve the FF toward the boost of photovoltaic performance in NF-OSCs.

Quantifying bulk and surface recombination processes in nanostructured water splitting photocatalysts via in situ ultrafast spectroscopy.

A quantitative description of recombination processes in nanostructured semiconductor photocatalysts-one that distinguishes between bulk (charge transport) and surface (chemical reaction) losses-is critical for advancing solar-to-fuel technologies. Here we present an in situ experimental framework that determines the bias-dependent quantum yield for ultrafast carrier transport to the reactive interface. This is achieved by simultaneously measuring the electrical characteristics and the subpicosecond charge dynamics of a heterostructured photoanode in a working photoelectrochemical cell. Together with direct measurements of the overall incident-photon-to-current efficiency, we illustrate how subtle structural modifications that are not perceivable by conventional X-ray diffraction can drastically affect the overall photocatalytic quantum yield. We reveal how charge carrier recombination losses occurring on ultrafast time scales can limit the overall efficiency even in nanostructures with dimensions smaller than the minority carrier diffusion length. This is particularly true for materials with high carrier concentration, where losses as high as 37% are observed. Our methodology provides a means of evaluating the efficacy of multifunctional designs where high overall efficiency is achieved by maximizing surface transport yield to near unity and utilizing surface layers with enhanced activity.

Long-distance coherent tunneling effect on the charge and heat currents in serially coupled triple quantum dots

The effect of long distance coherent tunneling (LDCT) on the charge and heat currents in serially coupled triple quantum dots (TQDs) connected to electrodes is illustrated by using a combination of the extended Hurbbard model and Anderson model. The charge and heat currents are calculated with a closed-form Landauer expression for the transmission coefficient suitable for the Coulomb blockade regime. The physical parameters including bias-dependent quantum dot energy levels, electron Coulomb interactions, and electron hopping strengths are calculated in the framework of effective mass theory for semiconductor TQDs. We demonstrate that the effect of LDCT on the charge and heat currents can be robust. In addition, it is shown that prominent heat rectification behavior can exist in the TQD system with asymmetrical energy levels.

Voltage and light bias dependent quantum efficiency measurements of GaInP/GaInAs/Ge triple junction devices

III–V monolithic multi-junction solar cells reach efficiencies exceeding 40% and have applications in space and terrestrial concentrator systems. Due to the series connection of the junctions, the external quantum efficiency (EQE) measurement of multi-junction cells presents additional challenges compared to EQE measurement procedures for single-junction devices. Previous work has shown that optimization of bias light and voltage is necessary to minimize measurement artifacts that usually appear during the measurement of the Germanium (Ge) junctions with low shunt resistance. This paper aims to contribute to the improvement of multi-junction device characterization by reporting the effects of low and high shunt resistance junctions on EQE for various voltage and light bias conditions. Voltage and light bias are shown to have different effects on junctions with different shunt resistance. Therefore further investigation of junctions with different shunt resistance under variable light and voltage conditions is needed to provide better understanding of the precise test conditions for EQE measurements. Our results showed that in the high shunt resistance junctions the bias voltage technique is not necessary. However, in the case of junctions with low shunt resistance, a combination of voltage and light bias is required during EQE measurements. Furthermore, the luminescence coupling effect was observed during EQE measurements at very intense light bias conditions confirming previous indications. Finally, an alternative approach for measuring the EQE is proposed.

Band alignment at Sb2S3/Cu(In,Ga)Se2 heterojunctions and electronic characteristics of solar cell devices based on them

Band offsets at Sb2S3/Cu(In,Ga)Se2 heterojunctions have been studied by x-ray and ultraviolet photoemission spectroscopy. The valence and conduction band offset have been estimated to −(0.6±0.3) eV and (0.2±0.3) eV, respectively. This result suggests Sb2S3 as a potential buffer layer material for chalcopyrite based solar cells. However, Cu(In,Ga)Se2/Sb2S3/ZnO solar cells have been investigated. While the open circuit voltage ranged up to ∼0.4–0.5 V, the short circuit current was limited to ∼1.8–4.9 mA/cm2. A photocurrent of about 30 mA/cm2 was found for negative bias. On the basis of bias dependent quantum efficiency measurements and calculations, limiting mechanisms are discussed.

In-line deposited Cu(In,Ga)Se2 solar cells: influence of deposition temperature and Na co-evaporation on carrier collection

We investigated the influence of deposition temperature and sodium co-evaporation on the carrier collection properties of Cu(In,Ga)Se 2 solar cells. I – V and bias-dependent quantum efficiency measurements were performed on cells deposited at 550 and 420 °C with and without sodium co-evaporation. A reduction of deposition temperature leads to a deterioration of solar cell output parameters due to reduced sodium diffusion from the substrate as well as an increased grain boundary density. Bias-dependent quantum efficiency measurements reveal that these effects lead to a decrease of minority carrier diffusion length. When sodium is co-evaporated during absorber growth at low T dep , solar cells with V OC and FF values in the range of high-temperature-deposited devices are realised. However, j sc is not enhanced. Bias-dependent quantum efficiency measurements show that an applied bias voltage does not influence the quantum efficiency curve when sodium is co-evaporated during CIGS growth. The origin of this behaviour is not completely understood.

In-line deposited Cu(In,Ga)Se 2 solar cells: influence of deposition temperature and Na co-evaporation on carrier collection

We investigated the influence of deposition temperature and sodium co-evaporation on the carrier collection properties of Cu(In,Ga)Se 2 solar cells. I– V and bias-dependent quantum efficiency measurements were performed on cells deposited at 550 and 420 °C with and without sodium co-evaporation. A reduction of deposition temperature leads to a deterioration of solar cell output parameters due to reduced sodium diffusion from the substrate as well as an increased grain boundary density. Bias-dependent quantum efficiency measurements reveal that these effects lead to a decrease of minority carrier diffusion length. When sodium is co-evaporated during absorber growth at low T dep, solar cells with V OC and FF values in the range of high-temperature-deposited devices are realised. However, j sc is not enhanced. Bias-dependent quantum efficiency measurements show that an applied bias voltage does not influence the quantum efficiency curve when sodium is co-evaporated during CIGS growth. The origin of this behaviour is not completely understood.

Demonstration of Si homojunction far-infrared detectors

A 48 μm cutoff wavelength (λc) Si far-infrared (FIR) detector is demonstrated. Internal photoemission over a Si interfacial work-function of a homojunction consisting of molecular beam epitaxy grown multilayers (p+ emitter layers and intrinsic layers) is employed. The detector shows high responsivity over a wide wavelength range with a peak responsivity of 12.3±0.1 A/W at 27.5 μm and detectivity D* of 6.6×1010 cmHz/W. The λc and bias dependent quantum efficiency agree well with theory. Based on the experimental results and the model, Si FIR detectors (40–200 μm) with high performance and tailorable λcs can be realized using higher emitter layer doping concentrations.

Numerical simulation of HgCdTe detector characteristics

We discuss analytic and numerical models for HgCdTe photodiodes and present examples of their application. Analytic models can account for the performance obtained by many device architectures. Numerical and analytic models agree in predicting several aspects of device performance, such as diffusion limited dark current, confirming the approximations used in deriving the analytic models. Areas are noted where improvement in the numerical models would allow application to a wider range of device simulations. Useful results are obtained from the numerical simulators that cannot be obtained from our analytic model. Flux dependent R0A products are shown to be a direct result of bias dependent quantum efficiency, a mechanism that is much more evident in heterojunction device architectures. Material compositional grading is demonstrated to lead to lower signal to noise ratio in devices designed to detect a particular infrared wavelength. We also show, particularly for high temperature operation, that heterojunction detectors can at best equal the performance of well-designed homojunction detectors; so, for photodetector design, heterojunctions do not offer any inherent performance advantages over homojunctions. Nevertheless, heterostructures, though ideally not required, may be helpful in achieving high performance in practice.

Effect of a valence-band barrier on the quantum efficiency and background-limited dynamic resistance of compositionally graded HgCdTe P-on-n heterojunction photodiodes

A new analytical model for the bias-dependent quantum efficiency of a HgCdTe P-on-n heterojunction photodiode with a valence band barrier elucidates the important physics of the phenomenon and shows that the background-induced shunt resistance is a result of the same mechanism, that is, a tendency of the light-induced carriers to pile up in the base layer due to the retarding field produced by the barrier. A parameterized version of the model agrees well with experimental current-vs-voltage and noise measurements.

Microcrystalline and wide band gap p + window layers for a-Si p-i-n solar cells

Amorphous silicon p-i-n solar cells have been made with amorphous silicon, a-Si:B:H, amorphous silicon carbide, a-SiC:B:H, and microcrystalline silicon, μc-Si:B:H p + layers. The open circuit voltage increases with the introduction of a-SiC or μc-Si p + layers due to an increase in the built-in field of the p-i-n junction. Bias dependent quantum efficiency and temperature dependent current voltage measurements indicate that the open circuit voltage is limited by a combination of interface and bulk recombination.