Photovoltaic Mechanism Research Articles

Towards High Performance Organic Photovoltaic Cells: A Review of Recent Development in Organic Photovoltaics

Organic photovoltaic cells (OPVs) have been a hot topic for research during the last decade due to their promising application in relieving energy pressure and environmental problems caused by the increasing combustion of fossil fuels. Much effort has been made toward understanding the photovoltaic mechanism, including evolving chemical structural motifs and designing device structures, leading to a remarkable enhancement of the power conversion efficiency of OPVs from 3% to over 15%. In this brief review, the advanced progress and the state-of-the-art performance of OPVs in very recent years are summarized. Based on several of the latest developed approaches to accurately detect the separation of electron-hole pairs in the femtosecond regime, the theoretical interpretation to exploit the comprehensive mechanistic picture of energy harvesting and charge carrier generation are discussed, especially for OPVs with bulk and multiple heterojunctions. Subsequently, the novel structural designs of the device architecture of OPVs embracing external geometry modification and intrinsic structure decoration are presented. Additionally, some approaches to further increase the efficiency of OPVs are described, including thermotics and dynamics modification methods. Finally, this review highlights the challenges and prospects with the aim of providing a better understanding towards highly efficient OPVs.

Photovoltaic mechanism in Na-substituted BiFeO3 films

Na-substituted BiFeO3 films were prepared on FTO/glass substrates using the sol–gel method. XRD results showed that all films adopted random orientation and an R3m perovskite structure. UV–Vis absorption results indicated that the films exhibited intense absorbance around 450 nm. The band gaps were 2.59 eV, 2.63 eV, 2.62 eV and 2.52 eV for the films substituted with 0%, 5%, 10% and 20% Na, respectively. A substantially enhanced photovoltaic effect was observed in the 20% Na substituted BiFeO3 film; its short-circuit current density was 1.26 µA cm−2 and the open circuit voltage was −0.66 V. Meanwhile, polarization-modulated Jsc and Voc were observed in the 20% Na substituted BiFeO3 film and the mechanism was studied using the Schottky barrier combined with the ferroelectric polarization model.

Exploring the structure as well as electrical and photovoltaic mechanism in PrFe0.5Ni0.5O3/GaAs heterojunction

Structural and electrical transport study of PrFe0.5Ni0.5O3 heterojunction prepared by pulsed laser deposition (PLD) on GaAs (001) substrate is presented. The film deposited on this substrate is polycrystalline and has orthorhombic structure with space group Pbnm. The surface roughness was estimated around 3.4nm and this increase in surface roughness results essentially from the grain formation. This heterojunction shows usual semiconducting behavior. Resistivity data have been fitted with variable range hopping (VRH) model, and small Polaron Hopping (SPH) model, from which different parameters like density of state at Fermi level N(Ef), hopping energy (Eh), hopping distance (Rh), and Mott׳s characteristic temperature (T0) were estimated. The photovoltaic behavior of this heterojunction was investigated via current–voltage (I–V) characteristics. A mechanism based on the well established model for photovoltaics in conventional semiconductors has been utilized to explain the observed trend in the present perovskite-type metal oxides. This observed behavior in the perovskite-type metal oxide may open a new window for its application in the modern optical based electronics.

Effects of Germanium Tetrabromide Addition to Zinc Tetraphenyl Porphyrin / Fullerene Bulk Heterojunction Solar Cells

The effects of germanium tetrabromide addition to tetraphenyl porphyrin zinc (Zn-TPP)/fullerene (C60) bulk heterojunction solar cells were characterized. The light-induced charge separation and charge transfer were investigated by current density and optical absorption. Addition of germanium tetrabromide inserted into active layer of Zn-TPP/C60 as bulk heterojunction had a positive effect on the photovoltaic and optical properties. The photovoltaic mechanism of the solar cells was discussed by experimental results. The photovoltaic performance was due to light-induced exciton promoted by insert of GeBr4 and charge transfer from HOMO of Zn-TPP to LUMO of C60 in the active layer.

General Working Principles of CH3NH3PbX3 Perovskite Solar Cells

Organometal halide perovskite-based solar cells have recently realized large conversion efficiency over 15% showing great promise for a new large scale cost-competitive photovoltaic technology. Using impedance spectroscopy measurements we are able to separate the physical parameters of carrier transport and recombination in working devices of the two principal morphologies and compositions of perovskite solar cells, viz. compact thin films of CH3NH3PbI(3-x)Clx and CH3NH3PbI3 infiltrated on nanostructured TiO2. The results show nearly identical spectral characteristics indicating a unique photovoltaic operating mechanism that provides long diffusion lengths (1 μm). Carrier conductivity in both devices is closely matched, so that the most significant differences in performance are attributed to recombination rates. These results highlight the central role of the CH3NH3PbX3 semiconductor absorber in carrier collection and provide a new tool for improved optimization of perovskite solar cells. We report for the first time a measurement of the diffusion length in a nanostructured perovskite solar cell.

Photothermoelectric Effect in Suspended Semiconducting Carbon Nanotubes

We have performed scanning photocurrent microscopy measurements of field-effect transistors (FETs) made from individual ultraclean suspended carbon nanotubes (CNTs). We investigate the spatial-dependence, polarization-dependence, and gate-dependence of photocurrent and photovoltage in this system. While previous studies of surface-bound CNT FET devices have identified the photovoltaic effect as the primary mechanism of photocurrent generation, our measurements show that photothermoelectric phenomena play a critical role in the optoelectronic properties of suspended CNT FETs. We have quantified the photothermoelectric mechanisms and identified regimes where they overwhelm the photovoltaic mechanism.

Photovoltaic effect of a bilayer thin film with (Na0.5Bi0.5)1−xBaxTiO3/BiFeO3 heterostructure

A bilayer film containing (Na0.5Bi0.5)1−xBaxTiO3 (NBT–BT)/BiFeO3(BFO) heterostructure was produced by chemical solution deposition. An enhancement of photovoltaic performance of the bilayer film was discovered. It leads to a short circuit current density of 18.9 µA cm−2 and an open circuit voltage of 0.5 V, these values are much higher than that of BFO film. When transparent indium tin oxide (ITO) was used to substitute gold (Au) as top electrode, the photovoltaic voltage and photocurrent density, respectively, improved to ∼0.71 V and ∼45 µA cm−2. A theoretical band diagram model considering the ferroelectric polarization, heterostructure on the interface, and energy band bending effect was constructed to depict the mechanism of photovoltaic enhancement of the bilayer film. Temperature-dependent I–V measurements were carried out to explore the barrier height of the Au/BFO/NBT–BT/FTO heterostructure, and a total barrier height of 0.54 eV was obtained. Our work opens up a promising new method that has the potential for enhancing the photovoltaic performance for ferroelectric thin film, and provides deeper understanding of the photovoltaic mechanism of ferroelectric films.

Effect of a-Si:H–polymer interface on the behavior of hybrid solar cells

Organic photovoltaics (OPVs) are primary candidates for economical and flexible next-generation solar electricity conversion devices. However, efficiencies of the OPVs are limited, among other reasons, by poor charge transport and limited acceptor materials. To overcome this, hybrid organic–inorganic solar cells have been proposed. In this study, the authors investigated the photovoltaic characteristics of hybrid amorphous silicon (a-Si:H) films with varying doping nature with poly(3-hexylthiophene) (P3HT) only and P3HT:phenyl-C61-butyric acid methyl ester (PCBM) bulk-heterojunction–based solar cells. Both intrinsic and doped a-Si:H films were used. Despite the higher short circuit current density of P3HT/intrinsic a-Si:H device, the charge collection and open circuit voltage in P3HT/n+ a-Si:H device were the highest. This was attributed to favorable band bending and high built-in electric field in P3HT. Previously proposed model for photovoltaic mechanism in polymer/a-Si:H devices was revisited, and a revision was proposed. P3HT:PCBM cells on the three types of cells largely followed the same trend as the P3HT-only devices, except that intrinsic device did not have a photocurrent contribution from the organic layer.

Analysis of governing factors for photovoltaic loss mechanism of n-CdS/p-CdTe heterojunction via multi-way data decomposition

An analytical framework for identifying key factors of the degradation of photovoltaic efficiency over time is presented. We demonstrate that, in many photovoltaic experimental settings, reliability data sets are easily cast in a multi- or N-way format. We adopt a statistical technique, N-way partial least squares, that generates a multi-linear model using all of the data simultaneously. With this approach, we are able to model variables of interest such as cell efficiency while representing the data in a lower-dimensional space in which salient features are more easily identified. We illustrate our approach with reliability data for CdS/CdTe heterojunction solar cell devices. Even with the inclusion of a noisy parameter such as the net acceptor density, and with a relatively small number of devices, we automatically identify key factors that are highly related to performance degradation. In particular, the conductance at the back contact is related to short stress-time degradation (0–300 h), whereas the net acceptor density near the junction (at +0.08 V DC bias) is correlated with more gradual, long stress-time degradation (300–1000 h). These notable degradation modes are explained with respect to our processing conditions and Cu-diffusion in the cells. Copyright © 2013 John Wiley & Sons, Ltd.

Scanning probe study on the photovoltaic characteristics of a Si solar cell by using Kelvin force microscopy and photoconductive atomic force microscopy

Poly-Si-based solar cells, prepared via conventional Si processes including phosphoryl chloride(POCl3) doping and diffusion, were investigated in this study in terms of their electrical and optical properties, including open-circuit voltage (Voc), short-circuit current (Isc), fill factor, external quantum efficiency and efficiency, employing a few recognized test methods. Also, we compared the experiment results from an identical specimen via Kelvin force microscopy (KFM) and photoconductive atomic force microscopy (PC-AFM), respectively, verifying that the scanning probe technique is very effective both in photovoltaic effect measurement and mechanism establishment. When the results of both conventional and nano-probing techniques are compared, the behavior of the surface potential property is similar to the Voc, and that of the photoinduced current property is similar to the Isc. Through this study, we have demonstrated that the KFM and the PC-AFM are effective tools to monitor and evaluate the properties of solar energy–producing materials and devices.

Cálculo de una tarifa de alimentación para instalaciones fotovoltaicas residenciales en Colombia

The objective of this article is to estimate a residential feed-in rate that will encourage the production of photovoltaic energy in Colombia. For this calculation, the used methodology was one that combines the point of equilibrium analysis by Rigter and Vidican (2010) with the representation of a photovoltaic mechanism designed by the Homer Energy software. The feed-in rate is calculated for three different locations in the country, and for Bogota the rate is 4,359 pesos per kilowatt-hour for a 3 kW photovoltaic system. Even though the estimated rate can seem high according to the economic and institutional constrains of the Colombian State, the space to explore alternative mechanisms and pilot programsexists. The article also discusses the barriers and legal possibilities for stimulating of renewable energies in Colombia.

Optimal Design of the Mechanical Device for a Photovoltaic Tracking Mechanism

Increasing the conversion efficiency of the solar energy into electricity is one of the most addressed topics in the field of renewable energy systems. In this paper, the pseudo-azimuthal tracking mechanism for a PV platform it was selected for presentation. The study is focused on the optimization of the mechanical device model of the tracking mechanism (developed with the MBS software environment ADAMS of MSC). The optimization is based on the parametrization of the model by using the design points that define the locations of the geometric constraints (i.e. the joints). The objective of the optimization is to minimize the motor force developed by the driving source, which is a linear actuator. In addition, a design constraint is used to limit (restrict) the value of the pressure angle. The optimization study leads to the minimization of the energetic consumption during tracking, with positive effect on the energy balance of the photovoltaic system with sun tracker.

Fabrication and characterization of tetracyanoquinodimethane/phthalocyanine solar cells

Abstract Fabrication and characterization of heterojunction solar cells of tetracyanoquinodimethane (TCNQ)/copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) were carried out. The light-induced charge separation with charge transfer was investigated by light-induced current density and optical absorption. In both cases of the TCNQ/CuPc and TCNQ/ZnPc solar cells, the TCNQ thin film worked for strong electron-accepting layer as n-type semiconductor. These behaviors would be originated in charge transfer of excited electron from CuPc and ZnPc to TCNQ. The photovoltaic mechanism was discussed on the basis of the experimental results.

Local Mapping of Generation and Recombination Lifetime in BiFeO3 Single Crystals by Scanning Probe Photoinduced Transient Spectroscopy

Carrier lifetime in photoelectric processes is the average time an excited carrier is free before recombining or trapping. Lifetime is directly related to defects and it is a key parameter in analyzing photovoltaic effects in semiconductors. We show here a scanning probe method combined with photoinduced current spectroscopy that allows mapping with nanoscale resolution of the generation and recombination lifetimes. Using this method we have analyzed the mechanism of the abnormal photovoltaic effect in multiferroic bismuth ferrite, BiFeO(3). We found that generation and recombination lifetimes in BiFeO(3) are large due to complex generation and recombination processes that involve shallow energy levels in the band gap. The domain walls do not play a major role in the photovoltaic mechanism.

Energy Saving Evaluation of the Ventilated BIPV Walls

This study integrates photovoltaic (PV) system, building structure, and heat flow mechanism to propose the notion of ventilated Building-Integrated Photovoltaic (BIPV) walls. The energy-saving potential of the ventilated BIPV walls was investigated via engineering considerations and computational fluid dynamics (CFD) simulations. The results show that the heat removal rate and indoor heat gain of the proposed ventilated BIPV walls were dominantly affected by outdoor wind velocity and airflow channel width. Correlations for predicting the heat removal rate and indoor heat gain, the reduction ratio of the indoor heat gain, CO2 reduction, and induced indoor air exchange are introduced.

Fabrication and Characterization of Fullerene / Dibenzo-Tetrathiafulvalene Solar Cells

Fabrication and characterization of fullerene (C60) / dibenzotetrathiafulvalene (DBTTF) solar cells were carried out. Photovoltaic and optical properties of the organic solar cells were investigated. Transmission electron microscopy, x-ray and electron diffraction confirmed that the bulk heterojunction thin films had microstructure of C60 crystal phase in DBTTF amorphous phase. The photovoltaic performance of the bulk heterojunction solar cell would be originated in the extent of electron diffusion across interface around the microstructure. Photovoltaic mechanism was discussed on the basis of experimental results.

Fabrication and characterization of porphyrin dye-sensitized solar cells

Solid dye-sensitized solar cells of porphyrin adsorbed on TiO 2 particles with iodine have been fabricated and characterized. Influence of the porphyrin in the dye-sensitized solar cells on photovoltaic properties was investigated by experimental results. Microstructure and X-ray diffractions suggested that TiO 2 molecules were ordered to form the tetragonal crystal. A slight amount of the crystal disorder caused to inhibit carrier diffusion with recombination at interface between TiO 2 and amorphous porphyrin. Photovoltaic mechanism of the solar cells was discussed by the experimental results using quantum calculation.

High-performance Schottky solar cells using ZrS2 nanobelt networks

The transition metal dichalcogenide ZrS2 is an important semiconductor and should be a promising material in the optoelectronic field, however, its optoelectronic properties and applications have seldom been reported. In this paper, we synthesized single-crystalline ZrS2 nanobelts for interesting applications in solar cells. A facile and effective approach to fabricate ZrS2 nanobelt network-embedded solar cells was represented by forming Schottky junctions between the contacts of ZrS2 nanobelts and metal Au electrodes. The solar cell devices showed a high short-circuit current Jsc of 21.8 mA cm−2 and a cell efficiency of 1.2%. The photovoltaic mechanism was discussed by constructing their energy band diagrams. Present results would be important for the field of transition-metal-dichalcogenide nanostructure optoelectronic applications.

PbSe Nanocrystal Excitonic Solar Cells

We report the design, fabrication, and characterization of colloidal PbSe nanocrystal (NC)-based photovoltaic test structures that exhibit an excitonic solar cell mechanism. Charge extraction from the NC active layer is driven by a photoinduced chemical potential energy gradient at the nanostructured heterojunction. By minimizing perturbation to PbSe NC energy levels and thereby gaining insight into the "intrinsic" photovoltaic properties and charge transfer mechanism of PbSe NC, we show a direct correlation between interfacial energy level offsets and photovoltaic device performance. Size dependent PbSe NC energy levels were determined by cyclic voltammetry and optical spectroscopy and correlated to photovoltaic measurements. Photovoltaic test structures were fabricated from PbSe NC films sandwiched between layers of ZnO nanoparticles and PEDOT:PSS as electron and hole transporting elements, respectively. The device current-voltage characteristics suggest a charge separation mechanism that is distinct from previously reported Schottky devices and consistent with signatures of excitonic solar cells. Remarkably, despite the limitation of planar junction structure, and without film thickness optimization, the best performing device shows a 1-sun power conversion efficiency of 3.4%, ranking among the highest performing NC-based solar cells reported to date.

Fabrication and characterization of inorganic-organic hybrid solar cells based on CuInS2

Chalcopyrite compounds/fullerene solar cells based on copper-indium-disulphide and C60 were produced. Two-types of devices with heterojunction and bulk heterojunction structures were examined and characterized. The bulk heterojunction structure provided better efficiency compared to that of the heterojunction structure. Nanostructures of the solar cells were confirmed as mixed nanocrystals by transmission electron microscopy. A photovoltaic mechanism of the present solar cells was discussed on the basis of the experimental results.