Flat Heterojunction Research Articles

InP Crystal Phase Heterojunction Transistor with a Vertical Gate-All-Around Structure.

Crystal phase transitions can form a new type of heterojunction with different atomic arrangements in the same material: crystal phase heterojunction (CPHJ). The CPHJ has an inherently strong impact on band engineering without concerns over critical thicknesses with misfit dislocations and a semiconductor-metal transition. In-plane CPHJ was recently demonstrated in two-dimensional (2D) transition-metal dichalcogenide (TMD) materials and utilized for conventional planar field-effect transistor applications. However, scalability such as gate electrostatic control, miniaturization, and multigate structure have been limited because of the geometrical issue. Here, we demonstrated a transistor using the CPHJ with a vertical gate-all-around structure by forming a CPHJ in conventional III-V semiconductors. The CPHJ, composed of wurtzite InP nanowires with zincblende InP substrates, showed an atomically flat heterojunction without dislocations and indicated a Type-II band discontinuity across the junction. The CPHJ transistor had moderate to good gate electrostatic controllability with high on-state currents and transconductance. The CPHJ offer will provide a new switching mechanism and add a new junction and device design choice to the long history of transistors.

Flexoelectricity‐Enhanced Self‐Powered Photodetection in 2D van der Waals Heterojunctions With Large Curvatures

AbstractThe unique morphology of 2D van der Waals materials enables them to withstand large deformations and significant nonuniform strain, potentially inducing a strong flexoelectric effect. Despite the size‐dependent flexoelectric effect showing potential for modulating the optoelectronic performance of 2D van der Waals materials, it is far from being fully exploited owing to various challenges. Herein, the use of nanowires with different diameters as the bending media to fabricate 2D α‐In2Se3/β‐InSe heterojunctions with large curvatures of 0.1–1 µm−1 is proposed. The significant band alignment modulation in α‐In2Se3 resulting from the bending‐induced flexoelectric effect is verified through Kelvin probe force microscopy. The strain‐induced piezoelectric effect can be negated because of the weak vdW forces at the interface. The flexoelectric polarization in β‐InSe is screened via the accumulated electrons in the unilateral depleted heterojunction. Compared to the flat heterojunction, the curved heterojunction with an average curvature of 0.9 µm−1 shows 2.48‐fold and 7.62‐fold increases in open‐circuit voltage and zero‐biased responsivity, respectively. This study demonstrates the first successful modulation of photodetection in 2D heterojunctions by exploiting the flexoelectric effect, providing a new perspective for high‐performance 2D vdW optoelectronic devices.

Spectroscopic near-infrared photodetectors enabled by strong light-matter coupling in (6,5) single-walled carbon nanotubes.

Strong light-matter coupling leads to the formation of mixed exciton-polariton states, allowing for a rigorous manipulation of the absorption and emission of excitonic materials. Here, we demonstrate the realization of this promising concept in organic photodetectors. By hybridizing the E11 exciton of semiconducting (6,5) single-walled carbon nanotubes (SWNTs) with near-infrared cavity photons, we create spectrally tunable polariton states within a photodiode. In turn, we are able to red-shift the detection peak that coincides with the lower polariton band. Our photodiodes comprise a metal cavity to mediate strong coupling between light and SWNTs and utilize P3HT and PC70BM as the electron donor and acceptor, respectively. The diodes are formed either via mixing of SWNTs, P3HT, and PC70BM to create a bulk heterojunction or by sequential processing of layers to form flat heterojunctions. The resulting near-infrared sensors show tunable, efficient exciton harvesting in an application-relevant wavelength range between 1000 nm and 1300 nm, with optical simulations showing a possible extension beyond 1500 nm.

Simulation and application of external quantum efficiency of solar cells based on spectroscopy

In this study, a method for optical simulation of external quantum efficiency (EQE) spectra of solar cells based on spectroscopy is proposed, which is based on the tested transmittance and reflectance spectra. First, to obtain a more accurate information of refractive index and extinction coefficient values, we modified the reported optical constants from the measured reflectance and transmittance spectra. The obtained optical constants of each layer were then collected to simulate the EQE spectra of the device. This method provides a simple, accurate and versatile way to obtain the actual optical constants of different layers. The EQE simulation approach was applied to the flat and textured heterojunctions with intrinsic layers (HIT) solar cells, respectively, which showed a perfect matching between the calculation results and the experimental data. Furthermore, the specific optical losses in different devices were analyzed.

Thermally Induced Fullerene Domain Coarsening Process in Organic Solar Cells

The recent advancements in power conversion efficiency for organic solar cells is still complained by their reliability and stability remaining the main bottlenecks for organic photovoltaics large scale production and commercialization. In this paper, we aim to provide further insights understanding in degradation processes affecting stability in small molecule flat heterojunction (Glass/ITO/MoO3/ZnPc/C60/BCP/Ag) solar cells through a systematic aging study coupled with optoelectrical characterizations. In particular, the burn-in phenomenon affecting short-circuit current in thermal-stressed samples has been clearly correlated with the C60 domain coarsening process and eventually to the decreased exciton lifetime.

The improvement in organic photovoltaic response by inserting an interlayer between MoO3 and mixed layer of C60:5 wt% TAPC

We demonstrate organic photovoltaic (OPV) cells with structure of ITO/MoO3 (5nm)/boron subphthalocyanine chloride (SubPc) (d nm)/C60:5wt% 1,1-bis-(4-methyl-phenyl)-aminophenyl-cyclohexane (TAPC) (40nm)/BCP (8nm)/Al, where d=0, 1, 3, 5, 7 and 13nm. We found that a 5-nm-thick SubPc based cell had a peak power conversion efficiency (PCE) of 3.75% and a voltage (Voc) of 1.03V, which was an increase of 35.4% and 0.19V, respectively, compared with the reference cell without SubPc. We found that the spectral response of cells with 1–5nm SubPc mostly corresponded to the C60 absorption and the cell with 5nm SubPc had the highest responsivity. The spectral response of the cell with 13nm SubPc corresponded to both C60 and SubPc absorptions, which is analogous to a previously reported flat heterojunction cell. The excellent PV parameter of the cell with 5nm SubPc was attributed to a built-in field induced by the Schottky barrier contact of MoO3/5-nm SubPc. To confirm this hypothesis, another two series of OPV cells with different interlayers and donors instead of SubPc and TAPC were developed.

Modeling materials and processes in hybrid/organic photovoltaics: from dye-sensitized to perovskite solar cells.

CONSPECTUS: Over the last 2 decades, researchers have invested enormous research effort into hybrid/organic photovoltaics, leading to the recent launch of the first commercial products that use this technology. Dye-sensitized solar cells (DSCs) have shown clear advantages over competing technologies. The top certified efficiency of DSCs exceeds 11%, and the laboratory-cell efficiency is greater than 13%. In 2012, the first reports of high efficiency solid-state DSCs based on organohalide lead perovskites completely revolutionized the field. These materials are used as light absorbers in DSCs and as light-harvesting materials and electron conductors in meso-superstructured and flat heterojunction solar cells and show certified efficiencies that exceed 17%. To effectively compete with conventional photovoltaics, emerging technologies such as DSCs need to achieve higher efficiency and stability, while maintaining low production costs. Many of the advances in the DSC field have relied on the computational design and screening of new materials, with researchers examining material characteristics that can improve device performance or stability. Suitable modeling strategies allow researchers to observe the otherwise inaccessible but crucial heterointerfaces that control the operation of DSCs, offering the opportunity to develop new and more efficient materials and optimize processes. In this Account, we present a unified view of recent computational modeling research examining DSCs, illustrating how the principles and simulation tools used for these systems can also be adapted to study the emerging field of perovskite solar cells. Researchers have widely applied first-principles modeling to the DSC field and, more recently, to perovskite-based solar cells. DFT/TDDFT methods provide the basic framework to describe most of the desired materials and interfacial properties, and Car-Parrinello molecular dynamics allow researchers the further ability to sample local minima and dynamical fluctuations at finite temperatures. However, conventional DFT/TDDFT has some limitations, which can be overcome in part by tailored solutions or using many body perturbation theory within the GW approach, which is however more computationally intensive. Relativistic effects, such as spin-orbit coupling, are also included in simulations since they are fundamental for addressing systems that contain heavy atoms. We illustrate the performance of the proposed simulation toolbox along with the fundamental modeling strategies using selected examples of relevant isolated device constituents, including dye and perovskite absorbers, metal-oxide surfaces and nanoparticles, and hole transporters. We critically assess the accuracy of various computational approaches against the related experimental data. We analyze the representative interfaces that control the operational mechanism of the devices, including dye-sensitized TiO2/hole transporter and organohalide lead perovskite/TiO2, and the results reveal fundamental aspects of the device's operational mechanism. Although the modeling of DSCs is relatively mature, the recent "perovskite storm" has presented new problems and new modeling challenges, such as understanding exciton formation and dissociation at interfaces and carrier recombination in these materials.

Design and fabrication of a SiOx/ITO double-layer anti-reflective coating for heterojunction silicon solar cells

In this contribution optical simulations of both flat and textured heterojunction silicon solar cells are presented and verified experimentally. Using Advanced Semiconductor Analysis (ASA) software, we optimize a double-layer anti-reflective (AR) coating, which has an additional SiOx film on the top of the existing indium tin oxide (ITO) coating. Our approach is based on maximizing the absorbance of the crystalline silicon (c-Si) wafer, which is strongly correlated with the solar cell′s short circuit current (Jsc). Our simulations show that for a flat heterojunction silicon solar cell c-Si absorbance can increase by using a double-layer AR coating instead of a single-layer AR coating. As predicted by the simulations, experimental devices show corresponding Jsc increase, leading to the increase of the solar cell efficiency. On a textured heterojunction silicon solar cell the incident light travels an oblique path through the AR coating and we use an advanced ray-tracing model to optimize the single and double-layer AR coating for this case. Our simulations show that for the textured heterojunction silicon solar cell, reflection losses are lower but parasitic absorption losses in the ITO and amorphous silicon layers play a more important role. Using a double-layer AR coating not only reduces reflection losses further, but because a thinner ITO layer can be used it also reduces parasitic absorption losses. Experimentally, our textured heterojunction silicon solar cell with a double-layer AR coating shows that the Jsc (active area) of 40.5mA/cm2 and an efficiency of 19.0%.

Phthalocyanine with Trifluoroethoxy Substituents for Organic Solar Cells

The properties of soluble zinc phthalocyanines with trifluoroethoxy substituents (TFEO–ZnPcs) were studied to evaluate their possible use in organic thin-film solar cells. We prepared five TFEO–ZnPcs with different numbers and positions of the TFEO substituents. The solubilities, UV–visible absorption spectra, and cyclic voltammograms of these ZnPcs varied with their substituents. TFEO–ZnPc thin-films fabricated by spin-casting exhibited photocurrent generation at wavelengths between 600 and 850 nm. Flat heterojunction solar cells consisting of a poly(3-hexylthiophene) (P3HT) layer as a donor and a TFEO–ZnPc layer as an acceptor exhibited photocurrent generation over a wide range of wavelengths corresponding to both the P3HT and TFEO–ZnPc absorption spectra. The best solar cell performance was obtained with (β-TFEO)4ZnPc film.

Photoconductivity as loss mechanism in organic solar cells

AbstractWe observe that the reverse current under illumination in solar cells containing C60 and ZnPc is dominated by a photoshunt. This shunt, not present in the dark, causes a linear current–voltage relation under illumination showing no saturation. Although observable in bulk heterojunctions, this effect is more pronounced in the presence of a pristine C60 layer. An internal quantum efficiency larger than unity under an applied negative voltage and in the spectral range where C60 absorbs identifies charges which are injected in addition to those photogenerated. The photoshunt is also present in the power‐generating region and represents a loss mechanism limiting the fill factor in particular for flat heterojunction devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of Driving Forces for Charge Extraction in Organic Solar Cells: Transient Photocurrent Measurements on Solar Cells Showing S‐Shaped Current–Voltage Characteristics

AbstractThe role of drift and diffusion as driving forces for charge carrier extraction in flat heterojunction organic solar cells is examined at the example of devices showing intentional S‐shaped current–voltage (J‐V) characteristics. Since these kinks are related to energy barriers causing a redistribution of the electric field and charge carrier density gradients, they are suitable for studying the limits of charge extraction. The dynamics of this redistribution process are experimentally monitored via transient photocurrents, where the current response on square pulses of light is measured in the μs to ms regime. In combination with drift‐diffusion simulation data, we demonstrate a pile‐up of charge carriers at extraction barriers and a high contribution of diffusion to photocurrent in the case of injection barriers. Both types of barrier lead to S‐kinks in the J‐V curve and can be distinguished from each other and from other reasons for S‐kinks (e.g. imbalanced mobilities) by applying the presented approach. Furthermore, it is also helpful to investigate the driving forces for charge extraction in devices without S‐shaped J‐V curve close to open circuit to evaluate whether their electrodes are optimized.

Temperature dependent behavior of flat and bulk heterojunction organic solar cells

ABSTRACTThe open-circuit voltage of an organic solar cell is increasing with decreasing temperature and with increasing illumination intensity. These dependencies are quantitatively investigated for two types of organic solar cells, one with a flat donor-acceptor heterojunction and one with a mixed layer bulk heterojunction. Zinc-phthalocyanine and C60 are used as donor and acceptor, respectively. A qualitative difference is found for the two geometries. We find that a logarithmic illumination intensity dependence with temperature as a linear pre-factor of the logarithm, which is commonly reported and observed, is applicable for the bulk heterojunction. The flat heterojunction, in contrast, shows a constant illumination intensity pre-factor which is independent of the temperature, and the temperature can be modeled as additional linear summand.

Dicyanovinyl substituted oligothiophenes: Thermal stability, mobility measurements, and performance in photovoltaic devices

A series of dicyanovinyl-oligothiophenes are investigated concerning their thermal stability, absorption in thin film, and hole mobility. Due to very high extinction coefficients, these materials are interesting for application as donor in solar cells. The quinquethiophene DCV 2-5T, which shows a hole mobility of 2.2×10 –5 cm 2/Vs, is used as donor material in a flat heterojunction organic small molecule solar cells. Despite a very thin donor layer of only 6 nm, these devices exhibit in a planar heterojunction with 15 nm C 60 an efficiency of up to 2.8% with a fill factor of up to 58%, a short circuit current density of 5.2 mA/cm 2, an open circuit voltage of 1.03 V, and an external quantum efficiency of 30% in the green spectral range.

Influence of Hole‐Transport Layers and Donor Materials on Open‐Circuit Voltage and Shape of I–V Curves of Organic Solar Cells

AbstractThe effect of injection and extraction barriers on flat heterojunction (FHJ) and bulk heterojunction (BHJ) organic solar cells is analyzed. The barriers are realized by a combination of p‐type materials with HOMOs varying between –5.0 and –5.6 eV as hole‐transport layer (HTL) and as donor in vacuum‐evaporated multilayer p‐i‐metal small‐molecule solar cells. The HTL/donor interface can be seen as a model for the influence of contacts in organic solar cells in general. Using drift‐diffusion simulations we are well able to reproduce and explain the experimental I–V curves qualitatively. In FHJ solar cells the open‐circuit voltage (Voc) is determined by the donor and is independent of the HTL. In BHJ solar cells, however, Voc decreases if injection barriers are present. This different behavior is caused by a blocking of the charge carriers at a spatially localized donor/acceptor heterojunction, which is only present in the FHJ solar cells. The forward current is dominated by the choice of HTL. An energy mismatch in the HOMOs leads to kinks in the I–V curves in the cases for which Voc is independent of the HTL.

Tetrapropyl-tetraphenyl-diindenoperylene derivative as a green absorber for high-voltage stable organic solar cells

We present small molecule organic solar cells (SMOSC) based on flat heterojunctions (FHJ) of the alternative green donor 2,3,10,11-tetrapropyl-1,4,9,12-tetraphenyl-diindeno[1,2,3-cd:1',2',3'-lm]perylene (P4-Ph4-DIP) and the fullerene ${\mathrm{C}}_{60}$. P4-Ph4-DIP absorbs in the green spectral range and thus fills the spectral gap that standard absorber materials (zinc or copper phthalocyanine for red and ${\mathrm{C}}_{60}$ for blue absorption) leave, thus allowing broad coverage of the sun spectrum, which is of major interest for tandem devices. The materials properties of P4-Ph4-DIP are studied, and SMOSC are characterized by current voltage, external quantum efficiency, and aging measurements. The solar cells display very high fill factors FF $g$ 76% and open circuit voltages ${V}_{\mathrm{OC}}$ of close to 1 V. Mismatch-corrected efficiencies of up to 1.9% are obtained. Aging measurements show that ${\mathrm{C}}_{60}$ in conjunction with P4-Ph4-DIP yields extremely stable devices. We observe $\ensuremath{\approx}$ 88% of the initial efficiency after 2500 h illumination at 999 mW/cm${}^{2}$ illumination intensity, with no observable change in short-circuit current density. Furthermore, we also show that a systematic variation of donor thickness in FHJ can be combined with transfer matrix formalism-based optical simulations and the continuity equation for excitons to reliably determine the exciton diffusion length ${L}_{\mathrm{D}}$. A value of 9 $\ifmmode\pm\else\textpm\fi{}$ 1 nm is found for P4-Ph4-DIP.

Aging of flat heterojunction zinc phthalocyanine/fullerene C 60 organic solar cells

This work addresses the long-term aging of organic solar cells based on a flat zinc phthalocyanine (ZnPc)/C 60 heterojunction. We investigate both the typical degradation behavior of the short circuit current and of the saturated photocurrent, defined as I(−1 V). The latter remains constant after a relatively small initial decay, which is directly related to a substantial reduction of the contribution of excitons generated in C 60 to the external quantum efficiency. Mass spectroscopy analysis of the organic material after aging revealed oxidized C 60 and sub-products thereof. The reduction of I(−1 V) is thus attributed to the reaction of C 60 molecules with oxygen impurities. The results strongly suggest that ZnPc/C 60 photovoltaic cells are intrinsically very stable on a time scale of thousand of hours if such reactions are prevented.

Solution-processed polymer-free photovoltaic devices consisting of PbSe colloidal quantum dots and tetrabenzoporphyrins

Polymer-free photovoltaic devices were fabricated via a solution process using PbSe colloidal quantum dots and an organic semiconductor, tetrabenzoporphyrin, that can be derived from a soluble precursor. Flat heterojunction (FHJ) and bulk heterojunction (BHJ) devices were fabricated and the current-voltage characteristics of the devices were measured. Consequently, we observed photovoltaic conversion for both devices and found the energy conversion efficiency of the BHJ device (1.8×10−3%) is 40 times that of the FHJ device under 100mWcm−2 illumination at 800nm.

Model for a-Si:H/c-Si interface recombination based on the amphoteric nature of silicon dangling bonds

The performance of many silicon devices is limited by electronic recombination losses at the crystalline silicon c-Si surface. A proper surface passivation scheme is needed to allow minimizing these losses. The surface passivation properties of amorphous hydrogenated silicon a-Si:H on monocrystalline Si wafers are investigated here. We introduce a simple model for the description of the surface recombination mechanism based on recombination through amphoteric defects, i.e. dangling bonds, already established for bulk a-Si:H. In this model, the injection-dependent recombination at the a-Si:H/c-Si interface is governed by the density and the average state of charge of the amphoteric recombination centers. We show that with our surface recombination model, we can discriminate between the respective contribution of the two main mechanisms leading to improved surface passivation, which is achieved by a the minimization of the density of recombination centers and b the strong reduction of the density of one carrier type near the interface by field effect. We can thereafter reproduce the behaviors experimentally observed for the dependence of the surface recombination on the injection level on different wafers, i.e., of both p and n doping type as well as intrinsic. Finally, we are able to exploit the good surface passivation properties of our a-Si:H layers by fabricating flat heterojunction solar cells with open-circuit voltages exceeding 700 mV.

Improved efficiency of zinc phthalocyanine/C60 based photovoltaic cells via nanoscale interface modification

The photovoltaic performance of zinc phthalocyanine (ZnPc)∕C60 heterojunctions is improved by alternately depositing ultrathin active layers between the two bulk layers. Using a proper multilayer structure, a 60% improvement in conversion efficiency under AM1.5 illumination is obtained, in comparison with that of flat heterojunctions. Transmission electron microscope investigations reveal an effective phase separation of ZnPc and C60 and a high degree of crystalline ordering of C60 in this system. The authors propose that an interpenetrating interface is formed, which facilitates both exciton separation and charge transport, and enhances the photocurrent from ZnPc due to an extended photoactive region.

Isotype heterojunctions with flat valence or conduction band

We show that isotype heterojunctions with a perfectly flat majority or minority carrier band edge can be realized by modulation doping of arbitrary continuous alloy grading. The required impurity distribution is obtained analytically from the knowledge of the compositional grading and band structure parameters in the grading. This analytic relationship is exact for heterojunctions in which the grading fields are negligible in comparison with the atomic fields. We illustrate the design of flat valence-band heterojunctions for application in high-reflectivity low-resistance distributed Bragg reflectors for vertical-cavity lasers. The presented formalism enables the design of isotype heterojunctions with arbitrary band-edge profiles.