Back Contact Barrier Height Research Articles

Quasi-ohmic contact formation assisted by the back contact with Cu2Te nanoparticles@reduced graphene oxide composites for highly efficient CdTe solar cells

Composite materials based on reduced graphene oxide decorated with cuprous telluride nanoparticles (Cu2Te NPs@rGO) were fabricated using a facilitated two-step method. A back-contact layer was formed using a spin-coating method for cadmium telluride (CdTe) solar cells. This research is aimed to improve the photovoltaic performance of a device containing Cu2Te NPs@rGO/Au back contacts. Cu2Te NPs, as the active copper diffusion source, dope and passivate the interfaces and grain boundaries of CdTe to form the p+-layer, which can reduce the recombination centers and back-contact barrier height. Simultaneously, the reduced graphene oxide (rGO), as a buffer and carrier transport layer, prevents excessive copper diffusion during the annealing process and improves the hole collection capacity. The p-type layer formed by the rGO contact with the Au film enhances hole transport and facilitates a low-resistance contact formation, forming a quasi-ohmic contact, and increases the open-circuit voltage and short-circuit current density. Cu2Te NPs@rGO/Au back contacts significantly improved the photovoltaic performance (Eff: 16.5%, Voc: 830 mV, Jsc: 27.13 mA/cm2, fill factor: 73.3%, area: 0.24 cm2). The composites retain properties of Cu2Te NPs and rGO. Their multifunctional properties reveal the potential of Cu2Te NPs@rGO as the back contact and broaden the application of graphene-based composites in photovoltaic devices. Additionaly, the preparation of graphene-based composites and their application as back contact is simple and convenient.

Effect of contact barrier height on performances of BaSi2 heterojunction and homojunction solar cells

The effects of contact barrier height on performances of Si/BaSi2 p–n heterojunction, BaSi2 p–n homojunction and Si/BaSi2/Si p–i–n heterojunction were numerical calculated. Band energy diagram, built-in electric field, carrier generation and carrier transportation distributed in the devices are comprehensively investigated. BaSi2 p–n homojunction solar cells are very sensitive to front contact barrier height due to the high light absorption coefficient of front p-BaSi2 layer. Si/BaSi2 p–n heterojunction and BaSi2 p–n homojunction solar cells with donor concentration [Formula: see text] less than [Formula: see text] are apparently affected by back contact barrier height. The ideal [Formula: see text]-Si/BaSi2/[Formula: see text]-Si p–n solar cell achieves a high [Formula: see text] of 1.131 V, suggesting a promising and alternative structure to gain excellent BaSi2-based solar cells once the Urbach tail states and defects can be effectively eliminated. The results help to fundamentally understand operation mechanism and provide intuitive guidance for achieving high-efficiency BaSi2 solar cells.

Numerical Insights into the Influence of Electrical Properties of n-CdS Buffer Layer on the Performance of SLG/Mo/p-Absorber/n-CdS/n-ZnO/Ag Configured Thin Film Photovoltaic Devices

A CdS thin film buffer layer has been widely used as conventional n-type heterojunction partner both in established and emerging thin film photovoltaic devices. In this study, we perform numerical simulation to elucidate the influence of electrical properties of the CdS buffer layer, essentially in terms of carrier mobility and carrier concentration on the performance of SLG/Mo/p-Absorber/n-CdS/n-ZnO/Ag configured thin film photovoltaic devices, by using the Solar Cell Capacitance Simulator (SCAPS-1D). A wide range of p-type absorber layers with a band gap from 0.9 to 1.7 eV and electron affinity from 3.7 to 4.7 eV have been considered in this simulation study. For an ideal absorber layer (no defect), the carrier mobility and carrier concentration of CdS buffer layer do not significantly alter the maximum attainable efficiency. Generally, it was revealed that for an absorber layer with a conduction band offset (CBO) that is more than 0.3 eV, Jsc is strongly dependent on the carrier mobility and carrier concentration of the CdS buffer layer, whereas Voc is predominantly dependent on the back contact barrier height. However, as the bulk defect density of the absorber layer is increased from 1014 to 1018 cm−3, a CdS buffer layer with higher carrier mobility and carrier concentration is an imperative requirement to a yield device with higher conversion efficiency and a larger band gap-CBO window for realization of a functional device. Most tellingly, simulation outcomes from this study reveal that electrical properties of the CdS buffer layer play a decisive role in determining the progress of emerging p-type photo-absorber layer materials, particularly during the embryonic device development stage.

How Oxygen Exposure Improves the Back Contact and Performance of Antimony Selenide Solar Cells.

The improvement of antimony selenide solar cells by short-term air exposure is explained using complementary cell and material studies. We demonstrate that exposure to air yields a relative efficiency improvement of n-type Sb2Se3 solar cells of ca. 10% by oxidation of the back surface and a reduction in the back contact barrier height (measured by J-V-T) from 320 to 280 meV. X-ray photoelectron spectroscopy (XPS) measurements of the back surface reveal that during 5 days in air, Sb2O3 content at the sample surface increased by 27%, leaving a more Se-rich Sb2Se3 film along with a 4% increase in elemental Se. Conversely, exposure to 5 days of vacuum resulted in a loss of Se from the Sb2Se3 film, which increased the back contact barrier height to 370 meV. Inclusion of a thermally evaporated thin film of Sb2O3 and Se at the back of the Sb2Se3 absorber achieved a peak solar cell efficiency of 5.87%. These results demonstrate the importance of a Se-rich back surface for high-efficiency devices and the positive effects of an ultrathin antimony oxide layer. This study reveals a possible role of back contact etching in exposing a beneficial back surface and provides a route to increasing device efficiency.

Self-Organized Back Surface Field to Improve the Performance of Cu2ZnSn(S,Se)4 Solar Cells by Applying P-Type MoSe2:Nb to the Back Electrode Interface.

Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells have been encountering a bottleneck period since the champion power conversion efficiency (PCE) of 12.7% was achieved by Kim et al. in 2014. One of the critical factors that impede its further development is the relatively low open-circuit voltage (VOC) caused by serious interface carrier recombination. In this regard, back surface field (BSF) employment is a feasible strategy to address the VOC issue of CZTSSe solar cells to some extent. Here, we demonstrated a self-organized BSF introduced by prompting interfacial MoSe2 layer transition from inherent n-type to desirable p-type with Nb doping (p-MoSe2:Nb). The BSF application can significantly reduce the carrier recombination at the back electrode interface (BEI) and lower down the back contact barrier height. The PCE of the corresponding cell was improved from 4.72 to 7.15% because of the enhancement of VOC and fill factor, primarily stemming from the doubling aspects of increased shunt resistance (RSh), decreased series resistance (RS), and alleviative recombination velocity of the BEI induced by the BSF. Our results suggest that introducing a BSF fulfilled with p-MoSe2:Nb is a facile and promising route to improve the performance of CZTSSe thin-film solar cells.

Buffer/absorber interface recombination reduction and improvement of back-contact barrier height in CdTe solar cells

Electronic properties of a CdTe solar cell are reported using temperature-dependent capacitance spectroscopy and current-voltage characteristics, the latter in dark and illuminated conditions. The baseline solar cell material stack investigated is comprised of soda-lime-glass/SnO2:F/SnO2/CdS:O-buffer/CdTe-absorber/Cu/Au. Device properties are compared with CdTe solar cells in which the back surface was hydroiodic acid etched, before the back-contact formation, and a CdTe device in which Mg-doped ZnO (MZO) replaces buffer layers. Reduced back-contact barrier height and grain boundary barrier height are observed in the hydroiodic acid treated CdTe cell. Improved device performance in the MZO-based CdTe device is attributed to reduced emitter/absorber interface recombination when using the MZO window layer.

Apparent quantum efficiency effects under forward bias in MZO/CdS/CdTe solar cells

The apparent quantum efficiency measurement under forward bias can be used to analyze the window layer inter-diffusion, the photocurrent loss mechanism and the battery back-contact barrier height. In this article, quantum efficiency measurement and apparent quantum efficiency measurement under forward bias are performed on the MgxZn1−xO/CdS/CdTe solar cells. The results show that the introduction of MZO thin film is benefit to reduce CdS thickness which improves the short-wavelength response of quantum efficiency. The inter-diffusion occurs at the interface between CdS and CdTe, where CdS0.844Te0.156 polycrystalline thin film possibly forms. However, there might be an obvious electron barrier at the conduction band interface between CdS0.844Te0.156 and MZO, which reduces the carrier collection efficiency. In addition, more photo-generated carriers are collected with increasing forward bias in the space charge region of the back contact barrier to generate a current opposite to the primary photocurrent. So the AQE response for the cell without the back contact decreases monotonously in the wavelength range of 300–600 nm and has a lower negative peak in the range of 800–900 nm when the forward bias exceeds 0.6 V.

Electrical and optical characterization of CdTe solar cells with CdS and CdSe buffers—A comparative study

A study is reported comparing the electrical and optical properties of CdTe solar cells, prepared using CdS and CdSe buffer layers, to investigate defects in the bulk and interface, carrier transport, and recombination. Temperature dependent capacitance–voltage measurement and admittance spectroscopy were used to extract carrier concentration, resistivity, charge carrier mobility, and their temperature dependence. The authors identify the presence of two defect signatures corresponding to carrier freeze-out and the formation of a Schottky back-contact barrier. The back-contact barrier height (≈300 meV) extracted from the temperature dependent current density–voltage (JVT) experiment was confirmed by conventional admittance spectroscopy. The activation energies of mobility (resistivity) are 101.2 ± 2.5 meV (92.6 ± 2.3 meV) and 84.7 ± 2.7 meV (77.6 ± 4.5 meV) for CdS and CdSe buffer layers, respectively. Intensity dependent photoluminescence analysis demonstrates that the CdSe/CdTe device exhibits lower radiative efficiency than the CdS/CdTe device. This confirms the presence of higher defects in the CdSe/CdTe device corroborated by temperature dependent VOC analysis. The comparative electrical and optical analysis provides insight into improving the performance of CdTe solar cell device by selenization.

A comparison of the effect of CdCl2 and MgCl2 processing on the transport properties of n-CdS/p-CdTe solar cells and a simple approach to determine their back contact barrier height

A simple approach, which can estimate the barrier height of non-Ohmic back contacts for CdS/CdTe solar cell by using its temperature dependent forward biased current-voltage data, is explained. The method involves modelling the forward J–V characteristics using a double exponential expression for the main junction and by a reverse biased Schottky barrier for the back contact. Cells processed with both CdCl2 and MgCl2 are compared, with the current transport phenomena in both kinds of cells also being analysed. Performance loss due to limitation of the forward bias hole current, and its dependence on the post-deposition chloride processing, is discussed. The forward current transport is mainly dominated by recombination at CdS/CdTe interfacial region with pronounced tunnelling effects. Classical Schottky-type conduction, as described by the Richardson-Schottky formula, is a good fit to the reverse biased current-voltage behaviour of an Au/CdTe junction above ∼240K. Below this temperature, the current limiting effect due to the increasing contribution from interfacial defect states can be satisfactorily explained by Bardeen’s model for a modified Schottky type barrier at back contact interface.

Understanding photo‐degradation mechanism in P3HT:PCBM bulk heterojunction solar cells: AMPS‐1D simulation study

The photo‐degradation mechanism of P3HT:PCBM bulk heterojunction solar cells is investigated by using the AMPS‐1D (analysis of microelectronic and photonic structures) computer program. An effective medium model with exponential tail states and Gaussian mid‐gap states is used to simulate the J–V characteristics of the photovoltaic devices. By using the proposed model, the J–V characteristics of both fresh and degraded devices could be nicely reproduced. The simulation results suggest that the main factors governing the mechanism of photo‐degradation are the increase in the density of the mid‐gap states, the increase of the p‐type doping concentration, the slight increase in back contact barrier height and the reduction of charge carriers mobility. The deterioration of the photovoltaic performance is attributed to the increased recombination rate and insufficient charge collection.

Progression of metalorganic chemical vapour‐deposited CdTe thin‐film PV devices towards modules

AbstractThis paper reports important developments achieved with CdTe thin‐film photovoltaic devices produced using metalorganic chemical vapour deposition at atmospheric pressure. In particular, attention was paid to understand the enhancements in solar cell conversion efficiency, to develop the cell design, and assess scalability towards modules. Improvements in the device performance were achieved by optimising the high‐transparency window layer (Cd0.3Zn0.7S) and a device‐activation anneal. These increased the fill factor and open‐circuit voltage to 77 ± 1% and 785 ± 7 mV, respectively, compared with 69 ± 3% and 710 ± 10 mV for previous baseline devices with no anneal and thicker Cd0.3Zn0.7S. The enhancement in these parameters is associated with the two fold to three fold increase in the net acceptor density of CdTe upon air annealing and a decrease in the back contact barrier height from 0.24 ± 0.01 to 0.16 ± 0.02 eV. The optimum thickness of the window layer for maximum photocurrent was 150 nm. The cell size was scaled from 0.25 to 2 cm2 in order to assess its impact on the device series resistance and fill factor. Finally, micro‐module devices utilising series‐connected 2‐cm2 sub‐cells were fabricated using a combination of laser and mechanical scribing techniques. An initial module‐to‐cell efficiency ratio of 0.9 was demonstrated for a six‐cell module with the use of the improved device structure and processing. Prospects for CdTe photovoltaic modules grown by metalorganic chemical vapour deposition are commented on. Copyright © 2015 John Wiley & Sons, Ltd.

A detailed study of the effect of Schottky barrier on the dark current density-voltage characteristics of CdS/CdTe solar cells

Numerical modeling is used to obtain insight into the details of the effect of back contact barrier height (φb) on the dark current density-voltage characteristics of CdS/CdTe solar cell. And relation between the roll-over and the barrier height is obtained. Analytic simulations are fitted to the measured current density-voltage curve in a temperature range from 220 to 300 K. And the influence of barrier height on J-V of the CdS/CdTe thin film solar cell with Cu/Mo back contact fitted parameters is discussed. The equation between back contact barrier height (φb) and the reverse saturation current density (Jb0) is revised and the experimental data are consistent with the simulation results very well.

Towards Ultra Thin and High Efficiency ZnxCd1-xS/CdTe Solar Cell by AMPS 1D

The main motivation of this work was to obtain high efficiency at reduced CdTe absorber layer thickness and replacing ZnxCd1-xS as window layer in conventional CdS/CdTe solar cells. The conventional CdTe baseline case was the starting point of this investigation to analyze ultra thin and high efficiency ZnxCd1-xS/CdTe solar cell for optimal value of x. The initial step of the analysis was to decrease the CdTe absorber layer to the extreme limit of 1 µm and at this thickness the proposed cell has shown satisfactory level of efficiencies. The ultimate step was to insert a suitable back surface field (BSF) with As2Te3 to reduce the back contact barrier height and back surface recombination loss of the ultra thin cell. All the analysis was done using the widely used simulator Analysis of Microelectronic and Photonic Structures (AMPS 1D). The conversion efficiency of 18.02% (Voc = 0.89 V, Jsc = 25.34 mA/cm2, FF = 0.78) without BSF and an efficiency of 20.3% (Voc = 0.93 V, Jsc = 25.97 mA/cm2, FF = 0.825) with As2Te3 BSF were achieved for the proposed cells from 1 µm and 0.6 µm CdTe absorber layer respectively. Moreover, the normalized efficiency of the proposed ultra thin cells linearly decreased with the increasing operating temperature at the gradient of -0.35%/°C, which indicates better stability of the ultra thin cells.

Discussion of some “trap signatures” observed by admittance spectroscopy in CdTe thin-film solar cells

Considerable ambiguity and controversy exist concerning the defect signatures (H1, H2, and H3) frequently observed in admittance spectroscopy of thin-film CdTe solar cells. We prove that the commonly labeled H1 defects, observed in all devices in this study, are actually due to the freeze-out of the majority carriers in the neutral CdTe absorber. This freeze-out is evident in the temperature dependencies of capacitance, carrier concentration, and depletion region width. Contrary to intuitive expectation, the activation energy of freeze-out is less than, not identical to, that of the conductivity. In some other cases, H2 or H3 are observed and attributed to the back-contact potential barrier, rather than to the carrier emission from the traps. We extract the back-contact barrier height from the activation energy of the saturation current determined from the temperature-dependent current-voltage curves using the back-to-back diode model. The back-contact barrier height agrees well with the H2 or H3 energy determined by admittance spectroscopy. We present a more comprehensive and realistic equivalent circuit that includes the admittances from both the back-contact and the neutral absorber.

Electrical properties of the CdTe back contact: A new chemically etching process based on nitric acid/acetic acid mixtures

The performance of the back contact is one of the major issues of CdTe solar cell research. Standard nitric–phosphoric (NP) acid chemical etching before metallization is widely used to improve contact formation. However, previous studies of this traditional etching method indicated a blocking Schottky barrier at the back contact, and a roll-over phenomenon was found in the J– V curves of the CdTe solar cells. In this work, a new etching solution, i.e. a nitric–acetic (NA) acid was employed. The etching rate was slow and a Te-rich layer was formed on the surface, which was less than 1 nm. The CdTe solar cell with this new etching method showed no roll-over phenomenon and displayed a good ohmic back contact performance. XPS analysis demonstrated that the back contact barrier height was close to those of CdTe with standard NP etching. A possible mechanism was presented for the improvement of back contact properties.

Exploring Back Contact Technology to Increase CdS/CdTe Solar Cell Efficiency

AbstractThe primary routes for increasing CdS/CdTe solar cell efficiency involve increasing free carrier density, reducing bulk and interface recombination, and/or reducing back contact barrier height. This paper focuses on the role of the back contact barrier in increasing cell efficiency. Measurement of barrier height and back surface recombination are outlined and three CdTe/MX/M back contact prototypes, each with particular strengths, are discussed to bring out important issues.

Admittance spectroscopy of CdTe∕CdS solar cells subjected to varied nitric-phosphoric etching conditions

In this work we investigate the electric and structural properties of CdTe∕CdS solar cells subjected to a nitric-phosphoric (NP) acid etching procedure, employed for the formation of a Te-rich layer before back contacting. The etching time is used as the only variable parameter in the study, while admittance spectroscopy is employed for the characterization of the cells’ electric properties as well as for the analysis of the defect energy levels. Particular attention was also given to the characteristics of unetched devices and it is shown that despite the larger height of back-contact barrier such samples show well defined admittance spectra, as well as allow for extraction of as much as five defect levels in the range of 0.08–0.9eV above the valence band. In contrast, admittance characteristics of the etched samples show a decrease of the number of the detectable trap levels with increasing etching time. (Hence it is usual for only one or two trap levels to be reported in the literature for finished devices.) The latter leads to the anomalous Arrhenius energy plots as well as the breakdown of low-frequency capacitance characteristics for samples etched with times larger than 30s. The observed effects are attributed to physical thinning of the cells, the etching out of grain boundaries, and the tellurium enrichment of the CdTe surface by NP etching. We also perform analysis of the back-contact barrier height as extracted from dark I-V measurements at different temperatures. The dependence of this barrier height on NP etching time is compared with that of conversion efficiency, from which conclusions are drawn about both positive and negative effects of the nitric-phosphoric etch.

Effect of back-contact barrier on thin-film CdTe solar cells

The presence of a back-contact barrier affects the current–voltage characteristics of thin-film CdS/CdTe/metal solar cells primarily by impeding hole transport, a current-limiting effect commonly referred to as “rollover.” In this work, the CdS/CdTe solar cell with a CdTe/metal back-contact barrier is modeled by two opposite polarity diodes in series. Analytic simulations are fitted to the measured current–voltage curve, the voltage distribution between the two diodes is shown under different conditions, and the back-contact barrier height is extracted. Room-temperature barrier heights exceeding 0.5 eV will generally result in significant fill-factor reduction.