Interface In Solar Cells Research Articles

Determination of band alignment at the CdTe/SnTe heterojunction interface for CdTe thin-film solar cells

The Ohmic back contact for CdTe is a key issue to realize high-efficiency CdTe thin film solar cells because of the high work function of CdTe. CdTe/SnTe heterojunctions (HJs) have been implemented to address this issue which shows promising potential, but the band alignment at the HJs is unknown. The valence band offsets in MBE-grown cadmium telluride (CdTe)/tin telluride (SnTe) (111) heterostructures were measured with X-ray photoelectron spectroscopy (XPS). The XPS results show that the heterostructure has an ideal type-I band structure for CdTe solar cells applications, with a valence band offset of and a conduction band offset of , which expedites hole transport from the CdTe absorber to the hole electrode and improves the Ohmic contact for CdTe. Experimental determination of the band structure of CdTe/SnTe HJs can help improve the photovoltaic performance of CdTe thin film solar cells and facilitate the design and fabrication of CdTe/SnTe related devices. Furthermore, we inserted a SnTe back contact buffer layer into the CdTe thin film solar cells, and it was compared with the cell structure without the SnTe buffer layer. The feasibility of using SnTe as a solar cell back contact is confirmed.

From Bulk to Surface: Sodium Treatment Reduces Recombination at the Nickel Oxide/Perovskite Interface

AbstractThe effect of sodium doping in NiO as a contact layer for perovskite solar cells is investigated. A combined X‐ray diffraction and X‐ray photoelectron spectroscopy analysis reveals that Na+ mostly segregates as NaOx/NaCl species around NiO crystallites, with the effect of reducing interface capacitance as revealed by impedance spectroscopy. Inspired by this finding, the NiO/perovskite interface in perovskite solar cells is modified via insertion of an ultrathin NaCl interlayer, which increases the NiO work‐function by 0.3 eV. This leads to an increase of power conversion efficiency, approaching 18%, and open‐circuit voltage due to a remarkable suppression of surface recombination, as revealed by photoluminescence analysis and light intensity–dependent electrical measurements.

Fabrication of textured substrates for dye-sensitized solar cells using polydimethylsiloxane nanoimprint lithography

Abstract We proposed a fabrication of nanoimprinted textures on a front glass/transparent conductive oxide interface for dye-sensitized solar cells (DSSCs). These textures were fabricated through polydimethylsiloxane (PDMS) nanoimprint lithography on organosilsesquioxane solution. The texture structures were estimated via optical simulation. Master molds were anodic aluminum oxide templates with nano-texture (N-Tx) and micro-nano double texture (D-Tx). Meanwhile, replicate molds used a hard PDMS. Fluorine-doped tin oxide and titanium dioxide were deposited on textured glass substrates to generate electrodes for DSSCs. Unlike the DSSCs without texture, textured DSSCs realized 11.4% (N-Tx) and 10% (D-Tx) improvement in conversion efficiency.

Effect of hot-casted NiO hole transport layer on the performance of perovskite solar cells

NiO is extensively studied as a hole transport layer in perovskite solar cells but syntheses of NiO precursor involves toxic chemicals and time-consuming processes. Moreover, the synthesized NiO contains surface defects acting as trapping sites at the NiO/perovskite interfaces, resulting in poor charge extraction, hysteresis and light soaking. In this manuscript, we developed a non-toxic methodology for NiO precursor solution by using a simple mixture of NiO powder and HCl in an air environment. In addition, a new hot-casting technique was developed to successfully fabricate densely-packed, less defective NiO films. Interestingly, the hot-casting temperature was found to significantly affect morphology, film coverage and surface defects of NiO films. When a hot-casting temperature was below 100 °C, non-uniform NiO films were sparsely formed on the FTO surface and were characterized by defects in the form of hydroxyl groups and water on the surface. Such defective NiO films resulted in severe hysteresis and light soaking effect due to the trapped charges at the defective NiO/perovskite interface of perovskite solar cells. In contrast, when the hot-casting temperature was 120 °C, the NiO film formed densely-packed morphologies, covering the FTO surface. Furthermore, this film exhibited an ordered chemistry with strong Ni-O octahedral bonding and facilitated charge extraction at NiO/perovskite interface, resulting in a negligible hysteresis and light soaking. Finally, this non-toxic and simple method of fabricating NiO film will assist further development of perovskite solar cells.

Nanostructural Modification of PEDOT:PSS for High Charge Carrier Collection in Hybrid Frontal Interface of Solar Cells.

In this work, we propose poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) material to form a hybrid heterojunction with amorphous silicon-based materials for high charge carrier collection at the frontal interface of solar cells. The nanostructural characteristics of PEDOT:PSS layers were modified using post-treatment techniques via isopropyl alcohol (IPA). Atomic force microscopy (AFM), Fourier-transform infrared (FTIR), and Raman spectroscopy demonstrated conformational changes and nanostructural reorganization in the surface of the polymer in order to tailor hybrid interface to be used in the heterojunctions of inorganic solar cells. To prove this concept, hybrid polymer/amorphous silicon solar cells were fabricated. The hybrid PEDOT:PSS/buffer/a-Si:H heterojunction demonstrated high transmittance, reduction of electron diffusion, and enhancement of the internal electric field. Although the structure was a planar superstrate-type configuration and the PEDOT:PSS layer was exposed to glow discharge, the hybrid solar cell reached high efficiency compared to that in similar hybrid solar cells with substrate-type configuration and that in textured well-optimized amorphous silicon solar cells fabricated at low temperature. Thus, we demonstrate that PEDOT:PSS is fully tailored and compatible material with plasma processes and can be a substitute for inorganic p-type layers in inorganic solar cells and related devices with improvement of performance and simplification of fabrication process.

Improving photovoltaic performance of carbon-based CsPbBr3 perovskite solar cells by interfacial engineering using P3HT interlayer

Carbon-based CsPbBr3 perovskite solar cell is an emerging inorganic perovskite solar cell with the advantages of simple fabrication process and excellent stability. However, power conversion efficiency of carbon-based CsPbBr3 perovskite solar cells is still unsatisfactory up to now, as the direct contact of the CsPbBr3 with carbon is plagued with interfacial recombination sites and undesirable hole extraction barrier. Here, we report an effective strategy that employs poly (3-hexylthiophene) (P3HT) to modify the CsPbBr3/carbon interface in carbon-based CsPbBr3 perovskite solar cells and enable higher efficiency. The systematic tests and analyses demonstrate that the P3HT interlayer can remarkably suppress the charge recombination and enhance the hole extraction capability via formation of favorable energy level alignment between CsPbBr3 film and carbon electrode, and passivation of the surface defect states of CsPbBr3 film. As a result, the carbon-based CsPbBr3 perovskite solar cell with P3HT interlayer achieves a high conversion efficiency of 6.49%, exhibiting an increase by 27% compared to pristine device. Moreover, the carbon-based CsPbBr3 perovskite solar cells with P3HT interlayer exhibits excellent stability in ambient air with almost no change in the power conversion efficiency of the unsealed device over 40 days.

Challenge in Cu-rich CuInSe2 thin film solar cells: Defect caused by etching

Thin-film solar cells consist of several layers. The interfaces between these layers can provide critical recombination paths and consequently play a vital role in the efficiency of the solar cell. One of the main challenges for polycrystalline semiconductor absorber materials is the absorber-buffer interface. The $\mathrm{Cu}(\mathrm{In},\mathrm{Ga})\mathrm{S}{\mathrm{e}}_{2}$ system is particularly interesting in this context, since Cu-rich absorbers are dominated by recombination at the interface, while Cu-poor ones are not. This paper unveils the root cause of the challenge in the interface of Cu-rich solar cells in terms of a Se-related defect with an activation energy of $200\ifmmode\pm\else\textpm\fi{}20\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. This defect causes interface recombination and is responsible for the deficiency of open-circuit voltage in Cu-rich cells. Moreover, this paper demonstrates that the origin of this defect is due to the etching step necessary to remove secondary phases. Postdeposition surface treatments or modified buffer layers are shown to passivate this defect, to reduce interface recombination, and to increase the efficiency.

Yttrium-doped TiO2 compact layers for efficient perovskite solar cells

To improve the electron transporting and suppress the charge recombination at the interface of TiO2/perovskite, the smooth and compact yttrium(Y)-doped TiO2 compact layers were successfully prepared via a hydrolysis-pyrolysis method. The influences of Y-doped on the microstructure, crystal phase, chemical composition, optical absorption of the TiO2 compact layers were investigated, and the electron transporting and charge recombination at the TiO2/perovskite interface of solar cells were systemically analyzed. The planar perovskite solar cells based on the Y-doped TiO2 compact layers with a Y/Ti molar ratio of 5% obtained a best photoelectric conversion efficiency (PCE) of 15.52%, comparing with a PCE of 14.05% from the undoped devices. When the 200-nm length 3% Y-doped TiO2 nanorod arrays were introduced between the 5% Y-doped TiO2 compact layers and the perovskite thin film, the champion cell achieved a PCE of 18.32% under illumination of simulated AM 1.5 sunlight (100 mA cm−2).

Energy Level Bending of Organic‐Inorganic Halide Perovskite by Interfacial Dipole

Cesium carbonate (Cs2CO3) is a widely used interfacial modification material in organic solar cells (OSCs), organic light‐emitting diode (OLED) and so on, which can: 1) lower the work function of metal electrode; 2) induce n‐doping effect; and 3) alter the energy level of semiconductor materials under the action of dipole. Cs2CO3 is selected in this study to modify the interface of perovskite solar cells (PSCs) and therefore improves the device performance. Further studies indicate that this performance improvement is due to the diffusion of Cs2CO3 through [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) to form dipoles with perovskite to bend its energy level, resulting in an increased electron collection efficiency. This proposed mechanism for the interfacial modification of PSCs by bending the perovskite energy level through interfacial dipoles deepens the understanding of the role of interfacial layer.

Resolving the detrimental interface in co-evaporated MAPbI3 perovskite solar cells by hybrid growth method

Abstract Among various processing methods for growing perovskite thin layers, dual-source vacuum deposition was once regarded as the most economical and high throughput method. However, co-evaporation of MAI and PbI2 often leads to an inevitable PbI2 interfacial layer formed by the initial growth of evaporation resulting in a compromised efficiency of vacuum deposited MAPbI3 solar cell. Here, two modified growth methods based on vacuum co-evaporation, namely solution-vacuum hybrid method (SVHM) and soaking-assistant fully-vacuum method (SAFVM), are developed to grow a completely converted MAPbI3 perovskite layer at the interface prior to the co-evaporation of MAPbI3 perovskite layer. Upon these two methods, the unwanted PbI2 layer can be removed from the interface resulting in much improved power conversion efficiency up to 14.35%. Our result shows that, while unwanted interfacial PbI2 layer is the main cause of low performing co-evaporated perovskite solar cell devices, by removing the PbI2 interfacial layer we can unlock the full potential of vacuum-based perovskite solar cells to industrial mass production.

SnO2-Catalyzed Oxidation in High-Efficiency CdTe Solar Cells.

Interfaces at the front of superstrate CdTe-based solar cells are critical to carrier transport, recombination, and device performance, yet determination of the chemical structure of these nanoscale regions has remained elusive. This is partly due to changes that occur at the front interfaces during high temperature growth and substantive changes occurring during postdeposition processing. In addition, these buried interfaces are extremely difficult to access in a way that preserves chemical information. In this work, we use a recently developed thermomechanical cleaving technique paired with X-ray photoelectron spectroscopy to probe oxidation states at the SnO2 interface of CdTe solar cells. We show that the tin oxide front electrode promotes the formation of nanometer-scale oxides of tellurium and sulfur. Most oxidation occurs during CdCl2/O2 activation. Surprisingly, we show that relatively low-temperature anneals (180-260 °C) used to diffuse and activate copper acceptors in a doping/back contact process also cause significant changes in oxidation at the front of the cell, providing a heretofore missing aspect of how back contact processes can modify device transport, recombination, and performance. Device performance is shown to correlate with the extent of tellurium and sulfur oxidation within this nanometer-scale region. Mechanisms responsible for these beneficial effects are proposed.

Controlling Band Alignment at the Back Interface of Cadmium Telluride Solar Cells using ZnTe and Te Buffer Layers

ABSTRACTFormation of a low barrier back contact plays a critical role in improving the photoconversion efficiency of the CdTe solar cells. Incorporating a buffer layer to minimize the band bending at the back of the CdTe device can significantly lower the barrier for the hole current, improving open circuit voltage (VOC) and the fill factor. Over the past years, researchers have incorporated the both ZnTe and Te as buffer layers to improve CdTe device performance. Here we compare device performance using these two materials as buffer layers at the back of CdTe devices. We show that using Te in contact to CdTe results in higher performance than using ZnTe in contact to the CdTe. Low temperature current density-voltage measurements show that Te results is a lower barrier with CdTe than ZnTe, indicating that Te has better band alignment, resulting in less downward bending in the CdTe at the back interface, than ZnTe does.

Terminal Modulation in Search of a Balance between Hole Transport and Electron Transfer at the Interface for BODIPY-Based Organic Solar Cells

Organic solar cells (OSCs) have made rapid advances in power conversion efficiency during the past decades, which is boosted partly by the various designs of new materials, especially in donor mate...

Characterization of Cd-Free Zn1- xMg xO:Al/Zn1- xMg xO/Cu(In,Ga)(S,Se)2 Solar Cells Fabricated by an All Dry Process Using Ultraviolet Light Excited Time-Resolved Photoluminescence.

Cd-free Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells with a structure of glass/Mo/CIGSSe/Zn1- xMg xO (buffer)/Zn1- xMg xO:Al (TCO), fabricated by an all dry process, are characterized using ultraviolet light excited time-resolved photoluminescence (UV-TRPL). The impact of bandgap energy ( Eg) values of buffer and transparent conductive oxide (TCO) layers, denoted by Eg of buffer and TCO, is examined. The Eg values of buffer and TCO layers are kept almost similar and varied from 3.30 to 3.94 eV. In this work, UV-TRPL measurement is performed to examine the UV-TRPL carrier lifetimes near the Zn1- xMg xO buffer/CIGSSe interface in the solar cell structure. It is revealed that the UV-TRPL carrier lifetimes near the Zn1- xMg xO buffer/CIGSSe interface in Cd-free solar cells are increased upon enhancing the Eg of buffer and TCO from 3.30 to 3.94 eV, thus increasing the open-circuit voltage and fill factor. Additionally, short-circuit current density is enhanced up to about 38 mA/cm2 owing to the highly transparent Zn1- xMg xO/Zn1- xMg xO:Al layers. Ultimately, an 18.5%-efficient Cd-free solar cell with the Eg of buffer and TCO of 3.94 eV, prepared by an all dry process, is fabricated, which has the same level of 18.3% for the reference solar cell (glass/Mo/CIGSSe/CdS/ZnO/ZnO:Al).

Optical Properties and Dielectric Functions of Grain Boundaries and Interfaces in CdTe Thin-Film Solar Cells

CdTe thin-film solar cells have complex microstructures, such as grain boundaries within the absorber layer, as well as CdS window, and Au back contact interfaces, where the local structure and chemistry undergo significant changes. The optical properties at these nanoscale defects are unknown, but their accurate measurement is required in order to identify potential losses in device efficiency. Here monochromated electron energy loss spectroscopy (EELS) in an aberration corrected scanning transmission electron microscope (STEM) is used to measure the complex dielectric function for the CdTe1–xSx interdiffusion layer at the CdS–CdTe interface, high angle CdTe grain boundaries and Au–CdTe interface. CdTe1–xSx is shown to have a lower absorption coefficient than CdTe, but its refractive index is more closely matched to CdS. Grain boundaries have a negligible effect on the light absorption profile within CdTe, despite significant changes in the local structure and chemistry (i.e., Te depletion) at the grain boundary. Delocalization in inelastic scattering is the dominant systematic error in the above measurements. Finally a light backscattering mechanism via surface plasmon polaritons at the Au–CdTe interface is uncovered, which could potentially increase the photocurrent extracted from incident light at energies just above the CdTe band gap.

Different Dissociation Rates of Singlet and Triplet Excitons of Pentacene at the Interface in Solar Cells

Fission of the lowest-energy singlet exciton (S1) to two lowest-energy triplet excitons (T1) in pentacene has been expected to be a promising means for increasing the quantum efficiency of solar cells. Experiments find that S1 and T1 dissociate at quite different time scales at the donor/acceptor interface. Using the pentacene/TiO2 heterojunction as the model, we investigate the dissociation of pentacene excitons by a combination of many-body Green’s function theory and the time-dependent Schrodinger equation. Singlet and higher-energy triplet (Tn) excitons of pentacene could dissociate at the same timescale of ∼100 fs, benefiting from their capability to scatter into charge-transfer (CT) states and weakly bound charge-separated (CS) states across the interface. Resonance of pentacene excitons with CS states could facilitate the creation of free charge carriers. However, dissociation of T1 is hampered due to its poor density of states projected onto the interfacial states, preventing its scattering into C...

Numerical analysis of the back interface for high efficiency wide band gap chalcopyrite solar cells

Significant efforts have been made to improve the performance of the Cu(In1-xGax)Se2 (CIGS) solar cells by tuning the band gap of the CIGS absorber to match it with the solar spectrum. However, the performance of the current record-holding CIGS solar cells is still far from theoretical expectations. Various researchers reported that the open circuit voltage (Voc) and the fill factor (FF) degrade in wide band gap CIGS solar cells. However, the limiting factors on further boosting the efficiency are still a matter of debate. In this study, we focus on tuning the properties of the interfacial layer between the rear contact and the wide-gap CIGS absorber to lower the contact resistance and recombination rate. Based on the numerical simulation using SCAPS (a solar cell capacitance simulator), we find that a MoO3 interfacial layer with high work function is more effective than its MoSe2 counterpart in reducing the back barrier, which in turn increases the Voc and the FF of the solar cell. We further predict that an overall efficiency of 24% can be achieved by reducing the back surface recombination and Schottky barrier with sub-micrometer a thick CIGS absorber. This work puts forward a strategy to improve the efficiency of wide band gap CIGS solar cells whilst reducing the raw materials consumption.

Nickel oxide and polytetrafluoroethylene stacked structure as an interfacial layer for efficient polymer solar cells

An efficient polymer solar cell (PSC) has been demonstrated by incorporating an ultrathin interfacial insulating organic layer of polytetrafluoroethylene (PTFE) between a photoactive layer and hole-collecting buffer layer (NiOx). The photoactive layer is made with bulk heterojunction composites of poly[N-9’’-hepta-decanyl-2,7-carbazolealt-5,5-(4′,7′-di-2-thienyl- 2′,1′,3′-ben-zothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC71BM). The PTFE layer not only improves the energy level alignment by forming an interfacial dipole at the interface but also reduces the inherent incompatibility between the hydrophobic active layer and hydrophilic NiOx layer, thereby benefiting to the charge extraction and transport in the solar cells. As a result, with the NiOx/PTFE stacked structure, all the photovoltaic performance parameters are significantly improved, leading to a higher power conversion efficiency (PCE) of up to 7.11% compared to the control device without PTFE layer (PCE 5.50%). The PTFE layer provides a superior alternative to interfacial engineering of the metal oxide/organic semiconductor interface in polymer solar cells and other organic electronic devices.

Increased charge transfer state separationviareduced mixed phase interface in polymer solar cells

For the first time, the mixed phase is quantified within a polymer solar cell and correlated to CT state separation and charge extraction efficiency. A causal relationship is revealed that a narrow mixed interphase between pure donor and pure acceptor domains is a key driver in device efficiency.

Phosphomolybdic acid as an efficient hole injection material in perovskite optoelectronic devices.

Efficient perovskite devices consist of a perovskite film sandwiched between charge selective layers, in order to avoid non-radiative recombination. A common metal oxide used as a p-type or hole transport layer is molybdenum oxide. MoO3 is of particular interest for its very large work function, which allows it to be used both as an interfacial charge transfer material and a dopant for organic semiconductors. However, high quality and high work function MoO3 is typically thermally evaporated in a vacuum. An alternative solution-processable high work function material is phosphomolybdic acid (PMA), which is stable, commercially available and environmentally friendly. In this Communication, we show the first application of PMA in efficient vacuum processed perovskite devices. We found that the direct growth of perovskite films onto PMA leads to strong charge carrier recombination, hindering the solar cell photovoltage. Using an energetically suitable selective transport layer placed between PMA and the perovskite film, solar cells with efficiency >13% as well as LEDs with promising quantum efficiency can be obtained.