Interface In Solar Cells Research Articles

Assessing the energy offset at the electron donor/acceptor interface in organic solar cells through radiative efficiency measurements

The radiative efficiency of non-fullerene devices is modulated by the energy offset, making electroluminescence a powerful tool for energy offset evaluation.

Revealing working mechanisms of PFN as a cathode interlayer in conventional and inverted polymer solar cells.

Energy level alignments at the PC71BM/PFN/Ag interface in conventional polymer solar cells (c-PSCs) and the ITO/PFN/PC71BM interface in inverted polymer solar cells (i-PSCs) are systematically investigated via ultraviolet photoelectron spectroscopy and by using the integer charge transfer (ICT) model. The findings demonstrate that PFN as a cathode interlayer is able to effectively reduce the electron extraction barriers from 0.72 eV to 0.38 eV for the c-PSCs and from 0.58 eV to 0.36 eV for the i-PSCs, respectively. In the c-PSCs, the final modified electron extraction barrier matches the predicted value (∼0.4 eV) using the ICT model. In the i-PSCs, there exists an intermixing layer of PFN and the active layer above PFN because some PFN is dissolved by the organic solvent in the active layer solution, resulting in a special energy level alignment at the PFN/PC71BM interface. ITO/PFN (2 nm)/PC71BM (20 nm) in the i-PSCs actually forms such an interface as ITO/PFN/PFN:PC71BM with an energy level alignment like Al/LiF/PC71BM/PFN (0.65 nm), which rationalizes a higher short circuit and fill factor in the i-PSCs than c-PSCs. Finally, a general model to simulate the intermixing layer between the organic cathode interlayer and the active layer has been proposed to describe the energy level alignment of the complicated interfaces in the i-PSCs.

Modifications of the CZTSe/Mo back-contact interface by plasma treatments.

Molybdenum (Mo) is the most commonly used back-contact material for copper zinc tin selenide (CZTSe)-based thin-film solar cells. For most fabrication methods, an interfacial molybdenum diselenide (MoSe2) layer with an uncontrolled thickness is formed, ranging from a few tens of nm up to ≈1 μm. In order to improve the control of the back-contact interface in CZTSe solar cells, the formation of a MoSe2 layer with a homogeneous and defined thickness is necessary. In this study, we use plasma treatments on the as-grown Mo surface prior to the CZTSe absorber formation, which consists of the deposition of stacked metallic layers and the annealing in selenium (Se) atmosphere. The plasma treatments include the application of a pure argon (Ar) plasma and a mixed argon–nitrogen (Ar–N2) plasma. We observe a clear impact of the Ar plasma treatment on the MoSe2 thickness and interfacial morphology. With the Ar–N2 plasma treatment, a nitrided Mo surface can be obtained. Furthermore, we combine the Ar plasma treatment with the application of titanium nitride (TiN) as back-contact barrier and discuss the obtained results in terms of MoSe2 formation and solar cell performance, thus showing possible directions of back-contact engineering for CZTSe solar cells.

Tetra‐ammonium Zinc Phthalocyanine to Construct a Graded 2D–3D Perovskite Interface for Efficient and Stable Solar Cells

Summary of main observation and conclusionThe crystallographic defects inevitably incur during the solution processed organic‐inorganic hybrid perovskite film, especially at surface and the grain boundaries (GBs) of perovskite film, which can further result in the reduced cell performance and stability of perovskite solar cells (PSCs). Here, a simple defect passivation method was employed by treating perovskite precursor film with a hydrophobic tetra‐ammonium zinc phthalocyanine (ZnPc). The results demonstrated that a 2D‐3D graded perovskite interface with a capping layer of 2D (ZnPc)0.5MAn − 1PbnI3n + 1 perovskite together with 3D MAPbI3 perovskite was successfully constructed on the top of 3D perovskite layer. This situation realized the efficient GBs passivation, thus reducing the defects in GBs. As expected, the corresponding PSCs with modified perovskite revealed an improved cell performance. The best efficiency reached 19.6%. Especially, the significantly enhanced long‐term stability of the responding PSCs against humidity and heating was remarkably achieved. Such a strategy in this work affords an efficient method to improve the stability of PSCs and thus probably brings the PSCs closer to practical commercialization.

Dye Sensitization of Titania Compact Layer for Efficient and Stable Perovskite Solar Cells

In this work, the dye-sensitization approach was used to optimize the compact layer/perovskite interface in planar perovskite solar cells (PSCs). The chemisorption of the triphenylamine-based metal-free organic (E)-3-(5-(4-(bis(2′,4′-dibutoxy-[1,1′-biphenyl]-4yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid (D35) sensitizer via a bidentate anchor group on the titania surface resulted in both enhanced electron transfer and controlled growth of the top MAPbI3 perovskite layer. This modification led to planar PSC devices presenting significantly improved performance and increased stability. Using the Spiro-OMeTAD molecular hole conductor, stabilized output power conversion efficiencies (PCEs) of 13% were obtained for cells with titania sensitization, clearly outperforming those of cells without D35. This investigation gives new insights into how the molecular engineering via dye sensitization of the functional interfaces affects the performance in PSCs.

Modifying the Semiconductor/Electrolyte Interface in CuO p-Type Dye-Sensitized Solar Cells: Optimization of Iodide/Triiodide-Based Electrolytes

Studies related to novel electrolyte concepts for p-type dye-sensitized solar cells (p-DSSCs) set focus on developing alternatives to the iodide/triiodide (I–/I3–) redox couples, while attempts to improve the performance of the latter are scarce. In this study, we report on the first complete optimization of an I–/I3–-based electrolyte for p-DSSCs employing CuO-based photocathodes. Optimization of the electrolyte solvent, the effect of ionic liquids, and the concentration-dependent properties of various electrolyte additives were at the focal point. Electrochemical impedance spectroscopy and intensity-modulated photovoltage and photocurrent spectroscopies complemented basic electrochemical and device characterization, leading to a sound understanding of charge injection, charge recombination, and charge transport. To this end, optimization of the aforementioned parameters enabled 2-fold device efficiencies of 0.156% compared to 0.078% for nonoptimized devices.

Energy Level Alignment at Interfaces in Metal Halide Perovskite Solar Cells

AbstractThe rapid progress of organic–inorganic metal halide perovskite solar cells (PSCs) has attracted broad interest in photovoltaic community. A typical PSC consists of anode/cathode, a perovskite layer as absorber, and carrier transport layer(s) (electron/hole transport layer(s)), which are stacked together, resulting in multi‐interfaces between these layers. Charge extraction and transport in these solar cell devices are strongly influenced by the interfaces and in particular the energy level alignment (ELA). It is the synergy of multiple interfaces and bulk films embedded in the cell architecture that has led to the extraordinary success of PSCs. Here, the authors review the progress of the studies on energy level alignment in PSCs, including several sections: methods for deriving ELA, semiconductor type of perovskite, bottom layer–dependent energy level shift of perovskite, density of states–governed ELA, ELA for specific interfaces, instability‐induced ELA variation, and defects and ion migration–induced ELA variation. Perspective and outlook for precisely determining ELA, designing the device architecture, and fabricating high performance PSCs are discussed.

Efficient planar heterojunction perovskite solar cells employing a solution-processed Zn-doped NiOX hole transport layer

Optimal interface of planar heterojunction perovskite solar cells determines efficient charge transport and collection for achieving high power conversion efficiency. Nickel oxide (NiOX) is one of the most well-known hole transport materials for perovskite solar cells because of its wide band gap, p-type transport characteristic, good chemical stability and high optical transparency. However, pristine NiOX possesses unsatisfactory electrical properties such as high surface trap density or low electrical conductivity which could deteriorate the device performance. We thus adopted Zn as a dopant to employ the Zn-doped NiOX as a hole transport layer in the planar heterojunction PSCs as considering that not only Zn has a similar atomic size with Ni but also Zn forms highly crystalline oxide materials. As a result, 5% Zn-doped devices achieved a power conversion efficiency up to 13.72% with improvements of a VOC by 4.0%, a JSC by 16.7% and a fill factor by 8.9% as compared to the un-doped NiOX counterpart.

Enhanced performance of amorphous silicon solar cells (110 °C) on flexible substrates with a-SiC:H(p) window layer and H2 plasma treatment at n/i and i/p interface

The a-Si:H based n-i-p solar cells were fabricated on flexible polyethylene terephthalate (PET), polyimide (PI) and rigid glass substrates with a-SiC:H(p) as a window layer using radio frequency plasma enhanced chemical vapor deposition technique. The effect of methane flow rate (MFR) on the structural and optoelectronic properties of a-SiC:H(p) films deposited at a low substrate temperature (Ts) 110 °C were studied on Corning 1737 glass (glass) substrate. The influence of hydrogen plasma treatment (HPT) at n/i and i/p interface of solar cells was also investigated. The HPT at interfaces was found to minimize interface defects and enhance the optical and electrical properties of the solar cell. The solar cell with a-SiC:H(p) as a window layer with HPT at interfaces have shown improvement in the series resistance (Rs), shunt resistance(Rsh), open circuit voltage (Voc) and fill factor (FF) as compared to the solar cells fabricated with conventional a-Si:H(p). The efficiencies of fabricated solar cell at 110 °C on flexible PET and PI are 3.36 and 4.06%, which is close to 4.42% on rigid glass. Our results demonstrate that, a-SiC:H(p) as a window layer with HPT at n/i and i/p interfaces can be considered as a practical method to produce the a-Si:H based solar cells on low cost flexible substrates at low substrate temperature.

Relationship between bandgap grading and carrier recombination for Cu(In,Ga)Se2-based solar cells

To understand the effect of bandgap grading on carrier recombination for Cu(In,Ga)Se2 (CIGS)-based solar cells in detail, samples with different bandgaps at the CIGS surface were fabricated by changing the Ga/(Ga + In) (GGI) ratio from 0.4 to 0 at the third stage of the conventional three-stage growth process. Optoelectronic characterizations, such as photoluminescence, temperature-dependent open-circuit voltage measurement and light-intensity-dependent current–voltage measurement, indicate that the photo-generated carriers move rapidly towards the location of the bandgap minimum, and the carrier recombination occurs mainly at this location. From simulation using a one-dimensional solar cell capacitance simulator (SCAPS-1D), a single-grade sample with the smallest bandgap on the surface of CIGS showed high recombination current at the surface, while the location of the maximum recombination current moved from the surface to the bulk for double-grade samples. This study suggests that controlling the bandgap grading is one way of suppressing recombination at the interface in CIGS-based solar cells.

Influence of organic interface modification layer on the photoelectric properties of ZnO-based hybrid solar cells

The organic-inorganic heterojunction interface of hybrid solar cells plays an important role on the cell performance. The properties of heterojunction interface were modulated by employing two small organic molecules. The results showed that the introduction of organic molecules could improve the compatibility between the organic and inorganic materials, accelerate excitons separation efficiency and suppress carriers recombination. The power conversion efficiency of device was improved from 0.23% to 1.41%. Furthermore, the charge lifetime and interface charge transfer/recombination mechanisms were analyzed in detail. The intrinsic factors affecting the cell performance were investigated by electrochemical impedance and open circuit voltage decay measurements.

The influence of ionic radius of interfacial molecule on device performances of polymer solar cells

Ionic radius of interfacial molecular can affect the interface property and key electron transfer processes occurring at interface in polymer solar cells. To study the effects of different ionic radius of electron transfer layer on device performances, we introduce various stearate salts (Na+, K+ and Cs+) as electron transfer layer to fabricate polymer solar cells. Interestingly, it is found that the key device parameters such as Voc, Jsc, fill factor and PCE of polymer solar cells present regular change with the ionic radius increasing. We systematically study the effects of different ionic radius of electron transfer layer on the various parameters of polymer solar cells and believe this regularity paves a way for further understanding and improving the performances of polymer solar cells.

(Invited) Improved Interfaces in Multilayered Organic-Inorganic Hybrid Solar Cells with π-Conjugated Polymers-Antimony Sulfide-Strontium Titanate-Titanium Oxide

Organic-inorganic hybrid solar cells composed of glass/F-doped SnO2/TiO2/ SrTiO3/Sb2S3/ poly[(3-hexylthiophene)-2,5-diyl]/poly[3-(3-carboxypropyl)thiophene-2,5-diyl]/Au have been prepared and evaluated their photovoltaic performance. Insertion of SrTiO3 layer between TiO2 and Sb2S3 interface resulted in the enhancement of the open circuit voltage (Voc ) from 0.61 V to 0.70 V. The thickness, surface roughness, and sea-island structures of the Sb2S3 layer were optimized in terms of transmittance, reflectivity, short circuit current density (Jsc ), fill factor (FF) and power conversion efficiency (PCE) by changing the precursor concentration and rotation speed for spin-coating of the Sb2S3 layer. Through the above optimization, enhancements of Jsc from 8.6 mA cm-2 to 13.6 mA cm-2 and FF from 0.56 to 0.61 in addition to the improvement of PCE from 3.0% to 6.1% as compared with the device without the SrTiO3 layer were achieved.

Characterization of Interface Between Accurately Controlled Cu‐Deficient Layer and Cu(In,Ga)Se2 Absorber for Cu(In,Ga)Se2 Solar Cells

We recently succeeded in controlling a Cu‐deficient Cu(In,Ga)Se2 layer (CDL) on a Cu(In,Ga)Se2 (CIGS) surface by introducing an Se irradiation after the completion of the second stage in a three‐stage process during CIGS growth. The CDL on the surface causes the formation of a valence band offset (ΔEV) between the CDL and CIGS because the Cu vacancies decrease the valence band maximum of the CDL. Therefore, we can expect the suppression of recombination at the CdS/CIGS interface in CIGS solar cells due to the repelling of holes by ΔEV. The amount of knowledge regarding the properties of CDL/CIGS interfaces is observed to be quite small because a control technique for CDL has not been developed so far. In this study, the compositional and structural properties of an accurately controlled CDL/CIGS interface are investigated in detail. The composition of the interface between the CDL and CIGS is observed using an energy dispersive X‐ray spectroscopy with the help of a transmission electron microscope. Using nanobeam electron diffraction and Fourier transfer mapping analysis, it is confirmed that the plane in the CDL continuously grows on the plane in CIGS. Further, these results indicate that a high‐quality interface is formed between the CDL and CIGS, which contains only a small amount of dangling bonds. Finally, a high conversion efficiency of 19.4% is achieved in the CIGS solar cell, which can be attributed to the formation of CDL and effect of ΔEV.

Controllable Spatial Configuration on Cathode Interface for Enhanced Photovoltaic Performance and Device Stability.

The molecular structure of cathode interface modification materials can affect the surface morphology of the active layer and key electron transfer processes occurring at the interface of polymer solar cells in inverted structures mostly due to the change of molecular configuration. To investigate the effects of spatial configuration of the cathode interfacial modification layer on polymer solar cells device performances, we introduced two novel organic ionic salts (linear NS2 and three-dimensional (3D) NS4) combined with the ZnO film to fabricate highly efficient inverted solar cells. Both organic ionic salts successfully decreased the surface traps of the ZnO film and made its work function more compatible. Especially NS4 in three-dimensional configuration increased the electron mobility and extraction efficiency of the interfacial film, leading to a significant improvement of device performance. Power conversion efficiency (PCE) of 10.09% based on NS4 was achieved. Moreover, 3D interfacial modification could retain about 92% of its initial PCE over 160 days. It is proposed that 3D interfacial modification retards the element penetration-induced degradation without impeding the electron transfer from the active layer to the ZnO film, which significantly improves device stability. This indicates that inserting three-dimensional organic ionic salt is an efficient strategy to enhance device performance.

(Invited) Improved Interfaces in Multilayered Organic-Inorganic Hybrid Solar Cells with π-Conjugated Polymers-Antimony Sulfide-Strontium Titanate-Titanium Oxide

We have developed various organic-inorganic hybrid solar cells using Sb2S3-based composites with π-electronic molecules [1,2]. For examples, utilization of zinc phthalocyanine (ZnPC) for the hybrid solar cell composed of glass/indium-tin-oxide (ITO)/TiO2/Sb2S3/ZnPC/Au achieved long durability with keeping the relative power conversion efficiency (PCE) of 90% after the stability test under 1 sun at 63 °C at a relative humidity of 50% for 1,500 h [1]. Using La-decorated Sb2S3 for hybrid solar cell composed of glass/F-doped SnO2 (FTO)/TiO2/La-decorated Sb2S3/poly[(3-hexylthiophene)-2,5-diyl] (P3HT)/poly[3-(3-carboxypropyl)thiophene-2,5-diyl] (P3CT)/Au resulted in 33% improvement of PCE as compared to that of the non-decorated Sb2S3 because of lowering the resistance at the interface, which was confirmed by the impedance analyses [2]. While doping of Zn into Sb2S3 attained 57% enhancement of PCE. This result is ascribed to the effective lowering the bulk-resistance [2]. In this context, we newly inserted SrTiO3 layer between TiO2 and Sb2S3 interface and found the enhancement of the open circuit voltage (VOC ) from 0.61 V to 0.74 V. Temperature dependence of VOC for SrTiO3-inserted device indicated the maximum voltage was 1.7 V, which had been estimated by extrapolation of the line to 0 K. For further improvement of the photovoltaic performance, we modified the thin-film making process for Sb2S3 layer from the previously reported procedures [1,2]. After the spin-coating of the precursor solution of SbCl3-thiourea complex in DMF onto the SrTiO3 layer, heat treatment at 220 °C under vacuum was newly performed. The thickness, surface roughness, and sea-island structure were optimized by their uv-vis measurement, AFM, TEM observations, and current density-voltage (J-V) characteristics in terms of transmittance, reflectivity, and short circuit current density (JSC ) by changing the spin-coating speed, duration of heating, and so on. Through the optimization, remarkable enhancement of JSC from 10.7 mA cm-2 to 13.6 mA cm-2 and improvement of PCE from 4.7% to 6.1% were achieved. [1] A. Hayakawa, M. Yukawa, and T. Sagawa, ECS J. Solid-State Technology, 6(4), Q35-Q38 (2017). [2] M. Yukawa, A. Hayakawa, and T. Sagawa, ECS Trans., 77(11), 653-659 (2017).

Analysis of silicon wafer surface preparation for heterojunction solar cells using X-ray photoelectron spectroscopy and effective minority carrier lifetime

We report a systematic study on the optimal conditions for silicon surface preparation to ensure excellent passivation at the crystalline-amorphous silicon (c-Si/a-Si:H) interface of silicon heterojunction solar cells for both untextured and chemically textured samples. X-ray photoelectron spectroscopy (XPS) was utilized to analyse the elemental composition of known silicon impurities on the wafer surface. Surface purity and passivation quality, characterized by effective minority carrier life time (τeff) and implied open circuit voltage (iVOC), were estimated using either quinhydrone-methanol solution or 10 nm intrinsic a-Si:H layers deposited using DC plasma process. This study confirms that surface damage etch (SDE), tetra-methyl ammonium hydroxide (TMAH) texturing and the subsequent TMAH residue removal are the most critical steps in the cleaning process, supporting a simplified wafer cleaning approach that is concise, repeatable and uses minimal volume of chemicals.

Improving performance by Na doping of a buffer layer—chemical and electronic structure of the InxSy:Na/CuIn(S,Se)2 thin‐film solar cell interface

AbstractDoping an indium sulfide buffer layer with sodium is a promising route to replace the “state‐of‐the‐art” CdS buffer layer in chalcopyrite‐based thin‐film solar cells, as it achieves efficiencies as high as 17.9% for large‐area devices (30 cm × 30 cm). We report on the chemical and electronic structure of the InxSy:Na/CuIn(S,Se)2 (CISSe) interface for thin‐film solar cells by means of photoelectron, soft x‐ray emission, and inverse photoemission spectroscopy. For as‐deposited InxSy:Na buffer layers, we find a sulfur‐poor surface and, in comparison to undoped InxSy and the standard CdS buffer, derive a large electronic surface band gap of 2.60 ± 0.11 eV. The conduction band offset at the buffer/absorber interface is a spike of 0.32 ± 0.10 eV. After annealing at 200°C to simulate the thermal load of subsequent cell manufacturing processes, an additional diffusion of copper and selenium from the absorber towards the buffer layer surface is observed, leading to a distinct electronic surface band gap decrease of the InxSy:Na buffer layer (to 2.11 ± 0.11 eV). We speculate that the diffusion of absorber elements causes a band gap widening at the former absorber surface and that both effects lead to a reduction of the conduction band spike for the buried InxSy:Na/CISSe interface after annealing.

Influence of Cu and Na incorporation on the thermodynamic stability and electronic properties of β-In2S3

We studied the incorporation of a Na and Cu in β-In2S3 buffer layer and addressed the stability of the absorber/buffer interface in CIGS-based thin film solar cells.

The direct observation of electron backflow in an organic heterojunction formed by two n-type materials.

Many physical processes such as exciton interfacial dissociation, exciton interfacial recombination, and exciton-electron and exciton-hole interactions coexist at the interface of organic solar cells (OSC). In this study, the direction of free charge generation is defined as the direction from the interface to the side where free charges are left. For a p-n type device, the direction of free electron (hole) generation from exciton dissociation at the donor/accepter (D/A) interface is the same as the subsequent transportation direction under the built-in electric field. However, the direction of free electron (hole) generation from exciton-exciton recombination across the D/A interface is opposite to the direction of free charge transportation. Both free charges generated from exciton interfacial dissociation and recombination are contributed to the photocurrent for a p-n type device. In a device with a heterojunction formed by two n-type materials (here it is defined as an n-n type device), the direction of free electron (hole) generation from exciton recombination across the interface is also the same as the subsequent free charge transportation. At the same time, there are also some free electrons (free holes) generated by exciton interfacial dissociation. The direction of free charge generation from exciton dissociation for this n-n type device is also opposite to the direction of free charge transportation. However, only free charges generated from exciton interfacial recombination are contributed to the photocurrent for an n-n type device. But so far there has been no direct experimental evidence to prove the above theories. In this work, an NPB interfacial layer with a high LUMO was introduced in an n-n type OSC to inhibit the backflow of electrons, which are generated from exciton dissociation at the heterojunction formed by two n-type materials, enhancing the device performance accordingly. This work is conducive to interfacial engineering in an OSC to further improve its performance.