Voc Of Device Research Articles

Photo-ferroelectric perovskite interfaces for boosting VOC in efficient perovskite solar cells

Interface engineering is the core of device optimization, and this is particularly true for perovskite photovoltaics (PVs). The steady improvement in their performance has been largely driven by careful manipulation of interface chemistry to reduce unwanted recombination. Despite that, PVs devices still suffer from unavoidable open circuit voltage (VOC) losses. Here, we propose a different approach by creating a photo-ferroelectric perovskite interface. By engineering an ultrathin ferroelectric two-dimensional perovskite (2D) which sandwiches a perovskite bulk, we exploit the electric field generated by external polarization in the 2D layer to enhance charge separation and minimize interfacial recombination. As a result, we observe a net gain in the device VOC reaching 1.21 V, the highest value reported to date for highly efficient perovskite PVs, leading to a champion efficiency of 24%. Modeling depicts a coherent matching of the crystal and electronic structure at the interface, robust to defect states and molecular reorientation. The interface physics is finely tuned by the photoferroelectric field, representing a new tool for advanced perovskite device design.

Interfacial energy-band engineering for high open-circuit voltage of rear emitter p-type crystalline silicon solar cells

Rear emitter p-type crystalline silicon (c-Si) tunnel oxide passivating contact (P-TOPCon) devices with low-resistivity (≤1 Ω.cm) c-Si wafer and high-conductivity phosphorus doped (n-type) polysilicon passivating contact (n-poly-Si PC) are expected to achieve high built-in potential (Vbi) and thus open-circuit voltage (Voc). However, the results indicate that the high conductivity of n-poly-Si PC considerably deteriorates the interface passivation quality and lowers the device's Voc. A crucial factor to consider is that the hole and electron can tunnel and recombine together at the c-Si/n-poly-Si PC contact, resulting in a decrease in the quality of interfacial passivation. To inhibit carrier tunnel diffusion and enable energy band engineering at the interface for achieving high Voc, a thin intrinsic poly-Si buffer layer (10 nm) is inserted between SiO2 and n-poly-Si PC. When compared to a device without a buffer, a device with the buffer exhibits a significant improvement in passivation quality and hence Voc. The device with buffer has a Voc of 705 mV and an efficiency of 23.5 %, while that without buffer is 681 mV and 22.6 %, respectively. The introduced buffer has little effect on the industrial manufacturing process, but it does significantly enhance Voc. It has the potential to improve the efficiency of the P-TOPCon devices.

Crystal reconstruction and defect healing enabled high-quality Sb2Se3 films for solar cell applications

A post-sulfurization technique has been developed to refine Sb2Se3 crystallization. This process enables high-quality Sb2Se3 with a smooth surface, large compact crystal grains and benign [hkl] growth orientation. It also mitigates the device VOC deficit, owing to the healed deep level defects.

In situ artificial wide-bandgap Cs-based recrystallized-arrays for optical optimization of perovskite solar cells

Coordinated light management as one of the most essential strategies to further improve the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs), which can achieve stronger light trapping, coupling and absorption through submicron nanostructured morphologies relative to planar structure. However, the interfacial preparation of textured-structures has been facing great challenges, which has severely limited the research progress in the field of optical structures of PSCs. Here, in situ growth of inorganic Cs-based perovskite recrystallized-arrays (CPRA) on the perovskite surface was realized by pseudo-halide anion solvent engineering. This preparation procedure of the CPRA is very facile and cost-effective. The induced recrystallization involved in the CPRA synthesis process is non-invasive to the underlying perovskite. It is experimentally and theoretically shown that highly recrystallized-arrays can provide excellent optoelectronic performance. The open circuit voltage (VOC) of the champion PSC modified by this wide-bandgap CPRA can exceed 1.20 V, and the PCE can reach 23.23%. Moreover, this scheme has the function of tuning the bandgap, which can increase the VOC of device up to 1.23 V (PCE 22.12%). The long-term stability of the CPRA-modified PSCs is also prominent based on the inherent lotus hydrophobic properties of the bionic arrays.

Bifunctional hole-shuttle molecule for improved interfacial energy level alignment and defect passivation in perovskite solar cells

Perovskite solar cells have reached a power conversion efficiency over 25%, and the engineering of the interface between the perovskite and hole transport layer (HTL) has been crucial to achieve high performance. Here we design a bifunctional molecule CBz-PAI with carbazole-triphenylamine and phenylammonium iodide units to passivate defects at the perovskite/HTL interface. Owing to a favourable energy level alignment with the perovskite, the CBz-PAI acts as a hole shuttle between the perovskite layer and the HTL. This minimizes the difference between the quasi-Fermi level splitting of the perovskite, or ‘internal’ Voc, and the external device Voc, thus reducing voltage losses. As a result, solar cells incorporating CBz-PAI reach a stabilized power conversion efficiency of 24.7% and maintain 92.3% of the initial efficiency after 1,000 h under damp heat test (85 °C and 85% relative humidity) and 94.6% after 1,100 h under maximum power point-tracking conditions.

Dually Modified Wide-Bandgap Perovskites by Phenylethylammonium Acetate toward Highly Efficient Solar Cells with Low Photovoltage Loss.

Wide-bandgap perovskites as a class of promising top-cell materials have shown great promise in constructing efficient perovskite-based tandem solar cells, but their intrinsic relatively low radiative efficiency results in a large open-circuit voltage (VOC) deficit and thereby limits the whole device performance. Reducing film flaws or optimizing interfacial energy level alignments in wide-bandgap perovskite devices can efficiently inhibit nonradiative recombination to boost device VOC and efficiency. However, the simultaneous regulation on both sides and their underlying mechanism are less explored. Herein, a bifunctional modification approach is proposed to optimize the wide-bandgap perovskite surface with an ultrathin layer of phenylethylammonium acetate (PEAAc) to synchronously decrease the surface imperfection and mitigate the interfacial energy barrier. This treatment effectively heals under-coordinated surface defects through the formation of chemical interaction between the perovskite and PEAAc, bringing about a much slower charge trapping process and dramatically decreasing nonradiative recombination losses. Meanwhile, the passivation-induced upshifted Fermi level of the perovskite contributes to accelerated electron extraction and larger Fermi-level splitting under illumination. Consequently, the PEAAc-modified wide-bandgap (1.68 eV) device achieves an optimal efficiency of 20.66% with a high VOC of 1.25 V, among the highest reported VOC values for wide-bandgap perovskite devices, enormously outperforming that (18.86% and 1.18 V) of the device without passivation. In addition, the radiative limit of VOC for both cells is determined to be 1.42 V, delivering nonradiative recombination losses of 0.24 and 0.17 V for the control and PEAAc-modified devices, respectively. These results highlight the significance of the bifunctional modification strategy in achieving high-performance wide-bandgap perovskite devices.

The Energy Level Conundrum of Organic Semiconductors in Solar Cells.

The frontier molecular energy levels of organic semiconductors are decisive for their fundamental function and efficiency in optoelectronics. However, the precise determination of these energy levels and their variation when using different techniques makes it hard to compare and establish design rules. In this work, the energy levels of 33 organic semiconductors via cyclic voltammetry (CV), density functional theory, ultraviolet photoelectron spectroscopy, and low-energy inverse photoelectron spectroscopy are determined. Solar cells are fabricated to obtain key device parameters and relate them to the significant differences in the energy levels and offsets obtained from different methods. In contrast to CV, the photovoltaic gap measured using photoelectron spectroscopy (PES) correlates well with the experimental device VOC . It is demonstrated that high-performing systems such as PM6:Y6 and WF3F:Y6, which are previously reported to have negligible ionization energy (IE) offsets (ΔIE), possess sizable ΔIE of ≈0.5eV, determined by PES. Using various D-A blends, it is demonstrated that ΔIE plays a key role in charge generation. In contrast to earlier reports, it is shown that a vanishing ΔIE is detrimental to device performance. Overall, these findings establish a solid base for reliably evaluating material energetics and interpreting property-performance relationships in organic solar cells.

Nanoscale small molecule self-assembled ITO for photon harvesting in polymer and perovskite solar cells

Benefit of small molecule self-assembled monolayers as a buffer layer at the light entering electrode in a photovoltaic device improves the transparency. This results in improved JSC, as a result increased photovoltaics performance. In This work, 4-nitro benzoic acid (NBA) was investigated as an ITO work function modifier, in place of PEDOT:PSS, both in organic and perovskite photovoltaics. Self-assembled monolayer of NBA helps in work function reduction of ITO. Self-assembled monolayer of NBA forms a permanent dipole on the ITO surface consequently increases WF. Using NBA as an ITO work function modifier does not influence the device VOC. From obtained results it is noted that light absorption enhancement is the primary factor for PCE improvement. Results indicates that NBA can be used as work function modifier both in organic–inorganic hybrid perovskite photovoltaics and organic photovoltaics.

Identifying correlation between the open-circuit voltage and the frontier orbital energies of non-fullerene organic solar cells based on interpretable machine-learning approaches

The open-circuit voltage (Voc) of the non-fullerene acceptors-based organicsolarcells(NFAs-OSCs)under device operation conditions, as an essential photovoltaic parameter, is extensively studied for further power conversion efficiency (PCE) improvement. Generally, the Voc of binary bulk-heterojunction (BHJ) OSCs is roughly estimated by the energy level offset between the highest occupied molecular orbital of the donor (HOMO(D)) and the lowest unoccupied molecular orbital of the acceptor (LUMO(A)). Existing simulation and experimental approaches focus on studying the correlation between the Voc of PC61BM- and PC71BM-based OSCs with various donors. In solution-processed NFAs-OSCs, however, providing a numerical method for accurate estimates of Voc prediction is very difficult due to the extremely large pool of possible design donor-NFA combinations. It is also noted that using a conventional statistical model is challenging to accurately predict Voc for many different blends where the behavior between (|HOMO(D)|-|LUMO(A)|) offset and VOC is usually not a simple linear relationship. Herein, two tree-based ensemble machine-learning models of Random Forest and XGBoost are proposed to predict the Voc of NFAs-OSCs with a reasonable accuracy based on the intrinsic electronic parameters. In addition, the Shapley Additive Explanations (SHAP) analysis is applied not only for demonstrating the interpretability of the XGBoost model but also for visualizing the correlation between the Voc and the frontier orbital energies of NFAs-OSCs. This study demonstrates that the machine-learning approaches provide an empirical relation for accurately predicting the Voc of NFAs-OSCs, which might offer a strategy for efficiently designing new donor-NFA pairs to improve the Voc in devices.

Combating open circuit voltage loss in Sb2Se3 solar cell with an application of SnS as a back surface field layer

Recently, Sb2Se3 based solar cells have shown severe open-circuit voltage (VOC) loss that results in low device efficiency. In the present work, a model of Sb2Se3 solar cell with tin sulfide (SnS) as a back surface field layer (BSF) layer was designed and investigated via a solar cell capacitance simulator (SCAPS)-1D simulation software. The influence of the ultrathin BSF layer was investigated as a function of Sb2Se3 absorber thickness, and the corresponding device performance was analyzed. Apart from this, the effect of high BSF layer thickness, doping concentration, interface defect density, and resistance were also examined. Applying the BSF layer decreased the valence band offset value, which eases the hole transport at the back interface. At the same time, the high electric field region generated at the back interface creates a barrier for the minority carriers. Synergistically, in the presence of the BSF layer, the carrier recombination current densities were found to be almost negligible. As a result, the introduction of the 10 nm thin BSF layer significantly improved the device's VOC and power conversion efficiency (PCE) from 0.416 to 0.603 V and 9.61 to 17.75%, respectively for the absorber layer thickness of 700 nm. The present simulation study performed with the full earth-abundant element-based low-cost materials may assist the photovoltaic community in combating the VOC loss and further improving the PCE of Sb2Se3 based solar cells.

Insights into the Effects of RbF‐Post‐Deposition Treatments on the Absorber Surface of High Efficiency Cu(In,Ga)Se2 Solar Cells and Development of Analytical and Machine Learning Process Monitoring Methodologies Based on Combinatorial Analysis

AbstractThe latest record efficiencies of the Cu(In,Ga)Se2 (CIGSe) photovoltaic technology are driven by heavy alkali post‐deposition treatments (PDTs). Despite their positive effect, especially in the Voc of the devices, their underlying mechanisms are still under discussion. This work sheds light on this topic by performing a high statistics analysis on 620 high efficiency CIGSe solar cells submitted to four different PDT processes (different RbF source temperature) employing a combinatorial approach based on Raman and photoluminescence (PL) spectroscopies. This reveals with statistical confidence subtle differences in the spectroscopic data that relate to the redistribution of defects between the ordered vacancy compound (OVC) and the chalcopyrite phases at the absorber surface. In particular, there is an intertwined decrease of the OVC phase and increase of the so‐called “defective chalcopyrite phase.” Additionally, two industry‐compatible methodologies for the assessment of the RbF‐PDT process and prediction of the Voc of the final devices with a ±2% error and an efficacy of ≈95% are developed based on analytical and machine learning approaches. These results show that the combined Raman and PL spectroscopic techniques represent a powerful tool for the future development of the CIGSe technology at a research level and for more efficient industrial manufacturing.

Suppressing PEDOT:PSS Doping-Induced Interfacial Recombination Loss in Perovskite Solar Cells.

PEDOT:PSS is widely used as a hole transport layer (HTL) in perovskite solar cells (PSCs) due to its facile processability, industrial scalability, and commercialization potential. However, PSCs utilizing PEDOT:PSS suffer from strong recombination losses compared to other organic HTLs. This results in lower open-circuit voltage (VOC) and power conversion efficiency (PCE). Most studies focus on doping PEDOT:PSS to improve charge extraction, but it has been suggested that a high doping level can cause strong recombination losses. Herein, we systematically dedope PEDOT:PSS with aqueous NaOH, raising its Fermi level by up to 500 meV, and optimize its layer thickness in p-i-n devices. A significant reduction of recombination losses at the dedoped PEDOT:PSS/perovskite interface is evidenced by a longer photoluminescence lifetime and higher magnitude of surface photovoltage, leading to an increased device VOC, fill factor, and PCE. These results provide insights into the relationship between doping level of HTLs and interfacial charge carrier recombination losses.

π-π conjugate structure enabling the channel construction of carrier-facilitated transport in 1D–3D multidimensional CsPbI2Br solar cells with high stability

All-inorganic CsPbIxBr3−x (0 ≤x ≤ 3) perovskites are expected to fundamentally solve the thermal stability issue of perovskite solar cell toward its final commercialization. However, the humidity stability of this class of all-inorganic perovskite is still not intrinsically tackled out. Herein, aminoquinoline is introduced to construct a thus far unreported one-dimensional (1D) perovskite, in which the distorted [PbI6]4- octahedra display a shoulder-by-shoulder arrangement along the [001] direction, and A-site quinoline (Aq) organic groups with π-π conjugate structure orderly distribute among the octahedra space. The structural incorporation of 1D Aq-PbI3(II) perovskite inside 3D CsPbI2Br perovskite is proved to be capable of improving the carrier transport ability, whilst enhancing its hydrophobic performances, which is associated with the unique structure of 1D Aq-PbI3(II) and lattice-matching heterojunction in 1D–3D perovskite. Consequently, the as-fabricated 1D–3D hybrid CsPbI2Br perovskite solar cell allows to increase the device VOC by a factor of 12%, yielding a power conversion efficiency (PCE) as high as 16.1%. Equally importantly, the corresponding 1D–3D perovskite solar cells demonstrate remarkably high stability more than 1300 h in comparison to the control 3D CsPbI2Br perovskite solar cell. This study provides a new strategy for the rational structure design of 1D–3D hybrid perovskite with high PCE and stability.

Double Side Interfacial Optimization for Low‐Temperature Stable CsPbI2Br Perovskite Solar Cells with High Efficiency Beyond 16%

CsPbI2Br perovskite solar cells have achieved rapid development owing to their exceptional optoelectronic properties and relatively outstanding stability. However, open‐circuit voltage (Voc) loss caused by band mismatch and charge recombination between perovskite and charge transporting layer is one of the crucial obstacles to further improve the device performance. Here, we proposed a bilayer electron transport layer ZnO(bottom)/SnO2(top) to reduce the Voc loss (Eloss) and promote device Voc by ZnO insert layer thickness modulation, which could improve the efficiency of charge carrier extraction/transfer and suppress the charge carrier recombination. In addition, guanidinium iodide top surface treatment is used to further reduce the trap density, stabilize the perovskite film and align the energy levels, which promotes the fill factor, short‐circuit current density (Jsc), and stability of the device. As a result, the champion cell of double‐side optimized CsPbI2Br perovskite solar cells exhibits an extraordinary efficiency of 16.25% with the best Voc as high as 1.27 V and excellent thermal and storage stability.

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

An Enhanced Conversion Efficiency of Metal Insulator Semiconductor Solar Cells by Using Different High-K Dielectrics

In this paper, the role of different high-k materials in enhancing the efficiency of a metal insulator semiconductor (MIS) solar cell is investigated in detail. In order to overcome the traditional SiO2 disadvantages, alternative dielectrics, namely TiO2, Si3N4, and Ta2O5, with a thickness of 10 A are used. Furthermore, the effect of the interface states density, doping density, and oxide fixed charge on the electrical outputs of the proposed MIS solar cell is assessed. The simulation results indicate that Ta2O5 could be a good candidate to replace SiO2. The optimized design of a 250-μm-thick cell structure, with a p-type substrate with a doping density of 7 × 1015 cm−3 and a Ta2O5layeras dielectric, performs a short circuit currentdensity JSC = 44.35 mA/cm2, an open circuit voltage VOC = 0.59 V,a fill factor FF = 81.19%, and a conversion efficiency η = 21.54%. Besides, to get a broad perspective on the device optimization possibility, an investigation on a suitable metal work function is also carried out in this study. In particular, a metal electrode with a low work function could efficiently improve the device VOC value.

Identifying the origin of the Voc deficit of kesterite solar cells from the two grain growth mechanisms induced by Sn2+ and Sn4+ precursors in DMSO solution

The large Voc deficit of kesterite solar cell mainly comes from the defective surface caused by multi-phase fusion grain growth; direct phase transformation grain growth produces high quality absorber with clean surface and thus high device Voc.

A New Strategy for Increasing the Efficiency of Inverted Perovskite Solar Cells to More than 21%: High‐Humidity Induced Self‐Passivation of Perovskite Films

The performance of perovskite solar cells (PSCs) is known to be extremely sensitive to humidity in the preparation environment. However, the main mechanism by which the moisture influences the quality of the perovskite film and the device performance is not yet fully understood. Herein, a new strategy is established to obtain inverted PSCs with a remarkabll high VOC by including a high‐humidity treatment and sufficient DMSO‐atmosphere annealing in the preparation process. It is found that the lattice distortion on the surface of perovskite grains caused by the high‐humidity treatment plays a key role in the self‐passivation of perovskite. Inverted (p‐i‐n) PSCs based on the self‐passivated perovskite films show effective suppression of nonradiative recombination, which increase the device VOC to 1.17 V and achieve the highest efficiency of 21.38%. It is expected that the findings of this work shed more light on the currently proposed mechanism governing the action of moisture on the performance of the PSCs.

Solution preparation of molybdenum oxide on graphene: a hole transport layer for efficient perovskite solar cells with a 1.12 V high open-circuit voltage

Recently, based on the tremendous progresses in performance of perovskite solar cells (PSCs), the inverted planar PSCs have attracted attentions for future applications. Since the traditional poly(3,4-ethylenedioxythiophene) hole transport layer could cause a loss of open-circuit voltage (Voc), some organic and inorganic alternatives have been widely investigated. However, the Voc is still below for the demand of optimized value. Here, we demonstrate that the reduced graphene oxide (RGO) doping is a facile and effective method to make molybdenum oxide (MoOx) as a promising hole transport layer (HTL) for high-performance PSCs. The conductive MoOx:RGO HTL can facilitate perovskite crystallization and reduce the potential losses of Voc in devices. Thus, a high PCE of up to 18.15% is achieved which simultaneously possesses a high Voc of 1.12 V. With this strategy, the MoOx:RGO provides a novel hole transport layer for high performance and decently stable optoelectronic devices. More importantly, it opens up the research significance of doping in inorganic buffer interlayers for the perovskite solar cells.

Enhanced Efficiency of Carbon-Based Mesoscopic Perovskite Solar Cells through a Tungsten Oxide Nanoparticle Additive in the Carbon Electrode

This paper presents perovskite solar cells employed with WO3 nanoparticles embedded carbon top electrode. WO3 nanoparticles works as an inorganic hole-transport material (HTM) to promote the hole-extraction in the perovskite/carbon interface as revealed by efficiency, electrochemical impedance and external quantum efficiency measurements. As a result, a 40% enhancement of energy conversion efficiency has been achieved compared to the reference devices with the energy conversion efficiency of 10.77% under standard conditions. In addition, the Li-TFSI can modify the interface between electron-transport material (ETM) and perovskite, which may inhibit the recombination at the ETM/perovskite interface. The VOC of devices upon the modification of Li-TFSI is increased from 887.9 to 934.2 mV. This work highlights about the enlightenment of the effective performance of carbon-based mesoscopic PSCs by the introduction of HTM and the modification of interfaces.