quasi-Fermi Level Splitting Research Articles

(Digital Presentation) Surface Photovoltage Provides Quasi Fermi Level Splitting and Minority Carrier Electrochemical Potential of BiVO4 /Liquid Junctions Under Illumination

The solar energy conversion efficiency of semiconductor-liquid junctions is determined by their photovoltage and by their photocurrent under illumination. Photoelectrochemistry (PEC) gives information on the current, but not on the photovoltage or the quasi Fermi level splitting (qFLS), which for slow redox couples is obscured by kinetic overpotentials and surface states. Here we show that surface photovoltage spectroscopy (SPS) provides direct measurement of the qFLS potential in BiVO4-electrolyte contacts. Measurements were done with a vibrating Kelvin probe on FTO deposited BiVO4 films in contact with aqueous electrolytes containing triiodide/iodide, oxygen/ hydrogen peroxide, or sulfate/sulfite redox couples and for iron-oxyhydroxide modified BiVO4 films. The triiodide/iodide couple, for example, yields qFLS=0.78 V under 49 mW/cm-2 (400 nm LED) illumination, comparable to the photovoltage from PEC (0.79 V). Furthermore, the reversibility of the redox reactions determines the reversibility of the SPS transients. For example, the triiodide/iodide couple gives nearly complete SPS reversibility on the 20 sec timescale, while the sulfate/sulfite couple gives an irreversible photovoltage signal. Lastly, by combining the SPS data with the majority carrier potential from open circuit potential measurements, the minority carrier electrochemical potentials, Efp can be obtained for all systems. For instance, Efp = 1.0 V vs RHE for the BiVO4 - triiodide/iodide junction and Efp = 0.26 V vs RHE for the sulfate/sulfite junction. These shows that BiVO4 can photoelectrochemically oxidize both redox couples, without applied bias. These results shed new light on the photoelectrochemistry of BiVO4, an important photoanode material for solar hydrogen production from water. Figure 1

Efficiency Potential and Voltage Loss of Inorganic CsPbI2Br Perovskite Solar Cells

Inorganic perovskite solar cells show excellent thermal stability, but the reported power conversion efficiencies are still lower than for organic–inorganic perovskites. This is mainly caused by lower open‐circuit voltages (VOCs). Herein, the reasons for the low VOC in inorganic CsPbI2Br perovskite solar cells are investigated. Intensity‐dependent photoluminescence measurements for different layer stacks reveal that n–i–p and p–i–n CsPbI2Br solar cells exhibit a strong mismatch between quasi‐Fermi level splitting (QFLS) and VOC. Specifically, the CsPbI2Br p–i–n perovskite solar cell has a QFLS–e ·VOC mismatch of 179 meV, compared with 11 meV for a reference cell with an organic–inorganic perovskite of similar bandgap. On the other hand, this study shows that the CsPbI2Br films with a bandgap of 1.9 eV have a very low defect density, resulting in an efficiency potential of 20.3% with a MeO–2PACz hole‐transporting layer and 20.8% on compact TiO2. Using ultraviolet photoelectron spectroscopy measurements, energy level misalignment is identified as a possible reason for the QFLS–e ·VOC mismatch and strategies for overcoming this VOC limitation are discussed. This work highlights the need to control the interfacial energetics in inorganic perovskite solar cells, but also gives promise for high efficiencies once this issue is resolved.

(Invited) Surface Photovoltage Spectroscopy on BiVO4, Gallium Phosphide, and CuGa3Se5 Photoelectrodes in Contact with Aqueous Electrolytes

The quasi-Fermi level splitting energy (photovoltage) is important for the solar to fuel conversion efficiency of photocatalysts and photoelectrodes. While electrochemical measurements can provide the electrochemical potential of the majority carriers at the semiconductor back-contact, the photovoltage, and the potential of the minority carriers at the front semiconductor/electrolyte are more difficult to determine experimentally, due to the lack of a direct electrical connection. Here we show that surface photovoltage spectroscopy (SPS) on photoelectrode films in contact with aqueous electrolytes can provide photovoltage information. For the measurements the semiconductor films (BiVO4, CuGa3Se5) or wafers (n-GaP or p-GaP) are immersed in aqueous electrolytes and placed underneath a vibrating Kelvin probe to monitor the light induced contact potential difference change. We find that the photovoltage (quasi-Fermi level splitting) of all photoelectrodes is increased by the aqueous electrolyte, and strongly depends on the electrochemical potential of the redox couple (TEOA, Na2SO3, KI, K4[Fe(CN)6] in the electrolyte, the back contact, and the light intensity. Electrochemical potentials of the minority carriers at the solid-liquid interface can be estimated by combining the photovoltage data with open circuit potential measurements that yield the quasi-Fermi levels of the majority carriers. This study provides new insight on the factors that affect photochemical charge separation at solid-liquid interfaces. Figure 1

Additive MXene and dominant recombination channel in perovskite solar cells

Two-dimensional (2D) MXenes have emerged as attractive materials for application in perovskite solar cells (PSCs). Their unique properties arising from terminating functional groups to tune the work function (WF) of the layers in a PSC lead to a notable improvement in the photovoltaic efficiency. The ideality factor (nid) is used to infer the dominant recombination type in the PSCs. We study the effect of the added MXene in the bulk absorber and the electron transport layer (ETL), and at the interfaces on the ideality factor (nid), and then the obtained nid is used to infer the dominant recombination type in the PSCs. By using the concept of intensity-dependent quasi-Fermi level splitting (QFLS), conduction and valence band offsets, and drift–diffusion simulations, we show that the bulk recombination in the MXene-free cells dominates. Considering the obtained nid values, we explain that adding MXene just into the absorber layer leads to a suppressed bulk recombination compared with the case of MXene-free device. Meanwhile, devices with added MXene into the ETL and at the ETL/absorber interface show a notable reduced interface recombination. The study of the interfacial energy offsets and their correlation with the interface and bulk recombination currents reveals that in the cells with MXene, higher nid values are desirable and correlate with better cell performance.

Imaging and quantifying non-radiative losses at 23% efficient inverted perovskite solar cells interfaces

Interface engineering through passivating agents, in the form of organic molecules, is a powerful strategy to enhance the performance of perovskite solar cells. Despite its pivotal function in the development of a rational device optimization, the actual role played by the incorporation of interfacial modifications and the interface physics therein remains poorly understood. Here, we investigate the interface and device physics, quantifying charge recombination and charge losses in state-of-the-art inverted solar cells with power conversion efficiency beyond 23% - among the highest reported so far - by using multidimensional photoluminescence imaging. By doing that we extract physical parameters such as quasi-Fermi level splitting (QFLS) and Urbach energy enabling us to assess that the main passivation mechanism affects the perovskite/PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) interface rather than surface defects. In this work, by linking optical, electrical measurements and modelling we highlight the benefits of organic passivation, made in this case by phenylethylammonium (PEAI) based cations, in maximising all the photovoltaic figures of merit.

Perovskite Solar Cells with Carbon‐Based Electrodes – Quantification of Losses and Strategies to Overcome Them

AbstractCarbon‐based electrodes represent a promising approach to improve stability and up‐scalability of perovskite photovoltaics. The temperature at which these contacts are processed defines the absorber grain size of the perovskite solar cell: in cells with low‐temperature carbon‐based electrodes (L‐CPSCs), layer‐by‐layer deposition is possible, allowing perovskite crystals to be large (>100 nm), while in cells with high‐temperature carbon‐based contacts (H‐CPSCs), crystals are constrained to 10–20 nm in size. To enhance the power conversion efficiency of these devices, the main loss mechanisms are identified for both systems. Measurements of charge carrier lifetime, quasi‐Fermi level splitting (QFLS) and light‐intensity‐dependent behavior, supported by numerical simulations, clearly demonstrate that H‐CPSCs strongly suffer from non‐radiative losses in the perovskite absorber, primarily due to numerous grain boundaries. In contrast, large crystals of L‐CPSCs provide a long carrier lifetime (1.8 µs) and exceptionally high QFLS of 1.21 eV for an absorber bandgap of 1.6 eV. These favorable characteristics explain the remarkable open‐circuit voltage of over 1.1 V in hole‐selective layer‐free L‐CPSCs. However, the low photon absorption and poor charge transport in these cells limit their potential. Finally, effective strategies are provided to reduce non‐radiative losses in H‐CPSCs, transport losses in L‐CPSCs, and to improve photon management in both cell types.

GaAs/GaInP nanowire solar cell on Si with state-of-the-art Voc and quasi-Fermi level splitting

With their unique structural, optical and electrical properties, III-V nanowires (NWs) are an extremely attractive option for the direct growth of III-Vs on Si for tandem solar cell applications. Here, we introduce a core-shell GaAs/GaInP NW solar cell grown by molecular beam epitaxy on a patterned Si substrate, and we present an in-depth investigation of its optoelectronic properties and limitations. We report a power conversion efficiency of almost 3.7%, and a state-of-the-art open-circuit voltage (VOC) for a NW array solar cell on Si of 0.65 V. We also present the first quantification of the quasi-Fermi level splitting in NW array solar cells using hyperspectral photoluminescence measurements. A value of 0.84 eV is obtained at 1 sun (1.01 eV at 81 suns), which is significantly higher than qVOC. It indicates NWs with a better intrinsic optoelectronic quality than what could be expected from TEM images or deduced from electrical measurements. Optical and electronic simulations provide insights into the main absorption and electrical losses, and guidelines to design and fabricate higher-efficiency devices. It suggests that improvements at the n-type contact (GaInP/ITO) are key to unlocking the potential of next generation NW solar cells.

Light induced quasi-Fermi level splitting in molecular semiconductor alloys

Quasi-Fermi-level splitting in molecular semiconductor alloys was studied.

Understanding the Limitations of Charge Transporting Layers in Mixed Lead–Tin Halide Perovskite Solar Cells

Lead–tin (Pb/Sn) mixed perovskites are considered as promising photovoltaic materials owing to their adjustable bandgap and excellent optoelectronic properties. The low‐bandgap perovskite solar cells (PSCs) based on lead–tin mixed perovskites play a critical role in the overall performance of perovskite‐based tandem devices. Nevertheless, the current record efficiencies for Pb/Sn PSCs are mostly reported in devices with p–i–n configuration rather than n–i–p, which restricts the further development of conventional perovskite‐based tandem solar cells. Herein, this work systematically investigates the influence of the interlayers on the performance of low‐bandgap PSCs by analyzing the energy losses in both n–i–p and p–i–n devices. Quasi‐Fermi level splitting (QFLS) analysis of pristine films and films covering charge extraction layers reveals that the electron transport layer/perovskite interface is dominating the VOC losses. A joint experimental–simulative approach quantitatively determines the interface defect density to be more than one order in magnitude larger for the n–i–p architecture. Among the polymeric hole transport layers investigated for n–i–p devices, poly(3‐hexylthiophen‐2,5‐diyl) (P3HT) exhibits the most favorable energy‐level alignment to Pb/Sn perovskites. These results clarify the nature of VOC losses in Pb/Sn perovskites and highlight the necessity to develop electron extraction layers with a significantly reduced interface defect density.

Fermi-Surface Modulation of Graphene Synergistically Enhances the Open-Circuit Voltage and Quantum Efficiency of Photovoltaic Solar-Blind Ultraviolet Detectors.

Increasing the open-circuit voltage (VOC) is of a great significance to achieve high photoelectric conversion efficiency in photovoltaic applications. Here, we present a simple NO2 doping strategy that can significantly modulate the VOC of graphene-based solar-blind ultraviolet photodetectors from 0.96 to 1.84 V. The intriguing result can be demonstrated by the fact that NO2 doping lowers the Fermi surface of graphene and thus enhances quasi-Fermi level splitting of the whole device under illumination. The >103% increase of both external quantum efficiency and photoresponsivity compared to before doping is the result of a 0.88 V increase in the VOC. Our work sheds light on the forming mechanism of VOC in graphene-based photovoltaic detectors and further suggests alternative pathways to enhance the VOC of photovoltaic devices with high efficiency.

Efficient and stable mesoscopic perovskite solar cell in high humidity by localized Dion-Jacobson 2D‐3D heterostructures

Mesoscopic perovskite solar cells (PSCs) suffer from poor stability despite high efficiency, due to irreversible decomposition of perovskite in moisture. Herein, we demonstrate localized Dion−Jacobson (DJ) 2D–3D heterostructure (L2D–3DH), in which DJ 2D perovskite is formed on the localized surface of 3D perovskite without fully covering it, to improve the efficiency and stability of PSCs simultaneously. Different from conventional 2D–3D composite, L2D–3DH does not hinder the charge transfer between perovskite and hole transport layer due to partial coverage of DJ 2D yet effectively improve the moisture stability. Moreover, control of crystal orientation is not critical for efficient charge transport. As a result, L2D–3DH yields a power conversion efficiency of 20.1%, which is higher than that obtained from pure 3D perovskite or conventional 2D–3D composite. The improved PCE can be attributed to the passivation of 3D perovskite with DJ 2D plates, which reduces the trap density, suppresses non-radiative recombination, and enhances quasi-Fermi level splitting. The L2D–3DH-based device retains 86% of the initial efficiency after 1300 h under high humidity (70% RH). This work offers a simple and effective way to improve the efficiency and stability of PSCs by incorporating the DJ 2D in 3D perovskite.

Semiconductor thermionics for next generation solar cells: photon enhanced or pure thermionic?

Semiconductors have been used in solar energy conversion for decades based on the photovoltaic effect. An important challenge of photovoltaics is the undesired heat generated within the device. An alternative approach is thermionics, which uses the thermal excitation of electrons from an emitter to a collector across a vacuum gap. If the emitter is a p-type semiconductor, the photogeneration-induced quasi-Fermi level splitting can reduce the effective barrier for electron emission—a mechanism used by a photon enhanced thermionic emission device. Here, we evaluate the prospects of this alternative solar conversion technology considering different semiconductor materials and thermionic device configurations. We also reveal that whether such a device operates in the photon enhanced or purely thermionic mode, depends on the complex interplay among materials properties, device physics and solar concentration level.

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

Direct Quantification of Quasi-Fermi-Level Splitting in Organic Semiconductor Devices

Non-radiative losses to the open-circuit voltage are a primary factor in limiting the power conversion efficiency of organic photovoltaic devices. The dominant non-radiative loss is intrinsic to the active layer and can be determined from the quasi-Fermi level splitting (QFLS) and the radiative thermodynamic limit of the photovoltage. Quantification of the QFLS in thin film devices with low mobility is challenging due to the excitonic nature of photoexcitation and additional sources of nonradiative loss associated with the device structure. This work outlines an experimental approach based on electro-modulated photoluminescence, which can be used to directly measure the intrinsic non-radiative loss to the open-circuit voltage; thereby, quantifying the QFLS. Drift-diffusion simulations are carried out to show that this method accurately predicts the QFLS in the bulk of the device regardless of device-related non-radiative losses. State-of-the-art PM6:Y6-based organic solar cells are used as a model to test the experimental approach, and the QFLS is quantified and shown to be independent of device architecture. This work provides a method to quantify the QFLS of organic solar cells under operational conditions, fully characterizing the different contributions to the non-radiative losses of the open-circuit voltage. The reported method will be useful in not only characterizing and understanding losses in organic solar cells, but also other device platforms such as light-emitting diodes and photodetectors.

2D/3D perovskite engineering eliminates interfacial recombination losses in hybrid perovskite solar cells

<h2>Summary</h2> Interface engineering and design is paramount in the optimization of a multilayer device stack. This stands true for multi-dimensional (2D/3D) perovskite-based solar cells, in which high efficiency can be combined with promising device durability. However, the complex function of the 2D/3D device interfaces remains vague. Here, we provide the exact knowledge on the interface energetics and demonstrate that the 2D/3D perovskite interface forms a <i>p-n</i> junction that is capable of reducing the electron density at the hole transport layer interface and ultimately suppresses interfacial recombination. As a consequence, we demonstrate photovoltaic devices with an enhanced fill factor (FF) and open-circuit voltage (V_OC) of 1.19 V, which approaches the potential internal quasi-Fermi level splitting (QFLS) voltage of the perovskite absorber, nullifying the interfacial losses. We thus identify the essential parameters and energetic alignment scenario required for 2D/3D perovskite systems to surpass the current limitations of hybrid perovskite solar cell performances.

Reconciliation of dipole emission with detailed balance rates for the simulation of luminescence and photon recycling in perovskite solar cells.

A theoretical description of light emission, propagation and re-absorption in semiconductor multilayer stacks is derived based on the transverse Green's function of the electromagnetic field in the presence of a complex dielectric. The canonical dipole emission model is parametrized in terms of the local optical material constants and the local quasi-Fermi level splitting using the detailed balance relation between local absorption and emission rates. The framework obtained in this way is shown to reproduce the generalized Kirchhoff relations between the luminescent emission from metal halide perovskite slabs under uniform excitation and the slab absorptance of light with arbitrary angle of incidence. Use of the proper local density of transverse photon states in the local emission rate includes cavity effects in the generalized Planck law for internal spontaneous emission, which are neglected in the conventional Van Roosbroeck-Shockley formalism and avoids spurious divergencies due to non-radiative energy transfer via longitudinal modes. Finally, a consistent treatment of re-absorption provides the local rate of secondary photogeneration required for the consideration of photon recycling in an opto-electronic device simulator that includes the effects of charge transport.

Impact of Photoluminescence Reabsorption in Metal‐Halide Perovskite Solar Cells

The precise quantification of the impact of photoluminescence reabsorption (PLr) in metal‐halide perovskite solar cells (PSCs) remains challenging. Herein, the PLr effect is examined by combined time‐resolved photoluminescence (TRPL) spectroscopy and time‐resolved terahertz spectroscopy (TRTS) and a model is proposed that relates both, the PLr and nonradiative recombination rate (knr) to the quasi‐Fermi‐level splitting (QFLS). PLr is shown to be beneficial for QFLS when the nonradiative recombination rate (knr) is below a critical value of ≈7 × 105 s−1; at high knr PLr is detrimental to QFLS. By incorporating PLr into a two‐diode model that allows extraction of the effective knr, the series resistance (rs), and the shunt resistance (rsh) in PSCs, it is found that neglecting PLr overestimates the effective knr, although it does not affect the value of rs and rsh. The findings herein provide insight into the impact of the PLr effect on metal‐halide PSCs.

Small grains as recombination hot spots in perovskite solar cells

Summary Non-radiative recombination in the perovskite bulk and at its interfaces prohibits the photovoltaic performance from reaching the Shockley-Queisser limit. While interfacial recombination has been widely discussed and demonstrated, bulk recombination and especially the influence of grain boundaries remain under debate. Most studies explore the role of grain boundaries on perovskite films rather than devices, making it difficult to link the film properties with those of the devices. Here, we systematically investigate the effects of grain boundaries on the performance of perovskite solar cells by two different methods. By combining experimental characterization with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and hence is inversely proportional to the grain area to the power of 3/2. Consequently, the prevalence of small grains—which act as recombination hot spots—across the perovskite active layer dictates the photovoltaic performance of the perovskite solar cells.

Reducing Energy Disorder of Hole Transport Layer by Charge Transfer Complex for High Performance p-i-n Perovskite Solar Cells.

Solution-processed organic semiconductor charge-transport layers (OS-CTLs) with high mobility, low trap density, and energy level alignment have dominated the important progress in p-i-n planar perovskite solar cells (pero-SCs). Unfortunately, their inevitable long chains result in weak molecular stacking, which is likely to generate high energy disorder and deteriorate the charge-transport ability of OS-CTLs. Here, a charge-transfer complex (CTC) strategy to reduce the energy disorder in the OS-CTLs by doping an organic semiconductor, 4,4'-(4,8-bis(5-(trimethylsilyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (BDT-Si), in a commercial hole-transport layer (HTL), poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine (PTAA), is proposed. The formation of the CTC makes the PTAA conjugated backbone electron-deficient, resulting in a quinoidal and stiffer character, which is likely to planarize the PTAA backbone and enhance the ordering of the film in nanoscale. The resultant HTL exhibits a reduced energy disorder, which simultaneously promotes hole transport in the HTL, hole extraction at the interface, energy level alignment, and quasi-Fermi level splitting in the device. As a result, the p-i-n planar pero-SCs with optimized HTL exhibit the best power conversion efficiency of 21.87% with good operating stability. This finding demonstrates that the CTC strategy is an effective way to reduce the energy disorder in HTLs and to improve the performance of planar pero-SCs.

Lifetime, quasi-Fermi level splitting and doping concentration of Cu-rich CuInS2 absorbers

Cu(In,Ga)S2–based solar cells have been shown by Hiroi et al (Hiroi et al 2015 IEEE Journal of Photovoltaics 6 309–312) to achieve higher efficiencies with absorbers processed at high deposition temperatures. Additionally, it is known for CuInS2 cells that the main improvement from higher deposition temperatures is the reduction in the density of deep defects and increased quasi-Fermi level splitting. The increased quasi-Fermi level splitting could result from a reduction in the rate of recombination or from an increase in doping concentration. To investigate which effect is the dominant one, we perform time-resolved photoluminescence measurements and estimate the doping concentration from carrier lifetime and quasi-Fermi level splitting. We find no changes in the effective lifetime, which is in the range of 200 ps. The doping concentration increases from 1016 cm−3 to 1017 cm−3. Our study shows that the increase in quasi-Fermi level splitting with higher deposition temperatures is not due to reduction in non-radiative recombination but due to increased doping concentration.