Photocurrent Loss Research Articles

One-micron-thick organic indoor light harvesters with low photocurrent loss and fill factors over 67%

A diluting strategy with insulating polystyrene is adopted in indoor light harvesters, and achieves high-performance 1-micron-thick photovoltaic device with low trap state density and a record fill factor over 67% under light-emitting diode lights.

A universal tactic of using Lewis-base polymer-CNTs composites as additives for high performance cm2-sized and flexible perovskite solar cells

Lewis-base polymers have been widely utilized as additives to act as a template for the perovskite nucleation/crystal growth and passivate the under-coordinated Pb2+ sites. However, it is uncovered in this work that the polymer on the perovskite grain boundaries would significantly hinder the charge transport due to its low conductivity, which brings about free carrier recombination and photocurrent losses. To circumvent this issue while fully exploiting the benefits of polymers in passivating the trap states in perovskite, we incorporate highly conductive multiwall carbon nanotubes (CNTs) with Lewis-base polymers as co-additives in the perovskite film. Functionalizing the CNTs with -COOH group enables a selective hole-extraction and charge transport from perovskite to the hole transporting materials (HTM). By studying the charge transporting and recombination dynamics, we revealed the individual role of the polymer and CNTs in passivating the trap states and facilitating the charge transport, respectively. As a result, the perovskite solar cells (PSCs) with polymer-CNTs composites exhibit an impressive PCE of 21.7% for a small-area device (0.16 cm2) and 20.7% for a large-area device (1.0 cm2). Moreover, due to the superior mechanical flexibility of both polymer and CNTs, the polymer-CNTs composites incorporation in the perovskite film encourages the fabrication of flexible PSCs (f-PSCs) with an impressive PCE of 18.3%, and a strong mechanical durability by retaining 80% of the initial PCE after 1,000 times bending. In addition, we proved that the selection criteria of the polymers can be extended to other long-chain Lewis-base polymers, which opens new possibilities in design and synthesis of inexpensive material for this tactic towards the fabrication of high performance large-area PSCs and f-PSCs.

Minimization of Optical Losses in Colorful Semitransparent Organic Solar Cells Using Antimony Oxide Cavity-Based Fabry-Pérot Etalon Electrodes

Semitransparent organic solar cells (STOSCs) are a technology that combines the benefits of visible light transparency and light-to-electrical energy conversion. One of the greatest opportunities for STOSCs is their integration into windows and skylights in energy-sustainable buildings. For this application, the aesthetic aspects of solar cells may be as important as their electrical performance. Here, our strategy enables to achieve high-quality and colorful STOSCs using Fabry-Pérot etalon-type electrodes. These electrodes are composed of an antimony oxide (Sb2O3) cavity layer and two thin Ag mirrors. These dichroic tri-layer structures perform two functions as top conducting electrodes and color filters. The electrodes are fabricated by thermal evaporation of smooth sheets of Ag and metal oxide thin films, with appropriate thicknesses calculated by the transfer matrix methodology. In this manner, dichroic electrodes with a high spectral purity were achieved with transmission wavelengths that could be accurately tuned throughout the visible spectrum by varying the Sb2O3 cavity thickness. These dual-function electrodes were applied to photovoltaic devices and displayed vivid colors, natural transparency, and good performance as compared to devices that use conventional metal electrodes. Furthermore, to achieve saturated colors and low photocurrent losses, active layer materials were selected such that their transmission peaks matched the transmission maxima of the electrodes. These strategies for colorful STOSCs resulted in power conversion efficiencies (PCEs) of up to 12.1% and maximum transmittances (T MAX) of 22.5% in blue devices, PCEs of up to 9.71% and T MAX of 35.4% in green devices, and PCEs of up to 7.63% and T MAX of 34.7% in red devices.

Metallo-graphene enhanced upconversion luminescence for broadband photodetection under polychromatic illumination

We report electrostatically conjugated core- silica (SiO2) shell upconversion nanoparticle ([email protected]2), and gold nanorod (AuNR) nanocomposite (NC) combined with graphene to demonstrate > 200-fold UC fluorescence enhancement. Plasmonic AuNR and graphene, resulted in enhanced fluorescence in UCNP with a maximum for the SiO2 shell thickness of 7 nm supported by finite difference time domain simulation of electric field distribution. In addition to the conventionally reported spectroscopic evidence, the plasmon aided UC fluorescence enhancement was demonstrated by direct confocal fluorescence imaging also, which was corroborated by a ~40% decrease in fluorescence lifetime. Finally, we have fabricated a NC/graphene hybrid photodetector (PD) that showed broadband (455–980 nm) photoresponse, with photoresponsivity of ~5000 AW−1, and response times of 80–200 ms under 980 nm illumination. The multiphoton infrared (IR, ~980 nm) absorbing UCNPs also show interesting high energy (blue (B), green (G), and red (R)) photoresponse which is now conclusively attributed to one-photon absorption in the UCNPs. We report, probably for the first time, the performance of the hybrid PD under monochromatic and polychromatic illumination of B, G, R, B + G, B + R, G + R, and B + G + R, among others. The photocurrent under polychromatic conditions is dominated by the strongest monochromatic response, and does not scale with net power of the illumination. The loss in photocurrent is attributed to saturation in absorption, and photothermal heating. The fast response of the PD device has been demonstrated while detecting high frequency modulated AC remote controller signals, and attributed to the fast charge sweeping by the AuNRs.

Y-shaped organic dyes with D2–π–A configuration as efficient co-sensitizers for ruthenium-based dye sensitized solar cells

Three new Y-shaped organic dyes, coded CD1–3, have been synthesized for dye-sensitized solar cells (DSSCs). They have a D2–π–A configuration and been molecularly engineered to modulate the spectral properties and energy levels. For comparison, a D–π–A type sensitizer with one donor group (CD4) is synthesized as well. We have performed a combined experimental and theoretical investigation of their optical, electrochemical, photovoltaic, geometrical and electronic characteristics. These new Y-shaped dyes display intensive UV–vis absorption with high molar extinction coefficients. So their roles as co-sensitizers for ruthenium-based DSSCs are further evaluated. As a consequence, the co-sensitized devices exhibit improved power conversion efficiencies (PCEs) in the range of 7.24–7.96% due to the enhanced short-circuit photocurrents (Jsc) and open-circuit voltages (Voc), which are higher than the DSSCs sensitized with N719 (PCE = 6.75%) or Z907 (PCE = 6.55%) alone. In addition, the mechanisms of these improvements have been elucidated by incident photon to current efficiency (IPCE) and electrochemical impedance spectroscopy (EIS) studies. The Jsc is improved because CD1–3 can compensate for the photocurrent loss by I−/I3−; while the Voc is enhanced because the electron recombination process is retarded by these Y-shaped dyes upon co-sensitization.

Aesthetic and colorful: Dichroic polymer solar cells using high-performance Fabry-Pérot etalon electrodes with a unique Sb2O3 cavity

Abstract Semitransparent organic solar cells (STOSCs) are a technology that combines the benefits of visible light transparency and light-to-electrical energy conversion. One of the greatest opportunities for STOSCs is their integration into windows and skylights in energy-sustainable buildings. For this application, the aesthetic aspects of solar cells may be as important as their electrical performance. Here, our strategy enables to achieve high-quality and colorful STOSCs using Fabry-Perot etalon-type electrodes. These electrodes are composed of an antimony oxide (Sb2O3) cavity layer and two thin Ag mirrors. These dichroic tri-layer structures perform two functions as top conducting electrodes and color filters. These dual-function electrodes were applied to photovoltaic devices and displayed vivid colors, natural transparency, and good performance as compared to devices that use conventional metal electrodes. Furthermore, to achieve saturated colors and low photocurrent losses, active layer materials were selected such that their transmittance peaks matched the transmittance maxima of the electrodes. These strategies for colorful STOSCs result in power conversion efficiencies (PCEs) of up to 13.3% and maximum transmittances (TMAX) of 24.6% in blue devices, PCEs of up to 9.71% and TMAX of 35.4% in green devices, and PCEs of up to 7.63% and TMAX of 34.7% in red devices.

Dominant recombination mechanism in perovskite solar cells: A theoretical study

Different recombination losses have been shown to play a role in the photocurrent and photovoltage loss of lead halide perovskite solar cells. While it is believed that trap density is extremely small in high-efficiency perovskite cells, trap-assisted recombination cannot be ruled out due to the existence of tail states, for instance at the grain boundaries. In this study, the optical and electrical performance of a perovskite solar cell of the structure: ITO/PEDOT:PSS/CH3NH3PbI3/PC61BM/LiF/Al was modelled using numerical drift-diffusion simulations with embedded optical modelling via the transfer matrix method. The total recombination was calculated as the sum of bimolecular recombination and recombination via tail states. The influence of charge carrier mobility on photovoltaic parameters (short-circuit current, fill factor, open-circuit voltage, and power conversion efficiency) of perovskite solar cells was studied. Next, the impact of Urbach energy on the recombination process was investigated by simulation of bimolecular and tail state recombination rates as a function of voltage for various values of Urbach energy. It was shown that by increasing the Urbach energy, the bimolecular recombination rate decreases, but the tail state recombination rate increases at all voltages. Furthermore, the slope of the open-circuit voltage-light intensity curve, the light ideality factor, demonstrates the dominant recombination at different Urbach energy levels. It was found that the slope increases from 1.04 at EU = 25 meV to1.65 at EU = 150 meV. It can be concluded that the bimolecular recombination is the dominant recombination at low EU, while the tail state recombination overcomes the bimolecular recombination at high EU.

Colloidal AgBiS2 nanocrystals with reduced recombination yield 6.4% power conversion efficiency in solution-processed solar cells

Abstract AgBiS2 nanocrystals have recently emerged as a RoHS compliant material for photovoltaics. This heavy-metal-free ternary chalcogenide can be prepared from earth-abundant precursors, is solution-processable and presents a high absorption coefficient as well as a suitable bandgap for solar cell applications. However, the full potential of AgBiS2 nanocrystals is yet to be realised; the highest efficiencies reported so far used very thin (~35 nm) absorbing layers due to the limited carrier transport and trap-assisted recombination that hinder the performance of thicker layers. In this work we implement a synthetic route to obtain larger size colloidal AgBiS2 nanocrystals, which in turn allows to fabricate thin film solar cells with higher mobility and reduced trap-assisted recombination, resulting in a power conversion efficiency of 6.4%, due to a photocurrent increase of 4 mA/cm2 compared to prior reports. We conclude by discussing on the main current challenges underpinning the photovoltaic performance of this material as well as strategies to further reduce the voltage and photocurrent losses.

Distinguishing Energy- and Charge-Transfer Processes in Layered Perovskite Quantum Wells with Two-Dimensional Action Spectroscopies.

Interest in photovoltaic devices based on layered perovskites is motivated by their tunable optoelectronic properties and stabilities in humid conditions. In these systems, quantum wells with different sizes are organized to direct energy and charge transport between electrodes; however, these relaxation mechanisms are difficult to distinguish based on conventional transient absorption techniques. Here, two-dimensional "action spectroscopies" are employed to separately target processes that lead to the production of photocurrent and energy loss due to fluorescence emission. These measurements show that energy transfer between quantum wells dominates the subnanosecond time scale, whereas electron transfer occurs at later times. Overall, this study suggests that while the intense exciton transitions promote light harvesting, much of the absorbed energy is lost by way of spontaneous emission. This limitation may be overcome with alternate layered perovskite systems that combine smaller exciton binding energies with large absorbance cross sections in the visible spectral range.

The Critical Role of AlInP Window Design in III–V Rear-Emitter Solar Cells

This article highlights the critical role of window design on short circuit carrier collection in rear-emitter solar cells, as demonstrated through modeling and experiment using metamorphic GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> . Ultimately, if the window design is not carefully considered, surface depletion caused by Fermi level pinning at the window/air interface can extend into regions of active collection resulting in a large increase in effective window/base interface recombination velocity. This was experimentally shown here to result in a potential AM1.5G photocurrent loss of 8.1 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , or nearly 50%, based on integrated internal quantum efficiency (IQE). Associated IQE modeling and curve fitting indicate that the effective IRV at the GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.64</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.36</sub> P interface is increased by multiple orders of magnitude when the window is not sufficiently thick or doped to fully contain the surface depletion to within the window layer. Calculations of the surface depletion depth as a function of doping and the surface Fermi pinning energy level provides insight into the fundamental limits of window thickness. This allows the estimation of the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.64</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.36</sub> P surface pinning level to be at least 1.25 eV below the conduction band edge, or in the bottom half of the bandgap.

New organic dyes with varied arylamine donors as effective co-sensitizers for ruthenium complex N719 in dye sensitized solar cells

Three new organic dyes (TD1–TD3) featured with donor–π–acceptor architecture are synthesized and applied to dye-sensitized solar cells (DSSCs). In these sensitizers, an alkyl oligothiophene π-spacer is used to link the 2-cyanoacrylic acid acceptor with varied arylamines donors, i.e. carbazole (TD1), phenothiazine (TD2) or triphenylamine (TD3). The effects of different donor units on the photophysical, electrochemical, and photovoltaic properties of sensitizers are investigated in detail. They all show intensive UV–vis absorption around 400 nm with rod-like molecular structures. The co-sensitizations of TD1–TD3 with ruthenium complex N719 are further evaluated. Consequently, the co-sensitized DSSCs demonstrate superior photovoltaic performances with power conversion efficiencies (PCEs) ranging from 7.60% to 8.02% in relation to the cell with N719 (PCE = 7.29%) alone, which is mainly ascribed to the higher short-circuit photocurrents (Jsc) in combination with larger open-circuit voltages (Voc). Moreover, the origins of these improvements are examined by incident photon to current efficiency (IPCE) spectra and electrochemical impedance spectroscopy (EIS) investigations. The Jsc is improved because TD1–TD3 could compensate for the photocurrent loss induced by I−/I3−; whereas, the Voc is enhanced owing to the fact that the electron recombination process is retarded upon co-sensitization.

Tail state limited photocurrent collection of thick photoactive layers in organic solar cells

We analyse organic solar cells with four different photoactive blends exhibiting differing dependencies of short-circuit current upon photoactive layer thickness. These blends and devices are analysed by transient optoelectronic techniques of carrier kinetics and densities, air photoemission spectroscopy of material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We conclude that, for the device series studied, the photocurrent loss with thick active layers is primarily associated with the accumulation of photo-generated charge carriers in intra-bandgap tail states. This charge accumulation screens the device internal electrical field, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail state distribution and density and increase the maximal photoactive thickness for efficient operation. Our work suggests that selecting organic photoactive layers with a narrow distribution of tail states is a key requirement for the fabrication of efficient, high photocurrent, thick organic solar cells.

Cold Crystallization Temperature Correlated Phase Separation, Performance, and Stability of Polymer Solar Cells

Summary Fabrication of non-fullerene (NF)-based polymer solar cells (PSCs) has been an arduous task involving various approaches to optimize the morphology of blend film for superior performance. In this work, unique exothermic peaks can be observed in differential scanning calorimetry first heating scan of the blend films, which are assigned to the cold crystallization temperature (TCC) of NF acceptors when blended with different degree of crystallinity polymers, and TCC contains the phase information at quenched stage of the film development. A linear correlation between the phase separation parameters and TCC was established, which helps to select the appropriate donor for a specific acceptor. Utilizing multiple polymer-NF blend systems, we report an expectation TCC value between 210°C and 230°C, which would promise balanced domain size, purity, and consequentially superior performance. Further investigation revealed that TCC can also be used to evaluate the thermal stability.

Photovoltage loss in semi-transparent organic photovoltaic devices

Abstract The use of semi-transparent photovoltaic devices causes an inevitable loss of photocurrent, as light transmitted has not been absorbed. This trivial effect also leads to a loss of photovoltage, an effect partially due to the lower photocurrent but also due to the geometry of the semi-transparent photovoltaic device. We here demonstrate and evaluate this photovoltage loss in semi-transparent organic photovoltaic devices, compared with non-transparent solar cells of the same material. Semi-transparent solar cells in addition introduce photovoltage loss when formed by lamination. We document and analyze these effects for a number of polymer blends in the form of bulk heterojunctions.

Origin of Photocurrent and Voltage Losses in Organic Solar Cells

AbstractOrganic solar cells (OSCs) have recently achieved power conversion efficiencies (PCEs) of over 16%. However, there still exist large losses in photocurrent and/or voltage in state‐of‐the‐art OSCs, making the PCEs still far below those of inorganic counterparts. Here, the factors and electronic processes for photocurrent and voltage losses are identified and discussed in the framework of device physics and photophysics. To simultaneously obtain both high photocurrent density and low voltage loss toward 20% PCEs, it is crucial to suppress the non‐radiative (NR) recombination of the lowest charge‐transfer (CT) state at the donor–acceptor interface. In principle, theoretical simulations can provide molecular insight into the origin of these losses, which is essential to guide material design. In particular, the authors highlight the importance of the local interface morphologies and the vibronic couplings on suppressing the NR decay of the lowest CT state according to recent theoretical studies.

Coalescence-Driven Simultaneous Enhancement and Quenching of the Excited States of Silver Nanoclusters

Despite the successful launch of Au nanoclusters (NCs) in light energy conversion applications, the utilization of Ag NCs has been limited by their instability under continuous illumination. The main cause of photoinduced degradation pathways remains elusive. Hence, understanding the underlying mechanism behind the low stability is an urgent task to provide a new impetus for the development of Ag NCs. The slow regeneration of Ag NCs by a redox couple leads to holes in the NCs, which could result in either the photoetching of NCs or the transformation into plasmonic nanoparticles. Transient absorption spectroscopy reveals that Ag NCs coalesce into plasmonic nanoparticles and begin to experience two conflicting effects (plasmonic enhancement and quenching effect) during this in situ transformation. It also discloses that the quenching effect prevails over the plasmonic enhancement, which eventually leads to photocurrent loss under illumination.

Electrical Signatures of Corrosion and Solder Bond Failure in c-Si Solar Cells and Modules

Moisture- and temperature-activated corrosion of metal fingers, mechanical stress induced delamination, and failure of solder bonds rank among the leading failure mechanisms of solar modules. The physics of moisture ingress, diffusion, and reaction have been explored in detail, but the electrical implications of corrosion and delamination on specific front-surface grid geometry is not fully understood. In this paper, we show that the module efficiency loss due to corrosion, delamination, and solder bond failure (CDS) involves a complex interplay of voltage/current redistribution, reflected as a loss in photocurrent, as well as decrease/increase in shunt/series resistances. The analysis presented in this paper will redefine the interpretation of experimental I–V characteristic features due to degradation mechanisms, integrate a variety of scattered and counter-intuitive experimental results within a common theoretical framework, and inform CDS-resistant grid design for solar modules.

Dual Protection Layer Strategy to Increase Photoelectrode–Catalyst Interfacial Stability: A Case Study on Black Silicon Photoelectrodes

AbstractPhotoelectrode degradation under harsh solution conditions continues to be a major hurdle for long‐term operation and large‐scale implementation of solar fuel conversion. In this study, a dual‐layer TiO2 protection strategy is presented to improve the interfacial durability between nanoporous black silicon and photocatalysts. Nanoporous silicon photocathodes decorated with catalysts are passivated twice, providing an intermediate TiO2 layer between the substrate and catalyst and an additional TiO2 layer on top of the catalysts. Atomic layer deposition of TiO2 ensures uniform coverage of both the nanoporous silicon substrate and the catalysts. After 24 h of electrolysis at pH = 0.3, unprotected photocathodes layered with platinum and molybdenum sulfide retain only 30% and 20% of their photocurrent, respectively. At the same pH, photocathodes layered with TiO2 experience an increase in photocurrent retention: 85% for platinum‐coated photocathodes and 91% for molybdenum sulfide–coated photocathodes. Under alkaline conditions, unprotected photocathodes experience a 95% loss in photocurrent within the first 4 h of electrolysis. In contrast, TiO2‐protected photocathodes maintain 70% of their photocurrent during 12 h of electrolysis. This approach is quite general and may be employed as a protection strategy for a variety of photoabsorber–catalyst interfaces under both acidic and basic electrolyte conditions.

Dethermalization of carriers in GaAs solar cells with quantum objects

Temperature photovoltaic dependencies have been measured at 90–350 K for a single-junction GaAs-based p-i-n solar cell and for a similar solar cell with imbedded in i-region In0.4Ga0.6As quantum objects. A theoretical approach for analyzing temperature dependencies of open circuit voltage has been proposed. A dethermalization of quantum objects has been observed at temperatures below 170 K, and the temperature of full dethermalization is estimated to be 80 K. It has been shown that dethermalized quantum objects are not involved in the photovoltaic process and may cause photocurrent losses, while, when thermalized, quantum objects give a photocurrent increment. It has been found that the energy difference between optic transitions in GaAs and in quantum objects is independent on temperature. Analysis of the temperature dependencies has allowed for the estimation of effective band gap energy of quantum objects which, at room temperature, was found to be 1.2 eV.

Quantifying Photocurrent Loss of a Single Particle-Particle Interface in Nanostructured Photoelectrodes.

Particle-particle interfaces are ubiquitous in nanostructured photoelectrodes and photovoltaics, which are important devices for solar energy conversion. These interfaces are expected to cause performance losses in these devices, but how much loss they would incur is poorly defined. Here we use a subparticle photoelectrochemical current measurement, in combination with specific photoelectrode configurations, to quantify the current losses from single particle-particle interfaces formed between individual TiO2 nanorods operating as photoanodes in aqueous electrolytes. We find that a single interface leads to ∼20% photocurrent loss (i.e., ∼80% retention of the original current). Such quantitative, first-of-its-kind, information provides a metric for guiding the optimization and design of nanostructured photoelectrodes and photovoltaics.