Photocurrent Loss Research Articles

High‐Performance and Stable All‐Polymer Solar Cells Using Donor and Acceptor Polymers with Complementary Absorption

To explore the advantages of emerging all‐polymer solar cells (all‐PSCs), growing efforts have been devoted to developing matched donor and acceptor polymers to outperform fullerene‐based PSCs. In this work, a detailed characterization and comparison of all‐PSCs using a set of donor and acceptor polymers with both conventional and inverted device structures is performed. A simple method to quantify the actual composition and light harvesting contributions from the individual donor and acceptor is described. Detailed study on the exciton dissociation and charge recombination is carried out by a set of measurements to understand the photocurrent loss. It is unraveled that fine‐tuned crystallinity of the acceptor, matched donor and acceptor with complementary absorption and desired energy levels, and device architecture engineering can synergistically boost the performance of all‐PSCs. As expected, the PBDTTS‐FTAZ:PNDI‐T10 all‐PSC attains a high and stable power conversion efficiency of 6.9% without obvious efficiency decay in 60 d. This work demonstrates that PNDI‐T10 can be a potential alternative acceptor polymer to the widely used acceptor N2200 for high‐performance and stable all‐PSCs.

Photovoltaic Devices: Opto-Electro-Thermal Physics and Modeling.

An opto-electro-thermal simulation of solar cells (SCs) is presented by addressing optoelectronic and thermodynamic responses simultaneously. The photocurrent losses due to carrier recombinations and the intrinsic heat generation (thermalization/Joule/Peltier/recombination heat) and dissipation (convective/radiative cooling) processes in the SCs are investigated quantitatively.

Electric Field and Mobility Dependent First‐Order Recombination Losses in Organic Solar Cells

The origin of photocurrent losses in the power‐generating regime of organic solar cells (OSCs) remains a controversial topic, although recent literature suggests that the competition between bimolecular recombination and charge extraction determines the bias dependence of the photocurrent. Here the steady‐state recombination dynamics is studied in bulk‐heterojunction OSCs with different hole mobilities from short‐circuit to maximum power point. It is shown that in this regime, in contrast to previous transient extracted charge and absorption spectroscopy studies, first‐order recombination outweighs bimolecular recombination of photogenerated charge carriers. This study demonstrates that the first‐order losses increase with decreasing slower carrier mobility, and attributes them to either mobilization of charges trapped at the donor:acceptor interface through the Poole–Frenkel effect, and/or recombination of photogenerated and injected charges. The dependence of both first‐order and higher‐order losses on the slower carrier mobility explains why the field dependence of OSC efficiencies has historically been attributed to charge‐extraction losses.

Polycrystalline Cu2O photovoltaic devices incorporating Zn(O,S) window layers

The tunability of the Zn(O,S) conduction band edge makes it an ideal, earth-abundant heterojunction partner for Cu2O, whose low electron affinity has limited photovoltaic performance with most other heterojunction candidates. However, to date Cu2O/Zn(O,S) solar cells have exhibited photocurrents well below the entitled short-circuit current in the detailed balance limit. In this work, we examine the sources of photocurrent loss in Cu2O/Zn(O,S) solar cells fabricated by sputter deposition of Zn(O,S) on polycrystalline Cu2O substrates grown by thermal oxidation of Cu foils. X-ray photoelectron spectra reveal that Zn(O,S) deposited at room temperature leads to a thin layer of ZnSO4 at the Zn(O,S)/Cu2O interface that impedes current collection and limits the short circuit current density to 2mA/cm2. Deposition of Zn(O,S) at elevated temperatures decreases the presence of interfacial ZnSO4 and therefore the barrier to photocurrent collection. Optimal photovoltaic performance is achieved at a Zn(O,S) deposition temperature of 100°C, which enables an increase in the short circuit current density to 5mA/cm2, although a small ZnSO4 layer is still present. Deposition at temperatures above 100°C leads to a reduction in photovoltaic performance. Spectral response measurements indicate the presence of a barrier to photocurrent and exhibit a strong dependence on voltage and light bias, likely due to the photodoping of Zn(O,S) layer.

Cu2O Photocathode for Low Bias Photoelectrochemical Water Splitting Enabled by NiFe-Layered Double Hydroxide Co-Catalyst.

Layered double hydroxides (LDHs) are bimetallic hydroxides that currently attract considerable attention as co-catalysts in photoelectrochemical (PEC) systems in view of water splitting under solar light. A wide spectrum of LDHs can be easily prepared on demand by tuning their chemical composition and structural morphology. We describe here the electrochemical growth of NiFe-LDH overlayers on Cu2O electrodes and study their PEC behavior. By using the modified Cu2O/NiFe-LDH electrodes we observe a remarkable seven-fold increase of the photocurrent intensity under an applied voltage as low as −0.2 V vs Ag/AgCl. The origin of such a pronounced effect is the improved electron transfer towards the electrolyte brought by the NiFe-LDH overlayer due to an appropriate energy level alignment. Long-term photostability tests reveal that Cu2O/NiFe-LDH photocathodes show no photocurrent loss after 40 hours of operation under light at −0.2 V vs Ag/AgCl low bias condition. These improved performances make Cu2O/NiFe-LDH a suitable photocathode material for low voltage H2 production. Indeed, after 8 hours of H2 production under −0.2 V vs Ag/AgCl the PEC cell delivers a 78% faradaic efficiency. This unprecedented use of Cu2O/NiFe-LDH as an efficient photocathode opens new perspectives in view of low biasd or self-biased PEC water splitting under sunlight illumination.

Impact of sequential annealing step on the performance of Cu2ZnSn(S,Se)4 thin film solar cells

In this study, we investigated influence of sequential heat treatment on the structural, morphological and optical properties of Cu2ZnSn(S,Se)4 (CZTSSe) thin films. Specifically, after preparation of sputtered Cu, ZnS, and Sn stacked layers, CZTSSe thin films were synthesized by sequential heat treatment (sulfurization follow by selenization). Analyses of the processed films show that non-uniform selenium distribution through the depth of the films can be obtained by introducing a controlled amount of selenium in the second heat treatment step. The top region of the film becomes selenium rich, while the bottom region is selenium poor. This structure has higher absorption coefficients than the uniform ones. In addition, the uneven composition distribution may lead to a bandgap gradient in the film, which can reduce the photocurrent loss. Finally, numerical modeling corroborates the potential of selenium gradient films for high efficiency CZTSSe solar cells.

Correlation between electrical parameters and defect states of polythiophene:fullerene based solar cell

We investigated the degradation of the poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C61-butyric acid methyl ester (PCBM) organic solar cells (OSCs) induced by several agents (ambient air and humid air in dark, and photo-oxidation in ambient air). Most of previous studies have focused on such a high degree of the organic film degradation which was far behind the OSC functionality. This study correlates degradation of OSC electrical parameters in time with defect states measured with charge deep-level transient spectroscopy on the same samples. The basic electrical parameters of the OSC, and the generation rate and the dissociation probability of exciton were determined from dark and light current–voltage measurements. Degradation agents were applied to the complete samples or directly to the active P3HT:PCBM layer before back Ca/Ag contact evaporation with the aim to discriminate contributions of particular solar cell regions to the degradation. The results show that a decrease of the OSC power conversion efficiency is mainly caused by the photocurrent loss in all kinds of degradation treatment. We observed the formation of oxygen-related defect states with the energy around 0.1eV above highest occupied molecular orbital of P3HT, which is in accordance with other published data. The increase of the defect states with the degradation time correlates with the photocurrent loss and enhancing exciton recombination. The OSC performance is substantially influenced by the degradation of the back contact and its interface with the active layer. This effect is even more serious than the P3HT:PCBM layer exposure to the ambient or humid air in dark. The fastest degradation was observed in the case of the active layer photo-oxidation.

Chalcogenization-Derived Band Gap Grading in Solution-Processed CuIn(x)Ga(1-x)(Se,S)₂ Thin-Film Solar Cells.

Significant enhancement of solution-processed CuIn(x)Ga(1-x)(Se,S)2 (CIGSSe) thin-film solar cell performance was achieved by inducing a band gap gradient in the film thickness, which was triggered by the chalcogenization process. Specifically, after the preparation of an amorphous mixed oxide film of Cu, In, and Ga by a simple paste coating method chalcogenization under Se vapor, along with the flow of dilute H2S gas, resulted in the formation of CIGSSe films with graded composition distribution: S-rich top, In- and Se-rich middle, and Ga- and S-rich bottom. This uneven compositional distribution was confirmed to lead to a band gap gradient in the film, which may also be responsible for enhancement in the open circuit voltage and reduction in photocurrent loss, thus increasing the overall efficiency. The highest power conversion efficiency of 11.7% was achieved with J(sc) of 28.3 mA/cm(2), V(oc) of 601 mV, and FF of 68.6%.

Protection of Si photocathode using TiO2 deposited by high power impulse magnetron sputtering for H2 evolution in alkaline media

Si is an excellent absorber material for use in photoelectrochemical (PEC) hydrogen production. Only a few studies have been done using Si in alkaline electrolyte for hydrogen evolution due to its poor chemical stability in high pH electrolyte, indicating that a chemically stable protection layer is essential. Here we investigate thin TiO2 films deposited by high power impulse magnetron sputtering (HiPIMS) as a protection layer for a p-type silicon photocathode for photoelectrochemical H2 evolution in a high pH electrolyte. The X-ray reflectometry analysis reveals that the HiPIMS process provides improved film density for TiO2 films (4.15g/cm3), and consequently results in a significantly less corroded Si surface. The Si photocathode protected by the HiPIMS grown TiO2 film along with Pt as co-catalyst produced a photocurrent onset potential of ~0.5V vs. RHE in 1M KOH and showed a 4% decay over 24h in KOH. In contrast, the sample with the TiO2 deposited using conventional DC sputtering technique of similar thickness shows 20% loss in photocurrent for the same time interval. Considering the fact that the experiments were carried out not in the cleanroom, much less corrosion loss can be obtained if done in dust-free condition. Hence, these results suggest the HiPIMS technique as an improved approach for the protection of photoelectrodes, which are unstable in alkaline solution.

Improvement of CH₃NH₃PbI₃ Formation for Efficient and Better Reproducible Mesoscopic Perovskite Solar Cells.

High-performance perovskite solar cells (PSCs) are obtained through optimization of the formation of CH3NH3PbI3 nanocrystals on mesoporous TiO2 film, using a two-step sequential deposition process by first spin-coating a PbI2 film and then submerging it into CH3NH3I solution for perovskite conversion (PbI2 + CH3NH3I → CH3NH3PbI3). It is found that the PbI2 morphology from different film formation process (thermal drying, solvent extraction, and as-deposited) has a profound effect on the CH3NH3PbI3 active layer formation and its nanocrystalline composition. The residual PbI2 in the active layer contributes to substantial photocurrent losses, thus resulting in low and inconsistent PSC performances. The PbI2 film dried by solvent extraction shows enhanced CH3NH3PbI3 conversion as the loosely packed disk-like PbI2 crystals allow better CH3NH3I penetration and reaction in comparison to the multicrystal aggregates that are commonly obtained in the thermally dried PbI2 film. The as-deposited PbI2 wet film, without any further drying, exhibits complete conversion to CH3NH3PbI3 in MAI solution. The resulting PSCs reveal high power conversion efficiency of 15.60% with a batch-to-batch consistency of 14.60 ± 0.55%, whereas a lower efficiency of 13.80% with a poorer consistency of 11.20 ± 3.10% are obtained from the PSCs using thermally dried PbI2 films.

Removal Mechanism and Defect Characterization for Glass-Side Laser Scribing of CdTe/CdS Multilayer in Solar Cells

Laser scribing is an important manufacturing process used to reduce photocurrent and resistance losses and increase solar cell efficiency through the formation of serial interconnections in large-area solar cells. High-quality scribing is crucial since the main impediment to large-scale adoption of solar power is its high-production cost (price-per-watt) compared to competing energy sources such as wind and fossil fuels. In recent years, the use of glass-side laser scribing processes has led to increased scribe quality and solar cell efficiencies; however, defects introduced during the process such as thermal effect, microcracks, film delamination, and removal uncleanliness keep the modules from reaching their theoretical efficiencies. Moreover, limited numerical work has been performed in predicting thin-film laser removal processes. In this study, a nanosecond (ns) laser with a wavelength at 532 nm is employed for pattern 2 (P2) scribing on CdTe (cadmium telluride) based thin-film solar cells. The film removal mechanism and defects caused by laser-induced micro-explosion process are studied. The relationship between those defects, removal geometry, laser fluences, and scribing speeds are also investigated. Thermal and mechanical numerical models are developed to analyze the laser-induced spatiotemporal temperature and pressure responsible for film removal. The simulation can well-predict the film removal geometries, transparent conducting oxide (TCO) layer thermal damage, generation of microcracks, film delamination, and residual materials. The characterization of removal qualities will enable the process optimization and design required to enhance solar module efficiency.

An effective way to analyse the performance limiting parameters of poly-crystalline silicon solar cell fabricated in the production line

This article focuses on the identification of key features that are responsible for the discrepancies in the performance of silicon solar cells fabricated on multicrystalline silicon under the identical conditions. In an experimental approach, direct current (DC) measurement coupled with alternating current (AC) characterisation technique has been employed. The scanning electron microscope analysis reveals an average grain size of few micrometres for all the solar cells and the top surface of least efficient solar cell contains the impurity precipitates with deep cone shaped holes or pits. The DC measurement reveals that the photocurrent density loss follows an exponential behaviour with respect to the current–voltage characteristics for all the solar cells. The analysis of −dV/dJ versus (JSC−J)−1 plot and the variation of ideality factor with junction voltage demonstrate that the higher resistive and recombination losses dominate the performance of least efficient solar cell. Impedance Spectroscopy (IS) technique is used to quantify and decouple the various photovoltaic parameters associated with the different physical processes. A lower value of shunt resistance and minority carrier lifetime along with the higher value of series resistance contribute to the higher resistive loss and surface recombination. The experimental results along with the analytical model provide an insight into the loss mechanisms and the use of a simple tool that can be integrated with the conventional photovoltaic testing.

Analysis of photo-current potentials and losses in thin film crystalline silicon solar cells

We present a detailed analysis of the photo-current potentials and losses in thin film crystalline silicon solar cells on glass. The effects of texturing the silicon backside, applying a diffuse back reflector and a textured anti-reflection foil were analysed. Light beam induced current measurements were used to determine the losses due to local effects like the absorber contact, cracks in the absorber and grain boundaries. Detailed loss analysis in combination with ray-tracing simulations showed that the maximum light trapping potential imposed by geometrical optics has nearly been achieved. The photocurrent losses due to incomplete carrier collection and parasitic absorption were accounted for using a theoretical model. For the investigated, textured, n-doped cell with reflector and anti-reflection foil, the short circuit current density (JSC) was 28.9mA/cm2 and the main loss factors were direct reflection (3.4mA/cm2), electrical shading effects due to the absorber contact (3.1mA/cm2) and incomplete carrier collection due to surface/bulk recombination (1.6mA/cm2). Using the presented light trapping scheme we obtained the following efficiencies: 11.8% for a p-doped and 12.1% for an n-doped crystalline silicon absorber. Finally, the potentials for efficiencies beyond 14% are discussed.

Experimental study of liquid-immersion III–V multi-junction solar cells with dimethyl silicon oil under high concentrations

In order to better apply direct liquid-immersion cooling (LIC) method in temperature control of solar cells in high concentrating photovoltaic (CPV) systems, electrical characteristics of GaInP/GaInAs/Ge triple-junction solar cells immersed in dimethyl silicon oil of 1.0–30.0mm thickness were studied experimentally under 500 suns and 25°C. Theoretical photocurrent losses caused by spectrum transmittance decrease from spectral absorption of silicon oil were estimated for three series sub-cells, and an in-depth analysis of the electrical performances changes of the operated cell in silicon oil was performed. Compared with cell performances without liquid-immersion, the conversion efficiency and the maximum output power of the immersed solar cell in silicon oil of 1.0mm thickness has increased from 39.567% and 19.556W to 40.572% and 20.083W respectively. However, the cell electrical performances decrease with increasing silicon oil thickness in the range of 1.0–30.0mm, and the efficiency and the maximum output power of the cell have become less than those without liquid-immersion when the silicon oil thickness exceeds 6.3mm.

Carrier depletion and electrical optimization of gallium arsenide plasmonic solar cell with a rear metallic grating

Plasmonic nanostructures have been extensively considered for photovoltaics due to the outstanding light-trapping capability; however, the intrinsic processes of carrier transport, recombination and collection have seldom been concerned. We report a complete optoelectronic investigation for plasmonic gallium arsenide solar cells (SCs) with a rear silver grating, by especially quantifying the plasmonics-induced photocurrent loss. It is found that, although the plasmonic design shows indeed much improved output photocurrent, its potential in improving the performance of SCs has not been fully exploited since a lot of energy has been wasted in the form of carrier depletion. A further design which electrically separates the plasmonic nanostructure from the core PN junction is verified to be an effective solution in improving the electrical performance of the SCs. The complete optoelectronic consideration is expected to advance the design of plasmonic SCs for thin-film and high-efficiency.

Improvement of Performance of Amorphous Silicon-Germanium Thin-Film Solar Modules With Large Width P2 Process Technology

In this paper, the laser power density is varied to increase the P2 laser spot size to investigate its effects on the performance of hydrogenated amorphous silicon-germanium (a-SiGe:H) thin-film solar modules. A larger P2 line spot size is expected to lower the series resistance of the modules, due to the increased contact area between the front and rear electrodes . However, increasing the laser power density faces several problems , which are: 1) damage of the SnO2 front electrode if the used power density exceeds the ablation threshold; 2) reduced quality of a-SiGe:H films; and 3) low spot size increasing rate at high power densities. The first can further lead to the presence of silicon oxide formation at the bottom of the P2 scribes. This paper demonstrates another method to obtain a larger spot size. A multiple P2 line scribing process has been performed to increase the spot size without deteriorating the film's quality and the module performance. Finally, a 6-P2 linewidth of about 160 $\mu $ m leads to a balance between the gain in fill factor and the loss in photocurrent. The optimal module conversion efficiency of 8.82%, which is 8.7% higher than that using a single P2 line process, can be obtained.

Root-cause failure analysis of photocurrent loss in polythiophene:fullerene-based inverted solar cells.

Metal oxide transport layers have played a crucial role in recent progress in organic photovoltaic (OPV) device stability. Here, we measure the stability of inverted and encapsulated polythiophene:fullerene cells with MoO3/Ag/Al composite anode in operational conditions combining solar radiation and 65 °C. Performance loss of over 50% in the first 100 h of the aging is dominated by a drop in the short-circuit current (Jsc). We reveal a concurrent loss in reflectance from 85% to 50% above 650 nm, which is below the optical gap of the used photoactive materials, hence, excluding any major degradation in the bulk of this layer. Correlating the responses of aged devices to a series of test structures comprised of ITO/ZnO cathode, MoO3/Ag, and MoO3/Ag/Al anodes and their combinations with the active layer allowed us to identify that the presence of Al causes the reduced reflectance in these devices, independent of the presence of the active layer. Systematic single-stress aging on the test structures further indicates that elevated heat is the cause of the reflectance loss. Cross-section transmission electron microscopy coupled with elemental analysis revealed the unsuspected role of Al; notably, it diffuses through the entire 150 nm thick Ag layer and accumulates at the MoO3/Ag interface. Moreover, XRD analysis of the aged MoO3/Ag/Al anode indicates the formation of Ag2Al alloy. Depth profiling with X-ray photoelectron spectroscopy advanced our understanding by confirming the formation of Ag-Al intermetallic alloy and the presence of oxidized Al only at the MoO3/Ag interface suggesting a concomitant reduction of MoO3 to most probably MoO2. This latter compound is less reflective than MoO3, which can explain the reduced reflectance in aged devices as proven by optical simulations. On the basis of these results, we could estimate that 20% of the loss in Jsc is ascribed to reduction of MoO3 triggered by its direct contact with Al.

The impact of hot charge carrier mobility on photocurrent losses in polymer-based solar cells.

A typical signature of charge extraction in disordered organic systems is dispersive transport, which implies a distribution of charge carrier mobilities that negatively impact on device performance. Dispersive transport has been commonly understood to originate from a time-dependent mobility of hot charge carriers that reduces as excess energy is lost during relaxation in the density of states. In contrast, we show via photon energy, electric field and film thickness independence of carrier mobilities that the dispersive photocurrent in organic solar cells originates not from the loss of excess energy during hot carrier thermalization, but rather from the loss of carrier density to trap states during transport. Our results emphasize that further efforts should be directed to minimizing the density of trap states, rather than controlling energetic relaxation of hot carriers within the density of states.

Improved photo-stability of silicon nanobelt arrays by atomic layer deposition for efficient photocatalytic hydrogen evolution

Silicon nanostructures have recently drawn great interest for use as photocathodes to produce hydrogen through water photoelectrolysis. Despite the high photocurrent observed, nanostructured Si photocathodes usually exhibit poor photo-stability in aqueous solution and rapidly deactivate. Herein, we report that by coating a thin titania protection layer using atomic layer deposition (ALD), the photo-stability of silicon nanobelt arrays fabricated by metal assisted chemical etching can be markedly improved. The photocurrent loss of the silicon nanobelt array photoelectrode coated with a 3 nm titania layer is found to be much lower than that of the electrode without a titania coating. We also demonstrate that the 3 nm titania coated Si nanobelt arrays can sustain more than twelve hours without a significant loss in photocurrent under operation conditions before it eventually fails. The possible failure mechanism is preliminarily investigated.

Fine tuning of fluorene-based dye structures for high-efficiency p-type dye-sensitized solar cells.

We report on an experimental study of three organic push-pull dyes (coded as zzx-op1, zzx-op1-2, and zzx-op1-3) featuring one, two, and three fluorene units as spacers between donors and acceptors for p-type dye-sensitized solar cells (p-DSSC). The results show increasing the number of spacer units leads to obvious increases of the absorption intensity between 300 nm and 420 nm, a subtle increase in hole driving force, and almost the same hole injection rate from dyes to NiO nanoparticles. Under optimized conditions, the zzx-op1-2 dye with two fluorene spacer units outperforms other two dyes in p-DSSC. It exhibits an unprecedented photocurrent density of 7.57 mA cm(-2) under full sun illumination (simulated AM 1.5G light illumination, 100 mW cm(-2)) when the I(-)/I3(-) redox couple and commercial NiO nanoparticles were used as an electrolyte and a semiconductor, respectively. The cells exhibited excellent long-term stability. Theoretical calculations, impedance spectroscopy, and transient photovoltage decay measurements reveal that the zzx-op1-2 exhibits lower photocurrent losses, longer hole lifetime, and higher photogenerated hole density than zzx-op1 and zzx-op1-3. A dye packing model was proposed to reveal the impact of dye aggregation on the overall photovoltaic performance. Our results suggest that the structural engineering of organic dyes is important to enhance the photovoltaic performance of p-DSSC.