Reduction In Fill Factor Research Articles

Delay of the potential-induced degradation of n-type crystalline silicon photovoltaic modules by the prior application of reverse bias

We investigated the influence of the pre-application of reverse bias on the potential-induced degradation (PID) of n-type front-emitter (n-FE) crystalline Si (c-Si) photovoltaic modules. Applying a prior positive reverse bias to n-FE cells delays charge-accumulation-type PID (PID-1), decreases in short-circuit current density (J sc) and open-circuit voltage (V oc). The prior positive bias accumulation may accumulate negative charges in the SiN x , which leads to an increase in a duration for the positive charge accumulation by the PID test applying a negative bias. We also found that sufficiently long prior positive bias application results in the delay of PID-2, a fill factor reduction by the incursion of Na ions into the depletion region of the p–n junction of n-FE cells.

PbS-TBAI / MoSe2-grating as new hole transport layer in perovskite solar cells: Superior efficiency enhancement thru simulation and optimization

For the first time, we increased the power conversion efficiency (PCE) of a perovskite solar cell (PSC) from 13.82% to 19.45%, by presenting a new structure of hole transport layer (HTL) consisting of spherical PbS-QD layer treated with tetrabutylammonium iodide (PbS-TBAI) and MoSe2 in the form of grating. MoSe2 with smaller bandgap than perovskite (CH3NH3PbI3) when used instead of CuSCN in the hole transport layer (HTL), can help absorb higher wavelengths and generate electron-hole pairs, which increases Jsc (short-circuit current density), Voc (open-circuit voltage) and PCE respectively from 17.2 to 20.8 mA/cm2, 0.97–1.21 V and 13.82–17.67%. However, due to the reduction of fill factor (FF), the arrangement of different layers to block electrons was investigated, which we found that employing grating structure can improve FF by 4%, while the other parameters do not change much (PCE: 17.67→17.3). Then, a spherical PbS-QD layer treated with tetrabutylammonium iodide (PbS-TBAI) was implemented inside the MoSe2 layer in grating form structure with Spiro to enhance the absorption and consequently the PCE. As a result, this change caused the increase of Jsc, FF and PCE, from 20.82 to 24.15 mA/cm2, 0.73–0.79 and 17.3–19.45%, respectively.

Second-stage potential-induced degradation of n-type front-emitter crystalline silicon photovoltaic modules and its recovery

We investigate the second-stage potential-induced degradation (PID) of n-type front-emitter (n-FE) crystalline silicon (c-Si) photovoltaic (PV) modules. The PID of n-FE c-Si PV modules is known to occur in three stages under negative bias stress. The second-stage PID is characterized by a reduction in fill factor (FF), due to the invasion of sodium (Na) into the depletion region of a p+–n junction and the resulting increase in recombination current. The second-stage PID shows a curious independence from a negative bias voltage for the PID stress. This may indicate that the Na inducing the FF reduction comes not from the cover glass but originally existed on and/or near the cell surface. The FF reduction is recovered quite rapidly, within a few seconds, by applying a positive bias to the degraded cell. The recovered n-FE c-Si PV modules show more rapid degradation if they receive the negative bias stress again, which can be explained by Na remaining in the p+ emitter.

Polishing the Lead-Poor Surfacefor Efficient Inverted CsPbI3 Perovskite Solar Cells.

Triiodide cesium lead perovskite (CsPbI3 ) has promising prospects in the development of efficient and stable photovoltaics in both single-junction and tandem structures. However, achieving inverted devices that provide good stability and are compatible to tandem devices remains a challenge, and the deep insights are still not understood. This study finds that the surface components of CsPbI3 are intrinsically lead-poor and the relevant traps are of p-type with localized states. These deep-energy-level p traps induce inferior transfer or electrons and serious nonradiative recombination at the CsPbI3 /PCBM interface, leading to the considerable open-circuit voltage (Voc ) loss and reduction of fill factor (FF). Compared to molecular passivation, polishing treatment with 1,4-butanediamine can eliminate the nonstoichiometric components and root these intrinsically lead-poor traps for superior electron transfer. The polishing treatment significantly improves the FF and Voc of the inverted CsPbI3 photovoltaics, creating an efficiency promotion from 12.64% to 19.84%. Moreover, 95% of the initial efficiency of the optimized devices is maintained after the output operation for 1000 h.

Reversible Degradation in Hole Transport Layer‐Free Carbon‐Based Perovskite Solar Cells

The rationale for ambient degradation behaviors in hole transport layer (HTL)‐free carbon‐based perovskite solar cells (PSCs) still remains a mystery, although carbon‐based PSCs have been the frontrunner among the emerging next‐generation photovoltaics owing to their low cost of materials and fabrication process, as well as the exceptional durability. Herein, the long‐term stability of HTL‐free carbon‐based PSCs (H‐C‐PSCs) in the ambient air environment is investigated to thoroughly understand and identify the governing chemistry during the degradation. Specifically, a reversible degradation phenomenon is observed along with an anomalous S‐shape of current–voltage curves involving only reduction of fill factor (FF), almost without impact for short‐circuit current density and open‐circuit voltage. Furthermore, a minute‐heating treatment will eliminate the reversible degradation which can be attributed to the reversible formation of intermediate hydrate at the interface between perovskite and carbon contact. This provides a new perspective for the stability issue of H‐C‐PSCs.

Effect of a Back-Surface Field and Passivation Layer on a Silicon Schottky Solar Cell

In this work, a numerical simulation of a silicon based solar cell (SC) is carried out using Silvaco-Atlas software. The back contact and the back surface field (BSF) combined with a passivation layer (PL) realized by using SiO2 tunneling layer, is addressed in this paper. It is demonstrated that a proper choice of the BSF and PL can enhance a Schottky back contact based solar cell compared to its ohmic counterpart. BSF has to be properly doped to reduce the barrier of the Schottky contact. The tunnel oxide is a vital part in this solar cell. It is required to achieve excellent interface passivation and has to have an optimum thickness and below this thickness, the SC performance is enhanced by a tunneling effect, while it is deteriorated by the fill factor reduction above this optimum thickness.

Performance Analysis of 0.4–1.2-μm CdTe Solar Cells

Thin-film cadmium telluride (CdTe) solar cells with an absorber thickness range of 0.4–1.2 μm were fabricated by close-space sublimation on a transparent MgZnO window layer. Microscopy cross sections showed that a significant fraction of the CdTe crystallites extended throughout the absorber layer, and capacitance measurements demonstrated that the absorbers were depleted into forward bias. Current and fill factor loss analyses were used to identify dominant loss mechanisms in the devices as a function of absorber thickness, and photoluminescence measurements were increasingly influenced by back-surface recombination as the absorber was made thinner. The thinner CdTe devices showed modest current and fill-factor reduction but overall good diode quality, and the highest efficiency of 15% was achieved with 1.2-μm CdTe.

Thermal stability analysis of buffered layer P3HT/P3HT:PCBM organic solar cells

Here to determine the thermal stability of buffered layer organic solar cell (BL-OSC), the effect of post anneal treatment has been studied. To investigate the effect of post annealing, the organic solar cells (OSCs) are annealed at the 120°C for different time duration. It has been observed that the BL-OSC structure exhibits the better thermal stability. Further, as the authors vary the post-annealing time duration from 0 min to 20 min, the power conversion efficiency (PCE) in the case BL-OSC drops by ~20%, whereas in conventional OSC, the PCE drops by ~35%. This annealing dependent study shows that, in conventional OSC structure an increase in phase segregation between donor and acceptor molecules reduces exciton dissociation and charge separation, this leads to sharp increase in series resistance and significant reduction in fill factor of the device. Whereas in the case of BL-OSS, there is a minimum reduction in the fill factor, which also determines the superior carrier collection and low recombination on the elevated annealing conditions. Further, the experimental results show that, in comparison with the conventional OSC structure, inserting the pure P3HT interlayer between PEDOT:PSS (hole transport layer) and P3HT:PCBM (photoactive layer) improves the PCE of the device by 34%.

Low sunlight concentration properties of InAs ultrahigh-density quantum-dot solar cells

InAs ultrahigh-density quantum-dot (UHD-QD) solar cells with type-I GaAs or type-II GaAsSb capping layers were fabricated by molecular beam epitaxy. The solar cell properties of these UHD-QD cells were measured as a function of quasi-sunlight concentration (QX) and were compared with that of the GaAs reference cell. For UHD-QD cells, the reduction of fill factor at high QX was suppressed by the low resistance of those cells, despite the wide separation width of 500 μm between comb electrodes. The open circuit voltage increased logarithmically with increasing QX, and the diode ideal factor was especially close to 1 for the type-II UHD-QD cell. These results could be mainly explained by carrier separation and lateral carrier transport through the UHD-QD layer. As a result, power conversion efficiency of UHD-QD cells increased with increasing QX and exceeded that of GaAs reference cell at over 4 suns in QX.

The Role of Bulk and Interface Recombination in High-Efficiency Low-Dimensional Perovskite Solar Cells.

2D Ruddlesden-Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite-based cells. Herein, 2D (CH3 (CH2 )3 NH3 )2 (CH3 NH3 )n -1 Pbn I3 n +1 perovskite cells with different numbers of [PbI6 ]4- sheets (n = 2-4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open-circuit voltage (VOC ) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C60 interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi-Fermi level splitting matches the device VOC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.

Optimization of Electrically Conductive Adhesive Bonds in Photovoltaic Modules

Gluing ribbons to silicon solar cells by using electrically conductive adhesives (ECAs) is an alternative interconnection technology for module integration to the state-of-the-art soldering process. We reveal cost reduction potentials by analyzing the influence of volume and contact resistivity, as well as the bond design of ECAs on the fill factor of photovoltaic modules. Solar cells with structured busbars are considered in the analysis. The volume resistivity is controlled by the cure temperature. We contact individual cells at different curing conditions and measure their fill factors. A volume resistivity of 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> Ω · cm does not cause a significant fill factor reduction compared with an ECA with around 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> Ω · cm. The contact resistivity is varied by using different ribbon coatings. Ag and Sn coatings achieve almost identical fill factors. A bare Cu surface reduces the fill factor. We lower the consumed ECA from 40 to 3 mg/cell by modifying the bond design. A design with 16 mg/cell achieves similar fill factors as with 40 mg/cell. A finite-element model is developed to study the combined influence of electrical properties and the bond design. We propose an optimized contact design for high fill factors and reduced material consumption.

Overcoming Fill Factor Reduction in Ternary Polymer Solar Cells by Matching the Highest Occupied Molecular Orbital Energy Levels of Donor Polymers

AbstractDespite the potential of ternary polymer solar cells (PSCs) to improve photocurrents, ternary architecture is not widely utilized for PSCs because its application has been shown to reduce fill factor (FF). In this paper, a novel technique is reported for achieving highly efficient ternary PSCs without this characteristic sharp decrease in FF by matching the highest occupied molecular orbital (HOMO) energy levels of two donor polymers. Our ternary device—made from a blend of wide‐bandgap poly[4,8‐bis(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐2,5‐dioctyl‐4,6‐di(thiophen‐2‐yl)pyrrolo[3,4‐c]pyrrole‐1,3(2H,5H)‐dione) (PBDT‐DPPD) polymer, narrow‐bandgap poly[4,8‐bis[5‐(2‐ethylhexyl)‐2‐thienyl]benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐ 6‐diyl)] (PTB7‐Th) polymer, and [6,6]‐phenyl C70‐butyric acid methyl ester (PC70BM)—exhibits a maximum power conversion efficiency of 10.42% with an open‐circuit voltage of 0.80 V, a short‐circuit current of 17.61 mA cm−2, and an FF of 0.74. In addition, this concept is extended to quaternary PSCs made by using three different donor polymers with similar HOMO levels. Interestingly, the quaternary PSCs also yield a good FF (≈0.70)—similar to those of corresponding binary PSCs. This study confirms that the HOMO levels of the polymers used on the photoactive layer of PSCs are a crucial determinant of a high FF.

Quantification of pn-Junction Recombination in Interdigitated Back-Contact Crystalline Silicon Solar Cells

Interdigitated back-contact (IBC) solar cells based on diffused crystalline silicon comprise a series of pn -junctions which border at the rear surface of the wafer. In this work, it is established that the presence of these pn -junctions can induce significant additional charge-carrier recombination, which affect the conversion efficiency of IBC cells through a reduction in fill factor and open-circuit voltage. Using specialized test structures with varying length of pn -junctions per area of solar cell (i.e., with varying junction density), the magnitude of the recombination at the pn -junction was determined. For nonpassivated rear surfaces, a second-diode recombination current density per unit of junction density J 02 of ∼61 nA·junction–1·cm–1 was measured, whereas for surfaces which were passivated by either SiN $_{x}$ or Al 2O3/SiN $_{x}$ , J 02 was reduced to ∼0.4 nA·junction–1·cm–1. Therefore, passivation of defects at the rear surface was proven to be vital in reducing this characteristic recombination current. Finally, by optimizing the p - and n -type dopant diffusion process recipes, J 02 recombination could be suppressed. The improved doping recipes led to an increase in conversion efficiency of industrial “mercury” IBC solar cells by ∼1% absolute.

Potassium post deposition treatment of solution-processed kesterite solar cells

Potassium post deposition treatment (K-PDT) has been a major breakthrough for CIGSSe (Cu(In,Ga)(S,Se)2) solar cells yielding world record efficiencies. K-PDT yields significant improvements in open circuit voltage, fill factor and allows thinning CdS thus leading to efficiencies above 20% for CIGSSe material. Here we present a similar K-PDT approach for solution processed kesterite solar cells. K-PDT improves the open circuit voltage of kesterite solar cells, however a severe blocking of the short circuit current and reduction of fill factor reduces the overall device performance. Furthermore, quantum efficiency measurements indicate that K-PDT alters the properties of the CdS layer whereas time-resolved photoluminescence measurements exhibit an additional third exponential component, which can be attributed to minority carrier trapping at the interface.

Photodegradation of Metal Oxide Interlayers in Polymer Solar Cells

The effect of 1 sun illumination for 24 h on encapsulated inverted organic photovoltaic devices is studied through a series of electrical and optical characterizations for donor/acceptor polymer solar cells based on poly(dithienogermole‐alt‐thienopyrrolodione)/[6,6]‐phenyl C71 butyric acid methyl ester blends. Power conversion efficiency is significantly reduced after light exposure due to a 45% drop in the device fill factor and a 40% drop in both the open circuit voltage and short circuit current. The decrease in open circuit voltage is attributed to a reduction in the built‐in field in the device due to photodegradation of the ZnO interlayer. The reduction in fill factor and short circuit current is shown to be due to the formation of an extraction barrier for hole collection as a result of photodegradation of the MoOx interlayer. Using a 400 nm long pass filter to block the UV light, device stability is significantly enhanced, resulting in a decrease in power conversion efficiency of less than 5% under the same operation conditions.

Nondestructive characterization of encapsulant discoloration effects in crystalline-silicon PV modules

Encapsulant discoloration is a common type of degradation in photovoltaic (PV) modules, which significantly affects its performance and reliability under field conditions. This paper presents the nondestructive characterization techniques for investigation of encapsulant discoloration in crystalline-silicon PV modules. These characterization techniques have been channelized and applied to investigate various aspects of discoloration effects on 20 years old field-exposed PV modules. Dark lock-in thermography (DLIT) has been exploited for investigation of temperature variation and defects caused by uneven discoloration over the cells, while electroluminescence (EL) imaging has been proposed for relative quantification of extent of discoloration in a PV module. The spatially-resolved images obtained from both the techniques provided the qualitative and quantitative information about the optical and electrical effects of discoloration in a module, which is not possible by the conventional visual inspection method alone. The electrical methods including proposed differential current analysis, I–V measurement and insulation resistance test have also been used to aid this investigation. The results obtained from these techniques show that the power degradation due to discoloration was attributed to significant reduction in fill factor by non-uniform discoloration and increase in series resistance of cell contacts, and to some extent by its direct effect of light reduction. Electrical mismatch appeared to play an important role in accelerating the encapsulant discoloration in the module. These nondestructive characterization approaches can enable to inspect large number of PV modules in their actual encapsulated form by fast and efficient manner.

A simulation study on the impact of band gap profile variations and secondary barriers on the temperature behavior, performance ratio, and energy yield of Cu(In,Ga)(Se,S)2solar cells

Using device simulations, we investigate the change of the temperature behavior of Cu(In,Ga)(Se,S)2 solar cells. Our goal is to understand the behavior of the performance ratio (PR), normalized energy yield (Y), and the temperature coefficient of the open-circuit voltage as a contribution to the PR and Y. Therefore, we simulate temperature-dependent current–voltage (IVT) curves and apply weather data of a hot climate location to calculate PR and Y. For differently widened absorber band gap profiles, we observe an increase of the open circuit voltage leading to an enhancement of the relative temperature coefficient of the open circuit voltage and of the PR. In addition to that, we investigate the impact of three different barrier locations within the solar cell: at the back contact (a), at the hetero-interface buffer/absorber (b) and a conduction band barrier in the absorber space charge region (c). For all barrier locations, the PR improves with increasing barrier height. For (a) and (b), this improvement is accompanied by a power reduction at standard test conditions (STC) due to a fill factor reduction. In case (c), a small barrier improves STC power and the PR simultaneously. However, increasing barrier height beyond an optimum again leads to a decrease in STC power.

The effect of calcium-induced fullerene migration on the performance of thermally stable nanoparticle organic solar cells

The impact of a calcium interface layer in combination with a thermal annealing treatment on the performance of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-buteric acid methylester (PCBM) nanoparticle photovoltaic devices is investigated. Annealing is found to disrupt the microstructure of the nanoparticle active layer leading to a reduction in fill factor. However, X-ray photoelectron spectroscopy measurements show that the calcium interface layer causes PCBM to preferentially migrate to the cathode interface upon annealing, resulting in better charge extraction from the PCBM moiety, an increase in the built-in voltage, open-circuit voltage, and power conversion efficiency. Moreover, the annealing trends could be completely explained by the observed PCBM migration. Unlike P3HT:PCBM bulk heterojunction devices, the P3HT:PCBM nanoparticle devices showed a remarkable thermal stability up to 120 °C. As such, OPVs fabricated from aqueous nanoparticle inks provide an attractive alternative to conventional organic solvent based bulk heterojunction devices.

Investigation of the s-shape caused by the hole selective layer in bulk heterojunction solar cells

A common failure mechanism of organic solar cells is the development of an s-shaped current voltage curve. Herein, we investigated the origin of this degradation mechanism by replacing the commonly used hole selective layer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) with a co-evaporated layer of N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) and Dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN). By varying the ratio of TPD to HATCN we are able to tune both the mobility and work function of the hole selective layer. Using a combination of field effect mobility measurements, Kelvin-Probe work function measurements, and numerical modeling we demonstrate that a degraded mobility of the hole selective layer leads to a buildup of charge within the device and reduction of fill factor.

Origin of the fill factor loss in bulk-heterojunction organic solar cells

In this paper, the decrease of fill factor with increase of light illumination intensity was investigated through light assisted impedance studies based on ITO/poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate)/[poly(3-hexylthiophene):1-(3-methyloxycarbonyl)-propyl-1-phenyl(6,6)C61 (P3HT:PCBM)]/Ca/Al solar cells. It was found that the charge accumulation at the electrode interface causes a decrease in parallel resistance (Rp), consequently a reduction in fill factor. Moreover, it has also been found that the photocurrent loss near the open-circuit voltage through bimolecular recombination also leads to the reduction of fill factor with increasing light intensity in P3HT:PCBM solar cells.