Reduction In Fill Factor Research Articles

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.

Equivalent-circuit and Transport-based Mobility Models of Microcrystalline Silicon Solar Cells

Microcrystalline silicon thin film solar cells exhibit optimal PV efficiency when the absorber layer contains similar proportions of crystalline and amorphous phases. When the crystalline fraction is reduced below 30%, efficiency falls very steeply, from around 8% to as low as 2%, and does not recover until fully amorphous growth conditions are established. We demonstrate that an electrical model, comprising two parallel-connected diodes scaled to reflect material composition, qualitatively predicts the features observed in the PV parameters. However the scale of the reduction in fill-factor is not reproduced. As an alternative approach, a homogeneous transport model is proposed in which carrier mobilities are scaled in accordance with values determined by the time-of-flight experiment. This model predicts a large reduction in fill-factor for low-crystallinity absorbers more in keeping with measurement. A novel carrier transport landscape is proposed to account for mobility variations.

Analysis of the degradation mechanism of ITO-free organic solar cells under UV radiation

This work reports on the stability of encapsulated ITO-free bulk heterojunction organic solar cells (BHJ-OSC) under UV radiation in ambient air with the layer sequence Cr/Al/Cr/photoactive layer (PAL)/poly (3, 4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS)/metal-grid. The PAL consisted of poly(3-hexylthiophene) (P3HT) as a donor and (6,6)-phenyl-C60 butyric acid methyl ester (PCBM) as an acceptor. BHJ-OSC with this setup showed remarkable stability under continuous illumination (1000W/m2) with a low UV content. In contrast, the devices degraded significantly under UV radiation which was characterized by a reduction in fill factor and short-circuit current density. Additional experiments revealed an increase of the sheet resistance of the PEDOT:PSS layer which was interestingly much more pronounced in pure PEDOT:PSS samples as compared to samples where a PAL was deposited underneath. In addition, current extraction by linear increasing voltage (CELIV) measurements indicated a decrease of the effective charge carrier mobility of the PAL. Numerical simulations based on the experimentally determined parameters showed good agreement of the solar cell performance as a function of UV exposure duration. This suggests that the increase of the sheet resistance of the polymeric hole contact and to a lesser extent the change of the effective mobility of the PAL are the main factors governing the deterioration of the photovoltaic performance upon UV exposure. A comparison to devices with a setup ITO/ZnO/PAL/PEDOT:PSS and a full metallization showed clearly improved UV stability, although the absorption of UV in the PAL is very similar. This further supports our interpretation that the degradation of the PAL plays a very minor role. The issue with the degradation of the PEDOT:PSS can easily be solved by incorporating an UV-filter into the device or preferably the use of UV-stabilized PEDOT:PSS formulations.

Loss Mechanisms in Thick‐Film Low‐Bandgap Polymer Solar Cells

AbstractPolymer bulk heterojunction solar cells based on low bandgap polymer:fullerene blends are promising for next generation low‐cost photovoltaics. While these solution‐processed solar cells are compatible with large‐scale roll‐to‐roll processing, active layers used for typical laboratory‐scale devices are too thin to ensure high manufacturing yields. Furthermore, due to the limited light absorption and optical interference within the thin active layer, the external quantum efficiencies (EQEs) of bulk heterojunction polymer solar cells are severely limited. In order to produce polymer solar cells with high yields, efficient solar cells with a thick active layer must be demonstrated. In this work, the performance of thick‐film solar cells employing the low‐bandgap polymer poly(dithienogermole‐thienopyrrolodione) (PDTG‐TPD) was demonstrated. Power conversion efficiencies over 8.0% were obtained for devices with an active layer thickness of 200 nm, illustrating the potential of this polymer for large‐scale manufacturing. Although an average EQE > 65% was obtained for devices with active layer thicknesses > 200 nm, the cell performance could not be maintained due to a reduction in fill factor. By comparing our results for PDTG‐TPD solar cells with similar P3HT‐based devices, we investigated the loss mechanisms associated with the limited device performance observed for thick‐film low‐bandgap polymer solar cells.

Degradation behavior of hydrogenated amorphous/microcrystalline silicon tandem solar cells

AbstractThe degradation behaviors of hydrogenated amorphous/microcrystalline silicon (a‐Si:H/µc‐Si:H) tandem solar cells are systematically investigated. It is found that the degradation of the tandem solar cells takes place not only under the light soaking but also after dark storage. The degradation in the efficiency of tandem solar cells is mainly due to a reduction of fill factor (FF) in our study. Different from light‐induced degradation (LID) behaviors for the tandem solar cells, the degradation behaviors under dark storage, i.e., the dark degradation could be mainly caused by the cracks or voids formed in the bottom µc‐Si:H cell. These defects could lead to an oxidization of internal silicon surfaces by the oxygen atoms. The stabilities of the top a‐Si:H cells and the ZnO:B electrodes are also investigated. Furthermore, adjusting the crystallinity of intrinsic µc‐Si:H material and inserting an intermediate light reflective (ILR) layer between the top cell and bottom cell can improve the performances of the novel structure tandem solar cells. A gain of 0.7% in efficiency is achieved, and meanwhile no dark degradation is observed for the novel tandem solar cells.

Effect of a-Si：H interface buffer layer on the performance of hydrogenated amorphous silicon germanium thin film solar cell

In the light of the open circuit voltage and fill factor reduction resulting from band gap discontinuities and high defect densities at interfaces when more germanium is mixed into the intrinsic layer of hydrogenated amorphous silicon germanium solar cell, the insertion of a-Si:H buffer layer with proper band gap into PI interface not only mitigates band gap discontinuities and interface recombination, but also improves the electric field distribution by reducing the defect densities at PI interface, thus the collection efficiency of a-SiGe:H solar cell is enhanced. By inserting a-Si:H buffer layer into IN interface and designing band gap profile along the a-SiGe:H intrinsic layer further, the 8.72% conversion efficiency of single junction a-SiGe:H solar cell is achieved when only Al back reflector is added as back contact.

Effect of nanotextured back reflectors on light trapping in flexible silicon thin-film solar cells

Nanotextured Ag back reflectors were used to enhance the short-circuit current of flexible silicon thin-film solar cells with an n-i-p configuration by means of optical confinement. A nanotextured topography with a root-mean-square (σrms) surface roughness of 88.0nm was successfully induced by abnormal grain growth of Ag films, which was controlled by varying the deposition temperature and film thickness in a direct current (dc) magnetron sputtering process. Effective light scattering of the long wavelengths over 600nm was achieved on the nano-textured Ag back reflectors, resulting in enhanced absorption of weakly absorbing, long-wavelength light in the hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) thin-film solar cells. Compared with similar reference cells on flat back reflectors, a 34.4% increase in the short-circuit current density (Jsc) for an a-Si:H solar cell and an 11.9% for μc-Si:H were observed in the solar cells on the nanotextured back reflectors, with little reduction in fill factor (FF) and open circuit voltage (Voc). Due to the increase of the Jsc by the improved light absorption at the long wavelengths, the conversion efficiency (η) of the a-Si:H and μc-Si:H solar cells increased from 5.59% to 7.60% and from 4.31% to 4.64%, respectively.

Organic photovoltaic devices with a crosslinkable polymer interlayer

Organic photovoltaic devices with a photo-crosslinkable interlayer were fabricated. This photo-crosslinkable interlayer acted as a leakage current reducing buffer layer. The performance of the small area OPV cell (0.04 cm 2) was enhanced by the increase in the short circuit current and the fill factor. When a larger area cell (1 cm 2) was used, the performance of OPV cell increased when the appropriate interlayer thickness was used. In the case of a 10 cm×10 cm module, the power conversion efficiency was about double than that without the interlayer. The insertion of the interlayer increased the current extraction by lowering the barrier height and attenuated the fill factor reduction by enhancing the rectification with a better leakage current sealing. From this study, it is clearly proved that the insertion of the appropriate photo-crosslinkable layer improves the performance of OPV devices, the effect was especially evident for large area cells.

Efficient interdigitated back‐contacted silicon heterojunction solar cells

AbstractWe present back‐contacted amorphous/crystalline silicon heterojunction solar cells (IBC‐SHJ) on n‐type substrates with fill factors exceeding 78% and high current densities, the latter enabled by a SiNx /SiO2 passivated phosphorus‐diffused front surface field. Voc calculations based on carrier lifetime data of reference samples indicate that for the IBC architecture and the given amorphous silicon layer qualities an emitter buffer layer is crucial to reach a high Voc, as known for both‐side contacted silicon heterojunction solar cells. A back surface field buffer layer has a minor influence. We observe a boost in solar cell Voc of 40 mV and a simultaneous fill factor reduction introducing the buffer layer. The aperture‐area efficiency increases from 19.8 ± 0.4% to 20.2 ± 0.4%. Both, efficiencies and fill factors constitute a significant improvement over previously reported values. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of back-contact barrier on thin-film CdTe solar cells

The presence of a back-contact barrier affects the current–voltage characteristics of thin-film CdS/CdTe/metal solar cells primarily by impeding hole transport, a current-limiting effect commonly referred to as “rollover.” In this work, the CdS/CdTe solar cell with a CdTe/metal back-contact barrier is modeled by two opposite polarity diodes in series. Analytic simulations are fitted to the measured current–voltage curve, the voltage distribution between the two diodes is shown under different conditions, and the back-contact barrier height is extracted. Room-temperature barrier heights exceeding 0.5 eV will generally result in significant fill-factor reduction.

Light trapping and optical losses in microcrystalline silicon pin solar cells deposited on surface-textured glass/ZnO substrates

Influence of front TCO thickness, surface texture and different back reflectors on short-circuit current density and fill factor of thin film silicon solar cells were investigated. For the front TCO studies, we used ZnO layers of different thickness and applied wet chemical etching in diluted HCl. This approach allowed us to adjust ZnO texture and thickness almost independently. Additionally, we used optical modeling to calculate optical absorption losses in every layer. Results show that texture and thickness reduction of front ZnO increase quantum efficiency over the whole spectral range. The major gain is in the red/IR region. However, the higher sheet resistance of the thin ZnO causes a reduction in fill factor. In the back reflector studies, we compared four different back reflectors: ZnO/Ag, Ag, ZnO/Al and Al. ZnO/Ag yielded the best, Al the worst light trapping properties. Furthermore, the Ag back contact turned out to be superior to ZnO/Al for microcrystalline cells. Finally, the smooth ZnO/Ag back contact showed a higher reflectivity than the rough one. We prepared pin cells with rough and smooth ZnO/Ag interface, leaving the roughness of all other interfaces unchanged.

Fill factor losses in ZnO/CdS/CuGaSe2 single-crystal solar cells

Abstract Current–voltage characteristics of ZnO/CdS/CuGaSe 2 single-crystal solar cells with solar conversion efficiency values of η =3.5%, 6.0%, 6.7% and 9.7% were analyzed using the single diode equation. The effect of each of the achieved parameters on the fill factor was calculated. The calculations revealed that the fill factor reduction due to the series resistance remained below Δ ff =4.4% under illumination, while this effect would have been much higher if the illumination had not reduced the series resistance markedly. The calculation furthermore revealed that the fill factor reduction due to the shunt resistance remained below Δ ff =3.6% under illumination. This effect would have been negligible if the illumination had not also reduced the shunt resistance in all studied cells. The increase of the saturation current density under illumination has brought about considerably high fill factor losses (at least Δ ff =8.3%) in all studied cells. Already the dark saturation current density and the diode ideality factor in such cells have been found to be much higher than the ones in the cells based on CuInSe 2 . This seems to be the most substantial restriction to the fill factor, and thus to the performance, of solar cells based on CuGaSe 2 . An explanation for this different behavior seems to lie in the different band structures of these cells.

Impact of light‐induced recombination centres on the current–voltage characteristic of czochralski silicon solar cells

AbstractWe have investigated the effect of the light‐induced deep‐level recombination centre specific to boron‐doped, oxygen‐contaminated Czochralski (Cz) silicon on the current–voltage characteristic of Cz silicon solar cells by means of numerical simulation and experiment. The device simulation predicts the occurrence of a shoulder in the current–voltage curve after activating the characteristic recombination centre. The physical reason for the non‐ideal diode behaviour, characterised by a local ideality factor greater unity, is the strongly injection‐level‐dependent bulk lifetime produced by the deep‐level centre. The increased ideality factor causes a degradation in fill factor with the magnitude of degradation depending on the doping concentration of the Cz silicon base. In order to verify the theoretical predictions experimentally, we have performed measurements on high‐efficiency Cz silicon solar cells. Current–voltage curves recorded before and after light degradation clearly show the theoretically predicted change in shape and the reduction in fill factor. An excellent quantitative agreement between calculation and experiment is obtained for the subtracted current–voltage curves measured after and before illumination. Copyright © 2001 John Wiley & Sons, Ltd.