Drop In Fill Factor Research Articles

Traps and transport resistance are the next frontiers for stable non-fullerene acceptor solar cells

Stability is one of the most important challenges facing material research for organic solar cells (OSC) on their path to further commercialization. In the high-performance material system PM6:Y6 studied here, we investigate degradation mechanisms of inverted photovoltaic devices. We have identified two distinct degradation pathways: one requires the presence of both illumination and oxygen and features a short-circuit current reduction, the other one is induced thermally and marked by severe losses of open-circuit voltage and fill factor. We focus our investigation on the thermally accelerated degradation. Our findings show that bulk material properties and interfaces remain remarkably stable, however, aging-induced defect state formation in the active layer remains the primary cause of thermal degradation. The increased trap density leads to higher non-radiative recombination, which limits the open-circuit voltage and lowers the charge carrier mobility in the photoactive layer. Furthermore, we find the trap-induced transport resistance to be the major reason for the drop in fill factor. Our results suggest that device lifetimes could be significantly increased by marginally suppressing trap formation, leading to a bright future for OSC.

Rubidium Fluoride Absorber Treatment for Wide‐Gap (Ag,Cu)(In,Ga)Se2Solar Cells

This contribution studies the potential of an RbF postdeposition treatment (RbF‐PDT) of wide‐gap (Ag,Cu)(In,Ga)Se2(ACIGS) absorbers to improve the corresponding solar cell performance. While a higher open‐circuit voltage (VOC) and short‐circuit current density are achieved, a lower fill factor (FF) is observed for most of the devices subjected to an RbF‐PDT. However, the drop in FF can be avoided for some close‐stoichiometric samples, leading to maximum efficiencies beyond 16% (without antireflection coating) at a bandgap energy (Eg) of 1.43 eV. For off‐stoichiometric ACIGS, a recordVOCvalue of 926 mV atEg = 1.44 eV is reached. LowerVOCdeficits likely require enhanced bulk quality of wide‐gap chalcopyrite absorbers. Extensive material analysis shows that the heavy alkali PDT of ACIGS with high Ag and Ga contents leads to similar absorber modifications as commonly observed for low‐gap Cu(In,Ga)Se2(CIGS). Rubidium is continuously distributed at “internal” (grain boundaries) and “external” (buffer and back contact) absorber interfaces. The results indicate that Rb diffusion into the absorber bulk (including 1:1:2 and 1:3:5 compounds) is restricted. Furthermore, the formation of a very thin RbInSe2surface layer is suggested. It remains open, which effects alter the device characteristics after RbF‐PDT.

Passivation, conductivity, and selectivity in solar cell contacts: Concepts and simulations based on a unified partial-resistances framework

Passivation, conductivity, and selectivity are often acknowledged as the three requirements for optimal contacts to photovoltaic solar cells. Although there are generally accepted definitions and metrics for passivation and conductivity, a common understanding of the concept of selectivity is emerging only now. In this contribution, we present a generalized model of solar cell contacts based on the distinct lumped resistances encountered by electrons and holes traversing a contact, which we refer to as partial specific contact resistances. The relations between electron and hole partial current densities, quasi-Fermi level separation, and external voltage are derived from these partial specific contact resistances, leading to simple metrics for the aforementioned contact properties: the sum of the electron and hole resistances is a metric for passivation, their ratio is a metric for selectivity, and the majority-carrier resistance is a metric for conductivity. Using PC1D, we validate our model by simulating 10 500 cases of homojunction contacts to crystalline silicon solar cells, although our framework is material agnostic and can be equally applied to any other type of absorber. In these simulations, the hole contact and absorber are assumed to be ideal, whereas we vary the partial specific contact resistances in the electron contact by orders of magnitude by adjusting the electron and hole mobilities, their densities (through variations of the donor doping density), and the contact thickness. The simulations confirm the finding of the model that, when the contact fraction cannot be adjusted—as is the case with full-area contacts—combined passivation and conductivity are necessary and sufficient for optimal solar cell performance, and they imply selectivity. However, the reciprocal is not true: contacts can be selective but lack conductivity—causing a deleterious drop in fill factor—or can be selective but provide poor passivation—leading to a reduction in implied open-circuit voltage and, hence, actual open-circuit voltage. Thus, selectivity is a meaningful metric in the sole case of partial-area contacts, where the contact fraction can be adjusted arbitrarily.

On the Influence of the Photo-Induced Leakage Current in Monolithically Interconnected Modules

This paper describes the influence of an irradiance-dependent photo-induced leakage current through a semi-insulating GaAs substrate on the performance of photovoltaic monolithically series-interconnected multisegment laser power converters. A reciprocal relation between the resistivity of a semi-insulating GaAs substrate and irradiance of monochromatic light is experimentally observed. A reduced resistivity of the substrate with an increasing irradiation results in a substantial increase of a leakage current through the semi-insulating GaAs substrate between adjacent segments. For a multisegment laser power converter, this photo-induced leakage current is identified as a major shunting mechanism between adjacent segments that arises under high irradiances. Open-circuit voltage $V_{{\rm{oc}}}$ , fill factor (FF), and consequently conversion efficiency of a multisegment laser power converters are highly affected by the shunting mechanism. Based on a shading experiment, we observed that $V_{{\rm{oc}}}$ drops up to 21.5 mV per segment at a short-circuit current density $J_{{\rm{sc}}}= 47.3{\rm{\,A/ cm}}^{2}$ for the studied six-segment MIM specimen. For the same device, FF drops by 4.1% absolute at $J_{{\rm{sc}}}= 40.5{\rm{\,A/ cm}}^{2}$ . For the two-segment specimen, 5.8 mV drop of $V_{{\rm{oc}}}$ per segment and 1.5% absolute drop in FF is reported at $J_{{\rm{sc}}}= 47.3$ and $43.7{\rm{\,A/ cm}}^{2}$ , respectively.

Multi-walled carbon nanotube incorporated nanoporous titanium dioxide electrodes for hybrid polymer solar cells

This study focuses on incorporating multi-walled carbon nanotubes (MWNT) in nanoporous titanium dioxides (TiO2) thin film for enhancing the performance of hybrid poly(3-hexylthiophene)(P3HT)/TiO2 solar cells. MWNTs were successfully incorporated in the nanoporous TiO2 electrodes either by systematically blending or dipping prior to the deposition of P3HT. Current density–voltage (J-V) characteristic of the corresponding fabricated devices showed that the short-circuit current density (JSC) and fill factor are strongly influenced by the amount of MWNT incorporated into TiO2 nanoparticles. Solar cells with optimum amount of MWNT incorporated by blending in nanoporous TiO2 layer showed a factor of two efficiency enhancement, mainly due to improvement in JSC compared to the corresponding control device. Further increment in MWNT reduces the efficiency mainly due to a sharp drop in fill factor and VOC, which can be attributed to shunting by excess amount of MWNTs. Devices with MWNT dip-coated TiO2 and polymer too showed enhanced efficiency over 2.5% under AM 1.5 illumination (100 mW cm−2), which is a factor of three higher than the corresponding control device.

Enhanced Current Collection in 1.7 eV GaInAsP Solar Cells Grown on GaAs by Metalorganic Vapor Phase Epitaxy

Quaternary GaInAsP solar cells with a bandgap of ∼1.7 eV offer an attractive Al-free alternative to AlGaAs solar cells for integration in next generation of III–V multijunction solar cells with five or more junctions. Development of a high quality 1.7 eV solar cell is also highly sought for III–V/Si tandem solar cells. In this work, we systematically investigate the impact of varying base thicknesses and doping concentrations on the carrier collection and performance of 1.7 eV GaInAsP solar cells. The photoresponse of these cells is found to be very sensitive to p-type zinc doping concentration in the base layer. Prototype 1.7 eV GaInAsP n-i-p solar cell designs are demonstrated that leverage enhanced depletion width as an effective method to achieve peak quantum efficiency exceeding 90%. We also show the importance of optimal i-layer thickness as a critical parameter to reduce the drop in fill-factor (FF) due to field-aided collection. Furthermore, we demonstrate substantial improvement in the cell performance when the GaInAsP base layer is grown at 650 °C instead of 600 °C. The best GaInAsP solar cell ( Eg ∼ 1.65 eV) in this study achieved J SC of 21.1 mA/cm2, V OC of 1.18 V, FF of 83.8%, and an efficiency of 20.8 ± 1% under AM1.5D spectrum (21.5 ± 1% under AM1.5G spectrum). These results highlight the potential of Al-free GaInAsP solar cells for integration in the next generation of III–V multijunction solar cells.

Highly Efficient Organic Solar Cells Consisting of Double Bulk Heterojunction Layers.

An organic solar cell (OSCs) containing double bulk heterojunction (BHJ) layers, namely, double-BHJ OSCs is constructed via stamp transferring of low bandgap BHJ atop of mediate bandgap active layers. Such devices allow a large gain in photocurrent to be obtained due to enhanced photoharvest, without suffering much from the fill factor drop usually seen in thick-layer-based devices. Overall, double-BHJ OSC with optimal ≈50 nm near-infrared PDPP3T:PC71 BM layer atop of ≈200 nm PTB7-Th:PC71 BM BHJ results in high power conversion efficiencies over 12%.

Enhanced photovoltaic performance of photoanodes based on Eu-doped ZnO nanowire arrays for dye-sensitized solar cells

Eu-doped ZnO nanowire (NW) arrays were grown on fluorine-doped tin oxide (FTO) substrates with assistance of hexamethylenetetramine (HMTA) and polyethylenimine (PEI) through Mannich reactions and used as photoanodes for dye-sensitized solar cells (DSCs). Structural characterizations indicate the Eu ions have been incorporated into the ZnO crystalline lattice. An overall light-to-electricity conversion efficiency (η) of 2.64 % was achieved for the DSC based on the 6 % Eu-doped ZnO NW array under AM 1.5 G illumination, and this η was found to significantly increase compared with that of the DSC with undoped ZnO NW array as the photoanode (2.01 %). Meanwhile, the fill factor (FF) of the DSC was upgraded from 63.2 to 78 %. Furthermore, the enhanced electron injection and transport abilities of Eu-doped ZnO NW arrays based on the DSC were revealed by the incident photon-to-electron conversion efficiency (IPCE) spectrum. This research demonstrates an effective way to resolve the low efficiency of DSCs due to the FF drop as well as a great potential for the expanding practical application of rare earth metals.

InGaAsP/InGaAs tandem photovoltaic devices for four-junction solar cells**Project supported by the National Natural Science Foundation of China (No. 61376065).

Lattice-matched InGaAs(P) photovoltaic devices were grown on InP substrates by metal-organic chemical vapor deposition. InGaAsP/InGaAs (1.07/0.74 eV) dual-junction (DJ) solar cells were fabricated and characterized by quantum efficiency and I–V measurements. The open circuit voltage, short circuit current density, fill factor, and efficiency of InGaAsP/InGaAs DJ solar cell are 0.977 V, 10.2 mA/cm2, 80.8%, and 8.94%, respectively, under one sun illumination of the AM 1.5D spectrum. For the InGaAsP/InGaAs DJ solar cell, with increasing concentration, the conversion efficiency first increases steadily and reaches 13% around 280 suns, and finally decreases due to the drop in fill factor at higher concentration ratios. These experimental results demonstrate the promising prospect of GaInP/GaAs/InGaAsP/InGaAs four-junction solar cells.

Overcoming interface losses in organic solar cells by applying low temperature, solution processed aluminum-doped zinc oxide electron extraction layers

Intrinsic zinc oxide (ZnO) is widely used as an electron extraction layer (EEL) for inverted polymer solar cells. Despite the excellent device performance, a major drawback for large area production is its low conductivity. Using microscopic simulations, we derived a technically reasonable threshold value of 10−3 S cm−1 for the conductivity required to overcome transport limitations. For conductivity values typical for ZnO we observed the interface layer thickness restriction at only a few tens of nanometers, either as a fill factor drop due to serial resistance, eventually accompanied by a second diode behavior, or by the need for light soaking. Higher conductive aluminum-doped zinc oxide (AZO), which was introduced earlier, meets the desired conductivity threshold, however, at the cost of high temperature processing. High annealing temperatures (>150 °C) significantly improve the electrical properties of ZnO, but prohibit processing on plastic substrates or organic active layers. Here we report on AZO layers from a sol–gel precursor, which has been already reported to give sufficiently high conductivities at lower processing temperatures (<150 °C). We investigate the influence of different precursor compositions on the electrical properties of the thin films and their performance in inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells. Low temperature AZO layers with thicknesses up to 680 nm maintained comparable performance to devices with thin AZO layers.