Improved efficiency and stability of inverted polymer solar cells with a solution-processed BPhen interlayer and polystyrene beads

We demonstrate improved power conversion efficiency (PCE) and strongly enhanced stability of inverted organic solar cells (OSCs) with Cs halides by solution casting BPhen (4,7-di(phenyl)-1,10-phenanthroline) on the halide layer and ∼100 nm polystyrene beads (PSB) on the blank side of the OSC’s substrate. The PCE of ITO/CsCl/P3HT:PCBM/MoO₃/Al (where P3HT is poly 3-hexylthiophene and PCBM is [6,6]-phenyl-C₆₀-butyric acid methyl ester) improves by up to 46%, from 2.5% to ∼3.7%, by adding a solution-processed BPhen layer between the CsCl and the active layer. For such cells with CsI (PCE ∼3.3–3.4%) the increase was only 6–9%, to 3.5–3.7%. The PCE of cells devoid of the halides but with BPhen was ∼3.3%. The cells were optimized by varying the BPhen concentration in a chlorobenzene solution. The results are consistent with reduced charge recombination at the ITO interface in the presence of the hole blocking BPhen interlayer. The use of hole blocking BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline), as a substitute for BPhen, also showed an enhancement (though lower due to its lower electron mobility), verifying the effect of these materials as hole blocking interlayers. Interestingly, the stability of such non-encapsulated devices with CsCl/BPhen or CsI/BPhen improved significantly. For example, the PCE of unencapsulated cells with CsCl/BPhen kept in the dark under ambient conditions dropped by less than 2% after more than 3 weeks; the PCE of similar cells devoid of the BPhen layer dropped by ∼60% during the same period. The PCE of the cell with CsCl/BPhen dropped by ∼16% after 2 months. High humidity, as expected, resulted in faster deterioration in cell performance. The PCE, however, was restored to within ∼10% of the original value for 2 week old cells by solution–application of a PSB layer on the blank side of the cell’s glass substrate. These beads direct and scatter the light to enhance absorption in the active layer. The results demonstrate that a simple approach such as casting a film of ∼100 nm diameter PSB from an aqueous suspension on the blank side of the OSC substrate can improve long-term performance, and that spin coating BPhen is a low-cost and easy approach to reduce charge recombination at the cathode in inverted structures for increased PCE and stability.     

The neglected influence of zinc oxide light-soaking on stability measurements of inverted organic solar cells

Although zinc oxide (ZnO) is one of the most commonly used materials for electron transport layers in organic solar cells (OSCs), it also comes with disadvantages such as the so-called light-soaking issues, i.e., its need for exposure to UV light to reach its full potential in OSCs. Here, the impact of ZnO light-soaking issues on stability measurements of OSCs is investigated. It is found that in the absence of UV light a reversible degradation occurs, which is independent of the used active layer material and accelerates at higher temperatures but can be undone with a short UV exposure. This reversible aging is attributed to the re-adsorption of oxygen, which for manufacturing reasons is trapped at the interface of ZnO, even in an oxygen-free environment. This oxygen can be removed with a UV pretreatment of the ZnO but at the expense of device efficiency and production that has to take place in an oxygen-free environment. This study establishes that stability measurements of ZnO-containing OSCs must be performed exclusively with a light source including a UV part since the usage of a simple white light source – as often reported in the literature – can lead to erroneous results.     

Spontaneous growth by sol-gel process of low temperature ZnO as cathode buffer layer in flexible inverted organic solar cells

In this study, the sol–gel method was employed to prepare zinc oxide (ZnO) thin films as cathode buffer layers for inverted organic solar cells (IOSCs). We used a low temperature sol-gel process for the synthesis of ZnO thin films, in which the molar ratio of zinc acetate dihydrate (ZAD) to ethanolamine (MEA) was varied; subsequently, using the thin films, we successfully fabricated inverted solar cells on flexible plastic substrates. A ZnO sol–gel was first prepared by dissolving ZAD and MEA in ethylene glycol monomethyl ether (EGME). The molar ratios of ZAD to MEA were set as 1:1.2, 1:1, and 1:0.8, and we investigated the characteristics of the resulting ZnO thin films. We investigated the optical transmittance, surface roughness, and surface morphology of the films. Then, we discussed the reasons about the improvement of the device efficiency. The devices were fabricated using the ZnO thin films as cathode buffer layers. The results indicated that the morphology of the thin films prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 changed to a rippled nanostructure after two-step annealing. The PCE was enhanced because of the higher light absorption in the active layer caused by the nanostructure. The structure of the inverted device was ITO/ZnO/P3HT:PC₆₁BM/MoO₃/Ag. The short-circuit current densities (8.59 mA/cm² and 8.34 mA/cm²) of the devices with films prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 ratios, respectively, and annealed at 125 °C were higher than that of the device containing the ZnO thin film that was annealed at 150 °C. Inverted solar cells with ZnO films that were prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 and annealed at 125 °C exhibited PCEs of 3.38% and 3.30%, respectively. More than that, PCEs of the flexible device can reach up to 1.53%.     

Light management in PCPDTBT:PC70BM solar cells: A comparison of standard and inverted device structures

We compare standard and inverted bulk heterojunction solar cells composed of PCPDTBT:PC₇₀BM blends. Inverted devices comprising 100 nm thick active layers exhibited short circuit currents of 15 mA/cm², 10% larger than in corresponding standard devices. Modeling of the optical field distribution in the different device stacks proved that this enhancement originates from an increased absorption of incident light in the active layer. Internal quantum efficiencies (IQEs) were obtained from the direct comparison of experimentally derived and modeled currents for different layer thicknesses, yielding IQEs of ∼70% for a layer thickness of 100 nm. Simulations predict a significant increase of the light harvesting efficiency upon increasing the layer thickness to 270 nm. However, a continuous deterioration of the photovoltaic properties with layer thickness was measured for both device architectures, attributed to incomplete charge extraction. On the other hand, our optical modeling suggests that inverted devices based on PCPDTBT should be able to deliver high power conversion efficiencies (PCEs) of more than 7% provided that recombination losses can be reduced.     

Effect of PEI cathode interlayer on work function and interface resistance of ITO electrode in the inverted polymer solar cells

In this paper, we investigated the effect of PEI cathode interlayer on the work function and the interface resistance of ITO electrode in the inverted polymer solar cells (PSCs) based on PBDTTT-C-T:PC₇₀BM. It is found that a very thin layer of PEI (⩽5.5 nm), either linear PEI (l-PEI) or branched PEI (b-PEI) with different molecular weights, is enough to lower the work function of the ITO electrode and to enhance the photovoltaic performance of the devices. The champion power conversion efficiency (PCE) of the devices with the PEI cathode interlayer is 7.84%, more than doubled of that without the interlayer. However, a thicker PEI interlayer (⩾10 nm) results in abrupt decrease of the PCEs due to the increase of the resistance. Interestingly, for the thicker interlayers, the l-PEI shows high photovoltaic performance than that of b-PEI, which can also be explained by their difference in the resistances. This work supplies an insight into the function of PEI cathode interlayer on improving the work function and resistance of ITO electrode in the inverted PSCs, and provides some instructions on the future design of interlayer materials in PSCs.     

Stability enhancement of normal-geometry organic solar cells in a highly damp condition: A study on the effect of top electrodes

We present our efforts to enhance the stability of normal-geometry organic solar cells (n-OSCs), which are generally considered inferior to their inverted-geometry counterparts in terms of stability. Upon the identification of the vulnerability of top electrode/buffer layer interfaces under a humid environment, various top electrode combinations are assessed under an extremely damp condition (27 °C, 90%) for n-OSCs based on a bulk-heterojunction of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C70-butyric acid methylester (PCBM70). Based on the experimental results, we propose an Al/Cu bilayer top electrode and demonstrate a 30-fold enhancement in the T₈₀-lifetime values. Our study reveals that the enhanced lifetime with an Al/Cu bilayer electrode can be attributed to its water vapor transmission rate (WVTR), which is significantly lower than that of the Al electrodes typically used in conventional organic solar cells. The enhanced normal OSCs yielded stability comparable to that of the previously reported inverted OSCs.     

Large area roll-to-roll sputtering of transparent ITO/Ag/ITO cathodes for flexible inverted organic solar cell modules

We fabricated solution-processed flexible inverted organic solar cell (IOSC) modules (10 cm × 10 cm) on roll-to-roll (RTR) sputtered ITO/Ag/ITO multilayer cathodes. By using a pilot-scale RTR sputtering system equipped with mid-range frequency power for dual ITO targets and direct current power for the Ag target, we were able to continuously deposit a high-quality ITO/Ag/ITO multilayer on PET substrate with a width of 700 mm and length of 20,000 mm as a function of Ag thickness. At the Ag thickness of 12 nm, the ITO/Ag/ITO multilayer had a very low sheet resistance of 3.03 Ohm/square and high transmittance of 88.17%, which are better values than those of amorphous ITO film. A strip-type ITO/Ag/ITO cathode was successfully patterned using a RTR wet etching process. Successful operation of flexible IOSC modules on RTR sputtered ITO/Ag/ITO cathodes indicate that the RTR sputtering technique is a promising coating process for fabrication of high-quality transparent and flexible cathodes and can advance the commercialization of cost-efficient flexible IOSCs.     

Energy levels in dilute-donor organic solar cell photocurrent generation: A thienothiophene donor molecule study

To investigate photocurrent generation mechanisms in these organic solar cells (OSCs), we design and synthesize four thienothiophene (TT)-based small-molecule donors with the highest occupied molecular orbital (HOMO) levels varying from −6.4 eV to −5.1 eV, which span across the HOMO value of the [6,6]-phenyl-C70-butyric acid methyl ester (PC₇₁BM) acceptor. We measure TT-based donor:PC₇₁BM films’ electronic and optical properties, OSC current density-voltage characteristic, and external quantum efficiency, and perform density functional theory (DFT) calculations. Our results show that photocurrent generation depends strongly on the substitutions of the center TT groups, cyano (-CN) versus hexyloxy (-OHex). With 1 wt% donor, TTOHex:PC₇₁BM devices produce seven times, increasing to twelve times for 5 wt % donor, higher photocurrent than neat PC₇₁BM devices. In contrast, TTCN:PC₇₁BM devices do not generate additional photocurrent even with 10 wt% donor. The photocurrent generation in TT-based donor:PC₇₁BM devices depends critically on the HOMO value of the donor molecule with respect to that of PC₇₁BM, indicating the importance of type II energy level alignment to facilitate exciton dissociation at the donor-acceptor interface. The photovoltage of all TT:PC₇₁BM devices are comparable to neat PC₇₁BM devices, 0.85–0.90 V, with a low voltage loss due to non-radiative recombination. The fill factor of TTOHex:PC₇₁BM devices are low due to the low hole mobility, ~10⁻⁸ cm²/V. Following exciton dissociation, hole transport is analyzed according to three possible mechanisms: tunneling, percolation pathways, and hole back transfer. We find that the hole back transfer mechanism can explain all experimental results and therefore is the primary hole transport mechanism for photocurrent generation in TT-based donor:PC₇₁BM dilute-donor OSCs.     

银纳米颗粒界面层对有机太阳能电池光敏层相分离的影响

通过原子力显微镜(AFM)和X射线光电子能谱(XP S)表征,研究了在光敏层与阳极缓冲层界面引入银(Ag)纳米颗粒界面层后对PTB7:PC70BM光 敏层相分离的影响,并通过接触角测试对其影响机理进行了分的,结果表明,当引入Ag纳米 颗粒界面层后,光敏层的表面形貌并没有发生 明显的改变,但是光敏层在横向与纵向方向有了更好的相分离;并发现,引入Ag纳米颗粒引 起 的光敏层优化的相分离,本质上源于其底部PEDOT:PSS层表面能的减小,其中起关键作用的 是乙醇溶剂;通过紫外-可见吸收光谱以及相关电池性能测试确认,乙醇溶剂处理PEDOT:P SS表面有助于提高电池的电性能。 本文研究表明,在有机太阳能电池(OSCs)中引入金属纳米颗粒,不仅要考虑其 光学效应,还要考虑由于光敏层相分离的变化引起的电学效应。     

Effects of using multi‐component electrolytes on the stability and properties of solar cells sensitized with simple organic dyes

In this paper we present experimental results for electrochemical (dye‐sensitized) solar cells that were prepared in our laboratory in order to examine some of the major factors affecting the efficiency and the stability of such cells. Nanostructured TiO₂ thin films were prepared and sensitized using an organic dye. For the purpose of this study three different types of electrolytes were developed: a standard‐type electrolyte containing potassium iodide and iodine in propylene carbonate (PC) and two novel, multi‐component electrolytes containing potassium iodide and iodine dissolved in varying mixtures of PC and EG (ethylene glycol). It was demonstrated that the combined properties of the two solvents in the multi‐component electrolytes enhance the efficiency and improve considerably the stability of the cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and characterization of a wide bandgap polymer based on a weak donor-weak acceptor structure for dual applications in organic solar cells and organic photodetectors

We synthesized a novel wide bandgap polymer, PDTFBT, forming a weak donor (WD)-weak acceptor (WA) structure for use in organic photodetectors (OPDs) and organic solar cells (OSCs). The fluorination in the D unit and the alkoxy substitution in the A unit induced WD and WA properties, respectively. The WD-WA structure of PDTFBT effectively broadened the bandgap compared to typical D-A structures, and the S-F and S-O dipole-dipole interactions induces a highly planar backbone structure with excellent π-π stacking in the vertical direction. In OPDs, conformationally less disordered PDTFBT polymer retained the constant responsivity and significantly improved the detectivity of PDTFBT:PC₇₁BM devices even with a thick active layer of 470 nm, contrary to the variation in the responsivity of P3HT:PC₆₁BM devices depending on the thickness. In OSCs, the deep HOMO energy level (−5.57 eV) of PDTFBT led to high Voc of 0.92 V in PDTFBT:PC₇₁BM devices, which was 0.3 eV higher than that of P3HT:PC₆₁BM devices (0.62 V), resulting in 1.8-fold enhanced power conversion efficiency. We demonstrated that the WD-WA structure with S-F and S-O interactions is highly promising strategy to make wide bandgap polymers for organic photodetectors and for the bottom cell of tandem architecture.     

Imaging of contamination on wafers using a scanning surface potential difference measurement technique

The sensitivity of the ChemetriQ^® method toward various contaminants was evaluated. Offering high resolution mapping without edge exclusion, the method based on surface work function lateral non-uniformities, i.e. the so called surface potential difference imaging (SPDI), appears to be very sensitive to traces of metals on Si wafers with native oxide. Using intentional contamination protocols and comparative methods such as total-reflection X-ray fluorescence (TXRF), different dynamics have been seen element to element. As an example, the signal for Fe and Cu contamination on Si is very strong, offering the capability to detect levels in the 10¹⁰ at/cm² range, while it is in the 10¹² at/cm² range for Al. This does not obviously agree with work function differences between Si and the metals; and the exact reason for signal variations must be further investigated. Practical results show that the method can be applied indifferently to CZ wafer front side or backside, whatever the polish finish; can be applied to silicon on insulator (SOI) wafers; and can be very useful for the control of wafer edge contamination left after deposition and further bevel and edge etch steps.     

Influence of Ti dopant on the properties and dye sensitized solar cell performance of ZnO chunk-shaped nanostructures

This paper presents the results of photoanode constructed with undoped and Ti-doped ZnO chunk shaped nanostructures for dye sensitized solar cells (DSSC). Nanostructures are prepared by simple chemical method and moreover the structural, morphological, optical and photovoltaic performances are investigated by XRD, SEM, UV–Vis, PL, and I–V measurements, respectively. The experimental observations demonstrate that the Ti is effectively doped into Zn site, which increases free electron concentration, hereby expected an enhancement in the DSSC performance. Fabricated DSSCs are tested and the results reveal that undoped ZnO shows conversion efficiency of 0.42% with better photocurrent density and photovoltage but the fill factor gets enhance up to 0.447 with 3 mole% (dilute) of Ti doping. Comparably, PEO/KI/LiI/I₂ electrolyte matches well for Ti doped ZnO DSSC due to the easier diffusion path offered by KI salt rather than TiO₂ and PEG additives.     

Influence of temperature on luminescent coupling and material quality evaluation in inverted lattice‐matched and metamorphic multi‐junction solar cells

Inverted metamorphic multi‐junction solar cells have reached efficiencies close to 46%. These solar cells contain very high‐quality materials that exhibit strong luminescent coupling between the junctions. The presence of luminescent coupling has a significant impact on the behavior of multi‐junction solar cells affecting the optimal design of these devices. Because of the importance of studying devices under real operating conditions, the temperature dependence of the luminescent coupling is analyzed over a range of 25–120°C. Luminescent coupling analysis results show a reduction of the luminescent coupling current as a function of temperature in two tandem components of an inverted metamorphic triple junction solar cell such as GaInP/GaAs and GaAs/GaInAs solar cells. This reduction is quantified and examined by means of luminescent coupling analysis and modeling, electroluminescence measurements and optical modeling at the device and subcell level. The results of the models are verified and discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of boron clustering on the emitter quality of implanted silicon solar cells: an atom probe tomography study

The use of ion implantation doping instead of the standard gaseous diffusion is a promising way to simplify the fabrication process of silicon solar cells. However, difficulties to form high‐quality boron (B) implanted emitters are encountered when implantation doses suitable for the emitter formation are used. This is due to a more or less complete activation of Boron after thermal annealing. To have a better insight into the actual state of the B distributions, we analyze three different B emitters prepared on textured Si wafers: (1) a BCl₃ diffused emitter and two B implanted emitters (fixed dose) annealed at (2) 950°C and at (3) 1050°C (less than an hour). Our investigations are in particular based on atom probe tomography, a technique able to explore 3D atomic distribution inside a material at nanometer scale. Atom probe tomography is employed here to characterize B atomic distribution inside textured Si solar cell emitters and to quantify clustering of B atoms. Here, we show that implanted emitters annealed at 950 °C present maximum clusters due to poor solubility at lower temperature and also highest emitter saturation current density (J_0e = 1000 fA/cm²). Increasing the annealing temperature results in greatly improved J_0e (131 fA/cm²) due to higher solubility and a consequently lower number of clusters. BCl₃ diffused emitters do not contain any B clusters and presented the best emitter quality. From our results, we conclude that clustering of B atoms is the main reason behind higher J_0e in the implanted boron emitters and hence degraded emitter quality. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of silver paste rheology and screen parameters on the front side metallization of silicon solar cell

Four commercially available silver pastes for silicon solar cells front side metallization were examined and compared. Both rheological and printing experiments were used to correlate pastes properties and screens characteristics with printing results. Theoretical printability and spreading behavior were first characterized by means of rotational and oscillatory rheological experiments. Further on, the pastes were printed, dried, and fired in a standard solar cell process on an industrial line using a full factorial design of experiments. This methodology allows to study the effect of the paste, the screen, the aperture, and their interactions on the printing results. Especially, the finger aspect ratio, the finger cross section and its relative standard deviation, which are respectively linked to solar cell efficiency, silver consumption, and process stability, were systematically measured. This work allows finding general guidelines for process optimization. Finally, a confirmation test was carried out, leading to 19% efficiency on monocrystalline silicon solar cells in an industrial environment.     

PET柔性衬底上生长绒面ZnO-TCO薄膜及其在薄膜太阳电池中的应用

利用低压金属有机化学气相沉积(LP-MOCVD)技术在PET柔性衬底上低温生长绒面结构ZnO-TCO薄膜,DEZn和H2O作为源材料,B2H6作为掺杂剂.详细研究了薄膜掺杂流量对ZnO薄膜微观结构以及光电性能影响.优化获得的PET/ZnO:B薄膜厚约为1 500nm时,绒面结构PET/ZnO薄膜的方块电阻约为10Ω,可...     

Industrial solar cell tester: study and improvement of I‐V curves and analysis of the measurement uncertainty

Determination of solar cell parameters by illuminated I–V measurement is a standard characterisation technique in the photovoltaic industry. These measurements are carried out under standard conditions (STC: 25 °C, 1000 W/m² AM1.5G spectrum). It can be considered as the most crucial in‐line test for solar cells as it provides the industry with the conversion efficiency, and it is also a reliable quality control test. Reference cells are mainly used in testing equipment to set irradiance and working conditions in the tester/sorter, the rest of the cells being measured and classified by comparison with that reference. An accurate calibration of the irradiance at STC in cell testers and high precision in determining the main parameters of the I–V curve are required; a suitable design of the mechanical components and an adequate selection of different programme options should be made (distribution of the points measured, temperature correction or classification method). Here, we have studied the accuracy of an industrial solar simulator whose mechanical, electrical, electronic and software components were analysed with an individual solar device and a production sample. An uncertainty analysis was carried out in order to determine the power uncertainty and which components to improve in order to reduce it. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of multi-intermediate bands on the behavior of InAs1-xNx/GaAs1-ySby quantum dot solar cell

A mathematical model of quantum dot intermediate band solar cells (QDIBSCs) is investigated using two intermediate bands (IBs). These two IBs arise from the quantum dot (QD) semiconductor material within the bandgap energy. Some parameters such as the width of the QD (W_QD) and the barrier thickness or the inter-dot distances between the QDs (B_T) are studied to show their influence on the performance of the QDIBSC. The time-independent Schrödinger equation, which is solved using the Kronig-Penney model, is used to determine the position and bandwidth energies of the two IBs. In our proposed model, the cubic shape of the QDs from InAs_0.9N_0.1 and the barrier or host semiconductor material from GaAs_0.98Sb_0.02 are utilized. It is shown from the results obtained that changing the parameters W_QD and B_T has more influence on the bandwidth energy for the first IB, Δ₁, than in the case of the second IB, Δ₂. The optimum power conversion efficiencies (PCEs) of the QDIBSCs with two IBs for the model under study are 58.01% and 73.55% at 1 Sun and maximum solar concentration, respectively. One can observe that, in the case of the two IBs, an improvement of the PCE is achieved.