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The paper presents an overview of studies of the multi-junction (MJ) solar cells (SC) with IV characteristics having nonlinear peculiarities. It is shown that such peculiarities can arise either due to hetero-interface barriers (HB) for majority charge carriers or due to problems with tunnel diodes (TD). It is usually difficult to identify which of these hetero-structural objects is the cause of this nonlinearity, since the peculiarities appear similarly in the IV characteristics of MJ SC. The experimental method presented in this work is based on the study of IV characteristics during the overheating of MJ SCs. The behavior of nonlinear peculiarities caused by an imperfect TD or an HB is fundamentally different during the overheating of SCs, which makes it possible to identify the type of hetero-structural object. Since the number of hetero-interfaces in MJ SC is large, a method determining the position of problematic objects in the MJ structure was also developed. The method is based on the illumination of individual subcells using different laser wavelengths. Both methods considered were tested experimentally on GaInP/GaAs/Ge SCs. • Heterobarriers and tunnel diodes can be responsible for nonlinear peculiarities arising in the IV characteristics of MJ SC. • The search for non-optimized layers position inside the SC structure can be performed using specialized structures. • Non-optimized tunnel diode could lead to the presence of an open-circuit voltage drop. • Temperature reaction of IV characteristics allows determining the type of object that gives a nonlinear peculiarity. • Analysis of such a reaction allows us to determine the position of non-optimized hetero-barriers. [ABSTRACT FROM AUTHOR]

Open-circuit voltage (V OC) losses due to various recombination mechanisms are well-recognized limitations towards high-performance solar cells. Thin film Cu(In,Ga)Se 2 (CIGSe)-based solar cells have been well-developed and commercially used in the photovoltaic community due to their high power conversion efficiency (PCE) with high chemical stability and cost-effectiveness. Recently, numerous works on modification of the CIGSe/buffer band alignment and alkali post-deposition treatment on CIGSe have shown obvious enhancement of the solar cell efficiency. In this work, we explore the effects of subgap absorption driven by quasi-Fermi level splitting (QFLS) and Urbach tail (E U) states in CIGSe on V OC losses. In particular, we take all subgap absorption contributions into account to precisely fit the full photoluminescence (PL) spectra and obtain the QFLS and E U energies using an integrated model. We found that for CIGSe films with different Ga contents, the model can fit the PL spectra perfectly, which allows the extraction of reliable values of QFLS and E U. Using these values, we further calculated the V OC in both the SQ and radiative limit, as well as the non-radiative loss due to subgap absorption. We also showed that sample with a larger QFLS and a lower E U exhibits a smaller V OC loss and thus a higher PCE. Although the effects of QFLS on V OC is much smaller than Urbach tail, a more accurate estimation of V OC loss due to non-radiative recombination can be achieved when QFLS is considered. These results shed light on the origin of V OC losses in solar cells, which is potentially useful for the advancement of PV technology. • Cu(In,Ga)Se 2 solar cells were fabricated by 3-stage co-evaporation. • Urbach tail energy and QFLS energy are extracted by fitting the full PL spectrum. • Low Urbach tail energy and large QFLS energy minimize the V OC loss. • Accurate non-radiative V OC loss can be calculated when QFLS is considered. [ABSTRACT FROM AUTHOR]

The optimization of present multijunction junction solar cells and the development of new concepts is an important task for modem photovoltaics. For its development, special attention should be paid to the potential quality of photoactive p-n junctions, as well as to their properties when operating at various temperatures. The present research has shown that for two-diode model of the p-n junction it is convenient to use current invariants J z1 and J z2 for calculating the diffusion and recombination saturation dark currents J 01 and J 02 for p-n junctions based on semiconductor materials with any band-gap energy. It has been experimentally found that J z1 and J z2 are constants relating the semiconductor material band gap energy and temperature with J 01 and J 02. It has also been shown that J z1 and J z2 determine the open circuit voltage and the value of voltage offset W OC. Current invariant suggested to be useful tool for evaluating the quality of a p-n junction and for modeling the characteristics of solar cells because it allows making calculations taking into account both main mechanisms of current flow and the operating temperature of the device. • Current invariant is a new approach for multijunction solar cell optimization. • Current invariant allows calculating dependence of saturation currents on band gap. • The approach takes into account both the diffusion and recombination current flows. • Experimentally shown that current invariant value doesn't depend on temperature. • Voltage offset linearly depends on temperature and is about 0.46 V at T = 300K [ABSTRACT FROM AUTHOR]

Photovoltaic device requires light-absorbing materials that are highly efficient, low cost, lightweight, and stable during operation. Here we demonstrate the use of a hydrogenated amorphous silicon (a-Si:H) top cell and a crystal silicon heterojunction (HIT) bottom cell to form a double-junction solar cell with a high open circuit voltage ( V OC ), which is potentially functioned in the solar-to-hydrogen generation process and the replacement of chemical battery. Our photovoltaic device shows a high overall V OC of 1.561 V and a short current density of over 14 mA/cm 2 , which outperforms all other reported pure silicon-based double-junction solar cells. The newly developed a-Si:H/HIT double-junction solar cells hold the promise for some potential applications. [ABSTRACT FROM AUTHOR]

A direct mapping of photovoltage in a complete Mo/CZTSSe/CdS/ZnO/Al:ZnO solar cell device is carried out using Kelvin probe force microscopic (KPFM) measurements in surface and junction modes. Four cells having different values of open circuit voltage (V OC ) have been studied, and nanoscale variation of photovoltage have been obtained from the difference of surface potential (SP) images in the two modes. The maps exhibit a higher photovoltage at grain boundaries in general. Observed SP image of pristine CZTSe layer reveals downward band bending resulting in reduction of carrier recombination, and thus, lower J dark at grain boundaries (GB). The observed downward band bending resulting in low J dark at GB in CZTSe layer is used to explain the increase in photovoltage (V OC ) at the GB. KPFM measurements of CZTSe and CZTSe/CdS layers show that the surface potential of CZTSe layer and its variation can be affected by CdS deposition and further device processing, in addition to surface adsorption and contamination effects. The photovoltage mapping obtained from Kelvin probe force microscopic measurements on the final device without any interfering effects, is an important advantage of the present method. [ABSTRACT FROM AUTHOR]

This paper presents an original investigation method to highlight the differences of mobilities and transport mechanisms of charge carriers in organic materials solar cells. Two types of polymer: fullerene solar cells were investigated: ITO/ PEDOT:PSS/P3HT:PCBM /Al and ITO/PEDOT:PSS/PCDTBT:PCBM/Al. The I–V characteristics were done under standard illumination (1000 W/m 2 ) using a mask placed at different distances from the collecting cathode position. The influence of the distance between the irradiation area and the position of the collecting electrode on the performances of solar cells was studied. It has been observed that the short-circuit current and the open-circuit voltage considerably increase with the decrease of the distance between the cathode and the irradiation area. This study method gives also the opportunity to find the suitable collecting grid layout (distance between electrodes) depending on the nature of the organic materials. Moreover it can also explain the differences observed between the reported values in literature for the conversion efficiencies of organic solar cells. [ABSTRACT FROM AUTHOR]

A novel 56 π-electron fullerene acceptor material, benzocyclobutene-C 60 bisadducts (BCBCBA), was developed via the [2+2] cycloaddition of in-situ generated benzyne with C 60 to raise the lowest unoccupied molecular orbital (LUMO) energy level of the fullerene. This four-membered linkages to C 60 of BCBCBA resulted in the fullerene bisadducts with very addends, thus minimizing the adverse effects of their regioisomers on the intermolecular packing between fullerene bisadducts. In addition, the LUMO level of BCBCBA was as high as −3.70 eV, which is 0.16 eV higher than that of phenyl-C 61 -butyric acid methyl ester (PCBM) (−3.86 eV). This result indicates the significant potential for improving open-circuit voltage ( V OC ) values in polymer solar cells (PSCs). The poly(3-hexylthiophene) (P3HT):BCBCBA based PSCs show V OC of 0.73 and power conversion efficiency (PCE) of 3.51%, which are similar to the P3HT:bis-PCBM based PSCs. [ABSTRACT FROM AUTHOR]

Physics ruling the temperature sensitivity of photovoltaic (PV) cells is discussed. Dependences with temperature of the fundamental losses for single junction solar cells are examined and fundamental temperature coefficients (TCs) are calculated. Impacts on TCs of the incident spectrum and of variations of the bandgap with temperature are highlighted. It is shown that the unusual behavior of the bandgaps of perovskite semiconductor compounds such as CH 3 NH 3 PbI 3- x Cl x and CsSnI 3 will ultimately, in the radiative limit, give PV cells made of these materials peculiar temperature sensitivities. The different losses limiting the efficiency of present commercial cells are depicted on a p–n junction diagram. This representation provides valuable information on the energy transfer mechanisms within PV cells. In particular, it is shown that an important fraction of the heat generation occurs at the junction. A review of the loss mechanisms driving the temperature coefficients of the different cell parameters (open circuit voltage V oc , short circuit current density J sc , fill factor FF ) is proposed. The temperature sensitivity of open circuit voltage is connected to the balance between generation and recombination of carriers and its variation with temperature. A general expression that relates the temperature sensitivity of V oc to the External Radiative Efficiency (ERE) of a solar cell is provided. Comparisons with experimental data are discussed. The impacts of bandgap temperature dependence and incident spectrum on the temperature sensitivity of short circuit current are demonstrated. Finally, it is argued that if the fill factor temperature sensitivity is ideally closely related to the open circuit voltage temperature sensitivity of the cell, it depends for some cells strongly on technological issues linked to carrier transport such as contact resistances. [ABSTRACT FROM AUTHOR]

The open circuit voltage (V OC) deficit in Cd(Se,Te)-based photovoltaics remains a critical obstacle for pushing the technology closer to theoretical performance limits. Arsenic doping has become a dominant and promising route to achieve the higher p-type carrier concentrations necessary for higher V OC , but challenges associated with this alternate defect chemistry and higher doping density have hindered progress. Here we show that while arsenic doping enables high carrier concentrations (>1016 cm−3), co-doping with copper can provide a boost to V OC without a significant change to carrier concentration. A large data set is initially used to explore current-voltage and capacitance-voltage trends associated with arsenic doped devices with and without copper. A smaller subset is then used to probe these trends using a wide variety of characterization techniques. Copper is found to facilitate reduced interface recombination and potentially improved bulk absorber characteristics, though the mechanisms for these improvements are not yet clear. Despite the improved performance of co-doped devices, V OC is still far below its potential especially for highly doped devices. Low emitter doping in conjunction with high absorber doping seems to be a plausible cause for this significant deficit, though other device properties may exacerbate this problem. • Cu does not (necessarily) compensate high carrier concentrations in As:Cd(Se,Te). • Advanced characterization of As and Cu related performance limitations in 4 regimes. • High carrier concentration Cd(Se,Te) absorbers are more sensitive to recombination. • Reducing doping near the interface may limit recombination-related Voc loss. [ABSTRACT FROM AUTHOR]

Abstract: Polymer solar cells (PSCs) based on alkyl-substituted thienylenevinylene and phthalimide-based D-A copolymer (PTVPhI-Eh) as a donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor were investigated with the intent of gaining high performance photovoltaic properties. PTVPhI-Eh had a deep highest-occupied molecular orbital (HOMO) level, ~0.3eV lower than rr-P3HT. As a result, PTVPhI-Eh:PC71BM devices showed a high open circuit voltage (V oc ) of 0.87V, which is over 0.2V higher compared to that of P3HT:PCBM-based PSCs. The maximum power conversion efficiency (PCE) of PSCs was 4.14% with V oc of 0.87V, short circuit current (J sc ) of 7.32mA/cm2, and fill factor (FF) of 65%, under optimized conditions of a 1:1wt/wt ratio and treatment with a 2vol% 1-chloronaphthalene solvent additive. The V oc and consequent photovoltaic performance of this PSC was significantly affected by the concentration of the solvent additive. To determine the relationship between V oc and concentration of solvent additive, PTVPhI-Eh:PC71BM devices were investigated with various analyses such as optical spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). [Copyright &y& Elsevier]

Sandwich configurations consisting of hole-only and electron-only devices are employed to study the transport properties of polymer/fullerene blended material. Upon thermal annealing at 150°C and subsequent device characterisations a direct correlation between solar cell device power conversion efficiency and charge carrier mobilities is identified. The highest power conversion efficiency reported herein resulted from the electron to hole mobility ratio of 4.78. This ratio further reduces to 0.357 at 175°C as a result of the deterioration of electron mobility at the same time as enhancement in hole carrier mobility with no further improvement in PCE. Effects of thermal annealing on trap density reveal that the trap assisted recombination is more dominant as compared to the bimolecular recombination. Furthermore such a mechanism enhances as a result of increasing trap density at higher annealing temperatures. The effect of light intensity on the open circuit voltage V OC further verifies that the dominant recombination process is through trap assisted recombination. [ABSTRACT FROM AUTHOR]

Abstract: A novel C70 fullerene derivative ICBM was rationally designed and synthesized by [4+2] cyclic addition reaction between an indene derivative (methyl 4-(1H-inden-3-yl)butanoate) and C70. The absorption and photoluminescence of the mixed films of ICBM with different polymer donor materials were characterized. The electrochemical reduction behavior of ICBM in the range of 0 to −2.5V was investigated by the cyclic voltammetry method, and the result shows that the lowest unoccupied molecular orbital (LUMO) level of ICBM is 0.07eV higher than that of PC70BM. The photovoltaic devices based on poly (3-hexylthiophene) (P3HT) and a low band gap polymer (PTB7) as donors, with ICBM as an acceptor gave power conversion efficiencies of 5.00% and 6.67%, respectively. The result demonstrates that this novel C70 fullerene derivative is a much better acceptor for wide-band gap as well as low-band gap donor polymers in the BHJ polymer photovoltaics. [Copyright &y& Elsevier]

Abstract: In this study, we investigate the mechanisms leading to the power conversion efficiency improvement in poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) based organic solar cells using calcium (Ca) in cathode structures. Ultraviolet and x-ray photoemission spectroscopy (UPS and XPS) indicate that chemical reactions occur at the P3HT/Ca interface. Upon Ca deposition, UPS results illustrate a 0.8eV-downward shift in energy levels of P3HT, but not in those of PCBM. In addition to forming an ohmic contact at the cathode the presence of Ca widens the energy difference between the HOMO of P3HT and the LUMO of PCBM at the cathode interfaces, which results in the increase of open circuit voltage and the enhancement of device efficiency. [Copyright &y& Elsevier]

Abstract: Photovoltaic devices made from metallo-phthalocyanine/fullerene (M-Pc/C60) with 5,6,11,12-tetraphenylnaphthacene (rubrene) as an additive are fabricated and characterized. The effect of rubrene is examined for 4 different M-phthalocyanines – H2Pc, ZnPc, ClInPc, and VOPc – to represent 4 different valencies of the central moiety of M-Pc. In each case, rubrene has shown a notable increase in the open circuit voltage and in the case of the ClInPc and VOPc results in an increase in the overall power conversion efficiency. Through measurement of external quantum efficiency, it is shown that the increased efficiency is due to increased V oc and not due to the photocurrent contribution from the complementary absorption profile of rubrene. Finally, the photostability of rubrene-based cells is studied, showing that unencapsulated devices decay rapidly in air as a result of the formation of rubrene peroxide, as evidenced by significant decay of the UV–vis absorption and direct measurement of the cell performance over a time period as short as several minutes. [Copyright &y& Elsevier]

Abstract: 1, 2-dicyano-methanofullerene (C60(CN)2) is a soluble fullerene derivative that has been reported to have stronger electron affinity than parent C60. Ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) experiments were carried out on C60(CN)2 thin films spin coated on heavily doped n-type Si substrate. UPS spectra enabled the determination of the vacuum shift at the fullerene derivative/Si interface and the onset of the highest occupied molecular orbital (HOMO). From the UV–vis absorption spectra of C60(CN)2 thin films spin coated on quartz substrates, the optical band gap (E g ) and the onset of absorption were determined. These measurements allowed the determination of the lowest occupied molecular orbital (LUMO) position. The morphology of the deposited film was probed by AFM and reveals non-uniformity of the thin film. Open circuit voltage (V oc ) measurements on P3HT/C60(CN)2 based organic solar cell device are compared to the commonly used P3HT/PCBM device. [Copyright &y& Elsevier]

Abstract: “Bulk heterojunctions” from polymers and inorganicmolecules or nanoparticles offer simple internal structures for organic photovoltaic cells. Systems proposed so far need quite expensive or instable materials, often with limited availability; open-circuit voltages are well below 1V. In this work, a system prepared from poly-3-hexyl-thiophene in contact with ZnS nanoparticles is described. Its almost perfect match of the electronic levels leads to a very high open-circuit voltage, not available until now in similar systems. [Copyright &y& Elsevier]

Abstract: A series of heterojunctions consisting of intrinsic zinc oxide (ZnO) films and p-type Si substrates have been prepared by DC reactive sputtering. The ZnO films were grown at different conditions, and the influence of growth conditions on photovoltaic (PV) property was discussed. It was found that both growth temperature and oxygen partial pressure play important roles for enhancing the PV effect of the samples. By optimizing growth conditions, the PV efficiency has been improved and also by more magnitudes. The open circuit voltage (V oc) and short circuit current (I sc) per square centimeter arrived at 350mV and 2.5mA, respectively. The variation mechanism of PV effect with growth conditions has been investigated in order to understand the photoelectric conversion behavior of the ZnO/Si heterojunction. [Copyright &y& Elsevier]

Abstract: TiO2-overcoated SnO2:F transparent conductive oxide films were prepared by atmospheric pressure chemical vapor deposition (APCVD) and an effect of TiO2 layer thickness on a-Si solar cell properties was investigated. The optical properties and the structure of the TiO2 films were evaluated by spectroscopic ellipsometry and X-ray difractometry. a-Si thin film solar cells were fabricated on the SnO2:F films over-coated with TiO2 films of various thicknesses (1.0, 1.5 and 2.0nm) and I–V characteristics of these cells were measured under 1 sun (100mW/cm2 AM-1.5) illumination. It was found that the TiO2 film deposited by APCVD has a refractive index of 2.4 at 550nm and anatase crystal structure. The conversion efficiency of the a-Si solar cell fabricated on the 2.0nm TiO2-overcoated SnO2:F film increased by 3%, which is mainly attributed to an increase in open circuit voltage (V oc) of 30mV. [Copyright &y& Elsevier]

Abstract: Open circuit voltage (V oc) and other photovoltaic parameters from fluorine tin oxide (FTO) P3OT/TiO2 composite solar cells have been investigated in comparison with those from the indium tin oxide (ITO) devices with the same device structure and fabrication process. From the experimental results, the performance of FTO-based devices is better than that of ITO devices in terms of V oc, short circuit current density (J sc), and power conversion efficiency. The origin of V oc and the higher V oc of FTO can be explained and estimated by metal–insulator–metal model with a non-ohmic cathode contact. [Copyright &y& Elsevier]

Motivated by possible application of InGaN/GaN multi-quantum well solar cells in hybrid concentrated photovoltaic / solar thermal power conversion systems, we have analyzed the temperature and intensity dependence of the open-circuit voltage of such devices up to 725 K and more than 1000 suns. We show that the simple ABC model routinely used to analyze the measured quantum efficiency data of InGaN/GaN LEDs can accurately reproduce the temperature and intensity dependence of the measured open-circuit voltage if a temperature-dependent Shockley–Read–Hall lifetime is used and device heating is taken into account. • Open-circuit voltage of InGaN quantum-well solar cells is well described by recombination models. • Temperature dependence is captured by a temperature dependent Shockley–Read–Hall model. • Self-heating becomes relevant at high solar concentration. • A transition to a radiatively limited regime is predicted below 200 K. [ABSTRACT FROM AUTHOR]

Silicon solar cells are now less than 3% absolute from the theoretical efficiency limit. Advanced passivated contact architectures have demonstrated surface saturation current densities close to 1 fA/cm2. We have optimized the thin intrinsic hydrogenated amorphous silicon layer by controlling the deposition temperature and the silane-to-hydrogen dilution ratio. Thin wafers were used as a testbed to increase the sensitivity to surface passivation. By optimizing the intrinsic layer, we reduced the surface saturation current densities from 1.7 fA/cm2 to 0.6 fA/cm2 on textured wafers with thicknesses ranging between 40 and 180 μm. Implied open-circuit voltages over 760 mV were accomplished on p-i/c-Si/i-n structures deposited on n-type CZ wafers with wafer thicknesses below 50 μm. Further, we demonstrated experimentally the potential of using very thin wafers by manufacturing screen-printed silicon heterojunction solar cells on 40 μm thick standalone wafers while achieving an efficiency of 20.48%. • Optimized i-a-Si:H layer by controlling the deposition temperature and the silane-to-hydrogen dilution ratio. • 40 μm thin wafers were used as a testbed to increase the sensitivity to the surface passivation. • Average surface saturation current densities of 0.6 fA/cm2 were measured on textured wafers. • Implied open-circuit voltages over 760 mV were accomplished on n-type CZ wafers with wafer thicknesses below 50 µm. • We manufactured screen-printed silicon heterojunction solar cells on 40 µm thick wafers with efficiencies close to 21%. [ABSTRACT FROM AUTHOR]

In this work we investigate the effect of rapid thermal annealing (RTA) on the performance of solar cells consisting of different GaAsSbN-based structures and correlate the device results with modifications of the optical and structural properties of the alloy. In particular, bulk layers grown at different growth rates and type-II GaAsSb/GaAsN superlattices with different period thickness are analyzed. We find evidences of material quality improvement after the annealing process such as a reduction of N-related radiative defects and Sb clusters. These RTA-induced changes lead to a notable enhancement of the open circuit voltage (V OC), which results in values of the bandgap-voltage offset (W OC = E G /q-V OC) comparable to that of a non-optimized reference GaAs solar cell with the same device structure (W OC ∼0.63 eV). The decrease in W OC after annealing shows a correlation with the reduced radiative recombination at low energy N-related sub-bandgap states. These results suggest that radiative recombination in a broad band of deep defect states is a source of V OC degradation in GaAsSbN solar cells. • Sub-bandgap deep radiative states induced by N are present in GaAs(Sb)N. • Reduction of the N-related radiative defect density after rapid thermal annealing. • Elimination of sub-bandgap optically active states correlates with enhanced V OC. • W OC values of GaAsSbN solar cells comparable to reference GaAs cell (∼0.63 eV). [ABSTRACT FROM AUTHOR]