Voltage Of Solar Cells Research Articles

Improvement of Ga distribution in Cu(In, Ga)(S, Se)2 film by pretreated Mo back contact

In the two-step selenization process, Ga accumulation near back contact is a common problem, resulting in a lower band gap near front surface and then lower open circuit voltage (VOC) of solar cell. In this work, Ga accumulation in Cu(In, Ga)(S, Se)2 (CIGSSe) film was weakened by pre-sulfurized and selenized (PSS) Mo back contacts. The sputtered dense Mo back contacts were sulfurized and selenized at different temperatures before Cu-In-Ga precursor deposition. Raman results show that MoS2, MoSe2 and pure Se phases were formed on the PSS Mo surface. These selenide and sulfide phases will become the bottom Se and S sources in the structure of PSS Mo/metal precursor during selenization process. The thermodynamic calculation confirms the bottom S and Se sources are effective. SIMS measurement shows the Ga back grading in CIGSSe films decreases when the PSS Mo back contacts are used. For higher substrate temperature (Tsub) we find reduced steepness of the Ga back gradient. The S atomic percentage in CIGSSe is so small that the band gaps of CIGSSe are mainly determined by the Ga/(Ga + In) distribution. Therefore, the band gap in the space charge region (SCR) of solar cell increased when PSS Mo was used. As expected, VOC increased about 10 mV when TSub increased to 600 °C. Due to the enhancement of VOC and fill factor (FF), the conversion efficiencies (η) of solar cells was improved.

Impact of carrier doping on electrical properties of laser-induced liquid-phase-crystallized silicon thin films for solar cell application

Large-grain-size (>1 mm) liquid-phase-crystallized silicon (LPC-Si) films with a wide range of carrier doping levels (1016–1018 cm−3 either of the n- or p-type) were prepared by irradiating amorphous silicon with a line-shaped 804 nm laser, and characterized for solar cell applications. The LPC-Si films show high electron and hole mobilities with maximum values of ∼800 and ∼200 cm2 V−1 s−1, respectively, at a doping level of ∼(2–4) × 1016 cm−3, while their carrier lifetime monotonically increases with decreasing carrier doping level. A grain-boundary charge-trapping model provides good fits to the measured mobility–carrier density relations, indicating that the potential barrier at the grain boundaries limits the carrier transport in the lowly doped films. The open-circuit voltage and short-circuit current density of test LPC-Si solar cells depend strongly on the doping level, peaking at (2–5) × 1016 cm−3. These results indicate that the solar cell performance is governed by the minority carrier diffusion length for the highly doped films, while it is limited by majority carrier transport as well as by device design for the lowly doped films.

Reciprocity and Open-Circuit Voltage in Solar Cells

The equation for open-circuit voltage of a solar cell based on optoelectronic reciprocity is combined with the standard textbook formula to obtain a general result that includes photon recycling as well as losses by carrier transport. It is shown that, in indirect-gap semiconductors, the general expression reduces to the “conventional” and “optoelectronic” expressions in the limits of low and high radiative efficiency.

Different methods to fabricate efficient planar perovskite solar cells based on solution-processing Nb2O5 as electron transporting layer

Organo-lead perovskites as light harvesters have represented a hot field of research on high-efficiency perovskite solar cells. Previous approaches to increasing the solar cell efficiency have focused on optimization of the morphology of perovskite film. In fact, the electron transporting layer (ETL) also has a significant impact on solar cell performance. Herein, we introduce a facile and low temperature solution-processing method to deposit Nb2O5 film as ETL for PSCs. Based on Nb2O5 ETL, we investigate the effect of the annealing time for the perovskite films via different solution processing, relating it to the perovskite film morphology and its influence on the device working mechanisms. These results shed light on the origin of photovoltaic performance voltage in perovskite solar cells, and provide a path to further increase their efficiency.

Optimum driving energy for achieving balanced open-circuit voltage and short-circuit current density in organic bulk heterojunction solar cells.

Organic bulk heterojunction solar cells generally suffer from a trade-off between the open circuit voltage (Voc) and the short circuit current density (Jsc) under a given donor/acceptor (D/A) interfacial energetic offset (or the so-called driving force). Here we theoretically investigate the optimum driving energy required for achieving the balanced Jsc and Voc simultaneously. To this end, the Jscversus the driving force ΔE curves are calculated under two different charge separation mechanisms by employing the drift-diffusion method. For the Marcus incoherent mechanism, the curve features a high plateau in a broad range of ΔE starting from 0.2 eV, which is due to the accumulation of undissociated excitons within their lifetime and signifies the possibility of obtaining a sizable Jsc under a ΔE value much smaller than the reorganization energy. After incorporating both the electron and hole transfer pathways into the device model, the calculated J-V curves are comparable to experimentally measured ones foractual blended systems of different driving forces. For the coherent mechanism, it is demonstrated that the maximum Jsc can also be achieved under the ΔE of 0.2 eV if a large proportion of the high-lying delocalized states are harvested through tuning the density of states for the charge transfer excitons to reduce the sub-gap states. This theoretical work revealed quantitatively the relationship between the interfacial energy offsets and device performance, and also provides some guidelines for identifying the macroscopic features of the actual charge separation mechanisms in bulk heterojunction solar cells.

High voltage vacuum-deposited CH3NH3PbI3–CH3NH3PbI3 tandem solar cells

High open circuit voltage solar cells are obtained employing two methyl-ammonium lead iodide absorbers in a monolithic tandem configuration joined by a molecular charge recombination layer.

Roles of Pseudo-Closed s2 Orbitals for Different Intrinsic Hole Generation between Tl-Bi and In-Bi Bromide Double Perovskites.

Although metal halide double perovskites A2B(I)B(III)X6 are expected as nontoxic alternatives for lead halide perovskites, recent studies have shown that only Tl(I)-Bi(III) and In(I)-Bi(III) bromides are thermodynamically stable and possess optoelectronic properties suitable for photovoltaic absorbers. Here, we show, through density functional theory calculations, that Tl-Bi and In-Bi bromide double perovskites exhibit significantly different semiconducting behaviors due to the different energy levels of the highest-occupied pseudoclosed s2 orbitals of Tl(I) and In(I). While Tl-Bi double perovskites can exhibit semiconducting p-type properties, In-Bi bromide double perovskites exhibit metallic p-type ones regardless of the synthesis condition due to the extremely low formation energy of In vacancy. Such difference makes Tl-Bi bromide double perovskites suitable for optoelectronic applications, but not In-Bi bromide double perovskites. Furthermore, there is a high probability for In to substitute a Bi site, forming a local In-In bromide double perovskite structure with a lower local conduction band minimum, detrimentally affecting the open circuit voltage of In-Bi bromide double perovskite-based thin film solar cells.

CdTe thin film solar cells with a SnTe buffer layer in back contact

In recent years CdTe thin film solar cells (SCs) have demonstrated promising market application foreground. However, a relatively large gap between the achieved SCs efficiencies and the theoretically predicted maximum efficiency still exists. In particular, the open circuit voltage (Voc) of the CdTe thin film SCs could be further increased, which is limited by high-work-function of CdTe. In this work, p-type narrow-gap SnTe thin films are proposed as a buffer layer in the back contact of CdTe SCs. The band alignment at SnTe/CdTe interface renders a small valance band offset, which expedites hole transport from the CdTe absorber to the hole electrode and improves the Ohmic contact for CdTe. With the insertion of a SnTe single buffer layer at the back contact, a Cu-free CdTe SC has been developed, which displays the same cell efficiency as the traditional cells using CuxTe as the back contact. The Voc of cells with ZnTe: Cu/SnTe double-layer at back contact is notably higher than the CdTe SCs fabricated with CuxTe or ZnTe: Cu single-layer at the back contact. Furthermore, accretion of hole concentration is also demonstrated by the diffusion of Sn in the CdTe absorber layer. This work shows the potential to improve CdTe SCs by using a narrow-gap IV-VI compound semiconductor.

Optimized efficiency in InP nanowire solar cells with accurate 1D analysis

Semiconductor nanowire arrays are a promising candidate for next generation solar cells due to enhanced absorption and reduced material consumption. However, to optimize their performance, time consuming three-dimensional (3D) opto-electronics modeling is usually performed. Here, we develop an accurate one-dimensional (1D) modeling method for the analysis. The 1D modeling is about 400 times faster than 3D modeling and allows direct application of concepts from planar pn-junctions on the analysis of nanowire solar cells. We show that the superposition principle can break down in InP nanowires due to strong surface recombination in the depletion region, giving rise to an IV-behavior similar to that with low shunt resistance. Importantly, we find that the open-circuit voltage of nanowire solar cells is typically limited by contact leakage. Therefore, to increase the efficiency, we have investigated the effect of high-bandgap GaP carrier-selective contact segments at the top and bottom of the InP nanowire and we find that GaP contact segments improve the solar cell efficiency. Next, we discuss the merit of p-i-n and p–n junction concepts in nanowire solar cells. With GaP carrier selective top and bottom contact segments in the InP nanowire array, we find that a p–n junction design is superior to a p-i-n junction design. We predict a best efficiency of 25% for a surface recombination velocity of 4500 cm s−1, corresponding to a non-radiative lifetime of 1 ns in p–n junction cells. The developed 1D model can be used for general modeling of axial p–n and p-i-n junctions in semiconductor nanowires. This includes also LED applications and we expect faster progress in device modeling using our method.

Effects of Nonradiative Losses at Charge Transfer States and Energetic Disorder on the Open‐Circuit Voltage in Nonfullerene Organic Solar Cells

AbstractThe considerable improvement on the power conversion efficiency (PCE) for emerging nonfullerene polymer solar cells is still limited by considerable voltage losses that have become one of the most significant obstacles in further boosting desired photovoltaic performance. Here, a comprehensive study is reported to understand the impacts of charge transport, energetic disorder, and charge transfer states (CTS) on the losses in open‐circuit voltage (Voc) based on three high performing bulk heterojunction solar cells with the best PCE exceeding 11%. It is found that the champion poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene)‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)‐benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione))] (PBDB‐T):IT‐M solar cell (PCE = 11.5%) is associated with the least disorder. The determined energetic disorder in part reconciles the difference in Voc between the solar cells. A reduction is observed in the nonradiative losses (ΔVnonrad) coupled with the increase of energy of CTS for the PBDB‐T:IT‐M device, which may be related to the improved balance in carrier mobilities, and partially can explain the gain in Voc. The determined radiative limit for Voc combined with the ΔVnonrad generates an excellent agreement for the Voc with the experimental values. The results suggest that minimizing the energetic disorder related to transport and CTS is critical for the mitigation of Voc losses and improvements on the device performance.

Determination of the electronic properties of Cu2ZnSnSe4-based solar cells by impedance spectroscopy and current–voltage characteristics analysis

Cu2ZnSnSe4 (CZTSe) thin films were synthesized by a cosputtering–annealing process and characterized as absorber layer of solar cells. Electrical characteristics of the solar cells were analyzed by coupling current–voltage measurements with impedance spectroscopy. The slightly p-type nature of the CZTSe absorber is evidenced with a doping level as low as 1.4 × 1015 cm−3. The solar cells exhibit a dominant recombination pathway in the space charge region in which it was revealed an apparent increase of the doping due to the ionization of deep level in the bandgap. Moreover, open circuit voltage of solar cells is found to be mainly limited by the low built-in voltage at the CdS/CZTSe interface. We conclude that the device architecture can play a major role in the electrical properties of solar cells.

Separating grain-boundary and bulk recombination with time-resolved photoluminescence microscopy

Two-photon excitation (2PE) microscopy allows contactless and non-destructive cross-sectional analysis of grain-boundary (GB) and grain-interior (GI) properties in polycrystalline solar cells, with measurements of doping uniformity, space-charge field distribution, and carrier dynamics in different regions of the device. Using 2PE time-resolved microscopy, we analyzed charge-carrier lifetimes near the GBs and in the GI of polycrystalline thin-film CdTe solar cells doped with As. When the grain radius is larger than the minority-carrier diffusion length, GI lifetimes are interpreted as the bulk lifetimes τB, and GB recombination velocity SGB is extracted by comparing recombination rates in the GI and near GBs. In As-doped CdTe solar cells, we find τB = 1.0–2.4 ns and SGB = (1–4) × 105 cm/s. The results imply the potential to improve solar cell voltage via GB passivation and reduced recombination center concentration in the GI.

Dithienosilole–phenylquinoxaline‐based copolymers with A‐D‐A‐D and A‐D structures for polymer solar cells

ABSTRACTTwo copolymers having D‐A‐D‐A (P1) and D‐A (P2) structures with quinoxaline acceptor unit and dithienosilole donor unit were synthesized and their optical and electrochemical (both experimental and theoretical) properties were investigated. The optical properties showed that these copolymers P1 and P2 exhibit optical bandgaps of 1.54 and 1.62 eV, respectively, with broader absorption profiles extending up to 800 nm and 770 nm, respectively. The electrochemical investigation of these two copolymers indicates that they exhibit suitable highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels for efficient exciton dissociation and high open circuit voltage in the resultant polymer solar cells (PSCs). These copolymers were used as donors along with the PC71BM as acceptor for the fabrication of solution processed bulk heterojunction PSCs. The optimized P1:PC71BM and P2:PC71BM active layers treated with solvent vapor treatment showed overall power conversion efficiency (PCE) of 7.16% and 6.57%, respectively. The higher PCE of P1‐based device as compared to P2 might be attributed to higher crystallinity of P1 and good hole mobility resulting more balanced charge transport. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 376–386

CsPbBr3 Solar Cells: Controlled Film Growth through Layer-by-Layer Quantum Dot Deposition

All inorganic cesium lead bromide (CsPbBr3) perovskite is a more stable alternative to methylammonium lead bromide (MAPbBr3) for designing high open-circuit voltage solar cells and display devices. Poor solubility of CsBr in organic solvents makes typical solution deposition methods difficult to adapt for constructing CsPbBr3 devices. Our layer-by-layer methodology, which makes use of CsPbBr3 quantum dot (QD) deposition followed by annealing, provides a convenient way to cast stable films of desired thickness. The transformation from QDs into bulk during thermal annealing arises from the resumption of nanoparticle growth and not from sintering as generally assumed. Additionally, a large loss of organic material during the annealing process is mainly from 1-octadecene left during the QD synthesis. Utilizing this deposition approach for perovskite photovoltaics is examined using typical planar architecture devices. Devices optimized to both QD spin-casting concentration and overall CsPbBr3 thickness produce...

High Quality CdS/CdTe P-N Junction Diode With a Noncontinuous Resistive SnO2 Buffer Layer

In order to reduce light absorption of the n-type CdS window layer in high efficiency CdTe thin film solar cells, the window layer should be as thin as possible but enough thickness should be left to keep function of the p-n junction. However, many random shunting/shorting micropaths are unavoidably formed at the CdS/CdTe interface due to the CdS consumption up by the reaction/interdiffusion, or due to a noncontinuous coverage of CdS on transparent-conducting-oxide (TCO) thin films. These shunting/shorting paths are the main cause for the low open-circuit voltage and low efficiency of CdTe thin film solar cells. In this study, we propose to employ a noncontinuous highly resistive tin oxide (SnO2) layer between the TCO and the CdS window layer as an electronic shunting blocking layer. The experimental results demonstrated that the insertion of a noncontinuous SnO 2, which had a thickness of less than 10 nm, led to an enhancement in efficiency by 20% and an increase in open-circuit voltage ( $V_{{\rm{oc}}}$ ) by 14% compared to the one without SnO2. This study demonstrated an important design strategy for thin film solar cell fabrication to effectively reduce microshunting/shorting paths and to improve fabrication reproducibility of devices as well.

Efficient kesterite solar cells with high open-circuit voltage for applications in powering distributed devices

Simultaneously achieving high voltage and high efficiency in thin-film solar cells is of paramount importance for real-world applications. While solar cells fabricated from the Earth-abundant kesterite absorber Cu2ZnSn(S x Se1−x )4 provide an attractive, non-toxic, energy harvesting solution, their utilization has been constrained by relatively low open-circuit voltages that limit efficiency. Increasing the sulfur content to widen the bandgap boosts the voltage, but usually at the expense of efficiency. Here, we report important progress on this fundamental problem by fabricating solar cells with high sulfur content that exhibit efficiencies up to 11.89% with open-circuit voltages as high as 670 mV. In a multistep process, fully functional solar cells are separated from their growth substrate, and a high-work-function back contact is subsequently deposited. With this approach, we fabricated a series-connected device that produces 5.7 V under 1 Sun illumination and ~2 V under low lighting conditions, below 10−3 Suns. Kesterite thin-film solar cells feature abundant non-toxic elements. Here, Antunez et al. present a process to simultaneously optimize the conversion efficiency and voltage over a wide range of light intensities appropriate for small-scale, distributed and indoor applications.

Measurement of subcell depletion layer capacitances in multijunction solar cells

A method for measuring subcell capacitance voltage (C–V) in a multijunction solar cell is introduced. The subcell of interest is illuminated by a monochromatic light pulse with a ns rise time. The subcell capacitance is calculated from the measured rise time of the solar cell voltage. The effect of optical coupling is eliminated by introducing a high intensity bias illumination to all subcells below the one measured. The method is verified by comparing the subcell capacitance obtained from four junction solar cells with the results from corresponding component cells, which can be measured using well-established methods. From the C–V curves, the built-in voltage and the base layer doping density for each subcell are calculated.

Design Rule for Improved Open-Circuit Voltage in Binary and Ternary Organic Solar Cells.

Mixing different compounds to improve functionality is one of the pillars of the organic electronics field. Here, the degree to which the charge transport properties of the constituent materials are simply additive when materials are mixed is quantified. It is demonstrated that in bulk heterojunction organic solar cells, hole mobility in the donor phase depends critically on the choice of the acceptor material, which may alter the energetic disorder of the donor. The same holds for electron mobility and disorder in the acceptor. The associated mobility differences can exceed an order of magnitude compared to pristine materials. Quantifying these effects by a state-filling model for the open-circuit voltage (VOC) of ternary Donor:Acceptor1:Acceptor2 (D:A1:A2) organic solar cells leads to a physically transparent description of the surprising, nearly linear tunability of the VOC with the A1:A2 weight ratio. It is predicted that in binary OPV systems, suitably chosen donor and acceptor materials can improve the device power conversion efficiency (PCE) by several percentage points, for example from 11 to 13.5% for a hypothetical state-of-the-art organic solar cell, highlighting the importance of this design rule.

Star-shaped electron acceptors containing a truxene core for non-fullerene solar cells

Abstract A series of new electron acceptors containing a truxene core with intense optical absorption were synthesized and used for non-fullerene organic solar cells. Due to the weak electron-donating characteristic of truxene core and thereby weak intramolecular charge transfer interaction from electron-donating core to electron-withdrawing end groups, the resulting new acceptors show relatively wide optical band-gap and high-lying lowest unoccupied molecular orbitals (LUMOs), which consequently lead to complementary light spectra with narrow band-gap donor polymers and high open circuit voltage (Voc) in solar cells. Particularly, Tr(Hex)6-3BR, a star-shaped planar acceptor, produced the highest power conversion efficiency of 2.1% with a high Voc of 1.02 V when blended with PTB7-Th.

Ultrafast Charge Transfer and Upconversion in Zinc β‐Tetraaminophthalocyanine‐Functionalized PbS Nanostructures Probed by Transient Absorption Spectroscopy

We functionalize PbS nanocrystals with the organic semiconductor Zn β-tetraaminophthalocyanine to design a nanostructured solid-state material with frequent organic-inorganic interfaces. By transient absorption and fluorescence spectroscopy, we investigate the optoelectronic response of this hybrid material under near-infrared excitation to find efficient charge transfer from the nanocrystals to the molecules. We demonstrate that the material undergoes cooperative sensitization of two nanocrystals followed by photon upconversion and singlet emission of the organic semiconductor. The upconversion efficiency resembles that of comparable systems in solution, which we attribute to the large amount of interfaces present in this solid-state film. We anticipate that this synthetic strategy has great prospects for increasing the open-circuit voltage in PbS nanocrystal-based solar cells.