Carrier Collection Losses Research Articles

Charge Carrier Collection Losses in Lead‐Halide Perovskite Solar Cells

AbstractThe collection of photogenerated charges in halide perovskite solar cells depends on the thickness of the absorber layer, with larger thicknesses leading to a reduced collection efficiency. This observation has traditionally been associated with insufficiently high electron and hole diffusion lengths in the absorber layers. However, it is shown that in the presence of low‐mobility contact layers, charge collection can be thickness‐dependent, even if the absorber layer has infinite mobility. Here, analytical equations are derived for the thickness dependence of charge collection losses in situations where recombination is bulk or interface‐limited and show how to relate these equations to voltage‐dependent photoluminescence data. The analytical equations are compared to experimental data and numerical simulations and it is observed that experimental data on triple‐cation perovskite devices with different thicknesses approximately follows the case, where bulk recombination dominates.

Optical properties of thin film Sb2Se3 and identification of its electronic losses in photovoltaic devices

Sb2Se3 has become a promising absorber layer material for photovoltaic applications because of its attractive optical properties and rapid improvement in Sb2Se3 solar cell efficiency in recent years. The indirect bandgap and Urbach energy of Sb2Se3 have been determined to be 1.12 eV and 21.1 meV, respectively, using photothermal deflection spectroscopy. Above bandgap critical points at 1.48, 2.00, 2.35, 2.87, and 3.86 eV are identified from the complex dielectric function (ε = ε1 + iε2) spectra of thin film Sb2Se3 obtained from spectroscopic ellipsometry. Spectra in ε serve as input for external quantum efficiency (EQE) simulations of substrate type Sb2Se3 solar cells. Comparison of experimental and simulated EQE quantifies carrier collection losses and identifies a carrier collection length in the absorber layer of 439 ± 4 nm and a 96.4 ± 0.2 % carrier collection probability near the heterojunction.

InAs/GaAs quantum dot solar cells with quantum dots in the base region

In this work, the influence of quantum dot (QD) position on the performance of solar cells was studied. The presence of QDs within the base regions leads to improved open circuit voltage (Voc) from 0.73 to 0.90 V. Despite a slight reduction in short-circuit current (Jsc) due to carrier collection loss, the enhancement of the Voc of QDSCs with QDs in base region is significant enough to ensure that power conversion efficiencies (η) are higher than the reference quantum dot solar cell (QDSC) of which QDs are embedded in the intrinsic region. Moreover, sample with QDs in deep base region achieved the highest η of 9.75%, an increase of 29% with regard to the reference quantum dot solar cell.

Controlling a crystalline seed layer for mirocrystalline silicon oxide window layer in rear emitter silicon heterojunction cells

Remarkable progress has been made in the improvement of rear-emitter silicon heterojunction (RE-SHJ) solar cells with the use of very thin n-type front contact layers. However, further reducing the thickness of the front window layers while maintaining high conductivity for mitigating the parasitic absorption and carrier collection loss has proven challenging. In this study, we implement controlling a seed layer for achieving ultra-thin, high crystalline and conductivity of n-type hydrogenated microcrystalline silicon oxide (n-µc-SiOx:H) front window layer in RE-SHJ solar cells. By using a seed layer, the crystallinity confirmed by Raman and TEM measurements, and the conductivity of the n-µc-SiOx:H front layers are significantly enhanced compared with that without using the seed layer. This leads to a remarkable increase in the open-circuit voltage (Voc) by 6 mV and fill factor (FF) by 4.11% while maintaining a high short-circuit current density (Jsc) in range of 38 mA/cm2. A high cell performance of 21.1% is obtained with the use of an optimised seed layer.

Refractive indices of layers and optical simulations of Cu(In,Ga)Se2 solar cells

Cu(In,Ga)Se2-based solar cells have reached efficiencies close to 23%. Further knowledge-driven improvements require accurate determination of the material properties. Here, we present refractive indices for all layers in Cu(In,Ga)Se2 solar cells with high efficiency. The optical bandgap of Cu(In,Ga)Se2 does not depend on the Cu content in the explored composition range, while the absorption coefficient value is primarily determined by the Cu content. An expression for the absorption spectrum is proposed, with Ga and Cu compositions as parameters. This set of parameters allows accurate device simulations to understand remaining absorption and carrier collection losses and develop strategies to improve performances.

Surface Recombination and Collection Efficiency in Perovskite Solar Cells from Impedance Analysis.

The large diffusion lengths recurrently measured in perovskite single crystals and films signal small bulk nonradiative recombination flux and locate the dominant carrier recombination processes at the outer interfaces. Surface recombination largely determines the photovoltaic performance, governing reductions under short-circuit current and open-circuit voltage. Quantification of recombination losses is necessary to reach full understanding of the solar cell operating principles. Complete impedance model is given, which connects capacitive and resistive processes to the electronic kinetics at the interfaces. Carrier collection losses affecting the photocurrent have been determined to equal 1%. Photovoltage loss is linked to the decrease in surface hole density, producing 0.3 V reduction with respect to the ideal radiative limit. Our approach enables a comparison among different structures, morphologies, and processing strategies of passivation and buffer layers.

A flatter gallium profile for high-efficiency Cu(In,Ga)(Se,S)2 solar cell and improved robustness against sulfur-gradient variation

Co-optimization of the gallium and sulfur profiles in penternary Cu(In,Ga)(Se,S)2 thin film solar cell and its impacts on device performance and variability are investigated in this work. An absorber formation method to modulate the gallium profiling under low sulfur-incorporation is disclosed, which solves the problem of Ga-segregation in selenization. Flatter Ga-profiles, which lack of experimental investigations to date, are explored and an optimal Ga-profile achieving 17.1% conversion efficiency on a 30 cm × 30 cm sub-module without anti-reflection coating is presented. Flatter Ga-profile gives rise to the higher Voc × Jsc by improved bandgap matching to solar spectrum, which is hard to be achieved by the case of Ga-accumulation. However, voltage-induced carrier collection loss is found, as evident from the measured voltage-dependent photocurrent characteristics based on a small-signal circuit model. The simulation results reveal that the loss is attributed to the synergistic effect of the detrimental gallium and sulfur gradients, which can deteriorate the carrier collection especially in quasi-neutral region (QNR). Furthermore, the underlying physics is presented, and it provides a clear physical picture to the empirical trends of device performance, I–V characteristics, and voltage-dependent photocurrent, which cannot be explained by the standard solar circuit model. The parameter “FGa” and front sulfur-gradient are found to play critical roles on the trade-off between space charge region (SCR) recombination and QNR carrier collection. The co-optimized gallium and sulfur gradients are investigated, and the corresponding process modification for further efficiency-enhancement is proposed. In addition, the performance impact of sulfur-gradient variation is studied, and a gallium design for suppressing the sulfur-induced variability is proposed. Device performances of varied Ga-profiles with front sulfur-gradients are simulated based on a compact device model. Finally, an exploratory path toward 20% high-efficiency Ga-profile with robustness against sulfur-induced performance variability is presented.

Carrier collection losses in interface passivated amorphous silicon thin-film solar cells

In silicon thin-film solar cells the interface between the i- and p-layer is the most critical. In the case of back diffusion of photogenerated minority carriers to the i/p-interface, recombination occurs mainly on the defect states at the interface. To suppress this effect and to reduce recombination losses, hydrogen plasma treatment (HPT) is usually applied. As an alternative to using state of the art HPT we apply an argon plasma treatment (APT) before the p-layer deposition in n-i-p solar cells. To study the effect of APT, several investigations were applied to compare the results with HPT and no plasma treatment at the interface. Carrier collection losses in resulting solar cells were examined with spectral response measurements with and without bias voltage. To investigate single layers, surface photovoltage and X-ray photoelectron spectroscopy (XPS) measurements were conducted. The results with APT at the i/p-interface show a beneficial contribution to the carrier collection compared with HPT and no plasma treatment. Therefore, it can be concluded that APT reduces the recombination centers at the interface. Further, we demonstrate that carrier collection losses of thin-film solar cells are significantly lower with APT.

Dependence of the photovoltaic performance of pseudomorphic InGaN/GaN multiple-quantum-well solar cells on the active region thickness

We investigate the photovoltaic performance of pseudomorphic In0.1Ga0.9N/GaN multiple-quantum well (MQW) solar cells as a function of the total active region thickness. An increase in the number of wells from 5 to 40 improves the short-circuit current and the open-circuit voltage, resulting in a 10-fold enhancement of the overall conversion efficiency. Further increasing the number of wells leads to carrier collection losses due to an incomplete depletion of the active region. Capacitance-voltage measurements point to a hole diffusion length of 48 nm in the MQW region.

Design of surface nanowire arrays for high efficiency thin (≤10 μm) Si solar cells

We investigated the optical characteristics and expected photocurrent of 10-μm-thick Si solar cells with surface nanowire arrays. The diameter and filling ratio of the array influenced not only the optical absorption, but also the spatial distribution of the optical generation rate. Proper design of the nanowire array could locate the high optical field region far from the surface, avoiding serious carrier collection loss and raising the energy conversion efficiency.

A Study of Light-Sensitive Ideality Factor and Voltage-Dependent Carrier Collection of CdTe Solar Cells in Forward Bias

Two features that distinguish CdS/CdTe solar cells from traditional c-silicon cells have been widely confirmed by experiment: 1) The ideality factor of the diode current is sensitive to photon flux input, and 2) the photocurrent is voltage dependent. Here, numerical simulation of one-sided p-type CdTe junctions has been performed in order to interpret the experimental characteristics under forward bias. In these calculations, we assumed a distribution of trapping states within the CdTe bandgap. Detailed discussions have been divided into two parts: light-dependent forward current and voltage-dependent carrier collection. The former has demonstrated that recombination loss increases with increased optical generation. As a result, the diode current, in a certain forward bias region, depends on illumination. With this mechanism, the ideality factor of simulated J-V curves shows a variation consistent with experimental curves. Carrier collection loss under voltage bias is the combined result of carrier diffusion, drift, and recombination during transport. Simulations that are based on our assumption have demonstrated that collection loss increases at higher forward bias and depends on wavelength, carrier mobility, and carrier lifetime.

Carrier collection losses in amorphous silicon and amorphous silicon–germanium alloy solar cells

The carrier collection losses in amorphous silicon (a-Si) and amorphous silicon–germanium (a-SiGe) alloy single-junction nip cells have been experimentally investigated by measuring the biased quantum efficiency and color (blue and red) fill factor values. The study has identified losses near the p/i junction and the bulk of the i layer. The extent of the losses have been found to be dependent on the quality and thickness of the i layer and the nature of the n-layer surface. The results suggest a broad interrelationship between the initial film properties and deposition conditions and an inhomogeneity in the direction of film growth.

Determination of carrier collection length and prediction of fill factor in amorphous silicon solar cells

Using a carrier collection length lc, defined for penetrating light, a convenient technique for measuring lc, in pin cells is demonstrated. We show experimentally that lc, predicts the fill factor measured with white light under AM1 conditions for a wide variety of pin cells. A simple model is presented which fits the experimental results. The implications of these results on the carrier collection loss for strongly absorbed light are discussed.

Study of Drift Type Photovoltaic Effect in Amorphous Silicon p-i-n Junction Structure

Taking account of continuously distributed gap states and a considerably small carrier mobility inherent to amorphous material, a unique photovoltaic behavior in an amorphous silicon has been examined in a p-i-n junction structure. Geminate and non-geminate recombination would play a significant role in the photo-carrier generation and photovoltaic carrier collection process, respectively. It has been shown that in the case of a properly designed a-Si:H p-i-n junction photovoltaic cell the carrier collection loss can be almost saved with the aid of high internal electric field in the i-layer despite of a very small hole diffusion length of about 500Å. While, owing to the geminate recombination, as high as 20% of absorbed photons are still thrown away without being converted to photocurrent.