Back Surface Recombination Research Articles

Simulation study of cadmium-free CIGS solar cell for efficiency enhancement by minimizing recombination losses through back surface field mechanism

Thin-film photovoltaic cells provide benefits over conventional first-generation technology, including lighter weight, greater flexibility, and lower power generation costs. Chalcogenide solar cells offer good efficiency and technological maturity among thin-film technology. In this work, a solar cell device structure, Al/FTO/SnS2/CIGS/CuO/Ni, is examined using SCAPS-1D. Further, by incorporating the back surface field (BSF) layer, the conversion efficiency increased from 18.93 % to 29.88 %, followed by VOC of 0.96 V, JSC of 37.07 mA cm−2, and FF of 83.71 %. This increment in device performance is owing to the lowering back surface recombination velocity and the additional hole tunnelling activity offered by the BSF layer by forming a quasi-ohmic contact, i.e. metal-semiconductor contact with negligible junction resistance relative to the total resistance of the device. Throughout this research work, the authors studied several factors such as surface recombination velocity, temperature impact, front and back contact metal work functions, parasitic resistance impact, gallium proportion, and doping density impact on CIGS solar cells. The work also included a calibration with experimental data from published sources to validate the simulation results.This paper presents a new approach for producing high-efficiency, eco-friendly CIGS solar cells with CuO back surface field mechanism.

BiVO4 Photoanode with NiV2O6 Back Contact Interfacial Layer for Improved Hole-Diffusion Length and Photoelectrochemical Water Oxidation Activity.

The short hole diffusion length (HDL) and high interfacial recombination are among the main drawbacks of semiconductor-based solar energy systems. Surface passivation and introducing an interfacial layer are recognized for enhancing HDL and charge carrier separation. Herein, we introduced a facile recipe to design a pinholes-free BiVO4 photoanode with a NiV2O6 back contact interfacial (BCI) layer, marking a significant advancement in the HDL and photoelectrochemical activity. The fabricated BiVO4 photoanode with NiV2O6 BCI layer exhibits a 2-fold increase in the HDL compared to pristine BiVO4. Despite this improvement, we found that the front surface recombination still hinders the water oxidation process, as revealed by photoelectrochemical (PEC) studies employing Na2SO3 electron donors and by intensity-modulated photocurrent spectroscopy measurements. To address this limitation, the surface of the NiV2O6/BiVO4 photoanode was passivated with a cobalt phosphate electrocatalyst, resulting in a dramatic enhancement in the PEC performance. The optimized photoanode achieved a stable photocurrent density of 4.8 mA cm-2 at 1.23 VRHE, which is 12-fold higher than that of the pristine BiVO4 photoanode. Density Functional Theory (DFT) simulations revealed an abrupt electrostatic potential transition at the NiV2O6/BiVO4 interface with BiVO4 being more negative than NiV2O6. A strong built-in electric field is thus generated at the interface and drifts photogenerated electrons toward the NiV2O6 BCI layer and photogenerated holes toward the BiVO4 top layer. As a result, the back-surface recombination is minimized, and ultimately, the HDL is extended in agreement with the experimental findings.

Shifting the Paradigm: A Functional Hole‐Selective Transport Layer for Chalcopyrite Solar Cells

High‐efficiency Cu(In,Ga)Se2 solar cells rely on Ga grading to mitigate back surface recombination. However, the inhomogeneous absorber has drawbacks, including increased non‐radiative loss and inadequate absorption. Therefore, literatures demand a paradigm shift of using a hole‐transport layer to passivate the back surface. Herein, a functional hole‐transport layer is demonstrated as an alternative to Ga grading. The novel hole‐transport layer is prepared as a double‐layer: co‐evaporated CuGaSe2 covered by solution combustion synthesis prepared In2O3. As demonstrated by micrographs, elemental mapping, and photoluminescence spectroscopy, the oxide layer improves thermal stability and prevents Ga diffusion. However, during the absorber deposition, a complete ion exchange of In and Ga converts CuGaSe2/In2O3 into CuInSe2/GaOx. Incorporating this hole‐transport layer in co‐evaporated nongraded CuInSe2 solar cells leads to significantly increased minority carrier lifetime from 5 to 113 ns, yielding an 80 meV improvement in quasi‐Fermi‐level splitting. The devices exhibit improved open‐circuit voltage, as well as a promising fill factor of over 71%, indicating good hole‐transport properties. In these results, the passivation effect and good hole‐transport properties of the hole‐transport layer are experimentally demonstrated. Thus, high‐efficiency solar cells can be achieved by using a functional hole‐transport layer without relying on Ga grading.

Optimizing photovoltaic performance: Strategic enhancement of Ag-doped graded CAZTS solar cells achieving 27.3% efficiency

The Ag-doped Cu2ZnSnS4 (CAZTS) based solar cell has been explored, with varying Ag concentrations (x = 0, 0.1, 0.2, 0.3, 0.4) in the composition (Cu1−xAgx)2ZnSnS4. In order to mitigate back surface recombination in CAZTS-based solar cells, a Sn2S3 layer has been incorporated as a back surface field (BSF). Further, the efficiency enhancement employs a bandgap grading approach, elevating efficiency from 15.8 % to 24.7 % compared to devices without the graded structure. Additionally, the thickness and total defect density of the CAZTS layer are optimized. The result demonstrates that the optimized value of the CAZTS layer’s thickness is 1.1 µm, and the defects are 1 × 1012 cm−3. To further enhance the performance, the doping variation of the CAZTS layer has also been investigated. The findings indicate that the maximum efficiency of 27.32 % is attained with a doping value of 1 × 1016 cm−3.

Approach to determining the limiting recombination mechanism in CdTe-based solar cells

Determining exactly how the performance of a thin-film photovoltaic device is limited by a particular recombination mechanism can be difficult, particularly in the case of CdTe solar cells. As a result, efforts are being made to improve all parts of the device without good knowledge of which improvements are necessary. To understand where the device limitation is, the recombination current densities of at least one on of the interfaces must be known. Here, we present a method to determine which recombination mechanisms is limiting. First, back illuminated quantum efficiency measurements are used to determine the key parameters of the back interface – the back surface recombination velocity and the band bending near the back surface. Once these back interface parameters are determined, the recombination current densities can be calculated for a front illuminated device to determine the limiting mechanism. The validity of the approach is tested using previously reported data.

An Investigation of The J-V Curve for N-AlGaAs/p-GaAs Heterojunction Solar Cell Due to Influence of Surface Recombination Velocity

Theoretical analysis can substantially reduce the time and costs required for developing a specific device such as solar cell, by allowing the designer to take a suitable geometry and doping profile prior to the fabrication stage. In this paper, theoretical analysis of electric field and electric potential within depletion layer of N-AlGaAS/p-GaAs has been made by solving Poisson's equation numerically using finite difference method (FDM) with Fortran program. Continuity equations of charge carriers in quasi- nature regions in N and p-layers were solved numerically to obtain the current density curve (J-V curve). Boundary conditions related to surface recombination velocity were implemented in this analysis. The effect of front and back surface recombination velocity on the excess minority carriers distribution and photocurrent have been studied. The aim of this work is to find the influence of surface recombination velocity in the output of N-AlGaAs/p-GaAs heterojunction solar cell. The analysis showed that, the low surface recombination velocity produce high efficient solar cell.

Performance Analysis of CdS‐Free, Sb2Se3/ZnSe p–n Junction Cells with Various Hole Transport Layers and Contacts

AbstractSb2Se3, a promising thin‐film photovoltaic (TFPV) absorber material known for its non‐toxic nature, abundance on Earth, and stability has the inherent limitations of low doping density of 1013 cm−3 and poor carrier mobility of 1.5 cm2 V−1 s−1, leading to a low built‐in potential and inefficient carrier collection. To address these challenges and enhance the built‐in potential and carrier collection, an effective hole‐transport layer (HTL), e.g. CdS, which contains toxic components is used. A less‐toxic alternative is explored: ZnSe as the electron‐transport layer (ETL) to mitigate this issue. In this study, the performance is evaluated of Sb2Se3/ZnSe solar cells using six different HTLs (CuSCN, MoSe2, NiO, Spiro‐OMeTAD, SnS, MoOx) through the utilization of the SCAPS‐1D modeling tool. The HTL plays a critical role in improving the effectiveness of the built‐in potential, enhancing carrier collection, and suppressing back surface recombination. The analysis involves varying the thickness and doping concentration of the absorber, buffer, and HTLs, and exploring the impact of temperature, series and shunt resistance, and the metal work function of the back contact. Among the tested devices, the one with NiO as the HTL demonstrates the highest efficiency, exhibiting a VOC (open‐circuit voltage) of 0.81 V and a PCE (power conversion efficiency) of 24.07%.

Derivative of Ac back surface recombination velocity as applied to n+/p/p+ silicon solar cell optimum base thickness determination: Effect of both temperature and frequency

Derivative of Ac back surface recombination velocity as applied to n+/p/p+ silicon solar cell optimum base thickness determination: Effect of both temperature and frequency

AC back surface recombination, as applied to determine (p) base thickness of both conventional and vertical series junction (n+/p/p+) silicon solar cells under white illumination

AC back surface recombination, as applied to determine (p) base thickness of both conventional and vertical series junction (n+/p/p+) silicon solar cells under white illumination

Machine learning for enhanced semiconductor characterization from time-resolved photoluminescence

<h2>Summary</h2> To enhance the accuracy and effectiveness of optoelectronic characterization, Bayesian inference has been applied to the statistical analysis of time-resolved photoluminescence (TRPL) data with large-scale graphics-processing unit (GPU)-based simulations of electron dynamics. A simulated TRPL dataset, derived from a CH₃NH₃PbI_3-xCl_x perovskite absorber, was used as a case study. From a power scan, Bayesian inference extracts values of the (ambipolar) carrier mobility, free-carrier density, radiative recombination rate, and the overall lifetime of the nonradiative recombination processes. Analysis of the experimental TRPL data yields similar parameter values. The independent contributions of bulk and surface recombination can be distinguished via the introduction of an additional sample thickness. Further experiments separate the front and back surface recombination velocities and can distinguish the electron and hole mobilities. Ultimately, Bayesian inference enables a significant increase in the information yield from TRPL measurements.

BIFACIAL (N+/P/P+) SILICON SOLAR CELL BASE THICKNESS OPTIMIZATION, WHILE ILLUMINATED BY THE REAR FACE WITH MONOCHROMATIC LIGHT OF SHORTWAVELENGTHS

The monochromatic absorption coefficient of silicon, inducing the depth of penetration of light into the base of the solar cell, is used through back surface recombination velocity expressions, to determine the optimum thickness necessary for the production of a large photocurrent. The absorption-generation-diffusion and recombination (bulk and surface) phenomena are taken into account in determining the optimum thickness of an n+ -p-p+ bifacial solar cell, for it manufacture process optimization.

Diode Factor in Solar Cells with Metastable Defects and Back Contact Recombination

AbstractTo achieve a high fill factor, a small diode factor close to 1 is essential. The optical diode factor determined by photoluminescence is the diode factor from the neutral zone of the solar cell and thus a lower bound for the diode factor. Due to metastable defects transitions, the optical diode factor is higher than 1 even at low excitation. Here, the influence of the backside recombination and the doping level on the optical diode factor are studied. First, photoluminescence and solar cell capacitance simulator (SCAPS) simulations are used to determine the back surface recombination velocity of Cu(In, Ga)Se2 with various back contacts and different doping levels. Then, experimental results and simulations show that both back surface recombination and high doping density reduce the optical diode factor. The back surface recombination reduces the optical diode factor with undesirable extra nonradiative recombination. The smaller value achieved by higher doping can increase quasi‐Fermi level splitting at the same time. The simulations show that the back surface recombination reduces the optical diode factor due to an illumination‐dependent recombination rate. In addition, a higher majority carrier doping reduces the influence of majority carrier gain from metastable defect transitions, thus reducing the optical diode factor.

Effectiveness of band discontinuities between CIGS absorber and copper-based hole transport layer in limiting recombination at the back contact

In this work, different copper-based thin-film materials as a hole transport layer (HTL) are introduced in Cu(In,Ga)Se2 (CIGS) solar cells to limit the recombination rate at the back contact and improve efficiency. To survey the impact of different HTLs on the efficiency of CIGS solar cells, an experimental cell with a record efficiency of 23.35% has been modeled. Then, cuprous oxide (Cu2O), cuprous thiocyanate (CuSCN), and copper sulfide (CuS) thin films as an HTL were added separately to the CIGS cell. It was demonstrated that the open-circuit voltage (Voc) is enhanced owing to the reduction of the back interface recombination, and short-circuit current density (Jsc) is improved thanks to the enhancement of carrier collection when Cu2O or CuSCN HTLs are added to the conventional CIGS solar cell. External quantum efficiency (EQE) has also shown a significant increase in long wavelengths of approximately 1000–1200 nm through this approach. The simulated impedance spectroscopy confirms that the use of Cu2O HTL in the CIGS solar cells significantly reduces back surface recombination, which could improve Voc and fill factor (FF) values. In addition, the utilization of Cu2O HTL in the CIGS solar cells reduces Urbach energy (EU), which is attributed to suitable band alignment at Cu2O/CIGS interface. High efficiency of 27.3% was achieved by adding the Cu2O HTL in CIGS solar cell, which is mainly attributed to the additional carrier extraction routes through HTL.

Sputter-deposited CdMgTe for rear contact to CdSeTe/CdTe solar cells

CdTe-based photovoltaic device performance improvement is predominantly limited by open-circuit voltage. This work approaches the voltage deficit challenge through the incorporation of a cadmium magnesium telluride (CdMgTe) electron reflector layer at the back of 1.5- μm CdSeTe/CdTe absorbers to reduce back-surface recombination through a conduction band offset. A sputtered CdMgTe electron reflector layer permitted low fabrication temperatures and circumvented the intolerance of CdMgTe to thermal processing present in traditional close-space sublimation (CSS)-deposited CdMgTe devices. We demonstrate that sputtered CdMgTe simultaneously promotes the retention of chlorine passivation and magnesium which is not feasible with CSS fabrication. We also show the importance of the Cu and Te back contact order in CdMgTe devices. Through the optimization of the sputtered CdMgTe substrate temperature, doping, and back structure configuration, 1.5 μm CdSeTe/CdTe devices with a sputtered CdMgTe layer achieved an efficiency of 16.0%, significantly higher than with CSS CdMgTe, and demonstrated partial enactment of electron-reflector behavior. • Sputtered CdMgTe electron reflector solar cells achieve higher efficiencies than close-space sublimation-deposited. • Low temperatures of sputtered CdMgTe reduce magnesium diffusion and chlorine loss. • CdTe devices with CdMgTe electron reflectors require back contact optimization. • CdTe voltage improvement with CdMgTe limited by a partial negative back valence band offset.

Surface recombination property of silicon wafers determined accurately by self-normalized photocarrier radiometry

Surface recombination has a great impact on the performance of solar cells and photodetectors, and thus is necessary to be accurately determined. In this paper, a self-normalized photocarrier radiometry (PCR) with mean square variance graph is employed to improve the determination accuracy of the surface recombination velocities of silicon wafers and to reduce the influence of initial values on multi-parameter estimations. In this method data obtained from both the illuminated front surface and the illuminated rear surface are employed. The uncertainties of the estimated carrier recombination parameters, especially for the surface recombination velocities are improved significantly. Moreover, the influence of instrumental frequency response (IFR) on the multi-parameter estimation is totally eliminated. Theoretical simulations and experiments are performed to confirm the theoretical predictions. The estimated average uncertainties of the front and back surface recombination velocities for the sample used are approximately ± 7.60% and ± 5.71% by the proposed method, which are much improved than ± 13.65% and >±50% by the conventional method.

Carrier transport and performance limit of semi-transparent photovoltaics: CuIn1−xGaxSe2 as a case study

Semi-transparent photovoltaic devices for building integrated applications have the potential to provide simultaneous power generation and natural light penetration. CuIn1−xGaxSe2 has been established as a mature technology for thin-film photovoltaics; however, its potential for Semi-Transparent Photovoltaics (STPV) is yet to be explored. In this paper, we present its carrier transport physics explaining the trend seen in recently published experiments. STPV requires deposition of films of only a few hundred nanometers to make them transparent and manifests several unique properties compared to a conventional thin-film solar cell. Our analysis shows that the short-circuit current, Jsc, is dominated by carriers generated in the depletion region, making it nearly independent of bulk and back-surface recombination. The bulk recombination, which limits the open-circuit voltage Voc, appears to be higher than usual and attributable to numerous grain boundaries. When the absorber layer is reduced below 500 nm, grain size reduces, resulting in more grain boundaries and higher resistance. This produces an inverse relationship between series resistance and absorber thickness. We also present a thickness-dependent model of shunt resistance showing its impact in these ultra-thin devices. For various scenarios of bulk and interface recombinations, shunt and series resistances, AVT, and composition of CuIn1−xGaxSe2, we project the efficiency limit, which—for most practical cases—is found to be ≤10% for AVT≥25%.

Distinguishing bulk and surface recombination in CdTe thin films and solar cells using time-resolved terahertz and photoluminescence spectroscopies

Understanding the nature of recombination and its dependence on defects and interfaces is essential for engineering materials and contacts for a higher open-circuit voltage (Voc) and power conversion efficiency in photovoltaic (PV) devices. Time-resolved photoluminescence (TRPL) has conventionally been used to evaluate recombination, but carrier redistribution often dominates the response at short times. Here, we report on the quantification of carrier dynamics and recombination mechanisms by complementary use of both time-resolved terahertz spectroscopy and TRPL combined with numerical modeling of the continuity equations and Poisson's equation. We have demonstrated this approach using CdTe thin films. A thin-film stack with CdTe fabricated by vapor transport deposition and treated with CdCl2 exhibited a bulk lifetime of 1.7 ± 0.1 ns, a negligible CdTe/CdS interface recombination velocity, and a back surface recombination velocity of 6.3 ± 1.3 × 104 cm/s. In contrast, a film stack without CdCl2 treatment had a bulk lifetime of only 68 ± 12 ps and a higher interface recombination velocity of 4 ± 2 × 108 cm/s. By determining the locus and mechanisms of performance-limiting recombination, we can accelerate the development of thin-film PVs with higher Voc and efficiency. While the method has been demonstrated here using CdTe, it is also applicable to perovskites, Cu(InGa)Se2, Cu2ZnSn(S,Se)4, and emerging technologies.

Efficiency enhancement of WSe2 heterojunction solar cell with CuSCN as a hole transport layer: A numerical simulation approach

In this research work, the tungsten diselenide (WSe2)-based thin-film solar cell with a copper thiocyanate (CuSCN) hole transport layer (HTL) has been designed and studied by using the Solar Cell Capacitance Simulator in One Dimension software program (SCAPS-1D). A comparative numerical study between Al/ITO/CdS/WSe2/Ni and Al/ITO/CdS/WSe2/CuSCN/Ni has been done. The SCAPS-1D simulator has been utilized to investigate photovoltaic parameters such as open circuit voltage, short-circuit current density, fill-factor, power conversion efficiency and quantum efficiency of heterojunction solar cell due to the variation of thickness, doping density, bulk defect density, defect density at buffer/absorber and absorber/HTL interfaces, operating temperature, band alignment and back surface recombination velocity. The conversion efficiency of 17.33% has been determined for the WSe2 solar cell without HTL. On the contrary, the efficiency of the proposed solar cell with CuSCN HTL has been found to be 24.20% at the optimal device structure. These numerical findings of the present study may therefore provide an intuitive approach to fabricate an economically feasible and highly efficient WSe2-based heterojunction thin-film solar cell.

Characterization of predictable quantum efficient detector in terms of optical non-linearity in the visible to near-infrared range

The characteristics of a predictable quantum efficient detector (PQED) in terms of optical non-linearity in the visible to near-infrared range were investigated under zero-bias and reverse-bias voltage conditions. In the zero-bias condition, linear behavior was observed in the wavelength from 405 nm to 1060 nm in the photocurrent range of 1 nA to 10 μA, and saturation occurred for photocurrents over 10 μA for all wavelengths. In the reverse-bias voltage of 10 V, the linear behavior was observed in the photocurrent range of 64 nA to 1 mA except for the wavelength of 1060 nm, and the saturation photocurrent increased up to 1 mA. The supralinearity value at 1060 nm increased sharply from the photocurrent of 100 μA, and its maximum value reached up to 0.34% at the photocurrent of 1 mA because of the back surface recombination and the longer absorption length. The spectral linearity results of the PQED help us to understand the charge-carrier loss mechanism in the PQED, and would lead to more accurate optical measurement with it.

Towards high sensitivity infrared detector using Cu2CdxZn1-xSnSe4 thin film by SCAPS simulation

Infrared photodetectors have been used widely in various applications. Significant efforts have been made to improve the performance of the commercial infrared photodetectors including HgCdTe and InGaAs for lowering the fabrication cost, simplifying the fabrication processes and increasing the production yield. Cu2CdxZn1-xSnSe4 (CCZTSe) kesterite material is a good candidate for infrared photodetectors application due to its low cost, abundant raw materials on the earth, tunable band gap. The detection rate is an important performance index for infrared detectors, which is inversely proportional to the square root of the dark current, so reducing the dark current is an effective way to improve the detection rate. In order to minimize the dark current and achieve high responsivity and detectivity for CCZTSe thin film infrared detectors (TFID), intermediate layers (between the CCZTSe and back contact) including MoO3, MoSe2 and MoS2 are adpoted and their effect on the performance of the detector are studied via SCAPS-1D (a Solar Cell Capacitance Simulator) software. SCAPS simulation shows that the high-functional MoO3 interfacial layer can reduce the back barrier and dark current of the detector more effectively than the other two. In addition, we find that the carrier concentration of the CCZTSe layer plays a great role on the dark current of the device. The dark current can be reduced significantly to the lowest value when the carrier concentration is 1017 cm−3. Finally, we further predict that the dark current can be reduced to ~ pA/cm2 level, by reducing the back surface recombination and Schottky barrier.