Searching of new,cheap, air- and thermally stable hole transporting materials for perovskite solar cells

In this work, two thermal- and air-stable, hole transporting materials (HTM) in perovskite solar cells are analyzed. Those obtained and investigated materials were two polyazomethines: the first one with three thiophene rings and 3,3′-dimethoxybenzidine moieties (S9) and the second one with three thiophene rings and fluorene moieties (S7). Furthermore, presented polyazomethines were characterized by Fourier transform infrared spectroscopy (FTIR), UV–vis spectroscopy, atomic force microscopy (AFM) and thermogravimetric analysis (TGA) experiments. Both polyazomethines (S7 and S9) possessed good thermal stability with a 5% weight loss at 406 and 377 °C, respectively. The conductivity of S7 was two orders of magnitude higher than for S9 polymer (2.7 × 10^?8 S/cm, and 2.6 × 10^?10 S/cm, respectively). Moreover, polyazomethine S9 exhibited 31 nm bathochromic shift of the absorption band maximum compared to S7.Obtained perovskite was investigated by UV–vis and XRD. Electrical parameters of perovskite solar cells (PSC) were investigated at Standard Test Conditions (STC). It was found that both polyazomethines protect perovskite which is confirmed by ageing test where V_oc did not decrease significantly for solar cells with HTM in contrast to solar cell without hole conductor, where V_oc decrease was substantial. The best photoconversion efficiency (PCE = 6.9%), among two investigated in this work polyazomethines, was obtained for device with the following architectures FTO/TiO₂/TiO₂ + perovskite/S7/Au. Stability test proved the procreative effects of polyazomethines on perovskite absorber.     

Enhancement of output performance of Cu2ZnSnS4 thin film solar cells—A numerical simulation approach and comparison to experiments

The formation of stable, low resistance and nonrectifying contacts to Cu₂ZnSnS₄ (CZTS) thin film photovoltaic material are the major and critical challenges associated with its effect over the output performance of fabricated solar cells. The solution of continuity equation in one dimension for a soda lime glass substrates (SLG) |Mo | CZTS | CdS | ZnO:Al cell structure is considered in the simulation of its current–voltage characteristics that is governed by the back contact material, acceptor concentration as well as thickness of the CZTS layer. Our primary simulation shows a 6.44% efficiency of the CZTS solar cell which is comparable to reported experimental data if these parameters are not optimized. However, by optimizing them a simulated conversion efficiency as high as 13.41% (V_oc=1.002 V, J_sc=19.31 mA/cm², fill factor (FF)=69.35%) could be achievable. The solar cell with a back contact metal work function of 5.5 eV, an absorber layer's thickness of 2.68 μm and an acceptor concentration of 5×10¹⁶ cm⁻³ were optimum. The presented optimization is ideal and subject to experimental verification with a precise control of the process parameters along with reduced surface as well as bulk recombination, secondary phases and thermalization losses.     

Comprehensive loss modeling in Cu2ZnSnS4 solar cells

Thin film solar cells based on Cu₂ZnSnS₄ (CZTS) absorber material suffers from performance issues arising due to the presence of a non-optimal back contact barrier, low carrier lifetime, acceptor/donor point defects in bulk, interface defects at the absorber-buffer junction and grain boundaries within the absorber. We perform comprehensive simulations enumerating the impact of these performance limiting factors on CZTS solar cells. These simulations capture the experimentally observed anomalies in current-voltage (I–V) characteristics and the open-circuit voltage (V_OC) pinning in CZTS solar cells. These cause-effect relationships as elaborated in the findings are expected to be of great interest to the experimentalists working in this field.     

Deep level transient spectroscopy investigation of deep levels in CdS/CdTe thin film solar cells with Te:Cu back contact

Deep levels in Cds/CdTe thin film solar cells have a potent influence on the electrical property of these devices. As an essential layer in the solar cell device structure, back contact is believed to induce some deep defects in the CdTe thin film. With the help of deep level transient spectroscopy (DLTS), we study the deep levels in CdS/CdTe thin film solar cells with Te:Cu back contact. One hole trap and one electron trap are observed. The hole trap H1, localized at E_v+0.128~eV, originates from the vacancy of Cd (V_Cd. The electron trap E1, found at E_c-0.178~eV, is considered to be correlated with the interstitial Cu_i⁼ in CdTe.     

新型空穴传输材料在钙钛矿太阳能电池中的研究进展

钙钛矿太阳能电池是一种全新的全固态薄膜电池. 报道的能量转换效率已提高到19.3%, 成为可再生能源领域的热点研究方向. 空穴传输材料是构成高效钙钛矿太阳能电池的重要组分之一. 本文介绍了钙钛矿太阳能电池的基本结构, 对空穴传输材料的分子结构、能级水平和迁移率等对电池性能的影响进行了详细的总结和评述.     

Dopant‐free back contact silicon heterojunction solar cells employing transition metal oxide emitters

The present study investigates the electrical properties of transition metal oxide (TMO) emitters in dopant‐free n‐Si back contact solar cells by comparing the properties of solar cells employing three TMOs (WO_x, MoO_x and V₂O_x) with varying electrical properties acting as p‐type contacts. The TMOs are found to induce large band bending in n‐Si, which reduces the injection level dependent interfacial recombination speed S_eff and contact resistivity ρ_c. Among the TMO/n‐Si contacts considered, the V₂O_x/n‐Si contact achieves the lowest S_eff of 138 cm/s and ρ_c of 0.034 Ω cm², providing the significant advantages over heavily doped a‐Si:H(p)/n‐Si contacts. The best device performance was achieved by the V₂O_x/n‐Si solar cell, demonstrating an efficiency of 16.59% and an open‐circuit voltage of 610 mV relative to solar cells based on MoO_x/n‐Si (15.09%, 594 mV) and WO_x/n‐Si (12.44%, 539 mV). Furthermore, the present work is the first to employ WO_x, V₂O_x and Cs₂CO₃ in back contact solar cells. The fabrication process employed offers great potential for the mass production of back contact solar cells owing to simple, metal mask patterning with high alignment quality and dopant‐free steps conducted at a lower temperature.     

Performance assessment of Cu2SnS3 (CTS) based thin film solar cells by AMPS-1D

A relative performance assessment of copper tin sulfide (CTS) thin film solar cells with different phases such as, cubic, tetragonal, and orthorhombic as an absorber layer has been carried out by AMPS-1D simulation software. Based on the proposed device architecture, the effects of thickness and carrier concentration for the absorber layer as well as the back metal contact with various work function are studied in order to improve the performance of CTS solar cell. It is found that 10¹⁸ cm⁻³ and 2500–3000 nm are optimum values for carrier concentration and thickness for all the investigated CTS absorber layer phases, respectively. On the other hand, back contact metal work function of 5.28 eV, 5.67 eV and 5.71 eV are identified to be the optimal values for cubic, tetragonal, and orthorhombic phases, respectively. We have analyzed in detail the output performance of CTS thin film solar cell with respect to its fabrication, which can serve a constructive research pathway for the thin film photovoltaic industry.     

Characterization of tunnel oxide passivated contact with n-type poly-Si on p-type c-Si wafer substrate

The junction properties of tunnel silicon oxide (SiO_x) passivated contact (TOPCon) with n-type poly-Si on p-type c-Si wafer are characterized using current-voltage (J-V) and capacitance-voltage (C-V) measurements. The dark J-V curves show a standard diode characteristic with a turn-on voltage of ～0.63 V, indicating a p-n junction is formed. While the C-V curve displays an irregular shape with features of 1) a slow C increase with the decrease of the magnitude of reverse bias voltage, being used to estimate the built-in potential (V_bi), 2) a significant increase at a given positive bias voltage, corresponding to the geometric capacitance crossing the ultrathin SiO_x, and 3) a sharp decrease to negative values, resulting from the charge tunneling through the SiO_x layer. The C of depleting layer deviates from the normal linear curve in the 1/C²-V plot, which is caused by the diffusion of P dopants from the n-type poly-Si into the p-type c-Si wafer as confirmed by the electrochemical capacitance-voltage measurements. However, the 1/C^2+γ-V plots with γ > 0 leads to linear curves with a proper γ and the V_bi can still be estimated. We find that the V_bi is the range of 0.75–0.85 V, increases with the increase of the doping ratio during the poly-Si fabrication process, and correlates with the passivation quality as measured by the reverse saturated current and implied open circuit voltage extracted from transient photoconductivity decay.     

Self-consistent theory for the built-in voltage in metal–organic semiconductor–metal structures

A self-consistent theory for calculation of built-in voltage (U_bi) of metal–organic semiconductor–metal (MOSM) structures is developed based on Gaussian energy distribution of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). It is shown that the built-in voltage depends not only on the work function difference of the two electrodes, but also on the mean energy level of HOMO and LUMO, as well as the Gaussian width of the energy distribution. The theory predicts that the spreading of HOMO and LUMO levels will results in an increase of U_bi, and that U_bi decreases with increasing temperature.     

Tuning of the open‐circuit voltage by wide band‐gap absorber and doped layers in thin film silicon solar cells

We present an experimental study combined with computer simulations on the effects of wide band‐gap absorber and window layers on the open‐circuit voltage (V_oc) in single junction thin film silicon solar cells. The quantity ΔE_p, taking as the difference between the band gap and the activation energy in ?p? layer, is treated as a measure of the p‐layer properties and shows a linear relation with V_oc over a range of 100 mV with a positive slope of around 430 mV/eV. Two limiting mechanisms of V_oc are identified: the built‐in potential at lower ΔE_p and the band gap of the absorber layer at higher ΔE_p. The results of the experimental findings are confirmed by computer simulations. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Impurity photovoltaic effect in silicon solar cell doped with sulphur: A numerical simulation

The impurity photovoltaic effect (IPV) has mostly been studied in various semiconductors such as silicon, silicon carbide and GaAs in order to increase infrared absorption and hence cell efficiency. In this work, sulphur is used as the IPV effect impurity incorporated in silicon solar cells. For our simulation we use the numerical device simulator (SCAPS). We calculate the solar cell performances (short circuit current density J_sc, open circuit voltage V_oc, conversion efficiency η and quantum efficiency QE). We study the influence of light trapping and certain impurity parameters like impurity concentration and position in the gap on the solar cell performances. Simulation results for IPV effect on silicon doped with sulphur show an improvement of the short circuit current and the efficiency for sulphur energy levels located far from the middle of the band gap especially at E_c-E_t=0.18 eV.     

Prediction and optimization of the performance characteristics of CZTS thin film solar cell using band gap grading

The results of the numerical simulation of the performance characteristics of (Cu₂ZnSnS₄) CZTS thin film solar cell due to bandgap grading is presented in this work. The investigation of the performance of this solar cell was carried out using the Analysis of Microelectronics and Photonics software (AMPS-1D). A substrate cell structure FTO/CdS/CZTS/Mo(SLG) was used as the base model. An efficiency of 8.33% was obtained from the simulation with baseline parameters while an intentional grading of the device was carried out on the device both at the front and back interface of the absorber. Front grading was observed to degrade the device performance while significant improvement of the device performance was observed with back grading. An intentional double grading of the device further enhanced the efficiency up to 12.26%.     

Charge carrier decay and diffusion in organic–inorganic CH3NH3PbI3–xClx perovskite based solar cell

In recent years, organic–inorganic lead halides attracted widespread interest, mainly due to their impressive photoconversion properties and low‐cost solution processing. In this study, we employed small amplitude transient photovoltage and photocurrent spectroscopy to investigate charge transport and recombination properties of perovskite CH₃NH₃PbI_3–xCl_x solar cell under realistic light harvesting conditions (<1 sun). Cell structure resembles outlay commonly found in organic photovoltaics, with perovskite absorber being sandwiched between two thin layers of organic polymers. Tested device displayed high power conversion efficiency (10.3%), good fill factor and negligible hysteresis effect. Fundamental device parameters were characterized at various open‐circuit voltages (V_oc) by examination of small voltage and current perturbations created by the low intensity pulsed laser excitations. The obtained results exhibit long charge carrier lifetimes and fast charge transport over the full range of applied optical bias, as well as remarkable diffusion lengths exceeding 1 μm. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Inverted structure perovskite solar cells: A theoretical study

We analysed perovskite CH₃NH₃PbI_3-xCl_x inverted planer structure solar cell with nickel oxide (NiO) and spiro-MeOTAD as hole conductors. This structure is free from electron transport layer. The thickness is optimized for NiO and spiro-MeOTAD hole conducting materials and the devices do not exhibit any significant variation for both hole transport materials. The back metal contact work function is varied for NiO hole conductor and observed that Ni and Co metals may be suitable back contacts for efficient carrier dynamics. The solar photovoltaic response showed a linear decrease in efficiency with increasing temperature. The electron affinity and band gap of transparent conducting oxide and NiO layers are varied to understand their impact on conduction and valence band offsets. A range of suitable band gap and electron affinity values are found essential for efficient device performance.     

Analysis of current–voltage and capacitance–voltage characteristics of perylene-monoimide/n-Si Schottky contacts

The electrical and interface state properties of Au/perylene-monoimide (PMI)/n-Si Schottky barrier diode have been investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements at room temperature. A good rectifying behavior was seen from the I–V characteristics. The series resistance (R_s) values were determined from I–V and C–V characteristics and were found to be 160 Ω and 53 Ω, respectively. The barrier height (Φ_b) of Au/PMI/n-Si Schottky diode was found to be 0.694 eV (I–V) and 0.826 eV (C–V). The ideality factor (n) was obtained to be 4.27 from the forward bias I–V characteristics. The energy distribution of interface state density (N_ss) of the PMI-based structure was determined, and the energy values of N_ss were found in the range from E_c ? 0.508 eV to E_c ? 0.569 eV with the exponential growth from midgap toward the bottom of the conduction band. The values of the N_ss without R_s are 2.11 × 10¹² eV^?1 cm^?2 at E_c ? 0.508 eV and 2.00 × 10¹² eV^?1 cm^?2 at E_c ? 0.569 eV. Based on the above results, it is clear that modification of the interfacial potential barrier for metal/n-Si structures has been achieved using a thin interlayer of the perylene-monomide.     

Internal cooling efficiency of a junction diode

The three thermal rate equations were built newly up at both ends and at the junction of a p–n diode, in order to derive analytically the temperature difference ΔT (between a junction and both ends) and the internal cooling efficiency η defined newly for a homojunction diode. The maxima ΔT and η of a diode were derived analytically as a function of V _j within the short-length approximation and calculated numerically as a function of V _j or V _bi, where V _j is a voltage across the junction and V _bi is a built-in voltage at the junction. As a result, ΔT increases abruptly with an increase of V _j below V _j=0.050 V or of V _bi below V _bi=0.10 V, while above their values, it increases slowly with an increase of V _j or V _bi to saturate a certain value. For example, ΔT was estimated as 14.6 K for Hg_0.8Cd_0.2Te diode with V _bi=0.36 V. η has a local maximum of 63% at V _j≈0.01 V or at V _bi≈0.03 V, while above their respective values, it decreases abruptly with an increase of V _j or V _bi and falls to 4.4% at V _bi=0.80 V which is equivalent to that of a diode emitting a laser for fiber optical communication. However, the greater enhancements in ΔT and η of a diode are required to apply the internal cooling system to a laser-emitting diode which needs the exact control of temperature. These results should be useful for the application of the internal cooling system to the double heterojunction diode used in the optical communication.     

Numerical analysis of ultrathin Sb2Se3-based solar cells by SCAPS-1D numerical simulator device

Antimony selenide is considering as an emerging photovoltaic solar cell absorber. In this paper, Solar Cell Capacitance Simulator in 1 Dimension (SCAPS-1D) is used to investigate the possibility of realizing ultrathin Sb₂Se₃-based solar cells. The comparison of the current-voltage characteristic and output performances simulation results of CdS/Sb₂Se₃ solar cells with and without HTL are in agreement with the experimental results. In the first step, by considering the cell without HTL, the best PCE of 5.29% is obtained with WS₂ buffer layer. Thereafter, we simulated the impact of the charge carriers diffusion length and the doping concentration on the output performances. By combining a high quality absorber and doping concentration in the order of 10¹⁵ cm⁻³, Sb₂Se₃ solar cell achieves high PCE above 10%. Secondly, we introduced a HTL between the absorber layer and back metal contact, which led to n-i-p configuration. This configuration with CZ-TA HTL shows a best PCE of 6.29%. For a high quality absorber, Sb₂Se₃-based solar cell achieves best PCE of 11.10% and better stability for a thickness of 250 nm and doping concentration of 10¹⁴ cm⁻³ of the Sb₂Se₃ absorber layer. Our numerical solar cell design provides an approach to further improve the efficiency of Sb₂Se₃-based solar cells.     

Comparison of D.C. and A.C. electro-analytical methods for measuring diode ideality factors and series resistances of silicon solar cells

The diode ideality factor (m) and the series resistance (R_s) of a Si solar cell represent two critical performance-indicator parameters of the device. Since both m and R_s are functions of voltage (V) and temperature (T), simultaneous electrical measurements of these parameters under variable conditions of V and T can often be difficult with traditional direct current (D.C.) techniques. Using the electro-analytical method of linear sweep voltammetry (LSV) and a commonly available Si solar cell, we explore these specific confines of such D.C. measurements. The results are compared with those obtained from a parallel set of alternating current (A.C.) measurements using impedance spectroscopy (IS). LSV provides the main D.C. parameters (open circuit voltage, short circuit current, fill factor, and efficiency) of the cell, but is limited in terms of independently measuring m and R_s beyond strong forward biased conditions. The IS approach is free of the latter experimental constraints, and at the same time can provide several other important electrical parameters of the solar cell. Specifically, IS detects the presence of a low-high (p–p⁺) junction at the back surface of the cell, and serves as an efficient probe of certain electrical characteristics of this junction.     

高绒度掺硼氧化锌透明导电薄膜用作非晶硅太阳电池前电极的研究

将自行研制的具有优异陷光能力的掺硼氧化锌用作p-i-n型非晶硅太阳电池的前电极,并且将传统商业用U型掺氟二氧化锡作为对比电极.相比表面较为平滑的掺氟二氧化锡,掺硼氧化锌表面大类金字塔的绒面结构会在本征层生长过程中触发阴影效应,形成大量的高缺陷材料区和漏电沟道,进而恶化电池的开路电压和填充因子.在不修饰掺硼氧化锌表面形貌的情况下,通过调节非晶硅本征层的沉积温度来消弱高绒度表面形貌引起的这种不利影响,对应的电池开路电压和填充因子均出现提升.在仅有铝背电极的情况下,在本征层厚度为200 nm的情况下,以掺硼氧化锌为前电极的非晶硅太阳电池转换效率达7.34%(开路电压为0.9 V,填充因子为70.1%,短路电流密度11.7 mA/cm2).     

Potential‐induced optimization of ultra‐thin rear surface passivated CIGS solar cells

Ultra‐thin Cu(In,Ga)Se₂ (CIGS) solar cells with an Al₂O₃ rear surface passivation layer between the rear contact and absorber layer frequently show a “roll‐over” effect in the J–V curve, lowering the open circuit voltage (V_OC), short circuit current (J_SC) and fill factor (FF), similar to what is observed for Na‐deficient devices. Since Al₂O₃ is a well‐known barrier for Na, this behaviour can indeed be interpreted as due to lack of Na in the CIGS absorber layer. In this work, applying an electric field between the backside of the soda lime glass (SLG) substrate and the SLG/rear‐contact interface is investi‐gated as potential treatment for such Na‐deficient rear surface passivated CIGS solar cells. First, an electrical field of +50 V is applied at 85 °C, which increases the Na concentration in the CIGS absorber layer and the CdS buffer layer as measured by glow discharge optical emission spectroscopy (GDOES). Subsequently, the field polarity is reversed and part of the previously added Na is removed. This way, the J –V curve roll‐over related to Na deficiency disappears and the V_OC (+25 mV), J_SC(+2.3 mA/cm²) and FF (+13.5% absolute) of the rear surface passivated CIGS solar cells are optimized. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)