Recombination In Space Charge Region Research Articles

Space charge region recombination in highly efficient silicon solar cells

The recombination rate in the space charge region (SCR) of a silicon-based barrier structure with a long Shockley–Reed–Hall lifetime is calculated theoretically by taking into account the concentration gradient of excess electron-hole pairs in the base region. Effects of the SCR lifetime and applied voltage on the structure ideality factor have been analyzed. The ideality factor is significantly reduced by the concentration gradient of electron-hole pairs. This mechanism provides an increase of the effective lifetime compared to the case when it is insignificant, which is realized at sufficiently low pair concentrations. The theoretical results have been shown to be in agreement with experimental data. A method of finding the experimental recombination rate in SCR in highly efficient silicon solar cells (SCs) has been proposed and implemented. It has been shown that at the high excess carrier concentration exceeding 1015 cm–3 the contribution to the SCR recombination velocity from the initial region of SCR that became neutral is significant. From a comparison of theory with experiment, the SCR lifetime and the ratio of the hole to the electron capture cross sections are determined for a number of silicon SCs. The effect of SCR recombination on the key characteristics of highly efficient silicon SCs, such as photoconversion efficiency and open-circuit voltage, has been evaluated. It has been shown that they depend not only on the charge carrier lifetime in SCR, but also on the ratio of hole to electron capture cross sections σp /σn. When σp /σn < 1, this effect is significantly strengthened, while in the opposite case σp /σn > 1 it is weakened. It has been ascertained that in a number of highly efficient silicon SCs, the distribution of the inverse lifetime in SCR is described by the Gaussian one. The effect described in the paper is also significant for silicon diodes with a thin base, p-i-n structures, and for silicon transistors with p-n junctions. In Appendix 2, the need to take into account the lifetime of non-radiative excitonic Auger recombination with participation of deep impurities in silicon is analyzed in detail. It has been shown, in particular, that considering it enables to reconcile the theoretical and experimental dependences for the effective lifetime in the silicon bulk.

Science in 2025-2027 and the SPQEO journal

Relevance of recent research is important for scientists and journals reporting research results. There are many sources of prognoses and one of them is the Report of European Commission "Looking into the R&I future priorities 2025-2027". It predicts the importance of the following areas for users: healthcare, energy, climate, sustainability and digitalization. The Ukrainian journal Semiconductor Physics, Quantum Electronics and Optoelectronics (SPQEO) actually focuses on these areas and contributes to the development of related knowledge. Monitoring of last SPQEO issues shows some interesting results: (i) the effect of local field amplification, which causes emergence of ponder motive forces acting on viruses until destruction of viral envelopes; (ii) the methods of malignant tumors treatment taking into account their genesis mechanisms and focusing on correction of definite pathogenesis components, while being nontoxic for other organs and tissues; and (iii) manipulation of the spectral characteristics of a “polycarbonate matrix – gold nanostructures – HTTH dye” system due to influence of gold nanostructures. SPQEO paid attention to the improvement of solar cells (SCs) by considering physical effects such as the effect of space charge region (SCR) recombination on the key characteristics of high-efficiency silicon solar cells, such as photovoltaic conversion efficiency and open-circuit voltage, is not only dependent on the charge-carrier lifetime in the SCR, but also on the ratio of hole-to-electron-capture cross section, σp/σn. Non- traditional SCs were also considered: SCs with perovskite thin films, SCs comprising CdS/CIGS heterojunctions, and vitamin B12-patterned silicon hybrids based SCs. Moreover, SPQEO also covers research results in the fields of quantum devices, diamond- like and oxide films, and light-emitting diodes.

Space charge region recombination, non-radiative exciton recombination and the band-narrowing effect in high-efficiency silicon solar cells

An expression for finding the dependence of narrowing the bands in silicon ΔEg on the level of illumination from the intrinsic absorption band (or short-circuit current) has been proposed. This expression is used to find experimental values of ΔEg in high-efficient silicon solar cells. The dependence ΔEg (J) or dependence ΔEg (JI), where JI is the short-circuit current density, has been rebuilt into the ΔEg (ΔnOC) dependence, where ΔnOC is the excitation level in open-circuit conditions. With this aim, the generation-recombination balance equation was solved taking into account six recombination mechanisms in silicon, including Shockley–Reed–Hall recombination, radiative recombination, interband Auger recombination, surface recombination, non-radiative exciton recombination, and recombination in the space charge region. The latter two recombination terms are not taken into account in studies of the key parameters of silicon solar cells and in programs for simulating the characteristics of these solar cells. Therefore, in this work their correct definition was performed, their contribution was compared with the contribution of other recombination mechanisms, and it has been shown that the description of the characteristics and key parameters of silicon SC without taking them into account is insufficiently correct. The experimental dependences ΔEg (ΔnOC) obtained in the work were compared with Schenk’s theory. It has been shown that there is a good agreement between them.

Modeling of characteristics of highly efficient textured solar cells based on c-silicon. The influence of recombination in the space charge region

Theoretical modeling of the optical and photovoltaic characteristics of highly efficient textured silicon solar cells (SC), including short-circuit current, open-circuit voltage and photoconversion efficiency, has been performed in this work. In the modeling, such recombination mechanisms as non-radiative exciton recombination relative to the Auger mechanism with the participation of a deep recombination level and recombination in the space charge region (SCR) was additionally taken into account. In a simple approximation, the external quantum efficiency of the photocurrent for the indicated SC in the long-wavelength absorption region has been simulated. A theory has been proposed for calculating the thickness dependences of short-circuit current, open-circuit voltage and photoconversion efficiency in them. The calculated dependences are carefully compared with the experimental results obtained for SC with the p+-i-α-Si:H/n-c-Si/i-n+-α-Si:H architecture and the photoconversion efficiency of about 23%. As a result of this comparison, good agreement between the theoretical and calculated dependences has been obtained. It has been ascertained that without taking into account recombination in SCR, a quantitative agreement between the experimental and theoretical light I V characteristics and the dependence of the output power in the SC load on the voltage on it cannot be obtained. The proposed approach and the obtained results can be used to optimize the characteristics of textured SC based on monocrystalline silicon.

Heterojunction Annealing Enabling Record Open-Circuit Voltage in Antimony Triselenide Solar Cells.

Despite the fact that antimony triselenide (Sb2 Se3 ) thin-film solar cells have undergone rapid development in recent years, the large open-circuit voltage (VOC ) deficit still remains as the biggest bottleneck, as even the world-record device suffers from a large VOC deficit of 0.59V. Here, an effective interface engineering approach is reported where the Sb2 Se3 /CdS heterojunction (HTJ) is subjected to a post-annealing treatment using a rapid thermal process. It is found that nonradiative recombination near the Sb2 Se3 /CdS HTJ, including interface recombination and space charge region recombination, is greatly suppressed after the HTJ annealing treatment. Ultimately, a substrate Sb2 Se3 /CdS thin-film solar cell with a competitive power conversion efficiency of 8.64% and a record VOC of 0.52V is successfully fabricated. The device exhibits a much mitigated VOC deficit of 0.49V, which is lower than that of any other reported efficient antimony chalcogenide solar cell.

Application Of The Sah-noyce-shockley Recombination Mechanism To The Model Of The Voltagecurrent Relationship Of Led Structures With Quantum Wells

The Sah-Noyce-Shockley (SNS) space charge region recombination theory is applied to build the mathematic model of the voltage-current relationships (VCR) of light emitting diodes with quantum wells. Unlike the mathematic model of VCR, for SNS in the proposed model, non-uniformity of recombination centres distribution over the space charge region and dependence of their mean concentration on voltage are assumed as well as the fact that the nonideality factor of forward current dependence on bias voltage may have a continuous series of values from 1 to 5 and is defined by the dependence on bias voltage of both saturation current and exponent of the VCR mathematical model.

The effect of electron-phonon interaction on the formation of reverse currents of p-n-junctions of silicon-based power semiconductor devices

The article represents the physical processes and mechanisms that accompany the work of the real p-n-junction. The nature of the reverse and forward currents of p-n-junctions is studied as the basis for the operation of these devices. The new model of recombination in space charge region of p-n-junction has been developed. This model made it possible to develop a new method for processing current-voltage characteristics and determining the parameters of recombination centers in semiconductor devices including electron-phonon interaction parameters. The probability of electronic transitions is calculated. It takes into account the electron-phonon interaction and explains the rapid build-up of reverse currents as the applied voltage increases. The mechanisms of formation of the reverse current of the p-n-junction were discussed and it was concluded that the current is determined by the generation with the participation of deep centers and electron-phonon interaction.

Hydrogen soaking irradiation acceleration method: application to and damage mechanism analysis on 3DG111 transistors

Bipolar devices are extremely sensitive to ionization effects, and their low dose rate radiation damage is more serious than their high dose rate radiation damage, which phenomenon is especially named enhanced low dose rate sensitivity. In the actual space radiation environment, the radiation dose rate of the device is extremely low. Currently, the enhanced low dose rate sensitivity effect has become a key factor of evaluating the reliability of spacecraft and its electronic systems, due to the fact that the low dose rate irradiation test needs longer time. The method to speed up the test on the ground is one of the hottest topics in this research area. In recent years, some researches have suggested that the use of hydrogen immersion irradiation for accelerating the test can simulate low dose rate radiation damage to some extent, but the damage mechanism has not been analyzed in detail. In this paper, the mechanisms of electrical properties and deep level defects for the 3DG111 transistor by <sup>60</sup>Co gamma ray under high and low dose rates in the cases with and without hydrogen are investigated. In order to analyze the damage mechanism of bipolar junction transistor, the excess base current and deep level transient spectrum are measured by using semiconductor parameter analyzer and deep level transient spectroscopy. The experimental results show that the current gain degradation of 3DG111 transistor is more serious under low dose rate radiation than under high dose rate radiation, at the same time, the excess base current of transistor increases significantly. This shows that in the device there appears the enhanced low dose rate sensitivity. Under both high dose rate radiation and low dose rate irradiation, the radiation damage defects are the traps for both oxide positive charge and interface state. Under the low dose rate irradiation, there are two main reasons for the increase in transistor damage. First, the oxide charge concentration increases under low dose rate irradiation, and the oxide charge and interface state energy levels move toward the middle band. Eventually, the space charge region recombination of the transistor is intensified, and thus causing the excessive base current of the transistor to increase and transistor performance to degrade. The comparison shows that the number and type of defects under the high dose rate irradiation are the same as those under the low dose rate irradiation. Based on the analysis, the hydrogen treatment can be used as an effective method of accelerating the assessment of radiation damage enhancement effect at low dose rates.

Modeling Edge Recombination in Silicon Solar Cells

A new approach to model edge recombination in silicon solar cells is presented. The model accounts for recombination both at the edge of the quasi-neutral bulk as well as at an exposed space-charge-region (SCR), the latter via an edge-length-specific diode property with an ideality factor of 2: a localized J02, edge. The model is implemented in Quokka3, where the $J_{02,edge}$ is applied locally to the edges of the three-dimensional geometry, imposing less simplifying assumptions compared with the common way of applying it as an external diode. A “worst-case” value for $J_{02,{\rm{edge}}}$ , assuming very high surface recombination, is determined by fitting to full detailed device simulations which resolve the SCR recombination. A value of $\sim \text{19 nA/cm}$ is found, which is shown to be largely independent of device properties. The new approach is applied to model the impact of edge recombination on full cell performance for a substantial variety of device properties. It is found that recombination at the quasi-neutral bulk edge does not increase the $J_{02}$ of the dark J–V curve, but still shows a nonideal impact on the light J–V curve similar to the SCR recombination. This needs to be considered in the experimental evaluation of edge losses, which is commonly performed via fitting $J_{02}$ to dark J–V curves.

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.

The role of Na incorporation in the low-temperature processed CIGS thin film solar cells using post deposition treatment

Na incorporation is widely considered to play an important role in low-temperature processed Cu(In,Ga)Se2 (CIGS) solar cells. In this work, we used post deposition treatment (PDT) method for Na incorporation in CIGS films, which involved the deposition of a thin NaF layer accompanied by thermal annealing. It was found that thermal annealing drove the diffusion of Na into CIGS absorbers without causing loss of elements and change of crystallinity. The dependence of the photovoltaic performance and recombination of CIGS solar cells on the Na PDT temperature was investigated. Impedance spectroscopy and admittance spectroscopy were used to characterize the recombination and defects of CIGS solar cells. The results indicated that the CIGS solar cells with Na PDT exhibited significant improvement of photovoltaic performance compared to those with Na-free. The enhanced transport performances of Na-treated cells were attributed to the decreased space-charge region (SCR) recombination. Both Na-free and Na-treated at 300 °C CIGS devices showed a donor defect with activation energy of about 295 meV, while the Na-treated devices demonstrated lower density of states, which resulted in an improved Voc. A maximum conversion efficiency of 11.9% was achieved for CIGS solar cells with Na PDT temperature at 300 °C.

Improved Model for Increased Surface Recombination Current in Irradiated Bipolar Junction Transistors

Current gain degradation in irradiated bipolar junction transistors is primarily due to excess base current caused by enhanced carrier recombination in the emitter-base space-charge region (SCR). Radiation-induced traps at the interface between silicon and the bipolar base oxide facilitate the recombination process primarily above the sensitive emitter-base junction. This leads to an increase in surface recombination current in the SCR, which is a non-ideal component of the BJT's base current characteristic under active bias conditions. In this paper, we derive a precise analytical model for surface recombination current that captures bias dependencies typically omitted from traditional models. This improved model is validated by comparisons to these traditional approaches.

Analytical device-physics framework for non-planar solar cells

Non-planar solar cell devices have been promoted as a means to enhance current collection in absorber materials with charge-transport limitations. This work presents an analytical framework for assessing the ultimate performance of non-planar solar cells based on materials and geometry. Herein, the physics of the p-n junction is analyzed for low-injection conditions, when the junction can be considered spatially separable into quasi-neutral and space-charge regions. For the conventional planar solar cell architecture, previously established one-dimensional expressions governing charge carrier transport are recovered from the framework established herein. Space-charge region recombination statistics are compared for planar and non-planar geometries, showing variations in recombination current produced from the space-charge region. In addition, planar and non-planar solar cell performances are simulated, based on a semi-empirical expression for short-circuit current, detailing variations in charge carrier transport and efficiency as a function of geometry, thereby yielding insights into design criteria for solar cell architectures. For the conditions considered here, the expressions for generation rate and total current are shown to universally govern any solar cell geometry, while recombination within the space-charge region is shown to be directly dependent on the geometrical orientation of the p-n junction.

Electrical characteristics of thin-film CdS/CdMgTe heterostructure for tandem solar cells

Charge transport mechanism in thin-film CdS/Cd1−xMgxTe (x=0.08) heterostructure are investigated. It is shown that the measured I–V characteristics and their temperature dependence are described well in terms of the Sah–Noyce–Shockley theory of generation–recombination in the diode space-charge region. A comparison of the experimental data with the theoretical results allowed determining the material and diode parameters such as the resistivity and hole density in the valence band of the film, the energy of the Fermi level and the barrier height at the interface, the lifetimes of charge carriers and the ionization energy of the generation–recombination center. It is also shown that low resistivity of the Cd1−xMgxTe absorber layer causes too low barrier height in the heterostructure and unacceptably high electrical losses in the solar cell.

Influence of band gradients on Cu(In,Ga)Se2 solar cell diode factors

The influence of band gap gradients on the charge collection and diode quality factor of solar cells is investigated by device simulation. A back surface band gap gradient manifested as a gradient of the conduction band is found to lead to an increased diode quality factor. Thus, the positive influence of the gradient on the fill factor is partially counterbalanced by the diode quality factor increase. The reason for the latter is the enhanced contribution of space charge region recombination. If the cell is equipped with a double gradient at front and back surfaces, the detrimental diode factor increase can be suppressed. The relevance of the findings is investigated using different carrier lifetimes and doping levels.

Current transport in ZnO/Si heterostructure grown by laser molecular beam epitaxy

The n-ZnO/p-Si heterojunction was fabricated by depositing high quality single crystalline aluminium-doped n-type ZnO film on p-type Si using the laser molecular beam epitaxy technique. The heterojunction exhibited a good rectifying behavior. The electrical properties of the heterojunction were investigated by means of temperature dependence current density—voltage measurements. The mechanism of the current transport was proposed based on the band structure of the heterojunction. When the applied bias V is lower than 0.15 V, the current follows the Ohmic behavior. When 0.15 V < V < 0.6 V, the transport property is dominated by diffusion or recombination in the junction space charge region, while at higher voltages (V > 0.6 V), the space charge limited effect becomes the main transport mechanism. The current—voltage characteristic under illumination was also investigated. The photovoltage and the short circuit current density of the heterojunction aproached 270 mV and 2.10 mA/cm2, respectively.

Impact of screen printing silver paste components on the space charge region recombination losses of industrial silicon solar cells

An approved measurement method for evaluating space charge region recombination (SCR-recombination) induced by the screen printed front side metallization is applied for industrial solar cells with aluminum back surface field (Al-BSF) on monocrystalline Czochralski silicon (Cz-Si). The H-patterned base grid of all processed solar cells is screen printed with a commercially available front side silver paste, ensuring sufficiently good contacting and therefore good electrical performance of the solar cells. Additionally, irregular contact fingers are realized between the regular contact fingers of the base grid by applying different screen printing layouts with commercially available front side pastes to increase the front side metallization fraction linearly as well as by applying various test pastes for the layout with the highest metallization fraction. Well-known components of glass frits as lead oxide, zinc oxide, bismuth oxide and boron oxide can affect SCR-recombination. Therefore we fabricated test pastes with these specific paste/glass frit components. By applying this test structure, a comparable qualitative evaluation of SCR-recombination with respect to the specific paste component is demonstrated. Quantitative evaluations regarding the electrical performance of the solar cells as well as related microstructural investigations are presented.

Separation of Series Resistance and Space Charge Region Recombination in Crystalline Silicon Solar Cells From Dark and Illuminated Current–Voltage Characteristics

The measurement of current–voltage (J–V) characteristics is one of the most straightforward methods for the characterization of solar cells. Consequently, an accurate knowledge of its meaning is of high relevance for the comprehension and technological feedback of these devices. The internal series resistance is one limiting parameter of the fill factor and the efficiency of these devices. A second limiting parameter is the p-n junction space charge region recombination. In this paper, we present a method to determine the lumped series resistance by combining the J–V characteristics in the dark and under 1-sun illumination. As a first approximation, the lumped series resistance under illuminated conditions is used for the dark J–V characteristic at small currents. Based on this, we present a method to quantify resistive losses and space charge region recombination only from the dark and illuminated J–V curves so that a simple separation of both losses becomes possible with all inline cell testers.

Influence of the absorber doping for p-type polycrystalline silicon thin-film solar cells on glass prepared by electron beam evaporation and solid-phase crystallization

A systematic investigation of the influence of the absorber doping on the performance of planar, p-type, evaporated, solid-phase crystallized polycrystalline silicon thin-film solar cells on glass is presented. It is found that the optimum Suns-Voc parameters (open-circuit voltage and pseudo fill factor) are achieved at intermediate absorber doping of Nabs∼1–2 × 1017 cm−3, while high short-circuit currents are achieved at the lowest absorber doping of Nabs ≤ 6 × 1015 cm−3. Since the short-circuit current is the dominating factor to achieve high conversion efficiencies for evaporated polycrystalline silicon cells, the maximum pseudo efficiencies are achieved at very low absorber doping. The Suns-Voc characteristics of lightly doped cells can be adequately described by a modified two-diode model with n1=1 and n2≈1.5, which is in contrast to the value of 2 for n2 commonly quoted in the literature. PC1D modeling demonstrates that such a low ideality factor for space charge region recombination can be modeled by a single trap energy level located at ∼0.18 eV away from midgap. Although the achievable short-circuit current densities and the conversion efficiencies can be higher for textured cells, planar cells are chosen intentionally to allow accurate modeling and extraction of relevant material parameters, such as minority carrier diffusion length.

Optoelectronic characteristics of Cu(In,Ga)(S,Se) 2 thin film solar cells obtained from varied chalcogenization processes

We investigated industrially produced chalcopyrite solar cells based on the absorber modifications Cu(In,Ga)Se 2 and Cu(In,Ga)(S,Se) 2 in order to study the nature of the experimentally verified efficiency improvement, mainly caused by an increased open circuit voltage. We show that the introduction of sulfur during the absorber formation via rapid thermal processing leads to a substantial lowering of the surface doping concentration and widening of the space charge region (SCR). Temperature dependent diode analysis revealed a reduction of the SCR recombination in (Se,S) devices which would lead to a larger splitting of quasi-Fermi levels and hence to an increased open circuit voltage as compared to neat Cu(In,Ga)Se 2 devices.