Enhancing Low Illumination Response of SHJ Solar Cells for More Sustainable Systems: A Device Simulation Study

Research Article Enhancing Low Illumination Response of SHJ Solar Cells for More Sustainable Systems: A Device Simulation Study Rupendra Kumar Sharma and Jakub Holovský Centre for Advanced Photovoltaics at the Department of Electrotechnology, Faculty of Electrical Engineering, Czech Technical University in Prague, Technická2, Prague 166 27, Czech Republic Correspondence should be addressed to Rupendra Kumar Sharma; [email protected] Received 21 October 2024; Revised 1 August 2025; Accepted 12 August 2025 Academic Editor: Guojun Yu Copyright ©2025 Rupendra Kumar Sharma and Jakub Holovský. International Journal of Energy Research published by John Wiley &Sons Ltd. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. In this work, we used calibrated numerical simulation models to optimize silicon heterojunction (SHJ) solar cells, with a focus principally on higher efficiency at lower illumination. The low-light analysis is important because photovoltaic (PV) modules are exposed to varied illumination conditions depending on location, weather, and climate. Recently, we have established that for lower illumination, the SHJ configuration with thin and lower-doped front p-type emitter contact is a viable option for providing higher efficiency; however, that study was only performed for a low-doped (~10 15 cm −3 ) c-Si(n) absorber. In this work, we comprehensively optimized the SHJ configuration for a wide variation of absorber doping/resistivity (5 ×10 14 –5×10 17 cm −3 /9.05–0.032 Ωcm) and observed that for highly doped (~5 ×10 16 cm −3 /0.141 Ωcm) absorbers, efficiency drops for thin and low-doped p-type emitters. On the contrary, a moderate to high doped (2 ×10 16 –5×10 16 cm −3 /0.292–0.141 Ωcm) absorber with a standard p-type emitter enhances efficiency most effectively under low light illumination, where the generated carrier density is low. Additionally, a combination of a moderate to high doped absorber, together with a doped a-Si:H(p) contact layer for an optimized front electrode workfunction, further boosts efficiency irrespective of illumination. The other advantage of our optimization is the relaxed requirements of a higher work function (WF) of the front electrode necessary for a hole-selective contact in SHJ solar cells. We achieved a remarkable 3.2%absolute increase at low illumination (0.01 suns), and a 1.4%absolute increase at 0.1 and 1.0 suns compared to an STC-optimized cell (optimized experimentally for best efficiency at 1.0 sun). This analysis suggests designing PV modules providing energy production that is slightly better matched to the actual people’s energy needs throughout the day and year. Keywords: device simulation; hetero-junctions; irradiance intensity; low illumination; optimization; photovoltaic devices; silicon solar cells 1. Introduction In the last few decades, the efficiency of silicon solar cells has been pushed by different technological and structural changes, such as passivated emitter and rear contact (PERC) [1, 2], silicon heterojunction (SHJ) [3, 4], tunnel oxide passivating contacts (TOPCon) [5, 6], and interdigitated back contact (IBC) [7, 8]. The PERC technology is approaching its efficiency limit [9, 10], and IBC technology is considered complicated and costly [11]. Recent advances in SHJ and TOPCon technologies have significantly enhanced efficiencies and are shaping the future of the photovoltaic (PV) industry [12]. The n-type bifacial i-TOPCon technology offers a record-high efficiency of 26.58%, reported by Trinasolar [13]. Recently, for the first time, a new record efficiency of 27.08%was achieved by Trinasolar for an industrial larger-area n-type total passivation (TOPAS) solar cell, based on front and back contact heterojunction (HJT or SHJ) technology. The other competitor, Longi, breaks its record of 26.81%efficiency reported in 2023 [14] and sets a new world record of 27.09%efficiency for crystalline SHJ back-contact solar cells utilizing laser patterning techniques [15]. SHJ technology has several advantages over Wiley International Journal of Energy Research Volume 2025, Article ID 9969335, 11 pages https://doi.org/10.1155/er/9969335 TOPCon, particularly higher efficiency, low temperature coefficient, excellent surface passivation, i.e., low surface recombination, and higher bifaciality ratio (excellent for bifacial designs) [16]. Additionally, SHJ technology is well-suited for ultra-thin silicon wafers, reducing material cost [17] and also performs better under low-light conditions [18]. The next step is the testing of these developed solar cells, and it is usually performed on an industry-wide specification, so-called standard test conditions (a cell temperature of 25°C, an irradiance of 1000 W/m 2 with an air mass of 1.5 (AM1.5) spectrum). However, in reality, these cells often experience variations in outdoor illumination intensity depending on time, location, weather, and climate [19, 20]. Recent studies on temperature and illumination dependance of silicon solar cells highlight the importance of low illumination analysis [21, 22]. Furthermore, the importance of the analysis at varied illumination conditions draws attention from the comparison of energy from PV and energy spot prices for Germany. The peaks of energy cost happen exactly when the PV is at the beginning or end of its daily production [23]. Most of the studies on low illumination are linked to indoor illumination [24, 25], which is different from the real outdoor conditions. The conversion efficiency of solar cells drops with reduced illumination for fundamental reasons and also for specific design decisions [26], and the latter one gives room to improve the low-light performance, thus increasing the overall energy yield. For SHJ technology, we have recently demonstrated experimentally and through numerical simulation that a thinner and low-doped a-Si:H(p) contact can considerably improve cell efficiency under low light illumination (~0.1 suns) while slightly sacrificing the efficiency at STC [18]. This improvement is related to the mechanism of modulation doping of the absorber by a doped p + contact layer with a higher bandgap. The analysis was performed at a fixed absorber doping (1 ×10 15 cm −3 /4.59 Ωcm resistive), and variation of c-Si absorber doping was not considered. In the present analysis, we observe that with increasing c-Si absorber doping, the efficiency at low illumination for thin and low-doped p contact decreases. This motivates us to perform a thorough optimization of all material parameters in conjunction. In this work, through more extensive mapping of the material parameter space, we improve efficiency at lower illumination without compromising efficiency at STC. We optimized the doping of the a-Si:H(p) layer and the front electrode work function (WF), in conjunction with c-Si(n) absorber doping and its thickness, within a set of material parameters. Higher c-Si doping (~10 17 cm −3 /0.086 Ωcm) is detrimental to cell performance at STC due to reduced mobility [27, 28]. and consequently to the current density. Nevertheless, higher doping helps to boost efficiency at lower illumination, where the generated carrier density is low. Additionally, with higher doping in the absorber, we can enhance efficiency by increasing doping in the a-Si:H(p) contact layer while maintaining the charge balance at the c-Si(n)/a-Si:H(p) interface. The electrode WF plays a crucial role in determining the other technological parameters for optimum performance at low illumination. We achieved 21.35%efficiency at 0.1 suns and 23.7%at STC (1.0 sun), which is a 1.4%absolute increase compared to baseline efficiency (19.95%–0.1 suns and 22.3%–1.0 sun). A notable gain of 3.2%in efficiency (19.2%compared to 16%) is achieved at very low illumination (0.01 suns), thus increasing the total energy yield. We can conclude that all structural parameters need to be optimized in conjunction rather than individually. This analysis presents a step forward for future solar cell technology optimized for better matching the energy demand, not necessarily optimized for STC. 2. Experiments and Computer Simulation Our SHJ solar cell structure consists of an a-Si:H(i)/a-Si:H(p)/ ITO stack on an n-type c-Si wafer (Figure 1). The details about the experimental devices and results we used for calibration can be found in our recently published work [29]. Silvaco TCAD was used for computer simulations, and experimental data from the real device were used for simulation calibration. The ITO is treated as a transparent metal-like layer denoted by its WF. The model uses the Schottky thermionic emission, accounting for field-dependent barrier-lowering caused by image forces, and possible static dipole layers at the metalsemiconductor interface are activated [30]. Quantum tunneling models for both the electrons and holes are introduced to take care of carrier transportation through ultra-thin a-Si layers. To accurately model the defects present in highly defective amorphous silicon, the defect density of states (DOS) is defined as a combination of exponentially decaying band tail states and Gaussian distributions of mid-gap states. In amorphous silicon, for a doping concentration up to 10 19 cm −3 , the ratio between defect density and doping concentration is constant. For higher doping concentration, all the dopant atoms are compensated by the dangling bonds, leading to a pinning of the Fermi level and thus to the saturation of the conductivity [31, 32]. The ITO (70 nm) ITO (70 nm) a-Si:H(p): 10 nm a-Si:H(i): 5 nm c-Si(n) (50–200 μm) a-Si:H(i): 5 nm a-Si:H(n): 10 nm FIGURE 1: The SHJ solar cell structure designed in the TCAD device simulator (layer thickness and doping are not to scale). 2 International Journal of Energy Research ijer, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/er/9969335 by Czech Technical University in Prague, Wiley Online Library on [21/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License details of the calibrated model parameters are provided in Table 1. We used a planar structure for our simulation instead of a textured one. The textured surface reduces reflection losses, thus slightly improving current density and consequently, efficiency. The simulation of a textured 3-D structure is timeconsuming and complicated, while not altering the findings of our current investigations. As a continuation of our recently published results [18], in which we observed that a thin and less doped a-Si:H(p) contact layer increases efficiency at low illumination for a low-doped cSi absorber (Figure 2a). We increase the doping of the absorber while keeping thin and less doped a-Si:H(p) contact, and notice adropintheefficiency to its baseline values at low illumination (Figure 2b), because of not having enough charges in a-Si:H(p) contact layer to compensate for the highly doped absorber. In the next sections, we comprehensively analyze the impact of the c-Si absorber doping on cell performance. From the experimental point of view, with varying wafer doping, one of the important parameters that must be discussed is wafer resistivity. Recently, Longi, for their world record SHJ back contact solar cells, refer to the resistivity values between 0.4 and 1.6 <Ωcm >, which corresponds to a wafer doping concentration of 1.5 ×10 16 –3×10 15 cm −3 [15]. Liu et al. [33] have also reported wafer resistivity of 0.3−2.1 Ωcm (corresponding wafer doping of 2 ×10 16 –2.25 ×10 15 cm −3 ) for over 25%efficiency silicon solar cells. However, for the sake of understanding, we choose a wide range of wafer doping (5 ×10 14 –5×10 17 cm −3 /9.05–0.032 Ωcm). The dependance of resistivity and mobility on wafer doping is shown in Figure 3. The resistivity shows linear dependance on the c-Si (n) wafer doping; however, mobility is decreasing rapidly for higher TABLE 1: Parameter values adopted in simulations. Parameters c-Si(n) a-Si:H(p) a-Si:H(n) a-Si:H(i) Layer thickness (nm) 0.5–2×10 5 10 10 5 Dielectric constant 11.9 11.9 11.9 11.9 Electron affinity (eV) 4.07 3.86 3.86 3.86 Band gap (eV) 1.12 1.7 1.7 1.7 Nc (cm –3 ) 2.08 ×10 19 1×10 20 1×10 20 1×10 20 Nv (cm –3 ) 1.04 ×10 19 1×10 20 1×10 20 1×10 20 Doping concentration (cm –3 )5×10 14 –5×10 17 5×10 18 –4×10 19 1×10 21 1×10 14 Defects: donors (conduction tail) 1 ×10 12 5×10 19 –4×10 20 1×10 20 1×10 17 Defects: acceptors (valence tail) 1 ×10 12 4×10 19 –3×10 20 1×10 20 1×10 17 Urbach E of donors (eV) 0.02 0.12 0.12 0.09 Urbach E of acceptors (eV) 0.01 0.07 0.07 0.06 0.01 0.1 1 16 18 20 22 24 26 Baseline A (p+ 10 nm_1.5e19) Reduced thickness (p+ 5 nm_1.5e19) Reduced thickness and doping (p+ 5 nm_1e19) Baseline B (p+ 10 nm_1e19) Efficiency (%) Illumination intensity (suns) c-Si_1e15 cm–3 ðaÞ 0.01 0.1 1 16 18 20 22 24 [18] p+ (a-Si)_1e19 cm–3 p+ 10 nm_c-Si_1e15 (baseline A) p+ 5 nm_c-Si_1e15 p+ 5 nm_c-Si_5e16 p+ 5 nm_c-Si_1e17 Efficiency (%) Illumination intensity (suns) ðbÞ FIGURE 2: (a) The comparison of thin and less doped p + contact with baseline cases A and B [18]. (b) Impact of c-Si absorber doping on the efficiency of thin and low-doped a-Si:H(p) contact layer. International Journal of Energy Research 3 ijer, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/er/9969335 by Czech Technical University in Prague, Wiley Online Library on [21/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License wafer doping (~1 ×10 17 cm −3 /0.086 Ωcm), which also reflects in the degradation of current density (Figure 4d). As a first step, we checked the impact of c-Si doping variations on the energy band diagram at equilibrium. The simulated equilibrium band diagram shows that the overall integrated band bending equals the WF difference between the front and back electrodes (Figure 5a). Since we maintain the condition of the flat band at the back contact through a highly doped n + layer, the dependance of overall band bending reduces to the WF difference between the front electrode and absorber (Figure 5a); therefore, band bending occurs either in the absorber or a-Si:H(p) layer. The role of the a-Si:H(p)layeris to facilitate the extraction of holes toward the front electrode by locally inverting the absorber front surface from n-type to ptype; therefore, most of the band bending shifts inside the absorber [18]. This is important because additional band bending inside a-Si:H(p) contact layer would increase the barrier for holes above an acceptable level (~0.55 eV, refer to our previous paper [29]). Therefore, an increase in the n-type doping of the absorber above a certain level may also increase the requirements on the level of a-Si: H(p) layer doping. Since the doping in the absorber shifts the electron quasi-Fermi level (QFL) and reduces WF, thus in most cases of moderate doping, the increase in absorber n-type doping will reduce the requirements of higher electrode WF for hole selective contacts in SHJ solar cells. In our previously published works [29, 34], we defined contact strength (the ability of a contact to efficiently collect the photo-generated carriers through electrode WF or a-Si:H (p) contact layer doping/thickness, or a combination of both, for a given illumination). We investigated these requirements and their relaxation for lower irradiance for a low-doped absorber (1 ×10 15 cm −3 ). Usually, going above the required level of contact strength (5.19 eV for hole selective contacts, equivalent to the valence band energy of c-Si) is not beneficial for cell performance. In our recent analysis [18], we noticed that reducing the contact strength can increase efficiency at low irradiances, only for low-doped absorbers (1 ×10 15 cm −3 / 4.594 Ωcm), as shown in Figure 2a. The question remains what happens in the case of higher absorber doping. First, due to the strong dependance of the electron QF level on absorber doping and only weak dependance of the hole QF level results in an overall larger QFL splitting (QFLS) (Figure 5b). The QFLS was calculated 10 nm deep from the surface of the c-Si absorber. We evaluated ideality factor (n) as a slope of QFLS vs. the logarithm of illumination (Figure 5b) and noticed that the baseline device (full lines) operates mostly in the regime of n=1.5 (highlighted by blue), while the highly doped device operates in the regime of n=1. 3. Optimization at Low Illuminations To deeply analyze the impact of c-Si doping on the SHJ cell performance, short circuit current density (J SC ), open circuit voltage (V OC ), fill factor (FF), and cell efficiency were extracted from device simulation (Figure 4) and compared with the technological baseline (experimental data [29]). At STC, the V OC , which depends on the QFLS in the absorber, slightly decreases with increasing absorber doping (Figure 4a); however, at low illumination (0.01 suns), where the generated carrier density is low, the trend is the opposite, and V OC gradually and slowly improves. At high doping (>1×10 17 cm −3 /0.086 Ωcm), V OC at all illumination levels starts dropping because a-Si:H(p) layer is not anymore able to provide enough charge to keep the band banding in the absorber and, consequently, the barrier for holes ϕ(Figure 5a) becomes larger than 0.55 eV, which is the value where the extracted voltage become limited [29]. Apart from this extreme case, the trends in V OC can be described by the concept of the ideality factor, and we know that the theoretical optimum is n=1.Wetrytogettheidealityfactorascloseto1 1015 1016 1017 0.01 0.1 1 10 Electron Hole Resistivity (Ω cm) c-Si absorber doping (cm–3) ðaÞ 1015 1016 1017 Electron Hole 200 400 600 Mobility (cm2/Vs) 800 1000 1200 1400 c-Si absorber doping (cm–3) ðbÞ FIGURE 3: (a) Resistivity values for the variation of doping in n-type c-Si absorber. (b) Electron and hole mobilities. 4 International Journal of Energy Research ijer, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/er/9969335 by Czech Technical University in Prague, Wiley Online Library on [21/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License (minimize recombination) using the variation of c-Si doping (Figure 5b) and evaluate it from the voltage at the maximum power point V MPP . The FF, on the other hand, increases with increasing c-Si doping, more strongly at lower illumination because the impact of series resistance becomes negligible. Note that (Figure 4c) by just optimizing absorber doping, we can boost >2%efficiency at 0.01 suns (18.4%compared to 16%), 0.6%at 0.1 suns (20.55%compared to 19.95%), and 0.4%at 1.0 sun (22.7%compared to 22.3%). A large portion of this improvement in efficiency comes from the FF. To better understand it, we plot the excess carrier density as a function of depth for various absorber dopings. The increase in c-Si absorber doping modified the carrier density close to its interface (Figure 6a) as compared to the bulk (Figure 6b) 0.01 0.1 1 400 500 600 700 800 n = 1.5 n = 1 c-Si_5e14, c-Si_1e15 c-Si_5e15, c-Si_1e16 c-Si_2e16, c-Si_5e16 c-Si_1e17, c-Si_5e17, Experimental, [21] VOC (mV) Illumination (suns) ðaÞ 0.01 0.1 1 60 65 70 75 80 85 90 FF (%) Illumination (suns) ðbÞ c-Si_5e14, c-Si_1e15 c-Si_5e15, c-Si_1e16 c-Si_2e16, c-Si_5e16 c-Si_1e17, c-Si_5e17, Experimental 0.01 0.1 1 14 16 18 20 22 24 26 Efficiency (%) Illumination (suns) ðcÞ JSC (mA/cm2) 1E15 1E16 1E17 0.30 0.32 0.34 0.36 0.38 30 32 34 36 38 40 0.01 suns 1.0 sun Absorber doping (cm–3) ðdÞ FIGURE 4: The impact of c-Si doping variation on (a) open circuit voltage (V OC ), (b) FF, (c) efficiency, and (d) short circuit current density. aSi:H(p) layer doping and front electrode WF are fixed to 1 ×10 19 cm –3 and 5.1 eV, respectively. International Journal of Energy Research 5 ijer, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/er/9969335 by Czech Technical University in Prague, Wiley Online Library on [21/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License because of induced band bending by the WF difference between the doped a-Si:H(p)/electrode and c-Si absorber. For the low-doped (~intrinsic) absorber, the maximum band bending is defined by the mid-gap energy of the bandgap of the c-Si absorber, that is, 0.56eV, and is reached for the highest a-Si:H (p) doping/WF of the electrode. This is not the case for doped absorbers. The fundamental rule is that band bending is either provided by the electrode WF/doped a-Si:H(p) layer or a combination of both. For doped absorbers (n-type), the Fermi level moves close to the conduction band, thus relaxing the requirement of strong doping in the a-Si:H(p) layer or higher WF materials for the electrode. With increasing c-Si(n) doping, the excess hole (minority carrier) concentration decreases (Figure 6a,b), thus increasing the minority carrier lifetime/ 0.06 0.08 200.08 200.10 –1.6 –1.2 –0.8 –0.4 0.0 0.4 0.8 1.2 1.1 eV EV EC (i) a-Si:H(n) ITO c-Si_1e15, c-Si_1e16 c-Si_5e16, c-Si_1e17 c-Si_5e17 c-Si(n) EV EFϕB Energy (eV) Depth (μm) EC a-Si:H(p i) ITO ðaÞ Energy (eV) 10–5 10–4 10–3 10–2 10–1 100 –1.2 –1.0 –0.8 –0.6 –0.4 –0.2 0.0 c-Si_1e15 c-Si_1e17 Efp Efn QFLS Illumination (suns) n ≈ 1.5 ðbÞ FIGURE 5: (a) Simulated equilibrium band diagram of the a-Si:H(p)/a-Si:H(i)/c-Si(n) interface for doping variation in the c-Si absorber. (b) Simulated positions of quasi-Fermi levels (extracted 10 nm below the top surface of c-Si absorber) with varying illumination intensity; the slopes of the lines correspond to ideality factor n≈1, except for the sections indicated by blue bars with n≈1.5. 0.00 0.01 0.02 0.03 0.04 0.05 105 107 109 1011 1013 1015 1017 c-Si(n) a-Si(p) a-Si(i) c-Si_1e15 c-Si_1e16 c-Si_5e16 c-Si_1e17 c-Si_5e17 Electron and hole concentration (cm–3) Depth (μm) ðaÞ c-Si_1e15 c-Si_1e16 c-Si_5e16 c-Si_1e17 c-Si_5e17 105 107 109 1011 1013 1015 1017 Electron and hole concentration (cm–3) Depth (μm) 0.0 0.1 0.2 0.3 0.4 Solid line: electron concentration Dotted line: hole concentration ðbÞ FIGURE 6: Excess electron and hole concentration at maximum power point (MPP) for different doping concentrations of c-Si absorber. (a) Zoom view and (b) full-scale view. 6 International Journal of Energy Research ijer, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/er/9969335 by Czech Technical University in Prague, Wiley Online Library on [21/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License diffusion length, which results in enhancing the cell efficiency. This is valid until the excess carrier concentration remains higher than the doping of the c-Si absorber. Further increasing the doping (~5 ×10 17 cm −3 /0.032 Ωcm) of the c-Si absorber reduces the generated current density, thus efficiency, irrespective of illumination (Figure 4c,d). This is because of the accumulation of electrons close to the a-Si:H(i)/c-Si(n) interface (Figure 6), thus limiting the movement of generated holes toward the a-Si:H(p) layer, which results in reducing short circuit current density (Figure 4d). Another factor contributing to the degradation of current density is the steep degradation of mobility of minority carriers (Figure 3b). We have also analyzed the impact of c-Si absorber thickness (Figure 7) and noticed that for a low-doped absorber (~10 15 cm −3 /4.594 Ωcm), reducing the c-Si thickness reduces efficiency at 1.0 sun and marginally improves it at 0.01 suns. This reduction at 1.0 sun mainly comes from the current reduction for thinner absorbers (optical losses, Figure 7a). The voltage and FF slightly 50 100 150 200 33 34 35 36 37 38 1.0 sun, c-Si_1e15 0.01 suns × 100, c-Si_1e15 1.0 sun, c-Si_5e16 1.0 sun, exp JSC (mA/cm2) c-Si absorber thickness ðaÞ 0.01 0.1 1 600 650 700 750 800 c-Si_200 um, 1e15 cm–3 c-Si_100 um, 1e15 cm–3 c-Si_50 um, 1e15 cm–3 c-Si_200 um, 2e16 cm–3 c-Si_200 um, 5e16 cm–3 Experimental VOC (mV) Illumination (suns) ðbÞ 0.01 0.1 1 60 65 70 75 80 85 90 FF (%) Illumination (suns) c-Si_200 um, 1e15 cm–3 c-Si_100 um, 1e15 cm–3 c-Si_50 um, 1e15 cm–3 c-Si_200 um, 2e16 cm–3 c-Si_200 um, 5e16 cm–3 Experimental ðcÞ 0.01 0.1 1 16 18 20 22 24 Efficiency (%) Illumination (suns) c-Si_200 um, 1e15 cm–3 c-Si_100 um, 1e15 cm–3 c-Si_50 um, 1e15 cm–3 c-Si_200 um, 2e16 cm–3 c-Si_200 um, 5e16 cm–3 Experimental ðdÞ FIGURE 7: Showing the impact of c-Si thickness on (a) short circuit current density: at 0.01 suns, we multiplied it by 100 to make it comparable to 1.0 sun illumination, (b) open circuit voltage, (c) fill factor, and (d) efficiency. a-Si:H(p) layer doping and front electrode WF are fixed to 1×10 19 cm –3 and 5.1 eV, respectively. International Journal of Energy Research 7 ijer, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/er/9969335 by Czech Technical University in Prague, Wiley Online Library on [21/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License improve with reduced absorber thickness (Figure 7b,c). We compared the most promising c-Si doping cases (2 ×10 16 and 5 × 10 16 cm −3 ) with the baseline case. For a moderately doped (2 ×10 16 cm −3 /0.292 Ωcm) absorber, efficiency increases to 18.2%, 20.3%,and22.7%compared to the technological baseline (16%,19.95%, and 22.3%) at 0.01, 0.1, and 1.0 sun, respectively (Figure 7d). For a 5 ×10 16 cm −3 absorber doping case, efficiency further enhanced to 18.4%, and 20.55%at 0.01 and 0.1 suns, respectively, while unaffected at 1.0 sun. Furthermore, we vary the a-Si:H(p) contact layer doping and examine the impact of both c-Si doping and illumination on cell performance (Figure 8). For the efficient collection of holes, we require a WF equal to the valence band energy of the c-Si absorber (~5.19 eV). With the a-Si: H(p)dopingof2e19cm −3 , which is well within the experimental limits [35], we noticed a substantialgainintheefficiency (M 2%) at low illumination (0.01 suns). For further increasing a-Si:H(p) doping (M 2 × 10 19 cm −3 ), which corresponds to the higher WF compared to 5.19 eV, the impact of a-Si:H(p) doping, irrespective of illumination, ceases to exist. This is because at very high doping in a-Si:H (p), all the dopant atoms are compensated by the dangling bonds, leading to a pinning of the Fermi level and thus to the 1 × 1019 2 × 1019 3 × 1019 4 × 1019 16 18 20 22 24 0.01 suns c-Si_1e15 c-Si_1e16 c-Si_2e16 c-Si_5e16 c-Si_1e17 Experimental Efficiency (%) Contact layer doping (cm–3) ðaÞ 1 × 1019 2 × 1019 3 × 1019 4 × 1019 c-Si_1e15 c-Si_1e16 c-Si_2e16 c-Si_5e16 c-Si_1e17 Experimental 16 18 20 22 24 0.1 suns Efficiency (%) Contact layer doping (cm–3) ðbÞ c-Si_1e15 c-Si_1e16 c-Si_2e16 c-Si_5e16 c-Si_1e17 Experimental 1 × 1019 2 × 1019 3 × 1019 4 × 1019 Contact layer doping (cm–3) 16 18 20 22 24 1.0 sun Efficiency (%) ðcÞ FIGURE 8: The impact of a-Si:H(p) layer doping and absorber doping on the efficiency of SHJ solar cell (a) 0.01 suns, (b) 0.1 suns, and (c) 1.0 sun. The experimental data corresponds to a-Si(p) doping: 1 ×10 19 cm –3 , front electrode WF: 5.1 eV, c-Si doping: 1 ×10 15 cm –3 , c-Si thickness: 200 μm. 8 International Journal of Energy Research ijer, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/er/9969335 by Czech Technical University in Prague, Wiley Online Library on [21/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License saturation of the conductivity. It is fascinating that at 1.0 sun illumination, the impact of both c-Si doping and a-Si:H(p) doping is small, in comparison to the impact at lower illumination (0.01 suns). The higher doping in the absorber facilitates maintaining higher doping in the a-Si:H(p) contact layer due to charge balance at the a-Si:H(i)/c-Si(n) interface. Furthermore, in the case of low illumination where carrier generation is low, higher doping boosts the voltage together with a considerable gain in FF (not shown here), thus improving efficiency. It is quite clear that optimizing these structural parameters is relatively important for low illumination. Using our comprehensive optimization approach, we achieved a notable 3.2 %absolute increase in efficiency at 0.01 suns, and 1.4%absolute increase at 0.1 and 1.0 suns, compared to baseline efficiency (Figure 8). Finally, the variation in efficiency with front electrode WF is shown in Figure 9. For lower a-Si:H(p) doping, the electrode WF has a strong influence, specifically at 1.0 sun illumination. The increase in absorber doping only improves efficiency at lower illumination; however, the higher doping in the a-Si:H(p) layer, together with higher doping in the absorber, increases the efficiency irrespective of illumination and electrode WF. The requirement of a higher WF is relaxed at lower illumination for hole-selective contacts in SHJ solar cells. 4.8 5.0 5.2 5.4 5.6 16 18 20 22 24 Dotted line: P+_1e19 Solid line: P+_2e19 c-Si_1e15 c-Si_1e16 c-Si_2e16 c-Si_5e16 Experimental 0.01 suns Efficiency (%) Electrode WF (eV) ðaÞ c-Si_1e15 c-Si_1e16 c-Si_2e16 c-Si_5e16 Experimental 4.8 5.0 5.2 5.4 5.6 16 18 20 22 24 Dotted line: P+_1e19 Solid line: P+_2e19 0.1 suns Efficiency (%) Electrode WF (eV) ðbÞ c-Si_1e15 c-Si_1e16 c-Si_2e16 c-Si_5e16 Experimental 4.8 5.0 5.2 5.4 5.6 16 18 20 22 24 Dotted line: P+_1e19 Solid line: P+_2e19 1.0 sun Efficiency (%) Electrode WF (eV) ðcÞ FIGURE 9: The impact of front electrode work function, a-Si:H(p) contact layer doping and absorber doping on the efficiency (a) 0.01 suns, (b) 0.1 suns, and (c) 1.0 sun. The symbol only represents experimental data. International Journal of Energy Research 9 ijer, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/er/9969335 by Czech Technical University in Prague, Wiley Online Library on [21/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License