On of the most fascinating properties of tetrahedrally coordinated solids is their negative thermal expansion. For Si, Ge and a number of binary compounds (cubic materials crystallizing in the diamond-type and sphalerite- or wurtzite-type crystal structure) it was shown that their thermal expansion coefficient becomes negative at temperatures between 50 to 100 K [1-3]. In ternary A^IB^IIIX₂^VI chalcogenides, crystallizing in the tetragonal chalcopyrite-type structure, the linear thermal expansion behaviour is described by the both independent linear thermal expansion coefficients α_a and α_c, which are anisotropic. It was shown that CuBX₂ chalcopyrites (B=In,Ga) exhibit negative linear thermal expansion at temperatures below 30 K, for both a and c. The lattice parameter in these compounds first decrease with decreasing temperature going through a minimum and increase at low temperatures [4-8]. In the exceptional case of  AgBX₂ semiconductors the lattice parameter c increases with decreasing temperature in the whole temperature range [9].

Because of negative linear thermal expansion coefficients α_a and α_c, negative Grueneisen parameters may be expected in the low-temperature region and as a consequence the existence of low-energy lattice vibrational modes [10]. Moreover high-pressure induced structural phase transitions are caused by low-energy lattice vibrational modes with negative Grueneisen parameters. It is known from literature that the binary compounds ZnS and ZnSe as well as ternary CuInSe₂ show a pressure induced structural phase transition to the rocksalt-type structure [11-12].

Not much is known about the low temperature behaviour of quaternary chalcogenide compound semiconductors, like A₂^IB^IIC^IVX₄^VI. According to the tetrahedrally coordinated crystal structure, a negative thermal expansion can be expected. This assumption is strenghtened by the observation of a pressure induced structural phase transition in Cu₂ZnSnS₄ [13]. This compound shows a transition from the tetragonal kesterite-type structure to ta distorted rocksalt-type structure at ~ 15 GPa [13].

The compound semiconductors discussed above are used as absorber layers in thin film solar cells. Photovoltaic (PV) devices with chalcopyrite-type Cu(In,Ga)Se₂ absorbers show very high efficiencies [14]. The quaternary semiconductors Cu₂ZnSn(S,Se)₄, crystallizing in the tetragonal kesterite-type structure, are the absorbers in the only critical raw material free PV technology. Record efficiencies have been reached with an (Ag,Cu)₂ZnSnSe₄ absorber layer [15].

Thin film solar cells are the most ubiquitous and reliable energy generation systems for aerospace applications, because of their appealing properties such as lightweightness, flexibility, cost-effective manufacturing, and exceptional radiation resistance [16]. For  longer  missions,  PV devices  in  conjunction  with  rechargeable  batteries  are  the  only  available  option  to  provide  uninterrupted, sustainable and stable  electrical  power. Especially satellites  on  inner  planets  missions  employ  solar  cells, because at  these  distances  the  power  density  of  sunlight  is  sufficient  for  the  production  of  electricity [16]. For these applications the harsh conditions in space, like radiation and low temperatures, have to be taken into account. The temperature in outer space far away from earth is just 3 K [17], and extreme temperature swings occur. Thus the low temperature behaviour of absorber materials in solar cells potentially used in space applications, are of extreme importance. 

The presentation will give an overview of our detailed in situ neutron diffraction based structural investigation comparing the thermal expansion behaviour of Cu₂ZnSnSe₄, Ag₂ZnSnSe₄ and (Ag,Cu)₂ZnSnSe₄ mixed crystals from room temperature to 3 K. These materials crystallize in the tetragonal kesterite-type structure in the whole temperature range studied. The linear thermal expansion coefficients α_a and α_c are highly anisotropic. The end member Cu₂ZnSnSe₄ shows negative thermal expansion coefficients below 50K. In case of Ag₂ZnSnSe₄ the lattice parameter c increases with decreasing temperature thus showing a negative thermal expansion coefficient over the whole temperature range studied, indicating the special behaviour of Ag-containing tetrahedrally coordinated semiconductors.

The synthesis of multinary semiconductors for solar energy conversion applications such as kesterite (Cu₂ZnSn(S,Se)₄, CZTSSe) is extremely challenging due to the complexity of this type of compounds. In particular, quaternary kesterite-type compounds are not the exception, and all these detrimental issues explain why during almost 10 years the world record efficiency was unchanged. But the very recent development of molecular inks route with special precursors, allows the accurate control of single kesterite phase with high crystalline quality, contributing to increase the conversion e"ciency record of kesterite based solar cells up to 15% in a short time.

This presentation will be focused first in demonstrating how the molecular inks synthesis route was of key relevance for the control of high-quality single phase kesterite, through the modification of the synthesis mechanisms. The relevance of the composition of the ink, the precursor salts, and the interaction between the solvent and the cations in the solution is key for a reliable and reproducible high efficiency kesterite production baseline. Then, diluted alloying/doping strategies will be presented including Cu, Zn and Sn partial substitution with elements such as Ag, Li, Cd or Ge, that allowed e"ciencies close to 15%. The positive impact of these cation substitutions will be discussed in regards of their impact on the kesterite quality, as well as on the annihilation of detrimental punctual defects, allowing for new efficiency records at 15% level. In addition, the main characteristics and challenges of key kesterite interfaces (front, back and grain boundaries) will be discussed. Finally, very recent, and innovative interface passivation strategies will be discussed, showing the pathway to increase the record efficiency beyond 20%.

With the motivation of absorber layers which contain Earth-abundant, cheap and low-toxicity elements, the material Cu₂ZnSn(S,Se)₄ (CZTSe) has been a popular choice in the inorganic thin-film photovoltaics (PV) research community. After an extended period with no improvements to efficiency, recent records of ~13.8% [1] suggest a possible resurgence in research. Here we explore quantifying the environmental impacts of fabricating a CZTSe solar cell at the laboratory-scale using a mix of vacuum techniques (sputtering, electron beam evaporation, tube furnace annealing) and non-vacuum techniques (CZTS nanocrystal synthesis, slot-die coating, chemical bath deposition) with a structure of glass/Mo/CZTSe/CdS/i-ZnO/ITO/Ag,Ni. The intention is to determine which processes or layers are the most significant on the overall environmental impact of the fabrication and hence which may inhibit future scale-up. A life cycle assessment (LCA) is conducted which includes the materials used, electricity consumed as well as the waste produced. Previously we have used LCA to show how to reduce environmental impacts during the synthesis of CZTS nanocrystals [2]. We now extend the LCA analysis to the whole device. The selenization step (where selenium substitutes for sulfur), whilst essential for improved device performance, contributes significantly to the overall environmental impacts of cell fabrication. This results in the impacts from the solution-processed absorber layer to be similar to the vacuum-deposited transparent conducting oxide (TCO) layer due to the necessity of a high temperature, low pressure anneal. This work is hence a timely contribution to discussions surrounding the environmental impacts of vacuum versus non-vacuum deposition. In future work, a functional unit of 1 kWh is chosen such that device performance is included. This will allow for comparison to other research-scale solar cells with absorber layers such as Sb₂(S,Se)₃ and BaZrS₃.

A new power conversion efficiency record of 15.1% was reported just recently for a CZTSSe-based thin film device in which the polycrystalline CZTSSe absorber layer shows an off-stoichiometric composition. Deviations from stoichiometry cause intrinsic point defects which determine the electronic properties of a semiconductor significantly. A special kind of structural disorder, the Cu/Zn disorder, is always present in these compounds and is discussed as a possible reason for band tailing as well.

To minimize or avoid Cu/Zn disorder cation mutation strategies can be applied. In this way the crystal structure of the material can change from kesterite- to stannite-type to completely avoid this disorder. Substituting Zn²⁺ with Cd²⁺ in CZTS is one of the options. In the resulting solid solution series, both end members adopt different crystal structures: Cu₂ZnSnS₄ crystallizes in the kesterite-type structure whereas Cu₂CdSnS₄ adopts the stannite-type crystal structure.

We studied crystal structure, cation distribution and intrinsic point defect scenario in Cu₂(Zn_1-xCd_x)SnS₄ monograins  by neutron diffraction. This method enables us to differentiate the isoelectronic cations Cu⁺ and Zn²⁺ in the crystal structure analysis. At the same time the presence of Cd²⁺ in the samples introduces a huge challenge for neutron diffraction, as Cd is absorbing neutrons, in this way increasing the measuring times significantly.

These investigations enabled us to deduce that in the range between x=0 and 0.38 the mixed crystals adopt kesterite type structure with increasing Cu/II disorder with increasing Cd content. Starting with x=0.57 and until x=1.0 the material adopt stannite type structure, with a complete absence of Cu/II disorder. The change in the cation distribution being so abrupt suggests us that in this solid solution the complex cation re-distribution process within the crystal structure is happening in a very narrow compositional range 0.38 < x < 0.57.

Chalcogenide-based solar cells are regarded as promising due to their use of abundant, low-toxicity materials
and their scalability for cost-efficient manufacturing. However, their performance is often constrained by
efficiency challenges, with non-radiative recombination identified as a significant limitation. The potential
to overcome these challenges is seen in the exploration of emerging chalcogenides, offering pathways toward
improved stability and higher efficiencies in next-generation photovoltaic devices. A Voc deficit, largely
attributed to non-radiative recombination, has been identified in prior analyses as a major obstacle for these
materials. In this presentation, findings will be extended to recently reported high-efficiency devices, with
key photovoltaic metrics compared to historical benchmarks, and future research directions proposed. The
influence of Zn substitutions in CXTS (where X includes Mn, Mg, Sr, Ba, Ni, Co, Fe) on structural,
optoelectronic, and photovoltaic properties will be discussed, with particular attention given to the role of Ag
and other cation substitutions in Cu₂CdSnS₄ photoabsorbers. Directional growth of high-mobility [hk1]
planes in Sb₂(S,Se)₃, which has enabled power conversion efficiencies exceeding 9%, will also be presented.
Lastly, the application of CZTS and Sb₂(S,Se)₃ as photocathodes in solar water splitting will be explored.

The evolution of Sb₂Se₃ heterojunction devices away from CdS electron transport layers (ETL) to wide band gap metal oxide alternatives is a critical target in the development of this emerging photovoltaic material. Metal oxide ETL/Sb₂Se₃ device performance has historically been limited by relatively low fill-factors (FF), despite offering clear advantages with regards to photocurrent collection. In this work, TiO₂ ETLs were fabricated via direct current (DC) reactive sputtering and tested in complete Sb₂Se₃ devices. A strong correlation between TiO₂ ETL processing conditions and the Sb₂Se₃ solar cell device response under forward bias conditions was observed and optimised. Ultimately, a SnO₂:F/TiO₂/Sb₂Se₃/P3HT/Au device with the reactively sputtered TiO₂ ETL delivers an 8.12% power conversion efficiency (η), the highest cadmium-free Sb₂Se₃ device reported to-date. This is achieved by a substantial reduction in series resistance (Rs), driven by improved crystallinity of the reactively sputtered anatase-TiO₂ ETL, whilst maintaining almost maximum current collection for this device architecture. This paper will also discuss the role of organic hole transport materials ‐ namely P3HT, PCDTBT, and spiro‐OMeTAD to modify device performance. By comparing these against one another, and to a reference device, their role in the device stack are clarified. These organic HTM layers are found to serve a dual purpose, increasing both the average and peak efficiency by simultaneously blocking pinholes and improving the band alignment at the back contact.

Antimony selenide (Sb₂Se₃) has emerged as a promising photoelectric material owing to its excellent material properties. The power conversion efficiency of Sb₂Se₃ thin-film solar cells has reached an impressive 10.57% within a decade. Despite rapid development, the efficiency of Sb₂Se₃ thin-film solar cells remains significantly below the theoretical prediction of 30%. Therefore, considerable efforts are still required to enhance the material quality of Sb₂Se₃, a critical factor in boosting solar cell efficiency. Post-deposition annealing treatments have been carried out as an effective method to improve the qualities of Sb₂Se₃ thin films, such as crystallinity and optical properties. However, these treatments typically require tens of minutes or more of thermal annealing at high temperatures (>300 °C), which severely limits both the throughput and substrate choice. For the first time, a low thermal budget annealing technique was used for post-deposition annealing of Sb₂Se₃, which was carried out to enhance the film properties of Sb₂Se₃ thin film solar cells. Photonic curing uses pulsed light annealing with a broadband light source and Sb₂Se₃ samples annealed with single pulse light in pulse lengths of 1 to 15 ms. This process heats the Sb₂Se₃ film above 400 °C within milliseconds without damaging the underlying layers of the material stack. Short pulses (less than 5 ms) having higher radiant power damage the Sb₂Se₃ thin films compared to longer pulses. In contrast, during longer pulses, the temperature rises quickly and decreases gradually, even while the light remains on. This is due to the power in the capacitor bank draining, which reduces the lamp’s output and limits the peak temperature at the sample surface. As a result, longer pulses cause less damage and lead to more crystalline films. Therefore, increasing pulse duration or reducing radiant exposure can minimise damage to the Sb₂Se₃ film. Photonic curing has increased the crystal orientations in the hkl, l ≠ 0 ([211] and ([221]) direction of Sb₂Se₃, reduced the surface roughness and decreased the leakage current of the solar cells. The reduced open circuit voltage effectively lowers recombination rates of charge carriers and improves carrier transport. These enhance the power conversion efficiency of Sb₂Se₃ by 46%. Hence, photonic curing shows the capability of curing Sb₂Se₃ thin films and creating high-quality Sb₂Se₃ thin films with high crystallinity without sacrificing surface coverage. This eliminates the rate-limiting annealing step and opens up new opportunities for Sb₂Se₃ photovoltaics.

Low-dimensional antimony-chalcogenide materials have received an outstanding interest for photovoltaic (PV) devices in the last years. They show high stability, low environmental impact, low cost, low carbon footprint and high technological flexibility. Currently, efficiencies above 10% have already been achieved for Sb2(S,Se)3-based solar cells [1]. On the other hand, Sb-Ge chalcogenide material is well studied as phase change material for different applications, especially when using Te as chalcogen [2]. However, up to our knowledge, the combinations of Sb-Ge chalcogenide semiconductors have not been integrated in solar cell devices. In this work, Sb-(Ge)-Se thin films are grown by selenization of co-evaporated Sb-(Ge) films on Mo/SLG and SLG substrates. The properties of Sb-Ge-Se layers are compared with those of Sb2Se3 thin films. The effect of the selenization process on the structural, morphological, compositional and optical properties of the Sb-(Ge)-Se compounds is investigated. Different growth parameters, such as the maximum selenization temperature and Se added during the thermal treatment, have been investigated in these devices. Independently from the used growth parameters, all Sb2Se3 absorbers show an orthorhombic structure with [hk1] preferred orientation, and a compact structure free of pinholes, while the Sb-Ge-Se active layers show the co-existence of Sb2Se3 and GeSe2 phases. The co-existence of these two phases is corroborated by Fourier-transform infrared spectroscopy (FTIR). Sb-Ge-Se thin films´ band gap energy Eg varies from 1.4 to 1.8 eV depending on the Ge content, as determined by ellipsometry spectroscopy. First efficient Sb-Ge-Se thin-film solar cells have been fabricated using these new absorbers and device efficiencies of 5.4 % and 1.3 % are achieved for Sb2Se3 and Sb26Ge6Se68-based films respectively. C-V and DLCP measurements performed in both type of solar cells, indicate a high defects concentration when introducing Ge in the absorber layer, in agreement with the device performance. This is an indication that the (GeSe2)x(Sb2Se3)1-x system is more defective than the Sb2Se3 material. TEM investigation of Sb-Ge-Se and Sb2Se3 solar cells reveal a different CdS/Sb-(Ge)-Se heterojunction that can explain the limitation of the PV devices when Ge is incorporated in the structure.
More investigations are carried out to understand performance limitation in the solar cells based on these new thin-film absorbers. Finally, we will discuss the possible
improvements of the current devices and the potential of these promising and sustainable chalcogenide material for outdoor/indoor/thin film PV applications

New materials of interest for photovoltaics or other applications are invariably multi-cation compounds or alloys. Therefore, to properly explore their properties, synthesis experiments must be carried out over a broad parameter defined by several compositional variables, core thermodynamic variables of temperature and pressure, and kinetic variables such as reaction time and heating rate. Each of these can affect the outcomes of synthesis: the phases formed and their microstructural and defect characteristics that are so important for functional properties. A special challenge in the context of inorganic materials such as multinary chalcogenides is that they can be grown over a very wide range of temperatures and pressures as well as off-stoichiometric compositions. Thus, the parameter space to be explored is particularly extensive.  

In this contribution, we will present our concept for automated exploration of new inorganic chalcogenides in a self-driving lab. This combines a powerful and rapid PVD-based synthetic method with automation and machine learning, to rapidly map the phase space and functional properties of new materials, without needing prior information. The developments to-date will be presented, starting with our approach for automated generation of co-sputtering processes. This involves two machine-learning stages coupled to a geometrical model of the sputter flux, fitted in real time using input from a trio of QCM sensors. Outputs of the trained models can be used to produce co-sputtering recipes that yield a specified composition, while allowing other parameters (e.g. pressure) to vary. In addition, compositional maps for each sample are obtained directly without time-consuming mapping in an external system.  We will preview coming developments in automation of the subsequent process stage – rapid thermal sulfurization – and some initial work in image-based high-throughput characterisation combined with simulations, to derive optical properties of the target materials.  Perspectives for the application of self-driving labs in development of new device materials will be discussed. 

Chalcogenide perovskites, in particular BaZrS3, have gained a lot of popularity in the last few years due to its great potential as an alternative lead-free photovoltaic absorber material. This is due to promising optoelectronic properties such as defect tolerance, strong dielectric screening, and light absorption [1]. However some of the fundamental material physics, in particular polymorphic phase transitions, have not been explored in detail. Experimental studies have given conflicting results with Raman spectroscopy showing no signs of a phase transition[2], whilst XRD studies show an orthorhombic-to-tetragonal phase transition at 800K [3].
In this talk, we will introduce our machine learning potential model trained on perovskite structures with the neuroevolution potential method [4]. Through molecular dynamics calculations, we heat the experimentally reported orthorhombic Pnma phase and observe a first-order phase transition to a tetragonal I4/mcm phase at 610K. Upon further heating, we observe a second-order phase transition from the tetragonal phase to the cubic Pm-3m phase at 880K. We explain the order of these phase transitions through group-subgroup relationships and Landau theory.
Further analysis shows that the phase transitions are mediated through the M and R phonon modes associated with octahedral tilting, as is typically found in perovskite structures [5]. We analyze all possible Glazer tiltings to show that for the BaZrS3 perovskite only the Pnma --> I4/mcm --> Pm-3m phase transition route is accessible through heating. We also show temperature-dependent static structure factors and compare them to published experimental work[3]. To end, we highlight the dependence of stability of different polymorphs of the perovskite across various pressures and temperatures through a phase diagram. 

Barium sulphide (BaS) serves as a crucial precursor for advanced barium-based materials, including the emerging perovskite absorber BaZrS₃.^[1] However, conventional BaS production methods are highly energy-intensive, requiring temperatures exceeding 1000 °C ^[2,3] and emitting large quantities of CO₂ and SO₂,^[3,4] raising environmental concerns.

This work presents a novel solid-state synthesis route for BaS that drastically reduces the environmental and energy demands. By employing a finely milled mixture of barium hydroxide [Ba(OH)₂] and elemental sulphur, we achieve an efficient conversion (85%) to BaS at a remarkably low annealing temperature of 500°C.

The process is enabled by a unique low-pressure annealing environment, which facilitates the rapid vaporization of H₂O byproducts while maintaining a controlled sulphur partial pressure. This balance prevents unwanted side reactions and enhances the conversion efficiency. Furthermore, this method is highly scalable and compatible with industrial processes, offering a sustainable and economically viable pathway for BaS production.

Ternary chalcogenide nanocrystals have emerged as a promising material in the field of renewable energy, particularly as absorber materials for solar cells. In recent years, there has been a notable focus on the development of environmentally friendly materials, such as AgBiS₂ and AgSbS₂, as an alternative to traditional quantum dots containing heavy metals, including lead and cadmium. The components of these materials are plentiful and less toxic, reflecting the growing emphasis on sustainability and green energy solutions. These materials display high absorption coefficients, tunable band gaps, efficient charge separation, and impressive stability, rendering them ideal for emerging applications in solar cells, photodetectors, photocatalysis, and thermoelectrics. [1-3]
Nevertheless, the synthesis of quantum dots has traditionally been performed using the hot injection method, which involves prolonged high-temperature and vacuum processes. This approach presents significant challenges in terms of scalability, cost, and reproducibility, which must be overcome for these materials to be suitable for commercial applications. In this study, we present a straightforward and low-temperature synthesis of AgBiS₂ and AgSbS₂ quantum dots via cation exchange. This study presents a novel approach to the synthesis of Ag₂S nanoparticles (NPs) that employs unconventional sulfur precursors and a sequential exchange of silver ions for bismuth and antimony ions. This method allows the preparation of high-quality ternary quantum dots with precise control of size and atomic ratio. Furthermore, the incorporation of the ternary nanocrystals into photovoltaic devices provides evidence of the viability of the novel synthetic approach for the fabrication of high-performance solar cells.

Despite its great potential, lead halide perovskite technology draws major scepticism from supporters of established optoelectronic technologies due to long-term stability and environmental compatibility issues. One of the suggested solutions in literature is the large family of non-conventional (other than CIGS and CdTe), defect-tolerant, non-toxic chalcogenide compounds which possess desirable optical band gaps in the visible range. However, the fabrication of highly-efficient photovoltaic devices is still challenging, as there is a clear gap in efficiency between metal halide and chalcogenide-based materials. 
Here we present our efforts to fabricate thin films of acceptable optoelectronic properties and then build operational devices, where possible. We focus our experiments only on solution-processing methods, working on semiconductors at the nano- or poly-crystalline scale.  We deal with compounds which have cubic crystal structure, such as AgBiS2, possessing 3D electron dimensionality and, thus, can work as effective absorbers in around 2% efficient photovoltaics. On the other hand, few of those materials can also present strong radiative recombination (PbS and CuInS2) and may act equally well as emitters in solar cells and LEDs. There are also cases where sulfides (or selenides) with lower dimensionality (such as Sb2S3) are adopted with very promising results (PCEs of over 5%). Additionally, we are also targeting at chalcogenide semiconductors which crystallize in the perovskite structure and are able to fabricate proof-of-concept liquid solar cells based on BaZrS3-modified TiO2 photoelectrodes [1]. Our ultimate goal is to replace Ba2+ with a di-protonated amine and fabricate, for the first time in literature, hybrid inorganic-organic chalcogenide perovskites.  
 

Bismuth sulfide (Bi₂S₃) is a nontoxic and inexpensive semiconductor that has potential for use in low-cost thin film photovoltaics owing to its near optimal 1.2eV bandgap and high absorption coefficients greater than 1x10⁴cm^-1 within the IR to UV range [1][2]. Although Bi₂S₃ has potential in photovoltaics, devices have performed poorly with typical efficiencies not surpassing 2% [3][4][5]. One of the challenges of creating high performance cells is from the bulk defects that can exist due to stoichiometric imbalances acting as effective recombination centres [6]. These defects such as the sulfur vacancy and interstitials act mostly as donors making p-type doping difficult [6][7]. The reported properties of Bi₂S₃ are also inconsistent and depend on how the film was prepared. Its lack of crystallinity for example can blue shift the band gap to as high as 1.8eV due to quantum confinement effects [9][4]. Carrier mobilities have varied greatly from 5 cm²V^-1s^-1 prepared by chemical bath to much higher values of 257 cm²V^-1s^-1 and 588 cm²V^-1s^-1 for rapid thermal evaporation and spray pyrolysis [9][4][10]. The latter tow is greater than the typical carrier mobilities reported of 21 cm²V^-1s^-1 to 50cm²V^-1s^-1 for monocrystalline Bi₂S₃ grown via the Bridgman technique, suggesting the importance of purity [12][11]. The choice of substrate can influence the charge transfer efficiency in photovoltaic devices due how the bands bend at the interface. Bi₂S₃ deposited onto fluorine doped tin oxide (FTO) for example exhibited better photocurrent than those on tin doped indium oxide (ITO), Molybdenum or Gold [8]. This research has embarked on a systematic study of Bi₂S₃ thin films for photovoltaic devices. These thin films were synthesised via sulfurization of sputtered bismuth films on FTO substrates within a tube furnace. By varying the holding temperatures, ramp rates and holding times different optical and structural properties can be obtained.  The effect of different film properties such as crystallite size, stichometry and thickness will be tested in a ITO/ZnO/CdS/Bi₂S₃/FTO cell architecture. By gaining control over the film properties, they can be tuned to improve the performance as an absorbing layer in photovoltaic devices.

The currently  demand for renewable energy sources, it is imperative to explore new configurations of solar cells that use multiple photosensitizers for enhanced efficiencies. Considering the aforementioned factors, the present study entails the fabrication of an innovative n/p/p heterojunction solar cell with a unique design, and it consists of an n/p-type photoanode made from TiO₂/CdS/Cu₂ZnSnS₄(CZTS) and a p-type photocathode composed of Ag₃SbS₃/NiO/C/Ni foam, using a polysulfide gel electrolyte. This design captures light across the visible to near-infrared spectrum, achieving a high-power conversation efficiency (PCE) of 9.76%, significantly higher than standalone p-type (Ag₃SbS₃ ~1.6%) and n-type (CdS/CZTS, ~7%) solar cells. The alignment of various band gaps within the photoactive semiconductors, along with well-matched the energy levels at both photo-electrodes, effectively suppresses undesirable recombination process and enables efficient charge separation and transfer. An electrically conductive carbon interlayer, with a high work function of 5 eV, and it facilitates rapid and efficient electron transport from Ni to NiO. The p-type conduction of CZTS enables efficient hole extraction from CdS and their transfer to polysulfide species, while the narrow band gap of Ag₃SbS₃ quantum dots (~1.46 eV) facilitates broad-spectrum absorption, contributing to the high PCE. The n/p/p heterojunction solar cell also demonstrates impressive durability, retaining ~72% of its initial efficiency after 90 days of intermittent light exposure and storage. This resilience underscores its practical viability as a cost-effective, streamlined, and lead-free alternative to conventional perovskite, quantum dot, or Si/Ru-based solar cells.