Third-generation solar cells
Perovskite solar cells
Perovskite solar cells utilize hybrid organic-inorganic lead halide perovskite materials as the light-absorbing layer, characterized by the general ABX₃ crystal structure where A is a monovalent organic or inorganic cation (e.g., methylammonium, MA⁺), B is a divalent metal cation (typically Pb²⁺), and X is a halide anion (e.g., I⁻, Br⁻, or Cl⁻). A prototypical example is methylammonium lead iodide (MAPbI₃), which exhibits a suitable direct bandgap for photovoltaic absorption. These materials are typically fabricated via low-temperature solution-processing techniques such as spin-coating, where perovskite precursors are dissolved in solvents and deposited onto substrates, or vapor deposition methods like thermal evaporation for more uniform films.[49][50][51]
The technology originated in 2009 with an initial power conversion efficiency (PCE) of 3.8% reported by Kojima et al., who used organolead halide perovskites as sensitizers in dye-sensitized solar cells. A pivotal breakthrough occurred in 2012 by researchers at the University of Oxford, led by Henry Snaith, who developed a solid-state device architecture that achieved 9.7% efficiency and demonstrated the perovskite's viability as a primary absorber rather than a mere sensitizer. Since then, rapid advancements have propelled lab-scale single-junction perovskite cells to certified PCEs exceeding 25%, with records reaching 27.0% as of 2025 verified by the National Renewable Energy Laboratory (NREL). In tandem configurations, particularly with silicon bottom cells, efficiencies have surpassed 33%, with a certified 34.85% achieved by LONGi in 2025.[52][53][54]
Key advantages of perovskite solar cells include their potential for low-cost fabrication due to simple solution-based processing that avoids high-vacuum or high-temperature requirements typical of silicon production. The bandgap is tunable across 1.5–2.3 eV by adjusting the A-site cation, halide composition, or dimensionality, enabling optimization for single-junction or multi-junction applications. Additionally, these materials exhibit high defect tolerance, allowing efficient charge carrier extraction despite imperfections in the crystal lattice; this stems from Rashba band splitting, a spin-orbit coupling effect that creates an indirect bandgap and reduces non-radiative recombination at defects.[55][56]
Despite these strengths, perovskite solar cells face significant challenges, including instability under environmental stressors such as moisture, which hydrolyzes the material leading to phase decomposition; elevated temperatures above 85°C, causing thermal degradation; and UV light, which accelerates ion migration and trap formation. The use of lead in most formulations also raises toxicity concerns, as leaching from damaged modules could pose environmental and health risks. Efforts to mitigate these issues include encapsulation and compositional engineering, but long-term operational stability remains below commercial silicon standards. Recent stability tests verified by NREL in November 2025 demonstrate retention of 100% initial efficiency after 1,200 hours of continuous operation.[57][58][59]
As of 2025, commercial pilots for perovskite-silicon tandem modules are underway by companies like LONGi and TrinaSolar, with early production lines targeting efficiencies above 30%, though scalability issues such as uniform large-area deposition and yield consistency persist. Commercial production by Oxford PV and others has begun scaling tandem modules toward market entry. Perovskite cells are often integrated in tandems with silicon to leverage complementary bandgaps, as detailed in the tandem solar cells section.[60][61][62]
Organic photovoltaic cells
Organic photovoltaic (OPV) cells, also known as polymer solar cells, utilize organic semiconductors, typically conjugated polymers, to convert sunlight into electricity through the photovoltaic effect. The active layer in these devices commonly employs a bulk heterojunction (BHJ) architecture, where a donor material such as poly(3-hexylthiophene) (P3HT) is blended with an acceptor like [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to form an interpenetrating network that facilitates efficient exciton dissociation and charge transport.[63] This structure is solution-processed, enabling low-cost fabrication techniques like spin-coating, printing, or blade coating at temperatures below 150°C, which contrasts with the high-temperature requirements of inorganic solar cells.[64]
Laboratory efficiencies for single-junction OPV cells have reached up to 20.82% as of early 2025, driven by optimized morphologies and material innovations, while large-area modules typically achieve around 12-15% under standard conditions.[65] A key limitation in BHJ designs arises from the short exciton diffusion length in organic materials, typically around 10 nm, which necessitates nanoscale phase separation to ensure excitons reach donor-acceptor interfaces before recombining. This length is described by the equation
where LLL is the diffusion length, DDD is the exciton diffusion coefficient, and τ\tauτ is the exciton lifetime.[66]
OPV cells offer significant advantages, including ultra-lightweight construction with active layers weighing less than 1 g/m², inherent semi-transparency for applications like building-integrated photovoltaics, and compatibility with low-temperature printing on flexible substrates such as textiles or plastics.[67] However, they suffer from drawbacks like reduced stability due to degradation from oxygen and moisture exposure, leading to lifetimes of only a few thousand hours under operational conditions, and a relatively narrow absorption spectrum that limits utilization of the solar spectrum compared to inorganic counterparts.[68]
Pioneered in the 1980s with early bilayer devices, OPV technology has evolved rapidly, particularly through the adoption of non-fullerene acceptors (NFAs) since the 2010s, which have broadened absorption ranges and improved voltage outputs, contributing to efficiency gains beyond 20%. By 2025, these advances have enabled practical deployment in wearables, such as flexible sensors, and Internet of Things (IoT) devices, where OPVs power low-energy electronics under ambient indoor lighting without batteries.[67][69]
Dye-sensitized solar cells
Dye-sensitized solar cells (DSSCs), also known as Grätzel cells, were invented by Brian O'Regan and Michael Grätzel in 1991, introducing a biomimetic approach inspired by photosynthesis for photovoltaic energy conversion.[70] The core structure consists of a mesoporous titanium dioxide (TiO₂) anode, typically a nanoporous film with high surface area, sensitized by ruthenium-based dyes such as N719 or N3 that absorb visible light.[71] This photoanode is paired with a liquid electrolyte, commonly an iodide/triiodide (I⁻/I₃⁻) redox couple, and a platinum-coated counter electrode on a transparent conductive substrate like fluorine-doped tin oxide (FTO).[71] The design enables efficient charge separation without relying on a traditional p-n junction, making it suitable for low-cost fabrication via solution processing.
The key operating mechanism involves photo-injection, where incident photons excite electrons in the dye molecule from its ground state (HOMO) to an excited state (LUMO), followed by rapid injection into the conduction band of the wide-bandgap TiO₂ semiconductor.[70] The injected electrons diffuse through the mesoporous network to the external circuit, while the oxidized dye is regenerated by the electrolyte, which in turn is reduced at the counter electrode.[71] This process yields power conversion efficiencies (PCEs) in the laboratory ranging from 10% to 14%, with certified records around 14.2% under standard AM1.5 illumination. By 2025, stable module efficiencies have reached approximately 11%, though scaling remains challenging due to material interactions.
DSSCs offer several advantages, including the use of abundant, low-cost materials like TiO₂ and simple printing techniques that reduce manufacturing expenses compared to silicon-based cells. They perform well under diffuse or low-light conditions, such as indoor lighting, retaining over 70% of peak efficiency at 200 lux, which suits applications like sensors or wearables.[72] Additionally, their tunable colors and semi-transparency enable aesthetically pleasing designs for building-integrated photovoltaics (BIPV), allowing integration into windows or facades without compromising visual appeal.[73] However, drawbacks include the volatility and leakage of liquid electrolytes, leading to degradation over time, as well as corrosion of electrodes by the iodide species, which limits long-term stability to hundreds of hours under operation. Their PCEs also lag behind crystalline silicon cells, which exceed 25%.
Efforts to address these issues have led to emerging solid-state variants by 2025, replacing liquid electrolytes with quasi-solid or solid hole conductors like Spiro-OMeTAD or polymers to prevent leakage and enhance durability.[72] These configurations achieve PCEs around 10-12% in labs but face commercialization hurdles due to higher interface recombination and processing complexities, with modules still below 8% stability.[74] Despite progress, widespread adoption remains limited, focusing primarily on niche indoor or flexible applications.[75]
Quantum dot solar cells
Quantum dot solar cells utilize semiconductor nanocrystals known as quantum dots (QDs), which exhibit quantum confinement effects that alter their electronic properties at the nanoscale. These zero-dimensional structures confine electrons and holes in all three spatial dimensions, leading to discrete energy levels and a size-dependent bandgap, enabling enhanced light absorption and potential for hot carrier extraction. The quantum confinement effect can be described by the approximate equation for the bandgap energy of a QD:
where Eg(bulk)E_g(\text{bulk})Eg(bulk) is the bulk bandgap, ℏ\hbarℏ is the reduced Planck's constant, μ\muμ is the reduced mass of the electron-hole pair, and rrr is the QD radius.[76]
Early research on QDs dates to the 1980s, with pioneering work by Alexei Ekimov and Louis Brus demonstrating quantum confinement in colloidal semiconductor nanocrystals such as CdSe.[77] By the 2020s, colloidal QDs like lead sulfide (PbS) and cadmium selenide (CdSe) have become central to solar cell architectures, typically assembled into Schottky junctions or p-i-n configurations where the QD layer acts as the absorber. In these devices, long insulating ligands on the QDs are exchanged for short conductive ones to facilitate charge transport while minimizing recombination.[78]
A key advantage of QD solar cells is the tunable bandgap, which ranges from 0.5 to 2 eV by varying QD size, allowing optimization for different solar spectrum regions, including infrared (IR) sensitivity for broader absorption. Additionally, their solution-processable nature supports low-cost, scalable fabrication via techniques like spin-coating or printing. Laboratory efficiencies for colloidal QD solar cells reached 12-14% by 2025, with PbS-based p-i-n devices achieving a certified value of 13.62% as of August 2025. Theoretically, efficiencies exceeding 40% are possible through multiple exciton generation (MEG), where a single high-energy photon creates multiple electron-hole pairs, surpassing the Shockley-Queisser limit for single-junction cells.[79][80][81]
Despite these benefits, challenges persist, including surface trap states that reduce open-circuit voltage (Voc) by promoting non-radiative recombination, and the toxicity of heavy metals like lead and cadmium in common QDs such as PbS and CdSe. By 2025, advancements in QD inks have enabled flexible photovoltaics with improved processability, though long-term stability under operational conditions remains a limitation, often lagging behind established technologies. Recent QD-perovskite hybrids have shown promise for enhanced IR harvesting, sharing an emerging status with perovskite cells but emphasizing nanoscale confinement for MEG.[82][83]