The escalating global energy demand, coupled with the environmental imperative to reduce carbon emissions, has intensified the search for efficient and sustainable energy sources. Among renewable options, solar energy stands out due to its abundance and universal availability. The core technology for harnessing this energy, the thin film solar panel, offers advantages like lower material usage, flexibility, and the potential for lower-cost manufacturing compared to traditional silicon-based panels. Within this domain, kesterite Cu2ZnSn(S,Se)4 (CZTSSe) has emerged as a highly promising absorber material for the next generation of thin film solar panels. Its constituent elements are earth-abundant, low-toxic, and it possesses a tunable direct bandgap (approximately 1.0–1.5 eV) and a high absorption coefficient exceeding 104 cm−1, making it theoretically suitable for high-efficiency photovoltaic devices.
Despite over a decade of intensive research, the record power conversion efficiency (PCE) for CZTSSe-based thin film solar panels remains at 13.6% (for pure-selenide) and 11% (for pure-sulfide), significantly lower than its theoretical Shockley-Queisser limit (~30%) and far behind its predecessor, Cu(In,Ga)Se2 (CIGS, ~23.4%). This efficiency bottleneck is primarily attributed to two intrinsic challenges: a large open-circuit voltage (VOC) deficit and a high defect density within the absorber layer. The VOC deficit, defined as (Eg/q − VOC), where Eg is the bandgap and q is the elementary charge, is exceptionally high for CZTSSe (>0.5 V) compared to high-performance CIGS (<0.4 V). This deficit stems from severe non-radiative recombination losses caused by various point defects and defect complexes with low formation energies. The most detrimental among these are the CuZn anti-site defect (a deep acceptor) and the [2CuZn + SnZn] defect cluster, which act as strong recombination centers. Furthermore, band misalignment at the critical p-n heterojunction (typically with a CdS buffer layer) can lead to unfavorable band offsets, further hindering carrier collection.
To overcome these fundamental limitations, partial cation substitution has been developed as a highly effective strategy. This approach involves intentionally replacing a fraction of the native cations (Cu+, Zn2+, or Sn4+) in the kesterite lattice with other foreign cations. The primary goals are to: 1) suppress the formation of harmful defect states, particularly disorder between Cu and Zn sites; 2) modify the electronic band structure, including the bandgap and band edge positions, to achieve a more favorable heterojunction alignment; and 3) improve the crystallinity and grain growth of the absorber layer. This article comprehensively reviews recent research progress in optimizing thin film solar panel performance through partial cation substitution in CZTSSe, categorizing the strategies into isovalent and heterovalent substitutions, and discussing their mechanisms, benefits, and limitations.
Isovalent Cation Substitution
Isovalent substitution involves replacing a host cation with a dopant cation of the same formal charge state. This strategy often aims to introduce steric or chemical effects to reduce cationic disorder without introducing additional charge carriers that could form compensating defects.
Substitution for Cu+
The substitution of monovalent Cu+ is primarily targeted at reducing CuZn anti-site defects. The small ionic radius difference between Cu+ (≈0.74 Å) and Zn2+ (≈0.74 Å) facilitates cation disorder. Introducing a larger or chemically distinct monovalent cation can disrupt this tendency.
Li+ Substitution: Theoretical calculations suggest LiCu has a relatively low formation energy (~0.25 eV). Experimentally, Li incorporation (e.g., forming (Cu1−xLix)2ZnSn(S,Se)4) has been shown to widen the bandgap slightly and improve grain boundary properties. It is believed that Li+ segregates at grain boundaries, creating a positive barrier that repels minority carriers (electrons), thereby reducing recombination. Efficiencies up to 11.6% have been achieved. However, a major practical limitation is the incompatibility with standard soda-lime glass (SLG) substrates. When Li-doped absorbers are deposited on SLG, Li+ often exchanges with Na+ from the glass rather than incorporating into the lattice, nullifying the intended effect.
Ag+ Substitution: Substituting Cu+ with the larger Ag+ (≈1.15 Å) is one of the most successful strategies. The larger size mismatch effectively suppresses Cu-Zn disorder. Furthermore, Ag incorporation significantly influences the band structure. The valence band maximum (VBM) of CZTSSe is dominated by Cu 3d and anion p orbitals. Replacing Cu with Ag, whose 4d orbitals are higher in energy, raises the VBM, thereby reducing the bandgap according to the formula:
$$E_g(x) = E_{g,\text{CZTSSe}} – \Delta E_v(x)$$
where $\Delta E_v(x)$ is the increase in the VBM with Ag fraction $x$. This bandgap reduction, counterintuitively, often leads to a higher VOC because it improves the conduction band alignment with the CdS buffer layer, reducing interface recombination. Advanced device engineering using Ag-graded absorber profiles has pushed the efficiency of (Ag,Cu)2ZnSn(S,Se)4 thin film solar panels to 12.5%. The downsides are the higher cost and relative scarcity of silver.
Substitution for Zn2+
Substituting the smaller Zn2+ cation is a direct route to physically impede Cu from occupying the Zn site. Various divalent cations have been explored.
Mg2+ Substitution: Mg2+ has a similar ionic radius (≈0.72 Å) to Zn2+. Its incorporation in Cu2(Zn1−xMgx)Sn(S,Se)4 reduces the formation of ZnS secondary phases and can introduce shallow acceptor states, maintaining good p-type conductivity. It enhances hole carrier density and grain growth. Optimal doping (x ≈ 0.03–0.05) has yielded devices with ~7.8% efficiency. Mg is abundant and low-cost, making it attractive for sustainable thin film solar panel production.
Cd2+ Substitution: This is arguably the most effective single-element substitution strategy for pure-sulfide kesterite. Cd2+ (≈0.95 Å) is significantly larger than Zn2+. Alloying Cd into Cu2(Zn1−xCdx)SnS4 (CZCTS) drastically reduces the bandgap from ~1.55 eV (x=0) to ~1.09 eV (x=1.0), as described by a bowing parameter b:
$$E_g(x) = x E_{g,\text{CCTS}} + (1-x) E_{g,\text{CZTS}} – b x(1-x)$$
More importantly, it effectively suppresses deep-level defects and promotes large-grained microstructure. The current record efficiency for a solution-processed CZCTS thin film solar panel is 12.6%. The major drawback is the toxicity of Cd, which contradicts one of the core virtues of kesterite absorbers.
Mn2+ Substitution: Mn2+ (≈0.80 Å) incorporation in Cu2(Zn1−xMnx)Sn(S,Se)4 also suppresses CuZn defects and can induce a phase transition from kesterite to stannite at high concentrations. It improves hole concentration and mobility. Devices with ~8.9% efficiency have been reported. Mn is a low-cost and benign element.
Ba2+ Substitution: The very large Ba2+ (≈1.56 Å) creates significant lattice strain when substituting for Zn. Even small amounts (x=0.01) can promote grain growth, but higher doping levels cause severe lattice distortion and poor interface quality with CdS. Efficiencies around 9.1% have been achieved with careful doping.
Substitution for Sn4+
Substituting tetravalent Sn4+ primarily aims to tune the bandgap and modify the conduction band minimum (CBM), which is derived from Sn 5s and anion p orbitals.
Ge4+ Substitution: Ge4+ (≈0.53 Å) is smaller than Sn4+ (≈0.69 Å). Alloying in Cu2Zn(Sn1−xGex)(S,Se)4 (CZTGSSe) increases the bandgap linearly by raising the CBM energy. This is highly effective in reducing the VOC deficit, as the increased Eg directly translates to a higher potential VOC, provided recombination is controlled. Ge incorporation also passivates deep Sn-related defects. This strategy has produced thin film solar panels with efficiencies up to 12.3%. Germanium is non-toxic but more expensive than Sn.
Heterovalent Cation Substitution
Heterovalent substitution involves cations with a different charge state than the host ion. This can introduce intentional doping effects but risks creating charge-compensating defect complexes.
Ga3+ Co-Substitution: A sophisticated strategy involves substituting two Ga3+ ions for one Zn2+ and one Sn4+ (i.e., 2Ga3+ → Zn2+ + Sn4+). This maintains charge neutrality while introducing Ga at both sites. First-principles calculations show this suppresses the deep SnZn defect and forms shallower defect clusters like (GaZn + CuZn). Implementing this in a “sandwich” structured absorber has yielded a thin film solar panel with 12.3% efficiency.
In3+ Substitution for Zn2+: Replacing Zn2+ with In3+ in Cu2InxZn1−xSn(S,Se)4 reduces the bandgap and can improve carrier transport. However, the efficiency gains have been modest (~4.8%), and the scarcity of indium undermines the economic advantage of kesterite-based thin film solar panels.
Comparative Analysis and Summary
The efficacy of various cation substitution strategies is summarized in the table below, highlighting their primary effects, advantages, and disadvantages in the context of thin film solar panel development.
| Substitution Type | Typical Formula | Key Effects on Absorber | Best Reported PCE | Advantages | Disadvantages |
|---|---|---|---|---|---|
| Ag+ for Cu+ | (Cu1−xAgx)2ZnSn(S,Se)4 | Suppresses CuZn disorder; Raises VBM, reduces $E_g$, improves band alignment. | 12.5% | Very effective for $V_{OC}$ boost; good grain growth. | Expensive; Ag can oxidize. |
| Cd2+ for Zn2+ | Cu2(Zn1−xCdx)SnS4 | Strongly reduces $E_g$; suppresses deep defects; promotes large grains. | 12.6% | Highest efficiency for sulfide; excellent defect passivation. | Toxic (Cd); contradicts eco-friendly premise. |
| Ge4+ for Sn4+ | Cu2Zn(Sn1−xGex)(S,Se)4 | Increases $E_g$ by raising CBM; reduces $V_{OC}$ deficit; passivates Sn defects. | 12.3% | Effective for $V_{OC}$; earth-abundant and low-toxicity. | Germanium cost is higher than Sn. |
| Li+ for Cu+ | (Cu1−xLix)2ZnSn(S,Se)4 | Widens $E_g$ slightly; passivates grain boundaries. | 11.6% | Low formation energy; improves GB properties. | Incompatible with Na from SLG substrates. |
| Mg2+ for Zn2+ | Cu2(Zn1−xMgx)Sn(S,Se)4 | Reduces ZnS secondary phases; introduces shallow acceptors. | ~7.8% | Abundant, low-cost, non-toxic. | Moderate efficiency gain; can form MgS/MgO. |
| Ga3+ Co-Substitution | Cu2ZnSn(Ga)(S,Se)4 | Suppresses SnZn; forms benign defect clusters. | 12.3% | Intelligent design; suppresses deep-level defects. | Synthesis control is more complex. |
The defect chemistry governing these improvements can often be modeled using equations related to defect formation energies and carrier statistics. For instance, the net hole concentration $p$ in a p-type absorber under typical doping conditions is influenced by the concentration of acceptors [A] and compensating donors [D]:
$$p \approx \frac{[A] – [D]}{N_v} \exp\left(-\frac{E_A}{kT}\right)$$
where $N_v$ is the valence band density of states, $E_A$ is the acceptor energy level, $k$ is Boltzmann’s constant, and $T$ is temperature. Successful cation substitution strategies lower [D] (e.g., by suppressing CuZn) or introduce benign shallow acceptors, thereby increasing $p$ and improving the fill factor and $V_{OC}$ of the thin film solar panel.
Conclusion and Future Perspectives
Partial cation substitution has proven to be an indispensable and powerful strategy for advancing the performance of kesterite Cu2ZnSn(S,Se)4 based thin film solar panels. By strategically altering the cationic sublattice, researchers have made significant strides in mitigating the two core issues: high defect density and large $V_{OC}$ deficit. Isovalent substitutions like Ag+ for Cu+, Cd2+ for Zn2+, and Ge4+ for Sn4+ have delivered the most efficient devices to date, each operating through distinct mechanisms—steric hindrance, bandgap engineering, and defect passivation. Heterovalent strategies, such as Ga3+ co-substitution, demonstrate the potential for sophisticated defect engineering.
Looking forward, the development of thin film solar panels using kesterite absorbers will likely focus on several key areas. First, combinatorial or double-cation substitution (e.g., simultaneous Ag and Ge doping, or Cd and Ge doping) could synergistically address multiple limitations, potentially pushing efficiencies beyond 15%. Second, the search for novel, non-toxic substituents with effects similar to Cd remains crucial to fulfill the promise of an entirely benign thin film solar panel technology. Elements like Mn, Mg, and certain alkaline earth metals warrant further investigation. Third, integrating these optimized absorbers with alternative buffer layers (e.g., Zn1−xSnxOy, In2S3) that offer better band alignment could further reduce interface recombination losses. Finally, advanced characterization and computational materials design will be vital for pinpointing the exact role of dopants and designing the next generation of high-performance, low-cost, and sustainable thin film solar panels for widespread terrestrial and building-integrated photovoltaic applications.