PDs can be categorized into photoconductors, photodiodes, and phototransistors (Figure 2) (Huo and Konstantatos, 2018; Li et al., 2019; Xu et al., 2020).

Graphene QDs (GQDs) and carbon QDs have attracted immense interest as promising candidates for PD application because of their nontoxicity, biocompatibility, and excellent electrical, optical, and physicochemical properties, such as tunable bandgap, high absorptivity, nontoxicity, high stability, and cost-effectiveness (Sun et al., 2006; Pan et al., 2010; Zhang et al., 2015; Tajik et al., 2020; Ghosh et al., 2021). Generally, GQDs have layered structures and possess strong quantum confinement effect with sizes down to 100 nm. On the contrary, carbon QDs have a spherical shape with a diameter up to 10 nm. In addition, GQDs are a derivation of graphene/graphite and other graphitic materials, whereas carbon QDs are generally produced by the bottom-up processes (Pan et al., 2010). Both carbon QDs and GQDs have large bandgap with ability to tune bandgap, and therefore, PDs based on carbon QDs and GQDs are generally applied to detect light near UV (Hai et al., 2018; Jia et al., 2020; Wang et al., 2020).

Copper (Cu) is one of the earth-abundant materials and mostly exists in the forms of compounds in nature. In recent years, a variety of Cu-based QDs have been employed for heavy metal–free CQD-based PDs.

One example of Cu-based CQDs is copper tin sulfide (Cu₂SnS₃), which is a p-type semiconductor, consisting of inexpensive and nontoxic elements, with a high absorption coefficient greater than 10⁴ cm⁻¹. Dias et al. (2017) utilized Cu₂SnS₃ CQDs for NIR photodetection, as shown in Figure 6A. Poly(vinylpyrrolidone) (PVP) was used as a surfactant to effectively prevent the agglomeration of CQDs. Another example of Cu-based CQDs was reported by Liu et al. (2019). The author employed copper oxide (Cu₂O) CQDs to develop a graphene/Cu₂O QD/Cu PD based on an internal photocurrent gain mechanism. Driven by the built-in electric field, photogenerated electrons in Cu₂O CQDs transferred to the Cu electrode, while holes migrated toward the graphene layer. Accordingly, the Fermi level of graphene decreased from equilibrium, and consequently, carriers were injected from electrodes to the graphene layer. This led to the increase in the carrier density, resulting in an amplified photocurrent. The device exhibited high responsivity of 1.2 × 10¹⁰ AW⁻¹ and the capacity of detecting femtowatt-scale light at room temperature. In addition, after applying excessive bending, the device still showed decent photoresponse, demonstrating its promising potential for flexible PDs.

Copper iron sulfide (CuFeS₂) CQD is another preferable semiconductor for PDs owing to its abundancy and tunable bandgap from ∼ 0.55 to 1.0 eV by controlling its particle size. Kumar et al. (2020) fabricated a PD based on a bilayer heterojunction of CuFeS₂ QDs and ZnO layers. Compared to PDs of single-layer CuFeS₂ QDs, the effective separation of electron–hole pairs driven by the electric field at the heterojunction considerably reduced carrier recombination (Figures 6B–E). As a result, the detectivity of the device reached above 10¹² Jones in visible and NIR regions at a bias of 10 V. Similarly, Sugathan et al. (2020) fabricated a hybrid PD based on CuFeS₂ QDs and Si with an extended photodetection spectrum from 460 to 2,200 nm. In addition, the device exhibited fall time of 20 μs in the NIR region, which is faster than that of PDs based on bulk Si (typically few milliseconds to seconds).

Copper indium selenide (CuInSe₂) QDs possess a stable phase structure, high extinction coefficient, and narrow bandgap and have demonstrated great potential for photodetection. Shen et al. (2020) hybridized CuInSe₂ QDs with ML molybdenum disulfide (MoS₂) and fabricated the first MoS₂-based PD with broadband photoresponse up to 1,064 nm (Figure 6F). Benefiting from the photogating effect and the favorable type-II energy band alignment between CuInSe₂ QDs and MoS₂, the device showed enhanced responsivity and detectivity which is about 30 and 20 times higher than that of the pristine MoS₂ PD, respectively, as shown in Figures 6G,H. Additionally, the depletion region formed at the CuInSe₂ QDs and MoS₂ interface enabled the slow minority carriers (i.e., holes) to escape from the localized states, improving rise time from 58 to 0.8 s. In addition, Guo et al. (2021) doped transition metal manganese ions (Mn²⁺) into CuInSe₂ QDs to improve the low responsivity of the QDs caused by poor crystal quality and high trap density and verified the effectiveness of this strategy in a PD. The optimized device exhibited remarkable improvement of photoresponse compared to PDs based on pristine CuInSe₂ QDs, such as high detectivity of 4.2 × 10¹² Jones and fast response of 0.76 μs. Such performance is comparable to some Pb-based CQD PDs, demonstrating the great potential of CuInSe₂ QDs as environmentally friendly alternatives for NIR photodetection. The parameters of PDs in Figure 6 are collected in Table 1.

We now turn our attention to the newly emerging perovskite CQDs (PQDs). PQDs have attracted tremendous attention worldwide as a promising photoactive material due to their excellent electrical and optoelectrical properties, such as long lifetime and diffusion length, superiorities in luminescent, tunable bandgap, and the facile processing technique (Jellicoe et al., 2016; Kajari-Schröder, 2019). Because of this, optoelectronic devices based on various dimensional perovskite materials, namely, 3D, 2D, and 0D PQDs, have demonstrated outstanding device performance comparable to their counterparts. For example, Dou et al. (2014) demonstrated the solution-processed hybrid perovskite PDs using an inverted device configuration with high device performances for the first time. However, PQDs and other perovskite materials contain heavy metals, Pb and/or Cd, which are highly toxic to the environment and human causing cancer (Babayigit et al., 2016; Kajari-Schröder, 2019). Therefore, there have been efforts to synthesize Pb-free PQDs in recent years, as shown in Figures 7A,B (Wang et al., 2016; Tong et al., 2017; Yang et al., 2017; Zhang et al., 2017; Ji et al., 2018; Zhou et al., 2018; Ke and Kanatzidis, 2019; Yang et al., 2019; Ma et al., 2020). Compared to other optoelectronic devices based on perovskite materials, however, heavy metal–free PQDs and their PD applications have attracted less attention so far. This might be due to the difficulties in the synthesis of Pb-free PQDs as well as low device performance compared to Pb-free 2D/3D perovskite films, resulting from the low crystallinity, prominent charge-carrier trapping, and short charge-carrier lifetimes of these PQDs (Bai et al., 2021). In spite of this fact, it is still of importance to develop nontoxic and nonhazardous PQD-based PDs as they offer facile bandgap tunability and consequently good selectivity of response range. So far, some achievements have been made in PDs based on Pb-free PQDs as well as other dimensional perovskite materials. It is worth noting that as nanostructured Pb-free perovskite-based PDs are still under intensive development in recent years, noncolloidal methods are also included to broaden a readership and our perspectives.

Ghosh et al. (2018) synthesized cesium–tin–iodine (Cs₂SnI₆)–based PQDs with different morphologies (nanocubes, nanorods, and small and large 2D nanosheets) and fabricated PDs based on each morphology. The author reported high photocurrent gain in PDs, demonstrating 15, 20, and 475 for nanocubes, nanosheets, and nanorods, respectively. In particular, the PD with the nanorods exhibited the best photoresponse time, achieving 1 s of both rise and fall time of the photocurrent, whereas the PD with the nanosheet showed the highest responsivity and detectivity values, approximately, 130 AW^-1 and 10¹³ Jones, respectively. Similarly, Ghosh et al. (2020) reported all-inorganic bismuth (Bi) (III)–based PQDs to achieve better stability and lower toxicity, which are attributed to the absence of Pb compound and the absence of the volatile nature of the methylammonium cation (Leng et al., 2016; Lou et al., 2018; Nelson et al., 2018; Tran et al., 2020). Using the stable PQDs in an ambient condition, the author demonstrated the photocurrent gain (I_Photo/I_Dark) up to 53 at the applied bias 5 V from Cs₃Br₂I₉ PQDs under the light illumination (solar simulator, 0.65 W/cm⁻²). Recently, Bai et al. (2021) have reported, for the first time, quadruple colloidal PQDs, such as Cs₄MnSb₂Cl₁₂ and Cs₄MnBi₂Cl₁₂, for highly sensitive PDs to overcome high instability of Sn-based PQDs. The author reported 96-fold and 77-fold increase in PLQY and PL lifetime in Cs₄MnBi₂Cl₁₂ PQDs, and the PD based on this PQD demonstrated high responsivity, detectivity, and EQE up to 860 AW^-1, 4.5 × 10¹⁰ Jones, and 2.9 × 10⁵%, respectively, under 365 nm wavelength light source.

In addition to colloidal PQDs, Waleed et al. (2017) synthesized 1D perovskite nanowires (NWs) using a chemical vapor method to enhance the stability. The vulnerability of the Sn-based perovskites to moistures and oxygen is due to the fast lateral diffusion of the water and oxygen molecules along the grain boundaries in perovskite films (Frost et al., 2014; Manser et al., 2016). 1D-structured Sn-based perovskites effectively prevented the lateral diffusion of the oxygen and water molecules, demonstrating high material and device stability compared to the Sn-based perovskite film, as shown in Figures 7C,D. The PD (Figure 7E) exhibited responsivity up to 0.47 AW^-1 with the specific detectivity of 8.80 × 10¹⁰ Jones and photocurrent rise and fall time of 1500 and 400 ms, respectively, under the periodically switched halogen light, as shown in Figures 7F,G. The parameters of PDs in Figure 7 are collected in Table 1.

These results demonstrated promising future of the heavy metal–free nanostructured perovskites and their PD applications. However, the device performance still lags behind that of the other counterparts, such as PDs based on PbS QDs and Pb-based perovskite films. So as to achieve green electronics and optoelectronics, intensive investigation on heavy metal–free PQDs is needed.

Xing et al. (2018) reported a photoelectrochemical (PEC)-type PD based on Bi QDs. Due to the strong interactions between Bi QDs and light, a significant number of electron–hole pairs were generated, and consequently, the device produced a strong photocurrent without any external bias, showing its excellent self-driven performance. In addition, even after immersing the device in alkaline electrolytes for 1 month without any protection, the PD still remained 50% of the original photocurrent, which demonstrated its long-term stability in liquid. Following this work, Premkumar et al. (2020) constructed PDs based on three types of silver bismuth iodide QDs including AgBiI₄, Ag₂BiI₅, and AgBi₂I₇ (Figure 8A). Oleylamine and oleic acid were used as ligands for controlling the crystal size and minimizing the defect states. Due to the long excited-state lifetime in QDs and the suitable energy band alignment between QDs and TiO₂, photogenerated electrons transferred to TiO₂ with minimum energy losses. The PD based on AgBi₂I₇ exhibited the best photoresponse performance among the fabricated three types, with responsivity of 0.15 AW^-1 at the wavelength of 390 nm.

MIR photodetection based on CQDs has reached a high level of maturity with various Hg chalcogenides compounds, denoted as HgX where X indicates chalcogenides compounds, for example, HgS, HgSe, and HgTe. Due to the metal-rich surface feature, excess electrons are located at the conduction band in the HgX compounds. The optical transitions of electrons within the conduction band result in strong absorption in MIR spectrum. Similarly, Ag₂Se CQDs, which possess very similar metal-rich surface chemistry to HgX compounds, have demonstrated MIR absorption and therefore been considered as a green alternative for MIR PDs (Martinez et al., 2017; Qu et al., 2018). Qu et al. (2018) integrated Ag₂Se QDs into an electrolyte-gated transistor. However, the device exhibited very limited photoresponse performance and bias stability below 200 mV, which might be caused by the reduction of Ag⁺. Similarly, Hafiz et al. (2019) fabricated a PD based on Ag₂Se QDs (Figure 8B) and investigated the effect of ligand exchange on photoresponse performance. The fabricated Ag₂Se QDs had excess electrons which were located at the first conduction energy level (1S_e). The authors reported that the optical transition from 1S_e to the second conduction energy level (1P_e) led to the stronger MIR absorbance (Figure 8C). According to measured absorbance and spectral photoresponse, the ligand exchange of QDs with 1,2-ethanedithiol (EDT) reduced the concentration of excess electrons and had a detrimental influence on their capacity as light absorbers. To improve the responsivity, the authors suggested increasing the doping concentration in QDs for providing more electrons in the 1S_e state, which led to a stronger MIR absorbance. In addition to MIR photodetection, Ag₂Se QDs demonstrated NIR light harvesting. Similarly, Graddage et al. (2020) reported the first Ag₂Se QD–based photodiode (Figure 8D) with high sensitivity ranging from 1,000 to 1,400 nm. To improve the precursor reactivity, the authors applied secondary phosphine and realized accurate QD size control. To fabricate a PD, QDs were deposited into a mesoporous TiO₂ scaffold to enhance optical absorption and charge carrier separation. In particular, 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-OMeTAD) was used as a hole transport layer because its HOMO energy level (−5.0 eV) matches well with the valence band of Ag₂Se QDs (−4.86 eV), which ensured efficient carrier collection. The device exhibited responsivity of 4.17 mAW⁻¹ and a high on and off ratio of 490 at the wavelength of 1,200 nm.

Furthermore, metallic oxide–based CQDs have been employed in PDs as a hole blocking layer (HBL) or light absorber as well (Kannan et al., 2020; Lin et al., 2020). Kannan et al. (2020) deposited CeO₂ QD film on an amorphous selenium (α-Se) film as the HBL for avalanche photodiodes (APDs). Due to the large bandgap of CeO₂ QDs, ∼ 3.77 eV, the CeO₂ QD layer effectively blocked holes. As a result, the author achieved an avalanche gain of 42 using 40 nm CeO₂ HBL and the lowest dark current density of J_D ∼ 30 pA/cm² ever reported (Figures 8E,F). In 2020, another type of metallic oxide QDs, Ga₂O₃ CQDs, was introduced to overcome the low photoresponse of all-Si–based PDs in the DUV region. Lin et al. (2020) developed a vertically stacked graphene/Ga₂O₃ QD/Si photodiode, where Ga₂O₃ QDs were used as a DUV sensitive layer. Upon illumination, the separation of photogenerated electron–hole pairs was promoted by the Ga₂O₃ QD/Si heterojunction. In addition, graphene layer significantly enhanced the absorption area of the PD for harvesting more photons. As a result, the device showed high photoresponsivity under DUV illumination and the highest EQE reached 80.2% without external bias, which is twice that of all-Si PDs. The parameters of PDs in Figure 8 are collected in Table 1.

The 0D/2D structure using various environmentally friendly QDs has been widely adopted to further improve PD performances as well. For example, Si is a principal material used in commercial PDs due to its abundance, nontoxicity, and stability. However, the application of Si PDs is largely limited by the small and indirect bandgap and weak light absorption over the visible-NIR range, resulting in low photoresponse of Si-based PDs. As an alternative to overcome the drawbacks of bulk Si, the combination of Si QDs (SQDs) and 2D materials have been demonstrated as an effective strategy for a future Si-based high-performance PD. The hybrid 0D/2D structure greatly improves light absorption capacity and dissociation of photogenerated charge carriers, which is attributed to the QD layer and the formation of the heterojunction, respectively (Konstantatos et al., 2012; Hu et al., 2016; Ra et al., 2016; Huo et al., 2017; Pak et al., 2018; Qiao et al., 2020). Shin et al. (2015) fabricated the first PD based on a SQD-embedded SiO₂ (SQD:SiO₂) and graphene heterojunction. The bandgap energy of SQDs within the SiO₂ matrix was measured to be varied in the range of 1.4–1.8 eV, enabling photodetection of a higher energy range compared to bulk Si. The fabricated heterostructure 0D/2D PD showed high detectivity of approximately 10⁹ cm Hz^1/2W⁻¹, which is about ten times higher than that of bulk-Si/graphene Schottky diodes (An et al., 2013). In addition, Yu et al. (2016) demonstrated a SQD/graphene heterojunction on a Si substrate. The Fermi level of graphene was lowered after coupled with SQDs because of electron transfer from graphene to SQDs, resulting in a larger built-in potential and a wider depletion region. Therefore, the separation of photogenerated carriers was effectively promoted, leading to the improved photoresponse. Following this work, Ni et al. (2017) demonstrated a broadband phototransistor based on a heterostructure of boron (B)-doped SQDs and graphene (Figure 9A). According to ultraviolet photoelectron spectroscopy (UPS) study, the work function of SQDs and graphene are -4.62 and -4.78 eV, respectively, indicating the electron transfer from SQDs to graphene (Figure 9B). After the Fermi level of SQDs and graphene reached equilibrium, upward bending of energy bands occurred at the interface, resulting in the transportation of photogenerated holes from SQDs to graphene, as shown in Figure 9C. Two optical phenomena related to SQDs can be exploited upon illumination. In the MIR region, the localized surface plasmon resonance (LSPR) of B-doped Si QDs induced an electric field, which enhanced light absorption of the underlying graphene. The optical absorption of SQDs from the UV to NIR region led to a photogating effect, resulting in improved photoconductive gain. As a result, the fabricated PD showed maximum responsivity of 2.2 × 10⁹ and 44.9 AW^-1, and the highest detectivity on the orders of 10¹³ and 10⁵ Jones under UV–NIR and MIR illumination, respectively, as shown in Figures 9D,E. Hu et al. (2019) fabricated a heterostructure utilizing InGaN QD and 2D graphene sheet. The resulting hybrid PD featured ultrahigh responsivity and detectivity of 1.6 × 10⁹ AW⁻¹ and 5.8 × 10¹⁴ Jones, respectively. The authors demonstrated its high sensitivity of detecting light on the femtowatt level at room temperature without extra signal processing.

As another nontoxic semiconductor of group IV, germanium (Ge) shares many similar characteristics with Si. In addition, Ge has a narrower bandgap (0.67 eV), which enables photodetection to IR range. Liu et al. (2015) fabricated an IR PD by decorating Ge QDs onto reduced graphene oxide (RGO), and the QD-decorated RGO/ZnO heterostructure is shown in Figure 9F. The authors achieved the most effective energy transfer between QDs and RGO by tailoring the diameter of Ge QDs. In addition, the Schottky barrier formed between ZnO and RGO contributed to the high on and off ratio (∼10³) of the IR Ge-based PD. Dark current was reduced due to the formation of the energy barrier which restricted the backflow of carriers. Furthermore, when the PD was exposed to incident light, the energy barrier virtually disappeared, which facilitated the charge transfer from RGO to ZnO under reverse bias. As a result, large photocurrent was generated in the device. The hybrid PD exhibited high responsivity (9.7 AW^-1) and detectivity (7.98×10¹² Jones) under IR illumination of 1,400 nm, as shown in Figure 9G. The parameters of PDs in Figure 9 are collected in Table 1.

Finally, we turn our attention to another type of the heterojunction employing new types of QDs which are based on 2D TMDCs. 2D TMDCs have drawn increasing research interest in the field of photodetection because of their excellent electrical and optical properties (Jiang et al., 2017). To further control lateral quantum confinement of 2D nanostructures, in recent years, TMDC QDs have been synthesized and introduced to optoelectronic applications (Gopalakrishnan et al., 2015; Jin et al., 2016; Jin et al., 2017). For example, Singh et al. (2019) reported a UV PD based on WS₂ QD/graphene heterojunction in 2019. The fabricated PD had high sensitivity in UV photodetection, which was attributed to the strong absorption of WS₂ QDs in the UV region. Upon UV illumination, a large number of electron–hole pairs were generated in QDs. As illustrated in Figure 10A, the photogenerated electrons moved toward the graphene layer while holes remained in WS₂ QDs. These electrons were transported to an electrode with a high speed due to excellent electron mobility of graphene, resulting in a large photocurrent and fast response time. In this device configuration, the WS₂ QDs acted as an effective light-harvesting material, whereas the graphene layer provided a pathway for high-speed carrier transportation. Under incident light of 365 nm, the PD showed high responsivity and detectivity of 1,814 AW^-1 and 7.47 × 10¹² Jones, respectively, as shown in Figure 10B. Following this work, Abid et al. (2020) developed a heterojunction of reduced graphene oxide (RGQ) and WS₂ QDs on e-textile fabric (Figure 10C). Compared to graphene, easily processable RGO offered simplified synthesis procedures and consequently device fabrication. The device showed no significant changes after bending, twisting, and stretching experiments, demonstrating its robust performance as a flexible PD. The parameters of PDs in Figure 10 are collected in Table 1.