High Efficiency Formamidinium‐Cesium Perovskite‐Based Radio‐Photovoltaic Cells

Radio‐photovoltaic cell is a micro nuclear battery for devices operating in extreme environments, which converts the decay energy of a radioisotope into electric energy by using a phosphor and a photovoltaic converter. Many phosphors with high light yield and good environmental stability have been developed, but the performance of radio‐photovoltaic cells remains far behind expectations in terms of power density and power conversion efficiency, because of the poor photoelectric conversion efficiency of traditional photovoltaic converters under low‐light conditions. This paper reports an radio‐photovoltaic cell based on an intrinsically stable formamidinium‐cesium perovskite photovoltaic converter exhibiting a wide light wavelength response from 300 to 800 nm, high open‐circuit voltage (VOC), and remarkable efficiency at low‐light intensity. When a He ions accelerator is adopted as a mimicked α radioisotope source with an equivalent activity of 0.83 mCi cm−2, the formamidinium‐cesium perovskite radio‐photovoltaic cell achieves a VOC of 0.498 V, a short‐circuit current (JSC) of 423.94 nA cm−2, and a remarkable power conversion efficiency of 0.886%, which is 6.6 times that of the Si reference radio‐photovoltaic cell, as well as the highest among all radio‐photovoltaic cells reported so far. This work provides a theoretical basis for enhancing the performance of radio‐photovoltaic cells.


Introduction
12] Instead, radio-photovoltaic (RPV) cells consisting of a radiation source, a phosphor, and a photovoltaic cell are much more reliable.In an RPV cell, the phosphor converts the decay energy of the radioisotope into optical energy, which is collected by the PV cell to generate electric power output. [2,4,13]Because of the much better radiation resistance of phosphors in comparison to semiconductor materials, RPV cells offer excellent operational stability and long service life.
To obtain high-performance RPV cells, it is important to improve the phosphor's light yield, the degree of coupling between the radioluminescence (RL) spectrum of phosphor and the spectral response of the PV converter, and the PCE of the PV converter.Considerable advances in RPV cells have been reported since the last century.The first RPV cell was demonstrated in the late 1950s based on a silicon PV converter; unfortunately, it was quickly replaced by lithium batteries due its poor output performance. [14,15]The low efficiency of RPV cells has hampered for a long time their practical application.[18][19] In 1990, Walko et al. conducted an in-depth analysis of the material selection for RPV cells.They found that zinc sulfide (ZnS) based phosphors with high light yield were the ideal choice for light generation, and III-V semiconductors such as gallium arsenide (GaAs), gallium phosphide (GaP), and silicon might also be suitable to act as PV converters. [20]Subsequently, different phosphors and different PV converters were further researched and reported.Sychov et al.

DOI: 10.1002/eem2.12513
Radio-photovoltaic cell is a micro nuclear battery for devices operating in extreme environments, which converts the decay energy of a radioisotope into electric energy by using a phosphor and a photovoltaic converter.Many phosphors with high light yield and good environmental stability have been developed, but the performance of radio-photovoltaic cells remains far behind expectations in terms of power density and power conversion efficiency, because of the poor photoelectric conversion efficiency of traditional photovoltaic converters under low-light conditions.This paper reports an radio-photovoltaic cell based on an intrinsically stable formamidinium-cesium perovskite photovoltaic converter exhibiting a wide light wavelength response from 300 to 800 nm, high open-circuit voltage (V OC ), and remarkable efficiency at low-light intensity.When a He ions accelerator is adopted as a mimicked a radioisotope source with an equivalent activity of 0.83 mCi cm À2 , the formamidinium-cesium perovskite radio-photovoltaic cell achieves a V OC of 0.498 V, a short-circuit current (J SC ) of 423.94 nA cm À2 , and a remarkable power conversion efficiency of 0.886%, which is 6.6 times that of the Si reference radio-photovoltaic cell, as well as the highest among all radio-photovoltaic cells reported so far.This work provides a theoretical basis for enhancing the performance of radiophotovoltaic cells.
developed long-life a-RPV cell arrays based on 238 Pu a-radioisotope and AlGaAs PV cells with a power output of 21 lW (enough to drive an electronic calculator).This relatively high output was explained by the high ionization density of a-particles in the ZnS phosphor. [16][23][24] Further, they achieved a higher PCE of 0.87% in their latest work by using an electron accelerator to match the high-activity of 147 Pm. [13] Weaver and Schott studied RPV cells with excimer gases (Ar and Xe) and Si PV converters. [25]Radioactive sources such as 210 Po a-radioisotope and 90 Sr b-radioisotope were loaded into the gas aiming to increase the surface interaction between the radioactive source and the phosphor; thus the conversion efficiency of radiation to light could be greatly improved.Similarly, Russo integrated a 63 Ni radioactive source in a ZnS solid phosphor with chloride solution, and the PCE of the 3D configuration achieved 0.289%, which was 4.5 times higher than the traditional planar structure. [19]28] At present, the performance of RPV cells remains far behind the expectations in terms of power density and PCE, [15,29,30] because it is difficult to improve the photoelectric conversion efficiency of the traditional PV converters at low-intensity illumination from the most commonly used phosphors, ranging from 1 lW cm À2 to 1 mW cm À2 . [13,14]Therefore, overcoming the bottleneck of the low photoelectric conversion efficiency of PV converters in dim light is the key to further improving the performance of RPV cells.Besides, phosphors usually have light emission spectra in the range from 400 to 800 nm with narrow emission peaks.It is thus desirable to use PV converters that work well at a low-light intensity and show a well-matched light-response wavelength range.33][34] Herein, we report a high-efficiency RPV cell based on FACs perovskite solar cell as a PV converter, which has a wide light wavelength response from 300 to 800 nm, which matches the spectra of the phosphors (LYSO:Ce, CsI:Tl, and ZnS:Ag).The electrical characteristics of the FACs perovskite RPV cell were measured using mimicked radioactive a and b sources.When the equivalent activity of the a source was 0.83 mCi cm À2 , the FACs perovskite RPV cell shows an ultra-high PCE of 0.886%, with a remarkable open-circuit voltage (V OC ) of 0.498 V and a maximum power output (P max ) of 423.94 nA cm À2 , which are 1.8 and 6.6 times that of the Si reference RPV cell, respectively.

Fabrication and Properties of the FACs Perovskite PV Converter
High-efficiency RPV cells, as presented in Figure 1a, require a PV converter with excellent performance under low-intensity light and wide light wavelength response matching that of common phosphors.Here, we used perovskite solar cells with a configuration of ITO/PTAA/ Cs 0.15 FA 0.85 Pb(I 0.95 Br 0.05 ) 3 /PEAI/PCBM/BCP/Bi/Ag as PV converter in our RPV cells because of the intrinsic good stability of MA-free FACs perovskite-based inverted structure device. [32,35]The device fabrication details are described in Section 4. The typical current density-voltage (J-V) curve for the solar cells under the standard AM 1.5G illumination at 100 mW cm À2 is shown in Figure 1b, giving a PCE of 20.91%, with an open-circuit voltage (V OC ) of 1.12 V, a short-circuit current density (J SC ) of 22.74 mA cm À2 , and a fill factor (FF) of 82.1%.The corresponding external quantum efficiency (EQE) result of the device is shown in Figure 1c.It is found that our solar cells show a wide light wavelength response range from 300 to 800 nm, which matches well with the light wavelength range of common phosphors.The main reason for us to use perovskite solar cells as PV converters in our RPV cells is their excellent photovoltaic performance under low-intensity light. [33,34]To study this point, we measured the J-V curves of our FACs perovskite PV converter and silicon reference solar cells under a xenon lamp and a light-emitting diode with different light intensities.
Figures 2 and 3 show the J-V curves and performances for the Si and FACs perovskite solar cells under light-emitting diode (emission wavelength, 450-460 nm) and xenon lamp (emission wavelength, 290-800 nm) illumination with different low-light intensities ranging from 70 to 0.1 mW cm À2 .It should be mentioned that due to the measuring system limitation, the light power below 0.1 mW cm À2 is difficult to be calibrated accurately, the characteristics of solar cells under lower light power will be studied in the near future.It is obvious that the efficiency of perovskite cells increases more than that of the silicon cell, mainly due to the minor changes in V OC and FF.Both solar cells also show very low hysteresis (Tables S1 and S2, Supporting Information).Under the condition of AM1.5 illumination, the efficiency limit of single junction PVs do not exceed 33% according to Shockley-Queisser limit.Under indoor light, the efficiency of perovskite solar cells could exceed 40%.This is because the light absorption range of the absorber with a bandgap of 1.5 eV almost perfectly converts the indoor light source spectrum (200-700 nm), making the photons fully utilized by active materials under indoor light sources with a narrow spectrum. [36]In contrast, commercialized single-crystalline silicon solar cells with the best performance in full direct sunlight do not work well at low-light intensity. [33]Compared with the sunlight case, PSCs under indoor light can deliver higher efficiency because the narrower spectral band reduces the transparency loss and thermal loss associated with the broadband solar spectrum.

Preparation and RL Characteristics of Phosphors
In an efficient RPV cell, the phosphor is required to have a high light yield (LY), strong irradiation resistance, long-term stability, and appropriate light wavelength matching the PV converter.For this purpose, we utilize as phosphors three types of commercial inorganic scintillators, LYSO:Ce, CsI:Tl, and ZnS:Ag.Their technical details are shown in Table 1.It is easy to recognize these phosphors from the X-ray excitation luminescence corresponding to distinct wavelengths as shown in Figure 4a.When comparing the spectra of phosphors with the external quantum efficiency (EQE) spectra of FACs PV converter, it is founded that the RL spectra of CsI:Tl and ZnS:Ag are in the region of optimal wavelength sensitivity, with a high EQE of nearly 90% (Figure 4b). Figure 4c shows the LY of three phosphors at room temperature, with values of 60 000 ph MeV À1 for ZnS:Ag, [37] 56 000 ph MeV À1 for CsI:Tl, [38] and 27 000 ph MeV À1 for LYSO:Ce, [38] respectively.Meanwhile, the selected phosphors exhibit strong irradiation resistance and environmental stability, which are suitable for creating a durable and efficient RPV cell.

Electrical Characteristics of the FACs Perovskite RPV Cell
The electrical properties of the FACs perovskite b-RPV cell were measured by using a miniature Xray tube (MAGPRO 60 kV 12 W X-ray Source; Moxtek), which can be recognized as an electron source.As shown in Figure 5a, phosphors were added in a plate holder 15 mm away from the Xray tube in a vertical position, with the FACs PV converter 5 mm parallel behind it.The X-ray tube was operated under direct current mode (tube current, 200 lA), with the tube voltages (V T ) set as 30, 45, and 60 kV, respectively.It is worth noting that the X-ray tube emits X-ray in a spectrum of energies ranging from 0 to V T keV, with an estimated average energy of 10, 15, and 20 keV, respectively.During X-ray illumination, the background signal of the RPV cell was measured with the phosphor completely shielded by black paper.
The electrical properties of the FACs perovskite a-RPV cell were measured with a 4UH Pelletron (Shanghai Institute of Applied Physics, Shanghai, China), providing a stable beam of He ions with a diameter of 1-3 mm, total yields of 10 nA to 5 lA (6.25 9 10 10 to 3.13 9 10 13 ions s À1 ), and mono-energy ranging from 1 to 3 MeV.Figure 5b gives the photograph of the experimental setup.A piece of gold foil (~50 nm in thickness) was used to scatter and reduce the intensity of the He ions beam irradiating the phosphor.The equivalent activity of alpha radioisotope sources can thus be tuned from around 0.1 to 10 mCi cm À2 .A black V-type acrylic bracket was suspended vertically in the center of the irradiation chamber for carrying the FACs perovskite RPV cell with its right tail, and gold foil was attached to the left top of the bracket.When the He ions beam travels vertically through the gold foil, the scattered He ions will irradiate the phosphor's upper surface uniformly.The beam intensity of He ions was obtained by monitoring the direct-current signal with a SiC PIN detector according to our previous report. [39]etailed information about the structure of the Vtype acrylic bracket is presented in the inset of Figure 5b.The acceleration voltage was set as 3 MV; the average energy and intensity of He ions irradiating the FACs perovskite a-RPV cell were 2.95 MeV and 3.07 9 10 7 cm À2 (equivalent activity, 0.83 mCi cm À2 , power density, 14.75 lW cm À2 ) throughout the test.
The energy deposition rates (simulated by Geant4 software) of the X-ray in the phosphors are presented in Figure S1, Supporting Information.According to the results, the LYSO:Ce and CsI:Tl phosphors are capable of depositing the vast majority of the X-ray energy within 60 keV, while the energy deposition rate of ZnS:Ag is 60% lower above 30 keV and reduces to 12% at 60 keV due to its insufficient thickness.Figure 6a-c present the J-V curves of the FACs perovskite b-RPV cell with the phosphor layer of LYSO:Ce, CsI:Tl, and ZnS:Ag, respectively.Obviously, the solid lines with hollow shapes corresponding to the background signal are much lower than the solid lines with solid shapes corresponding to the total signal, confirming that the electrical signals generated by FACs perovskite RPV cells were excited by radioluminescence rather than X-ray.The current density is in a near steady-state of J SC as the voltage increases from 0 to 0.3 V, then displays a smooth exponential drop to zero as the voltage increases to V OC .Significant improvements in the J SC and V OC were observed after the average energy increased from 10 to 15 and 20 keV.Comparing the J-V curves of FACs perovskite RPV cells with different phosphors under the unified average energy, CsI: Tl exhibited the best electrical properties, and ZnS: Ag performs slightly better than LYSO:Ce.These results show that the impact of phosphors on the output performance of FACs perovskite RPV cell is primarily determined by three factors: energy deposition, light yield, and spectral matching with the FACs PV converter.Figure 4d compares the P-V curves between FACs perovskite RPV cell and Si RPV cell with different phosphors at the average energy of 20 keV.It shows that the performance rankings of the three phosphors are still valid for the Si RPV cell, and it is worth noting that the PV curve of the FACs perovskite RPV cell is totally covered with that of the Si RPV cell for every phosphor.The corresponding electrical parameters comparison of FACs perovskite RPV cell and Si RPV cell with different phosphors are exhibited in Table 2.The optimal V OC of FACs perovskite RPV cell is 0.55 V, which is much higher than that of the Si RPV cell (0.39 V).J SC and P out of the FACs perovskite RPV cell with the CsI:Tl phosphor are 14.19 lA cm À2 and 4.30 lW cm À2 , respectively, 1.75 and 1.85 times that of Si RPV cell.Undoubtedly, the electrical properties of FACs perovskite RPV cells in this work are significantly superior to traditional Si RPV cells.It is worth noting that the absolute values of PCE  are not presented because the input power density of the X-ray exposed to the phosphors cannot be measured with enough precision.Figure 7a,b show the J-V and P-V curves of the FACs perovskite a-RPV cell with different phosphors, as well as the Si reference RPV cell based on ZnS:Ag phosphor.It indicates that the performance of FACs RPV cell with ZnS:Ag phosphor clearly surpassed the others.Thus, the advantage of high light yield of ZnS:Ag phosphor was fully demonstrated under He ions excitation.Table 3 compares the electrical parameters of FACs perovskite RPV cell to Si reference RPV cell.When the input power density (P in ) of He ions into the ZnS:Ag phosphor is 14.75 lW cm À2 , the V OC and J SC of FACs RPV cell are 0.498 V and 423.94 nA cm À2 , respectively 1.80 and 4.06 times that of Si reference RPV cell.Furthermore, the P max of the FACs perovskite RPV cell is 130.71 nW cm À2 , which is 6.56 times that of Si reference RPV cell, resulting in an ultra-high PCE of 0.886%.As shown in Table 3, all of the J SC , V OC , and FF parameters decrease drastically when the phosphor is changed from ZnS:Ag to CsI:Tl and then to LYSO:Ce.We can find that the decrease is along with the decrease in the emission power of the phosphor.The input power of the He ion is about 14.7 lW cm À2 , which is 0.000147 sun.Thus, the emission power of these phosphors should be much lower.The decrease of J SC , V OC , and FF parameters under lower light illumination has been reported for perovskite solar cells in previous works. [34,40,41]It is easy to understand that the decrease in J SC is mainly because less light is illuminated on the PV cell when the illumination power is reduced.The decrease of V OC and FF is mainly due to the decrease of shunt resistance (R sh ) with the decrease of light intensity under low-intensity conditions. [34,42]The origin of the significant decrease in FF is likely due to the high density of the interfacial traps within the device.These interfacial trap states could be filled with excess carriers at only higher light intensities, and then induce positive effects such as photodoping.However, there are no sufficient excess carriers to fill the traps under low-intensity light, thus the traps act as recombination centers and lead to largely decreased R sh , [34,40,41] causing drastically decrease in V OC and FF.This suggests that further effective passivation is required to improve the weak light performances of PSCs.

Comparisons and Output Power Extrapolation of a-RPV Cell
As shown in Figure 8a, the PCE of FACs perovskite a-RPV cell was compared to the reported RPV cells with different PV converters.When the equivalent   activity of the a-radioisotope in our work is only 0.83 mCi cm À2 , the perovskite a-RPV cell exhibits the highest PCE compared to the other reported values.This is because of the high efficiency of the FACs perovskite PV converter in dim light, as well as the higher kinetic energy of the He ions over the beta particles.
To evaluate the upper limit of P max of FACs perovskite a-RPV cell, Geant4 software was used to model americium dioxide ( 241 AmO 2 , a commonly used a-radioisotope for nuclear batteries, q = 11.66 g cm À3 , E a = 5.49 MeV) for simulating the variation of activity (A a ) and output power of the surface (P s ) with its thickness, as shown in Figure 8b.It indicates that the A a (blue line) increases linearly with increasing thickness, whereas the P s (red line) increases logarithmically due to the energy self-absorption effect of the a-radioisotope.The P s reaches a saturation level of 400 lW cm À2 at 12 lm, with the A a calculated to be 50 mCi cm À2 .Usually, the transport efficiency (g tr ) is used to characterize the degree of self-absorption by constructing the ratio of P s compared to the total power density of the a-radioisotope (P total ).Furthermore, a balance factor (BF) is introduced to determine the optimal thickness range of a-radioisotope for achieving high P s and outstanding g tr simultaneously, which can be calculated as BF = P s 9 g tr .
The relationship between BF and the thickness of the a-radioisotope is represented by the black dotted line in Figure 8b.It shows the BF increases as a function of quadratic approximately with the thickness ranging from 0 to 4 lm, and starts to drop gradually for greater thickness.Obviously, the P s (~300 lW cm À2 ) corresponding to the peak of BF is about 20 times that of the He ions beam adopted in this work, implying the P max can reach up to 2.66 lW cm À2 at the optimal level.

Conclusion
This work proposed a new RPV cell based on a novel FACs perovskite PV converter which operates outstandingly well at low-intensity illumination.Electrical properties between the FACs perovskite RPV cell and Si RPV cell with three types of phosphors were compared under irradiation with X-ray and He ions.Under X-ray irradiation with an average energy of 30 keV, the FACs perovskite RPV cell showed an optimal P max of 4.30 lW cm À2 , when the J SC and V OC were 14.19 lA cm À2 and 0.55 V, respectively, which were 1.75 and 1.85 times that of the Si reference RPV cell, respectively.When a He ions accelerator was used equivalent the a-radioisotope with an activity of 0.83 mCi cm À2 , a PCE of 0.886% was achieved, which is 6.6 times that of the Si reference RPV cell, reaching the highest PCE of an RPV cell reported so far.Moreover, this work provides a theoretical base to improve the performance and the PCE of FACs perovskite RPV cell could be further improved by optimizing the structure and size of the PV converter and phosphor, which will be investigated in our future work.

Figure 1 .
Figure 1.a) Schematic structure of the FACs perovskite RPV cell.b) The J-V curve of FACs perovskite PV converter under one sun AM 1.5G illumination, inset: Photograph of the top view of the FACs perovskite PV converter.c) EQE spectrum of FACs perovskite PV converter.

Figure 2 .
Figure 2. Comparison of the device performances of Si and FACs perovskite solar cells measured under light-emitting diode illumination with different light intensities.a, b) J-V curves of Si and perovskite cells under different light intensities.c-f) V OC , J SC , FF, and PCE of Si and perovskite cells under different light intensities.

Figure 3 .
Figure 3.Comparison of the device performances of Si and FACs perovskite solar cells measured under xenon lamp illumination with different light intensities.a, b) J-V curves of Si and perovskite cells under different light intensities.c-f) V OC , J SC , FF, and PCE of Si and perovskite cells under different light intensities.

Figure 4 .
Figure 4. RL characteristics of the commercial inorganic scintillators utilized as phosphors.a) Photograph of the glowing phosphors under X-ray irradiation.b) Spectral response of FACs PV converter and the RL of phosphors.c) Comparison of the light yields of phosphors.

Figure 5 .
Figure 5. Photograph of the experimental setup used in the measurement of electrical characteristics of FACs perovskite RPV cell under the irradiation of a) X-ray and b) He ions beam.

Figure 6 .
Figure 6.J-V curves of the FACs perovskite RPV cell under different tube voltages and currents, with different phosphors of a) LYSO:Ce, b) CsI:Tl, and c) ZnS:Ag, respectively.d) Comparison of P-V curves between FACs perovskite RPV cell and Si RPV cell with different phosphors irradiated by Xray (tube voltage, 60 kV, tube current, 200 lA).

Figure 7 .
Figure 7.Comparison of a) J-V curves and b) P-V curves between the FACs perovskite RPV cell and Si RPV cell with different phosphors irradiated by He ions.

Figure 8 .
Figure 8. a) PCE comparison with reported RPV cells with different PV converters.b) A a , P s , and BF of the AmO 2 a-radioisotope as a function of its thickness.

Table 1 .
Information on the phosphors used in the experiment.

Table 2 .
Performance comparison of Si and FACs perovskite b-RPV cells.

Table 3 .
Comparison of electrical characteristics of a-RPV cells.