Organic laser power converter for efficient wireless micro power transfer

Wireless power transfer with collimated power transmission and efficient conversion provides an alternative charging mode for off-grid and portable micro-power electronics. However, charging micro-power electronics with low photon flux can be challenging for current laser power converters. Here we show laser power converters with organic photovoltaic cells with good performance for application in laser wireless power transfer. The laser selection strategy is established and the upper limit of efficiency is proposed. The organic laser power converters exhibit a 36.2% efficiency at a 660 nm laser with a photon flux of 9.5 mW cm−2 and achieve wireless micro power transfer with an output of 0.5 W on a 2 meter scale. This work shows the good performance of organic photovoltaic cells in constructing organic laser power converters and provides a potential solution for the wireless power transfer of micro-power electronics.

show the results of time-resolved and low temperature photocarrier experiments, while the results can be interesting to better understand the carrier dynamics in these materials, they do not demonstrate the "superiority" of the power converters or the efficient power transfer. Fig. 4e shows an I-V curve of a 20cm^2 module at 2m with an efficiency of 26%, the author should clearly mention the input wavelength for Fig. 4e, as well as the interconnection architecture that leads to a Voc of ~12V. I do not see any superiority of the results presented in Fig 4e compared to other state-of-the-art power converters, other of course than potentially the low cost of the organic materials used.
Reviewer #3 (Remarks to the Author): Re: Organic Beamed Power Converter for Efficient Wireless Micro Power Transfer, by Yafei Wang, et al. Since the first days of organic photovoltaics (OPVs), our community has searched for applications that might demonstrate their unique advantages. Typically, OPV arguments have been based on proposed cost benefits. But over the years, some have also noted that OPVs can outperform conventional cells at low brightness.
The physical foundation for interest in performance at low brightness is the localization of optical and electrical excited states in organic materials. This can provide protection from traps, defects and recombination losses. But localization also creates problems at higher optical densities, like lower charge carrier mobilities, and losses such as exciton-exciton and exciton-charge annihilation.
To the best of my knowledge, arguments for the use of OPVs at low brightness never caught on. The question is why would anyone care about a solar cell at low brightness? Maybe there is some benefit in the morning or evening, or maybe the objective is energy harvesting from ambient light? But on the other hand (at least for OPVs), we can't ignore performance at high optical flux either because those are the crucial conditions for actually generating power.
This manuscript presents a variation on this old argument, but applied now to wireless power transfer. The OPVs are indeed outstanding: 17% under AM1.5, and even better under monochromatic excitation closer to the absorption edge. Two main materials are employed, with the wider gap choice unsurprisingly outperforming the conventional OPV structure. The OPV material system is also characterized using TA, which is nice although not especially relevant to the PB2:GS-ISO system that is highlighted as the optimal solution at 660nm.
My main concern with the manuscript is the engineering argument, especially the justification for the key conclusion 'We demonstrate the superiority of constructing OBPC with OPV cell for WPT technology.' I couldn't find any comparisons between the OPV systems and conventional cells, and while the performance sounds great in isolation, one has to wonder what GaAs can do at 660nm. Are we going to exclude the use of Si or GaAs based on cost? I wonder because WPT may not need a large area cell. I also don't understand whether low power, visible spectrum WPT is demanded by any practical application. The last line of the preceding paragraph seems crucial 'The performance of the LED charging system can fulfill the requirement of micro-power electronics such as passive electronic tag, on-board ETC and microfluid chip.' It struck me that a different way to present this work might have been to start with that target application, justify its significance with citations, explain why it needs to be visible spectrum, LED-powered, etc... and then work through the quantitative benefits of this OPV system versus conventional alternatives. I would really like to believe that OPV has a compelling application in WPT, but this manuscript unfortunately does not make that case.
Reviewer #1 (Remarks to the Author): Wang et al. demonstrated a beamed power converter using organic solar cells under laser illumination. They employed transient spectroscopic measurements and theoretical calculations to investigate the physical mechanism underlying the power converter. This work expands the potential applications of organic solar cells and is of significant interest to the community of organic optoelectronics. However, before publication, the following issues should be addressed: 1) While the authors mention the wireless power transfer with low photon flux as a merit of the device performance, it would be helpful if they compared the performance with other available techniques. Response: We appreciate the comment. We compared the performance in low photon flux WPT by using single-junction gallium arsenide (GaAs), perovskite (PVK) and monocrystalline silicon (Si) photoelectric converter. The results are listed here (reput in supporting information Section S9).  As shown in Fig. R1, in the view point of PCE under low photon flux, GaAs and perovskite LPC (laser power convertor) exhibit better performance than OLPC (organic laser power convertor); while the Si LPC shows even lower PCE than OLPC. Nevertheless, OLPC is still indispensable owing to the absolute advantage in energy budget and laser wavelength adaptability. Based on detail investigation, we summarized the bandgap tunability, flexibility, contain heavy metals or not, power per weight and cost in Table R1. Due to the highly adjustable bandgap, ultra-flexible, free of heavy metals, high power per weight, and low cost, OLPCs have unique advantages in wireless power transfer. As shown in Figure R2, the large-quantities preparation of OLPC will further reduce the cost of OLPC, which will promote the application of OLPC in wireless power transfer.
To demonstrate the ultra-flexible and lightweight of OPV devices, we prepared the ultra-thin substrate devices based on polyimide (PI). The ultra flexible OPV cells in the literature are also shown in the   Large-quantities preparation of OPV cells. A schematic illustration of knife coating where excess ink is kept ahead of the knife that is in close proximity to the web (left). Slot-die coating relies on the meniscus standing between a coating head with a slot from which ink is supplied to the standing meniscus thus forming a continuous (or striped) wet film (right). 6 2) On page 1, line 35-36, the authors state that "As photon flux decreases, the PCE of silicon or gallium arsenide photovoltaic cells drops due to the Auger process induced intrinsic losses". The authors should explain why this happens. In principle, the Auger process is a many-body effect that should be more significant under higher photon flux excitation. Additionally, the authors should clarify if there is any particular reason why the Auger process is different in organic devices. Response: We appreciate the reminding. The statement in the mentioned sentence is replaced in the revised one. We noticed that the intrinsic losses in photovoltaic laser power converters include the entropic loss produced during the radiative recombination and nonradiative recombination. 7,8 Radiation recombination includes absorption and emission process, while non radiative processes include the free carrier absorption and Auger recombination. In the widely investigated about Si and GaAs, the entropic loss produced during the absorption and emission of radiation is the major loss mechanism for all bandgap energies. In real semiconductor materials, the free carrier absorption and Auger recombination are also unavoidable. Since the free-carrier absorption is a minor effect in Si and GaAs devices, it can be neglected in the calculation. 9,10 However, Auger recombination can be comparable with radiative recombination even in thin-film materials and thereby is regarded as the sole intrinsic nonradiative recombination in the calculation. It is assumed that the semiconductor material is intrinsic or highly excited. Finite Auger recombination rates will result in a volumetric entropy generation term that determines the entropy generation rate via Auger recombination. Moreover, under higher incident irradiances, the fraction of input power via emission loss is almost constant, while the proportion of entropic loss diminishes logarithmically, as shown in Fig. R3(a).
For diminishing the intrinsic losses with respect to the Auger process: by intensifying the laser irradiance, the proportion of entropic loss in input power can be arbitrarily reduced; by using spectral and angular filters, the intrinsic losses can be diminished via absorption enhancement or emission restriction, as shown in Fig. R3(b). In this case, the excess energy cannot excite new carrier pairs, so that, Auger generation can be neglected. Therefore, the sentence "As photon flux decreases, the PCE of Si and GaAs photovoltaic cells drops due to the Auger process induced intrinsic losses" has been revised to "As photon flux decreases, the PCE of silicon or gallium arsenide photovoltaic cells drops due to the entropic loss induced intrinsic losses". The sentences have been revised and highlighted at line 47-48 in the manuscript. 3) On line 107-108, the authors mention that "Only the exciton diffuses to the donor/acceptor interface can be dissociated into free carrier," which is true in conventional devices. However, in OPV devices based on non-fullerene acceptors, charge separation is ready in individual acceptor domains, as evidenced by recent publications such as Nat. Commun. 2022Commun. , 13, 2827Commun. (2022 or Ref. 26. Therefore, the authors should consider this point. Response: We appreciate the comment. The regions where exciton dissociates locate at both the bulk and boundary acceptor. As the diffusion length of Frenkel exciton is usually less than tens of nanometers, the exciton dissociation at donor/acceptor interface will take place with lower energy consuming than the one in bulk. Although exciton dissociation can be observed in Y6 based non fullerene receptors (J. Am. Chem. Soc. 142, 12751 (2020).), the efficiency of single component devices is suppressed by bimolecular and rapid minority carrier charge recombination in the absence of donor. Overall, to diminish the ambiguity, we change the sentence to the new one: "The exciton dissociation into free charge carriers mainly occurs at the donor/acceptor interface." at line 109-110 in the manuscript.
4) The authors performed transient absorption measurements with excitation fluence of ~10 microJ/cm 2 , which is far beyond the working condition of the device. In this regime, exciton annihilation cannot be avoided. The authors should rationalize this disparity. Additionally, the authors note that the probe spectrum covers 750-1600 nm, but the data is only plotted in the range <1100 nm. However, some essential features appear in the longer-wavelength range, and the authors should carefully check for them. Response: We appreciate the comment. In this work, the radiative intensity of the lasers can reach 300 mW/cm 2 , which equals to 300 mJ/(s·cm 2 ). So the transient absorption measurements with excitation fluence of ~10 mJ/cm 2 are comparable to the working condition of the devices.
According the suggestion, we retested the TA spectrum of PBDB-TF:BTP-eC9 BHJ under excitation of 809 nm, and plotted the spectra at 800-1600nm in Fig. R5 (reput in Fig. 3c and Fig. 3e in the manuscript). As described in the literature, 11 the excited-state absorption (ESA) appears at 1400-1600nm, which is considered to be caused by the intermediate intra-moiety excited (i-EX) state of the hole transfer channel. In addition, the TA spectrum of PB2:GS-ISO BHJ under excitation of 660 nm is shown in Fig. R6 (added in supporting information Supplementary Fig. 7). However, it is not to appear the feature absorption in the longer-wavelength in PB2:GS-ISO.  The authors obtained interesting results with organic power converters for low input power conditions at different wavelengths. Unfortunately, I cannot recommend the paper for publication and significant revision would be required. Furthermore, I do not believe the results are noteworthy or significant enough to warrant publication in Nature Communications. 1) The grammar and the style needs significant revisions. For example, the authors use "beamed power converter" => I think they should remove "beamed". Instead of "beamed", maybe use "laser" or "optical", as it is usual used in the field. Grammatically, I think "beamed power converter" does not make sense. Response: We appreciate the comment. We have revised the language used in our manuscript as your requirement, and the revised part is marked red in the manuscript. The editing certificate is as follows and the original file is attached as Attachment. In previous literatures, "laser power beaming", "laser beam", "laser power converter" and "optical power converter" were used to describe the wireless power transfer. [12][13][14][15] According the suggestion, we have replaced "beamed" with "laser" in this manuscript.
2) The authors used "highly collimated power transmission" => why "highly", for example further towards the end of the manuscript, the authors proposed "With respect to practical application, the laser can be alternated by cheap but safe monochromatic light-emitting diode (LED).", so the collimation certainly need not to be "high". Response: Due to the 10 W-660-nm laser generator is very expensive, it is not a common-used instrument in our lab. The use of LED spotlight is to imitate the 10 W-660-nm laser and test the performance of laser power converters (OLPC). In practical application of wireless power transfer (WPT), laser generator will be used rather than LED. In addition, the "highly" is removed in corresponding sentence.
3) The authors further claim, "current beamed power converters are incapable of charging micro-power electronics with low photon flux" => I believe this statement to be not true nor substantiated. Response: We appreciate the comment. This question is the same as comment proposed by Reviewer 1# question 1). Please see the response. 4) They then use the sentence "The principle of laser selecting is established and the up limit of efficiency is prospected." => I find this sentence has no meaning or its meaning is very unclear. Response: We appreciate the comment. We change the origin sentences to the new one: "The laser selection principle is established and the upper limit of efficiency is proposed." at line 17-18 in the manuscript. 5) In addition, the authors mentioned "36.18% efficiency under 660 nm laser with photon flux of 9.46 mW/cm2" => there are too many digits of precision, all throughout the manuscript, including in Table 1, etc. For example, the real efficiency accuracy is probably at best 1 decimal digit of precision. For example, "0.52 W in 2 meters scale" should be replace with "0.5 W for a distance of 2 m". Response: We appreciate the comment. We changed the significant digits after the decimal point to one digit. But for short circuit voltage (Voc), due to it changes little with the illumination intensity or temperature, it still retains three significant digits after the decimal point.
6) The authors claim, "Our work for the first time manifests the superiority of organic photovoltaic cell in constructing organic beamed power converters and provides a solution for the wireless power transfer of micro-power electronics" I find this claim is misleading and unsubstantiated, for example, the literature shows that it is not superior. For example, Komuro et al published "A 43.0% efficient GaInP photonic power converter under high-power 638 nm laser irradiation of 17 W cm-2," or Fafard et al have shown "At 100 mW of input power, an efficiency of Eff = 59.7% is obtained … The output power (Pmpp) reaches 242 mW at 500 mW of input power and 415 mW at 1 W ". Response: We appreciate the comment. The potential application of organic photovoltaic (OPV) cells in laser wireless power transfer has not been explored. For sure, our reporting is the first one in the field of OLPC. In this work, "Our work for the first time manifests the superiority of organic photovoltaic cell in constructing organic beamed power converters and provides a solution for the wireless power transfer of micro-power electronics" refers to our systematic study of the application of OPV cells in laser wireless power transfer and the achievement of a PCE exceeding 36% under micro-power conditions. Moreover, we analyzed the advantages of OLPC in cost and laser wavelength adaptability. Consequently, our work opened up a bright future for OLPC-based WPT. With updates in materials and device, the PCE of OLPC in WPT would emerge as a promising technology. 7) Furthermore, the authors probably have not demonstrated a real "solution", because the reliability is still ambiguous. Some stability results are shown in the supplementary material, but a real solution would require a solid reliability study. In addition, the device area considered in this study is order of magnitudes much greater than the area used of the typical semiconductor power converters, for example 20 cm^2 vs < ~0.2cm^2, therefore the arguments and the comparisons must be adjusted accordingly. Response: We appreciate the comment. Our work illustrated the potential of organic semiconductor in converting laser into electricity. Focusing on this point, we carried out systematic researches. We think the "real solution" which expected by the reviewer is complete engineering of OLPC device or even a whole WPT system. This is not the aim of this work, especially for the OPV society which is still undergoing a potential industrialization. What we did is exploit a new application for OPV, and we find that in low-energy laser conversion, the cost of OLPC is the lowest. The solution for the complete technology contains efforts highly beyond the material engineering organized in this work. The finish of encapsulation, optical control, anti-reflection, pin hole, hydrocooling system, and even the intergration with energy storage module construct a real solution for OLPC. These items are obviously out the work we can initialize now because our work is just the first example of OLPC.
According to the suggestion, we compared GaAs and OLPC(PB2:GS-ISO) with an area of 0.2cm 2 and the results are listed here. As shown in Table R2, GaAs cells are capable of stronger laser illumination intensity, resulting in higher power output at 0.2 cm 2 . Due to the highly adjustable bandgap, ultra-flexible, free of heavy metals, high power per weight, and low cost, OLPCs have unique advantages in wireless power transfer. some micro sensors, detectors and chips only require a few milliwatts or even microwatts power per use, and they maybe do not require long-term power supply, such as anti-theft or anticounterfeiting of keys, bank cards (or other important card), passive electronic tags (Ink screen does not consume power during daily display) and on-board ETC etc. Except these consumer electronics, the energy consuming of long-term underwater detectors are usually as low as milliwatt-scale.
8) The authors mentioned, "As photon flux decreases, the PCE of silicon or gallium arsenide photovoltaic cells drops due to the Auger process induced intrinsic losses". I believe this statement is not true nor substantiated: at low photon flux, good state-ofthe-art power converters still have good PCE, and Auger process are not expected to be significant at low densities of charged carriers. The output voltage of state-of-the-art semiconductor power converters is only slightly lower at lower at low photon fluxes, as expected from the slightly lower output voltage expected from an ideal diode behavior. Response: We appreciate the comment. This question is the same as comment proposed by reviewer 1# question 2). Please see that response.
9) The authors mentioned, "it is of great importance to develop the BPC based on new photovoltaic technology with low cost, broad laser wavelength applicability and high PCE under micro-power density." I find the only aspect of this statement that could be valid in the context of the results presented in the paper is the "low cost" aspect. Although, no data as such to substantiate the relative low cost aspect is presented in the manuscript. Response: We appreciate the comment. The description of "low cost" is detailed in reviewer 1# question 1). Please see the response.
10) It would probably be interesting if the authors elaborated more on the stability of the results, for example, in Fig. 1, the FF decreases at optical intensities as low as a few tens of milliwatts per cm^2. Can the authors comment regarding if the devices show degradation at these intensities? Can the I-V or FF curves be retraced multiple times over extended periods of exposure times. Response: We appreciate the comment. Under low illuminance, the larger energetic disorder causes severe trap-assist recombination, rapidly decreasing FF in organic photovoltaic cells (OPV). 17 This phenomenon is also common in OPV under indoor lighting test. 18 According the suggestion, the FF curves be retraced 5 times over extended periods of exposure times, and the results are as follows:  Table  3, and the curves of FF plotted as a function of optical intensity is shown in Figure R9. When the optical intensities decreases, the FF also decreases.  11) Why use the scientific notation for the vertical scale of Fig. 2d?

Response:
We appreciate the comment. The temperature-dependent EQE values for all the devices can be fitted using the following equation 20 :

equation (R1)
To get Ea, we rewrite the above formula as: where EQE0 is the EQE value at infinite temperature, Ea is the activation energy, is the Boltzmann constant, and T is the temperature. The Ea value indicates the energy required for the geminate pair separation. Therefore, the Ea can be fitted with temperature dependent J-V carves. Fig. 2 and 3 show the results of time-resolved and low temperature photocarrier experiments, while the results can be interesting to better understand the carrier dynamics in these materials, they do not demonstrate the "superiority" of the power converters or the efficient power transfer. Fig. 4e shows an I-V curve of a 20cm^2 module at 2m with an efficiency of 26%, the author should clearly mention the input wavelength for Fig. 4e, as well as the interconnection architecture that leads to a Voc of ~12V. I do not see any superiority of the results presented in Fig 4e compared to other state-of-the-art power converters, other of course than potentially the low cost of the organic materials used. Response: We appreciate the comment. The results of time-resolved and low temperature photocarrier experiments in this work shown the feasibility of OPV cells application in WPT. What is more important, we explored the exciton annihilation and carrier recombination process of OPV cells under different monochromatic excitation wavelengths.

12)
Regarding the "superiority" of the OLPC, we summarized the bandgap tunability, flexibility, contain heavy metals or not, power per weight and cost of different power converters in Table R1 (we have elaborated on the advantages of OPV at reviewer 1# question 1), please see the response). Due to the highly adjustable bandgap, ultraflexible, free of heavy metals, high power per weight, and relatively low cost, OLPCs have unique advantages in wireless power transfer.
As shown in Fig. 1e and Fig. 4e in manuscript, the efficiency of OLPC in WPT can achieve to ~30% PCE at ~100 mW/cm 2 , and the illumination intensity is equivalent to AM 1.5G. Although the PCEs of the PBDB-TF:BTP-eC9-based OLPC have demonstrated the application potential of OPV cells in WPT, there is still much room for PCE improvement. Under the optimal condition of λ809 for the PBDB-TF:BTP-eC9based OLPC, the Although the PCEs of the PBDB-TF:BTP-eC9-based OLPC have demonstrated the application potential of OPV cells in WPT, there is still much room for PCE improvement. Under the optimal condition of λ809 for the PBDB-TF:BTP-eC9-based OLPC, the Vloss is as large as 560.0 mV. Moreover, the EQE and FF are still insufficient. Here, the ideal PCE for each laser wavelength is mapped in Fig. 4a and Fig. 4b. By using BHJ materials with lower Vloss, higher EQE and larger Eg, the PCE of the OLPC can potentially be significantly improved. loss is as large as 560.0 mV. Moreover, the EQE and FF are still insufficient. Here, the ideal PCE for each laser wavelength is mapped in Fig. R10 (reput Fig. R10 (a) and Fig. R10 (b) in Fig. 4a and Fig. 4b in the manuscript). By using BHJ materials with lower Vloss, higher EQE and larger Eg, the PCE of the OLPC can potentially be significantly improved. According to the suggestion, the schematic diagram is shown in Fig. R11 (reput in supporting information Supplementary Fig. 12), and module fabrication process of 20 cm2 module have been added at line 253-259 in the manuscript. Fabrication of 20 cm 2 OPV modules. Glass/ITO/PEDOT:PSS/PB2:GS-ISO/PFN-Br/Ag were fabricated by blade coating method. The ITO substrates were purchased from Huananxiangcheng Inc, and patterned with 12 μm P1. ITO substrates were cleaned by above method. The area of mask is 20 cm 2 . After coating and annealing the active layer, the PFN-Br was coated. P2 pattern was formed by a mechanical scribing machine with 50 μm scribing blade. The samples were transferred to thermal evaporator and 150 nm of Ag were deposited. Then, the P3 patterns were formed by a mechanical scribing machine. Notable, to prevent the etched silver from sticking and causing a short circuit, the air knife was applied during the etching process.
Reviewer #3 (Remarks to the Author): Re: Organic Beamed Power Converter for Efficient Wireless Micro Power Transfer, by Yafei Wang, et al. Since the first days of organic photovoltaics (OPVs), our community has searched for applications that might demonstrate their unique advantages. Typically, OPV arguments have been based on proposed cost benefits. But over the years, some have also noted that OPVs can outperform conventional cells at low brightness.
1) The physical foundation for interest in performance at low brightness is the localization of optical and electrical excited states in organic materials. This can provide protection from traps, defects and recombination losses. But localization also creates problems at higher optical densities, like lower charge carrier mobilities, and losses such as exciton-exciton and exciton-charge annihilation.

Response:
We appreciate the comment. We believe that the illumination intensity at 10 -1 to 10 2 mW/cm 2 is the best working condition for OLPC. For sure, localization will affect the performance of OLPC under high illumination intensity. However, the illumination intensity of 10 -1 to 10 2 mW/cm 2 has limited impact on performance of OLPC. Within this illumination intensity range, the impact of localization on the performance of OLPC is limited 2) To the best of my knowledge, arguments for the use of OPVs at low brightness never caught on. The question is why would anyone care about a solar cell at low brightness? Maybe there is some benefit in the morning or evening, or maybe the objective is energy harvesting from ambient light? But on the other hand (at least for OPVs), we can't ignore performance at high optical flux either because those are the crucial conditions for actually generating power. Response: We appreciate the comment. Based on this comment, we have mapped various applications of laser wireless power transfer at different laser wavelengths and output powers in Fig. R12 (reput in supporting information Section S1). Fig. R12. Illustration of the various WPT applications organized according to the output power (horizontal axis-log scale) and the wavelength of monochromatic (vertical axis-not to scale). The commercial aspects, the reliability, and the technical attributes of the available laser diode products often predominantly guide the selection of the optical input wavelength. We classify the LPC devices into low/regular/medium/high power, based on their output power capabilities. 21 As shown in Fig. R12, some micro sensors, detectors and chips only require a few milliwatts or even microwatts power per use, and they maybe do not require long-term power supply, such as anti-theft or anti-counterfeiting of keys, bank cards (or other important card), passive electronic tags (Ink screen does not consume power during daily display) and on-board ETC etc. Except these consumer electronics, the energy consuming of long-term underwater detectors are usually as low as milliwatt-scale. The charge for these electronics could be convenient when using power transfer. Therefore, the importance of developing BPC suitable for 10 -1 to 10 2 mW/cm 2 (Low power) WPT will be unfolded rapidly as the applications of IoTs expand. On the other hand, the use of OPV cells at low brightness, such as the application of OPV cells in indoor LED light, has attracted the attention of relevant researchers and received systematic research in recent years. 18,[22][23][24][25] In order to obtain higher output power, the laser intensity used can reach tens or even thousands of suns at present. But in our daily life, we need low power intensity to avoid laser damage human bodies and the environment (such as fires, light pollution, etc.). Due to the advantages of light weight, flexibility, low price and high power per weight, OPV cells are more suitable for powering low-power electronics under low light intensity. As shown in Fig. R10 (at reviewer 2# question 11), please see the response), owning to the adjustable band gap of OPV materials, we firmly believe that the power conversion efficiency of OPV in wireless power transfer system will be further improved with the development of OPV materials.
3) This manuscript presents a variation on this old argument, but applied now to wireless power transfer. The OPVs are indeed outstanding: 17% under AM1.5, and even better under monochromatic excitation closer to the absorption edge. Two main materials are employed, with the wider gap choice unsurprisingly outperforming the conventional OPV structure. The OPV material system is also characterized using TA, which is nice although not especially relevant to the PB2:GS-ISO system that is highlighted as the optimal solution at 660nm. Response: We appreciate the comment. As shown in Fig. R12, with the development of science and technology, the requirements for low power and visible wavelengths of WPT are gradually being proposed. Based on the advantages of OLPC described in reviewer 1# question 1). (please see the response), OLPC is expected to become another highly anticipated devices.
The TA characterization of PB2:GS-ISO have been added in the manuscript Fig.  4f to Fig. 4j, and the relevant description was added at line 175 to 182. The exciton diffusion length (LD) in λ533-excited PB2 (Fig. R14(a)) and λ809-excited GS-ISO ( Fig.  R14(b)) are 11.8 and 27.6 nm, respectively. The same as BTP-eC9， the longer LD of GS-ISO accounts for a more efficient exciton diffusion and illustrates the superiority of λ660.
In the existing OPV material system, PB2: GS-ISO is the best choice at 660 nm. However, as shown in Fig. R12 (at reviewer 2# question 11), please see the response), when the EQE and Vloss of OPV materials decrease, there will be a higher PCE for OLPC at 660 nm.  4) My main concern with the manuscript is the engineering argument, especially the justification for the key conclusion 'We demonstrate the superiority of constructing OBPC with OPV cell for WPT technology.' I couldn't find any comparisons between the OPV systems and conventional cells, and while the performance sounds great in isolation, one has to wonder what GaAs can do at 660nm. Are we going to exclude the use of Si or GaAs based on cost? I wonder because WPT may not need a large area cell. I also don't understand whether low power, visible spectrum WPT is demanded by any practical application. The last line of the preceding paragraph seems crucial 'The performance of the LED charging system can fulfill the requirement of micro-power electronics such as passive electronic tag, on-board ETC and microfluid chip.' It struck me that a different way to present this work might have been to start with that target application, justify its significance with citations, explain why it needs to be visible spectrum, LED-powered, etc... and then work through the quantitative benefits of this OPV system versus conventional alternatives. I would really like to believe that OPV has a compelling application in WPT, but this manuscript unfortunately does not make that case. Response: We appreciate the comment. At the response to Reviewer 1# question 1), we compared the performance in low photon flux WPT by using single-junction GaAs, PVK and monocrystalline Si photoelectric converter at λ660 and λ809. Based on detail investigation, we summarized the bandgap tunability, flexibility, contain heavy metals or not, power per weight and cost in Table R1. Due to the highly adjustable bandgap, ultra-flexible, free of heavy metals, high power per weight, and low cost, OLPCs have unique advantages in wireless power transfer. Please see the response.
Divided by the order of WPT photon flux, the target application scenario varies a lot: 10 3 to 10 5 mW/cm 2 -WPT techniques often participate in military and spatial applications; 10 2 to 10 3 mW/cm 2 -WPT techniques are suitable for powering highenergy-consuming loads; as for 10 -1 to 10 2 mW/cm 2 , few actual products appear in the past years because the low-energy-consuming flexible electronics just enter the explosive period of developing. Compared with the laser power density in other target application, the low-energy-consuming as for 10 -1 to 10 2 mW/cm 2 is "micro power".
The low power applications for WPT: 1. Power for micro sensors, detectors and chips.
Some micro sensors, detectors and chips only require a few milliwatts or even microwatts power per use, and they maybe do not require long-term power supply, such as anti-theft or anti-counterfeiting of keys, bank cards (or other important card), passive electronic tags (Ink screen does not consume power during daily display) and on-board ETC etc. 2. Power beaming for low power electronics.
The energy consuming of long-term underwater detectors are usually as low as milliwatt-scale. The charge for these electronics could be convenient when using power beaming. The advantages of WPT within the visible spectrum lasers are: 1. higher PCE: Similar to the trend of GaAs-series cells that exhibiting higher PCE when replacing IR laser/GaAs cell by green laser/InGaP cell, OPV cells based on BHJs with higher band gaps can output higher PCE if we chose appropriate laser.

Higher identification:
When applied to anti-theft or anti-counterfeiting of keys and bank cards (or other important card) etc., it is better to use lasers within the visible spectral in order to visually monitor the process when the holder unlocks or swipes the card. After integration with OLPT, it cannot be used normally even if the thieves copy the key or bank card.

Higher safety:
Lasers within the visible spectral range can remind people to avoid the laser. 4. Underwater adaptability: The penetration of visible spectrum lasers is stronger underwater, especially the blue-green spectrum. The visible spectrum lasers could power for underwater electrical appliances. According the suggestion, we revised the introduction in this manuscript and marked in highlight at line 33-44, as below: The target application scenario is divided by the order of WPT photon flux and varies greatly: 10 3 to 10 5 mW/cm 2 -for WPT techniques that often participate in military and spatial applications; 10 2 to 10 3 mW/cm 2 -for WPT techniques are suitable for powering high-energy-consuming loads such as cameras or small unmanned aerial vehicles; and 10 -1 to 10 2 mW/cm 2 for a few products that have appeared in recent years due to the low-energy-consuming flexible electronics that have entered rapid development period. Some micro sensors, detectors and chips only require a few milliwatts or even microwatts of power per use, and they do not potentially require a long-term power supply, such as anti-theft or anti-counterfeiting keys, bank cards (or other important cards), passive electronic tags (the ink screen does not consume power during daily display) and onboard ETC etc. Except for these consumer electronics, the energy consuming of long-term underwater detectors are usually as low as milliwattscale. The charge for these electronics could be convenient when using power beaming. Therefore, the importance of developing LPC suitable for 10 -1 to 10 2 mW/cm 2 WPT will unfold rapidly as the applications of IoT expand.