Efficient Planar Hybrid N-Si/PEDOT:PSS Solar Cells with Power Conversion Efficiency up to 13.31% Achieved by Controlling The

energies

Article Efﬁcient Planar Hybrid n-Si/PEDOT:PSS Solar Cells with Power Conversion Efﬁciency up to 13.31% Achieved by Controlling the SiOx Interlayer

Chenxu Zhang 1, Yuming Zhang 1,*, Hui Guo 1, Qubo Jiang 2, Peng Dong 1 and Chunfu Zhang 1,* ID

1 Wide Bandgap Semiconductor Technology Disciplines State Key Laboratory, School of Microelectronics, Xidian University, Xi’an 710071, China; [email protected] (C.Z.); [email protected] (H.G.); [email protected] (P.D.) 2 School of Electronic Engineering and Automation, Guilin University of Electronic Technology, No. 1 Jinji Road, Guilin 541004, China; [email protected] * Correspondence: [email protected] (Y.Z.); [email protected] (C.Z.)

Received: 25 April 2018; Accepted: 21 May 2018; Published: 30 May 2018

Abstract: In this work, the effects of the SiOx interface layer grown by exposure in air on the performance of planar hybrid n-Si/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solar cells are investigated. Compared to the cell with a hydrogen-terminated Si surface, the cell with an oxygen-terminated Si surface reveals improved characteristics in power conversion efﬁciency, increased from 10.44% to 13.31%. By introducing the SiOx, the wettability of the Si surface can be improved, allowing an effective spread of the PEDOT:PSS solution and thus a good contact between the PEDOT:PSS ﬁlm and Si. More importantly, it can change the polarity of the Si surface from a negative dipole to a positive dipole, owing to the introduction of the SiOx interface. The Si energy band will bend up and give rise to a favorable band alignment between Si and PEDOT:PSS to promote carrier separation. These results could be potentially employed to further development of this simple, low-cost heterojunction solar cell.

Keywords: SiOx interface; n-Si/PEDOT:PSS solar cells; surface polarity

1. Introduction Si/organic hybrid solar cells utilizing poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) combined with an n-type silicon (n-Si) have attracted significant research interest in recent years due to their obvious advantages of a low-temperature process, remarkably low fabrication cost, and potential high efficiency [1]. In this type of hybrid solar cells, as shown in Figure1, the PEDOT:PSS layer acts as a hole transporting path [2] when forming a heterojunction with the n-type Si substrate. Si has a strong absorption ability in a very wide spectrum range and excellent carrier transport ability. PEDOT:PSS is a water-soluble polymer which has a high conductivity, a transmission window in the visible spectral range, and excellent chemical and thermal stability. This type of Si/PEDOT:PSS hybrid solar cell combines the superior absorption property of Si in a wide spectrum range and the advantage of aqueous solution-based processes of PEDOT:PSS. Over the past few years, considerable progress towards creating highly efficient hybrid n-Si/PEDOT:PSS solar cells has been achieved by adjusting major factors such as electrical conductivity, chemical affinity, and passivation on the device surface [3–9]. Sun et al. introduced perovskite nanoparticles to the PEDOT:PSS layer to generate a positive electrical surface field [10]. Pham et al. presented adding functionalized graphene to the PEDOT:PSS solution based on Silicon nanowire (SiNW), and demonstrated an enhancement of photoelectric conversion efficiency (PCE). Wen et al. [11] fabricated an n+ amorphous silicon layer on

Energies 2018, 11, 1397; doi:10.3390/en11061397 www.mdpi.com/journal/energies Energies 2018, 11, 1397 2 of 10

Energies 2018, 11, x FOR PEER REVIEW 2 of 10 the back of the c-Si, which could significantly improve the extraction of the photon-generated carrier andextraction suppress of the the recombination photon-generated of hole carrier electrons and suppress at the rear the cathode recombination [12]. Despite of hole significant electrons effortsat the to improverear cathode solar cell[12]. performance, Despite significant the highest efforts power to improve conversion solar efficiencycell performance, (PCE) reported the highest to date power is still lowconversion when compared efficiency to (PCE) the value reported theoretically to date is estimated still low when by Shockley–Queisser compared to the value for a single-junctiontheoretically deviceestimated [13]. by Shockley–Queisser for a single-junction device [13].

Figure 1. Schematic structure of planar hybrid n-Si/PEDOT:PSS solar cells. Figure 1. Schematic structure of planar hybrid n-Si/PEDOT:PSS solar cells.

To further improve the performance of the hybrid n-Si/PEDOT:PSS solar cells, the interface propertiesTo further between improve n-Si theand performance PEDOT:PSS are of theof great hybrid importance. n-Si/PEDOT:PSS This is because solar cells,photogenerated the interface propertiesholes in n-Si between are required n-Si and to PEDOT:PSSbe effectively are transmitted of great importance.through the heterojunction This is because interface photogenerated [1] into holesthe inPEDOT:PSS n-Si are required layer and to then be effectively collected by transmitted the anode.through Therefore, the a heterojunctionhigh-quality interface interface between [1] into n- the PEDOT:PSSSi and PEDOT:PSS layer and is then critical collected for high-performance by the anode. Therefore, hybrid solar a high-quality cells. However, interface usually, between there n-Si are and PEDOT:PSSnumerous ismicrovoids critical for at high-performance the Si/PEDOT:PSS hybridinterface solar because cells. of However, the insufficient usually, wettability there are of numerous the Si microvoidssurface, which at the leads Si/PEDOT:PSS to a bad surface interface coverage because and severe of the recombination. insufficient wettability A proper ofinterface the Si surface,layer whichwith leadsa hydrophilic to a bad nature surface between coverage the andPEDOT:PSS severe recombination. and n-Si can improve A proper the interfacewettability layer of the with Si a hydrophilicsurface, which nature is helpful between in the the formation PEDOT:PSS of ideal and interface n-Si can contacts improve between the wettability the PEDOT:PSS of the film Si surface, and whichthe Si is substrate, helpful in and the thus formation a more ofideal ideal p–n interface junction, contacts resulting between in less charge the PEDOT:PSS recombination film and and a the Silarger substrate, photocurrent and thus density a more [14]. ideal At p–n the junction,same time, resulting the inserted in less interface charge layer recombination may help to andpassivate a larger photocurrentthe surface of density n-Si, which [14]. Atcould the further same time, enhance the insertedthe device interface performance. layer mayAs an help effective to passivate interface the surfacelayer between of n-Si, which PEDOT:PSS could and further n-Si, enhance its thickness the device is a critical performance. parameter. As When an effective the interface interface layer layer is betweentoo thin, PEDOT:PSS it is insufficient and n-Si, to provide its thickness good issurface a critical wettability parameter. and When passivation. the interface However, layer once is too the thin, it isinterface insufficient layer to is providetoo thick, good the photogenerated surface wettability carriers and have passivation. difficulty However, when they once tunnel the interfacethrough the layer insulating layer. is too thick, the photogenerated carriers have difficulty when they tunnel through the insulating layer. Previous work has shown improved solar cell performance when using a thin atomic layer Previous work has shown improved solar cell performance when using a thin atomic layer deposition (ALD) Al2O3 layer [15,16], which was achieved based on increased build-in potential and deposition (ALD) Al2O3 layer [15,16], which was achieved based on increased build-in potential improved charge collection by the electron blocking of Al2O3. However, the ALD instrument is and improved charge collection by the electron blocking of Al O . However, the ALD instrument expensive, and the process is complex. Other oxide layers [17,18]2 3are also reported to provide the is expensive,crucial interface and thelayer process for the is hybrid complex. solar Other cells. oxideHowever, layers all [ 17the,18 reported] are also deposition reported methods to provide are the crucial interface layer for the hybrid solar cells. However, all the reported deposition methods are complicated. As is well known, when Si is exposed to the air, a native oxide (SiOx) will grow easily. complicated.As an efficient As interface is well known, layer, its when thickness Si is exposedis also important. to the air, We a native could oxideremove (SiO thex )unintentionally will grow easily. Asgrown an efficient oxide interfacelayer by dipping layer, its the thickness Si substrate is also in a important. dilute HF solution We could and remove intentionally the unintentionally regrow the grownSiOx layer oxide by layer exposing by dipping the Si substrate the Si substrate to air in a in simple a dilute and HF controllable solution andway intentionallyso that the wanted regrow SiOx the SiOthicknessx layer bycould exposing be obtained. the Si However, substrate whether to air in this a simple oxide layer and can controllable effectively way improve so that the theinterface wanted SiOqualityx thickness and enhance could bethe obtained.device performance However, or whether not is still this not oxide well investigated. layer can effectively improve the interfaceIn qualitythis work, and the enhance effects the of devicethe SiO performancex layer intentionally or not isgrown still not though well investigated.exposure to air on the performanceIn this work, of planar the effects hybrid of n-Si/PEDOT:PSS the SiOx layer solar intentionally cells are investigated. grown though Compared exposure to the to cell air with on the performancethe bare Si surface, of planar the hybrid cell with n-Si/PEDOT:PSS an oxygen-terminated solar cells Si surface are investigated. reveals improved Compared characteristics to the cell in with thepower bare Siconversion surface, theefficiency cell with (PCE), an oxygen-terminated increased from 10.44% Si surfaceto 13.31%. reveals It is inferred improved that characteristics the formation in powerof oxygen-terminated conversion efficiency bonds (PCE), helps increased to build a from net 10.44%positive to surface 13.31%. dipole, It is inferred which promotes that the formationcarrier ofseparation oxygen-terminated and suppresses bonds the helps Si surface to build recombination. a net positive Our surfaceresults suggest dipole, promising which promotes strategies carrier to separationfurther exploit and suppresses the efficiency the potential Si surface for recombination. the simple, low-cost Our resultshybrid Si/organic suggest promising solar cells. strategies to further exploit the efficiency potential for the simple, low-cost hybrid Si/organic solar cells.

Energies 2018, 11, 1397 3 of 10

Energies 2018, 11, x FOR PEER REVIEW 3 of 10 2. Results and Discussion 2. Results and Discussion Figure2 shows the oxide thickness (d ox) of the Si substrates after exposure in air for different time Figure 2 shows the oxide thickness (dox) of the Si substrates after exposure in air for different periods. The thickness is measured by using the J. A. Woollam spectroscopic ellipsometer by fitting time periods. The thickness is measured by using the J. A. Woollam spectroscopic ellipsometer by the ellipsometric data in the wavelength range of 300–1200 nm under an angle of incidence of 75◦. fitting the ellipsometric data in the wavelength range of 300–1200 nm under an angle of incidence of When75°. the When substrate the substrate is immersed is immersed in the dilutein the dilute HF solution, HF solution, the nativethe native oxide oxide is easily is easily removed removed and and the Si surfacethe becomes Si surface the becomes hydrogen-terminated the hydrogen-terminated (H-terminated) (H-terminated) surface. surface. The wettability The wettability of the H-terminatedof the H- Si surfaceterminated is poor, Si andsurface it willis poor, lead and to ait bad will surface lead to coveragea bad surface when coverage the PEDOT:PSS when the PEDOT:PSS is deposited is later. Oncedeposited the Si substrate later. Once is exposedthe Si substrate in the is air, exposed the oxygen in the air, tends the tooxygen react tends with to the react Si andwith formthe Si theand SiOx. Afterform theSi the substrate SiOx. After is takenthe Si outsubstrate from is the taken HF solution,out from the we HF try solution, to measure we try the to SiO measurex thickness the SiO asx soon as possible.thickness However, as soon as it ispossible. inevitable However, that the it Siis substrateinevitable willthat comethe Si intosubstrate contact will with come the into air, contact and this is with the air, and this is why there is about 0.87 nm SiOx for the sample with the 0 min exposure, as why there is about 0.87 nm SiOx for the sample with the 0 min exposure, as shown in Figure2. For the shown in Figure 2. For the first 60 min of the Si substrate exposure in the air, the SiOx thickness first 60 min of the Si substrate exposure in the air, the SiOx thickness increases nearly linearly as the increases nearly linearly as the time progresses (see the inset in Figure 2). After 360 min, the SiOx time progresses (see the inset in Figure2). After 360 min, the SiO thickness tends to saturate at about thickness tends to saturate at about 3.1 nm. For better clarificationx in the following parts, we still 3.1 nm.name For the better sample clarification with 0 min in exposure the following in air parts,as the H-terminated we still name sample the sample and other with samples 0 min exposure as the in air asoxygen-terminated the H-terminated samples. sample and other samples as the oxygen-terminated samples.

3.5

3.0

2.5 2.0

2.0 1.5

1.0

1.5 (nm) Thickness Thickness (nm) Thickness 0.5 0 15 30 45 60 1.0 Times (mins)

0.5 0 300 600 900 1200 1500 Times (mins)

Figure 2. SiOx thickness for the Si substrates exposed in air for different times. Figure 2. SiOx thickness for the Si substrates exposed in air for different times.

Based on the formed SiOx layer, the planar hybrid n-Si/PEDOT:PSS solar cells are fabricated, and BasedFigure on3a shows the formed the current SiOx density–voltagelayer, the planar (J–V) hybrid characteristics n-Si/PEDOT:PSS of the devices solar under cells 100 are mW/cm fabricated,2 and Figureillumination3a shows (AM(air the current mass) density–voltage 1.5G), measured(J–V) from characteristics a solar simulator. of the The devices J–V characteristics under 100 mW/cm of 2 illuminationphotovoltaic (AM(air (PV) cells mass) can be 1.5G), approximately measured described from a by solar the Shockley simulator. equation: The J–V characteristics of photovoltaic (PV) cells can be approximately푞(푉 − described푅퐽) by the푉 − Shockley푅퐽 equation: 퐽 = 퐽 exp − 1 + − 퐽 (1) 푛푘 푇 푅 q(V − Rs J) V − RS J where J0 is the saturationJ =current,J0 exp Jph the photo current,− R1s the+ series resistance,− Jph Rsh the shunt resistance, (1) nkBT Rsh n the ideality factor, q the electron charge, kB the Boltzmann constant, and T the temperature. By using Equation (1) with our proposed explicit analytic expression method [19], the experimental data can where J0 is the saturation current, Jph the photo current, Rs the series resistance, Rsh the shunt resistance, be well rebuilt, as shown in Figure 3a, which confirmed the validity of the extracted parameters. By n the ideality factor, q the electron charge, kB the Boltzmann constant, and T the temperature. By using fitting the curves, the photovoltaic parameters of short-circuit current density (Jsc), open circuit voltage Equation (1) with our proposed explicit analytic expression method [19], the experimental data can (Voc), fill factor (FF), and PCE are extracted and summarized in Table 1. Figure 3b shows the variation be well rebuilt, as shown in Figure3a, which confirmed the validity of the extracted parameters. of device parameters with different exposure times. For the H-terminated device, a PCE of 10.44% is By fittingachieved the with curves, a Jsc theof 26.46 photovoltaic mA/cm2, a parametersVoc of 0.605 V, of and short-circuit an FF of 65.22%. current For densitythe device (J scwith), open 15 min circuit voltageexposure (Voc), in fill air factor where (FF), the andSiOx PCEthickness are extractedis 1.12 nm, and all the summarized device parameters in Table of1 .J Figuresc, Voc, and3b showsFF are the variationimproved of device and parameters the optimized with device different performance exposure is times. achieved. For The the H-terminated optimized device device, shows a PCE a of 2 2 10.44%remarkable is achieved Jsc of with 33.72 a mA/cm Jsc of 26.46, Voc of mA/cm 0.607 V,, and a V ocFF ofof 0.60565.01%, V, which and an improves FF of 65.22%. the overall For PCE the to device with 15 min exposure in air where the SiOx thickness is 1.12 nm, all the device parameters of Jsc,Voc, and FF are improved and the optimized device performance is achieved. The optimized device shows Energies 2018, 11, 1397 4 of 10

2 a remarkable Jsc of 33.72 mA/cm ,Voc of 0.607 V, and FF of 65.01%, which improves the overall PCE Energies 2018, 11, x FOR PEER REVIEW 4 of 10 to 13.31%. For a much longer exposure time, the device performance begins to decrease. At 60 min 2 exposure13.31%. where For the a muchSiOx thickness longer exposure is 1.73 time, nm, the the device device performance only shows begins a Jsc to of decrease. 26.45 mA/cm At 60 min,V oc of 2 0.594 V,exposure and FF ofwhere 59.58%, the SiO resultingx thickness in is a 1.73 PCE nm, of the 9.36%. device It only is guessed shows a thatJsc of the26.45 SiO mA/cmx can, helpVoc of enhance0.594 the V, and FF of 59.58%, resulting in a PCE of 9.36%. It is guessed that the SiOx can help enhance the uniformity of PEDOT:PSS, increase the built-in voltage (Vbi), and suppress recombination between uniformity of PEDOT:PSS, increase the built-in voltage (Vbi), and suppress recombination between the the Si and PEDOT:PSS interface with a proper thickness, and thus improve the device performance. Si and PEDOT:PSS interface with a proper thickness, and thus improve the device performance. All the All the possiblepossible reasons reasons will will be discussed be discussed in the infollowing the following part. part.

20 )

2 0 min 15 min 30 min 0 45 min 60 min Fitting Curve

-20

Current Density (mA/cm Density Current -40 -0.2 0.0 0.2 0.4 0.6 Voltage (V)

(a)

36 0.620

34 0.615 ) 2 32 0.610 0.605 30 (V)

0.600 oc 28 V (mA/cm 0.595 sc 26 J 0.590 7224 14 68 13

64 12

60 11 FF(%) PCE(%) 10 56 3.2 9 3.0

2.8 2.5 ) 2.0 2 2.4 1.5 n

1.0 (Ω·cm

2.0 s

0.5 R

1.6 0.0 0 10 20 30 40 50 60 Exposure time (min)

(b)

Figure 3. (a) J–V curves of the planar hybrid Si/PEDOT:PSS solar cells with different exposure times Figure 3. (a) J–V curves of the planar hybrid Si/PEDOT:PSS solar cells with different exposure under 100 mW/cm2 illumination (AM 1.5 G). (b) Variation of device parameters with different 2 times underexposure 100 times. mW/cm illumination (AM 1.5 G). (b) Variation of device parameters with different exposure times. Table 1. Extracted parameters of the planar hybrid Si/PEDOT:PSS solar cells at a temperature of 27 °C. ◦ Table 1. Extracted parametersTime of Jsc the planar Voc hybrid FF Si/PEDOT:PSS PCE solarRs cells at R ash temperature of 27 C. n (min) (mA/cm2) V % % Ω·cm2 kΩ·cm2 Time0 J sc 26.46V oc 0.605 FF 65.22 10.44PCE 1.99 3.9 R 0.37s Rsh 2 n 2 2 (min) (mA/cm15 ) 33.72V%% 0.607 65.01 13.31 1.79 4.5 Ω·cm 0.17 kΩ·cm 0 26.4630 28.97 0.605 0.599 65.22 64.42 10.44 11.19 2.26 1.99 3.2 3.9 0.36 0.37 15 33.72 0.607 65.01 13.31 1.79 4.5 0.17 30 28.97 0.599 64.42 11.19 2.26 3.2 0.36 45 28.784 0.596 60.17 10.33 2.74 5.9 0.32 60 26.45 0.594 59.58 9.36 2.8 27.3 0.48 Energies 2018, 11, x FOR PEER REVIEW 5 of 10

Energies 2018, 11, 1397 45 28.784 0.596 60.17 10.33 2.74 5.9 0.32 5 of 10 60 26.45 0.594 59.58 9.36 2.8 27.3 0.48

InIn order to to investigate investigate the the reason reason for for the the better better performance performance of theof the optimized optimized device, device, the thecapacitance–voltage capacitance–voltage (C–V) (C–V) characteristics characteristics of the of devices the devices are measured are measured by an by Agilent an Agilent 1500 1500A at Aa atfrequency a frequency of 1 ofMHz. 1 MHz. Figure Figure 4 shows4 shows the the 1/C 1/C2–V 2plot–V plotof the of H-terminated the H-terminated and andoxygen-terminated oxygen-terminated (15 (15min min and and 60 60min) min) hybrid hybrid solar solar cells, cells, and and there there is is a alinear linear relationship relationship between 1/C1/C22 andand the applied voltage. The The intercept intercept of of the the line line and and the the x-axisx-axis corresponds corresponds to V tobi V[20].bi [ 20All]. the All devices the devices show show the same the sameVbi ofV aroundbi of around 0.8 V; 0.8 this V; thismeans means that that there there is no is nodifference difference of ofthe the built-in built-in voltage voltage for for the the different exposure times, which which can can be be partially partially verified veriﬁed by by observing observing almost almost the the same same Voc V aroundoc around 0.60 0.60 V, as V, asshown shown in inTable Table 1.1 .Since Since the the different different exposure exposure times times cannot change the VVbibi, ,there there should should be otherother reasons forfor theirtheir differentdifferent performances,performances, whichwhich willwill bebe investigatedinvestigated inin thethe followingfollowing sections.sections.

1.0

0.8 0 min exposure

) 0.6 15 mins exposure -2

F 60 mins exposure 14

0.4 (×10 -2 p C

0.2

0.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage(V)

Figure 4.4. C–V measurements ofof the planarplanar Si/PEDOT:PSSSi/PEDOT:PSS solar cells with different exposure times.

We measure the contact angles of PEDOT:PSS aqueous solution drops on the H-terminated and We measure the contact angles of PEDOT:PSS aqueous solution drops on the H-terminated and oxygen-terminated substrates to investigate the wettability of the samples. As shown in Figure 5, oxygen-terminated substrates to investigate the wettability of the samples. As shown in Figure5, although the PEDOT:PSS solution is mixed with Triton X-100 as a surfactant to help the film although the PEDOT:PSS solution is mixed with Triton X-100 as a surfactant to help the ﬁlm formation, the fresh Si substrate (with 0 min exposure in air) still shows a large contact angle of 68.56°. formation, the fresh Si substrate (with 0 min exposure in air) still shows a large contact angle of With oxygen-terminated bonds instead of the H-terminated bonds, the contact angle begins to 68.56◦. With oxygen-terminated bonds instead of the H-terminated bonds, the contact angle begins to decrease, as indicated in Table 2. When the exposure time is 60 min, the contact angle is only 27.37°. decrease, as indicated in Table2. When the exposure time is 60 min, the contact angle is only 27.37 ◦. This shows that a longer exposure time will produce thicker SiOx, and then the more H-terminated This shows that a longer exposure time will produce thicker SiO , and then the more H-terminated bonds will be replaced by the oxygen-terminated bonds. Thus, xthe wettability of the Si surface is bonds will be replaced by the oxygen-terminated bonds. Thus, the wettability of the Si surface is improved. The remarkably increased wettability of the Si surface after the exposure in air promotes improved. The remarkably increased wettability of the Si surface after the exposure in air promotes easy spreading of the PEDOT:PSS solution, allowing for uniform covering of the surfaces. As a result, easy spreading of the PEDOT:PSS solution, allowing for uniform covering of the surfaces. As a result, the number of surface defects in PEDOT:PSS, such as microvoids, which are easily formed during the the number of surface defects in PEDOT:PSS, such as microvoids, which are easily formed during spin-coating process and give rise to carrier recombination, are significantly reduced. Although a the spin-coating process and give rise to carrier recombination, are signiﬁcantly reduced. Although a longer exposition time shows a better wettability, it is should be noted that the main operation longer exposition time shows a better wettability, it is should be noted that the main operation principle principle of our hybrid device is the tunneling of holes through the interface oxide layer. A thicker of our hybrid device is the tunneling of holes through the interface oxide layer. A thicker SiOx normally SiOx normally prevents a tunneling process, and eventually holes are likely to be recombined with prevents a tunneling process, and eventually holes are likely to be recombined with electrons. A thicker electrons. A thicker SiOx leads to a high series resistance because of its electrically insulating nature, SiO leads to a high series resistance because of its electrically insulating nature, which induces a whichx induces a decreased FF, as shown in Table 1. This could partially explain why we could obtain decreased FF, as shown in Table1. This could partially explain why we could obtain the highest PCE the highest PCE with high values of Jsc and FF when the interlayer had a thickness of 1.12 nm, even with high values of J and FF when the interlayer had a thickness of 1.12 nm, even though this sample though this sample scdoes not show the best wettability. From this point of view, it seems that the does not show the best wettability. From this point of view, it seems that the wettability change may wettability change may be one reason for their different performance. be one reason for their different performance.

Energies 2018, 11, 1397 6 of 10

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87.86° 27.37°

(a) (b)

Figure 5. Contact angle images of PEDOT:PSS aqueous solution drops on the (a) H-terminated and Figure 5. Contact(b) SiOx-terminated angle images silicon of substrates. PEDOT:PSS aqueous solution drops on the (a) H-terminated and (b) SiOx-terminated silicon substrates.87.86° 27.37° Table 2. Contact angles of PEDOT:PSS drops on the H-terminated and oxygen-terminated silicon substrates. Table 2. Contact angles(a) of PEDOT:PSS drops on the H-terminated (b) and oxygen-terminated silicon substrates.Time (min) 0 15 30 45 60 Figure 5. Contactθ angle(°) images87.86 of PEDOT:PSS 68.56 aqueous solution 48.51 drops 39.80 on the (a) H-terminated 27.37 and (b) SiOx-terminated silicon substrates. Time (min) 0 15 30 45 60 Table 3. RMS values of the planar n-Si/PEDOT:PSS films with different exposure times. Table 2. Contactθ (angles◦) of PEDOT:PSS87.86 drops 68.56 on the H-terminated 48.51 and 39.80 oxygen-terminated 27.37 silicon substrates. Time (min) 0 15 30 45 60 RMS (nm) 2.64 2.57 2.57 2.55 2.29 Time (min) 0 15 30 45 60 TableThe 3. atomicRMSθ (°) valuesforce microscope of the87.86 planar (AFM) n-Si/PEDOT:PSS images 68.56 of PEDOT:PSS 48.51 ﬁlms layers with 39.80 coated different on the exposure 27.37 Si substrates times. are shown in Figure 6. For all the films, the surfaces are relatively uniform with small root-mean-square (RMS) valuesTableTime 3.between RMS (min) values 2.29 nmof the and 0 planar 2.64 nm,n-Si/PEDOT:PSS15 although the 30films wettability with different 45of the samplesexposure 60 are times. different, as shown in Table 3. The samples exposed in air for 15 min, 30 min, 45 min, and 60 min after being HF- TimeRMS (min) (nm) 2.640 2.5715 2.5730 2.5545 2.2960 etched only present slightly smaller RMS values than those of samples exposed in air for 0 min. Usually, theRMS smooth (nm) surface would2.64 deliver fewer 2.57 defects 2.57between the Si 2.55 substrate and 2.29 the PEDOT:PSS The atomicfilm to improve force microscope the efficiency(AFM) of photogenerated images of carrier PEDOT:PSS transportation. layers However, coated this on should the Si not substrates be are theThe main atomic reason force for microscope the difference (AFM) in device images performance, of PEDOT:PSS since layers the differences coated on of the their Si substratesRMS values are shown inshown Figureare relativelyin 6Figure. For small. all6. For the all films, the films, the surfaces the surfaces are are relatively relatively uniform uniform with with small small root-mean-squareroot-mean-square (RMS) values between(RMS) values 2.29 between nm and 2.29 2.64 nm nm, andalthough 2.64 nm, although the wettability the wettability of the of samples the samples are different,are different, as as shown in Table3.shown The samples in Table exposed3. The samples in air exposed for 15 min,in air 30for min,15 min, 45 30 min, min, and 45 min, 60 minand 60 after min being after being HF-etched HF- only presentetched slightly only smaller present RMS slightly values smaller than RMS those values of samples than those exposed of samples in air exposed for 0 min. in air Usually, for 0 min. the smooth Usually, the smooth surface would deliver fewer defects between the Si substrate and the PEDOT:PSS surface would deliver fewer defects between the Si substrate and the PEDOT:PSS film to improve the film to improve the efficiency of photogenerated carrier transportation. However, this should not be efficiencythe ofmain photogenerated reason for the difference carrier transportation. in device performance, However, since this the should differences not beof their themain RMS values reason for the differenceare inrelatively device small. performance, since the differences of their RMS values are relatively small.

（a）（b）（c）

（a）（b）（c）（d）（e）

（d）（e）

Figure 6. AFM images of PEDOT:PSS ﬁlms with different exposure times of (a) 0 min; (b) 15 min; (c) 30 min; (d) 45 min; and (e) 60 min. Energies 2018, 11, x FOR PEER REVIEW 7 of 10

EnergiesFigure2018, 116. ,AFM 1397 images of PEDOT:PSS films with different exposure times of (a) 0 min; (b) 15 min; (c)7 of 10 30 min; (d) 45 min; and (e) 60 min.

We also investigate investigate the the optical optical properties properties of of n-Si/PEDOT:PSS n-Si/PEDOT:PSS structures by measuring the reflectancereflectance spectra. Figure Figure 7 7 compares compares thethe reflectance reflectance of of polished polished bare bare n-Si n-Si and and n-Si/PEDOT:PSS n-Si/PEDOT:PSS samples with with exposure exposure to to air air for for different different time time periods periods from from 0 to 60 0 min. to 60 For min. all the For samples, all the samples,with the withaddition the addition of the PEDOT:PSS of the PEDOT:PSS film, the film, reflectance the reflectance is greatly is greatly decreased, decreased, which which shows shows that that the PEDOT:PSSthe PEDOT:PSS film film has has the thefunction function of ofbeing being an an antireflection antireflection layer. layer. Compared Compared to to the the samples samples with different exposureexposure times,times, the the reflectance reflectance is is lowest lowest for for the the H-terminated H-terminated and and oxygen-terminated oxygen-terminated (15 min) (15 min)devices. devices. The two The samples two samples show almost show similar almost optical similar properties, optical properties, and this means and this that means the absorption that the absorptiondifference is difference not the main is not reason the main for their reason difference for their in difference performance. in performance.

0min 60 15min 30min 50 45min 60min 40 bare-Si

20 Reflectance (%) Reflectance 10

0 400 600 800 1000 1200 Wavelength (nm)

Figure 7. ReflectionReﬂection spectra of n-Si/PEDOT:PSS samples samples exposed exposed to to air air for for different different time time periods from 0 to 60 min; the reflection reﬂection spectrum of the polished bare n-Sin-Si isis alsoalso shownshown forfor aa reference.reference.

According to the above discussion, there are no obvious differences of Vbi, surface morphologies, According to the above discussion, there are no obvious differences of V , surface morphologies, and optical properties for the samples with different exposure times. The wettabilitybi change may be and optical properties for the samples with different exposure times. The wettability change may be one reason for their different performance. However, the different wettability does not lead to the one reason for their different performance. However, the different wettability does not lead to the obviously different film morphologies of PEDOT:PSS, since the differences of their RMS values are obviously different film morphologies of PEDOT:PSS, since the differences of their RMS values are relatively small. We believe that beyond the difference in wettability, there are other reasons for the relatively small. We believe that beyond the difference in wettability, there are other reasons for the different device performances. In order to interpret the performance differences for the H-terminated different device performances. In order to interpret the performance differences for the H-terminated and oxygen-terminated (15 min) devices, the possible mechanism is proposed here. Figure 8a,b shows and oxygen-terminated (15 min) devices, the possible mechanism is proposed here. Figure8a,b the energy band diagram of the H-Si/PEDOT:PSS and oxygen-Si/PEDOT:PSS heterojunctions, shows the energy band diagram of the H-Si/PEDOT:PSS and oxygen-Si/PEDOT:PSS heterojunctions, respectively. The electron affinity of bulk Si as shown is 4.05 eV [21]. Surface properties can either respectively. The electron affinity of bulk Si as shown is 4.05 eV [21]. Surface properties can either increase or decrease the effective electron affinity (Xeff) at the Si surface relative to that in the bulk Si, increase or decrease the effective electron affinity (X ) at the Si surface relative to that in the bulk Si, depending on the polarity of the associated dipole eff[22]. It is reported that the H-Si surface would depending on the polarity of the associated dipole [22]. It is reported that the H-Si surface would pose pose a net negative surface dipole upon the formation of the covalent H-Si surface bonds [21]. This a net negative surface dipole upon the formation of the covalent H-Si surface bonds [21]. This leads leads to an increase in Xeff at the Si/PEDOT:PSS surface and thus a bending down of the Si energy to an increase in X at the Si/PEDOT:PSS surface and thus a bending down of the Si energy band band near the Si/PEDOT:PSSeff interface, as shown in Figure 8a. As a result, a blocking internal electrical near the Si/PEDOT:PSS interface, as shown in Figure8a. As a result, a blocking internal electrical field field exists and prevents the photoexcited holes in the Si from injecting into PEDOT:PSS, resulting in exists and prevents the photoexcited holes in the Si from injecting into PEDOT:PSS, resulting in higher higher carrier recombination at the interface. However, the oxygen-terminated Si surface is reported carrier recombination at the interface. However, the oxygen-terminated Si surface is reported to pose a to pose a net positive surface dipole, and this will lead to the drop of Xeff. Accordingly, the Si energy net positive surface dipole, and this will lead to the drop of X . Accordingly, the Si energy band will band will bend up and give rise to a favorable band alignmenteff between oxygen-Si and PEDOT:PSS bend up and give rise to a favorable band alignment between oxygen-Si and PEDOT:PSS to promote to promote carrier separation. Moreover, the SiOx layer can passivate the Si surface and help in carrier separation. Moreover, the SiOx layer can passivate the Si surface and help in suppressing the suppressing the Si surface recombination. This could be partially verified by the increased Jsc, as Si surface recombination. This could be partially verified by the increased Jsc, as shown in Figure3. shown in Figure 3. However, a thicker SiOx layer would become a barrier for charge transport in the However, a thicker SiOx layer would become a barrier for charge transport in the cells, leading to a cells, leading to a higher series resistance. This is why there is an optimized SiOx thickness. higher series resistance. This is why there is an optimized SiOx thickness.

Energies 2018, 11, 1397 8 of 10 Energies 2018, 11, x FOR PEER REVIEW 8 of 10

Ec=4.05eV

HOMO=5.0eV

Ev=5.17eV X H-terminated Si PEDOT:PSS

（a）

Ec=4.05eV

HOMO=5.0eV

Ev=5.17eV

SiOx-Si PEDOT:PSS

（b）

FigureFigure 8. The 8. energyThe energy band diagram band diagram of the (a of) H-Si/PEDOT:PSS the (a) H-Si/PEDOT:PSS and (b) SiO andx-Si/PEDOT:PSS (b) SiOx-Si/PEDOT:PSS hybrid heterojunctions.hybrid heterojunctions.

3. Materials3. Materials and and Methods Methods

3.1. 3.1.Film Film Formation Formation and andDevice Device Fabrication Fabrication TheThe devices devices were were fabricated fabricated according according to the to thesequences sequences below: below: The The n-type n-type (100) (100) single-sided single-sided polishedpolished silicon silicon wafers wafers (resistivity: (resistivity: 0.05–0.1 0.05–0.1 Ω·cm)Ω with·cm) thickness with thickness of 300 of± 10 300 μm± were10 µ mcut were into cut15 × into 15 mm15 × squares15 mm as squares the substrates. as the substrates. The Si substrates The Si were substrates ultrasonically were ultrasonically cleaned in acetone, cleaned alcohol, in acetone, andalcohol, ionized and water ionized sequentially water sequentially for 15 min. for The 15 min. native The oxide native on oxide the substrates on the substrates were etched were etched by immersingby immersing the Si substrates the Si substrates in a dilute in aHF dilute (5%) HF solution (5%) solutionfor 30 s and for 30dried s and by driedN2 for by later N2 use.for laterAfter use. the Afterremoval the removalof the native of the oxide native layer, oxide there layer, were there H-terminated were H-terminated bonds bonds on the on Si the surface. Si surface. A highly A highly conductiveconductive PEDOT:PSS PEDOT:PSS solution solution (PH1000, (PH1000, Heraeus Clevios) Clevios) was was filtered filtered with with a polyvinylidene a polyvinylidene fluoride fluoridemembrane membrane (0.45 µ(0.45m porosity) μm porosity) to remove to remove agglomerations. agglomerations. 7% ethylene 7% ethylene glycol andglycol 1% and Triton 1% X-100 Triton were X-100added were to added the PEDOT:PSS to the PEDOT:PSS solution solution to improve to improve the wettability the wettability and conductivity. and conductivity. All the All Si substrates the Si substrateswere divided were divided into five into groups five and groups exposed and toexposed air for theto air different for the time different periods time of 0periods min, 15 of min, 0 min, 30 min, 15 min,45 min, 30 min, or 60 45 min, min, respectively. or 60 min, respectively. The PEDOT:PSS The PEDOT:PSS solution was solution then spin-coated was then spin-coated on the H-terminated on the H-terminated(0 min) and (0 SiOmin)x-terminated and SiOx-terminated (exposed to(exposed air for 15to min,air for 30 15 min, min, 45 30 min, min, or 45 60 min, min, or respectively) 60 min, respectively)substrates substrates at a speed at of a 2500 speed rpm of for2500 60 rpm s. After for 60 the s. PEDOT:PSS After the PEDOT:PSS deposition, deposition, the samples the were samples annealed wereon annealed a hot plate on ata 190hot ◦plateC for at 15 190 min °C to for remove 15 min the to solvent remove to formthe solvent a uniform to form and conductivea uniform and p-type conductiveorganic thinp-type film. organic A 150-nm thin film. thick A Ag 150-nm film was thick thermally Ag film evaporated was thermally to form evaporated a grid as to the form front a grid contact. as theFinally, front thecontact. eutectic Finally, In–Ga the alloy eutectic was usedIn–Ga to alloy fully was cover used the to rear fully side cover of the the Si rear substrate side of to the form Si an substrateohmic to contact. form an ohmic contact.

3.2. Device Characterization The morphology measurements of the PEDOT:PSS films were measured by atomic force microscopy (AFM, Agilent 5500). The UV–visible reflection spectra were recorded with a UV–visible reflection spectrophotometer (Perkin-Elmer Lambda 950). The thickness of the SiOx layer was

Energies 2018, 11, 1397 9 of 10

3.2. Device Characterization The morphology measurements of the PEDOT:PSS films were measured by atomic force microscopy (AFM, Agilent 5500). The UV–visible reflection spectra were recorded with a UV–visible reflection spectrophotometer (Perkin-Elmer Lambda 950). The thickness of the SiOx layer was measured by spectroscopic ellipsometry (Jawoolam M-2000). Photovoltaic parameters were measured using a Keithley 2400 source meter under simulated sunlight from a XES-70S1 solar simulator matching the AM 1.5G standard with an intensity of 100 mW/cm2. The system was calibrated against a National Renewable Energy Laboratory (NREL)-certified reference solar cell. The capacitance–voltage characteristics were measured by an Agilent 1400 semiconductor parameter analyzer. All the measurements of the solar cells were performed under an ambient atmosphere at room temperature without any encapsulation.

4. Conclusions In summary, we demonstrated that the exposure-oxidation treatment of an H-terminated Si substrate could enhance the performance of planar hybrid Si/PEDOT:PSS solar cells. After the oxidation, the Si surface wettability could be improved, which is ascribed to the hydrophilicity of the SiOx layer, allowing an effective spread of the PEDOT:PSS and thus a good contact between the PEDOT:PSS ﬁlm and Si. More importantly, it can change the polarity of the Si surface from a negative dipole to a positive dipole, owing to the formation of covalent oxygen–Si bonds. The Si energy band bends up and gives rise to a favorable band alignment between oxygen-terminated Si and PEDOT:PSS to promote carrier separation. These results could be potentially employed to further the development of this simple, low-cost heterojunction solar cell.

Author Contributions: C.Z. (Chunfu Zhang) and Y.Z. conceived the idea and guided the experiment; C.Z. (Chenxu Zhang) conducted most of the device fabrication and data collection; C.Z. (Chunfu Zhang) revised the manuscript; H.G., Q.J., and P.D. helped with the device measurements. All authors read and approved the manuscript. Acknowledgments: We thank the Natural Science Foundation of China (61604119), Fundamental Research Funds for the Central Universities (Grant No. JBX171103, JB161101, JB161102), Class General Financial Grant from the China Postdoctoral Science Foundation (Grant No. 2016M602771), the Funds for University from Department of Education of Guangxi (KY2015YB109), and the Funds for the Key Laboratory of Automatic Measurement Technology and Instruments (YQ16103). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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