# Development of Thin Film Amorphous Silicon Tandem Junction Based Photocathodes Providing High Open-Circuit Voltages for Hydrogen Production

Authors: F. Urbain; K. Wilken; V. Smirnov; O. Astakhov; A. Lambertz; J.-P. Becker; U. Rau; J. Ziegler; B. Kaiser; W. Jaegermann; F. Finger
Journal: International Journal of Photoenergy
 (2014)

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## Abstract

Hydrogenated amorphous silicon thin film tandem solar cells (a-Si:H/a-Si:H) have been developed with focus on high open-circuit voltages for the direct application as photocathodes in photoelectrochemical water splitting devices. By temperature variation during deposition of the intrinsic a-Si:H absorber layers the band gap energy of a-Si:H absorber layers, correlating with the hydrogen content of the material, can be adjusted and combined in a way that a-Si:H/a-Si:H tandem solar cells provide open-circuit voltages up to 1.87 V. The applicability of the tandem solar cells as photocathodes was investigated in a photoelectrochemical cell (PEC) measurement set-up. With platinum as a catalyst, the a-Si:H/a-Si:H based photocathodes exhibit a high photocurrent onset potential of 1.76 V versus the reversible hydrogen electrode (RHE) and a photocurrent of 5.3 mA/cm2 at 0 V versus RHE (under halogen lamp illumination). Our results provide evidence that a direct application of thin film silicon based photocathodes fulfills the main thermodynamic requirements to generate hydrogen. Furthermore, the presented approach may provide an efficient and low-cost route to solar hydrogen production.


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## Body

## 1.Introduction

Solar based hydrogen production via photoelectrochemical water splitting provides an alluring route to convert solar energy into a storable and clean chemical fuel and, therefore, increasingly plays a decisive role in sustainable energy concepts [1, 2]. Among the current variety of photoelectrode absorber materials for integrated water splitting devices, including III—V semiconductors [3–6], metal oxides [7–9], or crystalline silicon [10–12], amorphous silicon thin films emerge as a novel promising candidate [9, 13–16]. Overall, thin film silicon technology stands out as an attractive choice for water splitting applications, because it combines low-cost production, earth abundance, and versatility [17].Amorphous silicon tandem solar cells have been intensively developed in the past, with major focus on high conversion efficiencies [18]. In our approach here, however, the focus is on the development of high V
OC devices, suitable for water splitting applications. Therefore, we will describe a development route to achieve high voltage devices and emphasize which particular parameter regimes are optimal in order to achieve high open-circuit voltages in a-Si:H/a-Si:H tandem junction solar cells. Additionally it will be shown that the parameters can be systematically adjusted to tune the V
OC values in the range between 1.60 V and 1.87 V, which is relevant if one considers various PEC systems with different overpotential losses, that is, when different voltage requirements are implied. The major part of the reported a-Si:H tandem solar cells for water splitting applications was merely designed to provide additional bias to a photoactive metal oxide electrode [9, 14, 16]. In contrast, this study reports on the direct application of the a-Si:H tandem solar cells as all-silicon photoelectrodes for driving the water splitting reactions.Regarding solar assisted water electrolysis and the concomitant requirement of electrochemical potentials, that is, photovoltages above 1.23 V (disregarding overpotentials) [19–21], photocathodes providing high photocurrent at high voltages are desirable. Concerning this, great importance is given to the versatility of the a-Si:H material. By changing deposition process parameters, the electronic and optical properties of the a-Si:H absorber material can be adjusted, such that a wide range of high open-circuit voltages is provided in a-Si:H based single junction solar cells, without significantly impairing current densities [22, 23]. In comparison, other photocathode material systems listed above often suffer from rather low photovoltages and therefore limit the solar to hydrogen efficiency. The open-circuit voltage of a properly designed solar cell mainly depends on the band gap of the absorber layer. Thus, attempts for the variation in band gap energy of intrinsic a-Si:H absorber layers were made in this study. In real photoelectrochemical assemblies, the potential losses inevitably exceed 200 mV, depending on the current densities and catalysts used [24, 25]. Thus, for self-contained solar water splitting devices photovoltages of at least 1.43 V are needed. In this manner, a-Si:H based solar cells are very beneficial, because the electronic properties of individual a-Si:H absorber layers can be combined by connecting two single junction cells (subcells) in series. In this configuration a-Si:H/a-Si:H tandem junction solar cells promote a further extended range of achievable high voltages [26, 27]. Thus, they may satisfy the specific thermodynamic requirements of different photoelectrochemical systems.Based on (photo-)electrochemical experiments, the performance and stability of the developed a-Si:H/a-Si:H tandem junction based photocathodes were evaluated with respect to the hydrogen evolution reaction (HER). Moreover, to fully explore the performance of the photocathode, platinum was evaporated as a catalyst.


## 2.Experimental Details

### 2.1. Preparation of a-Si:H Layers and Solar Cells

All a-Si:H layers were deposited by a plasma enhanced chemical vapor deposition (PECVD) technique in a multichamber system. For the intrinsic a-Si:H absorber layers a mixture of silane (SiH4) and hydrogen (H2) gases was used. For the n- and p-type layers, trimethylborane (TMB), methane (CH4), and phosphine (PH3) gases were added to the silane-hydrogen mixture. An excitation frequency of 13.56 MHz was applied for all depositions. The a-Si:H material optimization was mainly performed as a function of the substrate temperature T
s and silane concentration SC, defined as the ratio between the SiH4 flow and the total gas flow. Single junction and tandem junction solar cells were made in a p-i-n and in a p-i-n-p-i-n superstrate configuration, respectively, with a sputtered zinc oxide/silver (ZnO/Ag) reflecting back contact [28] defining the area (1 cm2) of the individual cells. The thickness of the intrinsic a-Si:H layer in single junction solar cells was kept constant at 400 nm. In the standard tandem device, the bottom cell thickness was 400 nm and the top cell thickness was 110 nm. The p- and n-doped layers are the same for all fabricated solar cells and are deposited at a substrate temperature of 180°C. Since T
s is varied for the deposition of the intrinsic a-Si:H layer, a waiting time of 30 minutes between the layer deposition was implemented in order to maintain a required temperature of the substrate. All solar cells were deposited on 100 cm2 fluorine-doped tin oxide (SnO2:F) coated glass substrates with a native texture (Asahi U). Figure 1 schematically shows the tandem junction solar cell configuration used in the photovoltaic arrangement. For the investigation of the intrinsic a-Si:H layers alone, glass substrate (type Corning Eagle XG) and crystalline silicon wafers were used as substrates and all respective single a-Si:H layers had a thickness of approximately 500 nm. For the photoelectrochemical arrangement, 50 nm of platinum was deposited on the ZnO/Ag back contact by electron beam evaporation.Figure 1
Schematic drawing of the layer stack used for the deposition of a-Si:H/a-Si:H tandem solar cells.


### 2.2. Characterization of Materials and Solar Cells

Infrared (IR) absorption measurements were carried out, using a Fourier transform spectrometer (type Nicolet 5700), to evaluate the hydrogen contentc
H in the intrinsic a-Si:H layer. IR transmission was measured between 400 and 2400 cm−1. This spectrum was normalized to the spectrum of the crystalline silicon wafer substrate.Photothermal deflection spectroscopy (PDS) was performed to measure the absorption spectra of the intrinsic a-Si:H layers, which were deposited on glass substrates. From these spectra the gap energyE
04 with a PDS absorption coefficient of 104 cm−1 was extracted, which serves as a measure for the optical band gap of the intrinsic a-Si:H layer.The constant photocurrent method (CPM) was used to evaluate the absorption within the band gap (e.g., subgap absorption), which can be linked to the defect density of the a-Si:H material. The corresponding defect absorption densities of the a-Si:H intrinsic layers were determined by using the absorption coefficientα
1
.
2 at an energy of 1.2 eV [29].Solar cells were characterized by current-voltage (J
-
V) measurements under AM 1.5 illumination using a double source (Class A) sun simulator and by quantum efficiency (QE) measurements.


### 2.3. Photoelectrochemical Measurements

The photoelectrochemical performance of the electrodes was evaluated at room temperature in an aqueous 0.1 M sulfuric acid (H2SO4) electrolyte solution (pH 1) using a three-electrode configuration [30]. The potential of the working electrode was controlled by a potentiostat (μAutolab Type III). For illumination, a 150 W xenon-halogen lamp was used, which was calibrated (by adjusting the current) with a crystalline silicon diode to provide a light intensity of 100 mW cm−2. The solar cells were illuminated through the glass substrate. The set-up of the electrochemical cell (type Zahner PECC-2) consists of a Teflon cell body and three electrodes: a working electrode (optimized a-Si:H/a-Si:H tandem junction solar cell, 8 mm diameter), a coiled platinum wire as a counter electrode, and a Ag/AgCl reference electrode being in contact with 3 M sodium chloride (NaCl) [0.268 V versus normal hydrogen electrode (NHE)]. For simplicity, the entire set-up will be denoted as photoelectrochemical cell (PEC) arrangement hereafter. Cyclic voltammetry measurements were performed with a scan rate of 30 mV s−1. Electrical contact to the SnO2 coated substrate, that is, the front contact of the solar cell, was made by a silver paste.


## 2.1. Preparation of a-Si:H Layers and Solar Cells

All a-Si:H layers were deposited by a plasma enhanced chemical vapor deposition (PECVD) technique in a multichamber system. For the intrinsic a-Si:H absorber layers a mixture of silane (SiH4) and hydrogen (H2) gases was used. For the n- and p-type layers, trimethylborane (TMB), methane (CH4), and phosphine (PH3) gases were added to the silane-hydrogen mixture. An excitation frequency of 13.56 MHz was applied for all depositions. The a-Si:H material optimization was mainly performed as a function of the substrate temperature T
s and silane concentration SC, defined as the ratio between the SiH4 flow and the total gas flow. Single junction and tandem junction solar cells were made in a p-i-n and in a p-i-n-p-i-n superstrate configuration, respectively, with a sputtered zinc oxide/silver (ZnO/Ag) reflecting back contact [28] defining the area (1 cm2) of the individual cells. The thickness of the intrinsic a-Si:H layer in single junction solar cells was kept constant at 400 nm. In the standard tandem device, the bottom cell thickness was 400 nm and the top cell thickness was 110 nm. The p- and n-doped layers are the same for all fabricated solar cells and are deposited at a substrate temperature of 180°C. Since T
s is varied for the deposition of the intrinsic a-Si:H layer, a waiting time of 30 minutes between the layer deposition was implemented in order to maintain a required temperature of the substrate. All solar cells were deposited on 100 cm2 fluorine-doped tin oxide (SnO2:F) coated glass substrates with a native texture (Asahi U). Figure 1 schematically shows the tandem junction solar cell configuration used in the photovoltaic arrangement. For the investigation of the intrinsic a-Si:H layers alone, glass substrate (type Corning Eagle XG) and crystalline silicon wafers were used as substrates and all respective single a-Si:H layers had a thickness of approximately 500 nm. For the photoelectrochemical arrangement, 50 nm of platinum was deposited on the ZnO/Ag back contact by electron beam evaporation.Figure 1
Schematic drawing of the layer stack used for the deposition of a-Si:H/a-Si:H tandem solar cells.


## 2.2. Characterization of Materials and Solar Cells

Infrared (IR) absorption measurements were carried out, using a Fourier transform spectrometer (type Nicolet 5700), to evaluate the hydrogen contentc
H in the intrinsic a-Si:H layer. IR transmission was measured between 400 and 2400 cm−1. This spectrum was normalized to the spectrum of the crystalline silicon wafer substrate.Photothermal deflection spectroscopy (PDS) was performed to measure the absorption spectra of the intrinsic a-Si:H layers, which were deposited on glass substrates. From these spectra the gap energyE
04 with a PDS absorption coefficient of 104 cm−1 was extracted, which serves as a measure for the optical band gap of the intrinsic a-Si:H layer.The constant photocurrent method (CPM) was used to evaluate the absorption within the band gap (e.g., subgap absorption), which can be linked to the defect density of the a-Si:H material. The corresponding defect absorption densities of the a-Si:H intrinsic layers were determined by using the absorption coefficientα
1
.
2 at an energy of 1.2 eV [29].Solar cells were characterized by current-voltage (J
-
V) measurements under AM 1.5 illumination using a double source (Class A) sun simulator and by quantum efficiency (QE) measurements.


## 2.3. Photoelectrochemical Measurements

The photoelectrochemical performance of the electrodes was evaluated at room temperature in an aqueous 0.1 M sulfuric acid (H2SO4) electrolyte solution (pH 1) using a three-electrode configuration [30]. The potential of the working electrode was controlled by a potentiostat (μAutolab Type III). For illumination, a 150 W xenon-halogen lamp was used, which was calibrated (by adjusting the current) with a crystalline silicon diode to provide a light intensity of 100 mW cm−2. The solar cells were illuminated through the glass substrate. The set-up of the electrochemical cell (type Zahner PECC-2) consists of a Teflon cell body and three electrodes: a working electrode (optimized a-Si:H/a-Si:H tandem junction solar cell, 8 mm diameter), a coiled platinum wire as a counter electrode, and a Ag/AgCl reference electrode being in contact with 3 M sodium chloride (NaCl) [0.268 V versus normal hydrogen electrode (NHE)]. For simplicity, the entire set-up will be denoted as photoelectrochemical cell (PEC) arrangement hereafter. Cyclic voltammetry measurements were performed with a scan rate of 30 mV s−1. Electrical contact to the SnO2 coated substrate, that is, the front contact of the solar cell, was made by a silver paste.


## 3. Results and Discussion

### 3.1. Material Properties of Intrinsic a-Si:H Absorber Layer

Since the band gap of amorphous a-Si:H correlates with the hydrogen content of the material [22], FTIR measurements were conducted to estimate the amount of hydrogen in the samples. Figure 2 shows the hydrogen content as a function of the substrate temperature for a-Si:H layers with different silane concentrations. The general trend indicates that a decrease in T
s results in an increase of c
H. This behavior can be attributed to the increased desorption of hydrogen at higher substrate temperatures. As deposition temperature increases, the desorption of hydrogen atoms is energetically favored and thus less hydrogen is incorporated into the layer [31]. No clear dependency of c
H on the silane concentration in the chosen range was visible in FTIR data. The hydrogen content in all samples investigated is between 10.6% and 16.8%.Figure 2
Hydrogen content as a function of the substrate temperature during deposition of the a-Si:H layers, with different silane concentrations.For the determination of the band gap energy of the a-Si:H material, absorption spectra for differentT
s and SC are measured and subsequently evaluated, with respect to E
04 as an estimation of the band gap. The evaluated E
04 values for all samples are shown in Figure 3. In the considered temperature range the band gap energy E
04 decreases by approximately 80 meV with increasing T
s for a given SC. Additionally, a slight dependency of the E
04 on SC can be extracted. For constant T
s, an average difference of approximately 30 meV between SC of 4% and 10% was observed. Considering all a-Si:H absorber layers, the optical band gap could be varied by 120 meV from 1.86 eV to 1.98 eV.Figure 3
E
04 band gap as a function of silane concentrations for a-Si:H layers deposited at different substrate temperatures T
s. The dotted lines are to guide the eye.Figure4 presents the evaluated E
04 values as a function of c
H for different silane concentrations. The data of this study is in line with the commonly observed trend for an increase of the band gap energy with increasing fraction of bound hydrogen in the a-Si:H material [32–34]. A possible explanation suggested that the hydrogen could be involved in breaking of weak Si–Si bonds responsible for the states in the top of the valence band. As a consequence, by increasing c
H, stronger Si–H bonds emerge, leading to states deep in the valence band and resulting in the valence band edge being shifted down. The conduction band remains unaffected and thus, the band gap energy increases [22, 32]. Other microscopic models involve a discussion of the effects of nanosized void configurations on the hydrogen incorporation in a-Si:H material [35, 36].Figure 4
Band gap energyE
04 as a function of the hydrogen content of deposited a-Si:H layers with different silane concentrations.The defect density is an important parameter for absorber materials, because electronic defects can act as recombination centers and lead to a deterioration of the device performance [37, 38]. In general, it is well accepted that a reduction in the deposition temperature (below 250°C) results in the increase in the defect density of a-Si:H material [31]. Here, by adjusting SC together with T
s, we can maintain a material with a reasonable electronic quality (low defect density) even at a very low deposition temperature of around 70°C. Figure 5 shows the absorption at an energy of 1.2 eV as a function of T
s for different SC of the a-Si:H layers. It is apparent that for substrate temperatures between 110°C and 250°C the absorption in the subgap region is similar for different silane concentrations investigated here and α
1
.
2 ranges between 4.2 cm−1 and 8.5 cm−1. At a temperature of 70°C, a distinct trend of decreasing α
1
.
2 with decreasing silane concentration was observed. The defect absorption in this region rapidly increases up to 28 cm−1 for SC of 10%. By reducing SC of the a-Si:H layers deposited at 70°C, α
1
.
2 continuously decreases to 6.5 cm−1 for SC of 4%. This value is similar to α
1
.
2 values of the layers deposited at higher temperatures. Hence, this result emphasizes that the electronic quality of the a-Si:H material deposited at low substrate temperatures (around 70°C) can be improved, in terms of defect absorption density, by using lower silane concentrations during the deposition. We note that even at low SC of 4%, the materials are still fully amorphous as evident from the Raman scattering experiments.Figure 5
Absorption coefficient at 1.2 eV as a function of substrate temperature for different silane concentrations of the intrinsic a-Si:H layer.


### 3.2. Solar Cells

#### 3.2.1. Single Junction Solar Cells

The investigated intrinsic a-Si:H absorber layers with different band gaps were subsequently applied in single junction solar cells in a p-i-n configuration. The solar cells were prepared with intrinsic a-Si:H layers at silane concentrations of 4% and 10%, in order to consider the entire range ofE
04 absorber layer band gaps (from 1.86 eV to 1.98 eV) for the relevant temperatures, shown in Figure 3.The performance of the a-Si:H solar cells as a function ofT
s is shown in Figure 6. Reducing T
s from 250°C down to 110°C, the open-circuit voltage V
OC increases with decreasing T
s and can be varied from 802 mV up to a value of 946 mV, achieved in the case of an absorber layer with SC of 4% and T
s of 130°C (Figure 6(b)). In this temperature range (250°C–110°C), the fill factor FF is only slightly affected and varies between 68.5% and 73.8% for SC of 10% and between 70.2% and 74.2% for SC of 4% (Figure 6(c)). The best efficiency η of 10.3% is obtained with an intrinsic a-Si:H layer with SC of 10%, deposited at 180°C (Figure 6(a)).Photovoltaic parameters of single junction solar cells (efficiency (a), open-circuit voltage (b), fill factor (c), and short circuit current density (d)) as a function of the substrate temperature of the intrinsic a-Si:H layer, with 10% and 4% silane concentration, respectively. The lines are to guide the eye.
(a)
(b)
(c)
(d)For even lowerT
s of 70°C all performance parameters tend to deteriorate considerably, as compared with the solar cells deposited at T
s of 110°C or above. In the 70°C temperature region increased defect absorption density α
1
.
2 (Figure 5.) and higher optical band gap E
04 (Figure 3) significantly impair the electronic material quality. Nevertheless, as already apparent in the previous section, through the use of lower SC during low-temperature deposition, a significant improvement of the optoelectronic properties of a-Si:H material can be achieved. When SC is reduced down to 4%, a significant improvement in V
OC and FF is observed in the case of solar cells deposited at temperatures below 180°C. At T
s of 70°C and SC of 4%, all parameters are enhanced, compared to the 70°C cell with SC of 10%, and η yields 7.7% (Figure 6(a)). This underlines that a combination of low deposition temperature together with reduced SC of the absorber layer is necessary to obtain high voltage solar cells with appropriate efficiency level.Figure6(d) presents the J
SC data of the solar cell series as a function of T
s. From 70°C to 180°C, J
SC increases with increasing T
s and, except for T
s of 70°C, solar cells with SC of 10% promote higher J
SC compared to cells with SC of 4%. This can be explained by the wider E
04 gap of a-Si:H layers with SC of 4%, in comparison to layers with SC of 10% (Figure 3). For T
s of 250°C however, J
SC decreases for both SC.The quantum efficiencies for the solar cell series (the same as in Figure6) deposited at SC of 4% and 10% at different T
s are shown in Figures 7(a) and 7(b), respectively. In the longer wavelength region the expected increase in QE with increasing T
s (i.e., decreasing E
04) is apparent for both SC. The QE in the shorter wavelength region reflects the trend of J
SC data shown in Figure 6(d). Thus, the decrease in total J
SC for T
s of 250°C (Figure 6(d)) is caused by the drop in current under short wavelength illumination and can be localized to the region of the solar cell where light of short wavelengths is mainly absorbed, that is, the region of the p-doped/intrinsic a-Si:H layer interface. At T
s of 250°C for the deposition of the intrinsic absorber layer, boron diffusion from the p-doped layer (deposited at 180°C) into the intrinsic a-Si:H layer is assisted, leading to a deterioration of the p-/i-interface, which can result in a reduction of the short wavelength QE [39]. In addition, as can be deduced from Figure 7(a), the QE for T
s of 70°C deteriorates over the entire wavelength range at SC of 10% but can be improved considerably by using a lower SC of 4% during the deposition of the intrinsic a-Si:H layer (Figure 7(b)).Quantum efficiency curves for single junction solar cells at silane concentration of (a) 10% and (b) 4% for differentT
s of the absorber layer.
(a)
(b)


#### 3.2.2. Tandem Junction Solar Cells

Several tandem junction combinations of two a-Si:H single junction solar cells have been realized. The top and bottom cell combinations along with the absorber layer parameters (SC and T
s) and photovoltaic device parameters are listed in Table 1. The bottom cell thickness was 400 nm and the top cell thickness was 110 nm, respectively, for all the tandem junction cells from Table 1.Table 1
Photovoltaic parameters of a-Si:H/a-Si:H tandem junction solar cells fabricated with varying SC andT
s of top and bottom cell.
Top cell
Bottom cell
S
C
top [%]
T
s
,
top     [°C]
S
C
bot  [%]
T
s
,
bot      [°C]
η [%]
V
OC [mV]
FF [%]
J
SC [mA/cm2]
V
MPP [mV]
4
250
4
250
8.3
1602
71.8
7.2
1325
10
180
4
250
8.7
1700
68.5
7.4
1410
4
110
4
110
9.5
1870
77.5
6.6
1580
10
180
10
180
9.8
1796
78.8
7
1555
4
130
10
180
10.5
1816
73.5
7.9
1520
4
130
4
130
9.7
1855
78.2
6.7
1595The data in Table1 confirms that a broad range of high open-circuit voltages can be achieved, ranging from 1601 mV to 1870 mV. In addition, the V
OC values of the tandem solar cells are in good agreement with the summed V
OC values of the respective individual subcells, proving a stable process control. To ensure efficient hydrogen production, the a-Si:H/a-Si:H photocathode should preferably provide high photocurrent and operate near its maximum power point (MPP) during photoelectrochemical water splitting. Thus, J
SC and V
MPP play an important role here. In this regard, the best two tandem cells listed in Table 1, with respect to J
SC and V
MPP, were chosen to be further optimized by means of current matching. The MPP voltage for all tandem cells is presented in the last column of Table 1. The highest V
MPP of 1595 mV is achieved by the tandem cell combining two subcells with SC of 4% and both T
s
,
top and T
s
,
bot of 130°C. The tandem cell consisting of a 4% SC top cell with T
s
,
top of 130°C and a 10% SC bottom cell with T
s
,
bot of 180°C exhibits the highest J
SC of 7.9 mA/cm2 and an η of 10.5%.In order to match the two subcells, with respect to the current, the thickness of the absorber layer of the top celld
top has been optimized. The two chosen tandem cells were deposited with d
top of 70 nm and 90 nm, respectively, and compared with the tandem cell comprising the 110 nm thick top cell. Figure 8 presents the respective photovoltaic parameters as a function of the top cell thickness.Photovoltaic parameters of the current matching series of two tandem junction solar cells deposited at different SC andT
s, as a function of their top cell thickness. The lines are to guide the eye.
(a)
(b)
(c)
(d)The matching series of the tandem cells with top and bottom absorber layers deposited at differentT
s and SC (filled squares in Figure 8) shows that V
OC was only marginally affected by a reduced top cell thickness and increased by 15 mV for d
top of 110 nm to 70 nm (Figure 8(b)). The J
SC decreased with decreasing d
top from 7.9 mA/cm2 for d
top of 110 nm to 6.7 mA/cm2 for d
top of 70 nm (Figure 8(d)). As current matching should enhance the J
SC [40], one can conclude that the tandem cell was better matched at d
top of 110 nm and became top cell limited by further reducing d
top. The fill factor is expected to decrease when top and bottom cells are matched [39]. This is observed in Figure 8(c) where FF decreased by 4.9% with increasing d
top. The highest η of 10.5% was achieved when bottom and top cell are current matched (Figure 8(a)).In the matching series of the second tandem cell, in which both top and bottom cell were deposited atT
s of 130°C and SC of 4% (open squares in Figure 8), the reduction of d
top leads to an increase in J
SC of 0.4 mA/cm2 from d
top of 110 nm to 70 nm (Figure 8(d)). The FF decreased by 2.6% with decreasing d
top (Figure 8(c)). The highest V
OC of 1871 mV was achieved for the tandem cell with d
top of 90 nm (Figure 8(b)), which also exhibits the highest η of 9.9%.The best two tandem cells, with respect toη, are summarized in Table 2.Table 2
Photovoltaic parameters of a-Si:H/a-Si:H tandem junction solar cells resulting from the current matching series.
Top cell
Bottom cell
S
C
top [%]
T
s
,
top     [°C]
S
C
bot  [%]
T
s
,
bot      [°C]
η [%]
V
OC [mV]
FF [%]
J
SC [mA/cm2]
V
MPP [mV]
4
130
10
180
10.5
1816
73.5
7.9
1520
4
130
4
130
9.9
1872
77.8
6.8
1600


### 3.3. Photoelectrochemical Measurements

The performance of the developed a-Si:H/a-Si:H tandem junction based photocathodes was examined and compared to the photovoltaic performance of the corresponding solar cells. For this purpose, four tandem cells covering up theV
OC range from 1600 mV to 1870 mV (from Tables 1 and 2) were chosen and the onset potential for cathodic current (E
onset), the potential at maximum power point (E
MPP), and the photocurrent density at 0 V versus RHE (J
RHE) were determined. Table 3 provides the photovoltaic and PEC device parameters of the respective photocathodes. E
MPP and J
RHE (from Table 3.) are extracted from cyclic voltammetry (CV) measurements depicted in Figure 9. The measurements were conducted with the four a-Si:H/a-Si:H photocathodes without any further surface or back contact modification.Table 3
Photovoltaic and PEC parameters of a-Si:H/a-Si:H tandem junction based photocathodes.
Photocathode
η                 [%]
V
OC [mV]
FF   [%]
J
SC [mA/cm2]
V
MPP [mV]
E
onset  [mV versus RHE]
E
MPP  [mV versus RHE]
J
RHE [mA/cm2]
A
8.3
1602
71.8
7.2
1325
1196
832
5.9
B
8.7
1700
68.5
7.4
1410
1338
960
6.1
C
9.9
1872
77.8
6.8
1600
1495
1127
5.3
D
10.5
1816
73.5
7.9
1520
1450
1098
6.3Figure 9
Cyclic voltammetry measurement of a-Si:H/a-Si:H photocathodes (listed in Table3). Measurements were performed in 0.1 M H2SO4 solution (pH 1) under 100 mW/cm2 illumination intensity at a scan rate of 30 mV s−1. The dark current is shown as a black dashed curve.The disparity inJ
SC and J
RHE values originates from the different illumination source used in photovoltaic and PEC arrangement, respectively. AM 1.5 illumination provides higher light intensities in the wavelength range between 300 nm and 800 nm, where a-Si:H possesses the highest light absorption, compared to the halogen lamp used in the PEC arrangement. Notwithstanding this, photovoltaic and photoelectrochemical performances show the same trends. The higher the V
MPP of a photocathode is, the higher its E
MPP is. A similar correlation was observed for the current densities J
SC andJ
RHE. This result provides evidence that the photoelectrochemical performance of the photocathode can be partly deduced from the photovoltaic performance of the solar cell, which is highly important for further development.Photocathode D exhibits the highest photocurrent density of 6.3 mA/cm2 at 0 V versus RHE. E
MPP of photocathode D is 1098 mV versus RHE and thus slightly lower than the highest E
MPP value of 1127 mV versus RHE and is promoted by photocathode C, which in contrast has a lower J
RHE of 5.3 mA/cm2. The onset potential of cathodic current was taken as the value at a photocurrent density of −0.5 mA/cm2. Hereby, E
onset data (from Table 3) reflects V
OC data obtained in photovoltaic arrangement and shifts anodically from photocathodes A and B (1196 mV versus RHE and 1338 mV versus RHE, resp.) to photocathodes D and C (1450 mV versus RHE and 1495 mV versus RHE, resp.). Efficient self-contained solar water splitting requires a photocathode with high photocurrent at a positive potential over 1.23 V versus RHE [13]. Thus, within this study photocathodes C and D are the most suitable candidates for PEC water splitting devices.To describe the effect of different back contact catalysts, photocathode C was further investigated, as it exhibits the highestE
onset. Figure 10 shows the cyclic voltammograms measured for a-Si:H/a-Si:H photocathodes with no metallic back contact (solid grey curve, silicon in contact to the electrolyte), with ZnO/Ag back contact (solid black curve, photocathode C from Table 3) and with ZnO/Ag/Pt back contact (dashed curve). The low photocurrent (3.1 mA/cm2 at 0 V versus RHE) of the back contact-free photocathode is caused by optical losses, due to the missing metallic back-reflector. Additionally, the injection of charge carriers into the electrolyte is kinetically limited for metal-free surfaces, resulting in high overpotentials for water reduction [24]. This is confirmed by the significant shift of E
onset in negative bias direction (950 mV versus RHE). Unlike the photocathodes with metallic back contacts, forward and reverse scan of the bare a-Si:H/a-Si:H photocathode are shifted, revealing a protective or passivation effect of the metallic back contact [12]. No effect on J
RHE is observed when a thin 50 nm layer of platinum is deposited on the ZnO/Ag back contact. Both photocathodes with metallic back contact provide 5.3 mA/cm2 at 0 V versus RHE, proving that the optics of the photocathode remains mainly unaffected by the platinum layer.Figure 10
Cyclic voltammetry measurement of a-Si:H/a-Si:H photocathodes with no metallic back contact (solid grey curve), with ZnO/Ag back contact (solid black curve, photocathode C from Table3) and with ZnO/Ag/Pt back contact (dashed curve), in 0.1 M H2SO4 solution (pH 1) under 100 mW/cm2 illumination intensity at a scan rate of 30 mV s−1.However, with respect to the ZnO/Ag back contact photocathode, the photocathode with ZnO/Ag/Pt back contact shows a significant shift inE
onset in positive bias direction (270 mV) and is measured to be 1765 mV versus RHE. Furthermore, an impressively high E
MPP of 1457 mV versus RHE is exhibited by the platinized photocathode, with a photocurrent density of 4.4 mA/cm2, which accentuates both, the excellent catalytic activity of platinum as a catalyst for the hydrogen evolution reaction (HER) and its viable combination with the a-Si:H/a-Si:H/ZnO/Ag tandem device. In fact, the photocurrent density of 4.4 mA/cm2 at 1457 mV versus RHE is the highest reported value at such positive potentials for a-Si:H single and tandem solar cell based photoelectrodes [13, 41–43]. Photocathodes, which can provide high photocurrents at more positive potentials, could effectively attenuate the catalytic activity requirement (i.e., reduction of overpotential losses) of the anode in a two-electrode PEC system. Taking additionally into account the nonideal illumination of the a-Si:H/a-Si:H photocathode by the halogen lamp, our result a fortiori demonstrates the capability for a direct application of the developed photocathodes in efficient and self-contained PEC devices. A prior study on amorphous tandem junction solar cell used as photocathode reported E
onset values of 1.35 V versus the saturated calomel electrode (SCE) along with a photocurrent density of 2 mA/cm2 at 0 V versus SCE [41]. A similar approach led to 1.43 V versus SCE and 3 mA/cm2 at 0 V versus SCE [42]. Furthermore, this study reported on a tandem junction a-Si:H solar cell deposited on a crystalline silicon wafer. This triple cell concept promoted a high E
onset of 1.84 V versus RHE, but a rather low photocurrent density of 2.4 mA/cm2. The most recent work focusing on the direct application of a-Si:H solar cells as photoelectrodes was done by Lin et al. [13]. Here, single junction a-Si:H solar cells were tested as photocathodes under the irradiation by a simulated sunlight (AM 1.5 light spectrum, 100 mW/cm2) and exhibited an E
onset of 0.93 V versus RHE and photocurrent densities of 6.1 and 9.4 mA/cm2 at 0.8 and 0.7 V versus RHE, respectively.As integrated water splitting devices require chemical-resistant electrodes, stability issues of the silicon solar cells in contact with aqueous solutions need to be addressed and are currently under investigation. Furthermore, light induced degradation plays a major role for a-Si:H based devices and has been extensively investigated [18, 43]. In the present study this was not yet a focus of our work. It should however be noticed that the important parameter V
OC is typically the least affected parameter upon prolonged illumination and in some cases even increases [44].


## 3.1. Material Properties of Intrinsic a-Si:H Absorber Layer

Since the band gap of amorphous a-Si:H correlates with the hydrogen content of the material [22], FTIR measurements were conducted to estimate the amount of hydrogen in the samples. Figure 2 shows the hydrogen content as a function of the substrate temperature for a-Si:H layers with different silane concentrations. The general trend indicates that a decrease in T
s results in an increase of c
H. This behavior can be attributed to the increased desorption of hydrogen at higher substrate temperatures. As deposition temperature increases, the desorption of hydrogen atoms is energetically favored and thus less hydrogen is incorporated into the layer [31]. No clear dependency of c
H on the silane concentration in the chosen range was visible in FTIR data. The hydrogen content in all samples investigated is between 10.6% and 16.8%.Figure 2
Hydrogen content as a function of the substrate temperature during deposition of the a-Si:H layers, with different silane concentrations.For the determination of the band gap energy of the a-Si:H material, absorption spectra for differentT
s and SC are measured and subsequently evaluated, with respect to E
04 as an estimation of the band gap. The evaluated E
04 values for all samples are shown in Figure 3. In the considered temperature range the band gap energy E
04 decreases by approximately 80 meV with increasing T
s for a given SC. Additionally, a slight dependency of the E
04 on SC can be extracted. For constant T
s, an average difference of approximately 30 meV between SC of 4% and 10% was observed. Considering all a-Si:H absorber layers, the optical band gap could be varied by 120 meV from 1.86 eV to 1.98 eV.Figure 3
E
04 band gap as a function of silane concentrations for a-Si:H layers deposited at different substrate temperatures T
s. The dotted lines are to guide the eye.Figure4 presents the evaluated E
04 values as a function of c
H for different silane concentrations. The data of this study is in line with the commonly observed trend for an increase of the band gap energy with increasing fraction of bound hydrogen in the a-Si:H material [32–34]. A possible explanation suggested that the hydrogen could be involved in breaking of weak Si–Si bonds responsible for the states in the top of the valence band. As a consequence, by increasing c
H, stronger Si–H bonds emerge, leading to states deep in the valence band and resulting in the valence band edge being shifted down. The conduction band remains unaffected and thus, the band gap energy increases [22, 32]. Other microscopic models involve a discussion of the effects of nanosized void configurations on the hydrogen incorporation in a-Si:H material [35, 36].Figure 4
Band gap energyE
04 as a function of the hydrogen content of deposited a-Si:H layers with different silane concentrations.The defect density is an important parameter for absorber materials, because electronic defects can act as recombination centers and lead to a deterioration of the device performance [37, 38]. In general, it is well accepted that a reduction in the deposition temperature (below 250°C) results in the increase in the defect density of a-Si:H material [31]. Here, by adjusting SC together with T
s, we can maintain a material with a reasonable electronic quality (low defect density) even at a very low deposition temperature of around 70°C. Figure 5 shows the absorption at an energy of 1.2 eV as a function of T
s for different SC of the a-Si:H layers. It is apparent that for substrate temperatures between 110°C and 250°C the absorption in the subgap region is similar for different silane concentrations investigated here and α
1
.
2 ranges between 4.2 cm−1 and 8.5 cm−1. At a temperature of 70°C, a distinct trend of decreasing α
1
.
2 with decreasing silane concentration was observed. The defect absorption in this region rapidly increases up to 28 cm−1 for SC of 10%. By reducing SC of the a-Si:H layers deposited at 70°C, α
1
.
2 continuously decreases to 6.5 cm−1 for SC of 4%. This value is similar to α
1
.
2 values of the layers deposited at higher temperatures. Hence, this result emphasizes that the electronic quality of the a-Si:H material deposited at low substrate temperatures (around 70°C) can be improved, in terms of defect absorption density, by using lower silane concentrations during the deposition. We note that even at low SC of 4%, the materials are still fully amorphous as evident from the Raman scattering experiments.Figure 5
Absorption coefficient at 1.2 eV as a function of substrate temperature for different silane concentrations of the intrinsic a-Si:H layer.


## 3.2. Solar Cells

### 3.2.1. Single Junction Solar Cells

The investigated intrinsic a-Si:H absorber layers with different band gaps were subsequently applied in single junction solar cells in a p-i-n configuration. The solar cells were prepared with intrinsic a-Si:H layers at silane concentrations of 4% and 10%, in order to consider the entire range ofE
04 absorber layer band gaps (from 1.86 eV to 1.98 eV) for the relevant temperatures, shown in Figure 3.The performance of the a-Si:H solar cells as a function ofT
s is shown in Figure 6. Reducing T
s from 250°C down to 110°C, the open-circuit voltage V
OC increases with decreasing T
s and can be varied from 802 mV up to a value of 946 mV, achieved in the case of an absorber layer with SC of 4% and T
s of 130°C (Figure 6(b)). In this temperature range (250°C–110°C), the fill factor FF is only slightly affected and varies between 68.5% and 73.8% for SC of 10% and between 70.2% and 74.2% for SC of 4% (Figure 6(c)). The best efficiency η of 10.3% is obtained with an intrinsic a-Si:H layer with SC of 10%, deposited at 180°C (Figure 6(a)).Photovoltaic parameters of single junction solar cells (efficiency (a), open-circuit voltage (b), fill factor (c), and short circuit current density (d)) as a function of the substrate temperature of the intrinsic a-Si:H layer, with 10% and 4% silane concentration, respectively. The lines are to guide the eye.
(a)
(b)
(c)
(d)For even lowerT
s of 70°C all performance parameters tend to deteriorate considerably, as compared with the solar cells deposited at T
s of 110°C or above. In the 70°C temperature region increased defect absorption density α
1
.
2 (Figure 5.) and higher optical band gap E
04 (Figure 3) significantly impair the electronic material quality. Nevertheless, as already apparent in the previous section, through the use of lower SC during low-temperature deposition, a significant improvement of the optoelectronic properties of a-Si:H material can be achieved. When SC is reduced down to 4%, a significant improvement in V
OC and FF is observed in the case of solar cells deposited at temperatures below 180°C. At T
s of 70°C and SC of 4%, all parameters are enhanced, compared to the 70°C cell with SC of 10%, and η yields 7.7% (Figure 6(a)). This underlines that a combination of low deposition temperature together with reduced SC of the absorber layer is necessary to obtain high voltage solar cells with appropriate efficiency level.Figure6(d) presents the J
SC data of the solar cell series as a function of T
s. From 70°C to 180°C, J
SC increases with increasing T
s and, except for T
s of 70°C, solar cells with SC of 10% promote higher J
SC compared to cells with SC of 4%. This can be explained by the wider E
04 gap of a-Si:H layers with SC of 4%, in comparison to layers with SC of 10% (Figure 3). For T
s of 250°C however, J
SC decreases for both SC.The quantum efficiencies for the solar cell series (the same as in Figure6) deposited at SC of 4% and 10% at different T
s are shown in Figures 7(a) and 7(b), respectively. In the longer wavelength region the expected increase in QE with increasing T
s (i.e., decreasing E
04) is apparent for both SC. The QE in the shorter wavelength region reflects the trend of J
SC data shown in Figure 6(d). Thus, the decrease in total J
SC for T
s of 250°C (Figure 6(d)) is caused by the drop in current under short wavelength illumination and can be localized to the region of the solar cell where light of short wavelengths is mainly absorbed, that is, the region of the p-doped/intrinsic a-Si:H layer interface. At T
s of 250°C for the deposition of the intrinsic absorber layer, boron diffusion from the p-doped layer (deposited at 180°C) into the intrinsic a-Si:H layer is assisted, leading to a deterioration of the p-/i-interface, which can result in a reduction of the short wavelength QE [39]. In addition, as can be deduced from Figure 7(a), the QE for T
s of 70°C deteriorates over the entire wavelength range at SC of 10% but can be improved considerably by using a lower SC of 4% during the deposition of the intrinsic a-Si:H layer (Figure 7(b)).Quantum efficiency curves for single junction solar cells at silane concentration of (a) 10% and (b) 4% for differentT
s of the absorber layer.
(a)
(b)


### 3.2.2. Tandem Junction Solar Cells

Several tandem junction combinations of two a-Si:H single junction solar cells have been realized. The top and bottom cell combinations along with the absorber layer parameters (SC and T
s) and photovoltaic device parameters are listed in Table 1. The bottom cell thickness was 400 nm and the top cell thickness was 110 nm, respectively, for all the tandem junction cells from Table 1.Table 1
Photovoltaic parameters of a-Si:H/a-Si:H tandem junction solar cells fabricated with varying SC andT
s of top and bottom cell.
Top cell
Bottom cell
S
C
top [%]
T
s
,
top     [°C]
S
C
bot  [%]
T
s
,
bot      [°C]
η [%]
V
OC [mV]
FF [%]
J
SC [mA/cm2]
V
MPP [mV]
4
250
4
250
8.3
1602
71.8
7.2
1325
10
180
4
250
8.7
1700
68.5
7.4
1410
4
110
4
110
9.5
1870
77.5
6.6
1580
10
180
10
180
9.8
1796
78.8
7
1555
4
130
10
180
10.5
1816
73.5
7.9
1520
4
130
4
130
9.7
1855
78.2
6.7
1595The data in Table1 confirms that a broad range of high open-circuit voltages can be achieved, ranging from 1601 mV to 1870 mV. In addition, the V
OC values of the tandem solar cells are in good agreement with the summed V
OC values of the respective individual subcells, proving a stable process control. To ensure efficient hydrogen production, the a-Si:H/a-Si:H photocathode should preferably provide high photocurrent and operate near its maximum power point (MPP) during photoelectrochemical water splitting. Thus, J
SC and V
MPP play an important role here. In this regard, the best two tandem cells listed in Table 1, with respect to J
SC and V
MPP, were chosen to be further optimized by means of current matching. The MPP voltage for all tandem cells is presented in the last column of Table 1. The highest V
MPP of 1595 mV is achieved by the tandem cell combining two subcells with SC of 4% and both T
s
,
top and T
s
,
bot of 130°C. The tandem cell consisting of a 4% SC top cell with T
s
,
top of 130°C and a 10% SC bottom cell with T
s
,
bot of 180°C exhibits the highest J
SC of 7.9 mA/cm2 and an η of 10.5%.In order to match the two subcells, with respect to the current, the thickness of the absorber layer of the top celld
top has been optimized. The two chosen tandem cells were deposited with d
top of 70 nm and 90 nm, respectively, and compared with the tandem cell comprising the 110 nm thick top cell. Figure 8 presents the respective photovoltaic parameters as a function of the top cell thickness.Photovoltaic parameters of the current matching series of two tandem junction solar cells deposited at different SC andT
s, as a function of their top cell thickness. The lines are to guide the eye.
(a)
(b)
(c)
(d)The matching series of the tandem cells with top and bottom absorber layers deposited at differentT
s and SC (filled squares in Figure 8) shows that V
OC was only marginally affected by a reduced top cell thickness and increased by 15 mV for d
top of 110 nm to 70 nm (Figure 8(b)). The J
SC decreased with decreasing d
top from 7.9 mA/cm2 for d
top of 110 nm to 6.7 mA/cm2 for d
top of 70 nm (Figure 8(d)). As current matching should enhance the J
SC [40], one can conclude that the tandem cell was better matched at d
top of 110 nm and became top cell limited by further reducing d
top. The fill factor is expected to decrease when top and bottom cells are matched [39]. This is observed in Figure 8(c) where FF decreased by 4.9% with increasing d
top. The highest η of 10.5% was achieved when bottom and top cell are current matched (Figure 8(a)).In the matching series of the second tandem cell, in which both top and bottom cell were deposited atT
s of 130°C and SC of 4% (open squares in Figure 8), the reduction of d
top leads to an increase in J
SC of 0.4 mA/cm2 from d
top of 110 nm to 70 nm (Figure 8(d)). The FF decreased by 2.6% with decreasing d
top (Figure 8(c)). The highest V
OC of 1871 mV was achieved for the tandem cell with d
top of 90 nm (Figure 8(b)), which also exhibits the highest η of 9.9%.The best two tandem cells, with respect toη, are summarized in Table 2.Table 2
Photovoltaic parameters of a-Si:H/a-Si:H tandem junction solar cells resulting from the current matching series.
Top cell
Bottom cell
S
C
top [%]
T
s
,
top     [°C]
S
C
bot  [%]
T
s
,
bot      [°C]
η [%]
V
OC [mV]
FF [%]
J
SC [mA/cm2]
V
MPP [mV]
4
130
10
180
10.5
1816
73.5
7.9
1520
4
130
4
130
9.9
1872
77.8
6.8
1600


## 3.2.1. Single Junction Solar Cells

The investigated intrinsic a-Si:H absorber layers with different band gaps were subsequently applied in single junction solar cells in a p-i-n configuration. The solar cells were prepared with intrinsic a-Si:H layers at silane concentrations of 4% and 10%, in order to consider the entire range ofE
04 absorber layer band gaps (from 1.86 eV to 1.98 eV) for the relevant temperatures, shown in Figure 3.The performance of the a-Si:H solar cells as a function ofT
s is shown in Figure 6. Reducing T
s from 250°C down to 110°C, the open-circuit voltage V
OC increases with decreasing T
s and can be varied from 802 mV up to a value of 946 mV, achieved in the case of an absorber layer with SC of 4% and T
s of 130°C (Figure 6(b)). In this temperature range (250°C–110°C), the fill factor FF is only slightly affected and varies between 68.5% and 73.8% for SC of 10% and between 70.2% and 74.2% for SC of 4% (Figure 6(c)). The best efficiency η of 10.3% is obtained with an intrinsic a-Si:H layer with SC of 10%, deposited at 180°C (Figure 6(a)).Photovoltaic parameters of single junction solar cells (efficiency (a), open-circuit voltage (b), fill factor (c), and short circuit current density (d)) as a function of the substrate temperature of the intrinsic a-Si:H layer, with 10% and 4% silane concentration, respectively. The lines are to guide the eye.
(a)
(b)
(c)
(d)For even lowerT
s of 70°C all performance parameters tend to deteriorate considerably, as compared with the solar cells deposited at T
s of 110°C or above. In the 70°C temperature region increased defect absorption density α
1
.
2 (Figure 5.) and higher optical band gap E
04 (Figure 3) significantly impair the electronic material quality. Nevertheless, as already apparent in the previous section, through the use of lower SC during low-temperature deposition, a significant improvement of the optoelectronic properties of a-Si:H material can be achieved. When SC is reduced down to 4%, a significant improvement in V
OC and FF is observed in the case of solar cells deposited at temperatures below 180°C. At T
s of 70°C and SC of 4%, all parameters are enhanced, compared to the 70°C cell with SC of 10%, and η yields 7.7% (Figure 6(a)). This underlines that a combination of low deposition temperature together with reduced SC of the absorber layer is necessary to obtain high voltage solar cells with appropriate efficiency level.Figure6(d) presents the J
SC data of the solar cell series as a function of T
s. From 70°C to 180°C, J
SC increases with increasing T
s and, except for T
s of 70°C, solar cells with SC of 10% promote higher J
SC compared to cells with SC of 4%. This can be explained by the wider E
04 gap of a-Si:H layers with SC of 4%, in comparison to layers with SC of 10% (Figure 3). For T
s of 250°C however, J
SC decreases for both SC.The quantum efficiencies for the solar cell series (the same as in Figure6) deposited at SC of 4% and 10% at different T
s are shown in Figures 7(a) and 7(b), respectively. In the longer wavelength region the expected increase in QE with increasing T
s (i.e., decreasing E
04) is apparent for both SC. The QE in the shorter wavelength region reflects the trend of J
SC data shown in Figure 6(d). Thus, the decrease in total J
SC for T
s of 250°C (Figure 6(d)) is caused by the drop in current under short wavelength illumination and can be localized to the region of the solar cell where light of short wavelengths is mainly absorbed, that is, the region of the p-doped/intrinsic a-Si:H layer interface. At T
s of 250°C for the deposition of the intrinsic absorber layer, boron diffusion from the p-doped layer (deposited at 180°C) into the intrinsic a-Si:H layer is assisted, leading to a deterioration of the p-/i-interface, which can result in a reduction of the short wavelength QE [39]. In addition, as can be deduced from Figure 7(a), the QE for T
s of 70°C deteriorates over the entire wavelength range at SC of 10% but can be improved considerably by using a lower SC of 4% during the deposition of the intrinsic a-Si:H layer (Figure 7(b)).Quantum efficiency curves for single junction solar cells at silane concentration of (a) 10% and (b) 4% for differentT
s of the absorber layer.
(a)
(b)


## 3.2.2. Tandem Junction Solar Cells

Several tandem junction combinations of two a-Si:H single junction solar cells have been realized. The top and bottom cell combinations along with the absorber layer parameters (SC and T
s) and photovoltaic device parameters are listed in Table 1. The bottom cell thickness was 400 nm and the top cell thickness was 110 nm, respectively, for all the tandem junction cells from Table 1.Table 1
Photovoltaic parameters of a-Si:H/a-Si:H tandem junction solar cells fabricated with varying SC andT
s of top and bottom cell.
Top cell
Bottom cell
S
C
top [%]
T
s
,
top     [°C]
S
C
bot  [%]
T
s
,
bot      [°C]
η [%]
V
OC [mV]
FF [%]
J
SC [mA/cm2]
V
MPP [mV]
4
250
4
250
8.3
1602
71.8
7.2
1325
10
180
4
250
8.7
1700
68.5
7.4
1410
4
110
4
110
9.5
1870
77.5
6.6
1580
10
180
10
180
9.8
1796
78.8
7
1555
4
130
10
180
10.5
1816
73.5
7.9
1520
4
130
4
130
9.7
1855
78.2
6.7
1595The data in Table1 confirms that a broad range of high open-circuit voltages can be achieved, ranging from 1601 mV to 1870 mV. In addition, the V
OC values of the tandem solar cells are in good agreement with the summed V
OC values of the respective individual subcells, proving a stable process control. To ensure efficient hydrogen production, the a-Si:H/a-Si:H photocathode should preferably provide high photocurrent and operate near its maximum power point (MPP) during photoelectrochemical water splitting. Thus, J
SC and V
MPP play an important role here. In this regard, the best two tandem cells listed in Table 1, with respect to J
SC and V
MPP, were chosen to be further optimized by means of current matching. The MPP voltage for all tandem cells is presented in the last column of Table 1. The highest V
MPP of 1595 mV is achieved by the tandem cell combining two subcells with SC of 4% and both T
s
,
top and T
s
,
bot of 130°C. The tandem cell consisting of a 4% SC top cell with T
s
,
top of 130°C and a 10% SC bottom cell with T
s
,
bot of 180°C exhibits the highest J
SC of 7.9 mA/cm2 and an η of 10.5%.In order to match the two subcells, with respect to the current, the thickness of the absorber layer of the top celld
top has been optimized. The two chosen tandem cells were deposited with d
top of 70 nm and 90 nm, respectively, and compared with the tandem cell comprising the 110 nm thick top cell. Figure 8 presents the respective photovoltaic parameters as a function of the top cell thickness.Photovoltaic parameters of the current matching series of two tandem junction solar cells deposited at different SC andT
s, as a function of their top cell thickness. The lines are to guide the eye.
(a)
(b)
(c)
(d)The matching series of the tandem cells with top and bottom absorber layers deposited at differentT
s and SC (filled squares in Figure 8) shows that V
OC was only marginally affected by a reduced top cell thickness and increased by 15 mV for d
top of 110 nm to 70 nm (Figure 8(b)). The J
SC decreased with decreasing d
top from 7.9 mA/cm2 for d
top of 110 nm to 6.7 mA/cm2 for d
top of 70 nm (Figure 8(d)). As current matching should enhance the J
SC [40], one can conclude that the tandem cell was better matched at d
top of 110 nm and became top cell limited by further reducing d
top. The fill factor is expected to decrease when top and bottom cells are matched [39]. This is observed in Figure 8(c) where FF decreased by 4.9% with increasing d
top. The highest η of 10.5% was achieved when bottom and top cell are current matched (Figure 8(a)).In the matching series of the second tandem cell, in which both top and bottom cell were deposited atT
s of 130°C and SC of 4% (open squares in Figure 8), the reduction of d
top leads to an increase in J
SC of 0.4 mA/cm2 from d
top of 110 nm to 70 nm (Figure 8(d)). The FF decreased by 2.6% with decreasing d
top (Figure 8(c)). The highest V
OC of 1871 mV was achieved for the tandem cell with d
top of 90 nm (Figure 8(b)), which also exhibits the highest η of 9.9%.The best two tandem cells, with respect toη, are summarized in Table 2.Table 2
Photovoltaic parameters of a-Si:H/a-Si:H tandem junction solar cells resulting from the current matching series.
Top cell
Bottom cell
S
C
top [%]
T
s
,
top     [°C]
S
C
bot  [%]
T
s
,
bot      [°C]
η [%]
V
OC [mV]
FF [%]
J
SC [mA/cm2]
V
MPP [mV]
4
130
10
180
10.5
1816
73.5
7.9
1520
4
130
4
130
9.9
1872
77.8
6.8
1600


## 3.3. Photoelectrochemical Measurements

The performance of the developed a-Si:H/a-Si:H tandem junction based photocathodes was examined and compared to the photovoltaic performance of the corresponding solar cells. For this purpose, four tandem cells covering up theV
OC range from 1600 mV to 1870 mV (from Tables 1 and 2) were chosen and the onset potential for cathodic current (E
onset), the potential at maximum power point (E
MPP), and the photocurrent density at 0 V versus RHE (J
RHE) were determined. Table 3 provides the photovoltaic and PEC device parameters of the respective photocathodes. E
MPP and J
RHE (from Table 3.) are extracted from cyclic voltammetry (CV) measurements depicted in Figure 9. The measurements were conducted with the four a-Si:H/a-Si:H photocathodes without any further surface or back contact modification.Table 3
Photovoltaic and PEC parameters of a-Si:H/a-Si:H tandem junction based photocathodes.
Photocathode
η                 [%]
V
OC [mV]
FF   [%]
J
SC [mA/cm2]
V
MPP [mV]
E
onset  [mV versus RHE]
E
MPP  [mV versus RHE]
J
RHE [mA/cm2]
A
8.3
1602
71.8
7.2
1325
1196
832
5.9
B
8.7
1700
68.5
7.4
1410
1338
960
6.1
C
9.9
1872
77.8
6.8
1600
1495
1127
5.3
D
10.5
1816
73.5
7.9
1520
1450
1098
6.3Figure 9
Cyclic voltammetry measurement of a-Si:H/a-Si:H photocathodes (listed in Table3). Measurements were performed in 0.1 M H2SO4 solution (pH 1) under 100 mW/cm2 illumination intensity at a scan rate of 30 mV s−1. The dark current is shown as a black dashed curve.The disparity inJ
SC and J
RHE values originates from the different illumination source used in photovoltaic and PEC arrangement, respectively. AM 1.5 illumination provides higher light intensities in the wavelength range between 300 nm and 800 nm, where a-Si:H possesses the highest light absorption, compared to the halogen lamp used in the PEC arrangement. Notwithstanding this, photovoltaic and photoelectrochemical performances show the same trends. The higher the V
MPP of a photocathode is, the higher its E
MPP is. A similar correlation was observed for the current densities J
SC andJ
RHE. This result provides evidence that the photoelectrochemical performance of the photocathode can be partly deduced from the photovoltaic performance of the solar cell, which is highly important for further development.Photocathode D exhibits the highest photocurrent density of 6.3 mA/cm2 at 0 V versus RHE. E
MPP of photocathode D is 1098 mV versus RHE and thus slightly lower than the highest E
MPP value of 1127 mV versus RHE and is promoted by photocathode C, which in contrast has a lower J
RHE of 5.3 mA/cm2. The onset potential of cathodic current was taken as the value at a photocurrent density of −0.5 mA/cm2. Hereby, E
onset data (from Table 3) reflects V
OC data obtained in photovoltaic arrangement and shifts anodically from photocathodes A and B (1196 mV versus RHE and 1338 mV versus RHE, resp.) to photocathodes D and C (1450 mV versus RHE and 1495 mV versus RHE, resp.). Efficient self-contained solar water splitting requires a photocathode with high photocurrent at a positive potential over 1.23 V versus RHE [13]. Thus, within this study photocathodes C and D are the most suitable candidates for PEC water splitting devices.To describe the effect of different back contact catalysts, photocathode C was further investigated, as it exhibits the highestE
onset. Figure 10 shows the cyclic voltammograms measured for a-Si:H/a-Si:H photocathodes with no metallic back contact (solid grey curve, silicon in contact to the electrolyte), with ZnO/Ag back contact (solid black curve, photocathode C from Table 3) and with ZnO/Ag/Pt back contact (dashed curve). The low photocurrent (3.1 mA/cm2 at 0 V versus RHE) of the back contact-free photocathode is caused by optical losses, due to the missing metallic back-reflector. Additionally, the injection of charge carriers into the electrolyte is kinetically limited for metal-free surfaces, resulting in high overpotentials for water reduction [24]. This is confirmed by the significant shift of E
onset in negative bias direction (950 mV versus RHE). Unlike the photocathodes with metallic back contacts, forward and reverse scan of the bare a-Si:H/a-Si:H photocathode are shifted, revealing a protective or passivation effect of the metallic back contact [12]. No effect on J
RHE is observed when a thin 50 nm layer of platinum is deposited on the ZnO/Ag back contact. Both photocathodes with metallic back contact provide 5.3 mA/cm2 at 0 V versus RHE, proving that the optics of the photocathode remains mainly unaffected by the platinum layer.Figure 10
Cyclic voltammetry measurement of a-Si:H/a-Si:H photocathodes with no metallic back contact (solid grey curve), with ZnO/Ag back contact (solid black curve, photocathode C from Table3) and with ZnO/Ag/Pt back contact (dashed curve), in 0.1 M H2SO4 solution (pH 1) under 100 mW/cm2 illumination intensity at a scan rate of 30 mV s−1.However, with respect to the ZnO/Ag back contact photocathode, the photocathode with ZnO/Ag/Pt back contact shows a significant shift inE
onset in positive bias direction (270 mV) and is measured to be 1765 mV versus RHE. Furthermore, an impressively high E
MPP of 1457 mV versus RHE is exhibited by the platinized photocathode, with a photocurrent density of 4.4 mA/cm2, which accentuates both, the excellent catalytic activity of platinum as a catalyst for the hydrogen evolution reaction (HER) and its viable combination with the a-Si:H/a-Si:H/ZnO/Ag tandem device. In fact, the photocurrent density of 4.4 mA/cm2 at 1457 mV versus RHE is the highest reported value at such positive potentials for a-Si:H single and tandem solar cell based photoelectrodes [13, 41–43]. Photocathodes, which can provide high photocurrents at more positive potentials, could effectively attenuate the catalytic activity requirement (i.e., reduction of overpotential losses) of the anode in a two-electrode PEC system. Taking additionally into account the nonideal illumination of the a-Si:H/a-Si:H photocathode by the halogen lamp, our result a fortiori demonstrates the capability for a direct application of the developed photocathodes in efficient and self-contained PEC devices. A prior study on amorphous tandem junction solar cell used as photocathode reported E
onset values of 1.35 V versus the saturated calomel electrode (SCE) along with a photocurrent density of 2 mA/cm2 at 0 V versus SCE [41]. A similar approach led to 1.43 V versus SCE and 3 mA/cm2 at 0 V versus SCE [42]. Furthermore, this study reported on a tandem junction a-Si:H solar cell deposited on a crystalline silicon wafer. This triple cell concept promoted a high E
onset of 1.84 V versus RHE, but a rather low photocurrent density of 2.4 mA/cm2. The most recent work focusing on the direct application of a-Si:H solar cells as photoelectrodes was done by Lin et al. [13]. Here, single junction a-Si:H solar cells were tested as photocathodes under the irradiation by a simulated sunlight (AM 1.5 light spectrum, 100 mW/cm2) and exhibited an E
onset of 0.93 V versus RHE and photocurrent densities of 6.1 and 9.4 mA/cm2 at 0.8 and 0.7 V versus RHE, respectively.As integrated water splitting devices require chemical-resistant electrodes, stability issues of the silicon solar cells in contact with aqueous solutions need to be addressed and are currently under investigation. Furthermore, light induced degradation plays a major role for a-Si:H based devices and has been extensively investigated [18, 43]. In the present study this was not yet a focus of our work. It should however be noticed that the important parameter V
OC is typically the least affected parameter upon prolonged illumination and in some cases even increases [44].


## 4. Conclusion

We presented the development of a-Si:H/a-Si:H based photocathodes for efficient hydrogen production. By varying the substrate temperature and SiH4 to total gas-flow concentration during the deposition of intrinsic a-Si:H absorber layers, we demonstrated that, in the case of a-Si:H/a-Si:H tandem cells, the optical and electrical properties of the a-Si:H subcells can be tuned and provide an extended V
OC range in tandem devices. It was found that the V
OC of tandem solar cells, with efficiencies around 10%, could systematically be adjusted between 1600 mV and 1870 mV.Furthermore, the performance of the developed tandem junction solar cells as photocathodes was demonstrated in a PEC arrangement. In particular, a-Si:H/a-Si:H photocathodes with ZnO/Ag/Pt back contact exhibited excellent onset potentials over 1760 mV versus RHE with a photocurrent density of 5.3 mA/cm2 at 0 V versus RHE and thus fulfill the main thermodynamic requirements to generate H2. The presented approach exploits a strategy of an efficient and low-cost route to solar hydrogen production based on amorphous thin film silicon tandem junction solar cells at a level which is sufficient for technological use.



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